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.