Lattice Vibrational Modes in Si/Ge Core-shell Nanowires

We present a first-principles study on lattice vibrational modes of Si/Ge core-shell nanowires (NWs). In addition to quantum confinement, the internal strain induced by the lattice mismatch between core and shell contributes to significant frequency shifts of characteristic optical modes. More importantly, our simulation shows that these frequency shifts can be detected by Raman scattering experiments, providing convenient and nondestructive ways to obtain structural information of core-shell materials. Meanwhile, another type of collective modes, radial breathing modes (RBMs), are identified in Si-core/Ge-shell NWs and their frequency dependence is explained by an elastic media model.