5족 원소로 이루어진 2차원 흑린 구조 물질의 원자 구조 및 전자 구조 특성

Abstract

The burgeoning demand on portable and flexible electronic devices requires the intensive study on low-dimensional materials with appropriate electronic properties for fabricating the devices. Graphene is the first discovered two-dimensional material having fascinating electronic properties. However, the absence of band gap in graphene has limited its applications to electronics despite of the high carrier mobility. Recently, the study on two-dimensional graphene alternative has been actively conducted by many research groups. Phosphorene, which has a moderate band gap and high carrier mobility, is regarded as one of the most promising graphene alternative. The materials composed of elements in the same group generally share structural and electronic properties. The elemental analogues of graphene such as silicene, germanene and stanene are the most well-known example. Due to the suitable electronic properties for electronics and the layered crystal structure of black phosphorus, two-dimensional black phosphorus-like group V materials (P, As, Sb and Bi) have been intensively studied. We study two-dimensional group V materials in puckered honeycomb structure using first-principles calculations. Two factors, the degree of puckering and buckling characterize not only the atomic structure but also the electronic structure and its topological phase. By analyzing the lone-pair character of constituent elements and the softening of the phonon mode, we clarify the origin of the buckling. We show that the phonon softening leads the second-order type structural phase transition from a flat to a buckled configuration. The inversion symmetry breaking associated with the structural transition induces the spontaneous polarization in these homogenous materials. Our calculations suggest that external strains or n-type doping are effective methods to control the degree of buckling. We find that the ferroelectric and non-trivial topological phase can coexist in puckered Bi when tensile strains are applied.