(color online) Raman study of CDW phenomena in 2D materials. (a)–(c), Temperature maps of the Raman scattering intensity of bulk, 2L, and 1L NbSe2, respectively, for the perpendicular polarization configuration. The two arrows in each map indicate the low-frequency feature and the weak high-frequency feature. Dashed lines approximately indicate TCDW. (d)–(f) Raman spectra of each sample at selected temperatures. Reproduced with permission from Ref. [31]. Copyright 2015, Nature Publishing Group.