Interlayer breathing and shear modes in NbSe2 atomic layers
  Atomically thin NbSe2 is a metallic layered transition metal dichalcogenide (TMD) with considerably different crystallographic structure and electronic properties from other TMDs, such as MoS2, MoSe2, WS2 and WSe2. Properties of TMD 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 to 15 layers. Their frequency, Raman activity, and environmental instability depend systematically on the layer number. We account for these results utilizing a combination of the linear-chain model, group-theory analysis and first-principles calculations. Although NbSe2 possesses different stacking order from MoS2, MoSe2, WS2 and WSe2, it exhibits the same symmetry and Raman selection rules, as well as similar interlayer coupling strength and thickness dependence of interlayer phonon modes. 
Rui He, Jeremiah van Baren, Jia-An Yan, Xiaoxiang Xi, Zhipeng Ye,   Gaihua Ye, I-Hsi Lu, S. M. Leong, C. H. Lui
cond-mat.mes-hall