Significantly enhanced superbroadband nearinfrared emission in bismuth/aluminum doped
high-silica zeolite derived nanoparticles
Hong-Tao Sun,1,* Takashi Hasegawa,2 Minoru Fujii,1 Fumiaki Shimaoka,1 Zhenhua
Bai,1 Minoru Mizuhata, 2 Shinji Hayashi 1 and Shigehito Deki 2
1

Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai,
Nada, Kobe 657-8501, Japan2
Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokkodai,
Nada, Kobe 657-8501, Japan*
Corresponding author: timothyhsun@gmail.com

Abstract: Significantly enhanced superbroadband near infrared emission
has been realized in bismuth/aluminum doped high-silica zeolite derived
nanoparticles. The emission intensity can be easily tailored by the
introduction of aluminum. The luminescence lifetime can reach up to 695 µs.
The results reveal that the existence of charge imbalance environment
caused by [AlO4/2]- units in host materials is requisite to the formation of
infrared-active Bi+. The finding presents a feasible route to design highefficient bismuth activated infrared luminescent nanoparticles. Thesebismuth doped nanoparticles may find applications as superbroadband nearinfrared nano optical sources.
2009 Optical Society of America
OCIS codes: (160.0160) Materials; (300.2140) Emission; (300.0300) Spectroscopy.

References and links
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3325-3326 (2003).
2. X. Meng, J. Qiu, M. Peng, D. Chen, Q. Zhao, X. Jiang, and C. Zhu, “Infrared broadband emission of
bismuth-doped barium-aluminum-borate glasses,” Opt. Express 13, 1635-1642 (2005),
http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-5-1635.
3. V. V. Dvoyrin, V. M. Mashinsky, L. I. Bulatov, I. A. Bufetov, A. V. Shubin, M. A. Melkumov, E. F.
Kustov, E. M. Dianov, A. A. Umnikov, V. F. Khopin, M. V. Yashkov, and A. N. Guryanov, “Bismuthdoped-glass optical fibers-a new active medium for lasers and amplifiers,” Opt. Lett. 31, 2966-2968(2006).
4. Y. Arai, T. Suzuki, Y. Ohishi, S. Morimoto, and S. Khonthon, “Ultrabroadband near-infrared emission
from a colorless bismuth-doped glass,” Appl. Phys. Lett. 90, 261110-1–3 (2007).
5. V. O. Sokolov, V. G. Plotnichenko, and E. M. Dianov, “Origin of broadband near-infrared luminescencein bismuth-doped glasses,” Opt. Lett. 33, 1488-1490 (2008).
6. Q. Qian, Q. Y. Zhang, G. F. Yang, Z. M. Yang, and Z. H. Jiang, “Enhanced broadband near-infraredemission from Bi-doped glasses by codoping with metal oxides,” J. Appl. Phys. 104, 043518-1-3 (2008).
7. Okhrimchuk, L. Butvina, E. Dianov, N. Lichkova, V. Zagorodnev, and K. Boldyrev, “Near-infrared
luminescence of RbPb2Cl5 : Bi crystals,” Opt. Lett. 33, 2182-2184 (2008).
8. P. Li, X. H. Sun, N. B. Wong, C. S. Lee, S. T. Lee, and B. Teo, “Ultrafine and uniform silicon nanowires
grown with zeolites,” Chem. Phys. Lett. 365, 22-26 (2002).
9. M. Ryo, Y. Wada, T. Okubo, T. Nakazawa, Y. Hasegawa, and S. Yanagida, “Spectroscopic study onstrongly luminescent Nd(III) exchanged zeolite: TMA+-containing FAU type zeolite as a suitable host for
ship-in-bottle synthesis,” J. Mater. Chem. 12, 1748-1753 (2002).
10. H. Sun, Y. Miwa, F. Shimaoka, M. Fujii, A. Hosokawa, M. Mizuhata, S. Hayashi, and S. Deki,“Superbroadband near infrared nano optical source based on bismuth doped high-silica nanocrystalline
zeolites,” Opt. Lett. in press.
1.

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11. S. Sen and R. E. Youngman, “High-Resolution Multinuclear NMR Structural Study of Binary
Aluminosilicate and Other Related Glasses,” J. Phys. Chem. B 108, 7557-7564 (2004).

1. Introduction
In recent years, extensive studies on bismuth related superbroadband near-infrared (NIR)
photoluminescence (PL) have been carried out in traditional glass materials [1-6].
Concurrently, different authors tentatively assigned the NIR emission to the electronictransition of Bi5+, Bi+, Bi2+, or to cluster of Bi atoms dispersed in glass host, and the final
conclusion has not been made yet [1-6]. In comparison with the studies of Bi doped glasses,
little attention has been paid on the investigation of Bi doped crystals [7]. It is well known thatspectroscopy of active ions is much more understandable in crystals than in glasses because
there is a definite majority of sites for doping ions in crystal. Therefore, finding a suitable
crystal as the host of bismuth infrared-active (BiIRA) centres may pave the way forunderstanding the PL origin.
Zeolites, as smart crystalline materials mainly consisting of [SiO4] and [AlO4] structure
units, possess pore structures and these enable them to act as hosts for molecules and ions oras templates for nanostructures synthesis [8]. Recently, their potential as host materials forrare-earth ions has been evaluated [9]. However, the efficiency of the emitters is very small in
the NIR region due to the fast relaxation of the excitation energy through nonradiative
vibrational deactivation. Moreover, the obtained samples were not air-stable, which wereusually kept in vacuum to avoid the adsorption of coordinated water [9]. Thus, it is aninteresting topic to find a strategy to increase the NIR PL efficiency in active ions doped
zeolites.
Very recently, we realized above two purposes by using bismuth doped crystalline
nanozeolites [10]. The emission band covered the range of 930~1620 nm, with a maximum
peak at 1146.3 nm, a full width at the half maximum (FWHM) of 152 nm and a lifetime ofover 300 µs under the excitation of a 488 nm laser line. In this paper, we studied the effect ofaluminum doping on the NIR emission of bismuth doped zeolite derived nanoparticles. NIR
emission has been enhanced significantly by the introduction of aluminum in thesenanoparticles. The samples were characterized by X-ray diffraction (XRD), field emission
scanning electron microscopy (FE-SEM), steady-state and time-resolved PL measurements.
Based on these results, the origin and enhancement mechanism of PL were discussed in detail.
2. Experimental
The H form of FAU-type zeolites were purchased from Tosoh Co. Japan (Zeolite Y, grain
size: 200 ~ 400 nm). Zeolites was stirred in a 0.01 M aqueous solution of Bi3+ prepared from
Bi(NO3)3·5H2O at 80 oC for 72 h to exchange H ions with Bi3+ ions. The products were
removed by centrifugation, and dried in air. The Bi3+ embedded zeolites were further stirred in
0.01M Bi3+ and xM Al3+ (x=0.015, 0.025, 0.035) mixed solution at 80 oC for 24 h to dope Al3+
ions. The products were collected by centrifugation, then washed with deionized water, anddried in air at 120 oC. The Bi3+ and Bi3+/Al3+ doped zeolites were calcined at 1150 oC for 20
min in N2 atmospheric condition. All samples were exposed to the laboratory atmosphereprior to measurements. The prepared products were first characterized by X-ray
diffractometer (Rigaku-TTR/S2, λ=0.154056 nm). The morphologies of the prepared products
were characterized using a FE-SEM (JEOL, JSM-6335F) operating at an accelerating voltageof 15 kV. Bismuth and aluminum concentrations were measured by X-ray fluorescence (XRF)
and energy-dispersive X-ray spectroscopy (EDS). The atomic ratios of Bi and Al to(Si+Al+Bi) were summarized in Table 1. Note that the Al content in sample 1 is from zeolites.
Luminescence measurements were carried out at room temperature with the excitation of a
488 nm line of an Ar+ laser. The signal was analyzed by a single grating monochromator anddetected by a liquid-nitrogen-cooled InGaAs detector. For all the spectra, the spectral#108147 - $15.00 USD(C) 2009 OSA

Received 27 Feb 2009; revised 25 Mar 2009; accepted 26 Mar 2009; published 1 Apr 200913 April 2009 / Vol. 17, No. 8 / OPTICS EXPRESS 6240

response of the detection system was corrected by the reference spectrum of a standard
tungsten lamp. Time-resolved luminescence measurements were performed by detecting themodulated luminescence signal with a photomultiplier tube (Hamamatsu, R5509-72), and thenanalyzing the signal with a photon-counting multichannel scaler. The excitation source for the
lifetime measurements was the 488 nm light from an optical parametric oscillator pumped by
the third harmonic of a Nd:YAG laser.
Table 1. Determined Bi and Al Ratios of the Samples

Sample no.

Bi ratio (%)

Al ratio (%)

Doped ions

1

0.70

0.27

Bi3+

2

0.47

0.53

Bi3+/Al3+

3

0.29

0.68

Bi3+/Al3+

4

0.24

0.73

Bi3+/Al3+

3. Results and discussion
Figure 1 shows X-ray diffraction (XRD) patterns of Bi3+ and Bi3+/Al3+ doped zeolites annealed
at 1150 oC. The crystalline structure of Bi3+ singly doped sample keeps intact after thermal
treatment. However, the amorphization takes place for Bi3+/Al3+ doped samples, althoughsome weak diffraction peaks of zeolite phase can be observed. This indicates that the
introduced bismuth and aluminum content existing in zeolite pores strongly affect the eutectic
temperature of the samples.

Fig. 1. XRD spectra of Bi3+ and Bi3+/Al3+ doped samples. The peaks denoted by the asterisks
are ascribed to the reflections of zeolite phase.

Figure 2(a) displays a typical FE-SEM image of sample 3. The morphology andmonodispersity of the annealed zeolites remain almost unchanged. The chemical compositionof the above sample was analyzed by EDS as shown in Fig. 2(b), indicating that the particle iscomposed of Si, Al, O, and Bi. In combination with the corresponding XRD result, it is clearthat the product is silica-alumina amorphous nanoparticles.

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Fig. 2. (a) FE-SEM image of sample 3. (b) EDS of the above sample.

Figure 3 shows the NIR PL spectra of the samples. All samples show strong NIR
emission from 930 to 1620 nm. It is noteworthy that the Bi3+/Al3+ doped samples show muchstronger emission than Bi3+ singly doped one; sample 3 displays the strongest emission, which
is about eight times stronger than that of sample 1. In Ref. 10, we proposed that subvalent Bi(Bi+) ions is the NIR PL origin in doped zeolites: Bi+ ions act as charge compensators of[AlO4/2]- units. Thus, the existence of charge imbalance environment in host materials is
requisite to the formation of infrared-active Bi+. It was revealed that the homogeneous binary
SiO2-Al2O3 glasses with 0.4 to up to 12.0 wt % Al2O3 contain a mixture of 4-, 5-, and 6-foldcoordinated Al sites (AlIV, AlV, and AlVI) [11]. The relative proportions of these sites arestrongly dependent on composition with AlIV being most dominant in glasses with <1wt %Al2O3. It was hypothesized that the tetrahedral [AlO4/2]- units in glasses with <1 wt % Al2O3
are predominantly charge balanced by the formation of oxygen triclusters, and addition ofeither low field strength alkali ions such as K+ or high field strength rare earth ions such as
La3+ to these glasses results in charge balance and stabilization of AlIV sites [11]. Therefore, it
is reasonable to assume that more [AlO4/2]- units will be formed after annealing when Al
content was introduced into the matrix of zeolites, and more Bi+ ions act as charge
compensators of [AlO4/2]- units. Owing to the extended nature of 6p orbitals of Bi+, the crystal
field plays an important role in the luminescent properties. Thus, the host composition canaffect the NIR emission as revealed before [2, 6]. For example, Meng et al. reported the peak
position of the broadband infrared emission shifts from 1252 nm to 1300 nm with increasing
BaO concentration from 20 mol% to 40 mol% in barium-aluminum-borate glasses.
Interestingly, the shape of all spectra shown in Fig. 3 is nearly identical, indicating that Bi+
ions has similar local environments in zeolites and their derived amorphous nanoparticles.

Fig. 3. PL spectra of the samples under the excitation of a 488 nm laser line. All samples were
measured under the same condition.

To further know the PL enhancement mechanism, we measured the fluorescence decay
curves of the samples (Fig. 4). The lifetimes of Bi3+/Al3+ doped samples are much longer than#108147 - $15.00 USD(C) 2009 OSA

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that of bismuth doped one: with increasing aluminum and decreasing bismuth contents, thelifetime monotonously increases; sample 4 shows the longest 1/e lifetime of 695 µs, which iscomparable to those reported in bulk glasses [1-5]. It appears that the changes of PL lifetimes
are closely related to the structural evolution of zeolites. As revealed by the XRD result (Fig.
1), the microporous structure of the samples 2, 3 and 4 were destroyed, resulting in no room
for the admission of water molecules. However, the crystalline structure of the sample 1 keeps
well, thus water molecules have a chance to interact with active Bi+ ions, leading to a much
shorter lifetime and a fast part of the decay curve.
As is known, in the low-excitation regime the luminescence intensity I is proportional toσΦNτ/τrad, where σ is the excitation cross section, Φ the photon flux, N the content of optically
active centres, τ the lifetime, and τrad the radiative lifetime. Assuming that τrad and σ of Bi+
ions are same in these samples, the ratios of active Bi+ ions in these samples can be estimatedbased on the following equation:

Ni Ii × τ1=
N1 I1 × τ i(1)where i is the sample no., Ni the number of Bi+ ions in sample i, I the integrated PL intensity,and τi the measured 1/e lifetime. The calculated result is shown in Fig. 5. The number of Bi+
ions in sample 3 is over twenty times more than that in sample 1. As shown in Table 1, the Alconcentration monotonously increases from sample 1 to 4, while Bi concentration decreases.
These results reveal that the increase of aluminum content is favorable to the formation of Bi
active centres owing to the existence of more [AlO4/2]- units. The number decrease of Bi+ ionsin sample 4 arises from the decreased bismuth concentration. These results clearly indicatethat some bismuth in the samples is not infrared active, and the creation of a charge imbalanceenvironment in the matrix is requisite to activate bismuth. The above result presents a feasible
route to design high-efficient bismuth activated infrared luminescent nanoparticles.

Fig. 4. Fluorescence decay curves of the samples. The detected wavelength is 1146 nm.

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Received 27 Feb 2009; revised 25 Mar 2009; accepted 26 Mar 2009; published 1 Apr 200913 April 2009 / Vol. 17, No. 8 / OPTICS EXPRESS 6243

Fig. 5. The calculated ratios of active Bi+ ions in these samples.

4. Conclusion
In summary, enhanced superbroadband near infrared emission has been realized inbismuth/aluminum doped high-silica zeolite derived nanoparticles. The emission covers therange of 930 ~ 1620 nm, with a lifetime of as long as 695 µs under the excitation of a 488 nm
laser line. The results further reveal that the NIR emission should be from subvalent bismuth
infrared active centre, i.e., Bi+, instead of others. The existence of a charge imbalanceenvironment caused by [AlO4/2]- units in host materials is requisite to the formation of
infrared-active Bi+, which is the main reason for the enhanced emission in Bi3+/Al3+ doped
high-silica zeolite derived nanoparticles. The finding may pave the way for the applications ofthese NIR luminescent nanoparticles as superbroadband near infrared nano optical sources.
Acknowledgments
This work is supported by the Japan Society of Promotion of Science (JSPS), and the Grantin-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science andTechnology (Grant No. 19007322), Japan. H.S thanks the support from JSPS in the form of afellowship.

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Received 27 Feb 2009; revised 25 Mar 2009; accepted 26 Mar 2009; published 1 Apr 200913 April 2009 / Vol. 17, No. 8 / OPTICS EXPRESS 6244