Compared to the non-annealed EDC NPs, it can be observed that the bandgap is biased towards 3 eV, which is approximately the bandgap energy for Ce2O3.

Thus, there is a high concentration of Ce3+ and oxygen vacancies [10], after the anneal at 700°C. The bandgap energy of the EDC NPs is slightly larger following the 800°C anneal, Vactosertib nmr indicative of a lower concentration of Ce3+ in the nanoparticles [21]. However, there is a significant shift in the bandgap of the EDC NPs annealed at 900°C, which suggests that the cerium ions in the EDC NPs have been almost completely converted from the Ce3+ ions into Ce4+ states during the 900°C anneal, similar to the unannealed composition. Figure 3 Absorbance dispersion curves (a), graphs to calculate direct bandgap (b), SEM image (c), and XRD pattern. (a) Absorbance dispersion curves for the EDC NPs annealed at 700°C, 800°C, and 900°C; (b) the graphs used to calculate the direct bandgap of the annealed EDC NPs, and MDV3100 order (c) a SEM image of and (d) XRD pattern from a sample of the EDC NPs following the 800°C anneal, as a representative example (AS, as-synthesized or unannealed). The annealed EDC NPs are imaged using TEM and compared

to that of the unannealed EDC NPs. A representative image is shown in Figure 3c; it is an image of the EDC NPs after an 800°C anneal. Following the anneal temperature range between 700°C to 900°C, the mean diameter is found to be in the range of 9 to 13 nm as compared to a mean diameter of 7 nm for the unannealed (as-synthesized) EDC NPs. The synthesized EDC NPs have mean diameter smaller than other optical nanoparticles

that have been studied as an optical active medium for down- or up-conversion [22–25]. An X-ray diffraction (XRD) pattern is presented in Figure 3d, measured on a sample of the EDC NPs annealed at 800°C, to demonstrate that the predominant nanostructure of the EDC NPs is cerium dioxide [10, 26]. The diffraction find more peaks in the XRD patterns measured on the as-synthesized EDC NPs and the nanoparticles annealed at 700°C and 900°C also are characteristics of ceria. Under near-UV (λ = 430 nm) excitation, the visible emission from the EDC NPs is centered around 520 nm, as shown in Figure 4a. As can be seen, the anneal conditions at 700°C and 800°C are optimum for the down-conversion process, which involves the radiative relaxation of 5d to 4f transition of an excited Ce3+ ions in Ce2O3 that results in broadband emission in the green wavelength [10, 27]. A further explanation of the down-conversion process is as follows: When the EDC NPs containing some fraction of Ce2O3 are illuminated with near-UV light, some fraction of the valence band electrons are excited to an oxygen vacancy defect state located within the CeO2 bandgap. From the defect state, the electron undergoes multiple transitions as it returns to the ground state. Only one of the transitions results in radiative emission and the other transitions are non-radiative.