Understanding the\nUltralow Thermal Conductivity and\nStrong Anharmonicity of a Lanthanum-Based Germanium Halide Monolayer\nfor Possible Thermoelectric Applications

Low-dimensional materials outperform their bulk equivalents\nin\nterms of thermal and electronic charge transport phenomena. Ultralow\nthermal conductivity in thermoelectric (TE) semiconductors is rare\nand plays a crucial role in obtaining promising TE performances. Their\nperformance can be effectively improved via strain engineering, which\nallows the modulation of geometrical parameters as well as electronic\nenergy levels of a material. With this concept in mind, we systematically\nstudied the effect of biaxial tensile strain on the structure, stability,\nmechanics, and thermoelectric properties of a novel La<sub>2</sub>GeI<sub>2</sub> monolayer by using the hybrid density functional\ntheory and solving Boltzmann transport equations. The strain-induced\ndistortion manipulates the electronic band characteristics with an\nincrease in the band gap, effective mass, and relaxation time of carriers.\nIn principle, La<sub>2</sub>Ge is a metal, while the functionalized\nLa<sub>2</sub>GeI<sub>2</sub> structure becomes a semiconductor. Two\ntemperature-dependent adsorption structures have been reported in\nexperiments with the <i>R</i>3̅<i>m</i> phase\nas the most stable ground-state structure. HSE06 calculations predict\nan indirect gap of 0.69 eV appearing at the Γ–M symmetry\npoints of the Brillion zone in this monolayer. La–Ge bands\nbeing prominent around the Fermi level emerge out of p–d covalent\nhybridization, providing an edge to enhanced conductivities. The calculated\ntransport coefficients and thermal conductivity (<i>k</i><sub>l</sub>) seem to be better than those of available two-dimensional\nTE materials such as phosphorene, arsenene, etc. We find that a significantly\nlow <i>k</i><sub>l</sub> value (3.22 W/mK) at 300 K can\nbe reduced to an ultralow value of 0.57 W/mK under strain. Owing to\nthe strain-engineered low thermal conductivity, small band gap, significant\nSeebeck coefficient (∼1100 μV/K), and ZT(∼2),\nwe can rule out the enhanced TE conversion potentials of this monolayer\nin comparison to traditional TE materials.