Phonon Dynamics and Carrier-Mobility Engineering in Gd-Doped Bi2Se3 Nanoflakes for Thermoelectric Applications

Bismuth selenide (Bi₂Se₃) has attracted considerable interest as a lead-free, environmentally benign thermoelectric (TE) material for near-room-temperature applications. Despite extensive research, Bi₂Se₃ has yet to achieve a thermoelectric figure of merit (zT) exceeding unity, primarily due to intrinsic limitations in its thermal and electronic transport properties. In this work, we report on the thermoelectric performance of Gd-doped Bi₂Se₃ nanoflakes synthesized via a hydrothermal process and spark plasma sintering (SPS). Temperature-dependent thermal transport measurements revealed a significant reduction in total thermal conductivity, to nearly ∼30%, reaching 0.69 W/mK at 453 K for the 2 mol % Gd-doped Bi₂Se₃. Raman spectroscopy confirmed significant optical phonon softening and a decrease in the interatomic force constant upon Gd doping. Further, sound velocity measurements confirmed the enhanced lattice anharmonicity, contributing to the suppression of lattice thermal conductivity. The 1 mol % Gd-doped Bi₂Se₃ exhibited an enhanced power factor of 256 × 10⁻⁶ W/mK² due to optimized carrier mobility and an improved balance between electrical conductivity and Seebeck coefficient. This results in a peak zT of 0.14 at 453 K. These findings demonstrate the importance of rare-earth doping in simultaneously tuning phonon and charge transport, offering a promising pathway for improving the performance of Bi₂Se₃-based thermoelectric materials.