fluorescence of rare earth ions in binary zirconia-silica sol-gel glasses fluorescence of rare earth...

Fluorescence of Rare Earth Ions in Binary Zirconia-Silica Sol-Fluorescence of Rare Earth Ions in Binary Zirconia-Silica Sol-Gel GlassesGel Glasses

Jessica R. Callahan, Karen S. Brewer, Ann J. SilversmithDepartments of Chemistry and Physics

Hamilton College, Clinton, NY

Mix 4.90 mL TMOS with 2.5 mL ethanol; stir for 10 minutes

Add 0.50 mL deionized H2O and 20 µL conc. HCl; stir for 90 min

Stir 10 minutes or until all light precipitate has dissolved; cast into 12

75 mm tightly capped polypropylene test tubes

Add solution of 1% RE ions dissolved in

2.5 mL H2O

… Zr(OPr)4 via syringe, stir 10

minutes

Add 2.5 mL ethanol

simultaneously with…

Often using two stir bars was helpful

5D0

partial energy diagram for Eu3+

0

5

10

15

20

25

x103cm-1

En

erg

y

7F0

7F27F1

5D1

5D2

5D3

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

synthesis and processingsynthesis and processing

sample qualitysample 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 resultsspectroscopic results

referencesreferences

acknowledgementsacknowledgementsThis 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 glasssol-gel glass vs. melt glassAdvantages3

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

introductionintroductionOur 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?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 goalsproject goalsSynthesize 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 ionschallenges in doping sol-gel glasses with rare earth ionsClustering 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

excitation spectrum of Eu-doped zirconia-silica glass

250 350 450 550

wavelength (nm)

fluorescence (arb. units)

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 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 7F05D3 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.

partial energy diagram for Ho3+

5

10

15

20

25

x103cm-1

En

erg

y

5I8

5F5

5G4

3K8

5S2

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

fluorescence of holmium-doped zirconia-silica glass

520 570 620 670wavelength (nm)


10% Zr, 12h dwell at 750 ˚Cexcite 457 nm

emission spectrum comparing Eu-doped glasses

570 590 610 630

wavelength (nm)


exc 254nm, RT 2%Al 1%Eu

25% Zr

5D0→7F0

5D0→7F1

5D0→7F2

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

Homogeneous sol Reaction

hydrolysis and condensation,ambient conditions,pH 1.5 to 3.5

Gelationpolymeric gel forms “wet” gel

2 days, 40°C

Agingsolvents escape,pore contraction

1-3 days, 60°C

Dryingshrinkage,densification,pore collapse,

2-4 days, 80°C

europium fluorescenceeuropium fluorescence

enhanced fluorescence in thulium and enhanced fluorescence in thulium and holmiumholmium

02468

1012141618

202224

2628

Energ

y (

10

00cm

-

1)

3H6

3F4

1G4

1D2

650

nm

476

nm

3H5

3H4

3F2,3

790

nm

partial energy diagram for Tm3+

partial energy diagram for Ho3+

Pr Nd

Er Eu

550

nm

663

nm

our collaboratorsour collaborators

Ann SilversmithHamilton College

Physics

Dan BoyeDavidson CollegePhysics

Ken KrebsFranklin & Marshall

CollegePhysics

Karen BrewerHamilton College

Chemistry

comparison of Tm-doped glasses

600 650 700 750 800

wavelength (nm)

intensity (arbitrary units)

Tm/Al glass 750˚C x5

Tm/10%Zr, 750˚C for 12 hrs

476 nm exc

comparison of fluorescence lifetimes

0 1 2 3 4 5 6

time (ms)

ln(fluorescence)

25%Zr

Al co-dopeno co-dope

temperature program for zirconia-silica glasses

0

100

200

300

400

500

600

700

800

0 1 2 3 4

time (days)

temperature (˚C)

fluorescence line narrowing results

590 600 610 620 630 640 650

wavelength (nm)

intensity (arbitrary units)

577nm

581nm

579.5nm10%Zr/1% Ho7.5%Zr/1% Er10%Zr/1% Nd

1% Thulium Glass

7.5%Zr 10%Zr 12.5%Zr

20%Zr

1% Europium Glass Under UV light

2%Zr 12.5%Zr 20%Zr 30%Zr

579.5 nm

573.2 nm

600 610 620 630 640

575.1nm

581.6nm

575nm

577nm578nm

579nm

wavelength (nm)

SiO2 glass Al3+ co-doped

SiO2 glass Ti4+ co-doped

SiO2 glass no co-dopants