## 3 Results
### Photoenhancement of PbS QDs and PbS/MnS Core Shell QDs  
Figures 4 and 5 show the PL spectra of PbS QDs and PbS/MnS core shell QDs CS 0.3, CS 0.6, CS 0.9, CS 1.2 and CS 1.5. Generally, the PL peak intensities of the PbS QDs and PbS/MnS core shell QDs were continuously increased over a time of 40 min with no energy shifts observed in core shell samples except in CS 1.5. Figure 4: PL spectra of PbS core QDs (**a**) showing enhancement of PL intensity, including the intensity trend over time (inset). PL spectra of PbS/MnS CS 0.3 (**b**) exposed to laser of power \(p=100\) mW and time exposure for 40 min at room temperature. It is well understood that the PL intensity enhancement depends on several factors, including the intensity of laser light, capping ligands and the presence of water [32]. Several experiments have been carried out with the aim of determining the atmospheric constituents involved in the activation procedure [33]. From our results, due to the fact that the PbS QDs and PbS/MnS core shell QDs were synthesized in an aqueous medium, it is likely because of the presence of oxygen in the aqueous solvent. As discussed in the sample preparation section, the samples were synthesized in aqueous solution and the QDs were capped by the thiol ligands (TGL and DTG). Thiol ligands cause the PL intensity to be enhanced by smoothing the surface of the core QDs and removing surface defects. Previous research also reported the enhancement of PL intensity when water soluble QDs were exposed to continuous laser irradiation. This includes research from [PERSON] et al., [PERSON] et al., and [PERSON] et al., where they synthesized citrate-CdSe, 2-mercaptoethanolpropionic acid-CdSe and thioglycolic acid-CdTe, respectively [34; 35; 36]. However, their PL emission showed an inconsistent shift after continuous irradiation with a large energy shift of \(\sim\)1.80-3.60 meV due to photocorrosion. Unlike in our samples that were capped by thiol ligands, PL peak energy of PbS showed a small shift (0.34 meV) and the behavior was improved with a MnS shell which showed a constant position that can be seen in Figure 6a. Figure 5: PL spectra of PbS/MnS (**a**) CS 0.6, (**b**) CS 0.9, (**c**) CS 1.2 and (**d**) CS 1.5 exposed to laser of power \(p=100\) mW and time exposure up to 40 min at room temperature. In this work, the enhancement of PL intensity may be due to the adsorption of H\({}_{2}\)O molecules on the QD surface, and this pathway is the consequence of the stabilizing effect of H\({}_{2}\)O in coordination with the surface. In this case, at early illumination times, the water molecules adsorb on the surface of the QDs and passivate surface charge carrier traps thus increasing the exciton emission. The presence of oxygen in the aqueous solution speeds up PL activation as it causes a slow photocorrosion. This results in releasing Pb\({}^{2+}\) based on the following reaction:  
\[\text{PbS}+2\text{H}_{2}\text{O}+\text{O}_{2}\rightarrow\text{Pb}^{2+}+\text{ SO}4^{2-}+4\text{H}^{+}\]  
where the presence of oxygen in water gives rises to the PL intensity [37]. While photoenhancement can be a simple phenomenon to improve the PL intensity of the QDs, this process is usually accompanied by a blueshift of the PL peak energy [33, 38]. These effects are less pronounced in our samples, which could be advantageous particularly for biological marker applications where samples displayed promising constant positions of the PL peak energies after continuous irradiation with laser light. Figure 6a shows a graph of PL peak energies of all samples with respect to time. It was noticed that the constant PL peak energies as the time increased were due to the passivation of capping ligands on the QDs. For PbS QDs, small shifts of 0.34 meV in the PL spectrum were detected after 40 min of continuous irradiation. Meanwhile, constant PL peak energies were obtained for PbS/MnS core shell QDs (CS 0.3, CS 0.6, CS 0.9, CS 1.2). However, the PL peak energy of PbS/MnS CS 1.5 was redshifted after 25 min.