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For the first time, we studied the effect of structural relaxation on the NIR spectroscopic properties of bismuth-activated germanium glasses below glass transition temperature. Interestingly, distinct change behavior of NIR luminescence is observed at two different heat-treatment temperature ranges corresponding to two different relaxation behavior of glass structure. Besides, when structural modified by partly substituting B2O3 for GeO2, a narrower and more thermal sensitive luminescence is observed, which is inexplicable by “inhomogeneous broadening” and we tentatively attribute it to a defect-involved reason. Fundamentally the results here not only provide us a deeper insight into the optical property of bismuth-activated materials but also increase our understanding of the glassy state, and practically it delivers some valuable guidance in designing bismuth-activated glasses with superior NIR optical properties.
© 2014 Optical Society of America
1. Introduction
The last decade has witnessed a mass of significant research achievements in bismuth activated photonic materials, which have been considered as a novel candidate for application in areas ranging from telecommunication [1, 2], biomedicine [3], white LEDs [4] to lasers [5]. Among them, bismuth-doped glasses have been intensively investigated as a promising optical amplification material in near-infrared (NIR) telecommunication window due to its ultrabroad and tunable luminescence in this important spectral region. Accordingly, optical amplification and laser generation have been achieved [2, 5–8]. However, which valence state of bismuth accounts for the NIR luminescence is still in controversy, which presents as a major obstacle for optimization of technology and compositions to improve the performance of Bi-doped fibers [7, 9]. So far, two main models are suggested, high-valence (Bi5+ [10, 11]) and low-valence (Bi+ [12], Bi0 [13], Bi cluster [14]) bismuth, as the origin, while most researchers are inclined to agree on bismuth in low-valence as the NIR active center based on abundant experimental evidences [2, 3, 5, 15–21].
Bismuth, the so-called “wonder metal”, shows various oxidation states, such as 0, + 1, + 2, + 3, + 5, and a great tendency to form bismuth clusters [7, 22]. When melt at high temperature, bismuth oxidation will proceed reduction reactions extensively, and results in various forms in glasses, including Bi3+, Bi2+, Bi+, Bi0, Bi clusters, Bi nanocrystal [7]. With further fiber drawing process, NIR luminescence degradation happens as a result of aggregation of Bi NIR active centers, induced by the thermal activation during fiber drawing [23]. Considering fiber drawing usually performs at a temperature largely above the glass transition temperature (Tg) when glass structure is in a floppy state [24, 25], the migration of Bi ions and/or atoms is easy to happen. However, fundamentally more interestingly, provided the high reactivity and thermal sensitive of bismuth, what will the scenario like if bismuth-doped glasses heat-treated below Tg when glass structural relaxation is also attendant to some extent as a process of releasing excess free energy to get thermodynamic equilibrium [26]? This is of great significance with respect to a better understanding of NIR luminescence in Bi-doped materials and the specialty of the element bismuth as well as an iconic image about glass structural evolution behavior below Tg. To the best of our knowledge, there have been no reports about this issue yet.
In this contribution, for the first time, we studied the effect of structural relaxation on the NIR spectroscopic properties of Bi-doped germanium glasses. Interestingly, we found that NIR luminescence intensity increases first and then decreases as heat-treat temperature approaching Tg and glass structural relaxation becoming intense. A slight increase again was observed at ~Tg. While, no such increase at ~Tg was observed when GeO2 was partly substituted by B2O3. Besides, it seems that the germanium borate glass is more sensitive to structural relaxation. What’s more, the luminescence spectrum of germanium borate glass shows a blueshift and relative narrow bandwidth compared to germanium glass, which is exactly opposite to the case in germanium silicate glass [2], the possible reasons are discussed. The above results provide a deeper insight into the NIR luminescence properties of Bi-doped glass and might shed some light upon the outstanding issue of luminescence mechanism.
2. Experimental
Glass samples with compositions of 75GeO2-20MgO-5Al2O3-0.5Bi2O3 (GMA) and 60GeO2-15B2O3-20MgO-5Al2O3-0.5Bi2O3 (GBMA) were fabricated by conventional melt-quenching method. The raw materials were analytical grade MgO, H3BO3, Al2O3, Bi2O3 and high-purity GeO2 (5N). Approximately 30 g batches corresponding to each sample were mixed throughly and then melted in a corundum crucible at 1500°C for 30 min in air. The melts were then poured onto a stainless steel plate. The as-obtained glasses were then cut into pieces and heat-treated at various temperatures for 2 h according to the thermal analysis results. At last, the heat-treated glass samples were polished into slices of an appropriate size with a thickness of 1.5 mm before optical measurements.
The differential thermal analysis (DTA) was performed on a CRY-Z Differential Thermal Analyzer with a heating rate of 10°C/min. The NIR photoluminescence (PL) spectra were recorded using ZOLIX SBP300 spectrophotometer with an InGaAs detector under an 800 nm excitation. The luminescence decay curves were performed on an Edinburgh FLS920 spectrometer with a μs flash Xe lamp as the excitation source. Absorption spectra were measured using UV3600 UV-Vis-NIR spectrophotometer (Shimadzu Corp., Japan). The structure of glass samples were analyzed on a Raman spectrometer (Jobin Yvon Corp., France) using Ar+-ion laser (514.5 nm) as the irradiation source. The electron spin resonance (ESR) spectra were performed on an ESRA-300 Electron Paramagnetic Resonance spectrometer (Bruker Corp., Germany) operating in the X-band frequency ( = 9.4428 GHz). All the measurements were conducted at room temperature.
3. Results and discussion
As a common sense, glassy materials are in unstable thermodynamic equilibrium. When annealed at certain temperature that is low enough to avoid rapid crystallization but still allow some atomic motion, glassy materials will relax to low free-energy states accompanied with annihilation of excess free volume, which is known as structural relaxation [27]. On the other hand, it is identified that Tg is an indicator of glass network structure from rigid state to floppy state, yet translational molecular motions still occur below Tg [24, 26]. In order to confirm the glass transition temperature of these two glasses, we performed DTA on both of the as-made glasses, and Tg of glass GMA and GBMA were estimated to be 666°C and 634°C, respectively. Based on the Tg of each glass sample, we heat-treated both of them at various temperatures below Tg with the purpose to study the effect of structural relaxation on the optical properties of Bi-doped glasses.
NIR PL spectra of glass sample GMA and GBMA heat-treated at different temperatures, under an 800 nm excitation, are presented in Figs. 1(a) and 1(c), respectively. Figure 1(b) shows the dependence of emission intensity on heat-treatment temperature corresponding to GMA. Interestingly, the emission intensity firstly increases with heat-treatment temperature up to around 500°C (enhancement ratio of 40% for GMA, 60% for GBMA), then decreases rapidly before a slight increase again when heat-treated at ~Tg, but soon follows a fall. Similar scenario is observed in glass GBMA except for such an increase at ~Tg [Fig. 1(d)]. Besides, luminescence of glass GBMA seems more thermal sensitive as a more striking drop at 550°C presents in glass GBMA, the reasons will be discussed later.
Fig. 1 NIR emission spectra of glass GMA (a) and GBMA (c) heat-treated at different temperatures; and corresponding dependences of emission intensity on heat-treated temperature (b and d).
To reveal the possible internal variations that cause this interesting optical properties change under thermal activation, optical absorption spectra are recorded in Figs. 2(a) and 2(b) corresponding to glass GMA and GBMA, respectively. Photographs of each sample are also presented in corresponding inset. Clearly, the color of both samples turned from reddish-brown to yellow-brown then to dark-brown within the studied temperature range. Besides, two absorption peaks at around 500 and 700 nm could be seen in both samples which can be ascribed to Bi related species [28, 29]. As shown in Fig. 2(a), with increasing heat-treatment temperature to about 550°C, the peak at 508 nm gradually blueshifts to 487 nm, indicating some new absorption centers peaking at lower wavelength may come into formation. With higher heat-treatment temperature, characteristic absorption peak at around 500 nm vanishes, accompanied with a distinct redshift of absorption edge. This redshift of absorption edge and darkening effect of glass color have been interpreted as a result of formation of Bi metallic colloid and subsequent Mie scattering [23, 30]. Similar case is also happened in glass GBMA, except that the absorption peak position at around 500 nm is almost unchanged when heat-treated below 500°C. It seems that the change of absorption properties with temperature well coincides with that of NIR luminescence properties.
Fig. 2 Absorption spectra of glass GMA (a) and GBMA (b) under different heat-treatment temperatures. Insets are corresponding glass photographs.
According to the Adam and Gibbs’s hypothesis, structural relaxation is temperature dependent cooperative relaxation, involving cooperative motion of molecules: when one molecule moves, another molecule moves by closely following the first [31–33]. As temperature decreases, the molecular motions slow down, and the timescale of molecule rearrangement will be prolonged, which could be simply expressed by empirical Volgel-Fulcher-Tamman (VFT) function [34]
where A, B, and C are constants, τ is the molecular relaxation time constants representing the average time taken for a single molecular motion of a particular type to occur [22]. According to Eq. (1), τ significantly increases as temperature decreasing, one can expect that negligible motions could be detected over the timescale of our experiment under a temperature well below Tg [26, 35]. On the other hand, it has been identified by Xu that the electrons trapped on GeO4 can combine with high valent Bi ions to form low valent NIR active centers [28]. Besides, the aggregation of Bi ions to from metallic colloids accounts for the NIR luminescence degradation [23]. Based on above considerations, we propose that when heat-treated below 500°C, a temperature well below Tg, τ is already too high to detect obvious molecular motions except for some in situ vibrations, while more electrons are released from the trapped-electron centers or dangling bonds of the network under thermal activation and thus more high valent Bi ions reduced to low valent NIR active centers, as a result, the luminescence intensity increases; further approaching Tg, τ decreases rapidly and structural cooperative molecule motions become more and more significant, which in turn provides chance and space for Bi ions to move, thus part of luminous Bi species aggregate to form nonluminous Bi clusters and/or metallic colloid, resulting in the decrease of emission intensity and a distinct redshift of absorption edge band. However, when heat-treated at ~Tg, glass structure switches to a under-constraint “floppy” state with more dangling bonds and free electrons [24], consequently more high valent Bi ions might be reduced to Bi active centers and a slight rise of emission intensity is observed. Meanwhile, at this temperature, the migration of Bi ions is also largely enhanced, so a quick fall is followed because of luminescence quenching from aggregation. The reason for the absent of such a slight rise in glass GBMA will be covered in the following analysis.On the other hand, with a closer look on the PL spectra of glass GMA and GBMA, we found that the PL spectroscopic property of glass GMA (germanium glass) is somewhat different from that of glass GBMA (germanium borate glass). Take the as-made sample of each glass for example, as shown in Figs. 3(a) and 3(b). The full width at half maximum of the luminescence of glass GMA and GBMA are 286 and 245 nm, respectively. In other word, the luminescence spectrum of glass GBMA is narrower than that of glass GMA, which is opposite to a broader luminescence achieved by Zhou in germanium silicate glass compared to germanium glass due to “inhomogeneous broadening” by the presence of multiple structural units in glass matrix [2]. What’s more, the emission peak position of glass GBMA ( = 1230 nm) shows a blueshift compared to glass GMA ( = 1256 nm) which is also on the contrary. In addition, the spectrum of glass GMA is not Gaussian type, which can be fitted into two Gaussian peaks at 1256 and 1495 nm, respectively, while that of glass GBMA is nearly Gaussian fitted, as seen in Fig. 3(b). The luminescence decay curves at 1200 nm and 1500 nm are presented in Fig. 3(c), different decay kinetics (with luminescence lifetime of 336 and 415 μs, respectively) indicates different emission centers coexist in glass GMA. The former emission peak at around 1250 nm most probably come from Bi0 [28, 36], while the latter peak at around 1500 nm may origin from Bi dimers, like Bi2- [37] and Bi22- [14]. Further, ESR spectra were performed [Fig. 3(d)], and a signal was found at g = 2.2106 on glass GMA heat-treated at 500°C, which can be ascribed to Bi2- [28, 37]. While no apparent signal was observed on as-made glass may be from the low concentration of Bi2- cluster in as-made glass and more dimers are formed by thermal activation in the sample heat-treated at 500°C as indicated by the enhanced luminescence intensity. So, Bi2- most probably contributes to the emission at ~1495 nm in glass GMA, and no emission at ~1500 nm in glass GBMA is consistent with the absent of ESR signal of Bi2- on glass GBMA. Moreover, according to Sokolov et al [38], Bi2- have a absorption band near 460 nm, thus a blueshift at ~500 nm absorption peak when heat-treated blow 500°C is observed in glass GMA.
Fig. 3 PL spectrum of glass GMA (a) and GBMA (b); (c) luminescence decay curves of glass GMA; (d) ESR and (e) raman spectra of glass GMA and GBMA.
Considering the “inhomogeneous broadening” theory can’t explain above “abnormalities”, different mechanisms may exist. We measured raman spectra of the two glasses to analyze the structural differences, as presented in Fig. 3(e). Peaks at ~465 cm−1 and ~890 cm−1 can be ascribed to the bending and symmetric vibrations of Ge-O-Ge linkages, respectively [2]. Peak at ~804 cm−1 could be attributed to the symmetric vibration of the non-bridging oxygen of Ge-O-, or ring-breathing vibration of the boroxol ring [39], and the shoulder at ~535 cm−1 could be attributed to the symmetric breathing mode of regular rings, most probably four- and three-membered rings, which involves oxygen motions only [40]. A new band in glass GBMA at ~1425 cm−1 belongs to the vibrational modes of B-O- terminal bonds of [BO3] metaborate triangles [41]. It’s noteworthy that peak at ~890 cm−1 is weakened with part of GeO2 substituted by B2O3, indicating a less rigid structure is formed with relative more dangling bonds in glass GBMA, which is also confirmed by the lower Tg of glass GBMA, therefore a higher enhancement ratio is observed at 500°C for more dangling bonds and a slight PL intensity rise is not present when heat-treated at ~Tg due to the relative more floppy network structure of glass GBMA compared to glass GMA. Besides, the value of B in Eq. (1) is related to the structural rigidity of the system studied [26], and a lower rigidity may cause a more intense structural relaxation in glass GBMA and consequently more thermal sensitive luminescence. Considered the structural unit [GeO4] is usually tetrahedral coordinated to form a three-dimensional network while [BO3] tends to be planar three-coordinated to form two-dimensional sheet-like network, more structural defect and inhomogeneity will be induced in the germanium borate glass; besides, Bi dimers or clusters are more likely to locate at defective sites due to their relative large radius. On the other hand, according to Duffy [42], the optical basicity value of B2O3 (0.42) is lower than GeO2 (0.60) and more low-valence Bi will be present in glass GBMA due to its relative lower optical basicity. From this point of view, low-valence Bi species are more easy and probable to aggregate in glass GBMA and may thus be inclined to form bigger nonluminous clusters, in other word, luminous dimers Bi2- maybe unstable in glass GBMA, and emission at~1450 nm is absent, the detailed local structural characteristic is being under investigation. Additionally, the peak blueshift of glass GBMA compared to glass GMA may due to the lower covalence of B-O bond compared to Ge-O and the Bi active centers in B-O network with a higher field strength would emit higher energy photons [43, 44].
4. Conclusion
In conclusion, for the first time, the effect of structural relaxation below Tg on the NIR spectroscopic properties of bismuth-activated glasses is studied. We found that at low temperature when molecule motions are restricted to local vibrations, NIR luminescence intensity increases with temperature for more high-valence bismuth ions receive electrons released from trapped centers by thermal activation to form low-valence NIR luminous bismuth species, e.g. Bi0; with heat-treatment temperature approaching Tg, structure relaxation becomes more and more significant and bismuth active centers would aggregate to form nonluminous Bi clusters or metallic colloids, resulting in the NIR luminescence degradation. Besides, when structural modified by partly substituting B2O3 for GeO2, a narrower and more thermal sensitive luminescence is observed, which is inexplicable by “inhomogeneous broadening” and we tentatively attribute it to a defect-involved reason. As is generally believed that no significant structural change will happen below Tg let alone migration of dopants among the glass matrix, thus fundamentally the results here not only provide us a deeper insight into the optical property of bismuth-activated materials but also increase our understanding of the glassy state, and practically it delivers some valuable guidance in designing bismuth-activated glasses with superior NIR optical properties.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant. 51102209), Chinese Program for New Century Excellent Talents in University (Grant NCET-13-0221), Guangdong Natural Science Funds for Distinguished Young Scholar (Grant S2013050014549), Fundamental Research Funds for the Central University, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
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