Strain-tunable elastic, electronic, and thermal transport properties of two-dimensional CaI2 monolayer: A first-principles study

Abstract

Despite the increasing attention on two-dimensional (2D) metal iodides in recent years for their appropriate bandgaps and exceptional optical properties, there is limited understanding of their intriguing thermal transport properties. Here, the strain-tunable mechanic, electronic, and transport properties of the monolayer CaI2 are calculated in detail by combining first-principles calculations with Boltzmann transport theory. We have checked its thermal, dynamic, and mechanical stability by computing ab-initio molecular dynamics, phonon spectra and elastic constants, respectively. The calculation results from the PBE-GGA and HSE06 methods indicate that the monolayer CaI2 is an indirect semiconductor, with band gaps of 3.92 eV and 5.12 eV, respectively. Additionally, the effective masses and Fermi velocities for both holes and electrons are also obtained, which indicates that the monolayer CaI2 owns the excellent electronic transport property. These mechanical constants and electronic parameters are also highly sensitive to the external strain. The calculated real lattice thermal conductivity of the monolayer CaI2 at 300 K is about 1.01 W/mK, which is rather low among 2D materials, implying its potential application in heat insulation materials or thermal insulation material. With the biaxial tensile strain increasing, its lattice thermal conductivity would further decrease. The difference in lattice thermal conductivities between 0 % and 2 % strain, as well as between 4 % and 6 % strain, is much smaller than that between 2 % and 4 % strains. Subsequently, the group velocities, phonon scattering rates, and cumulative lattice thermal conductivities of the monolayer CaI2 are discussed under the different strains.