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Published on January 9, 2008

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Fluorescence of Rare Earth Ions in Binary Zirconia-Silica Sol-Gel Glasses Jessica R. Callahan, Karen S. Brewer, Ann J. Silversmith Departments of Chemistry and Physics Hamilton College, Clinton, NY:  Fluorescence of Rare Earth Ions in Binary Zirconia-Silica Sol-Gel Glasses Jessica R. Callahan, Karen S. Brewer, Ann J. Silversmith Departments of Chemistry and Physics Hamilton College, Clinton, NY synthesis and processing sample quality optically clear were monoliths obtained for zirconia content from 2% to 30% some cracking can occur during drying if water and solvent evaporated too quickly annealing above 750 ˚C can cause phase separation of the zirconia, producing opaque glassy materials spectroscopic results references acknowledgements This work sponsored in part by the Research Corporation through a Cottrell College Science Award JRC thanks the General Electric Fund at Hamilton College for summer research stipends sol-gel glass vs. melt glass Advantages3 high purity starting materials & lower processing temperatures higher concentrations of RE3+ possible simple manipulations & greater homogeneity of samples chemical composition can be varied & precisely controlled processing parameters can be readily changed & optimized Disadvantages3 heating must be carefully & consistently controlled processing times can be long (> 2 weeks) cracking during aging, drying, or densification can be extensive residual hydroxyl groups & RE clustering in samples quench fluorescence introduction Our success in the synthesis of rare earth-doped TiO2-SiO2 glasses and their spectroscopic results1 led us to re-examine our preliminary work on the synthesis of the zirconium analogs. In this project, rare earth-doped zirconia-silica glasses have been successfully produced through the co-hydrolysis of Zr(OiPr)4 with Si(OMe)4 in ethanol. Careful drying and aging of the gels produced clear, crack-free glass monoliths. Optical properties were then studied via laser and fluorescence spectroscopy. Synthetic obstacles rapid hydrolysis of the zirconium alkoxide precursor vs. that of TMOS precipitation of the zirconia as a opaque solid during synthesis choosing processing temperatures & programs to limit the precipitation of zirconia during transformation from gel to glass why dope glasses with rare earth ions? In the lanthanide series, the optically active electrons are shielded by filled s and p shells producing narrow spectral lines long fluorescence lifetimes energy levels that are insensitive to the environment Applications of rare earth-doped materials2 phosphors solid state lasers optical fibers waveguides antireflective coatings project goals Synthesize glasses doped with Eu3+ and other rare earth cations including erbium, neodymium, holmium, and thulium Optimize processing parameters to obtain clear, crack-free glass monoliths Match concentrations of Zr with Ti glasses for direct spectroscopic comparison Increase the percentage of zirconium in the glass samples (up to 30% vs. SiO2) Compare optical properties of the zirconia-silica glasses with other sol-gel glasses (e.g., silica, titiania-silica, and chelated rare earth dried gels) challenges in doping sol-gel glasses with rare earth ions Clustering of the rare earth cations in the glass4 only a limited number of non-network oxygen atoms for the RE3+ to bond within the glass clusters formed through RE-O-RE bonding in the glass matrix energy migration is facilitated in the clusters fluorescence is quenched through a cross relaxation mechanism Residual hydroxyl (OH) groups5 present even after annealing to high temperatures give reduced fluorescence lifetimes through a non-radiative decay mechanism when close to the rare earth cation in the glass fluorescence occurs from the 5D0 level in Eu3+ sample excited in the charge-transfer region Al co-doped sample must be annealed at 1000˚C before significant fluorescence is observed Zr co-doped glass annealed only to 750 ˚C and gave comparable fluorescence in general, the Zr co-doped glasses fluoresce more brightly than Al co-doped & about the same as Ti co-doped europium in zirconia-silica glass annealed at 750 ˚C has a longer decay time (~1.4 ms) compared to aluminum co-doped silica glass annealed to 1000 ˚C glasses without co-dopants have very short lifetimes different spectral profiles when excitation l is changed little energy migration between the different RE3+ sites in the glass shows declustering of the Eu3+ in the glass similar to results in Al co-doping Ti results show enhanced peak at 613 nm with longer exc indicating reduced energy migration and more uniform site distribution note that Tm/Al fluorescence spectrum is multiplied by 5 in the above spectrum Zr co-doped glass fluoresces more efficiently than Al co-doped & about the same as Ti co-doped closely spaced energy levels prevents efficient luminescence here, however, in glass annealed at 750 ˚C, we observe fairly strong fluorescence monitored at 612 nm strongest excitation occurs at 393 nm corresponding to the 7F05D3 excitation (1) Boye, D.M.; Silversmith, A.J.; Nolen, J.; Rumney, L.; Shaye, D.; Smith, B.C.; Brewer, K.S. J. Lumin. 2001, 94-95, 279. Silversmith, A.J.; Boye, D.M.; Anderman, R.E.; Brewer, K.S. J. Lumin. 2001, 94-95, 275. (2) Steckl, A.J.; Zavada, J.M., eds. MRS Bulletin, 1999, 24, 16-56. Scheps, R. Prog. Quantum Electron. 1996, 20, 271. Reisfeld, R. Opt. Mater. 2001, 16, 1. Weber, M.J. J. Non-Cryst. Solids, 1990, 123, 208. (3) Brinker, C.J.; Scherer, G.W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press, Boston, 1990. (4) Almeida, R.M. et al. J. Non-Cryst. Solids 1998, 232-234, 65. Arai, K.; Namikawa, H.; Kumata, K.; Honda, T.; Ishii, Y.; Handa, T. J. Appl. Phys. 1986, 59, 3430. (5) Lochhead, M.J.; Bray, K.L. Chem. Mater. 1995, 7, 572. Stone, B.T.; Costa, V.C.; Bray, K.L. Chem. Mater. 1997, 9, 2592. Nogami, M. J. Non-Cryst. Solids 1999, 259, 170. compare to our previous work in Al and Ti co-doped silica glasses1 addition of 1% RE3+ is the critical step high Zr amounts often gelled upon contact with the RE3+(aq) solution after cast into tubes, sols were gelled at 40 ˚C (24 h), 60 ˚C (24 h) and 80 ˚C (48 h) before processing in furnace dried gels heated from ambient temperature to 750 ˚C over a period of 72 h heating rate = 1 ˚C/min to preserve integrity of sample dwell temperatures = 250 and 500 ˚C to remove organics and residual water/OH groups europium fluorescence enhanced fluorescence in thulium and holmium partial energy diagram for Ho3+ Pr Nd Er Eu 550 nm 663 nm our collaborators

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