Research

Minoru FUJII （藤井　稔） : Research on Mesoscopic Materials

RESEARCH ACTIVITIES

Bismuth-doped Near Infrared Luminescent Materials

		170.　Kenji Imakita, Yuya Tsuchihashi, Ryo Naruiwa, Minoru Fujii, Hong-Tao Sun, Jianrong Qiu, and Shinji Hayashi, Ultrafast nonlinear optical responses of bismuth doped silicon-rich silica films Applied Physics Letters, Volume 101,Issue 19, 191106, pp. 1-4 (2012).
		Nonlinear optical responses of bismuth (Bi) doped silicon-rich silicon dioxide (Si-rich SiO2) films were studied by a z-scan and an optical Kerr gate method under femtosecond excitation around 800 nm. It was found that the Bi-doping enhances the nonlinear optical response of Si-rich SiO2 films by several orders of magnitudes. The nonlinear refractive index was of the order of 10−11 cm2/W and the response time was shorter than our time resolution of 100 fs. The nonlinear refractive index was independent of the wavelength in the range from 750 to 835 nm, suggesting that virtual transitions are involved in the nonlinear optical processes.

		169.　Hong-Tao Sun, Yoshio Sakka, Naoto Shirahata, Minoru Fujii, and Tetsu Yonezawa, Near-infrared photoluminescence from molecular crystals containing tellurium Journal of Materials Chemistry, Vol. 22, pp. 24792–24797 (2012).
		We report the observation of near-infrared photoluminescence from Te4(Ga2Cl7)2 and Te4(Al2Cl7)2 molecular crystals containing Te42+ polycations. The experimental and theoretical results clearly revealed that the Te42+ polycation is a smart near-infrared emitter with characteristic emission peaks at 1252 and 1258 nm for Te4(Ga2Cl7)2 and Te4(Al2Cl7)2 crystals, respectively, resulting from the intrinsic electronic transitions of Te42+. Furthermore, it was also found that the emissions strongly depend on the excitation wavelengths for both Te4(Ga2Cl7)2 and Te4(Al2Cl7)2 samples, most possibly due to the co-existence of other Te-related optically active centers. This research not only enriches the species of luminescent charged p-block element polyhedra and deepens the understanding of Te-related photophysical behaviors, but also may stimulate efforts for designing novel material systems using such active centers. It is also greatly expected that these sub-nanometer optically active species could exist in other systems such as glasses, polymers, and bulk optical crystals, and the stabilization of these centers in widely used hosts will pave the way for their practical applications.

		167.　Satoshi Morimoto, Minoru Fujii, Hong-Tao Sun, Yuji Miwa, Kenji Imakita, Jianrong Qiu, and Shinji Hayashi, Broadband near-infrared emission from bismuth-doped multilayer films Journal of Applied Physics, Vol. 112, 073511, pp. 1-4 (2012).
		Multilayer structures consisting of bismuth (Bi)-doped silica thin films and different kinds of spacer, i.e., Si, silica, Si-rich silica, layers are grown and the luminescence properties are studied. When samples were annealed at a low temperature, Bi-related near infrared active centers (BRACs) were formed at interfaces between Bi-doped silica and Si-rich silica (or silicon) due to the reduction of Bi3þ to BRACs by silicon. On the other hand, films annealed at a high temperature showed similar emission behaviors to bulk glasses. The results demonstrated here establish a new strategy for the control of BRACs and building peculiar Bi activated film structures.

		166.　Hong-Tao Sun, Tetsu Yonezawa, Miriam M. Gillett-Kunnath, Yoshio Sakka, Naoto Shirahata, Sa Chu Rong Gui, Minoru Fujii and Slavi C. Sevov, Ultra-broad near-infrared photoluminescence from crystalline (K-crypt)₂Bi₂ containing [Bi₂]^2- dimers Journal of Materials Chemistry, Vol.22, Issue 38, pp. 20175-20178 (2012).
		For the first time, we report that a single crystal of (K-crypt)2Bi2 containing [Bi2]2− displays ultra-broad near-infrared photoluminescence (PL) peaking at around 1190 nm and having a full width at the half maximum of 212 nm, stemming from the inherent electronic transitions of [Bi2]2−. The results not only add to the number of charged Bi species with luminescence, but also deepen the understanding of Bi-related near-infrared emission behavior and lead to the reconsideration of the fundamentally important issue of Bi-related PL mechanisms in some material systems such as bulk glasses, fibers, and conventional optical crystals.

		165.　Hong-Tao Sun, Beibei Xu, Tetsu Yonezawa, Yoshio Sakka, Naoto Shirahata, Minoru Fujii,Jianrong Qiu and Hong Gao, Photoluminescence from Bi₅(GaCl₄)₃ molecular crystal Dalton Transactions, Vol. 41, pp.11055–11061 (2012).
		Bi5(GaCl4)3 sample has been synthesized through the oxidation of Bi metal by gallium chloride (GaCl3) salt. Powder X-ray diffraction as well as micro-Raman scattering results revealed that, in addition to crystalline Bi5(GaCl4)3 in the product, an amorphous phase containing [GaCl4]− and [Ga2Cl7]− units also exists. The thorough comparison of steady-state and time-resolved photoluminescent behaviors between the Bi5(GaCl4)3 product and Bi5(AlCl4)3 crystal leads us to conclude that Bi53+ is the dominant emitter in the product, which gives rise to the ultrabroad emission ranging from 1 to 2.7 μm. Detailed quantum chemistry calculation helps us assign the observed excitations to some electronic transitions of the Bi53+ polycation, especially at shorter wavelengths. It is believed that our work shown here is not only helpful to solve the confusions on the luminescent origin of bismuth in other material systems, but also serves to develop novel broadband tunable laser materials.

		159.　Sa chu rong gui, Kenji Imakita, Minoru Fujii, Zhenhua Bai, Shinji Hayashi, Luminescence properties of Bi-doped oxidized porous silicon thin films　 Optical Materials, Vol. 34, pp. 1161-1164 (2012).
		Luminescence properties of Bi-doped oxidized porous silicon (OPS) thin films were studied. It was found that this material shows two broad luminescence bands centered at 845 nm with the FWHM of 120 nm and at 1410 nm with that of 220 nm under 488 nm excitation. A detailed analysis of the 3D plot of PL intensities versus excitation and emission wavelengths revealed that these luminescence bands arise from at least two different kinds of Bi luminescence centers. The broad luminescence covering the whole telecommunication window (1.2–1.6 μm) suggests that Bi-doped OPS thin films can be a candidate material for a broadband waveguide-type optical amplifier at optical telecommunication wavelengths.

		156.　Zhenhua Bai, Minoru Fujii, Takashi Hasegawa, Shohei Kitanoa, Kenji Imakita, Minoru Mizuhatab, and Shinji Hayashi, Co-existence of Bi with multiple valence states in zeolites-controlling the optical properties by annealing atmosphere　 Optical Materials, Vol. 34 , pp. 821-825 (2011).
		We investigate the effect of annealing atmosphere on the optical properties of Bi doped zeolites by diffuse reflectance, steady state and time-resolved photoluminescence (PL), and PL excitation measurements. The results reveal that zeolites can be used as an excellent host material to stabilize multiple Bi centers (Bi3+, Bi2+, and Bi-related near-infrared (NIR) active centers) in the framework, which shows ultra-broadband emission from visible to NIR range. Annealing in N2 leads to the partial conversion of Bi3+ ions into Bi2+ and Bi-related NIR active centers. Our results demonstrate that the broadband NIR emission may be attributed to the electronic transition of Bi low valence state, rather than a higher valence state.

		154.　Yuji Miwa, Hong-Tao Sun, Kenji Imakita, Minoru Fujii, Yu Teng, Jianrong Qiu, Yoshio Sakka, and Shinji Hayashi, Sensitized broadband near-infrared luminescence from bismuth-doped silicon-rich silica films　 Optics Letters, Vol. 36, No. 21, pp. 4221-4223, November 1 (2011).
		Developing Si compatible optical sources has attracted a great deal of attention owing to the potential for forming inexpensive, monolithic Si-based integrated devices. In this Letter, we show that ultra broadband near-IR (NIR) luminescence in the optical telecommunication window of silica optical fibers was obtained for Bi-doped silicon-rich silica films prepared by a co-sputtering method. Without excess Si, i.e., Bi-doped pure silica films, no luminescence was observed in the NIR range. A broad Bi-related NIR photoluminescence appears when excess Si was doped in the Bi-doped silica. The luminescence properties depended strongly on the amount of excess Si and the annealing temperature. Photoluminescence results suggest that excess Si acts as an agent to activate Bi NIR luminescence centers and also as an energy donor to transfer excitation energy to the centers. It is believed that this peculiar structure might find some important applications in Si photonics.

		153.　Zhenhua Bai, Minoru Fujii, Takashi Hasegawa, Kenji Imakita,Minoru Mizuhata and Shinji Hayashi, Efficient ultraviolet-blue to near-infrared downconversion in Bi-Dy-Yb doped zeolites　 Journal of Physics D: Applied Physics, Vol. 44, pp. 455301 1-5 (2011).
		Ultraviolet-blue to near-infrared (NIR) downconversion is investigated for the Dy3+–Yb3+ couple in zeolites by steady-state and time-resolved photoluminescence (PL) spectra, and PL excitation spectra. Upon excitation of the 4F9/2 level of Dy3+, NIR quantum cutting could occur through a two-step energy transfer from one Dy3+ ion to two neighbouring Yb3+ ions via an intermediate level. The energy transfer efficiency from the 4F9/2 level is estimated to be 42%, and the intrinsic PL quantum efficiency of Yb3+ emission reaches 54%. The findings may have potential application in enhancing the energy efficiency of silicon-based solar cells.

		147.　Zhenhua Bai, Minoru Fujii, Takashi Hasegawa, Kenji Imakita, Yuji Miwa, Minoru Mizuhata, and Shinji Hayashi Effect of doping concentration on broadband near-infrared emission of Bi doped zeolites　 Microporous and Mesoporous Materials, Vol. 145, pp. 21-25 (2011).
		A series of Bi doped zeolites are prepared by an ion-exchange process by changing doping concentration in a wide range, and the optical properties are investigated. The near-infrared photoluminescence (PL) intensity, full width at half maximum and decay time depend strongly on Bi doping concentration. The PL intensity increases 178 times when the concentration is changed from 0.3 to 1.5 at. %. At the same time, the lifetime increases from 83 to 527 μs. The results prove the model that doped Bi acts not only as optically active centers, but also as pore-sealing substances to isolate the centers. The comparison of PL and Raman data suggests that in addition to previously proposed Bi₂O₃, other Bi-related materials, probably Bi metal, play an important role to isolate the active centers.

		146.　Hong-Tao Sun, Yoshio Sakka, Hong Gao, Yuji Miwa, Minoru Fujii, Naoto Shirahata, Zhenhua Bai and Ji-Guang Li, Ultrabroad near-infrared photoluminescence from Bi5(AlCl4)3 crystal　 Journal of Materials Chemistry, Vol. 21, pp. 4060-4063 (2011).
		The Bi₅(AlCl₄)₃ crystal, synthesized by a environmentally friendly room-temperature method using ionic liquids as reaction solvents, exhibits extremely broad near-infrared photoluminescence (PL) with a full width at the half maximum (FWHM) of >510 nm and an effective PL lifetime of 4.1 µs at 1160 nm. We envision that this study not only extends the understanding of photophysical properties of materials containing subvalent bismuth, but also may have promise for the design of novel photonic materials containing a wide array of p-block elements.

		145.　Zhenhua Bai, Hong-Tao Sun, Minoru Fujii, Yuji Miwa,Takashi Hasegawa, Minoru Mizuhata and Shinji Hayashi, Bismuth-sensitized Efficient Near-infrared Luminescence from Ytterbium in Zeolites　 Journal of Physics D: Applied Physics, Vol. 44, 155101, pp. 1-5 (2011).
		Yb–Bi co-doped zeolites are prepared by a method consisting of a simple ion-exchange process and subsequent high-temperature annealing. We observe two strong near-infrared (NIR) emission bands overlapping in the range 930–1480 nm, corresponding to the electronic transitions of bismuth-related active centres (BiRAC) and Yb3+ ions, respectively. In the obtained products, the excitation wavelength of Yb3+ is extended to the range 420–850 nm, and the lifetime reaches 665 µs. In the zeolite matrix, Bi ions exist as BiRAC and Bi compound agglomerates. The former act as a sensitizer of Yb3+ ions, and the latter act as a blockage to seal the pores of zeolites, which enables Yb3+ ions to show efficient NIR emission even if the zeolites contain a large amount of coordinated water. The excellent optical and structural properties make these NIR-emitting nanoparticles promising in applications as biological probes.

		144.　Zhenhua Bai, Minoru Fujii, Yuki Mori, Yuji Miwa, Minoru Mizuhata, Hong-Tao Sun, and Shinji Hayashi, Efficient Near-infrared Emission from Neodymium by Broadband Sensitization of Bismuth in Zeolites　 Optics Letters, Vol. 36, No. 6, pp. 1017-1019 (2011) .
		Nd-Bi codoped zeolites were prepared by an ion-exchange process, and the optical properties were investigated by photoluminescence (PL) and PL excitation spectra, and decay time measurements. The results show that the NIR emission of Nd3+ ions is significantly enhanced by the introduction of bismuth in codoped samples, and the lifetime reaches 246 μs. It is also observed that NIR-active Bi acts as a sensitizer of Nd3+ ions. The energy transfer efficiency is also estimated. The peculiar optical properties make Nd-Bi codoped zeolites promising for potential application in biological probes.

		141.　Hong-Tao Sun, Junjie Yang, Minoru Fujii, Yoshio Sakka, Yufang Zhu, Takayuki Asahara, Naoto Shirahata, Masaaki Ii, Zhenhua Bai, Ji-Guang Li, and Hong Gao, Highly Fluorescent Silica-Coated Bismuth-Doped Aluminosilicate Nanoparticles for Near-Infrared Bioimaging　 Small, Vol. 7, No. 2, pp. 199-203 (2011) .
		The combined advantages of bismuth and aluminosilicates lead to a new type of nanosized biolabel. The near-infrared (NIR)-emitting silica-coated bismuth-doped aluminosilicate nanoparticles are simple to prepare and exhibit a broad excitation band in the first biological window, efficient long-lived photoluminescence (PL) covering the second window, high photostability, low cytotoxicity, and easy penetration into living tissues. In vivo PL bioimaging and decay are tested in living mice.

		140.　Hong-Tao Sun, Yoshio Sakka, Minoru Fujii, Naoto Shirahata, and Hong Gao, Ultrabroad Near-infrared Photoluminescence from Ionic Liquids Containing Subvalent Bismuth　 Optics Letters, Vol. 36, No. 2, pp. 100-102 (2011) .
		We have shown that Lewis-acidic halogenoaluminate ionic liquid (IL) containing subvalent bismuth can be used as a near-IR (NIR) luminescent material. Raman and absorption spectra evidence the coexistence of Bi3+5 and Bi+ in the liquid. The Bi3+5 and Bi+ emitters, stabilized by this Lewis-acidic liquid, demonstrate ultrabroad NIR photoluminescence with a lifetime of around 1μs. We envisage that the bismuth activated ILs would not only enrich the well-established spectrum of soft luminescent materials but also might promote the design of novel photonic materials activated by other p-block elements.

		137.　Hong-Tao Sun, Yoshio Sakka, Yuji Miwa, Naoto Shirahata, Minoru Fujii, and Hong Gao Spectroscopic Characterization of Bismuth Embedded Y Zeolites Applied Physics Letters, Vol. 97, 131908 pp. 1-3 (2010).
		Bismuth embedded Y zeolites were studied by using UV-vis- near infrared (NIR) diffuse reflectance, Raman, and steady-state NIR photoluminescence spectroscopy. The results suggest that Bi53+ and Bi+ active centers coexist in the dehydrated and hydrated zeolite framework, both of which contribute to NIR emission. Furthermore, it was revealed that the high-temperature annealing leads to the formation of Bi2O3 clusters, which act as blocks for selectively closing down the “in-out windows” of H2O and O2 molecules in the zeolites. It is believed that these materials can find a wide array of applications as active media of broadly tunable micro or nano-optical sources.

		132.　Hong-Tao Sun, Fumiaki Shimaoka, Yuji Miwa, Jian Ruan, Minoru Fujii, Jianrong Qiu, and Shinji Hayashi, Sensitized superbroadband near-IR emission in bismuth glass/Si nanocrystal superlattices　 Optics Letters, Vol. 35, Issue 13, pp. 2215-2217 (2010) .
		We show that sensitized superbroadband near-IR (NIR) emission in bismuth glass/Si nanocrystal superlattices can be realized. Photoluminescence is enhanced by 1 order of magnitude in this structure. We observed that the excitation wavelength dependence of the NIR emission does not show any distinct structure corresponding to the direct transition of bismuth IR-active centers. Our results suggest that the enhanced emission might result from the energy transfer from Si nanocrystals to IR-active bismuth. This structure may find broad applications for broadband amplifiers and broadly tunable laser sources.

		131.　Zhenhua Bai, Hong-Tao Sun, Takashi Hasegawa, Minoru Fujii, Fumiaki Shimaoka, Yuji Miwa, Minoru Mizuhata, and Shinji Hayashi, Efficient near infrared luminescence and energy transfer in erbium/bismuth co-doped zeolites　 Optics Letters, Vol. 35, Issue 11, pp. 1926 - 1928 (2010) .
		We have shown that tunable and highly efficient broadband near-IR (NIR) luminescence can be realized in erbium/bismuth codoped zeolites. The emission covers the ranges of 930–1450nm and 1450–1630nm. The intensity ratio of the two bands can be tuned by adjusting the concentration of erbium and the excitation wavelength. Steady-state and time-resolved photoluminescence (PL), and PL excitation measurements indicate that two kinds of emitters coexist in the pores of zeolites, and that NIR active bismuth simultaneously acts as a sensitizer of erbium. The present results demonstrate an important rational strategy for the design of a tunable NIR-emitting zeolite-based nanosystem.

		129.　Hong-Tao Sun, Minoru Fujii, Yoshio Sakka, Zhenhua Bai, Naoto Shirahata, Liyan Zhang, Yuji Miwa, and Hong Gao Near-infrared photoluminescence and Raman characterization of bismuth-embedded sodalite nanocrystals　 Optics Letters, Vol. 35, Issue 11, pp. 1743-1745 (2010) .
		Ultrabroadband near-IR (NIR) emission has been realized in bismuth-embedded sodalite nanocrystals. Steady-state and time-resolved photoluminescence and Raman results suggest that Bi+ active centers contribute to the NIR emission. This study demonstrates that sodalite nanocrystals can serve as excellent hosts for bismuth NIR active centers, thus paving the way for their wide applications in nanophotonics.

		125. Hong-Tao Sun, Akinobu Hosokawa, Yuji Miwa, Fumiaki Shimaoka,Minoru Fujii, Minoru Mizuhata, Shinji Hayashi, and Shigehito Deki “Strong Ultrabroadband Near-Infrared Photoluminescence from Bismuth Embedded Nanocrystalline Zeolites and Their Derivatives,” Advanced Materials, Vol. 21, pp. 3694–3698 (2009).
		Zeolites with embedded bismuth compounds display strong, air-stable, long-lived, ultrabroadband, and tunable near-infrared photoluminescence (see picture). Bismuth ions not only act as luminescence-active centers, but also as blocks for selectively closing the “in–out windows” of water molecules. Bismuth active centers can be sealed in a low-vibrational environment by bismuth agglomerates even when the sample still contains a large amount of water.

		117. Hong-Tao Sun, Takashi Hasegawa, Minoru Fujii, Fumiaki Shimaoka, Zhenhua Bai, Minoru Mizuhata, Shinji Hayashi, and Shigehiko Deki, "Highly-Efficient and Air-stable Near Infrared Emission in Erbium/Bismuth Codoped Zeorites,” Applied Physics Letters, Vol. 94, 141106, pp. 1-3 (2009).
		We have shown that highly efficient and air-stable 1531 nm emission can be realized in erbium/bismuth codoped zeolites. The luminescence lifetime of Er3+ at 1531 nm can reach up to 4.2 ms. Bismuth compounds formed by high temperature annealing can act as blockages of selectively closing down the “in-out windows” of water molecules, i.e., Er3+ ions are sealed in a low-vibrational environment by bismuth agglomerates even when the samples contain a large amount of water. This finding may pave the way for the applications of active ions doped porous materials in infrared photonics.

		116. Hong-Tao Sun, Y. Miwa, Fumiaki Shimaoka, Minoru Fujii, A. Hosokawa, Minoru Mizuhata, Shinji Hayashi, and Shigehito Deki, "Superbroadband Near Infrared Nano Optical Source Based on Bismuth Doped High-silica Nanocrystalline,” Optics Letters, Vol. 34, Issue 8, pp. 1219-1221 (2009).
		We have shown that efficient superbroadband near-IR luminescence can be realized in bismuth-doped high-silica nanocrystalline zeolites. The emission band covered the range of 930-1620 nm, with a maximum peak at 1146.3 nm, an FWHM of 152 nm, and a lifetime of over 300 μs under the excitation of a 488 nm laser line. The observed luminescence was attributed to subvalent Bi (Bi+) ions formed in the annealed zeolites. These Bi-doped nanozeolites may find applications as superbroadband near-IR nano-optical sources.

		115. Hong-Tao Sun, Takashi Hasegawa, Minoru Fujii, Fumiaki Shimaoka, Zhenhua Bai, Minoru Mizuhata, Shinji Hayashi, and Shigehito Deki, "Significantly Enhanced Superbroadband Near Infrared Emission in Bismuth/Aluminum Doped High-Silica Zeolite Derived Nanoparticles,” Optics Express, Vol. 17, No. 8, pp. 6239-6244 (2009).
		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 [AlOV4/2]- 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. These bismuth doped nanoparticles may find applications as superbroadband near infrared nano optical sources.