The characterization of nano-structures with spectroscopical methods is a fundamental step in the production of nanotechnological devices. We developed a symmetry based method to extract Raman active, infrared active and optically inactive (silent) phonon modes from molecular dynamics simulations. Our method is conceptually simple and applicable to structures of any dimension, like quantum-dots, nanowires, thin films and bulk material.
We apply our method to bulk silicon and to silicon nanowires. We show that thermal anharmonicity plays a major role for silicon phonons at room temperature and we show that surface stress influences the Raman peak shift of thin silicon nanowires by 10% to 20%.