In addition, it should be noted that the ZnS shell growth on CIS, Cd–CIS, and Zn–CIS causes a blue shift and QY enhancement. Both blue shift and QY enhancement caused by the ZnS shell growth on CIS cores have been observed by others (Park and Kim 2011; Trizio et al. 2012). In their reports, both Park and Trizio agree that cation exchange between Zn ions (from Zn precursor) with Cu or In in NCs causes blue shift and QY enhancement; however, they have different explanations. Park et al. suggest that during ZnS shell growth, Zn ions etch cores, reducing core size, causing a PL blue shift and resulting in the formation of a thin ZnS shell around cores to enhance QY. Trizio et al. claim that no shell is formed in the core etching process by Zn ions but only Zn–CIS, and the blue shift is due to the nature of Zn–CIS (the more
Zn in NC, the more blue shift). Moreover, Zn atoms fill vacancies in CIS NCs to enhance QY. Due to the limited availability of scientific tools in profiling composites in small NCs of several nanometers, both Park and Trizio did not investigate Zn distribution in NCs after core etching.