Surface vibrational modes of the topological insulator Bi2Se3 observed by Raman spectroscopy

We present polarization resolved Raman scattering study of surface vibration modes in the topological insulator Bi₂Se₃ single crystal and thin films. Besides the 4 Raman active bulk phonons, we observed 4 additional modes with much weaker intensity and slightly lower energy than the bulk counterparts. By symmetry analysis and comparison with theoretical calculation, we assigned these additional modes to out-of-plane surface phonons, where the frequency is slightly modified from the bulk phonon due to out-of-plane lattice distortion near the crystal surface. In particular, two of the surface modes at 60 and 173 cm^-1 are associated with Raman active A_1g bulk phonon modes, the other two at 136 and 158 cm^-1 are associated with infrared active bulk phonons with A_2u symmetry. The latter become Raman allowed due to restriction of crystalline symmetry from D_3d in the bulk to C_3v at the surface of Bi₂Se₃. The 158 cm^-1 surface phonon mode show a Fano line shape, suggesting interaction with an electronic continuum at the crystal surface. In addition, we observed two weak features at 67 and 126 cm^-1 likely corresponding to in-plane surface vibrational modes.