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Photoluminescent, undoped ZnO films have been fabricated using spray pyrolysis of zinc nitrate solution. The luminescent films had a polycrystalline hexagonal wurtzite type structure with no preferred orientation. Photoluminescence intensity was critically dependent on substrate temperature during spray pyrolysis and on post-annealing temperature. Green, photoluminescent films possessed a porous structure while orange films possessed a close packed granular morphology. Green luminescence appears to be due to oxygen vacancies in a layer just below the crystallite surface.
Studenikin et al. (1998) studied this question.