Joshua A. Robinson

Pennsylvania State University, University Park, Maryland, United States

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Publications (43)203.61 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Few-layer tungsten diselenide (WSe2) is attractive as a next-generation electronic material as it exhibits modest carrier mobilities and energy band gap in the visible spectra, making it appealing for photovoltaic and low-powered electronic applications. Here we demonstrate the scalable synthesis of large-area, few-layer WSe2 via replacement of oxygen in hexagonally stabilized tungsten oxide films using dimethyl selenium. Cross-sectional transmission electron microscopy reveals successful control of the final WSe2 film thickness through control of initial tungsten oxide thickness, as well as development of layered films with grain sizes up to several hundred nanometers. Raman spectroscopy and atomic force microscopy confirms high crystal uniformity of the converted WSe2, and time domain thermo-reflectance provide evidence that near record low thermal conductivity is achievable in ultra-thin WSe2 using this method.
    02/2015; 2(1).
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    ABSTRACT: In this work, we demonstrate abrupt, reversible switching of resistance in 1T-TaS2 using DC and pulsed sources, corresponding to an insulator-metal-transition between the insulating Mott and equilibrium metallic states. This transition occurs at a constant critical resistivity of 7 mohm-cm regardless of temperature or bias conditions and the transition time is significantly smaller than abrupt transitions by avalanche breakdown in other small gap Mott insulating materials. Furthermore, this critical resistivity corresponds to a carrier density of 4.5x10^19 cm-3, which compares well with the critical carrier density for the commensurate to nearly commensurate charge density wave transition. These results suggest that the transition is facilitated by a carrier driven collapse of the Mott gap in 1T-TaS2 which results in a fast (3ns) switching.
    Nano letters. 01/2015;
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    ABSTRACT: Tungsten diselenide (WSe2) is a two-dimensional material that is of interest for next-generation electronic and optoelectronic devices due to its direct bandgap of 1.65 eV in the monolayer form and excellent transport properties. However, technologies based on this 2D material cannot be realized without a scalable synthesis process. Here, we demonstrate the first scalable synthesis of large-area, mono and few-layer WSe2 via metal-organic chemical vapor deposition using tungsten hexacarbonyl (W(CO)6) and dimethylselenium ((CH3)2Se). In addition to being intrinsically scalable, this technique allows for the precise control of the vapor-phase chemistry, which is unobtainable using more traditional oxide vaporization routes. We show that temperature, pressure, Se:W ratio, and substrate choice have a strong impact on the ensuing atomic layer structure, with optimized conditions yielding >8 μm size domains. Raman spectroscopy, atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM) confirm crystalline mono-to-multilayer WSe2 is achievable. Finally, TEM and vertical current/voltage transport provide evidence that a pristine van der Waals gap exists in WSe2/graphene heterostructures.
    ACS Nano 01/2015; · 12.03 Impact Factor
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    ABSTRACT: Conspectus In the wake of the discovery of the remarkable electronic and physical properties of graphene, a vibrant research area on two-dimensional (2D) layered materials has emerged during the past decade. Transition metal dichalcogenides (TMDs) represent an alternative group of 2D layered materials that differ from the semimetallic character of graphene. They exhibit diverse properties that depend on their composition and can be semiconductors (e.g., MoS2, WS2), semimetals (e.g., WTe2, TiSe2), true metals (e.g., NbS2, VSe2), and superconductors (e.g., NbSe2, TaS2). The properties of TMDs can also be tailored according to the crystalline structure and the number and stacking sequence of layers in their crystals and thin films. For example, 2H-MoS2 is semiconducting, whereas 1T-MoS2 is metallic. Bulk 2H-MoS2 possesses an indirect band gap, but when 2H-MoS2 is exfoliated into monolayers, it exhibits direct electronic and optical band gaps, which leads to enhanced photoluminescence. Therefore, it is important to learn to control the growth of 2D TMD structures in order to exploit their properties in energy conversion and storage, catalysis, sensing, memory devices, and other applications. In this Account, we first introduce the history and structural basics of TMDs. We then briefly introduce the Raman fingerprints of TMDs of different layer numbers. Then, we summarize our progress on the controlled synthesis of 2D layered materials using wet chemical approaches, chemical exfoliation, and chemical vapor deposition (CVD). It is now possible to control the number of layers when synthesizing these materials, and novel van der Waals heterostructures (e.g., MoS2/graphene, WSe2/graphene, hBN/graphene) have recently been successfully assembled. Finally, the unique optical, electrical, photovoltaic, and catalytic properties of few-layered TMDs are summarized and discussed. In particular, their enhanced photoluminescence (PL), photosensing, photovoltaic conversion, and hydrogen evolution reaction (HER) catalysis are discussed in detail. Finally, challenges along each direction are described. For instance, how to grow perfect single crystalline monolayer TMDs without the presence of grain boundaries and dislocations is still an open question. Moreover, the morphology and crystal structure control of few-layered TMDs still requires further research. For wet chemical approaches and chemical exfoliation methods, it is still a significant challenge to control the lateral growth of TMDs without expansion in the c-axis direction. In fact, there is plenty of room in the 2D world beyond graphene. We envisage that with increasing progress in the controlled synthesis of these systems the unusual properties of mono- and few-layered TMDs and TMD heterostructures will be unveiled.
    Accounts of Chemical Research 12/2014; · 24.35 Impact Factor
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    ABSTRACT: The use of graphene as a template layer for the heteroepitaxy of III-nitrides (GaN and AlN) has gained interest due to the hexagonal arrangement of the sp2 hybridized carbon atoms being similar to the (0001) c-plane of wurtzite GaN. In this study, the nucleation of GaN and AlN by metalorganic chemical vapor deposition on quasi-free standing epitaxial graphene (EG) was investigated. We observed that the nucleation of AlN and GaN was preferential along the periodic (1View the MathML source0n) EG coated step edges and at defects sites on the (0001) terraces due to the enhanced chemical reactivity at those regions. The density of nuclei on the (0001) terraces of EG increased with the incorporation of nitrogen defects into the graphene lattice via NH3 exposure as was evident from surface chemical analysis by XPS. Raman spectral mapping showed that GaN selectively nucleates on regions of few-layered EG as opposed to regions of multi-layered EG. HR-TEM also revealed that the EG underlayers were highly defective in the region of GaN nucleation, however, the GaN nuclei were single crystalline, c-axis oriented and were free of threading dislocations. In contrast, polycrystalline islands of AlN were found to nucleate on EG without producing disorder in the underlying EG.
    Surface Science 11/2014; 634. · 1.87 Impact Factor
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    ABSTRACT: Heterogeneous engineering of two-dimensional layered materials, including metallic graphene and semiconducting transition metal dichalcogenides, presents an exciting opportunity to produce highly tunable electronic and optoelectronic systems. In order to engineer pristine layers and their interfaces, epitaxial growth of such heterostructures is required. We report the direct growth of crystalline, monolayer tungsten diselenide (WSe2) on epitaxial graphene (EG) grown from silicon carbide. Raman spectroscopy, photoluminescence, and scanning tunneling microscopy confirm high-quality WSe2 monolayers; while transmission electron microscopy shows an atomically sharp interface, and low energy electron diffraction confirms near perfect orientation between WSe2 and EG. Vertical transport measurements across the WSe2/EG heterostructure provides evidence that an additional barrier to carrier transport beyond the expected WSe2/EG band offset exists due to the inter-layer gap, which is supported by theoretical local density of states (LDOS) calculations using self-consistent density functional theory (DFT) and non-equilibrium Green's function (NEGF).
    Nano Letters 11/2014; · 12.94 Impact Factor
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    ABSTRACT: Hexagonal boron nitride (h-BN) atomic layers are synthesized on polycrystalline copper foils via a novel chemical vapor deposition (CVD) process that maintains a vapor-phase copper overpressure during growth. Compared to h-BN films grown without a copper overpressure, this process results in a >10x reduction of 3-dimensional BN fullerene-like surface features, a reduction of carbon and oxygen contamination of 65% and 62%, respectively, an increase in h-BN grain size of >2x, and an 89% increase in electrical breakdown strength.
    ACS Applied Materials & Interfaces 09/2014; · 5.90 Impact Factor
  • APL Materials. 09/2014; 2(9):092508.
  • Ganesh R Bhimanapati, Daniel Kozuch, Joshua A Robinson
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    ABSTRACT: A simple and inexpensive method to functionalize hexagonal boron nitride (hBN) was achieved by using an acid mixture of phosphoric and sulphuric acid. This functionalization induced the exfoliation of the layered structure of hBN into monolayer to few-layer sheets where the sizes of the sheets were dependent on the parent hBN powder used. Exfoliated hBN was shown to be stable in solvents such as ethanol, acetone, deionized water and isopropyl alcohol, and this stability was linked to sulfur functionalization that was induced during the exfoliation process. Further evidence of the functionalization was observed using transmission electron spectroscopy (TEM) and X-ray photoelectron spectroscopy (XPS). By deconvoluting the high resolution peaks for B 1s, the bonding of boron to oxygen and sulfur was confirmed. The exfoliated hBN nanosheets were crystalline as confirmed from X-ray diffraction and they also exhibited an optically active defect related to sulfur functionalization at 320 nm (3.9 ± 0.1 eV).
    Nanoscale 08/2014; · 6.74 Impact Factor
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    ABSTRACT: Graphene's unique symmetry between p- and n-branches has enabled several interesting device applications; however, short-channel devices often exhibit degraded symmetry. We examine how graphene nanoribbon geometries can improve transfer characteristics and p–n symmetry, as well as reduce Dirac point shift for highly scaled graphene devices. RF graphene transistors utilizing a multiribbon channel are fabricated with channel length down to 100 nm, achieving 4.5-fold improved transconductance, 3-fold improved cutoff frequency, and 2.4-fold improved symmetry compared with sheet devices. The improved performance is linked to reduced contact effects by modeling the extent of charge transfer into the channel as a function of graphene width.
    Applied Physics Express 04/2014; 7(5):055103. · 2.73 Impact Factor
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    ABSTRACT: The stacking of two-dimensional layered materials such as semiconducting transition metal dichalcogenides (TMDs), insulating hexagonal boron nitride (h-BN), and semi-metallic graphene has been theorized to produce tunable electronic and optoelectronic properties. Here we demonstrate the direct growth of MoS2, WSe2, and hBN on epitaxial graphene to form large area van der Waal heterostructures. We reveal that the properties of the underlying graphene dictate properties of the heterostructures, where strain, wrinkling, and defects on the surface of graphene act as nucleation centers for lateral growth of the overlayer. Additionally, we demonstrate that the direct synthesis of TMDs on epitaxial graphene exhibits atomically sharp interfaces. Finally we demonstrate that direct growth of MoS2 on epitaxial graphene can lead to a 103 improvement in photoresponse compared to MoS2 alone.
    ACS Nano 03/2014; · 12.03 Impact Factor
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    ABSTRACT: The structural evolution of thick polycrystalline gadolinium oxide (Gd2O3) films deposited by reactive electron beam-physical vapor deposition (EB-PVD) is investigated. High deposition rates (> 5 Angstroms/s) lead to the growth of mixed phase films which are of the cubic phase near the film/substrate interfaces before forming monoclinic phase as distance from the interface increases. By decreasing the deposition rate to < 1 Angstroms/s for films grown at temperatures of 650C, films up to one micron thick have been grown in the pure cubic phase. The growth of the thermodynamically stable cubic phase under these conditions is attributed to both higher surface mobility of the adatoms during growth and to increased tensile stress within the film. Ion beam assisted deposition (IBAD) was then performed to introduce compressive stress into the film resulting in the formation of the monoclinic phase. Wafer curvature, x-ray diffraction, confocal Raman spectroscopy, and scanning electron microscopy are utilized to characterize the film and present evidence for the existence of a stress-induced phase transition in the Gd2O3 films.
    Surface and Coatings Technology 03/2014; · 2.20 Impact Factor
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    ABSTRACT: We report the realization of top-gated graphene nanoribbon field effect transistors (GNRFETs) of ~10 nm width on large-area epitaxial graphene exhibiting the opening of a band gap of ~0.14 eV. Contrary to prior observations of disordered transport and severe edge-roughness effects of GNRs, the experimental results presented here clearly show that the transport mechanism in carefully fabricated GNRFETs is conventional band-transport at room temperature, and inter-band tunneling at low temperature. The entire space of temperature, size, and geometry dependent transport properties and electrostatics of the GNRFETs are explained by a conventional thermionic emission and tunneling current model. Our combined experimental and modeling work proves that carefully fabricated narrow GNRs behave as conventional semiconductors, and remain potential candidates for electronic switching devices.
    10/2013;
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    ABSTRACT: We present a route for direct growth of boron nitride via a polyborazylene to h-BN conversion process. This two-step growth process ultimately leads to a >25x reduction in the RMS surface roughness of h-BN films when compared to a high temperature growth on Al2O3(0001) and Si(111) substrates. Additionally, the stoichiometry is shown to be highly dependent on the initial polyborazylene deposition temperature. Importantly, CVD graphene transferred to direct-grown boron nitride films on Al2O3 at 400{\deg}C results in a >1.5x and >2.5x improvement in mobility compared to CVD graphene transferred to Al2O3 and SiO2 substrates, respectively, which is attributed to the combined reduction of remote charged impurity scattering and surface roughness scattering. Simulation of mobility versus carrier concentration confirms the importance of limiting the introduction of charged impurities in the h-BN film and highlights the importance of these results in producing optimized h-BN substrates for high performance graphene and TMD devices.
    Journal of Materials Research 10/2013; 29(03). · 1.82 Impact Factor
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    ABSTRACT: We present a comprehensive study on the integration of hexagonal boron nitride (h-BN) with epitaxial graphene (EG) and bilayer hydrogen intercalated EG. Charged impurity scattering is the dominant scattering mechanism for as-grown and h-BN coated graphene. Use of h-BN dielectrics leads to a 2.6× improvement in Hall mobility relative to HfO2 by introducing less charged impurities and negligible additional remote surface optical scattering beyond that introduced by the substrate. Temperature dependent mobility measurement is used to link the surface morphology of the silicon carbide substrate (i.e., step-edge density) with charge carrier transport, showing that significant degradation in mobility can result from increased remote charged impurity as well as remote surface optical scattering at the SiC step-edges. Furthermore, we demonstrate that the integration of h-BN with EG and bilayer graphene presents unique challenges compared to previous works on exfoliated graphene, where the benefits of h-BN as a dielectric is highly dependent on the initial quality of the EG. To this end, modeling of the carrier mobility as a function of impurity density is used to identify the regimes where h-BN dielectrics outperform conventional dielectrics and where they fail to surpass them. Modeling indicates that h-BN can ultimately lead to a >5× increase in mobility relative to HfO2 dielectrics due to higher energy surface optical phonon (SOP) modes.
    Physica Status Solidi (A) Applications and Materials 06/2013; 210(6). · 1.53 Impact Factor
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    ABSTRACT: In this work, we investigate the effect of nano-ribbon geometries on graphene device performance and explain its effect on reducing the negative impact of Dirac point shift due to charge transfer into the graphene channel from the metal-graphene contact thereby leading to improved device performance and balanced n, p FET performance at submicron channel lengths.
    2013 71st Annual Device Research Conference (DRC); 06/2013
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    ABSTRACT: In recent years, there has been great interest in utilizing graphene based electronics for high frequency RF applications. To this end, researchers have demonstrated several key applications using graphene devices [1-3]. One application of interest is the low noise amplifier (LNA), where graphene's high mobility and high velocity saturation can potentially allow for very high frequency of operation as well as low noise. To this end, [4] has demonstrated a 10dB graphene amplifier, yet there exists no experimental study investigating the linearity of a graphene LNA. In this study we analyze the third order intermodulation product, gain compression and high frequency noise performance of graphene transistors for LNA application and benchmark it with other RF device technologies. The graphene amplifier (un-matched) exhibits an output third order intercept (OIP3) of 19dBm and input 1dB gain compression (Pin, 1dB) of 5.6dBm. We also report excellent noise performance for the graphene transistor, with intrinsic NFmin of 0.26dB (extrinsic 1.26dB) at 1GHz.
    2013 71st Annual Device Research Conference (DRC); 06/2013
  • 223th ECS Meeting; 05/2013
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    ABSTRACT: A key limitation to graphene based electronics is graphene's interaction with dielectric interfaces. SiO2 and various high-k gate dielectrics can introduce scattering from charged surface states, impurities, and surface optical phonons; degrading the transport properties of graphene. Hexagonal boron nitride (h-BN) exhibits an atomically smooth surface that is expected to be free of dangling bonds, leading to an interface that is relatively free of surface charge traps and adsorbed impurities. Additionally, the decreased surface optical phonon interaction from h-BN is expected to further reduce scattering. While h-BN gated graphene FETs have been demonstrated on a small scale utilizing CVD grown or exfoliated graphene, integrating quasi-freestanding epitaxial graphene (QFEG) with h-BN gate dielectrics on a wafer scale has not been explored. We present results from the first large scale CVD growth of h-BN and its subsequent transfer to a 75mm QFEG wafer. The effects of growth conditions on the thickness and quality of the h-BN film and its potential and limitations as a gate dielectric to QFEG are discussed. The introduction of charged impurities during the transfer process resulted in an average degradation in mobility of only 9%. Despite the slight degradation, we show that h-BN is highly beneficial compared to high-k dielectrics when the charged impurity concentration of QFEG is below 5x1012cm-2. Here we show improvements in mobility of >3x and intrinsic cutoff frequency of >2x compared to HfO2.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2012; · 0.20 Impact Factor
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    ABSTRACT: The effects of growth temperature, film thickness, and oxygen flux on the microstructure, phase transition, and interfacial chemistry of gadolinium oxide (Gd{sub 2}O{sub 3}) films grown on Si(111) substrates by electron-beam physical vapor deposition were investigated using a combination of transmission electron microscopy (TEM), electron diffraction, scanning TEM, x-ray energy dispersive spectrometry, and electron energy loss spectrometry. The authors find that a low growth temperature (250 Degree-Sign C) and a high oxygen flux (200 sccm) led to a small grain size and a high porosity of the Gd{sub 2}O{sub 3} film. Lowering the oxygen flux to 50 sccm led to reduced film porosity, presumably due to the increased diffusion length of the Gd atoms on the surface. Increasing the growth temperature to 650 Degree-Sign C resulted in a film with large columnar grains and elongated pores at the grain boundaries. Thin films grown at 250 Degree-Sign C consisted of cubic Gd{sub 2}O{sub 3}, but thermodynamically less stable monoclinic phase formed as the film thickness increased. Lowering the oxygen flux apparently further promoted the formation of the monoclinic phase. Furthermore, monoclinic phase dominated in the films grown at 650 Degree-Sign C. Such phase transitions may be related to the stress evolution of the films at different temperatures, thicknesses, and oxygen fluxes. Enhanced Gd{sub 2}O{sub 3}/Si interfacial reaction was observed as the growth temperature, film thickness, and oxygen flux increased. Moreover, oxygen was found to play a crucial role in the Gd{sub 2}O{sub 3}/Si interfacial reaction and the formation of Gd-Si-O interface layers, which proceeded by the reaction of excess oxygen with Si followed by the intermixing of SiO{sub x} and Gd{sub 2}O{sub 3}.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 07/2012; 30(4). · 2.14 Impact Factor

Publication Stats

480 Citations
203.61 Total Impact Points

Institutions

  • 2009–2014
    • Pennsylvania State University
      • • Department of Materials Science and Engineering
      • • Center for Electro-Optics (EOC)
      University Park, Maryland, United States