# Fabrication and Luminescent Properties of Zn-Cu-In-S/ZnS Quantum Dot Films under UV Excitation
## 4 Discussion  
In this research, we present a simple method for the fabrication of QDs/PMMA films. Each step of the manufacturing procedure is described in detail. Forward and backward luminescent light showed a linear increase with the increase of the incident UV radiation. Forward luminescence light, as well as backward luminescent light, saturated when the concentration reached QD150. Both phenomena are explained with well-known mechanisms, such as self-absorption and FRET. It is worth noting that for the ZnCuInS/ZnS QDs the concentration of QD100 might be a marginal concentration, after which the dominant luminescence is reversed from forward to backward, even though more measurements around the concentration of QD100 could optimize the accuracy of that value. The latter is an essential parameter for the design of novel dual-mode imaging modalities such as hybrid UV-X-ray equipment or for in vivo imaging, as well as in microscopy [48; 49; 50]. This conclusion might also be of value for the design of non-medical applications such as light-emitting diodes (LEDs), displays, and photovoltaics. The measurement of the reflected UV radiation revealed that the concentration of the QDs affects the optical properties of the compound films, such as refractive index, back scattering, etc. [46; 47]. Fabrication of compound films with layers of different QD concentrations could lead to the minimization of the reflected UV radiation, thus increasing the amount of radiation that contributes to the production of luminescent light. Furthermore, the EQE of the compound films was calculated when exposed to UV radiation. Transmitted UV radiation and reflected UV radiation were not taken into account for the calculation of the EQE since they do not contribute to luminescence. Due to experimental limitations, we only considered the forward and backward luminescence but not the lateral luminescence in order to calculate the EQE. Thus, it is expected that the actual EQE would be higher than 45.52%. Homogeneity of the PMMA matrix and QD dispersion was evaluated by means of X-ray irradiation. Excellent PMMA homogeneity was achieved; however, the formation of QD clusters as a result of aggregations was also revealed. QD25, QD100, and QD150 developed a significant amount of QD clusters while QD250 performed much better due to the use of sonication. Excessive aggregation can lead to the formation of QD clusters, which behave like energy traps and can trigger several excitation energy transfer processes, such as FRET, due to short interparticle distances. Photoluminescence quenching and energy trapping due to energy migration in the QD lattice have been well described [51; 52; 53; 54]. Photoluminescence quenching was not observed in our experiments, not even in QD100 and QD150, where strong QD aggregation was present. However, a photoluminescence plateau appeared with the QD250 sample, which exhibited minimum CV and QD aggregation. The latter may lead to the conclusion that even though QD clustering, due to increased QD concentration,  
Figure 9: **(a)** The variation of the energy quantum efficiency (EQE) by increasing the concentration of the QDs in the compound films and **(b)** the variation of EQE by increasing the power of the incident UV radiation. may play a role in luminescence efficiency, it might not be as significant as the non-radiative excitation energy transfer mechanisms. These mechanisms tend to be quite prominent in our experiments, and the observed plateau is attributed to them. The luminescence efficiency can be also affected by QD composition. During the fabrication phase of QDs, oxygen (O\({}_{2}\)) pressure can also affect the defect formation and oxygen vacancies on the QD film properties [55; 56]. Inhomogeneities in QD height, diameter, volume, and strain that usually appear in ensembles of QDs can also affect the luminescence efficiency [57]. The emission wavelength of QDs is directly linked to their quantum confinement, which is also directly affected by their shape and size.