Study on the dynamics of photo-induced structural change and valence change phenomena

1999年，我们首次在Eu_2O_3中发现了室温下可逆的紫外光诱导现象。自发现以来，我们已经在许多物质中发现了类似的现象。利用块体、薄膜和纳米晶样品，对几种稀土金属氧化物(Eu_2O_3、Sm_2O_3)、几种金属氧化物(Y_2O_3、SiO_2、Al_2O_3、锐钛矿型二氧化钛)、量子顺电(SrTiO_3)和几种AgO纳米颗粒复合材料的紫外光诱导结构变化和价数变化现象的机理和动力学进行了实验研究。特别是，我们注意到样品系统的大小可能决定动力学。上述样品都清楚地显示了可逆光致光谱变化和室温下的记忆效应。这种可逆现象很可能产生用于白光发光器件和可擦除光存储的材料。从理论上讲，粒子尺寸的减小可能导致激发波和长波声子模在粒子中的限制。然而，在实验中没有观察到尺寸与动力学的关系，所得到的结果用激子理论来解释。如果激子是在体相中形成的，则由光照射引起的激子不稳定性导致晶格分解为电子中心和空穴中心。在表面上，空穴中心从表面发射，因此只留下电子中心。因此，这种激子不稳定性可能导致氧在样品表面脱附。这种平衡可以在样品和辐射场组成的系统中实现。

In 1999, we discovered a reversible ultraviolet-laser-light-induced phenomenon at room temperature in Eu_2O_3 for the first time. Since the discovery, we have found similar phenomena for many substances.Experimental studies have been done about the mechanism and dynamics of the ultraviolet- laser-light-induced structural change and valence number change phenomenon for several rare-earth metal oxides (Eu_2O_3,Sm_2O_3), several metal oxides (Y_2O_3,SiO_2,Al_2O_3, anatase TiO_2), quantum paraelectric (SrTiO_3) and several AgI-oxide nanoparticle composites, using bulk-, film- and nanocrystal specimens. Especially, we noted that the size of specimen system may determine the dynamics. All of the above specimens showed clearly both the reversible photo-induced spectral change and the memory effect at room temperature. Such reversible phenomenon may well yield materials for white-light-emitting devices and erasable optical storage. Theoretically, particle-size reduction may give rise to the confinement of excitation waves and long-wavelength phonon modes in the particle in the particle. However, the size dependence on the dynamics has been not observed in the experiments.The obtained results are explained by the exciton theory. The exciton instability induced by photo-irradiation leads to lattice decomposition into an electron centre and a hole centre, if the exciton is formed in the bulk. On surfaces, the hole centre is emitted from the surface, thus leaving only the electron centre. Therefore, such exciton instability may result in oxygen desorption at specimen surfaces. The equilibrium may be realized in a system composed of specimen and a radiation field.