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. (letter)