Abstract

Based on a bandgap thermodynamic approach, the thickness-dependent bandgap of two-dimensional transition metal dichalcogenides is modeled without any adjustable parameter. An efficient expansion in bandgap upon lowering the thickness is predicted. The thickness-dependence of bandgap is believed originated from the interlayer van der Waals (vdW) interaction, while the surface effect is characterized by the difference in atomic thermal vibration between the surface and the interior. Due to the suppression role of the interlayer vdW interaction on the thermal vibration of interior chalcogen atoms, the surface effect is variable, which changes from monotonic increase for sulfides to decrease for tellurides. The role of the interlayer vdW interaction depends on the polarity of metal-chalcogen bonds. The model predictions agree with available experiment and simulation results.

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