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Sensors and Materials, Vol. 30, No. 7 (2018) 1533–1538
MYU Tokyo
S & M 1608

Relationship between Near-infrared Emission ofBi-doped Glass and Preparation Conditions
Hirokazu Masai,1* Takenobu Suzuki,2 and Yasutake Ohishi2
1National Institute of Advanced Industrial Science and Technology,
1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
2
Future-industry Oriented Basic Science and Materials, Toyota Technological Institute,2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan(Received January 22, 2018; accepted March 27, 2018)

Keywords: glass, emission, near infrared, bismuthWe have examined the effects of Bi concentration, annealing in air, and addition of reducing
agent on the near-infrared (NIR) emission of Bi-doped TiO2–ZnO–B2O3–Al2O3 (TZBA) glass.
Since the decay constants of the NIR emission are independent of the Bi concentration andthe preparation conditions, it is expected that a small part of Bi species can act as effective
activators in the glass. Annealing affects the blue shift and broadening of the NIR emission.
The change in the bandwidth upon Bi or AlN addition can be explained from the viewpointof the viscosity of the glass melt, which seems to be one of the important factors for obtaining
broadband-emitting materials.

1.

Introduction

Recently, Bi-containing glass has attracted attention as an optical functional material
because of its unique characteristics. In particular, broad emission in the near-infrared (NIR)
region is one of the recent attractive characteristics of Bi glass for a wavelength divisionmultiplex system.(1–11) Although the origin of the broad NIR emission of Bi-containing glass isstill under discussion, because of the variety of Bi species, such as Bi2,(12) BiO,(13) and Bi2−,(12)such Bi-containing glasses exhibiting broadband emission are promising candidates for superbroadband amplification gain media.
Our group previously reported the NIR emission in CaO–B2O3–Bi2O3–Al2O3–TiO2(CaBBAT) glass.(14) Since the activation energy of the NIR emission and that of the electron
spin resonance signal in CaBBAT glass show similar values, it is strongly suggested thatNIR emission correlates with Bi radical species possessing low valence states. Considering
the energy diagram of Bi-related species,(12,13) we assume that the reducing condition during
melting is preferable for obtaining better emitting glasses.
To examine the effects of various factors on NIR emission, three factors are chosen. The
first is Bi concentration, because there is no clear data on concentration quenching of NIR*Corresponding author: e-mail: hirokazu.masai@aist.go.jp
http://dx.doi.org/10.18494/SAM.2018.1924
ISSN 0914-4935 © MYU K.K.
http://myukk.org/

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emission for the various Bi-doped glasses. The second is annealing in air, because annealing
in air may induce both the structural rearrangement of the glass network and the oxidation ofBi activators, whose valence states are conventionally thought to be lower than that of Bi(III).(11–14)
The third is the addition of a reducing agent, AlN, because it is thought that a reducing
condition is favorable for NIR emission and the addition of AlN can induce a strong reducing
atmosphere that can induce the precipitation of Bi nanoparticles during melting.(15) Since theBi concentration is a changeable parameter in the study, a TiO2-ZnO-B2O3-Al2O3 (TZBA)
system,(16) which exhibits similar physical properties and crystallization behavior to those ofCaBBAT glass, is selected as the host glass. In this study, we discuss several important factors
regarding NIR emission of Bi species from the spectroscopic viewpoint.

2.

Experimental Procedure

2.1

Preparation of Bi-doped oxide glasses

Bi-doped TZBA glasses were prepared by a conventional melt-quenching method.(16)Batches consisted of TiO2 (15 mol%), ZnO (25 mol%), B2O3 (60 mol%), and Bi2O3. The Biconcentrations were 1–5 mol%, which were added as an additional amount. These chemicals
were mixed and melted in an alumina crucible in an electric furnace at 1350 °C for 40 min
in air. Some samples were added additional AlN (5 mol%) as reducing agents. Since AlN
strongly reacts with Pt crucibles, alumina crucibles are suitable for the present examination.
The glass melt was quenched on a steel plate at 200 °C, and then annealed at the glass transition
temperature Tg (approximately 570 °C) for 60 min. After annealing, the glass samples were cut
10 × 10 × ≈1 mm3 in size and mechanically polished to obtain a mirror surface. In addition,
we have also prepared an annealed sample in order to discuss the effect of annealing in air
on the emission property. The annealing was done at Tg + 30 °C for 5 h in air, at which thecrystallization of TiO2 was observed in a previous study.(16)
2.2

Analytical methods

XRD measurements were performed using Ultima IV (Rigaku). Bulk glass samples wereexcited with a Ti3+:sapphire laser (Coherent, 890) with the wavelength of 800 nm and the laserpower of approximately 70 mW. The emission measurement was mechanically chopped at 107
Hz. Emission from the samples was dispersed by a single monochromator [CT-25 (JASCO),blaze wavelength, 1.0 nm; grating, 600 grooves/mm] and detected using a photomultiplier(Hamamatsu Photonics, H10330A-75) with a long-path filter of 800 nm. The emission lifetime
τ1/e of the TZBA glasses was defined as the first e-folding time of the decay curves, detectedusing a digital oscilloscope (Yokogawa, DL-1640). The absorption spectra were measured
within the wavelength range from 300 to 850 nm using a spectrometer (Shimadzu, UV-3150).

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3.

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Results and Discussion

Absorption
coefficient / cm-1

30
2520

4.0
3.0
2.0
2.0

3.0

15

1Bi
2Bi
5Bi

10
50

1.0

2.0

3.0

Photon energy / eV(a)4.0Intensity (arb. unit)

Figure 1(a) shows the optical absorption spectra of the Bi-doped TZBA glasses. The inset
shows the enlarged spectra of the Bi-doped glass in the visible region. The absorption band at
2.6 eV, attributed to the absorption of Bi species,(14) increases with increasing Bi concentration.
By chemical analysis, the Al2O3 amount eluted from the crucible was estimated to beapproximately 20 mol%. Figure 1(b) shows the XRD patterns of Bi-doped TZBA glasses after
annealing at Tg + 30 °C for 5 h. It is notable that these Bi-containing glasses exhibit no clear
diffraction peak attributable to the TiO2 phase, although similar glasses without Bi addition
exhibited precipitation of TiO2 after a similar heat treatment.(16) Therefore, we assume thatthe addition of Bi increases the thermal stability of these glasses against crystallization. For
practical application, annealing without the precipitation of crystallites is important, because
the precipitation of crystallites increases the propagation loss due to the scattering at theinterface. Since there was no linear relationship between the NIR emission intensities and theBi concentrations, we here mainly focus on the spectral shapes. By comparison with the datain the previous report,(11) we can roughly estimate the quantum efficiency of the Bi-dopedglasses. From the NIR intensity, it is speculated that the efficiency is approximately 10%. The
ambiguity of the Bi concentration dependence suggests that a small number of Bi cations canwork as an activator, i.e., not all Bi cations work as activators of the NIR emission.
Figure 2(a) shows the normalized NIR emission spectra of Bi-doped TZAB as-preparedglasses (dashed lines) and annealed glasses (solid lines) obtained by excitation with thewavelength of 800 nm. We observe asymmetric broad emission bands in the NIR region,
suggesting the existence of (1) different coordination sites of a Bi species and/or (2) different
Bi species. The deviation from the Gaussian peak is observed at the higher photon energy,
and most of the emission spectra at least three components. Figure 2(b) shows the emission
peak energy and bandwidth (half-width at half-maximum) of NIR emission in Bi-doped TZAB
glasses as a function of Bi concentration. These parameters are estimated by Gaussian peakfitting at the normalized intensity region of 0.5–1.0. Although no significant differences

5Bi2Bi
Anatase Rutile

20301Bi402θ / degree

5060(b)Fig. 1. (Color online) (a) Optical absorption spectra of Bi-doped TZAB as-prepared glasses. The inset shows theenlarged spectra of Bi-doped glass in the visible region. (b) XRD patterns of Bi-doped TZAB glass annealed at Tg +30 °C for 5 h, along with diffractions of anatase and rutile.

1Bi
2Bi
5Bi

As-prepared
Annealed

Photon energy / eV(a)Annealed

As-prepared

Bandwidth of emission / eVSensors and Materials, Vol. 30, No. 7 (2018)

Peak energy ofemission / eV

Normalized intensity(arb. unit)

1536

Bi concentration (mol%)(b)

Fig. 2. (Color online) NIR emission of Bi-doped TZAB as-prepared and annealed glasses. (a) PL spectra of Bidoped TZAB glasses excited with the wavelength of 800 nm. (b) Peak energy and width of emission in Bi-doped
TZAB glasses as a function of Bi concentration. The dashed and solid lines correspond to the as-prepared andannealed samples, respectively.

between the two parameters are observed among as-prepared glasses, a slight redshift of theemission peak and the narrowing of the emission bandwidth are observed in annealed glasses
with increasing Bi concentration. In addition, after annealing, a blue shift of the emission peak
and a broadening of the bandwidth are observed. Since Bi2O3 is a low-melting metal oxide,
the viscosity of the glass melt is expected to decrease with increasing Bi concentration. Suchlower viscosity can induce structural arrangement during the quenching process, which may be
one cause of the narrowing of the emission bandwidth. Although the annealing process did not
induce the crystallization of TiO2 nanocrystallites, we assume that other emission sites locatedat higher peak energies were generated during the heat treatment.
The emission decay curves of Bi-doped TZAB as-prepared and annealed glasses are shown
on a linear scale in Fig. 3(a) and on a logarithmic scale in Fig. 3(b). These decay curves takesimilar decay shapes independent of the Bi concentration and annealing process. As shownin Fig. 3(b), these decay curves are nonexponential, indicating that several activator sites arecorrelated with the emission, which is consistent with our prediction mentioned above. Table 1
shows the emission decay constant τ1/e of Bi-doped TZBA glasses. Considering the error barsof the measurements, we assume that these values are within the error bars, and no significant
decrease in τ1/e owing to concentration quenching is observed. It also indicates that theconcentration of Bi species, which can act as effective NIR emission centers, is independent of
the additional Bi2O3 fraction in this concentration range. The idea that some Bi acts as effectiveemission centers will be adaptable in the Bi4Ge3O12 scintillator, in which Bi3+ species act asemission centers.(17)
Next, we examined another relationship between the addition of AlN and the NIR
emission properties of the Bi-doped glasses. Figure 4(a) shows the NIR emissions of AlN,
Bi codoped TZAB as-prepared and annealed glasses excited with the wavelength of 800 nm.
Although the spectral shapes are similar to those of AlN nondoped samples, the blue shiftof the emission peak, which is also observed in nondoped samples, is observed. Similarlyto Fig. 2(b), the emission peak energy and bandwidth of the glasses are plotted in Fig. 4(b).
Although the relationships with Bi concentration and annealing are similar to those of AlN

Sensors and Materials, Vol. 30, No. 7 (2018)

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1Bi
2Bi
5Bi

As-prepared
Annealed

Ln(Intensity)

Normalized intensity

As-prepared
Annealed

1Bi
2Bi
5Bi

Time / μs

Time / μs(a)(b)

Fig. 3. (Color online) Emission decay curves of Bi-doped TZAB as-prepared and annealed glasses. The dashedand solid lines represent the as-prepared and annealed samples, respectively.

Photon energy / eV(a)Annealed

As-prepared

Bi concentration (mol%)(b)

Fig. 4. (Color online) NIR emissions of AlN, Bi codoped TZAB as-prepared and annealed glasses. (a) PL spectraof Bi-doped TZAB glasses excited with the wavelength of 800 nm. (b) Peak energy and width of emission in Bidoped TZAB glasses as functions of Bi concentration. The dashed and solid lines represent the as-prepared andannealed samples, respectively.

nondoped glasses, both the peak energy and the bandwidth are slightly larger than those ofthe nondoped glasses. Since the NIR emission intensities and decay constants are within theerror bars, the addition of AlN is not effective for the marked improvement of luminescentperformance, i.e., the local coordination state of the activators. Here, we discuss the cause of
the broadening of the emission band. During melting, an oxidation reaction from AlN to Al2O3
is expected to occur. The generated Al2O3 increases the viscosity of the melt, which prevents
the structural arrangement during the quenching process. The addition of AlN, therefore,

Bandwidth of emission / eV

As-prepared
Annealed

1Bi5BiPeak energy ofemission / eV

Normalized intensity(arb. unit)

Table 1Emission decay constant τ1/e of Bi-doped TZBA glasses. The error bars are ±20 µs.
1Bi-TZBA
2Bi-TZBA
5Bi-TZBA
As-prepared
180
150
160
Annealed
150
150
170

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induces the broadening of the bandwidth in contrast to increasing the Bi2O3 concentration.
From these results, we believe that Al2O3 or a metal oxide, which induces the high viscosityof the glass melt, is effective for broadening the emission. We have reported a relationshipbetween different quenching conditions and the luminescence properties of glasses(18–20) andemphasized that the cooling process from the supercooled liquid state should be affected by
the local coordination state of activators. Compared with the previous glass systems,(18–20) the
present glass is stronger (viscosity changes slowly above Tg). The present results suggest thatthe viscosity of the glass melt is important for understanding not only the NIR emission of Bicontaining materials but also other emitting solid-state matters.

4.

Conclusions

We examined the effect of the preparation conditions on the NIR emission of Bi-dopedoxide glasses. The Bi concentration, the annealing process, and the addition of AlN affect theNIR emission of the glasses. No concentration quenching of NIR emission was observed in
the 5Bi-doped TZBA glass, and the relationship between the NIR emission intensity and the Biconcentration is unclear. These results indicate that the concentration of Bi emission centers forNIR emission is much lower than 1 mol%, and their local coordination states are affected by thechemical composition.

Acknowledgments
This work was partially supported by a Grant-in-Aid for Young Scientists (A) 26709048, andthe Cooperative Research Project of the Research Institute of Electronics, Shizuoka University.

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