摘要
The thickness-dependent surface states of MoS 2 thin films grown by the chemical vapor deposition process on the SiO 2 -Si substrates are investigated by X-ray photoelectron spectroscopy. Raman and high-resolution transmission electron microscopy suggest the thicknesses of MoS 2 films to be ranging from 3 to 10 layers. Both the core levels and valence band edges of MoS 2 shift downward â0.2 eV as the film thickness increases, which can be ascribed to the Fermi level variations resulting from the surface states and bulk defects. Grainy features observed from the atomic force microscopy topographies, and sulfur-vacancy-induced defect states illustrated at the valence band spectra imply the generation of surface states that causes the downward band bending at the n-type MoS 2 surface. Bulk defects in thick MoS 2 may also influence the Fermi level oppositely compared to the surface states. When Au contacts with our MoS 2 thin films, the Fermi level downshifts and the binding energy reduces due to the hole-doping characteristics of Au and easy charge transfer from the surface defect sites of MoS 2 . The shift of the onset potentials in hydrogen evolution reaction and the evolution of charge-transfer resistances extracted from the impedance measurement also indicate the Fermi level varies with MoS 2 film thickness. The tunable Fermi level and the high chemical stability make our MoS 2 a potential catalyst. The observed thickness-dependent properties can also be applied to other transition-metal dichalcogenides (TMDs), and facilitates the development in the low-dimensional electronic devices and catalysts.