## 3 Results and Discussion
### Characterization and Optical Properties of Nanophosphor
The quantum efficiency can be calculated for the resulting nanosheets by the following equation [44]:  
\[\eta=\frac{\varepsilon}{\alpha}=\frac{\int\text{L}_{\text{S}}}{\int\text{E}_ {\text{R}}-\int\text{E}_{\text{S}}} \tag{3}\]  
In the formula, \(\varepsilon\) is the number of photons emitted by the sample; \(\alpha\) is the number of photons absorbed by the sample; \(\text{L}_{\text{S}}\) is the emission spectrum of the sample; \(\text{E}_{\text{S}}\) is the excitation spectrum of the standard light source, and \(\text{E}_{\text{R}}\) is the excitation spectrum of the sample without the sample in the integrating sphere. After calculation, the absorption efficiency of nanosheet strontium aluminate is 23.40%, while the absorption efficiency of commercial strontium aluminate luminous powder is 31.86%. The quantum efficiency of nanosheet strontium aluminate luminescent powder was lower than that of commercialized bulk strontium aluminate luminescent powder, and the level of quantum efficiency essentially reflects the luminescence intensity of luminescent powder; therefore, the luminescence intensity of nanosheet luminescent powder was also slightly lower than that of commercialized strontium aluminate luminescent material. The results were shown to correspond to the intensity of the emission peaks of both in the fluorescence spectrogram (Figure 10d). Afterglow performance is an important indicator of the optical properties of phosphors, and in this work, it was observed that the afterglow decay curves of the samples contained two processes (Figure 10e): a fast-decay phase and a slow-decay phase. The fast decay stage is a stage in which the electron inventory inside the luminescent center Eu\({}^{2+}\) is short, and thus, a rapid decrease in the afterglow intensity occurs; the slow decay stage was caused by the Dy\({}^{3+}\) ions, which provide a deeper trap energy level in the SAOED, and the electrons were trapped in the process of irradiation of excitation source, and after the excitation source was cut off, the trapped electrons were slowly released, slowing down the rate of decrease in the afterglow intensity of the sample. The afterglow decay curves of SAOED bulk luminescent materials, as well as nanophosphors, were fitted using Equation (4):  
\[\text{Y}=\text{Y}_{0}+\text{A}_{1}\text{exp}(-\text{t}/\text{t}_{1})+\text{A} _{2}\text{exp}(-\text{t}/\text{t}_{2}) \tag{4}\]  
where Y is the photoluminescence intensity; \(\text{Y}_{0}\), \(\text{A}_{1}\), \(\text{A}_{2}\) are the material correlation constants; t is the desired afterglow time; and t\({}_{1}\), t\({}_{2}\) are the fitting parameters related to the phosphor decay rate, which correspond to the two decay processes. From the results, it can be seen that the fluorescence brightness of the nano phosphor was 53.06% of the original sample (Figure 10d), and the afterglow lifetime was 74.47% of the original sample, but it was still better compared with the afterglow performance of other nano-sized phosphors obtained by the top-down method [25; 45]. Similarly, we studied the variation of afterglow intensity with temperature by ramping up from 100 K to 460 K at 45 K intervals (Figure 10f). When the temperature was less than 150 K, the afterglow intensity was weak due to the lower temperature and weaker thermal perturbation; subsequently, when the temperature increases to 325 K, the process of trapping and releasing electrons becomes more and more active with the increase of temperature, and thus the afterglow intensity shows a linear increase in this temperature band; when 320 K-460 K, the afterglow intensity shows a linear decay relationship (Figure 10g), which was due to the thermal disturbance caused by the ambient temperature. Induced thermal disturbances were too strong, and internal carrier storage in different traps became difficult. In this case, electrons escape or are lost, and thus, the emission intensity of the afterglow becomes weaker.