Large in-plane and out-of-plane piezoelectricity in 2D?-LiMX2 (M=Al, Ga and In; X=S, Se and Te) monolayers

Atomically thin two-dimensional (2D) materials have attracted tremendous attention owing to their unique properties compared to bulk counterparts and great application potential in the field of piezoelectric nano devices. In present work, the non-centrosymmetric gamma-phase 2D Li-based ternary chalcogenide monolayer structures (LiMX2; M = Al, Ga and In; X = S, Se and Te) have been studied through the first-principles calculation method. According to the results, gamma-LiAlS2, gamma-LiAlSe2, gamma-LiAlTe2, gamma-LiGaSe2, gamma-LiInSe2 and gamma-LiInTe2 monolayers were stable structures, being direct bandgap semiconductors with a bandgap greater than 0.1 eV. The charges of six stable monolayers were distributed in a mirror asymmetry manner, meanwhile, they possessed electrostatic potential gradients. Consequently, the gamma-LiMX2 monolayers showed the better in-plane piezoelectric coefficients (d11 = 14.48 p.m./V for LiGaSe2) than most of other 2D piezoelectric materials, as well as the plurality of bulk piezoelectric systems. In the meanwhile, some candidates (gamma-LiAlS2 and gamma-LiAlSe2) also had superior out-of-plane piezoelectric coefficients (d(31)), which were up to twice those of the corresponding beta-phase monolayers. These outstanding properties imply the gamma-LiMX2 structures are very promising transducer materials for lightweight and high-performance piezoelectric-nanodevices.