Theoretical insights into strong intrinsic piezoelectricity of blue- phosphorus-like group-Ⅳ monochalcogenides

On the basis of known structures of β-GeTe bulk and the derived monolayer, we proposed a series of structural analogues MXs (M = Ge, Sn; X = S, Se, Te) with an intrinsic built-in electric field via a substitution strategy. Using first-principles calculations, we demonstrated that these MX monolayers and bulks are thermodynamically, dynamically and mechanically stable, and the stabilities of bulks are more robust than the monolayer counterparts. Electronic calculations showed that the monolayers have large band gaps ranging from 2.38 to 3.27 eV while the bulks have pronounced small band gaps ranging from 0.06 to 0.78 eV. The calculated piezoelectric coefficients d₁₁ for the MX monolayers are in the range from 6.6 to 10.9 pm/V. Strikingly, the calculated d₃₃ for the MX bulks are as high as 40.3–213.7 pm/V. By correlating atomic polarizability, atomic mass, relative ion motion, Bader charge and lattice parameters, we proposed an empirical model to estimate the piezoelectric coefficients for the two-dimensional (2D) MXs, where a nice match between the estimated ones and the calculated ones was found. The versatile electronic properties and large piezoelectric coefficients endow MXs a broad prospect of application in optoelectronic and piezoelectric devices, and the revealed underlying mechanisms offer valuable guidelines for seeking novel piezoelectrics.