Our Publications

  • Interfacial Properties of Monolayer MoSe2-Metal Contacts. The Journal of Physical Chemistry C. 2016

    Monolayer (ML) transition-metal dichalcogenides are considered as promising channel materials in next-generation transistors. Using ab initio energy band calculations and more reliable ab initio quantum transport simulations, we study the interfacial properties of ML MoSe2–metal interfaces (metals = Al, Ag, Pt, Cr, Ni, and Ti). Weak or medium adsorption is found between ML MoSe2 and the Al, Ag, and Pt surfaces with the band structure of ML MoSe2 preserved, while strong adsorption is found between ML MoSe2 and the Ni, Ti, and Cr surfaces with the band structure of ML MoSe2 destroyed. The two methods give similar polarity and height of Schottky barriers for ML MoSe2 with Al, Ag, Pt, and Ti electrodes. ML MoSe2 forms an n-type Schottky contact with Ag, Ti, and Al electrodes with electron Schottky barrier heights (SBH) of 0.25, 0.29, and 0.56 eV, respectively, and a p-type Schottky contact with Pt electrode with hole SBH of 0.78 eV according to ab initio quantum transport simulations. Our study offers a guidance for the choices of suitable metal electrodes in ML MoSe2 devices.

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  • Magnetoresistance in Co/2D MoS 2/Co and Ni/2D MoS 2/Ni junctions. Physical Chemistry Chemical Physics. 2016

    Semiconducting single-layer (SL) and few-layer MoS2 have a flat surface, free of dangling bonds. Using density functional theory coupled with non-equilibrium Green's function method, we investigate the spin-polarized transport properties of Co/2D MoS2/Co and Ni/2D MoS2/Ni junctions with MoS2 layer numbers of N = 1, 3, and 5. Well-defined interfaces are formed between MoS2 and metal electrodes. The junctions with a SL MoS2 spacer are almost metallic owing to the strong coupling between MoS2 and the ferromagnets, while those are tunneling with a few layer MoS2 spacer. Both large magnetoresistance and tunneling magnetoresistance are found when fcc or hcp Co is used as an electrode. Therefore, flat single- and few-layer MoS2 can serve as an effective nonmagnetic spacer in a magnetoresistance or tunneling magnetoresistance device with a well-defined interface.

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  • Effect of aging-induced disorder on the quantum transport properties of atomically thin WTe2. arXiv:1608.04801. 2016

    Atomically thin layers of transition-metal dicalcogenides (TMDCs) are often known to be metastable in the ambient atmosphere. Understanding the mechanism of degradation is essential for their future applications in nanoelectronics, and thus has attracted intensive interest recently. Here, we demonstrate a systematic study of atomically thin WTe$_{2}$ in its low temperature quantum electronic transport properties. Strikingly, while the temperature dependence of few layered WTe$_{2}$ showed clear metallic tendency in the fresh state, degraded devices first exhibited a re-entrant insulating behavior, and finally entered a fully insulating state. Correspondingly, a crossover from parabolic to linear magnetoresistance, and finally to weak anti-localization was seen. Real-time Raman scattering measurement, together with transmission electron microscopy studies done before and after air degradation of atomically thin WTe$_{2}$ further confirmed that the material gradually form amorphous islands. It thus leads to localized electronic states and explains the low temperature Coulomb gap observed in transport measurements. Our study reveals for the first time the correlation between the unusual magnetotransport and disorder in few-layered WTe2, which is indispensable in providing guidance on its future devices application.

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  • Mechanical and electronic coupling in few-layer graphene and hBN wrinkles: a first-principles study. Nanotechnology. 2016

    Wrinkle engineering is an important pathway to develop novel functional devices of two-dimensional materials. By combining first-principles calculations and continuum mechanics modelling, we have investigated the wrinkling of few-layer graphene and hexagonal boron nitride (hBN) and provide a way to estimate their bending stiffness. For few-layer wrinkles under the same strain, the magnitude of structural deformation of each constituent layer gradually decreases from bottom to top layers, while interlayer interaction increases with increasing layer number. Comparing with monolayer wrinkles, the electronic properties of few-layer wrinkles are more sensitive to bending deformation as mechanical and electronic coupling induce charge redistribution at the wrinkles, making few-layer graphene and hBN wrinkles suitable for electromechanical system application.

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  • Reduction of interfacial friction in commensurate graphene/h-BN heterostructures by surface functionalization. Nanoscale. 2016

    The reduction of interfacial friction in commensurately stacked two-dimensional layered materials is important for their application in nanoelectromechanical systems. Our first-principles calculations on the sliding energy corrugation and friction at the interfaces of commensurate fluorinated-graphene/h-BN and oxidized-graphene/h-BN heterostructures show that the sliding energy barriers and shear strengths for these heterostructures are approximately decreased to 50% of those of commensurate graphene/h-BN. The adsorbed F and O atoms significantly suppress the interlayer electrostatic and van der Waals energy corrugations by modifying the geometry and charge redistribution of the graphene layers. Our empirical registry index models further reveal the difference between the roles of the F and O atoms in affecting the sliding energy landscapes, and are also utilized to predict the interlayer superlubricity in a large-scale oxidized-graphene/h-BN system. Surface functionalization is a valid way to control and reduce the interlayer friction in commensurate graphene/h-BN heterostructures.

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  • Wettability of Supported Monolayer Hexagonal Boron Nitride in Air. Advanced Functional Materials. 2016

    Hexagonal boron nitride (h-BN), a wide band gap monolayer crystal with structure similar to graphene, is optically transparent with exceptionally high thermal and chemical stability, and should be ideal to serve as an atomically thin coating. However, limited by the challenges in fabricating h-BN of high quality in large area, the wetting performance of h-BN has seldom been studied. Here, it is shown that the water contact angle of freshly grown h-BN film is nearly independent of the underlying materials as well as the h-BN layer number, but increases gradually to a saturated stable value in air due to the spontaneous adsorption of airborne hydrocarbon. First-principles calculations and molecular interaction modeling confirm that a monolayer h-BN coating does efficiently tune the interaction of a water molecule with different substrates to a converging level. The saturated wettability of h-BN coating is robust against variation of several factors, facilitating its practical applications.

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  • Chemical vapor deposition of ultra-thin molybdenum dioxide nanosheets. Materials Letters. 2016

    We report the growth of ultra-thin molybdenum dioxide nanosheets on SiO2/Si substrate via chemical vapor deposition using molybdenum trioxide and sublimated sulfur as precursors. The thicknesses of the obtained MoO2 nanosheets show notable dependence on the baking temperature of the sulfur precursor. At sulfur temperature of 90 °C, the obtained nanosheets can be 5.5 nm thin, more than one order of magnitude thinner than that previously reported, in a narrow scatter ranging from 5.5 to 11.5 nm. Twoprobe electrical measurements show that the as-prepared ultrathin MoO2 nanosheets preserve a high electrical conductivity of 3600 S/cm with thermal stability up to 200 °C. Above 250 °C, metallic MoO2 nanosheets are oxidized into insulating MoO3 flakes in air.

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  • Growth of Polar Hexagonal Boron Nitride Monolayer on Nonpolar Copper with Unique Orientation. Small. 2016

    Suppressing the oppositely orientated hexagonal boron nitride (h-BN) domains during the growth is of great challenge due to its bipolar structure. It is found that h-BN domains grown on onefold symmetric Cu(102) or (103) share a unique orientation, with one zigzag edge of the h-BN triangles perpendicular to the symmetry axis of the substrate surface.

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  • Tunable Electrical Performance of Few-Layered Black Phosphorus by Strain. Small. 2016

    Strain engineering shows promising applications in low-dimensional materials. It is demonstrated that the bandgap of few-layered black phosphorus can be effectively reduced by out-of-plane compressive strain, resulting in a significant modulation of the vertical electrical performance of black phosphorus and even inducing a nonlinear current–voltage curve to linear current–voltage curve transition.

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  • Substrate-Sensitive Graphene Oxidation. J. Phys. Chem. Lett. 2016

    The inertness of graphene toward reaction with ambient molecules is essential for realizing durable devices with stable performance. Many device applications require graphene to contact with substrates, but whose impact on the chemical property of graphene has been largely overlooked. Here, we combine comprehensive first-principles analyses with experiments to show that graphene oxidation is highly sensitive to substrates. Graphene remains inert on SiO2 and hexagonal boron nitride but becomes increasingly weak against oxidation on metal substrates because of enhanced charge transfer and chemical interaction between them. In particular, Ni and Co substrates lead to spontaneous oxidation of graphene, while a Cu substrate maximally promotes the oxygen diffusion on graphene, with an estimated diffusivity 13 orders of magnitude higher than that on freestanding graphene. Bilayer graphene is revealed to have high oxidation resistance independent of substrate and thus is a better choice for high-performance nanoelectronics. Our findings should be extendable to a wide spectrum of chemical functionalizations of two-dimensional materials mediated by substrates.

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  • Analytical solutions for elastic binary nanotubes of arbitrary chirality. Acta Mechanica Sinica. 2016

    Analytical solutions for the elastic properties of a variety of binary nanotubes with arbitrary chirality are obtained through the study of systematic molecular mechanics. This molecular mechanics model is first extended to chiral binary nanotubes by introducing an additional out-of-plane inversion term into the so-called stick-spiral model, which results from the polar bonds and the buckling of binary graphitic crystals. The closed-form expressions for the longitudinal and circumferential Young’s modulus and Poisson’s ratio of chiral binary nanotubes are derived as functions of the tube diameter. The obtained inversion force constants are negative for all types of binary nanotubes, and the predicted tube stiffness is lower than that by the former stick-spiral model without consideration of the inversion term, reflecting the softening effect of the buckling on the elastic properties of binary nanotubes. The obtained properties are shown to be comparable to available density functional theory calculated results and to be chirality and size sensitive. The developed model and explicit solutions provide a systematic understanding of the mechanical performance of binary nanotubes consisting of III–V and II–VI group elements.

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  • Tunable electronic and magnetic properties of two-dimensional materials and their one-dimensional derivatives. WIREs Comput Mol Sci. 2016

    Low-dimensional materials exhibit many exceptional properties and functionalities which can be efficiently tuned by externally applied force or fields. Here we review the current status of research on tuning the electronic and magnetic properties of low-dimensional carbon, boron nitride, metal-dichalcogenides, phosphorene nanomaterials by applied engineering strain, external electric field and interaction with substrates, etc, with particular focus on the progress of computational methods and studies. We highlight the similarities and differences of the property modulation among one- and two-dimensional nanomaterials. Recent breakthroughs in experimental demonstration of the tunable functionalities in typical nanostructures are also presented. Finally, prospective and challenges for applying the tunable properties into functional devices are discussed.

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  • Boron Nitride Nanostructures: Fabrication, Functionalization and Applications. Small.2016

    Boron nitride (BN) structures are featured by their excellent thermal and chemical stability and unique electronic and optical properties. However, the lack of controlled synthesis of quality samples and the electrically insulating property largely prevent realizing the full potential of BN nanostructures. A comprehensive overview of the current status of the synthesis of two-dimensional hexagonal BN sheets, three dimensional porous hexagonal BN materials and BN-involved heterostructures is provided, highlighting the advantages of different synthetic methods. In addition, structural characterization, functionalizations and prospective applications of hexagonal BN sheets are intensively discussed. One-dimensional BN nanoribbons and nanotubes are then discussed in terms of structure, fabrication and functionality. In particular, the existing routes in pursuit of tunable electronic and magnetic properties in various BN structures are surveyed, calling upon synergetic experimental and theoretical efforts to address the challenges for pioneering the applications of BN into functional devices. Finally, the progress in BN superstructures and novel B/N nanostructures is also briefly introduced.

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  • Tuning the energy gap of bilayer alpha-graphyne by applying strain and electric field. Chin. Phys. B 2016

    Chin. Phys. B
    Yang Hang, Wen-Zhi Wu, Jin Yu, and Wan-Lin Guo
    DOI: 10.1088/1674-1056/25/2/023102
    State Key Laboratory of Mechanics and Control of Mechanical Structures, Key Laboratory for Intelligent Nano Materials and Devices of Ministry of Education,
    Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

    Abstract
    Our density functional theory calculations show that the energy gap of bilayer α-graphyne can be modulated by a vertically applied electric field and interlayer strain. Like bilayer graphene, the bilayer α-graphyne has electronic properties
    that are hardly changed under purely mechanical strain, while an external electric field can open the gap up to 120 meV. It is of special interest that compressive strain can further enlarge the field induced gap up to 160 meV, while tensile strain
    reduces the gap. We attribute the gap variation to the novel interlayer charge redistribution between bilayer α-graphynes. These findings shed light on the modulation of Dirac cone structures and potential applications of graphyne in mechanicalelectric devices.

    http://www.cnki.com.cn/Article/CJFDTotal-ZGWL201602016.htm

  • Fast and large-area growth of uniform MoS2 monolayers on molybdenum foils. Nanoscale. 2016

    Nanoscale
    Guoan Tai, Tian Zeng, Jin Yu, Jianxin Zhou, Yuncheng You, Xufeng Wang,
    Hongrong Wu, Xu Sun, Tingsong Hua, and Wanlin Guo
    DOI: 10.1039/c5nr07226c
    Fast and large-area growth of uniform MoS2 monolayers on molybdenum foils.
    The State Key Laboratory of Mechanics and Control of Mechanical Structures, Key
    Laboratory for Intelligent Nano Materials and Devices of Ministry of Education and
    Institute of Nanoscience, College of Aerospace Engineering, Nanjing University of
    Aeronautics and Astronautics, Nanjing 210016, China.
    E-mail: taiguoan@nuaa.edu.cn, wlguo@nuaa.edu.cn
    School of Material Science and Technology, Nanjing University of Aeronautics and
    Astronautics, Nanjing 210016, China

    Abstract
    A controllable synthesis of two-dimensional crystal monolayers in a large area is a prerequisite for potential applications, but the growth of transition metal dichalcogenide monolayers in a large area with spatial homogeneity remains a great challenge. Here we report a novel and efficient method to fabricate largescale MoS2 monolayers by direct sulfurization of pre-annealed molybdenum foil surfaces with large grain boundaries of more than 50 µm in size at elevated temperatures. Continuous MoS2 monolayers can be formed uniformly by sulfurizing the Mo foils in sulfur vapor at 600 °C within 1 min. At a lower temperature even down to 500 °C, uniform MoS2 monolayers can still be obtained but in a much longer sulfurizing duration. It is demonstrated that the formed monolayers can be nondestructively transferred onto arbitrary substrates by removing the Mo foil using diluted ferric chloride solution and can be successfully fabricated into photodetectors. The results show a novel avenue to efficiently fabricate two-dimensional crystals in a large area in a highly controllable way and should have great potential for the development of large-scale applications of two-dimensional crystals in electrophotonic systems

    http://pubs.rsc.org/is/content/articlehtml/2015/nr/c5nr07226c

  • Reduction of interfacial friction in commensurate graphene/h-BN heterostructures by surface functionalization. Nanoscale. 2016

    Nanoscale
    Yufeng Guo, Jiapeng Qiu and Wanlin Guo
    DOI: 10.1039/c5nr05806f
    Received 26th August 2015,
    Accepted 22nd November 2015
    State Key Laboratory of Mechanics and Control of Mechanical Structures and MOE
    Key Laboratory for Intelligent Nano Materials and Devices, Institute of Nanoscience,
    Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China.
    E-mail: yfguo@nuaa.edu.cn; Fax: +86 25 84895827; Tel: +86 25 84890513

    Abstract
    The reduction of interfacial friction in commensurately stacked two-dimensional layered materials is important for their application in nanoelectromechanical systems. Our first-principles calculations on the sliding energy corrugation and friction at the interfaces of commensurate fluorinated-graphene/h-BN and oxidized-graphene/h-BN heterostructures show that the sliding energy barriers and shear strengths for these heterostructures are approximately decreased to 50% of those of commensurate graphene/ h-BN. The adsorbed F and O atoms significantly suppress the interlayer electrostatic and van der Waals energy corrugations by modifying the geometry and charge redistribution of the graphene layers. Our empirical registry index models further reveal the difference between the roles of the F and O atoms in
    affecting the sliding energy landscapes, and are also utilized to predict the interlayer superlubricity in a large-scale oxidized-graphene/h-BN system. Surface functionalization is a valid way to control and reduce the interlayer friction in commensurate graphene/h-BN heterostructures.

    http://pubs.rsc.org/en/content/articlehtml/2015/nr/c5nr05806f