More optical investigations were obtained using photoluminescence (PL) spectroscopy. First, it was noticed that the observed PL peaks for all the samples were red-shifted with increasing the excitation power Pexc. This may be due to the possible heating of the NPs. It should be noticed that the observed peaks initially were growing in intensity with increasing the Pexc, and after a certain threshold, the peaks started to be red-shifted. We have obtained the PL spectra at a constant Pexc for both the ZnO NPs and the ZnO@ZnS CSNPs. Therefore, we can compare the absolute emission intensities of the different samples assuming that the same amount of material was irradiated. Otherwise, the ratio of the near band edge (NBE) to the deep level emission (DLE) emissions intensity can be used for comparison . According to Fig. 5, pure ZnO NPs demonstrates a narrow peak of ultraviolet (UV) emission at 378 nm. The UV emission peak is due to the free excitonic recombination (FX), possibly assisted by remaining donor bound excitonic emission (D0X) and even by longitudinal optical replicas of several orders (1st LO(FX), 2nd LO(FX), etc.) . The next to the lower Hexanoylcarnitine peak is centered at 513 nm and is much broader. This is so called “green-yellow” band of the visible luminescence or deep level emission (DLE)  which is due to the discrete deep energy levels in the energy band formed by the point defects in the lattice. The ZnO@ZnS samples demonstrate the same qualitative character of luminescence, having two peaks. However, their spectral position is slightly different for both the NBE and the DLE. First, for the ZnO@ZnS1 the NBE peak shifts slightly toward lower energies and is located at 381 nm, while for the ZnO@ZnS2 and the ZnO@ZnS3 the NBE peak appear at 380 and 379 nm, respectively. As it can be seen, there is a slight red-shift for all the ZnO@ZnS CSNPs and this behavior could be expected from type II system. But by enhancing the sulfur source and therefore increasing the shell thickness, the red-shift stars to be less. This could be due to the difference between the sizes of the samples. For covering the ZnO NPs with ZnS shell the pH was adjusted to 10 and in this pH the surface of the ZnO NPs starts to dissolve and the size of the particles reduces slightly but after covering the particles and increasing the concentration of the sulfur source to reach a dense shell, the size will be increased. The DLE peaks in general follow the same trend, being first blue-shifted to 487 nm for ZnO@ZnS1, and then red-shifted to 497 and 490 nm respectively for ZnO@ZnS2 and ZnO@ZnS3. Also, the DLE intensity sample ZnO@ZnS1 is higher than that of the ZnO NPs and decreases with increasing the shell thickness. This type of measurements is random and depends on the specific point on the sample, the nanostructures density over the samples surface, and their non-uniformity, etc. Therefore, we have applied also the macro study, where the excitation area was approximately 10 times larger and hundreds of CSNPs were irradiated. Thus, the PL was averaged over a set of the CSNPs. According to Fig. 6 pancreas is clear that the observed red-shift behavior of the samples is the same as in the micro-study but the behavior of the DLE intensity is different. Since this spectra come from hundreds of CSNPs, it is more accurate to use to interpret the DLE intensity behavior. Table 1 shows the ratio of the NBE to the DLE intensity of the all samples and the data indicate that the defects are decreasing gradually by filling up the oxygen vacancies via the sulfur atoms for the case of ZnO@ZnS1, and then it is increased even further for the ZnO@ZnS2 and for ZnO@ZnS3 is decreased again. Coating the surface of the ZnO NPs with ZnS shell leads to fill up the oxygen vacancies on the surface of the NPs with the S atoms. But for the ZnO@ZnS1 the concentration of the sulfur source is not enough to form a full shell, therefore more S atoms fill up more oxygen vacancies and decrease the DLE intensity in sample ZnO@ZnS2. Although a lot of oxygen vacancies have been filled up in this case but this ZnS layer has more sulfur vacancies. With further increasing of the sulfur ions, more uniform ZnS layer sediments on the ZnO NPs forms a shell with fewer defects. The inset of Fig. 5a–d shows the CIE 1931 color coordinate measurements for the ZnO NPs and the ZnO@ZnS CSNPs. As it can be seen when covering the ZnO NPs with ZnS the emission light turns from green to blue.