Raman Characterization of the Charge Density Wave Phase of 1T-TiSe₂: From Bulk to Atomically Thin Layers

Raman scattering is a powerful tool for investigating the vibrational properties of two-dimensional materials. Unlike the 2H phase of many transition metal dichalcogenides, the 1T phase of TiSe₂ features a Raman-active shearing and breathing mode, both of which shift toward lower energy with increasing number of layers. By systematically studying the Raman signal of 1T-TiSe₂ in dependence of the sheet thickness, we demonstrate that the charge density wave transition of this compound can be reliably determined from the temperature dependence of the peak position of the E_g mode near 136 cm^-1. The phase transition temperature is found to first increase with decreasing thickness of the sheets, followed by a decrease due to the effect of surface oxidation. The Raman spectroscopy-based method is expected to be applicable also to other 1T-phase transition metal dichalcogenides featuring a charge density wave transition and represents a valuable complement to electrical transport-based approaches.