  Niobium chloride (Nb3Cl8) is a layered 2D 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 Nb3Cl8 in 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, 5 of which
are A1g modes while the remaining 7 are Eg modes, 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 FWHM 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
temperature P3m1 phase to the low-temperature R3m phase. Magneto-Raman
measurements carried out at 140 and 2 K between -2 to 2 T show that the Raman
modes are not magnetically coupled. Overall, our study presented here
significantly advances the fundamental understanding of layered Nb3Cl8 material
which can be further exploited for future applications.
