Temperature-dependent optical and vibrational properties of PtSe₂ thin films

PtSe₂ has received substantial research attention because of its intriguing physical properties and potential practical applications. In this paper, we investigated the optical properties of bilayer and multilayer PtSe₂ thin films through spectroscopic ellipsometry over a spectral range of 0.73-6.42 eV and at temperatures between 4.5 and 500 K. At room temperature, the spectra of refractive index exhibited several anomalous dispersion features below 1000 nm and approached a constant value in the near-infrared frequency range. The thermo-optic coefficients of bilayer and multilayer PtSe₂ thin films were (4.31 ± 0.04) × 10^-4/K and (-9.20 ± 0.03) × 10^-4/K at a wavelength of 1200 nm. Analysis of the optical absorption spectrum at room temperature confirmed that bilayer PtSe₂ thin films had an indirect band gap of approximately 0.75 ± 0.01 eV, whereas multilayer PtSe₂ thin films exhibited semimetal behavior. The band gap of bilayer PtSe₂ thin films increased to 0.83 ± 0.01 eV at 4.5 K because of the suppression of electron-phonon interactions. Furthermore, the frequency shifts of Raman-active E_g and A_1g phonon modes of both thin films in the temperature range between 10 and 500 K accorded with the predictions of the anharmonic model. These results provide basic information for the technological development of PtSe₂-based optoelectronic and photonic devices at various temperatures.