Interlayer breathing and shear modes in NbSe2 atomic layers

Atomically thin transition metal dichalcogenide (TMD) niobium diselenide (NbSe2) has recently stimulated strong scientific interest due to its distinctive electronic and magnetic properties. 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 to 15 layers. Their frequency, Raman activity, and environmental instability depend systematically on the layer number. In addition, the interlayer phonon modes evolve smoothly from T = 300 K to 8 K, with no observable response to the charge density wave formation in NbSe2. Our results are useful to understand the thickness-dependent properties of NbSe2.