Prof. Dr. Gerd Bacher

Electrical Engineering and Information Technology
University of Duisburg-Essen

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  • Direct growth of graphene on GaN via plasma-enhanced chemical vapor deposition under N2 atmosphere
    Mischke, J. and Pennings, J. and Weisenseel, E. and Kerger, P. and Rohwerder, M. and Mertin, W. and Bacher, G.
    2D Materials 7 (2020)
    One of the bottlenecks in the implementation of graphene as a transparent electrode in modern opto-electronic devices is the need for complicated and damaging transfer processes of high-quality graphene sheets onto the desired target substrates. Here, we study the direct, plasma-enhanced chemical vapor deposition (PECVD) growth of graphene on GaN-based light-emitting diodes (LEDs). By replacing the commonly used hydrogen (H2) process gas with nitrogen (N2), we were able to suppress GaN surface decomposition while simultaneously enabling graphene deposition at <800 °C in a single-step growth process. Optimizing the methane (CH4) flow and varying the growth time between 0.5 h and 8 h, the electro-optical properties of the graphene layers could be tuned to sheet resistances as low as ∼1 kΩ/D with a maximum transparency loss of ∼12%. The resulting high-quality graphene electrodes show an enhanced current spreading effect and an increase of the emission area by a factor of ∼8 in operating LEDs. © 2020 The Author(s).
    view abstract10.1088/2053-1583/ab8969
  • Direct growth of graphene on Ge(100) and Ge(110) via thermal and plasma enhanced CVD
    Bekdüz, B. and Kaya, U. and Langer, M. and Mertin, W. and Bacher, G.
    Scientific Reports 10 (2020)
    The integration of graphene into CMOS compatible Ge technology is in particular attractive for optoelectronic devices in the infrared spectral range. Since graphene transfer from metal substrates has detrimental effects on the electrical properties of the graphene film and moreover, leads to severe contamination issues, direct growth of graphene on Ge is highly desirable. In this work, we present recipes for a direct growth of graphene on Ge via thermal chemical vapor deposition (TCVD) and plasma-enhanced chemical vapor deposition (PECVD). We demonstrate that the growth temperature can be reduced by about 200 °C in PECVD with respect to TCVD, where usually growth occurs close to the melting point of Ge. For both, TCVD and PECVD, hexagonal and elongated morphology is observed on Ge(100) and Ge(110), respectively, indicating the dominant role of substrate orientation on the shape of graphene grains. Interestingly, Raman data indicate a compressive strain of ca. − 0.4% of the graphene film fabricated by TCVD, whereas a tensile strain of up to + 1.2% is determined for graphene synthesized via PECVD, regardless the substrate orientation. Supported by Kelvin probe force measurements, we suggest a mechanism that is responsible for graphene formation on Ge and the resulting strain in TCVD and PECVD. © 2020, The Author(s).
    view abstract10.1038/s41598-020-69846-7
  • 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 (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 abstract10.1021/acs.nanolett.9b05136
  • Exciton-driven change of phonon modes causes strong temperature dependent bandgap shift in nanoclusters
    Muckel, F. and Lorenz, S. and Yang, J. and Nugraha, T.A. and Scalise, E. and Hyeon, T. and Wippermann, S. and Bacher, G.
    Nature Communications 11 (2020)
    The fundamental bandgap Eg of a semiconductor—often determined by means of optical spectroscopy—represents its characteristic fingerprint and changes distinctively with temperature. Here, we demonstrate that in magic sized II-VI clusters containing only 26 atoms, a pronounced weakening of the bonds occurs upon optical excitation, which results in a strong exciton-driven shift of the phonon spectrum. As a consequence, a drastic increase of dEg/dT (up to a factor of 2) with respect to bulk material or nanocrystals of typical size is found. We are able to describe our experimental data with excellent quantitative agreement from first principles deriving the bandgap shift with temperature as the vibrational entropy contribution to the free energy difference between the ground and optically excited states. Our work demonstrates how in small nanoparticles, photons as the probe medium affect the bandgap—a fundamental semiconductor property. © 2020, The Author(s).
    view abstract10.1038/s41467-020-17563-0
  • Flexible Large-Area Light-Emitting Devices Based on WS2 Monolayers
    Andrzejewski, D. and Oliver, R. and Beckmann, Y. and Grundmann, A. and Heuken, M. and Kalisch, H. and Vescan, A. and Kümmell, T. and Bacher, G.
    Advanced Optical Materials 8 (2020)
    Strong covalent in-plane bonds and a tiny thickness in the nanometer range make two-dimensional (2D) materials ideally suited for flexible electronic or optoelectronic applications. Despite this exciting perspective, only a few prototypes of such flexible devices—photodetectors and transistors—have been reported until now. The first large-area flexible light-emitting device (LED) based on 2D materials is realized by integrating a transition metal dichalcogenide (TMDC) monolayer synthesized by metal organic chemical vapor deposition (MOCVD) into a p–n architecture on conductive polymer foil. This flexible LED demonstrates homogeneous red light emission from a few square millimeter area in a scalable design. Uniquely, the electroluminescence can be tuned over 30 meV simply by bending the devices, i.e., by applying a defined strain. This approach combines the flexibility of organic semiconductor device concepts with the durability of inorganic semiconductor technology. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adom.202000694
  • MOVPE of Large-Scale MoS2/WS2, WS2/MoS2, WS2/Graphene and MoS2/Graphene 2D-2D Heterostructures for Optoelectronic Applications
    Grundmann, A. and McAleese, C. and Conran, B.R. and Pakes, A. and Andrzejewski, D. and Kümmell, T. and Bacher, G. and Khin Teo, K.B. and Heuken, M. and Kalisch, H. and Vescan, A.
    MRS Advances 5 (2020)
    Most publications on (opto)electronic devices based on 2D materials rely on single monolayers embedded in classical 3D semiconductors, dielectrics and metals. However, heterostructures of different 2D materials can be employed to tailor the performance of the 2D components by reduced defect densities, carrier or exciton transfer processes and improved stability. This translates to additional and unique degrees of freedom for novel device design. The nearly infinite number of potential combinations of 2D layers allows for many fascinating applications. Unlike mechanical stacking, metal-organic vapour phase epitaxy (MOVPE) can potentially provide large-scale highly homogeneous 2D layer stacks with clean and sharp interfaces. Here, we demonstrate the direct successive MOVPE of MoS2/WS2 and WS2/MoS2 heterostructures on 2 sapphire (0001) substrates. Furthermore, the first deposition of large-scale MoS2/graphene and WS2/graphene heterostructures using only MOVPE is presented and the influence of growth time on nucleation of WS2 on graphene is analysed. Copyright © 2020 Materials Research Society.
    view abstract10.1557/adv.2020.104
  • WS2 monolayer based light emitting devices fabricated by scalable deposition techniques
    Kümmell, T. and Andrzejewski, D. and Beckmann, Y. and Abdelbaky, M. and Yeow, T. and Grundmann, A. and Heuken, M. and Kalisch, H. and Vescan, A. and Musselman, K. and Bacher, G.
    Proceedings of SPIE - The International Society for Optical Engineering 11302 (2020)
    Transition metal dichalcogenides (TMDC) have become attractive candidates for 2D electronics and optoelectronics. While several concepts for light emitting devices have been reported, many of them realized using exfoliated TMDC flakes of micrometer size, only few approaches tackle the challenge of upscaling to relevant device sizes. We demonstrate a light emitting diode based on WS2 monolayers in a scalable design. The devices are fabricated by combining two industrially relevant deposition processes in a vertical p-n architecture: Metal organic CVD (MOCVD) is used to realize the optically active WS2 monolayers, while ZnO deposited by spatial atomic layer deposition (sALD) is employed as an electron injection layer on the cathode side. Organic layers spin-coated on an ITO covered glass substrate provide hole injection and transport. The resulting devices exhibit rectifying behavior and red electroluminescence from an area of 6 mm2. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2544926
  • H2S-free Metal-Organic Vapor Phase Epitaxy of Coalesced 2D WS2 Layers on Sapphire
    Grundmann, A. and Andrzejewski, D. and Kümmell, T. and Bacher, G. and Heuken, M. and Kalisch, H. and Vescan, A.
    MRS Advances 4 (2019)
    The 2D transition metal dichalcogenide (TMDC) tungsten disulfide (WS2) has attracted great interest due to its unique properties and prospects for future (opto)electronics. However, compared to molybdenum disulfide (MoS2), the development of a reproducible and scalable deposition process for 2D WS2 has not advanced very far yet. Here, we report on the systematic investigation of 2D WS2 growth on hydrogen (H2)-desorbed sapphire (0001) substrates using a hydrogen sulfide (H2S)-free metal-organic vapor phase epitaxy (MOVPE) process in a commercial AIXTRON planetary hot-wall reactor in 10 × 2" configuration. Tungsten hexacarbonyl (WCO, 99.9 %) and di-tert-butyl sulfide (DTBS, 99.9999 %) were used as MO sources, nitrogen (N2) was selected as carrier gas for the deposition processes (standard growth time 10 h). In an initial study, the impact of growth temperature on nucleation and growth was investigated and an optimal value of 820 °C was found. The influence of the WCO flow on lateral growth was investigated. The aim was to maximize the edge length of triangular crystals as well as the total surface coverage. Extending gradually the growth time up to 20 h at optimized WCO flow conditions yields fully coalesced WS2 samples without parasitic carbon-related Raman peaks and with only sparse bilayer nucleation. After substrate removal, a fully coalesced WS2 film was implemented into a light-emitting device showing intense red electroluminescence (EL). © Materials Research Society 2018.
    view abstract10.1557/adv.2018.669
  • 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 abstract10.1063/1.5129391
  • Scalable Large-Area p-i-n Light-Emitting Diodes Based on WS2 Monolayers Grown via MOCVD
    Andrzejewski, D. and Myja, H. and Heuken, M. and Grundmann, A. and Kalisch, H. and Vescan, A. and Kümmell, T. and Bacher, G.
    ACS Photonics 6 (2019)
    Transition metal dichalcogenides (TMDCs) represent a novel and sustainable material basis for ultrathin optoelectronic devices. Although various approaches toward light-emitting devices, e.g., based on exfoliated or chemical vapor deposited (CVD) TMDC monolayers, have been reported, they all suffer from limited scalability and reproducibility required for industrial fabrication. Here, we demonstrate a light-emitting device in a scalable approach by embedding metal-organic (MO-)CVD WS2 monolayers into a vertical p-i-n device architecture using organic and inorganic injection layers. Red electroluminescence is emitted from an active area of 6 mm2 starting already at a driving voltage of about 2.5 V. © 2019 American Chemical Society.
    view abstract10.1021/acsphotonics.9b00311
  • Tailoring exchange interactions in magnetically doped II-VI nanocrystals
    Fainblat, R. and Muckel, F. and Bacher, G.
    Chalcogenide: From 3D to 2D and Beyond (2019)
    The flexibility of colloidal synthesis allows engineering size, shape, and composition - and thus electronic states - of semiconductor nanocrystals with a high degree of perfection. Intentional incorporation of transition metal impurities on lattice sites expands the nanocrystal functionality enabling an unprecedented adjustment of magnetic exchange interactions. Hereby, an enhanced control over spatial location and concentration of dopants down to the single atom level paves the way for tailoring giant magneto-optical effects through the precise adjustment of the wavefunction overlap between charge carriers in the host matrix and localized magnetic impurities. This chapter will review recent advances on giant magneto-optical effects in size and shape engineered colloidal II-VI nanostructures doped with Mn2+ impurities. Both, two-dimensional (2D) nanocrystals (i.e. nanoribbons, nanoplatelets) with well-defined valence band states, as well as zero-dimensional (0D) nanocrystals (quantum dots, magic-size cluster) exhibiting a complex valence band structure are considered. We will discuss how the magneto-optical response of higher excited states gives insight into fundamental properties of nano-scaled materials such as the nature of excited state transitions, valence band mixing or the presence of magneto-optically active and passive states. In addition, the chapter will include important advances in the field of solotronics, showing that magnetic functionality can be associated to the incorporation of single magnetic impurities into zero-dimensional colloidal nanostructures, yielding unique discoveries like huge zero-field exchange splittings or digital doping effects. © 2020 Elsevier Ltd. All rights reserved.
    view abstract10.1016/B978-0-08-102687-8.00014-2
  • WS 2 monolayer-based light-emitting devices in a vertical p-n architecture
    Andrzejewski, D. and Hopmann, E. and John, M. and Kümmell, T. and Bacher, G.
    Nanoscale 11 (2019)
    2D semiconductors represent an exciting new material class with great potential for optoelectronic devices. In particular, WS 2 monolayers are promising candidates for light-emitting devices (LEDs) due to their direct band gap with efficient recombination in the red spectral range. Here, we present a novel LED architecture by embedding exfoliated WS 2 monolayer flakes into a vertical p-n layout using organic p- and inorganic n-supporting layers. Laser lithography was applied to define the current path perpendicular to the WS 2 flake. The devices exhibit rectifying behavior and emit room temperature electroluminescence with luminance up to 50 cd m -2 in the red spectral range. © The Royal Society of Chemistry 2019.
    view abstract10.1039/c9nr01573f
  • A light-emitting electrochemical cell (LEC) containing a hole-blocking layer of TmPyPB
    Di Marcantonio, M. and Vollkommer, F. and Bacher, G. and Nannen, E.
    Journal of Materials Chemistry C 6 (2018)
    Light-emitting electrochemical cells (LECs) are attractive candidates for low-cost light-emitting devices fabricated using solution-based processes on flexible substrates. Despite promising luminance levels, their efficiency needs further improvement to reach the requirements of the lighting market. Thereby one of the reasons for the efficiency loss is the imbalance between the electrons and holes typical within such devices. Here we present a hybrid solution-based device architecture comprising a thin film of 1,3,5-tri(m-pyridin-3-ylphenyl)benzene (TmPyPB) on top of the emissive layer of an LEC, targeting to improve the charge carrier balance within the device. The hybrid LEC in constant voltage mode shows an efficiency improvement of almost a factor of 2 compared to the reference device, reaching maximum luminous and current efficacy values of 4.2 lm W−1 and 5.4 cd A−1, respectively. The measurements conducted using the hybrid and reference devices suggested a pronounced hole-blocking effect of the additional organic supporting layer as the origin of improvement. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/C8TC02314J
  • 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 (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 (>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 abstract10.1021/acs.nanolett.8b03627
  • 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 (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 abstract10.1021/acsami.8b15100
  • Graphene growth through a recrystallization process in plasma enhanced chemical vapor deposition
    Bekdüz, B. and Beckmann, Y. and Mischke, J. and Twellmann, J. and Mertin, W. and Bacher, G.
    Nanotechnology 29 (2018)
    Thermal chemical vapor deposition (TCVD) is the current method of choice to fabricate high quality, large area graphene films on catalytic copper substrates. In order to obtain sufficiently high growth rates at reduced growth temperatures an efficient dissociation of the precursor molecules already in the gas phase is required. We used plasma enhanced chemical vapor deposition (PECVD) to fabricate high quality graphene films at various temperatures. The efficient, plasma-induced dissociation of the precursor molecules results in an activation energy of 2.2 eV for the growth rate in PECVD, which is reduced by almost a factor of 2 compared to TCVD growth in the same reactor. By varying the growth time, we demonstrate that crystalline graphene grains surrounded by amorphous carbon formed during the early stage of growth merge into an almost defect-free graphene film with growth time via a recrystallization process. Almost defect-free graphene is prepared with negligible (I D/I G < 0.1) contributions of the D peak in Raman spectroscopy and with a sheet resistance down to 470 Ω/sq. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/aadd74
  • Improved luminescence properties of MoS2 monolayers grown via MOCVD: Role of pre-treatment and growth parameters
    Andrzejewski, D. and Marx, M. and Grundmann, A. and Pfingsten, O. and Kalisch, H. and Vescan, A. and Heuken, M. and Kümmell, T. and Bacher, G.
    Nanotechnology 29 (2018)
    Fabrication of transition metal dichalcogenides (TMDCs) via metalorganic chemical vapor deposition (MOCVD) represents one of the most attractive routes to large-scale 2D material layers. Although good homogeneity and electrical conductance have been reported recently, the relation between growth parameters and photoluminescence (PL) intensity - one of the most important parameters for optoelectronic applications - has not yet been discussed for MOCVD TMDCs. In this work, MoS2 is grown via MOCVD on sapphire (0001) substrates using molybdenum hexacarbonyl (Mo(CO)6, MCO) and di-tert-butyl sulphide as precursor materials. A prebake step under H2 atmosphere combined with a reduced MCO precursor flow increases the crystal grain size by one order of magnitude and strongly enhances PL intensity with a clear correlation to the grain size. A decrease of the linewidth of both Raman resonances and PL spectra down to full width at half maxima of 3.2 cm-1 for the E 2g Raman mode and 60 meV for the overall PL spectrum indicate a reduced defect density at optimized growth conditions. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/aabbb9
  • Influence of atmospheric species on the electrical properties of functionalized graphene sheets
    Bekdüz, B. and Kampermann, L. and Mertin, W. and Punckt, C. and Aksay, I.A. and Bacher, G.
    RSC Advances 8 (2018)
    We report on the time-dependent influence of atmospheric species on the electrical properties of functionalized graphene sheets (FGSs). When exposed to laboratory air, FGSs exhibit a significant, irreversible decrease in electrical conductance with time, strongly depending on the oxygen content of the FGSs. To separate the roles of charge carrier density and mobility in this aging process, we performed electron transport measurements using a back-gate field-effect transistor architecture. Investigating the position of the Dirac point under different atmospheres, we found that adsorbed atmospheric species result in pronounced p-doping, which-on a short time scale-can be reversed under nitrogen atmosphere. However, on a time scale of several days, the resistance increases irreversibly, while the Dirac point voltage remains constant. From these experiments, we conclude that the aging of FGSs is related to the chemisorption of atmospheric species leading to enhanced carrier scattering due to an increasing amount of sp 3 - regions and thus to a reduced charge carrier mobility. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c8ra08227h
  • Low injection losses in InGaN/GaN LEDs: The correlation of photoluminescence, electroluminescence, and photocurrent measurements
    Quitsch, W.-A. and Sager, D. and Loewenich, M. and Meyer, T. and Hahn, B. and Bacher, G.
    Journal of Applied Physics 123 (2018)
    Time-resolved photoluminescence spectroscopy and photocurrent measurements at quasi-resonant laser excitation are combined with electroluminescence studies to get access to low injection losses in high power InGaN/GaN LEDs. A direct relation between electroluminescence and photoluminescence efficiencies with photocurrent is found, indicating that tunneling losses play a key role in the low injection regime. This assertion is confirmed by comparing photoluminescence efficiencies under open and closed circuit conditions. Experiments under various excitation wavelengths hint at the role of resonant tunneling processes in the efficiency losses. © 2018 Author(s).
    view abstract10.1063/1.5022026
  • Metalorganic Vapor-Phase Epitaxy Growth Parameters for Two-Dimensional MoS2
    Marx, M. and Grundmann, A. and Lin, Y.-R. and Andrzejewski, D. and Kümmell, T. and Bacher, G. and Heuken, M. and Kalisch, H. and Vescan, A.
    Journal of Electronic Materials 47 (2018)
    The influence of the main growth parameters on the growth mechanism and film formation processes during metalorganic vapor-phase epitaxy (MOVPE) of two-dimensional MoS2 on sapphire (0001) have been investigated. Deposition was performed using molybdenum hexacarbonyl and di-tert-butyl sulfide as metalorganic precursors in a horizontal hot-wall MOVPE reactor from AIXTRON. The structural properties of the MoS2 films were analyzed by atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. It was found that a substrate prebake step prior to growth reduced the nucleation density of the polycrystalline film. Simultaneously, the size of the MoS2 domains increased and the formation of parasitic carbonaceous film was suppressed. Additionally, the influence of growth parameters such as reactor pressure and surface temperature is discussed. An upper limit for these parameters was found, beyond which strong parasitic deposition or incorporation of carbon into MoS2 took place. This carbon contamination became significant at reactor pressure above 100 hPa and temperature above 900°C. © 2017, The Minerals, Metals & Materials Society.
    view abstract10.1007/s11664-017-5937-3
  • Phonon Interaction and Phase Transition in Single Formamidinium Lead Bromide Quantum Dots
    Pfingsten, O. and Klein, J. and Protesescu, L. and Bodnarchuk, M.I. and Kovalenko, M.V. and Bacher, G.
    Nano Letters (2018)
    Formamidinium lead bromide (FAPbBr3) quantum dots (QDs) are promising materials for light emitting applications in the visible spectral region because of their high photoluminescence (PL) quantum yield (QY) and the enhanced chemical stability as compared to, for instance, methylammonium based analogues. Towards practical harnessing of their compelling optical characteristics, the exciton recombination process - in particular the exciton-phonon interaction and the impact of crystal phase transition - has to be understood in detail. This is addressed in this contribution by PL studies on single colloidal FAPbBr3 QDs. Polarization resolved PL measurements reveal a fine structure splitting of excitonic transitions due to the Rashba effect. Distinct phonon replica have been observed within energetic distances of 4.3 ± 0.5 meV, 8.6 ± 0.9 meV and 13.2 ± 1.1 meV from the zero phonon line, which we attribute to vibrational modes of the lead bromide lattice. Additional vibrational modes of 18.6 ± 0.3 meV and 38.8 ± 1.1 meV are found and related to liberation modes of the formamidinium (FA) cation. Temperature depended PL spectra reveal a line broadening of the emission caused by exciton phonon interaction as well an unusual energy shift which is attributed to a crystal phase transition within the single QD. © 2018 American Chemical Society.
    view abstract10.1021/acs.nanolett.8b01523
  • Photocatalytic Methanol Oxidation by Supported Vanadium Oxide Species: Influence of Support and Degree of Oligomerization
    Kortewille, B. and Wachs, I.E. and Cibura, N. and Pfingsten, O. and Bacher, G. and Muhler, M. and Strunk, J.
    European Journal of Inorganic Chemistry 2018 (2018)
    Supported isolated vanadium oxide (VO4) species on silica have recently been shown to photocatalytically oxidize methanol selectively to formaldehyde. Insights into support effects and the reactivity of the different supported vanadium oxide species in photocatalytic methanol oxidation are obtained in the present study by varying the support, surface vanadium oxide loading, and synthesis procedure. Isolated and oligomeric surface vanadium oxide species supported on alumina can also photocatalytically oxidize methanol to formaldehyde. Crystalline V2O5 nanoparticles are inactive for photocatalytic conversion of methanol irrespective of the support, but they further convert the formaldehyde produced by the surface vanadium oxide species to surface formate species. The formation of surface formate species is also observed on the bare alumina support. Thermal catalyzed reactions take place at elevated temperatures, leading to product degradation, when attempting to desorb and quantify the photocatalysis products adsorbed on the alumina-supported samples. This study is a step forward in the directed development of active and selective sites for photocatalysis, and highlights the importance of limitation by desorption in the kinetics of photoreactions. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ejic.201800490
  • Proof of Equivalent Catalytic Functionality upon Photon-Induced and Thermal Activation of Supported Isolated Vanadia Species in Methanol Oxidation
    Kortewille, B. and Wachs, I.E. and Cibura, N. and Pfingsten, O. and Bacher, G. and Muhler, M. and Strunk, J.
    ChemCatChem 10 (2018)
    In this study, evidence is provided that isolated surface vanadia (VO4) species on SiO2 can similarly act as a thermal heterogeneous catalyst and as a heterogeneous photocatalyst. Structurally identical surface VO4 species catalyze the selective oxidation of methanol both by thermal activation and by UV-light induction. Selectivity to formaldehyde appears to be unity. For the photocatalytic reaction at room temperature, formaldehyde desorption is rate limiting. With larger agglomerates or V2O5 nanoparticles, on the contrary, only the thermal reaction is feasible. This is tentatively attributed to the different positions of electronic states (HOMO/LUMO, valence/conduction band) on the electrochemical energy scale owing to the quantum size effect. Besides providing new fundamental insight into the mode of action of nanosized photocatalysts, our results demonstrate that tuning the photocatalytic reactivity of supported transition-metal oxides by adjusting the degree of agglomeration is feasible. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201800311
  • Sp-d Exchange Interactions in Wave Function Engineered Colloidal CdSe/Mn:CdS Hetero-Nanoplatelets
    Muckel, F. and Delikanli, S. and Hernández-Martínez, P.L. and Priesner, T. and Lorenz, S. and Ackermann, J. and Sharma, M. and Demir, H.V. and Bacher, G.
    Nano Letters 18 (2018)
    In two-dimensional (2D) colloidal semiconductor nanoplatelets, which are atomically flat nanocrystals, the precise control of thickness and composition on the atomic scale allows for the synthesis of heterostructures with well-defined electron and hole wave function distributions. Introducing transition metal dopants with a monolayer precision enables tailored magnetic exchange interactions between dopants and band states. Here, we use the absorption based technique of magnetic circular dichroism (MCD) to directly prove the exchange coupling of magnetic dopants with the band charge carriers in hetero-nanoplatelets with CdSe core and manganese-doped CdS shell (CdSe/Mn:CdS). We show that the strength of both the electron as well as the hole exchange interactions with the dopants can be tuned by varying the nanoplatelets architecture with monolayer accuracy. As MCD is highly sensitive for excitonic resonances, excited level spectroscopy allows us to resolve and identify, in combination with wave function calculations, several excited state transitions including spin-orbit split-off excitonic contributions. Thus, our study not only demonstrates the possibility to expand the extraordinary physical properties of colloidal nanoplatelets toward magneto-optical functionality by transition metal doping but also provides an insight into the excited state electronic structure in this novel two-dimensional material. © 2018 American Chemical Society.
    view abstract10.1021/acs.nanolett.8b00060
  • The Role of Excitation Energy in Photobrightening and Photodegradation of Halide Perovskite Thin Films
    Quitsch, W.-A. and Dequilettes, D.W. and Pfingsten, O. and Schmitz, A. and Ognjanovic, S. and Jariwala, S. and Koch, S. and Winterer, M. and Ginger, D.S. and Bacher, G.
    Journal of Physical Chemistry Letters 9 (2018)
    We study the impact of excitation energy on the photostability of methylammonium lead triiodide (CH3NH3PbI3 or MAPI) perovskite thin films. Light soaking leads to a transient increase of the photoluminescence efficiency at excitation wavelengths longer than 520 nm, whereas light-induced degradation occurs when exciting the films with wavelengths shorter than 520 nm. X-ray diffraction and extinction measurements reveal the light-induced decomposition of CH3NH3PbI3 to lead iodide (PbI2) for the high-energy excitation regime. We propose a model explaining the energy dependence of the photostability that involves the photoexcitation of residual PbI2 species in the perovskite triggering the decomposition of CH3NH3PbI3. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpclett.8b00212
  • Carrier transfer across a 2D-3D semiconductor heterointerface: The role of momentum mismatch
    Kümmell, T. and Hutten, U. and Heyer, F. and Derr, K. and Neubieser, R.-M. and Quitsch, W. and Bacher, G.
    Physical Review B - Condensed Matter and Materials Physics 95 (2017)
    Two-dimensional (2D) transition metal dichalogenides exhibit a unique band structure: In contrast to many direct-gap classical semiconductors, their band-gap minimum is not at the center of the Brillouin zone, but at finite values of the k vector. We report on clear indications that this momentum mismatch fundamentally influences the carrier transfer between a 2D WS2 crystal and a three-dimensional (3D) GaN layer: Populating different local band extrema of the WS2 in k space by selective laser excitation leads to a pronounced difference in the WS2 photoluminescence signal. These findings may be of high importance for future 2D-3D semiconductor devices. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.95.081304
  • 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 (2017)
    view abstract10.1021/jacs.7b02953
  • 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 (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 abstract10.1021/acs.nanolett.7b01496
  • Green-yellow emitting hybrid light emitting electrochemical cell
    Di Marcantonio, M. and Namanga, J.E. and Smetana, V. and Gerlitzki, N. and Vollkommer, F. and Mudring, A.-V. and Bacher, G. and Nannen, E.
    Journal of Materials Chemistry C 5 (2017)
    Light-emitting electrochemical cells (LECs) are attractive candidates for future low-cost lighting applications such as light-emitting smart tags, thanks to their simplicity, fully solution-based fabrication and flexibility. However, high brightness and efficiency in combination with satisfactory operation lifetimes need to be achieved for different emission colours bearing future device commercialization in mind. LECs emitting in the yellow-green spectral range, where the human eye is most sensitive are thereby particularly attractive. Here we present an improved hybrid LEC based on an Ir-iTMC, [Ir(4-Fppy)2(pbpy)][PF6] (4-Fppy = 2-(4-fluorophenyl)pyridinato, pbpy = 6-phenyl-2,2′-bipyridine) emitting at 557 nm. It features a luminance of 2400 cd m-2 when driven at a constant voltage of 4 V, and a lifetime of 271 h at a luminance of 1500 cd m-2 under pulsed current operation. The hybrid LEC shows an enhanced performance compared to a LEC solely based on the Ir-ITMC where operation lifetimes of 165 h at a luminance above 1200 cd m-2 under pulsed current operation conditions were observed. The performance improvement was achieved by addition of a solution-processed ZnO nanoparticle film on top. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7tc02976d
  • Low Resistive Edge Contacts to CVD-Grown Graphene Using a CMOS Compatible Metal
    Shaygan, M. and Otto, M. and Sagade, A.A. and Chavarin, C.A. and Bacher, G. and Mertin, W. and Neumaier, D.
    Annalen der Physik (2017)
    view abstract10.1002/andp.201600410
  • Performance Enhancement by ZnO Nanoparticle Layer in Hybrid Ionic Transition Metal Complex-Light-Emitting Electrochemical Cells (iTMC-LECs)
    Marcantonio, M.D. and Gellner, S. and Namanga, J.E. and Frohleiks, J. and Gerlitzki, N. and Vollkommer, F. and Bacher, G. and Nannen, E.
    Advanced Materials Technologies 2 (2017)
    Light-emitting electrochemical cells (LECs) are solution processable solid-state light sources comprising in their simplest architecture an ionic emissive layer in between of two electrodes. Although LECs possess several advantages that make them promising candidates for future large-area low-cost lighting technologies, their device wall-plug efficacies remain so far moderate on the order of a few lumens per watt. One of the reasons therefore is considered to be the charge imbalance within the device. Here, a hybrid LEC device concept is introduced, whereby an additional layer of zinc oxide (ZnO) nanoparticles at the cathode side supports electron injection into the active light-emitting layer and boosts the performance of the Ir-based ionic transition metal complex LEC (iTMC-LEC). The brightness and efficacy of the devices can be increased in average by more than 70% by the implementation of the additional inorganic layer. The time to reach the maximum brightness can be reduced in average by a factor of 7, which is attributed to an improved electron/hole balance in the device due to enhanced electron injection into the active iTMC layer. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/admt.201600215
  • Relation between growth rate and structure of graphene grown in a 4″ showerhead chemical vapor deposition reactor
    Bekdüz, B. and Beckmann, Y. and Meier, J. and Rest, J. and Mertin, W. and Bacher, G.
    Nanotechnology 28 (2017)
    The chemical vapor deposition (CVD) growth of graphene on copper is controlled by a complex interplay of substrate preparation, substrate temperature, pressure and flow of reactive gases. A large variety of recipes have been suggested in literature, often quite specific to the reactor, which is being used. Here, we report on a relation between growth rate and quality of graphene grown in a scalable 4″ CVD reactor. The growth rate is varied by substrate pre-treatment, chamber pressure, and methane to hydrogen (CH4:H2) ratio, respectively. We found that at lower growth rates graphene grains become hexagonal rather than randomly shaped, which leads to a reduced defect density and a sheet resistance down to 268 Ω/sq. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/aa68a8
  • 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 (2017)
    view abstract10.1021/acsami.6b15660
  • 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 (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 abstract10.1039/c6tc05571k
  • Digital Doping in Magic-Sized CdSe Clusters
    Muckel, F. and Yang, J. and Lorenz, S. and Baek, W. and Chang, H. and Hyeon, T. and Bacher, G. and Fainblat, R.
    ACS Nano 10 (2016)
    Magic-sized semiconductor clusters represent an exciting class of materials located at the boundary between quantum dots and molecules. It is expected that replacing single atoms of the host crystal with individual dopants in a one-by-one fashion can lead to unique modifications of the material properties. Here, we demonstrate the dependence of the magneto-optical response of (CdSe)13 clusters on the discrete number of Mn2+ ion dopants. Using time-of-flight mass spectrometry, we are able to distinguish undoped, monodoped, and bidoped cluster species, allowing for an extraction of the relative amount of each species for a specific average doping concentration. A giant magneto-optical response is observed up to room temperature with clear evidence that exclusively monodoped clusters are magneto-optically active, whereas the Mn2+ ions in bidoped clusters couple antiferromagnetically and are magneto-optically passive. Mn2+-doped clusters therefore represent a system where magneto-optical functionality is caused by solitary dopants, which might be beneficial for future solotronic applications. © 2016 American Chemical Society.
    view abstract10.1021/acsnano.6b03348
  • 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 (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 abstract10.1021/acs.nanolett.6b02775
  • Large-area MoS2 deposition via MOVPE
    Marx, M. and Nordmann, S. and Knoch, J. and Franzen, C. and Stampfer, C. and Andrzejewski, D. and Kümmell, T. and Bacher, G. and Heuken, M. and Kalisch, H. and Vescan, A.
    Journal of Crystal Growth 464 (2016)
    The direct deposition of the 2D transition metal dichalcogenide MoS2 via metal-organic vapour phase epitaxy (MOVPE) is investigated. Growth is performed in a commercial AIXTRON horizontal hot-wall reactor. Molybdenum hexacarbonyl (MCO) and Di-tert-butyl sulfide (DTBS) are used as metal-organic precursors for molybdenum and sulfur, respectively. The successful deposition of MoS2 is demonstrated via Raman spectroscopy on various substrates such as sapphire and Si as well as AlN and GaN templates. The influence of growth time on the evolution of layer morphology is investigated. Variation of carrier gas reveals that a pure nitrogen growth atmosphere and a growth temperature of 750C improve layer quality. Additionally, a post-deposition annealing process of the grown samples is examined. It is shown that annealing in a pure nitrogen atmosphere at temperatures between 650C and 750C strongly increases the Raman intensities. © 2016 Elsevier B.V.
    view abstract10.1016/j.jcrysgro.2016.11.020
  • On the origin of contact resistances in graphene devices fabricated by optical lithography
    Chavarin, C.A. and Sagade, A.A. and Neumaier, D. and Bacher, G. and Mertin, W.
    Applied Physics A: Materials Science and Processing 122 (2016)
    The contact resistance is a key bottleneck limiting the performance of graphene-based electronic and optoelectronic devices. Using a combined approach of atomic force microscopy patterning, Kelvin probe force microscopy and micro-Raman mapping, we study the influence of optical lithography resists on the contact resistance in graphene devices. We find that devices fabricated by optical lithography show a significantly larger contact resistance compared to devices produced by electron beam lithography using polymethylmethacrylate as resist. This difference is attributed to a 3–4-nm-thick residual layer remaining in between the contact metal and the graphene after optical lithography. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00339-015-9582-5
  • 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 (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 abstract10.1021/acsami.6b06833
  • Gate control of carrier distribution in k -space in MoS2 monolayer and bilayer crystals
    Kümmell, T. and Quitsch, W. and Matthis, S. and Litwin, T. and Bacher, G.
    Physical Review B - Condensed Matter and Materials Physics 91 (2015)
    We demonstrate gate control of the carrier distribution in k-space in bilayer and monolayer MoS2 devices, probed by microphotoluminescence spectroscopy on a contacted single flake. The characteristic emission lines of the neutral and the negatively charged exciton act as a sensor for electron depletion/agglomeration via gate voltage. Gate-induced carrier depletion enhances the sensitivity to defects in monolayers, whereas in bilayers the indirect transition becomes more weight. The specific band structure of bilayers results in a thermal dissociation of trions at 200 K and above, in contrast to monolayers where trion emission is observed up to room temperature. We show that these findings are a consequence of a bias-driven redistribution of charge carriers between the different band minima. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.91.125305
  • High-Speed GaN/GaInN Nanowire Array Light-Emitting Diode on Silicon(111)
    Koester, R. and Sager, D. and Quitsch, W.-A. and Pfingsten, O. and Poloczek, A. and Blumenthal, S. and Keller, G. and Prost, W. and Bacher, G. and Tegude, F.-J.
    Nano Letters 15 (2015)
    (Graph Presented). The high speed on-off performance of GaN-based light-emitting diodes (LEDs) grown in c-plane direction is limited by long carrier lifetimes caused by spontaneous and piezoelectric polarization. This work demonstrates that this limitation can be overcome by m-planar core-shell InGaN/GaN nanowire LEDs grown on Si(111). Time-resolved electroluminescence studies exhibit 90-10% rise- and fall-times of about 220 ps under GHz electrical excitation. The data underline the potential of these devices for optical data communication in polymer fibers and free space. © 2015 American Chemical Society.
    view abstract10.1021/nl504447j
  • Implementation of graphene multilayer electrodes in quantum dot light-emitting devices
    Wolff, S. and Jansen, D. and Terlinden, H. and Kelestemur, Y. and Mertin, W. and Demir, H.V. and Bacher, G. and Nannen, E.
    Applied Physics A: Materials Science and Processing 120 (2015)
    Graphene is a highly attractive candidate for implementation as electrodes in next-generation large-area optoelectronic devices thanks to its high electrical conductivity and high optical transparency. In this study, we show all-solution-processed quantum dot-based light-emitting devices (QD-LEDs) using graphene mono- and multilayers as transparent electrodes. Here, the effect of the number of graphene layers (up to three) on the QD-LEDs performance was studied. While the implementation of a second graphene layer was found to reduce the turn-on voltage from 2.6 to 1.8 V, a third graphene layer was observed to increase the turn-on voltage again, which is attributed to an increased roughness of the graphene layer stack. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00339-015-9304-z
  • Route to the Smallest Doped Semiconductor: Mn2+-Doped (CdSe)13 Clusters
    Yang, J. and Fainblat, R. and Kwon, S.G. and Muckel, F. and Yu, J.H. and Terlinden, H. and Kim, B.H. and Iavarone, D. and Choi, M.K. and Kim, I.Y. and Park, I. and Hong, H.-K. and Lee, J. and Son, J.S. and Lee, Z. and Kang, K. and Hwang, S.-J. and Bacher, G. and Hyeon, T.
    Journal of the American Chemical Society 137 (2015)
    Doping semiconductor nanocrystals with magnetic transition-metal ions has attracted fundamental interest to obtain a nanoscale dilute magnetic semiconductor, which has unique spin exchange interaction between magnetic spin and exciton. So far, the study on the doped semiconductor NCs has usually been conducted with NCs with larger than 2 nm because of synthetic challenges. Herein, we report the synthesis and characterization of Mn2+-doped (CdSe)13 clusters, the smallest doped semiconductors. In this study, single-sized doped clusters are produced in large scale. Despite their small size, these clusters have semiconductor band structure instead of that of molecules. Surprisingly, the clusters show multiple excitonic transitions with different magneto-optical activities, which can be attributed to the fine structure splitting. Magneto-optically active states exhibit giant Zeeman splittings up to elevated temperatures (128 K) with large g-factors of 81(±8) at 4 K. Our results present a new synthetic method for doped clusters and facilitate the understanding of doped semiconductor at the boundary of molecules and quantum nanostructure. © 2015 American Chemical Society.
    view abstract10.1021/jacs.5b07888
  • Current-induced control of the electron-nuclear spin system in semiconductors on a micrometer scale
    Chen, Y. and Kim, J. and Puls, J. and Henneberger, F. and Bacher, G.
    Physica Status Solidi (B) Basic Research 251 (2014)
    The ability of using onchip microcoils to control the electron-nuclear spin system in semiconductors is demonstrated. Electrically generated magnetic fields of several tens of mT can be obtained on a micrometer length scale, which are switchable on a sub-ns time scale due to the low complex coil impedance. This allows one to electrically (i) manipulate the nuclear spins by means of nuclear magnetic resonance in n-GaAs and (ii) control the hyperfine flip-flop rate in CdSe/ZnSe quantum dots. © 2014 The Authors. Phys. Status Solidi B is published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201350255
  • Electrically driven single photon emission from a CdSe/ZnSSe single quantum dot at 200 K
    Quitsch, W. and Kümmell, T. and Gust, A. and Kruse, C. and Hommel, D. and Bacher, G.
    Applied Physics Letters 105 (2014)
    High temperature operation of an electrically driven single photon emitter based on a single epitaxial quantum dot is reported. CdSe/ZnSSe/MgS quantum dots are embedded into a p-i-n diode architecture providing almost background free excitonic and biexcitonic electroluminescence from individual quantum dots through apertures in the top contacts. Clear antibunching with g 2(τ = 0) = 0.28 ± 0.20 can be tracked up to T = 200 K, representing the highest temperature for electrically triggered single photon emission from a single quantum dot device. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4894729
  • Electrically driven single photon emission from a CdSe/ZnSSe/MgS semiconductor quantum dot
    Quitsch, W. and Kümmell, T. and Gust, A. and Kruse, C. and Hommel, D. and Bacher, G.
    Physica Status Solidi (C) Current Topics in Solid State Physics 11 (2014)
    We demonstrate electrically driven single photon emission from a CdSe/ZnSSe/MgS single quantum dot embedded in a resonant cavity light emitting diode. Patterned Pd/Au top contacts are used to inject the charge carriers locally. Single quantum dot electroluminescence is obtained at T = 4 K from nanoapertures lithographically defined in the top contacts. At low current densities, antibunching with a value of g(2)(0) = 0.16 is achieved. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssc.201300627
  • Highly luminescent ZnO quantum dots made in a nonthermal plasma
    Felbier, P. and Yang, J. and Theis, J. and Liptak, R.W. and Wagner, A. and Lorke, A. and Bacher, G. and Kortshagen, U.
    Advanced Functional Materials 24 (2014)
    Nonthermal plasmas allow the preparation of ligand-free quantum dots combining high production rates with superior crystalline quality and luminescence properties. Here, ZnO quantum dots are produced in a radiofrequency capacitively-coupled plasma, exhibiting size dependent photoluminescent quantum yields up to 60% after air exposure - the highest reported to date for any compound semiconductor quantum dots prepared in the gas phase. Systematic studies indicate the importance of the surface for the observed luminescence behavior. The high luminescent quantum yields in the visible range of the spectrum and the ligand-free, scalable synthesis make these quantum dots good candidates for light emitting applications. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201303449
  • 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 (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 abstract10.1021/nn505610e
  • Low-temperature MOCVD of crystalline Ga2O3 nanowires using tBu3Ga
    Schulz, S. and Bendt, G. and Assenmacher, W. and Sager, D. and Bacher, G.
    Chemical Vapor Deposition 19 (2013)
    Crystalline Ga2O3 nanowires are synthesized via an Au-catalyzed, as well as a self-catalyzed, growth by a low-temperature metal-organic (MO)CVD process using tBu3Ga as a novel Ga source. Morphology, elemental composition, and crystallinity of the resulting nanowires are analyzed by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and selected area electron diffraction (SAED). Photoluminescence (PL) spectra of the Ga2O3 nanowires show efficient defect-luminescence in the visible and UV ranges with blue and green emission peaks at 430nm and 512 nm, respectively, at room temperature. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cvde.201307060
  • Measurement of gain and device performance of a 1050-nm vertical external cavity surface emitting laser
    Haupt, S. and Furitsch, M. and Lindberg, H. and Pietzonka, I. and Strauss, U. and Bacher, G.
    IEEE Journal of Quantum Electronics 49 (2013)
    In this paper, we measure and analyze the pump power and wavelength dependent gain of an optically pumped infrared 1050-nm vertical external-cavity surface-emitting laser vertical external-cavity surface-emitting laser (VECSEL) developed for an application as a frequency doubled green laser in mobile projectors. Increasing the reflectivity of the chip coating from 3% to 20% for 1050-nm is found to result in a maximal gain of 7.9%, more than twice as high as that of the 3% coating, and in a spectral narrowing of the wavelength-dependent gain as a consequence of the enhanced coupling between the optical field and the quantum well gain medium. From our gain measurements, the wavelength dependent laser threshold can directly be obtained and compared to the VECSEL device characteristics. A very good agreement between the data extracted from the gain measurements and the measured laser threshold in VECSELs with different output mirrors is found. © 1965-2012 IEEE.
    view abstract10.1109/JQE.2013.2239958
  • Recombination dynamics in single GaAs-nanowires with an axial heterojunction: N- versus p-doped areas
    Sager, D. and Gutsche, C. and Prost, W. and Tegude, F.-J. and Bacher, G.
    Journal of Applied Physics 113 (2013)
    The recombination dynamics of vapor-liquid-solid grown GaAs-nanowires with an axial p-n heterojunction is investigated by spatially and time-resolved photoluminescence spectroscopy. By scanning across the doping transition of single p-n and n-p doped nanowires, respectively, the particular influence of surface losses in differently doped areas is studied. We found a significantly reduced non-radiative recombination for the n-doped region compared to the p-doped one, which can be attributed to suppressed surface losses because of the characteristic band bending at the surface. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4803488
  • Subnanosecond magnetization dynamics induced by a pulsed magnetic field in diluted magnetic semiconductor quantum wells
    Chen, Y.S. and Wiater, M. and Karczewski, G. and Wojtowicz, T. and Bacher, G.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    The magnetization dynamics induced by a pulsed magnetic field is investigated by time- and polarization-resolved photoluminescene measurements in (Cd,Mn)Te/(Cd,Mg)Te quantum wells. The magnetization dynamics of Mn2 + ions is found to be strongly dependent on the external static magnetic field. A dynamical response of the magnetization on a subnanosecond time scale is observed at zero static magnetic field, while it drastically slows down and approaches the spin-lattice relaxation time constant for a nonzero static field. Theoretical calculations emphasize the importance of local spin interactions that interplay with the Zeeman interaction for the observed magnetization dynamics. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.155301
  • The effect of degree of reduction on the electrical properties of functionalized graphene sheets
    Punckt, C. and Muckel, F. and Wolff, S. and Aksay, I.A. and Chavarin, C.A. and Bacher, G. and Mertin, W.
    Applied Physics Letters 102 (2013)
    We study the effect of carbon to oxygen ratio (C/O) on the electrical resistance of functionalized graphene sheets prepared by thermal exfoliation and reduction of graphite oxide at various temperatures. Using a 2-probe technique in conjunction with Kelvin probe force microscopy, we observe a transition from high-resistance (>400 kΩ/sq) nonlinear current/voltage characteristics at low C/O to low-resistance (<10 kΩ/sq) linear behavior at high C/O, indicating a transition from hopping to diffusive electron transport. Simultaneously, the metal-graphene contacts change from high-resistance Schottky-type behavior to nearly non-invasive metal-metal contact characteristics. © 2013 American Institute of Physics.
    view abstract10.1063/1.4775582
  • Analysis of the pump wavelength dependence of a 1060-nm VECSEL
    Haupt, S. and Furitsch, M. and Lindberg, H.H. and Pietzonka, I. and Strau, U. and Bacher, G.
    IEEE Photonics Technology Letters 24 (2012)
    We have developed an optically pumped infrared 1060-nm vertical-external- cavity surface-emitting laser for an application as a frequency doubled green laser in mobile projectors. The efficiency has been investigated as a function of pump wavelength and a differential efficiency η diff of 50% has been demonstrated using an 808-nm pump laser source. The usage of a longer pump wavelength decreases the quantum defect and increases the differential efficiency to over 52%. The pump light absorption in the chip is investigated as a function of pump wavelength and absorber thickness. A thicker absorber layer enables the usage of a longer pump wavelength and hence reduces the quantum defect further. © 2011 IEEE.
    view abstract10.1109/LPT.2011.2178823
  • Electric field tuning of spin splitting in a quantum dot coupled to a semimagnetic quantum dot
    Lyanda-Geller, Y. and Reinecke, T.L. and Bacher, G.
    Journal of Applied Physics 111 (2012)
    We develop an approach for tuning the spin splitting and g-factor of a quantum dot by coupling it to semi-magnetic quantum dot and tuning the electric field. We show that spin splittings and g-factors of the states of a non-magnetic quantum dot coupled to semimagnetic quantum dot can be enhanced orders of magnitude. Evaluations are made for coupled CdTe/CdMnTe quantum dots. These effects are caused by electric field control of repulsion of spin sublevels in the non-magnetic dot due to tunnel coupling of quantum dots. Electric field control of spin splittings in quantum dots is of potential interest in connection with spin qubit rotations for quantum computation. © 2012 American Institute of Physics.
    view abstract10.1063/1.4705287
  • Electron-nuclei spin coupling in GaAs-Free versus localized electrons
    Huang, J. and Chen, Y.S. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Bacher, G.
    Applied Physics Letters 100 (2012)
    We report on an experimental evidence of a significantly different dynamic nuclear polarization (DNP) for localized and itinerant electrons in n-GaAs. Optically injected spin-polarized electrons are used to generate dynamic nuclear polarization via electron-nucleus hyperfine interaction. Using time resolved Kerr rotation measurements for probing the transient Overhauser field, the DNP time constants for itinerant and localized electrons are extracted to be 10 min and less than 1 min, respectively. This is attributed to a rapid DNP occurring in the vicinity of the donors followed by a delayed nuclear spin polarization in between the donor sites. © 2012 American Institute of Physics.
    view abstract10.1063/1.3699261
  • High-frequency electrical charge and spin control in a single InGaAs quantum dot
    Nannen, J. and Quitsch, W. and Eliasson, S. and Kümmell, T. and Bacher, G.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    We report on the charging behavior of a single self-assembled InGaAs quantum dot with unpolarized and spin-polarized electrons under direct current (DC) and high-frequency biasing. The tunnel coupling of the quantum dot to a spin-polarized electron reservoir leads to characteristic voltage dependence of the polarization of the neutral and the negatively charged exciton emissions in a magnetic field under DC biasing conditions. Via high-frequency adaptation of the device, electrical control of the charge state of the single quantum dot in the gigahertz regime is achieved. A technique for optical preparation of single holes and subsequent electrical charging via high-frequency voltage pulses allows for an ultrafast injection and readout of spin-polarized electrons on a subnanosecond timescale. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.035325
  • Material and doping contrast in III/V nanowires probed by Kelvin probe force microscopy
    Vinaji, S. and Bacher, G. and Mertin, W.
    NanoScience and Technology 79 (2012)
    We have studied the local surface potential and the voltage drop along individual VLS grown GaAs nanowires using Kelvin probe force microscopy. With the obtained information, we identify a core-shell structure inGaAs/GaP hetero structure nanowires, which we attribute to the difference in radial and vertical growth between the two semiconductor materials. In p-doped GaAs nanowires, qualitative and quantitative doping levels are estimated. Furthermore, we find a better incorporation of the zinc compared to the carbon to realize doping in partially p-doped GaAs nanowires by localizing the doping transitions and estimating the width of their depletion layers. Additionally, the p-n junction can be localized with a resolution better than 50nm and the bias dependence of the depletion layer width can be studied. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-29656-7_8
  • Optical spectroscopy on magnetically doped semiconductor nanoparticles
    Schneider, L. and Bacher, G.
    NanoScience and Technology 79 (2012)
    Semiconductor nanoparticles doped with magnetic ions represent an exciting class of materials with unique optical, electronic, and magnetic properties and potential applications in the field of spintronics. A key feature required is the exchange interaction between magnetic ions and charge carriers, which finally controls the magneto-optical response of these materials. In this contribution, some recent advances for two classes of magnetically doped nanoparticles, namely, ZnO doped with Cr and Co, respectively, and CdSe doped with Mn, are summarized. We found that chromium is incorporated as Cr 3+ in ZnO. With increasing Cr concentration, the quantum efficiency is being reduced while themagnetic properties observed can be attributed to a phase separation between ZnO and ZnCr 2O 4. In contrast, cobalt apparently exists in the Co 2+ configuration in the nanocrystals as demonstrated via optical spectroscopy. No enhanced magneto-optical properties have been obtained for both classes of magnetically doped ZnO nanoparticles. This is completely different in case of Mn-doped CdSe nanocrystals. A giant Zeeman effect is found as a consequence of a pronounced sp-d exchange interaction. The strong 3D carrier confinement finally results in a significantly enhanced exchange field leading to the observation of optically induced magnetism up to room temperature. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-28546-2_12
  • 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 abstract10.1143/APEX.5.035001
  • Quantum confinement-controlled exchange coupling in manganese(II)-doped CdSe two-dimensional quantum well nanoribbons
    Fainblat, R. and Frohleiks, J. and Muckel, F. and Yu, J.H. and Yang, J. and Hyeon, T. and Bacher, G.
    Nano Letters 12 (2012)
    The impact of quantum confinement on the exchange interaction between charge carriers and magnetic dopants in semiconductor nanomaterials has been controversially discussed for more than a decade. We developed manganese-doped CdSe quantum well nanoribbons with a strong quantum confinement perpendicular to the c-axis, showing distinct heavy hole and light hole resonances up to 300 K. This allows a separate study of the s-d and the p-d exchange interactions all the way up to room temperature. Taking into account the optical selection rules and the statistical distribution of the nanoribbons orientation on the substrate, a remarkable change in particular of the s-d exchange constant with respect to bulk is indicated. Room-temperature studies revealed an unusually high effective g-factor up to ∼13 encouraging the implementation of the DMS quantum well nanoribbons for (room temperature) spintronic applications. © 2012 American Chemical Society.
    view abstract10.1021/nl302639k
  • Room temperature single photon emission from an epitaxially grown quantum dot
    Fedorych, O. and Kruse, C. and Ruban, A. and Hommel, D. and Bacher, G. and Kümmell, T.
    Applied Physics Letters 100 (2012)
    Single photon emission from an epitaxially grown quantum dot at room temperature is presented. CdSe/ZnSSe quantum dots are embedded into MgS barriers, providing dominant radiative recombination up to 300 K. Under continuous wave optical excitation, the autocorrelation function g (2)(t) exhibits a sharp dip at (t = 0) with g (2)(0) = 0.16 ± 0.15 at T = 300 K, revealing excellent suppression of multiphoton emission even at room temperature. © 2012 American Institute of Physics.
    view abstract10.1063/1.3683498
  • Dynamic nuclear spin resonance in n-GaAs
    Chen, Y.S. and Reuter, D. and Wieck, A.D. and Bacher, G.
    Physical Review Letters 107 (2011)
    The dynamics of optically detected nuclear magnetic resonance is studied in n-GaAs via time-resolved Kerr rotation using an on-chip microcoil for rf field generation. Both optically allowed and optically forbidden NMR are observed with a dynamics controlled by the interplay between dynamic nuclear polarization via hyperfine interaction with optically generated spin-polarized electrons and nuclear spin depolarization due to magnetic resonance absorption. Comparing the characteristic nuclear spin relaxation rate obtained in experiment with master equation simulations, the underlying nuclear spin depolarization mechanism for each resonance is extracted. © 2011 American Physical Society.
    view abstract10.1103/PhysRevLett.107.167601
  • Gain and reflectance measurements of a 1050nm VECSEL chip
    Haupt, S. and Furitsch, M. and Lindberg, H. and Pietzonka, I. and Strauß, U. and Bacher, G.
    2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 (2011)
    Optically pumped Vertical External Cavity Surface Emitting Lasers (VECSEL) have a wide range of applications due to a combination of output power, high efficiency, excellent beam quality and a broad spectral range [1-4]. Furthermore the visible spectrum is accessible with intra cavity frequency-doubling [2]. As frequency-doubling requires precise spectral stabilization a detailed knowledge of the spectral gain curve of the VECSEL chip is essential. The exact value of gain depends on the resonant periodic gain (RPG) structure, Bragg mirror reflectance, pump power and wavelength. © 2011 IEEE.
    view abstract10.1109/CLEOE.2011.5942634
  • Local voltage drop in a single functionalized graphene sheet characterized by Kelvin probe force microscopy
    Yan, L. and Punckt, C. and Aksay, I.A. and Mertin, W. and Bacher, G.
    Nano Letters 11 (2011)
    We studied the local voltage drop in functionalized graphene sheets of subμm size under external bias conditions by Kelvin probe force microscopy. Using this noninvasive experimental approach, we measured ohmic current-voltage characteristics and an intrinsic conductivity of about 3.7 × 10 5 S/m corresponding to a sheet resistance of 2.7 kΩ/sq under ambient conditions for graphene produced via thermal reduction of graphite oxide. The contact resistivity between functionalized graphene and metal electrode was found to be < 6.3 × 10-7 Ωcm2. © 2011 American Chemical Society.
    view abstract10.1021/nl201070c
  • Manipulation of nuclear spin dynamics in n-GaAs using an on-chip microcoil
    Chen, Y.S. and Huang, J. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Bacher, G.
    Journal of Applied Physics 109 (2011)
    We present an approach for electrically manipulating nuclear spins in n-GaAs using an on-chip microcoil. Optically injected spin-polarized electrons are used to generate a dynamic nuclear polarization via electron-nucleus hyperfine interaction with a characteristic time constant of ∼10 min. The saturated Overhauser field amplitude is on the order of several 10 mT and proportional to the spin polarization degree of the injected electrons. Applying an rf field resonant for the 75As nuclei, complete depolarization of 75As nuclear spins is observed. © 2011 American Institute of Physics.
    view abstract10.1063/1.3530731
  • Optically detected nuclear magnetic resonance in n-GaAs using an on-chip microcoil
    Chen, Y.S. and Huang, J. and Reuter, D. and Ludwig, Ar. and Wieck, A.D. and Bacher, G.
    Applied Physics Letters 98 (2011)
    Optically detected nuclear magnetic resonance (NMR) with micrometer resolution is demonstrated in n-GaAs using an on-chip microcoil. To trace the Overhauser field, the electron Larmor frequency is monitored via time-resolved magneto-optical Kerr rotation. Sweeping the frequency of the rf magnetic field induced by an on-chip microscale current loop, nuclear spin depolarization is achieved for each isotope species. The experimental data indicate an impact of a local quadrupole field, most likely caused by ionized donors, on the amplitude and linewidth of the NMR spectrum. By applying rf pulse sequences, the Rabi oscillation of 75As nuclear spins is obtained with an effective dephasing time of ∼200 μs. © 2011 American Institute of Physics.
    view abstract10.1063/1.3553503
  • Potential distribution in functionalized graphene devices probed by Kelvin probe force microscopy
    Yan, L. and Punckt, C. and Aksay, I.A. and Mertin, W. and Bacher, G.
    AIP Conference Proceedings 1399 (2011)
    Kelvin probe force microscopy was used to study the impact of contacts and topography on the local potential distribution on contacted, individual functionalized graphene sheets (FGS) deposited on a SiO 2/Si substrate. Negligible contact resistance is found at the graphene/Ti interface and a graphene resistance of 2.3 kΩ is extracted for a single sheet with sub-μm size. Pronounced steps in the topography, which we attribute to a variation of the spacing between graphene and substrate, result in a significant change of the local resistivity. © 2011 American Institute of Physics.
    view abstract10.1063/1.3666628
  • Spatially resolved photoelectric performance of axial GaAs nanowire pn-diodes
    Lysov, A. and Vinaji, S. and Offer, M. and Gutsche, C. and Regolin, I. and Mertin, W. and Geller, M. and Prost, W. and Bacher, G. and Tegude, F.-J.
    Nano Research 4 (2011)
    The spatially resolved photoelectric response of a single axial GaAs nanowire pn-diode has been investigated with scanning photocurrent and Kelvin probe force microscopy. Optical generation of carriers at the pn-junction has been shown to dominate the photoresponse. A photocurrent of 88 pA, an open circuit voltage of 0.56 V and a fill factor of 69% were obtained under AM 1.5 G conditions. The photocurrent followed the increasing photoexcitation with 0.24 A/W up to an illumination density of at least 90 W/cm2, which is important for potential applications in concentrator solar cells. © 2011 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s12274-011-0155-4
  • Coherent dynamics of localized spins in an inhomogeneous magnetic field
    Hohage, P.E. and Nannen, J. and Halm, S. and Puls, J. and Henneberger, F. and Bacher, G.
    Journal of Superconductivity and Novel Magnetism 23 (2010)
    We studied the influence of the inhomogeneity of fringe fields originating from nanostructured ferromagnets on the coherent dynamics of localized Mn 2+ spins in a (Zn,Cd,Mn)Se/ZnSe quantum well by time-resolved Kerr rotation. By studying hybrid systems with different geometry, we demonstrate that the spatially inhomogeneous fringe field results in two main consequences for the localized spin system: First, a temporal variation of the ensemble precession frequency is obtained, and second, a reduced ensemble spin dephasing time T 2 * is observed in comparison to an unstructured reference area of the sample. © 2009 Springer Science+Business Media, LLC.
    view abstract10.1007/s10948-009-0572-8
  • Electron-nuclear spin control in charged semiconductor quantum dots by electrical currents through micro-coils
    Kim, J. and Puls, J. and Chen, Y.S. and Bacher, G. and Henneberger, F.
    Applied Physics Letters 96 (2010)
    We have fabricated micrometer-sized single-turn coils on top of charged CdSe/ZnSe quantum dot heterostructures by lithographical techniques. Current injection creates magnetic fields in the some 10 mT range, strong enough to modulate the hyperfine interaction. The very low coil inductance allows for generation of fast field transients. We demonstrate local control of the resident electron spin as well as read-out of the nuclear spin state on the 10 ns time scale by electrical current pulses. © 2010 American Institute of Physics.
    view abstract10.1063/1.3373591
  • Erratum: Publisher's Note: "Ultrafast electrical charging and discharging of a single InGaAs quantum dot" (Applied Physics Letters (2010) 97 (173108))
    Nannen, J. and Kümmell, T. and Bartsch, M. and Brunner, K. and Bacher, G.
    Applied Physics Letters 97 (2010)
    view abstract10.1063/1.3525279
  • Exciton states and tunneling in semimagnetic asymmetric double quantum wells
    Zaitsev, S.V. and Brichkin, A.S. and Yu Tarakanov, A. and Bacher, G.
    Physica Status Solidi (B) Basic Research 247 (2010)
    Exciton level structure and interwell relaxation are studied in Cd(Mn,Mg)Te-based asymmetric double quantum wells (ADQWs) by a steady-state optical spectroscopy in magnetic fields up to B = 10T. The as grown heterostructures with CdTe QWs and nonmagnetic interwell CdMgTe barrier were subjected to a rapid temperature annealing to introduce Mn and Mg atoms from opposite barriers inside the QWs which results in a formation of the ADQW with completely different magnetic field behavior of the intrawell excitons. The giant Zeeman effect in the QW with magnetic Mn ions gives rise to a crossing of the ground exciton levels in two QWs at BC ~ 3-6T which is accompanied by a reverse of the interwell tunneling direction. In a single-particle picture the exciton tunneling is forbidden at B < 1T as supported by calculations. Experimentally, nevertheless, a very efficient interwell relaxation of excitons is found at resonant excitation in the whole magnetic field range, regardless of the tunneling direction, emphasizing importance of excitonic correlations in the interwell tunneling. At nonresonant excitation an unexpectedly slow relaxation of the σ--polarized excitons from the nonmagnetic QW to the σ+-polarized ground state in the semimagnetic QW is observed at B &gt; BC, giving rise to a nonequilibrium distribution of excitons in ADQW. A strong dependence of the total circular polarization degree on the hh-lh splitting Δhh-lh in the nonmagnetic QW is found and attributed to the spin dependent interwell tunneling controlled by an exciton spin relaxation. Different charge-transfer mechanisms are analyzed in details and an elastic scattering due to a strong disorder is suggested as the main tunneling mechanism with the underlying influence of the valence band-mixing. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.200945374
  • Local definition of spin polarization in a semiconductor by micro-scale current loops
    Chen, Y.S. and Halm, S. and Kümmell, T. and Bacher, G. and Wiater, M. and Wojtowicz, T. and Karczewski, G.
    Journal of Superconductivity and Novel Magnetism 23 (2010)
    We present an approach to electrical control of the spin polarization in a diluted magnetic semiconductor (DMS) structure. A variable magnetic field induced by a micro-scale current loop magnetizes the Mn 2+ ions in a CdMnTe/CdMgTe DMS quantum well, which via the sp-d exchange interaction polarizes photo-generated electron-hole pairs confined in the well. A maximum spin polarization degree of ±8.5% is obtained at 4.2 K without external magnetic field. The current-induced magnetic field and the current-generated heating of the spin system are quantitatively extracted by micro magneto-luminescence measurements. © 2009 Springer Science+Business Media, LLC.
    view abstract10.1007/s10948-009-0559-5
  • Radiative recombination dynamics of CdSe/Zn(S,Se)/MgS quantum dots up to room temperature
    Zaitsev, S.V. and Arians, R. and Kümmell, T. and Bacher, G. and Gust, A. and Kruse, C. and Hommel, D.
    Physica Status Solidi (B) Basic Research 247 (2010)
    The recombination dynamics and its temperature dependence are studied in detail in epitaxially grown CdSe/Zn(S,Se) quantum dots (QDs) with additional wide-band MgS barriers. Such design allows to preserve a very high quantum yield and to track the QD recombination dynamics up to room temperature. At low temperatures, a fast initial decay ~0.6 ns is observed which is followed by a long decay with a time constant ~25 ns. The fast initial decay disappears with increasing temperature and above 100K only a single-exponential decay is observed with a decay time τ of 1.2-1.5 ns, which is weakly dependent on temperature up to 300 K. A two-level model including bright and dark exciton states and temperature dependent spin-flip between them describes well the experimental findings. According to the model, the long decay tail results from a thermally activated population of the bright exciton state from the energetically lowest dark state. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.200983250
  • Radiative recombination in photoexcited quantum dots up to room temperature: The role of fine-structure effects
    Kümmell, T. and Zaitsev, S.V. and Gust, A. and Kruse, C. and Hommel, D. and Bacher, G.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    We demonstrate dominant radiative recombination in high-quality self-assembled quantum dots all the way up to room temperature. This allows a proof of the theoretically predicted characteristic doubling of the radiative recombination time with increasing temperature, entirely caused by fine-structure effects. Over the whole temperature range, the transient decay of the photoluminescence can be described by temperature-independent radiative decay rates of dark and bright excitons, respectively, taking into account a thermally activated interplay between dark and bright states and dark-exciton accumulation. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.241306
  • Sub-ns electrical control of spin polarization in a semiconductor by microscale current loops
    Chen, Y.S. and Wiater, M. and Karczewski, G. and Wojtowicz, T. and Bacher, G.
    Physica Status Solidi (B) Basic Research 247 (2010)
    We demonstrate a method to electrically control the spin polarization in a diluted magnetic semiconductor (DMS) quantum well (QW) on a micrometer length scale. A variable magnetic field generated from a microscale current loop atop a single CdMnTe/CdMg TeQ Wmagnetizes the Mn2+ ion spins in the DMS QW and, via the sp-d exchange interaction, polarizes the spins of photon-generated carriers. In the absence of an external field, the spin polarization can be switched on a time scale clearly below 1 ns. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.200983279
  • Temperature dependence of radiative recombination in CdSe quantum dots with enhanced confinement
    Zaitsev, S.V. and Kümmell, T. and Bacher, G. and Hommel, D.
    JETP Letters 92 (2010)
    We studied in details the recombination dynamics and its temperature dependence in epitaxially grown neutral CdSe/ZnSSe quantum dots with additional wide-band gap MgS barriers. Such design allows to preserve a very high quantum yield and track the radiative recombination dynamics up to room temperature. A fast initial decay of ~0.6 ns followed by a slow decay with a time constant ~30-50 ns is observed at low temperature T < 50 K. The fast decay gradually disappears with increasing temperature while the slow decay shortens and above 100 K predominantly a single-exponential decay is observed with a time constant ~1. 3 ns, which is weekly temperature dependent up to 300 K. To explain the experimental findings, a two-level model which includes bright and dark exciton states and a temperature dependent spin-flip between them is considered. According to the model, it is a thermal activation of the dark exciton to the bright state and its consequent radiative recombination that results in the long decay tail at low temperature. The doubling of the decay time at high temperatures manifests a thermal equilibrium between the dark and bright excitons. © 2010 Pleiades Publishing, Ltd.
    view abstract10.1134/S0021364010130114
  • Ultrafast electrical charging and discharging of a single InGaAs quantum dot
    Nannen, J. and Kümmell, T. and Bartsch, M. and Brunner, K. and Bacher, G.
    Applied Physics Letters 97 (2010)
    We report on ultrafast control of the charge state of a single InGaAs quantum dot in a charge-tunable p-i-n diode structure. Focused ion beam etching is employed to decrease the capacitance of the device to enable radio frequency operation. A time-resolved photoluminescence technique is demonstrated that allows monitoring the charge state with a time-resolution which is limited only by the radiative lifetime of the charged and neutral exciton, respectively. Experimental data show that the charge state can be manipulated on time scales shorter than the radiative lifetime of approximately 1.4 ns. © 2010 American Institute of Physics.
    view abstract10.1063/1.3505358
  • 2D materials

  • graphene

  • magnetism

  • optoelectronics

  • quantum dots

  • quantum information technology

  • semiconductors

  • spintronics

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