Understanding the Ultralow Thermal Conductivity and Strong Anharmonicity of a Lanthanum-Based Germanium Halide Monolayer for Possible Thermoelectric Applications

Abstract

Low-dimensional materials outperform their bulk equivalents in terms of thermal and electronic charge transport phenomena. Ultralow thermal conductivity in thermoelectric (TE) semiconductors is rare and plays a crucial role in obtaining promising TE performances. Their performance can be effectively improved via strain engineering, which allows the modulation of geometrical parameters as well as electronic energy levels of a material. With this concept in mind, we systematically studied the effect of biaxial tensile strain on the structure, stability, mechanics, and thermoelectric properties of a novel La 2 GeI 2 monolayer by using the hybrid density functional theory and solving Boltzmann transport equations. The strain-induced distortion manipulates the electronic band characteristics with an increase in the band gap, effective mass, and relaxation time of carriers. In principle, La 2 Ge is a metal, while the functionalized La 2 GeI 2 structure becomes a semiconductor. Two temperature-dependent adsorption structures have been reported in experiments with the R3̅ m phase as the most stable ground-state structure. HSE06 calculations predict an indirect gap of 0.69 eV appearing at the Γ−M symmetry points of the Brillion zone in this monolayer. La−Ge bands being prominent around the Fermi level emerge out of p−d covalent hybridization, providing an edge to enhanced conductivities. The calculated transport coefficients and thermal conductivity (k l ) seem to be better than those of available two-dimensional TE materials such as phosphorene, arsenene, etc. We find that a significantly low k l value (3.22 W/mK) at 300 K can be reduced to an ultralow value of 0.57 W/mK under strain. Owing to the strain-engineered low thermal conductivity, small band gap, significant Seebeck coefficient (∼1100 μV/K), and ZT(∼2), we can rule out the enhanced TE conversion potentials of this monolayer in comparison to traditional TE materials.