Tip-Mediated Bandgap Tuning for Monolayer Transition Metal Dichalcogenides.

Monolayer transition metal dichalcogenides offer an appropriate 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, MoS 2 and PtTe 2 , with the tunneling current as a control parameter. The bandgap of monolayer MoS 2 decreases logarithmically by the increasing tunneling current, indicating an electric-field-induced gap renormalization effect. Monolayer PtTe 2 , 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 PtTe 2 , not seen in bulk semimetallic PtTe 2 , can be attributed to its surface electronic structure that can readily couple to the tunneling tip, as demonstrated by theoretical calculations.