Scanning tunneling microscopy (STM) images of transition-metal-dichalcogenide (TMD) surfaces are studied theoretically. First, STM images of the MoS2 surface are calculated by the first-principles method with a plane-wave basis, where it has not been settled whether the first or second layer is observed in STM. It is determined theoretically that the bright spots observed in the experimental STM images correspond to the S atoms of the outermost layer. Second, the superstructures observed in the STM images of the heteroepitaxial TMDs such as the MoSe2/MoS2 surface are investigated. It is found that the effect of the atomic displacement is an important factor in explaining the features of the superstructures.
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