Role of Optical Fiber Drawing in Radioluminescence Hysteresis of Yb

Jun 10, 2015 - The results demonstrate that the fiber-drawing process is responsible for modifications of the defectiveness of the glass network, with...
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Role of Optical Fiber Drawing in Radioluminescence Hysteresis of Yb-Doped Silica Ivan Veronese,*,† Cristina De Mattia,† Mauro Fasoli,‡ Norberto Chiodini,‡ Marie Claire Cantone,† Federico Moretti,§ Christophe Dujardin,§ and Anna Vedda‡ †

Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca, Via Cozzi 55, 20125 Milano, Italy § Institut Lumière Matière, UMR5306, Université Claude Bernard Lyon1-CNRS bâtiment Kastler, 10 rue Ada Byron, 69622 Villeurbanne Cedex, France ‡

ABSTRACT: Point defects in the host lattice of a scintillator material can trap carriers, slowing down their migration or even preventing their transfer to luminescent centers. Such competition schemes between defects and luminescent centers may explain also the hysteresis effect, which consists of a progressive enhancement of scintillation efficiency with accumulated dose. We propose a comparison between the scintillation hysteresis effect of Yb-doped sol−gel silica glasses in bulk and fiber forms, and we correlate them with traps monitored by wavelength-resolved thermally stimulated luminescence in both materials. The results demonstrate that the fiberdrawing process is responsible for modifications of the defectiveness of the glass network, with a change of the local distribution of the traps surrounding the luminescent center. The consequence of such modifications is the removal, in the fiber samples, of the thermally stimulated luminescence peak ascribed to traps closer to Yb ions and unstable at room temperature. We highlight that suitable postdensification thermal treatments can significantly modify the concentration and spatial distribution of defects around a luminescent center and can therefore be used as a tool for the engineering of scintillating glasses.

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scintillators like CsI:Tl,7−9 Al2O3,10 crystalline silicates,11,12 Ceand Tb- doped glasses,13,14 and BaAl4O7:Eu2+.15 Besides scintillation, it was detected also in the photocurrent of chemical vapor deposition (CVD) diamond which was shown to be influenced by traps too.16 It has been recently shown in a detailed way with doped phosphates that the relevance of the bright burn effect and the shape of the scintillation sensitivity increase depend upon several parameters like trap and luminescent center concentration, trap parameters (trap depth and frequency factor), and temperature.17 On the basis of the crucial role of traps, whose type and concentration is often correlated to the preparation route, we report here the case of sol−gel silica doped with ytterbium, both in bulk and fiber form. This material is of applicative importance because it has been recently identified as nearinfrared emitting material for real time remote dosimetry by radio-luminescence (RL).18 Indeed, optical-fiber-based detectors proved to be promising tools for ionizing radiation monitoring and dosimetry.19−22 The hysteresis effect present in “as prepared” fibers can be minimized by preirradiating them with a rather high dose

INTRODUCTION Scintillation is a complex phenomenon including the conversion of high-energy radiation into low-energy free electrons and holes, their transfer to luminescent centers, and finally the radiative relaxation of the excited center. The transfer stage entails carrier migration over several tens or even hundreds of nanometers,1,2 making the quality of the host lattice in terms of carrier trap concentration of critical importance. Point defects can trap carriers, slowing down their migration or even preventing their transfer to luminescent centers. Therefore, they affect scintillator performances because they either cause slow tails in the scintillation time decay or decrease the scintillation efficiency. Presently, the role of defects in scintillation is widely recognized; several strategies have been considered to lower their concentrations or to limit their influence, and a number of examples can be found in recent literature.3−6 Interestingly, the same competition scheme between defects and luminescent centers in the scintillation process can explain also the occurrence of the hysteresis effect also named “brightburn”, which consists of a progressive enhancement of scintillation efficiency with accumulated dose. The effect has been assigned to the progressive filling of traps which become inefficient from the point of view of carrier-trapping capability. Such effect was evidenced, with different magnitude, in several © 2015 American Chemical Society

Received: May 25, 2015 Revised: June 10, 2015 Published: June 10, 2015 15572

DOI: 10.1021/acs.jpcc.5b04987 J. Phys. Chem. C 2015, 119, 15572−15578

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The Journal of Physical Chemistry C (approximately 7 kGy) which fills all stable competing traps. The traps are deep enough to remain filled at room temperature (RT), enabling a good reproducibility of the dosimeter afterward, which is one of the main quality criteria for dosimetry. In this work we correlate the hysteresis curves of bulk glass and of fiber with traps monitored by wavelengthresolved thermally stimulated luminescence (WR-TSL) in both materials. In particular, we show that the fiber-drawing process is responsible for modifications of the defectiveness of the glass network and for the removal of traps unstable at RT, resulting in the observed good stability of the postirradiation sensitivity. Moreover, TSL data allow us to investigate the properties of hole traps and the characteristics of the TSL recombination mechanism operating in this case and to compare their TSL features with those of electron traps previously investigated in Ce- and Tb-doped silica.23

fabricated by Tecna s.r.l., and the heating rate was 1 K/s. The detection system was a back-illuminated, nitrogen-cooled, and UV-enhanced CCD (Jobin-Yvon Symphony) coupled with a spectrograph operating in the 200−1100 nm range (Jobin-Yvon μ-HR). Photoluminescence (PL) measurements were performed by stimulating the samples at λexc = 245 nm with an Energetiq LDLS EQ-99x lamp coupled with a Jobin-Yvon Triax monochromator. PL spectra were collected by an Andor Shamrock 500i spectrometer coupled to an Andor Newton 970 EMCCD camera. The temperature of the samples during the PL measurements was controlled by a Linkam HFS600 stage.

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RESULTS AND DISCUSSION The results of the TSL measurements performed on Yb-doped silica samples in bulk and fiber form are shown in Figure 1. A

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MATERIALS AND METHODS Yb-doped silica bulk samples (surface, approximately 25 mm2; height, 1 mm), with Yb molar concentration in the range of 0− 1 mol % (hereinafter indicated as %) were prepared by means of the sol−gel technique. The procedure for sample preparation, described in detail elsewhere,24 is here briefly summarized. A volume of 2.0 mL of Tetraethyl orthosilicate (TEOS, Aldrich, 99.999%) was mixed with ethanol (HPLC grade reagent) and with a suitable volume of 0.1 M ethanol solution of Yb(NO3)3·6H2O (Aldrich, 99.99%). Afterward, 1.20 mL of water (Merck analytical grade) was added under stirring (H2O:TEOS molar ratio 7.4), and the solution was sealed in a polypropylene cylindrical container for gelation. The resulting gel samples were aged for 2 days; then, small holes were produced in the container covers to induce slow drying of the alcogel. Drying of the alcogel occurred in approximately 1−3 weeks at 35 °C, yielding transparent xerogels. After a couple of months of aging, the xerogels were densified up to 1050 °C. For some of the samples, a rapid thermal treatment (RTT) at 1800 °C for few seconds was performed after densification to improve the scintillation efficiency of the doped glasses. Yb-doped silica fiber samples (diameter 200 μm) with Yb molar concentration of 0.04% were also prepared. Indeed, a similar method to the bulk synthesis described above can be used to produce small cylinders of glass that can be assembled in a suitable preform for optical fiber production. Typical dimensions of such cylinders are 25−70 mm in length and 10− 15 mm in diameter. Optical fibers were obtained without any cladding by drawing the cylinder welded on a couple of silica handles. Light guiding in such fibers is due to the interface with air. ICP-MS-LA analysis (inductively coupled plasma mass spectrometer, PerkinElmer DRC-e, equipped with New Wave UP 213 laser ablation sampler) on RE-doped silica glasses confirmed the nominal compositions and Yb concentration. A NIST 610 glass sample was used as the standard for the analysis. RL measurements were performed with homemade equipment by irradiating the samples at RT using a Philips 2274 Xray tube. RL spectra were collected by means of a backilluminated, nitrogen-cooled, and ultraviolet (UV)-enhanced CCD detector (Jobin-Yvon Spectrum One 3000) operating in the 200−1100 nm interval, coupled with a Jobin-Yvon Triax 180 monochromator. WR-TSL measurements were carried out using homemade equipment. The heater section was custom designed and

Figure 1. TSL glow curves of bulk silica and fibers doped with Yb obtained after X-ray irradiation at RT. A heating rate of 1 K/s was adopted. The curves were obtained by integration of wavelengthresolved TSL measurements from 958 to 981 nm. The frequency factors and RT (290 K) decay times of the trap distribution calculated by the analysis of the curves (see text) are also reported.

dependence of the shape of the glow curves on the dopant concentration, as well as on the thermal treatment can be clearly observed. A dominant peak at approximately 370 K and a lower peak around 450 K characterized the glow curve of the bulk sample with an Yb concentration equal to 1%. The relative intensity of these TSL peaks changed with decreasing Yb concentration from 1% to 0.05%. The RTT on the bulk samples proved to have an effect on the trap distributions. Indeed, for a fixed Yb concentration, the TSL peak at lower temperature almost disappeared in the bulk samples subjected to this thermal treatment. At the same time, a broadening of the higher-temperature emission was detected, which extends up to 600 K after RTT. The glow curve of the fiber was characterized by a single broad peak around 470 K and by the absence of any TSL components at lower temperature. Therefore, the heating process required for the fiber drawing influences the TSL trap distribution in a way that is qualitatively similar to the effect of RTT on the bulk samples. 15573

DOI: 10.1021/acs.jpcc.5b04987 J. Phys. Chem. C 2015, 119, 15572−15578

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The Journal of Physical Chemistry C

Figure 2. Contour plots of the WR-TSL measurements of bulk silica and Yb-doped fiber obtained after X-ray irradiation at RT. A heating rate of 1 K/s was adopted.

As introduced above, intrinsic defects in a scintillating material impair the scintillation properties. In this context, a significant increase of the scintillation yield with cumulated dose was previously observed in Yb-doped silica fiber samples.18 A thorough investigation of this feature can be achieved with Yb-doped silica samples characterized by different trap distributions. Therefore, those cases presenting the most different glow curves, i.e., the fiber and the bulk sample with a dopant concentration of 1% and not subjected to the RTT, were taken into account for further measurements. The wavelength-resolved glow curves of these samples are shown in Figure 2. In both spectra, only the emission band centered at approximately 975 nm due to the 2F5/2−2F7/2 transition of Yb3+can be observed. No other emissions were detected in the investigated wavelength interval extending from 200 to 1100 nm. Similarly, the RL spectra of both fiber and bulk glass were characterized by the same emission lines. Figure 3 shows the RL spectrum of the SiO2:Yb bulk as a representative example. The RL efficiency of the bulk glass and fiber increased as a consequence of a prolonged exposure to ionizing radiation. The hysteresis curves are shown in the inset of Figure 3, where the relative intensity of the Yb3+ RL signal is plotted versus the cumulated absorbed dose during the irradiation with 20 kV Xrays at a nominal dose rate (i.e., dose rate in air at the sample position) of 31.2 mGy/s. Each RL spectrum was obtained by irradiating the samples with a test dose of 156 mGy. This value was considered sufficiently high to produce a well detectable RL signal and, at the same time, much lower than the dose cumulated over the entire RL measurement. In both cases the intensity of the RL signal increased up to a saturation value nearly equal to four times the initial value. However, the shapes of the two curves were clearly different. In general, the shape of the hysteresis curve depends on several parameters.17 Some of the parameters are related to the experimental conditions used to investigate the phenomenon, like temperature and dose rate. Others are strictly correlated with the nature of the competing traps and thus the material. Because the experimental conditions were kept identical for both samples, we assign the different shape of the hysteresis curves measured for the

Figure 3. RL measurement obtained at RT for the bulk sample (Yb concentration 1%). The spectrum is representative of the emission of all considered samples. The inset shows the increase of RL sensitivity versus accumulated dose for both fiber and bulk SiO2.

bulk and fiber to the different distributions of the competing traps in the two samples. The two samples exhibit as well a different “restoring” time scale, as shown in Figure 4. For each point of the plot, the irradiation was stopped after the sample reached its saturation level, and the RL intensity was measured during a short time after the respective delay. In this way, possible memory effects due to the partial filling of the competing traps during the RL measurements were avoided. The rate of the decrease corresponding to the recovery proved to be significantly different for the two samples. At 67 h postsaturation, the RL efficiency of the fiber sample decreased approximately 8%; the same decrease was observed in the bulk samples after only 120 s. We also highlight that the loss is on the order of few percent, while the increase during irradiation was about 400%. The progressive decrease of the RL efficiency is explained by the detrapping phenomena involving the shallower competing traps. The observed behavior is in agreement with the fact that the relative concentration of the 15574

DOI: 10.1021/acs.jpcc.5b04987 J. Phys. Chem. C 2015, 119, 15572−15578

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The Journal of Physical Chemistry C

Figure 4. Bulk and fiber RL intensity as a function of the time elapsed after the end of the trap-filling irradiation. The inset shows the first 120 s data in a larger scale.

Figure 5. Arrhenius plots of the glow curves of the bulk and fiber samples following partial cleaning of the data at the temperatures indicated for each curve. The red lines are single exponential fits of the data from which the trap depth could be determined. The inset displays the trap depth value versus partial cleaning temperature for the bulk sample.

lower-temperature traps in the bulk sample is higher than that in the fiber sample, as suggested by their different glow curves. To better characterize the competing traps giving rise to the TSL signal above RT, the trap depth was evaluated through partial cleaning TSL measurements followed by the analysis of the glow curves with the initial rise method.25 Both fiber and bulk were first irradiated at RT then heated to a fixed partial cleaning (PC) temperature, cooled, and heated again to the temperature of 600 K, recording the glow curve. The same procedure was repeated using progressively higher PC temperatures. For each glow curve, the trap depth (E) was assessed by the fit of the initial portion of the TSL intensity (ITL) using the equation ITL(T ) = n0se−E / kT

(1)

where n0 is the initial number of trapped charges, s the frequency factor, T the temperature, and k Boltzmann’s constant. The Arrhenius plots of some selected glow curves of bulk and fiber samples, measured after partial cleanings at 350, 447, and 465 K, are shown in Figure 5. The exponential fits performed on the initial rising parts of the curves are also displayed. The inset of Figure 5 reports the trap depth values obtained for all the partial cleanings performed on the bulk sample. Despite the significant differences in the detrapping temperature, a nearly constant value of the trap depth equal to 1.2 eV (with an uncertainty around 10%) was obtained for all traps in this temperature range for bulk and fiber samples. To discard potential effect of thermal quenching phenomena able to bias our results (i.e., decrease of the radiative efficiency of the luminescence center with increasing the temperature), PL measurements at different temperatures were carried out. The bulk sample was maintained at a fixed and controlled temperature, and the PL signal due to the 2F5/2−2F7/2 transition of Yb3+ was monitored. The temperature interval from 250 to 513 K was investigated. Figure 6 shows two PL spectra collected at selected temperatures. A dependence of the shape of the PL spectra on the temperature was observed. In particular, the amplitude of the main PL peak at 975 nm decreased with increasing temperature. By contrast, the tail of the emission band at lower

Figure 6. Photoluminescence emission of the bulk sample at 273 and 493 K for λexc = 245 nm. The inset shows the dependence of the emission intensity evaluated by integration of the emission spectrum from 958 to 981 nm.

wavelength, (i.e., the 875−960 nm interval) proved to increase systematically with increasing temperature. As a whole, the radiative efficiency of Yb3+ remained almost constant up to 450 K, and only a slight decrease of approximately 15% was detected between 450 and 500 K, as shown in the inset of Figure 6. Such a small decrease cannot lead to significant bias in our trap depth estimation. It suggests that the main difference regarding the TSL peak position is due to significant changes of the frequency factor. The commonly accepted mechanism for the Yb-related TSL is the capture of an electron followed by the hole recombination at Yb2+ sites. Therefore, we expect that the detected glow peaks are to be ascribed to hole traps, according to the following scheme: Yb2 + + hole (freed from hole trap) → Yb3 + * (excited state) → Yb3 + + hν (TSL emission) 15575

DOI: 10.1021/acs.jpcc.5b04987 J. Phys. Chem. C 2015, 119, 15572−15578

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that RE aggregation phenomena and cluster formation were observed in silica highly doped with several RE, including Yb, by electron microscopy and vibrational spectroscopy.33,34 It was recognized that the nature and crystalline degree of the aggregates depend on the RE ion type and, interestingly, that RTT strongly influences the concentration and physical dimensions of such aggregates, giving rise also to significant improvement of the material transparency and of its luminescence efficiency.33 Such results further corroborate our hypothesis that RTT (and fiber drawing) modifies the network that surrounds Yb and changes the spatial distribution of intrinsic defects around it. The positive influence of RTT on defects demonstrated here proves once more that this kind of treatment can be considered in glass preparation technology to improve its optical performance. Finally, we also note that previous studies performed on silica glasses doped with rare earths different from Yb (e.g., Tb and Ce) displayed glow curves similar to those observed here for Yb doping, with peaks at around 360 and 440 K and a third peak at about 550 K. However, a different and rather opposite effect of the RTT was noticed because it did not influence significantly the relative intensities of the two lower temperature peaks, while it reduced that of the 550 K peak.23 Both bulk glasses and fibers doped by Ce displayed broad signals representative of a continuous distribution of trap parameters.23,35 However, in those cases the TSL analysis revealed the presence of a distribution of trap depths (spanning from 8 meV to 1.8 eV, considering also data at cryogenic temperatures) rather than of frequency factors as evidenced for Yb (spanning from 1010 to 1015 s−1 considering only data above RT). Such differences are in agreement with the fact that defects monitored by TSL in the cases of Ce and Tb doping are different from those monitored in the case of Yb-doped silica. Indeed, Ce3+ and Tb3+ are the natural valence state in silica, and the other stable states are Ce4+ and Tb4+. This indicates that Ce3+ and Tb3+ act first as hole traps, while Yb3+ acts as an electron trap. Therefore, the TSL reveals electron release in the first case while it reveals hole release when doped with Yb3+.

Because the partial cleaning experiment revealed the same trap depth for the different glow peaks, we suggest the occurrence of thermally stimulated tunneling recombination processes, as previously observed in other luminescent materials.26,27 The trapped hole can thus reach the tunneling level of the trap by thermal activation and recombines with Yb2+ ion. In general, the charge carrier escape probability (P) is an exponential function of the temperature, and depends on the trap depth (E) and on the frequency factor (s) according to P = se−E / kT

(2)

In classical recombination processes via the delocalized bands, the trap depth (E) is the energy difference between the trap level and the conduction or valence band. In the case of thermally assisted tunneling, E is the energy difference between the tunneling state and the ground state of the trap, and the frequency factor depends as an exponential function on the trap-to-luminescence center distance.25 Using the well-known relation linking the trap depth (E), the peak maximum temperature (Tm), and the frequency factor (s)25 ⎛ −E ⎞ βE = s exp⎜ ⎟ 2 kTm ⎝ kTm ⎠

(3)

the frequency factors of the trap distributions, and consequently their RT (290 K) lifetimes τ = (1/s) exp(E/ kT), can be calculated. Such values are reported in separate scales in Figure 1. For times longer than 10−2 h, the decay rate of the RL sensitivity of the bulk sample has the same order of magnitude as the decay time of the peak at 370 K; for shorter times, a faster decay is noticed (see inset of Figure 4). This could be due to the presence of lower temperature traps, as evidenced in preliminary TSL measurements below RT. Although such traps decay too fast to be detected by means of TSL measurements above RT, they may contribute to the rapid decrease of the RL efficiency for short times. The TSL distribution above 400 K is characterized by extremely long decay times, and it cannot contribute to the decay of RL sensitivity in our time scale. This explains the much higher stability of the RL sensitivity of the fiber sample with respect to bulk. Coming back to the thermally assisted tunneling mechanism, it is found that a single trap gives rise to different glow peaks according to its distance from the recombination center. In this framework, it is reasonable to assume that the heating process of the Yb-doped silica samples during fiber drawing (or similarly during RTT of the bulk samples) modifies the local distribution of the traps surrounding the luminescent centers, thus explaining the differences observed in the RL measurements of the bulk and fiber. Previous Raman measurements performed on Ce-doped bulk silica and fibers showed that both the RTT process and fiber drawing are responsible for an increase of D1 and D2 peaks at 490 and 610 cm−1 due to symmetric modes of 4-fold and 3-fold rings of SiO 2 tetrahedra.28 A very similar phenomenology was found to occur for the present Yb-doped glasses and fiber, also in agreement with recent literature findings obtained on silica fibers.29,30 The increase of such peaks was related to the occurrence of material densification29,31 with possible consequences on defect generation.32 All these findings support the suggestion that RTT and fiber drawing modify the traps in accordance with our TSL results. Moreover, it is worth noting

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CONCLUSIONS The hysteresis of the RL signal of Yb-doped silica in bulk and fiber forms was investigated and correlated with traps monitored by wavelength-resolved TSL measurements. The results demonstrated that the fiber-drawing process affects the defectiveness of the glass network with a change of the local distribution of the traps surrounding the luminescent centers. A qualitative agreement between RL sensitivity increase during irradiation, its decay after irradiation, and trap parameters could be reached in spite of the difficulties due to the amorphous nature of the glass leading to inhomogeneous broadening of defect levels. Our study provides evidence that suitable postdensification thermal treatments can modify the concentration and spatial distribution of defects around a luminescent center; therefore, such treatments can be considered as a tool for the engineering of scintillating glasses. From a practical point of view, we highlight the removal of the TSL peak ascribed to traps closer to Yb ions and unstable at RT in the fiber samples. Indeed, the remaining traps proved to be deep enough to give a satisfactory stability of the RL sensitivity, which is a key element in the case of using Yb-doped silica optical fiber for dosimetric applications. 15576

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(14) Fasoli, M.; Chiodini, N.; Lauria, A.; Moretti, F.; Vedda, A. Effect of Deep Traps on the Optical Properties of Tb3+ Doped Sol-Gel Silica. Phys. Status Solidi C 2007, 4, 1056−1059. (15) Patton, G.; Moretti, F.; Belsky, A.; Al Saghir, K.; Chenu, S.; Matzen, G.; Allix, M.; Dujardin, C. Light Yield Sensitization by X-Ray Orradiation of the BaAl4O7:Eu2+ Ceramic Scintillator Obtained by Full Crystallization of Glass. Phys. Chem. Chem. Phys. 2014, 16, 24824− 24829. (16) Manfredotti, C.; Fizzotti, F.; Vittone, E.; Paolini, C.; Olivero, P.; Lo Giudice, A. Photocurrent Study of Beta-Ray Priming in CVD Diamond. Diamond Relat. Mater. 2004, 13, 914−917. (17) Moretti, F.; Patton, G.; Belsky, A.; Fasoli, M.; Vedda, A.; Trevisani, M.; Bettinelli, M.; Dujardin, C. Radioluminescence Sensitization in Scintillators and Phosphors: Trap Engineering and Modeling. J. Phys. Chem. C 2014, 118, 9670−9676. (18) Veronese, I.; De Mattia, C.; Fasoli, M.; Chiodini, N.; Mones, E.; Cantone, M. C.; Vedda, A. Infrared Luminescence for Real Time Ionizing Radiation Detection. Appl. Phys. Lett. 2014, 105, 061103-1− 061103-4. (19) Veronese, I.; Cantone, M. C.; Catalano, M.; Chiodini, N.; Fasoli, M.; Mancosu, P.; Mones, E.; Moretti, F.; Scorsetti, M.; Vedda, A. Study of the Radioluminesence Spectra of Doped Silica Optical Fibre Dosimeters for Stem Effect Removal. J. Phys. D: Appl. Phys. 2013, 46, 015101-1−015101-7. (20) Veronese, I.; Cantone, M. C.; Chiodini, N.; Fasoli, M.; Mones, E.; Moretti, F.; Vedda, A. The Influence of the Stem Effect in EuDoped Silica Optical Fibres. Radiat. Meas. 2013, 56, 316−319. (21) Avino, S.; D’Avino, V.; Giorgini, A.; Pacelli, R.; Liuzzi, R.; Cella, L.; De Natale, P.; Gagliardi, G. Detecting Ionizing Radiation with Optical Fibers Down to Biomedical Doses. Appl. Phys. Lett. 2013, 103, 184102-1−184102-5. (22) Di Francesca, D.; Girard, S.; Agnello, S.; Marcandella, C.; Paillet, P.; Boukenter, A.; Gelardi, F. M.; Ouerdane, Y. Near Infrared RadioLuminescence of O2 Loaded Radiation Hardened Silica Optical Fibers: A Candidate Dosimeter for Harsh Environments. Appl. Phys. Lett. 2014, 105, 183508-1−183508-4. (23) Vedda, A.; Chiodini, N.; Di Martino, D.; Fasoli, M.; Griguta, L.; Moretti, F.; Rosetta, E. Thermally Stimulated Luminescence of Ce and Tb Doped SiO2 Sol-Gel Glasses. J. Non-Cryst. Solids 2005, 351, 3699− 3703. (24) Chiodini, N.; Fasoli, M.; Martini, M.; Rosetta, E.; Spinolo, G.; Vedda, A.; Nikl, M.; Solovieva, N.; Baraldi, A.; Cappelletti, R. HighEfficiency SiO2:Ce3+ Glass Scintillators. Appl. Phys. Lett. 2002, 81, 4374−4376. (25) McKeever, S. W. S. Thermoluminescence of Solids; Cambridge Solid State Science Series; Cambridge University Press: Cambridge, U.K., 1985. (26) Vedda, A.; Martini, M.; Meinardi, F.; Chval, J.; Dusek, M.; Mares, J. A.; Mihokova, E.; Nikl, M. Tunneling Process in Thermally Stimulated Luminescence of Mixed Lux(Y3+)1−xAlO3:Ce Crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 8081−8086. (27) Vedda, A.; Nikl, M.; Fasoli, M.; Mihokova, E.; Pejchal, J.; Dusek, M.; Ren, G.; Stanek, C. R.; McClellan, K. J.; Byler, D. D. Thermally Stimulated Tunnelling in Rare-Earth Doped Lu-Y Oxyorthosilicates. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 195123-1− 195123-8. (28) Vedda, A.; Chiodini, N.; Di Martino, D.; Fasoli, M.; Keffer, S.; Lauria, A.; Martini, M.; Moretti, F.; Spinolo, G.; Nikl, M.; et al. Ce3+Doped Optical Fibers for Remote Radiation Dosimetry. Appl. Phys. Lett. 2004, 85, 6536−6358. (29) Alessi, A.; Girard, S.; Marcandella, C.; Cannas, M.; Boukenter, A.; Ouerdane, Y. Raman Investigation of the Drawing Effects on GeDoped Fibers. J. Non-Cryst. Sol. 2011, 357, 24−27. (30) Alessi, A.; Girard, S.; Cannas, M.; Agnello, S.; Boukenter, A.; Ouerdane, Y. Influence of Drawing Conditions on the Properties and Radiation Sensitivities of Pure-Silica-Core Optical Fibers. J. Lightwave Technol. 2012, 30, 1726−1732. (31) Burgin, J.; Guillon, C.; Langot, P.; Vallée, F.; Hehlen, B.; Foret, M. Vibrational Modes and Local Order in Permanently Densified

AUTHOR INFORMATION

Corresponding Author

*Università degli Studi di Milano, Dipartimento di Fisica Via Celoria 16, 20133 Milano, Italy. E-mail: ivan.veronese@unimi. it. Phone: +39 0250317432. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.C. acknowledges financial support by NANOMED project (PRIN 2010-2011, 2010FPTBSH_003) from Ministero dell’Istruzione, dell’Università e della Ricerca.

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DOI: 10.1021/acs.jpcc.5b04987 J. Phys. Chem. C 2015, 119, 15572−15578