Direct Synthesis of Highly Conductive tert‐Butylthiol‐Capped CuInS₂ Nanocrystals

Abstract

tert-butylthiol (tBuSH) is used as the sulfur source, surface ligand and co-solvent in the synthesis of CuInS2 nanocrystals (NCs). The presented method gives direct access to short-ligand-capped NCs without post-synthetic ligand exchange. The obtained 5 nm CuInS2 NCs crystallize in the cubic sphalerite phase with space group F-43m and a lattice parameter a=5.65 Å. Their comparably large optical and electrochemical band gap of 2.6-2.7 eV is attributed to iodine incorporation into the crystal structure as reflected by the composition Cu1.04 In0.96 S1.84 I0.62 determined by EDX. Conductivity measurements on thin films of the tBuSH-capped NCs result in a value of 2.5(.) 10(-2) S m(-1) , which represents an increase by a factor of 400 compared to established dodecanethiol-capped CuInS2 NCs.

Doping and Surface Effects of CuFeS2 Nanocrystals Used in Thermoelectric Nanocomposites

“…Scattering elements such as impurities or grain boundaries regularly integrated in the material's structure at a periodicity of 20–100 nm can induce a selective drop in κ L . However, when going to even smaller dimensions, e. g. colloidal nanocrystals (NCs) with sizes below 10 nm, reduced electrical conductivity in thin films and in compacted pellets is observed ,. Concerning chalcopyrite NCs for thermoelectric applications, very few reports exist.…”

“…The present synthesis of large NCs thus tends to yield the thermodynamically favored stoichiometric Cu/Fe 1 : 1 composition. This stands out against previous works on chalcopyrite NCs of smaller size yielding mostly Cu‐enriched crystals ,. The sulfur anion content is expected to adjust the global charge balance and is presented in Figure B as the anion/cation charge ratio A/C=2[S 2− ]/([Cu + ]+3[Fe 3+ ]), where [S], [Cu] and [Fe] are the atomic sulfur, copper and iron concentrations, respectively.…”

“…Having this layer at the grain boundaries may strongly impact the electronic and thermal conduction paths in the thermoelectric material. The nature and bulkiness of the organic ligand layer strongly influences the electronic properties and transport in solids made of chalcopyrite NCs ,. To improve the electrical conductivity, the organic ligand shell was chemically stripped off from the NC surface to avoid residual carbonaceous species issued from alkyl chain decomposition upon thermal annealing.…”

“…The nature and bulkiness of the organic ligand layer strongly influences the electronic properties and transport in solids made of chalcopyrite NCs. [12,26] To improve the electrical conductivity, the organic ligand shell was chemically stripped off from the NC surface to avoid residual carbonaceous species issued from alkyl chain decomposition upon thermal annealing. This strategy, pioneered by Kovalenko and Talapin, [27] is based on surface ligand exchange with inorganic ions.…”

“…[11] However, when going to even smaller dimensions, e. g. colloidal nanocrystals (NCs) with sizes below 10 nm, reduced electrical conductivity in thin films and in compacted pellets is observed. [7,12] Concerning chalcopyrite NCs for thermoelectric applications, very few reports exist. Liang et al described the synthesis of 6.4 nm nanocrystals with a composition of CuFe 0.99 S 2.08 , exhibiting p-type behavior and a high Seebeck coefficient (> 800 mV/K at 500 K), leading to ZT values of 0.22-0.26 at 500 K. [7] Very recently, Gabka et al reported first thermoelectric measurements on n-type chalcopyrite NCs, yielding a maximum ZT of 0.13 at 616 K. [13] In this case, the initial surface ligands (oleylamine) had been exchanged by treatment with inorganic salts (ammonium sulfide or Meerwein's salt) to increase electrical conductivity within the solid materials processed from the NCs.…”