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On the origin of near-IR luminescence in SiO2 glass with bismuth as the single dopant. Formation of the photoluminescent univalent bismuth silanolate by SiO2 surface modification
Author’s Accepted Manuscript On the origin of near-IR luminescence in SiO2 glass with bismuth as the single dopant. Formation of the photoluminescent univalent bismuth silanolate by SiO2 surface modification A.N. Romanov, E.V. Haula, D.P. Shashkin, D.N. Vtyurina, V.N. Korchak
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To appear in: Journal of Luminescence Received date: 29 November 2015 Revised date: 14 October 2016 Accepted date: 18 November 2016 Cite this article as: A.N. Romanov, E.V. Haula, D.P. Shashkin, D.N. Vtyurina and V.N. Korchak, On the origin of near-IR luminescence in SiO 2 glass with bismuth as the single dopant. Formation of the photoluminescent univalent bismuth silanolate by SiO2 surface modification, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.11.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
On the origin of near-IR luminescence in SiO2 glass with bismuth as the single dopant. Formation of the photoluminescent univalent bismuth silanolate by SiO2 surface modification. A.N. Romanov*, E.V. Haula, D.P. Shashkin, D.N. Vtyurina, V.N. Korchak
N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 4 Kosygina Str., 119991 Moscow, Russia *Corresponding author, e-mail: [email protected]; Phone: +7(905)5970866
Abstract Near infrared photoluminescent bismuth(I) silanolate centers ((≡Si-O)3Si–O-Bi) were prepared on the surface of SiO2 xerogel, by the treatment in the vapors of bismuth(I) chloride. The optical properties of these groups are almost identical to that of photoluminescent centers in the bulk SiO2 glasses with bismuth as the single dopant. Keywords: Photoluminescence; Univalent bismuth; Surface silanol
Highlights: ● univalent bismuth silanolate can be prepared on SiO2 surface by treatment in BiCl vapors. ● univalent bismuth silanolate is responsible for NIR photoluminescence in Bi-doped SiO2 glass. ● univalent bismuth silanolate is the active center in laser, operating on Bi-doped SiO2 fiber.
Introduction Bismuth doped glasses and crystals demonstrate the interesting broadband photoluminescence (PL) in the near infrared (NIR) [1-3]. The characteristics of this luminescence depend on the respective composition and it seems that distinct bismuth containing species participate in the emission from the different materials [3-5]. Previously, we had investigated the NIR PL from Bi-doped leucite and pollucite aluminosilicates and demonstrated, that Bi+ monocation is the source of NIR emission at 1100-1300 nm [6]. The same Bi+ interstitial monocation, charge-balanced by the nearby [AlO4/2]- tetrahedral aluminium unit, seems to be the
NIR emitter at the similar wavelengths in bismuth and aluminium codoped SiO2 glasses at low bismuth concentrations. Contrarily to these materials, the SiO2 glasses, with bismuth as the single dopant display the very distinct NIR photoluminescence with the emission maxima at 830 and 1430 nm [5,714]. The respective photoluminescence excitation spectra display maxima at 1420, 820, 420, 370 and 240 nm (emission at 1430 nm) or 820 and 420 nm (emission at 830 nm) [11,14]. The detailed discussion on the spectral properties of this luminescent center can be found elsewhere [8-14]. The similar NIR PL was also observed in SiOxNy siliconoxynitride glass films [15]. The origin of the PL remains elusive, although the observed emission from SiO2 glasses with very low bismuth doping level seems to rule out the cluster bismuth species as the possible luminescent centers. Recently, as a result of quantum chemical simulations, Sokolov et al. postulated the Bi··· ≡Si–Si≡ structure for PL bismuth-containing center in SiO2 glasses [14]. Yet, it should be noted that simulation results can not be considered as a proof for any structural hypothesis because of the possible spurious coincidence with experimental data, and nonexhaustive search in the space of all possible PL structures. That’s why it is highly desirable to provide a more reasonable ground for the hypothesis on the structure of bismuth-related NIR PL center in SiO2. Assuming, that interstitial Bi+ is the NIR emitter in the aluminosilicate glasses, it is instructive to find the examples of univalent cations, resembling Bi+, in similar environment. The closest approach to Bi+ is, certainly, the univalent thallium Tl+, with the roughly equal value of crystal ionic radius (1.84 Å for Tl+ vs. 1.9 Å for Bi+, coordination number 12) and (probably) similar polarizability. Alexander et al. studied the structure of Tl+-related photoluminescent centers in sodium aluminosilicate glasses [16,17]. They had concluded, that in glasses with aluminium excess (Al/Na > 1) monocation Tl+ exist as an interstitial charge compensator for the tetrahedral aluminium [AlO4/2]- units. In glasses with Al/Na < 1 all the charges of tetrahedral aluminium units can be compensated by Na+ ions and some Tl+ exist as network modifier, bonded to the non-bridging oxygen anionic unit (≡Si-O)3Si– O-. In parallel with the Tl+-containing glasses, the Cu+-related photoluminescent centers exist as charge compensator for [AlO4/2]- in Al2O3-rich SiO2, while they are also bound to (≡Si-O)3Si–Oanions in aluminium-poor SiO2 [18]. Also, it was shown, that univalent ions (Me+ = Li+, Na+, K+) in alkali silicate glasses exist in the form of the so called L-centers (≡Si-O)3Si–O- Me+ [19]. Therefore, in Al2O3-rich silicate compositions all the monocations Bi+, Tl+, Cu+ act as the interstitial charge compensator for [AlO4/2]- unit, while in the Al2O3-poor glasses or pure SiO2 the ions Tl+, Cu+ and alkali monocations are bonded to negatively charged non-bridging oxygen. Although the structure of bismuth-related PL center in pure SiO2 is still unknown it is
reasonable to suggest, that in parallel with Tl+, Cu+ and alkali monocations it consists of Bi+, charge compensated by (≡Si-O)3Si–O- anions. The properties of such hypothetical center should be rather different from ones of Bi+ in aluminosilicate glasses and crystals [20]. In the aluminosilicate glass the charge of interstitial Bi+ is neutralized by tetrahedral [AlO4/2]- with electron density, distributed over several adjacent oxygens. With this diffuse countercharge the Bi+ monocation can be regarded as mainly ionic. The non-bridging oxygen anionic unit (≡Si-O)3Si–O- have all the negative charge concentrated at one oxygen and its complex with Bi+ should have a high degree of covalence, like the similar known univalent bismuth molecules: BiF [21], BiCl [22,23], BiBr [24,25], BiI [26,27] and BiOH [28-30]. In analogous way, the structure of such hypothetical NIR PL center in SiO2 can be better represented by the molecular fragment (≡Si-O)3Si–O-Bi. From the point of view of a chemist it is the univalent bismuth silanolate with the free bonds at silicon, connected to the bridging oxygens from the rest of the glass network. Of all the known molecules, containing univalent bismuth, BiOH seems to be the most appropriate model for the proposed (≡Si-O)3Si– O-Bi pseudomolecule center, since the electronegativities of H and Si are similar and the nonlinear BiOH may reproduce well the spectroscopic effects from the bent Si–O-Bi motif. Fig. 1 represents the comparison of energy level diagrams for BiOH (based on quantum chemical simulation [29] and experimental results [28,30]) and bismuth-related photoluminescent center in SiO2 (based on the NIR PL emission and excitation spectra [11]). The similarity in energies is evident and support greatly the hypothesis of univalent bismuth silanolate (≡Si-O)3Si–O-Bi pseudomolecule as the emitting center in SiO2 glass with bismuth as the single dopant. Although the above mentioned consideration looks reasonable it is advisable to find experimental evidences for the participation of (≡Si-O)3Si–O-Bi centers in NIR PL at 830 and 1430 nm, typical for SiO2 glass with bismuth as the single dopant. The most straightforward way is to prepare (≡Si-O)3Si–O-Bi pseudomolecule fragments by some well established selective reaction and explore its spectral properties. It is known, that SiO2 network terminates at the surface by silanol groups (≡Si-O)3Si–O-H and these surface silanols can be functionalized in many ways [31]. For example, the hydrogens can be exchanged for some metals by the reaction with the respective metal halides in liquid or gas phase. It is possible to prepare the univalent bismuth halides in the gas phase at elevated temperatures (in equilibrium with the vapors of Bi3+ halide)[21-27]: 2Bi + BiX3 ↔ 3BiX; X = F, Cl, Br, I (1) So, the surface univalent bismuth silanolate can be prepared by the reaction of silanols with the vapors of BiX: (≡Si-O)3Si–O-H + BiX ↔ (≡Si-O)3Si–O-Bi + HX; X = F, Cl, Br, I (2)
This surface modification should proceed at rather mild conditions (400-800˚C) and the sufficient quantity of the surface univalent bismuth silanolates can be prepared in the silica samples with the high specific surface area values. The preparation of silica, bearing the surface univalent bismuth silanolate groups and characterization of NIR PL from such specimens (in relation to NIR PL from bulk bismuth-doped SiO2) are the main goals of this work.
Materials and methods The SiO2 xerogel samples, used in the surface modification experiments were produced by the acid hydrolysis of tetraethoxysilane (TEOS). TEOS (25 см3) was vigorously stirred with the mixture of water (25 cm3) and concentrated nitric acid. The initially biphasic mixture quickly turns homogeneous and the heat evolution is noticeable during the reaction progress. After 0.5 h of the additional agitation the liquid was poured into the Petri dishes and allowed to stand at 60˚C for gelation. The gelation time depends on the amount of nitric acid, used in the preparation and varies from 0.5 to 48 h. The resulted gel was dried slowly (for up to several days) to produce the thin pieces of SiO2 xerogel. The xerogel samples from preparations with large amount of nitric acid (6 cm3 for 25 cm3 of H2O) appears opaque (specimen 1), while the low nitric acid concentrations (0.6 cm3 for 25 cm3 of H2O) gives completely clear specimens (specimen 2). The specific surface area of the specimens was determined by BET method and it turns to be the similar for all samples (377 m2/g for the specimen 1 vs. 387 m2/g for the specimen 2). To eliminate the adsorbed water and organics the xerogel samples was heated to 700˚C for several hours in the air, than it was placed in fused silica container (221 cm3 inner volume) and heated additionally to 300˚C for 24 h in vacuum. Then, after the container cooling, BiCl3 (24 mg) and Bi metal (in excess) was also added to the same reactor in such a way, that they do not form the direct contact with the xerogel pieces. The container was evacuated and sealed. The surface modification of xerogel samples was attained by the uniform heating of the container to 500 or 700˚C for 4 hours. At such temperatures the BiCl3 vaporizes and its vapors react with Bi metal to form the equilibrium mixture of BiCl and BiCl3 via reaction (1). Starting from the known volume of the container and the initial amount of BiCl3, the partial pressures of BiCl3 and BiCl inside the container can be calculated, using the equilibrium constants [22]. The respective values are presented in the Table 1 for different temperatures. After the thermal treatment, the container had cooled to the room temperature. The part of the reactor, distant from the xerogel samples, was cooled firstly, to prevent the condensation of BiCl3 and the dark products of BiCl disproportionation on the studied samples. NIR PL spectra (λ>1000 nm) were obtained with SDH-IV spectrometer (Solar LS), equipped with G9212-512 InGaAs linear image sensor (Hamamatsu). Spectra of the emission
with λ<1000 nm was recorded with Ocean Optics USB4000 spectrometer. Laser modules with the emission wavelengths of 405, 445, 532, 635, 660, 690, 800 and 820 nm were used for the excitation of NIR PL. The 445 nm laser diode, modulated by pulse train with low duty cycle, was used as an excitation source for the time-resolved studies of NIR PL. The duration of excitation pulse was 1 ms and the decaying of PL signal after the each pulse was detected by InGaAs photodiode module with the home-build preamplifier. The output signal was digitized and averaged (over several thousands pulses) by Picoscope 5242A oscilloscope. The response time of the system was 0.5 μs. The IKS-7 optical filter was placed before the photodiode, so the spectral response for the timeresolved measurements (1020-1660 nm) can be obtained as the product of filter transmission and detector sensitivity functions. No other emissions, besides the photoluminescence of interest, were observed from our specimens in this spectral range. The elemental analysis of the surface modified xerogel samples was performed on Varian 820 ICP MS spectrometer. The probes for ICP MS were prepared by xerogel dissolution in diluted HNO3 and HF.
Results and discussion The NIR PL with maxima at 1450 nm and 830 nm was detected from the specimens 1 and 2, modified with bismuth chloride vapors at 700˚C (Fig. 2), while the modification at 500˚C afforded no photoluminescence. The NIR PL spectra for the both specimens (1 and 2) are identical. The photoluminescence near 1450 nm can be excited by the light with λex=405, 445, 800, 820 nm, while the luminescence at 830 nm can be observed under excitation at λex=405, 445 nm. FWHM values are 35 and 74 nm for 830 and 1450 nm emission bands respectively. Excitation at 532, 635, 660 and 690 nm afforded no detectable NIR PL. It is evident, that the emission maxima and its excitation profiles matches well the corresponding properties of NIR PL centers in SiO2 glass with bismuth as the single dopant [7-13]. The time resolved study of NIR PL after pulse excitation reveals biexponential decay process with characteristic times of 221 and 927 μs (Fig. 3). These values are of the same order with those of the NIR PL centers in the bulk bismuth-doped SiO2 glasses and biexponential photoluminescence decay was also observed for the bulk specimens [32]. So, the temporal characteristics of NIR PL from SiO2 xerogel, modified with bismuth chloride vapors are also similar to the corresponding values for the bismuth-doped SiO2 glass. The temperature of NIR PL centers formation in our experiments is well below the onset of SiO2 network rebuilding, so, all the produced NIR PL centers should be on the surface. Formation of NIR PL centers by surface modification at 700˚C and its absence after the analogous procedure at 500˚C is well understood if we assume that only univalent bismuth
chloride (which has sufficiently larger partial pressure at 700˚C, see Table 1) produces the NIR PL centers at the surface of SiO2 via reaction 2 (X=Cl). The partial pressure of BiCl3 (which is also present in the gas phase) is comparable at 500 and 700˚C, so it’s participation in the formation of NIR PL centers can be ruled out. All these facts demonstrate that surface center is indeed the source of NIR PL in our experiments. The near identity of the prepared surface centers (≡Si-O)3Si–O-Bi with the respective centers in bulk Bi-doped SiO2 glass signify that, as expected above, the univalent bismuth silanolate pseudomolecule (≡Si-O)3Si–O-Bi is the source of NIR PL from the bulk SiO2 glass with bismuth as the single dopant and it can be the lasing group in bismuth-doped fiber laser at 1430 nm [10]. Focusing on the diagram of the excited states of closely related BiOH molecule (as the simplified model for (≡Si-O)3Si–O-Bi group) we can see, that according to quantum chemical calculations its lowest excited state was predicted to be the quasidegenerate doublet, with energy gap of only 24 cm-1 (Fig. 1). In accordance with this fact, the recent Magnetic Circularly Polarized Luminescence (MCPL) studies of luminescent centers in bismuth doped SiO2 glass reveal, that its lowest excited state is an isolated non-Kramers doublet of the even-electron system with 6.18 cm-1 zero-field splitting [33]. The variation of energy gap value can be the result of difference in the molecules structures (BiOH vs. (≡Si-O)3Si–O-Bi pseudomolecule). Comparing the MCPL and NIR PL spectra of Bi-doped SiO2 glass at 1440 nm authors noticed the shift of MCPL spectrum relative to the NIR PL one, and this shift is attributed to the presence of A -term in MCPL spectrum due to the quasidegeneracy in the ground state. In pseudomolecule (≡Si-O)3Si–O-Bi the ground state is nondegenerate an this fact seems to contradict the results of MCPL experiments. Yet, it should be noted, that the shift observed in the MCPL spectra [33] can be originated not from the A-term, but also from the effects of inhomogeneous broadening in glass matrix or non-Boltzmann population of emissive quasidegenerate doublet sublevels. It is clear from the Fig. 2, that the dependency of PL bands shape on the excitation wavelength for (≡Si-O)3Si–O-Bi pseudomolecule is not as prominent as for the Bi+ centers in aluminosilicate glasses and crystal phases [6,11]. This can be rationalized by the fact, that electronic transitions in (≡Si-O)3Si–O-Bi center involve the localized molecular orbitals. The geometry of this pseudomolecule only weakly perturbed by the surrounding disordered media, resulted in the relatively feeble inhomogeneous broadening of the emission bands. In other words, the electrostatic crystal field of the disordered glass network disturbs the energy levels of charged monocation Bi+ (in aluminosilicates) to a much greater extent in comparison with the electronic terms of the neutral (≡Si-O)3Si–O-Bi pseudomolecule.
It is known, that the silanol groups on the surface of SiO2 can be either isolated, geminal or vicinal, so the different types of bismuth silanolate center can be produced by BiCl vapor modification (Fig. 4). Although it is too early to make any conclusions, the isolated bismuth (I) silanolate (case a) in Fig. 4) should be considered as the probable NIR PL emitter. Firstly, the thermal pretreatment of SiO2 xerogel diminishes the number of vicinal and geminal silanols to a large extent. And secondly, an efficient non-radiative quenching of the excited states of geminal and vicinal bismuth (I) silanolates can be resulted from the presence of nearby hydrogen atom. Considering the mechanism of the surface NIR PL centers formation, besides the direct functionalization of the surface silanols, we should mention here another possible route. It is the strained siloxane bonds cleavage by BiCl (case d) in Fig 4). Elemental analysis provides the additional information on the nature of the surface bismuth species. In fact, it gives 0.206±0.022 at.% of Bi and 0.089±0.01 at.% of chlorine content in the analyzed xerogel samples. The presence of the substantial chlorine amount in xerogel samples demonstrates the possible involvement of the surface functionalization by the strained siloxane cleavage reaction (case d) in Fig 4). On the other hand, the concentration of chlorine in our samples is more than twice lower, than that of bismuth. That is why, the direct functionalization of the surface silanols (for example, case a) in Fig 4) can also be considered as the source of bismuth-containing surface centers. The calculated coverage of SiO2 surface by bismuth-bearing groups is 0.053 Bi atoms per nm2. It is significantly less than the surface density of silanol groups, which is 1.1 groups per nm2 at 700°C [34]. So, the silanol groups’ consumption via the surface modification reactions should be low. The 2-nd overtone band of OH vibration in the non-reacted silanol groups is noticeable as the sharp optical absorption at 1365 nm, superimposed on the broad NIR PL spectrum (Fig 2). Recently, the participation of oxygen deficient centers (ODC) in the formation of bismuth-related NIR PL centers was suggested [14, 35]. But, this involvement of ODC can hardly be considered in our experiments. Indeed, the special treatment (for example, silanol methoxylation with subsequent vacuum pyrolysis or high-energy mechanical activation) is needed to produce the SiO2 with the noticeable concentration of surface ODC (in order of 10-2 – 10-3 groups per nm2) – the so called reactive silica [36]. The processes of such kind can not take place throughout SiO2 surface modification by bismuth halide vapors. All the possible processes of NIR PL centers formation can be examined in future by the numerous techniques of surface species characterization. Also, the unexplored reactivity of the surface univalent bismuth silanolates can be studied with an attention to its PL properties. So, the formation of surface bismuth (I) silanolate can be viewed as an alternative mild technology of NIR PL centers preparation and also as a flexible tool for its investigation.
Conclusions The univalent bismuth compounds display diverse photoluminescence properties. In aluminosilicate glasses and crystal hosts, the monocation Bi+, emits in 1100-1300 nm range, while the covalently bound bismuth (I) silanolate pseudomolecule ((≡Si-O)3Si–O-Bi) is characterized by the distinct photoluminescence spectrum with the maximum at 1450 nm. These bismuth (I) silanolates are responsible for NIR PL of the bulk SiO2 glasses, with bismuth as the single dopant.
Acknowledgments This work was supported by Russian Science Foundation (RSF, research project number 15-13-30034). The authors are also thankful to V.A. Radzig for the valuable discussion.
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Table 1. Equilibrium partial pressures of BiCl and BiCl3 vapors in reaction cell (cell volume Vcell=221 cm3, initial BiCl3 amount mBiCl3=24 mg). Equilibrium constants were taken from [22]. Temperature (˚С)
500
600
700
Partial pressure
P(BiCl)
0.403
2.46
10.41
(Torr)
P(BiCl3)
16.43
17.42
17.04
Figure captions
Fig.1 Comparison of the energy level diagrams for BiOH molecule (left) and bismuth-related PL center in SiO2 glass (right). Diagram for BiOH molecule based mainly on the experimental results [28,30]. The positions of the “dark” non-emissive states (shown by dash lines) were inferred from the quantum chemical calculations [29]. Positions of energy levels for bismuthrelated PL center in SiO2 glass were taken from [11]. Fig.2 Photoluminescence spectra (bands at 830 and 1450 nm) of BiCl-treated SiO2 xerogel, excited by the laser diodes with the different emission wavelengths. The superimposed absorption at 1365 nm is due to the 2ν SiO-H vibrational band of the remaining surface silanols. The rise of PL signal at λ<775 nm is due to the presence of photoluminescence band with maxima at ~ 600 nm. This band of unknown origin was also observed in the bulk Bi-doped SiO2 glasses [8,11].
Fig.3 Temporal decay of NIR PL from BiCl-treated SiO2 xerogel sample (excitation wavelength is 445 nm, interval of PL registration is 1350-1550 nm), fitted by biexponential curve. Fitting parameters (time constants and amplitudes) are also shown.
Fig.4 Possible routes to the surface bismuth-related NIR PL centers. Reactions of different surface groups with bismuth monocloride (BiCl): a) isolated silanol; b) vicinal silanol; c) geminal silanol; d) insertion of BiCl into the strained surface-exposed siloxane bond.
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S0022-2313(15)30764-X http://dx.doi.org/10.1016/j.jlumin.2016.11.045 LUMIN14383
To appear in: Journal of Luminescence Received date: 29 November 2015 Revised date: 14 October 2016 Accepted date: 18 November 2016 Cite this article as: A.N. Romanov, E.V. Haula, D.P. Shashkin, D.N. Vtyurina and V.N. Korchak, On the origin of near-IR luminescence in SiO 2 glass with bismuth as the single dopant. Formation of the photoluminescent univalent bismuth silanolate by SiO2 surface modification, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.11.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
On the origin of near-IR luminescence in SiO2 glass with bismuth as the single dopant. Formation of the photoluminescent univalent bismuth silanolate by SiO2 surface modification. A.N. Romanov*, E.V. Haula, D.P. Shashkin, D.N. Vtyurina, V.N. Korchak
N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 4 Kosygina Str., 119991 Moscow, Russia *Corresponding author, e-mail: [email protected]; Phone: +7(905)5970866
Abstract Near infrared photoluminescent bismuth(I) silanolate centers ((≡Si-O)3Si–O-Bi) were prepared on the surface of SiO2 xerogel, by the treatment in the vapors of bismuth(I) chloride. The optical properties of these groups are almost identical to that of photoluminescent centers in the bulk SiO2 glasses with bismuth as the single dopant. Keywords: Photoluminescence; Univalent bismuth; Surface silanol
Highlights: ● univalent bismuth silanolate can be prepared on SiO2 surface by treatment in BiCl vapors. ● univalent bismuth silanolate is responsible for NIR photoluminescence in Bi-doped SiO2 glass. ● univalent bismuth silanolate is the active center in laser, operating on Bi-doped SiO2 fiber.
Introduction Bismuth doped glasses and crystals demonstrate the interesting broadband photoluminescence (PL) in the near infrared (NIR) [1-3]. The characteristics of this luminescence depend on the respective composition and it seems that distinct bismuth containing species participate in the emission from the different materials [3-5]. Previously, we had investigated the NIR PL from Bi-doped leucite and pollucite aluminosilicates and demonstrated, that Bi+ monocation is the source of NIR emission at 1100-1300 nm [6]. The same Bi+ interstitial monocation, charge-balanced by the nearby [AlO4/2]- tetrahedral aluminium unit, seems to be the
NIR emitter at the similar wavelengths in bismuth and aluminium codoped SiO2 glasses at low bismuth concentrations. Contrarily to these materials, the SiO2 glasses, with bismuth as the single dopant display the very distinct NIR photoluminescence with the emission maxima at 830 and 1430 nm [5,714]. The respective photoluminescence excitation spectra display maxima at 1420, 820, 420, 370 and 240 nm (emission at 1430 nm) or 820 and 420 nm (emission at 830 nm) [11,14]. The detailed discussion on the spectral properties of this luminescent center can be found elsewhere [8-14]. The similar NIR PL was also observed in SiOxNy siliconoxynitride glass films [15]. The origin of the PL remains elusive, although the observed emission from SiO2 glasses with very low bismuth doping level seems to rule out the cluster bismuth species as the possible luminescent centers. Recently, as a result of quantum chemical simulations, Sokolov et al. postulated the Bi··· ≡Si–Si≡ structure for PL bismuth-containing center in SiO2 glasses [14]. Yet, it should be noted that simulation results can not be considered as a proof for any structural hypothesis because of the possible spurious coincidence with experimental data, and nonexhaustive search in the space of all possible PL structures. That’s why it is highly desirable to provide a more reasonable ground for the hypothesis on the structure of bismuth-related NIR PL center in SiO2. Assuming, that interstitial Bi+ is the NIR emitter in the aluminosilicate glasses, it is instructive to find the examples of univalent cations, resembling Bi+, in similar environment. The closest approach to Bi+ is, certainly, the univalent thallium Tl+, with the roughly equal value of crystal ionic radius (1.84 Å for Tl+ vs. 1.9 Å for Bi+, coordination number 12) and (probably) similar polarizability. Alexander et al. studied the structure of Tl+-related photoluminescent centers in sodium aluminosilicate glasses [16,17]. They had concluded, that in glasses with aluminium excess (Al/Na > 1) monocation Tl+ exist as an interstitial charge compensator for the tetrahedral aluminium [AlO4/2]- units. In glasses with Al/Na < 1 all the charges of tetrahedral aluminium units can be compensated by Na+ ions and some Tl+ exist as network modifier, bonded to the non-bridging oxygen anionic unit (≡Si-O)3Si– O-. In parallel with the Tl+-containing glasses, the Cu+-related photoluminescent centers exist as charge compensator for [AlO4/2]- in Al2O3-rich SiO2, while they are also bound to (≡Si-O)3Si–Oanions in aluminium-poor SiO2 [18]. Also, it was shown, that univalent ions (Me+ = Li+, Na+, K+) in alkali silicate glasses exist in the form of the so called L-centers (≡Si-O)3Si–O- Me+ [19]. Therefore, in Al2O3-rich silicate compositions all the monocations Bi+, Tl+, Cu+ act as the interstitial charge compensator for [AlO4/2]- unit, while in the Al2O3-poor glasses or pure SiO2 the ions Tl+, Cu+ and alkali monocations are bonded to negatively charged non-bridging oxygen. Although the structure of bismuth-related PL center in pure SiO2 is still unknown it is
reasonable to suggest, that in parallel with Tl+, Cu+ and alkali monocations it consists of Bi+, charge compensated by (≡Si-O)3Si–O- anions. The properties of such hypothetical center should be rather different from ones of Bi+ in aluminosilicate glasses and crystals [20]. In the aluminosilicate glass the charge of interstitial Bi+ is neutralized by tetrahedral [AlO4/2]- with electron density, distributed over several adjacent oxygens. With this diffuse countercharge the Bi+ monocation can be regarded as mainly ionic. The non-bridging oxygen anionic unit (≡Si-O)3Si–O- have all the negative charge concentrated at one oxygen and its complex with Bi+ should have a high degree of covalence, like the similar known univalent bismuth molecules: BiF [21], BiCl [22,23], BiBr [24,25], BiI [26,27] and BiOH [28-30]. In analogous way, the structure of such hypothetical NIR PL center in SiO2 can be better represented by the molecular fragment (≡Si-O)3Si–O-Bi. From the point of view of a chemist it is the univalent bismuth silanolate with the free bonds at silicon, connected to the bridging oxygens from the rest of the glass network. Of all the known molecules, containing univalent bismuth, BiOH seems to be the most appropriate model for the proposed (≡Si-O)3Si– O-Bi pseudomolecule center, since the electronegativities of H and Si are similar and the nonlinear BiOH may reproduce well the spectroscopic effects from the bent Si–O-Bi motif. Fig. 1 represents the comparison of energy level diagrams for BiOH (based on quantum chemical simulation [29] and experimental results [28,30]) and bismuth-related photoluminescent center in SiO2 (based on the NIR PL emission and excitation spectra [11]). The similarity in energies is evident and support greatly the hypothesis of univalent bismuth silanolate (≡Si-O)3Si–O-Bi pseudomolecule as the emitting center in SiO2 glass with bismuth as the single dopant. Although the above mentioned consideration looks reasonable it is advisable to find experimental evidences for the participation of (≡Si-O)3Si–O-Bi centers in NIR PL at 830 and 1430 nm, typical for SiO2 glass with bismuth as the single dopant. The most straightforward way is to prepare (≡Si-O)3Si–O-Bi pseudomolecule fragments by some well established selective reaction and explore its spectral properties. It is known, that SiO2 network terminates at the surface by silanol groups (≡Si-O)3Si–O-H and these surface silanols can be functionalized in many ways [31]. For example, the hydrogens can be exchanged for some metals by the reaction with the respective metal halides in liquid or gas phase. It is possible to prepare the univalent bismuth halides in the gas phase at elevated temperatures (in equilibrium with the vapors of Bi3+ halide)[21-27]: 2Bi + BiX3 ↔ 3BiX; X = F, Cl, Br, I (1) So, the surface univalent bismuth silanolate can be prepared by the reaction of silanols with the vapors of BiX: (≡Si-O)3Si–O-H + BiX ↔ (≡Si-O)3Si–O-Bi + HX; X = F, Cl, Br, I (2)
This surface modification should proceed at rather mild conditions (400-800˚C) and the sufficient quantity of the surface univalent bismuth silanolates can be prepared in the silica samples with the high specific surface area values. The preparation of silica, bearing the surface univalent bismuth silanolate groups and characterization of NIR PL from such specimens (in relation to NIR PL from bulk bismuth-doped SiO2) are the main goals of this work.
Materials and methods The SiO2 xerogel samples, used in the surface modification experiments were produced by the acid hydrolysis of tetraethoxysilane (TEOS). TEOS (25 см3) was vigorously stirred with the mixture of water (25 cm3) and concentrated nitric acid. The initially biphasic mixture quickly turns homogeneous and the heat evolution is noticeable during the reaction progress. After 0.5 h of the additional agitation the liquid was poured into the Petri dishes and allowed to stand at 60˚C for gelation. The gelation time depends on the amount of nitric acid, used in the preparation and varies from 0.5 to 48 h. The resulted gel was dried slowly (for up to several days) to produce the thin pieces of SiO2 xerogel. The xerogel samples from preparations with large amount of nitric acid (6 cm3 for 25 cm3 of H2O) appears opaque (specimen 1), while the low nitric acid concentrations (0.6 cm3 for 25 cm3 of H2O) gives completely clear specimens (specimen 2). The specific surface area of the specimens was determined by BET method and it turns to be the similar for all samples (377 m2/g for the specimen 1 vs. 387 m2/g for the specimen 2). To eliminate the adsorbed water and organics the xerogel samples was heated to 700˚C for several hours in the air, than it was placed in fused silica container (221 cm3 inner volume) and heated additionally to 300˚C for 24 h in vacuum. Then, after the container cooling, BiCl3 (24 mg) and Bi metal (in excess) was also added to the same reactor in such a way, that they do not form the direct contact with the xerogel pieces. The container was evacuated and sealed. The surface modification of xerogel samples was attained by the uniform heating of the container to 500 or 700˚C for 4 hours. At such temperatures the BiCl3 vaporizes and its vapors react with Bi metal to form the equilibrium mixture of BiCl and BiCl3 via reaction (1). Starting from the known volume of the container and the initial amount of BiCl3, the partial pressures of BiCl3 and BiCl inside the container can be calculated, using the equilibrium constants [22]. The respective values are presented in the Table 1 for different temperatures. After the thermal treatment, the container had cooled to the room temperature. The part of the reactor, distant from the xerogel samples, was cooled firstly, to prevent the condensation of BiCl3 and the dark products of BiCl disproportionation on the studied samples. NIR PL spectra (λ>1000 nm) were obtained with SDH-IV spectrometer (Solar LS), equipped with G9212-512 InGaAs linear image sensor (Hamamatsu). Spectra of the emission
with λ<1000 nm was recorded with Ocean Optics USB4000 spectrometer. Laser modules with the emission wavelengths of 405, 445, 532, 635, 660, 690, 800 and 820 nm were used for the excitation of NIR PL. The 445 nm laser diode, modulated by pulse train with low duty cycle, was used as an excitation source for the time-resolved studies of NIR PL. The duration of excitation pulse was 1 ms and the decaying of PL signal after the each pulse was detected by InGaAs photodiode module with the home-build preamplifier. The output signal was digitized and averaged (over several thousands pulses) by Picoscope 5242A oscilloscope. The response time of the system was 0.5 μs. The IKS-7 optical filter was placed before the photodiode, so the spectral response for the timeresolved measurements (1020-1660 nm) can be obtained as the product of filter transmission and detector sensitivity functions. No other emissions, besides the photoluminescence of interest, were observed from our specimens in this spectral range. The elemental analysis of the surface modified xerogel samples was performed on Varian 820 ICP MS spectrometer. The probes for ICP MS were prepared by xerogel dissolution in diluted HNO3 and HF.
Results and discussion The NIR PL with maxima at 1450 nm and 830 nm was detected from the specimens 1 and 2, modified with bismuth chloride vapors at 700˚C (Fig. 2), while the modification at 500˚C afforded no photoluminescence. The NIR PL spectra for the both specimens (1 and 2) are identical. The photoluminescence near 1450 nm can be excited by the light with λex=405, 445, 800, 820 nm, while the luminescence at 830 nm can be observed under excitation at λex=405, 445 nm. FWHM values are 35 and 74 nm for 830 and 1450 nm emission bands respectively. Excitation at 532, 635, 660 and 690 nm afforded no detectable NIR PL. It is evident, that the emission maxima and its excitation profiles matches well the corresponding properties of NIR PL centers in SiO2 glass with bismuth as the single dopant [7-13]. The time resolved study of NIR PL after pulse excitation reveals biexponential decay process with characteristic times of 221 and 927 μs (Fig. 3). These values are of the same order with those of the NIR PL centers in the bulk bismuth-doped SiO2 glasses and biexponential photoluminescence decay was also observed for the bulk specimens [32]. So, the temporal characteristics of NIR PL from SiO2 xerogel, modified with bismuth chloride vapors are also similar to the corresponding values for the bismuth-doped SiO2 glass. The temperature of NIR PL centers formation in our experiments is well below the onset of SiO2 network rebuilding, so, all the produced NIR PL centers should be on the surface. Formation of NIR PL centers by surface modification at 700˚C and its absence after the analogous procedure at 500˚C is well understood if we assume that only univalent bismuth
chloride (which has sufficiently larger partial pressure at 700˚C, see Table 1) produces the NIR PL centers at the surface of SiO2 via reaction 2 (X=Cl). The partial pressure of BiCl3 (which is also present in the gas phase) is comparable at 500 and 700˚C, so it’s participation in the formation of NIR PL centers can be ruled out. All these facts demonstrate that surface center is indeed the source of NIR PL in our experiments. The near identity of the prepared surface centers (≡Si-O)3Si–O-Bi with the respective centers in bulk Bi-doped SiO2 glass signify that, as expected above, the univalent bismuth silanolate pseudomolecule (≡Si-O)3Si–O-Bi is the source of NIR PL from the bulk SiO2 glass with bismuth as the single dopant and it can be the lasing group in bismuth-doped fiber laser at 1430 nm [10]. Focusing on the diagram of the excited states of closely related BiOH molecule (as the simplified model for (≡Si-O)3Si–O-Bi group) we can see, that according to quantum chemical calculations its lowest excited state was predicted to be the quasidegenerate doublet, with energy gap of only 24 cm-1 (Fig. 1). In accordance with this fact, the recent Magnetic Circularly Polarized Luminescence (MCPL) studies of luminescent centers in bismuth doped SiO2 glass reveal, that its lowest excited state is an isolated non-Kramers doublet of the even-electron system with 6.18 cm-1 zero-field splitting [33]. The variation of energy gap value can be the result of difference in the molecules structures (BiOH vs. (≡Si-O)3Si–O-Bi pseudomolecule). Comparing the MCPL and NIR PL spectra of Bi-doped SiO2 glass at 1440 nm authors noticed the shift of MCPL spectrum relative to the NIR PL one, and this shift is attributed to the presence of A -term in MCPL spectrum due to the quasidegeneracy in the ground state. In pseudomolecule (≡Si-O)3Si–O-Bi the ground state is nondegenerate an this fact seems to contradict the results of MCPL experiments. Yet, it should be noted, that the shift observed in the MCPL spectra [33] can be originated not from the A-term, but also from the effects of inhomogeneous broadening in glass matrix or non-Boltzmann population of emissive quasidegenerate doublet sublevels. It is clear from the Fig. 2, that the dependency of PL bands shape on the excitation wavelength for (≡Si-O)3Si–O-Bi pseudomolecule is not as prominent as for the Bi+ centers in aluminosilicate glasses and crystal phases [6,11]. This can be rationalized by the fact, that electronic transitions in (≡Si-O)3Si–O-Bi center involve the localized molecular orbitals. The geometry of this pseudomolecule only weakly perturbed by the surrounding disordered media, resulted in the relatively feeble inhomogeneous broadening of the emission bands. In other words, the electrostatic crystal field of the disordered glass network disturbs the energy levels of charged monocation Bi+ (in aluminosilicates) to a much greater extent in comparison with the electronic terms of the neutral (≡Si-O)3Si–O-Bi pseudomolecule.
It is known, that the silanol groups on the surface of SiO2 can be either isolated, geminal or vicinal, so the different types of bismuth silanolate center can be produced by BiCl vapor modification (Fig. 4). Although it is too early to make any conclusions, the isolated bismuth (I) silanolate (case a) in Fig. 4) should be considered as the probable NIR PL emitter. Firstly, the thermal pretreatment of SiO2 xerogel diminishes the number of vicinal and geminal silanols to a large extent. And secondly, an efficient non-radiative quenching of the excited states of geminal and vicinal bismuth (I) silanolates can be resulted from the presence of nearby hydrogen atom. Considering the mechanism of the surface NIR PL centers formation, besides the direct functionalization of the surface silanols, we should mention here another possible route. It is the strained siloxane bonds cleavage by BiCl (case d) in Fig 4). Elemental analysis provides the additional information on the nature of the surface bismuth species. In fact, it gives 0.206±0.022 at.% of Bi and 0.089±0.01 at.% of chlorine content in the analyzed xerogel samples. The presence of the substantial chlorine amount in xerogel samples demonstrates the possible involvement of the surface functionalization by the strained siloxane cleavage reaction (case d) in Fig 4). On the other hand, the concentration of chlorine in our samples is more than twice lower, than that of bismuth. That is why, the direct functionalization of the surface silanols (for example, case a) in Fig 4) can also be considered as the source of bismuth-containing surface centers. The calculated coverage of SiO2 surface by bismuth-bearing groups is 0.053 Bi atoms per nm2. It is significantly less than the surface density of silanol groups, which is 1.1 groups per nm2 at 700°C [34]. So, the silanol groups’ consumption via the surface modification reactions should be low. The 2-nd overtone band of OH vibration in the non-reacted silanol groups is noticeable as the sharp optical absorption at 1365 nm, superimposed on the broad NIR PL spectrum (Fig 2). Recently, the participation of oxygen deficient centers (ODC) in the formation of bismuth-related NIR PL centers was suggested [14, 35]. But, this involvement of ODC can hardly be considered in our experiments. Indeed, the special treatment (for example, silanol methoxylation with subsequent vacuum pyrolysis or high-energy mechanical activation) is needed to produce the SiO2 with the noticeable concentration of surface ODC (in order of 10-2 – 10-3 groups per nm2) – the so called reactive silica [36]. The processes of such kind can not take place throughout SiO2 surface modification by bismuth halide vapors. All the possible processes of NIR PL centers formation can be examined in future by the numerous techniques of surface species characterization. Also, the unexplored reactivity of the surface univalent bismuth silanolates can be studied with an attention to its PL properties. So, the formation of surface bismuth (I) silanolate can be viewed as an alternative mild technology of NIR PL centers preparation and also as a flexible tool for its investigation.
Conclusions The univalent bismuth compounds display diverse photoluminescence properties. In aluminosilicate glasses and crystal hosts, the monocation Bi+, emits in 1100-1300 nm range, while the covalently bound bismuth (I) silanolate pseudomolecule ((≡Si-O)3Si–O-Bi) is characterized by the distinct photoluminescence spectrum with the maximum at 1450 nm. These bismuth (I) silanolates are responsible for NIR PL of the bulk SiO2 glasses, with bismuth as the single dopant.
Acknowledgments This work was supported by Russian Science Foundation (RSF, research project number 15-13-30034). The authors are also thankful to V.A. Radzig for the valuable discussion.
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Table 1. Equilibrium partial pressures of BiCl and BiCl3 vapors in reaction cell (cell volume Vcell=221 cm3, initial BiCl3 amount mBiCl3=24 mg). Equilibrium constants were taken from [22]. Temperature (˚С)
500
600
700
Partial pressure
P(BiCl)
0.403
2.46
10.41
(Torr)
P(BiCl3)
16.43
17.42
17.04
Figure captions
Fig.1 Comparison of the energy level diagrams for BiOH molecule (left) and bismuth-related PL center in SiO2 glass (right). Diagram for BiOH molecule based mainly on the experimental results [28,30]. The positions of the “dark” non-emissive states (shown by dash lines) were inferred from the quantum chemical calculations [29]. Positions of energy levels for bismuthrelated PL center in SiO2 glass were taken from [11]. Fig.2 Photoluminescence spectra (bands at 830 and 1450 nm) of BiCl-treated SiO2 xerogel, excited by the laser diodes with the different emission wavelengths. The superimposed absorption at 1365 nm is due to the 2ν SiO-H vibrational band of the remaining surface silanols. The rise of PL signal at λ<775 nm is due to the presence of photoluminescence band with maxima at ~ 600 nm. This band of unknown origin was also observed in the bulk Bi-doped SiO2 glasses [8,11].
Fig.3 Temporal decay of NIR PL from BiCl-treated SiO2 xerogel sample (excitation wavelength is 445 nm, interval of PL registration is 1350-1550 nm), fitted by biexponential curve. Fitting parameters (time constants and amplitudes) are also shown.
Fig.4 Possible routes to the surface bismuth-related NIR PL centers. Reactions of different surface groups with bismuth monocloride (BiCl): a) isolated silanol; b) vicinal silanol; c) geminal silanol; d) insertion of BiCl into the strained surface-exposed siloxane bond.