Joshua A. Robinson

Pennsylvania State University, University Park, Maryland, United States

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Publications (36)191.18 Total impact

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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; · 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
  • 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
  • 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.
    Proc SPIE 09/2012;
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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
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    ABSTRACT: Current state-of-the-art nanotechnology offers multiple benefits for radiation sensing applications. These include the ability to incorporate nano-sized radiation indicators into widely used materials such as paint, corrosion-resistant coatings, and ceramics to create nano-composite materials that can be widely used in everyday life. Additionally, nanotechnology may lead to the development of ultra-low power, flexible detection systems that can be embedded in clothing or other systems. Graphene, a single layer of graphite, exhibits exceptional electronic and structural properties, and is being investigated for high-frequency devices and sensors. Previous work indicates that graphene-oxide (GO) - a derivative of graphene - exhibits luminescent properties that can be tailored based on chemistry; however, exploration of graphene-oxide's ability to provide a sufficient change in luminescent properties when exposed to gamma or neutron radiation has not been carried out. We investigate the mechanisms of radiation-induced chemical modifications and radiation damage induced shifts in luminescence in graphene-oxide materials to provide a fundamental foundation for further development of radiation sensitive detection architectures. Additionally, we investigate the integration of hexagonal boron nitride (hBN) with graphene-based devices to evaluate radiation induced conductivity in nanoscale devices. Importantly, we demonstrate the sensitivity of graphene transport properties to the presence of alpha particles, and discuss the successful integration of hBN with large area graphene electrodes as a means to provide the foundation for large-area nanoscale radiation sensors.
    Proc SPIE 05/2012;
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    ABSTRACT: Hexagonal boron nitride (h-BN) is a promising dielectric material for graphene-based electronic devices. Here we investigate the potential of h-BN gate dielectrics, grown by chemical vapor deposition (CVD), for integration with quasi-freestanding epitaxial graphene (QFEG). We discuss the large scale growth of h-BN on copper foil via a catalytic thermal CVD process and the subsequent transfer of h-BN to a 75 mm QFEG wafer. X-ray photoelectron spectroscopy (XPS) measurements confirm the absence of h-BN/graphitic domains and indicate that the film is chemically stable throughout the transfer process, while Raman spectroscopy indicates a 42% relaxation of compressive stress following removal of the copper substrate and subsequent transfer of h-BN to QFEG. Despite stress-induced wrinkling observed in the films, Hall effect measurements show little degradation (<10%) in carrier mobility for h-BN coated QFEG. Temperature dependent Hall measurements indicate little contribution from remote surface optical phonon scattering and suggest that, compared to HfO(2) based dielectrics, h-BN can be an excellent material for preserving electrical transport properties. Graphene transistors utilizing h-BN gates exhibit peak intrinsic cutoff frequencies >30 GHz (2.4× that of HfO(2)-based devices).
    ACS Nano 04/2012; 6(6):5234-41. · 12.03 Impact Factor
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    ABSTRACT: Chemical vapor deposition on copper substrates is a primary technique for synthesis of high quality graphene films over large areas. While well-developed processes are in place for catalytic growth of graphene on bulk copper substrates, chemical vapor deposition of graphene on thin films could provide a means for simplified device processing through the elimination of the layer transfer process. Recently, it was demonstrated that transfer-free growth and processing is possible on SiO(2). However, the Cu/SiO(2)/Si material system must be stable at high temperatures for high quality transfer-free graphene. This study identifies the presence of interdiffusion at the Cu/SiO(2) interface and investigates the influence of metal (Ni, Cr, W) and insulating (Si(3)N(4), Al(2)O(3), HfO(2)) diffusion barrier layers on Cu-SiO(2) interdiffusion, as well as graphene structural quality. Regardless of barrier choice, we find the presence of Cu diffusion into the silicon substrate as well as the presence of Cu-Si-O domains on the surface of the copper film. As a result, we investigate the choice of a sapphire substrate and present evidence that it is a robust substrate for synthesis and processing of high quality, transfer-free graphene.
    Nanotechnology 03/2012; 23(13):135601. · 3.67 Impact Factor
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    ABSTRACT: Thick polycrystalline gadolinium oxide (Gd2O3) films up to 11 μm in thickness were deposited via reactive electron beam-physical vapor deposition (EB-PVD) on silicon (111) substrates for use in neutron radiation detection. The effects of coating thickness, substrate temperature, and oxygen flow on film structural, electrical and optical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), capacitance–voltage (C–V) measurements, and ultraviolet–visible (UV–Vis) spectroscopy. Films were characterized as either monoclinic or mixed monoclinic and cubic phase depending on deposition parameters. Increasing the deposition temperature resulted in increased film crystallinity and cubic phase volume while decreasing the O2 flow rate resulted in increased volume of the monoclinic phase. Evidence of a thickness dependent crystallography is also presented. Electrical property measurements showed thin film dielectric constant could be tailored between 12 and 20 at 1 MHz frequency by decreasing the oxygen flow rate at deposition temperatures of 250 °C which is attributed to an increased presence of the monoclinic phase and increased film density. Band gap values were calculated from transmission measurements and ranged between 5.44 and 5.96 eV.
    Surface and Coatings Technology 02/2012; 206(13):3094–3103. · 2.20 Impact Factor
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    ABSTRACT: The fundamental structural properties of multilayer epitaxial graphene (MEG) on C-face SiC(000) were revealed in a straightforward manner using cross-sectional transmission electron microscopy (TEM) and scanning TEM (STEM). The AB-stacking and the azimuthal rotational disorder of the graphene layers were directly identified by selected area electron diffraction and high-resolution TEM. The directly interpretable STEM revealed that the interlayer spacing between the first graphene layer and the top SiC bilayer is substantially larger than that of the bulk graphite. Such a large interlayer spacing combined with the regional partially decomposed top bilayers of the SiC substrate provides a plausible explanation to the weak bonding between the MEG film and the SiC(000) substrate.
    Applied Physics Letters 01/2012; 100(3). · 3.52 Impact Factor
  • H. Madan, Matthew J. Hollander, Joshua A. Robinson, S. Datta
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    ABSTRACT: Graphene as a material has created a lot of interest due to properties like high saturation velocity, high current carrying capacity, ambipolar characteristics and high transconductance. These properties make graphene based transistors a promising candidate for high frequency applications. Recently, there have been demonstration of RF mixers with graphene transistors. Traditional DC measurements are not sufficient when considering graphene transistors for high frequency circuit design, making it essential to study the transistor IV performance at operating frequencies >;GHz. In this work we outline an RF IV extraction technique and use physics based analytical model to evaluate the performance of graphene transistors with HfO2 high-κ dielectric.
    Device Research Conference (DRC), 2012 70th Annual; 01/2012

Publication Stats

279 Citations
191.18 Total Impact Points

Institutions

  • 2009–2014
    • Pennsylvania State University
      • • Department of Materials Science and Engineering
      • • Department of Physics
      University Park, Maryland, United States