Electronic properties of monolayer molybdenum dichalcogenides under strains
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Electronic properties of monolayer molybdenum dichalcogenides under strains J. Sugimoto and K. Shintani Department of Mechanical Engineering and Intelligent Systems, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
ABSTRACT The electronic band structures of monolayer molybdenum dichalcogenides, MoS2, MoSe2, and MoTe2 under either uniaxial or biaxial strain are calculated using first-principles calculation with the GW method. The imposed uniaxial strain is in the zigzag direction in the honeycomb lattice whereas the imposed biaxial strain is in the zigzag and armchair directions. It is found that the band gaps of these dichalcogenides almost linearly increase with the decrease of the magnitude of compressive strain, reach their maxima at some compressive strain, and then decrease almost linearly with the increase of tensile strain. It is also found their maximum band gaps are direct bandgaps. INTRODUCTION When monolayer graphene was exfoliated from graphite by Novoselov et al. [1], the scientific community was surprised at the disproof of the established theories and experiments. Researchers have now great expectations of applicability of graphene to nanoelectronics and nanomechanics because of its existence in two-dimension and of its superior electronic and mechanical properties; its two-dimensionality matches well the conventional processes of thin film technology. Since pristine graphene is a zero-bandgap semiconductor, methods of opening its bandgap have been developed. On the other hand, the success of exfoliation of graphene stimulated researchers to seek other graphene-like two-dimensional materials such as transition metal dichalcogendies [2], among which molybdenum dichalcogenides have attracted much attention because they are direct-bandgap semiconductors. Mechanical and solution-based exfoliation methods are used to fabricate monolayer MoS2; a method of solution-based exfoliation for fabricating large-area two-dimensional flakes of MoS2 has recently been reported by Pachuri et al. [3], and its device applications are expected to become a real thing. The research regarding the bandgaps of MoS2, MoSe2, α-MoTe2 in their layered structures date back to 2001 [4] where the valence-band structures of these molybdenum dichalcogenides were investigated resorting to both experimental techniques such as angle-resolved photoelectron spectroscopy and synchrotron radiation and ab initio calculations. A decade later, Yun et al. [5] studied the thickness and strain effects on the electronic structures of the molybdenum dichalcogenides. They found the indirect bandgaps of these materials in bulk change into the direct bandgaps with larger magnitudes when the number of the layers reduces to one; the direct band gaps of single layers of dichalcogenides also change into indirect bandgaps if the single leyers are under strains. Recently, Chang et al. [6] performed the orbital analysis of the electronic structure and phonon dispersion of MoS2, MoSe2, WS2, and WSe2 monolayers under strain, a
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