The flexible Janus X2PAs (X = Si, Ge and Sn) monolayers with in-plane and out-of-plane piezoelectricity

The 2D piezoelectric materials have attracted growing attention due to their potential application in energy harvesters, strain sensors and active flexible electronics. Now, the challenge is to find flexible 2D material with piezoelectricity in both in-plane and out-of-plane. To address this challenge, this paper demonstrates the Janus X2PAs (X = Si, Ge and Sn) monolayers through density functional theory (DFT) calculation for the first time. The dynamical stability, stiffness and piezoelectric tensors of these monolayers are systematically investigated. The results show that these monolayers are stable and extremely flexible. The X2PAs monolayers exhibit piezoelectric effect in both in-plane and out-of-plane. Especially, among them, the Sn2PAs monolayer has the largest piezoelectric coefficient |d31| (1.42 pm/V) that is two times larger than the |d31| of group III–V buckled honeycomb (GaP, GaAs, InP and InAs) monolayers (0.40 ∼ 0.74 pm/V) and MoSO monolayer (0.7 pm/V). These facts are due to an inversion symmetry breaking (along the out-of-plane direction) in both atomic structures and charge distribution of X2PAs monolayers. The out-of-plane piezoelectric and flexible characters of these 2D Janus X2PAs monolayers could enhance their performance in multifunctional sensing and controlling of the nanodevices.