Preparation of Upconversion Fluorescent Glasses by the Sol-Gel Method and Their Properties

用溶胶-凝胶法制备了掺Er~(2+)和Nb_2O_5、Al_2O_3-GeO_2多晶玻璃，观察到了近红外到可见光的上转换荧光。上转换荧光和电子自旋共振(ESR)的结果被解释为Er^&lt；3&gt；离子在这些材料中的聚集。100Nb_2O_5-1Er_2O_3干凝胶在500ﾟ℃热处理得到玻璃，而干凝胶在550ﾟC热处理形成TT-Nb_2O_5相。在805 nm激发下，晶体在544 nm处有微弱的绿色上转换荧光，而玻璃没有。在晶体和玻璃中，在4.2K下测得Er^&lt；3&gt；在约80mT处都有Er^&lt；3&gt；信号，表明Er^&lt；3&gt；离子可以很好地分散在这些材料中而没有聚集。在玻璃xAl_2O_3-(99-x)GeO_2-1Er_2O_3(x=0-5)中，观察到Er^&lt；3&gt；离子的上转换荧光位于525,550和660 nm，归属于从激发态^2H&lt；到基态^4i&lt；15/2&gt；的跃迁。分别为11/2&gt；、^4S_&lt；3/2&gt；和^4F_&lt；9/2&gt；。660 nm荧光的相对强度随着x值的减小而增加，这是因为Er^&lt；3&gt；离子发生聚集，而团簇中紧密相连的Er^&lt；3&gt；离子之间的能量传递交叉弛豫增加了^4F_&lt；9/2&gt；能级的布居。Er^&lt；3&gt；产生的80mT ESR信号强度随着x值的减小而减小，这意味着它也与星团有关。Er~(2+)离子间的顺磁性偶极-偶极相互作用或反铁磁性相互作用可使ESR信号减弱或消失。

Near infrared to visible upconversion fluorescence has been observed in Er^<3+> doped polycrystalline Nb_2O_5 and Al_2O_3-GeO_2 glasses prepared by the sol-gel method. The results of upconversion fluorescence and electron spin resonance (ESR) for Er^<3+> ions have been interpreted in terms of clustering of Er^<3+> ions in these materials. The details are described below.Heat-treatment of a dried gel 100Nb_2O_5-1Er_2O_3 at 500ﾟC gave a glass, whereas the dried gel crystallized into the TT-Nb_2O_5 phase by the heat-treatment at 550ﾟC.The crystal showed weak green upconversion fluorescence at 544 nm under 805 nm excitation, while the glass did not. In both the crystal and glass, ESR spectra measured at 4.2 K showed signals due to Er^<3+> at about 80 mT,indicating that Er^<3+> ions may be well dispersed in these materials without clustering.In glasses xAl_2O_3-(99-x)GeO_2-1Er_2O_3(x=0-5), upconversion fluorescence due to Er^<3+> ions was observed at 525,550, and 660nm, which is assigned to the transition to the ground state ^4I_<15/2> from excited states ^2H_<11/2>, ^4S_<3/2>, and ^4F_<9/2>, respectively. The relative intensity of the 660-nm fluorescence increases with decreasing the value of x. This is because clustering of Er^<3+> ions occurs and then the populations of the ^4F_<9/2> level increases by the energy transferring cross relaxation between close-lying Er^<3+> ions in a cluster. An ESR signal at 80 mT due to Er^<3+> decreases in intensity with decreasing the value of x, implying that it is also associated with clustering. The ESR signal may weaken or disappear due to the paramagnetic dipole-dipole interaction or the antiferromagnetic interaction between clustered Er^<3+> ions.