We observed that Zn alloyed in CIS causes a blue shift but it also quenches the NC PL. Moreover, ZnS shell growth on Zn–CIS cores (or Zn–CIS core etching by Zn ions) enhances NC QY and causes a further blue shift. In the shell growth or core etching process, according to Park and Trizio, Zn atoms enter into NCs by exchanging with Cu or In in cores (Cu or In is
dominant in alloyed NCs as shown in Table 1). Both Park’s and Trizio’s theories can be applied to explain the blue shift—the reduction of NC core size or the Zn increment in Zn–CIS. On the other hand, as shown in Table 2, Zn in CIS quenches NC PL, but Zn entering into CIS or Zn–CIS in the ZnS shell growth causes NC QY enhancement. We suggest that a Zn increment in NCs through cation exchange introduces extra defects but also minimizes defects near NC surfaces where the latter process is dominant and thus the overall effect is to enhance NC QY.