Composition Control and Thermoelectric Properties of Quaternary Chalcogenide Nanocrystals: The Case of Stannite Cu₂CdSnSe₄

Abstract

A high-yield and upscalable colloidal synthesis route for the production of quaternary I 2 −II−IV−VI 4 nanocrystals, particularly stannite Cu 2+x Cd 1−x SnSe 4 , with narrow size distribution and precisely controlled composition is presented. It is also shown here how the diversity of valences in the constituent elements allows an effective control of their electrical conductivity through the adjustment of the cation ratios. At the same time, while the crystallographic complexity of quaternary chalcogenides is associated with intrinsically low thermal conductivities, the reduction of the lattice dimensions to the nanoscale further reduces the materials thermal conductivity. In the specific case of the stannite crystal structure, a convenient slab distribution of the valence band maximum states permits a partial decoupling of the p-type electrical conductivity from both the Seebeck coefficient and the thermal conductivity. Combining these features, we demonstrate how an initial optimization of the nanocrystals Cd/Cu ratio allowed us to obtain low-temperature solution-processed materials with ZT values up to 0.71 at 685 K.

Band convergence in the non-cubic chalcopyrite compounds Cu2MGeSe4

“…[21][22][23][24][25][26] Furthermore Zeier et al have shown that a restructuring process in the series of solid solutions Cu 2 Zn 1Àx Fe x GeSe 4 changes the c/2a axis ratio, which leads to enhanced point defect scattering and a reduction in the thermal conductivity due to the high local anisotropic structural disorder. 27 The changing bond angles and bond lengths in these materials strongly inuence the thermal transport properties in these materials.…”

Electron Microscopy for Characterization of Thermoelectric Nanomaterials

“…Other Method to Enhance ZT Bulk materials with complex structure (such as skutterudites [62], half-Heusler [63], Stannite [64], Zintl phases [65]) are promising approach to obtain high TE performance because of their intrinsic special structures [22]. Here, we can take skutterudite structure, for example.…”

“…Indeed, for n-type PbS we showed dramatic improvements in ZT from 0.4 to 1.1 at 923 K by combining nanoprecipitates of Bi 2 S 3 and Sb 2 S 3 s phases with heavy PbCl 2 doping [58]. A comparison of some physical properties for optimized, highperformance n-and p-type Pb-based chalcogenide TE materials is listed in Table 10.2. At the same time, complex bulk materials [22] (such as skutterudites [62], half-Heusler [63], Stannite [64], Zintl phases [65]) have been explored and found that high efficiencies could indeed be obtained. Many of the recent high-ZTTE materials similarly achieve a reduced k lat through disordering the unit cell.…”

Improved thermoelectric performance of solid solution Cu4Sn7.5S16 through isoelectronic substitution of Se for S

“…Because of the p-d hybridization of Cu3 d and Se4 p orbitals, the density of states (DOS) near the Fermi level would be localized. In this way, we would be able to improve the electrical conductivity 12 .…”

“…Although such investigations with respect to the hierarchical chemical bonds contributing to the low thermal conductivity are so far limited and are mostly focused on the cubic I-V-VI 2 semiconductors 15 and Nowotny–Juza compounds like α-MgAgSb with a non-caged structure 13 , we believe that this strategy is important for engineering the phonon transport of Cu-Sn-S compounds. In this regard, we specially substitute Se for S in the compound Cu 4 Sn 7.5 S 16 , aiming to engineer the band structure, phonon transport, and introduce lattice disorder, similar to the cases of Cu substitution for Cd 12 in Cu 2 CdSnSe 4 and Se for Te in n-type PbTe based solid solutions 17,18 .…”