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A new type of bismuth doped Ba2B5O9Cl crystal is reported to exhibit broadband near infrared (NIR) photoluminescence at room temperature, which has been identified here originating from elementary bismuth atom. Rietveld refining, static and dynamic spectroscopic properties reveal two types of Bi0 centers in the doped compound due to the successful substitution for two different nine-coordinated barium lattice sites. These centers can be created only in a reducing condition, and when treated in air and N2/H2 flow in turn, they can be removed and restored reversely. As the dwelling time is prolonged in N2/H2 at high temperature, conversion from Bi2+ to Bi0, as reflected by changes of their relative emission intensities, is witnessed in the crystal of Ba2B5O9Cl:Bi. The lifetime of the NIR luminescence was observed in a magnitude of ~30 μs, rather different from bismuth doped either glasses or crystals reported previously.
©2012 Optical Society of America
1. Introduction
New laser materials are essential to develop unprecedented light sources. Emerging in 1990’s, bismuth activated materials fascinate scientists in communities of optics and materials science, primarily because of the capability to provide net optical gain in a spectral range of NIR inaccessible to traditional rare earth [1–10]. In 2005 with bismuth doped glass fibers, Dianov, Bufetov, Dvoyrin and their colleagues in Fiber Optics Research Center, Russian Academy of Sciences have demonstrated CW lasering, and consequently fabricated fiber laser devices with high efficiency even without additional cooling [11–17]. For instance, bismuth doped pure silica fiber laser demonstrates at room temperature an efficiency up to 58% [15]. These pioneer works have convincingly proved the potency of the new material and simultaneously promoted the extremely rapid transition, namely less than a decade, from the very first finding of it to practical application [11–18]. And this seldom appears in history.
Nowadays, bismuth doped materials have been experiencing intensive researches worldwide, but mostly on doped glasses [18–21]. The reports on bismuth doped crystals are rather limited [18]. In 2005 Peng et al found broad NIR luminescence from bismuth doped SrB4O7, later confirmed by Su et al [22,23]. In 2008, similar luminescence was observed in RbPb2Cl5:Bi [24], and subsequently found in BaF2:Bi, α-BaB2O4:Bi, zeolite:Bi, Ba2P2O7:Bi, CsI:Bi and Ba10(PO4)6Cl2:Bi [25–31]. Peng et al reported, in a series of compounds M2P2O7 where M stands for Ca, Sr, and Ba, that NIR emission center can only be accommodated in the Ba compound rather than Ca or Sr compound even prepared in a reducing atmosphere such as CO [30]. Inspecting the crystal compounds where NIR emission center of bismuth can reside, we can find that a large host cation, such as Ba2+ or Cs+, seems necessary to stabilize the bismuth center [18,25–31]. For proving this, a compound of Ba2B5O9Cl is selected in the study.
So far, inspiring and series reports have appeared subsequently on the laser devices and new bismuth containing materials [32–36]. Nevertheless, the nature of the optically active NIR emission centers remains disputable [1–10,18,37,38]. The seemingly elemental problem is complicated by the diversity of potential valence states of bismuth, the easy conversion between each other, and the strong proneness to form into clusters. Different models have been proposed tentatively, such as Bi5+, Bi+, Bi0, Bi clusters with positive, neutral or even negative charges [1–10,18–40]. However, for each of them, direct and unequivocal experimental supports are highly desired. It is found here that spectroscopic data of Ba2B5O9Cl:Bi match well the Bi0 model.
For NIR emissive bismuth doped glasses and crystals [1–40], the luminescence lifetime is usually in an order from several hundred microseconds to even milliseconds. For instance, for CsI:Bi, two lifetimes of 130 and 220 μs were reported for the emission at 1216 nm and 1560 nm, respectively [29]. And Bi doped silicate glass shows an emission lifetime longer than 400 μs [8,40]. Differently, Ba2B5O9Cl:Bi of this work exhibits a lifetime of ~30 μs for an NIR emission.
2. Experimental procedure
The crystal samples of Ba2B5O9Cl:Bi were synthesized by a standard solid state reaction at high temperature. For this, analytical reagents BaCO3, H3BO3, BaCl2.2H2O and Bi2O3 were selected as raw materials. Individual batches of 10 g were weighed according to Ba2(1-x)B5O9Cl: 2x%Bi (x = 0, 0.1, 0.3, 0.7, 1.0, 1.5, 3.0, 5.0) and mixed. To compensate for the volatilization loss, H3BO3 was added in an excess of 3%. Heating rate was set 50 K/h to prevent batch foaming. The complete reaction comprised the steps as follows: (1) all batches were preheated at 500° C for 5 h in air; (2) the pre-reacted samples were ground afterwards to improve homogeneity; (3) the samples were sintered at 850° C for 10 h in air; (4) after an intermediate grinding, they were sintered at 850° C again for 10 h in air to form solid cylinders; (5) the cylinder samples were taken out of crucible and put into an alumina boat and treated in 95%N2/5%H2 for 2.5 h at 850° C. To optimize the dwelling time of step (5) and study the mechanism of NIR emission of bismuth, 12 batches of Ba2(1-x)B5O9Cl: 2x%Bi (x = 0.3) were first prepared as steps 1-4. Five of 12 were further sintered at 850° C in 95%N2/5%H2 for 0, 0.5, 1.0, 2.0 and 2.5 h, respectively. The rest of seven batches were treated as: (a) all were treated in air at 850° C for 0.5 h; (b) six of (a) were sintered in 95%N2/5%H2 at 850° C for 2.5 h; the rest five samples of (b) were treated alternately in air and 95%N2/5%H2 as (a) and (b) for two cycles. After that, the last sample was annealed again as (a). At each stage one sample was saved as reference for measurements.
X-ray diffraction (XRD) patterns of the samples were recorded with a Rigaku D/max-IIIA X-ray diffractometer (40 kV, 1.2° min−1, 40 mA, Cu-Kα1, λ = 1.5405 Å). Static excitation and emission spectra and dynamic emission decay spectra were measured with a high resolution spectrofluorometer Edinburgh Instruments FLS 920 equipped with a red sensitive single photon counting photomultiplier (Hamamatsu R928P) in Peltier air-cooled house for ultraviolet to visible range and a liquid nitrogen cooled photomultiplier (Hamamatsu R5509-72) for NIR range. A microsecond pulsed xenon flashlamp μF900 with an average power of 60 W applicable to a lifetime of 1 μs to 10 s was used to measure the decay curves. All the measurements were performed at room temperature.
3. Results and discussion
3.1 Determination of optimal dwell time in N2/H2 flow
For the samples of Ba2(1-x)B5O9Cl: 2x%Bi sintered only in air, no NIR luminescence can be detected, as shown for instance in Fig. 1(a) . When treated, however, in N2/H2, NIR luminescence appears peaking at 1055 nm, and it is intensified as the dwell time increases (see Fig. 1(a)), and meanwhile, the shape keeps constant. The intensification becomes less marked when the dwelling is further prolonged from 2 to 2.5 h. And we therefore determine the duration of treating in N2/H2 as 2.5 h.
Fig. 1 (a, λex = 478 nm) NIR and (b, λex = 273 nm) red emission spectra of Ba2(1-x)B5O9Cl: 2x%Bi (x = 0.3) treated in H2 at 850°C for different times: (black line) 0.0 h, (red line) 0.5 h, (green line) 1.0 h, (blue line) 2.0 h and (pink line) 2.5 h.
3.2 Evaluation of crystal structure
XRD patterns show that the samples prepared in air and N2/H2 are all single phase of Ba2B5O9Cl. For example, Fig. 2 depicts the XRD pattern of Ba2B5O9Cl: 0.1%Bi prepared in N2/H2. The Rietveld refining starts with the crystallographic data of Ba2B5O9Cl [41] and it converges with residual values of Rp = 8.66%, Rwp = 11.40%, Rexp = 9.52%, RBragg = 3.33% and GOF = 1.44. The structural refining results are listed in Fig. 2 along with the different profile between experimental and calculated values. The comparison confirms that the sample can be indexed in an orthorhombic Pnn2 space group. Noteworthy for the following discussion, in the compound, there are two types of barium sites Ba(1) and Ba(2), both of which are surrounded by seven oxygen and two chlorine atoms, shown as inset of Fig. 2. The average bond lengths are 2.8671 and 2.8781 Å for Ba(1)-O(Cl) and Ba(2)-O(Cl), respectively. Three [BO4] tetrahedra are corner linked by common oxygen atoms, and bridged up by two planar [BO3], and in this way they compose pentaborate group [B5O9]. Sharing oxygen atoms, the [B5O9] groups form into wavelike infinite chains [B5O9]∞ parallel to the c axis as shown in Fig. 2, and the chains are interlinked along a and b axis by [BO3] units, and then large channels, the centers of which are occupied by Ba atoms, come into being along the b axis [41]. It has been noticed that such infrastructure can effectively accommodate Eu2+ even when europium doped Ba2B5O9Cl is sintered in air [42]. So in Ba2B5O9Cl: Bi, it is not surprised to find the existence of Bi2+ labeled by typical red luminescence as shown in Fig. 1.
Fig. 2 XRD pattern (-o-) of Ba2(1-x)B5O9Cl: 2x%Bi (x = 0.1) prepared in N2/H2, Rietveld refining results (−), Bragg reflections (|) and the profile difference between experimental and calculated values (−). Inset left: Double cell of Ba2B5O9Cl viewed along b; Inset right up and down: Coordination environment around Ba (1) and Ba (2), respectively. Blue ball: Ba; green ball: Cl; red ball: O; and cyan ball: B.
The refining calculation produces lattice parameters of a = 11.6287 Å, b = 11.5783 Å, c = 6.6817 Å. And they are slightly smaller than a = 11.6359 Å, b = 11.5831 Å, c = 6.6823 Å reported for the blank crystal [41]. It has also been noticed that, when bismuth content increases to 5% from 0.1%, the cell parameters further shrink to a = 11.6191 Å, b = 11.5711 Å, c = 6.6762 Å. This is possibly due to the successful substitution of smaller bismuth ions, such as Bi3+ or Bi2+, for larger Ba ions.
3.3 Spectroscopic properties of Ba2B5O9Cl:Bi
When monitoring the emission at 1055 nm, similar excitation spectrum can be achieved for Ba2(1-x)B5O9Cl: 2x%Bi. As exemplarily shown as curve 1 of Fig. 3(a) , it comprises four well separated group peaks at 298 nm, 478 nm, 665 nm and 850 nm, respectively. At the lower energy sides of the peaks at 298 nm or 478 nm, a shoulder band at ~378 and ~534 nm can be traced, respectively. When excited with these wavelengths, emission spectra show notable dependence. For instance, upon excitations at 298 nm, 378 nm, 478 nm, 534 nm and 665 nm, the emission peak shifts correspondingly to 1030 nm, 1061 nm, 1055 nm, 1061 nm and 1041 nm, respectively. The FWHM (full width at half maximum) of these emissions is larger than 200 nm. The excitation spectrum of the emission at 1061 nm is almost same as the emission at 1055 nm. The excitation spectra of the emissions at 1030 and 1041 nm are same, and they are very similar to that of the 1055 nm except the relative intensity of the strongest peak at 298 nm (see curve 2 of Fig. 3(a)). The spectroscopic data shows that there are at least two emission centers in the bismuth doped compound, which correspond the emissions at 1030 and 1061 nm, respectively. The emissions at 1041 and 1055 nm can be raised up by the different contributions from the two emission centers at 1030 and 1061 nm, since they share absorptions almost in the same spectral range as we can see from Fig. 3(a). The two kinds of emission centers, due to the substitution of bismuth for Ba(2) and Ba(1), can also be confirmed by the decay curves of the emissions at 1030 and 1061 nm as illustrated in Fig. 4 . And they comply well with single exponential decay equation, fitting to which produces a lifetime of 30.19 and 35.92 μs. The lifetime is shorter for the emission at 1030 nm.
Fig. 3 (a) Excitation (curve 1: λem = 1055 nm; 2: λem = 1030 nm) and emission spectra (1: λex = 298 nm; 2: λex = 378 nm; 3: λex = 478 nm; 4: λex = 534 nm; 5: λex = 665 nm) of Ba2(1-x)B5O9Cl: 2x%Bi (x = 0.7).
Fig. 4 Decay and fit (with simple exponential decay equation) curves of Ba2(1-x)B5O9Cl: 2x%Bi (x = 0.7): (1) and (2) for the case of the emission at 1061 nm upon 478 nm excitation, and (3) and (4) for the emission at 1030 nm upon the excitation of 298 nm. Inset shows the dependence of lifetime on the bismuth content x%.
The intensity of NIR emission depends on bismuth content (see Fig. 5 ). When x increases from 0.1% to 0.3%, the emission is enhanced more than eight times. The enhancement persists until x = 1.5% and afterwards the emission begins to decrease. Contrary to the intensity, the lifetimes of the emissions at 1030 and 1061 nm show no clear dependence on the content of bismuth (see inset of Fig. 4). When x varies the lifetime of the 1030 nm emission slightly changes between 29.63 and 31.16 μs, and that of the emission at 1061 nm varies in 33.21-36.88 μs.
Fig. 5 Emission spectra of Ba2(1-x)B5O9Cl: x%Bi (x = 0.1, 0.3, 0.7, 1.0, 1.5, 3.0, 5.0) upon excitation of 478 nm.
3.4 Origin of NIR emission from Ba2B5O9Cl:Bi
When treating Ba2B5O9Cl:Bi alternatively in air and N2/H2, bismuth NIR and red emission centers can be removed and restored reversibly, as shown clearly by Fig. 6 . As the treatment is prolonged in N2/H2 (see Fig. 1), the NIR emission increases monotonically at consumption of red emission of Bi2+ (Note: no meaningful emission can be detected from Bi3+ within instrument limit. And lifetime of Bi2+ emission is 8.60 μs). This evidences the valence conversion of bismuth from + 2 to even lower state. Since the NIR emission apparates with the conversion, it should correlate to the lower valent bismuth. Previous studies have confirmed that solely Bi2+ or Bi3+ doped samples [30,43,44] do not exhibit any NIR emission, so in view of the possibility to substitute for barium sites in the compound, Bi+ and Bi0 are primarily considered as the candidates.
Fig. 6 Reversely removing and recovering NIR (-●-, λex = 478 nm) and red (-●-, λex = 273 nm) emission centers of bismuth by repeatable treating Ba2(1-x)B5O9Cl: 2x%Bi (x = 0.3) in air and H2 in turn. The emission was excited by 478 nm.
Spectroscopic data are rather scarce on Bi+ and Bi0 ions [45–47]. In 1967, Bjerrum et al reported Bi+ in molten salts of AlCl3-NaCl at 130° C and found five absorptions at 308, 333, 585, 658, 694 and 901 nm with oscillator strengths of 1.5 × 10−4, 0.3 × 10−4, 37 × 10−4, 5 × 10−4, 4.5 × 10−4, and 1.5 × 10−4, respectively [45]. The peak at 585 nm is the strongest, and the peaks at 658 and 694 nm are the second and the third. So usually these three peaks are observed in the absorption spectrum, for instance, only absorptions at 585, 657 and 694 nm were observed in Bi+ benzene solution at room temperature [47]. Sometimes the peak at 694 nm immerges into the 658 nm peak, for example, only absorption peaks at ~580 and 660 nm were recently found from Bi+ in bismuth doped chloride glass at room temperature [34]. In the meantime, it can be noticed that the absorption of Bi+ is very weak in UV range, and it is even absent in blue spectral range [45,46]. Bjerrum et al assigned 308 and 333 nm to 3P0→1D2, 585, 658 and 694 nm to 3P0→3P2, and 901 nm to 3P0→3P1 [45,46], and they did not study the luminescence from Bi+. Romanov et al recently found an NIR luminescence at 1060 nm with a lifetime of 266 μs from Bi+ in bismuth doped chloride glass at room temperature [34]. Exact same emission peak was observed in Bi+ containing ionic liquid [10]. Okhrimchuk et al reported an NIR luminescence peaking at 1080 nm with a lifetime of 140 μs from Bi+ doped RbPb2Cl5:Bi at room temperature [24]. These works show that the temperature and the medium in which Bi+ can exist can influence the absorption and emission peak positions, but not in a significant way.
Comparing spectroscopic data of Bi+ to the case of Ba2B5O9Cl:Bi, though the emission peak location is similar, we can find they are really different. The differences are:
- (a) The feature of excitation spectrum:
1) most of the absorption peaks of Ba2B5O9Cl:Bi cannot fit well those of Bi+ as reported by Bjerrum et al [45,46].
2) though the absorptions at 298 nm and 665 nm in Ba2B5O9Cl:Bi are close to 308 nm and 658 nm of Bi+, their intensity ratio is so different [45,46]. And typical strongest absorption of Bi+ around ~585 nm doesn’t exist in Ba2B5O9Cl:Bi [45].
3) As Bjerrum et al noticed, the absorption of Bi+ is very weak in UV and it is absent in blue range [45]; while in Ba2B5O9Cl:Bi, the absorptions at 298 nm and 478 nm dominate the spectrum as you can see from Fig. 3.
Radhakrishna and Setty reported strong Bi0 absorption lying at 290 nm for NaCl:Bi0, 295 nm for KCl:Bi0, 300 nm for KBr:Bi0, 305 nm for KI:Bi0, and 300 nm for RbCl:Bi0 [49]. It coincides with the absorption at 298 nm of Ba2B5O9Cl:Bi. Boundybey et al observed the absorptions of Bi0 in neon matrix peaking at 21825 cm−1 (458 nm), 15519 cm−1 (644 nm), and 11480 cm−1 (871 nm) [50]. And these absorptions agree well with 478 nm, 665 nm and 850 nm of Ba2B5O9Cl:Bi. Additionally, sample color can change from nearly colorless to light reddish when the ratio of bismuth to matrix was fine-tuned between 1/500 and 1/2500 [51]. These resemblances encourage us to consider Bi0 as the possible NIR emission centers in Ba2B5O9Cl:Bi. The comparison in Table 1 illustrates the well match between the energy levels of free Bi0 [52] and Ba2B5O9Cl:Bi. Slight discrepancy between them is probably due to the change of crystal field around Bi0 ion when it is embedded into the crystal compound of Ba2B5O9Cl. Therefore, the absorptions at 298 (378) nm, 478 (534) nm, 665 nm and 850 nm can be attributed to the transitions from 4S3/20 to (2P3/20, 4P1/2), 2P1/20, 2D5/20 and 2D3/20, respectively, and the NIR emission is to 2D3/20→4S3/20. The peaks at 378 nm and 534 nm are possibly the sidebands of the 298 nm and 478 nm band due to the enhanced electron-vibration interaction in the compound. The interaction also promotes the orbital admixing of higher energy states such as 4P, 2P or 2D into the state of 4S3/20, and it thereby lifts the theoretically forbidden transitions between 4S3/20 and 2D3/20 or 2D5/20. Considerable absorptions can then be observed at these transitions. This, at the same time, leads to the significant shortening of the luminescence lifetime corresponding to the transition of 2D3/20→4S3/20, from free state 690 μs to now ~30 μs in Ba2B5O9Cl:Bi. The stabilization of Bi0 species in the compound is enabled by the desirable size match between Bi0 (1.60 Å) and Ba2+ (1.61 Å) [30].Infrared luminescence of bismuth is rather complex. For instance, α-BaB2O4:Bi annealed in H2 shows an emission at 985 nm [28], CsI:Bi exhibits two emissions at 1216 nm and 1560 nm [29], Bi doped SiO2 glass shows an emission at 1430nm [12], and Bi5(AlCl4)3 even luminesces in 1000 to 4000 nm at room temperature [33]. This, in other content, increases the difficulty to determine the exact luminescence nature. In our opinion, it is physically improper to interpret all the complicated infrared luminescence properties of bismuth with only a single type of bismuth ions such as either Bi0 or Bi+. More precisely, the infrared luminescence in different spectral ranges doesn’t need necessarily to share same nature, and it may come from different valent bismuth ions, for instance, Bi0 or Bi+ responsible for near infrared luminescence while bismuth polycation such as Bi53+ for mid infrared luminescence. In similar spectral range, such as ~1060 nm, assignment of the luminescence should be paid more attentions to Bi0 or Bi+. For example, after comparison of absorptions, emission and lifetime with Bi0 or Bi+, the emission at 1060 nm in bismuth doped chloride glass should be due to Bi+ rather than Bi0 [34].
Table 1. Comparing energy levels of Bi0 [52] with Ba2B5O9Cl:Bi
4. Conclusions
In all, we have reported here a novel type of bismuth doped crystals Ba2B5O9Cl:Bi and confirmed the NIR luminescence at least in the compound from bismuth atom. The NIR emissive species, different from those previously reported in bismuth doped glasses and crystals, can be bleached and recovered reversely when the sample was treated in air and N2/H2 in turn. Static and kinetic spectroscopic analyses as well as crystal structure refining have implied two types of Bi0 centers in Ba2B5O9Cl:Bi due to the successful substitution for barium sites. As a consequence of Bi0 doping into the compound of Ba2B5O9Cl, the lifetime was shortened to only ~30 μs from 690 μs of free state Bi0. This work deepens our understanding the complex nature of bismuth NIR luminescence.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51072060, 51132004, 51102096, 11174085), Fundamental Research Funds for the Central Universities (Grant No. 2011ZZ0001), Guangdong Natural Science Foundation (Grant No. S2011030001349), SRF for ROCS SEM, Guangdong Undergraduate Innovative Research and Training Program (Grant No. 1056111036) of SCUT, Fok Ying Tong Education Foundation (Grant No. 132004) and Chinese Program for New Century Excellent Talents in University (Grant No. NCET-11-0158).
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