Infrared luminescent materials are widely used as laser media, for example, Nd:YAG and Er-doped silica fibers. In the infrared region, luminescence offers many advantages as laser media due to the variety of optics and excitation sources, such as semiconductor lasers and flash lamps. A frequency-doubled laser-beam with a nonlinear crystal can also obtained in the visible region. Therefore, infrared light source materials are very useful for scientific research and industrial uses.
Fujimoto et al. discovered a new infrared luminescent from bismuth-doped silica glass (BiSG)(Fujimoto & Nakatsuka 2001). Wide luminescent exists between 1000 and 1600 nm, and absorption is in the visible region. The lifetime is 630 µs at room temperature. Since this silica glass-based material has luminescence around 1.3 µm of zero-dispersion wavelength, it can be used as a core material of an optical fiber amplifier for telecommunication in the 1.3-µm range. Fujimoto et al. also demonstrated an optical amplification at 1.3 µm with 0.8-µm excitation(Fujimoto & Nakatsuka 2003). Therefore, BiSG is expected to be a promising substitution for Pr3+:ZBLAN fiber or Raman amplifiers in the 1.3-µm region.
The spectroscopic properties of BiSG are different from previously reported Bi luminescent material(Fujimoto & Nakatsuka 2001), because the previous Bi-doped glasses or crystals with low Bi2O3 concentration showed that luminescences exist in the visible region (400 ~ 600 nm) and that absorptions are in the ultraviolet region. The lifetimes (~4 µs) are much shorter than those of BiSG at room temperature. Such luminescences have been reported to originate from the Bi3+ ion in the media(M. J. Weber & Monchamp 1973; Parke & Webb 1973; van der Steen, van Hesteren et al. 1981). BiSG is also different from the Bi2O3-rich glass whose Bi2O3 concentration ranges from 18.7 to 42.5 mol%(Sugimoto, Kanbara et al. 1996) because the glass has no absorption in the visible region. Then the investigation of new glass compositions with Bi luminescence is also progressing vigorously in a past decade. Although the first discovered glass was a silica-based material with a SiO2 of 97.5 mol%(Fujimoto & Nakatsuka 2001), many glass compositions were tested and shown to effectively generate Bi luminescence, such as silicate(Peng, Qiu et al. 2005; Ren, Yang et al. 2006; Suzuki & Ohishi 2006; Arai, Suzuki et al. 2007; Murata & Mouri 2007; Peng, Chen et al. 2007; Ren, Qiu et al. 2007; Zhou, Feng et al. 2007; Peng, Wu et al. 2008), germanate(Peng, Qiu et al. 2004; Meng, Qiu et al. 2005; Peng, Meng et al. 2005; Peng, Qiu et al. 2005; Peng, Wang et al. 2005; Wang & Xia 2006; Xia & Wang 2006; Murata & Mouri 2007; Ren, Wu et al. 2007; Ren, Dong et al. 2007; Ren, Qiao et al. 2007; Ren, Qiu et al. 2007; Ren, Qiu et al. 2007; Peng, Wu et al. 2008; Qiu, Peng et al. 2008 ), borate(Meng, Qiu et al. 2005; Murata & Mouri 2007; Qiu, Peng et al. 2008), germanosilicate(Ren, Dong et al. 2007; Ren, Dong et al. 2008), and phosphate(Meng, Qiu et al. 2005; Qiu, Peng et al. 2008) glasses.
In this chapter, the author will introduce the basic properties of Bi doped silica glass (BiSG), such as a phase diagram and spectroscopic properties, and then mainly talk about the origine of luminescent center.
2. Basic properties of BiSG
2.1. Phase diagram
A phase diagram of BiSG was obtained for the Bi2O3-Al2O3-SiO2 glass system. Silica powder (Aerosil 50; SiO2, 99.8%), bismuth-oxide (Kojundo Chemical Lab. Co., Ltd.; !α-Bi2O3, 99.99%), and aluminum oxide (Kojundo Chemical Lab. Co., Ltd.; !α-Al2O3, 99.99%) were used as reagents. A properly mixed powder was inserted into a silica tube with an inner/outer diameter of 1/3 mmϕ, and then the tube was heated by a mixed gas burner of natural gas and oxygen. After heating, the powder was checked for a distinctive glassy wetting. If it had such wetting, the composition was determined to be the glassy phase in the Bi2O3-Al2O3-SiO2 glass system.
The measured phase diagram of the Bi2O3-Al2O3-SiO2 glass system is shown in Figure 1. The circle, triangle, and cross points show the glassy wetting states (pink to reddish-brown), unclear glassy states (pink to reddish-brown), and no glassy state (reddish-brown), respectively. This phase diagram shows a tendency where the glass whose Al2O3 content is larger than Bi2O3 is well glassified above a 90% SiO2 concentration. On the other hand, all the samples are glassified at a 80% SiO2 concentration without dependence on the Bi2O3 or Al2O3 content. The infrared luminescent spectrum is observed until 10 mol% of Bi2O3 concentration.
2.2. Influence of additives on luminescent intensitys
The Bi2O3-Al2O3-SiO2 glass system is affected by alkaline metal additive. Figure 2 shows an effect of Li2O additive in BiSG. The glass composition is Bi2O3(1.0 mol%)-Al2O3(7.0 mol%)-SiO2(92-x mol%)-Li2O(x mol%) glass system. The luminescent intensity decreases with increasing Li2O additive drastically (Seo, Fujimoto et al. 2006). Therefore, in order to get luminescent center, we should carefully choose the host composition.
On the other hand, GeO2 is the one of attractive additive to increase luminescent intensity of BiSG (Fujimoto, Hirata et al. 2007). Figure 3 shows the dependence of luminescent intensity on GeO2 concentration. All the samples contain 7.0 mol% of Al2O3. The nomenclature of the samples is as follows: for example, the sample name “BiSG_1.0_5.0” means that the composition is Bi2O3 = 1.0 mol%, GeO2 = 5.0 mol%. The luminescent intensity of BiSG grows with increasing GeO2 concentration almost linearly for BiSG_0.1_xx samples, and the luminescent intensity of BiSG_0.1_5.0 is 26.3 times larger than that of BiSG_0.1_0.0 only because of the GeO2 additive. The increment effect on BiSG with GeO2 additive is also observed for a Bi2O3 concentration of 1.0 mol% (BiSG_1.0_5.0), but it is less remarkable than for the lower Bi2O3 concentration of 0.1 mol%. Furthermore, the increment is only about 1.5 times larger than for BiSG without GeO2 (BiSG_1.0_0.0). On the other hand, the luminescent intensity of BiSG_0.1_5.0 is almost same as that of BiSG_1.0_0.0. According to these data, the GeO2 additive effectively increases the luminescent intensity of BiSG, and this effect is especially remarkable at low Bi2O3 concentrations.
2.3. Spectroscopic properties
The typical spectroscopic properties of BiSG are shown in Figure 4. Five main absorption bands exist between 250 and 2000 nm: 300 (A), 500 (B), 700 (C), 800 (D), and 1000 nm (E) (Figure 5(a)). It is considered that absorption band A is mainly derived from O2- in the base glass(Scholze 1991). The other four absorptions (B~E) are connected to the infrared emissions shown in Figure 5(b). Each luminescent peak wavelength and full width at half maximum (FWHM) were recognized as 1182 and 300 nm at 500-nm excitation, 1144 and 192 nm at 700 nm, 1265 and 336 nm at 800 nm, and 1130 and 188 nm at 1000 nm. A similarity of the luminescent spectra can be seen between 700- and 1000-nm excitations. These spectroscopic properties are different from the Bi3+ ones(M. J. Weber & Monchamp 1973; Parke & Webb 1973; van der Steen, van Hesteren et al. 1981)(Table 1).
3. The origin of luminescent center
After the discovery of a new infrared luminescent bismuth center, several research groups try to understand what is the origine of the luminescence. Although there have been many previous discussions on this distinctive luminescent center, it has remained unclear. Different authors tentatively assigned the different origins of this near infrared luminescence to the electronic transition derived from Bi5+(Fujimoto & Nakatsuka 2001; Dvoyrin, Mashinsky et al. 2006; Fujimoto & Nakatsuka 2006; Wang & Xia 2006 ; Xia & Wang 2006; Ohkura, Fujimoto et al. 2007), Bi2+(Ren, Qiu et al. 2007; Ren, Qiu et al. 2007; Ren, Qiu et al. 2007; Ren, Dong et al. 2008), and Bi+(Meng, Qiu et al. 2005; Meng, Qiu et al. 2005;
Dvoyrin, Mashinsky et al. 2006; Arai, Suzuki et al. 2007; Ren, Qiu et al. 2007; Ren, Qiu et al. 2007; Ren, Qiu et al. 2007; Yang, Chen et al. 2007; Zhou, Feng et al. 2007; Qiu, Peng et al. 2008; Ren, Dong et al. 2008; Truong, Bigot et al. 2008) or to that of BiO molecules(Ren, Yang et al. 2006; Murata & Mouri 2007; Peng, Chen et al. 2007; Peng, Wu et al. 2008) dispersed in the glass host. Since most of these proposals were however analogized from only spectroscopic mesurement analysis, uncertainty remains in the present discussions. In addition, even the same researchers offer several explanations, suggesting that spectroscpic analogy is insufficient to correctly understand the luminescent center or the luminescent mechanism.
Based on this viewpoint, our group discarded the spectroscopic analogical approach to understand the structure of the luminescent center or its mechanism and instead chose such instrumental analytical approaches as nuclear magnetic resonance (NMR), x-ray diffraction (XRD), electron spin resonance (ESR), x-ray photoelectron spectroscopy (XPS), and extended x-ray absorption fine structure (EXAFS). NMR data revealed the coordination structure of the specific ions in a material, ESR and XPS data provided the valence state of ions, and EXAFS offered the bonding distance between the specific and neighboring ions. Once these analytic instrumental data are integrated to build one possible figure of a luminescent center, then the luminescent mechanism will be discussed, because knowledge of the luminescent center is very useful for understanding the luminescent mechanism, which is obviously important for such optical applications as lasers or amplifiers.
In the following paragraphs, let’s try to build up an image of the local structure of the Bi luminescent center based on the analytical data, such as, spectroscopic properties (SPCT), luminesent intensity (LMI), NMR, XRD, ESR, XPS, and the EXAFS data of BiSG in a simple Bi2O3-Al2O3-SiO2 glass system without discrepancies. From this approach, we will reach one strongly possible local structure of a distinctive Bi luminescent center that consists of Bi and Al that exist closely to each other.
3.1. Sample preparation
In these analyses, composition of BiSG is very important, because Bi infrared luminescent center, as shown in section 2.2, is disappeared with increasing some additives in the host. This means that majority part of Bi ions in the host shifts to non-emissive site, thus even if we choose the multi-component glass system and analyze the Bi centers, we could not judge the results form an inherent luminescent center or the other non-emissive site, then we would go into the complicated maze. Therefore, the analyses should be carried out to concentrate on the simple three components system, Bi2O3-Al2O3-SiO2 glass system.
The samples for the instrumental analyses were prepared as follows.
A-series: The ratio of Bi2O3 to Al2O3 was kept at 3/7, and the Bi2O3 concentration varied from 0.1, 0.3, 0.5, 1.0, and 3.0 mol%. These samples were called A1-A5, respectively. Their compositions are inside the glassy phase area in Figure 1.
B-series: B1 was composed of Bi2O3 (1.0 mol%), Al2O3 (7.0 mol%), and SiO2 (92.0 mol%). This was used for the measurement of the spectroscopic properties because Al2O3-rich BiSG includes few bubbles. B2 was composed of Bi2O3 (0.3 mol%), Al2O3 (2.3 mol%), and SiO2 (97.4 mol%) and B3 of Bi2O3 (3.0 mol%), Al2O3 (7.0 mol%), and SiO2 (90.0 mol%). They were used for the ESR measurements.
C-series: C1 was composed of Bi2O3 (1.0 mol%) and SiO2 (99.0 mol%) without Al2O3, and C2 was composed of Al2O3 (2.3 mol%) and SiO2 (97.7 mol%) without Bi2O3. C1 and C2 were dark brown and colorless, respectively. C1 had no glassy wetting, but C2 did.
The chemical composition of the investigated BiSG samples and the several standards are listed in Table 2. Silica powder (Aerosil 50; SiO2, 99.8%), bismuth-oxide (Kojundo Chemical Lab. Co., Ltd.; α-Bi2O3, 99.99%), and aluminum oxide (Kojundo Chemical Lab. Co., Ltd.; α-Al2O3, 99.99%) were used as reagents to make the glass samples of the Bi2O3-Al2O3-SiO2 glass system. The mixtures of these powders were melted in an aluminum crucible in an electric furnace at 1750 C in air, and the melts were annealed by natural cooling. All of these glasses were reddish-brown. NaBiO3 (Kanto Chemical Co., Inc.; >80%), which was identified as NaBiO32H2O by XRD(PDF#30-1161 2000), α-Bi2O3, and Bi-metal (Kojundo Chemical Lab. Co., Ltd.; 99.9%) were used as standard materials for the analyses.
LMI: Luminescent Intensity, NMR: 27Al-NMR(Nuclear Magnetic Resonance), XRD: X-ray Diffraction, ESR: Electron Spin Resonance, XPS: X-ray Photoelectron Spectroscopy, EXAFS: Extended X-ray Absorption Fine Structure, SPCT: Spectroscopic Properties
The prepared samples were evaluated by instrumental analyses, including spectroscopic measurements, NMR, XRD, ESR, XPS, and EXAFS. Luminescent intensity and spectra were measured by a spectrophotometer (JASCO Corporation; SS-25). Transmission spectra were obtained by a spectrophotometer (HITACHI; U-4100). The aluminum coordination state (ACS) in BiSG was determined by the 27Al-NMR spectrum, and the 27Al-NMR spectra were obtained on a nuclear magnetic resonance spectrometer (JEOL Ltd.; JNM-GSX-400). The XRD patterns were measured by an X-ray diffractometer (RIGAKU; RINT2500). The ESR signals were measured by an electron spin resonance spectrometer (JEOL Ltd.; JES-ME-2X) at both room and liquid nitrogen temperatures.
The XPS spectra were recorded on a SHIMADZU (KRATOS) AXIS 165 using MgK radiation (1253.6 eV) with a beam size of 0.7 x 0.3 mm2 and 50 eV analyzer pass energy. The operating vacuum pressure was maintained from 10-8 to 10-9 Torr. An electron beam was used to reduce the electrical charging during the measurements. Any binding energy shifts due to electrical charging were corrected by the C1s (284.6 eV) peak as an internal reference. 3-minute Ar-sputtering was used to remove the oxidized surface on the Bi-metal powder.
The Bismuth LIII XAFS spectra were measured in the transmission mode using a BL-7C beam line at the Photon Factory (PF). The ring energy was 2.5 GeV. The x-ray was monochromized by a Si(111) double crystal monochrometer. Two ionization chambers were used as detectors. A 170-mm length chamber was filled with Ar/N2 (15%:85%) gas for incident x-ray energy (I0 ), and the other was 310-mm long with Ar-100% gas for transmitted x-ray energy (I). The EXAFS data of the Bi LIII edge (13426.5 eV) were collected between 12926 and 14526 eV with 481 energy points. Data analysis was carried out on UWXAFS. The back-scattering amplitude and the phase shift were theoretically calculated using FEFF 8.2 code. The Debye-Waller factor was estimated by the Debye code implemented in FEFF 8.2 based on Raman spectroscopy results(Narang, Patel et al. 1994).
3.3. Luminescent intensity
The dependence of luminescent intensity (LMI) on Bi2O3 concentration is illustrated in Figure 5. The measured samples were A-series (A1~A5) and C1. The excitation and detection wavelengths of the luminescence were at 500 and 1120 nm, respectively. The luminescent intensity nonlinearly increased with increased Bi2O3 concentration. At a 1.0 mol% of Bi2O3 concentration, the luminescent intensity from A4, which includes 2.3 mol% of Al2O3, is three orders of magnitude larger than that of C1 without Al2O3. Based on these results, we conclude the following:
3.4. 27Al-NMR spectra
27The Al-NMR spectra in BiSG are shown in Figure 6(Fujimoto & Nakatsuka 2006). 27Al chemical shifts were measured relative to Al(H2O)6 3+. The measured samples were A-series, C2, and α-Al2O3. α-Al2O3 with a 6-fold coordinated state of corundum structure was used as a standard sample, and a peak exists at 15 ppm. The peaks at 70 and –40 ppm (marked by asterisks) were derived from spinning sidebands. The peaks of 27Al-NMR from A1 to A3 only exist at 15 ppm and are the same as α-Al2O3. The A4 peak is still dominated by the 15 ppm peak, but a peak around 50 ppm begins to emerge, and then a peak of 56.4 ppm becomes dominant in A5 (Figure 6(b)).
Sample A4, which includes Al2O3 of 2.3 mol%, has a weak 50 ppm peak in the 27Al-NMR spectrum, while sample C2, which has the same amount of Al2O3 without Bi2O3, shows no signal around 50 ppm (Figure 6(b)), suggesting that the Bi ion affects ACS over a 1.0 mol% of Bi2O3 concentration. On the other hand, since the C2 spectrum is dominated by a peak at 15 ppm, the aluminum ions in the silica glass naturally configure the 6-fold coordinated state of the corundum structure up to 2.3 mol% of Al2O3 without Bi2O3. This is also supported by the work of Mysen et al., who concluded the aluminum ions in silica glass work as a network modifier rather than a network former up to a 6.1 mol% Al2O3 concentration in the measurement of Raman spectra(Mysen, Virgo et al. 1980). The Al corundum structure dominates ACS at lower Bi concentration up to 0.5 mol%, so the Al corundum structure clearly has certain important roles for the generation of the Bi luminescent center in BiSG.
3.5. XRD measurements
The XRD data of the BiSG samples were measured to check for existing crystallizations, including undissolved alumina, mullite, or crystbalite because crystallization influences the 27Al-NMR spectra. The measured samples are A-series. Figure 7 shows the XRD data on samples A4, α-Al2O3 (alumina), and pure silica with a range between 10 and 80 in 2. Sample A4 is substituted for the other BiSG ones because these XRD patterns are almost the same. The peaks due to any kind of crystallization are not recognized in Figure 7, especially undissolved alumina, where there is only a halo pattern. We previously confirmed an XRD pattern on a Nd2O3(3 wt%; 0.55 mol%)-SiO2(97 wt%; 99.45 mol%) system that included undissolved 0.55 mol% of Nd2O3 at best(Fujimoto & Nakatsuka 1997). 7.0 mol% of alumina (at maximum in this experiment) is probably an adequate quantity for XRD detection if the quantity is changed to undissolved alumina or other crystals in the sample. Therefore, it is concluded that all our samples are in the amorphous phase.
3.6. ESR measurements
The presence of unpaired electrons in BiSG was verified by ESR signal. The measured sample was B2 and B3. There was no ESR signal due to the unpaired electrons for both B2 and B3, even at liquid N2 temperature. The same phenomena without signals were also reported on Bi-doped multi-component glasses( Peng, Wang et al. 2005 ; Peng, Wu et al. 2008). According to Hund’s rule, the valence states of bismuth ions without unpaired electrons should be Bi3+(~5d106s2) or Bi5+ (~5d10)(Ohkura, Fujimoto et al. 2007 ).
3.7. XPS measurements
3.7.1. Analysis on chemical shift
The results of the XPS measurements are shown in Figure 8. The measured samples were A4, A5, and three standards, NaBiO3, Bi2O3, and Bi-metal. The main Bi(4f5/2, 4f7/2) peaks of Bi2O3 exist at 163.7 and 158.4 eV, respectively. Bi-metal was treated with 3-minute etching by Ar-beam in a vacuum chamber (1.010-7 Torr) to eliminate the oxidized Bi-metal surface before the measurement. Even after the treatment, weak residual peaks were found due to Bi2O3. The main Bi(4f5/2, 4f7/2) peaks of the Bi-metal exist at 162.4 and 157.1 eV, respectively. These Bi2O3 and Bi-metal peaks well agree with those previously reported(Wagner 1990; Saffarini & Saiter 2000), and the chemical shifts of Bi2O3 and Bi-metal are very stable in the XPS measurement. NaBiO3 is often used as a standard of the penta-valent state of Bi, but in our experiment, the main Bi(4f5/2, 4f7/2) peaks of NaBiO3 were obtained at 163.8 and 158.5 eV corresponding to the Bi2O3 ones, and the second Bi(4f5/2, 4f7/2) peaks exist at higher bonding energy at 165.9 and 160.6 eV, respectively. The peaks of both BiSGs, that is, A4 and A5, are located at almost the same position at the second NaBiO3 peaks. After arranging the chemical shifts for the measured samples, the order of the bonding energy is as follows:
[lower bonding energy] Bi metal (Bi0) -> Bi2O3 (Bi3+), first peaks of NaBiO3 (Bi3+) -> second peaks of NaBiO3 (Bi5+) = BiSG (A4, A5) [higher bonding energy]
In general, the valence state of the target ion becomes higher with increased bonding energy(Wagner 1990), and the same tendency is observed in my measurement. Therefore, Bi ions of the penta-valent state exist in BiSG.
The peak positions of NaBiO3, however, seem unstable. The previously reported Bi(4f5/2, 4f7/2) peaks(Kulkarni, Vijayakrishnan et al. 1990) were 164.1 and 158.7 eV, respectively, and they showed single peaks with almost the same binding energy of the Bi2O3 ones. In fact, although we measured the XPS data on the NaBiO3 several times, the ratio of the main peaks corresponding to Bi2O3 to the second peaks was unstable. Therefore, we put the most probable XPS data of NaBiO3 in Figure 8.
Kumada et al.(Kumada, Takahashi et al. 1996; Kumada, Kinomura et al. 1999) reported that NaBiO3 and LiBiO3 are synthesized at 120-200 C and that the Bi5+ state is changed to a Bi3+ state over 400 C; similar unstability may occur for the Bi5+ state in NaBiO3. The standard material of NaBiO3 was identified as NaBiO3•2H2O by XRD in Kumada’s experiment. They also reported that Na ions in the A-site (A+B5+O3) tend to be exchanged for Sr2+ or Ba2+ ions in NaBiO3•nH2O, and then the redistributed Bi ions in the A-site take the Bi3+ state(Kumada, Kinomura et al. 1999). Although they neglected to mention the redistribution of Bi ions in NaBiO3 themselves, a similar phenomenon may occur for NaBiO3 in their experiment.
3.7.2. Analysis on peak separation
By precisely observing the peak positions and the line widths, we recognized that the A4 peaks were slightly shifted to higher bonding energy than A5 and that the line widths of A4 and A5 were wider than the standards. Since the A4 and A5 peaks are composed of two or more peaks, we separated them with a Gaussian fitting curve to examine the origin of the peak shift. In this procedure, we make the following assumptions:
If such Bi ionic states as Bi0 or Bi3+ are considered identical, the line widths of Bi(4f5/2, 4f7/2) are also identical.
The line widths of Bi 4f5/2 and Bi 4f7/2 are the same.
The ratio of Bi 4f5/2 to Bi 4f7/2 is constant for different ionic states in a sample. This ratio is theoretically calculated as Bi4f5/2/Bi4f7/2=l/(l+1)=3/4(Seah 1983).
The results are shown in Table 3. The peak separation results show five peak positions for all Bi 4f5/2 and Bi 4f7/2 peaks that are normalized at 100. No. 5 corresponds to Bi0, No. 4 to Bi3+, and No. 1 to Bi5+. Nos. 2 and 3 are the intermediate states between numbers 1 and 4, and these states have intermediate coordination states rather than intermediate valence states such as Bi4+ due to the ESR measurements. These results show that all the Bi ions in BiSG are not the penta-valent state, and therefore the Bi valence states were mixed states of Bi3+ with Bi5+. This mixed valence state of Bi3+ and Bi5+ is also supported by EXAFS analysis
in the next section. The peak height ratio of Nos. 1 and 2 is counterchanged for A4 and A5 due to the existence ratio of Bi3+ and Bi5+. Thus, the peaks of A4 are slightly shifted due to higher bonding energy than A5.
3.8. Bi-O distance from EXAFS
Figure 9 shows the radial structure functions (RSF) to which the EXAFS oscillations were Fourier-transformed. The measured samples were A-series (A2~A5) and the two standards of α-Bi2O3 and NaBiO3. The peak shown in about 1.0 Å is derived from the XANES region because it is too short for any Bi-O distance. Therefore, it is a ghost peak, and we conclude that the largest peak around 1.6-1.7 Å (the first relevant peak) corresponds to the first neighboring Bi-O bond. α-Bi2O3 and NaBiO3 have the second peak at 3.5 and 3.2 Å, respectively. Since the BiSG ones have no secondary peak, the local environment of the Bi ion does not have any periodical structure; that is, BiSG should be an amorphous phase. These results are supported by the XRD data in Section 3.5. The RSF shows that all the BiSG peaks are about 0.1 Å shorter than α-Bi2O3. The NaBiO3 peak is also shifted to a shorter position, but the line width is wider than that of any of the BiSG ones. Since RSF |F(r)| includes a phase shift, the radial distance in RSF shifted a shorter range than the actual Bi-O distance. To determine the length of the first neighboring Bi-O, the RSF of α-Bi2O3 and the BiSG samples were analyzed by the curve-fitting method in r-space with FEFF 8.2. In this curve-fitting calculation, we only took two coordination spheres due to the parameter number limitation in FEFF 8.2.
The fitting results of the BiSG samples, α-Bi2O3 and NaBiO3, are listed in Table 4. We assumed amplitude reduction factor S0 2 = 0.9(Manzini, Lottici et al. 1998) and absorption edge energy E0 = 13426.5 eV. The fitting range was selected from 1.2 to 2.1 Å in RSF (Figure 9). The Bi-O distances of the first and second coordination spheres for BiSG were calculated as about 2.1 and 2.3 Å, respectively; on the other hand, the Bi-O distances for α-Bi2O3 were 2.2 and 2.4 Å, respectively. The Bi-O distance of 2.1 Å in BiSG is in good agreement with the previously reported Bi5+-O distance in LiBi(5+)O3(Kumada, Takahashi et al. 1996) and Bi2(3+,5+)O4(Kumada, Kinomura et al. 1995). Therefore, the existence of the Bi5+ state is also indicated from the Bi-O distance in BiSG, and the first coordination sphere corresponds to the Bi5+-O distance. The second coordination sphere of 2.3Å corresponds to the Bi3+-O distance(Ohkura, Fujimoto et al. 2007). Therefore, the EXAFS curve-fitting results also show that the mixed valence state of the Bi ions exists in BiSG as Bi3+ and Bi5+.
The Bi-O distance of the first coordination sphere in the A-series is slightly shifted to a longer range with increased Bi2O3 concentration in Table 4. The ratio of Bi3+ to Bi5+ increases with increased Bi2O3 concentration, and the change of the Bi3+ to Bi5+ ratio can also explain the non-linear increment of LMI. NaBiO3 is a well-known material as a standard for penta-valent state Bi ions. The first coordination state distance of NaBiO3 is longer than the expected value of the Bi5+-O distance. This valence state of Bi ions in NaBiO3 is also the mixed state of Bi3+ and Bi5+. These phenomena are also supported by the peak separation data of XPS.
3.9. Discussion (local structure of luminescent center)
In the previous section, several physical phenomena were observed in BiSG, especially regarding the local structure of the distinctive luminescent center. Now we consider the structural configuration on the Bi luminescent center.
First, the roles of the Al2O3 additive can be understood by luminescent intensity measurement. Based on Figure 5, the luminescent intensity of A4, which includes 2.3 mol% of Al2O3, is three orders of magnitude larger than that of C1 without Al2O3; clearly, the Al2O3 additive remarkably increases the generation of the Bi luminescent center. Second, Al2O3 assists the Bi ions to enter the silica glass network because C1 has no glassy wetting. This tendency is also supported by the phase diagram of the Bi2O3-Al2O3-SiO2 glass system, because the glassy phase is likely achieved at the Al2O3-rich composition. Therefore, aluminum ions have two roles in BiSG:
They assist the conﬁguration of the distinctive luminescent center of Bi ions with a coupling effect that denotes that an aluminum ion behaves like a “generator” of the luminescent center.
They increase compatibility with the silica network.
These aluminum ion roles imply that both the Bi and Al atoms should be close together in BiSG. Based on the above discussion, the image view between Bi and Al ions in BiSG is illustrated in Figure 10(a). Peng et al.(Peng, Qiu et al. 2005) reported that Ta ions also work as a “generator.” Although aluminum is not the only element that behaves as a generator, the aluminum ion accepts its important role in the Bi2O3-Al2O3-SiO2 glass system.
Next, the aluminum cordination state (ACS) should be close to the Bi ion in BiSG, as seen from the 27Al-NMR results. Since the relation between ACS and the chemical shift in the 27Al-NMR measurement has been well studied(Laussac, Enjalbert et al. 1983), ACS is determined by a chemical shift in comparison between the standard materials and the target samples. In the case of BiSG, ACS is dominated by the α-Al2O3 corundum structure at lower Bi2O3 concentration up to 0.5 mol%. Al ions with corundum structure are crucial to generate a distinctive luminescent center, and the Al coordination state located near Bi should be a 6-fold corundum structure. Based on the above discussion, the image view between Bi and Al is illustrated in Figure 10(b).
Next, information on the valence states of the Bi ions in BiSG is given by ESR measurements. Of course, it’s not only for Bi ions, but since no signal exists for the unpaired electrons in the whole BiSG, the valence states of Bi ions without unpaired electrons are Bi3+ or Bi5+. These results show Bi3+ or Bi5+ ions close to the 6-fold coordination state of the Al ions. Based on the above discussion, the image view between Bi and Al is illustrated in Figure 10(c).
Three types of coordination states of Bi3+ exist, including 5-, 6-, and 8-fold; on the other hand, only 6-fold coordination exists for Bi5+(Shannon 1976). Since BiSG is an oxide material, it is estimated that the neighboring ions of the Bi ion are oxygen. Although the ionic radius of O2- has few differences with the coordination number, variation exists between 1.35 and 1.42 Å. If the coordination number of O2- is 4, Bi3+(5)-O2-(4), Bi3+(6)-O2-(4), and Bi5+(6)-O2-(4) are calculated to be 2.34, 2.41, and 2.12 Å, respectively(Shannon 1976). The Bi-O distances in typical crystals including Bi3+ or Bi5+, such as LiBiO3(Kumada, Takahashi et al. 1996) or Bi2O4(Kumada, Kinomura et al. 1995), show that the Bi5+-O distance is 2.1 Å. This value agrees well with the 2.1 Å of the first coordination sphere for A4 and A5. But the Bi3+-O distance varies from 2.15 to 3.26 Å in α-Bi2O3(Harwig 1978) or Bi2O4(Kumada, Kinomura et al. 1995). Therefore, the EXAFS data show that the Bi5+ ionic state exists in BiSG, and this is also supported by the XPS data. The previously reported Bi3+ spectroscopic properties are quite different in luminescent and absorption spectra and lifetime. It is concluded that the Bi valence state of the Bi luminescent center is Bi5+, not Bi3+. Therefore, the luminescent center model of Bi5+ with 6-fold coordination is expected to be close to Al3+ with 6-fold coordination of the corundum structure (Figure 10(d)). Since the neighboring atom is oxygen, the local structure of the distinctive bismuth luminescent center is expected (Figure 10(e)).
After the discovery of a new infrared luminescent bismuth center, several research groups started to study its applications, such as optical amplification(Fujimoto & Nakatsuka 2003; Seo, Fujimoto et al. 2006; Seo, Fujimoto et al. 2006; Ren, Wu et al. 2007; Ren, Dong et al. 2007; Ren, Qiao et al. 2007; Seo, Lim et al. 2007), waveguide inscription(Psaila, Thomson et al. 2006), or laser oscillation(Dianov, Dvoyrin et al. 2005; Dianov, Shubin et al. 2007; Razdobreev, Bigot et al. 2007; Rulkov, Ferin et al. 2007; Truong, Bigot et al. 2008) using Bi luminescent materials.
With respects to device applications, optical fibers with Bi luminescent center in the core material are very curious. Optical amplification around 1.3 µm with Bi-doped multi-component glass fiber was achieved by Seo et al.(Seo, Fujimoto et al. 2006), and this is useful for metro area network optical amplifiers. Laser oscillation with Bi-doped optical fiber was firstly demonstrated by Dianov’s group in 2005(Dianov, Dvoyrin et al. 2005), then the possibility of Bi-doped fiber is actively developed, now the oscillation power has achieved at 15 W at 1160 nm(Bufetov & Diagram 2009 ). It is known that the 570 – 590 nm band is very promising for ophthalmology and dermatology applications, thus the second harmonic of Bi fiber laser can be used in medical use. And the broad luminescence in near infrared region is also useful for a light source of optical coherence tomography.
In this chapter, we introduce the basic properties of BiSG and the analyzed local structure of Bi luminescent center. Several instrumental analyses, such as spectroscopic properties (SPCT), LMI, NMR, XRD, ESR, XPS, and EXAFS were advanced on a Bi2O3-Al2O3-SiO2 glass system. The roles and the structure of the Al ions and the valence state of the luminescent Bi ions were examined.
The following are the roles and the structure of the Al ions: 1) to assist the conﬁguration of the distinctive luminescent center of Bi ions with a coupling effect, which means that the aluminum ion behaves like a “generator” of the luminescent center; 2) to increase compatibility with the silica network; 3) to be a 6-fold corundum structure. The valence state exmination of Bi ions in BiSG reveals the following: 1) Bi3+ or Bi5+; 2) a mixed state of Bi3+ and Bi5+; 3) Bi5+ for the distinctive Bi luminescent center. Therefore, the distinctive bismuth luminescent center model was investigated with a 6-fold coordination state of Bi5+ that is combined with the 6-fold corundum structure of Al3+ through an oxygen ion (Figure 10(e)). These results will bridge to verify the energy diagram and the mechanism of Bi luminescent center.
In the last place, this new infrared luminescent material, Bi-doped silica glass, which attains sensational progress in the past decade will continue to give us curious possibilities in the field of the optical science.