# Fabrication and Luminescent Properties of Zn-Cu-In-S/ZnS Quantum Dot Films under UV Excitation
## 3 Results  
For this study, various UV irradiation intensities were used as shown in Figure 3a. Spectral presentation of the forward luminescent light for the QD250 sample is presented in Figure 3b. Further analysis required the arithmetic measurement of the output UV radiation power. The Newport, ST 851-UV Power Meter was used, and the results are presented in Table 2.  
Figure 2: Experimental setups for the UV irradiation of the prepared films: (**a**) The experimental configuration for the measurement of the forward luminescent light produced by the sample, as well as the UV radiation that pass through the sample and (**b**) the experimental configuration for the measurement of the reflected UV, as well as the backward luminescent light produced by the sample. Compound films were irradiated with X-rays in order to assess the volume homogeneity as a result of QD dispersion. Figure 4 shows the resulting images. Many QD aggregations are apparent in compound films QD100 and QD150. Since the fabrication process was the same for QD25, it is assumed that there are aggregations in that sample also, but they are not visible due to the small QD concentration. On the contrary, the sample with the highest QD concentration, QD250, has no visible aggregations which is attributed to the 5 min sonication which was added in the fabrication process. The CV was measured in each sample. Table 3 shows the CV measurements obtained using the ImageJ software package. The region of interest (ROI) was equal for all samples (area 139.414 square pixels). The CV value increased as the QD concentration increased while for the QD250 sample the CV value dropped dramatically due to the 5 min sonication incorporated in the fabrication method. \begin{table}
\begin{tabular}{c c} \hline \hline
**Power Steps** & **Output (mW)** \\ \hline
1 & 7.3 \\
2 & 8.9 \\
3 & 10.4 \\
4 & 11.9 \\
5 & 13.3 \\
6 & 14.3 \\
7 & 14.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Measurements of the output UV radiation power. Figure 4: X-ray imaging of the four compound films. Figure 3: (**a**) Spectral presentation of the incident UV irradiation intensities and (**b**) spectral presentation of the forward luminescence of the QD250 when irradiated with the different incident UV intensities. Using the configuration described in Figure 2a, the forward luminescent light and the percentage of incident UV radiation that passed through the four compound films were measured, for the various steps of UV radiation power, as shown in Figure 5.  
From Figure 5a, it can be deduced that in the examined UV energy range, the power of the forward luminescent light is linearly increased by increasing the power of the incident UV radiation. Goodness of fit and slope characteristics for Figure 5a are shown in Table 4.  
Figure 5b shows the intensity variation of the forward luminescent light for the different QD concentrations. These values were obtained using 11.9 mW incident UV radiation power. This figure depicts that the forward luminescent light production rate is increasing almost linearly up to the concentration of QD150 while after that point it tends to reach a plateau. Such performance is quite justified by taking into consideration effects such as self-absorption and Forster resonance energy transfer (FRET), which are of major importance. The relation between FRET and QD concentration has been studied extensively and is in accordance with our experimental results [41; 42]. FRET involves the energy transfer between two neighbor chromophores. The donor chromophore at its excited state can transfer its excitation energy to the receptor chromophore via a non-radiative dipole-dipole coupling mechanism. Still, it is a prerequisite that the two chromophores must be in proximity, typically in a range of 3-10 nm. Generally, FRET efficiency increases with QD concentration until it saturates. The saturation level depends on the size of the chromophores, as well as on the distance between the donor and the receptor.