1. Enhanced catalytic activity in strained chemically exfoliated WS2 nanosheets for hydrogen evolution
    Damien Voiry, Hisato Yamaguchi, Junwen Li, Rafael Silva, Diego C. B. Alves, Takeshi Fujita, Mingwei Chen, Tewodros Asefa, Vivek B. Shenoy, Goki Eda, and Manish Chhowalla Nature Materials 12, 850 (2013)

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    Efficient evolution of hydrogen through electrocatalysis at low overpotentials holds tremendous promise for clean energy. Hydrogen evolution can be easily achieved by electrolysis at large potentials that can be lowered with expensive platinum-based catalysts. Replacement of Pt with inexpensive, earth-abundant electrocatalysts would be significantly beneficial for clean and efficient hydrogen evolution. To this end, promising results have been reported using 2H (trigonal prismatic) XS2 (where X = Mo or W) nanoparticles with a high concentration of metallic edges. The key challenges for XS2 are increasing the number and catalytic activity of active sites. Here we report monolayered nanosheets of chemically exfoliated WS2 as efficient catalysts for hydrogen evolution with very low overpotentials. Analyses indicate that the enhanced electrocatalytic activity of WS2 is associated with the high concentration of the strained metallic 1T (octahedral) phase in the as-exfoliated nanosheets. Our results suggest that chemically exfoliated WS2 nanosheets are interesting catalysts for hydrogen evolution.

  2. Structure and Electronic Properties of Grain Boundaries in Earth-Abundant Photovoltaic Absorber Cu2ZnSnSe4
    Junwen Li, David B. Mitzi, and Vivek B. Shenoy, ACS Nano 5, 8613 (2011)

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    We have studied the atomic and electronic structure of Cu2ZnSnSe4 and CuInSe2 grain boundaries using first-principles calculations. We find that the constituent atoms at the grain boundary in Cu2ZnSnSe4 create localized defect states that promote the recombination of photon-excited electron and hole carriers. In distinct contrast, significantly lower density of defect states is found at the grain boundaries in CuInSe2, which is consistent with the experimental observation that CuInSe2 solar cells exhibit high conversion efficiency without the need for deliberate passivation. Our investigations suggest that it is essential to effectively remove these defect states in order to improve the conversion efficiency of solar cells with Cu2ZnSnSe4 as photovoltaic absorber materials.

  3. Bonding Charge Density and Ultimate Strength of Monolayer Transition Metal Dichalcogenides
    Junwen Li, Nikhil V. Medhekar, and Vivek B. Shenoy, J. Phys. Chem. C 117, 15842 (2013)

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    We have carried out first-principles density functional calculations on the mechanical response of two-dimensional (2D) monolayer group VI transition metal dichalcogenides (TMDs) to large elastic deformation. We find that the ultimate strength and the overall stress response of these 2D materials is strongly influenced by their chemical composition and loading direction. We demonstrate that differences in the observed mechanical behavior can be attributed to the spatial redistribution of the occupied hybridized electronic states in the region between the transition metal atom and the chalcogens. Despite the strong covalent bonding between the transition metal and the chalcogens, we find that a simple linear relationship can be established to describe the dependence of the mechanical strength on the charge transfer from the transition metal atom to the chalcogens. Our studies suggest that these two-dimensional semiconducting TMDs can withstand large deformations, making them attractive for application in novel strain-engineered and flexible electronic and optoelectronic devices.

  4. Hidden One-Electron Interactions in Carbon Nanotubes Revealed in Graphene Nanostrips
    Carter T. White, Junwen Li, Daniel Gunlycke, and John W. Mintmire, Nano Lett. 7, 825 (2007)

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    Many single-wall carbon nanotube (SWNT) properties near the Fermi level were successfully predicted using a nearest-neighbor tight-binding model characterized by a single parameter, V1. We show however that this model fails for armchair-edge graphene nanostrips due to interactions directly across hexagons. These same interactions are found largely hidden in the description of SWNTs, where they renormalize V1 leaving previous nearest-neighbor model SWNT results largely intact while resolving a long-standing puzzle regarding the magnitude of V1.

  5. Graphene quantum dots embedded in hexagonal boron nitride sheets
    Junwen Li and Vivek B. Shenoy Appl. Phys. Lett. 98, 013105 (2011)

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    We have carried out first-principles calculations on electronic properties of graphene quantum dots embedded in hexagonal boron nitride monolayer sheets. The calculations with density functional theory show that the band gaps of quantum dots are determined by the quantum confinement effects and the hybridization of π orbitals from B, N, and C atoms. The energy states near the Fermi level are found to be strongly localized within and in the vicinity of the quantum dots.

  6. Edges Bring New Dimension to Graphene Nanoribbons
    Daniel Gunlycke, Junwen Li, John W. Mintmire and Carter T. White Nano Lett. 10, 3638 (2010)

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    Chemistry at the edges of saturated graphene nanoribbons can cause ribbons to leave the plane and form three-dimensional helical structures. Calculations, based on density functional theory and enabled by adopting helical symmetry, show that F-terminated armchair ribbons are intrinsically twisted in helices, unlike flat H-terminated strips. Twisting ribbons of either termination couple the conduction and valence bands, resulting in band gap modulation. This electromechanical response could be exploited in switches and sensor applications.

    Colloquia & Seminars

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    2. Department of Physics and Astronomy Colloquia
      4:00 PM every second Wednesday at Room A8 of David Rittenhouse Laboratory (DRL)
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    Research Interest

    I would like to understand and predict electronic and optical properties of materials using first principles calculations.


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