Polarized Raman Study of the Phonon Dynamics and Structural Transition of Kagome Lattice Semiconductor Nb₃Cl₈*

Niobium chloride (Nb₃Cl₈) is a layered 2D semiconductor that forms a kagome lattice of niobium trimers. Nb₃Cl₈ undergoes a temperature induced structural and magnetic phase transition below 90 K. In this study, we present an investigation of Nb₈Cl₈'s phonon modes in bulk and few layers at high and low temperatures via polarized Raman spectroscopy. Density-functional theory is used to model the crystal’s phonon dynamics and atomic displacements. 12 experimental phonon modes are resolved, 5 A_1g symmetric and 7 E_g symmetric modes. Layer-dependent Raman studies show that peak frequencies slightly shift with changes in layer thickness. Polarized Raman studies above and below the transition temperature show no differences in the symmetries of the phonon modes, suggesting that the structural phase transition is likely from the room temperature P3¯m1 phase to the low-temperature R3¯m phase. Magneto-Raman measurements carried out at 140 and 2 K between −2 and 2 T show that the Raman modes are not magnetically coupled. Excitation-dependent Raman spectroscopy reveals near-resonant effect when utilizing a 473 nm laser, resolving a new peak. Overall, our study significantly advances the fundamental understanding of Nb3Cl8’s phonon dynamics and structural transitions, paving the way for future research on the exotic quantum material.