Joshua A. Robinson

University of Texas at Dallas, Richardson, Texas, United States

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Publications (51)273.07 Total impact

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    ABSTRACT: Graphene-covered copper surfaces have been exposed to borazine, (BH)3(NH)3, with the resulting surfaces characterized by low-energy electron microscopy. Although the intent of the experiment was to form hexagonal boron nitride (h-BN) on top of the graphene, such layers were not obtained. Rather, in isolated surface areas, h-BN is found to form micrometer-size islands that substitute for the graphene. Additionally, over nearly the entire surface, the properties of the layer that was originally graphene is observed to change in a manner that is consistent with the formation of a mixed h-BN/graphene alloy, i.e. h-BNC alloy. Furthermore, following the deposition of the borazine, a small fraction of the surface is found to consist of bare copper, indicating etching of the overlying graphene. The inability to form h-BN layers on top of graphene is discussed in terms of the catalytic behavior of the underlying copper surface and the decomposition of the borazine on top of the graphene.
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    ABSTRACT: Tungsten ditelluride (WTe2) is a transition metal dichalcogenide (TMD) with physical and electronic properties that make it attractive for a variety of electronic applications. Although WTe2 has been studied for decades, its structure and electronic properties have only recently been correctly described. We experimentally and theoretically investigate the structure, dynamics and electronic properties of WTe2, and verify that WTe2 has its minimum energy configuration in a distorted 1T structure (Td structure), which results in metallic-like transport. Our findings unambiguously confirm the metallic nature of WTe2, introduce new information about the Raman modes of Td-WTe2, and demonstrate that Td-WTe2 is readily oxidized via environmental exposure. Finally, these findings confirm that, in its thermodynamically favored Td form, the utilization of WTe2 in electronic device architectures such as field effect transistors may need to be reevaluated.
    Scientific Reports 06/2015; 5:10013. DOI:10.1038/srep10013 · 5.58 Impact Factor
  • Daniel A. Grave · Joshua A. Robinson · Douglas E. Wolfe
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    ABSTRACT: One micron thick Gd2O3 films were grown on GaN/AlGaN heterostructures by reactive electron beam physical vapor deposition. The films were of cubic bixbyite phase with strong (222) out-of-plane and in-plane textures. The films showed a columnar microstructure with feather-like growth. Transmission electron microscopy analysis and selected area diffraction showed highly oriented single crystal like growth near the film interface which degraded as the film thickness increased. Capacitance–voltage (C–V) characteristics show that the Gd2O3 device results in a negative threshold shift of approximately 1.9 V. Hysteresis of 0.9 V was extracted from the C–V curve corresponding to a trapped charge density of 6.9 × 1010 cm− 2. The conduction mechanisms were found to be dominated by Poole–Frenkel conduction between 50 and 100 °C and Schottky emission between 125 and 200 °C. The trap height for Poole–Frenkel conduction was 0.46 eV and the Schottky barrier height was 0.79 eV.
    Thin Solid Films 05/2015; 589. DOI:10.1016/j.tsf.2015.04.072 · 1.76 Impact Factor
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    ABSTRACT: Vertical stacking of two-dimensional (2D) crystals has recently attracted substantial interest due to unique properties and potential applications they can introduce. Little is however known about their microstructure as fabrication of the two-dimensional (2D) heterostructures on a rigid substrate limits one's ability to directly study their atomic and chemical structures using electron microscopy. This study demonstrates a unique approach to create atomically thin freestanding van der Waals heterostructures: WSe2/graphene and MoS2/graphene, as ideal model systems to investigate the nucleation and growth mechanisms in heterostructures. In this study we use transmission electron microscopy (TEM) imaging and diffraction to show epitaxial growth of the freestanding WSe2/graphene heterostructure, while no epitaxy is maintained in the MoS2/graphene heterostructure. Ultra high-resolution aberration-corrected scanning transmission electron microscopy (STEM) shows growth of monolayer WSe2 and MoS2 triangles on graphene membrane and reveals their edge morphology and crystallinity. Photoluminescence (PL) measurements indicate a significant quenching of the photoluminescence response for the transition metal dichalcogenides (TMDs) on freestanding graphene, compared to those on a rigid substrate, i.e. sapphire and epitaxial graphene (EG). Using a combination of (S)TEM imaging and electron diffraction analysis, this study also reveals the significant role of defects on the heterostructure growth. The direct growth technique applied here enables us to investigate the heterostructure nucleation and growth mechanisms at the atomic level without sample handling and transfer. Importantly, this approach can be utilized to study a wide spectrum of van der Waals heterostructures.
    ACS Nano 04/2015; 9(5). DOI:10.1021/acsnano.5b01677 · 12.88 Impact Factor
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    ABSTRACT: Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalocogenides: MoS2, MoSe2, and WSe2.The realization of MoS2-WSe2-Graphene and WSe2-MoSe2-Graphene heterostructures leads toresonant tunneling in an atomically thin stack with spectrally narrow room temperature negative differential resistance characteristics.
    Nature Communications 03/2015; 6. DOI:10.1038/ncomms8311 · 11.47 Impact Factor
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    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.
    2D Materials 02/2015; 2(1). DOI:10.1088/2053-1583/2/1/014003
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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; 9(2). DOI:10.1021/nn5073286 · 12.88 Impact Factor
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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; 15(3). DOI:10.1021/nl504662b · 13.59 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; 48(1). DOI:10.1021/ar5002846 · 22.32 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. DOI:10.1016/j.susc.2014.11.020 · 1.93 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; 14(12). DOI:10.1021/nl503144a · 13.59 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; 6(19). DOI:10.1021/am503844u · 6.72 Impact Factor
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    ABSTRACT: The utilization of tungsten diselenide (WSe2) in electronic and optoelectronic devices depends on the ability to understand and control the process-property relationship during synthesis. We demonstrate that spectroscopic ellipsometry is an excellent technique for accurate, non-destructive determination of ultra-thin (<30 nm) WSe2 properties. The refractive index (n) and extinction coefficient (k) were found to be independent of thickness down to 1.3 nm, and were used to determine film thickness, which was confirmed to be within 9% of values found via atomic force microscopy. Finally, the optical bandgap was found to closely correlate with thickness, ranging from 1.2 to 1.55 eV as the WSe2 is thinned to the equivalent of 2 atomic layers. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
    APL Materials 09/2014; 2(9):092508. DOI:10.1063/1.4893961 · 2.79 Impact Factor
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    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(20). DOI:10.1039/c4nr01816h · 7.39 Impact Factor
  • Annual Review of Materials Research 08/2014; 45(1):150514142752001. DOI:10.1146/annurev-matsci-070214-021034 · 11.85 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. DOI:10.7567/APEX.7.055103 · 2.37 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; 8(4). DOI:10.1021/nn5003858 · 12.88 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; 242. DOI:10.1016/j.surfcoat.2014.01.020 · 2.00 Impact Factor
  • Journal of Materials Research 02/2014; 29(3):297-298. DOI:10.1557/jmr.2014.4 · 1.65 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 graphene nanoribbons (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.
    APL Materials 10/2013; 3(1). DOI:10.1063/1.4905155 · 2.79 Impact Factor

Publication Stats

583 Citations
273.07 Total Impact Points


  • 2015
    • University of Texas at Dallas
      • Department of Materials Science & Engineering
      Richardson, Texas, United States
  • 2009–2015
    • Pennsylvania State University
      • • Department of Materials Science and Engineering
      • • Center for Electro-Optics (EOC)
      University Park, Maryland, United States
  • 2009–2014
    • William Penn University
      Worcester, Massachusetts, United States