Bandgap Tuning for Monolayer TMD Materials by Using STM Tip*

Monolayer transition metal dichalcogenides (TMDs) offer a promising platform for developing advanced electronics beyond graphene. Similar to two-dimensional molecular frameworks, the electronic properties of such monolayers can be sensitive to perturbations from the surroundings; the implied tunability of electronic structure is of great interest. Using scanning tunneling microscopy/spectroscopy, we demonstrated a bandgap engineering technique in two monolayer materials, MoS₂ and PtTe₂, with the tunneling current as a control parameter [1]. The bandgap of monolayer MoS₂ decreases logarithmically by the increasing tunneling current, indicating an electric-field-induced gap renormalization effect. Monolayer PtTe₂, by contrast, exhibits a much stronger gap reduction, and a reversible semiconductor-to-metal transition occurs at a moderate tunneling current. This unusual switching behavior of monolayer PtTe₂, not seen in bulk semimetallic PtTe₂, can be attributed to its surface electronic structure that can readily couple to the tunneling tip, as demonstrated by theoretical calculations. Our findings reported herein have significant implications regarding methods and the principles for bandgap engineering of TMD monolayers.