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The known photo-thermo-refractive (PTR) glass is a photosensitive multi-component silicate glass containing fluorine and bromine and doped with cerium, antimony, and silver. The UV irradiation and subsequent heat treatment of this glass result in the crystalline phase precipitation and negative refractive index change up to 1000 ppm in the UV-irradiated area. This effect is used for holographic recording the volume Bragg’s gratings. In the present research, we developed and studied the new class of chlorine-containing PTR glasses in which the UV irradiation and subsequent heat treatment cause the precipitation of silver nanoparticles with a shell consisting of mixed silver and sodium chlorides. The growth of the shell on the silver nanoparticle was shown to lead to a local positive refractive index change in the UV-irradiated area against the unirradiated one up to 1500 ppm.
© 2016 Optical Society of America
Introduction
Today, fluoride photo-thermo-refractive (PTR) glasses are among the most promising materials for the photonic applications. These glasses are widely used for recording the volume Bragg gratings [1] and pictorial holograms [2], constructing laser and optical amplifiers [3–5], and also creating luminescent elements [6], fluidic devices [7], and waveguide structures [8].
The standard PTR glass is a photosensitive multi-component sodium–zinc–aluminosilicate glass containing fluorine (6 mol.%) and bromine (0.5 mol.%) and doped with small amounts of additives that are responsible for the photo-thermo-induced precipitation of silver nanoparticles and sodium fluoride crystals (cerium 0.02 mol.%, antimony– 0.02 mol.%, and silver 0.02 mol.% – see, for example [9–11],). The selective UV irradiation into the Ce3+ absorption band in the spectra of these glasses results in the formation of neutral silver molecular clusters [12] that provide a broadband luminescence in the visible and IR ranges [13]. The subsequent heat treatment of UV-irradiated PTR glasses near the glass transition temperature (Tg) leads to the silver nanoparticle formation [11]. The thermal treatment of these glasses at temperatures above Tg leads to the growth of silver bromide shell on a silver nanoparticle [13, 16] and then to the precipitation of sodium fluoride cone on it [11,14]. In [9], the authors showed, for the first time, the dramatic effect of bromine on the process of NaF crystal growth in fluoride PTR glasses. The paper has demonstrated that the growth of sodium fluoride crystals is possible only in the presence of bromide additives in the PTR glass composition. A generalized NaF crystallization mechanism which consists of three stages is proposed in [11,15].
At the first stage, the trivalent cerium ion donates an electron under the effect of the UV irradiation, thus increasing its own valency in accordance with the following reaction (Eq. (1)):
Released photoelectrons can be trapped partially by silver ions (~20%) with subsequent neutral silver atom and molecular cluster formation (Ag0, Ag20, Ag2+, Ag32+) but most photoelectrons are trapped by antimony according to the following reaction (Eq. (2)):At the second stage, the heat treatment at relatively low temperatures (300° - 450°C) leads to releasing the trapped electrons from antimony (Eq. (3)) with further formation of silver molecular clusters and colloidal particles (Eq. (4)): At the third stage, the heat treatment at temperatures above Tg results, first, in the growth of mixed silver bromide - sodium bromide shell on a silver nanoparticle and, further, in the coaxial growth of sodium fluoride crystalline phase on this shell.In [16], the authors showed that the UV irradiation and subsequent heat treatment of fluoride PTR glass lead to the refractive index change only in the UV-irradiated area.
There is still some uncertainty in the origin of refractive index change in PTR glass and several presumable mechanisms of the change exist. Classically, this effect is assumed to be caused by difference in the refractive index between NaF crystalline phase (n ~1.33) that sediments in the UV-irradiated area and unexposed glass area (n ~1.49). Although a difference between the refractive indices of sodium fluoride and vitreous phases is rather big, the refractive index change in the UV-irradiated area does not exceed 1000 ppm [16,17]. This can probably be due to the fact that, in addition to the NaF phase precipitation, there is the silver bromide shell with a high refractive index value (n ~2.3) on the silver nanoparticle. As shown in many sources (see, for example [10,18,19],), the maximum of surface plasmon resonance of silver nanoparticle in fluoride PTR glasses shifts to the greater wavelengths owing to the silver bromide shell growth.
On the other hand, the authors of [20] have proposed another possible mechanism of photo-thermo-induced refractive index change in PTR glass. They assumed that the transformation of Na and F distributed in PTR glass matrix into crystalline NaF (chemical changes) and structural relaxation process are not the main causes of photo-thermo-induced refractive index change and accounted it for high residual stresses around the NaF crystals. According to calculations presented in the paper, these stresses are the most important cause for photo-thermo-induced refractive index change in PTR glass.
In our research, we have synthesized, for the first time, chloride PTR glasses and studied their spectral properties and refractive indices.
Experimental
We developed chloride PTR glasses based on Na2O-ZnO-Al2O3-SiO2-NaF system with reduced fluorine concentration (from 6 to 2 mol.%) and variable batch concentration of Cl (0-2.2 mol.%) doped with photosensitizer such as CeO2 (0.01 mol.%), Sb2O3 (0.05 mol.%), and Ag2O (0.15 mol.%). For glass synthesis, the high purity reagents only were used. The glass synthesis was conducted in Gero electric furnace with air atmosphere using the platinum crucibles, the melts being homogenized with platinum stirrer. It should be noted that, during the synthesis, Ce and Sb partly change their valence states from IV to III and from III to V, respectively [21,22]. Glass samples synthesized were annealed at 495°C with further cooling in accord with a pre-set program (~0.3°C/min). The glass transition temperature Tg was measured with differential scanning calorimeter STA 449 F1 Jupiter (Netzsch), the Tg magnitude being found to be 495 ± 3°C for all chlorine concentrations. Because chlorine in glass is a highly volatile component, all glass compositions were controlled with the X-ray fluorescence analysis using ARL PERFORM'X (Thermo Scientific).
Samples to be investigated were prepared in the form of polished plane-parallel plates 0.3 - 0.4 mm thick. For all samples, the refractive index was measured using Abbe-type (IRF-454 2BM) refractometer at λ ≈590 nm (nd) with an error of 0.0002.
For glass photoactivating, a mercury lamp (EFOS Novacure N2001) was used. To pass a radiation with wavelengths corresponding to the selective absorption of Ce3+ ions and eliminate that capable to cause the photodestruction of silver molecular clusters (SMC), the 290-410 nm spectral range was cut off with optical filters. The digital interface of this lamp allows for supporting a constant power (19 W/cm2) and controlling the UV irradiation dose via the irradiation time variation in the 5-500 sec limits. The heat treatment of the samples was conducted at 550°C for 3 hours in a muffle furnace (Nabertherm) with program control.
The optical density spectra of PTR glass samples were recorded in the 200–800 and 200-2500 nm spectral regions using Lambda 650 and Lambda 900 (Perkin–Elmer) spectrophotometers, respectively. Also, the XRD analysis of the samples was performed using Ultima IV (Rigacu) X-ray diffractometer.
Results and discussion
Figure 1 presents the dependence of chlorine concentration remained in glass according to XRF analysis on its as-batch concentration. As seen from Fig. 1, the analytical chlorine concentration in glass is less, for all samples, by 20-30% than the as-batch one. The dependence shown can be linearly approximated. Therefore, we will refer further to the as-batch chlorine concentration.
Fig. 1 Chlorine concentration remained in PTR glass according to XRF analysis vs. its as-batch concentration.
As indicated in Experimental, the glass transition temperature (Tg) for all synthesized glasses was measured using DTA method. It was found that, independently of the chlorine concentration in the glasses, their Tg magnitudes were equal to 495°C ± 3°C (Fig. 2).
Figure 3 demonstrates the absorption spectra of untreated PTR glasses differing in the chlorine concentration. As seen, the occurrence of chlorine in glass composition does not affect its absorption spectrum and, in particular, generates no absorption in the region of Ce3+-related band involved into the glass photoactivation.
Fig. 3 Absorption spectra of untreated PTR glasses differing in the chlorine concentration (mol.%): (1) 0, (2) 1.0, (3) 2.2. An inset shows the 250-550 nm region on a larger scale and the photos of untreated PTR glasses differing in the chlorine concentration (mol.%): (1) 0, (2) 1.0, (3) 2.2.
At the same time, the incorporation of chlorine affects the PTR glass refractive index (nd). As seen from Fig. 4, an increase in the Cl concentration leads to a linear increase in its refractive index from 1.5009 to 1.5020 for chlorine-free glass and glass containing 2.2 mol.% Cl, respectively, which is in agreement with reference data for silica glasses [23,24].
Figure 5 illustrates the effect of UV irradiation on the absorption spectra of PTR glass. As seen, an increase in the UV irradiation dose results in shifting the UV edge of strong absorption and intensifying the absorption in the 300 – 500 nm region that corresponds to the locations of SMC absorption bands. An inset in Fig. 5 shows the difference absorption spectra of UV-irradiated PTR glass containing 2.2 mol.% Cl. As seen from the inset, an increase in the UV exposure duration results in intensifying the absorption in the 250-500 nm region. The absorption band around 260 nm assigned to the negatively charged (Sb5+)- center is responsible for the absorption in the 250-300 nm region. The trivalent cerium-related absorption band is located at 309 nm. The absorption in the 330-500 nm was taken to be due to SMC such as Ag2, Ag3, Ag2+, etc [13]. The intensification of absorption bands due to the charged antimony and SMC centers is caused by increasing their amounts during the Ce3+ ion photoionization by mercury lamp radiation in accord with mechanism described in [15]. Notably, as seen from Fig. 5 and its inset, the long-lasting UV irradiation (500 sec) leads to a decrease in the intensities of SMC-related absorption bands. This reduction in absorption can be caused by photodestruction of SMC by mercury lamp radiation, the radiation range of the latter matching the SMC absorption band.
Fig. 5 Effect of UV irradiation dose on the absorption spectra of PTR glass containing 0.75 mol.% Cl. The exposure duration (sec) that sets a dose is (1) 0, (2) 1, (3) 5, (4) 50, and (5) 500. An inset presents the difference absorption spectra of glass containing 2.2 mol.% Cl for various UV exposure durations, the durations (sec) being (1) 1, (2) 5, (3) 50, and (4) 500.
In Fig. 6, the absorption spectra of PTR glasses differing in the chlorine concentration that were subjected to the UV irradiation for 50 sec are shown. As seen, an increase in the Cl concentration results in the intensification of absorption in the 330-500 nm region covering the SMC absorption bands. From an inset in Fig. 6, one can also see that, for long-lasting (>5 sec) UV exposure durations, a significant growth of SMC-related absorption around 330 nm with an increase in the chlorine concentration is observed.
Fig. 6 Absorption spectra of UV-irradiated for 50 sec PTR glasses with the chlorine concentration (mol.%) varying as follows: (1) 0, (2) 0.75, (3) 2.2. An inset shows the effect of chlorine concentration and UV exposure duration (logarithmic scale) on the difference absorption of PTR glasses around 330 nm, the chlorine concentrations in the glasses (mol.%) being (1) 0.5, (2) 0.75, (3) 1.5, and (4) 2.2.
One can assume that an increase in the chlorine concentration should lead to an increase in the amount of inhomogeneities in the glass bulk that occur in the form of discontinuities of the glass network [24]. In this case, conditions for the formation of neutral silver molecular clusters should become more favorable and, consequently, their concentration should increase. Also, the negatively charged chlorine ions in glass can attach to the positively charged SMC, thus forming Agn-Cl molecular clusters. The possibility of forming Agn–Cl (n = 2–7) stable molecular clusters was demonstrated in [26] by numerical simulation.
The thermal treatment of UV-irradiated samples at temperatures higher than glass transition one leads to the formation of silver nanoparticles that manifest themselves in the surface plasmon resonance absorption band located near the 410 - 420 nm region.
Figure 7 shows the effects of UV irradiation, with various doses, and subsequent heat treatment on the absorption spectra of PTR glasses containing Cl in amounts less than 1 mol.%. As seen from Figs. 7(a) and 7(b), the plasmon resonance maximum of silver nanoparticles is located at 426 nm for PTR glass without chlorine and remains to lie near this wavelength until the chlorine concentration reaches 1.0 mol.%, the plasmon resonance maximum intensity varying only (Figs. 7(a) and 7(b)).
Fig. 7 Absorption spectra of UV-irradiated and heat-treated PTR glasses for various UV exposure durations. (a) Spectra of PTR glass with no chlorine, the exposure durations (sec) being (1) 0, (2) 1, (3) 5, (4) 50, and (5) 500. (b) Spectra of PTR glass containing 0.5 mol.% Cl, the exposure durations (sec) being (1) 0, (2) 1, (3) 5, (4) 50, and (5) 500. An inset shows the photos of the glass samples for the exposure durations (sec) of (1) 1, (2) 5, (3) 50, and (4) 500.
When reaching the chlorine concentration of 1.5 mol. %, the location of plasmon resonance band starts to shift gradually up to 434 nm; further, when the chlorine concentration increases from 1.5 to 2.2 mol. %, the absorption maximum shifts to 445 nm. As shown in [22], this shift of plasmon resonance band can be caused by local refractive index increase in an area surrounding the silver nanoparticle. Also, Fig. 6 shows that an increase in the chlorine concentration results in increasing the peak absorptivity of plasmon resonance band related to the silver nanoparticles up to 150 cm−1 for glass containing 2.2 mol. % Cl. Such increase in the plasmon resonance band intensity is apparently caused by forming the greater amount of SMC during the UV irradiation of glasses with higher chlorine concentration, these clusters serving as a supply for silver nanoparticles growing in the course of heat treatment.
XRD analysis of samples under study was conducted to determine phases precipitated in glass bulk after the UV irradiation and subsequent thermal treatment (Fig. 8).
Fig. 8 Absorption spectra of UV-irradiated (for 50 sec) and heat-treated PTR glasses with various chlorine concentrations (mol.%) such as (1) 0, (2) 0.15, (3) 0.5, (4) 0.75, (5) 1.0, (6) 1.5, and (7) 2.2.
XRD analysis showed that the heat treatment of UV-irradiated PTR glasses with high chlorine concentration (>1.0 to 2.2 mol.%) results in the precipitation of a phase consisting of mixed silver and sodium chlorides such as (0.903Na0.097Ag)Cl (see Fig. 9). Notably, the mixed silver and sodium chlorides can form, according to the AgCl - NaCl phase diagram [27], a continuous series of solid solutions at room temperature if silver or sodium chloride concentrations are low, which can explain the gradual long-wavelength shift of surface plasmon resonance band for the silver nanoparticle. Furthermore, using Scherrer formula [28] and XRD analysis data, we can numerically evaluate the size of (0.903Na0.097Ag)Cl nanocrystals, which equal to 27nm for the PTR glass doped with 2.2 mol.% Cl. As well it is possible to calculate the size of silver nanoparticles, applying Mie theory [29] to the absorption spectra, this size distribution being found to be 3 ± 0,2 nm for all UV-irradiated (50 s) and heat treated chloride PTR glasses.
Fig. 9 X-ray diffraction pattern of UV-irradiated and heat treated chloride PTR glass doped with 2.2 mol.% Cl.
In accord with the above size calculations for silver nanoparticles and mixed silver-sodium chlorides crystals, it is reasonable to draw a conclusion that the contribution of scattering to the optical losses spectra in the 1000-2500 nm range is rather low.
In accord with the above, we can conclude that the UV irradiation of all studied chloride PTR glasses results in the Ce3+ ions photoionization and the SMC formation, the latter playing the role of crystallization centers (Fig. 10(a)). Heating all studied chloride PTR glasses at temperatures above 250°C and less than Tg results in discharging electrons by Sb and capturing them by Ag ions with further formation of an extra amount of neutral silver atoms and molecular clusters (Fig. 10(b)) [13]. Further heat treatment of PTR glasses containing 0 – 1.0 mol.% Cl at temperatures above Tg leads to the precipitation of silver nanoparticles with no shell (Fig. 10(c)). At the same time, as we suppose, such treatment of PTR glasses containing >1.0 to 2.2 mol.% Cl results in the precipitation of silver nanoparticles with a shell consisting of mixed sodium and silver chlorides in a varied proportion (Fig. 10(c)).
Fig. 10 Scheme for the photo-thermo-induced crystallization mechanism inherent in chloride PTR glasses for various Cl concentrations (0 – 2.2 mol.%). a – Photoactivation of PTR glass (Ce3+ ion photoionization), formation of neutral silver molecular clusters, and capturing electrons by Sb5+ ions. b – Discharging electrons by Sb and capturing them by Ag ions with the formation of neutral silver atoms and clusters. c – Growth of shell-free silver nanoparticles in glasses containing 0 – 1.0 mol.% Cl and growth of silver nanoparticles with a shell composed of mixed silver and sodium chlorides in glasses containing 0 – 2.2 mol.% Cl.
Figure 11 illustrates the absorption spectra of PTR glass containing 2.2 mol.% Cl observable at all the stages of photo-thermo-induced crystallization. As seen from Fig. 11, the precipitation of silver nanoparticles and silver/sodium chloride crystalline phase occurs only in the UV-irradiated and then heat-treated area, whereas the thermal treatment of unirradiated area imposes no measurable effect on the absorption spectra of chloride PTR glass.
Fig. 11 Absorption spectra of PTR glass containing 2.2 mol.% Cl. (1) is the spectrum for initial untreated glass, (2) is that for glass after the UV irradiation for 50 sec alone, (3) is the spectrum for glass after the heat treatment alone, and (4) is the one for glass after the UV irradiation for 50 sec and subsequent heat treatment. An inset shows the photos and absorption spectra (700-2500nm) of treated chloride PTR glass samples containing 2.2 mol.% Cl. (1) is initial untreated glass, (2) is glass after the UV irradiation for 50 sec alone, (3) is glass after the heat treatment alone, and (4) is glass after the UV irradiation for 50 sec and subsequent heat treatment.
According to data [18], the plasmon resonance maximum of silver nanoparticles exhibits the treatment-induced long-wavelength shift. Also, both silver [30] and sodium [31] chlorides have the greater refractive index than chloride PTR glasses. Based on these facts, one can assume that the refractive index of mixed AgCl – NaCl crystals should exceed that of initial glass. We conducted some direct measurements of the refractive index in the UV-irradiated and unirradiated areas of heat-treated glass. The measurements showed that the UV irradiation and subsequent heat treatment of chloride PTR glasses result in an increase in the refractive index of the UV-irradiated areas compared to that of unirradiated ones, i.e., the sign of refractive index increment ∆n is positive.
Figure 12 shows the evolution of the refractive index of PTR glass with an increase in the chlorine concentration for initial, heat-treated, and UV-irradiated and then heat-treated glasses (curves 1 to 3). As seen, the incorporation of Cl results in a consecutive increase in the refractive index of glass irrespective of treatment applied. In particular, Curves 1 and 2 coincide with each other, i.e., the heat treatment of unirradiated chloride PTR glasses does not change their refractive index [9]. On the contrary, the UV irradiation and subsequent heat treatment of chloride PTR glasses result in a significant increase in their refractive index. For the maximum chlorine concentration, a difference Δn between the refractive index values of the UV-irradiated and unirradiated glasses after the heat treatment reaches magnitudes up to 1500 ppm.
Fig. 12 Effect of chlorine concentration on the refractive index (nd) of PTR glass. 1 – untreated glass samples, 2 – glass samples after the heat treatment, 3 – glass samples after the UV irradiation and subsequent heat treatment.
Thus, based on the results of studying the optical and spectral properties of chloride PTR glasses and also on the XRD analysis data, we may conclude that an increase in the glass refractive index is apparently caused by precipitation, in the UV-irradiated area, of a phase consisting of mixed sodium and silver chlorides. This phase has the greater refractive index than that of unirradiated glass, which also can be one of possible reasons for the long-wavelength shift of the plasmon resonance maximum related to the silver nanoparticles.
One can conclude that the positive refractive index increment of chloride PTR glass that forms in the course of photo-thermo-induced crystallization is the fundamental specific feature of the glass. This feature allows one to use chloride PTR glass for not only hologram recording but also creating waveguide structures via the photo-thermo-induced crystallization process (UV irradiation + heat treatment). Despite an intense surface plasmon resonance absorption band of silver nanoparticles centered at 445 nm, chloride PTR glass is transparent enough in the 1000-2500 nm region (the optical losses being no more than 0.35 cm−1), this fact making this glass a promising material for creating certain kinds of the near-IR optical devices.
Conclusions
Chloride PTR glasses were developed and synthesized for the first time. The incorporation of chlorine into the PTR glass compositions does not affect the glass absorption spectra and, in particular, generates no absorption in the region of Ce3+-related band responsible for the PTR glass photosensitivity. An increase in the PTR glass chlorine concentration leads to the linear growth of its refractive index. The presence of chlorine in glass results also in an increase in the amount of SMC formed during the UV irradiation as compared to the case of chlorine-free PTR glass.
For PTR glasses with chlorine concentration lower than 1 mol. %, the subsequent UV irradiation and heat treatment result in the formation of silver nanoparticles with no shell and an increase in the chlorine concentration within the 0 – 1.0 mol. % range leads to the intensification of plasmon resonance band of silver nanoparticles.
For PTR glasses with the higher chlorine concentrations (>1.0 to 2.2 mol. %), the UV irradiation and subsequent heat treatment result, as we presume, in the formation of silver nanoparticles with a shell consisting of sodium chloride and silver chloride solid solutions. The refractive index of this shell is greater than that of PTR glass matrix, which can be one of possible reasons for the long-wavelength shift of silver nanoparticle plasmon resonance maximum by 8 – 19 nm. The growth of mixed AgCl - NaCl crystals results in a significant increase in the refractive index of the UV-irradiated area compared to that of unirradiated one, i.e., the sign of refractive index increment is positive and its magnitude reaches + 1500 ppm for glasses with chlorine concentration in the >1.0 to 2.2 mol. % range.
These results allow for considering chloride PTR glasses developed as promising photosensitive materials for recording the volume Bragg gratings and also creating waveguide structures via the photo-thermo-induced crystallization process that combines the UV irradiation and subsequent heat treatment.
Acknowledgments
The authors are grateful to R. Nuryev for conducting the XRD measurement and Prof. A. Efimov for fruitful discussion and article reviewing. This work was financially supported by Russian Scientific Foundation (Agreement # 14-23-00136)
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