Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

Below, you can either scroll through the complete list of our annually published material, search for a specific author or term via the free text search, or use the interactive keyword cloud to get to know our research strengths. You can also review the publication record of every Materials Chain member via his or her personal member’s page.

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  • 2020 • 180 Hydrothermal Stability of High-Surface-Area α-Al2O3and Its Use as a Support for Hydrothermally Stable Fischer-Tropsch Synthesis Catalysts
    Amrute, A.P. and Jeske, K. and Łodziana, Z. and Prieto, G. and Schüth, F.
    Chemistry of Materials 32 4369-4374 (2020)
    Nanocrystalline corundum synthesized by ball milling of boehmite is found to be exceptionally robust toward chemical weathering, a common problem of transition aluminas in different applications, most notably in the case of supported catalysts, which are exposed to hydrothermal reaction environments. Detailed characterization and surface cation coordination analysis indicate that the absence of tetrahedral Al species on corundum makes it stable toward chemical weathering. A cobalt catalyst developed using nano-α-Al2O3 as the support showed Fischer-Tropsch synthesis activity and selectivity comparable to the benchmark Co/γ-Al2O3 and remained stable over 250 h on-stream. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.0c01587
  • 2020 • 179 Detection and amplification of spin noise using scattered laser light in a quantum-dot microcavity
    Kamenskii, A.N. and Petrov, M.Y. and Kozlov, G.G. and Zapasskii, V.S. and Scholz, S.E. and Sgroi, C. and Ludwig, Ar. and Wieck, A.D. and Bayer, M. and Greilich, A.
    Physical Review B 101 (2020)
    Fundamental properties of the spin-noise signal formation in a quantum-dot microcavity are studied by measuring the angular characteristics of the scattered light intensity. A distributed Bragg reflector microcavity was used to enhance the light-matter interaction with an ensemble of n-doped (In,Ga)As/GaAs quantum dots, which allowed us to study subtle effects of coherent scattering at the quantum dot ensemble. Detecting the scattered light outside of the aperture of the transmitted light, we measured the basic electron spin properties, such as g factor and spin dephasing time. Further, we investigated the influence of the microcavity on the scattering distribution and possibilities of signal amplification by additional resonant excitation. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.041401
  • 2020 • 178 Electron tunneling dynamics between two-dimensional and zero-dimensional quantum systems: Contributions of momentum matching, higher subbands, and phonon-assisted processes
    Korsch, A.R. and Ebler, C. and Nguyen, G.N. and Scholz, S. and Wieck, A.D. and Ludwig, Ar.
    Physical Review B 102 (2020)
    We investigate tunneling dynamics of electrons from an ensemble of self-assembled InAs quantum dots into the subbands of a two-dimensional electron gas (2DEG). LO-phonon-assisted tunneling processes and tunneling into higher subbands of the 2DEG electronic structure cause distinct resonances in the evolution of the tunneling rate as a function of the energy detuning between quantum dot and 2DEG ground state. By devising a semiquantitative model, we identify the momentum mismatch between the quantum dot and 2DEG wave function as the crucial quantity governing the evolution of the tunneling rate. In particular, we demonstrate that this mechanism along with the availability of tunneling into the second 2DEG subband allows for tuning of the tunneling rate by more than two orders of magnitude. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.035413
  • 2020 • 177 Compatible deformation and extra strengthening by heterogeneous nanolayer composites
    Li, J. and Lu, W. and Gibson, J. and Zhang, S. and Korte-Kerzel, S. and Raabe, D.
    Scripta Materialia 179 30-35 (2020)
    A topologically heterogeneous microstructure design is introduced in a Cu/Zr nanolayered composite, in which each soft 100 nm Cu or Zr layer is surrounded on both sides by several hard 10 nm Cu/Zr bilayers. This design aims to impose a full geometrical constraint on all of the soft layers. Micropillar compression tests demonstrate that the composite deforms in a compatible fashion among the layers, in which no extrusion of the soft layers occurs. An elevated strength of 730 MPa is achieved in the composite compared with the strength prediction based on the linear rule of mixtures. © 2020
    view abstractdoi: 10.1016/j.scriptamat.2020.01.006
  • 2020 • 176 Correlative chemical and structural investigations of accelerated phase evolution in a nanocrystalline high entropy alloy
    Li, Y.J. and Kostka, A. and Savan, A. and Ludwig, Al.
    Scripta Materialia 183 122-126 (2020)
    Based on our recently-developed combinatorial processing platforms for accelerated investigations of phase evolution in multinary alloys, a novel correlative atom probe tomography and transmission electron microscopy approach is proposed to study phase stability in a nanocrystalline CrMnFeCoNi alloy. We observed that the material can decompose at 250 °C for 5 h or 300 °C for 1 h, having the same decomposed products as in its coarse-grained counterpart after annealing at 500 °C for 500 days. A low apparent activation energy for the diffusion of Ni in the nanocrystalline alloy is derived and explains the fast kinetics of phase decomposition in nanocrystalline alloys. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2020.03.016
  • 2020 • 175 Phase decomposition in a nanocrystalline CrCoNi alloy
    Li, Y.J. and Kostka, A. and Savan, A. and Ludwig, Al.
    Scripta Materialia 188 259-263 (2020)
    Phase stability of a nanocrystalline CrCoNi alloy is investigated using the combinatorial processing platform approach, which enables synthesis, processing and direct atomic-scale characterizations of alloys by atom probe tomography and transmission electron microscopy. Phase decomposition with formation of CoNi-rich phase occurs faster in the smaller (10 nm) grain-sized region than the larger one (20 nm), both being present in the same sample. Chemical analyses indicate that diffusion of Co and Cr plays an important role in phase decomposition. Comparison of phase stability between CrMnFeCoNi and CrCoNi implies that elemental segregation may promote phase decomposition by providing an additional chemical driving force for it. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2020.07.054
  • 2020 • 174 Real-Time Detection of Single Auger Recombination Events in a Self-Assembled Quantum Dot
    Lochner, P. and Kurzmann, A. and Kerski, J. and Stegmann, P. and König, J. and Wieck, A.D. and Ludwig, Ar. and Lorke, A. and Geller, M.
    Nano Letters 20 1631-1636 (2020)
    Auger recombination is a nonradiative process, where the recombination energy of an electron-hole pair is transferred to a third charge carrier. It is a common effect in colloidal quantum dots that quenches the radiative emission with an Auger recombination time below nanoseconds. In self-assembled QDs, the Auger recombination has been observed with a much longer recombination time on the order of microseconds. Here, we use two-color laser excitation on the exciton and trion transition in resonance fluorescence on a single self-assembled quantum dot to monitor in real-time single quantum events of the Auger process. Full counting statistics on the random telegraph signal give access to the cumulants and demonstrate the tunability of the Fano factor from a Poissonian to a sub-Poissonian distribution by Auger-mediated electron emission from the dot. Therefore, the Auger process can be used to tune optically the charge carrier occupation of the dot by the incident laser intensity, independently from the electron tunneling from the reservoir by the gate voltage. Our findings are not only highly relevant for the understanding of the Auger process but also demonstrate the perspective of the Auger effect for controlling precisely the charge state in a quantum system by optical means. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.9b04650
  • 2020 • 173 Radiative Auger process in the single-photon limit
    Löbl, M.C. and Spinnler, C. and Javadi, A. and Zhai, L. and Nguyen, G.N. and Ritzmann, J. and Midolo, L. and Lodahl, P. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J.
    Nature Nanotechnology 15 558-562 (2020)
    In a multi-electron atom, an excited electron can decay by emitting a photon. Typically, the leftover electrons are in their ground state. In a radiative Auger process, the leftover electrons are in an excited state and a redshifted photon is created1–4. In a semiconductor quantum dot, radiative Auger is predicted for charged excitons5. Here we report the observation of radiative Auger on trions in single quantum dots. For a trion, a photon is created on electron–hole recombination, leaving behind a single electron. The radiative Auger process promotes this additional (Auger) electron to a higher shell of the quantum dot. We show that the radiative Auger effect is a powerful probe of this single electron: the energy separations between the resonance fluorescence and the radiative Auger emission directly measure the single-particle splittings of the electronic states in the quantum dot with high precision. In semiconductors, these single-particle splittings are otherwise hard to access by optical means as particles are excited typically in pairs, as excitons. After the radiative Auger emission, the Auger carrier relaxes back to the lowest shell. Going beyond the original theoretical proposals, we show how applying quantum optics techniques to the radiative Auger photons gives access to the single-electron dynamics, notably relaxation and tunnelling. This is also hard to access by optical means: even for quasi-resonant p-shell excitation, electron relaxation takes place in the presence of a hole, complicating the relaxation dynamics. The radiative Auger effect can be exploited in other semiconductor nanostructures and quantum emitters in the solid state to determine the energy levels and the dynamics of a single carrier. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41565-020-0697-2
  • 2020 • 172 Directed Exciton Magnetic Polaron Formation in a Single Colloidal Mn2+:CdSe/CdS Quantum Dot
    Lorenz, S. and Erickson, C.S. and Riesner, M. and Gamelin, D.R. and Fainblat, R. and Bacher, G.
    Nano Letters 20 1896-1906 (2020)
    One of the most prominent signatures of transition-metal doping in colloidal nanocrystals is the formation of charge carrier-induced magnetization of the dopant spin sublattice, called exciton magnetic polaron (EMP). Understanding the direction of EMP formation, however, is still a major obstacle. Here, we present a series of temperature-dependent photoluminescence studies on single colloidal Mn2+:CdSe/CdS core/shell quantum dots (QDs) performed in a vector magnetic field providing a unique insight into the interaction between individual excitons and numerous magnetic impurities. The energy of the QD emission and its full width at half-maximum are controlled by the interplay of EMP formation and statistical magnetic fluctuations, in excellent agreement with theory. Most important, we give the first direct demonstration that anisotropy effects - hypothesized for more than a decade - dominate the direction of EMP formation. Our findings reveal a pathway for directing the orientation of optically induced magnetization in colloidal nanocrystals. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.9b05136
  • 2020 • 171 Tantalum and zirconium induced structural transitions at complex [111] tilt grain boundaries in copper
    Meiners, T. and Duarte, J.M. and Richter, G. and Dehm, G. and Liebscher, C.H.
    Acta Materialia 190 93-104 (2020)
    Alloying nanocrystalline copper (Cu) with immiscible elements, such as tantalum (Ta) and zirconium (Zr), is a promising technique to manipulate grain boundary properties and by this suppress grain growth at elevated temperatures. However, insights on the atomistic origins on the influence of impurity elements on grain boundaries are lacking. In this study, the atomistic effects of Ta and Zr on [111] tilt grain boundaries in Cu are investigated by high resolution scanning transmission electron microscopy techniques. In case of Ta, the formation of spherical, nano-scale precipitates in close vicinity to the grain boundaries is observed, but no sign of segregation. The particles induce a repelling force to migrating boundaries and act as local pinning points. The segregation of Zr is observed to occur either at confined grain boundary steps or homogeneously along the boundaries without steps. In both cases a strong disordering of the defect or grain boundary structure is revealed. Furthermore, at low Zr concentrations it induces structural grain boundary transitions and partial atomic reordering of the grain boundary structural units. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.02.064
  • 2020 • 170 Coherence of a Driven Electron Spin Qubit Actively Decoupled from Quasistatic Noise
    Nakajima, T. and Noiri, A. and Kawasaki, K. and Yoneda, J. and Stano, P. and Amaha, S. and Otsuka, T. and Takeda, K. and Delbecq, M.R. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Loss, D. and Tarucha, S.
    Physical Review X 10 (2020)
    The coherence of electron spin qubits in semiconductor quantum dots suffers mostly from low-frequency noise. During the past decade, efforts have been devoted to mitigate such noise by material engineering, leading to substantial enhancement of the spin dephasing time for an idling qubit. However, the role of the environmental noise during spin manipulation, which determines the control fidelity, is less understood. We demonstrate an electron spin qubit whose coherence in the driven evolution is limited by high-frequency charge noise rather than the quasistatic noise inherent to any semiconductor device. We employ a feedback-control technique to actively suppress the latter, demonstrating a π-flip gate fidelity as high as 99.04±0.23% in a gallium arsenide quantum dot. We show that the driven-evolution coherence is limited by the longitudinal noise at the Rabi frequency, whose spectrum resembles the 1/f noise observed in isotopically purified silicon qubits. © 2020 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevX.10.011060
  • 2020 • 169 Microstructure characteristics of non-monodisperse quantum dots: On the potential of transmission electron microscopy combined with X-ray diffraction
    Neumann, S. and Menter, C. and Mahmoud, A.S. and Segets, D. and Rafaja, D.
    CrystEngComm 22 3644-3655 (2020)
    Although the concept of quantum confinement was introduced more than thirty years ago, a wide application of quantum dots is still limited by the fact that monodisperse quantum dots with controlled optoelectronic properties are typically synthesized on a relatively small scale. Larger scale synthesis techniques are usually not able to produce monodisperse nanoparticles yet. In this contribution, we illustrate the capability of the combination of transmission electron microscopy and X-ray diffraction to reveal detailed and scale-bridging information about the complex microstructure of non-monodisperse quantum dots, which is the first step towards further upscaling of the techniques for production of quantum dots with controlled properties. As a model system, CdSe quantum dots synthesized using an automated robotic hot-injection method at different temperatures were chosen. The combined microstructure analytics revealed the size and shape of the CdSe nanocrystals and the kind, density and arrangement of planar defects. The role of the planar defects in the particle coarsening by oriented attachment and the effect of the planar fault arrangement on the phase constitution, on the crystallographic coherence of the counterparts and on the optoelectronic properties are discussed. © The Royal Society of Chemistry 2020.
    view abstractdoi: 10.1039/d0ce00312c
  • 2020 • 168 High temperature creep resistance of a thermally stable nanocrystalline Fe-5 at.% Zr steel
    Shan, G.B. and Chen, Y.Z. and Li, Y.J. and Zhang, C.Y. and Dong, H. and Cong, Y.B. and Zhang, W.X. and Huang, L.K. and Suo, T. and Liu, F.
    Scripta Materialia 179 1-5 (2020)
    The application of nanocrystalline (NC) materials at high temperatures is challenging due to their poor thermal stability or low creep resistance. Here we report that a thermally stable NC Fe-5 at.% Zr steel produced by High-Pressure-Thermal-Compression sintering exhibits an excellent creep resistance (with a creep rate of 3.92 × 10−8 s−1 at 923 K and under the applied stress of 250 MPa). The excellent creep resistance is ascribed to its highly stable NC structure stabilized by nano-sized precipitates. Mechanical testing suggests that the creep of the NC Fe-5 at.% Zr steel is controlled by dislocation activities rather than diffusion dominated mechanisms. © 2019
    view abstractdoi: 10.1016/j.scriptamat.2019.12.036
  • 2020 • 167 Influence of the cathode microstructure on the stability of inverted planar perovskite solar cells
    Sirotinskaya, S. and Schmechel, R. and Benson, N.
    RSC Advances 10 23653-23661 (2020)
    One of the main challenges for perovskite solar cells (PSC) is their environmental stability, as oxygen and water induced aging may result in mobile decomposition compounds, which can enhance the recombination rate and react with charge carrier extraction layers or the contact metallization. In this contribution the importance of the microstructure of the contact metallization on the environmental cell stability is investigated. For this purpose, the storage stability of inverted planar methylammonium lead iodide (MAPI)-based perovskite solar cells without encapsulation is tested, using the metals aluminum (Al), silver (Ag), gold (Au) and nickel (Ni) as representative cathode materials. For this study, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis of the different electrodes as well as the perovskite is correlated with PSC device current-voltage (J-V) and impedance measurements. Our findings substantiate that the metal microstructure has a significant influence on the PSC aging properties. While a strong perovskite decomposition and iodide diffusion to the contacts were detected for devices using Al, Ag or Au cathodes with a polycrystalline microstructure, these effects were strongly reduced when Ni metallization was employed, where a nanocrystalline microstructure was exhibited under the chosen process conditions. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ra00195c
  • 2020 • 166 Fabrication and optical characterization of photonic crystal nanocavities with electrodes for gate-defined quantum dots
    Tajiri, T. and Sakai, Y. and Kuruma, K. and Ji, S.M. and Kiyama, H. and Oiwa, A. and Ritzmann, J. and Ludwig, Ar. and Wieck, A.D. and Ota, Y. and Arakawa, Y. and Iwamoto, S.
    Japanese Journal of Applied Physics 59 (2020)
    Among various solid-state systems, gate-defined quantum dots (QD) with high scalability and controllability for single electron spin qubits are promising candidates to realize quantum spin-photon interface. The efficiency of the spin-photon interface is expected to be significantly enhanced by optical coupling of gate-defined QDs with photonic crystal (PhC) nanocavities. As the first step towards this optical coupling, we designed and experimentally demonstrated a PhC nanocavity with electrodes. The electrodes, which can form a single QD, were introduced on the top surfaces of two-dimensional PhC nanocavities with a position accuracy of a few tens of nanometers. Despite the electrodes, a resonant mode was confirmed for the PhC nanocavities through micro-photoluminescence spectroscopy. This work marks a crucial step towards optical coupling between gate-defined QDs and PhC nanocavities. © 2020 The Japan Society of Applied Physics.
    view abstractdoi: 10.7567/1347-4065/ab5b62
  • 2020 • 165 On-Chip Nanomechanical Filtering of Quantum-Dot Single-Photon Sources
    Zhou, X. and Uppu, R. and Liu, Z. and Papon, C. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lodahl, P. and Midolo, L.
    Laser and Photonics Reviews 14 (2020)
    Semiconductor quantum dots in photonic integrated circuits enable scaling quantum-information processing to many single photons and quantum-optical gates. Obtaining high-purity and coherent single photons from quantum dots requires spectral filtering to select individual excitonic transitions. Here, an on-chip wavelength-tunable filter integrated with a single-photon source, which preserves the optical properties of the emitter, is demonstrated. Nanomechanical motion is used for tuning the resonant wavelength over 10 nm, enabling operation at cryogenic temperatures, and single-photon emission from a quantum dot under non-resonant excitation is demonstrated without resorting to free-space optical filters. These results are key for the development of fully integrated de-multiplexing, multi-path photon encoding schemes, and multi-emitter circuits. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/lpor.201900404
  • 2019 • 164 Photon Noise Suppression by a Built-in Feedback Loop
    Al-Ashouri, A. and Kurzmann, A. and Merkel, B. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Nano Letters 19 135-141 (2019)
    Visionary quantum photonic networks need transform-limited single photons on demand. Resonance fluorescence on a quantum dot provides the access to a solid-state single photon source, where the environment is unfortunately the source of spin and charge noise that leads to fluctuations of the emission frequency and destroys the needed indistinguishability. We demonstrate a built-in stabilization approach for the photon stream, which relies solely on charge carrier dynamics of a two-dimensional hole gas inside a micropillar structure. The hole gas is fed by hole tunneling from field-ionized excitons and influences the energetic position of the excitonic transition by changing the local electric field at the position of the quantum dot. The standard deviation of the photon noise is suppressed by nearly 50% (noise power reduction of 6 dB) and it works in the developed micropillar structure for frequencies up to 1 kHz. This built-in feedback loop represents an easy way for photon noise suppression in large arrays of single photon emitters and promises to reach higher bandwidth by device optimization. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.8b03486
  • 2019 • 163 Dental pulp stem cells in chitosan/gelatin scaffolds for enhanced orofacial bone regeneration
    Bakopoulou, A. and Georgopoulou, Α. and Grivas, I. and Bekiari, C. and Prymak, O. and Loza, Κ. and Epple, M. and Papadopoulos, G.C. and Koidis, P. and Chatzinikolaidou, Μ.
    Dental Materials 35 310-327 (2019)
    Objective: Biomimetic chitosan/gelatin (CS/Gel) scaffolds have attracted great interest in tissue engineering of several tissues. However, limited information exists regarding the potential of combining CS/Gel scaffolds with oral cells, such as dental pulp stem cells (DPSCs), to produce customized constructs targeting alveolar/orofacial bone reconstruction, which has been the aim of the present study. Methods: Two scaffold types, designated as CS/Gel-0.1 and CS/Gel-1, were fabricated using 0.1 and 1% (v/v) respectively of the crosslinker glutaraldehyde (GTA). Scaffolds (n = 240) were seeded with DPSCs with/without pre-exposure to recombinant human BMP-2. In vitro assessment included DPSCs characterization (flow cytometry), evaluation of viability/proliferation (live/dead staining, metabolic-based tests), osteo/odontogenic gene expression analysis (qRT-PCR) and structural/chemical characterization (scanning electron microscopy, SEM; energy dispersive X-ray spectroscopy, EDX; X-ray powder diffraction, XRD; thermogravimetry, TG). In vivo assessment included implantation of DPSC-seeded scaffolds in immunocompromised mice, followed by histology and SEM-EDX. Statistical analysis employed one/two-way ANOVA and Tukey's post-hoc tests (significance for p < 0.05). Results: Both scaffolds supported cell viability/proliferation over 14 days in culture, showing extensive formation of a hydroxyapatite-rich nanocrystalline calcium phosphate phase. Differential expression patterns indicated GTA concentration to significantly affect the expression of osteo/odontogenic genes, with CS/Gel-0.1 scaffolds being more effective in upregulating DSPP, IBSP and Osterix. In vivo analysis demonstrated time-dependent production of a nanocrystalline, mineralized matrix at 6, 8 and 10 weeks, being more prominent in constructs bearing rhBMP-2 pre-treated cells. The latter showed higher amounts of osteoid and fully mineralized bone, as well as empty space reduction. Significance: These results reveal a promising strategy for orofacial bone tissue engineering. © 2018 The Academy of Dental Materials
    view abstractdoi: 10.1016/j.dental.2018.11.025
  • 2019 • 162 Hyperfine Interactions and Slow Spin Dynamics in Quasi-isotropic InP-based Core/Shell Colloidal Nanocrystals
    Brodu, A. and Tessier, M.D. and Canneson, D. and Dupont, D. and Ballottin, M.V. and Christianen, P.C.M. and De Mello Donega, C. and Hens, Z. and Yakovlev, D.R. and Bayer, M. and Vanmaekelbergh, D. and Biadala, L.
    ACS Nano 13 10201-10209 (2019)
    Colloidal InP core nanocrystals are taking over CdSe-based nanocrystals, notably in optoelectronic applications. Despite their use in commercial devices, such as display screens, the optical properties of InP nanocrystals and especially their relation to the exciton fine structures remain poorly understood. In this work, we show that the ensemble magneto-optical properties of InP-based core/shell nanocrystals investigated in strong magnetic fields up to 30 T are strikingly different from other colloidal nanostructures. Notably, the mixing of the lowest spin-forbidden dark exciton state with the nearest spin-allowed bright state does not occur up to the highest magnetic fields applied. This lack of mixing in an ensemble of nanocrystals suggests an anisotropy tolerance of InP nanocrystals. This striking property allowed us to unveil the slow spin dynamics between Zeeman sublevels (up to 400 ns at 15 T). Furthermore, we show that the unexpected magnetic-field-induced lengthening of the dark exciton lifetime results from the hyperfine interaction between the spin of the electron in the dark exciton with the nuclear magnetic moments. Our results demonstrate the richness of the spin physics in InP quantum dots and stress the large potential of InP nanostructures for spin-based applications. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.9b03384
  • 2019 • 161 Functionalization of titania nanotubes with electrophoretically deposited silver and calcium phosphate nanoparticles: Structure, composition and antibacterial assay
    Chernozem, R.V. and Surmeneva, M.A. and Krause, B. and Baumbach, T. and Ignatov, V.P. and Prymak, O. and Loza, K. and Epple, M. and Ennen-Roth, F. and Wittmar, A. and Ulbricht, M. and Chudinova, E.A. and Rijavec, T. and Lapanje, A...
    Materials Science and Engineering C 97 420-430 (2019)
    Herein TiO2 nanotubes (NTs) were fabricated via electrochemical anodization and coated with silver and calcium phosphate (CaP) nanoparticles (NPs) by electrophoretic deposition. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) revealed that Ag and CaP NPs were successfully deposited onto the TiO2 NTs. Using X-ray diffraction, only anatase and Ti were observed after deposition of Ag and CaP NPs. However, X-ray photoelectron spectroscopy (XPS) analysis revealed that the binding energy (BE) of the Ag and CaP NP core levels corresponded to metallic Ag, hydroxyapatite and amorphous calcium phosphate, based on the knowledge that CaP NPs synthesized by precipitation have the nanocrystalline structure of hydroxyapatite. The application of Ag NPs allows for decreasing the water contact angle and thus increasing the surface free energy. It was concluded that the CaP NP surfaces are superhydrophilic. A significant antimicrobial effect was observed on the TiO2 NT surface after the application of Ag NPs and/or CaP NPs compared with that of the pure TiO2 NTs. Thus, fabrication of TiO2 NTs, Ag NPs and CaP NPs with PEI is promising for diverse biomedical applications, such as in constructing a biocompatible coating on the surface of Ti that includes an antimicrobial effect. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2018.12.045
  • 2019 • 160 Coherent Optical Control of a Quantum-Dot Spin-Qubit in a Waveguide-Based Spin-Photon Interface
    Ding, D. and Appel, M.H. and Javadi, A. and Zhou, X. and Löbl, M.C. and Söllner, I. and Schott, R. and Papon, C. and Pregnolato, T. and Midolo, L. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J. and Schröder, T. and Lodahl, P.
    Physical Review Applied 11 (2019)
    Waveguide-based spin-photon interfaces on the GaAs platform have emerged as a promising system for a variety of quantum information applications directly integrated into planar photonic circuits. The coherent control of spin states in a quantum dot can be achieved by applying circularly polarized laser pulses that may be coupled into the planar waveguide vertically through radiation modes. However, proper control of the laser polarization is challenging since the polarization is modified through the transformation from the far field to the exact position of the quantum dot in the nanostructure. Here, we demonstrate polarization-controlled excitation of a quantum-dot electron spin and use that to perform coherent control in a Ramsey interferometry experiment. The Ramsey interference reveals an inhomogeneous dephasing time of 2.2±0.1 ns, which is comparable to the values so far only obtained in bulk media. We analyze the experimental limitations in spin initialization fidelity and Ramsey contrast and identify the underlying mechanisms. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevApplied.11.031002
  • 2019 • 159 Impurity incorporation and exchange interactions in Co2+-doped CdSe/CdS core/shell nanoplatelets
    Fainblat, R. and Delikanli, S. and Spee, L. and Czerny, T. and Isik, F. and Sharma, V.K. and Demir, H.V. and Bacher, G.
    Journal of Chemical Physics 151 (2019)
    The intentional incorporation of transition metal impurities into colloidal semiconductor nanocrystals allows an extension of the host material's functionality. While dopant incorporation has been extensively investigated in zero-dimensional quantum dots, the substitutional replacement of atoms in two-dimensional (2D) nanostructures by magnetic dopants has been reported only recently. Here, we demonstrate the successful incorporation of Co2+ ions into the shell of CdSe/CdS core/shell nanoplatelets, using these ions (i) as microscopic probes for gaining distinct structural insights and (ii) to enhance the magneto-optical functionality of the host material. Analyzing interatomic Co2+ ligand field transitions, we conclude that Co2+ is incorporated into lattice sites of the CdS shell, and effects such as diffusion of dopants into the CdSe core or diffusion of the dopants out of the heterostructure causing self-purification play a minor role. Taking advantage of the absorption-based technique of magnetic circular dichroism, we directly prove the presence of sp-d exchange interactions between the dopants and the band charge carriers in CdSe/Co2+:CdS heteronanoplatelets. Thus, our study not only demonstrates magneto-optical functionality in 2D nanocrystals by Co2+ doping but also shows that a careful choice of the dopant type paves the way for a more detailed understanding of the impurity incorporation process into these novel 2D colloidal materials. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5129391
  • 2019 • 158 Nonequilibrium carrier dynamics in self-assembled quantum dots
    Geller, M.
    Applied Physics Reviews 6 (2019)
    Self-assembled quantum dots are still one of the best model systems for artificial atoms in a solid-state environment, where the electronic states can be accessed by electrical and optical means. This article focuses on nonequilibrium carrier dynamics in these quantum dots, using the ability of recent developments in electrical and optical spectroscopy techniques. All-electrical transconductance spectroscopy is introduced, where a two-dimensional electron gas serves as a fast and sensitive detector for the electron/hole dynamics and charge/spin state preparation and relaxation in an ensemble of dots. Latest results on single electron tunneling and nonequilibrium Auger recombination in a single quantum dot using a high-resolution optical experiment (the time-resolved resonance fluorescence) are summarized. This article concludes with a perspective view on a future combination of both techniques toward an electro-optical measurement toolbox to link the coherent control of quantum states by optical means with an electrical preparation of electron charge and spin states. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5091742
  • 2019 • 157 Gradients of microstructure, stresses and mechanical properties in a multi-layered diamond thin film revealed by correlative cross-sectional nano-analytics
    Gruber, D.P. and Todt, J. and Wöhrl, N. and Zalesak, J. and Tkadletz, M. and Kubec, A. and Niese, S. and Burghammer, M. and Rosenthal, M. and Sternschulte, H. and Pfeifenberger, M.J. and Sartory, B. and Keckes, J.
    Carbon 144 666-674 (2019)
    Thin diamond films deposited by chemical vapour deposition (CVD) usually feature cross-sectional gradients of microstructure, residual stress and mechanical properties, which decisively influence their functional properties. This work introduces a novel correlative cross-sectional nano-analytics approach, which is applied to a multi-layered CVD diamond film grown using microwave plasma-enhanced CVD and consisting of a ∼8 μm thick nanocrystalline (NCD) base and a ∼14.5 μm thick polycrystalline (PCD) top diamond sublayers. Complementary cross-sectional 30 nm beam synchrotron X-ray diffraction, depth-resolved micro-cantilever and hardness testing and electron microscopy analyses reveal correlations between microstructure, residual stress and mechanical properties. The NCD sublayer exhibits a 1.5 μm thick isotropic nucleation region with the highest stresses of ∼1.3 GPa and defect-rich nanocrystallites. With increasing sublayer thickness, a 110 fibre texture evolves gradually, accompanied by an increase in crystallite size and a decrease in stress. At the NCD/PCD sublayer interface, texture, stresses and crystallite size change abruptly and the PCD sublayer exhibits the presence of Zone T competitive grain growth microstructure. NCD and PCD sublayers differ in fracture stresses of ∼14 and ∼31 GPa, respectively, as well as in elastic moduli and hardness, which are correlated with their particular microstructures. In summary, the introduced nano-analytics approach provides complex correlations between microstructure, stresses, functional properties and deposition conditions. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.carbon.2018.12.093
  • 2019 • 156 Oxygen-mediated deformation and grain refinement in Cu-Fe nanocrystalline alloys
    Guo, J. and Duarte, M.J. and Zhang, Y. and Bachmaier, A. and Gammer, C. and Dehm, G. and Pippan, R. and Zhang, Z.
    Acta Materialia 166 281-293 (2019)
    Light elements play a crucial role on the microstructure and properties of conventional alloys and steels. Oxygen is one of the light elements which is inevitably introduced into nanocrystalline alloys during manufacturing. Here, we report that severe plastic deformation can fragment the oxides formed in powder processing and eventually cause oxygen dissolution in the matrix. A comparative investigation on Cu-Fe nanocrystalline alloys generated from different initial materials, blended powders and arc-melted bulk materials which have different oxygen contents, reveals that fragmented oxides at grain boundaries effectively decrease the grain boundary mobility, markedly facilitating grain refinement. In contrast, those oxygen atoms dissolved as interstitials in the Cu-Fe matrix lead to lattice expansion and significant decrease of stacking fault energy locally as validated by density functional theory. Such oxygen-mediated microstructure gives rise to enhanced strength and superior structural stability. The remarkable tailoring effect of oxygen can be employed to engineer nanocrystalline materials with desired properties for different applications. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.040
  • 2019 • 155 Long-Lived Negative Photocharging in Colloidal CdSe Quantum Dots Revealed by Coherent Electron Spin Precession
    Hu, R. and Wu, Z. and Zhang, Y. and Yakovlev, D.R. and Liang, P. and Qiang, G. and Guo, J. and Jia, T. and Sun, Z. and Bayer, M. and Feng, D.
    Journal of Physical Chemistry Letters 10 4994-4999 (2019)
    Photoinduced charging in CdSe colloidal quantum dots (QDs) is investigated by time-resolved pump-probe spectroscopy that is sensitive to electron spin polarization. This technique monitors the coherent spin dynamics of optically oriented electrons precessing around an external magnetic field. By addition of 1-octanethiol to the CdSe QD solution in toluene, an extremely long-lived negative photocharging is detected that lives up to 1 month in an N2 atmosphere and hours in an air atmosphere at room temperature. 1-Octanethiol not only acts as a hole acceptor but also results in a reduction of the oxygen-induced photo-oxidation in CdSe QDs, allowing air-stable negative photocharging. Two types of negative photocharging states with different spin precession frequencies and very different lifetimes are identified. These findings have important implications for understanding the photophysical processes in colloidal nanostructures. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.9b02341
  • 2019 • 154 Experimental realization of a quantum dot energy harvester
    Jaliel, G. and Puddy, R.K. and Sánchez, R. and Jordan, A.N. and Sothmann, B. and Farrer, I. and Griffiths, J.P. and Ritchie, D.A. and Smith, C.G.
    Physical Review Letters 123 (2019)
    We demonstrate experimentally an autonomous nanoscale energy harvester that utilizes the physics of resonant tunneling quantum dots. Gate-defined quantum dots on GaAs/AlGaAs high-electron-mobility transistors are placed on either side of a hot-electron reservoir. The discrete energy levels of the quantum dots are tuned to be aligned with low energy electrons on one side and high energy electrons on the other side of the hot reservoir. The quantum dots thus act as energy filters and allow for the conversion of heat from the cavity into electrical power. Our energy harvester, measured at an estimated base temperature of 75 mK in a He3/He4 dilution refrigerator, can generate a thermal power of 0.13 fW for a temperature difference across each dot of about 67 mK. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.123.117701
  • 2019 • 153 Temperature and bias anomalies in the photoluminescence of InAs quantum dots coupled to a Fermi reservoir
    Korsch, A.R. and Nguyen, G.N. and Schmidt, M. and Ebler, C. and Valentin, S.R. and Lochner, P. and Rothfuchs, C. and Wieck, A.D. and Ludwig, Ar.
    Physical Review B 99 (2019)
    We present anomalous behavior of temperature-dependent photoluminescence (PL) measurements on InAs quantum dot ensembles coupled to an electron reservoir in an n-i-p diode structure. When negative gate voltages are applied to the sample, an anomalous initial increase of the integrated PL signal with rising temperature is observed for the ground-state and first-excited-state emission peaks. In contrast, measurements at positive gate voltages show no such anomaly and are well described by the commonly used Arrhenius model. Unlike previous studies on uncoupled quantum dot ensembles, we show that in quantum dot diode structures the anomalous temperature dependence and its dependence on the applied bias voltage is dominated by electrons tunneling from the electron reservoir to the quantum dots. Tunneling electrons enhance the PL signal by recombining with holes stored in the quantum dots and the tunneling rate depends on temperature via the Fermi distribution in the electron reservoir. With the implementation of a rate-based tunnel coupling, we develop a modified Arrhenius model that takes the observed anomalies excellently into account. Gate voltage dependent PL measurements at 77 K are further compared to capacitance-voltage spectroscopy measurements on the same sample, supporting the proposed interpretation. The PL peak width shows a characteristic evolution as a function of temperature, which is discussed qualitatively in terms of our model. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.165303
  • 2019 • 152 Epitaxial InGaAs Quantum Dots in Al0.29Ga0.71As Matrix: Intensity and Kinetics of Luminescence in the Near Field of Silver Nanoparticles
    Kosarev, A.N. and Chaldyshev, V.V. and Kondikov, A.A. and Vartanyan, T.A. and Toropov, N.A. and Gladskikh, I.A. and Gladskikh, P.V. and Akimov, I. and Bayer, M. and Preobrazhenskii, V.V. and Putyato, M.A. and Semyagin, B.R.
    Optics and Spectroscopy 126 492-496 (2019)
    Abstract: Quantum dots of indium gallium arsenide buried in a thin layer of aluminum gallium arsenide were grown by means of molecular-beam epitaxy. The influence of silver nanoparticles grown on the surface of the semiconductor structure by vacuum thermal evaporation on photoluminescence of quantum dots was investigated. Photoluminescence spectra of quantum dots were obtained under stationary and pulsed excitation. The influence of silver nanoparticles exhibiting plasmon resonances on spectral distribution and kinetics of luminescence of the epitaxial quantum dots was studied. © 2019, Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S0030400X19050151
  • 2019 • 151 Photogeneration of a single electron from a single Zeeman-resolved light-hole exciton with preserved angular momentum
    Kuroyama, K. and Larsson, M. and Chang, C.Y. and Muramoto, J. and Heya, K. and Fujita, T. and Allison, G. and Valentin, S.R. and Ludwig, Ar. and Wieck, A.D. and Matsuo, S. and Oiwa, A. and Tarucha, S.
    Physical Review B 99 (2019)
    Quantum state transfer from a single photon to a single electron following selection rules can only occur for a spin-resolved light-hole excitation in GaAs quantum dots; however, these phenomena have yet to be experimentally realized. Here, we report on single-shot readout of a single electron spin via the Zeeman-resolved light-hole excitation using an optical spin blockade method in a GaAs quantum dot and a Pauli spin blockade method in a double GaAs quantum dot. The observed photoexcitation probability strongly depends on the photon polarization, an indication of angular momentum transfer from a single photon to an electron. Our demonstration will open a pathway to further investigation of fundamental quantum physics and applications of quantum networking technology. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.085203
  • 2019 • 150 Optical Detection of Single-Electron Tunneling into a Semiconductor Quantum Dot
    Kurzmann, A. and Stegmann, P. and Kerski, J. and Schott, R. and Ludwig, Ar. and Wieck, A.D. and König, J. and Lorke, A. and Geller, M.
    Physical Review Letters 122 (2019)
    The maximum information of a dynamic quantum system is given by real-time detection of every quantum event, where the ultimate challenge is a stable, sensitive detector with high bandwidth. All physical information can then be drawn from a statistical analysis of the time traces. We demonstrate here an optical detection scheme based on the time-resolved resonance fluorescence on a single quantum dot. Single-electron resolution with high signal-to-noise ratio (4σ confidence) and high bandwidth of 10 kHz make it possible to record the individual quantum events of the transport dynamics. Full counting statistics with factorial cumulants gives access to the nonequilibrium dynamics of spin relaxation of a singly charged dot (γ↑↓=3 ms-1), even in an equilibrium transport measurement. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.122.247403
  • 2019 • 149 Simulating nanocrystal-based solar cells: A lead sulfide case study
    Lin, W.M.M. and Yazdani, N. and Yarema, O. and Volk, S. and Yarema, M. and Kirchartz, T. and Wood, V.
    Journal of Chemical Physics 151 (2019)
    Nanocrystal-based solar cells are promising candidates for next generation photovoltaic applications; however, the most recent improvements to the device chemistry and architecture have been mostly trial-and-error based advancements. Due to complex interdependencies among parameters, determining factors that limit overall solar cell efficiency are not trivial. Furthermore, many of the underlying chemical and physical parameters of nanocrystal-based solar cells have only recently been understood and quantified. Here, we show that this new understanding of interfaces, transport, and origin of trap states in nanocrystal-based semiconductors can be integrated into simulation tools, based on 1D drift-diffusion models. Using input parameters measured in independent experiments, we find excellent agreement between experimentally measured and simulated PbS nanocrystal solar cell behavior without having to fit any parameters. We then use this simulation to understand the impact of interfaces, charge carrier mobility, and trap-assisted recombination on nanocrystal performance. We find that careful engineering of the interface between the nanocrystals and the current collector is crucial for an optimal open-circuit voltage. We also show that in the regime of trap-state densities found in PbS nanocrystal solar cells (∼1017 cm-3), device performance exhibits strong dependence on the trap state density, explaining the sensitivity of power conversion efficiency to small changes in nanocrystal synthesis and nanocrystal thin-film deposition that has been reported in the literature. Based on these findings, we propose a systematic approach to nanocrystal solar cell optimization. Our method for incorporating parameters into simulations presented and validated here can be adopted to speed up the understanding and development of all types of nanocrystal-based solar cells. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5129159
  • 2019 • 148 Correlations between optical properties and Voronoi-cell area of quantum dots
    Löbl, M.C. and Zhai, L. and Jahn, J.-P. and Ritzmann, J. and Huo, Y. and Wieck, A.D. and Schmidt, O.G. and Ludwig, Ar. and Rastelli, A. and Warburton, R.J.
    Physical Review B 100 (2019)
    A semiconductor quantum dot (QD) can generate highly indistinguishable single photons at a high rate. For application in quantum communication and integration in hybrid systems, control of the QD optical properties is essential. Understanding the connection between the optical properties of a QD and the growth process is therefore important. Here, we show for GaAs QDs, grown by infilling droplet-etched nanoholes, that the emission wavelength, the neutral-to-charged exciton splitting, and the diamagnetic shift are strongly correlated with the capture-zone area, an important concept from nucleation theory. We show that the capture-zone model applies to the growth of this system even in the limit of a low QD density in which atoms diffuse over μm distances. The strong correlations between the various QD parameters facilitate preselection of QDs for applications with specific requirements on the QD properties; they also suggest that a spectrally narrowed QD distribution will result if QD growth on a regular lattice can be achieved. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.100.155402
  • 2019 • 147 Block copolymer-directed synthesis of porous anatase for lithium-ion battery electrodes
    McRae, O.F. and Xia, Q. and Tjaberings, S. and Gröschel, A.H. and Ling, C.D. and Müllner, M.
    Journal of Polymer Science, Part A: Polymer Chemistry 57 1890-1896 (2019)
    A templating method is developed to produce porous nanocrystalline anatase materials for negative electrodes in lithium-ion batteries (LIBs). Amphiphilic diblock copolymers are used to generate template films with phase-separated internal structure. Subsequent swelling with acidified titanium(IV) bis(ammonium lactato) dihydroxide (TALH) solution yielded structured hybrid films. Upon heating, the formation of TiO2 nanocrystals is induced, resulting in a three-dimensional mesoporous structure directed by the bulk morphology of the polymer template. In comparison to commercial nanosized anatase, the structured anatase shows significant performance improvements in lithium-ion coin cell batteries in terms of capacity, stability, and rate capability. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1890–1896. © 2018 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/pola.29312
  • 2019 • 146 Iterative path-integral summations for the tunneling magnetoresistance in interacting quantum-dot spin valves
    Mundinar, S. and Stegmann, P. and König, J. and Weiss, S.
    Physical Review B 99 (2019)
    We report on the importance of resonant-tunneling processes on quantum transport through interacting quantum-dot spin valves. To include Coulomb interaction in the calculation of the tunneling magnetoresistance (TMR), we reformulate and generalize the recently developed, numerically exact method of iterative summation of path integrals (ISPI) to account for spin-dependent tunneling. The ISPI scheme allows us to investigate weak to intermediate Coulomb interaction in a wide range of gate and bias voltage and down to temperatures at which a perturbative treatment of tunneling severely fails. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.195457
  • 2019 • 145 Difference in charge and spin dynamics in a quantum dot-lead coupled system
    Otsuka, T. and Nakajima, T. and Delbecq, M.R. and Stano, P. and Amaha, S. and Yoneda, J. and Takeda, K. and Allison, G. and Li, S. and Noiri, A. and Ito, T. and Loss, D. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Physical Review B 99 (2019)
    We analyze time evolution of charge and spin states in a quantum dot coupled to an electric reservoir. Utilizing high-speed single-electron detection, we focus on dynamics induced by the first-order tunneling. We find that there is a difference between the spin and the charge relaxation: The former appears slower than the latter. The difference depends on the Fermi occupation factor and the spin relaxation becomes slower when the energy level of the quantum dot is lowered. We explain this behavior by a theory including the first-order tunneling processes and find a good agreement between the experiment and the theory. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.085402
  • 2019 • 144 Dual-emitting dot-in-bulk cdse/cds nanocrystals with highly emissive core-and shell-based trions sharing the same resident electron
    Pinchetti, V. and Shornikova, E.V. and Qiang, G. and Bae, W.K. and Meinardi, F. and Crooker, S.A. and Yakovlev, D.R. and Bayer, M. and Klimov, V.I. and Brovelli, S.
    Nano Letters (2019)
    Colloidal CdSe nanocrystals (NCs) overcoated with an ultrathick CdS shell, also known as dot-in-bulk (DiB) structures, can support two types of excitons, one of which is core-localized and the other, shell-localized. In the case of weak "sub-single-exciton" pumping, emission alternates between the core-and shell-related channels, which leads to two-color light. This property makes these structures uniquely suited for a variety of photonic applications as well as ideal model systems for realizing complex excitonic quasi-particles that do not occur in conventional core/shell NCs. Here, we show that the DiB design can enable an unusual regime in which the same long-lived resident electron can endow trionlike characteristics to either of the two excitons of the DiB NC (core-or shell-based). These two spectrally distinct trion states are apparent in the measured photoluminescence (PL) and spin dynamics of core and shell excitons conducted over a wide range of temperatures and applied magnetic fields. Low-Temperature PL measurements indicate that core-and shell-based trions are characterized by a nearly ideal (â 100%) emission quantum yield, suggesting the strong suppression of Auger recombination for both types of excitations. Polarization-resolved PL experiments in magnetic fields of up to 60 T reveal that the core-and the shell-localized trions exhibit remarkably similar spin dynamics, which in both cases are controlled by spin-flip processes involving a heavy hole. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.9b03676
  • 2019 • 143 Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment
    Rautert, J. and Shamirzaev, T.S. and Nekrasov, S.V. and Yakovlev, D.R. and Klenovský, P. and Kusrayev, Y.G. and Bayer, M.
    Physical Review B 99 (2019)
    The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a Γ-valley electron and the excitons in the indirect QDs contributed by an X-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.195411
  • 2019 • 142 A Z-Scheme-Inspired Photobioelectrochemical H2O/O2 Cell with a 1 V Open-Circuit Voltage Combining Photosystem II and PbS Quantum Dots
    Riedel, M. and Wersig, J. and Ruff, A. and Schuhmann, W. and Zouni, A. and Lisdat, F.
    Angewandte Chemie - International Edition 58 801-805 (2019)
    A biohybrid photobioanode mimicking the Z-scheme has been developed by functional integration of photosystem II (PSII) and PbS quantum dots (QDs) within an inverse opal TiO2 architecture giving rise to a rather negative water oxidation potential of about −0.55 V vs. Ag/AgCl, 1 m KCl at neutral pH. The electrical linkage between both light-sensitive entities has been established through an Os-complex-modified redox polymer (POs), which allows the formation of a multi-step electron-transfer chain under illumination starting with the photo-activated water oxidation at PSII followed by an electron transfer from PSII through POs to the photo-excited QDs and finally to the TiO2 electrode. The photobioanode was coupled to a novel, transparent, inverse-opal ATO cathode modified with an O2-reducing bilirubin oxidase for the construction of a H2O/O2 photobioelectrochemical cell reaching a high open-circuit voltage of about 1 V under illumination. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201811172
  • 2019 • 141 A Z-Scheme-Inspired Photobioelectrochemical H 2 O/O 2 Cell with a 1 V Open-Circuit Voltage Combining Photosystem II and PbS Quantum Dots
    Riedel, M. and Wersig, J. and Ruff, A. and Schuhmann, W. and Zouni, A. and Lisdat, F.
    Angewandte Chemie - International Edition 58 801-805 (2019)
    A biohybrid photobioanode mimicking the Z-scheme has been developed by functional integration of photosystem II (PSII) and PbS quantum dots (QDs) within an inverse opal TiO 2 architecture giving rise to a rather negative water oxidation potential of about −0.55 V vs. Ag/AgCl, 1 m KCl at neutral pH. The electrical linkage between both light-sensitive entities has been established through an Os-complex-modified redox polymer (P Os ), which allows the formation of a multi-step electron-transfer chain under illumination starting with the photo-activated water oxidation at PSII followed by an electron transfer from PSII through P Os to the photo-excited QDs and finally to the TiO 2 electrode. The photobioanode was coupled to a novel, transparent, inverse-opal ATO cathode modified with an O 2 -reducing bilirubin oxidase for the construction of a H 2 O/O 2 photobioelectrochemical cell reaching a high open-circuit voltage of about 1 V under illumination. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201811172
  • 2019 • 140 Nonlinear chiral refrigerators
    Sánchez, D. and Sánchez, R. and López, R. and Sothmann, B.
    Physical Review B 99 (2019)
    We investigate a mesoscopic refrigerator based on chiral quantum Hall edge channels. We discuss a three-terminal cooling device in which charge transport occurs only between a pair of voltage-biased terminals. The third terminal, which is to be cooled, is set as a voltage probe with vanishing particle flux. This largely prevents the generation of direct Joule heating, which ensures a high coefficient of performance. Cooling operation is based on energy-dependent quantum transmissions. The latter are implemented with the aid of two tunable scattering resonances (quantum dots). To find the optimal performance point and the largest temperature difference created with our refrigerator, it is crucial to address the nonlinear regime of transport, accounting for electron-electron interaction effects. Our numerical simulations show that the maximal cooling power can be tuned with the quantum dot couplings and energy levels. Further, we provide analytical expressions within a weakly nonlinear scattering-matrix formalism which allow us to discuss the conditions for optimal cooling in terms of generalized thermopowers. Our results are important for the assessment of chiral conductors as promising candidates for efficient quantum refrigerators with low dissipation. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.245304
  • 2019 • 139 A machine learning approach for automated fine-tuning of semiconductor spin qubits
    Teske, J.D. and Humpohl, S.S. and Otten, R. and Bethke, P. and Cerfontaine, P. and Dedden, J. and Ludwig, Ar. and Wieck, A.D. and Bluhm, H.
    Applied Physics Letters 114 (2019)
    While spin qubits based on gate-defined quantum dots have demonstrated very favorable properties for quantum computing, one remaining hurdle is the need to tune each of them into a good operating regime by adjusting the voltages applied to electrostatic gates. The automation of these tuning procedures is a necessary requirement for the operation of a quantum processor based on gate-defined quantum dots, which is yet to be fully addressed. We present an algorithm for the automated fine-tuning of quantum dots and demonstrate its performance on a semiconductor singlet-triplet qubit in GaAs. The algorithm employs a Kalman filter based on Bayesian statistics to estimate the gradients of the target parameters as a function of gate voltages, thus learning the system response. The algorithm's design is focused on the reduction of the number of required measurements. We experimentally demonstrate the ability to change the operation regime of the qubit within 3-5 iterations, corresponding to 10-15 min of lab-time. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5088412
  • 2019 • 138 Thermal transistor and thermometer based on Coulomb-coupled conductors
    Yang, J. and Elouard, C. and Splettstoesser, J. and Sothmann, B. and Sánchez, R. and Jordan, A.N.
    Physical Review B 100 (2019)
    We study a three-terminal setup consisting of a single-level quantum dot capacitively coupled to a quantum point contact. The point contact connects to a source and drain reservoir while the quantum dot is coupled to a single base reservoir. This setup has been used to implement a noninvasive, nanoscale thermometer for the bath reservoir by detecting the current in the quantum point contact. Here, we demonstrate that the device can also be operated as a thermal transistor where the average (charge and heat) current through the quantum point contact is controlled via the temperature of the base reservoir. We characterize the performances of this device both as a transistor and a thermometer and derive the operating condition maximizing their respective sensitivities. The present analysis is useful for the control of charge and heat flow and high precision thermometry at the nanoscale. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.100.045418
  • 2018 • 137 Intra- and inter-nanocrystal charge transport in nanocrystal films
    Aigner, W. and Bienek, O. and Falcão, B.P. and Ahmed, S.U. and Wiggers, H. and Stutzmann, M. and Pereira, R.N.
    Nanoscale 10 8042-8057 (2018)
    The exploitation of semiconductor nanocrystal (NC) films in novel electronic and optoelectronic applications requires a better understanding of charge transport in these systems. Here, we develop a model of charge transport in NC films, based on a generalization of the concept of transport energy level ET to nanocrystal assemblies, which considers both intra- and inter-NC charge transfer processes. We conclude that the role played by each of these processes can be probed from temperature-dependent measurements of charge carrier density n and mobility μ in the same films. The model also enables the determination of the position of the Fermi energy level EF with respect to ET, an important parameter of charge transport in semiconductor materials, from the temperature dependence of n. Moreover, we provide support to an essentially temperature-independent intra-NC charge carrier mobility, considered in the transport level concept, and consequently the frequently observed temperature dependence of the overall mobility μ in NC films results from a temperature variation of the inter-NC charge transport processes. Importantly, we also conclude that the temperature dependence of conductivity in NC films should result in general from a combination of temperature variations of both n and μ. By applying the model to solution-processed Si NC films, we conclude that transport within each NC is similar to that in amorphous Si (a-Si), with charges hopping along band tail states located below the conduction band edge. For Si NCs, we obtain values of ET - EF of ∼0.25 eV. The overall mobility μ in Si NC films is significantly further reduced with respect to that typically found in a-Si due to the additional transport constraints imposed by inter-NC transfer processes inherent to a nanoparticulate film. Our model accounting for inter- and intra-NC charge transport processes provides a simple and more general description of charge transport that can be broadly applied to films of semiconductor NCs. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8nr00250a
  • 2018 • 136 Studies of photon echo from exciton ensemble in (In,Ga)As quantum dots
    Babenko, I.A. and Yugova, I.A. and Poltavtsev, S.V. and Salewski, M. and Akimov, I.A. and Kamp, M. and Höfling, S. and Yakovlev, D.R. and Bayer, M.
    Journal of Physics: Conference Series 951 (2018)
    Photon echo from trions and excitons in (In,Ga)As/GaAs quantum dots has been studied theoretically and experimentally. Theoretical analysis allowed us to distinguish photon echo signals from excitons and trions measured in the same range of wavelength using different polarization configurations of laser excitation. The theoretical predictions are in good agreement with the experimental data. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/951/1/012029
  • 2018 • 135 Improved Models for Metallic Nanoparticle Cores from Atomic Pair Distribution Function (PDF) Analysis
    Banerjee, S. and Liu, C.-H. and Lee, J.D. and Kovyakh, A. and Grasmik, V. and Prymak, O. and Koenigsmann, C. and Liu, H. and Wang, L. and Abeykoon, A.M.M. and Wong, S.S. and Epple, M. and Murray, C.B. and Billinge, S.J.L.
    Journal of Physical Chemistry C 122 29498-29506 (2018)
    X-ray atomic pair distribution functions (PDFs) were collected from a range of canonical metallic nanomaterials, both elemental and alloyed, prepared using different synthesis methods and exhibiting drastically different morphological properties. Widely applied shape-tuned attenuated crystal (AC) fcc models proved inadequate, yielding structured, coherent, and correlated fit residuals. However, equally simple discrete cluster models could account for the largest amplitude features in these difference signals. A hypothesis testing based approach to nanoparticle structure modeling systematically ruled out effects from crystallite size, composition, shape, and surface faceting as primary factors contributing to the AC misfit. On the other hand, decahedrally twinned cluster cores were found to be the origin of the AC structure misfits for a majority of the nanomaterials reported here. It is further motivated that the PDF can readily differentiate between the arrangement of domains in these multiply twinned motifs. Most of the nanomaterials surveyed also fall within the sub-5 nm size regime where traditional electron microscopy cannot easily detect and quantify domain structures, with sampling representative of the average nanocrystal synthesized. The results demonstrate that PDF analysis is a powerful method for understanding internal atomic interfaces in small noble metallic nanomaterials. Such core cluster models, easily built algorithmically, should serve as starting structures for more advanced models able to capture atomic positional disorder, ligand induced or otherwise, near nanocrystal surfaces. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b05897
  • 2018 • 134 Efficient p-n junction-based thermoelectric generator that can operate at extreme temperature conditions
    Chavez, R. and Angst, S. and Hall, J. and Maculewicz, F. and Stoetzel, J. and Wiggers, H. and Thanh Hung, L. and Van Nong, N. and Pryds, N. and Span, G. and Wolf, D.E. and Schmechel, R. and Schierning, G.
    Journal of Physics D: Applied Physics 51 (2018)
    In many industrial processes, a large proportion of energy is lost in the form of heat. Thermoelectric generators can convert this waste heat into electricity by means of the Seebeck effect. However, the use of thermoelectric generators in practical applications on an industrial scale is limited in part because electrical, thermal, and mechanical bonding contacts between the semiconductor materials and the metal electrodes in current designs are not capable of withstanding thermal-mechanical stress and alloying of the metal-semiconductor interface when exposed to the high temperatures occurring in many real-world applications. Here we demonstrate a concept for thermoelectric generators that can address this issue by replacing the metallization and electrode bonding on the hot side of the device by a p-n junction between the two semiconductor materials, making the device robust against temperature induced failure. In our proof-of-principle demonstration, a p-n junction device made from nanocrystalline silicon is at least comparable in its efficiency and power output to conventional devices of the same material and fabrication process, but with the advantage of sustaining high hot side temperatures and oxidative atmosphere. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa9b6a
  • 2018 • 133 Size-selected Fe3O4-Au hybrid nanoparticles for improved magnetism-based theranostics
    Efremova, M.V. and Nalench, Y.A. and Myrovali, E. and Garanina, A.S. and Grebennikov, I.S. and Gifer, P.K. and Abakumov, M.A. and Spasova, M. and Angelakeris, M. and Savchenko, A.G. and Farle, M. and Klyachko, N.L. and Majouga, A....
    Beilstein Journal of Nanotechnology 9 2684-2699 (2018)
    Size-selected Fe3O4-Au hybrid nanoparticles with diameters of 6-44 nm (Fe3O4) and 3-11 nm (Au) were prepared by high temperature, wet chemical synthesis. High-quality Fe3O4 nanocrystals with bulk-like magnetic behavior were obtained as confirmed by the presence of the Verwey transition. The 25 nm diameter Fe3O4-Au hybrid nanomaterial sample (in aqueous and agarose phantom systems) showed the best characteristics for application as contrast agents in magnetic resonance imaging and for local heating using magnetic particle hyperthermia. Due to the octahedral shape and the large saturation magnetization of the magnetite particles, we obtained an extraordinarily high r2-relaxivity of 495 mM-1·s-1 along with a specific loss power of 617 W·gFe-1 and 327 W·gFe-1 for hyperthermia in aqueous and agarose systems, respectively. The functional in vitro hyperthermia test for the 4T1 mouse breast cancer cell line demonstrated 80% and 100% cell death for immediate exposure and after precultivation of the cells for 6 h with 25 nm Fe3O4-Au hybrid nanomaterials, respectively. This confirms that the improved magnetic properties of the bifunctional particles present a next step in magnetic-particle-based theranostics. © 2018 Efremova et al.
    view abstractdoi: 10.3762/bjnano.9.251
  • 2018 • 132 Design and Realization of White Quantum Dot Light-Emitting Electrochemical Cell Hybrid Devices
    Frohleiks, J. and Gellner, S. and Wepfer, S. and Bacher, G. and Nannen, E.
    ACS Applied Materials and Interfaces 10 42637-42646 (2018)
    The simple device architecture as well as the solution-based processing makes light-emitting electrochemical cells (LECs) a promising device concept for large-area flexible lighting solutions. The lack of deep-blue emitters, which are, at the same time, efficient, bright, and long-term stable, complementary to the wide variety of yellow-orange-emitting LECs, hampers the creation of white LECs. We present a hybrid device concept for the realization of white light emission by combining blue colloidal quantum dots (QDs) and an Ir-based ionic transition-metal complex (iTMC) LEC in a new type of white QD-LEC hybrid device (QLEC). By careful arrangement of the active layers, we yield light emission from both the blue QDs and the yellow iTMC emitter already at voltages below 3 V. The QLEC devices show homogeneous white light emission with high color rendering index (up to 80), luminance levels above 850 cd m-2, and a maximum external quantum efficiency greater than 0.2%. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acsami.8b15100
  • 2018 • 131 Four single-spin Rabi oscillations in a quadruple quantum dot
    Ito, T. and Otsuka, T. and Nakajima, T. and Delbecq, M.R. and Amaha, S. and Yoneda, J. and Takeda, K. and Noiri, A. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Applied Physics Letters 113 (2018)
    Scaling up qubits is a necessary step to realize useful systems of quantum computation. Here, we demonstrate coherent manipulations of four individual electron spins using a micro-magnet method in each dot of a quadruple quantum dot - the largest number of dots used for the single spin control in multiple quantum dots. We observe Rabi oscillations for each dot through electron spin resonance, evaluate the spin-electric coupling of the four dots, and finally discuss practical approaches to independently address single spins in multiple quantum dot systems containing even more quantum dots. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5040280
  • 2018 • 130 Far-field nanoscopy on a semiconductor quantum dot via a rapid-adiabatic-passage-based switch
    Kaldewey, T. and Kuhlmann, A.V. and Valentin, S.R. and Ludwig, Ar. and Wieck, A.D. and Warburton, R.J.
    Nature Photonics 12 68-72 (2018)
    The diffraction limit prevents a conventional optical microscope from imaging at the nanoscale. However, nanoscale imaging of molecules is possible by exploiting an intensity-dependent molecular switch 1-3 . This switch is translated into a microscopy scheme, stimulated emission depletion microscopy 4-7 . Variants on this scheme exist 3,8-13, yet all exploit an incoherent response to the lasers. We present a scheme that relies on a coherent response to a laser. Quantum control of a two-level system proceeds via rapid adiabatic passage, an ideal molecular switch. We implement this scheme on an ensemble of quantum dots. Each quantum dot results in a bright spot in the image with extent down to 30 nm (λ/31). There is no significant loss of intensity with respect to confocal microscopy, resulting in a factor of 10 improvement in emitter position determination. The experiments establish rapid adiabatic passage as a versatile tool in the super-resolution toolbox. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41566-017-0079-y
  • 2018 • 129 Understanding the Effect of Au in Au-Pd Bimetallic Nanocrystals on the Electrocatalysis of the Methanol Oxidation Reaction
    Kelly, C.H.W. and Benedetti, T.M. and Alinezhad, A. and Schuhmann, W. and Gooding, J.J. and Tilley, R.D.
    Journal of Physical Chemistry C 122 21718-21723 (2018)
    Pd or Pt alloyed with a secondary metal are the typical catalysts at the anode for the direct oxidation of methanol. The secondary metal is employed to diminish deactivation commonly ascribed to CO poisoning. Here we investigate the origin of the improved performance of Au-Pd core-shell and alloy nanocrystals as electrocatalysts for the methanol oxidation reaction (MOR), relative to Pd alone. Monodisperse Au-Pd core-shell nanocrystals were synthesized using H2 as a mild reducing agent followed by annealing under a 5% H2 atmosphere to produce the Au-Pd alloys. The nanocrystals were characterized using high-resolution electron microscopy to confirm their structures. The core-shell and alloy nanocrystals showed an improvement in specific activity with respect to pure Pd nanocrystals. Importantly, the stability was also improved by the inclusion of Au for both nanocrystals, being 2.7× higher for the alloy than for the core-shell after 30 min, while the activity is completely lost for the Pd nanocrystals within 10 min. We show that there is no evidence of CO formation for any of the Pd-based catalysts in an alkaline environment. The origin of the improvement in terms of both activity and stability results from positive shifts in the PdO formation/reduction potential caused by the presence of Au, which results in more Pd sites available for the MOR. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b05407
  • 2018 • 128 Revealing attractive electron-electron interaction in a quantum dot by full counting statistics
    Kleinherbers, E. and Stegmann, P. and König, J.
    New Journal of Physics 20 (2018)
    Recent experiments (2015 Nature 521 196; 2017 Nat. Commun. 8 395) have presented evidence for electron pairing in a quantum dot beyond the superconducting regime. Here, we show that the impact of an attractive electron-electron interaction on the full counting statistics of electron transfer through a quantum dot is qualitatively different from the case of a repulsive interaction. In particular, the sign of higher-order (generalized) factorial cumulants reveals more pronounced correlations, which even survive in the limit of fast spin relaxation. © 2018 The Author(s). Published by IOP Publishing Ltd on behalf of Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/aad14a
  • 2018 • 127 Advanced Materials through Assembly of Nanocelluloses
    Kontturi, E. and Laaksonen, P. and Linder, M.B. and Nonappa and Gröschel, A.H. and Rojas, O.J. and Ikkala, O.
    Advanced Materials 30 (2018)
    There is an emerging quest for lightweight materials with excellent mechanical properties and economic production, while still being sustainable and functionalizable. They could form the basis of the future bioeconomy for energy and material efficiency. Cellulose has long been recognized as an abundant polymer. Modified celluloses were, in fact, among the first polymers used in technical applications; however, they were later replaced by petroleum-based synthetic polymers. Currently, there is a resurgence of interest to utilize renewable resources, where cellulose is foreseen to make again a major impact, this time in the development of advanced materials. This is because of its availability and properties, as well as economic and sustainable production. Among cellulose-based structures, cellulose nanofibrils and nanocrystals display nanoscale lateral dimensions and lengths ranging from nanometers to micrometers. Their excellent mechanical properties are, in part, due to their crystalline assembly via hydrogen bonds. Owing to their abundant surface hydroxyl groups, they can be easily modified with nanoparticles, (bio)polymers, inorganics, or nanocarbons to form functional fibers, films, bulk matter, and porous aerogels and foams. Here, some of the recent progress in the development of advanced materials within this rapidly growing field is reviewed. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adma.201703779
  • 2018 • 126 Retention mechanisms of 1.7 nm ZnS quantum dots and sub-20 nm Au nanoparticles in ultrafiltration membranes
    Lee, H. and Segets, D. and Süß, S. and Peukert, W. and Chen, S.-C. and Pui, D.Y.H.
    Journal of Membrane Science 567 58-67 (2018)
    Membrane processes are considered to be a very effective and promising method for drinking water and wastewater treatments. However, particle removal mechanisms have not been fully elucidated due to complex surface interactions between colloids and membranes, especially for very small colloidal particles. In this study, a series of systematic filtration tests for eight different types of membrane filters, having nominal pore sizes from 0.005 to 0.1 µm, against 1.7 nm ZnS quantum dots (QDs) and 5, 10 and 20 nm Au nanoparticles (NPs) was performed to understand their retention mechanisms, including rejection in front of the filter surface and adsorption inside the filter. By comparing rejection, adsorption and recovery, it was found that the predominant retention mechanisms for retaining small NPs varied from filter to filter. For instance, electrostatic repulsion played a significant role for the rejection of NPs, i.e. impeding them entering the membrane pores in most membranes. In comparison, the Nylon membrane had a significant adsorption retention ability for Au NPs due to electrostatic attraction. Besides, it was found that filtration flow rate, or flux, was also an important parameter for the final retention because the enhanced hydrodynamic drag could trigger the detachment of deposited NPs or press NPs flowing through the superficial entrance leading to penetration. Tests of 10 nm Au NP retention using five different membranes with the same nominal pore size of 0.1 µm showed large variation of NP retention efficiencies demonstrating that pore size should not be used as the only criterion for rating filter performance, especially for small NPs. Our results provide not only detailed insights into the retention mechanisms of various membranes but also suggestions on how to select membrane filters for different filtration purposes. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2018.09.033
  • 2018 • 125 Densification of nanocrystalline NdFeB magnets processed by electro-discharge sintering – Microstructure, magnetic, and mechanical properties
    Leich, L. and Röttger, A. and Theisen, W. and Krengel, M.
    Journal of Magnetism and Magnetic Materials 460 454-460 (2018)
    This work investigates the densification process of nanocrystalline NdFeB powder by electro-discharge sintering (EDS) and the associated magnetic properties. The EDS technique is used as a fast and energy-saving compaction process for metal powders. A large current is discharged from capacitors into a pre-compacted loose powder, thus resulting in complete compaction. In this study, the microstructure, magnetic, and mechanical properties of the compacted, hard magnetic NdFeB specimens were investigated under variation of the energy EEDS and compression load pEDS. For all specimens, the intrinsic coercivity HcJ decreases on increasing the discharge energy. However, the compaction load has apparently no influence on the coercivity HcJ, whereas the residual induction Br decreases only with increasing discharge energy. An increase in the compression load pEDS causes an increase in the specimens’ density and thus promotes residual induction Br. The applied EDS parameters led to the formation of three different microstructures (insufficiently densified zone, fully densified zone, and remelted zone) along the cross-section of the EDS-densified specimens. Volume fractions of the three different microstructures that form during the EDS process determine the resulting mechanical and magnetic properties of the specimens. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2018.04.035
  • 2018 • 124 Ultrasound-mediated deposition and cytocompatibility of apatite-like coatings on magnesium alloys
    Liu, C.-N. and Böke, F. and Gebhard, M. and Devi, A. and Fischer, H. and Keller, A. and Grundmeier, G.
    Surface and Coatings Technology 345 167-176 (2018)
    A novel ultrasound-based approach for the deposition of an octacalcium phosphate (OCP) and nanocrystalline apatite (ncAp)-based coating on the magnesium alloy AZ31 as a biodegradable implant material is established. The studies consider both the structural analysis and the resulting corrosion protection and correlate the related findings with cytocompatibility. The ultrasound-based approach is shown to lead to the deposition of an OCP and ncAp-based coating with a trilayer structure on AZ31. The coatings consist of two inner compact layers and an open porous top layer. The formation of the functional OCP/ncAp coating and deposition on the AZ31 surface take place within minutes in a single processing step and do not require any pre-heating, mediators or post-deposition treatment for the achievement of corrosion protection and cytocompatibility. The obtained coatings were characterized by means of FTIR and Raman spectroscopy as well as FE-SEM and X-ray crystallography. Electrochemical impedance spectroscopy revealed corrosion inhibition especially in the initial phase after immersion in physiological electrolyte. Moreover, this corrosion protection resulted in a strongly improved cytocompatibility of AZ31 as verified by in vitro viability tests using human mesenchymal stromal cells. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2018.03.100
  • 2018 • 123 Zinc stannate by reactive laser sintering
    Mackert, V. and Gebauer, J.S. and Notthoff, C. and Winterer, M.
    Applied Surface Science 457 1174-1180 (2018)
    A novel procedure for producing polycrystalline zinc stannate (Zn 2 SnO 4 , ZTO) films is presented in this paper. Nanocrystals of zinc oxide (ZnO) and tin dioxide (SnO 2 ) are prepared by chemical vapor synthesis (CVS) and processed into stable aqueous dispersions including mixed colloids. These colloids are transformed into nanostructured films via electrophoretic deposition where the mixed colloid forms a homogeneous, nanoscaled composite. Ultraviolet (UV) laser sintering of these codeposited ZnO-SnO 2 nanocrystals generates the inverse cubic spinel Zn 2 SnO 4 phase by chemical reaction on the area of interest. The effects of UV laser sintering at a wavelength of 325 nm on the nanoscaled microstructure of pure deposited films are investigated by variation of laser power and scanning speed. The microstructure of composite films is compared to a film obtained by classical reactive sintering in a furnace. High-resolution scanning electron microscopy and energy dispersive X-ray spectroscopy are used to investigate film morphology and chemical composition. Structural characterization is performed by X-ray diffraction. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.06.304
  • 2018 • 122 Multifunctional Stimuli-Responsive Cellulose Nanocrystals via Dual Surface Modification with Genetically Engineered Elastin-Like Polypeptides and Poly(acrylic acid)
    Malho, J.-M. and Brand, J. and Pecastaings, G. and Ruokolainen, J. and Gröschel, A. and Sèbe, G. and Garanger, E. and Lecommandoux, S.
    ACS Macro Letters 7 646-650 (2018)
    Cellulose nanocrystals (CNCs) are promising candidates for a myriad of applications; however, successful utilization of CNCs requires balanced and multifunctional properties, which require ever more applied concepts for supramolecular tailoring. We present here a facile and straightforward route to generate dual functional CNCs using poly(acrylic acid) (PAA) and biosynthetic elastin-like polypeptides (ELPs). We utilize thiol-maleimide chemistry and SI-ATRP to harvest the temperature responsiveness of ELPs and pH sensitivity of PAA to confer multifunctionality to CNCs. Cryo-TEM and light microscopy are used to exhibit reversible temperature response, while atomic force microscopy (AFM) provides detailed information on the particle morphology. The approach is tunable and allows variation of the modifying molecules, inspiring supramolecular engineering beyond the currently presented motifs. The surge of genetically engineered peptides adds further possibilities for future exploitation of the potential of cellulose nanomaterials. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acsmacrolett.8b00321
  • 2018 • 121 Multiscale Characterization of Microstructure in Near-Surface Regions of a 16MnCr5 Gear Wheel After Cyclic Loading
    Medghalchi, S. and Jamebozorgi, V. and Bala Krishnan, A. and Vincent, S. and Salomon, S. and Basir Parsa, A. and Pfetzing, J. and Kostka, A. and Li, Y. and Eggeler, G. and Li, T.
    JOM 1-7 (2018)
    The dependence of the microstructure on the degree of deformation in near-surface regions of a 16MnCr5 gear wheel after 2.1 × 106 loading cycles has been investigated by x-ray diffraction analysis, transmission electron microscopy, and atom probe tomography. Retained austenite and large martensite plates, along with elongated lamella-like cementite, were present in a less deformed region. Comparatively, the heavily deformed region consisted of a nanocrystalline structure with carbon segregation up to 2 at.% at grain boundaries. Spheroid-shaped cementite, formed at the grain boundaries and triple junctions of the nanosized grains, was enriched with Cr and Mn but depleted with Si. Such partitioning of Cr, Mn, and Si was not observed in the elongated cementite formed in the less deformed zone. This implies that rolling contact loading induced severe plastic deformation as well as a pronounced annealing effect in the active contact region of the toothed gear during cyclic loading. © 2018 The Minerals, Metals & Materials Society
    view abstractdoi: 10.1007/s11837-018-2931-z
  • 2018 • 120 Crystallographic examination of the interaction between texture evolution, mechanically induced martensitic transformation and twinning in nanostructured bainite
    Morales-Rivas, L. and Archie, F. and Zaefferer, S. and Benito-Alfonso, M. and Tsai, S.-P. and Yang, J.-R. and Raabe, D. and Garcia-Mateo, C. and Caballero, F.G.
    Journal of Alloys and Compounds 752 505-519 (2018)
    The deformation mechanisms operating in nanostructured bainite, leading to its excellent combination of strength and ductility, are far from being understood. Its nanocrystalline nature and its multiphase-evolving structure underlie the plastic flow and the strain-hardening behaviour. In this work, the microstructural and crystallographic bulk changes of a high-C nanostructured bainite under tensile testing have been evaluated. The influence of the mechanically-induced transformation of the C-enriched retained austenite into α martensite and other deformation mechanisms on the texture evolution has been analysed by electron backscatter diffraction (EBSD). Additionally, the undeformed and the deformed conditions have been examined by electron channelling contrast imaging (ECCI) and transmission electron microscopy (TEM). Results reveal the presence of plate martensite and suggest a strong variant selection during the transformation, mainly responsible for the texture observed. Mechanical twinning in austenite seems to be basically the mechanism of accommodation of the displacive bainitic transformation, while some direct interaction with the applied stress also appears. © 2018
    view abstractdoi: 10.1016/j.jallcom.2018.04.189
  • 2018 • 119 Polymer brush guided templating on well-defined rod-like cellulose nanocrystals
    Morits, M. and Hynninen, V. and Nonappa and Niederberger, A. and Ikkala, O. and Gröschel, A.H. and Müllner, M.
    Polymer Chemistry 9 1650-1657 (2018)
    Cellulose is a natural biomaterial harvested from regrowing resources and it is one of the most attractive components for the construction of functional materials with low adverse ecological impact. Among various nanocelluloses, cellulose nanocrystals (CNC) are rod-like nanoparticles whose high crystallinity and stiffness make them viable candidates for templating materials. We report here on CNC-based polymer brushes used as templates for the synthesis of porous inorganic nanorods with tunable diameters and aspect ratios. The CNC were modified with initiation sites for surface-initiated polymerisation (SI-ATRP) to act as a backbone for the grafting of poly(2-(dimethyl amino)ethyl methacrylate) (PDMAEMA) brushes. Controlled polymerisation conditions allowed for adjusting the brush length and consequently the morphology of the hybrid nanomaterials. The PDMAEMA brush served as coordination and nucleation sites for the mineralisation of tetramethyl orthosilicate (TMOS) into SiO2@CNC-g-PDMAEMA hybrids. After calcination, microscopy and N2-sorption measurements revealed hollow silica nanorods with accessible micro-and meso-pores. We foresee that this strategy can be adapted to other nanocelluloses to create high-Aspect ratio porous silica nanotubes, or to achieve uniform depositions in the mineralisation of other inorganic metals or metal oxide compounds. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7py01814b
  • 2018 • 118 Subsurface characterization of high-strength high-interstitial austenitic steels after impact wear
    Mujica Roncery, L. and Agudo Jácome, L. and Aghajani, A. and Theisen, W. and Weber, S.
    Wear 402-403 137-147 (2018)
    The microstructure of the subsurface after impact wear of three high-strength high interstitial austenitic steels has been analysed using transmission electron microscopy (TEM) in alloys with C and N as interstitial elements. In all cases, a nanocrystalline region followed by a transition zone and a cold-worked area are present. Additionally, microhardness and nano-scratching tests were conducted to study the wear-related properties of the impact subsurface and the base material. The results of the microstructural analysis reveal that the following mechanisms are involved during impact wear: abrasion (ploughing), microcrack formation associated with contact fatigue, entrapment and adhesion of SiO2 particles. The analysis of the wear-related properties indicates that the subsurface acts as a self-protective layer that prevents the deterioration of the substrate. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2018.02.016
  • 2018 • 117 A Charge-Tunable Quantum Dot Deep in the Strong Coupling Regime of Cavity QED
    Najer, D. and Sollner, I. and Loebl, M.C. and Riedel, D. and Petrak, B. and Starosielec, S. and Dolique, V. and Valentin, S.R. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Warburton, R.J.
    IEEE Photonics Society Summer Topicals Meeting Series, SUM 2018 169-170 (2018)
    We present high-cooperativity (C up to 140) strong coupling of a charge-Tunable InAs quantum dot embedded in a tunable Fabry-Pérot microcavity (Q=500,000). Via second-order correlation measurements we show high single-photon purity in the photon-blockade regime and pronounced vacuum Rabi oscillations in the photon-induced tunneling regime. © 2018 IEEE.
    view abstractdoi: 10.1109/PHOSST.2018.8456757
  • 2018 • 116 Local Structure of Nanocrystalline Aluminum Nitride
    Ognjanović, S.M. and Zähres, M. and Mayer, C. and Winterer, M.
    Journal of Physical Chemistry C 122 23749-23757 (2018)
    The local structure of chemical-vapor-synthesized (CVS) crystalline AlN nanoparticles is investigated by combining magic angle spinning nuclear magnetic resonance and X-ray absorption spectroscopies. Extended X-ray absorption fine structure data are analyzed by reverse Monte Carlo method, and X-ray absorption near edge structure is interpreted by first principles FEFF calculations. The measurements show behavior characteristic of partially disordered systems. Nevertheless, combined analysis of the data, supported by Rietveld refinement of X-ray diffraction patterns, leads to the conclusion that the observed behavior is due to the small size (large surface to volume ratios) of the nanoparticles (dXRD &lt; 6 nm) and that highly crystalline wurtzite AlN is formed during the CVS process. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b06610
  • 2018 • 115 Germanium Template Assisted Integration of Gallium Arsenide Nanocrystals on Silicon: A Versatile Platform for Modern Optoelectronic Materials
    Schmitt, S.W. and Sarau, G. and Speich, C. and Döhler, G.H. and Liu, Z. and Hao, X. and Rechberger, S. and Dieker, C. and Spiecker, E. and Prost, W. and Tegude, F.J. and Conibeer, G. and Green, M.A. and Christiansen, S.H.
    Advanced Optical Materials (2018)
    Metal organic vapor phase epitaxy is used to grow gallium arsenide (GaAs) nanocrystals (NCs) on germanium (Ge) templates on nanoscopic silicon (Si) threads prepared by reactive ion etching. Scanning transmission electron microscopy with energy dispersive X-ray measurements shows an epitaxial growth of the GaAs on the Ge template that is supported by the Si thread, and that Ge doping is induced to the GaAs by the template. On Ge templates of about 60 nm diameter, as-grown GaAs NCs show a very regular rhombic-dodecahedral outer shape that can be explained by a preferential growth along the <110> plane. Photoluminescence measurements of the Ge/GaAs structures reveal radiative emission peaks on top of the GaAs band-to-band emission and at sub-band gap energies. While high energy peaks are originating from Ge acceptor levels in GaAs, sub-band gap peaks can be explained by radiation from Ge donor and acceptor bands that are amplified by photonic modes hosted in the rhombic-dodecahedral GaAs NCs. This study shows that a template-assisted crystal growth at the nanoscale opens up routes for a versatile integration of strongly emitting nanomaterials for a use in on-chip solid state lighting and photonics. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adom.201701329
  • 2018 • 114 Addressing the exciton fine structure in colloidal nanocrystals: The case of CdSe nanoplatelets
    Shornikova, E.V. and Biadala, L. and Yakovlev, D.R. and Sapega, V.F. and Kusrayev, Y.G. and Mitioglu, A.A. and Ballottin, M.V. and Christianen, P.C.M. and Belykh, V.V. and Kochiev, M.V. and Sibeldin, N.N. and Golovatenko, A.A. and...
    Nanoscale 10 646-656 (2018)
    We study the band-edge exciton fine structure and in particular its bright-dark splitting in colloidal semiconductor nanocrystals by four different optical methods based on fluorescence line narrowing and time-resolved measurements at various temperatures down to 2 K. We demonstrate that all these methods provide consistent splitting values and discuss their advances and limitations. Colloidal CdSe nanoplatelets with thicknesses of 3, 4 and 5 monolayers are chosen for experimental demonstrations. The bright-dark splitting of excitons varies from 3.2 to 6.0 meV and is inversely proportional to the nanoplatelet thickness. Good agreement between experimental and theoretically calculated size dependence of the bright-dark exciton splitting is achieved. The recombination rates of the bright and dark excitons and the bright to dark relaxation rate are measured by time-resolved techniques. © The Royal Society of Chemistry 2018.
    view abstractdoi: 10.1039/c7nr07206f
  • 2018 • 113 Electron and Hole g-Factors and Spin Dynamics of Negatively Charged Excitons in CdSe/CdS Colloidal Nanoplatelets with Thick Shells
    Shornikova, E.V. and Biadala, L. and Yakovlev, D.R. and Feng, D. and Sapega, V.F. and Flipo, N. and Golovatenko, A.A. and Semina, M.A. and Rodina, A.V. and Mitioglu, A.A. and Ballottin, M.V. and Christianen, P.C.M. and Kusrayev, Y...
    Nano Letters 18 373-380 (2018)
    We address spin properties and spin dynamics of carriers and charged excitons in CdSe/CdS colloidal nanoplatelets with thick shells. Magneto-optical studies are performed by time-resolved and polarization-resolved photoluminescence, spin-flip Raman scattering and picosecond pump-probe Faraday rotation in magnetic fields up to 30 T. We show that at low temperatures the nanoplatelets are negatively charged so that their photoluminescence is dominated by radiative recombination of negatively charged excitons (trions). Electron g-factor of 1.68 is measured, and heavy-hole g-factor varying with increasing magnetic field from -0.4 to -0.7 is evaluated. Hole g-factors for two-dimensional structures are calculated for various hole confining potentials for cubic- and wurtzite lattice in CdSe core. These calculations are extended for various quantum dots and nanoplatelets based on II-VI semiconductors. We developed a magneto-optical technique for the quantitative evaluation of the nanoplatelets orientation in ensemble. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b04203
  • 2018 • 112 Suspension- and powder-based derivation of Hansen dispersibility parameters for zinc oxide quantum dots
    Süß, S. and Lin, W. and Getmanenko, O. and Pflug, L. and Sobisch, T. and Peukert, W. and Lerche, D. and Segets, D.
    Particuology (2018)
    For most particle-based applications, formulation in the liquid phase is a decisive step, and thus, particle interactions and stability in liquid media are of major importance. The concept of Hansen solubility parameters (HSP) was initially invented to describe the interactions of (polymer) molecules and their solubility in different liquids and is increasingly being used in particle technology to describe dispersibility. Because dispersions are not thermodynamically stable, the term Hansen dispersibility parameters (HDP) is used instead of HSP (Süß Sobisch, Peukert, Lerche, & Segets, 2018). Herein, we extend a previously developed standardized and non-subjective method for determination of Hansen parameters based on analytical centrifugation to the important class of quantum materials. As a technically relevant model system, zinc oxide quantum dots (QDs) were used to transfer our methodology to nanoparticles (NPs) with sizes below 10 nm. The results obtained using the standard procedure starting from a dried powder were compared with those obtained through redispersion from the wet sediment produced during the typical washing procedure of QDs, and drying was observed to play an important role. In conclusion, our study reveals the high potential of HDP for quantifying the interfacial properties of NPs as well as their link to dispersibility. © 2018 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences
    view abstractdoi: 10.1016/j.partic.2018.05.010
  • 2018 • 111 Quantum Optics with Near-Lifetime-Limited Quantum-Dot Transitions in a Nanophotonic Waveguide
    Thyrrestrup, H. and Kiršanske, G. and Le Jeannic, H. and Pregnolato, T. and Zhai, L. and Raahauge, L. and Midolo, L. and Rotenberg, N. and Javadi, A. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Löbl, M.C. and Söllner, I....
    Nano Letters 18 1801-1806 (2018)
    Establishing a highly efficient photon-emitter interface where the intrinsic linewidth broadening is limited solely by spontaneous emission is a key step in quantum optics. It opens a pathway to coherent light-matter interaction for, e.g., the generation of highly indistinguishable photons, few-photon optical nonlinearities, and photon-emitter quantum gates. However, residual broadening mechanisms are ubiquitous and need to be combated. For solid-state emitters charge and nuclear spin noise are of importance, and the influence of photonic nanostructures on the broadening has not been clarified. We present near-lifetime-limited linewidths for quantum dots embedded in nanophotonic waveguides through a resonant transmission experiment. It is found that the scattering of single photons from the quantum dot can be obtained with an extinction of 66 ± 4%, which is limited by the coupling of the quantum dot to the nanostructure rather than the linewidth broadening. This is obtained by embedding the quantum dot in an electrically contacted nanophotonic membrane. A clear pathway to obtaining even larger single-photon extinction is laid out; i.e., the approach enables a fully deterministic and coherent photon-emitter interface in the solid state that is operated at optical frequencies. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b05016
  • 2018 • 110 Co2+-Doping of Magic-Sized CdSe Clusters: Structural Insights via Ligand Field Transitions
    Yang, J. and Muckel, F. and Choi, B.K. and Lorenz, S. and Kim, I.Y. and Ackermann, J. and Chang, H. and Czerney, T. and Kale, V.S. and Hwang, S.-J. and Bacher, G. and Hyeon, T.
    Nano Letters 18 7350-7357 (2018)
    Magic-sized clusters represent materials with unique properties at the border between molecules and solids and provide important insights into the nanocrystal formation process. However, synthesis, doping, and especially structural characterization become more and more challenging with decreasing cluster size. Herein, we report the successful introduction of Co2+ ions into extremely small-sized CdSe clusters with the intention of using internal ligand field transitions to obtain structural insights. Despite the huge mismatch between the radii of Cd2+ and Co2+ ions (&gt;21%), CdSe clusters can be effectively synthesized with a high Co2+ doping concentration of ∼10%. Optical spectroscopy and mass spectrometry suggest that one or two Co2+ ions are substitutionally embedded into (CdSe)13 clusters, which is known as one of the smallest CdSe clusters. Using magnetic circular dichroism spectroscopy on the intrinsic ligand field transitions between the different 3d orbitals of the transition metal dopants, we demonstrate that the Co2+ dopants are embedded on pseudotetrahedral selenium coordinated sites despite the limited number of atoms in the clusters. A significant shortening of Co-Se bond lengths compared to bulk or nanocrystals is observed, which results in the metastability of Co2+ doping. Our results not only extend the doping chemistry of magic-sized semiconductor nanoclusters, but also suggest an effective method to characterize the local structure of these extremely small-sized clusters. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.8b03627
  • 2017 • 109 Negatively Charged and Dark Excitons in CsPbBr3 Perovskite Nanocrystals Revealed by High Magnetic Fields
    Canneson, D. and Shornikova, E.V. and Yakovlev, D.R. and Rogge, T. and Mitioglu, A.A. and Ballottin, M.V. and Christianen, P.C.M. and Lhuillier, E. and Bayer, M. and Biadala, L.
    Nano Letters 17 6177-6183 (2017)
    The optical properties of colloidal cesium lead halide perovskite (CsPbBr3) nanocrystals are examined by time-resolved and polarization-resolved spectroscopy in high magnetic fields up to 30 T. We unambiguously show that at cryogenic temperatures the emission is dominated by recombination of negatively charged excitons with radiative decay time of 300 ps. The additional long-lived emission, which decay time shortens from 40 down to 8 ns and in which the decay time shortens and relative amplitude increases in high magnetic fields, evidences the presence of a dark exciton. We evaluate g-factors of the bright exciton gX = +2.4, the electron ge = +2.18, and the hole gh = -0.22. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b02827
  • 2017 • 108 Role of Composition and Size of Cobalt Ferrite Nanocrystals in the Oxygen Evolution Reaction
    Chakrapani, K. and Bendt, G. and Hajiyani, H. and Schwarzrock, I. and Lunkenbein, T. and Salamon, S. and Landers, J. and Wende, H. and Schlögl, R. and Pentcheva, R. and Behrens, M. and Schulz, S.
    ChemCatChem 9 2988-2995 (2017)
    Sub-10 nm CoFe2O4 nanoparticles with different sizes and various compositions obtained by (partial) substitution of Co with Ni cations have been synthesized by using a one-pot method from organic solutions by the decomposition of metal acetylacetonates in the presence of oleylamine. The electrocatalytic activity of CoFe2O4 towards the oxygen evolution reaction (OER) is clearly enhanced with a smaller size (3.1 nm) of the CoFe2O4 nanoparticles (compared with 4.5 and 5.9 nm). In addition, the catalytic activity is improved by partial substitution of Co with Ni, which also leads to a higher degree of inversion of the spinel structure. Theoretical calculations attribute the positive catalytic effect of Ni owing to the lower binding energy differences between adsorbed O and OH compared with pure cobalt or nickel ferrites, resulting in higher OER activity. Co0.5Ni0.5Fe2O4 exhibited a low overpotential of approximately 340 mV at 10 mA cm−2, a smaller Tafel slope of 51 mV dec−1, and stability over 30 h. The unique tunability of these CoFe2O4 nanocrystals provides great potential for their application as an efficient and competitive anode material in the field of electrochemical water splitting as well as for systematic fundamental studies aiming at understanding the correlation of composition and structure with performance in electrocatalysis. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201700376
  • 2017 • 107 Photocatalytic Polymerization of 3,4-Ethylenedioxythiophene over Cesium Lead Iodide Perovskite Quantum Dots
    Chen, K. and Deng, X. and Dodekatos, G. and Tüysüz, H.
    Journal of the American Chemical Society 139 12267-12273 (2017)
    The outstanding performance of halide perovskites in optoelectronic applications can be partly attributed to their high absorption coefficient and long carrier lifetime, which are also desirable for photocatalysts. Herein, we report that cesium lead iodide perovskite quantum dots (CsPbI3 QDs) can be used as catalysts to promote the polymerization of 2,2′,5′,2″-ter-3,4-ethylenedioxythiophene under visible light illumination while preserving the quantum dot in the desirable cubic crystal phase. Simultaneously, the generated conducting poly(3,4-ethylenedioxythiophene), PEDOT, encapsulates and stabilizes the morphology of the CsPbI3 QDs. The photocatalytic polymerization clearly depends on the concentration of the CsPbI3 QDs, and the CsPbI3 QDs maintain the desirable perovskite phase when the concentration of the QD increases. Molecular oxygen and 1,4-benzoquinone can serve as electron acceptors during the photocatalytic polymerization reaction. When molecular oxygen is used, the structure of the CsPbI3 QD transforms from cubic to orthorhombic, while usage of 1,4-benzoquinone preserves the cubic phase of CsPbI3 QD. This novel approach enables the one-step formation of CsPbI3/PEDOT composite, which could be promising for the preparation of novel optoelectronic materials and high performance devices. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b06413
  • 2017 • 106 Picosecond Control of Quantum Dot Laser Emission by Coherent Phonons
    Czerniuk, T. and Wigger, D. and Akimov, A.V. and Schneider, C. and Kamp, M. and Höfling, S. and Yakovlev, D.R. and Kuhn, T. and Reiter, D.E. and Bayer, M.
    Physical Review Letters 118 (2017)
    A picosecond acoustic pulse can be used to control the lasing emission from semiconductor nanostructures by shifting their electronic transitions. When the active medium, here an ensemble of (In,Ga)As quantum dots, is shifted into or out of resonance with the cavity mode, a large enhancement or suppression of the lasing emission can dynamically be achieved. Most interesting, even in the case when gain medium and cavity mode are in resonance, we observe an enhancement of the lasing due to shaking by coherent phonons. In order to understand the interactions of the nonlinearly coupled photon-exciton-phonon subsystems, we develop a semiclassical model and find an excellent agreement between theory and experiment. © 2017 American Physical Society. American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.118.133901
  • 2017 • 105 Water-free synthesis of ZnO quantum dots for application as an electron injection layer in light-emitting electrochemical cells
    Daumann, S. and Andrzejewski, D. and Di Marcantonio, M. and Hagemann, U. and Wepfer, S. and Vollkommer, F. and Bacher, G. and Epple, M. and Nannen, E.
    Journal of Materials Chemistry C 5 2344-2351 (2017)
    Large-area light emitters like organic (OLEDs) or quantum dot light-emitting devices (QLEDs) and light-emitting electrochemical cells (LECs) have gained increasing interest due to their cost-effective fabrication on various even flexible substrates. The implementation of ZnO nanoparticles as an electron injection layer in large-area emitters leads to efficient solution-based devices. However, ZnO support layers are frequently in direct contact with water-sensitive emitter materials, which requires ZnO nanoparticles with minimum water content. A water-free synthesis route (except for the small amount of water formed during the synthesis) of ligand-free ZnO nanoparticles is presented. The spherical ZnO nanoparticles have a diameter of 3.4 nm, possess a high crystallinity, and form stable dispersions in ethanol or 1-hexanol. Their application together with a transition metal complex (iTMC)-LEC as an additional electron injection layer resulted in an increase of the device efficiency from 1.6 to 2.4 lm W−1 as well as the reduction of the run-up time to one fifth, compared to the same system without ZnO nanoparticles. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6tc05571k
  • 2017 • 104 All-electrical measurement of the triplet-singlet spin relaxation time in self-assembled quantum dots
    Eltrudis, K. and Al-Ashouri, A. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Geller, M. and Lorke, A.
    Applied Physics Letters 111 (2017)
    We have measured the spin relaxation time of an excited two-electron spin-triplet state into its singlet ground state in self-assembled InAs/GaAs quantum dots. We use a time-resolved measurement scheme that combines transconductance spectroscopy with spin-to-charge conversion to address the |s↑,p↑〉 triplet state, where one electron is in the quantum dot s-shell and a second one in the p-shell. The evaluation of the state-selective tunneling times from the dots into a nearby two-dimensional electron gas allows us to determine the s- and p-shell occupation and extract the relaxation time from a rate equation model. A comparably long triplet-to-singlet spin relaxation time of 25 μs is found. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4985572
  • 2017 • 103 Dynamic Evolution from Negative to Positive Photocharging in Colloidal CdS Quantum Dots
    Feng, D. and Yakovlev, D.R. and Pavlov, V.V. and Rodina, A.V. and Shornikova, E.V. and Mund, J. and Bayer, M.
    Nano Letters 17 2844-2851 (2017)
    The optical properties of colloidal semiconductor nanocrystals are largely influenced by the trapping of charge carriers on the nanocrystal surface. Different concentrations of electron and hole traps and different rates of their capture to the traps provide dynamical charging of otherwise neutral nanocrystals. We study the photocharging formation and evolution dynamics in CdS colloidal quantum dots with native oleic acid surface ligands. A time-resolved technique with three laser pulses (pump, orientation, and probe) is developed to monitor the photocharging dynamics with picosecond resolution on wide time scales ranging from picoseconds to milliseconds. The detection is based on measuring the coherent spin dynamics of electrons, allowing us to distinguish the type of carrier in the QD core (electron or hole). We find that although initially negative photocharging happens because of fast hole trapping, it eventually evolves to positive photocharging due to electron trapping and hole detrapping. The positive photocharging lasts up to hundreds of microseconds at room temperature. These findings give insight into the photocharging process and provide valuable information for understanding the mechanisms responsible for the emission blinking in colloidal nanostructures. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b05305
  • 2017 • 102 A linear triple quantum dot system in isolated configuration
    Flentje, H. and Bertrand, B. and Mortemousque, P.-A. and Thiney, V. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Applied Physics Letters 110 (2017)
    The scaling up of electron spin qubit based nanocircuits has remained challenging up till date and involves the development of efficient charge control strategies. Here, we report on the experimental realization of a linear triple quantum dot in a regime isolated from the reservoir. We show how this regime can be reached with a fixed number of electrons. Charge stability diagrams of the one, two, and three electron configurations where only electron exchange between the dots is allowed are observed. They are modeled with the established theory based on a capacitive model of the dot systems. The advantages of the isolated regime with respect to experimental realizations of quantum simulators and qubits are discussed. We envision that the results presented here will make more manipulation schemes for existing qubit implementations possible and will ultimately allow to increase the number of tunnel coupled quantum dots which can be simultaneously controlled. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4984745
  • 2017 • 101 Nonagglomerated Iron Oxyhydroxide Akaganeite Nanocrystals Incorporating Extraordinary High Amounts of Different Dopants
    Fominykh, K. and Böhm, D. and Zhang, S. and Folger, A. and Döblinger, M. and Bein, T. and Scheu, C. and Fattakhova-Rohlfing, D.
    Chemistry of Materials 29 7223-7233 (2017)
    Dispersible nonagglomerated akaganeite (β-FeOOH) nanocrystals doped with various elements in different oxidation states such as Co(II), Ni(II), V(III), Ti(IV), Sn(IV), Si(IV), and Nb(V) were prepared using a microwave-assisted solvothermal synthesis in tert-butanol. The doping elements could be incorporated in very high concentrations of up to 20 at.â€"%, which is attributed to the kinetic control of the phase formation during the solvothermal reaction, together with the extremely small crystal size, which can stabilize the unusual structural compositions. The particle morphology is mostly anisotropic consisting of nanorods â1/44 nm in width and varying length. Depending on the doping element, the length ranges from â4 nm, resulting in an almost-spherical shape, to 90 nm, giving the highest aspect ratio. The particles are perfectly dispersible in water, giving stable colloidal dispersions that can be deposited on different substrates to produce thin films 35-250 nm thick. In addition, films up to 30 μm thick, consisting of interconnected mesoporous spheres, can be prepared in situ during the reactions. The nanostructures assembled from akaganeite nanocrystals are stable up to high temperatures of &gt;400 °C. They can be transformed to hematite (α-Fe2O3) by heating between 480 °C and 600 °C without losing the morphology, which can be used for the fabrication of doped hematite nanostructures. The tunable properties of the doped akaganeite nanoparticles make them excellent candidates for a wide range of applications, as well as versatile building blocks for the fabrication of doped hematite nanomorphologies. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.7b01611
  • 2017 • 100 Stability, phase separation and oxidation of a supersaturated nanocrystalline Cu-33 at.% Cr thin film alloy
    Harzer, T.P. and Dehm, G.
    Thin Solid Films 623 48-58 (2017)
    A binary nanocrystalline Cu67Cr33 thin film alloy consisting of columnar grains was synthesized via co-evaporation of the constituent elements under non-equilibrium ultra-high vacuum conditions using molecular beam epitaxy. In the as-deposited state, the alloy film is a supersaturated solid solution with a single-phase body-centered cubic structure. In order to study the thermal stability of the microstructure and phase separation behavior towards the two phase equilibrium structure, isothermal annealing experiments in a temperature range of 150 °C – 500 °C were conducted inside a transmission electron microscope and compared to data obtained by X-ray diffraction under protective N2 atmosphere. It is shown that the single-phase nature of the alloy film is maintained for annealing temperatures of ≤ 300 °C, whereas heat treatment at temperatures of ≥ 400 °C results in the formation of a second phase, i.e. the equilibrium face-centered cubic phase of Cu. Phase separation proceeds predominantly by a spinodal-type decomposition process but a simultaneous diffusion of Cr along the columnar grain boundaries to the surface of the alloy film is observed as well. Temperature dependent diffusion coefficients for volume and grain boundary diffusion along with the activation energy for volume diffusion of Cr within the crystal lattice of the alloy film in a temperature range between 400 °C – 500 °C are determined from analytical in situ transmission electron microscopy experiments. Moreover, grain boundary diffusion of Cr leads to the growth of an external Cr-rich oxide scale. It is found that the growth kinetics of this oxide scale exhibits a transition from a linear to a nearly parabolic growth rate. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2016.12.048
  • 2017 • 99 In–situ TEM study of diffusion kinetics and electron irradiation effects on the Cr phase separation of a nanocrystalline Cu–4 at.% Cr thin film alloy
    Harzer, T.P. and Duarte, M.J. and Dehm, G.
    Journal of Alloys and Compounds 695 1583-1590 (2017)
    The Cr phase separation process and kinetics of a metastable Cu96Cr4 alloy film were investigated by isothermal annealing at different temperatures of up to 500 °C using transmission electron microscopy. It is shown that the Cr phase separation proceeds predominantly via enrichment of Cr at grain boundaries and grain boundary diffusion. Temperature dependent diffusion coefficients and the activation energy for grain boundary diffusion of Cr in face–centered cubic Cu are determined from analytical in–situ transmission electron microscopy experiments. In addition, the influence of electron beam irradiation on the diffusion kinetics is considered. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2016.10.302
  • 2017 • 98 Fracture behavior of nanostructured heavily cold drawn pearlitic steel wires before and after annealing
    Jaya, B.N. and Goto, S. and Richter, G. and Kirchlechner, C. and Dehm, G.
    Materials Science and Engineering A 707 164-171 (2017)
    In situ micro-cantilever fracture testing is used to demonstrate changes in fracture behavior of nanostructured, heavily cold drawn pearlitic steel wires as a function of drawing strain and annealing conditions. It is shown that these steels exhibit a sharp transition in fracture behavior between a drawing strain of 320% and 520% with a drop in fracture toughness from 7.5 to 4 MPam1/2. This is confirmed from the nature of fracture which is stable with some degree of plasticity at drawing strains below 320% and changes to catastrophic cleavage fracture at drawing strains of 420% and above. This transition and associated brittleness is attributed to structural (cementite decomposition and strain induced increase in tetragonality) and microstructural (increasing nanocrystallinity and dislocation density) evolution that these steels undergo at higher drawing strains. On heat treating the 420% strained sample, brittle cleavage fracture continues for low temperature (200 °C) annealing with no visible changes in microstructure, while crack growth is suppressed and large-scale plasticity is recovered for high temperature (500 °C) annealing with accompanying grain coarsening, and re-precipitation of spherodized cementite at grain boundaries. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.09.010
  • 2017 • 97 Atomic scale characterization of white etching area and its adjacent matrix in a martensitic 100Cr6 bearing steel
    Li, Y.J. and Herbig, M. and Goto, S. and Raabe, D.
    Materials Characterization 123 349-353 (2017)
    Atom probe tomography was employed to characterize the microstructure and C distribution in the white etching area (WEA) of a martensitic 100Cr6 bearing steel subjected to rolling contact fatigue. Different from its surrounding matrix where a plate-like martensitic structure prevails, the WEA exhibits equiaxed grains with a uniform grain size of about 10 nm. Significant C grain boundary enrichment (>7.5at.%) and an overall higher C concentration than the nominal value are observed in the WEA. These results suggest that the formation of WEA results from severe local plastic deformation that causes dissolution of carbides and the redistribution of C. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.matchar.2016.12.002
  • 2017 • 96 Ultra-low charge and spin noise in self-assembled quantum dots
    Ludwig, Ar. and Prechtel, J.H. and Kuhlmann, A.V. and Houel, J. and Valentin, S.R. and Warburton, R.J. and Wieck, A.D.
    Journal of Crystal Growth 477 193-196 (2017)
    Self-assembled InxGa1−xAs quantum dots (QDs) are promising hosts for spin qubits with excellent coupling to photons. Nuclear spin and charge fluctuations lead to dephasing and limit the applicability of QDs as qubits. We show that charge noise can be minimized by high quality MBE growth of well-designed heterostructures yielding natural optical linewidths down to 1.15 µeV. To minimize the nuclear spin noise, one direction would be to reduce the wave function overlap with the nuclei. We show that this is indeed the case for a single hole spin in a QD that we embedded in the intrinsic region of an n-i-p-diode. For random nuclei, the heavy-hole limit is achieved down to neV energies, equivalent to dephasing times of microseconds. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jcrysgro.2017.05.008
  • 2017 • 95 Nanoindentation testing as a powerful screening tool for assessing phase stability of nanocrystalline high-entropy alloys
    Maier-Kiener, V. and Schuh, B. and George, E.P. and Clemens, H. and Hohenwarter, A.
    Materials and Design 115 479-485 (2017)
    The equiatomic high-entropy alloy (HEA), CrMnFeCoNi, has recently been shown to be microstructurally unstable, resulting in a multi-phase microstructure after intermediate-temperature annealing treatments. The decomposition occurs rapidly in the nanocrystalline (NC) state and after longer annealing times in coarse-grained states. To characterize the mechanical properties of differently annealed NC states containing multiple phases, nanoindentation was used. The results revealed besides drastic changes in hardness, also for the first time significant changes in the Young's modulus and strain rate sensitivity. Nanoindentation of NC HEAs is, therefore, a useful complementary screening tool with high potential as a high throughput approach to detect phase decomposition, which can also be used to qualitatively predict the long-term stability of single-phase HEAs. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2016.11.055
  • 2017 • 94 Insights into the deformation behavior of the CrMnFeCoNi high-entropy alloy revealed by elevated temperature nanoindentation
    Maier-Kiener, V. and Schuh, B. and George, E.P. and Clemens, H. and Hohenwarter, A.
    Journal of Materials Research 32 2658-2667 (2017)
    A CrMnFeCoNi high-entropy alloy was investigated by nanoindentation from room temperature to 400 °C in the nanocrystalline state and cast plus homogenized coarse-grained state. In the latter case a âŒ100)-orientated grain was selected by electron back scatter diffraction for nanoindentation. It was found that hardness decreases more strongly with increasing temperature than Young's modulus, especially for the coarse-grained state. The modulus of the nanocrystalline state was slightly higher than that of the coarse-grained one. For the coarse-grained sample a strong thermally activated deformation behavior was found up to 100-150 °C, followed by a diminishing thermally activated contribution at higher testing temperatures. For the nanocrystalline state, different temperature dependent deformation mechanisms are proposed. At low temperatures, the governing processes appear to be similar to those in the coarse-grained sample, but with increasing temperature, dislocation-grain boundary interactions likely become more dominant. Finally, at 400 °C, decomposition of the nanocrystalline alloy causes a further reduction in thermal activation. This is rationalized by a reduction of the deformation controlling internal length scale by precipitate formation in conjunction with a diffusional contribution. © 2017 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2017.260
  • 2017 • 93 Current-Induced Magnetic Polarons in a Colloidal Quantum-Dot Device
    Muckel, F. and Barrows, C.J. and Graf, A. and Schmitz, A. and Erickson, C.S. and Gamelin, D.R. and Bacher, G.
    Nano Letters 17 4768-4773 (2017)
    Electrical spin manipulation remains a central challenge for the realization of diverse spin-based information processing technologies. Motivated by the demonstration of confinement-enhanced sp-d exchange interactions in colloidal diluted magnetic semiconductor (DMS) quantum dots (QDs), such materials are considered promising candidates for future spintronic or spin-photonic applications. Despite intense research into DMS QDs, electrical control of their magnetic and magneto-optical properties remains a daunting goal. Here, we report the first demonstration of electrically induced magnetic polaron formation in any DMS, achieved by embedding Mn2+-doped CdSe/CdS core/shell QDs as the active layer in an electrical light-emitting device. Tracing the electroluminescence from cryogenic to room temperatures reveals an anomalous energy shift that reflects current-induced magnetization of the Mn2+ spin sublattice, that is, excitonic magnetic polaron formation. These electrically induced magnetic polarons exhibit an energy gain comparable to their optically excited counterparts, demonstrating that magnetic polaron formation is achievable by current injection in a solid-state device. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b01496
  • 2017 • 92 Robust Single-Shot Spin Measurement with 99.5% Fidelity in a Quantum Dot Array
    Nakajima, T. and Delbecq, M.R. and Otsuka, T. and Stano, P. and Amaha, S. and Yoneda, J. and Noiri, A. and Kawasaki, K. and Takeda, K. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Loss, D. and Tarucha, S.
    Physical Review Letters 119 (2017)
    We demonstrate a new method for projective single-shot measurement of two electron spin states (singlet versus triplet) in an array of gate-defined lateral quantum dots in GaAs. The measurement has very high fidelity and is robust with respect to electric and magnetic fluctuations in the environment. It exploits a long-lived metastable charge state, which increases both the contrast and the duration of the charge signal distinguishing the two measurement outcomes. This method allows us to evaluate the charge measurement error and the spin-to-charge conversion error separately. We specify conditions under which this method can be used, and project its general applicability to scalable quantum dot arrays in GaAs or silicon. © 2017 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.119.017701
  • 2017 • 91 A triangular triple quantum dot with tunable tunnel couplings
    Noiri, A. and Kawasaki, K. and Otsuka, T. and Nakajima, T. and Yoneda, J. and Amaha, S. and Delbecq, M.R. and Takeda, K. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Semiconductor Science and Technology 32 (2017)
    A two-dimensional arrangement of quantum dots (QDs) with finite inter-dot tunnel coupling provides a promising platform for studying complicated spin correlations as well as for constructing large-scale quantum computers. Here, we fabricate a tunnel-coupled triangular triple QD with a novel gate geometry in which three dots are defined by positively biasing the surface gates. At the same time, the small area in the center of the triangle is depleted by negatively biasing the top gate placed above the surface gates. The size of the small center depleted area is estimated from the Aharonov-Bohm oscillation measured for the triangular channel but incorporating no gate-defined dots, with a value consistent with the design. With this approach, we can bring the neighboring gate-defined dots close enough to one another to maintain a finite inter-dot tunnel coupling. We finally confirm the presence of the inter-dot tunnel couplings in the triple QD from the measurement of tunneling current through the dots in the stability diagram. We also show that the charge occupancy of each dot and that the inter-dot tunnel couplings are tunable with gate voltages. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6641/aa7596
  • 2017 • 90 Ion-induced interdiffusion of surface GaN quantum dots
    Rothfuchs, C. and Semond, F. and Portail, M. and Tottereau, O. and Courville, A. and Wieck, A.D. and Ludwig, Ar.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 409 107-110 (2017)
    In the flourishing fields of quantum technology gallium nitride (GaN) quantum dots (QDs) have great appeal by providing high stability and room-temperature operation. Here, we report on the ion implantation of surface GaN QDs grown in the hexagonal crystal structure. An uncapped sample (S1) and two samples capped by 8 ML (S2) and 16 ML (S3) of AlN are subjected to a 100 keV gallium (S1, S2) and a 210 keV erbium (S3) ion beam. The fluence ranged from 5×1010 cm−2 to 1×1015 cm−2 (S1, S2) and from 5×1010 cm−2 to 5×1013 cm−2 (S3). QD characterization is performed by cathodoluminescence measurements at 77 K and atomic force microscopy and scanning electron microscopy. Strong interdiffusion processes upon ion impact at the interfaces are evidenced leading besides other effects to a quenching of the quantum confined Stark effect. Moreover, a model for the QD morphology based on a fluence-dependent diffusion coefficient is developed. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.nimb.2017.04.036
  • 2017 • 89 Automated synthesis of quantum dot nanocrystals by hot injection: Mixing induced self-focusing
    Salaheldin, A.M. and Walter, J. and Herre, P. and Levchuk, I. and Jabbari, Y. and Kolle, J.M. and Brabec, C.J. and Peukert, W. and Segets, D.
    Chemical Engineering Journal 320 232-243 (2017)
    The hot injection technique for the synthesis of quantum dots (QDs) is a well-established and widely used method in the lab. However, scale-up rules do not exist. One reason is that in particular the role of process parameters like mixing on particle formation is largely unknown, as systematic examination of the latter is impossible for the laborious and complex manual synthesis. Herein we studied the mixing induced self-focusing of particle size distributions (PSDs) of CdSe QDs using automation in combination with a defined stirrer geometry. Basis for our study is a platform that allows parallelization with inline temperature monitoring, defined injection rate, accurate sampling times as well as controlled stirring. Reproducibility in terms of optical product properties was analyzed by absorption and emission whereas reproducibility in terms of the PSD was verified by deconvolution of UV/Vis absorbance spectra and especially by analytical ultracentrifugation (AUC) complemented by transmission electron microscopy (TEM). In line with previous results, AUC confirmed that even QDs made by hot injection in an automated setup are polydisperse with multimodal size distributions. Finally, reproducibility in combination with early stage sampling and controlled mixing allowed us for the first time to analyze the influence of stirring on focusing and defocusing of PSDs, that has been expressed in terms of the evolution of the relative standard deviation (RSD). Our work paves the way to gain in-depth understanding of often forgotten process-structure relationships of colloidal nanoparticles which eventually is a first step in the direction of the development of scalable synthesis and reliable application of high-quality QDs in technical applications. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.cej.2017.02.154
  • 2017 • 88 Local characterization of light trapping effects of metallic and dielectric nanoparticles in ultra-thin Cu(In,Ga)Se2 solar cells via scanning near-field optical microscopy
    Song, M. and Yin, G. and Fumagalli, P. and Schmid, M.
    Proceedings of SPIE - The International Society for Optical Engineering 10114 (2017)
    Plasmonic and photonic nanoparticles have proven beneficial for solar cells in the aspect of light management. For improved exploitation of nanoparticles in solar cells, it is necessary to reveal the absorption enhancement mechanism from the nanoparticles. In this study, we investigated the nanoparticle-enhanced solar cells in near-field regime with optic and opto-electric scanning near-field optical microscopy (SNOM). The near-field distribution of regularly arranged silver and polystyrene nanoparticles produced by nanosphere lithography on Cu(In,Ga)Se2 (CIGSe) solar cells is characterized using a custom-built SNOM, which gives insight into the optical mechanism of light trapping from nanoparticles to solar cells. On the other hand, the photocurrent of CIGSe solar cells with and without nanoparticles is studied with an opto-electric SNOM by recording the photocurrent during surface scanning, further revealing the opto-electrical influences of the nanoparticles. In addition, finite element method simulations have been performed and agree with the results from SNOM. We found the dielectric polystyrene spheres are able to enhance the absorption and benefit the generation of charge carriers in the solar cells. Copyright © 2017 SPIE.
    view abstractdoi: 10.1117/12.2253223
  • 2017 • 87 Microstructure and thermoelectric properties of Si-WSi2 nanocomposites
    Stoetzel, J. and Schneider, T. and Mueller, M.M. and Kleebe, H.-J. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Acta Materialia 125 321-326 (2017)
    Nanocomposites of n-doped Si/WSi2 were prepared and morphologically and thermoelectrically investigated. The composites were densified by spark-plasma-sintering of doped Si nanoparticles with WSi2 nanoinclusions. The nanoparticles were synthesized in a gas-phase process. The microstructure of the bulk nanocomposite shows an inhomogeneous distribution of the WSi2 nanoinclusions in form of WSi2-rich and -depleted regions. This inhomogeneity is not present in the starting material and is assigned to a self-organizing process during sintering. The inhomogeneities are in the micrometer range and may act as scattering centers for long-wavelength phonons. The WSi2 nanoinclusions grow during sintering from originally 3–7 nm up to 30–143 nm depending on the total W content and might act as scattering centers for the medium wavelength range of phonons. Further, the growth of Si grains is suppressed by the WSi2 inclusions, which leads to an enhanced grain boundary density. Adding 1 at% W reduces lattice thermal conductivity by almost 35% within the temperature range from 300 K to 1250 K compared to pure, nanocrystalline silicon (doped). By addition of 6 at% W a reduction of 54% in lattice thermal conductivity is achieved. Although little amounts of W slightly reduce the power factor an enhancement of the thermoelectric figure of merit of 50% at 1250 K compared to a tungsten-free reference was realized. © 2016
    view abstractdoi: 10.1016/j.actamat.2016.11.069
  • 2017 • 86 Incorporation of silver nanoparticles into magnetron-sputtered calcium phosphate layers on titanium as an antibacterial coating
    Surmeneva, M.A. and Sharonova, A.A. and Chernousova, S. and Prymak, O. and Loza, K. and Tkachev, M.S. and Shulepov, I.A. and Epple, M. and Surmenev, R.A.
    Colloids and Surfaces B: Biointerfaces 156 104-113 (2017)
    A three-layer system of nanocrystalline hydroxyapatite (first layer; 1000 nm thick), silver nanoparticles (second layer; 1.5 μg Ag cm−2) and calcium phosphate (third layer, either 150 or 1000 nm thick) on titanium was prepared by a combination of electrophoretic deposition of silver nanoparticles and the deposition of calcium phosphate by radio frequency magnetron sputtering. Scanning electron microscopy showed that the silver nanoparticles were evenly distributed over the surface. The adhesion of multilayered coating on the substrate was evaluated using the scratch test method. The resistance to cracking and delamination indicated that the multilayered coating has good resistance to contact damage. The release of silver ions from the hydroxyapatite/silver nanoparticle/calcium phosphate system into the phosphate-buffered saline (PBS) solution was measured by atomic absorption spectroscopy (AAS). Approximately one-third of the incorporated silver was released after 3 days immersion into PBS, indicating a total release time of the order of weeks. There were no signs of cracks on the surface of the coating after immersion after various periods, indicating the excellent mechanical stability of the multilayered coating in the physiological environment. An antimicrobial effect against Escherichia coli was found for a 150 nm thick outer layer of the calcium phosphate using a semi-quantitative turbidity test. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfb.2017.05.016
  • 2017 • 85 Solution-Processed CuInS2-Based White QD-LEDs with Mixed Active Layer Architecture
    Wepfer, S. and Frohleiks, J. and Hong, A.-R. and Jang, H.S. and Bacher, G. and Nannen, E.
    ACS Applied Materials and Interfaces 9 11224-11230 (2017)
    doi: 10.1021/acsami.6b15660
  • 2017 • 84 Control of quantum dot laser emission by coherent phonon wave packets
    Wigger, D. and Czerniuk, T. and Reiter, D.E. and Bayer, M. and Kuhn, T.
    Journal of Physics: Conference Series 906 (2017)
    Travelling coherent phonons can be actively used to manipulate the optical properties of semiconductor nanostructures on the picosecond time scale. Phonon wave packets that interact with a quantum dot (QD) ensemble can significantly vary the output intensity of a laser, which uses the QDs as active medium. Based on a recently developed theoretical model to describe this coupled phonon-QD-photon system, we here study how the laser response on phonon wave packets depends on several parameters, for example phonon pulse properties and laser pump rate. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/906/1/012025
  • 2017 • 83 Systematic study of the influence of coherent phonon wave packets on the lasing properties of a quantum dot ensemble
    Wigger, D. and Czerniuk, T. and Reiter, D.E. and Bayer, M. and Kuhn, T.
    New Journal of Physics 19 (2017)
    Coherent phonons can greatly vary light-matter interaction in semiconductor nanostructures placed inside an optical resonator on a picosecond time scale. For an ensemble of quantum dots (QDs) as active laser medium, phonons are able to induce a large enhancement or attenuation of the emission intensity, as has been recently demonstrated. The physics of this coupled phonon-exciton-light system consists of various effects, which in the experiment typically cannot be clearly separated, in particular, due to the complicated sample structure a rather complex strain pulse impinges on the QD ensemble. Here we present a comprehensive theoretical study how the laser emission is affected by phonon pulses of various shapes as well as by ensembles with different spectral distributions of the QDs. This gives insight into the fundamental interaction dynamics of the coupled phonon-exciton-light system, while it allows us to clearly discriminate between two prominent effects: the adiabatic shifting of the ensemble and the shaking effect. This paves the way to a tailored laser emission controlled by phonons. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/aa78bf
  • 2017 • 82 Chemical Synthesis, Doping, and Transformation of Magic-Sized Semiconductor Alloy Nanoclusters
    Yang, J. and Muckel, F. and Baek, W. and Fainblat, R. and Chang, H. and Bacher, G. and Hyeon, T.
    Journal of the American Chemical Society 139 6761-6770 (2017)
    doi: 10.1021/jacs.7b02953
  • 2017 • 81 In-situ tracking the structural and chemical evolution of nanostructured CuCr alloys
    Zhang, Z. and Guo, J. and Dehm, G. and Pippan, R.
    Acta Materialia 138 42-51 (2017)
    We report the thermal stability of supersaturated CuCr nanocrystallines alloys at the atomic resolution using modern spherical aberration-corrected transmission electron microscopy (TEM) via performing in-situ structural and spectroscopy experiments. It is found that CuCr nanocrystallines are not only subjected to a structural change but also undergo a chemical evolution upon annealing. Chemical destabilization of supersaturated CuCr nanocrystallines occurs at a quite low temperature. Heating triggers a rapid separation of Cu and Cr grains at the forced intermixing zone, accompanied by an obvious decrease of average interface width whereas the grain growth is not significant. Elemental profiles and images recorded in real time reveal that the local compositions vary with heating, which in turn permits to derive the concentration of excess vacancy generated by deformation and observe its evolution with temperature, further to analyze the dynamic behavior in nanocrystalline materials. Electronic structure changes at the interface forced intermixing zone are revealed by the fine structure analysis. The study uncovers the interplay between the thermal stability and chemical decomposition process of bulk nanostructured materials in real-time. © 2017
    view abstractdoi: 10.1016/j.actamat.2017.07.039
  • 2017 • 80 Nano-sized metal organic framework to improve the structural properties and desalination performance of thin film composite forward osmosis membrane
    Zirehpour, A. and Rahimpour, A. and Ulbricht, M.
    Journal of Membrane Science 531 59-67 (2017)
    In the present study, nano-sized metal-organic framework (MOF) particles consisting of silver (I) and 1,3,5-benzene tricarboxylic acid were synthesized and applied to improve the structural properties as well as desalination performance of thin-film composite (TFC) forward osmosis (FO) membranes. The MOF nanocrystals were incorporated into the polyamide layer of membranes through interfacial polymerization. Characterizations by Field emission scanning electron microscopy and X-ray photoelectron spectroscopy enabled the detection of MOF nanocrystals within the selective layer of the resultant membranes. The MOF incorporation led to changes of the membrane active layer in terms of hydrophilicity and transport properties, without detrimental effects on the layer selectivity. These features enhanced pure water permeability of the membranes to 129%, which was provided through 0.04% MOF loading of the organic phase during interfacial polymerization. As a result, the modified membrane exhibited an enhanced FO seawater desalination performance in comparison with the control membrane. The performance stability of TFC membrane was also improved by presence of MOF in active layer (as seen by a water flux decline of about 7% for modified membrane against about 18% for unmodified membrane when tested with real seawater). This study demonstrates the potential of MOF particles to enhance desalination performance of TFC membranes in FO systems. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2017.02.049
  • 2016 • 79 Injection of a single electron from static to moving quantum dots
    Bertrand, B. and Hermelin, S. and Mortemousque, P.-A. and Takada, S. and Yamamoto, M. and Tarucha, S. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Nanotechnology 27 (2016)
    We study the injection mechanism of a single electron from a static quantum dot into a moving quantum dot. The moving quantum dots are created with surface acoustic waves (SAWs) in a long depleted channel. We demonstrate that the injection process is characterized by an activation law with a threshold that depends on the SAW amplitude and on the dot-channel potential gradient. By sufficiently increasing the SAW modulation amplitude, we can reach a regime where the transfer has unity probability and is potentially adiabatic. This study points to the relevant regime to use moving dots in quantum information protocols. © 2016 IOP Publishing Ltd Printed in the UK.
    view abstractdoi: 10.1088/0957-4484/27/21/214001
  • 2016 • 78 Band-Edge Exciton Fine Structure and Recombination Dynamics in InP/ZnS Colloidal Nanocrystals
    Biadala, L. and Siebers, B. and Beyazit, Y. and Tessier, M.D. and Dupont, D. and Hens, Z. and Yakovlev, D.R. and Bayer, M.
    ACS Nano 10 3356-3364 (2016)
    We report on a temperature-, time-, and spectrally resolved study of the photoluminescence of type-I InP/ZnS colloidal nanocrystals with varying core size. By studying the exciton recombination dynamics we assess the exciton fine structure in these systems. In addition to the typical bright-dark doublet, the photoluminescence stems from an upper bright state in spite of its large energy splitting (∼100 meV). This striking observation results from dramatically lengthened thermalization processes among the fine structure levels and points to optical-phonon bottleneck effects in InP/ZnS nanocrystals. Furthermore, our data show that the radiative recombination of the dark exciton scales linearly with the bright-dark energy splitting for CdSe and InP nanocrystals. This finding strongly suggests a universal dangling bonds-assisted recombination of the dark exciton in colloidal nanostructures. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.5b07065
  • 2016 • 77 Thermal shift of the resonance between an electron gas and quantum dots: What is the origin?
    Brinks, F. and Wieck, A.D. and Ludwig, Ar.
    New Journal of Physics 18 (2016)
    The operation of quantum dots (QDs) at highest possible temperatures is desirable for many applications. Capacitance-voltage spectroscopy (C(V)-spectroscopy) measurements are an established instrument to analyse the electronic structure and energy levels of self-assembled QDs. We perform C(V) in the dark and C(V) under the influence of non-resonant illumination, probing exciton states up to X4+ on InAs QDs embedded in a GaAs matrix for temperatures ranging from 2.5 to 120 K. While a small shift in the charging spectra resonance is observed for the two spin degenerate electron s-state charging voltages with increasing temperature, a huge shift is visible for the electron-hole excitonic states resonance voltages. The s2-peak moves to slightly higher, the s1-peak to slightly lower charging voltages. In contrast, the excitonic states are surprisingly charged at much lower voltages upon increasing temperature. We derive a rate-model allowing to attribute and value different contributions to these shifts. Resonant tunnelling, state degeneracy and hole generation rate in combination with the Fermi distribution function turn out to be of great importance for the observed effects. The differences in the shifting behaviour is connected to different equilibria schemes for the peaks - s-peaks arise when tunnelling-in- and out-rates become equal, while excitonic peaks occur, when electron tunnelling-in- and hole-generation rates are balanced. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/aa4f63
  • 2016 • 76 An experimental study of ultrafiltration for sub-10nm quantum dots and sub-150 nm nanoparticles through PTFE membrane and Nuclepore filters
    Chen, S.-C. and Segets, D. and Ling, T.-Y. and Peukert, W. and Pui, D.Y.H.
    Journal of Membrane Science 497 153-161 (2016)
    Ultrafiltration techniques (pore size of membrane below 100nm) are widely used in chemical engineering, semiconductor, pharmaceutical, food and beverage industries. However, for small particles, which are more and more attracting interests, the pore size often does not correlate well with sieving characteristics of the ultra-membranes. This may cause serious issues during modeling and prediction of retention efficiencies. Herein, a series of liquid filtration experiments with unfavorable conditions were performed. PTFE membranes (50, 100nm) and Nuclepore filters (50, 400nm) were challenged with 1.7nm manganese doped ZnS and 6.6 nm ZnO quantum dots (QDs), 12.4, 34.4 and 50 nm Au and 150 nm SiO2 nanoparticles. For larger and medium sized particles, sieving and eventually pore blockage phenomena were observed. In comparison, for small QDs, a high initial retention efficiency (&gt;0.4) in both filters was monitored, followed by a reduced efficiency with ongoing particle loading. This high initial retention of small nanoparticles was attributed to diffusion deposition rather than to sieving since the ratio of pore size to particle size was significantly high (up to 58). Our experimental results allow a basic understanding of the deposition mechanism of small nanoparticles (diffusion vs. sieving) in different filter structures. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2015.09.022
  • 2016 • 75 Lattice dynamics and thermoelectric properties of nanocrystalline silicon-germanium alloys
    Claudio, T. and Stein, N. and Petermann, N. and Stroppa, D.G. and Koza, M.M. and Wiggers, H. and Klobes, B. and Schierning, G. and Hermann, R.P.
    Physica Status Solidi (A) Applications and Materials Science 213 515-523 (2016)
    The lattice dynamics and thermoelectric properties of sintered phosphorus-doped nanostructured silicon-germanium alloys obtained by gas-phase synthesis were studied. Measurements of the density of phonon states by inelastic neutron scattering were combined with measurements of the elastic constants and the low-temperature heat capacity. A strong influence of nanostructuring and alloying on the lattice dynamics was observed. The thermoelectric transport properties of samples with different doping as well as samples sintered at different temperature were characterized between room temperature and 1000°C. A peak figure of merit zT=0.88 at 900°C is observed and is comparatively insensitive to the aforementioned parameter variations. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532500
  • 2016 • 74 Quantum Dephasing in a Gated GaAs Triple Quantum Dot due to Nonergodic Noise
    Delbecq, M.R. and Nakajima, T. and Stano, P. and Otsuka, T. and Amaha, S. and Yoneda, J. and Takeda, K. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Physical Review Letters 116 (2016)
    We extract the phase coherence of a qubit defined by singlet and triplet electronic states in a gated GaAs triple quantum dot, measuring on time scales much shorter than the decorrelation time of the environmental noise. In this nonergodic regime, we observe that the coherence is boosted and several dephasing times emerge, depending on how the phase stability is extracted. We elucidate their mutual relations, and demonstrate that they reflect the noise short-time dynamics. © 2016 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.116.046802
  • 2016 • 73 Spent Tea Leaf Templating of Cobalt-Based Mixed Oxide Nanocrystals for Water Oxidation
    Deng, X. and Chan, C.K. and Tüysüz, H.
    ACS Applied Materials and Interfaces 8 32488-32495 (2016)
    The facile synthesis of nanostructured cobalt oxides using spent tea leaves as a hard template is reported. Following an impregnation-calcination and template removal pathway, sheetlike structures containing nanosized crystallites of Co3O4 are obtained. Co3O4 incorporated with Cu, Ni, Fe, and Mn (M/Co = 1/8 atomic ratio) are also prepared, and the materials are thoroughly characterized using X-ray diffraction, electron microscopy, and N2 sorption. The method is applicable to several commercial tea leaves and is successfully scaled up to prepare over 7 g of Co3O4 with the same nanostructure. The oxides are then tested for electrochemical water oxidation, and Cu, Ni, and Fe incorporations show beneficial effect on the catalytic activity of Co3O4, achieving performance comparable to levels from benchmark electrocatalysts. These data suggest that tea leaf templating can be utilized as a facile and promising approach to prepare nanostructured functional catalyst. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsami.6b12005
  • 2016 • 72 Are Mo2BC nanocrystalline coatings damage resistant? Insights from comparative tension experiments
    Djaziri, S. and Gleich, S. and Bolvardi, H. and Kirchlechner, C. and Hans, M. and Scheu, C. and Schneider, J.M. and Dehm, G.
    Surface and Coatings Technology 289 213-218 (2016)
    Mo2BC nanocrystalline coatings were deposited on Cu substrates to compare their mechanical performance with bench-mark TiAlN, and pure Mo, Al and Al2O3 reference coatings. The Mo2BC coatings were characterized by X-ray diffraction and transmission electron microscopy to analyze the microstructure. In order to study the damage behavior, the coatings were subjected to uniaxial tensile loading and the crack spacing with increasing strain was monitored using optical and scanning electron microscopy. Based on crack density measurements, the Mo2BC coatings were found to be significantly less prone to cracking than the bench-mark TiAlN coatings. The higher resistance to cracking arises from the electronic structure of the Mo2BC nanolaminates, which imparts moderate ductility to the deformation behavior. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.02.010
  • 2016 • 71 Giant Excitonic Exchange Splittings at Zero Field in Single Colloidal CdSe Quantum Dots Doped with Individual Mn2+ Impurities
    Fainblat, R. and Barrows, C.J. and Hopmann, E. and Siebeneicher, S. and Vlaskin, V.A. and Gamelin, D.R. and Bacher, G.
    Nano Letters 16 6371-6377 (2016)
    Replacing a single atom of a host semiconductor nanocrystal with a functional dopant can introduce completely new properties potentially valuable for "solotronic" information-processing applications. Here, we report successful doping of colloidal CdSe quantum dots with a very small number of manganese ions - down to the ultimate limit of one. Single-particle spectroscopy reveals spectral fingerprints of the spin-spin interactions between individual dopants and quantum-dot excitons. Spectrally well-resolved emission peaks are observed that can be related to the discrete spin projections of individual Mn2+ ions. In agreement with theoretical predictions, the exchange splittings are enhanced by more than an order of magnitude in these quantum dots compared to their epitaxial counterparts, opening a path for solotronic applications at elevated temperatures. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b02775
  • 2016 • 70 Quantum Dot/Light-Emitting Electrochemical Cell Hybrid Device and Mechanism of Its Operation
    Frohleiks, J. and Wepfer, S. and Kelestemur, Y. and Demir, H.V. and Bacher, G. and Nannen, E.
    ACS Applied Materials and Interfaces 8 24692-24698 (2016)
    A new type of light-emitting hybrid device based on colloidal quantum dots (QDs) and an ionic transition metal complex (iTMC) light-emitting electrochemical cell (LEC) is introduced. The developed hybrid devices show light emission from both active layers, which are combined in a stacked geometry. Time-resolved photoluminescence experiments indicate that the emission is controlled by direct charge injection into both the iTMC and the QD layer. The turn-on time (time to reach 1 cd/m2) at constant voltage operation is significantly reduced from 8 min in the case of the reference LEC down to subsecond in the case of the hybrid device. Furthermore, luminance and efficiency of the hybrid device are enhanced compared to reference LEC directly after device turn-on by a factor of 400 and 650, respectively. We attribute these improvements to an increased electron injection efficiency into the iTMC directly after device turn-on. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsami.6b06833
  • 2016 • 69 Thermochromic modulation of surface plasmon polaritons in vanadium dioxide nanocomposites
    Jostmeier, T. and Mangold, M. and Zimmer, J. and Karl, H. and Krenner, H.J. and Ruppert, C. and Betz, M.
    Optics Express 24 17321-17331 (2016)
    We propose and implement a new concept for thermochromic plasmonic elements. It is based on vanadium dioxide (VO2) nanocrystals located in the near field of surface plasmon polaritons supported by an otherwise unstructured gold thin film. When the VO2 undergoes the metal-insulator phase transition, the coupling conditions for conversion of light into propagating surface plasmon polaritons change markedly. In particular, we realize thermochromic plasmonic grating couplers with substantial switching contrast as well as tunable plasmonic couplers in a Kretschmann configuration. The use of VO2 nanocrystals permits highly repetitive switching and room temperature operation. Simulations based on the actual dielectric function of our VO2 nanocrystals agree well with the experiment. © 2016 Optical Society of America.
    view abstractdoi: 10.1364/OE.24.017321
  • 2016 • 68 Carrier relaxation in (In,Ga)As quantum dots with magnetic field-induced anharmonic level structure
    Kurtze, H. and Bayer, M.
    Applied Physics Letters 109 (2016)
    Sophisticated models have been worked out to explain the fast relaxation of carriers into quantum dot ground states after non-resonant excitation, overcoming the originally proposed phonon bottleneck. We apply a magnetic field along the quantum dot heterostructure growth direction to transform the confined level structure, which can be approximated by a Fock-Darwin spectrum, from a nearly equidistant level spacing at zero field to strong anharmonicity in finite fields. This changeover leaves the ground state carrier population rise time unchanged suggesting that fast relaxation is maintained upon considerable changes of the level spacing. This corroborates recent models explaining the relaxation by polaron formation in combination with quantum kinetic effects. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4955459
  • 2016 • 67 Optical Blocking of Electron Tunneling into a Single Self-Assembled Quantum Dot
    Kurzmann, A. and Merkel, B. and Labud, P.A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M. P.
    Physical Review Letters 117 (2016)
    Time-resolved resonance fluorescence (RF) is used to analyze electron tunneling between a single self-assembled quantum dot (QD) and an electron reservoir. In equilibrium, the RF intensity reflects the average electron occupation of the QD and exhibits a gate voltage dependence that is given by the Fermi distribution in the reservoir. In the time-resolved signal, however, we find that the relaxation rate for electron tunneling is, surprisingly, independent of the occupation in the charge reservoir - in contrast to results from all-electrical transport measurements. Using a master equation approach, which includes both the electron tunneling and the optical excitation or recombination, we are able to explain the experimental data by optical blocking, which also reduces the electron tunneling rate when the QD is occupied by an exciton. © 2016 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.117.017401
  • 2016 • 66 Photoelectron generation and capture in the resonance fluorescence of a quantum dot
    Kurzmann, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Applied Physics Letters 108 (2016)
    Time-resolved resonance fluorescence on a single self-assembled quantum dot (QD) is used to analyze the generation and capture of photoinduced free charge carriers. We directly observe the capture of electrons into the QD as an intensity reduction of the exciton transition. The exciton transition is quenched until the captured electron tunnels out of the dot again in the order of milliseconds. Our results demonstrate that even under resonant excitation, excited free electrons are generated and can negatively influence the optical properties of a QD. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4954944
  • 2016 • 65 Auger Recombination in Self-Assembled Quantum Dots: Quenching and Broadening of the Charged Exciton Transition
    Kurzmann, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Nano Letters 16 3367-3372 (2016)
    In quantum dots (QDs), the Auger recombination is a nonradiative process in which the electron-hole recombination energy is transferred to an additional carrier. It has been studied mostly in colloidal QDs, where the Auger recombination time is in the picosecond range and efficiently quenches the light emission. In self-assembled QDs, on the other hand, the influence of Auger recombination on the optical properties is in general neglected, assuming that it is masked by other processes such as spin and charge fluctuations. Here, we use time-resolved resonance fluorescence to analyze the Auger recombination and its influence on the optical properties of a single self-assembled QD. From excitation-power-dependent measurements, we find a long Auger recombination time of about 500 ns and a quenching of the trion transition by about 80%. Furthermore, we observe a broadening of the trion transition line width by up to a factor of 2. With a model based on rate equations, we are able to identify the interplay between tunneling and Auger rate as the underlying mechanism for the reduced intensity and the broadening of the line width. This demonstrates that self-assembled QDs can serve as an ideal model system to study how the charge recapture process, given by the band-structure surrounding the confined carriers, influences the Auger process. Our findings are not only relevant for improving the emission properties of colloidal QD-based emitters and dyes, which have recently entered the consumer market, but also of interest for more visionary applications, such as quantum information technologies, based on self-assembled quantum dots. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b01082
  • 2016 • 64 Formation of nanosized grain structure in martensitic 100Cr6 bearing steels upon rolling contact loading studied by atom probe tomography
    Li, Y.J. and Herbig, M. and Goto, S. and Raabe, D.
    Materials Science and Technology (United Kingdom) 32 1100-1105 (2016)
    To understand the origin of white etching cracks (WECs), a systematic microstructural characterisation in the regions affected from the near-surface region down to the subsurface layers where WECs occur is necessary. As a starting point, we focus on the near-surface region of an axial thrust bearing, made of martensitic 100Cr6 steel, to study the influence of rolling contact loading on the microstructure and the resulting distributions of the major alloying elements C and Cr using atom probe tomography. We find that upon rolling contact loading the original plate-like martensitic structure evolves into a nanosized equiaxed grain structure with C segregation up to 5 at.-% at the grain boundaries. Cementite particles, located at grain boundaries and triple junctions, undergo spheroidisation. The originally homogeneously distributed Cr becomes enriched in spheroidised cementite particles. The microstructural changes give strong hints that rolling contact loading induces plastic deformation and an increased temperature on the near-surface region. This paper is part of a Themed Issue on Recent developments in bearing steels. © 2016 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1080/02670836.2015.1120458
  • 2016 • 63 Silver-doped zinc oxide single nanowire multifunctional nanosensor with a significant enhancement in response
    Lupan, O. and Cretu, V. and Postica, V. and Ahmadi, M. and Cuenya, B.R. and Chow, L. and Tiginyanu, I. and Viana, B. and Pauporté, T. and Adelung, R.
    Sensors and Actuators, B: Chemical 223 893-903 (2016)
    Enhanced performances were obtained for nanosensors based on a single nanowire of silver-doped zinc oxide (ZnO:Ag). Arrays of crystalline ZnO:Ag nanowires were synthesized by electrodeposition on F-doped tin oxide coated substrates and studied by SEM, EDX, TEM, HRTEM, SIMS, XPS, PL and micro-Raman spectroscopy. Integration of a single nanowire or a single microwire on the chip was performed by employing metal maskless nanodeposition in the dual beam focused electron/ion beam instrument. The ultraviolet (UV) response and hydrogen (H2) gas response were studied for nanodevices and microdevices based on a single ZnO:Ag nanowire. We found that ZnO:Ag nanowire based nanosensor possesses a much faster response/recovery time and a higher response to UV radiation and hydrogen gas (∼50%) than those reported in literature. An increase in current value of about two orders in magnitude IUVON/IUVOFF was observed under exposure to UV light. Faster response/recovery times of about 0.98 s/0.87 s were observed. The ZnO:Ag nanowires and microwires can serve as nano-building materials for ultrasensitive and ultra-fast sensors with reduced power consumption. The mechanisms for such improved responses to UV and H2 were discussed. The developed nanomaterial is of great scientific interest for further studies as promising candidates for fabricating multifunctional nano-sensors, LEDs and photodetectors by bottom-up and hybrid nanotechnologies. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.snb.2015.10.002
  • 2016 • 62 Phase selection and nanocrystallization in Cu-free soft magnetic FeSiNbB amorphous alloy upon rapid annealing
    Morsdorf, L. and Pradeep, K.G. and Herzer, G. and Kovács, A. and Dunin-Borkowski, R.E. and Povstugar, I. and Konygin, G. and Choi, P. and Raabe, D.
    Journal of Applied Physics 119 (2016)
    Nucleation of soft magnetic Fe3Si nanocrystals in Cu-free Fe74.5Si15.5Nb3B7 alloy, upon rapid (10 s) and conventional (30 min) annealing, was investigated using x-ray diffraction, transmission electron microscopy, Mössbauer spectroscopy, and atom probe tomography. By employing rapid annealing, preferential nucleation of Fe3Si nanocrystals was achieved, whereas otherwise there is simultaneous nucleation of both Fe3Si and undesired Fe-B compound phases. Analysis revealed that the enhanced Nb diffusivity, achieved during rapid annealing, facilitates homogeneous nucleation of Fe3Si nanocrystals while shifting the secondary Fe-B crystallization to higher temperatures resulting in pure soft magnetic nanocrystallization with very low coercivities of ∼10 A/m. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4944595
  • 2016 • 61 Coherent electron-spin-resonance manipulation of three individual spins in a triple quantum dot
    Noiri, A. and Yoneda, J. and Nakajima, T. and Otsuka, T. and Delbecq, M.R. and Takeda, K. and Amaha, S. and Allison, G. and Ludwig, Ar. and Wieck, A.D. and Tarucha, S.
    Applied Physics Letters 108 (2016)
    Quantum dot arrays provide a promising platform for quantum information processing. For universal quantum simulation and computation, one central issue is to demonstrate the exhaustive controllability of quantum states. Here, we report the addressable manipulation of three single electron spins in a triple quantum dot using a technique combining electron-spin-resonance and a micro-magnet. The micro-magnet makes the local Zeeman field difference between neighboring spins much larger than the nuclear field fluctuation, which ensures the addressable driving of electron-spin-resonance by shifting the resonance condition for each spin. We observe distinct coherent Rabi oscillations for three spins in a semiconductor triple quantum dot with up to 25 MHz spin rotation frequencies. This individual manipulation over three spins enables us to arbitrarily change the magnetic spin quantum number of the three spin system, and thus to operate a triple-dot device as a three-qubit system in combination with the existing technique of exchange operations among three spins. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4945592
  • 2016 • 60 Coherent acoustic phonons in colloidal semiconductor nanocrystal superlattices
    Poyser, C.L. and Czerniuk, T. and Akimov, A. and Diroll, B.T. and Gaulding, E.A. and Salasyuk, A.S. and Kent, A.J. and Yakovlev, D.R. and Bayer, M. and Murray, C.B.
    ACS Nano 10 1163-1169 (2016)
    The phonon properties of films fabricated from colloidal semiconductor nanocrystals play a major role in thermal conductance and electron scattering, which govern the principles for building colloidal-based electronics and optics including thermoelectric devices with a high ZT factor. The key point in understanding the phonon properties is to obtain the strength of the elastic bonds formed by organic ligands connecting the individual nanocrystallites. In the case of very weak bonding, the ligands become the bottleneck for phonon transport between infinitively rigid nanocrystals. In the opposite case of strong bonding, the colloids cannot be considered as infinitively rigid beads and the distortion of the superlattice caused by phonons includes the distortion of the colloids themselves. We use the picosecond acoustics technique to study the acoustic coherent phonons in superlattices of nanometer crystalline CdSe colloids. We observe the quantization of phonons with frequencies up to 30 GHz. The frequencies of quantized phonons depend on the thickness of the colloidal films and possess linear phonon dispersion. The measured speed of sound and corresponding wave modulus in the colloidal films point on the strong elastic coupling provided by organic ligands between colloidal nanocrystals. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.5b06465
  • 2016 • 59 Photoluminescence of gallium ion irradiated hexagonal and cubic GaN quantum dots
    Rothfuchs, C. and Kukharchyk, N. and Koppe, T. and Semond, F. and Blumenthal, S. and Becker, H.-W. and As, D.J. and Hofsäss, H.C. and Wieck, A.D. and Ludwig, Ar.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 383 1-5 (2016)
    We report on ion implantation into GaN QDs and investigate their radiation hardness. The experimental study is carried out by photoluminescence (PL) measurements on molecular beam epitaxy-grown GaN quantum dots after ion implantation. Both quantum dots grown in the hexagonal (H) and the cubic (C) crystal structure were subjected to gallium ions with an energy of 400 kV (H) and 75 kV (C) with fluences ranging from 5×1010 cm−2 to 1×1014 cm−2 (H) and to 1×1015 cm−2 (C), respectively. Low-temperature PL measurements reveal a PL quenching for which a quantitative model as a function of the ion fluence is developed. A high degradation resistance is concluded. A non-radiative trap with one main activation energy is found for all QD structures by temperature-dependent PL measurements. Further analysis of fluence-dependent PL energy shifts shows ion-induced intermixing and strain effects. Particular for the hexagonal quantum dots, a strong influence of the quantum confined Stark effect is present. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.nimb.2016.06.004
  • 2016 • 58 Altering the luminescence properties of self-assembled quantum dots in GaAs by focused ion beam implantation
    Rothfuchs, C. and Kukharchyk, N. and Greff, M.K. and Wieck, A.D. and Ludwig, Ar.
    Applied Physics B: Lasers and Optics 122 (2016)
    Using quantum dots (QDs) as single-photon sources draws the attention in many quantum communication technologies. One pathway towards manufacturing single-photon sources is focused ion beam (FIB) implantation in molecular beam epitaxy-grown QD samples to disable all QDs around an intentional one for single photoluminescence (PL) emission. In this paper, we investigate the lattice disorders in the vicinity of InAs/GaAs QDs introduced by FIB implantation of gallium and indium ions. For high fluences, we achieve total elimination of the QDs photoluminescence. The impact of the different ion species and fluences is studied by low-temperature PL measurements. Furthermore, we deduce a simple model based on the trap-assisted recombination for the description of the degradation of the PL emission. It allows the determination of the fluences at which the PL emission is suppressed. Moreover, we identify the implantation-induced non-radiative defects by temperature-dependent PL measurements. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00340-015-6305-8
  • 2016 • 57 Silicon-based nanocomposites for thermoelectric application
    Schierning, G. and Stoetzel, J. and Chavez, R. and Kessler, V. and Hall, J. and Schmechel, R. and Schneider, T. and Petermann, N. and Wiggers, H. and Angst, S. and Wolf, D.E. and Stoib, B. and Greppmair, A. and Stutzmann, M. and B...
    Physica Status Solidi (A) Applications and Materials Science 213 497-514 (2016)
    Here we present the realization of efficient and sustainable silicon-based thermoelectric materials from nanoparticles. We employ a gas phase synthesis for the nanoparticles which is capable of producing doped silicon (Si) nanoparticles, doped alloy nanoparticles of silicon and germanium (Ge), SixGe1-x, and doped composites of Si nanoparticles with embedded metal silicide precipitation phases. Hence, the so-called "nanoparticle in alloy" approach, theoretically proposed in the literature, forms a guideline for the material development. For bulk samples, a current-activated pressure-assisted densification process of the nanoparticles was optimized in order to obtain the desired microstructure. For thin films, a laser annealing process was developed. Thermoelectric transport properties were characterized on nanocrystalline bulk samples and laser-sintered-thin films. Devices were produced from nanocrystalline bulk silicon in the form of p-n junction thermoelectric generators, and their electrical output data were measured up to hot side temperatures of 750°C. In order to get a deeper insight into thermoelectric properties and structure forming processes, a 3D-Onsager network model was developed. This model was extended further to study the p-n junction thermoelectric generator and understand the fundamental working principle of this novel device architecture. Gas phase synthesis of composite nanoparticles; nanocrystalline bulk with optimized composite microstructure; laser-annealed thin film. The authors fabricated thermoelectric nanomaterials from doped silicon and silicon and germanium alloy nanoparticles, as well as composites of Si nanoparticles with embedded metal silicide nanoparticles. Processing was performed applying a current-activated pressure-assisted densification process for bulk samples and a laser annealing process for thin film samples. Devices were produced in the form of pn junction thermoelectric generators. A 3D-Onsager network model was used to understand the fundamental working principle of this novel device architecture. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532602
  • 2016 • 56 Analysis of particle size distributions of quantum dots: From theory to application
    Segets, D.
    KONA Powder and Particle Journal 2016 48-62 (2016)
    Small, quantum-confined semiconductor nanoparticles, known as quantum dots (QDs) are highly important material systems due to their unique optoelectronic properties and their pronounced structure-property relationships. QD applications are seen in the emerging fields of thin films and solar cells. In this review, different characterization techniques for particle size distributions (PSDs) will be summarized with special emphasis on strategies developed and suggested in the past to derive data on the dispersity of a sample from optical absorbance spectra. The latter use the assumption of superimposed individual optical contributions according to the relative abundance of different sizes of a colloidal dispersion. In the second part, the high potential of detailed PSD analysis to get deeper insights of typical QD processes such as classification by size selective precipitation (SSP) will be demonstrated. This is expected to lead to an improved understanding of colloidal surface properties which is of major importance for the development of assumption-free interaction models. © 2016 Hosokawa Powder Technology Foundation.
    view abstractdoi: 10.14356/kona.2016012
  • 2016 • 55 Coherent Control of the Exciton-Biexciton System in an InAs Self-Assembled Quantum Dot Ensemble
    Suzuki, T. and Singh, R. and Bayer, M. and Ludwig, Ar. and Wieck, A.D. and Cundiff, S.T.
    Physical Review Letters 117 (2016)
    Coherent control of a strongly inhomogeneously broadened system, namely, InAs self-assembled quantum dots, is demonstrated. To circumvent the deleterious effects of the inhomogeneous broadening, which usually masks the results of coherent manipulation, we use prepulse two-dimensional coherent spectroscopy to provide a size-selective readout of the ground, exciton, and biexciton states. The dependence on the timing of the prepulse is due to the dynamics of the coherently generated populations. To further validate the results, we performed prepulse polarization dependent measurements and confirmed the behavior expected from selection rules. All measured spectra can be excellently reproduced by solving the optical Bloch equations for a 4-level system. © 2016 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.117.157402
  • 2016 • 54 TiO2 quantum dots embedded in bamboo-like porous carbon nanotubes as ultra high power and long life anodes for lithium ion batteries
    Tang, Y. and Liu, L. and Wang, X. and Jia, D. and Xia, W. and Zhao, Z. and Qiu, J.
    Journal of Power Sources 319 227-234 (2016)
    TiO2 quantum dots embedded in bamboo-like porous carbon nanotubes have been constructed through the pyrolysis of sulfonated polymer nanotubes and TiO2 hybrids. The TiO2 quantum dots are formed during the pyrolysis, due to the space confinement within the highly cross-linked copolymer networks. The sulfonation degree of the polymer nanotubes is a critical factor to ensure the formation of the unique interpenetrating structure. The nanocomposites exhibit high reversible capacity of 523 mAh g-1 at 100 mA g-1 after 200 cycles, excellent rate capability and superior long-term cycling stability at high current density, which could attain a high discharge capacity of 189 mAh g-1 at 2000 mA g-1 for up to 2000 cycles. The enhanced electrochemical performance of the nanocomposites benefit from the uniform distribution of TiO2 quantum dots, high electronic conductivity of porous carbons and unique interpenetrating structure, which simultaneously solved the major problems of TiO2 anode facing the pulverization, loss of electrical contact and particle aggregation. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jpowsour.2016.04.033
  • 2016 • 53 Polaron-induced lattice distortion of (In,Ga)As/GaAs quantum dots by optically excited carriers
    Tiemeyer, S. and Bombeck, M. and Göhring, H. and Paulus, M. and Sternemann, C. and Nase, J. and Wirkert, F.J. and Möller, J. and Büning, T. and Seeck, O.H. and Reuter, D. and Wieck, A.D. and Bayer, M. and Tolan, M.
    Nanotechnology 27 (2016)
    We report on a high resolution x-ray diffraction study unveiling the effect of carriers optically injected into (In,Ga)As quantum dots on the surrounding GaAs crystal matrix. We find a tetragonal lattice expansion with enhanced elongation along the [001] crystal axis that is superimposed on an isotropic lattice extension. The isotropic contribution arises from excitation induced lattice heating as confirmed by temperature dependent reference studies. The tetragonal expansion on the femtometer scale is tentatively attributed to polaron formation by carriers trapped in the quantum dots. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/27/42/425702
  • 2016 • 52 Advanced optical manipulation of carrier spins in (In,Ga)As quantum dots
    Varwig, S. and Evers, E. and Greilich, A. and Yakovlev, D.R. and Reuter, D. and Wieck, A.D. and Meier, T. and Zrenner, A. and Bayer, M.
    Applied Physics B: Lasers and Optics 122 1-11 (2016)
    Spins in semiconductor quantum dots have been considered as prospective quantum bit excitations. Their coupling to the crystal environment manifests itself in a limitation of the spin coherence times to the microsecond range, both for electron and hole spins. This rather short-lived coherence compared to atomic states asks for manipulations on timescales as short as possible. Due to the huge dipole moment for transitions between the valence and conduction band, pulsed laser systems offer the possibility to perform manipulations within picoseconds or even faster. Here, we report on results that show the potential of optical spin manipulations with currently available pulsed laser systems. Using picosecond laser pulses, we demonstrate optically induced spin rotations of electron and hole spins. We further realize the optical decoupling of the hole spins from the nuclear surrounding at the nanosecond timescales and demonstrate an all-optical spin tomography for interacting electron spin sub-ensembles. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00340-015-6274-y
  • 2015 • 51 Investigation of the size-property relationship in CuInS2 quantum dots
    Akdas, T. and Walter, J. and Segets, D. and Distaso, M. and Winter, B. and Birajdar, B. and Spiecker, E. and Peukert, W.
    Nanoscale 7 18105-18118 (2015)
    In this work we investigated fundamental properties of CuInS2 quantum dots in dependence of the particle size distribution (PSD). Size-selective precipitation (SSP) with acetone as poor solvent was performed as an adequate post-processing step. Our results provide deep insight into the correlation between particle size and various optical characteristics as bandgap energy, absorption and emission features and the broadness of the emission signal. These structure-property relationships are only achieved due to the unique combination of different analytical techniques. Our study reveals that the removal of 10 wt% of smallest particles from the feed results in an enhancement of the emission signal. This improvement is ascribed to a decreased quenching of the emission in larger particles. Our results reveal the impact of PSDs on the properties and the performance of an ensemble of multicomponent QDs and anticipate the high potential of controlling PSDs by well-developed post-processing. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5nr04291g
  • 2015 • 50 Enhanced biomedical heat-triggered carriers via nanomagnetism tuning in ferrite-based nanoparticles
    Angelakeris, M. and Li, Z.-A. and Hilgendorff, M. and Simeonidis, K. and Sakellari, D. and Filippousi, M. and Tian, H. and Van Tendeloo, G. and Spasova, M. and Acet, M. and Farle, M.
    Journal of Magnetism and Magnetic Materials 381 179-187 (2015)
    Biomedical nanomagnetic carriers are getting a higher impact in therapy and diagnosis schemes while their constraints and prerequisites are more and more successfully confronted. Such particles should possess a well-defined size with minimum agglomeration and they should be synthesized in a facile and reproducible high-yield way together with a controllable response to an applied static or dynamic field tailored for the specific application. Here, we attempt to enhance the heating efficiency in magnetic particle hyperthermia treatment through the proper adjustment of the core-shell morphology in ferrite particles, by controlling exchange and dipolar magnetic interactions at the nanoscale. Thus, core-shell nanoparticles with mutual coupling of magnetically hard (CoFe2O4) and soft (MnFe2O4) components are synthesized with facile synthetic controls resulting in uniform size and shell thickness as evidenced by high resolution transmission electron microscopy imaging, excellent crystallinity and size monodispersity. Such a magnetic coupling enables the fine tuning of magnetic anisotropy and magnetic interactions without sparing the good structural, chemical and colloidal stability. Consequently, the magnetic heating efficiency of CoFe2O4 and MnFe2O4 core-shell nanoparticles is distinctively different from that of their counterparts, even though all these nanocrystals were synthesized under similar conditions. For better understanding of the AC magnetic hyperthermia response and its correlation with magnetic-origin features we study the effect of the volume ratio of magnetic hard and soft phases in the bimagnetic core-shell nanocrystals. Eventually, such particles may be considered as novel heating carriers that under further biomedical functionalization may become adaptable multifunctional heat-triggered nanoplatforms. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmmm.2014.12.069
  • 2015 • 49 Quantum Manipulation of Two-Electron Spin States in Isolated Double Quantum Dots
    Bertrand, B. and Flentje, H. and Takada, S. and Yamamoto, M. and Tarucha, S. and Ludwig, Ar. and Wieck, A.D. and Bäuerle, C. and Meunier, T.
    Physical Review Letters 115 (2015)
    We studied experimentally the dynamics of the exchange interaction between two antiparallel electron spins in an isolated double quantum dot where coupling to the electron reservoirs can be ignored. We demonstrate that the level of control of such a double dot is higher than in conventional double dots. In particular, it allows us to couple coherently two electron spins in an efficient manner following a scheme initially proposed by Loss and DiVincenzo [Phys. Rev. A 57, 120 (1998)]. The present study demonstrates that isolated quantum dots are a possible route to increase the number of coherently coupled quantum dots. © 2015 American Physical Society. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.115.096801
  • 2015 • 48 Impact of nanomechanical resonances on lasing from electrically pumped quantum dot micropillars
    Czerniuk, T. and Tepper, J. and Akimov, A.V. and Unsleber, S. and Schneider, C. and Kamp, M. and Höfling, S. and Yakovlev, D.R. and Bayer, M.
    Applied Physics Letters 106 (2015)
    We use a picosecond acoustics technique to modulate the laser output of electrically pumped GaAs/AlAs micropillar lasers with InGaAs quantum dots. The modulation of the emission wavelength takes place on the frequencies of the nanomechanical extensional and breathing (radial) modes of the micropillars. The amplitude of the modulation for various nanomechanical modes is different for every micropillar which is explained by a various elastic contact between the micropillar walls and polymer environment. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4906611
  • 2015 • 47 Thermoelectric transport properties of boron-doped nanocrystalline diamond foils
    Engenhorst, M. and Fecher, J. and Notthoff, C. and Schierning, G. and Schmechel, R. and Rosiwal, S.M.
    Carbon 81 650-662 (2015)
    Natural diamond is known for its outstanding thermal conductivity and electrical insulation. However, synthetic production allows for doping and tailoring microstructural and transport properties. Despite some motivation in the literature and the ongoing search for abundant and non-toxic thermoelectric materials, the first experimental study on a set of eight substrate-free boron-doped nanocrystalline diamond foils is presented herein. All transport coefficients were determined in the same direction within the same foils over a broad temperature range up to 900 °C. It is found that nanostructuring reduces the thermal conductivity by two orders of magnitude, but the mobility decreases significantly to around 1 cm2 V-1 s-1, too. Although degenerate transport can be concluded from the temperature dependence of the Seebeck coefficient, charge carriers notably scatter at grain boundaries where sp2-carbon modifications and amorphous boron-rich phases form during synthesis. A detailed analysis of doping efficiency yields an acceptor fraction of only 8-18 at%, meaning that during synthesis excess boron thermodynamically prefers electrically inactive sites. Decent power factors above 10-4Wm-1 K-2 at 900 °C are found despite the low mobility, and a Jonker-type analysis grants a deeper insight into this issue. Together with the high thermal conductivity, the thermoelectric figure of merit zT does not exceed 0.01 at 900 °C. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2014.10.002
  • 2015 • 46 Epitaxial lift-off for solid-state cavity quantum electrodynamics
    Greuter, L. and Najer, D. and Kuhlmann, A.V. and Valentin, S.R. and Ludwig, Ar. and Wieck, A.D. and Starosielec, S. and Warburton, R.J.
    Journal of Applied Physics 118 (2015)
    We demonstrate an approach to incorporate self-assembled quantum dots into a Fabry-Pérot-like microcavity. Thereby, a 3λ/4 GaAs layer containing quantum dots is epitaxially removed and attached by van der Waals bonding to one of the microcavity mirrors. We reach a finesse as high as 4100 with this configuration limited by the reflectivity of the dielectric mirrors and not by scattering at the semiconductor-mirror interface, demonstrating that the epitaxial lift-off procedure is a promising procedure for cavity quantum electrodynamics in the solid state. As a first step in this direction, we demonstrate a clear cavity-quantum dot interaction in the weak coupling regime with a Purcell factor in the order of 3. Estimations of the coupling strength via the Purcell factor suggest that we are close to the strong coupling regime. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4928769
  • 2015 • 45 Deformation induced alloying in crystalline - metallic glass nano-composites
    Guo, W. and Yao, J. and Jägle, E.A. and Choi, P.-P. and Herbig, M. and Schneider, J.M. and Raabe, D.
    Materials Science and Engineering A 628 269-280 (2015)
    We study the mechanisms of deformation driven chemical mixing in a metallic nanocomposite model system. More specific, we investigate shear banding at the atomic scale in an amorphous CuZr/ crystalline Cu nanolaminate, deformed by microindentation. Three CuZr/Cu multilayer systems (100 nm Cu/100 nm CuZr, 50 nm Cu/100 nm CuZr, and 10 nm Cu/100 nm CuZr) are fabricated to study the effect of layer thickness on shear band formation and deformation induced alloying. The chemical and structural evolution at different strain levels are traced by atom probe tomography and transmission electron microscopy combined with nano-beam diffraction mapping. The initially pure crystalline Cu and amorphous CuZr layers chemically mix by cross-phase shear banding after reaching a critical layer thickness. The Cu inside the shear bands develops a high dislocation density and can locally undergo transition to an amorphous state when sheared and mixed. We conclude that the severe deformation in the shear bands in the amorphous layer squeeze Zr atoms into the Cu dislocation cores in the Cu layers (thickness <5 nm), resulting in local chemical mixing. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.01.062
  • 2015 • 44 Influence of the Nuclear Electric Quadrupolar Interaction on the Coherence Time of Hole and Electron Spins Confined in Semiconductor Quantum Dots
    Hackmann, J. and Glasenapp, Ph. and Greilich, A. and Bayer, M. and Anders, F.B.
    Physical Review Letters 115 (2015)
    The real-time spin dynamics and the spin noise spectra are calculated for p and n-charged quantum dots within an anisotropic central spin model extended by additional nuclear electric quadrupolar interactions and augmented by experimental data. Using realistic estimates for the distribution of coupling constants including an anisotropy parameter, we show that the characteristic long time scale is of the same order for electron and hole spins strongly determined by the quadrupolar interactions even though the analytical form of the spin decay differs significantly consistent with our measurements. The low frequency part of the electron spin noise spectrum is approximately 1/3 smaller than those for hole spins as a consequence of the spectral sum rule and the different spectral shapes. This is confirmed by our experimental spectra measured on both types of quantum dot ensembles in the low power limit of the probe laser. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.115.207401
  • 2015 • 43 Impact of ambient pressure on titania nanoparticle formation during spray-flame synthesis
    Hardt, S. and Wlokas, I. and Schulz, C. and Wiggers, H.
    Journal of Nanoscience and Nanotechnology 15 9449-9456 (2015)
    Nanocrystalline titania was synthesized via liquid-fed spray-flame synthesis in a hermetically closed system at various pressures. Titanium tetraisopropoxide dissolved in isopropanol was used as precursor. The size, crystal structure, degree of agglomeration, morphology and the band gap of the as-prepared particles were investigated ex situ by nitrogen adsorption, transmission electron microscopy, X-ray diffraction, and UV-VIS absorption spectroscopy. In comparison to synthesis at atmospheric pressure it was found that decreasing pressure has a significant influence on the particle size distribution leading to smaller particles with reduced geometric standard deviation while particle morphology and crystal structure are not affected. Computational fluid dynamics simulations support the experimental findings also indicating a significant decrease in particle size at reduced pressure. Although it is well known that decreasing pressure leads to smaller particle sizes, it is (to our knowledge) the first time that this relation was investigated for spray-flame synthesis. Copyright © 2015 American Scientific Publishers All rights reserved.
    view abstractdoi: 10.1166/jnn.2015.10607
  • 2015 • 42 Hybrid biocomposite with a tunable antibacterial activity and bioactivity based on RF magnetron sputter deposited coating and silver nanoparticles
    Ivanova, A.A. and Surmenev, R.A. and Surmeneva, M.A. and Mukhametkaliyev, T. and Loza, K. and Prymak, O. and Epple, M.
    Applied Surface Science 329 212-218 (2015)
    In this work, we describe fabrication techniques used to prepare a multifunctional biocomposite based on a hydroxyapatite (HA) coating and silver nanoparticles (AgNPs). AgNPs synthesized by a wet chemical reduction method were deposited on Ti substrates using a dripping/drying method followed by deposition of calcium phosphate (CaP) coating via radio-frequency (RF) magnetron sputter-deposition. The negatively charged silver nanoparticles (zeta potential -21 mV) have a spherical shape with a metallic core diameter of 50 ± 20 nm. The HA coating was deposited as a dense nanocrystalline film over a surface of AgNPs. The RF-magnetron sputter deposition of HA films on the AgNPs layer did not affect the initial content of AgNPs on the substrate surface as well as NPs size and shape. SEM cross-sectional images taken using the backscattering mode revealed a homogeneous layer of AgNPs under the CaP layer. The diffraction patterns from the coatings revealed reflexes of crystalline HA and silver. The concentration of Ag ions released from the biocomposites after 7 days of immersion in phosphate and acetate buffers was estimated. The obtained results revealed that the amount of silver in the solutions was 0.27 ± 0.02 μg mL-1 and 0.54 ± 0.02 μg mL-1 for the phosphate and acetate buffers, respectively, which corresponded well with the minimum inhibitory concentration range known for silver ions in literature. Thus, this work establishes a new route to prepare a biocompatible layer using embedded AgNPs to achieve a local antibacterial effect. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2014.12.153
  • 2015 • 41 Nanocellulose-Templated Porous Titania Scaffolds Incorporating Presynthesized Titania Nanocrystals
    Ivanova, A. and Fravventura, M.C. and Fattakhova-Rohlfing, D. and Rathouský, J. and Movsesyan, L. and Ganter, P. and Savenije, T.J. and Bein, T.
    Chemistry of Materials 27 6205-6212 (2015)
    Nanocrystalline cellulose (NCC) is an abundant biogenic nanomaterial with unique properties that enables the efficient synthesis of mesoporous crystalline titania. We significantly enhance the photocatalytic activity of titania thin films by introducing solvothermally synthesized preformed anatase nanoparticles into a sol-gel based biotemplated titania scaffold. The resulting dual source titania thin films containing different amounts of preformed crystalline species were investigated by time-resolved microwave conductivity (TRMC) measurements and tested in the photocatalytic conversion of 4-chlorophenol. The gradual addition of preformed nanoparticles leads to a consistent increase of the mean size of titania crystalline domains, whereas the porosity of the composite is well-preserved due to the shape-persistent nature of the NCC template. Microwave conductivity studies establish increased photoconductivity of the films containing preformed anatase nanoparticles in comparison to that of films made without the nanoparticles. The synergistic features of the dual source titania, namely the improved crystalline properties brought by the preformed nanocrystals in combination with the high surface area provided by the NCC-templated sol-gel titania, result in a very high photocatalytic activity of the films in the photocatalytic decomposition of 4-chlorophenol. In quantitative terms, the dual source titania films prepared with 75% nanoparticles exhibit a first order degradation rate constant of 0.53 h-1 (1.47 × 10-4 sec-1), which strongly outperforms the activity of commercial P90 nanopowder showing a rate constant of 0.17 h-1 (0.47 × 10-4 sec-1) under the same conditions. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.5b00770
  • 2015 • 40 Iron(II) disulfides as precursors of highly selective catalysts for hydrodeoxygenation of dibenzyl ether into toluene
    Ji, N. and Wang, X. and Weidenthaler, C. and Spliethoff, B. and Rinaldi, R.
    ChemCatChem 7 960-966 (2015)
    In this report, we show that nanocrystalline pyrite and marcasite (FeS2), supported on SBA-15, aerosil SiO2, activated carbon or Al2O3, are precursors of highly active catalysts for the hydrodeoxygenation of dibenzyl ether into toluene. High yields of toluene (up to 100 %) were achieved in experiments performed at 250 C under initial H2 pressure of 100 bar for 2 h. In the recycling experiments, results from XRD and XPS analyses indicate that a fresh surface, formed upon the chemical transformation of FeS2 into Fe(1-x)S, is responsible for the high activity and high selectivity achieved in the conversion of dibenzyl ether into toluene. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201500041
  • 2015 • 39 Photon-Statistics Excitation Spectroscopy of a Quantum-Dot Micropillar Laser
    Kazimierczuk, T. and Schmutzler, J. and Aßmann, M. and Schneider, C. and Kamp, M. and Höfling, S. and Bayer, M.
    Physical Review Letters 115 (2015)
    We introduce photon-statistics excitation spectroscopy and exemplarily apply it to a quantum-dot micropillar laser. Both the intensity and the photon number statistics of the emission from the micropillar show a strong dependence on the photon statistics of the light used for excitation of the sample. The results under coherent and pseudothermal excitation reveal that a description of the laser properties in terms of mean input photon numbers is not sufficient. It is demonstrated that the micropillar acts as a superthermal light source when operated close to its threshold. Possible applications for important spectroscopic techniques are discussed. © 2015 American Physical Society. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.115.027401
  • 2015 • 38 The effect of charged quantum dots on the mobility of a two-dimensional electron gas: How important is the Coulomb scattering?
    Kurzmann, A. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Journal of Applied Physics 117 (2015)
    We have investigated the influence of a layer of charged self-assembled quantum dots (QDs) on the mobility of a nearby two-dimensional electron gas (2DEG). Time-resolved transconductance spectroscopy was used to separate the two contributions of the change in mobility, which are: (i) The electrons in the QDs act as Coulomb scatterers for the electrons in the 2DEG. (ii) The screening ability and, hence, the mobility of the 2DEG decreases when the charge carrier density is reduced by the charged QDs, i.e., the mobility itself depends on the charge carrier concentration. Surprisingly, we find a negligible influence of the Coulomb scattering on the mobility for a 2DEG, separated by a 30nm tunneling barrier to the layer of QDs. This means that the mobility change is completely caused by depletion, i.e., reduction of the charge carrier density in the 2DEG, which indirectly influences the mobility. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4907217
  • 2015 • 37 Towards solar cell emitters based on colloidal Si nanocrystals
    Leendertz, C. and Chirvony, V.S. and García-Calzada, R. and Görög, L. and Töfflinger, J.A. and Korte, L. and Agouram, S. and Martínez-Pastor, J. and Petermann, N. and Wiggers, H. and Ulyashin, A.G.
    Physica Status Solidi (A) Applications and Materials Science 212 156-161 (2015)
    The application of layers of doped colloidal silicon nanocrystals sandwiched between hydrogenated amorphous silicon layers as emitters in silicon heterojunction solar cells is explored. It is shown that such emitters provide excellent interface passivation and reasonable conductivity. Final solar cells with such nanoparticle emitters reach conversion efficiencies on the same level as solar cells with conventional hetero emitters. Quantum efficiency measurements indicate that the light absorbed in the nanocrystals contributes to the current extracted from the solar cell. The remaining challenges that need to be addressed before the application of such colloidal silicon nanocrystals for the processing of low-cost and potentially printable emitter layers becomes feasible are discussed. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/pssa.201431264
  • 2015 • 36 A general approach to study the thermodynamics of ligand adsorption to colloidal surfaces demonstrated by means of catechols binding to zinc oxide quantum dots
    Lin, W. and Walter, J. and Burger, A. and Maid, H. and Hirsch, A. and Peukert, W. and Segets, D.
    Chemistry of Materials 27 358-369 (2015)
    A general strategy to study the thermodynamics of ligand adsorption to colloidal surfaces was established. The versatility of our approach is demonstrated by means of catechols binding to ZnO quantum dots (QDs). First, isothermal titration calorimetry (ITC) was used to extract all relevant thermodynamic parameters, namely association constant, enthalpy, entropy, and free energy of ligand binding. Noteworthy, the determined δG of -20.3 ± 0.4 kJ mol-1 indicates a strong, reproducible, and exothermic interaction between the catechol anchor group and the oxide particle surface. To confirm the characterization of ligand binding by measuring the heat of adsorption, the free energy was cross-validated by mass-based adsorption isotherms. A combination of inductively coupled plasma optical emission spectroscopy (ICP-OES) and UV/vis spectroscopy was developed to quantitatively determine the mass of bound catechols with respect to the available particle surface. The association constant K was determined by a Langmuir fit to be 2618 M-1 which leads to δG = -19.50 kJ mol-1 according to δG = -RTln K. To close the mass balance, analytical ultracentrifugation (AUC) was applied to detect the amount of the free, unbound catechol in solution. Finally, Raman spectroscopy and nuclear magnetic resonance spectroscopy (NMR) were performed to quantify the amount of remaining acetate from particle synthesis and to distinguish bound (chemisorbed) and unbound (physisorbed) catechol. Our results reveal that approximately 65 wt % of acetate is replaced, and physisorbed catechol will not affect the amount of remaining acetate on the ZnO surface. Moreover, no pronounced chemical shift peak as it would be expected for free catechol is observed by NMR at all. This indicates a highly dynamic adsorption-desorption equilibrium between the free and the physisorbed state of catechol on the particle surface. Our concept of combined analytics is seen to be a generally applicable strategy for particle-ligand interfacial studies. It gives detailed insight into thermodynamics, binding states, and ligand composition and is thus considered as an important step toward tailored colloidal surface properties. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cm504080d
  • 2015 • 35 Influence of Post-Implantation Annealing Parameters on the Focused Ion Beam Directed Nucleation of InAs Quantum Dots
    Mehta, M. and Reuter, D. and Kamruddin, M. and Tyagi, A.K. and Wieck, A.D.
    Nano 10 (2015)
    We present the effect of post-implantation annealing conditions on the structural and optical quality of InAs quantum dots (QDs) grown by combination of focused ion beam (FIB) and molecular beam epitaxy (MBE) approach. A FIB of Ga+ ion was employed to pattern a homogeneously GaAs buffer layers and then, an in situ annealing step followed by InAs deposition was performed. Three different post-implantation annealing conditions were tested and under well-optimized conditions, a dislocation and defect-free InAs QDs growth on FIB patterned surface was successfully achieved. Furthermore, using photoluminescence (PL) study, we demonstrate that our best sample shows almost similar optical quality as MBE grown QDs on unimplanted GaAs surface. The patterning technique described here can presumably be applied to systems other than InAs/GaAs and highly interesting for site-controlled nucleation of QDs that finds its potential applications in nanooptoelectronic devices. © 2015 World Scientific Publishing Company.
    view abstractdoi: 10.1142/S1793292015500496
  • 2015 • 34 Direct photonic coupling of a semiconductor quantum dot and a trapped ion
    Meyer, H.M. and Stockill, R. and Steiner, M. and Le Gall, C. and Matthiesen, C. and Clarke, E. and Ludwig, Ar. and Reichel, J. and Atatüre, M. and Köhl, M.
    Physical Review Letters 114 (2015)
    Coupling individual quantum systems lies at the heart of building scalable quantum networks. Here, we report the first direct photonic coupling between a semiconductor quantum dot and a trapped ion and we demonstrate that single photons generated by a quantum dot controllably change the internal state of a Yb+ ion. We ameliorate the effect of the 60-fold mismatch of the radiative linewidths with coherent photon generation and a high-finesse fiber-based optical cavity enhancing the coupling between the single photon and the ion. The transfer of information presented here via the classical correlations between the σz projection of the quantum-dot spin and the internal state of the ion provides a promising step towards quantum-state transfer in a hybrid photonic network. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.114.123001
  • 2015 • 33 Atomic scale study of CU clustering and pseudo-homogeneous Fe-Si nanocrystallization in soft magnetic FeSiNbB(CU) alloys
    Pradeep, K.G. and Herzer, G. and Raabe, D.
    Ultramicroscopy 159 285-291 (2015)
    A local electrode atom probe has been employed to trace the onset of Cu clustering followed by their coarsening and subsequent growth upon rapid (10s) annealing of an amorphous Fe73.5Si15.5Cu1Nb3B7 alloy. It has been found that the clustering of Cu atoms introduces heterogeneities in the amorphous matrix, leading to the formation of Fe rich regions which crystallizes pseudo-homogeneously into Fe-Si nanocrystals upon annealing. In this paper, we present the data treatment method that allows for the visualization of these different phases and to understand their morphology while still quantifying them in terms of their size, number density and volume fraction. The crystallite size of Fe-Si nanocrystals as estimated from the atom probe data are found to be in good agreement with other complementary techniques like XRD and TEM, emphasizing the importance of this approach towards accurate structural analysis. In addition, a composition driven data segmentation approach has been attempted to determine and distinguish nanocrystalline regions from the remaining amorphous matrix. Such an analysis introduces the possibility of retrieving crystallographic information from extremely fine (2-4nm sized) nanocrystalline regions of very low volume fraction (< 5Vol%) thereby providing crucial in-sights into the chemical heterogeneity induced crystallization process of amorphous materials. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.04.006
  • 2015 • 32 Transition from shear to stress-assisted diffusion of copper-chromium nanolayered thin films at elevated temperatures
    Raghavan, R. and Wheeler, J.M. and Harzer, T.P. and Chawla, V. and Djaziri, S. and Thomas, K. and Philippi, B. and Kirchlechner, C. and Jaya, B.N. and Wehrs, J. and Michler, J. and Dehm, G.
    Acta Materialia 100 73-80 (2015)
    The mechanical behavior of Cu-Cr nanolayered films and an alloy film of nominal composition Cu<inf>20</inf>Cr<inf>80</inf> at.% was studied by microcompression testing at temperatures from 25 °C to 300 °C. Comparing nanolayered films, plastic deformation and failure occurred at consistently higher stress levels in the film with the smaller layer thicknesses. Plasticity in the nanolayered films always initiated in the softer Cu layers followed by a finite strain-hardening response in the stress-strain curves. Failure indicated by a strain-softening response following the higher peak strength due to shearing and tearing at columnar boundaries of Cr was observed in the nanolayered films at 25 °C and 100 °C. A transition from shearing and crack formation across the Cu-Cr interfaces leading to anomalous grain growth or beading of the nanocrystalline Cu layers was observed at elevated temperatures of 200 °C and 300 °C. On the other hand, the Cu<inf>20</inf>Cr<inf>80</inf> at.% alloy film exhibited failure by columnar buckling consistently at elevated temperatures, but shearing promoted by buckling at the highest strengths among the films at ambient temperature. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.08.016
  • 2015 • 31 Role of developing L10 chemical order on the (0 0 1)-texture formation of (Fe1 - XCux)Pt films grown on amorphous substrates
    Scheibel, F. and Haering, F. and Ziemann, P. and Wiedwald, U.
    Journal of Physics D: Applied Physics 48 (2015)
    We study the technologically important (0 0 1)-texture formation in 10 nm thick (Fe0.9Cu0.1)52Pt48 and, as reference, Fe49Pt51 alloy films. The samples are grown on SiO2(200 nm)/Si(0 0 1) substrates at ambient temperature by pulsed laser deposition. Subsequent rapid thermal processing (RTP) at 650 °C for various time steps drives the initially nanocrystalline and chemically disordered films into the tetragonal L10 phase accompanied by a strong (0 0 1)-texture leading to perpendicular magnetic anisotropy. The fundamental role of the chemical order during short-time annealing as an additional source of strain in the films is experimentally addressed. The structural and magnetic results indicate selective grain growth leading to the (0 0 1)-texture. Strongly prolonged annealing, however, leads to a reorientation of grains towards the (1 1 1)-texture pointing to the increasing importance of surface energies when the initial strain has released. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/8/085001
  • 2015 • 30 From in situ characterization to process control of quantum dot systems
    Segets, D. and Peukert, W.
    Procedia Engineering 102 575-581 (2015)
    Quantum confined semiconductor nanoparticles (quantum dots, QDs) are promising candidates for various applications in emerging fields like electronics, solar cells, sensors and diagnostics. However, a larger scale production of QDs at high product quality is still missing. One of the key requirements to address this issue in the near future was identified to be a fast and in situ applicable characterization method. Suitable characterization requires knowledge on the full shape of the particle size distributions (PSDs) under investigation. Thus, determination of a mean particle size together with the width of the PSD is not sufficient. In the following, a method will be presented that allows the derivation of arbitrary shaped PSDs for QDs with direct band gap based on their optical absorbance spectra. After validation of the technique by means of ZnO nanoparticles the transfer of the concept to other QD materials like PbS and PbSe will be proven. Therefore we will extend our methodology and show how our approach can be used to derive spectral properties like the size dependent band gap energy. This is realized by proper calibration of the calculation results against PSDs determined by an independent analysis technique like transmission electron microscopy (TEM). © 2015 The Authors.
    view abstractdoi: 10.1016/j.proeng.2015.01.129
  • 2015 • 29 Process engineering of nanoparticles below 20 nm—a fundamental discussion of characterization, particle formation, stability and post processing
    Segets, D. and Peukert, W.
    Colloid Process Engineering 279-305 (2015)
    In the following chapter fundamental aspects that have to be considered during the processing of small nanoparticles will be addressed. We investigated quantum confined manganese doped ZnS, ZnO, PbS and PbSe semiconductor nanoparticles, so-called quantum dots (QDs) and silver noble metal nanorods. All materials were chosen due to their technical relevance for future applications in the emerging fields of solar cells, sensors and diagnostics as well as due to the possibility of their in situ characterization by UV/Vis absorbance spectroscopy. After a brief introduction to the specific prospects and challenges of these materials we will focus on the important processing issues that need to be solved for producing these particles at high quality on a larger scale: (i) the modelling of particle formation including nucleation, growth and ripening based on a mechanistic understanding and on experimentally derived data on solubility and surface energies, (ii) the stabilization of nanoparticles not only against agglomeration but also against shape changes and (iii) classification. The latter is realized by size selective precipitation which allows surprisingly sharp separations (κ= 0.75) of particles with only a few nm in diameter. Although the extremely small particle sizes (feed PSD between 1.5 and 3 nm), classification results were successfully analyzed by well-known concepts from particle technology. Our results are seen to be an essential contribution to colloidal processing. They enable a future optimization of process parameters by a knowledge-based design strategy that can be applied within continuous as well as automatized batch reactor concepts. © Springer International Publishing Switzerland 2015.
    view abstractdoi: 10.1007/978-3-319-15129-8_12
  • 2015 • 28 Classification of zinc sulfide quantum dots by size: Insights into the particle surface-solvent interaction of colloids
    Segets, D. and Lutz, C. and Yamamoto, K. and Komada, S. and Süß, S. and Mori, Y. and Peukert, W.
    Journal of Physical Chemistry C 119 4009-4022 (2015)
    We present a detailed study on the classification of ZnS quantum dots (QDs) by size selective precipitation (SSP). SSP allows the postsynthetic narrowing of a given feed distribution and is usually realized by titration of a poor solvent into a suspension of dispersed particles. Thereby preferred flocculation of larger structures is induced. Our results confirm that SSP is a highly robust process, following a law of mass action. That means a certain solvent composition always leads to the same ratio between coarse and fines particles with respect to a specific particle size xi. This ratio is independent of the particle size distribution (PSD) of the feed, the washing history of the particles, and the solid concentration of the particles. Regarding the illustration of our findings, we established a combined approach that takes Hansen solubility parameters (HSP) of solvent mixtures as well as changing van der Waals interactions into account. Relating both to each other, a size-dependent region of enhanced solubility is clearly identified. Our concept allows a differentiation between volume-related effects like van der Waals interactions and surface-related effects like the interaction of a ligand with a solvent mixture. A comprehensive interpretation of classification results obtained with different good solvents and poor solvents enables to deduce a general strategy for the demanding determination of HSP for small colloids. Our work makes an important contribution to the design of an appropriate colloidal postprocessing which is applicable to larger quantities. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jp508746s
  • 2015 • 27 Kinetics enhancement, reaction pathway change, and mechanism clarification in LiBH4 with Ti-catalyzed nanocrystalline MgH2 composite
    Shao, H. and Felderhoff, M. and Weidenthaler, C.
    Journal of Physical Chemistry C 119 2341-2348 (2015)
    A composite of 2LiBH4 + nano-MgH2* (Ti-catalyzed) shows significantly enhanced desorption kinetics compared to a conventional mixture of 2LiBH4 + MgH2. The desorption mechanism was studied in the temperature range between 304 and 383 °C and under different pressure conditions. Desorption temperatures are 50-70 °C lower compared to conventional 2LiBH4 + MgH2 mixtures. During the hydrogen release from a mixture of 2LiBH4 + nano-MgH2* at a hydrogen back-pressure of 0.4 MPa, MgB2 is formed and three different plateaus of equilibrium were detected. The reaction pathway is changed at 360 °C for the 2LiBH4 + MgH2 system when the nano-MgH2* is used. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jp511479d
  • 2015 • 26 Thermoelectric energy harvesting with quantum dots
    Sothmann, B. and Sánchez, R. and Jordan, A.N.
    Nanotechnology 26 (2015)
    We review recent theoretical work on thermoelectric energy harvesting in multi-terminal quantum-dot setups. We first discuss several examples of nanoscale heat engines based on Coulomb-coupled conductors. In particular, we focus on quantum dots in the Coulomb-blockade regime, chaotic cavities and resonant tunneling through quantum dots and wells. We then turn toward quantum-dot heat engines that are driven by bosonic degrees of freedom such as phonons, magnons and microwave photons. These systems provide interesting connections to spin caloritronics and circuit quantum electrodynamics. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/26/3/032001
  • 2015 • 25 Rabi oscillations in an InAs quantum dot ensemble observed in pre-pulse 2D coherent spectroscopy
    Suzuki, T. and Singh, R. and Akimov, I.A. and Bayer, M. and Reuter, D. and Wieck, A.D. and Cundiff, S.T.
    Springer Proceedings in Physics 162 271-274 (2015)
    We have observed Rabi oscillations in an InAs quantum dot ensemble by using optical pre-pulse 2D coherent spectroscopy. The polarization for 2D coherent spectroscopy is set to be cross-linear in order to obtain biexciton signal, which enables us to distinguish the signals from the ground and excited states. Furthermore, the spectral domain in 2D can reveal the coherent evolution in an inhomogeneously broadened ensemble. With increasing pre-pulse intensity, the signals attributed to the ground and excited states exhibit the sinusoidal decrease and increase, respectively. The observed excitation behavior is well reproduced by a damped oscillation model. From the fitting the pulse area achieved in this work is deduced to be 0.41p and the dipole moment is estimated as 29 Debye. © Springer International Publishing Switzerland 2015.
    view abstractdoi: 10.1007/978-3-319-13242-6_66
  • 2014 • 24 Effect of a side reaction involving structural changes of the surfactants on the shape control of cobalt nanoparticles
    Comesaña-Hermo, M. and Estivill, R. and Ciuculescu, D. and Li, Z.-A. and Spasova, M. and Farle, M. and Amiens, C.
    Langmuir 30 4474-4482 (2014)
    Cobalt nanoparticles with different sizes and morphologies including spheres, rods, disks, and hexagonal prisms have been synthesized through the decomposition of the olefinic precursor [Co(η3-C 8H13)(η4-C8H12)] under dihydrogen, in the presence of hexadecylamine and different rhodamine derivatives, or aromatic carboxylic acids. UV-vis spectroscopy, X-ray diffraction, low and high resolution transmission electron microscopy, and electron tomography have been used to characterize the nanomaterials. Especially, the Co nanodisks formed present characteristics that make them ideal nanocrystals for applications such as magnetic data storage. Focusing on their growth process, we have evidenced that a reaction between hexadecylamine and rhodamine B occurs during the formation of these Co nanodisks. This reaction limits the amount of free acid and amine, usually at the origin of the formation of single crystal Co rods and wires, in the growth medium of the nanocrystals. As a consequence, a growth mechanism based on the structure of the preformed seeds rather than oriented attachment or template assisted growth is postulated to explain the formation of the nanodisks. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/la5005165
  • 2014 • 23 Spin-flip Raman scattering of the Γ-X mixed exciton in indirect band gap (In,Al)As/AlAs quantum dots
    Debus, J. and Shamirzaev, T.S. and Dunker, D. and Sapega, V.F. and Ivchenko, E.L. and Yakovlev, D.R. and Toropov, A.I. and Bayer, M.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    The band structure of type-I (In,Al)As/AlAs quantum dots with band gap energy exceeding 1.63 eV is indirect in momentum space, leading to long-lived exciton states with potential applications in quantum information. Optical access to these excitons is provided by mixing of the Γ- and X-conduction-band valleys, for which their spins may be oriented by resonant spin-flip Raman scattering. This access is used to study the exciton spin-level structure by accurately measuring the anisotropic hole and isotropic electron g factors. The spin-flip mechanisms for the indirect exciton and its constituents as well as the underlying optical selection rules are determined. The spin-flip intensity is a reliable measure of the strength of Γ-X-valley mixing, as evidenced by both experiment and theory. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.90.125431
  • 2014 • 22 Nanojunction-mediated photocatalytic enhancement in heterostructured CdS/ZnO, CdSe/ZnO, and CdTe/ZnO nanocrystals
    Eley, C. and Li, T. and Liao, F. and Fairclough, S.M. and Smith, J.M. and Smith, G. and Tsang, S.C.E.
    Angewandte Chemie - International Edition 53 7838-7842 (2014)
    A series of highly efficient semiconductor nanocrystal (NC) photocatalysts have been synthesized by growing wurtzite-ZnO tetrahedrons around pre-formed CdS, CdSe, and CdTe quantum dots (QDs). The resulting contact between two small but high-quality crystals creates novel CdX/ZnO heterostructured semiconductor nanocrystals (HSNCs) with extensive type-II nanojunctions that exhibit more efficient photocatalytic decomposition of aqueous organic molecules under UV irradiation. Catalytic testing and characterization indicate that catalytic activity increases as a result of a combination of both the intrinsic chemistry of the chalcogenide anions and the heterojunction structure. Atomic probe tomography (APT) is employed for the first time to probe the spatial characteristics of the nanojunction between cadmium chalcogenide and ZnO crystalline phases, which reveals various degrees of ion exchange between the two crystals to relax large lattice mismatches. In the most extreme case, total encapsulation of CdTe by ZnO as a result of interfacial alloying is observed, with the expected advantage of facilitating hole transport for enhanced exciton separation during catalysis. That's a (quantum dot) wrap! A series of highly active semiconductor photocatalysts have been synthesized by growing wurtzite-ZnO tetrahedrons around pre-formed CdS, CdSe, and CdTe quantum dots. The resulting heterostructured CdX/ZnO nanocrystals with extensive type-II nanojunctions exhibit rapid photocatalytic decomposition of organic molecules in aqueous media. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201404481
  • 2014 • 21 Valence-band mixing effects in the upper-excited-state magneto-optical responses of colloidal Mn2+-doped CdSe quantum dots
    Fainblat, R. and Muckel, F. and Barrows, C.J. and Vlaskin, V.A. and Gamelin, D.R. and Bacher, G.
    ACS Nano 8 12669-12675 (2014)
    We present an experimental study of the magneto-optical activity of multiple excited excitonic states of manganese-doped CdSe quantum dots chemically prepared by the diffusion doping method. Giant excitonic Zeeman splittings of each of these excited states can be extracted for a series of quantum dot sizes and are found to depend on the radial quantum number of the hole envelope function involved in each transition. As seven out of eight transitions involve the same electron energy state, 1Se, the dominant hole character of each excitonic transition can be identified, making use of the fact that the g-factor of the pure heavy-hole component has a different sign compared to pure light hole or split-off components. Because the magnetic exchange interactions are sensitive to hole state mixing, the giant Zeeman splittings reported here provide clear experimental evidence of quantum-size-induced mixing among valence-band states in nanocrystals. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nn505610e
  • 2014 • 20 Ultrasmall dispersible crystalline nickel oxide nanoparticles as high-performance catalysts for electrochemical water splitting
    Fominykh, K. and Feckl, J.M. and Sicklinger, J. and Döblinger, M. and Böcklein, S. and Ziegler, J. and Peter, L. and Rathousky, J. and Scheidt, E.-W. and Bein, T. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 24 3123-3129 (2014)
    Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time, and it is shown that they are promising candidates as catalysts for electrochemical water oxidation. Using a solvothermal reaction in tert-butanol, very small nickel oxide nanocrystals can be made with sizes tunable from 2.5 to 5 nm and a narrow particle size distribution. The crystals are perfectly dispersible in ethanol even after drying, giving stable transparent colloidal dispersions. The structure of the nanocrystals corresponds to phase-pure stoichiometric nickel(ii) oxide with a partially oxidized surface exhibiting Ni(iii) states. The 3.3 nm nanoparticles demonstrate a remarkably high turn-over frequency of 0.29 s-1 at an overpotential of g = 300 mV for electrochemical water oxidation, outperforming even expensive rare earth iridium oxide catalysts. The unique features of these NiO nanocrystals provide great potential for the preparation of novel composite materials with applications in the field of (photo)electrochemical water splitting. The dispersed colloidal solutions may also find other applications, such as the preparation of uniform hole-conducting layers for organic solar cells. Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time using a solvothermal reaction in tert-butanol. These nanocrystals can be prepared with sizes tunable from 2.5 to 5 nm and are highly efficient catalysts for electrochemical oxygen generation. © 2014 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201303600
  • 2014 • 19 Antibacterial AgNPs/CaP biocomposites
    Ivanova, A.A. and Surmenev, R.A. and Surmeneva, M.A. and Mukhametkaliyev, T. and Sharonova, A.A. and Grubova, I.Y. and Loza, K. and Chernousova, S. and Prymak, O. and Epple, M.
    2014 9th International Forum on Strategic Technology, IFOST 2014 472-474 (2014)
    The modification of implant surface is in the focus of many scientists worldwide. In this study, multifunctional biocomposite on the basis of calcium phosphate coating and silver nanoparticles has been fabricated through the use of nanofabrication techniques. Dense nanocrystalline HA film was deposited over AgNPs. The properties as well as the in vitro behavior of the developed biocomposites have been studied. The diffraction patterns of the biocomposites revealed the peaks of crystalline HA and silver (Ag). The release of Ag from the developed biocomposites was evaluated. The concentration of the released silver ions for 7 days of dissolution was 0.27±0.02 μg/mL and 0.54±0.02 μg/mL for phosphate and acetate buffers, respectively. In order to estimate the cytotoxicity of the samples the functional activity of osteoclasts, in particular, cell morphology, multinuclearity, actin ring and resorption pit on the substrates coated with HA and AgNPs-HA have been evaluated. © 2014 IEEE.
    view abstractdoi: 10.1109/IFOST.2014.6991166
  • 2014 • 18 Optically imprinted reconfigurable photonic elements in a VO2 nanocomposite
    Jostmeier, T. and Zimmer, J. and Karl, H. and Krenner, H.J. and Betz, M.
    Applied Physics Letters 105 (2014)
    We investigate the optical and thermal hysteresis of single-domain vanadium dioxide nanocrystals fabricated by ion beam synthesis in a fused silica matrix. The nanocrystals exhibit a giant hysteresis, which permits to optically generate a long-time stable supercooled metallic phase persistent down to practically room temperature. Spatial patterns of supercooled and insulating nanocrystals feature a large dielectric contrast, in particular, for telecom wavelengths. We utilize this contrast to optically imprint reconfigurable photonic elements comprising diffraction gratings as well as on- and off-axis zone plates. The structures allow for highly repetitive (>104) cycling through the phase transition without structural damage. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4893570
  • 2014 • 17 Resonant electronic coupling enabled by small molecules in nanocrystal solids
    Pereira, R.N. and Coutinho, J. and Niesar, S. and Oliveira, T.A. and Aigner, W. and Wiggers, H. and Rayson, M.J. and Briddon, P.R. and Brandt, M.S. and Stutzmann, M.
    Nano Letters 14 3817-3826 (2014)
    The future exploitation of the exceptional properties of nanocrystal (NC) thin films deposited from liquid dispersions of nanoparticles relies upon our ability to produce films with improved electrical properties by simple and inexpensive means. Here, we demonstrate that the electronic conduction of solution-processed NC films can be strongly enhanced without the need of postdeposition treatments, via specific molecules adsorbed at the surfaces of adjacent NCs. This effect is demonstrated for Si NC films doped with the strong molecular oxidizing agent tetrafluoro-tetracyanoquinodimethane (F 4-TCNQ). Density functional calculations were carried out with molecule-doped superlattice solid models. It is shown that, when populated by electrons, hybrid molecule/NC states edge (and may actually resonate with) the conduction-band states of the NC solid. This provides extra electronic connectivity across the NC network as the molecules effectively flatten the electronic potential barriers for electron transfer across the otherwise vacuum-filled network interstitialcies. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nl500932q
  • 2014 • 16 Atom probe tomography study of ultrahigh nanocrystallization rates in FeSiNbBCu soft magnetic amorphous alloys on rapid annealing
    Pradeep, K.G. and Herzer, G. and Choi, P. and Raabe, D.
    Acta Materialia 68 295-309 (2014)
    Rapid annealing (4-10 s) induced primary crystallization of soft magnetic Fe-Si nanocrystals in a Fe73.5Si15.5Cu1Nb 3B7 amorphous alloy has been systematically studied by atom probe tomography in comparison with conventional annealing (30-60 min). It was found that the nanostructure obtained after rapid annealing is basically the same, irrespective of the different time scales of annealing. This underlines the crucial role of Cu during structure formation. Accordingly, the clustering of Cu atoms starts at least 50 C below the onset temperature of primary crystallization. As a consequence, coarsening of Cu atomic clusters also starts prior to crystallization, resulting in a reduction of available nucleation sites during Fe-Si nanocrystallization. Furthermore, the experimental results explicitly show that these Cu clusters initially induce a local enrichment of Fe and Si in the amorphous matrix. These local chemical heterogeneities are proposed to be the actual nuclei for subsequent nanocrystallization. Nevertheless, rapid annealing in comparison with conventional annealing results in the formation of ∼30% smaller Fe-Si nanocrystals, but of identical structure, volume fraction and chemical composition, indicating the limited influence of thermal treatment on nanocrystallization, owing to the effect of Cu. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.01.031
  • 2013 • 15 Structural changes in amorphous GexSiOy on the way to nanocrystal formation
    Nyrow, A. and Sternemann, C. and Sahle, Ch.J. and Hohl, A. and Zschintzsch-Dias, M. and Schwamberger, A. and Mende, K. and Brinkmann, I. and Moretti Sala, M. and Wagner, R. and Meier, A. and Völklein, F. and Tolan, M.
    Nanotechnology 24 (2013)
    Temperature induced changes of the local chemical structure of bulk amorphous GexSiOy are studied by Ge K-edge x-ray absorption near-edge spectroscopy and Si L2/3-edge x-ray Raman scattering spectroscopy. Different processes are revealed which lead to formation of Ge regions embedded in a Si oxide matrix due to different initial structures of as-prepared samples, depending on their Ge/Si/O ratio and temperature treatment, eventually resulting in the occurrence of nanocrystals. Here, disproportionation of GeOx and SiOx regions and/or reduction of Ge oxides by pure Si or by a surrounding Si sub-oxide matrix can be employed to tune the size of Ge nanocrystals along with the chemical composition of the embedding matrix. This is important for the optimization of the electronic and luminescent properties of the material. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/24/16/165701
  • 2013 • 14 Preparation of amorphous and nanocrystalline sodium tantalum oxide photocatalysts with porous matrix structure for overall water splitting
    Tüysüz, H. and Chan, C.K.
    Nano Energy 2 116-123 (2013)
    Herein, we report the preparation of a series of surfactant-free nanostructured sodium tantalum oxide using NaTa(OC3H7)6 as a single precursor. The reaction conditions for the novel synthetic method were optimized and the morphology and crystal structure of the prepared materials were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Condensation and polymerization of NaTa(OC3H7)6 under atmospheric pressure gave a porous amorphous structure that could be converted to crystalline NaTaO3 while crystalline Na2Ta2O6 nanocrystals with a 25nm average particle size could be obtained from a hydrothermal method using NH3 as a base catalyst. In addition, the photocatalytic behaviors of the prepared materials were investigated for overall water splitting into hydrogen and oxygen. Unexpectedly, porous amorphous sodium tantalum oxide showed much better catalytic activity over the crystalline one. The synthesized Na2Ta2O6 nanocrystals also indicated promising activity for overall water splitting without any co-catalyst in comparison to bulk NaTaO3. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.nanoen.2012.08.003
  • 2012 • 13 Photon statistics in quantum dot micropillar emission
    Aβmann, M. and Bayer, M.
    Quantum Dots: Optics, Electron Transport and Future Applications 9781107012585 169-184 (2012)
    The ever-growing demand for fast optical data transmission calls for lasers offering high modulation rates and low energy consumption at the same time. Advances in growth and processing methods make quantum dot (QD) based lasers better candidates for this challenge than ever before. Placed in microresonators able to confine light in regions roughly the size of their wavelength, QDs pave the way to ultra-low threshold lasing. The most common resonator geometries aimed at three-dimensional light confinement are microdisks, photonic crystal membrane cavities and micropillars. The latter are especially good candidates for realizing microlasers suitable for applications as they offer directed emission and allow for parallel device processing. However, this increased efficiency also results in modified emission properties of QD lasers [8]. Semiconductor-specific processes like Pauli-blocking of states, the composite nature of the carriers involved and Coulomb interactions between carriers cause deviations from the standard atomistic laser picture. The main aim of our studies is to characterize microlaser emission in terms of photon statistics and coherence properties. Following Glauber, the most detailed description of a light field is given in a series of correlation functions describing coherence in different orders [10].This chapter is organized as follows. Section 10.2 contains a brief review on the characteristic properties of micropillar lasers and discusses the emission properties of microlasers operated below and above threshold. Section 10.2.1 focuses on photon statistics and the classification of light fields. © Cambridge University Press 2012.
    view abstractdoi: 10.1017/CBO9780511998331.011
  • 2012 • 12 Nanoscale porous framework of lithium titanate for ultrafast lithium insertion
    Feckl, J.M. and Fominykh, K. and Döblinger, M. and Fattakhova-Rohlfing, D. and Bein, T.
    Angewandte Chemie - International Edition 51 7459-7463 (2012)
    Mesoporous lithium titanate has been prepared with a titanate morphology that leads to the fastest insertion of lithium. It features a gravimetric capacity of about 175 mAhg -1 and delivers up to 73% of the maximum capacity at up to 800 C (4.5 s) without deterioration over 1000 cycles. A key feature is a fully crystalline interconnected porous framework composed of spinel nanocrystals of only a few nanometers in size. Scale bar: 10 nm. Copyright © 2012 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201201463
  • 2012 • 11 P-Si/n-ZnO nanocrystal heterojunction light emitting device
    Nannen, E. and Kümmell, T. and Ebbers, A. and Bacher, G.
    Applied Physics Express 5 (2012)
    ZnO has a high potential for use in light-emitting devices in the visible and UV spectral range. One of the main challenges in an electrically driven device is the low energy of the valence band and, consequently, the difficult injection of holes. Here, we present an approach combining naturally n-type ZnO nanocrystals with intentionally p-doped Si nanoparticles in a solution-processable nanoparticle heterojunction multilayer. The heterojunction device exhibits an efficiency, that is more than one order of magnitude enhanced compared with the ZnO reference device. White electroluminescence with color rendering indices up to 98 is obtained. © 2012 The Japan Society of Applied Physics.
    view abstractdoi: 10.1143/APEX.5.035001
  • 2012 • 10 Exchange-coupled donor dimers in nanocrystal quantum dots
    Pereira, R.N. and Almeida, A.J. and Stegner, A.R. and Brandt, M.S. and Wiggers, H.
    Physical Review Letters 108 (2012)
    Doping of semiconductor nanocrystals (NCs) is expected to enable the control of key NC properties, yet its practical exploitation requires an understanding of exchange interactions when multiple dopants are incorporated in a single NC. Here, we experimentally probe the exchange of donor dimers in NCs via a deviation of their triplet-state magnetic resonance from Curie paramagnetism. We show that the exchange coupling of the closely spaced donors can be well described by effective mass theory, which allows the consideration of statistical effects crucial in NC ensembles. While a dimer induces discrete states in a NC, their energy splitting differs by up to 3 orders of magnitude for randomly placed dimers in a NC ensemble, due to an enormous dependence of the exchange energy on the dimer configuration. © 2012 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.108.126806
  • 2012 • 9 Nanoscopic NaCl crystals as water-soluble porogens for polymer membranes
    Range, S. and Epple, M.
    RSC Advances 2 6650-6654 (2012)
    Nanoscopic NaCl was prepared by elimination from sodium malonate and phenacyl chloride with a particle diameter of 100 nm. The NaCl crystals were added to either poly(d,l)lactide or polysulfone (dissolved in dichloromethane) as water-soluble porogens. The dispersion was applied to silicon wafers by dip-coating. After drying in air, the NaCl crystals were removed by washing with water, leaving behind a porous membrane with pore diameters around 100 nm. These free-standing membranes were prepared with a thickness from 1 to 10 μm. They can be used to separate small molecules from larger molecules like proteins. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2ra01237e
  • 2011 • 8 Solution-processed networks of silicon nanocrystals: The role of internanocrystal medium on semiconducting behavior
    Pereira, R.N. and Niesar, S. and You, W.B. and Da Cunha, A.F. and Erhard, N. and Stegner, A.R. and Wiggers, H. and Willinger, M.-G. and Stutzmann, M. and Brandt, M.S.
    Journal of Physical Chemistry C 115 20120-20127 (2011)
    We have produced networks of surface-oxidized and hydrogen-terminated silicon nanocrystals (Si-NCs), both intrinsic and n-type doped, on flexible plastic foil from nanoparticle inks. The charge transport in these networks was comprehensively studied by means of time-dependent conductivity, steady-state current versus voltage characteristics, and steady-state photocurrent measurements as a function of incident light intensity. These measurements were complemented by surface chemistry and structural/morphological analysis from Fourier transform infrared spectroscopy and electron microscopy. Whereas H-terminated Si-NC networks function as semiconductors (both in air and in vacuum), where conductivity enhancement upon impurity doping and photoconductivity were observed, these characteristics are not present in networks of surface-oxidized Si-NCs. For both network types, the observation of a power law behavior for steady-state current versus voltage and a current decaying with time at constant bias indicate that charge transport is controlled by space-charge-limited current (involving trap states) via percolation paths through the networks. We have also monitored the evolution of the networks (photo)conductivity when the internanocrystal separating medium formed by Si-H bonds is progressively replaced by a native oxide upon exposure to air. Although a decrease in the (photo)conductivity is observed, the networks still behave as semiconductors even after a long-term air exposure. From an analysis of all (photo)current data, we deduce that in networks of oxidized Si-NCs inter-NC charge transfer requires the participation of oxide-related electronic states, whereas in H-terminated Si-NC networks direct inter-NC charge transfer plays a major role in the overall long-range conduction process. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp205984m
  • 2011 • 7 Influence of hydrogen on thermally induced phase separation in GeO/SiO 2 multilayers
    Sahle, C.J. and Zschintzsch, M. and Sternemann, C. and Von Borany, J. and Mücklich, A. and Nyrow, A. and Jeutter, N.M. and Wagner, R. and Frahm, R. and Tolan, M.
    Nanotechnology 22 (2011)
    The influence of the annealing atmosphere on the temperature induced phase separation of Ge oxide in GeOx/SiO2 multilayers (x ≈ 1), leading to size controlled growth of Ge nanocrystals, is explored by means of x-ray absorption spectroscopy at the Ge K-edge. Ge sub-oxides contained in the as-deposited multilayers diminish with increasing annealing temperature, showing complete phase separation at approximately 450 °C using inert N 2 ambient. The use of reducing H2 in the annealing atmosphere influences the phase separation even at an early stage of the disproportionation. In particular, the temperature regime where the phase separation occurs is lowered by at least 50 °C. At temperatures above 400 °C the sublayer composition, and thus the density of the Ge nanocrystals, can be altered by making use of the reduction of GeO2 by H 2. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/12/125709
  • 2011 • 6 Exciton recombination dynamics in an ensemble of (In,Al)As/AlAs quantum dots with indirect band-gap and type-I band alignment
    Shamirzaev, T.S. and Debus, J. and Abramkin, D.S. and Dunker, D. and Yakovlev, D.R. and Dmitriev, D.V. and Gutakovskii, A.K. and Braginsky, L.S. and Zhuravlev, K.S. and Bayer, M.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    The dynamics of exciton recombination in an ensemble of indirect band-gap (In,Al)As/AlAs quantum dots with type-I band alignment is studied. The lifetime of confined excitons that are indirect in momentum space is mainly influenced by the sharpness of the heterointerface between the (In,Al)As quantum dot and the AlAs barrier matrix. Time-resolved photoluminescence experiments and theoretical model calculations reveal a strong dependence of the exciton lifetime on the thickness of the interface diffusion layer. The lifetime of excitons with a particular optical transition energy varies because this energy is obtained for quantum dots differing in size, shape, and composition. The different exciton lifetimes, which result in photoluminescence with nonexponential decay obeying a power-law function, can be described by a phenomenological distribution function G(τ), which allows one to fit the photoluminescence decay with one parameter only. © 2011 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.84.155318
  • 2011 • 5 Ge-Si-O phase separation and Ge nanocrystal growth in Ge:SiO x/SiO2 multilayers - A new dc magnetron approach
    Zschintzsch, M. and Sahle, C.J. and Borany, J.V. and Sternemann, C. and Mücklich, A. and Nyrow, A. and Schwamberger, A. and Tolan, M.
    Nanotechnology 22 (2011)
    Ge:SiOx/SiO2 multilayers are fabricated using a new reactive dc magnetron sputtering approach. The influence of the multilayer stoichiometry on the ternary Ge-Si-O phase separation and the subsequent size-controlled Ge nanocrystal formation is explored by means of x-ray absorption spectroscopy, x-ray diffraction, electron microscopy and Raman spectroscopy. The ternary system Ge-Si-O reveals complete Ge-O phase separation at 400 °C which does not differ significantly to the binary Ge-O system. Ge nanocrystals of < 5nm size are generated after subsequent annealing below 700°C. It is shown that Ge oxides contained in the as-deposited multilayers are reduced by a surrounding unsaturated silica matrix. A stoichiometric regime was found where almost no GeO2 is present after annealing. Thus, the Ge nanocrystals become completely embedded in a stoichiometric silica matrix favouring the use for photovoltaic applications. © IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/48/485303
  • 2010 • 4 Phase separation and Si nanocrystal formation in bulk SiO studied by x-ray scattering
    Feroughi, O.M. and Sternemann, C. and Sahle, Ch.J. and Schroer, M.A. and Sternemann, H. and Conrad, H. and Hohl, A. and Seidler, G.T. and Bradley, J. and Fister, T.T. and Balasubramanian, M. and Sakko, A. and Pirkkalainen, K. and ...
    Applied Physics Letters 96 (2010)
    We present an x-ray scattering study of the temperature-induced phase separation and Si nanocrystal formation in bulk amorphous SiOx with x≈1. X-ray Raman scattering at the Si LII,III -edge reveals a significant contribution of suboxides present in native amorphous SiO. The suboxide contribution decreases with increasing annealing temperature between 800-1200 °C pointing toward a phase separation of SiO into Si and SiO2 domains. In combination with x-ray diffraction and small angle x-ray scattering the SiO microstructure is found to be dominated by internal suboxide interfaces in the native state. For higher annealing temperatures above 900 °C growth of Si nanocrystals with rough surfaces embedded in a silicon oxide matrix can be observed. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3323106
  • 2010 • 3 Freestanding spherical silicon nanocrystals: A model system for studying confined excitons
    Goller, B. and Polisski, S. and Wiggers, H. and Kovalev, D.
    Applied Physics Letters 97 (2010)
    We report on the light emitting properties of freestanding hydrogen-terminated spherical silicon nanocrystals. The nanocrystals exhibit size-dependent tunable light emission properties. Many light emission properties of this system are found to be very similar to those known for other systems containing silicon nanocrystals. However, we found specific emission properties of this system ascribed to the spherical shape of silicon nanocrystals and their spatial separation. We attributed all observations to the spatial confinement of excitons within the crystalline core of the indirect band gap silicon nanocrystals. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3470103
  • 2010 • 2 Silicon nanocrystals dispersed in water: Photosensitizers for molecular oxygen
    Goller, B. and Polisski, S. and Wiggers, H. and Kovalev, D.
    Applied Physics Letters 96 (2010)
    We report on the synthesis of freestanding silicon spheres having sizes in the range of 3-10 nm. As-prepared luminescent silicon nanocrystals have H-passivated surface. Therefore, energy transfer from excitons confined in Si nanocrystals to oxygen molecules is found to be efficient. It is demonstrated that a termination of silicon nanocrystal H-passivated hydrophobic surface by lipids provides their water solubility. We found that this procedure preserves photosensitizing ability of silicon nanocrystals. Therefore, this material system can potentially be employed for a variety of biomedical applications. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3432349
  • 2010 • 1 Small gold particles supported on MgFe2O4 nanocrystals as novel catalyst for CO oxidation
    Jia, C.-J. and Liu, Y. and Schwickardi, M. and Weidenthaler, C. and Spliethoff, B. and Schmidt, W. and Schüth, F.
    Applied Catalysis A: General 386 94-100 (2010)
    We present the study on the catalytic performance of gold particles supported on spinel type MgFe2O4 nanocrystals (Au/MgFe2O4) which exhibit high activity for low temperature CO oxidation. Using XRD, TEM, XPS and CO titration techniques, we investigated the effect of the pretreatment atmosphere on the structure and catalytic properties of the Au/MgFe2O4 catalyst in CO oxidation. TEM, XPS and XRD showed that the pretreatment atmosphere had a negligible effect on the particle size distribution, chemical states of the gold, and the structure of the support. Among the various pretreated catalysts, O2-Au/MgFe2O4 exhibits superior activity, indicating that pretreatment in oxidative atmosphere induced the high capability of the catalyst to activate CO and supply active oxygen for CO oxidation as confirmed by CO titration experiments. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2010.07.036