Surface-Facet-Dependent Phonon Deformation Potential in Individual Strained Topological Insulator Bi₂Se₃ Nanoribbons

Strain is an important method to tune the properties of topological insulators. For example, compressive strain can induce superconductivity in Bi₂Se₃ bulk material. Topological insulator nanostructures are the superior candidates to utilize the unique surface states due to the large surface to volume ratio. Therefore, it is highly desirable to monitor the local strain effects in individual topological insulator nanostructures. Here, we report the systematical micro-Raman spectra of single strained Bi₂Se₃ nanoribbons with different thicknesses and different surface facets, where four optical modes are resolved in both Stokes and anti-Stokes Raman spectral lines. A striking anisotropy of the strain dependence is observed in the phonon frequency of strained Bi₂Se₃ nanoribbons grown along the <1120> direction. The frequencies of the in-plane E(Formula presented.) and out-of-plane A(Formula presented.) modes exhibit a nearly linear blue-shift against bending strain when the nanoribbon is bent along the <1120> direction with the curved {0001} surface. In this case, the phonon deformation potential of the E(Formula presented.) phonon for 100 nm-thick Bi₂Se₃ nanoribbon is up to 0.94 cm^-1/%, which is twice of that in Bi₂Se₃ bulk material (0.52 cm^-1/%). Our results may be valuable for the strain modulation of individual topological insulator nanostructures.