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Abstract Niobium chloride (Nb₃Cl₈) is a layered two-dimensional semiconducting material with many exotic properties including a breathing kagome lattice, a topological flat band in its band structure, and a crystal structure that undergoes a structural and magnetic phase transition at temperatures below 90 K. Despite being a remarkable material with fascinating new physics, the understanding of its phonon properties is at its infancy. In this study, we investigate the phonon dynamics of Nb₃Cl₈in bulk and few layer flakes using polarized Raman spectroscopy and density-functional theory (DFT) analysis to determine the material’s vibrational modes, as well as their symmetrical representations and atomic displacements. We experimentally resolved 12 phonon modes, five of which areA_1gmodes while the remaining seven areE_gmodes, which is in strong agreement with our DFT calculation. Layer-dependent results suggest that the Raman peak positions are mostly insensitive to changes in layer thickness, while peak intensity and full width at half maximum are affected. Raman measurements as a function of excitation wavelength (473–785 nm) show a significant increase of the peak intensities when using a 473 nm excitation source, suggesting a near resonant condition. Temperature-dependent Raman experiments carried out above and below the transition temperature did not show any change in the symmetries of the phonon modes, suggesting that the structural phase transition is likely from the high temperatureP $\overline{\text{3}m}$1 phase to the low-temperatureR $\overline{3m}$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. Overall, our study presented here significantly advances the fundamental understanding of layered Nb₃Cl₈material which can be further exploited for future applications. 

Abstract Kagome materials have become solid grounds to study the interplay among geometry, topology, correlation, and magnetism. Recently, niobium halide semiconductors Nb 3 X 8 ( X  = Cl, Br, I) have been predicted to be two-dimensional magnets and these materials are also interesting for their breathing kagome geometry. However, experimental electronic structure studies of these promising materials are still lacking. Here, we report the spectroscopic evidence of flat and weakly dispersing bands in breathing-kagome semiconductor Nb 3 I 8 around 500 meV binding energy, which is well supported by our first-principles calculations. These bands originate from the breathing kagome lattice of niobium atoms and have niobium d -orbital character. They are found to be sensitive to the polarization of the incident photon beam. Our study provides insight into the electronic structure and flat band topology in an exfoliable kagome semiconductor, thereby providing an important platform to understand the interaction of geometry and electron correlations in two-dimensional materials.