Tuning thermoelectric properties of Sb2Te3-AgSbTe2 nanocomposite thin film -- synergy of band engineering and heat transport modulation
Abstract
The present study demonstrates a large enhancement in the Seebeck coefficient and ultralow thermal conductivity (TE) in Sb2Te3-AgSbTe2 nanocomposite thin film. The addition of Ag leads to the in-situ formation of AgSbTe2 secondary phase nanoaggregates in the Sb2Te3 matrix during the growth resulting in a large Seebeck coefficient and reduction of the thermal conductivity. A series of samples with different amounts of minor AgSbTe2 phases are prepared to optimize the TE performance of Sb2Te3 thin films. Based on the experimental and theoretical evidence, it is concluded that a small concentration of Ag promotes the band flattening and induces a sharp resonate-like state deep inside the valence band of Sb2Te3, concurrently modifying the density of states (DOS) of the composite sample. In addition, the electrical potential barrier introduced by the band offset between the host TE matrix and the secondary phases promotes strong energy-dependent carrier scattering in the composite sample, which is also responsible for enhanced TE performance. A contemporary approach based on scanning thermal microscopy is performed to experimentally obtain thermal conductivity values of both the in-plane and cross-plane directions, showing a reduced in-plane thermal conductivity value by ~ 58% upon incorporating the AgSbTe2 phase in the Sb2Te3 matrix. Benefitting from the synergistic manipulation of electrical and thermal transport, a large ZT value of 2.2 is achieved at 375 K. The present study indicates the importance of a combined effect of band structure modification and energy-dependent charge carrier scattering along with reduced thermal conductivity for enhancing TE properties.
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