Nikolai B. Zhitenev

National Institute for Materials Science, Tsukuba, Ibaraki, Japan

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Publications (46)221.83 Total impact

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    ABSTRACT: We report on spatial measurements of the superconducting proximity effect in epitaxial graphene induced by a graphene-superconductor interface. Superconducting aluminum films were grown on epitaxial multilayer graphene on SiC. The aluminum films were discontinuous, with networks of trenches in the film morphology reaching down to exposed graphene terraces. Scanning tunneling spectra measured on the graphene terraces show a clear decay of the superconducting energy gap with increasing separation from the graphene-aluminum edges. The spectra were well described by BCS theory. The decay length for the superconducting energy gap in graphene was determined to be greater than 400 nm. Deviations in the exponentially decaying energy gap were also observed on a much smaller length scale of tens of nanometers.
    No preview · Article · Jan 2016
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    ABSTRACT: Superconductivity results from a Bose condensate of Cooper-paired electrons with a macroscopic quantum wave function. Dramatic effects can occur when the region of the condensate is shaped and confined to the nanometer scale. Recent progress in nanostructured superconductors has revealed a route to topological superconductivity, with possible applications in quantum computing. However, challenges remain in controlling the shape and size of specific superconducting materials. Here, we report a method to create nanostructured superconductors by partial crystallization of the half-Heusler material, YPtBi. Superconducting islands, with diameters in the range of 100 nm, were reproducibly created by local current annealing of disordered YPtBi in the tunneling junction of a scanning tunneling microscope. We characterize the superconducting island properties by scanning tunneling spectroscopic measurements to determine the gap energy, critical temperature and field, coherence length, and vortex formations. These results show unique properties of a confined superconductor and demonstrate that this method holds promise to create tailored superconductors for a wide variety of nanometer scale applications.
    Full-text · Article · Sep 2015 · Physical Review B
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    ABSTRACT: Electron beam induced current (EBIC) is a powerful technique which measures the charge collection efficiency of photovoltaics with sub-micron spatial resolution. The exciting electron beam results in a high generation rate density of electron-hole pairs, which may drive the system into nonlinear regimes. An analytic model is presented which describes the EBIC response when the {\it total} electron-hole pair generation rate exceeds the rate at which carriers are extracted by the photovoltaic cell, and charge accumulation and screening occur. The model provides a simple estimate of the onset of the high injection regime in terms of the material resistivity and thickness, and provides a straightforward way to predict the EBIC lineshape in the high injection regime. The model is verified by comparing its predictions to numerical simulations in 1 and 2 dimensions. Features of the experimental data, such as the magnitude and position of maximum collection efficiency versus electron beam current, are consistent with the 3 dimensional model.
    Full-text · Article · Jul 2015 · Nanotechnology
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    ABSTRACT: The observation of phonons in graphene by inelastic electron tunneling spectroscopy has been met with limited success in previous measurements arising from weak signals and other spectral features which inhibit a clear distinction between phonons and miscellaneous excitations. Utilizing a back-gated graphene device that allows adjusting the global charge carrier density, we introduce an averaging method where individual tunneling spectra at varying charge carrier density are combined into one representative spectrum. This method improves the signal for inelastic transitions while it suppresses dispersive spectral features. We thereby map the total graphene phonon density of states, in good agreement with density functional calculations. Unexpectedly, an abrupt change in the phonon intensity is observed when the graphene charge carrier type is switched through a variation of the back-gate electrode potential. This sudden variation in phonon intensity is asymmetric in the carrier type, depending on the sign of the tunneling bias.
    No preview · Article · Jun 2015 · Physical Review Letters
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    ABSTRACT: The design of high-finesse resonant cavities for electronic waves faces challenges due to short electron coherence lengths in solids. Complementing previous approaches to confine electronic waves by carefully positioned adatoms at clean metallic surfaces, we demonstrate an approach inspired by the peculiar acoustic phenomena in whispering galleries. Taking advantage of graphene’s gate-tunable light-like carriers, we create whispering-gallery mode (WGM) resonators defined by circular pn junctions, induced by a scanning tunneling probe. We can tune the resonator size and the carrier concentration under the probe in a back-gated graphene device over a wide range. The WGM-type confinement and associated resonances are a new addition to the quantum electron-optics toolbox, paving the way to develop electronic lenses and resonators.
    Preview · Article · May 2015 · Science
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    ABSTRACT: The local collection characteristics of grain interiors and grain boundaries in thin film CdTe polycrystalline solar cells are investigated using scanning photocurrent microscopy. The carriers are locally generated by light injected through a small aperture (50-300 nm) of a near-field scanning optical microscope in an illumination mode. Possible influence of rough surface topography on light coupling is examined and eliminated by sculpting smooth wedges on the granular CdTe surface. By varying the wavelength of light, nanoscale spatial variations in external quantum efficiency are mapped. We find that the grain boundaries (GBs) are better current collectors than the grain interiors (GIs). The increased collection efficiency is caused by two distinct effects associated with the material composition of GBs. First, GBs are charged, and the corresponding built-in field facilitates the separation and the extraction of the photogenerated carriers. Second, the GB regions generate more photocurrent at long wavelength corresponding to the band edge, which can be caused by a smaller local band gap. Resolving carrier collection with nanoscale resolution in solar cell materials is crucial for optimizing the polycrystalline device performance through appropriate thermal processing and passivation of defect and surfaces.
    Full-text · Article · Oct 2014 · ACS Nano
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    ABSTRACT: Electron beam induced current (EBIC) is a powerful characterization technique which offers the high spatial resolution needed to study polycrystalline solar cells. Ideally, an EBIC measurement reflects the spatially resolved quantum efficiency of the device. In this work, a model for EBIC measurements is presented which applies when recombination within the depletion region is substantial. This model is motivated by cross-sectional EBIC experiments on CdS-CdTe photovoltaic cells which show that the maximum efficiency of carrier collection is less than 100 \% and varies throughout the depletion region. The model can reproduce experimental results only if the mobility-lifetime product $\mu\tau$ is spatially varying within the depletion region. The reduced collection efficiency is speculated to be related to high-injection effects, and the resulting increased radiative recombination.
    Preview · Article · Sep 2014
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    ABSTRACT: Quantitative determination of electronic properties at high spatial resolution is crucial for the development of high-efficiency solar cells. Electron beam induced current (EBIC) is a powerful technique in which electron-hole pairs are created in proximity to an exposed surface, and the carrier collection efficiency is measured as a function of excitation position. Cross-sections of device are often created by focused ion beams (FIB) due to the flexibility of the patterning and milling processes. However, the irradiating Ga ions of the FIB fabrication may introduce unintended artifacts, affecting local electronic properties. In this study, we investigate the impact of the FIB process observed in EBIC measurements and two-dimensional finite element simulations.
    No preview · Conference Paper · Aug 2014
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    ABSTRACT: Recent experiments reveal that a scanning tunneling microscopy (STM) probe tip can generate a highly localized strain field in a graphene drumhead, which in turn leads to pseudomagnetic fields in the graphene that can spatially confine graphene charge carriers in a way similar to a lithographically defined quantum dot (QD). While these experimental findings are intriguing, their further implementation in nanoelectronic devices hinges upon the knowledge of key underpinning parameters, which still remain elusive. In this paper we first summarize the experimental measurements of the deformation of graphene membranes due to interactions with the STM probe tip and a back-gate electrode. We then carry out systematic coarse-grained (CG) simulations to offer a mechanistic interpretation of STM tip-induced straining of the graphene drumhead. Our findings reveal the effect of (i) the position of the STM probe tip relative to the graphene drumhead center, (ii) the sizes of both the STM probe tip and graphene drumhead, as well as (iii) the applied back-gate voltage, on the induced strain field and corresponding pseudomagnetic field. These results can offer quantitative guidance for future design and implementation of reversible and on-demand formation of graphene QDs in nanoelectronics.
    No preview · Article · Aug 2014 · Physical Review B
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    ABSTRACT: A thin film of quantum dots (QD) was used to visualize the local photo-response of polycrystalline CdTe solar cells by down-converting an electron beam of high energy to photons of visible light. The efficient photon generation in the QD film is compared to cathodoluminescence of the high-purity bulk semiconductors and phosphor.
    No preview · Article · Jun 2014
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    ABSTRACT: To boost the efficiency of thin-film polycrystalline solar cells that are microscopically inhomogeneous, it is imperative to understand how the grain interiors (GIs) and grain boundaries (GBs) within these materials affect its overall electronic properties. By using an apertured near-field scanning optical microscope in an illumination mode, we determined the local photocurrent that is generated within the GIs and at the GBs with nanoscale resolution and correlate the results with surface morphology and composition.
    Full-text · Article · Jan 2014 · IEEE Journal of Photovoltaics
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    B. H. Hamadani · J. Roller · P. Kounavis · N. B. Zhitenev · D. J. Gundlach
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    ABSTRACT: Modulated photocurrent spectroscopy was used to investigate the dynamic response of charge carrier transport in thin film CdTe/CdS solar cells. The impact of light bias and temperature over a broad excitation frequency range were measured. The observed features of the data, including a photocurrent ‘phase-lead’ and a ‘phase-lag’ over different regions of the frequency spectrum, were explored in the context of an equivalent circuit model. Comparisons between the model's predicted performance and the measured data suggest that charge carrier recombination at the cell's back metal/semiconductor contact is the main source of photocurrent loss in these devices.
    Full-text · Article · Sep 2013 · Solar Energy Materials and Solar Cells
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    ABSTRACT: We investigate local electronic properties of cadmium telluride solar cells using electron beam induced current (EBIC) measurements with patterned contacts. EBIC measurements are performed with a spatial resolution as high as ≈20 nm both on the top surface and throughout the cross-section of the device, revealing a remarkable degree of electrical inhomogeneity near the p–n junction and enhanced carrier collection in the vicinity of grain boundaries (GB). Simulation results of low energy EBIC suggest that the band bending near a GB is downward, with a magnitude of at least 0.2 eV for the most effective current-collecting GBs. Furthermore, we demonstrate a new approach to investigate local open-circuit voltage by applying an external bias across electrical contact with a point electron-beam injection. The length scale of the nanocontacts is on the length scale of a single or a few grains, confining current path with highly localized photo-generated carriers.
    No preview · Article · Aug 2013 · Solar Energy Materials and Solar Cells
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    ABSTRACT: We report a fast, versatile photocurrent imaging technique to visualize the local photo response of solar energy devices and optoelectronics using near-field cathodoluminescence (CL) from a homogeneous quantum dot layer. This approach is quantitatively compared with direct measurements of high-resolution Electron Beam Induced Current (EBIC) using a thin film solar cell (n-CdS / p-CdTe). Qualitatively, the observed image contrast is similar, showing strong enhancement of the carrier collection efficiency at the p-n junction and near the grain boundaries. The spatial resolution of the new technique, termed Q-EBIC (EBIC using quantum dots), is determined by the absorption depth of photons. The results demonstrate a new method for high-resolution, sub-wavelength photocurrent imaging measurement relevant for a wide range of applications.
    Full-text · Article · Jun 2013 · AIP Advances
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    ABSTRACT: We used the modulated photocurrent spectroscopy technique based on sinusoidal excitation of high-powered LEDs to investigate the dynamic response of charge carrier transport in thin film solar cells based on CdTe. The impact of light bias, voltage bias and the temperature over a broad excitation frequency bandwidth were studied. The observed features of the data, including a photocurrent phase-lead and a phase-lag over different regions of the frequency spectrum, were explored in the context of an equivalent circuit model. Comparisons between the model's predicted performance and the measured data suggest that charge carrier recombination at the cell's back metal/semiconductor contact is the main source of photocurrent loss in the cells that were investigated by our group.
    No preview · Article · Mar 2013
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    ABSTRACT: The power conversion efficiency of commercial solar modules based on thin-film chalcogenide materials is well below the theoretical limits. To understand the underlying physical mechanisms limiting the efficiency, we investigate local photovoltaic properties isolating the difference between the grain bulk (0.5-2 mkm in size) and the grain boundary in CdTe absorber. Local current-voltage measurements are performed using nano-contacts in conjunction with local electron-hole pairs generation comparing multiple injection techniques. First, the carriers are excited using variable energy electron beam enabling measurements with a spatial resolution down to 20 nm. Second, we have developed a novel approach for high-resolution and high-throughput photocurrent imaging downconverting electron beam into a near-field optical source using a thin film (50 nm) of phosphors. The electron beam is fully absorbed in the phosphors layer, and the cathodoluminescence is used as a local photon source. Third, we generate carriers using a near-filed optical microscope varying the excitation wavelength. The results show that, in a well-optimized material, a large fraction of grain boundaries displays higher photocurrent as compared to grain bulk effectively serving as a three-dimensional distributed photocurrent collector.
    No preview · Article · Mar 2013
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    ABSTRACT: In graphene, as in most metals, electron-electron interactions renormalize the properties of electrons but leave them behaving like noninteracting quasiparticles. Many measurements probe the renormalized properties of electrons right at the Fermi energy. Uniquely for graphene, the accessibility of the electrons at the surface offers the opportunity to use scanned probe techniques to examine the effect of interactions at energies away from the Fermi energy, over a broad range of densities, and on a local scale. Using scanning tunneling spectroscopy, we show that electron interactions leave the graphene energy dispersion linear as a function of excitation energy for energies within �200 meV of the Fermi energy. However, the measured dispersion velocity depends on density and increases strongly as the density approaches zero near the charge neutrality point, revealing a squeezing of the Dirac cone due to interactions.
    Full-text · Article · Sep 2012 · Physical Review Letters
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    ABSTRACT: We determined the electromechanical properties of a suspended graphene layer by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements, as well as computational simulations of the graphene-membrane mechanics and morphology. A graphene membrane was continuously deformed by controlling the competing interactions with a STM probe tip and the electric field from a back-gate electrode. The probe tip-induced deformation created a localized strain field in the graphene lattice. STS measurements on the deformed suspended graphene display an electronic spectrum completely different from that of graphene supported by a substrate. The spectrum indicates the formation of a spatially confined quantum dot, in agreement with recent predictions of confinement by strain-induced pseudomagnetic fields.
    Full-text · Article · Jun 2012 · Science
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    ABSTRACT: The relation between macroscopic charge transport properties and microscopic carrier distribution is one of the central issues in the physics and future applications of graphene devices (GDs). We find strong conductance enhancement at the edges of GDs using scanning gate microscopy. This result is explained by our theoretical model of the opening of an additional conduction channel localized at the edges by depleting accumulated charge by the tip.
    Full-text · Article · Mar 2012 · Nano Letters
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    ABSTRACT: We have performed scanning tunneling spectroscopy measurements on gated-graphene devices in the quantum Hall regime under varying disorder potential landscapes. Relatively thin hexagonal boron-nitride (h-BN) crystals are mechanically exfoliated on SiO2/Si substrates and single-layer graphene films are later transferred on pre-located h-BN crystals. In this device scheme, we can investigate the interactions of Dirac particles with local impurities ranging from strongly disordered to weakly perturbed environments by adjusting the thickness of h-BN crystals, while varying both the Fermi-energy with respect to a Dirac point and magnetic field. In the h-BN devices, we have observed that the electron-hole puddles are larger in lateral size than those observed on SiO2 devices, and resonance scatterings are significantly reduced due to weakened disorder potentials. Accordingly, we start observing well-defined Landau levels (LLs) as early as 0.5 T and the width of individual LLs, broadened by the scattering of charged carriers, is much narrower than those from graphene on SiO2. In high magnetic fields, we observe the electronic structure of graphene devices is significantly altered by the electron-electron interactions and the formation of large interaction energy gaps. We will discuss the spatial, orbital quantum number, and magnetic field dependence of the observed interaction gaps.
    No preview · Article · Feb 2012

Publication Stats

353 Citations
221.83 Total Impact Points

Institutions

  • 2015
    • National Institute for Materials Science
      • Advanced Materials Laboratory
      Tsukuba, Ibaraki, Japan
  • 2010-2014
    • National Institute of Standards and Technology
      • Center for Nanoscale Science and Technology (CNST)
      GAI, Maryland, United States