Colorless transparent fluorescence material: Sintered porous glass
containing rare-earth and transition-metal ions
Danping Chen, Hiroshi Miyoshi, Tomoko Akai, and Tetsuo Yazawa
Citation: Appl. Phys. Lett. 86, 231908 (2005); doi: 10.1063/1.1946897
View online: http://dx.doi.org/10.1063/1.1946897
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APPLIED PHYSICS LETTERS 86, 231908 共2005兲
Colorless transparent fluorescence material: Sintered porous glass
containing rare-earth and transition-metal ions
Danping Chen and Hiroshi Miyoshi
Conversion and Control by Advanced Chemistry, PRESTO, JST, Ikeda, Osaka 563-8577, Japan
Tomoko Akaia兲
National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan
Tetsuo Yazawa
Himeji Institute of Technology, Himeji, Hyogo 671-2201, Japan
共Received 4 January 2005; accepted 3 May 2005; published online 2 June 2005兲
Transparent fluorescence oxide glass with high emission yields has been prepared. Porous glass was
impregnated with rare-earth and transition-metal ions and consequently sintered at 1100 ° C into a
compact nonporous glass. Reduction sintering is indispensable for obtaining fluorescence glass with
high emission yield. Sintering of glass impregnated with Eu ions in a reducing atmosphere enhances
the emission intensity by about 15 times than that sintered in air. The Eu2+ and Ce3+ ions and Sn2+
and Cu+ ions incorporated in SiO2 glass obtained by reduction sintering exhibit intense fluorescence
in the near-ultraviolet and visible ranges, their emission yields are 97%, 70%, 100%, and 90%,
respectively. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1946897兴
Oxide glass is an attractive host matrix for the emission
ions of rare-earth and transition-metal ions because it has
excellent optical and mechanical properties, is very stable in
chemistry, and can easily be formed into any shape. However, a major obstacle to using such glasses as host matrices
is concentration quenching resulting from interaction among
emission activator ions in the glass. Despite the extensive
research on glasses doped with rare-earth ions and the wealth
of glass-forming systems that have been investigated, the
number of commercially available glasses doped with rareearth ions are quite limited; specifically only Nd3+-doped laser glass and Er3+-doped fiber lasers and amplifiers.1 The
fluorescence of heavy-metal and transition-metal ions in oxide glass has not received as much attention as that of rareearth ions because emissions from these ions in oxide glass
have usually been weak.2,3 Impurity traces 共⬍10 ppm兲 of
heavy-metal and transition-metal ions in sodium-silicate
glass exhibit fluorescence phenomenon,3 implicating that the
concentration quenching of these ions in oxide glass may be
very low. Concentration quenching is associated with the
relative proximities of activator ions in the glass. Rare-earth
and transition-metal ions are of low solubility in oxide glass
and easily lead to segregation or phase separation at very low
concentrations.4 Study of 29Si magic angle spinning nuclear
magnetic resonance spin-lattice relaxation has shown the
presence of clusters of Nd ions even at ppm concentrations in
silicate glass.5 Addition of Al2O3 to oxide glass can partially
inhibit phase separation and has a beneficial effect on concentration quenching.4–7 However, satisfactory methods to
suppress concentration quenching through glass synthesis are
still lacking and may be impossible.
To avoid concentration quenching, at present, we propose a physical method to uniformly distribute the rare-earth
and transition-metal ions in oxide glass by means of the microscopic voids in porous glass. Porous glass is very useful
a兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
0003-6951/2005/86共23兲/231908/3/$22.50
for impregnation with different ions for the purpose of making various functional materials,8–10 such as nonlinear optical
materials of quantum-confined semiconductor nanocrystals.
The emission properties of some rare-earth and transitionmetal ions adsorbed onto porous Vycor glass 关porous glass
which mainly consists of silica 共96%兲兴, has been investigated
by a number of researchers, including Anpo et al.,11 Reisfeld
et al.,12,13 and Hazenkamp and Blasse.14,15 They obtained
valuable information on the emission properties of some activator ions adsorbed onto porous Vycor glass; however, their
resulting materials do not seem to be strong fluorescent
glasses. We notice that in all these papers that, though the
activator ions are distributed uniformly in glass, the porous
Vycor glass was not sintered to a dense glass. The reason for
the low fluorescent intensity of the rare-earth ions adsorbed
onto porous Vycor glass may be impurity quenching from a
large number of residual OH− groups present in the porous
glass.1 In addition, the microscopic pores of porous glass
strongly scatter the UV light of excitation and also decrease
the emission intensity. Sintering at a high temperature can
eliminate the pores and most of the residual OH− groups.
The sintering process then, particularly, sintering in a reducing atmosphere, may be an important step in the process of
preparing strong emission glass. This deduction is supported
by the fact that 1% Al2O3 · 99% SiO2 共mol%兲 glass containing 1 wt % Eu2O3, prepared by sol-gel processing and heat
treated under reducing conditions at 800 ° C, exhibits strong
emission characteristics at an emission yield of 90%.16
In this letter, we propose a method of preparing highSiO2 glass containing rare-earth and transition-metal ions
through a process of impregnation and sintering in a reducing atmosphere, and report a significant enhancement of
emission due to the reduction sintering. The colorless transparent fluorescent glasses have potential for application in
lasers,17 fiber lasers and amplifiers, solar concentrators, displays, fluorescent lamps, and transparent phosphors used in
special places, such as high temperature and high humidity
environments.
86, 231908-1
© 2005 American Institute of Physics
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231908-2
Chen et al.
FIG. 1. Excitation 共left-hand side, monitored at 430 nm兲 and emission
共right-hand side兲 spectra of porous glasses impregnated with europium ions
and sintered in air 共a兲 共excited at 317 nm兲 and a reducing atmosphere 共b兲
共excited at 280 nm兲.
The porous glass in the present study was prepared using
waste glass as described elsewhere.18 This method is similar
to the treatment process used in manufacturing Vycor glass.6
Although the waste glass contains Na+, Ca2+, Al3+, and some
colored ions, all cations except Si4+ can be leached out after
the waste glass is remelted with B2O3 and leached by hot
acid solution. The analytical composition of the porous glass
obtained is 97.0% SiO2 · 2.1% B2O3 · 0.8% Al2O3 · 0.05%
Na2O · 0.05% CaO, which is close to that of Vycor glass or
high-SiO2 glass. The obtained porous glass is a transparent
material with pore sizes of less than 40 Å and the pores
nominally occupy about 40% of the volume of the glass. The
impurity of the transition-metal ions in the glass is less than
100 ppm and the 50% transmission value of the glass sintered at 1100 ° C in a reducing atmosphere is located near
225 nm for a thickness of 1 mm. The infrared 共IR兲 absorption band due to OH− stretching vibration 共⬃2.7 ␮m兲 共Ref.
1兲 showed that the quantity of the residual OH− groups in the
glass sintered at 1100 ° C is close to that of fused silica glass.
The obtained porous glass was immersed into 0.01– 0.1 M
solution of europium and cerium nitrate or copper and tin
chloride for 1 h and dried at room temperature. The porous
glasses impregnated with these activator ions were then sintered at 1100 ° C for 2 h in air or in a carbon crucible used
for reducing conditions.
The glasses impregnated with europium ions and heat
treated below 800 ° C in air showed a very weak red emission band at about 610 nm 共 5D0 → 7F2兲 due to Eu3+ ions12,16
when excited by UV light at a wavelength of around 250 nm.
When increasing heat treatment temperatures from
800 to 1100 ° C in air, the red light gradually weakened and
a blue light from an emission band at about 430 nm became
strong. Namely, the Eu3+ ions were reduced to Eu2+ ions
even though the porous glass had been sintered in air. Moreover, when the porous glass was sintered at 1100 ° C in a
reducing atmosphere, the blue light was further remarkably
enhanced and the emission band at about 610 nm vanished
completely. This result indicates that the nanosized, interconnected pores in porous glass are a favorable environment for
reducing Eu ions. Figure 1 shows excitation spectra and
emission spectra of porous glasses impregnated with europium ions and sintered in air and a reducing atmosphere.
The broad excitation and emission bands are attributed to the
4f 65d ↔ 4f 7共 8S7/2兲 transition of the Eu2+ ions.16 The emission
intensity of Eu2+ ions incorporated in the glass sintered in a
reducing atmosphere is 15 times larger than that of the glass
Appl. Phys. Lett. 86, 231908 共2005兲
FIG. 2. Excitation 共left-hand side兲 and emission 共right-hand side兲 spectra of
porous glasses impregnated with Ce ions and sintered in air 共a兲 共monitored
at 404 nm and excited at 314 nm兲 and a reducing atmosphere 共b兲 共monitored
at 402 nm and excited at 354 nm兲, and co-impregnated with Ce2O3 and
Al2O3 and sintered in a reducing atmosphere 共c兲 共monitored at 382 nm and
excited at 320 nm兲.
sintered in air. The Eu2+-ion-doped glass is colorless and
transparent, and exhibits a remarkably strong blue fluorescence, which can be observed even in sunlight. To evaluate
the emission properties, the emission yield of the
Eu2+-ion-doped glass was measured by comparison with
that of anhydrous quinine 共C20H24N2O2兲 in 1.0NH2SO4
solution.16 The observed emission yield of the glass sintered
in a reducing atmosphere is approximately 97%, which is
higher than that of Eu2+-doped Al2O3-SiO2 glass prepared by
the sol-gel process.16 The method of manufacturing highSiO2 glass 共Vycor glass兲 through glass phase separation has a
long history6 and lends itself readily to mass production. Further, the glass can be easily formed into various shapes, such
as plates, fibers, and tubes.
Figure 2 shows the excitation and emission spectra of
porous glasses impregnated with Ce ions and sintered in air
or a reducing atmosphere. The broad emission bands observed in both glasses are attributed to the 5d → 4f transition
of the Ce3+ ions.14 The emission intensity of Ce3+ ions incorporated in the glass is increased seven times by reduction
sintering. The absorption and emission of Ce3+ ions in glass
arises from transitions between 4f and 5d energy levels. Unlike 4f, 5d orbitals are exposed to significant interaction with
the orbitals of surrounding atoms and ions which in turn
influences the emission properties. As seen in Fig. 2, when
the porous glass was co-impregnated with Al3+ and Ce3+ ions
and sintered in a reducing atmosphere, the emission intensity
was further enhanced two times. Moreover, the Al3+ and
Ce3+ co-doped high-SiO2 glass also exhibits an intense fluorescence on par with that exhibited by the Eu2+-doped highSiO2 glass. This result indicates that the nanosized pores in
the porous glass can be modified through impregnation with
a compound to form favorable local circumstances for the
activator ions. However, as the peak from the Ce3+-doped
high-SiO2 glass is at about 405 nm and shifts to 385 nm with
the addition of Al2O3, we only observed a weak blue fluorescence in comparison with the Eu2+-doped glass when
these glasses were excited by a UV lamp at a wavelength
250 nm or 360 nm. The emission intensities of Eu2+ or Ce3+
ions incorporated in the glass are nearly independent of the
solution concentration in the range of 0.01– 0.1 M europium
and cerium nitrate. The observed emission yield of Ce3+ ions
incorporated in the glass is approximately 70%, as shown in
Table I.
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231908-3
Appl. Phys. Lett. 86, 231908 共2005兲
Chen et al.
TABLE I. Emission properties of various ions in porous glass sintered in a
reducing atmosphere.
Eu2+
Ce3+
Cu+
Sn2+
Excitation
wavelength 共nm兲
Emission
peak 共nm兲
Emission
yield 共%兲
280
320
258
262
435
385
478
395
97
70
100
90
Further, some heavy-metal and transition-metal ions,
such as Ag, Bi, Sn, In, Mn, Fe, Co, and Cu ions, were impregnated in the porous glasses. All of these glasses exhibited strong luminescence with UV irradiation after sintering
in a reducing atmosphere. These glasses did not exhibit such
strong luminescence when sintered in air. Figure 3 shows
photographs of the glasses impregnated with Eu2+, Cu+, and
Sn2+ ions and sintered in a reducing atmosphere before and
after irradiation by a black light at a wavelength of 254 nm.
It is seen that the luminescence from the glass with Cu+, and
Sn2+ is comparable to that with Eu2+. Indeed, the emission
bands of the glass with Cu and Sn are as intense as that with
Eu, as shown in Table I. The intense emission at around
478 nm is assigned to the electronic transition 3d104s1
→ 3d10 of the Cu+ ions.19 The glass containing Sn2+ shows
the band at around 395 nm, which may be attributable to
Sn-related defects in the SiO2 matrix.20 Fluorescent glass
with metal ions as phosphors has merit in the diversity of
positions of the emission band. The optical fluorescence of
the metal ions is mostly related to the s and d levels, and the
energy level of d orbitals is strongly correlated with the
chemical environment around the ions. Accordingly, the
emission intensities of metal ions incorporated in the glasses
are strongly dependent on the preparation conditions, including concentration, co-doped compound, and sintering temperature. If these conditions are adequately controlled, any
photoluminescence in the near-IR-UV and visible range
共330– 700 nm兲 is possible.
In conclusion, the porous glass is not only advantageous
for uniform distribution of the rare-earth and transition-metal
ions, but also favorable for redox reactions and modification
of local circumstances for the activator ions in the microscopic voids. The process through which the porous glass is
impregnated with activator ions and sintered in a special atmosphere provides a method for producing colorless transparent fluorescence materials. The very strong emissions
from the Eu2+, Ce3+, Cu+, and Sn2+ ions incorporated in the
high-SiO2 glasses may make them useful as colorless transparent fluorescence materials, such as for lasers, lamps, and
displays.
1
FIG. 3. 共Color兲 Photographs of glasses impregnated with Eu2+ ions 共right兲,
Cu+ ions 共middle兲, and Sn2+ ions 共left兲 and sintered in a reducing atmosphere: 共a兲 Without UV irradiation and 共b兲 under UV light 共254 nm兲.
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