A theoretical study of the controversial surface states of the 2D lead halide perovskites

Two-dimensional (2D) lead halide perovskites (LHPs) have gained great research interest in recent years. They are dominated by the strong quantum confinement effect as well as the surface effect. However, the location of their surface states remains controversial so far. Some experimental results imply that the surface state may be located in the band gap and lead to negative effects, but the anti-bonding nature of the valance band indicates the inexistence of surface state in the band gap. In this work, the location of the surface states of 2D LHPs was investigated by using the first-principles method. Two kinds of LHPs: 2D CsPbBr3 and 2D MAPbBr3(MA = CH3NH3) were used as the model systems, on behalf of weakly and badly distorted surface structures respectively, and the 2D LHPs in thickness of one, two and three octahedron layers were considered. The conclusion is that there is no surface state in the band gap in any situation even though the band gap is enlarged by the strong quantum confinement effect and the surface states always lay inside the valence band. This result may provide constructive suggestion to understand the experimental phenomena of 2D LHPs.