Bonding Heterogeneity Inducing Low Lattice Thermal Conductivity and High Thermoelectric Performance in 2D CdTe2

Two-dimensional (2D) materials have emerged as a broad platform for exploring promising thermoelectric materials. Motivated by the fabrication of diverse artificially designed Te-based 2D materials with high thermoelectric performance, here, we predicted 2D hexagonal CdTe and pentagonal CdTe2 for potential thermoelectric materials, using the particle swarm optimization (PSO) method combined with density functional theory. CdTe and CdTe2 show predicted direct/indirect band gaps of 1.82 and 1.96 eV, respectively. Chemical bonding analysis revealed that all the Te atoms in CdTe are coupled through uniform ionic bonding. CdTe2 exhibits bonding heterogeneity, arising from weak the Cd–Te ionic bonding and strong Te–Te covalent bonding. Based on Boltzmann transport theory, we found that the bonding heterogeneity in CdTe2 favors low lattice conductivity. The calculated lattice thermal conductivity of CdTe2 is 0.33 Wm–1 K–1 at 300 K, which was contributed by the weaker coupling between acoustic and optical phonon modes, low group velocities of the acoustic modes, and high lattice anharmonicity. On the other hand, the occupied π*5p, π5p, and σ5p bondings in Te–Te pairs significantly facilitate the electrical conductivity and enhance the Seebeck coefficient of p-type CdTe2. The low thermal conductivity and high power factor in CdTe2 give rise to a high thermoelectric performance at low temperature. Our findings should encourage the exploration of 2D materials for thermoelectric applications with strong bonding heterogeneity.