Accurate Assessments of the Electronic Structures of Ultrathin PtSe: Bandgap Quantification and Critical Thickness for the Metal-Semiconductor Transition.

Ultrathin PtSe, a member of the group-10 transition metal dichalcogenides, has emerged as a promising two-dimensional material due to its layer-dependent, tunable bandgap. Notably, a unique semiconductor-to-metal transition is predicted as the layer number of this material increases; however, pinpointing the exact critical thickness for this transition and reliably quantifying the energy gaps of the semiconducting layers remain formidable challenges. In this work, all-van der Waals assembled multiprobe schemes and planar tunnel junctions are employed to systematically investigate the thickness-sensitive charge transport properties and energy gaps of ultrathin PtSe films. Temperature-dependent measurements reveal that PtSe exhibits semiconducting behavior from monolayer to five layers, with a transition to a semimetallic state at six layers. Furthermore, using electron tunneling spectroscopy, we accurately quantify the energy gaps of monolayer, bilayer, and trilayer PtSe and identifies that PtSe in monolayer form behaves as an n-type semiconductor but intriguingly transitions to a p-type semiconductor in bilayer form. First-principles calculations highlight the importance of correctly evaluating interlayer distances to select the appropriate density functional theory functional, enabling reliable predictions of the critical thickness of ultrathin PtSe for the semiconductor-to-metal transition and corresponding electronic structures.