Band Alignment at Au/MoS₂ Contacts: Thickness Dependence of Exfoliated Flakes

We investigated the surface potential (V_surf) of exfoliated MoS₂ flakes on bare and Au-coated SiO₂/Si substrates using Kelvin probe force microscopy. The V_surf of MoS₂ single layers was larger on the Au-coated substrates than on the bare substrates; our theoretical calculations indicate that this may be caused by the formation of a larger electric dipole at the MoS₂/Au interface leading to a modified band alignment. V_surf decreased as the thickness of the flakes increased until reaching the bulk value at a thickness of a20 nm (a30 layers) on the bare and a80 nm (a120 layers) on the Au-coated substrates, respectively. This thickness dependence of V_surf was attributed to electrostatic screening in the MoS₂ layers. Thus, a difference in the thickness at which the bulk V_surf appeared suggests that the underlying substrate has an effect on the electric-field screening length of the MoS₂ flakes. This work provides important insights to help understand and control the electrical properties of metal/MoS₂ contacts.