- 商用荧光粉的要求:
- YAG:Ce3+, CaAlSiN3:Eu2+目前还是最好的商用荧光粉,outcompete rapidly developing alternatives。
- 优点:a close to 100% internal quantum yield and prolonged LED operating hours without severe degradation.
- 缺点:thermal quenching or saturation quenching (automotive head lights and projector devices)
- 对于high-power applications应用场景,目前只有YAG:Ce3+满足要求,没有可用的红色荧光粉。
Two-level systems $$ \frac{\mathrm{d} p}{\mathrm{~d} t}=k_{\mathrm{exc}}[1-p(t)]-k_{\mathrm{decay}} p(t) $$(1) \(p(t)\) is the probability to find a center in the excited state at some moment in time \(t\);
(2) \(k_{\mathrm{exc}}=\sigma I_{\mathrm{exc}} / \hbar \omega\) is the rate constant for excitation, depends on the absorption cross-section \(\sigma\) of a center, , the excitation intensity \(I_{\mathrm{exc}}\), , and on the excitation photon energy \( \hbar \omega\);
(3) \(k_{\text {decay }}\) is the rate constant for decay, \(=k_{\text {rad }}+k_{\text {nonrad }}\).
两种情况:
- \(t>t_{2}\)
\(p(t)=p\left(t_{2}\right) e^{-k\tiny{\text {decay }}\left(t-t\tiny{2}\right)}\) - \(t_{1}<t<t_{2}\)
\(p(t)=p\left(t_{2}\right)\left[1-e^{-k_{\text {rise }}\left(t-t_{1}\right)}\right]\)其中\(k_{\text {rise }}=k_{\text {exc }}+k_{\text {decay }}\)
可以被测定的参数:As the emission intensity from the sample is proportional to excited-state population, both \(k_{\text {exc }}\) and \(k_{\text {decay }}\) can be determined from a measurement of the luminescence dynamics with square-wave-modulated excitation.
KSF:Mn4+ Mild experiment conditions
(1) the grains were placed on an indium foil directly connected to a heat sink; (2) the peak excitation intensity was limited to a maximum of 0.7 W mm−2;
(3) the duty cycle was set to no more than 5%;
(4) the heat sink waskept at room temperature.
- Mn4+的激发态寿命为8.5 ms,和之前光谱仪测试的一致,而且这里没有随着激发功率的变化而变化,说明没有在这个实验条件下没有open nnradiative decay pathways。
- 激发光为blue,发射光为red,所以必然存在热的生成,但是寿命没有变化,说明在此条件下not sufficient to lead to thermal quenching.
- rise dynamics accelerate with increasing excitation intensity,这很容易理解,因为\(k_{\text {rise }}=k_{\text {exc }}+k_{\text {decay }}\)