Adding any access atoms to a crystal acts as impurity and that affects to decrease the energy gap

for details you can read the following book

optical process in semiconductors

by J K Pankove

Following

I would guess that some C atoms will dffuse into your ZnO. Then you have three possibilities:

i) neutral C stays interstitial, thereby increasing the local lattice parameter and decreasing the bandgap;

ii) C goes on Zn^2+ site, thereby adding as a donor

iii) C goes on O^2- site, thereby adding as an acceptor

In both doping cases, the bandgap will shrink from Ec - Ev, to either Ed-Ev (ii) or Ec-Ea (iii)

Thank you Thomas for your explanation. How can i draw this shift( decreased) ZnO@C-dots band gap relative to ZnO ?

Good luck.

why energy transfer from C-dots added to ZnO decreased the band gap of ZnO? how can i explain?