Ferro-piezoelectricity in emerging Janus monolayer BMX2 (M = Ga, In and X = S, Se): ab initio investigations

Nanoscale materials with inter-correlation characteristics are fundamental for developing high performance devices and applications. Hence theoretical research into unprecedented two-dimensional (2D) materials is crucial for improving understanding, especially when piezoelectricity is merged with other unique properties such as ferroelectricity. In this work, an unexplored 2D Janus family BMX2 (M = Ga, In and X = S, Se) corresponding to group-III ternary chalcogenides has been explored. The structural and mechanical stability, and optical and ferro-piezoelectric properties of BMX2 monolayers were investigated using first-principles calculations. We found that the lack of imaginary phonon frequencies in the phonon dispersion curves establishes the dynamic stability of the compounds. The monolayers BGaS2 and BGaSe2 are indirect semiconductors with bandgaps of 2.13 eV and 1.63 eV, respectively, while BInS2 is a direct semiconductor with a bandgap of 1.21 eV. BInSe2 is a novel zero-gap ferroelectric material with quadratic energy dispersion. All monolayers exhibit a high spontaneous polarization. The optical characteristics of the BInSe2 monolayer show high light absorption ranging from the infrared to the ultraviolet. The BMX2 structures exhibit in-plane and out-of-plane piezoelectric coefficients of up to 4.35 pm V-1 and 0.32 pm V-1. According to our findings, 2D Janus monolayer materials are a promising choice for piezoelectric devices.