| With the rapid development of information technology,the size of device becomes smaller and smaller.Since the heat produced during processor running is difficult to eliminate,the traditional semiconductor technology has almost reached the physical limit.Finding new materials that can replace semiconductor silicon has become extremely important in electronics.Two-dimensional(2D)materials are suggested as ideal candidates for the next generation of electronic devices due to their rich physical properties.Because of the weak interlayer vdW force in layered materials,it is easy to obtain two-dimensional monolayer materials by mechanical exfoliation method.The layered materials have thus attracted much attention in recent years.MoO3 studied in this paper is a kind of abundant layered semiconductor with intrinsic band gap,high dielectric constant and adjustable electronic and optical properties.It has been applied widely in electronics and optoelectronic devices.In this paper,the geometry structure,electronic structure,optical properties and carrier mobility of layered molybdenum trioxide and their few layer structures were studied,and the underlying mechanism was analyzed.Specifically,the present paper includes:(1)Based on density functional theory,two layered molybdenum trioxides with orthorhombic(?-MoO3)and monoclinic(MoO3-?)structure have been investigated comparatively using several state-of-the-art functionals including optB88-vdW and HSE06.The structure,cleavage energy,electronic structure,optical properties and band edges have been given in details.Our results indicate that PBE overestimated the interlayer distance and underestimated the band gap seriously.The non-local optB88-vdW can predict a reasonable crystal structure and the HSE06 functional can give consistent band gap with experiment.The low cleavage energy calculated by optB88-vdW indicated that single layer MoO3 can be easily exfoliated from the bulk crystal.The electronic structure calculation indicate that in-plane atomic orbitals dominated the electronic states of valence band maximum(VBM)and the conduction band minimum(CBM).In addition,we also found that layered MoO3 have highly anisotropic optical adsorption behavior,which can be understood by the state-resolved distributions of electrons near VBM.Furthermore,the band edges of these two structures are also estimated by band gap center approximation,which are in good agreement with recent ultraviolet and inverse photoemission spectroscopy experiment.(2)The novel two-dimensional semiconductors with high carrier mobility and excellent stability are essential to the next-generation high-speed and low-power nanoelectronic devices.Because of the natural abundance,intrinsic gap,and chemical stability,metal oxides were also recently suggested as potential candidates for electronic materials.However,their carrier mobilities are typically on the order of tens of square centimeters per volt per second,much lower than that for commonly used silicon.By using first-principles calculations and deformation potential theory,we have predicted few-layer MoO3 as chemically stable wide-band-gap semiconductors with considerably high acoustic-phonon-limited carrier mobility above 3000 cm2V-1s-1,which makes them promising candidates for both electron-and hole-transport applications.Moreover,we also find a large in-plane anisotropy of the carrier mobility with a ratio of about 20-30 in this unusual system.Further analysis indicates that,because of the unique charge density distribution of whole valence electrons and the states near the band edge,both the elastic modulus and deformation potential are strongly directionally dependent.Also,the predicted high-mobility transport anisotropy of few-layer MoO3 can be attributed to the synergistic effect of the anisotropy of the elastic modulus and deformation potential.Our results not only give an insightful understanding for the high carrier mobility observed in few-layer MoO3 systems but also reveal the importance of the carrier-transport direction to the device performance. |
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