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

Abstract

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.

Fig. 1. Side and top views of the optimized 2D Janus monolayer BMX₂ compounds, where B and M are shown in red and white, and X (S and Se) atoms are shown as green spheres, respectively.

Fig. 2. Calculated phonon dispersion curves along the high symmetry directions (Γ–M–K–Γ) in the Brillouin zone, for all of the 2D Janus monolayer BMX₂ compounds considered.

Fig. 3. Band structures of the 2D Janus monolayer BMX₂ compounds. Orange and black dotted lines correspond to results obtained using the HSE06 and PBE functionals, respectively. The dashed purple line represents the Fermi level, E_F.

Fig. 4. The total density of states (DOS) of the 2D Janus monolayers BMX₂ compounds.

Fig. 5. (a) The real (ε₁) and (b) imaginary (ε₂) parts of the dielectric function for the 2D Janus monolayer BMX₂ compounds.

Fig. 6. Polarization change as a function of strain for the 2D Janus monolayer BMX₂ compounds.

Fig. 7. (a) The transition barrier for the 2D Janus monolayer BMX₂ compounds. (b) Comparison between the predicted energy barriers of the BMX₂ compounds and other theoretical and experimental energy barriers for the 2D ferroelectrics.