(color online) Raman characterization of phosphorene. (a) Crystal structure of phosphorene. (b) Polarized Raman spectra of a 15 L phosphorene sample. (c) Angle-dependent Raman mode intensity extracted from the spectra in panel (b). (d) Crystalline orientation of the 15 L phosphorene sample. Figure reproduced from Refs. [15] and [20].