Coexistence of Large In-Plane and Out-of-Plane Piezoelectric Response in Group III–VI XMAY[sub.2] Monolayers

Flexible materials with both in-plane and out-of-plane piezoelectric coefficients are needed in the development of advanced nanoelectromechanical systems. However, the challenge is to find flexible materials with the coexistence of in-plane and -out-of-plane piezoelectric responses, which hinders the progress of high-performance piezoelectric sensor development. In this paper, we propose the flexible XMAY[sub.2] (X = I; M = Ti, Zr; A = Al, Ga; Y = S, Se) monolayers, which belong to the group III-VI XMAY[sub.2] family, which showcase notable in-plane and out-of-plane piezoelectric coefficients. The in-plane (d [sub.11]) and out-of-plane (d [sub.31]) piezoelectric coefficients of the XMAY[sub.2] monolayers vary from 5.20 to 7.04 pm/V and from −0.23 to 0.48 pm/V, respectively. The large in-plane and out-plane piezoelectric responses coexist (d [sub.11] = 7.04 pm/V; d [sub.31] = 0.48 pm/V) in the IZrGaS[sub.2] monolayer, which is larger than other materials in the XMAY[sub.2] family, such as SMoSiN[sub.2] (d [sub.11] = 2.51; d [sub.31] = 0.28 pm/V). In addition, the mechanical and transport properties of XMAY[sub.2] demonstrate its impressive flexibility characteristics as well as its efficient electrical conductivity. Due to inversion symmetry breaking in both atomic structure and charge distribution of XMAY[sub.2] monolayers, the group III-VI XMAY[sub.2] family exhibits a potentially rich scope of applications in the field of piezoelectricity.