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

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 La2GeI2 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, La2Ge is a metal, while the functionalized La2GeI2 structure becomes a semiconductor. Two temperature-dependent adsorption structures have been reported in experiments with the R3̅m 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 (kl) seem to be better than those of available two-dimensional TE materials such as phosphorene, arsenene, etc. We find that a significantly low kl 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.