Composition-dependent crystal phase, optical properties, and self-assembly of Cu-Sn-S colloidal nanocrystals

We demonstrate a robust protocol for preparing monodisperse copper-tin-sulfide (CTS) nanocrystals (NCs) of tunable composition and controlled crystal phase. We show that the crystal phase of CTS NCs is determined not only by the reactivity of chalcogenide precursors but also by the cation composition (Cu:Sn ratio) and identity of ligands bound to the NC surface. In contrast to previous studies, we demonstrate broad variation of the Cu:Sn ratio in the product NCs, in both directions from the stoichiometric Cu₂SnS₃ compound. Localized surface plasmon resonance in the CTS NCs was tuned by varying the Cu:Sn elemental ratio. This demonstrates that the doping level of such alloy semiconductor NCs can be manipulated by varying the cation composition. This result opens new possibilities for applying CTS and related materials for solution-processed photovoltaic devices, by taking advantage of controllable and variable doping. In addition, reversible gelation of colloidal CTS NCs in nonpolar solvents was observed and is discussed.