Tuning transport coefficients of monolayer $\mathrm{MoSi_2N_4}$ with biaxial strain

TL;DR

This study uses density functional theory to explore how biaxial strain influences the electronic structure and thermoelectric transport properties of monolayer MoSi2N4, revealing strain as an effective tuning parameter.

Contribution

It provides a systematic analysis of strain effects on MoSi2N4's electronic and transport properties, highlighting the role of spin-orbit coupling and conduction band convergence.

Findings

Strain causes non-monotonic changes in band gap.

SOC effects on Seebeck coefficient depend on strain.

Optimal strain (~0.96) enhances thermoelectric performance.

Abstract

Experimentally synthesized $\mathrm{MoSi_2N_4}$ (\textcolor[rgb]{0.00,0.00,1.00}{Science 369, 670-674 (2020)}) is a piezoelectric semiconductor. Here, we systematically study the large biaxial (isotropic) strain effects (0.90 to 1.10) on electronic structures and transport coefficients of monolayer $\mathrm{MoSi_2N_4}$ by density functional theory (DFT). With $a/a_0$ from 0.90 to 1.10, the energy band gap firstly increases, and then decreases, which is due to transformation of conduction band minimum (CBM). Calculated results show that the $\mathrm{MoSi_2N_4}$ monolayer is mechanically stable in considered strain range. It is found that the spin-orbital coupling (SOC) effects on Seebeck coefficient depend on the strain. In unstrained $\mathrm{MoSi_2N_4}$, the SOC has neglected influence on Seebeck coefficient. However, the SOC can produce important influence on Seebeck coefficient, when the strain is applied, for example 0.96 strain. The compressive strain can change relative position and numbers of conduction band extrema (CBE), and then the strength of conduction bands convergence can be enhanced, to the benefit of n-type $ZT_e$. Only about 0.96 strain can effectively improve n-type $ZT_e$. Our works imply that strain can effectively tune the electronic structures and transport coefficients of monolayer $\mathrm{MoSi_2N_4}$, and can motivate farther experimental exploration.