Cation exchange and shelling in ternary CuInS2 and quaternary CuIn(Zn)S2 quantum dots: Structural and optical properties

by Morales Orocu, Jean Carlos; Heyes, Colin D.

Synthesis of CuInS2 /ZnS quantum dots (QDs) is performed in a two-step process that includes the synthesis of the CuInS2 (CIS) core followed by addition of Zn and S precursors, which results in a combination of cation exchange and shelling. Recent reports have attempted to control the optical properties of CuInS2 /ZnS nanocrystals by modifying the composition and thickness of the "shell" during the second step. However, the effect of modifying the environment surrounding CuInS2 emitting centers prior to cation exchange/shelling has not been studied.In this study, we found that incorporating Zn into CIS QDs during the first step of synthesis results in quaternary CuIn(Zn)S2 (CIZS) alloyed cores that show interesting optical properties. Varying the Cu:Zn ratio (keeping Cu:In constant) leads to an improvement of ensemble PLQY of ~28% compared to ternary CIS cores of ~12%. The spectral width (FWHM) and polydispersity of CIZS were reduced compared to CIS, with minimal increases in fluorescence lifetime (~300-350ns) and nanoparticle size (~3-4nm).After addition of addnl. Zn in the second step in a one-pot, non-injection approach, quaternary CIZS QDs showed a much smaller improvement of PLQY to about 35% compared with the improvement when adding Zn to ternary CIS QDs, which can reach PLQY of ~70%, as well as showing variations in spectral shifts and lineshapes. Furthermore, even though negligible particle growth was observed for CIZS QDs, elemental anal. revealed that, similar to when zinc is added to ternary CIS cores, zinc comprises the vast majority (~98%) of their metal composition after this second step. These results suggest that the environment of the CIS emitting cores are different when cation exchange occurs in "pre-zinced" quaternary CIZS QDs compared to ternary CIS QDs, which affects their resulting optical properties.