I mean what the most possible sites for these TM ions are in the ZnO nanocrystal not ZnO crystal lattice. In other words, are the TM ions most likely to appear at the surface, or sub-surface, or center of ZnO nanocrystals?
It depends mainly on the energy and mass of doped ions and also on the volume of ZnO nanocrystals and their temperature. I dont`t think there is a simple answer for your question.
In principle there are many possible sites for transition metal. But a priori I do not think that it can be possible to give a definite answer. Too many factors are involved.