Lattice dynamics of layered kagome lattice material Nb ₃ Br ₈ investigated via Raman spectroscopy and DFT

In this paper, the lattice dynamics of bulk and few layer Nb₃Br₈ flakes are investigated via linearly polarized Raman spectroscopy and density-functional theory (DFT) to study the crystal's phonon modes and their symmetrical representations. 18 phonon modes are resolved in exfoliated bulk, 10 of which are E_g modes and 8 are A_1g modes, in close agreement with results obtained from DFT analysis. A near-resonant condition is revealed by excitation-dependent Raman experiments conducted with wavelengths between 473 nm-785 nm when a 532 nm source is used. It is shown via layer-dependent studies that the intensity of bands in the Raman spectra decrease and a slight shift in frequency is experienced in thin layers, however their symmetries remain consistent with bulk. Finally, no changes in Raman selection rules for the phonon modes are observed in a polarized Raman study conducted above the structural phase transition temperature of 387 K, suggesting their insensitivity to the phase transition. This study serves to establish a foundation for future studies of Nb₃Br₈ and its characteristic Raman-active phonon modes across different structural phases and thicknesses down to bilayer.