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Low-temperature sintering of bismuth-doped glass with high fluorescence properties from mesoporous silica SBA-15
Ceramics International 46 (2020) 1164–1170
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Low-temperature sintering of bismuth-doped glass with high fluorescence properties from mesoporous silica SBA-15
T
Lei Yuana,b, Shijia Gua,c,∗, Xin Zhanga, Yuye Zhaoa,b, Wei Luoa,b, Lianjun Wanga,b,∗∗, Wan Jianga,c,d a
Engineering Research Center of Advanced Glasses Manufacturing Technology, Ministry of Education, Donghua University, Shanghai, 201620, PR China State Key Laboratory for Modification of Chemical Fibers and Polymer Materials & College of Materials Science and Engineering, Donghua University, Shanghai, 201620, PR China c Institute of Functional Materials, Donghua University, Shanghai, 201620, PR China d School of Material Science and Engineering, Jingdezhen Ceramic Institute, Jindezhen, 333000, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Bismuth-doped Mesoporous powder NIR photoluminescence Low-temperature Silica glass
A kind of bismuth-doped silica glasses have been produced successfully by spark plasma sintering at 1070 °C using bismuth-doped SBA-15 powders. The samples were detected by X-ray diffraction, UV–vis–NIR, photoluminescence, fluorescent decay curve and X-ray photoelectron spectroscopy. The results showed that the samples possess favorable near-infrared fluorescence and the 0.9 mol% sample is the best one among these glasses. The emission peaks at ~1180, ~1150 and ~1250 nm can be observed when excited by 500, 700 and 800 nm. The average lifetimes of 0.9 mol% under 500, 700 and 800 nm excitation are 189.87, 311.46 and 132.16 μs. Furthermore, the value of the important optical parameter σemτ is 5.79 × 10−24 cm2s at 800 nm excitations, which is 36.8% and 9.2% larger than bismuth-doped germanate glasses and phosphate glasses that have been reported. These emission peaks could be decomposed into three bands at ~1150, ~1240 and ~1440 nm via Gaussian decomposition. Based on the results of this work, it is concluded that the NIR emission peaks at ~1150, ~1240 and ~1440 nm correspond to Bi0, Bi+ and (Bi2)2-, respectively. This glass with excellent luminescence properties could offer a practical material to improve the performance of broadband fiber amplifiers and tunable lasers.
1. Introduction Bismuth-doped glasses have attracted increasing attention because of its outstanding near infrared (NIR) photoluminescence (PL) within the spectral range from 1000 to 1600 nm, a remarkable lifetime and a wide full-width at half maximum (FWHM) of 300 nm under 800 nm excitation [1]. And this is evidently important for optical applications as amplifiers in optical communication. Currently, the most common materials used as optical fiber amplifiers are the traditional rare earth (RE) ions doped glasses. But the gain bandwidth of these glasses is only 70 nm, which is much smaller than bismuth-doped glasses. The emission range of bismuth-doped glass spans the telecommunication window [2,3], and it can be considered as one of the most promising candidates of ultra-broadband fiber amplifier that is used in the optical communication system [4–6]. In recent years, there have been a large number of researches on
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ultra-broadband NIR photoluminescence of bismuth-doped materials, such as bismuth-doped silicate [1,2,4,7,8], borate [9,10], phosphate [11], germanate [12–15], chloride [16], chalcohalide [17], fluoride [18] glass and even some unusual materials including CsI [19], Bi5(GaCl4)3 [20], Bi5(AlCl4)3 [21], Ge2Al6O13 [6], CsPbI3 [22], CsCdBr3 [23], zeolite [24,25], KAlSi2O6 [26], etc. However, the exact origin of NIR emission is still controversial. Although there are many kinds of bismuth-doped glasses that have been explored up to now, the silica glass is still the main matrix material owing to its outstanding high temperature resistance, optical transparency [27], stable chemical stability [28,29], electrical insulation and wide composition adjustability [12]. Furthermore, silica glass has low optical loss over the work window of fiber optic communication and it is used to make optical fiber until now [28]. Yet, the handicap is that producing pure silica glass by the traditional meltquenching method needs extremely high temperature as well as long
Corresponding author. No. 2999 North of Renmin Road, Songjiang District, Shanghai, PR China. Corresponding author. No. 2999 North of Renmin Road, Songjiang District, Shanghai, PR China. E-mail addresses: [email protected] (S. Gu), [email protected] (L. Wang).
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https://doi.org/10.1016/j.ceramint.2019.09.085 Received 25 July 2019; Received in revised form 8 September 2019; Accepted 9 September 2019 Available online 09 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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temperature was detected with infrared thermometer during the sintering process. Before the examination, these samples were polished into 1 mm thickness.
melting time. This may make it difficult in controlling the proportion of bismuth in silica glass owing to the obvious volatility of bismuth oxide at the temperature over 1750 °C [24]. Generally speaking, the alkali and alkali earth metal oxides could be added to reduce the melting temperature but this way can also reduce the NIR luminescence intensity [12]. So a method which can bring down the preparing temperature to prevent bismuth from volatilizing at high temperature would be helpful to obtain high-performance bismuth-doped glass. Solid phase sintering is a practical low-temperature method for producing glass, but the transparency of sample is low and limits its application before nanometer materials are discovered. In the past decade, a sample was successfully manufactured by solid phase sintering with nanometer-sized powder, and the transmission rate is about 80% at 600 nm. However, the intensive tendencies to agglomerate and crystallize restrict the application of nanoparticles [30,31]. Mesoporous materials that received a large number of observations in recent years were chosen due to large specific surface area and tunnel structure [32,33]. A kind of porous material called ZSM-5 could be sintered into transparent glass at 1300 °C by spark plasma sintering (SPS) compared to traditional melt-quenching method. The process is a transfer from solid powder to transparent glass without molten or fluid state which can be regarded as a solid-state sintering method [34]. SPS has been widely applied because of its fast heating rate, low sintering temperature and short sintering time [35–37], which are very helpful to produce glass compared to the traditional melt-quenching method [38,39]. A series of bismuth-doped glasses were prepared by SPS using ZSM-5, but the bismuth-doped concentration was low because of the high sintering temperature about 1330 °C, which led to the unsatisfactory photoluminescence of those samples [34]. Recently, highly transparent silica glass was prepared from mesoporous silica SBA-15 powder at 1020 °C using SPS [33]. As a mesoporous material with lower sintering temperature, SBA-15 may be a more suitable matrix powder for producing high-performance bismuth-doped glass. In this work, the uniform bismuth-doped SBA-15 powders were prepared by in-situ chemosynthesis method and a series of bismuthdoped silica glasses with various concentrations were successfully fabricated by SPS method at 1070 °C. X-ray diffraction (XRD), UV–vis–NIR and photoluminescence of these glass samples were measured in this study. The fluorescent decay curve and the stimulated emission cross section of 0.9 mol% sample were investigated in detail. Furthermore, luminescent mechanism of this system was discussed based on the data of X-ray photoelectron spectroscopy (XPS) and some phenomena.
2.3. Materials characterization The X-ray diffraction patterns of this series samples were measured with a D/max-2500 PC X-ray diffractometer (Rigaku, Japan). The transmission spectra were recorded by a UV3600 UV–vis–NIR spectrometer (Shimadzu, Japan). The fluorescence emission spectra were detected through a QM/TM/NIR steady-state and time-resolved fluorescence spectrometer (PTI, USA). The fluorescence decay curves were carried out with an FLS920 fluorescence spectrometer (Edinburgh Instrument Ltd., UK) that uses a xenon lamp as the pumping source. Xray photoelectron spectroscopy was acquired at an EscaLab 250Xi X-ray photoelectron spectrometer (Thermo Fisher Scientific Inc., USA). All measurements were conducted at room temperature. 3. Results and discussion 3.1. XRD analysis and photographs XRD patterns of bismuth-doped glasses with different concentrations are illustrated in Fig. 1. Obviously, all of the sintered samples show abroad diffraction peak that corresponds to amorphous phase of silica glass, proving that the main phase of these samples is amorphous. It also indicates that the bismuth-doped SBA-15 powders have transformed to the dense glasses smoothly. It means that these bismuthdoped glasses were produced by SPS successfully. However, the XRD pattern of 0.1 mol% exists a sharp diffraction peak which is corresponding to Bi-metal (JCPDS. No. 44–1246). And the intensity of the sharp diffraction peak decreases as the concentration of bismuth changes from x = 0.1 to 0.5. When the concentrations are 0.7 mol% and 0.9 mol%, the peak disappeared, but it reappeared at 1.1 mol%. The reason for this phenomenon is that the Bi-metal is deoxygenized during sintering process at low concentration. And for high concentration, the amount of bismuth ion is so numerous that it would aggregate and form Bi-metal. The photographs of these bismuth-doped glasses samples are also shown in Fig. 1. The color turns from pink to red wine with the increase of bismuth-doped concentration, except 0.1 mol%. The 0.1 mol % sample is gray-black and opaque. The 0.5–0.9 mol% samples are completely transparent and have a corking appearance, but the 1.1 mol % sample has a slight inhomogeneity of color, indicating that the system has an optimal doping range. And the color changes of these samples are corresponding to the XRD patterns. Compared with the
2. Experiment 2.1. Preparation of bismuth-doped SBA-15 powders The bismuth-doped SBA-15 powders were synthesized via hydrothermal method [40]. Bi(NO3)3·5H2O and Al(NO3)3·9H2O were selected as precursors to produce the bismuth-doped powders with nominal compositions was (100-6x) SiO2-5xAl2O3-xBi2O3, where x = 0.1, 0.3, 0.5, 0.7, 0.9 and 1.1, respectively. Polyethylene oxide–polypropylene oxide–polyethylene oxide (P123) and Tetraethyl Orthosilicate (TEOS) were applied as the template agent and silicon source. Above chemicals were mixed in hydrochloric acid solution before the hydrothermal reaction to synthesize bismuth-doped powders. The synthetic powders were filtered and then calcined at 550 °C in a muffle furnace to remove residual organic matter, and bismuth-doped SBA-15 powders were obtained. 2.2. Preparation of bismuth-doped glasses The treated powders were compacted into a Φ10-mm graphite mold and the glass samples were gained at 1070 °C with the heating rate of 100 °C/min by a Dr. Sinter 725 SPS system (Sumitomo Coal Mining Co., Japan). The graphite mold sustained a 50 MPa axial pressure and the
Fig. 1. XRD patterns and photographs of different concentrations of bismuthdoped glasses. 1165
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Fig. 2. Transmission spectra of different concentrations of bismuth-doped glasses.
Fig. 4. Fluorescence spectra of glasses excited by 700 nm.
FWHM of ~250 nm. It is worth noting that the emission peaks of samples have a slight redshift from 1180 to 1220 nm with the concentration of bismuth increasing. This phenomenon also existed at other bismuth-doped glasses that have been studied [1,7,13]. Fig. 4 shows the infrared emission spectra of samples under the excitation of 700 nm. With increasing bismuth content, the emission band firstly increases, then decreases at 1.1 mol%. The NIR emission peaks locate at ~1150 nm and the changing trend of emission intensity is same as the 500 nm excitation. The sample doped with 0.1 mol% bismuth shows almost no NIR emission peak, indicating that the bismuth metal is not the NIR-emitting center. What is more, the location of emission peak shifts toward longer wavelength with increasing concentration of bismuth. The peak position locates at 1150 nm for 0.3 mol % sample, and it shifts to 1170 nm when the concentration is 0.9 mol%. The change can also be observed in Fig. 3. The emission spectra of this kind of glasses excited by 800 nm are illustrated in Fig. 5, which possess a broad emission peak at ~1250 nm with the FWHM of ~245 nm. More interestingly, the emission intensity of 800 nm excitation shares the same trend with 500 and 700 nm excitations. The emission intensity achieves the best value when the concentration is 0.9 mol%. Furthermore, it is clearly observed that the position of the emission peak has a redshift with increasing concentration of bismuth. Based on these phenomena above, it can be concluded that the 0.9 mol% sample has the best fluorescence performance among these samples. To get more information about the luminescent center of these bismuth-doped glasses, Fig. 6(a)-(f) illustrate the Gaussian multipeak fit for the emission spectra of 0.5 mol% and 0.9 mol% samples that excited
bismuth-doped zeolite-derived silica glass as previous reported [24], the doping concentration of samples in this study is higher and the color is more uniform. One reason for this phenomenon is that bismuthdoped SBA-15 powder was obtained by in-situ chemical synthesis method which can ensure the large doping amount and uniform distribution. Another is the lower sintering temperature and it could inhibit the volatilization or decomposition of bismuth ion. 3.2. Optical performance The transmission spectra of samples are provided in Fig. 2. As can be seen, all the samples are optically transparent in the scanning region except for 0.1 mol%. As is clearly seen, there are two main absorption peaks at about 500 and 700 nm, with a weaker absorption band at about 800 nm. These three absorption bands are consistent with previous reports [8,10,14,41,42]. As bismuth increases from 0.3 to 1.1 mol %, the absorption intensity of sample firstly decreases, then increases and the 0.9 mol% sample has the relatively obvious absorption band. The trend of absorption intensity is almost corresponding to the change of color in Fig. 1. In summary, it can be concluded that bismuth ion is doped into these glasses successfully. Fig. 3 exhibits the emission spectra of samples with different bismuth doping concentration under the excitation of 500 nm. All samples display strong ultra-broadband NIR emission centered at ~1180 nm covering the wavelength range from 1000 to 1600 nm except for the 0.1 mol%. As bismuth-doped concentration increases from 0.3 mol% to 1.1 mol%, the emission intensity of glasses rapidly increases, then decreases at 1.1 mol%, and attains a maximum at 0.9 mol% with the
Fig. 3. Fluorescence spectra of glasses excited by 500 nm.
Fig. 5. Fluorescence spectra of glasses excited by 800 nm. 1166
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Fig. 6. Emission spectra deconvoluted into Gaussians of 0.5 mol% and 0.9 mol% samples excited at (a) (b) 500 nm, (c) (d) 700 nm, (e) (f) 800 nm.
by 500, 700 and 800 nm. These spectra can be fitted with two Gaussian peaks and the determination coefficient R2 > 0.996 guarantees the accuracy of results. It is revealed that the emission spectra under 500 nm excitation are deconvoluted into two peaks at ~1150 (peak 1) and ~1240 nm (peak 2). The emission spectra excited at 700 nm is composed of two Gaussian bands at ~1150 (peak 1) and ~1240 nm (peak 2). When excited by 800 nm, the emission spectra can be confirmed as the combination of two Gaussian bands. One of them appears at ~1240 (peak 2) and the other locates at ~1440 nm (peak 3). As the concentration increases, the relative area of ~1150 nm (peak 1) decreases but it increases at ~1240 nm (peak 2) under 500 and 700 nm excitation. As for 800 nm excitation, the relative area of ~1240 nm (peak 2) decreases, while the relative area of ~1440 nm (peak 3) increases with the concentration increases. This tendency may suggest that the three bands at ~1150, ~1240 and ~1440 nm originate from three different luminescence centers. At the same time, by using this change rule, the redshift of emission peaks can also be explained. To obtain further information about these glasses, the fluorescence lifetime was tested and the fluorescent decay curves of 0.9 mol% sample under 500, 700 and 800 nm excitations are revealed in Fig. 7. The fluorescent decay curves can be best fitted with the second-order exponential decay equation compared with the first-order exponential
Fig. 7. Fluorescent decay curves of emission at 1220, 1170 and 1250 nm of 0.9 mol% sample under 500, 700 and 800 nm excitations.
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decay equation. Two lifetimes under 500 nm excitation are 92.6 and 436.2 μs. And the fluorescence lifetimes under 700 nm excitation are 117.7 and 619.5 μs. In addition, the fluorescence lifetimes under 800 nm excitation are 60.5 and 294.3 μs, respectively. The existence of two lifetimes under one excitation wavelength indicates that there may coexist several NIR luminescent centers in the silica glass, so the emission peak is moving with the change of bismuth-doped concentration. The average fluorescence lifetime τ is calculated by the equation [43]:
τ=
∫ I (t ) tdt /∫ I (t ) dt
(1)
where I(t) is the luminescence intensity I at time t. The mean lifetimes for the emission band at 1220, 1170 and 1250 nm are 189.87, 311.46 and 132.16 μs when excited by 500, 700 and 800 nm, respectively. In order to research the NIR luminescence performance of these glasses better, the stimulated emission cross section σem at 500, 700 and 800 nm were calculated by the following formula [9,11,44]:
σem =
λ 02 In2 1/2 1 ⎛ ⎞ 2 4πn cτem ⎝ π ⎠ Δvem
Fig. 8. XPS spectra of 0.1 mol%, 0.9 mol%, Bi2O3 powder and Bi-metal.
lower valence bismuth ions such as Bi+, Bi0 and other clusters. As reported in recent studies, Zhang et al. researched the NIR emission of bismuth-doped silica glass and they concluded that the emission peaks at 1100 nm and 1260 nm were caused by the Bi+ and Bi0 [20,47]. Apart from this, Li et al. and Zhang et al. also suggested that the NIR emission bands were originated from the Bi+ and Bi0 [41,48]. Laguta et al. proposed that the source was a single Bi+ active center [49]. Based on our results, we present that these glasses have three emission centers rather than one or two centers. Xu et al. also supported that there were three NIR photoluminescence centers in their samples [50]. To know more about the NIR active center for NIR luminescence in our glass, XPS analyses of 0.1 mol%, 0.9 mol% and the two standards of Bi2O3 powder and Bi-metal are shown in Fig. 8. It can be clearly seen that the main peaks of the Bi-metal exist at 157.2 and 162.5 eV. While for the Bi2O3 powder, two peaks appear at 159.0 eV and 164.3 eV, the bonding energy of Bi3+ and Bi-metal differs by 1.8 eV. In order to expound the bonding energy of Bi-metal in our glass system, 0.1 mol% sample with high concentration of Bi-metal was detected. The bonding energy of 0.1 mol% sample increases 2.2 eV compared to pure Bi-metal, indicating that this glass matrix can strengthen binding energy of bismuth. Two peaks of 0.9 mol% sample appear at 160.3 eV and 165.5 eV, which are about 0.8 eV larger than 0.1 mol% sample with high concentration of Bi-metal, so it can be deduced that the bismuth ions in 0.9 mol% sample are low valence states. And the XPS spectrum of 0.9 mol% sample may be composed of several bismuth ions with low valence states. Combined with the previous results, it can be inferred that the emission peaks at ~1150, ~1240 and ~1440 nm originate from Bi0, Bi+ and (Bi2)2-, respectively. The energy level diagrams of Bi0, Bi+ and (Bi2)2- are shown in Fig. 9
(2)
where λ0 is the peak wavelength of emission spectrum when n is the refractive index under the excitation wavelength. And τem corresponds to emission lifetime while the meaning of c is the speed of light, and Δvem is the FWHM of emission peak. Using this formula, it can compute σem = 2.75 × 10−20 cm2 under 500 nm excitation, with λ0 = 1220 nm, n = 1.478, c = 3.0 × 108 m/s, τem = 189.9 μs and Δvem = 1626 cm−1. The σem under 700 and 800 nm excitations are 1.77 × 10−20 cm2 and 4.38 × 10−20 cm2. In addition, the product of the stimulated cross section and the lifetime that called σemτ is a significant parameter for estimating the performance of laser material. The σemτ at 500, 700 and 800 nm excitations are 5.23 × 10−24 cm2s, 5.51 × 10−24 cm2s and 5.79 × 10−24 cm2s, respectively. It is noteworthy that the σemτ at 800 nm excitation increases 36.8% and 9.2%, compared to bismuthdoped germanate (σemτ = 4.23 × 10−24 cm2s) and phosphate (σemτ = 5.3 × 10−24 cm2s) glass, and is about 4 times higher than Ti doped sapphire (σemτ = 1.4 × 10−24 cm2s) [9,44,45]. Therefore, this suggests that the glass prepared in this work is a promising material for broadband optical fiber amplifier. 3.3. Luminescent mechanism There are many deductions about the origin of NIR luminescence in bismuth-doped glasses because of its complex valence state. And it is practical to infer its origin by analyzing the fluorescence spectroscopy. As mentioned above, three Gaussian peaks locate at ~1150, ~1240 and ~1440 nm may correspond to three active centers in this series of glasses. Based on recent research, subvalent Bi centers whose valence state is below than 3+ are the origin of NIR luminescence in bismuthdoped glasses [46]. For the high valence of Bi, the Bi5+ has a strong oxidizability that can be easily reduced to Bi3+ by other Bi ions [10]. And the emission peaks of Bi3+ usually exist in ultraviolet and visible regions [41]. On the basis of the analysis, we suppose that Bi5+ and Bi3+ are not the NIR emission center of these glasses and the center should be the lower valence state. Combined with recent literatures, the low-valence bismuth ions like Bi0 and Bi+ can be formed by thermal activation when the high-valence bismuth ions receive electrons during the sintering process [15]. For instance, the Bi3+ can be deoxidized to Bi+ by bismuth metal as following change [18]: (3)
Bi3 + + 2Bi ↔ 3Bi+ 2+
As is reported before, the emission peak of Bi -doped glasses occurs in the visible region [9]. So we consider that the Bi2+ is also not the NIR emission center of these glasses. As already noted, the bismuth metal doesn't possess any NIR photoluminescence. Thence, it is obvious that the NIR luminescent centers of glass may come down to other
Fig. 9. The energy level diagrams of Bi0, Bi+ and (Bi2)2-. 1168
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which are based on our work and some previous reports [20,24,41,46,50]. In consideration of the redshift of emission peak, Gaussian decomposition, fluorescent decay curves as well as XPS results, we surmise that the NIR emission peaks at ~1150, ~1240 and ~1440 nm might be attributed to 3P1→3P0 transition of Bi+, 2D3/ 1 − 3 4 0 22→ S3/2 transition of Bi and ∑0 → ∏2g transition of (Bi2) , respectively. Based on this, all the emissions can be explained by the proposed model in Fig. 9. For example, the emission peak at ~1180 nm originates from 3P1→3P0 transition of Bi+ and 2D3/2→4S3/2 transition of Bi0 when excited by 500 nm.
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4. Conclusion Bismuth-doped silica glasses have been produced successfully at lower temperature by SPS method. This series of glasses possess excellent NIR luminescence properties. The emission peaks appear at ~1180, ~1150 and ~1250 nm when excited by 500, 700 and 800 nm. And the NIR emission intensity increases with bismuth-doped concentration increases but it decreases finally. The 0.9 mol% sample has the best fluorescence performance among these samples. All the emission spectra can be decomposed into three Gaussian bands peak at ~1150, ~1240 and ~1440 nm via Gaussian decomposition. The fluorescent decay curves of 0.9 mol% show a second-order exponential decay. The mean lifetimes for the emission band at 1220, 1170 and 1250 nm are 189.87, 311.46 and 132.16 μs. And the σemτ at 500, 700 and 800 nm excitations are 5.23 × 10−24 cm2s, 5.51 × 10−24 cm2s and 5.79 × 10−24 cm2s. And it increases 36.8% and 9.2% when compared to bismuth-doped germanate and phosphate glasses. Based on our results, we suppose that there are three NIR photoluminescence centers in our samples and the NIR emission peaks at ~1150, ~1240 and ~1440 nm are attributed to 3P1→3P0 transition of Bi+, 2D3/2→4S3/2 − transition of Bi0 and 1∑0 →3∏2g transition of (Bi2)2-, respectively. The NIR luminescence performance of the samples prepared in this work is greatly improved and it would be a useful material for high-performance broadband fiber amplifiers and tunable lasers in the future. Acknowledgement This work was funded by the Fundamental Research Funds for the Central Universities (2232018D3-38), Natural Science Foundation of China (Nos. 51672041, 51432004 and 51822202), Sponsored by Shanghai Sailing Program (18YF1400500 and 17YF1400400), Supported by Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-03-E00025). References [1] Y. Fujimoto, M. Nakatsuka, Infrared luminescence from bismuth-doped silica glass, Jpn. J. Appl. Phys. 40 (2001) L279–L281 https://doi.org/10.1143/JJAP.40.L279. [2] O. Laguta, H.E. Hamzaoui, M. Bouazaoui, V.B. Arion, I. Razdobreev, Magnetic circular polarization of luminescence in bismuth-doped silica glass, Optica 2 (2015) 663–666 https://doi.org/10.1364/OPTICA.2.000663. [3] J. Cao, L. Wondraczek, Y. Wang, L. Wang, J. Li, S. Xu, M. Peng, Ultrabroadband near-infrared photoemission from bismuth-centers in nitridated oxide glasses and optical fiber, ACS Photonics 5 (2018) 4393–4401 https://doi.org/10.1021/ acsphotonics.8b00814. [4] S. Zhou, N. Jiang, B. Zhu, H. Yang, S. Ye, G. Lakshminarayana, J. Hao, J. Qiu, Multifunctional bismuth-doped nanoporous silica glass: from blue-green, orange, red, and white light sources to ultra-broadband infrared amplifiers, Adv. Funct. Mater. 18 (2008) 1407–1413 https://doi.org/10.1002/adfm.200701290. [5] A.B. Rulkov, A.A. Ferin, S.V. Popov, J.R. Taylor, I. Razdobreev, L. Bigot, G. Bouwmans, Narrow-line, 1178nm CW bismuth-doped fiber laser with 6.4W output for direct frequency doubling, Opt. Express 15 (2007) 5473–5476 https:// doi.org/10.1364/OE.15.005473. [6] Z. Fang, S. Zheng, W. Peng, H. Zhang, Z. Ma, G. Dong, S. Zhou, D. Chen, J. Qiu, Bismuth-doped multicomponent optical fiber fabricated by melt-in-tube method, J. Am. Ceram. Soc. 99 (2016) 856–859 https://doi.org/10.1111/jace.14060. [7] X. Li, M. Peng, J. Cao, Z. Yang, S. Xu, Distribution and stabilization of bismuth NIR centers in Bi-doped aluminosilicate laser glasses by managing glass network structure, J. Mater. Chem. C 6 (2018) 7814–7821 https://doi.org/10.1039/ C8TC01042K.
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Low-temperature sintering of bismuth-doped glass with high fluorescence properties from mesoporous silica SBA-15
T
Lei Yuana,b, Shijia Gua,c,∗, Xin Zhanga, Yuye Zhaoa,b, Wei Luoa,b, Lianjun Wanga,b,∗∗, Wan Jianga,c,d a
Engineering Research Center of Advanced Glasses Manufacturing Technology, Ministry of Education, Donghua University, Shanghai, 201620, PR China State Key Laboratory for Modification of Chemical Fibers and Polymer Materials & College of Materials Science and Engineering, Donghua University, Shanghai, 201620, PR China c Institute of Functional Materials, Donghua University, Shanghai, 201620, PR China d School of Material Science and Engineering, Jingdezhen Ceramic Institute, Jindezhen, 333000, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Bismuth-doped Mesoporous powder NIR photoluminescence Low-temperature Silica glass
A kind of bismuth-doped silica glasses have been produced successfully by spark plasma sintering at 1070 °C using bismuth-doped SBA-15 powders. The samples were detected by X-ray diffraction, UV–vis–NIR, photoluminescence, fluorescent decay curve and X-ray photoelectron spectroscopy. The results showed that the samples possess favorable near-infrared fluorescence and the 0.9 mol% sample is the best one among these glasses. The emission peaks at ~1180, ~1150 and ~1250 nm can be observed when excited by 500, 700 and 800 nm. The average lifetimes of 0.9 mol% under 500, 700 and 800 nm excitation are 189.87, 311.46 and 132.16 μs. Furthermore, the value of the important optical parameter σemτ is 5.79 × 10−24 cm2s at 800 nm excitations, which is 36.8% and 9.2% larger than bismuth-doped germanate glasses and phosphate glasses that have been reported. These emission peaks could be decomposed into three bands at ~1150, ~1240 and ~1440 nm via Gaussian decomposition. Based on the results of this work, it is concluded that the NIR emission peaks at ~1150, ~1240 and ~1440 nm correspond to Bi0, Bi+ and (Bi2)2-, respectively. This glass with excellent luminescence properties could offer a practical material to improve the performance of broadband fiber amplifiers and tunable lasers.
1. Introduction Bismuth-doped glasses have attracted increasing attention because of its outstanding near infrared (NIR) photoluminescence (PL) within the spectral range from 1000 to 1600 nm, a remarkable lifetime and a wide full-width at half maximum (FWHM) of 300 nm under 800 nm excitation [1]. And this is evidently important for optical applications as amplifiers in optical communication. Currently, the most common materials used as optical fiber amplifiers are the traditional rare earth (RE) ions doped glasses. But the gain bandwidth of these glasses is only 70 nm, which is much smaller than bismuth-doped glasses. The emission range of bismuth-doped glass spans the telecommunication window [2,3], and it can be considered as one of the most promising candidates of ultra-broadband fiber amplifier that is used in the optical communication system [4–6]. In recent years, there have been a large number of researches on
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ultra-broadband NIR photoluminescence of bismuth-doped materials, such as bismuth-doped silicate [1,2,4,7,8], borate [9,10], phosphate [11], germanate [12–15], chloride [16], chalcohalide [17], fluoride [18] glass and even some unusual materials including CsI [19], Bi5(GaCl4)3 [20], Bi5(AlCl4)3 [21], Ge2Al6O13 [6], CsPbI3 [22], CsCdBr3 [23], zeolite [24,25], KAlSi2O6 [26], etc. However, the exact origin of NIR emission is still controversial. Although there are many kinds of bismuth-doped glasses that have been explored up to now, the silica glass is still the main matrix material owing to its outstanding high temperature resistance, optical transparency [27], stable chemical stability [28,29], electrical insulation and wide composition adjustability [12]. Furthermore, silica glass has low optical loss over the work window of fiber optic communication and it is used to make optical fiber until now [28]. Yet, the handicap is that producing pure silica glass by the traditional meltquenching method needs extremely high temperature as well as long
Corresponding author. No. 2999 North of Renmin Road, Songjiang District, Shanghai, PR China. Corresponding author. No. 2999 North of Renmin Road, Songjiang District, Shanghai, PR China. E-mail addresses: [email protected] (S. Gu), [email protected] (L. Wang).
∗∗
https://doi.org/10.1016/j.ceramint.2019.09.085 Received 25 July 2019; Received in revised form 8 September 2019; Accepted 9 September 2019 Available online 09 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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temperature was detected with infrared thermometer during the sintering process. Before the examination, these samples were polished into 1 mm thickness.
melting time. This may make it difficult in controlling the proportion of bismuth in silica glass owing to the obvious volatility of bismuth oxide at the temperature over 1750 °C [24]. Generally speaking, the alkali and alkali earth metal oxides could be added to reduce the melting temperature but this way can also reduce the NIR luminescence intensity [12]. So a method which can bring down the preparing temperature to prevent bismuth from volatilizing at high temperature would be helpful to obtain high-performance bismuth-doped glass. Solid phase sintering is a practical low-temperature method for producing glass, but the transparency of sample is low and limits its application before nanometer materials are discovered. In the past decade, a sample was successfully manufactured by solid phase sintering with nanometer-sized powder, and the transmission rate is about 80% at 600 nm. However, the intensive tendencies to agglomerate and crystallize restrict the application of nanoparticles [30,31]. Mesoporous materials that received a large number of observations in recent years were chosen due to large specific surface area and tunnel structure [32,33]. A kind of porous material called ZSM-5 could be sintered into transparent glass at 1300 °C by spark plasma sintering (SPS) compared to traditional melt-quenching method. The process is a transfer from solid powder to transparent glass without molten or fluid state which can be regarded as a solid-state sintering method [34]. SPS has been widely applied because of its fast heating rate, low sintering temperature and short sintering time [35–37], which are very helpful to produce glass compared to the traditional melt-quenching method [38,39]. A series of bismuth-doped glasses were prepared by SPS using ZSM-5, but the bismuth-doped concentration was low because of the high sintering temperature about 1330 °C, which led to the unsatisfactory photoluminescence of those samples [34]. Recently, highly transparent silica glass was prepared from mesoporous silica SBA-15 powder at 1020 °C using SPS [33]. As a mesoporous material with lower sintering temperature, SBA-15 may be a more suitable matrix powder for producing high-performance bismuth-doped glass. In this work, the uniform bismuth-doped SBA-15 powders were prepared by in-situ chemosynthesis method and a series of bismuthdoped silica glasses with various concentrations were successfully fabricated by SPS method at 1070 °C. X-ray diffraction (XRD), UV–vis–NIR and photoluminescence of these glass samples were measured in this study. The fluorescent decay curve and the stimulated emission cross section of 0.9 mol% sample were investigated in detail. Furthermore, luminescent mechanism of this system was discussed based on the data of X-ray photoelectron spectroscopy (XPS) and some phenomena.
2.3. Materials characterization The X-ray diffraction patterns of this series samples were measured with a D/max-2500 PC X-ray diffractometer (Rigaku, Japan). The transmission spectra were recorded by a UV3600 UV–vis–NIR spectrometer (Shimadzu, Japan). The fluorescence emission spectra were detected through a QM/TM/NIR steady-state and time-resolved fluorescence spectrometer (PTI, USA). The fluorescence decay curves were carried out with an FLS920 fluorescence spectrometer (Edinburgh Instrument Ltd., UK) that uses a xenon lamp as the pumping source. Xray photoelectron spectroscopy was acquired at an EscaLab 250Xi X-ray photoelectron spectrometer (Thermo Fisher Scientific Inc., USA). All measurements were conducted at room temperature. 3. Results and discussion 3.1. XRD analysis and photographs XRD patterns of bismuth-doped glasses with different concentrations are illustrated in Fig. 1. Obviously, all of the sintered samples show abroad diffraction peak that corresponds to amorphous phase of silica glass, proving that the main phase of these samples is amorphous. It also indicates that the bismuth-doped SBA-15 powders have transformed to the dense glasses smoothly. It means that these bismuthdoped glasses were produced by SPS successfully. However, the XRD pattern of 0.1 mol% exists a sharp diffraction peak which is corresponding to Bi-metal (JCPDS. No. 44–1246). And the intensity of the sharp diffraction peak decreases as the concentration of bismuth changes from x = 0.1 to 0.5. When the concentrations are 0.7 mol% and 0.9 mol%, the peak disappeared, but it reappeared at 1.1 mol%. The reason for this phenomenon is that the Bi-metal is deoxygenized during sintering process at low concentration. And for high concentration, the amount of bismuth ion is so numerous that it would aggregate and form Bi-metal. The photographs of these bismuth-doped glasses samples are also shown in Fig. 1. The color turns from pink to red wine with the increase of bismuth-doped concentration, except 0.1 mol%. The 0.1 mol % sample is gray-black and opaque. The 0.5–0.9 mol% samples are completely transparent and have a corking appearance, but the 1.1 mol % sample has a slight inhomogeneity of color, indicating that the system has an optimal doping range. And the color changes of these samples are corresponding to the XRD patterns. Compared with the
2. Experiment 2.1. Preparation of bismuth-doped SBA-15 powders The bismuth-doped SBA-15 powders were synthesized via hydrothermal method [40]. Bi(NO3)3·5H2O and Al(NO3)3·9H2O were selected as precursors to produce the bismuth-doped powders with nominal compositions was (100-6x) SiO2-5xAl2O3-xBi2O3, where x = 0.1, 0.3, 0.5, 0.7, 0.9 and 1.1, respectively. Polyethylene oxide–polypropylene oxide–polyethylene oxide (P123) and Tetraethyl Orthosilicate (TEOS) were applied as the template agent and silicon source. Above chemicals were mixed in hydrochloric acid solution before the hydrothermal reaction to synthesize bismuth-doped powders. The synthetic powders were filtered and then calcined at 550 °C in a muffle furnace to remove residual organic matter, and bismuth-doped SBA-15 powders were obtained. 2.2. Preparation of bismuth-doped glasses The treated powders were compacted into a Φ10-mm graphite mold and the glass samples were gained at 1070 °C with the heating rate of 100 °C/min by a Dr. Sinter 725 SPS system (Sumitomo Coal Mining Co., Japan). The graphite mold sustained a 50 MPa axial pressure and the
Fig. 1. XRD patterns and photographs of different concentrations of bismuthdoped glasses. 1165
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Fig. 2. Transmission spectra of different concentrations of bismuth-doped glasses.
Fig. 4. Fluorescence spectra of glasses excited by 700 nm.
FWHM of ~250 nm. It is worth noting that the emission peaks of samples have a slight redshift from 1180 to 1220 nm with the concentration of bismuth increasing. This phenomenon also existed at other bismuth-doped glasses that have been studied [1,7,13]. Fig. 4 shows the infrared emission spectra of samples under the excitation of 700 nm. With increasing bismuth content, the emission band firstly increases, then decreases at 1.1 mol%. The NIR emission peaks locate at ~1150 nm and the changing trend of emission intensity is same as the 500 nm excitation. The sample doped with 0.1 mol% bismuth shows almost no NIR emission peak, indicating that the bismuth metal is not the NIR-emitting center. What is more, the location of emission peak shifts toward longer wavelength with increasing concentration of bismuth. The peak position locates at 1150 nm for 0.3 mol % sample, and it shifts to 1170 nm when the concentration is 0.9 mol%. The change can also be observed in Fig. 3. The emission spectra of this kind of glasses excited by 800 nm are illustrated in Fig. 5, which possess a broad emission peak at ~1250 nm with the FWHM of ~245 nm. More interestingly, the emission intensity of 800 nm excitation shares the same trend with 500 and 700 nm excitations. The emission intensity achieves the best value when the concentration is 0.9 mol%. Furthermore, it is clearly observed that the position of the emission peak has a redshift with increasing concentration of bismuth. Based on these phenomena above, it can be concluded that the 0.9 mol% sample has the best fluorescence performance among these samples. To get more information about the luminescent center of these bismuth-doped glasses, Fig. 6(a)-(f) illustrate the Gaussian multipeak fit for the emission spectra of 0.5 mol% and 0.9 mol% samples that excited
bismuth-doped zeolite-derived silica glass as previous reported [24], the doping concentration of samples in this study is higher and the color is more uniform. One reason for this phenomenon is that bismuthdoped SBA-15 powder was obtained by in-situ chemical synthesis method which can ensure the large doping amount and uniform distribution. Another is the lower sintering temperature and it could inhibit the volatilization or decomposition of bismuth ion. 3.2. Optical performance The transmission spectra of samples are provided in Fig. 2. As can be seen, all the samples are optically transparent in the scanning region except for 0.1 mol%. As is clearly seen, there are two main absorption peaks at about 500 and 700 nm, with a weaker absorption band at about 800 nm. These three absorption bands are consistent with previous reports [8,10,14,41,42]. As bismuth increases from 0.3 to 1.1 mol %, the absorption intensity of sample firstly decreases, then increases and the 0.9 mol% sample has the relatively obvious absorption band. The trend of absorption intensity is almost corresponding to the change of color in Fig. 1. In summary, it can be concluded that bismuth ion is doped into these glasses successfully. Fig. 3 exhibits the emission spectra of samples with different bismuth doping concentration under the excitation of 500 nm. All samples display strong ultra-broadband NIR emission centered at ~1180 nm covering the wavelength range from 1000 to 1600 nm except for the 0.1 mol%. As bismuth-doped concentration increases from 0.3 mol% to 1.1 mol%, the emission intensity of glasses rapidly increases, then decreases at 1.1 mol%, and attains a maximum at 0.9 mol% with the
Fig. 3. Fluorescence spectra of glasses excited by 500 nm.
Fig. 5. Fluorescence spectra of glasses excited by 800 nm. 1166
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Fig. 6. Emission spectra deconvoluted into Gaussians of 0.5 mol% and 0.9 mol% samples excited at (a) (b) 500 nm, (c) (d) 700 nm, (e) (f) 800 nm.
by 500, 700 and 800 nm. These spectra can be fitted with two Gaussian peaks and the determination coefficient R2 > 0.996 guarantees the accuracy of results. It is revealed that the emission spectra under 500 nm excitation are deconvoluted into two peaks at ~1150 (peak 1) and ~1240 nm (peak 2). The emission spectra excited at 700 nm is composed of two Gaussian bands at ~1150 (peak 1) and ~1240 nm (peak 2). When excited by 800 nm, the emission spectra can be confirmed as the combination of two Gaussian bands. One of them appears at ~1240 (peak 2) and the other locates at ~1440 nm (peak 3). As the concentration increases, the relative area of ~1150 nm (peak 1) decreases but it increases at ~1240 nm (peak 2) under 500 and 700 nm excitation. As for 800 nm excitation, the relative area of ~1240 nm (peak 2) decreases, while the relative area of ~1440 nm (peak 3) increases with the concentration increases. This tendency may suggest that the three bands at ~1150, ~1240 and ~1440 nm originate from three different luminescence centers. At the same time, by using this change rule, the redshift of emission peaks can also be explained. To obtain further information about these glasses, the fluorescence lifetime was tested and the fluorescent decay curves of 0.9 mol% sample under 500, 700 and 800 nm excitations are revealed in Fig. 7. The fluorescent decay curves can be best fitted with the second-order exponential decay equation compared with the first-order exponential
Fig. 7. Fluorescent decay curves of emission at 1220, 1170 and 1250 nm of 0.9 mol% sample under 500, 700 and 800 nm excitations.
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decay equation. Two lifetimes under 500 nm excitation are 92.6 and 436.2 μs. And the fluorescence lifetimes under 700 nm excitation are 117.7 and 619.5 μs. In addition, the fluorescence lifetimes under 800 nm excitation are 60.5 and 294.3 μs, respectively. The existence of two lifetimes under one excitation wavelength indicates that there may coexist several NIR luminescent centers in the silica glass, so the emission peak is moving with the change of bismuth-doped concentration. The average fluorescence lifetime τ is calculated by the equation [43]:
τ=
∫ I (t ) tdt /∫ I (t ) dt
(1)
where I(t) is the luminescence intensity I at time t. The mean lifetimes for the emission band at 1220, 1170 and 1250 nm are 189.87, 311.46 and 132.16 μs when excited by 500, 700 and 800 nm, respectively. In order to research the NIR luminescence performance of these glasses better, the stimulated emission cross section σem at 500, 700 and 800 nm were calculated by the following formula [9,11,44]:
σem =
λ 02 In2 1/2 1 ⎛ ⎞ 2 4πn cτem ⎝ π ⎠ Δvem
Fig. 8. XPS spectra of 0.1 mol%, 0.9 mol%, Bi2O3 powder and Bi-metal.
lower valence bismuth ions such as Bi+, Bi0 and other clusters. As reported in recent studies, Zhang et al. researched the NIR emission of bismuth-doped silica glass and they concluded that the emission peaks at 1100 nm and 1260 nm were caused by the Bi+ and Bi0 [20,47]. Apart from this, Li et al. and Zhang et al. also suggested that the NIR emission bands were originated from the Bi+ and Bi0 [41,48]. Laguta et al. proposed that the source was a single Bi+ active center [49]. Based on our results, we present that these glasses have three emission centers rather than one or two centers. Xu et al. also supported that there were three NIR photoluminescence centers in their samples [50]. To know more about the NIR active center for NIR luminescence in our glass, XPS analyses of 0.1 mol%, 0.9 mol% and the two standards of Bi2O3 powder and Bi-metal are shown in Fig. 8. It can be clearly seen that the main peaks of the Bi-metal exist at 157.2 and 162.5 eV. While for the Bi2O3 powder, two peaks appear at 159.0 eV and 164.3 eV, the bonding energy of Bi3+ and Bi-metal differs by 1.8 eV. In order to expound the bonding energy of Bi-metal in our glass system, 0.1 mol% sample with high concentration of Bi-metal was detected. The bonding energy of 0.1 mol% sample increases 2.2 eV compared to pure Bi-metal, indicating that this glass matrix can strengthen binding energy of bismuth. Two peaks of 0.9 mol% sample appear at 160.3 eV and 165.5 eV, which are about 0.8 eV larger than 0.1 mol% sample with high concentration of Bi-metal, so it can be deduced that the bismuth ions in 0.9 mol% sample are low valence states. And the XPS spectrum of 0.9 mol% sample may be composed of several bismuth ions with low valence states. Combined with the previous results, it can be inferred that the emission peaks at ~1150, ~1240 and ~1440 nm originate from Bi0, Bi+ and (Bi2)2-, respectively. The energy level diagrams of Bi0, Bi+ and (Bi2)2- are shown in Fig. 9
(2)
where λ0 is the peak wavelength of emission spectrum when n is the refractive index under the excitation wavelength. And τem corresponds to emission lifetime while the meaning of c is the speed of light, and Δvem is the FWHM of emission peak. Using this formula, it can compute σem = 2.75 × 10−20 cm2 under 500 nm excitation, with λ0 = 1220 nm, n = 1.478, c = 3.0 × 108 m/s, τem = 189.9 μs and Δvem = 1626 cm−1. The σem under 700 and 800 nm excitations are 1.77 × 10−20 cm2 and 4.38 × 10−20 cm2. In addition, the product of the stimulated cross section and the lifetime that called σemτ is a significant parameter for estimating the performance of laser material. The σemτ at 500, 700 and 800 nm excitations are 5.23 × 10−24 cm2s, 5.51 × 10−24 cm2s and 5.79 × 10−24 cm2s, respectively. It is noteworthy that the σemτ at 800 nm excitation increases 36.8% and 9.2%, compared to bismuthdoped germanate (σemτ = 4.23 × 10−24 cm2s) and phosphate (σemτ = 5.3 × 10−24 cm2s) glass, and is about 4 times higher than Ti doped sapphire (σemτ = 1.4 × 10−24 cm2s) [9,44,45]. Therefore, this suggests that the glass prepared in this work is a promising material for broadband optical fiber amplifier. 3.3. Luminescent mechanism There are many deductions about the origin of NIR luminescence in bismuth-doped glasses because of its complex valence state. And it is practical to infer its origin by analyzing the fluorescence spectroscopy. As mentioned above, three Gaussian peaks locate at ~1150, ~1240 and ~1440 nm may correspond to three active centers in this series of glasses. Based on recent research, subvalent Bi centers whose valence state is below than 3+ are the origin of NIR luminescence in bismuthdoped glasses [46]. For the high valence of Bi, the Bi5+ has a strong oxidizability that can be easily reduced to Bi3+ by other Bi ions [10]. And the emission peaks of Bi3+ usually exist in ultraviolet and visible regions [41]. On the basis of the analysis, we suppose that Bi5+ and Bi3+ are not the NIR emission center of these glasses and the center should be the lower valence state. Combined with recent literatures, the low-valence bismuth ions like Bi0 and Bi+ can be formed by thermal activation when the high-valence bismuth ions receive electrons during the sintering process [15]. For instance, the Bi3+ can be deoxidized to Bi+ by bismuth metal as following change [18]: (3)
Bi3 + + 2Bi ↔ 3Bi+ 2+
As is reported before, the emission peak of Bi -doped glasses occurs in the visible region [9]. So we consider that the Bi2+ is also not the NIR emission center of these glasses. As already noted, the bismuth metal doesn't possess any NIR photoluminescence. Thence, it is obvious that the NIR luminescent centers of glass may come down to other
Fig. 9. The energy level diagrams of Bi0, Bi+ and (Bi2)2-. 1168
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which are based on our work and some previous reports [20,24,41,46,50]. In consideration of the redshift of emission peak, Gaussian decomposition, fluorescent decay curves as well as XPS results, we surmise that the NIR emission peaks at ~1150, ~1240 and ~1440 nm might be attributed to 3P1→3P0 transition of Bi+, 2D3/ 1 − 3 4 0 22→ S3/2 transition of Bi and ∑0 → ∏2g transition of (Bi2) , respectively. Based on this, all the emissions can be explained by the proposed model in Fig. 9. For example, the emission peak at ~1180 nm originates from 3P1→3P0 transition of Bi+ and 2D3/2→4S3/2 transition of Bi0 when excited by 500 nm.
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4. Conclusion Bismuth-doped silica glasses have been produced successfully at lower temperature by SPS method. This series of glasses possess excellent NIR luminescence properties. The emission peaks appear at ~1180, ~1150 and ~1250 nm when excited by 500, 700 and 800 nm. And the NIR emission intensity increases with bismuth-doped concentration increases but it decreases finally. The 0.9 mol% sample has the best fluorescence performance among these samples. All the emission spectra can be decomposed into three Gaussian bands peak at ~1150, ~1240 and ~1440 nm via Gaussian decomposition. The fluorescent decay curves of 0.9 mol% show a second-order exponential decay. The mean lifetimes for the emission band at 1220, 1170 and 1250 nm are 189.87, 311.46 and 132.16 μs. And the σemτ at 500, 700 and 800 nm excitations are 5.23 × 10−24 cm2s, 5.51 × 10−24 cm2s and 5.79 × 10−24 cm2s. And it increases 36.8% and 9.2% when compared to bismuth-doped germanate and phosphate glasses. Based on our results, we suppose that there are three NIR photoluminescence centers in our samples and the NIR emission peaks at ~1150, ~1240 and ~1440 nm are attributed to 3P1→3P0 transition of Bi+, 2D3/2→4S3/2 − transition of Bi0 and 1∑0 →3∏2g transition of (Bi2)2-, respectively. The NIR luminescence performance of the samples prepared in this work is greatly improved and it would be a useful material for high-performance broadband fiber amplifiers and tunable lasers in the future. Acknowledgement This work was funded by the Fundamental Research Funds for the Central Universities (2232018D3-38), Natural Science Foundation of China (Nos. 51672041, 51432004 and 51822202), Sponsored by Shanghai Sailing Program (18YF1400500 and 17YF1400400), Supported by Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-03-E00025). References [1] Y. Fujimoto, M. Nakatsuka, Infrared luminescence from bismuth-doped silica glass, Jpn. J. Appl. Phys. 40 (2001) L279–L281 https://doi.org/10.1143/JJAP.40.L279. [2] O. Laguta, H.E. Hamzaoui, M. Bouazaoui, V.B. Arion, I. Razdobreev, Magnetic circular polarization of luminescence in bismuth-doped silica glass, Optica 2 (2015) 663–666 https://doi.org/10.1364/OPTICA.2.000663. [3] J. Cao, L. Wondraczek, Y. Wang, L. Wang, J. Li, S. Xu, M. Peng, Ultrabroadband near-infrared photoemission from bismuth-centers in nitridated oxide glasses and optical fiber, ACS Photonics 5 (2018) 4393–4401 https://doi.org/10.1021/ acsphotonics.8b00814. [4] S. Zhou, N. Jiang, B. Zhu, H. Yang, S. Ye, G. Lakshminarayana, J. Hao, J. Qiu, Multifunctional bismuth-doped nanoporous silica glass: from blue-green, orange, red, and white light sources to ultra-broadband infrared amplifiers, Adv. Funct. Mater. 18 (2008) 1407–1413 https://doi.org/10.1002/adfm.200701290. [5] A.B. Rulkov, A.A. Ferin, S.V. Popov, J.R. Taylor, I. Razdobreev, L. Bigot, G. Bouwmans, Narrow-line, 1178nm CW bismuth-doped fiber laser with 6.4W output for direct frequency doubling, Opt. Express 15 (2007) 5473–5476 https:// doi.org/10.1364/OE.15.005473. [6] Z. Fang, S. Zheng, W. Peng, H. Zhang, Z. Ma, G. Dong, S. Zhou, D. Chen, J. Qiu, Bismuth-doped multicomponent optical fiber fabricated by melt-in-tube method, J. Am. Ceram. Soc. 99 (2016) 856–859 https://doi.org/10.1111/jace.14060. [7] X. Li, M. Peng, J. Cao, Z. Yang, S. Xu, Distribution and stabilization of bismuth NIR centers in Bi-doped aluminosilicate laser glasses by managing glass network structure, J. Mater. Chem. C 6 (2018) 7814–7821 https://doi.org/10.1039/ C8TC01042K.
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