This red shift has been already observed in Mn
doped ZnO crystallites [18] and nanorods [6] and it was attributed
to the strong exchange interactions between the d electrons of Mn
and the s and p electrons of the host bands. On the other hand, it
can also be due to an increase of the luminescence band related
to extended defects which is centered at 3.1 eV [19]. As we mention
before, XRD measurements show the incorporation of Mn2+
ions into ZnO lattice. Since Mn2+ ionic radius is higher than that of
Zn2+, it can be expected some deformation in the atomic lattice and,
hence, the formation of extended defects such as dislocations or
stacking faults that would lead to the appearance of this emission.