Biaxial strain tuned electronic structure, lattice thermal conductivity and thermoelectric properties of MgI2 monolayer

We systematically investigate the effect of strain engineering on the thermodynamic stability, electronic structure, Seebeck coefficient and other properties of two-dimensional (2D) MgI2 monolayer on the basis of first-principles. The increasing stress causes the maximum phonon frequency of the MgI2 monolayer to decrease gradually. With the increase of tensile strain, although the indirect-band structure remains the same from the Perdew-Burke-Eruzerhof (PBE) and Heyd-Scuseria-Ernzerhof (HSE06) levels with considering the spin-orbital coupling, the peaks of conduction band and valence band are closer of the MgI2 monolayer. In the process of tensile strain from 2% to 4%, the number of band valleys increases, and the multiple valley pockets caused by such strain increase the Seebeck coefficient. It is found that the Seebeck coefficient increased from 140.86 μV/K without strain to 231.58 μV/K under 4% tensile strain. It also makes the power factor reach its peak at 4% strain of the MgI2 monolayer. However, the lattice thermal conductivity of the MgI2 monolayer is 0.89 W/mK at 300 K in the case of no strain, and it decreases linearly with the increase of tensile strain. The results showed that the ZT value increased gradually with the increasing tensile strain, and it reaches 1.39 at 300 K for the MgI2 monolayer under the 9% tensile strain. Greatly stimulated further theoretical and experimental research on strain engineering and wish to improve thermoelectric conversion efficiency of two-dimensional (2D) materials.