Optical properties Figure illustrates the PL spectra of porous ZnO thin films annealed in oxygen at 400-800 °C. Three distinct areas can be identified in all spectra. Several peaks in the range of 360 -390 nm, next a significant broad peak centered at 525 nm and finally a very intensive peak centered around 680-700 nm. On top of the last peak, a second diffraction order of the structure from the 360-390 range is visible.(37-41) The UV emission at 360-390 nm, denoted as Near Band Edge Emission (NBE), is related to excitonic recombination at energies close to the ZnO band edge. A clear NBE emission visible at room temperature for all sample batches indicates a the very high crystal quality of the individual nanocrystallites. The dominating NBE lines, can be identified, based on the previous studies of nanoporous as free excitons (F X) at 3.41 eV and donor-bound excitons (D 0 X) at 3.36 eV. The lower intensity lines are LO phonon replicas of the D 0 X line positioned respectively one (72 meV) and two (144 meV) LO phonon energies in ZnO below the D 0 X line. A small peak on the high energy slope of the D 0 X line (3.37 eV) in samples annealed at 800°C can be attributed to the surface bound excitons (SBE) usually visible with samples with a highly developed surface like ours. This peak structure is the same, with same positions of the peaks for all the sample batches, however the general intensities of the emission are the highest in the batch B-800 °C. The intensity of the near band edge (N BE) UV emission region increases as the annealing temperature was raised from 400 °C to 800 °C, relating to the improved crystalline quality, which is consistent with the XRD data.