  Chemical vapor deposition has been most recently employed to fabricate
centimeter-scale high-quality single-layer MoSi2N4 (Science; 2020;369; 670).
Motivated by this exciting experimental advance, herein we conduct extensive
first-principles based simulations to explore the stability, mechanical
properties, lattice thermal conductivity, piezoelectric and flexoelectric
response, and photocatalytic and electronic features of MA2Z4 (M = Cr, Mo, W; A
= Si, Ge; Z = N, P) monolayers. The considered nanosheets are found to exhibit
dynamical stability and remarkably high mechanical properties. Moreover, they
show diverse electronic properties from antiferromagnetic metal to half metal
and to semiconductors with band gaps ranging from 0.31 to 2.57 eV. Among the
studied nanosheets, the MoSi2N4 and WSi2N4 monolayers yield appropriate band
edge positions, high electron and hole mobilities, and strong visible light
absorption, highly promising for applications in optoelectronics and
photocatalytic water splitting. The MoSi2N4 and WSi2N4 monolayers are also
predicted to show outstandingly high lattice thermal conductivity of 440 and
500 W/mK, respectively. For the first time we show that machine learning
interatomic potentials trained over small supercells can be employed to examine
the flexoelectric and piezoelectric properties of complex structures. As the
most exciting finding, WSi2N4, CrSi2N4 and MoSi2N4 are found to exhibit the
highest piezoelectric coefficients, outperforming all other-known 2D materials.
Our results highlight that MA2Z4 nanosheets not only undoubtedly outperform the
transition metal dichalcogenides family but also can compete with graphene for
applications in nanoelectronics, optoelectronic, energy storage/conversion and
thermal management systems.
