all core NCs, three PL lifetimes s1 (5–12 ns), s2
(40–90 ns), and s3 (204–362 ns) were observed
(Fig. 4a; Table 3). Considering s3 is in the range of
hundreds of nanoseconds and all PL emission spectra
have broad peaks, we also attribute s3 to the donor–
acceptor transition. In addition, upon comparing A1
and A2 parameters for each pair of a core and its
corresponding core–shell (Table 3), both parameters
are decreased after ZnS shell growth. The ZnS shell
growth minimizes surface defects and may be
involved in core etching (or core size reduction) due
to exchange of copper or indium in the core with Zn
ions from Zn precursor (Park and Kim 2011; Trizio
et al. 2012). It is reasonable to associate the decrease
of A1 and A2 with the minimization of surface defects
and intrinsic defects. In other words, s1 and s2
indicating two energy states between the conduction
band and the valence band of NCs for electron–hole
recombination could be related to intrinsic and surface
defects. However, it may not be appropriate to
correlate which lifetime parameter to which type of
defects, because both intrinsic and surface defects
could contribute to both energy states.