Electrical Bandgap Evolution and Carrier-Induced Transport Regimes in Ultrathin PtSe₂

Ionic-liquid gating enables electrostatic carrier accumulation, allowing transport-based band-edge determination while extending chemical potential into high-density regimes where interaction effects emerge strongly. PtSe₂ exhibits a pronounced thickness-dependent electronic structure, evolving from a gapped semiconductor in the ultrathin limit to a metallic state in thicker crystals. While the metallic regime has been widely studied, quantitative determination of the transport gap and controlled carrier tuning in the ultrathin semiconducting thickness range require electrostatic access beyond conventional dielectric gating. Here, by using ambipolar ionic liquid-gated transistors, we systematically probe transport in ultrathin PtSe₂. We extract electrical bandgaps of ∼1.1 eV (2L), ∼0.8 eV (3L), and ∼0.46 eV (4L), followed by complete gap closure at five layers. In 4L PtSe₂, high carrier densities drive a metal-insulator crossover and reveal distinct temperature-dependent regimes, including Fermi-liquid T² resistivity, extended T-linear behavior, and a low-temperature logarithmic upturn, establishing PtSe₂ as a thickness- and carrier-density-tunable correlated system.