Flexible MA2Z4 (M = Mo, W; A = Si, Ge and Z = N, P, As) monolayers with outstanding mechanical, dynamical, electronic, and piezoelectric properties and anomalous dynamic polarization

We systematically investigate the mechanical, dynamical, and piezoelectric properties of MA(2)Z(4) monolayers (M = Mo, W; A = Si, Ge and Z = N, P, As) based on first-principles calculations. The structural properties, cohesive energy and formation energy analyses show that all of the considered MA(2)Z(4) monolayers are dynamically stable. Ab initio molecular dynamics simulations further indicate that the MA(2)Z(4) monolayers can sustain stability at high temperatures. The MA(2)Z(4) monolayers exhibit isotropic mechanical properties with the bearable largest strains exceeding 25% and 30% in the armchair and zigzag directions. All MA(2)Z(4) monolayers exhibit semiconducting properties, and the band gaps change in a wide range. The piezoelectric constants e(11) and d(11) increase from 3.21 x 10(-10) to 8.17 x 10(-10) C m(-1) and 0.73 to 6.05 pm V-1, respectively. We reveal that the piezoelectric coefficients are closely related to the ratio of the polarizabilities of the isolated anions and cations. Infrared spectroscopy indicates that the piezoelectricity is the overlap of the intrinsic dipole moments existing in the inner MZ(2) monolayer and outer A(2)Z(2) bilayer. Besides, the Born effective charges quantificationally show the contribution of component atoms to polarization. The anomalous dynamic polarization around M atoms is found, which is generated from the anti-bonding of the last occupied orbital. Our results indicate that the MA(2)Z(4) monolayers have great potential in piezotronics and piezo-phototronics fields.