Thermoelectric Performance of the 2D Bi 2 Si 2 Te 6 Semiconductor.

Bi 2 Si 2 Te 6 , a 2D compound, is a direct band gap semiconductor with an optical band gap of ∼0.25 eV, and is a promising thermoelectric material. Single-phase Bi 2 Si 2 Te 6 is prepared by a scalable ball-milling and annealing process, and the highly densified polycrystalline samples are prepared by spark plasma sintering. Bi 2 Si 2 Te 6 shows a p-type semiconductor transport behavior and exhibits an intrinsically low lattice thermal conductivity of ∼0.48 W m -1 K -1 (cross-plane) at 573 K. The first-principles density functional theory calculations indicate that such low lattice thermal conductivity is derived from the interactions between acoustic phonons and low-lying optical phonons, local vibrations of Bi, the low Debye temperature, and strong anharmonicity result from the unique 2D crystal structure and metavalent bonding of Bi 2 Si 2 Te 6 . The Bi 2 Si 2 Te 6 exhibits an optimal figure of merit ZT of ∼0.51 at 623 K, which can be further enhanced by the substitution of Bi with Pb. Pb doping leads to a large increase in power factor S 2 σ, from ∼3.9 μW cm -1 K -2 of Bi 2 Si 2 Te 6 to ∼8.0 μW cm -1 K -2 of Bi 1.98 Pb 0.02 Si 2 Te 6 at 773 K, owing to the increase in carrier concentration. Moreover, Pb doping induces a further reduction in the lattice thermal conductivity to ∼0.38 W m -1 K -1 (cross-plane) at 623 K in Bi 1.98 Pb 0.02 Si 2 Te 6 , due to strengthened point defect (Pb Bi ') scattering. The simultaneous optimization of the power factor and lattice thermal conductivity achieves a peak ZT of ∼0.90 at 723 K and a high average ZT of ∼0.66 at 400-773 K in Bi 1.98 Pb 0.02 Si 2 Te 6 .