Thickness-Dependent Binding Energy Shift in Few-Layer MoS2 Grown by Chemical Vapor Deposition

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• 作者：Yu-Kai Lin ; Ruei-San Chen ; Tsu-Chin Chou ; Yi-Hsin Lee ; Yang-Fang Chen ; Kuei-Hsien Chen ; Li-Chyong Chen
• 刊名：ACS Applied Materials & Interfaces
• 出版年：2016
• 出版时间：August 31, 2016
• 年：2016
• 卷：8
• 期：34
• 页码：22637-22646
• 全文大小：639K
• 年卷期：0
• ISSN：1944-8252

文摘

The thickness-dependent surface states of MoS₂ thin films grown by the chemical vapor deposition process on the SiO₂–Si substrates are investigated by X-ray photoelectron spectroscopy. Raman and high-resolution transmission electron microscopy suggest the thicknesses of MoS₂ films to be ranging from 3 to 10 layers. Both the core levels and valence band edges of MoS₂ shift downward ∼0.2 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 MoS₂ surface. Bulk defects in thick MoS₂ may also influence the Fermi level oppositely compared to the surface states. When Au contacts with our MoS₂ 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 MoS₂. 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 MoS₂ film thickness. The tunable Fermi level and the high chemical stability make our MoS₂ 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.