Doping and Surface Effects of CuFeS2 Nanocrystals Used in Thermoelectric Nanocomposites

In the search for low-cost thermoelectric materials operating near room temperature, the potential of chalcopyrite (CuFeS2) nanocrystals is explored. Their colloidal synthesis is optimized to achieve around 40 nm sized nanocrystals with the goal to effectively reduce thermal conductivity via phonon scattering while maintaining high electrical conductivity. EDX and XPS analyses reveal that the nanocrystals are intrinsically nanostructured with a radial compositional gradient. Three strategies are explored to optimize the thermoelectric properties: i) Intrinsic doping by varying the Cu:Fe ratio. However, the effect of this variation is overcompensated by a global sulfur deficiency, making the chalcopyrite nanocrystals n-type. A high Seebeck coefficient, 5, up to -380 mu V/K is obtained, while the figure of merit remains comparably low (ZT-0.07 at 400 degrees C) because of low electrical conductivity sigma. ii) Removal of the native, insulating ligands by exchange with potassium selenide. This results in a better trade-off between S and a and hence a strongly improved ZT (0.18 at 400 degrees C). iii) Extrinsic doping via intimate mixture of chalcopyrite nanocrystals with metal nanoparticles. Sn (3 wt%) or Ag (16 wt%) nanoparticles give the best results (ZT=0.16 at 400 degrees C), inducing the concomitant reduction of thermal conductivity kappa and increase of sigma.