  Experimental realization of single-layer MoSi2N4 is among the latest
groundbreaking advances in the field of two-dimensional (2D) materials.
Inspired by this accomplishment, herein we conduct first-principles
calculations to explore the stability of MC2N4 (M= Cr, Mo, W, V, Nb, Ta, Ti,
Zr, Hf) monolayers. Acquired results confirm the desirable thermal, dynamical
and mechanical stability of MC2N4 (M= Cr, Mo, W, V) nanosheets. Interestingly,
CrC2N4, MoC2N4 and WC2N4 monolayers are found to be semiconductors with band
gaps of 2.32, 2.76 and 2.86 eV, respectively, using the HSE06 functional,
whereas VC2N4 lattice shows a metallic nature. The direct gap semiconducting
nature of CrC2N4 monolayer results in excellent absorption of visible light.
The elastic modulus and tensile strength of CrC2N4 nanosheet are predicted to
be remarkably high, 676 and 54.8 GPa, respectively. On the basis of iterative
solutions of the Boltzmann transport equation, the room temperature lattice
thermal conductivity of CrC2N4 monolayer is predicted to be 350 W/mK, among the
highest in 2D semiconductors. CrC2N4 and WC2N4 lattices are also found to
exhibit outstandingly high piezoelectric coefficients. This study introduces
CrC2N4 nanosheet as a novel 2D semiconductor with outstandingly high mechanical
strength, thermal conductivity, carrier mobility and piezoelectric coefficient.
