Prof. Dr. Axel Lorke

Experimental Physics
University of Duisburg-Essen


  • Laser- and Ion-Induced Defect Engineering in WS2 Monolayers
    Asaithambi, A. and Kozubek, R. and Prinz, G.M. and Reale, F. and Pollmann, E. and Ney, M. and Mattevi, C. and Schleberger, M. and Lorke, A.
    Physica Status Solidi - Rapid Research Letters 15 (2021)
    Tungsten disulfide is one of the prominent transition metal dichalcogenide materials, which shows a transition from an indirect to a direct bandgap as the layer thickness is reduced down to a monolayer. To use (Formula presented.) monolayers in devices, detailed knowledge about the luminescence properties regarding not only the excitonic but also the defect-induced contributions is needed. Herein, (Formula presented.) monolayers are irradiated with (Formula presented.) ions with different fluences to create different defect densities. Apart from the excitonic contributions, two additional emission bands are observed at low temperatures. These bands can be reduced or even suppressed, if the flakes are exposed to laser light with powers up to 1.5 mW. Increasing the temperature up to room temperature leads to recovery of this emission, so that the luminescence properties can be modified using laser excitation and temperature. The defect bands emerging after ion irradiation are attributed to vacancy defects together with physisorbed adsorbates at different defect sites. © 2020 The Authors. Physica Status Solidi (RRL) – Rapid Research Letters published by Wiley-VCH GmbH
    view abstract10.1002/pssr.202000466
  • Quantum Sensor for Nanoscale Defect Characterization
    Kerski, J. and Lochner, P. and Ludwig, Ar. and Wieck, A.D. and Kurzmann, A. and Lorke, A. and Geller, M.
    Physical Review Applied 15 (2021)
    The optical and electronic properties of semiconductors are strongly affected by structural and stoichiometric defects. The precise incorporation of dopants and the control of impurities are essentially what makes semiconductors useful materials for a broad range of devices. The standard defect and impurity characterization methods are sensitive only on a macroscopic scale, like the most widely used method of deep-level transient spectroscopy (DLTS). We perform time-resolved measurements of the resonance fluorescence of a single self-assembled (In,Ga)As quantum dot (QD) at low temperatures (4.2K). By pulsing the applied gate voltage, we are able to selectively occupy and unoccupy individual defects in the vicinity of the dot. We address the exciton transition of the QD with a tunable diode laser. Our time-resolved measurements exhibit a shift of the resonance energy of the optical transition. We attribute this to a change of the electric field in the dot's vicinity, caused by electrons tunneling from a reservoir to the defect sites. Furthermore, we are able to characterize the defects concerning their position and activation energy by modeling our experimental data. Our results thus demonstrate how a quantum dot can be used as a quantum sensor to characterize the position and activation energy of individual shallow defects on the nanoscale. © 2021 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevApplied.15.024029
  • Defect spectroscopy on the dielectric material aluminum oxide
    Oing, D. and Geller, M. and Stahl, L. and Kerski, J. and Lorke, A. and Wöhrl, N.
    Scientific Reports 10 (2020)
    A method for defect characterization is presented that allows to measure the activation energy, capture cross-section, and defect density in dielectric materials. This is exemplarily performed on aluminum oxide thin films deposited on hydrogen-terminated diamond. During the measurement, samples were illuminated using a 405 nm laser, charging the defects while simultaneously measuring the surface conductivity of the diamond at different temperatures. By applying the standard boxcar evaluation known from deep-level transient spectroscopy, we found five different defect levels in Al 2O 3. One can be identified as substitutional silicon in aluminum oxide, while the others are most likely connected to either aluminum interstitials or carbon and nitrogen impurities. © 2020, The Author(s).
    view abstract10.1038/s41598-020-69240-3
  • One-step synthesis of carbon-supported electrocatalysts
    Tigges, S. and Wöhrl, N. and Radev, I. and Hagemann, U. and Heidelmann, M. and Nguyen, T.B. and Gorelkov, S. and Schulz, S. and Lorke, A.
    Beilstein Journal of Nanotechnology 11 (2020)
    Cost-efficiency, durability, and reliability of catalysts, as well as their operational lifetime, are the main challenges in chemical energy conversion. Here, we present a novel, one-step approach for the synthesis of Pt/C hybrid material by plasma-enhanced chemical vapor deposition (PE-CVD). The platinum loading, degree of oxidation, and the very narrow particle size distribution are precisely adjusted in the Pt/C hybrid material due to the simultaneous deposition of platinum and carbon during the process. The as-synthesized Pt/C hybrid materials are promising electrocatalysts for use in fuel cell applications as they show significantly improved electrochemical long-term stability compared to the industrial standard HiSPEC 4000. The PE-CVD process is furthermore expected to be extendable to the general deposition of metal-containing carbon materials from other commercially available metal acetylacetonate precursors. © 2020 Tigges et al.; licensee Beilstein-Institut.
    view abstract10.3762/BJNANO.11.126
  • Real-Time Detection of Single Auger Recombination Events in a Self-Assembled Quantum Dot
    Lochner, P. and Kurzmann, A. and Kerski, J. and Stegmann, P. and König, J. and Wieck, A.D. and Ludwig, Ar. and Lorke, A. and Geller, M.
    Nano Letters 20 (2020)
    Auger recombination is a nonradiative process, where the recombination energy of an electron-hole pair is transferred to a third charge carrier. It is a common effect in colloidal quantum dots that quenches the radiative emission with an Auger recombination time below nanoseconds. In self-assembled QDs, the Auger recombination has been observed with a much longer recombination time on the order of microseconds. Here, we use two-color laser excitation on the exciton and trion transition in resonance fluorescence on a single self-assembled quantum dot to monitor in real-time single quantum events of the Auger process. Full counting statistics on the random telegraph signal give access to the cumulants and demonstrate the tunability of the Fano factor from a Poissonian to a sub-Poissonian distribution by Auger-mediated electron emission from the dot. Therefore, the Auger process can be used to tune optically the charge carrier occupation of the dot by the incident laser intensity, independently from the electron tunneling from the reservoir by the gate voltage. Our findings are not only highly relevant for the understanding of the Auger process but also demonstrate the perspective of the Auger effect for controlling precisely the charge state in a quantum system by optical means. © 2020 American Chemical Society.
    view abstract10.1021/acs.nanolett.9b04650
  • Single-Crystalline Optical Microcavities from Luminescent Dendrimers
    Iwai, K. and Yamagishi, H. and Herzberger, C. and Sato, Y. and Tsuji, H. and Albrecht, K. and Yamamoto, K. and Sasaki, F. and Sato, H. and Asaithambi, A. and Lorke, A. and Yamamoto, Y.
    Angewandte Chemie - International Edition 59 (2020)
    Microcrystallites are promising minute mirrorless laser sources. A variety of luminescent organic compounds have been exploited along this line, but dendrimers have been inapplicable owing to their fragility and extremely poor crystallinity. Now, a dendrimer family that overcomes these difficulties is presented. First-, second-, and third-generation carbazole (Cz) dendrimers with a carbon-bridged oligo(phenylenevinylene) (COPV2) core (GnCOPV2, n=1–3) assemble to form microcrystals. The COPV2 cores align uni/bidirectionally in the crystals while the Cz units in G2- and G3COPV2 align omnidirectionally. The dendrons work as light-harvesting antennas that absorb non-polarized light and transfer it to the COPV2 core, from which a polarized luminescence radiates. Furthermore, these crystals act as laser resonators, where the lasing thresholds are strongly coupled with the crystal morphology and the orientation of COPV2, which is in contrast with the conventional amorphous dendrimers. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.202000712
  • Tailoring of Bound Exciton Photoluminescence Emission in WS2 Monolayers
    Kaupmees, R. and Grossberg, M. and Ney, M. and Asaithambi, A. and Lorke, A. and Krustok, J.
    Physica Status Solidi - Rapid Research Letters 14 (2020)
    Temperature- and laser power-dependent photoluminescence (PL) properties of the asymmetric defect-bound exciton band XD in defective WS2 monolayers, grown by chemical vapor deposition, are studied. Based on PL mapping, a monolayer region with an intensive XD band emission at about 1.9 eV is chosen for further studies. The XD band is thermally quenched above 180 K, and the thermal activation energy is found to be Ea= 33 ± 4 meV. At T = 15 K, the XD band intensity reveals a sublinear dependence with increasing excitation power and the peak position shows a blueshift of about 15 meV per decade of laser power. It is shown that the XD band is related to the deep defect states within the band gap of WS2. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssr.201900355
  • The effect of metal-oxide incorporation on the morphology of carbon nanostructures
    Tigges, S. and Wöhrl, N. and Hagemann, U. and Ney, M. and Lorke, A.
    Journal of Physics D: Applied Physics 53 (2020)
    Metal-organic, single-source, low-temperature, morphology-controlled growth of carbon nanostructures is achieved, using an inductively coupled plasma-enhanced chemical vapor deposition system. Three distinctive morphologies, namely nanoflakes, nanowalls (CNWs) and nanorods (and intermediates between these morphologies), can be reproducibly deposited, depending on the process parameters. The synthesized structures can be described as hybrid materials consisting of metal oxide incorporated in a carbon matrix material. Since the incorporation of metal oxide into the carbon structure significantly influences their growth, the synthesis cannot be described solely with the existing models for the growth of CNWs. Optical emission spectroscopy is used to measure the relative number density of suspected growth and etching species in the plasma, while physical and chemical surface analysis techniques (scanning electron microscopy, Raman spectroscopy, scanning Auger microscopy and x-ray photoelectron spectroscopy) were employed to characterize the properties of the different nanostructures. Therefore, by using methods for both plasma and surface characterization, the growth process can be understood. The precursor dissociation in the plasma can be directly linked to the deposited morphology, as the incorporation of Al2O3 into the nanostructures is found to be a major cause for the transition between morphologies, by changing the dominant type of defect within the carbon structure. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstract10.1088/1361-6463/ab6946
  • Author Correction: Contrast of 83% in reflection measurements on a single quantum dot (Scientific Reports, (2019), 9, 1, (8817), 10.1038/s41598-019-45259-z)
    Lochner, P. and Kurzmann, A. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lorke, A. and Geller, M.
    Scientific Reports 9 (2019)
    In the original version of this Article, the authors Andreas D. Wieck and Arne Ludwig were incorrectly affiliated with ‘Solid State Physics Laboratory, ETH Zurich, 8093 Zurich, Switzerland’. The correct affiliation is listed below. Ruhr-Universität Bochum, Lehrstuhl für Angewandte Festkörperphysik, D-44780 Bochum, Germany This error has now been corrected in the PDF and HTML versions of the Article. © 2019, The Author(s).
    view abstract10.1038/s41598-019-54298-5
  • Contrast of 83% in reflection measurements on a single quantum dot
    Lochner, P. and Kurzmann, A. and Schott, R. and Wieck, A.D. and Ludwig, Ar. and Lorke, A. and Geller, M.
    Scientific Reports 9 (2019)
    We report on a high optical contrast between the photon emission from a single self-assembled quantum dot (QD) and the back-scattered excitation laser light. In an optimized semiconductor heterostructure with an epitaxially grown gate, an optically-matched layer structure and a distributed Bragg reflector, a record value of 83% is obtained; with tilted laser excitation even 885%. This enables measurements on a single dot without lock-in technique or suppression of the laser background by cross-polarization. These findings open up the possibility to perform simultaneously time-resolved and polarization-dependent resonant optical spectroscopy on a single quantum dot. © 2019, The Author(s).
    view abstract10.1038/s41598-019-45259-z
  • Optical Detection of Single-Electron Tunneling into a Semiconductor Quantum Dot
    Kurzmann, A. and Stegmann, P. and Kerski, J. and Schott, R. and Ludwig, Ar. and Wieck, A.D. and König, J. and Lorke, A. and Geller, M.
    Physical Review Letters 122 (2019)
    The maximum information of a dynamic quantum system is given by real-time detection of every quantum event, where the ultimate challenge is a stable, sensitive detector with high bandwidth. All physical information can then be drawn from a statistical analysis of the time traces. We demonstrate here an optical detection scheme based on the time-resolved resonance fluorescence on a single quantum dot. Single-electron resolution with high signal-to-noise ratio (4σ confidence) and high bandwidth of 10 kHz make it possible to record the individual quantum events of the transport dynamics. Full counting statistics with factorial cumulants gives access to the nonequilibrium dynamics of spin relaxation of a singly charged dot (γ↑↓=3 ms-1), even in an equilibrium transport measurement. © 2019 American Physical Society.
    view abstract10.1103/PhysRevLett.122.247403
  • Patterning of diamond with 10 nm resolution by electron-beam-induced etching
    Dergianlis, V. and Geller, M. and Oing, D. and Wöhrl, N. and Lorke, A.
    Nanotechnology 30 (2019)
    We report on mask-less, high resolution etching of diamond surfaces, featuring sizes down to 10 nm. We use a scanning electron microscope (SEM) together with water vapor, which was injected by a needle directly onto the sample surface. Using this versatile and low-damage technique, trenches with different depths were etched. Cross sections of each trench were obtained by focused ion beam milling and used to calculate the achieved aspect ratios. The developed technique opens up the possibility of mask- and resist-less patterning of diamond for nano-optical and electronic applications. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/ab25fe
  • Photon Noise Suppression by a Built-in Feedback Loop
    Al-Ashouri, A. and Kurzmann, A. and Merkel, B. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Nano Letters 19 (2019)
    Visionary quantum photonic networks need transform-limited single photons on demand. Resonance fluorescence on a quantum dot provides the access to a solid-state single photon source, where the environment is unfortunately the source of spin and charge noise that leads to fluctuations of the emission frequency and destroys the needed indistinguishability. We demonstrate a built-in stabilization approach for the photon stream, which relies solely on charge carrier dynamics of a two-dimensional hole gas inside a micropillar structure. The hole gas is fed by hole tunneling from field-ionized excitons and influences the energetic position of the excitonic transition by changing the local electric field at the position of the quantum dot. The standard deviation of the photon noise is suppressed by nearly 50% (noise power reduction of 6 dB) and it works in the developed micropillar structure for frequencies up to 1 kHz. This built-in feedback loop represents an easy way for photon noise suppression in large arrays of single photon emitters and promises to reach higher bandwidth by device optimization. © 2018 American Chemical Society.
    view abstract10.1021/acs.nanolett.8b03486
  • Polychromatic Photoluminescence of Polymorph Boron Dipyrromethene Crystals and Heterostructures
    Asaithambi, A. and Okada, D. and Prinz, G. and Sato, H. and Saeki, A. and Nakamura, T. and Nabeshima, T. and Yamamoto, Y. and Lorke, A.
    Journal of Physical Chemistry C 123 (2019)
    Micrometer-sized boron dipyrromethene crystalline rods were grown from solution. Fluorescence microscopy images reveal that each rod displays characteristic visible light emission of a different color. In a particular case, optical heterostructures with discrete, differently colored sections are observed within a single microrod. Microphotoluminescence (μ-PL) spectra of green and red rods at room temperature show multiple contributions, indicating the presence of microdomains. Temperature-dependent μ-PL measurements further confirm this, as red emission decreases and green emission increases at lower temperatures. These observations are discussed as a result of crystalline polymorphism, leading to a local variation of the highest occupied molecular orbital-lowest unoccupied molecular orbital energy difference. An Arrhenius plot quantifies the hopping barrier for the charge carriers to reach the low emission energy (red) regions. A line scan of a single rod further supports that microdomains of green- and orange-red-emitting crystal phases are present in a single rod. Time-resolved microwave conductivity studies clarify that microdomain-free green rods display 2 orders of magnitude longer photocarrier lifetime and 5-fold higher photoconductivity than the red rods with many small band-gap regions. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpcc.8b09202
  • Tunable carrier density and high mobility of two-dimensional hole gases on diamond: The role of oxygen adsorption and surface roughness
    Oing, D. and Geller, M. and Lorke, A. and Wöhrl, N.
    Diamond and Related Materials 97 (2019)
    The transport properties of two-dimensional hole gases (2DHGs) on chemical-vapor-deposition (CVD)-grown diamond are investigated. A hydrogen plasma treatment and exposure to ambient atmosphere are used to establish and tailor the properties of the 2DHG. The transport parameters of the 2DHGs (namely carrier density and mobility) are characterized by temperature-dependent Hall measurements. The importance of the surface oxygen adsorption, determined by X-ray photoelectron spectroscopy (XPS), on the carrier density and mobility is shown. Hall measurements reveal that for oxygen concentrations below 2.2% (relative XPS signal) the carrier density is increasing from 1.4 ∙ 1010 cm−2 to 1.5 ∙ 1013 cm−2 with increasing oxygen adsorption. For oxygen concentrations above 2.2%, the charge carrier density decreases again. The carrier density remains constant over a temperature range between 4.2 K and 325 K. At room temperature, the mobility increases with decreasing carrier concentration. The opposite behavior is observed for 4.2 K. By decreasing the surface roughness to 8.2 nm, we were able to increase the mobility to above 250 cm2/V s at room temperature for a carrier density of 1.2 ∙ 1013 cm−2. This is among the highest values reported for 2DHGs on diamond. © 2019 Elsevier B.V.
    view abstract10.1016/j.diamond.2019.107450
  • Analog Sauter-Schwinger effect in semiconductors for spacetime-dependent fields
    Linder, M.F. and Lorke, A. and Schützhold, R.
    Physical Review B 97 (2018)
    The Sauter-Schwinger effect predicts the creation of electron-positron pairs out of the quantum vacuum via tunneling induced by a strong electric field. Unfortunately, as the required field strength is extremely large, this fundamental prediction of quantum field theory has not been verified experimentally yet. Here, we study under which conditions and approximations the interband tunneling in suitable semiconductors could be effectively governed by the same (Dirac) Hamiltonian, especially for electric fields which depend on space and time. This quantitative analogy would allow us to test some of the predictions (such as the dynamically assisted Sauter-Schwinger effect) in this area by means of these laboratory analogs. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.97.035203
  • Stability of suspended monolayer graphene membranes in alkaline environment
    Miranda, A. and Lorke, A.
    Materials Research Letters 6 (2018)
    We report on the structural, chemical and electrical compatibility of graphene in an alkaline environment, namely diluted KOH solutions as those used in standard nanofabrication methods. Electron microscopy and Raman spectroscopy indicate that graphene preserves its structural properties during the process. The electrical transport properties such as conductance and shape of the Dirac point, appear degraded after KOH treatment, but can be fully recovered by current annealing. Electron beam-induced etching can provide further nanofabrication subsequent to KOH processing. These results demonstrate that, with the due precautions, graphene-based structures can be integrated in standard fabrication processes involving KOH.(Image presented)IMPACT STATEMENT To be implemented in novel (nano) devices, graphene must be compatible with standard fabrication techniques. We show how graphene’s intrinsic properties can be restored after exposure to an alkaline environment. © 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/21663831.2017.1390793
  • Structural and thermoelectrical characterization of epitaxial Sb2Te3 high quality thin films grown by thermal evaporation
    Bendt, G. and Kaiser, K. and Heckel, A. and Rieger, F. and Oing, D. and Lorke, A. and Rodriguez, N.P. and Schierning, G. and Jooss, C. and Schulz, S.
    Semiconductor Science and Technology 33 (2018)
    Thermal evaporation of Sb2Te3 powder was systematically studied under various pressure and temperature conditions. Low pressure experiments (5 •10-6 mbar) conducted inside a horizontal tube reactor at a temperature range of 500 °C-600 °C generated rough polycrystalline films on Si(100) substrates. Based on these experiments, the chemical composition of the resulting films were determined by the furnace temperature. Enhancing the reactor pressure to 20 mbar shifted the growth zone towards higher temperature ranges and yielded highly c-oriented Sb2Te3 films on Si(100) and Al2O3(0001) substrates. Additional experiments were conducted inside a special reactor containing two independent heaters to study the effects of the evaporator and substrate temperatures independently. In contrast to the samples generated in the previous reactor, a two-zone heating reactor allowed the growth of epitaxial Sb2Te3 films with a very smooth surface topology on Al2O3(0001) substrates, as shown by SEM, EDX, XPS, and HRTEM. The electrical in-plane conductivity of the Sb2Te3 films decreased with increasing temperature, ultimately reaching 3950 S •cm-1 at 300 K. The films showed a p-type carrier concentration of 4.3 •10-19 cm-3 at 300 K and a very high carrier mobility of 558 cm2 •V-1 •s-1. The Seebeck coefficient increased monotonically from 94 μV •K-1 at 270 K to 127 μV •K-1 at 420 K. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6641/aad7a3
  • All-electrical measurement of the triplet-singlet spin relaxation time in self-assembled quantum dots
    Eltrudis, K. and Al-Ashouri, A. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Geller, M. and Lorke, A.
    Applied Physics Letters 111 (2017)
    We have measured the spin relaxation time of an excited two-electron spin-triplet state into its singlet ground state in self-assembled InAs/GaAs quantum dots. We use a time-resolved measurement scheme that combines transconductance spectroscopy with spin-to-charge conversion to address the |s↑,p↑〉 triplet state, where one electron is in the quantum dot s-shell and a second one in the p-shell. The evaluation of the state-selective tunneling times from the dots into a nearby two-dimensional electron gas allows us to determine the s- and p-shell occupation and extract the relaxation time from a rate equation model. A comparably long triplet-to-singlet spin relaxation time of 25 μs is found. © 2017 Author(s).
    view abstract10.1063/1.4985572
  • Charge-driven feedback loop in the resonance fluorescence of a single quantum dot
    Merkel, B. and Kurzmann, A. and Schulze, J.-H. and Strittmatter, A. and Geller, M. P. and Lorke, A.
    Physical Review B - Condensed Matter and Materials Physics 95 (2017)
    We demonstrate a feedback loop that manifests itself in a strong hysteresis and bistability of the exciton resonance fluorescence signal. Field ionization of photogenerated quantum dot excitons leads to the formation of a charged interface layer that drags the emission line along over a frequency range of more than 30GHz. These measurements are well described by a rate equation model. With a time-resolved resonance fluorescence measurement we determined the buildup times for the hole gas in the orders of milliseconds. This internal charge-driven feedback loop could be used to reduce the spectral wandering in the emission spectra of single self-assembled quantum dots. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.95.115305
  • Electron dynamics in transport and optical measurements of self-assembled quantum dots
    Kurzmann, A. and Merkel, B. and Marquardt, B. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Physica Status Solidi (B) Basic Research 254 (2017)
    The tunneling dynamics between self-assembled quantum dots (QDs) and a charge reservoir can be measured in an all-electrical or optical detection scheme. In all-electrical transconductance spectroscopy, a two-dimensional electron gas is used to probe the evolution of the many-particle states inside an ensemble of QDs from non-equilibrium to equilibrium. The optical detection scheme measures the tunneling dynamic into a single self-assembled dot. The work done and results obtained using these different measurement techniques are reviewed and compared within this article. We will show that transconductance spectroscopy is sensitive to a time-dependent density of states and enables preparation of non-equilibrium charge and spin states for future applications in quantum information processing. The optical resonance fluorescence measurements on the electron dynamics demonstrates the influence of the exciton states on the charge-carrier dynamics and enables a systematic study of the Auger recombination in self-assembled dots. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201600625
  • Giant magneto-photoelectric effect in suspended graphene
    Sonntag, J. and Kurzmann, A. and Geller, M. P. and Queisser, F. and Lorke, A. and Schützhold, R.
    New Journal of Physics 19 (2017)
    We study the optical response of a suspended, monolayer graphene field-effect transistor structure in magnetic fields of up to 9 T (quantum Hall regime). With an illumination power of only 3 μW, we measure a photocurrent of up to 400 nA (without an applied bias) corresponding to a photo-responsivity of 0.13 A W-1, which we believe to be one of the highest values ever measured in single-layer graphene. We discuss possible mechanisms for generating this strong photo-response (17 electron-hole pairs per 100 incident photons). Based on our experimental findings, we believe that the most likely scenario is a ballistic two-stage process including carrier multiplication via Auger-type inelastic Coulomb scattering at the graphene edge. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/aa739d
  • Intermediate Product Regulation in Tandem Solid Catalysts with Multimodal Porosity for High-Yield Synthetic Fuel Production
    Duyckaerts, N. and Bartsch, M. and Trotuş, I.-T. and Pfänder, N. and Lorke, A. and Schüth, F. and Prieto, G.
    Angewandte Chemie - International Edition 56 (2017)
    Tandem catalysis is an attractive strategy to intensify chemical technologies. However, simultaneous control over the individual and concerted catalyst performances poses a challenge. We demonstrate that enhanced pore transport within a Co/Al2O3 Fischer–Tropsch (FT) catalyst with hierarchical porosity enables its tandem integration with a Pt/ZSM-5 zeolitic hydrotreating catalyst in a spatially distant fashion that allows for catalyst-specific temperature adjustment. Nevertheless, this system resembles the case of close active-site proximity by mitigating secondary reactions of primary FT α-olefin products. This approach enables the combination of in situ dewaxing with a minimum production of gaseous hydrocarbons (18 wt %) and an up to twofold higher (50 wt %) selectivity to middle distillates compared to tandem pairs based on benchmark mesoporous FT catalysts. An overall 80 % selectivity to liquid hydrocarbons from syngas is attained in one step, attesting to the potential of this strategy for increasing the carbon efficiency in intensified gas-to-liquid technologies. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201705714
  • Rendering Ti3C2Tx (MXene) monolayers visible
    Miranda, A. and Halim, J. and Lorke, A. and Barsoum, M.W.
    Materials Research Letters 5 (2017)
    Herein we report on how to render Ti3C2Tx (MXene) monolayers deposited on SiO2/Si wafers, with different SiO2 thicknesses, visible. Inputting the effective thickness of a Ti3C2Tx monolayer (1 ± 0.2 nm) measured by atomic force microscopy, and its refractive index into a Fresnel-law-based simulation software, we show that the optical contrast of Ti3C2Tx monolayers deposited on SiO2/Si wafers depends on the SiO2 thickness, number of MXene layers, and the light’s wavelength. The highest contrast was found for SiO2 thicknesses around 220 nm. Simulations for other substrates, namely, Al2O3/Si, HfO2/Si, Si3N4/Si and Al2O3/Al, are presented as supplementary information. © 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/21663831.2017.1280707
  • Auger Recombination in Self-Assembled Quantum Dots: Quenching and Broadening of the Charged Exciton Transition
    Kurzmann, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Nano Letters 16 (2016)
    In quantum dots (QDs), the Auger recombination is a nonradiative process in which the electron-hole recombination energy is transferred to an additional carrier. It has been studied mostly in colloidal QDs, where the Auger recombination time is in the picosecond range and efficiently quenches the light emission. In self-assembled QDs, on the other hand, the influence of Auger recombination on the optical properties is in general neglected, assuming that it is masked by other processes such as spin and charge fluctuations. Here, we use time-resolved resonance fluorescence to analyze the Auger recombination and its influence on the optical properties of a single self-assembled QD. From excitation-power-dependent measurements, we find a long Auger recombination time of about 500 ns and a quenching of the trion transition by about 80%. Furthermore, we observe a broadening of the trion transition line width by up to a factor of 2. With a model based on rate equations, we are able to identify the interplay between tunneling and Auger rate as the underlying mechanism for the reduced intensity and the broadening of the line width. This demonstrates that self-assembled QDs can serve as an ideal model system to study how the charge recapture process, given by the band-structure surrounding the confined carriers, influences the Auger process. Our findings are not only relevant for improving the emission properties of colloidal QD-based emitters and dyes, which have recently entered the consumer market, but also of interest for more visionary applications, such as quantum information technologies, based on self-assembled quantum dots. © 2016 American Chemical Society.
    view abstract10.1021/acs.nanolett.6b01082
  • Color-Tunable Resonant Photoluminescence and Cavity-Mediated Multistep Energy Transfer Cascade
    Okada, D. and Nakamura, T. and Braam, D. and Dao, T.D. and Ishii, S. and Nagao, T. and Lorke, A. and Nabeshima, T. and Yamamoto, Y.
    ACS Nano 10 (2016)
    Color-tunable resonant photoluminescence (PL) was attained from polystyrene microspheres doped with a single polymorphic fluorescent dye, boron-dipyrrin (BODIPY) 1. The color of the resonant PL depends on the assembling morphology of 1 in the microspheres, which can be selectively controlled from green to red by the initial concentration of 1 in the preparation process of the microspheres. Studies on intersphere PL propagation with multicoupled microspheres, prepared by micromanipulation technique, revealed that multistep photon transfer takes place through the microspheres, accompanying energy transfer cascade with stepwise PL color change. The intersphere energy transfer cascade is direction selective, where energy donor-to-acceptor down conversion direction is only allowed. Such cavity-mediated long-distance and multistep energy transfer will be advantageous for polymer photonics device application. © 2016 American Chemical Society.
    view abstract10.1021/acsnano.6b03188
  • Conjugated Polymer Blend Microspheres for Efficient, Long-Range Light Energy Transfer
    Kushida, S. and Braam, D. and Dao, T.D. and Saito, H. and Shibasaki, K. and Ishii, S. and Nagao, T. and Saeki, A. and Kuwabara, J. and Kanbara, T. and Kijima, M. and Lorke, A. and Yamamoto, Y.
    ACS Nano 10 (2016)
    Highly luminescent π-conjugated polymeric microspheres were fabricated through self-assembly of energy-donating and energy-accepting polymers and their blends. To avoid macroscopic phase separation, the nucleation time and growth rate of each polymer in the solution were properly adjusted. Photoluminescence (PL) studies showed that efficient donor-to-acceptor energy transfer takes place inside the microspheres, revealing that two polymers are well-blended in the microspheres. Focused laser irradiation of a single microsphere excites whispering gallery modes (WGMs), where PL generated inside the sphere is confined and resonates. The wavelengths of the PL lines are finely tuned by changing the blending ratio, accompanying the systematic yellow-to-red color change. Furthermore, when several microspheres are coupled linearly, the confined PL propagates the microspheres through the contact point, and a cascade-like process converts the PL color while maintaining the WGM characteristics. The self-assembly strategy for the formation of polymeric nano- to microstructures with highly miscible polymer blends will be advantageous for optoelectronic and photonic device applications. © 2016 American Chemical Society.
    view abstract10.1021/acsnano.6b02100
  • Electronic properties of freestanding Ti3C2Tx MXene monolayers
    Miranda, A. and Halim, J. and Barsoum, M.W. and Lorke, A.
    Applied Physics Letters 108 (2016)
    We report on the electrical characterization of single MXene Ti3C2Tx flakes (where T is a surface termination) and demonstrate the metallic nature of their conductivities. We also show that the carrier density can be modulated by an external gate voltage. The density of free carriers is estimated to be 8 ± 3 × 1021 cm-3 while their mobility is estimated to be 0.7 ± 0.2 cm2/V s. Electrical measurements, in the presence of a magnetic field, show a small, but clearly discernable, quadratic increase in conductance at 2.5 K. © 2016 AIP Publishing LLC.
    view abstract10.1063/1.4939971
  • Enwrapping conjugated polymer microspheres with graphene oxide nanosheets
    Aikyo, Y. and Kushida, S. and Braam, D. and Kuwabara, J. and Kondo, T. and Kanbara, T. and Nakamura, J. and Lorke, A. and Yamamoto, Y.
    Chemistry Letters 45 (2016)
    Water-dispersible conjugated polymer microspheres were obtained by enwrapping with graphene oxide (GO) nanosheets. Simply mixing the polymer microspheres and GO in water results in an exclusive formation of GO-wrapped microspheres. The photoluminescence (PL) spectra of the GO-wrapped single microsphere show whispering gallery modes, in which the PL lines are broadened in comparison with bare microspheres without GO. The broadening is attributed to scattering and reabsorption of the confined PL. © 2016 The Chemical Society of Japan.
    view abstract10.1246/cl.160504
  • Irradiation of graphene field effect transistors with highly charged ions
    Ernst, P. and Kozubek, R. and Madauß, L. and Sonntag, J. and Lorke, A. and Schleberger, M.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 382 (2016)
    In this work, graphene field-effect transistors are used to detect defects due to irradiation with slow, highly charged ions. In order to avoid contamination effects, a dedicated ultra-high vacuum set up has been designed and installed for the in situ cleaning and electrical characterization of graphene field-effect transistors during irradiation. To investigate the electrical and structural modifications of irradiated graphene field-effect transistors, their transfer characteristics as well as the corresponding Raman spectra are analyzed as a function of ion fluence for two different charge states. The irradiation experiments show a decreasing mobility with increasing fluences. The mobility reduction scales with the potential energy of the ions. In comparison to Raman spectroscopy, the transport properties of graphene show an extremely high sensitivity with respect to ion irradiation: a significant drop of the mobility is observed already at fluences below 15 ions/μm2, which is more than one order of magnitude lower than what is required for Raman spectroscopy. © 2016 Elsevier B.V.
    view abstract10.1016/j.nimb.2016.03.043
  • Optical Blocking of Electron Tunneling into a Single Self-Assembled Quantum Dot
    Kurzmann, A. and Merkel, B. and Labud, P.A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M. P.
    Physical Review Letters 117 (2016)
    Time-resolved resonance fluorescence (RF) is used to analyze electron tunneling between a single self-assembled quantum dot (QD) and an electron reservoir. In equilibrium, the RF intensity reflects the average electron occupation of the QD and exhibits a gate voltage dependence that is given by the Fermi distribution in the reservoir. In the time-resolved signal, however, we find that the relaxation rate for electron tunneling is, surprisingly, independent of the occupation in the charge reservoir - in contrast to results from all-electrical transport measurements. Using a master equation approach, which includes both the electron tunneling and the optical excitation or recombination, we are able to explain the experimental data by optical blocking, which also reduces the electron tunneling rate when the QD is occupied by an exciton. © 2016 American Physical Society.
    view abstract10.1103/PhysRevLett.117.017401
  • Optically induced mode splitting in self-assembled, high quality-factor conjugated polymer microcavities
    Braam, D. and Kushida, S. and Niemöller, R. and Prinz, G.M. and Saito, H. and Kanbara, T. and Kuwabara, J. and Yamamoto, Y. and Lorke, A.
    Scientific Reports 6 (2016)
    We investigate the whispering gallery modes (WGMs) of self-assembled single microspheres. They consist of a recently developed highly fluorescent €-conjugated copolymer and exhibit excellent optical properties with Q-factors up to 10 4. Under continuous laser irradiation, we observe a splitting of the highly degenerate spherical WGMs into a multiplet of lines. Comparison with the calculated spectral response of a weakly distorted sphere shows that the optical excitation induces a change of the optical path length in the microcavity so that it resembles a prolate spheroid. The separation of the lines is given by the ellipticity and the azimuthal mode number. Measurements in various gaseous environments suggest that the distortion is caused by light induced oxidation of the polymer. Our findings show that photooxidation can be a beneficial mechanism for in-situ tuning of optically active polymer structures.
    view abstract10.1038/srep19635
  • Photoelectron generation and capture in the resonance fluorescence of a quantum dot
    Kurzmann, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Applied Physics Letters 108 (2016)
    Time-resolved resonance fluorescence on a single self-assembled quantum dot (QD) is used to analyze the generation and capture of photoinduced free charge carriers. We directly observe the capture of electrons into the QD as an intensity reduction of the exciton transition. The exciton transition is quenched until the captured electron tunnels out of the dot again in the order of milliseconds. Our results demonstrate that even under resonant excitation, excited free electrons are generated and can negatively influence the optical properties of a QD. © 2016 Author(s).
    view abstract10.1063/1.4954944
  • Quantum confinement in EuO heterostructures
    Prinz, G.M. and Gerber, T. and Lorke, A. and Müller, M.
    Applied Physics Letters 109 (2016)
    Quantum wells are created from ultrathin single-crystalline EuO layers to study the evolution of the optical band gap down to the single nanometer regime. We find that the EuO band gap is indirect - independent of quantum well thickness - and increases from 1.19 eV for bulk-like (d = 32 nm) to ≈1.4 eV in the ultrathin films (d = 1.1 nm). The observed band-gap widening is a clear sign of a quantum confinement effect, which can be used to control and modify the band gap in EuO-based all-oxide heterostructures. © 2016 Author(s).
    view abstract10.1063/1.4966223
  • Charge storage in β -FeSi2 nanoparticles
    Theis, J. and Bywalez, R. and Küpper, S. and Lorke, A. and Wiggers, H.
    Journal of Applied Physics 117 (2015)
    We report on the observation of a surprisingly high specific capacitance of β-FeSi2 nanoparticle layers. Lateral, interdigitated capacitor structures were fabricated on thermally grown silicon dioxide and covered with β-FeSi2 particles by drop or spin casting. The β-FeSi2-nanoparticles, with crystallite sizes in the range of 10-30nm, were fabricated by gas phase synthesis in a hot wall reactor. Compared to the bare electrodes, the nanoparticle-coated samples exhibit a 3-4 orders of magnitude increased capacitance. Time-resolved current voltage measurements show that for short times (seconds to minutes), the material is capable of storing up to 1 As/g at voltages of around 1V. The devices are robust and exhibit long-term stability under ambient conditions. The specific capacitance is highest for a saturated relative humidity, while for a relative humidity below 40% the capacitance is almost indistinguishable from a nanoparticle-free reference sample. The devices work without the need of a fluid phase, the charge storing material is abundant and cost effective, and the sample design is easy to fabricate. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4906500
  • Edge magnetotransport in graphene: A combined analytical and numerical study
    Stegmann, T. and Lorke, A.
    Annalen der Physik 527 (2015)
    The current flow along the boundary of graphene stripes in a perpendicular magnetic field is studied theoretically by the nonequilibrium Green's function method. In the case of specular reflections at the boundary, the Hall resistance shows equidistant peaks, which are due to classical cyclotron motion. When the strength of the magnetic field is increased, anomalous resistance oscillations are observed, similar to those found in a nonrelativistic 2D electron gas [New. J. Phys. 15:113047 (2013)]. Using a simplified model, which allows to solve the Dirac equation analytically, the oscillations are explained by the interference between the occupied edge states causing beatings in the Hall resistance. A rule of thumb is given for the experimental observability. Furthermore, the local current flow in graphene is affected significantly by the boundary geometry. A finite edge current flows on armchair edges, while the current on zigzag edges vanishes completely. The quantum Hall staircase can be observed in the case of diffusive boundary scattering. The number of spatially separated edge channels in the local current equals the number of occupied Landau levels. The edge channels in the local density of states are smeared out but can be made visible if only a subset of the carbon atoms is taken into account. © 2015 by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/andp.201500124
  • Electron-beam induced nano-etching of suspended graphene
    Sommer, B. and Sonntag, J. and Ganczarczyk, A. and Braam, D. and Prinz, G. and Lorke, A. and Geller, M.
    Scientific Reports 5 (2015)
    Besides its interesting physical properties, graphene as a two-dimensional lattice of carbon atoms promises to realize devices with exceptional electronic properties, where freely suspended graphene without contact to any substrate is the ultimate, truly two-dimensional system. The practical realization of nano-devices from suspended graphene, however, relies heavily on finding a structuring method which is minimally invasive. Here, we report on the first electron beam-induced nano-etching of suspended graphene and demonstrate high-resolution etching down to ~7 nm for line-cuts into the monolayer graphene. We investigate the structural quality of the etched graphene layer using two-dimensional (2D) Raman maps and demonstrate its high electronic quality in a nano-device: A 25 nm-wide suspended graphene nanoribbon (GNR) that shows a transport gap with a corresponding energy of ~60 meV. This is an important step towards fast and reliable patterning of suspended graphene for future ballistic transport, nano-electronic and nano-mechanical devices.
    view abstract10.1038/srep07781
  • The effect of charged quantum dots on the mobility of a two-dimensional electron gas: How important is the Coulomb scattering?
    Kurzmann, A. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M.
    Journal of Applied Physics 117 (2015)
    We have investigated the influence of a layer of charged self-assembled quantum dots (QDs) on the mobility of a nearby two-dimensional electron gas (2DEG). Time-resolved transconductance spectroscopy was used to separate the two contributions of the change in mobility, which are: (i) The electrons in the QDs act as Coulomb scatterers for the electrons in the 2DEG. (ii) The screening ability and, hence, the mobility of the 2DEG decreases when the charge carrier density is reduced by the charged QDs, i.e., the mobility itself depends on the charge carrier concentration. Surprisingly, we find a negligible influence of the Coulomb scattering on the mobility for a 2DEG, separated by a 30nm tunneling barrier to the layer of QDs. This means that the mobility change is completely caused by depletion, i.e., reduction of the charge carrier density in the 2DEG, which indirectly influences the mobility. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4907217
  • The influence of different pre-treatments of current collectors and variation of the binders on the performance of Li4Ti5O12 anodes for lithium ion batteries
    Wennig, S. and Langklotz, U. and Prinz, G.M. and Schmidt, A. and Oberschachtsiek, B. and Lorke, A. and Heinzel, A.
    Journal of Applied Electrochemistry 45 (2015)
    In order to optimize the electron transfer between the Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf>-based active mass and the current collector, the surface of aluminum foil was modified either by alkaline etching or by a carbon coating. The as-modified aluminum foils were coated with an active mass of Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf> mixed with polyvinylidene fluoride, sodium carboxymethyl cellulose, or polyacrylic acid as binders. Untreated aluminum and copper foils served as reference current collectors. The corrosion reactions of aluminum foil with the applied binder solutions were studied and the electrode structure has been analyzed, depending on the binder. Finally, the electrochemical performance of the prepared electrodes was investigated. Based on these measurements, conclusions concerning the electrical contact between the different current collectors and the active masses were drawn. The energy density of the Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf> electrodes cast on carbon-coated aluminum foils was significantly increased, compared to the corresponding electrodes with a copper current collector. © 2015, Springer Science+Business Media Dordrecht.
    view abstract10.1007/s10800-015-0878-0
  • Tuning the tunneling probability between low-dimensional electron systems by momentum matching
    Zhou, D. and Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Geller, M. and Lorke, A.
    Applied Physics Letters 106 (2015)
    We demonstrate the possibility to tune the tunneling probability between an array of self- assembled quantum dots and a two-dimensional electron gas (2DEG) by changing the energy imbalance between the dot states and the 2DEG. Contrary to the expectation from Fowler-Nordheim tunneling, the tunneling rate decreases with increasing injection energy. This can be explained by an increasing momentum mismatch between the dot states and the Fermi-circle in the 2DEG. Our findings demonstrate momentum matching as a useful mechanism (in addition to energy conservation, density of states, and transmission probability) to electrically control the charge transfer between quantum dots and an electron reservoir. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4922738
  • Van der Waals epitaxial MOCVD-growth of (BixSb1-x)2Te3 (0 < x < 1) films
    Bendt, G. and Sonntag, J. and Lorke, A. and Assenmacher, W. and Hagemann, U. and Schulz, S.
    Semiconductor Science and Technology 30 (2015)
    Epitaxial (Bi<inf>x</inf>Sb<inf>1-x</inf>)<inf>2</inf>Te<inf>3</inf> films with (0 < x < 1) were grown by the metal-organic chemical vapour deposition (MOCVD) process at 400 °C using the tailor-made precursors Et<inf>2</inf>Te<inf>2</inf>, i-Pr<inf>3</inf>Sb and Et<inf>3</inf>Bi. The films grown on Al<inf>2</inf>O<inf>3</inf>(0001) substrates show a very smooth surface morphology as shown by a scanning electron microscope (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM), while those grown on Si(100) are rather polycrystalline. The chemical composition of the crystalline films (x-ray powder diffraction (XRD)) was investigated by energy-dispersive x-ray (EDX) and x-ray photoelectron spectroscopy (XPS), and the in-plane transport properties were measured, and a strong dependency from the bismuth content was found, which allows the tuning of the carrier concentration and mobility in a wide range. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0268-1242/30/8/085021
  • Whispering gallery mode photoemission from self-assembled poly-para-phenylenevinylene microspheres
    Kushida, S. and Braam, D. and Lorke, A. and Yamamoto, Y.
    AIP Conference Proceedings 1702 (2015)
    Poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMOPPV) self-assembles to form well-defined spheres with several micrometers in diameter upon addition of a methanol vapor into a chloroform solution of MDMOPPV. The single sphere of MDMOPPV with 5.7 μm diameter exhibits whispering gallery mode (WGM) photoemission upon excitation with focused laser beam. The periodic emission lines are characterized by transverse electric and magnetic WGMs, and Q-factor reaches ∼345 at the highest. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4938854
  • Whispering Gallery Resonance from Self-Assembled Microspheres of Highly Fluorescent Isolated Conjugated Polymers
    Kushida, S. and Braam, D. and Pan, C. and Dao, T.D. and Tabata, K. and Sugiyasu, K. and Takeuchi, M. and Ishii, S. and Nagao, T. and Lorke, A. and Yamamoto, Y.
    Macromolecules 48 (2015)
    Self-assembly of highly fluorescent isolated conjugated polymers (ICPs), comprising alternating phenylene moieties with an insulating cyclic side chain and different arylene moieties, was comprehensively studied. Two out of nine ICPs were identified to form well-defined spheres of 1-6 μm diameter. The degree of twisting of the main chains was found to be an important structural factor enabling formation of spheres, for which dihedral angles >50° between the neighboring arylene moieties were required. A single microsphere with high sphericity exhibited whispering gallery mode (WGM) photoemission upon excitation with a focused laser. In this emission, sharp and periodic emission lines were superimposed on a broad photoemission spectrum. The WGM spectral profiles were very sensitive to the integrity of the spherical geometries and surface smoothness, which depends on the self-assembling condition as well as the structure of the polymer backbone. Microspherical optical resonators consisting of such highly fluorescent conjugated polymers are novel. They also present advantages in that (i) there is no need for a light waveguide and fluorescent-dye doping, (ii) its high refractive index is beneficial for light confinement, and (iii) the fabrication process is simple, not requiring sophisticated, costly microfabrication technology. © 2015 American Chemical Society.
    view abstract10.1021/acs.macromol.5b00707
  • 3 ns single-shot read-out in a quantum dot-based memory structure
    Nowozin, T. and Beckel, A. and Bimberg, D. and Lorke, A. and Geller, M.
    Applied Physics Letters 104 (2014)
    Fast read-out of two to six charges per dot from the ground and first excited state in a quantum dot (QD)-based memory is demonstrated using a two-dimensional electron gas. Single-shot measurements on modulation-doped field-effect transistor structures with embedded InAs/GaAs QDs show read-out times as short as 3ns. At low temperature (T=4.2K) this read-out time is still limited by the parasitics of the setup and the device structure. Faster read-out times and a larger read-out signal are expected for an improved setup and device structure. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4864281
  • Asymmetry of charge relaxation times in quantum dots: The influence of degeneracy
    Beckel, A. and Kurzmann, A. and Geller, M. P. and Ludwig, Ar. and Wieck, A.D. and König, J. and Lorke, A.
    EPL 106 (2014)
    Using time-resolved transconductance spectroscopy, we study the tunneling dynamics between a two-dimensional electron gas (2DEG) and self-assembled quantum dots (QDs), embedded in a field-effect transistor structure. We find that the tunneling of electrons from the 2DEG into the QDs is governed by a different time constant than the reverse process, i.e., tunneling from the QDs to the 2DEG. This asymmetry is a clear signature of Coulomb interaction and makes it possible to determine the degeneracy of the quantum-dot orbitals even when the individual states cannot be resolved energetically because of inhomogeneous broadening. Our experimental data can be qualitatively explained within a master-equation approach. © CopyrightEPLA, 2014.
    view abstract10.1209/0295-5075/106/47002
  • Control of molecular orientation and morphology in organic bilayer solar cells: Copper phthalocyanine on gold nanodots
    Sasaki, T. and Tabata, K. and Tsukagoshi, K. and Beckel, A. and Lorke, A. and Yamamoto, Y.
    Thin Solid Films 562 (2014)
    Molecular orientation, morphology of donor (D)/acceptor (A) interface and photoabsorptivity in organic bilayer solar cells were controlled using Au nanodots with an ∼ 20 nm diameter inserted between the bottom electrode and the organic layer. Copper phthalocyanine (CuPc) molecules deposited onto the Au nanodot-coated electrode were mostly oriented face-on with large surface roughness, which is beneficial for photoabsorption, charge separation and transport. Furthermore, Au nanodots exhibit blue-shifted plasmon bands so that CuPc absorbs light more efficiently than that on thin Au layer. Bilayer C 60/CuPc solar cells containing Au nanodots exhibited 1.4 times higher photoelectric conversion efficiency than those without Au nanodots. Factors for the enhanced efficiency are (i) improvement of the optical absorption characteristics by face-on orientation of CuPc and (ii) increase of the D/A heterointerface area. In addition, the shift of the plasmon absorption band of Au by the formation of nanodots makes absorption of the CuPc layer much more efficiently, resulting in better photovoltaic output. © 2014 Elsevier B.V.
    view abstract10.1016/j.tsf.2014.03.090
  • GaSb quantum dots on GaAs with high localization energy of 710 meV and an emission wavelength of 1.3 μm
    Richter, J. and Strassner, J. and Loeber, T.H. and Fouckhardt, H. and Nowozin, T. and Bonato, L. and Bimberg, D. and Braam, D. and Lorke, A.
    Journal of Crystal Growth 404 (2014)
    We report on the epitaxial growth of GaSb quantum dots (QDs) that show photoluminescence at a wavelength of around 1.3 μm and a high hole localization energy of up to 710 meV with a capture cross-section of 1×10-13cm2. The QDs were grown in Stranski-Krastanov (SK) epitaxial mode with molecular beam epitaxy. The characteristics of these QDs are a high dot density of up to 2.6×1010cm-2as well as narrow dot size and dot density distributions. To achieve the desired values for emission wavelength, hole localization energy, and dot density, the influence of the growth parameters must be controlled precisely. The influence of the V/III (i.e. Sb/Ga) partial pressure or flux ratio, growth temperature, nominal coverage, and growth interruption after quantum dot deposition are investigated. The QD samples are analyzed with atomic force microscopy (AFM), photoluminescence (PL), and deep-level transient spectroscopy (DLTS). Theoretical simulations are performed with the nextnano++ software. © 2014 Elsevier B.V.
    view abstract10.1016/j.jcrysgro.2014.06.045
  • Growth and spectroscopy of semiconductor quantum rings
    Lei, W. and Lorke, A.
    NanoScience and Technology 87 (2014)
    Quantum rings are unique nanostructures as they are topologically not simply connected and therefore different from most other low-dimensional systems such as quantum dots, quantum wires or quantum wells. This topology gives rise to an intriguing energy structure, in particular, when a magnetic field is applied such that a flux can penetrate through the ring’s interior. Flux quantization will lead to a ground state, which has a non-vanishing angular momentum, and the intraband transitions are affected by the corresponding change in dipole-allowed transitions. Quantum rings, which are of the order of 10 nm in size are of particular interest, because they make it possible to study these systems in the true quantum limit. In this chapter, we will review the growth techniques, which lead to the selforganized formation of quantum rings of a few tens of nanometers in diameter. The mechanisms will be discussed, which ‘invert’ the geometry of InAs islands, grown in the Stranski-Krastanov mode on GaAs, when they are partially capped with GaAs. When the thus formed nanorings are embedded in a suitable heterostructure, they can be electrically tuned and carriers can be injected with single-electron/single-hole precision. We will discuss, how the number of carriers and the strength of the applied field influence the single-particle and many-particle ground states, which can be probed by capacitance-voltage measurements. Also, far-infrared absorption spectra will be presented, which show the influence of flux quantization on the intraband transitions. These spectroscopic techniques, together with photoluminescence data obtained on single rings as well as on ring ensembles, make it possible to obtain an in-depth view into the detailed energetic structure of nanoscopic rings. © Springer-Verlag Berlin Heidelberg 2014.
    view abstract10.1007/978-3-642-39197-2_2
  • 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
  • Self-assembled conjugated polymer spheres as fluorescent microresonators
    Tabata, K. and Braam, D. and Kushida, S. and Tong, L. and Kuwabara, J. and Kanbara, T. and Beckel, A. and Lorke, A. and Yamamoto, Y.
    Scientific Reports 4 (2014)
    Confinement of light inside an active medium cavity can amplify emission. Whispering gallery mode (WGM) is one of mechanisms that amplifies light effectively by confining it inside high-refractive-index microstructures, where light propagates along the circumference of a sphere via total internal reflection. Here we show that isolated single microspheres of 2-10 μ4m diameter, formed from self-assembly of π-conjugated alternating copolymers, display WGM photoemission induced by laser pumping. The wavelengths of the emission peaks depend sensitively on the sphere size, position of the excitation spot and refractive index of each polymer. The Q-factor increases with increasing sphere diameter and displays a linear correlation with the reciprocal radius, indicating that the small curvature increases the efficacy of the total internal reflection. WGM photoemission from π-conjugated polymer microspheres is unprecedented and may be of high technological impact since the microspheres fulfill the role of fluorophores, high-refractive-index media and resonators simultaneously, in addition to their simple fabrication process.
    view abstract10.1038/srep05902
  • Time-resolved transconductance spectroscopy on self-assembled quantum dots: Spectral evolution from single- into many-particle states
    Beckel, A. and Ludwig, Ar. and Wieck, A.D. and Lorke, A. and Geller, M. P.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    Using transconductance spectroscopy we study the tunneling dynamics of electrons from a two-dimensional electron gas (2DEG) into excited and ground states of a layer of self-assembled InAs quantum dots (QDs). From an initially selected nonequilibrium condition, we observe the charging dynamics of the QD states and their spectral evolution for one- and two-electron configurations. Furthermore, we measure the electron emission from the QD states into the 2DEG for the corresponding evolution of the QD-hydrogen and QD-helium spectra. The comparison with theoretically predicted energies, as well as the evaluation of the dynamics in charging and emission, allows us to separate and identify ground and excited electron configurations in the spectral evolution and discuss in detail the observed maxima in the different spectra. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.155430
  • Wave functions of elliptical quantum dots in a magnetic field
    Zhou, D. and Lorke, A.
    American Journal of Physics 83 (2014)
    We use the variational principle to obtain the wave functions of elliptical quantum dots under the influence of an external magnetic field. For the first excited states, whose wave functions have recently been mapped experimentally, we find a simple expression, based on a linear combination of the wave functions in the absence of a magnetic field. The results illustrate how a magnetic field breaks the x-y symmetry and mixes the corresponding eigenstates. The obtained eigenenergies agree well with those obtained by more involved analytical and numerical methods. © 2015 American Association of Physics Teachers.
    view abstract10.1119/1.4898790
  • Magnetotransport along a boundary: From coherent electron focusing to edge channel transport
    Stegmann, T. and Wolf, D.E. and Lorke, A.
    New Journal of Physics 15 (2013)
    We study theoretically how electrons, coherently injected at one point on the boundary of a two-dimensional electron system, are focused by a perpendicular magnetic field B onto another point on the boundary. Using the non-equilibrium Green's function approach, we calculate the generalized four-point Hall resistance Rxy as a function of B. In weak fields, Rxy shows the characteristic equidistant peaks observed in the experiment and explained by classical cyclotron motion along the boundary. In strong fields, Rxy shows a single extended plateau reflecting the quantum Hall effect. In intermediate fields, we find superimposed upon the lower Hall plateaus anomalous oscillations, which are neither periodic in 1/B (quantum Hall effect) nor in B (classical cyclotron motion). The oscillations are explained by the interference between the occupied edge channels, which causes beatings in Rxy. In the case of two occupied edge channels, these beatings constitute a new commensurability between the magnetic flux enclosed within the edge channels and the flux quantum. Introducing decoherence and a partially specular boundary shows that this new effect is quite robust. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/15/11/113047
  • Role of the ligand layer for photoluminescence spectral diffusion of CdSe/ZnS nanoparticles
    Braam, D. and Mölleken, A. and Prinz, G.M. and Notthoff, C. and Geller, M. P. and Lorke, A.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    The time-resolved photoluminescence (PL) characteristics of single CdSe/ZnS nanoparticles, embedded in a poly(methyl methacrylate) layer, is studied at room temperature. We observe a strong spectral jitter of up to 55 meV, which is correlated with a change in the observed linewidth. We evaluate this correlation effect using a simple model, based on the quantum confined Stark effect induced by a diffusing charge in the vicinity of the nanoparticle. This allows us to derive a mean distance between the center of the particle and the diffusing charge of approximately 5 nm on average, as well as a mean charge carrier displacement within the integration time. These results are reproducible, even for particles which exhibit strong blueing, with shifts of up to 150 meV. Both the statistics and its independence of core-shell alterations lead us to conclude that the charge causing the spectral jitter is situated in the ligands or on its surface. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.125302
  • Electrical transport in semiconductor nanoparticle arrays: Conductivity, sensing and modeling
    Hartner, S. and Schwesig, D. and Plümel, I. and Wolf, D.E. and Lorke, A. and Wiggers, H.
    NanoScience and Technology 79 (2012)
    Electrical properties of nanoparticle ensembles are dominated by interparticle transport processes, mainly due to particle-particle and particle-contact interactions. Thismakes their electrical properties dependent on the network properties such as porosity and particle size and is a main prerequisite for solid- state gas sensors, as the surrounding gas atmosphere influences the depletion layer surrounding each particle. Different kinds of nanoparticle arrays such as pressed pellets, printed layer, and thin films prepared by molecular beam-assisted deposition are characterized with respect to their electrical transport properties. Experimental results are shown for the electrical and sensing properties of several metal oxide nanoparticle ensembles and the influence of porosity is investigated during compaction of nanoparticle powders exposed to an external force. A model describing these properties is developed and it is shown that for a given material only porosity, geometry, and particle size influence the overall electrical properties. The model developed for the description of current transport in particulate matter can also be utilized to describe current-assisted sintering. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-28546-2_10
  • Manipulation of electronic transport in the Bi(111) surface state
    Jnawali, G. and Klein, C. and Wagner, T. and Hattab, H. and Zahl, P. and Acharya, D.P. and Sutter, P. and Lorke, A. and Horn-Von Hoegen, M.
    Physical Review Letters 108 (2012)
    We demonstrate the controlled manipulation of the 2D-electronic transport in the surface state of Bi(111) through the deposition of small amounts of Bi to generate adatoms and 2D islands as additional scatterers. The corresponding increase in resistance is recorded in situ and in real time. Model calculations based on mean-field nucleation theory reveal a constant scattering efficiency of adatoms and of small 2D Bi islands, independent of their size. This finding is supported by a detailed scanning tunneling microscopy and spectroscopy study at 5 K which shows a highly anisotropic scattering pattern surrounding each surface protrusion. © 2012 American Physical Society.
    view abstract10.1103/PhysRevLett.108.266804
  • Momentum matching in the tunneling between 2-dimensional and 0-dimensional electron systems
    Beckel, A. and Zhou, D. and Marquardt, B. and Reuter, D. and Wieck, A.D. and Geller, M. and Lorke, A.
    Applied Physics Letters 100 (2012)
    We investigate the tunneling rates from a 2-dimensional electron gas (2DEG) into the ground state of self-assembled InGaAs quantum dots. These rates are strongly affected by a magnetic field perpendicular to the tunneling direction. Surprisingly, we find an increase in the rates for fields up to 4 T before they decrease again. This can be explained by a mismatch between the characteristic momentum of the quantum dot ground state and the Fermi momentum k F of the 2DEG. Calculations of the tunneling probability can account for the experimental data and allow us to determine the dot geometry as well as k F. © 2012 American Institute of Physics.
    view abstract10.1063/1.4728114
  • Nanoparticles from the Gas Phase: Formation, Structure, Properties
    Lorke, A. and Winterer, M. and Schmechel, R. and Schulz, C.
    NanoScience and Technology 79 (2012)
    view abstract10.1007/978-3-642-28546-2
  • Optical properties of silicon Nanoparticles
    Meier, C. and Lorke, A.
    NanoScience and Technology 79 (2012)
    This chapter reviews recent results on optical spectroscopy on silicon nanoparticles. The quantum confinement effect causing a spectral shift of the photoluminescence together with an intensity enhancement is discussed. The small spatial dimensions lead not only to a change of the electronic states, but affect also the vibronic spectrum as is seen in results on first- and second-order Raman scattering. Using time-resolved spectroscopy, the excitonic fine structure of silicon nanoparticles is investigated and a crossover of bright and dark exciton states is found. The analysis of the recombination dynamics allows to determine the size-dependence of the oscillator strength, which is in the order of 10 -5 and increases with decreasing particle size. Finally,we demonstrate an electroluminescence device based on silicon particles using impact ionization. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-28546-2_9
  • Thermoreflectance imaging of percolation effects and dynamic resistance in indium tin oxide nanoparticle layers
    Chavez, R. and Angst, S. and Maize, K. and Gondorf, A. and Schierning, G. and Wolf, D.E. and Lorke, A. and Shakouri, A.
    Journal of Applied Physics 112 (2012)
    Thin films of indium tin oxide nanoparticles are studied using charge-coupled device thoermoreflectance. High resolution sub-micron thermal images confirm that percolation in current conduction induces strongly inhomogeneous heat loads on the thin film. We experimentally show that the inhomogeneous current densities induce thousands of micro-hotspots that can be 20 hotter than the average Joule heating in the thin film layer and show comparable behavior in a resistor network. In addition to the percolation induced micro-hotspots, we report major hotspots, with non-Joule behavior, whose temperature response is greater than I 2. We demonstrate that a temperature dependent resistor can account for an effective exponent larger than 2. Finally, it is shown that while ambient molecules modify the thin film conductivity by at least 20, current conduction and percolation effects remain largely unchanged, but such chemical reactions can be nonetheless detected with thermoreflectance. © 2012 American Institute of Physics.
    view abstract10.1063/1.4757960
  • Time-resolved detection of many-particle hole states in InAs/GaAs quantum dots using a two-dimensional hole gas up to 77 K
    Nowozin, T. and Marent, A. and Bimberg, D. and Beckel, A. and Marquardt, B. and Lorke, A. and Geller, M.
    Physica Status Solidi (C) Current Topics in Solid State Physics 9 (2012)
    We demonstrate the detection of many-particle hole states in self-organized InAs/GaAs quantum dots (QDs) using an adjacent two-dimensional hole gas (2DHG) as detector for temperatures up to 77 K. capacitance-voltage (C-V) measurements as well as time-resolved current measurements in the 2DHG resolve a structure of six distinct peaks which are related to the many-particle hole states in the QD ensemble. The time constants of the capture and emission processes for each individual many-particle hole state are extracted and the underlying emission processes are identified. An equivalent circuit model yields the gate voltage depedent lever-arm and the level-splittings of the many-particle hole states. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssc.201100218
  • Transverse rectification in density-modulated two-dimensional electron gases
    Ganczarczyk, A. and Rojek, S. and Quindeau, A. and Geller, M. P. and Hucht, A. and Notthoff, C. and König, J. and Lorke, A. and Reuter, D. and Wieck, A.D.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    We demonstrate tunable transverse rectification in a density-modulated two-dimensional electron gas (2DEG). The density modulation is induced by two surface gates, running in parallel along a narrow stripe of 2DEG. A transverse voltage in the direction of the density modulation is observed, i.e., perpendicular to the applied source-drain voltage. The polarity of the transverse voltage is independent of the polarity of the source-drain voltage, demonstrating rectification in the device. We find that the transverse voltage U y depends quadratically on the applied source-drain voltage and nonmonotonically on the density modulation. The experimental results are discussed in the framework of a diffusion thermopower model. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.085309
  • All-electrical transport spectroscopy of non-equilibrium many-particle states in self-assembled quantum dots
    Marquardt, B. and Geller, M. and Baxevanis, B. and Pfannkuche, D. and Wieck, A.D. and Reuter, D. and Lorke, A.
    AIP Conference Proceedings 1399 (2011)
    We demonstrate an all-electrical prepartion and detection of many-particle spin states in self-assembled QDs. The excited states of the first three 'QD elements' are measured, i. e. the degenerate s-, p- and d-shells of QD-Hydrogen (one electron), the many-particle triplet and singlet states of QD-Helium (two electrons) and the many-particle states of QD-Lithium (three electrons). Comparison with exact diagonalization calculations fully accounting for the effects of Coulomb interaction enables us to reveal unambiguously the different charge and spin configurations. © 2011 American Institute of Physics.
    view abstract10.1063/1.3666398
  • Edge-induced magnetoplasmon excitation in a two-dimensional electron gas under quantum Hall conditions
    Notthoff, C. and Reuter, D. and Wieck, A.D. and Lorke, A.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    The spectrally resolved terahertz photoconductivity between two separately contacted edge channels of a two-dimensional electron gas in the quantum Hall regime is investigated. We use a not-simply-connected sample geometry which is topologically equivalent to a Corbino disk. Due to the high sensitivity of our sample structure, a weak resonance situated on the high-energy side of the well known cyclotron resonance is revealed. The magnetic field as well as the carrier density dependence of this weak resonance, in comparison with different models, suggests that the additional resonance is an edge-induced magnetoplasmon. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.035311
  • Energy transport by neutral collective excitations at the quantum hall edge
    Deviatov, E.V. and Lorke, A. and Biasiol, G. and Sorba, L.
    Physical Review Letters 106 (2011)
    We use the edge of the quantum Hall sample to study the possibility for counterpropagating neutral collective excitations. A novel sample design allows us to independently investigate charge and energy transport along the edge. We experimentally observe an upstream energy transfer with respect to the electron drift for the filling factors 1 and 1/3. Our analysis indicates that a neutral collective mode at the interaction-reconstructed edge is a proper candidate for the experimentally observed effect. © 2011 American Physical Society.
    view abstract10.1103/PhysRevLett.106.256802
  • Mobility and carrier density in nanoporous indium tin oxide films
    Gondorf, A. and Geller, M. P. and Weißbon, J. and Lorke, A. and Inhester, M. and Prodi-Schwab, A. and Adam, D.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    The charge-carrier concentration and mobility of various nanoporous indium tin oxide films have been studied by Hall measurements and optical spectroscopy. The carrier density obtained by Hall measurements is found to be significantly lower than that derived from optical spectroscopy. The results of these complementary measurement techniques thus support recent theoretical work on granular conducting layers by Kharitonov and Efetov, which predicts that the Hall carrier concentration n of such a system is reduced by a geometrical factor. As expected, the mobility inside the particles, derived from optical transmission spectra (22 - 35 cm2Vs), is much higher than the macroscopic Hall mobility (0.12-0.7cm2Vs). © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.212201
  • Quantum Hall Mach-Zehnder interferometer far beyond equilibrium
    Deviatov, E.V. and Ganczarczyk, A. and Lorke, A. and Biasiol, G. and Sorba, L.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    We experimentally realize a quantum Hall Mach-Zehnder interferometer that operates far beyond equilibrium. The operation of the interferometer is based on allowed intraedge elastic transitions within the same Landau sublevel in the regime of high imbalances between the copropagating edge states. Since every edge state is definitely connected with a certain Landau sublevel, the formation of the interference loop can be understood as a splitting and a further reconnection of a single edge state. We observe an Aharonov-Bohm type interference pattern even for small-size interferometers. This interference scheme demonstrates high visibility even at millivolt imbalances and survives in a wide temperature range. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.235313
  • Synthesis and ink-jet printing of highly luminescing silicon nanoparticles for printable electronics
    Gupta, A. and Khalil, A.S.G. and Offer, M. and Geller, M. P. and Winterer, M. and Lorke, A. and Wiggers, H.
    Journal of Nanoscience and Nanotechnology 11 (2011)
    The formation of stable colloidal dispersions of silicon nanoparticles (Si-NPs) is essential for the manufacturing of silicon based electronic and optoelectronic devices using cost-effective printing technologies. However, the development of Si-NPs based printable electronics has so far been hampered by the lack of long-term stability, low production rate and poor optical properties of Si- NPs ink. In this paper, we synthesized Si-NPs in a gas phase microwave plasma reactor with very high production rate, which were later treated to form a stable colloidal dispersion. These particles can be readily dispersed in a variety of organic solvents and the dispersion is stable for months. The particles show excellent optical properties (quantum yields of about 15%) and long-term photoluminescence (PL) stability. The stable ink containing functionalized Si-NPs was successfully used to print structures on glass substrates by ink-jet printing. The homogeneity and uniformity of large-area printed film was investigated using photoluminescence (PL) mapping. Copyright © 2011 American Scientific Publishers.
    view abstract10.1166/jnn.2011.4184
  • The influence of charged InAs quantum dots on the conductance of a two-dimensional electron gas: Mobility vs. carrier concentration
    Marquardt, B. and Beckel, A. and Lorke, A. and Wieck, A.D. and Reuter, D. and Geller, M.
    Applied Physics Letters 99 (2011)
    Using time-resolved transport spectroscopy, we investigate the influence of charge-tunable InAs quantum dots (QDs) on the conductance of a nearby two-dimensional electron gas (2DEG). Loading successively electrons into the self-assembled QDs decreases the carrier concentration and mobility in the 2DEG. We are able to quantify how these transport properties change for each additional charge in the s- or p-shell. It is found that mobility and carrier concentration contribute equally to the overall change in conductance. © 2011 American Institute of Physics.
    view abstract10.1063/1.3665070
  • Time-resolved high-temperature detection with single charge resolution of holes tunneling into many-particle quantum dot states
    Nowozin, T. and Marent, A. and Hönig, G. and Schliwa, A. and Bimberg, D. and Beckel, A. and Marquardt, B. and Lorke, A. and Geller, M. P.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    We demonstrate detection of many-particle hole states in InAs/GaAs quantum dots with single charge resolution up to a temperature of 75 K. Capacitance-voltage measurements as well as time-resolved current measurements in an adjacent two-dimensional hole gas are used to determine the emission and capture time constants from 4 K up to 130 K. A transition from pure tunneling to thermally assisted tunneling is observed with increasing temperature. An equivalent circuit model gives access to the energy level splittings of the many-particle hole states and explains the broadening of the peaks at higher temperatures. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.075309
  • Transport spectroscopy of non-equilibrium many-particle spin states in self-assembled quantum dots
    Marquardt, B. and Geller, M. and Baxevanis, B. and Pfannkuche, D. and Wieck, A.D. and Reuter, D. and Lorke, A.
    Nature Communications 2 (2011)
    Self-assembled quantum dots (QDs) are prominent candidates for solid-state quantum information processing. For these systems, great progress has been made in addressing spin states by optical means. In this study, we introduce an all-electrical measurement technique to prepare and detect non-equilibrium many-particle spin states in an ensemble of self-assembled QDs at liquid helium temperature. The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method. An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum. These experiments are a starting point for an all-electrical control of electron spin states in self-assembled QDs above liquid helium temperature. © 2011 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms1205
  • XeF2 gas-assisted focused-electron-beaminduced etching of GaAs with 30 nm resolution
    Ganczarczyk, A. and Geller, M. P. and Lorke, A.
    Nanotechnology 22 (2011)
    We demonstrate the gas-assisted focused-electron-beam (FEB)-induced etching of GaAs with a resolution of 30 nm at room temperature. We use a scanning electron microscope (SEM) in a dual beam focused ion beam together with xenon difluoride (XeF2) that can be injected by a needle directly onto the sample surface. We show that the FEB-induced etching with XeF2 as a precursor gas results in isotropic and smooth etching of GaAs, while the etch rate depends strongly on the beam current and the electron energy. The natural oxide of GaAs at the sample surface inhibits the etching process; hence, oxide removal in combination with chemical surface passivation is necessary as a strategy to enable this high-resolution etching alternative for GaAs. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/22/4/045301
  • "Artificial atoms" in magnetic fields: Wave-function shaping and phase-sensitive tunneling
    Lei, W. and Notthoff, C. and Peng, J. and Reuter, D. and Wieck, A. and Bester, G. and Lorke, A.
    Physical Review Letters 105 (2010)
    We demonstrate the possibility to influence the shape of the wave functions in semiconductor quantum dots by the application of an external magnetic field Bz. The states of the so-called p shell, which show distinct orientations along the crystal axes for Bz=0, can be modified to become more and more circularly symmetric with an increasing field. Their changing probability density can be monitored using magnetotunneling wave function mapping. Calculations of the magnetotunneling signals are in good agreement with the experimental data and explain the different tunneling maps of the p+ and p- states as a consequence of the different sign of their respective phase factors. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevLett.105.176804
  • A 2D electron gas for studies on tunneling dynamics and charge storage in self-assembled quantum dots
    Marquardt, B. and Moujib, H. and Lorke, A. and Reuter, D. and Wieck, A.D. and Geller, M.
    Lecture Notes of the Institute for Computer Sciences, Social-Informatics and Telecommunications Engineering 36 LNICST (2010)
    The carrier tunneling dynamics of self-assembled InAs quantum dots (QD) is studied using time-resolved conductance measurements of a nearby two-dimensional electron gas (2DEG). The coupling strength (tunneling time) between the QDs and the 2DEG is adjusted by different thicknesses of the spacer layers. We demonstrate a strong influence of charged QDs on the conductance on the 2DEG, even for very weak coupling, where standard C-V spectroscopy is unsuitable to investigate the electronic structure of these QDs. © Institute for Computer Sciences, Social-Informatics and Telecommunications Engineering 2010.
    view abstract10.1007/978-3-642-11731-2_22
  • A two-dimensional electron gas as a sensitive detector for time-resolved tunneling measurements on self-assembled quantum dots
    Geller, M. and Marquardt, B. and Lorke, A. and Reuter, D. and Wieck, A.D.
    Nanoscale Research Letters 5 (2010)
    A two-dimensional electron gas (2DEG) situated nearby a single layer of self-assembled quantum dots (QDs) in an inverted high electron mobility transistor (HEMT) structure is used as a detector for time-resolved tunneling measurements. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG which allows us to probe the tunneling dynamics between the 2DEG and the QDs time resolved. Measurements of hysteresis curves with different sweep times and real-time conductance measurements in combination with an boxcar-like evaluation method enables us to unambiguously identify the transients as tunneling events between the s- and p-electron QD states and the 2DEG and rule out defect-related transients. © 2010 The Author(s).
    view abstract10.1007/s11671-010-9569-2
  • A two-dimensional electron gas as a sensitive detector to observe the charge carrier dynamics of self-assembled QDs
    Marquardt, B. and Geller, M. and Lorke, A. and Reuter, D. and Wieck, A.D.
    Physica E: Low-Dimensional Systems and Nanostructures 42 (2010)
    The carrier tunneling dynamics of self-assembled InAs quantum dots (QD) is studied using a time-resolved conductance measurement of a nearby two-dimensional electron gas (2DEG). The investigated heterostructures consist of a layer of QDs with different coupling strengths to a 2DEG, adjusted by different thicknesses of the spacer layers. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG, even for very weak coupling between the QD layer and the 2D system, where standard capacitance (C)voltage (V) spectroscopy is unsuitable to investigate the electronic structure of these QDs. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.physe.2010.02.010
  • A voltage-tunable in-plane diode in a two-dimensional-electron system
    Ganczarczyk, A. and Voßen, S. and Geller, M. and Lorke, A. and Reuter, D. and Wieck, A.D.
    Physica E: Low-Dimensional Systems and Nanostructures 42 (2010)
    We present a voltage-tunable in-plane diode in a nanoscale, two-dimensional electron system, whose functionality is mainly determined by the sample geometry. The diode consists of a narrow semiconductor channel, confined by etched insulating trenches. An applied voltage along the channel modulates the effective width of the conducting channel, depending on the sign of the applied voltage. This behavior results in a diode-like IV -characteristic. The tunability of the device is achieved by two in-plane side gates, which are able to widely tune the I (V) -characteristic of this rectifier. In the normally-off regime, this tunable in-plane diode works as a half-wave rectifier with sharply defined turn-on voltage. The value of the turn-on voltage depends on the side-gate voltage. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.physe.2009.10.018
  • Electroluminescence from silicon nanoparticles fabricated from the gas phase
    Theis, J. and Geller, M. and Lorke, A. and Wiggers, H. and Wieck, A. and Meier, C.
    Nanotechnology 21 (2010)
    Electroluminescence from as-prepared silicon nanoparticles, fabricated by gas phase synthesis, is demonstrated. The particles are embedded between an n-doped GaAs substrate and a semitransparent indium tin oxide top electrode. The total electroluminescence intensity of the Si nanoparticles is more than a factor of three higher than the corresponding signal from the epitaxial III-V semiconductor. This, together with the low threshold voltage for electroluminescence, shows the good optical properties of these untreated particles and the efficient electrical injection into the device. Impact ionization by electrons emitted from the top electrode is identified as the origin of the electrically driven light emission. © 2010 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/21/45/455201
  • Electronic structure of self-assembled InGaAs/GaAs quantum rings studied by capacitance-voltage spectroscopy
    Lei, W. and Notthoff, C. and Lorke, A. and Reuter, D. and Wieck, A.D.
    Applied Physics Letters 96 (2010)
    Self-assembled InGaAs quantum rings, embedded in a GaAs matrix, were investigated using magneto-capacitance-voltage spectroscopy. The magnetic-field dispersion of the charging energies exhibits characteristic features for both the first and second electron, which can be attributed to a ground state transition from l=0 into l=-1, and a ground state transition from l=-1 into l=-2, respectively. Furthermore, using a combination of capacitance-voltage spectroscopy and one-dimensional numerical simulations, the conduction band structure of these InGaAs quantum rings was determined. © 2010 American Institute of Physics.
    view abstract10.1063/1.3293445
  • Local investigation of the energy gap within the incompressible strip in the quantum Hall regime
    Deviatov, E.V. and Lorke, A. and Biasiol, G. and Sorba, L. and Wegscheider, W.
    JETP Letters 92 (2010)
    We experimentally study the energy gap within the incompressible strip at local filling factor νc = 1 at the quantum Hall edge for samples of very different mobilities. The obtained results indicate strong enhancement of the energy gap in comparison to the single-particle Zeeman splitting. We identify the measured gap as a mobility gap, so a pronounced experimental in-plane magnetic field dependence can both be attributed to the spin effects as well as to the change in the energy levels broadening. © 2010 Pleiades Publishing, Ltd.
    view abstract10.1134/S0021364010130138
  • Self-assembled quantum dots in a liquid-crystal-tunable microdisk resonator
    Piegdon, K.A. and Offer, M. and Lorke, A. and Urbanski, M. and Hoischen, A. and Kitzerow, H.-S. and Declair, S. and Frstner, J. and Meier, T. and Reuter, D. and Wieck, A.D. and Meier, C.
    Physica E: Low-Dimensional Systems and Nanostructures 42 (2010)
    GaAs-based semiconductor microdisks with high quality whispering gallery modes (Q>4000) have been fabricated. A layer of self-organized InAs quantum dots (QDs) served as a light source to feed the optical modes at room temperature. In order to achieve frequency tuning of the optical modes, the microdisk devices have been immersed in 4-cyano-4′-pentylbiphenyl (5CB), a liquid crystal (LC) with a nematic phase below the clearing temperature of TC≈34°C. We have studied the device performance in the temperature range of T=2050°C, in order to investigate the influence of the nematicisotropic phase transition on the optical modes. Moreover, we have applied an AC electric field to the device, which leads in the nematic phase to a reorientation of the anisotropic dielectric tensor of the liquid crystal. This electrical anisotropy can be used to achieve electrical tunability of the optical modes. Using the finite-difference time domain (FDTD) technique with an anisotropic material model, we are able to describe the influence of the liquid crystal qualitatively. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.physe.2009.12.051
  • THz-photoconductivity of quantum Hall systems in quasi-corbino-geometry
    Notthoff, C. and Lorke, A. and Rachor, K. and Heitmann, D. and Reuter, D.
    Journal of Low Temperature Physics 159 (2010)
    The THz-photoresponse (PR) between two separately contacted edge-channels of a two-dimensional electron gas in the quantum Hall regime is investigated. We use a not-simply-connected sample geometry, which is topologically equivalent to a ring shape (Corbino-geometry). At filling factors ν<2, spectrally resolved PR-measurements show a Lorentzian resonance, centered at the cyclotron-frequency, whereas above the integer filling factor, an asymmetric broadening is observed. Two independent contributions to the PR-signal can be resolved. One contribution clearly results from bolometric heating inside the bulk and the other one is caused by a non-bolometric mechanism. © 2010 Springer Science+Business Media, LLC.
    view abstract10.1007/s10909-009-0116-4
  • Tuning quantum-dot based photonic devices with liquid crystals
    Piegdon, K. A. and Declair, S. and Forstner, J. and Meier, T. and Matthias, H. and Urbanski, M. and Kitzerow, H. S. and Reuter, D. and Wieck, A. D. and Lorke, A. and Meier, C.
    Optics Express 18 (2010)
    Microdisks made from GaAs with embedded InAs quantum dots are immersed in the liquid crystal 4-cyano-4'-pentylbiphenyl (5CB). The quantum dots serve as emitters feeding the optical modes of the photonic cavity. By changing temperature, the liquid crystal undergoes a phase transition from the isotropic to the nematic state, which can be used as an effective tuning mechanism of the photonic modes of the cavity. In the nematic state, the uniaxial electrical anisotropy of the liquid crystal molecules can be exploited for orienting the material in an electric field, thus externally controlling the birefringence of the material. Using this effect, an electric field induced tuning of the modes is achieved. Numerical simulations using the finite-differences time-domain (FDTD) technique employing an anisotropic dielectric medium allow to understand the alignment of the liquid crystal molecules on the surface of the microdisk resonator. (C) 2010 Optical Society of America
    view abstract10.1364/OE.18.007946
  • Two-dimensional electron transport and scattering in Bi(111) surface states
    Jnawali, G. and Wagner, Th. and Hattab, H. and Möller, R. and Lorke, A. and Horn-Von Hoegen, M.
    e-Journal of Surface Science and Nanotechnology 8 (2010)
    The Bi(111) surface exhibits a pronounced surface state which acts as dominant transport channel for electric current. We performed in situ four-point probe resistance measurements for thin Bi(111) films on Si(001) to study electron scattering effects in this two-dimensional (2D) electron gas. The surface morphology was manipulated by additional deposition of Bi at 80 K. A linear increase of surface resistance was measured at extremely low coverage of less than 1 % of a bilayer (BL) and the slope gradually decreases with coverage up to about 0.5 BL. This behavior was qualitatively explained applying a simple picture of electron scattering at adatoms or small islands during the early stages of growth in Bi(111) homoepitaxy. Beyond 0.5 BL resistance changes periodically showing an antiphase correlation with roughness-induced LEED (00)-spot intensity oscillations, indicating the scattering of electrons at island edges. © 2010 The Surface Science Society of Japan.
    view abstract10.1380/ejssnt.2010.27
  • ballistic transport

  • nanostructures

  • optical properties

  • optical spectroscopy

  • quantum dots

  • quantum transport

  • self assembled quantum dots

  • self-organized nanostructures

  • semiconductors

  • transport

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