Electrical conduction and photoconduction in PtSe2 ultrathin films

Platinum diselenide (PtSe2) is one of the most studied materials of the emerging group-10 transition-metal dichalcogenides, with interesting chemical and physical properties as the semimetal-to-semiconductor transition when approaching the monolayer thickness. In this work, we investigate the electrical conduction and the photoconduction in ultrathin films of PtSe2 synthesized by direct selenization of platinum deposited onto SiO2/Si substrates. The PtSe2 ultrathin films are exploited as the channel of back-gated field-effect transistors (FETs) and their electric conductance is investigated at different temperatures and pressures as well as under the irradiation of a super-continuous white light source. The increasing conductance with raising temperature confirms the semiconducting nature of the PtSe2 film, while the gate modulation reveals p-type conduction with hole field-effect mobility up to 40 cm2/(Vs). The PtSe2 conductivity is higher in air than in high vacuum due to the p-doping effect of oxygen. Moreover, electrical conduction measured along different directions shows isotropic transport ascribed to the polycrystalline structure of the film. A reduction of the PtSe2 conductance (negative photoconductivity) is observed under exposure to light in air, while positive or negative photoconductivity is observed in vacuum depending on the intensity of the light and the pressure. Such a behavior can be explained by the combination of the photogating effect caused by photo-charge accumulation in the SiO2 dielectric and the adsorption/desorption of adsorbates.