Thickness evolution of phonon properties in ultrathin Bi (111) films

Abstract

Ultrathin Bi films have been theoretically predicted to be an elemental two-dimensional topological insulator recently. Investigation on phonon properties in topological insulators, especially down to a few-layer regime, is an interesting topic. In this paper, the evolution of structures and phonon properties of ultrathin Bi (111) films with thickness is studied from first-principles calculations by including spin–orbit coupling. The calculations of lattice dynamics show that both two Raman modes A1g and Eg in ultrathin Bi (111) films harden compared with those in bulk, which is contrary to other nanomaterials. The possible origin of this anomalous phonon behavior is discussed by considering the peculiar semimetallicity of Bi. With increasing thickness, the frequencies of these two Raman modes decrease gradually toward the values in the bulk. Our calculations also show that the softening and hardening of surface phonon modes coexist in ultrathin Bi films.