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

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Fluorescence of Rare Earth Ions in Binary
Zirconia-Silica Sol-Gel GlassesJessica R.
Callahan, Karen S. Brewer, Ann J.
SilversmithDepartments of Chemistry and Physics
Hamilton College, Clinton, NY

• spectroscopic results

Pr
Nd
europium 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

• 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 (gt 2 weeks)
• cracking during aging, drying, or densification
  can be extensive
• residual hydroxyl groups RE clustering in
  samples quench fluorescence

Er
Eu

• 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

• fluorescence occurs from the 5D0 level in Eu3
• sample excited in the charge-transfer region
• Al co-doped sample must be annealed at 1000C
  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

• monitored at 612 nm
• strongest excitation occurs at 393 nm
  corresponding to the 7F0?5D3 excitation

• 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)
compare to our previous work in Al and Ti
co-doped silica glasses1

• 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

• 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

• synthesis and processing

partial energy diagram for Ho3

• enhanced fluorescence in thulium and holmium

• 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

550 nm
663 nm

• 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

• 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

• references

(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.
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
our collaborators