Tunable structural and optical properties of Ag_xCu_yInS_² colloidal quantum dots

Facile stoichiometric and phase selective synthesis of doped/alloyed metal chalcogenide colloidal quantum dots has been an important pursuit because of the opportunity for tunable photoconductivity. Herein, the structural and optical properties of relatively monodispersed copper indium sulphide incorporated with Ag ⁺ ions, i.e., Ag _xCu _yInS ₂ (Ag:CIS) quantum dots (average diameter of 4.9 ± 0.6 nm) synthesized via a hot-injection colloidal method are investigated. The Ag:CIS quantum dots exhibit a degree of wurtzite to chalcopyrite phase change with increasing Ag ⁺ concentration (1.1-6.8%) under primarily kinetic synthetic conditions at 180 °C for 10 minutes using copper(ii) hexafluoroacetylacetonate hydrate, indium(iii) and silver(i) diethyldithiocarbamate precursors. The indium-rich and copper-deficient quantum dots are close to the CuInS ₂ stoichiometry with tunable bandgaps between 1.60 and 1.81 eV influenced by Ag ⁺ concentration, intrinsic defects, minimal quantum confinement and structural permutations. They exhibit broad photoluminescence emission via a dual radiative pathway with long decay lifetimes, τ ₁ (0.68-2.11 ± 0.02 μs) and τ ₂ (3.37-7.38 ± 0.20 μs), implicating donor-acceptor pair transitions of indium interstitials, and/or indium-copper antisites, to copper vacancies, for low Ag ⁺ concentrations but primarily silver interstitials, to for higher Ag ⁺ concentrations. Importantly, this study is the first involving Ag ⁺ ion-dependent wurtzite to chalcopyrite phase transformation of CIS quantum dots and with their long radiative emission lifetimes are potentially effective photo-absorbers in quantum dot solar cells.