J. Eroms

Universität Regensburg, Ratisbon, Bavaria, Germany

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Publications (51)132.25 Total impact

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    ABSTRACT: Graphene has unique optical and electronic properties that make it attractive as an active material for broadband ultrafast detection. We present here a graphene-based detector that shows 40-picosecond electrical rise time over a spectral range that spans nearly three orders of magnitude, from the visible to the far-infrared. The detector employs a large area graphene active region with interdigitated electrodes that are connected to a log-periodic antenna to improve the long-wavelength collection efficiency, and a silicon carbide substrate that is transparent throughout the visible regime. The detector exhibits a noise-equivalent power of approximately 100 µW·Hz–½ and is characterized at wavelengths from 780 nm to 500 µm.
    Full-text · Article · Nov 2015 · Optics Express
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    ABSTRACT: Experimental and theoretical studies on ratchet effects in graphene with a lateral superlattice excited by alternating electric fields of terahertz frequency range are presented. A lateral superlatice deposited on top of monolayer graphene is formed either by periodically repeated metal stripes having different widths and spacings or by inter-digitated comb-like dual-grating-gate (DGG) structures. We show that the ratchet photocurrent excited by terahertz radiation and sensitive to the radiation polarization state can be efficiently controlled by the back gate driving the system through the Dirac point as well as by the lateral asymmetry varied by applying unequal voltages to the DGG subgratings. The ratchet photocurrent includes the Seebeck thermoratchet effect as well as the effects of "linear" and "circular" ratchets, sensitive to the corresponding polarization of the driving electromagnetic force. The experimental data are analyzed for the electronic and plasmonic ratchets taking into account the calculated potential profile and the near field acting on carriers in graphene. We show that the photocurrent generation is based on a combined action of a spatially periodic in-plane potential and the spatially modulated light due to the near field effects of the light diffraction.
    No preview · Article · Oct 2015
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    ABSTRACT: Graphene samples can have a very high carrier mobility if influences from the substrate and the environment are minimized. Embedding a graphene sheet into a heterostructure with hexagonal boron nitride (hBN) on both sides was shown to be a particularly efficient way of achieving a high bulk mobility. Nanopatterning graphene can add extra damage and drastically reduce sample mobility by edge disorder. Preparing etched graphene nanostructures on top of an hBN substrate instead of SiO2 is no remedy, as transport characteristics are still dominated by edge roughness. Here we show that etching fully encapsulated graphene on the nanoscale is more gentle and the high mobility can be preserved. To this end, we prepared graphene antidot lattices where we observe magnetotransport features stemming from ballistic transport. Due to the short lattice period in our samples we can also explore the boundary between the classical and the quantum transport regime.
    Preview · Article · Sep 2015 · Nano Letters
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    ABSTRACT: Two-dimensional carbon nanomaterials ranging from single-layer graphene to defective structures such as chemically reduced graphene oxide were studied with respect to their use in electrodes and sensors. Their electrochemical properties and utility in terms of fabrication of sensing devices are compared. Specifically, the electrodes have been applied to reductive amperometric determination of hydrogen peroxide. Low-defect graphene (SG) was obtained through mechanical exfoliation of natural graphite, while higher-defect graphenes were produced by chemical vapor deposition (CVDG) and by chemical oxidation of graphite and subsequent reduction (rGO). The carbonaceous materials were mainly characterized by Raman microscopy. They were applied as electrode material and the electrochemical behavior was investigated by chronocoulometry, cyclic voltammetry, electrochemical impedance spectroscopy and amperometry and compared to a carbon disc electrode. It is shown that the quality of the graphene has an enormous impact on the amperometric performance. The use of carbon materials with many defects (like rGO) does not result in a significant improvement in signal compared to a plain carbon disc electrode. The sensitivity is 173 mA · M−1 · cm−2 in case of using CVDG which is about 50 times better than that of a plain carbon disc electrode and about 7 times better than that of rGO. The limit of detection for hydrogen peroxide is 15.1 μM (at a working potential of −0.3 V vs SCE) for CVDG. It is concluded that the application of two-dimensional carbon nanomaterials offers large perspectives in amperometric detection systems due to electrocatalytic effects that result in highly sensitive detection. Graphical abstract Graphene materials prepared by different techniques were studied as electrode material for the electrochemical detection of H2O2. Materials comprising a less defective structure showed a significantly higher sensitivity.
    No preview · Article · Aug 2015 · Microchimica Acta
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    ABSTRACT: We report on transport properties of monolayer graphene with a laterally modulated potential profile, employing striped top gate electrodes with spacings of 100 nm to 200 nm. Tuning of top and back gate voltages gives rise to local charge carrier density disparities, enabling the investigation of transport properties either in the unipolar (nn'n) or the bipolar (np'n) regime. In the latter pronounced single- and multibarrier Fabry-Perot (FP) resonances occur. We present measurements of different devices with different numbers of top gate stripes and spacings. The data is highly consistent with a phase coherent ballistic tight binding calculation and quantum capacitance model, whereas a superlattice effect and modification of band structure can be excluded.
    Full-text · Article · Jan 2014 · Physical Review B
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    ABSTRACT: In recent years, the dichalcogenide MoS2 has gained attention as an interesting material system for basic research and possible optoelectronic applications. Here, we report on optical spectroscopy of few- and single-layer MoS2 flakes. We use Raman spectroscopy to characterize our samples. The energy of the characteristic phonon modes in MoS2 depends on the number of layers, so that the thickness of individual flakes can be mapped in scanning Raman experiments. While bulk MoS2 is an indirect-gap semiconductor, single-layer MoS2 has a direct band gap and emits strong photoluminescence. We investigate the photoluminescence in single-layer MoS2 for different experimental conditions. Additionally, we study the photocarrier dynamics in time-resolved photoluminescence experiments.
    No preview · Chapter · Jan 2014
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    ABSTRACT: Theoretical and experimental studies on the ratchet effects in graphene and in quantum wells with a lateral superlattice excited by alternating electric fields of terahertz frequency range are presented. We discuss the Seebeck ratchet effect and helicity driven photocurrents and show that the photocurrent generation is based on the combined action of a spatially periodic in-plane potential and a spatially modulated light.
    No preview · Conference Paper · Dec 2013
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    ABSTRACT: MoS$_2$ is a highly interesting material system, which exhibits a crossover from an indirect band gap in the bulk crystal to a direct gap for single layers. Here, we perform a direct comparison between large-area MoS$_2$ films grown by chemical vapor deposition (CVD) and MoS$_2$ flakes prepared by mechanical exfoliation from natural bulk crystal. Raman spectroscopy measurements show differences between the in-plane and out-of-plane phonon mode positions in CVD-grown and exfoliated MoS$_2$. Photoluminescence (PL) mapping reveals large regions in the CVD-grown films that emit strong PL at room temperature, and low-temperature PL scans demonstrate a large spectral shift of the A exciton emission as a function of position. Polarization-resolved PL measurements under near-resonant excitation conditions show a strong circular polarization of the PL, corresponding to a valley polarization.
    No preview · Article · Oct 2013 · Semiconductor Science and Technology
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    ABSTRACT: We present an ultrafast terahertz detector suitable for wavelengths from 30 μm to 220 μm, which is based on a graphene flake. A logarithmic-periodic antenna is used to couple the radiation to the flake. The detector, characterized by a fast rise time combined with room temperature operation, is well suited for determining timing differences of THz laser pulses.
    No preview · Conference Paper · Sep 2013
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    ABSTRACT: We study a crystallographic etching process of graphene nanostructures where zigzag edges can be prepared selectively. The process involves heating exfoliated single-layer graphene samples with a predefined pattern of antidot arrays in an argon atmosphere at 820 C, which selectively removes carbon atoms located on armchair sites. Atomic force microscopy and scanning electron microscopy cannot resolve the structure on the atomic scale. However, weak localization and Raman measurements - which both probe intervalley scattering at armchair edges - indicate that zigzag regions are enhanced compared to samples prepared with oxygen based reactive ion etching only.
    No preview · Article · Aug 2013 · Applied Physics Letters
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    ABSTRACT: We present an ultrafast graphene-based detector, working in the THz range at room temperature. A logarithmic-periodic antenna is coupled to a graphene flake that is produced by exfoliation on SiO2. The detector was characterized with the free-electron laser FELBE for wavelengths from 8 um to 220 um. The detector rise time is 50 ps in the wavelength range from 30 um to 220 um. Autocorrelation measurements exploiting the nonlinear photocurrent response at high intensities reveal an intrinsic response time below 10 ps. This detector has a high potential for characterizing temporal overlaps, e. g. in two-color pump-probe experiments.
    Preview · Article · May 2013 · Applied Physics Letters
  • Jonathan Eroms
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    ABSTRACT: A Comment on the Letter by X. Hong et al., Phys. Rev. Lett. 108, 226602 (2012). The authors of the Letter offer a Reply.
    No preview · Article · Oct 2012 · Physical Review Letters
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    ABSTRACT: We present results of non-local and three terminal (3T) spin precession measurements on spin injection devices fabricated on epitaxial graphene on SiC. The measurements were performed before and after an annealing step at 150 degrees Celsius for 15 minutes in vacuum. The values of spin relaxation length L_s and spin relaxation time tau_s obtained after annealing are reduced by a factor 2 and 4, respectively, compared to those before annealing. An apparent discrepancy between spin diffusion constant D_s and charge diffusion constant D_c can be resolved by investigating the temperature dependence of the g-factor, which is consistent with a model for paramagnetic magnetic moments.
    Full-text · Article · Oct 2012 · Physical review. B, Condensed matter
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    ABSTRACT: The dichalcogenide MoS2, which is an indirect-gap semiconductor in its bulk form, was recently shown to become an efficient emitter of photoluminescence as it is thinned to a single layer, indicating a transition to a direct-gap semiconductor due to confinement effects. With its layered structure of weakly coupled, covalently bonded twodimensional sheets, it can be prepared, just as graphene, using mechanical exfoliation techniques. With these techniques, few- and single-layer flakes can be prepared. Raman spectroscopy is a sensitive tool to determine the number of layers of a flake, as two characteristic Raman modes in MoS2 shift to higher or lower frequency with the number of layers. In addition to previously reported Raman modes in MoS2, we observe an interlayer shear mode at very low frequencies, which also shifts with the number of layers. We use scanning Raman spectroscopy to map and characterize MoS2 flakes.
    No preview · Article · Oct 2012 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: The dichalcogenide MoS2, which is an indirect-gap semiconductor in its bulk form, was recently shown to become an efficient emitter of photoluminescence as it is thinned to a single layer, indicating a transition to a direct-gap semiconductor due to confinement effects. With its layered structure of weakly coupled, covalently bonded two-dimensional sheets, it can be prepared, just as graphene, using mechanical exfoliation techniques. Here, we present temperature-dependent and time-resolved photoluminescence (PL) studies of single-layer MoS2 flakes. Some of the flakes are covered with oxide layers prepared by atomic layer deposition (ALD). At low temperatures, we clearly see two PL peaks in the as-prepared flakes without oxide layers, which we may assign to bound and free exciton transitions. The lower-energy, bound exciton PL peak is absent in the oxide-covered flakes. In time-resolved PL measurements, we observe very fast photocarrier recombination on the few-ps timescale at low temperatures, with increasing photocarrier lifetimes at higher temperatures due to exciton-phonon scattering.
    No preview · Article · Sep 2012 · Proceedings of SPIE - The International Society for Optical Engineering
  • Source
    S. Minke · J. Bundesmann · D. Weiss · J. Eroms
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    ABSTRACT: We have experimentally investigated quantum interference corrections to the conductivity of graphene nanoribbons at temperatures down to 20 mK studying both weak localization (WL) and universal conductance fluctuations (UCF). Since in individual nanoribbons at millikelvin temperatures the UCFs strongly mask the weak localization feature we employ both gate averaging and ensemble averaging to suppress the UCFs. This allows us to extract the phase coherence length from both WL and UCF at all temperatures. Above 1 K, the phase coherence length is suppressed due to Nyquist scattering whereas at low temperatures we observe a saturation of the phase coherence length at a few hundred nanometers, which exceeds the ribbon width, but stays below values typically found in bulk graphene. To better describe the experiments at elevated temperatures, we extend the formula for 1D weak localization in graphene, which was derived in the limit of strong intervalley scattering, to include all elastic scattering rates.
    Preview · Article · Aug 2012 · Physical review. B, Condensed matter
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    ABSTRACT: Single- and few-layer MoS2 has recently gained attention as an interesting new material system for opto-electronics. Here, we report on scanning Raman measurements on few-layer MoS2 flakes prepared by exfoliation. We observe a Raman mode corresponding to a rigid shearing oscillation of adjacent layers. This mode appears at very low Raman shifts between 20 and 30 relative wavenumbers. Its position strongly depends on the number of layers, which we independently determine using AFM measurements and investigation of the other characteristic Raman modes. Raman spectroscopy of the shear mode therefore is a useful tool to determine the number of layers for few-layer MoS2 flakes.
    Full-text · Article · May 2012 · Applied Physics Letters
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    ABSTRACT: We present a photoluminescence study of single-layer MoS2 flakes on SiO2 surfaces. We demonstrate that the luminescence peak position of flakes prepared from natural MoS2, which varies by up to 25 meV between individual as-prepared flakes, can be homogenized by annealing in vacuum, which removes adsorbates from the surface. We use HfO2 and Al2O3 layers prepared by atomic layer deposition to cover some of our flakes. We clearly observe a suppression of the low-energy luminescence peak observed for as-prepared flakes at low temperatures, indicating that this peak originates from excitons bound to surface adsorbates. We also observe different temperature-induced shifts of the luminescence peaks for the oxide-covered flakes. This effect stems from the different thermal expansion coefficients of the oxide layers and the MoS2 flakes. It indicates that the single-layer MoS2 flakes strongly adhere to the oxide layers and are therefore strained.
    Full-text · Article · Mar 2012 · physica status solidi (RRL) - Rapid Research Letters
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    Full-text · Article · Mar 2012 · physica status solidi (RRL) - Rapid Research Letters
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    ABSTRACT: We observe photocurrents induced in single layer graphene samples by illumination of the graphene edges with circularly polarized terahertz radiation at normal incidence. The photocurrent flows along the sample edges and forms a vortex. Its winding direction reverses by switching the light helicity from left- to right-handed. We demonstrate that the photocurrent stems from the sample edges, which reduce the spatial symmetry and result in an asymmetric scattering of carriers driven by the radiation electric field. The developed theory is in a good agreement with the experiment. We show that the edge photocurrents can be applied for determination of the conductivity type and the momentum scattering time of the charge carriers in the graphene edge vicinity.
    Full-text · Article · Dec 2011 · Physical Review Letters

Publication Stats

1k Citations
132.25 Total Impact Points

Institutions

  • 1998-2015
    • Universität Regensburg
      • • Institute of Experimental and Applied Physics
      • • Intitute of Theoretical Physics
      Ratisbon, Bavaria, Germany
  • 2006
    • Delft University of Technology
      Delft, South Holland, Netherlands
  • 2002
    • University Hospital Regensburg
      Ratisbon, Bavaria, Germany