  Palladium diselenide (PdSe$_2$), a new type of two-dimensional noble metal
dihalides (NMDCs), has received widespread attention for its excellent
electrical and optoelectronic properties. Herein, high-quality continuous
centimeter-scale PdSe$_2$ films with layers in the range of 3L-15L were grown
using Chemical Vapor Deposition (CVD) method. The absorption spectra and DFT
calculations revealed that the bandgap of the PdSe$_2$ films decreased with
increasing number of layers, which is due to PdSe$_2$ enhancement of orbital
hybridization. Spectroscopic ellipsometry (SE) analysis shows that PdSe2 has
significant layer-dependent optical and dielectric properties. This is mainly
due to the unique strong exciton effect of the thin PdSe$_2$ film in the UV
band. In particular, the effect of temperature on the optical properties of
PdSe$_2$ films was also observed, and the thermo-optical coefficients of
PdSe$_2$ films with different number of layers were calculated. This study
provides fundamental guidance for the fabrication and optimization of
PdSe$_2$-based optoelectronic devices.
