Thickness-Dependent Ultrafast Photocarrier Dynamics in Selenizing Platinum Thin Films

The atomically thin layered transition metal dichalcogenide (TMDCs) PtSe2 is a new emerging two-dimension (2D) material, which has a transition from indirect-gap semiconductor to semimetal with increasing thickness. Defects in 2D TMDCs are very ubiquitous and play a crucial role in understanding many electronics and optoelectronics in TMDCs. In this article, PtSe2 films with different thickness are obtained by selenizing the variously thick Pt films. Extrapolation of the onset absorption from the visible-infrared spectrum demonstrates that the selenization of 1 and 3 nm Pt films shows semiconductor-like character, while those of thick Pt films (with thickness of 10 and 15 nm) show metallic behavior. The transient absorption (TA) spectroscopy reveals that all films show immediately photobleaching signals after photoexcitation at 800 nm, and the subsequent relaxation process shows strongly thickness dependence. The thinnest film shows biexponential relaxation with typical time constants of 1.28 and 101.2 ps. In contrast, the photobleaching signals are developed into photoinduced absorption signals in thicker films, and the absorption magnitude increases with the thickness of the films. The optical pump and terahertz (THz) probe spectroscopy reveals that all samples show positive photoconductivity after photoexcitation of 800 nm, the subsequent recovery is completed within 2.0 ps, and the recovery time constant decreases with the increase of the films' thickness. Our TA and THz spectroscopy results reveal that the defect states of the films play dominated role in the relaxation of photocarrier after photoexcitation, and edgesite states are inferred to make dominated contributions to the defect states in the selenization of platinum films.