Emergence of high piezoelectricity along with robust electron mobility in Janus structures in semiconducting Group IVB dichalcogenide monolayers†
Piezoelectric nanomaterials have been emerging as flagship materials for harvesting nanoelectromechanical energy. Pristine, semiconducting 1T-MX2 (M = Zr and Hf; X = S, Se, and Te) monolayers are intrinsically centrosymmetric, and hence non-piezoelectric. This inversion symmetry is broken in their Janus monolayer (non-centrosymmetric) structures, leading to the emergence of a high degree of piezoelectricity in them. This brings along a new dimension in nanoscale piezoelectricity, as the origin of this piezoelectricity is predominantly ionic in nature, in contrast to the 1H-MoS2 monolayer, where it is of electronic character. DFT calculations reveal the piezoelectric coefficient (d22 = 4.68–14.58 pm V−1) in these Janus monolayers to be much higher than that in single layer 1H-MoS2 (d11 = 2.99 pm V−1). 9% uniaxial tensile strain applied along the arm-chair direction is found to raise d22 in HfSSe Janus monolayers to 123.04 pm V−1, which reaches the level of piezoelectric coefficients in the state-of-the-art perovskites. The major contribution of the ionic component to the piezoelectric coefficient is attributable to the predominance of ionic character in the interatomic bonds in these monolayers, which arises from the decoupled band edges, i.e., no hybridization between the band edge states (chalcogen-p and metal-d). Contrarily, 1H-MX2 (M = Mo and W; X = S, Se, and Te) monolayers with coupled band edges are held together mainly by covalent bonds, resulting in the dominance of electronic contribution to piezoelectricity. The nature of band edges causes a lower deformation potential for electrons in 1T Hf and Zr based dichalcogenide monolayers and their Janus structures with respect to 1H-MX2 (M = Mo and W; X = S, Se, and Te) monolayers. This induces a much higher electron mobility in the former than in 1H-MX2 (M = Mo and W; X = S, Se, and Te) monolayers. The carrier mobility calculated using Lang et al.'s formalism [Phys. Rev. B, 2016, 94, 235306] agrees well with the experimentally measured electron mobility. Our predictive findings underscore the imminent need to synthesize these 1T-MX2 semiconducting Janus structures to induce a high level of piezoelectricity together with robust electron mobility.