Interlayer breathing and shear modes in NbSe2 atomic layers

Atomically thin NbSe2 is a metallic layered transition metal dichalcogenide with novel charge-density-wave (CDW) and superconductive phases. Properties of NbSe2 atomic layers are sensitive to interlayer coupling. Here we investigate the interlayer phonons of few-layer NbSe2 by ultralow-frequency Raman spectroscopy. We observe both the interlayer breathing modes and shear modes at frequencies below 40 cm−1 for samples of 2-15 layers. Their frequency, Raman activity, and environmental instability depend systematically on the layer number. We account for these results by a combination of linear-chain model, group theory and first-principles calculations. We find that, although NbSe2 has different stacking order from MoS2, MoSe2, WS2 and WSe2, they share the same crystal symmetry groups and exhibit similar Raman selection rules for interlayer phonons. In addition, the interlayer phonon modes evolve smoothly from T = 300 to 8 K, with no observable response to the CDW formation in NbSe2. This finding indicates that the atomic registry between adjacent NbSe2 layers is well preserved in the CDW transition.