Graphene-assisted synergistic electronic DOS modulation and phonon scattering to improve the thermoelectric performance of Mg3Sb2-based materials

Abstract

Grain boundary (GB) scattering has been widely reckoned as a primary restraint on room-temperature (RT) carrier mobility in Mg₃Sb₂-based materials. In this work, two-dimensional graphene (G) with varied contents was added to single phase Mg_3.24Sb_1.5Bi_0.49Te_0.01 materials in order to tailor the highly resistive space-charge region at GBs. The results indicate that introducing G effectively lowers the carrier transport energy barrier E_b from 42 meV (pristine sample) to 18 meV (Mg_3.24Sb_1.5Bi_0.49Te_0.01/1.0 vol% G sample), and correspondingly increases the drift mobility by 112.5% from 32 cm² V⁻¹ s⁻¹ to 68 cm² V⁻¹ s⁻¹ at RT, leading to an enhanced power factor and thus ZT value. Besides, first-principles calculations were also performed to qualitatively bridge the underlying negative correlation between the electronic density of states (DOS) and GB potential barrier E_b. Moreover, the equivalent nano-particle phonon scattering is also realized through introducing the two-dimensional G, contributing to a moderate reduction in lattice thermal conductivity as quantitatively illustrated based on the Debye–Callaway model. Consequently, the figure of merit ZT for G-added samples is greatly improved in the entire temperature range compared with that of the G-free sample. This study opens up a new avenue for rationally engineering the grain boundary, via regulating interfacial electronic DOS, to optimize electrical transport properties and ZT values.