In Situ XPS Investigation of Transformations at Crystallographically Oriented MoS2 Interfaces
Nanoscale transition-metal dichalcogenide (TMDC) materials, such as MoS 2, exhibit promising behavior in next-generation electronics and energy-storage devices. TMDCs have a highly anisotropic crystal structure, with edge sites and basal planes exhibiting different structural, chemical, and electronic properties. In virtually all applications, two-dimensional or bulk TMDCs must be interfaced with other materials (such as electrical contacts in a transistor). The presence of edge sites vs basal planes (i.e., the crystallographic orientation of the TMDC) could influence the chemical and electronic properties of these solid-state interfaces, but such effects are not well understood. Here, we use in situ X-ray photoelectron spectroscopy (XPS) to investigate how the crystallography and structure of MoS 2 influence chemical transformations at solid-state interfaces with various other materials. MoS 2 materials with controllably aligned crystal structures (horizontal vs vertical orientation of basal planes) were fabricated, and in situ XPS was carried out by sputter-depositing three different materials (Li, Ge, and Ag) onto MoS 2 within an XPS instrument while periodically collecting photoelectron spectra; these deposited materials are of interest due to their application in electronic devices or energy storage. The results showed that Li reacts readily with both crystallographic orientations of MoS 2 to form metallic Mo and Li 2 S, while Ag showed very little chemical or electronic interaction with either type of MoS 2 . In contrast, Ge showed significant chemical interactions with MoS 2 basal planes, but only minor chemical changes were observed when Ge contacted MoS 2 edge sites. These findings have implications for electronic transport and band alignment at these interfaces, which is of significant interest for a variety of applications.