Spectroscopic properties of Co/sup 2+/: ZnAl/sub 2/O/sub 4/ nanocrystals in sol-gel derived glass-ceramics

Transparent glass-ceramics containing zinc-aluminum spinel (Zn.41204) nanocrystals doped with tetrahedrally coordinated Co2+ ions were obtained by the sol-gel method. The gels of composition Si02-Al203-ZnO-CoO were prepared at room temperature and heat-treated at temperature ranging 80O-95O0C. When the gel samples were heated up to 900"C, ZnAlzO4 nanocrystals were precipitated. Co2+ ions were located in tetrahedral sites in Zn.41204 nanocrystals. X-ray diffraction analysis shows that the crystallite sizes of ZnAlzO4 crystal become large with the heat-treatment temperature and time, and the crystallite diameter is in the range of 10-15nm. After heat treatment at 900-950°C for 5hs or 12hs, new broad absorption bands centered at 1400nm clearly appeared in the NIR regions and the bands are attributed to the Co2+ ions in the znAlzo4 crystals formed in the ZAS glass. This absorption bands near IR region clearly indicated that the ZnAl204 nanocrystals were formed and the CO' ions are located in the tetrahedral sites of the spinel-structured Zn.41~04 crystals. The presence of Co2+ ions located in different crystalline and glassy sites in ZAS glass-ceramic leads to the fine structure of the luminescence bands. We assign the main peak at 994.8nm to 2E(2G) + 4T2(4F) of Co2+ occupying crystalline sites. The influence of heat-treatment temperature on the luminescence spectra demonstrates the importance of energy-transfer process. The crystal field parameter D, of 423cm-' and the Racah parameters B of 773cm-' and C of 3478.5cm-l were calculated for tetrahedral CO" ions. This glass-ceramic system has potential as a laser material in the visible and NIR regions.