Piezoelectric Effects in Surface-Engineered Two-Dimensional Group III Nitrides.

Piezoelectric effects of two-dimensional (2D) group III-V compounds have received considered attention in recent years because of their wide applications in semiconductor devices. However, they face a problem that only metastable or unstable structures are noncentrosymmetric with piezoelectricity, thus leading to the difficulty in experimental observation. Motivated by the recent advances in the synthesis of 2D group III nitrides, in this paper, for the first time, we study the piezoelectric properties of the 2D group III nitrides (XN, X = Al, Ga, and In) with buckled hexagonal configurations by surface passivation, which are thermodynamically stable. Unlike the previously reported planar graphitic structure, we demonstrate that the hydrogenated 2D nitrides (H-XN-H, X = Al, Ga, and In) exhibit both the in-plane and out-of-plane piezoelectric effects in their monolayer and multilayer structures under an external strain in the basal plane. We further elucidate the underlying mechanism of the piezoelectricity by analyzing the correlations between the piezoelectric coefficients and their structural, electronic, and chemical properties. In addition, we show that H-F cofunctionalization not only enhances the stability, but also significantly improves the ionic polarization because of the charge redistribution, thus leading to large in-plane piezoelectric coefficients in F-XN-H. Our study advances the research in 2D piezoelectric materials and would stimulate more theoretical and experimental efforts in developing effective piezoelectric materials for device applications.