Raman Spectroscopy of Graphene under Uniaxial Stress: Phonon Softening   and Determination of the Crystallographic Orientation
  We present a systematic study of the Raman spectra of optical phonons in graphene monolayers under tunable uniaxial tensile stress. Both the G and 2D bands exhibit significant red shifts. The G band splits into two distinct sub-bands (G+, G-) because of the strain-induced symmetry breaking. Raman scattering from the G+ and G- bands shows a distinctive polarization dependence that reflects the angle between the axis of the stress and the underlying graphene crystal axes. Polarized Raman spectroscopy therefore constitutes a purely optical method for the determination of the crystallographic orientation of graphene. 
Mingyuan Huang, Hugen Yan, Changyao Chen, Daohua Song, Tony F. Heinz,   and James Hone