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, MoS₂ and PtTe₂, with the tunneling current as a control parameter. 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.