Structures, stabilities and piezoelectric properties of Janus gallium oxides and chalcogenides monolayers

Due to losing out-of-surface symmetry and the induced built-in dipole, two-dimensional (2D) Janus structures were believed to have various potential applications in the fields of piezoelectric devices, photocatalysis and electrochemical catalysis. In this work, 2D gallium oxides and chalcogenides (GaX) derived Janus monolayers Ga₂XY (X/Y  =  O, S, Se, and Te) were designed. Our first-principles calculations showed that apart from Ga₂OTe, all the monolayers are thermodynamically, dynamically and mechanically stable. These monolayers are demonstrated to be semiconductors with the band gaps in a wide range of 1.00-3.24 eV. The calculated in-plane piezoelectric coefficients of Janus monolayers (d ₁₁  =  3.09-5.67 pm V^-1) are demonstrably enhanced with respect to the pristine monolayers (d ₁₁  =  0.41-3.04 pm V^-1). Meanwhile, the yielding additional out-of-plane piezoelectric coefficients range from 0.11 to 0.34 pm V^-1. Interestingly, O-participated Janus monolayers display distinctive properties: dipole direction flipping from O to S/Se, high stabilities and moderate direct-band-gaps. Particularly, Ga₂OSe monolayer was found to have the largest piezoelectric coefficient of 5.67 pm V^-1, which can be ascribed to the highly imbalanced charge distribution on O and Se as well as the largest bond length differences between Ga-O and Ga-Se. Our study revealed that the Janus Ga₂XY monolayers, especially O-related systems, could be pretty promising as candidates in optoelectronic, piezoelectric sensors and energy conversion devices.