We fabricated stand-alone trilayers (3L) and their heterostructures of MoS2 and WS2 on Al2O3 substrates. In particular, two kinds of heterostructures with different stacking sequences (i.e., 3L-MoS2/3L-WS2 and 3L-WS2/3L-MoS2) were prepared for comparison. The light-induced surface potential change suggested that the internal electric field along the thickness direction was present in the stand-alone (MoS2 and WS2) and heterostructure samples. Relative peak shifts of the Raman spectra supported the presence of the internal field in our samples. Physical origins to induce the internal electric field will be discussed in the presentation.
Understanding the band alignment at metal/2D semiconductor (SC) contacts is essential for electrical characterizations of 2D SC materials and for fabrication of high performance 2D SC devices. Many researchers have attempted to understand the electrical properties of metal/2D SC contacts and have revealed that they have unique features distinct from those of 3D SC counterparts. In this work, we investigated the surface potential (Vsurf) of exfoliated MoS2 flakes on bare and Au-coated SiO2/Si substrates using Kelvin probe force microscopy. The Vsurf 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. Vsurf decreased as the thickness of the flakes increased until reaching the bulk value at a thickness of ~20 nm on the bare and ~80 nm on the Au-coated substrates, respectively. This thickness-dependence of Vsurf was attributed to electrostatic screening in the MoS2 layers. Thus, a difference in the thickness at which the bulk Vsurf 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 understand the band alignment and the charge transport at the metal/2D SC interfaces.
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