Site selectively excited luminescence and energy transfer of X$_1$-Y$_2$SiO$_5$:Eu at nanometric scale

Two different types of structure called X/sub 1/ and X/sub 2/ are existing in Y/sub 2/SiO/sub 5/ at normal conditions. In X/sub 1/ type, Y/sup 3+/ ions occupy two sites where they are surrounded respectively by nine and seven oxygen ions, while in X/sub 2/ structure, only six and seven oxygen ions are involved. Nanometric scale X/sub 1/-Y/sub 2/SiO/sub 5/ crystals were prepared by sol-gel method with particle size around 50 nm. Site selective excitation at low temperature has shown four different luminescent centers. Two of them belong to Eu/sup 3+/ embedded in X/sub 1/-Y/sub 2/SiO/sub 5/, the other two are attributed to Y/sub 2/O/sub 3/:Eu phase and to a particular site on the surface. The occurrence of the latter site may be related to the nanometric size of the sample. A pronounced excitation energy transfer from site 2 to site 1 was also observed on excitation spectra. The energy transfer rate increases rapidly with increasing Eu/sup 3+/ concentration and for x=0.7 in Y/sub 2-x/SiO/sub 5/:Eu/sub x/, no fluorescence takes place in site 2 at 15 K. The lifetimes of the /sup 5/D/sub 0/ levels of Eu/sup 3+/ in the two sites were measured as a function of Eu/sup 3+/ concentration. The results have shown that the lifetime of the /sup 5/D/sub 0/ level of Eu/sup 3+/ in site 2 decreases with increasing Eu/sup 3+/ concentration. The energy transfer rate dependence upon temperature was studied in detail and compared to a theoretical simulation