A. W. Holleitner

Universität Regensburg, Ratisbon, Bavaria, Germany

Are you A. W. Holleitner?

Claim your profile

Publications (110)577.57 Total impact

  • M. A. Mangold · A. W. Holleitner · J. S. Agustsson · M. Calame
    [Show abstract] [Hide abstract]
    ABSTRACT: Arrays of metal nanoparticles in an organic matrix have attracted a lot of interest due to their diverse electronic and optoelectronic properties. By varying parameters such as the nanoparticle material, the matrix material, the nanoparticle size, and the interparticle distance, the electronic behavior of the nanoparticle array can be substantially tuned and controlled. For strong tunnel coupling between adjacent nanoparticles, the assembly exhibits conductance properties similar to the bulk properties of the nanoparticle material. When the coupling between the nanoparticles is reduced, a metal insulator transition is observed in the overall assembly. Recent work demonstrates that nanoparticle arrays can be further utilized to incorporate single molecules, such that the nanoparticles act as electronic contacts to the molecules. Furthermore, via the excitation of the surface plasmon polaritons, the nanoparticles can be optically excited and electronically read out.
    No preview · Chapter · Jan 2016
  • Source
    Christoph Kastl · Paul Seifert · Xiaoyue He · Kehui Wu · Yongqing Li · Alexander Holleitner
    [Show abstract] [Hide abstract]
    ABSTRACT: A strong coupling between the electron spin and its motion is one of the prerequisites of spin-based data storage and electronics. A major obstacle is to find spin-orbit coupled materials where the electron spin can be probed and manipulated on macroscopic length scales, for instance across the gate channel of a spin-transistor. Here, we report on millimeter-scale edge channels with a conductance quantized at a single quantum 1 $\times$ $e^2/h$ at zero magnetic field. The quantum transport is found at the lateral edges of three-dimensional topological insulators made of bismuth chalcogenides. The data are explained by a lateral, one-dimensional quantum confinement of non-topological surface states with a strong Rashba spin-orbit coupling. This edge transport can be switched on and off by an electrostatic field-effect. Our results are fundamentally different from an edge transport in quantum spin Hall insulators and quantum anomalous Hall insula-tors.
    Preview · Article · Dec 2015
  • [Show abstract] [Hide abstract]
    ABSTRACT: MoS2 crystals exhibit excellent catalytic properties and great potential for photo-catalytic production of solar fuels such as hydrogen gas. In this regard, the photocatalytic stability of exfoliated single- and few-layer MoS2 immersed in water is investigated by µ-Raman spectroscopy. We find that while the basal plane of MoS2 can be treated as stable under photocatalytic conditions, the edge sites and presumably also defect sites are highly affected by a photo-induced corrosion process. The edge sites of MoS2 monolayers are significantly more resistant to photocatalytic degradation compared to MoS2 multi-layer edge sites. The photo-stability of MoS2 edge sites depends on the photon energy with respect to the band gap in MoS2 and also on the presence of oxygen in the electrolyte. These findings are interpreted in the framework of an oxidation process converting MoS2 into MoOx in the presence of oxygen and photo-induced charge carriers. The high stability of the MoS2 basal plane under photocatalytic treatment under visible light irradiation of extreme light intensities in the order of P ≈ 10mW/µm(2) substantiates MoS2's potential as photocatalyst for solar hydrogen production.
    No preview · Article · Nov 2015 · ACS Nano
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the ultrafast optoelectronic properties of single Al0.3Ga0.7As/GaAs-core-shell-nanowires. The nanowires contain GaAs-based quantum wells. For a resonant excitation of the quantum wells, we find a picosecond photocurrent which is consistent with an ultrafast lateral expansion of the photogenerated charge carriers. This Dember-effect does not occur for an excitation of the GaAs-based core of the nanowires. Instead, the core exhibits an ultrafast displacement current and a photo-thermoelectric current at the metal Schottky contacts. Our results uncover the optoelectronic dynamics in semiconductor core-shell nanowires comprising quantum wells, and they demonstrate the possibility to use the low-dimensional quantum well states therein for ultrafast photoswitches and photodetectors.
    Preview · Article · Sep 2015 · Nano Letters
  • [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the optoelectronic properties of single indium arsenide nanowires, which are grown vertically on p-doped silicon substrates. We apply a scanning photocurrent microscopy to study the optoelectronic properties of the single heterojunctions. The measured photocurrent characteristics are consistent with an excess charge carrier transport through mid-gap trap states, which form at the Si/InAs heterojunctions. Namely, the trap states add an additional transport path across a heterojunction, and the charge of the defects changes the band bending at the junction. The bending gives rise to a photovoltaic effect at a small bias voltage. In addition, we observe a photoconductance effect within the InAs nanowires at large biases.
    No preview · Article · Sep 2015 · ACS Nano
  • Source

    Full-text · Dataset · Jul 2015
  • Christoph Kastl · Paul Seifert · Xiaoyue He · Kehui Wu · Yongqing Li · Alexander Holleitner
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the photocurrent properties of the topological insulator (Bi0.5Sb0.5)2Te3 on SrTiO3-substrates. We find reproducible, submicron photocurrent patterns generated by long-range chemical potential fluctuations, occurring predominantly at the topological insulator/substrate interface. We fabricate nano-plowed constrictions which comprise single potential fluctuations. Hereby, we can quantify the magnitude of the disorder potential to be in the meV range. The results further suggest a dominating photo-thermoelectric current generated in the surface states in such nanoscale constrictions.
    No preview · Article · Jun 2015 · 2D Materials
  • Source
    Christoph Kastl · Paul Seifert · Xiaoyue He · Kehui Wu · Yongqing Li · Alexander Holleitner
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the photocurrent properties of the topological insulator (Bi$_{0.5}$Sb$_{0.5}$)$_2$Te$_3$ on SrTiO$_3$-substrates. We find reproducible, submicron photocurrent patterns generated by long-range chemical potential fluctuations, occurring predominantly at the topological insulator/substrate interface. We fabricate nano-plowed constrictions which comprise single potential fluctuations. Hereby, we can quantify the magnitude of the disorder potential to be in the meV range. The results further suggest a dominating photo-thermoelectric current generated in the surface states in such nanoscale constrictions.
    Full-text · Article · May 2015
  • Source
    Christoph Kastl · Christoph Karnetzky · Helmut Karl · Alexander W. Holleitner
    [Show abstract] [Hide abstract]
    ABSTRACT: In recent years, a class of solid state materials, called three-dimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spin-polarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions yet at cryogenic temperatures. Here, we show that surface currents in Bi2Se3 can be controlled by circularly polarized light on a picosecond time scale with a fidelity near unity even at room temperature. We re-veal the temporal separation of such ultrafast helicity-dependent surface currents from photo-induced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators.
    Preview · Article · Mar 2015 · Nature Communications
  • Source
    Bastian Miller · Eric Parzinger · Anna Vernickel · Alexander W. Holleitner · Ursula Wurstbauer
    [Show abstract] [Hide abstract]
    ABSTRACT: We describe a photogating effect in mono- and few-layer MoS2, which allows the control of the charge carrier density by almost two orders of magnitude without electrical contacts. Our Raman studies are consistent with physisorbed environmental molecules, that effectively deplete the intrinsically n-doped charge carrier system via charge transfer, and which can be gradually removed by the exposure to light. This photogating process is reversible and precisely tunable by the light intensity. The photogating efficiency is quantified by comparison with measurements on electrostatically gated MoS2.
    Full-text · Article · Mar 2015 · Applied Physics Letters
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Arrays of metal nanoparticles in an organic matrix have attracted a lot of interest due to their diverse electronic and optoelectronic properties. Recent work demonstrates that nanoparticle arrays can be utilized as a template structure to incorporate single molecules. In this arrangement, the nanoparticles act as electronic contacts to the molecules. By varying parameters such as the nanoparticle material, the matrix material, the nanoparticle size, and the interparticle distance, the electronic behavior of the nanoparticle arrays can be substantially tuned and controlled. Furthermore, via the excitation of surface plasmon polaritons, the nanoparticles can be optically excited and electronically read-out. The versatility and possible applications of well-ordered nanoparticle arrays has been demonstrated by the realization of switching devices triggered optically or chemically and by the demonstration of chemical and mechanical sensing. Interestingly, hexagonal nanoparticle arrays may also become a useful platform to study the physics of collective plasmon resonances that can be described as Dirac-like bosonic excitations.
    Full-text · Article · Feb 2015 · Chemical Society Reviews
  • Source
    C Kastl · M Stallhofer · D Schuh · W Wegscheider · A W Holleitner
    [Show abstract] [Hide abstract]
    ABSTRACT: We use GaAs-based quantum point contacts as mesoscopic detectors to locally analyze the flow of photogenerated electrons in a two-dimensional electron gas (2DEG) at perpendicular, quantizing magnetic fields. The 2DEG is formed within a quantum well of a doped GaAs/AlGaAs-heterostructure. We find an optoelectronic signal along the lateral boundaries of the 2DEG, which is consistent with an optically induced quantum transport through quantum Hall edge channels. We demonstrate that photogenerated electrons can be directly injected into an edge channel, transported across several tens of micrometers and read-out on-chip by the quantum point contact.
    Preview · Article · Feb 2015 · New Journal of Physics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: InGaN nanowires are suitable building blocks for many future optoelectronic devices. We show that a linear grading of the indium content along the nanowire axis from GaN to InN introduces an internal electric field evoking a photocurrent. Consistent with quantitative band structure simulations we observe a sign change in the measured photocurrent as a function of photon flux. This negative differential photocurrent opens the path to a new type of nanowire-based photodetector. We demonstrate that the photocurrent response of the nanowires is as fast as 1.5 ps.
    Full-text · Article · Dec 2014 · Nano Letters
  • [Show abstract] [Hide abstract]
    ABSTRACT: Non-radiative transfer processes are often regarded as loss channels for an optical emitter because they are inherently difficult to access experimentally. Recently, it has been shown that emitters, such as fluorophores and nitrogen-vacancy centres in diamond, can exhibit a strong non-radiative energy transfer to graphene. So far, the energy of the transferred electronic excitations has been considered to be lost within the electron bath of the graphene. Here we demonstrate that the transferred excitations can be read out by detecting corresponding currents with a picosecond time resolution. We detect electronically the spin of nitrogen-vacancy centres in diamond and control the non-radiative transfer to graphene by electron spin resonance. Our results open the avenue for incorporating nitrogen-vacancy centres into ultrafast electronic circuits and for harvesting non-radiative transfer processes electronically.
    No preview · Article · Dec 2014 · Nature Nanotechnology
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.
    Full-text · Article · Nov 2014 · Applied Physics Letters
  • Source
    Nadine Erhard · Alexander Holleitner
    [Show abstract] [Hide abstract]
    ABSTRACT: Photocurrent spectroscopy is a versatile technique to identify and understand the optoelectronic dynamics occurring in semiconductor nanowires. Conventional photocurrent spectroscopy allows to explore the morphology and material properties of nanowires as well as their contact interfaces. Using time-resolved photocurrent spectroscopy one gets additional information on the multiple photocurrent generation mechanisms and their respective timescales. This chapter discusses various aspects of the photocurrent spectroscopy and it summarizes the physical mechanisms behind the photocurrent and photoconductance effects in semiconductor nanowires.
    Full-text · Article · Sep 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Nonradiative transfer processes are often regarded as loss channels for an optical emitter1, since they are inherently difficult to be experimentally accessed. Recently, it has been shown that emitters, such as fluorophores and nitrogen vacancy centers in diamond, can exhibit a strong nonradiative energy transfer to graphene. So far, the energy of the transferred electronic excitations has been considered to be lost within the electron bath of the graphene. Here, we demonstrate that the trans-ferred excitations can be read-out by detecting corresponding currents with picosecond time resolution. We electrically detect the spin of nitrogen vacancy centers in diamond electronically and con-trol the nonradiative transfer to graphene by electron spin resonance. Our results open the avenue for incorporating nitrogen vacancy centers as spin qubits into ultrafast electronic circuits and for harvesting non-radiative transfer processes electronically.
    Full-text · Article · Aug 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on the photoconductance in two-dimensional arrays of gold nanorods which is strongly enhanced at the frequency of the longitudinal surface plasmon of the nanorods. The arrays are formed by a combination of droplet deposition and stamping of gold nanorod solutions on SiO2 substrates. We find that the plasmon induced photoconductance is sensitive to the linear polarization of the exciting photons. We interpret the occurrence of the photoconductance as a bolometric enhancement of the arrays' conductance upon excitation of the longitudinal surface plasmon resonance of the nanorods.
    Full-text · Article · Mar 2014 · physica status solidi (RRL) - Rapid Research Letters
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Undoped diamond, a remarkable bulk electrical insulator, exhibits a high surface conductivity in air when the surface is hydrogen-terminated. Although theoretical models have claimed that a two-dimensional hole gas is established as a result of surface energy band bending, no definitive experimental demonstration has been reported so far. Here, we prove the two-dimensional character of the surface conductivity by low temperature characterization of diamond in-plane gated field-effect transistors that enable the lateral confinement of the transistor's drain-source channel to nanometer dimensions. In these devices, we observe Coulomb blockade effects of multiple quantum islands varying in size with the gate voltage. The charging energy and thus the size of these zero-dimensional islands exhibits a gate voltage dependence which is the direct result of the two-dimensional character of the conductive channel formed at hydrogen-terminated diamond surfaces.
    Full-text · Article · Oct 2013 · Physical Review B
  • [Show abstract] [Hide abstract]
    ABSTRACT: There's the rub: Friction of single polymers on solid bodies in a liquid environment was investigated. Apart from expected mechanisms, such as slip and stick, a third nanoscale friction mechanism exists that is independent of normal force, velocity, and adsorbed polymer length. A model is proposed for this mechanism that is based on measurements with various polymers on topographically and chemically nanostructured surfaces.
    No preview · Article · Jun 2013 · Angewandte Chemie International Edition

Publication Stats

2k Citations
577.57 Total Impact Points

Institutions

  • 2015
    • Universität Regensburg
      • Institute of Experimental and Applied Physics
      Ratisbon, Bavaria, Germany
  • 2008-2015
    • Technische Universität München
      • • Walter Schottky Institut (WSI)
      • • Faculty of Physics
      München, Bavaria, Germany
  • 2010
    • Philipps University of Marburg
      Marburg, Hesse, Germany
  • 1999-2009
    • Ludwig-Maximilians-University of Munich
      • Center for Nanoscience (CeNS)
      München, Bavaria, Germany
  • 2004-2006
    • University of California, Santa Barbara
      • Department of Physics
      Santa Barbara, CA, United States