Unraveling mechanical and electronic attributes of MN4 (M= Be, Mg, Zn, Cd) monolayers with anisotropic Dirac cone and their excellent supercapacitive performances

Abstract

The recent experimental realization of $Be{N}_4$$Be{N}_4$ monolayer, beryllonitrene (Phys. Rev. Lett. 126(2021), 175501), has unveiled a novel group of nitrogen-rich 2D materials, namely $M{N}_4$$M{N}_4$ (M = Be, Mg, Zn, Cd). In this present work, we theoretically explore the anisotropic mechanical and electronic responses of all systems based on the first principles calculations. Among the structures, $Be{N}_4$$Be{N}_4$ and $Mg{N}_4$$Mg{N}_4$ sheets are brittle while $Zn{N}_4$$Zn{N}_4$ and $Cd{N}_4$$Cd{N}_4$ are ductile in response to external strain along particular direction. Electronic band structure analysis reveals that all the structures exhibit anisotropic Dirac cones like features. We further propose a simple coupled-chain model to investigate the occurrence and robustness of such Dirac cones, thus validating our numerical results. In addition, the capacitive responses of all structures are probed by computing quantum capacitance for different electrochemical potential ranges and identifying $Cd{N}_4$$Cd{N}_4$ with the maximum value of 219 $\mu F/c{m}^2$$\mu F/c{m}^2$ for positive electrode potential. The adaptability of a particular electrode as cathode or anode is confirmed by evaluating surface charge density i.e. $\frac{Q_a}{Q_c}$$\frac{Q_a}{Q_c}$. The numerical results clearly demonstrate that the most prominent value of $\frac{Q_a}{Q_c}$$\frac{Q_a}{Q_c}$ is recorded for $Cd{N}_4$$Cd{N}_4$ monolayer (2.13), whereas other $M{N}_4$$M{N}_4$ analogues emerge as promising electrodes for symmetric supercapacitors ($Q_a\approx Q_c$$Q_a\approx Q_c$). We have also traced out the average value of $C_Q$$C_Q$ for both aqueous and ionic/organic systems and $Cd{N}_4$$Cd{N}_4$ is found to exhibit maximum value of 65 $\mu F/c{m}^2$$\mu F/c{m}^2$ for higher electrochemical ranges. Moreover, significant improvement in quantum capacitance is reported for metal atom adsorbed $Be{N}_4$$Be{N}_4$ structure at positive electrode potential. These explorations are expected to encourage future research in developing novel and intriguing nanoelectronics and energy storage devices.