Spectroscopic Investigation of van der Waals Contacts between Semiconducting-TMD Monolayers and Semimetals

Before two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) can replace silicon-based electronic devices, the high contact resistance between the 2D semiconductors and their metallic contacts must be overcome. Although the Schottky–Mott rule (SMR) provides a simple model for metal–semiconductor contacts, it rarely holds experimentally, with Fermi-level pinning (FLP) effects dominating metal–semiconductor contacts through defect- and metal-induced gap states. The use of van der Waals interfaces composed of 2D semiconductors and semimetal contacts has been proposed to suppress FLP and restore SMR behavior. To test this conjecture, we fabricate pristine TMD/semimetal heterostructures across three monolayer TMDs (MoS₂, WS₂, WSe₂) and two semimetals (Bi and graphite), then directly probe their interfacial electronic structures with scanning tunneling microscopy/spectroscopy, angle-resolved photoelectron spectroscopy, and field-emission resonance spectroscopy. We find that the SMR is recovered in TMD/Bi junctions, despite strong interlayer hybridization, while collapsing completely in weakly coupled TMD/graphite interfaces. With the aid of first-principles calculations, the underlying mechanism for this apparent contradiction will be discussed.