▎ 摘　　要

For two-dimensional (2D) materials, a pure large out-of-plane piezoelectric response, compatible with the nowadays bottom/top gate technologies, is highly desired. In this work, the piezoelectric properties of penta-graphene (CCC) monolayer are studied with pure out-of-plane piezoelectric effect by density functional theory awn. However, the d(36) is very small, and only -0.065 pm/V. Two strategies are proposed to enhance piezoelectric properties of CCC monolayer. Firstly, both biaxial and uniaxial strains are applied, but the enhancement is very small, and at -2% biaxial (-4% uniaxial) strain, the d(36) is increased only by 3.1% (13.9%). Secondly, a Janus penta-monolayer (CCB) is constructed by replacing the top C (B) atomic layer in monolayer CCC [pentagonal CB2 monolayer (CBB)] with B (C) atoms, which shows dynamic and mechanical stability. Fortunately, the pure out-of-plane piezoelectric effect of CCB monolayer still holds, and exhibits a band gap. The calculated d(31) and d(32) are -0.505 pm/V and 0.273 pm/V, respectively, which are very larger than d(36) of CCC monolayer. The out-of-plane piezoelectricity d(31) of CCB monolayer is obviously higher compared with many other 2D known materials. Moreover, its room-temperature electronic mobility along y direction is as high as 8865.23 cm(2)V(-1)s(-1). Our works provide a new way to achieve pure out-of-plane piezoelectric effect, which is highly desirable for ultrathin piezoelectric devices.