by Raman Spectroscopy

Tantalum diselenide (TaSe₂) is a metallic, layered transition metal dichalcogenide that can be cleaved into 2D layers. Bulk 2H-TaSe₂ undergoes a structural phase transition to an incommensurate charge density wave state (CDW) at 122 K and a commensurate CDW state at 90 K. The CDW amplitude modes are Raman-active and well understood. Additionally, there are previously unanalyzed Raman peaks associated with the CDW that can be observed. We report the temperature and polarization dependence of all CDW modes and the undistorted-state Raman modes of TaSe₂ for 633 nm, 515 nm, and 476 nm excitation. The intensity, frequency, and width of these peaks are monitored for samples from bulk down to monolayer thickness. We also track the transition temperature of the incommensurate and commensurate CDW states as a function of thickness. Finally, our DFT calculations provide insight into the nature and behavior of the CDW Raman modes.