Hot-injection synthesis of monodispersed Cu 2ZnSn(S xSe 1-x) 4 nanocrystals: Tunable composition and optical properties

In this study, we demonstrate that Cu ₂ZnSn(S _xSe _1-x) ₄ nanocrystals with a tunable bandgap could be synthesized by a "hot-injection" protocol. In this protocol, metal stearates dissolved in oleylamine were injected into a hot solution of anion precursors in 1-octadecene (ODE) at a given reaction temperature. ODE, which is a low-cost, low-hazard, and air-stable liquid, was used as the solvent. Oleylamine was chosen as both the reagent to activate the precursors and as the capping agent for the nanocrystals. The composition of the Cu ₂ZnSn(S _xSe _1-x) ₄ nanocrystals could be adjusted across the x range from 0 to 1 by varying the S/Se reactant ratio. The lattice parameters (a and c) measured from X-ray diffraction patterns decreased linearly with increasing Se content. This trend was consistent with Vegard's law, which confirmed the formation of homogeneous Cu ₂ZnSn(S _xSe _1-x) ₄ nanocrystals. The A ₁ symmetry modes of the Cu ₂ZnSn(S _xSe _1-x) ₄ nanocrystals seen by Raman spectroscopy gradually shifted with decreasing x (S content) to the lower frequency side and completely disappeared when x = 0. The absorption spectra of the Cu ₂ZnSn(S _xSe _1-x) ₄ nanocrystals revealed that the bandgaps of the nanocrystals could be adjusted over the range 1.0-1.5 eV by decreasing the S content. The relatively small value for the bowing parameter indicated that the synthesized Cu ₂ZnSn(S _xSe _1-x) ₄ nanocrystals had good miscibility.