Eu Incorporation into Sol–Gel Silica for Photonic Applications

Article pubs.acs.org/JPCC

Eu Incorporation into Sol−Gel Silica for Photonic Applications: Spectroscopic and TEM Evidences of α‑Quartz and Eu Pyrosilicate Nanocrystal Growth Andrea Baraldi,*,† Elisa Buffagni,†,∥ Rosanna Capelletti,† Margherita Mazzera,†,⊥ Mauro Fasoli,‡ Alessandro Lauria,‡,# Federico Moretti,‡,∇ Anna Vedda,‡ and Mauro Gemmi§ †

Department of Physics and Earth Sciences, University of Parma, Viale G.P. Usberti 7/A, 43124 Parma, Italy Department of Materials Science, University of Milano-Bicocca, Via R. Cozzi 55, 20125 Milano, Italy § Center for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia, Piazza S. Silvestro 12, 56127 Pisa, Italy ‡

S Supporting Information *

ABSTRACT: The problem of Eu incorporation into silica as dispersed dopants, clusters, separate-phase nanoparticles, or nanocrystals, which is of key importance for applications in the fields of lasers and scintillators, is faced by applying to sol− gel silica doped with nine different Eu3+ concentrations (0.001−10 mol % range) various spectroscopic techniques, including crystal field and vibrational mode analysis by means of Fourier transform absorption and microreflectivity (in the 200−6000 cm−1 and 9−300 K ranges), radioluminescence, and Raman scattering studies at 300 K. The variety of methods revealed the following concordant results: (1) amorphous Eu clusters grow when the Eu concentration is increased up to 3 mol % and (2) Si−OH groups are completely removed and ordered phase separation occurs at 10 mol % doping, as suggested by the remarkable narrowing of the spectral lines. Comparison with polycrystalline Eu oxide, Eu silicates, and αquartz spectra allowed the unequivocal identification of Eu2Si2O7 pyrosilicate and α-quartz as the main components of nanocrystals in 10 mol % Eu-doped silica. Such conclusions were brilliantly confirmed by transmission electron microscopy and electron diffraction analysis. Phonon coupling and anharmonicity were analyzed and are discussed for a few vibrational modes of nanocrystals.

1. INTRODUCTION Optical materials aimed at applications in the fields of lasers and scintillators often exploit the luminescence of foreign elements such as rare-earth (RE) or transition-metal ions that are introduced as dopants in crystalline hosts and act as luminescent centers. However, in some cases the growth of bulk single crystals is difficult and expensive as a result, for example, of a very high melting point of the considered system. Therefore, alternative preparation routes are pursued to obtain bulk samples of specific compounds. An interesting technology that has given satisfactory results in recent years is optical ceramics, where a powder of micro- or nanometric dimension is pressed (often at high temperature) to obtain a transparent or translucent bulk polycrystalline material. For example, optical ceramics were obtained for several systems, including YAG:Nd and Y2O3:Nd,1,2 YAG:Ce,3,4 Lu2O3:Eu5 and LuAG:Ce,6 LSO.7 This technology still faces some problems, as the presence of grain boundaries gives rise to surface defects causing afterglow processes, as shown for Lu2O3 and for YAG:Ce.8−10 Moreover, transparent materials can be obtained mostly with cubic structures that do not display anisotropy of the refractive index. Nanocomposites are also considered as an alternative morphological form for optical materials. In this case, © 2013 American Chemical Society

luminescent particles are embedded in a transparent supporting medium, which can be liquid or solid. Very recently, growing interest has been devoted to liquid nanocomposites for applications in the photodynamic therapy of cancer.11,12 More extended studies concern solid nanocomposites, such as simple RE oxides,13−18 silicates,19,20 CdTe nanoparticles embedded in BaFBr:Eu,21 nanoporous silica “soaked” by CdSe/ZnS luminescent nanoparticles or rhodamine,22 and organic−inorganic hybrids.23 For these materials, the particle dimensions should be lower than a few hundreds of nanometers to avoid scattering of the emitted light. The embedding stage can be performed after powder preparation; a critical point in this case is that particle aggregation into bigger, light-diffusing clusters can easily occur. Alternatively, the sol−gel process can be exploited to obtain doped xerogels and glasses in which luminescent nanoaggregates are spontaneously formed because of the low solubility of the dopants in the silica matrix. The main difficulty is control of the degree of crystallinity and the specific crystalline form of the aggregates, which are determined by thermodynamic conditions and cannot be directly tuned. Crystalline but not luminescent CeO2 nanoparticles form in Received: October 11, 2013 Published: November 25, 2013 26831

dx.doi.org/10.1021/jp4101174 | J. Phys. Chem. C 2013, 117, 26831−26848

The Journal of Physical Chemistry C

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Ce-doped sol−gel silica sintered in an oxidizing atmosphere,24 while amorphous clusters with a structure close to that of silicates are formed in Ce-doped silica sintered in a reducing atmosphere and in Gd-, Tb-, and Yb-doped silica.25,26 In all of these cases, the dopant concentrations are equal to or higher than 1 mol %. The particular behavior of Ce can be explained by its tendency to be incorporated with 4+ valence in the silica matrix under oxidizing conditions, at variance with the other mentioned REs. An exception to amorphous clustering is represented by silica-based glass ceramics obtained by heavy Gd doping (40 mol %) and codoping with Ce: crystalline aggregates either of Gd2Si2O7 pyrosilicate or with the Gd4.67O(SiO4)3 apatite-like structure were revealed.27 In the present work, the analysis is extended to incorporation of Eu into silica prepared by the sol−gel method. A wide dopant concentration range (from 0.001 up to 10 mol %) was considered, which allowed the clustering phenomena to be followed from the beginning up to the phase separation and devitrification. The Eu clustering in silica was previously faced by molecular dynamics simulation studies.28−30 For Eu concentrations ranging from 1 to 5 mol %, Afify and Mountjoy29 envisaged the onset of clustering already at concentrations as low as 1 mol %, but they complained about the lack of experimental evidence of a cluster structure, a deficiency that the present results try to fulfill. Up to now, indirect evidence of Eu clustering has been argued mainly from fluorescence spectra in the visible region, without providing any hint about the cluster and/or separate phase structure. For example, inhomogeneous line broadening due to the short distance between Eu ions in clusters favors energy transfer and compromises the site selectivity of an exciting laser,31,32 while concentrations higher than 3 mol % induce emission quenching.33 More recently, our preliminary Fourier transform infrared (FTIR) measurements34 showed that (1) the area under the Eu 7F0 → 7F4 absorption (in the 2500−3200 cm−1 range) exhibits a supralinear trend when plotted versus Eu concentration in the 0.1−3 mol % range, thus suggesting cluster formation, and (2) increasing the Eu doping level from 3 to 10 mol % drastically changes the absorption and Raman spectra, with the appearance of narrow lines, at variance with the behavior displayed by other REs.26,27 The crystal field (CF) and vibrational spectroscopy results available at that time34,35 were able to make apparent the growth of generic Eu silicate-like clusters but not to identify their specific structure. The peculiar behavior displayed by Eu in silica glass was the motivation for the present work, which aimed to understand the detailed nature of the clusters. Europium (1) works as a microscopic probe of its environment through the splitting of the electronic levels of the 4f6 configuration by the CF and (2) modifies, even heavily, the vibrational modes of the silica network. Thus, CF, vibrational absorption, and microreflectance spectra were measured, even at low temperature, by means of FT spectroscopy in the 200−6000 cm−1 range and were complemented by Raman and radioluminescence (RL) spectra. The spectroscopic approach was further integrated with transmission electron microscopy and electron diffraction analysis.

of ethanolic solutions of europium nitrate pentahydrate [EuIII(NO3)3·5H2O, 99.9%, Aldrich], ensuring the homogeneous dispersion of the dopant before addition of water. Doping levels from 0.001 up to 10 mol % were considered. The sols were put into polypropylene screw-cap flasks (Kartell) and tightly sealed. After gelation, bulk xerogels were obtained by slow solvent evaporation at 40 °C. A sintering process leading to fully densified composite materials was performed by heating at a rate of 4 °C/h up to 1050 °C. All samples were kept at this temperature for times never longer than 2 h, and then they were slowly cooled (the time required to reach 150−100 °C was roughly 6−7 h). The whole thermal treatment was performed in a pure O2 flux. Some samples were submitted to a further postdensification rapid thermal treatment (RTT) by an oxidizing oxygen−hydrogen flame. Such a treatment typically featured a very fast (2−4 s) temperature increase up to 1500− 1800 °C, where the sample was kept for approximately 10 s before rapid cooling in air. The temperature was monitored by an optical pyrometer (Impac IE 120) working at 5140 nm emission. Most of the samples were transparent, but a few of them (e.g., the 0.001 mol % sample) showed foggy regions. The 10 mol % sample was brittle after densification and almost completely milky. This feature is not unusual because brittle sol−gel samples were also obtained as a consequence of heavy (8−10 mol %) Pr doping.36 Reference powdered materials were also prepared by solidstate reaction, as Eu oxyorthosilicate (Eu2SiO5) and Eu pyrosilicate (Eu2Si2O7), or purchased, as crystalline europium oxide (Eu2O3, 99.99% from Aldrich), and natural smoky quartz. The Eu2O3 powders were submitted to a preliminary 1 h thermal treatment at ∼1000 °C to remove some adsorbed moieties (e.g., water) responsible for spurious lines at ∼850, 1087, 1494, 3410, and 3594 cm−1 detected in the absorption spectra; however, the treatment was unable to remove traces of carbonate groups (bands around 1500 cm−1), very likely introduced during the oxide synthesis procedure37,38 (see Figure 3, curve f). Reference slices of natural α-quartz and fused pure silica were also investigated. CsI-based pellets were prepared to measure the absorption spectra of either powdered compounds (oxyorthosilicates, pyrosilicates, and oxides) or silica (Eu3+-doped or undoped) and α-quartz in the spectral ranges where the massive samples exhibit very high absorption coefficients. In the last cases, chips of the samples were ground and then mixed with CsI powder. The sample/CsI weight ratio ranged between ∼0.6 mg/100 mg and 20.2 mg/100 mg (Table 1). The optical absorption measurements over the wavenumber range 200−6000 cm−1 were performed by means of a Bomem DA8 Fourier transform spectrophotometer capable of a resolution as fine as 0.02 cm−1. The microreflectance spectra were measured at room temperature (RT) in the 600−6000 cm−1 range by coupling the spectrophotometer to an IR PLAN microscope (Spectra Tech). As a reflectance standard, a gold mirror was used. Microreflectance measurements were chosen instead of the specular reflectance ones because some of the sol−gel samples were brittle, mainly those heavily RE3+-doped, and thus, only small-sized samples were available for measurements. The sample area investigated was less than 1 mm2. In the case of optically inhomogeneous samples, the analysis was extended to different areas. Diffuse reflectance spectra of powdered compounds were measured at RT by coupling the spectrophotometer to a Praying Mantis diffuse reflection accessory (Harrick Scientific Products). As a diffuse reflectance

2. EXPERIMENTAL SECTION Silica samples were obtained by the sol−gel method by hydrolizing tetraethyl orthosilicate (TEOS) in ethanol (Si/H2O 1:8 molar ratio; TEOS/ethanol 1:3 volume ratio). Eu doping was achieved in the sol by adding under stirring proper volumes 26832



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reducing the data collection time to a few minutes in the case of an area of 1 μm2 sampled every 5 nm.44 The recorded patterns are then compared via software with simulated patterns of the crystal phases that should be present in the sample. In this way it is possible to obtain a phase mapping by recognizing the phase to which each pattern belongs and an orientation mapping by determining the crystal orientation that corresponds to each pattern. In the Libra 120 microscope the resolution of the mapping is 10 nm because this is the minimum beam size that can be used.

Table 1. Sample/CsI Weight Ratios (WRs) for the Analyzed Pelletsa sample

WR (mg/100 mg)

undoped SiO2 smoky α-quartz SiO2:Eu3+ 3 mol % SiO2:Eu3+ 10 mol % (milky) SiO2:Eu3+ 10 mol % (transparent) Eu2SiO5 Eu2O3 Eu2Si2O7 YAlSi2O7

0.6 2.3; 19.5 0.66 2 3.8 2.4; 19.6 2.3; 20.2 3.8; 19.6 0.9; 9.7

3. RESULTS 3.1. FTIR Spectroscopy. Figure 1 compares the CF absorption spectra (measured at RT in the 2100−5500 cm−1

a

For samples with two WRs, the lower WR was used for the fundamental vibrational mode analysis and the higher WR for overtone modes and CF studies.

standard, powdered CsI was used. Standard RT absorption and reflectance spectra were measured at a resolution of 1 cm−1 by acquiring 500 scans, while for 9 K absorption spectra the resolution was improved to 0.5 cm−1 and the number of scans was increased to 1000. The temperature at which the absorption spectra were measured could be varied in the 9− 300 K range by assembling the samples in a model 22 Cryodine Refrigerator (CTI Cryogenics) equipped with KRS5 windows. Raman spectra were measured at RT using a micro-Raman spectrometer (Labram, Jobin-Yvon). The excitation source was either the 488 nm line of an Ar+ laser or the 632.8 nm line of a He−Ne laser. Unpolarized Raman spectra were collected in the backscattering configuration through a CCD detector. X-ray-excited RL measurements at RT were performed using a homemade apparatus featuring a CCD detector (Jobin-Yvon Spectrum One 3000) coupled to a monochromator (JobinYvon Triax 180) with 300 grooves/mm grating. The spectral resolution was 2 nm in the 300−700 nm range. X-ray irradiation was realized with a Philips 2274 tube operated at 20 kV. Transmission electron microscopy (TEM) was carried out on a FEI Tecnai F20ST microscope working at 200 kV at the Earth Science Department of the University of Milan and on a Zeiss Libra 120 microscope working at 120 kV at the Center for Nanotechnology Innovation@NEST in Pisa. The Libra microscope was equipped with an in-column omega filter, a NanoMEGAS DigiSTAR P1000 tool for precession electron diffraction and diffraction tomography, a NanoMEGAS ASTAR module for phase and orientation mapping, and a scanning TEM (STEM) unit with a high-angle annular dark-field (HAADF) detector. Electron diffraction tomography and the ASTAR module were used to study the crystal structure of nanoparticles. In electron diffraction tomography, a series of electron diffraction patterns is collected every 1° while the crystal is tilted around the goniometric axis39 in a variable range between 90° and 120°. In this way, a large portion of the reciprocal space is sampled and, via software, a 3D reconstruction of the crystal reciprocal space is obtained,40 allowing the unit cell parameters to be derived easily.41 PETS software was used for the unit cell determination and refinement,42 and ADT3D software was used for 3D visualization of the reconstructed reciprocal space.43 In ASTAR phase and orientation mapping, a series of diffraction patterns is collected while the beam is scanning a given area. The patterns are recorded by taking pictures of the fluorescent screen with a fast CCD that can collect tens of patterns per second, thus

Figure 1. Optical absorption spectra measured at room temperature on SiO2 samples doped with different amounts of Eu3+ in the region of the 7F0 → 7F4, 7F5, 7F6 Eu3+ transitions. Curve a, undoped sample; curve b, 0.1 mol %; curve c, 1 mol %; curve d, 3 mol %; curve e, 10 mol %. Curves a, d, and e were taken from ref 33. Inset: curve a coincides with curve d in the main frame; curve b is for a 3 mol % Eu-doped sample submitted to RTT (see section 2). For clarity, some spectra have been vertically shifted.

range) of sol−gel silica doped with different amounts of Eu3+ (curves b−e) with that of undoped silica (curve a). The spectra show that when the Eu3+ concentration is increased from 0.1 to 3 mol % (curves b−d), the following occur: (1) broad and weak bands due to the presence of Eu3+ grow at ∼2670, 2826, 3020, 3900, 4526, and 4760 cm−1; (2) the 3673 cm−1 peak, attributed to the Si−OH stretching mode,34,45 smoothly decreases; (3) a bump at ∼3600 cm−1, indicated by a vertical arrow, overlaps the Si−OH peak in the 1 and 3 mol % Eu3+ samples and is more marked in the latter (curve d, magnified as curve a in the inset). Dramatic changes were monitored in the 10 mol % Eu3+-doped samples (curve e): (1) the Eu3+-related absorptions are characterized by rather sharp lines; (2) some features related to typical SiO2 absorptions are heavily modified, for example, the O−Si−O asymmetric stretching overtone at ∼2260 cm−1 46 is replaced by a more structured peak at 2238 cm−1; (3) the Si− OH peak is no longer detected; (4) the baseline increases with increasing wavenumber, accounting for the light scattering caused by the milky sample (compare curves d and e in Figure 1). The structures displayed by Eu3+ peaks (curve e in Figure 1) represent a rather unusual feature for RE3+ absorptions in glasses. To get a better resolution of them, the spectra of 3 and 26833



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complex,25 no direct evidence of similar absorptions was found in the spectra of the Eu3+-doped samples. The modifications induced by Eu3+ doping on the intrinsic silica O−Si−O overtone mode at ∼2260 cm−1 (RT), portrayed in Figures 1 (RT) and 2A (9 K), suggested that such an analysis be extended to the silica fundamental modes by measuring in the 200−2000 cm−1 range the RT spectra of CsI pellets containing chips of doped and undoped silica samples (Table 1). The results are collected in Figure 3, where spectra related

10 mol % Eu3+-doped samples were measured at 9 K (Figure 2A, curves b and c) and compared with those of undoped silica

Figure 2. Optical absorption spectra measured at 9 K in the region of the 7F0 → 7F4, 7F5, 7F6 Eu3+ transitions. (A) SiO2 samples doped with 0, 3, and 10 mol % Eu3+ (curves a, b, and c, respectively) and CdF2:1 mol % Eu3+ (curve d). In curve c, the background due to light scattering (see curve e in Figure 1) has been subtracted. (B) CsI pellets containing Eu2SiO5 (curve e), Eu2O3 (curve f), and Eu2Si2O7 (curve g) powders (see Table 1). For clarity, some spectra have been vertically shifted.

Figure 3. Optical absorption spectra measured at room temperature on CsI pellets containing chips of samples or reference powders (Table 1). Curve a, undoped SiO2 sample; curve b, SiO2:3 mol % Eu3+ sample; curve c, SiO2:10 mol % Eu3+ sample (transparent fragment); curve d, SiO2:10 mol % Eu3+ sample (milky fragment); curve e, Eu2SiO5 powder; curve f, Eu2O3 powder; curve g, Eu2Si2O7 powder; curve h, smoky quartz sample. For clarity, some spectra have been vertically shifted.

(Figure 2A, curve a) and a 1 mol % Eu3+-doped CdF2 single crystal (Figure 2A, curve d). The absorptions in the 3 mol % Eu3+-doped sample remain broad even at 9 K (curve b), while the 10 mol % Eu3+-doped sample shows a line-rich spectrum (curve c): with decreasing temperature, the lines become narrower and their number increases (compare curve c in Figure 2A with curve e in Figure 1). The 9 K measurements confirmed the considerations of vibrational modes derived from the analysis of the RT spectra and displayed in Figure 1. In Figure 2A, the peak due to an overtone absorption of the O−Si−O asymmetric stretching (shifted from 2260 to ∼2275 cm−1 as a consequence of the temperature decrease from RT to 9 K) in the undoped sample (curve a) is replaced by a “triplet” (peaks at 2226, 2240, and 2257 cm−1) in the 10 mol % Eu3+ sample (curve c). The bands at ∼3646 and 4521 cm−1 due to the stretching and combination (stretching + bending) mode absorptions of Si−OH groups, respectively,47 monitored in the undoped sample (curve a) are still present in the spectrum of the 3 mol % Eu3+ sample (curve b); however, the latter is remarkably weaker than the homologous absorption detected in the corresponding RT spectrum (compare curve b in Figure 2A with curve d in Figure 1; also compare curves a and b in the bottom right panel of Figure 9). Both bands disappear for the 10 mol % Eu3+ sample (Figure 2A, curve c). The 3600 cm−1 bump detected at RT in the spectrum of the 3 mol % Eu3+ sample (Figure 1, curve d) contributes to the broadening and red shift of the ∼3646 cm−1 band at 9 K (Figure 2A, curve b). No trace of OH belonging to water molecules was monitored, as supported by the absence of the related stretching and combination-mode absorptions at ∼3440 and 5260 cm−1, respectively, thus proving that the 1050 °C annealing (section 2) was effective in removing water from the xerogel.47,48 At variance with Gd-doped sol−gel silica, where a peak at 4250 cm−1 was attributed to a Gd−OH

to two different fragments of the 10 mol % Eu3+-doped sample, one (curve c) being partially transparent and the other (curve d) being completely milky, are compared to the spectrum of an undoped sol−gel silica (curve a). Strong changes are induced by Eu3+ doping: additional absorptions appear, and the peaks due to intrinsic O−Si−O vibrational modes at ∼1100, 800, and 470 cm−1 experience a red shift and shape modifications. On the contrary, the major changes induced in the 3 mol % Eu3+doped sample are only broad and weak shoulders peaking around 240, 540, and 920 cm−1 (curve b). To complement the information acquired from absorption spectra concerning the changes induced by Eu3+ on the silica intrinsic fundamental vibration modes, microreflectance measurements were performed at RT in the 600−1400 cm−1 range. The spectra are collected in the top panel of Figure 4: curves b and c, related to the 1 and 3 mol % Eu3+ samples, respectively, show moderate modifications with respect to the spectrum of the undoped sample (curve a). To better catch the shape changes induced by 1 and 3 mol % Eu3+ doping, the spectra have been normalized to the reflectivity maximum occurring at ∼1123 cm−1. The position is in good agreement with the value of 1122 cm−1 measured by means of a specular reflectance accessory on extended-area, polished, undoped fused silica samples.46 The main features of the undoped sample (curve a) are a peak at 1123 cm−1 (peak A) and a shoulder at ∼1230 cm−1 (peak B), both related to O−Si−O asymmetric stretching modes, and a peak at ∼780 cm−1 related to the bending mode (peak C) (see section 4.1.3). The major modifications induced by Eu3+ doping levels up to 3 mol % are a small red shift of 26834



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Figure 5) and new peaks grow over the whole range investigated. Such features seem particular to Eu3+ doping because they do not appear in the spectra of silica doped with 10 mol % Ce3+ or Tb3+, for which even the shift in peak A is rather modest (see the open diamond and open star in Figure 5). A nonuniform Eu3+ distribution within the sample can be easily monitored by microreflectivity, as proven by Figure 6A

Figure 4. Top panel: RT microreflectance spectra of SiO2 samples doped with different amounts of Eu3+. Curve a, undoped SiO2; curve b, 1 mol %; curve c, 3 mol %; curve d, 10 mol %. The spectra are compared with the microreflectance spectrum of a natural α-quartz sample (curve e) and the diffuse reflectance spectra of Eu2Si2O7 (curve f) and smoky quartz (curve g) powders. All of the spectra are normalized to their maximum. Bottom panel: difference microreflectance spectra (ΔR) relative to the spectrum of the undoped SiO2 sample (curve a, top panel) for the 0.01, 1, and 3 mol % Eu3+-doped samples (curves h, i, and l, respectively).

Figure 6. (A) Microreflectance spectra measured at RT on an undoped SiO2 sample (curve a) and on different areas of the 0.001 and 10 mol % Eu3+-doped ones. Curve b, transparent 0.001 mol %; curve c, cloudy 0.001 mol %; curve d, bottom surface of milky 10 mol %; curve e, top surface milky 10 mol %; curve f, transparent 10 mol %. (B) Optical absorption spectra measured at 9 K on two different areas of the same SiO2:10 mol % Eu3+ sample in the region of the 7F0 → 7F4 Eu3+ CF transition. The spectrum measured at 9 K on a Eu2Si2O7 pellet (see Table 1) is also reported for comparison (curve c). The curves have been vertically shifted for clarity. The scale on the left is for curves a and b, and that on the right is for curve c.

peak A and a dip occurring between peaks A and B; the shift in peak A is nearly negligible for Eu concentrations lower than 1 mol % (Figure 5). Similar results were recently reported for Ce3+-, Gd3+-, Tb3+-, and Yb3+-doped silica.26 The spectrum related to the milky 10 mol % Eu3+ sample (curve d) shows a completely different pattern: peak A seemingly disappears (or is strongly red shifted; also see

both for very low (0.001 mol %) and high (10 mol %) Eu3+ concentrations. The spectra of a perfectly transparent area and a slightly cloudy area within the 0.001 mol % Eu3+ sample are portrayed by curves b and c, respectively, and compared with that of the undoped sample (curve a). Curves a and b coincide within the experimental error. However, with respect to curve a, curve c shows a meaningful increase in the 700−1100 cm−1 range that in part resembles the increase displayed in curve c in the top panel of Figure 4, which is related to the 3 mol % Eu3+ sample. In addition, a small red shift of the 1123 cm−1 peak was detected (Figure 5). In the case of the 10 mol % sample, the pattern described by curve d in the top panel of Figure 4 and reported again as curve d in Figure 6A is related to the bottom, rough face of the milky sample; curve e monitors the top, smooth face of the same milky piece, while curve f is related to a small, nearly transparent chip (the poor quality of the sample accounts for the noise, recorded mainly on the low-wavenumber side, where the dedicated detector is less sensitive). The shape of the last curve looks to be intermediate between that described by curve c in the top panel of Figure 4 (3 mol %) and those portrayed by curves d and e in Figure 6A. Further evidence of the nonuniform distribution of Eu in sol−gel silica is supplied by Figure 6B, where the 9 K CF spectra related to two different areas of the same 10 mol % sample are compared. Such a nonuniform Eu distribution might be related to an unpredictable early reaction of Eu during the gelation process followed by precipitation.

Figure 5. RT peak position of the main microreflectivity band as a function of the Eu3+ concentration in silica samples. The black solid line and open squares are related to as-prepared samples; the dashed line and open circles are for samples submitted to a further RTT (see section 2). Solid black squares are related to cloudy or milky asprepared samples and open squares to transparent ones. Straight lines indicate the peak positions in pure samples. For comparison, the open diamond and open star are related to 10 mol % Tb- and Ce-doped silica, respectively. Inset: RT radioluminescence spectra of silica doped with different Eu concentrations (0.01, 0.1, 3, and 10 mol %; curves a− d, respectively). The spectra have been vertically shifted for clarity. The wavenumber ranges of the 5D0 → 7FJ (J = 0, 1, ..., 4) transitions are indicated. 26835



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cm−1. In addition, relative to SiO2:10% mol Eu3+, a strong peak at 465 cm−1 is detected in all three spectra. 3.3. RL Measurements. For all Eu concentrations, the RL spectra measured at RT feature 5D0 → 7FJ transitions of Eu3+, while no evidence of Eu2+ emissions was found. Up to 3 mol % the lines are broad (curves a−c in the inset of Figure 5), while for the highest concentration (10 mol %) the spectrum is characterized by more numerous and narrower lines (curve d in the inset of Figure 5). Such a trend confirms that already suggested by the RT absorption spectra (Figure 1, curves b−e). A common feature displayed by the RT absorption and RL curves is the transition whose final state is 7F4 (7F0 → 7F4 absorption and 5D0 → 7F4 emission in Figure 1 and the Figure 5 inset, respectively). The RL intensity, obtained by integration of the RL signals in the 550−750 nm (18182−13333 cm−1) interval and plotted as a function of the Eu concentration, displays a minor maximum at about 0.01 mol % followed by a monotonic decrease and a sudden sharp increase for SiO2:10 mol % Eu. The ratio of the latter intensity to that of the minor maximum is about 50. 3.4. TEM Analysis. The marked differences exhibited by the optical spectra (absorption, microreflectance, Raman, and RL) of the 3 and 10 mol % Eu-doped silica samples suggested that such samples should be analyzed by means of TEM. Crystalline nanoparticles having sizes in the 10−30 nm range could be detected in some areas of the 3% mol sample (Figure 8a). The poor quality of the related diffraction patterns, due to the small nanoparticle size, allowed the claim that they are crystalline but could not be used to determine their crystal phases. Two different characteristic typologies of nanocrystalline areas were identified in the 10 mol % sample, giving a direct evidence of the inhomogeneity pointed out by the optical spectra. The former is characterized by a very dense distribution of crystalline nanoparticles, quite homogeneous in size and shape (elongated crystals whose size is in the 30−70 nm range), embedded in an amorphous matrix made up by lighter elements, as indicated by the darker contrast in the HAADF images (Figure 8b). The latter displays bigger nanocrystalline particles (variable size in the 100−500 nm range) embedded in a crystalline matrix formed by nanoparticles of different type (Figure 8c). As in the previous case, the matrix shows a darker HAADF contrast and therefore hosts lighter chemical elements.

3.2. Raman Spectroscopy. Raman investigations were also carried out to obtain a complementary picture of the vibrational properties of glasses. The Raman measurements are reported in Figure 7, where also the spectra of Eu oxide (Eu2O3), Eu silicate (Eu2SiO5), and Eu pyrosilicate (Eu2Si2O7) powders and crystalline quartz are displayed for comparison.

Figure 7. RT Raman spectra of SiO2 samples doped with different Eu concentrations and spectra of some reference compounds for comparison. Curve a, undoped SiO2 sample; curve b, 3 mol % Eudoped sample; curve c, 10 mol % Eu-doped sample, transparent area; curves d and e, 10 mol % Eu-doped sample, milky areas; curve f, smoky quartz powder; curve g, Eu2SiO5 powder; curve h, Eu2Si2O7 powder; curve i, Eu2O3 powder. Some curves have been vertically shifted for clarity.

The Raman profile of the undoped glass (curve a) shows typical modes of amorphous silica, including the structures at 440, 800, and 1060 cm−1 ascribed to rocking, bending, and asymmetric stretching of SiO2,49 respectively, and the peaks at 490 and 610 cm−1 related to symmetric stretching of fourfold and threefold rings of silicon dioxide tetrahedra, respectively.50 These features are also observed in the spectrum of the 3 mol % Eu3+ sample (curve b) which also contains additional vibrational modes at ∼880 cm−1. When the concentration of Eu rises to 10 mol % (curves c−e), the homogeneity of the sample is not preserved: some portions show a spectrum rather similar to that of SiO2:3% mol Eu3+ (curve c), while others (curves d and e) display a more complex spectrum in which a multitude of modes is clearly evidenced together with the disappearance of the broad low-frequency structure at 440

4. DISCUSSION 4.1. FTIR Spectroscopy. 4.1.1. Crystal Field Spectra (Including RL Results). According to the RL results, no Eu2+

Figure 8. (a) Bright-field image of a nanoparticle-rich area in the 3 mol % Eu-doped silica sample. (b, c) HAADF images of two different nanoparticle-rich areas in the 10 mol % Eu-doped silica sample. The contrast is proportional to the atomic number Z, and thus, the nanoparticles host heavier elements (as Eu) than the silica matrix does (i.e., ZEu = 63, ZSi = 14, and ZO = 8). In (b) the nanoparticles are small, while those in (c) are larger. The arrow in (c) indicates the crystal on which diffraction tomography was performed. 26836



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Figure 9. Absorption spectra at RT (curves a and c) and 9 K (curves b and d) to demonstrate the thermally activated population of the Eu3+7F1 level, as monitored by the 4579 cm−1 peak intensity. Top left: CsI/Eu2Si2O7 pellet (Table 1). Bottom left: 10 mol % Eu-doped silica. Bottom right, 3 mol % Eu-doped silica (curves a and b) and pure fused silica (curves c and d). Top right: 4579 cm−1 peak intensity (normalized to that measured at RT) vs temperature for the CsI/Eu2Si2O7 pellet (open squares) and 10 mol % Eu-doped silica (black stars); the red curve is the fit according to eq 2, and the green curve is the fit obtained by keeping fixed ΔE1−0 = 251 cm−1 and g1 = 18.

was detected in the present glasses (section 3.3), but only Eu3+, which is a non-Kramers ion characterized by a 4f 6 configuration.51 In silica, f−f electronic transitions are partially allowed by the “crystal field” originated by the nearest neighbors and probed by the RE3+ ion. The Eu3+ ground manifold is the 7F0 singlet (J = 0), and the first three CF transitions (i.e., 7F0 → 7F1, 7F2, 7F3) are expected to lie at energies below ∼2000 cm−1.51 Thus, they cannot be easily identified in the Eu3+-doped silica samples because of overlap with the dominant O−Si−O vibrational absorption peaks (section 4.1.3). The separations between the 5D0 → 7F0 emission and each of the 5D0 → 7FJ (J = 1−3) ones, suggested by the RL spectra displayed in the inset of Figure 5, confirm the above hint. Absorptions due to the CF transitions 7F0 → 7F4, 7 F5, 7F6 were detected, as shown by the 9 K spectra displayed in Figure 2A. They appear as broad bands in both the RT and 9 K spectra of samples containing Eu3+ concentrations up to 3 mol % and are well-structured in the spectra of 10 mol % Eu3+doped silica. Such a trend clearly appears even upon inspection of the RL spectra (Figure 5 inset). The absorption bands grow with increasing Eu3+ nominal concentration, thus supporting once more that they originate from Eu3+. The supralinear trend reported for the RT peak in the 3020−3050 cm−1 range as a function of the Eu concentration suggests that clustering may occur at high Eu3+ doping levels.34 The well-structured CF spectra at the 10 mol % Eu3+ level suggest that RE3+ is probing nearly ordered environments, as for Er3+ in SnO2 nanocrystals embedded in silica,52 at variance with the behavior of other RE3+ ions (e.g., Ce3+, Tb3+, and Yb3+) in silica, where the RE3+-associated absorptions remain structureless even for concentrations higher than 3 mol %.26 The results suggest that at 10 mol % Eu3+ doping some segregation of a Eu-rich, nearly ordered phase takes place, as supported by TEM images (Figure 8b,c). Indirect information about the separation of the lowest excited manifold 7F1 from the ground manifold 7F0 can be

extracted by comparing curve e in Figure 1 with curve c in Figure 2A, both of which related to the 10 mol % Eu3+-doped sample. For clarity, a magnification of the spectra in the 4410− 5160 cm−1 region is portrayed in the bottom left panel of Figure 9: the 4579 cm−1 peak appearing in the RT spectrum (curve a) is no longer detectable at 9 K (curve b). A similar trend is observed for the spectra related to the 1 and 3 mol % Eu3+-doped samples: the 4527 cm−1 peak occurring in the RT spectrum of the 3 mol % Eu3+-doped sample is strongly reduced in the spectrum measured at 9 K (Figure 9, bottom right). Such a feature is typical of Eu-doped silica, since in a pure fused silica the difference between the RT and 9 K spectra is absolutely negligible (Figure 9, bottom right, curves c and d): the ∼4520 cm−1 peak, accompanied by a lower-wavenumber shoulder, is due to an Si−OH combination mode (stretching + bending). It should be remarked that the 9 K spectra of the 3 and 10 mol % Eu-doped silica confirm a limited and a negligible concentration of Si−OH, as more clearly displayed by Figure 1 (curves d and e, respectively) and Figure 2A (curves b and c, respectively) in the region of the stronger Si−OH stretching band (∼3670 cm−1). The results portrayed by curves a and b in the bottom left and bottom right panels of Figure 9 suggest that the starting level of the CF transitions corresponding to the absorptions at 4579 and 4527 cm−1 in the 10 and 3 mol % Eudoped silica samples, respectively, is populated at RT but is practically empty at 9 K. Such a level cannot be a sublevel of the ground 7F0 manifold, which is not split by the CF (J = 0) and thus may be a sublevel of the first excited manifold, 7F1. The RT separation of 251 cm−1 between the 4579 cm−1 absorption and the first peak on its high-energy side (4830 cm−1) for the 10 mol % Eu3+-doped sample provides at least a rough estimate of the position of the 7F1 manifold with respect to the ground one. The values obtained for the separation between the two lowest CF levels are ΔE1−0 = 251, ∼231, and ∼237 cm−1 in the case of 10, 3, and 1 mol % Eu3+-doped samples, respectively, which account well for the population of the 7F1 manifold at 26837



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RT and are in agreement with a 240 cm−1 separation between the 7F0 → 5D0 and 7F1 → 5D0 fluorescence excitation peaks in the visible range of sol−gel silica doped with 2 mol % Eu.32 Unfortunately the present RL spectra do not provide a clear-cut value of the separation between the 5D0 → 7F0 and 5D0 → 7F1 emissions because of their partial overlapping in silica samples doped at Eu concentrations of ≤3 mol % (curves a−c in the Figure 5 inset) and the uncertain identification of the 5D0 → 7 F0 emission in the 10 mol % Eu3+-doped sample (curve d). Similar analyses of fluorescence spectra performed on various silicates provide values for the first Eu3+ CF transition (7F0 → 7 F1) that fall in the 175−284 cm−1 range.53−55 Indeed, peaks between 250 and 300 cm−1 are monitored in the spectrum of the present 10 mol % Eu3+-doped silica pellet (Figure 3, curve d), and a broad band appears around 240 cm−1 in the 3 mol % Eu3+ one (Figure 3, curve b); these should be ascribed to Eu because they are absent in the pure silica pellet (curve a). However, they cannot be unequivocally attributed to a Eu3+ CF transition (7F0 → 7F1) because they may be hidden under the much stronger absorptions related to Eu−O group vibrations (section 4.1.3), as shown by the spectra of Eu2O3, Eu2SiO5, and Eu2Si2O7 pellets (Figure 3, curves f, e, and g, respectively). According to calculations, the “terminal bending modes” ν6(A1) and ν21(B1) are expected to fall around 273 cm−1 in type-I pyrosilicates, as Eu2Si2O7 is.56 The sharpest among the Eu3+-related lines portrayed in Figure 2A by curve c (10 mol % Eu3+) is characterized by a full width at half-maximum (fwhm) of about 10 cm−1, which is much less than that of the broad bands displayed by curve b (3 mol % Eu3+) but larger than that (∼3.5 cm−1) exhibited by some of the Eu3+ lines in a single crystal of CdF2 (curve d). This suggests that, in the heavily doped sol−gel silica, Eu3+ probes environments that are ordered to some extent but not as ordered as those experienced in an extended single crystal, as supported by the TEM results (section 4.3). Moreover, the number of lines originated by a given 7F0 → 7FJ transition (J = 4−6) is much higher than that expected: the ground state is not split by the CF (J = 0), and thus, the maximum number of lines predicted at 9 K for Eu3+ probing a low-symmetry environment is 2J + 1, where J labels the excited manifold reached by the transition.57 For example, in the region of the 7F0 → 7F5 transition (3700−4000 cm−1),51 many more lines are observed than the expected number of 11. Similar considerations can be applied to the lines arising from the 7F0 → 7F4 transition (2600−3200 cm−1):51 the number of lines exceeds the predicted number of 9 (Figure 6B and Table S2 in the Supporting Information). This means that Eu3+ experiences different surroundings, as also supported by TEM findings (section 4.3). The supralinear dependence of a Eu3+ band amplitude (in the region of the 7F0 → 7F4 transition) on the nominal Eu3+ concentration indicates that Eu3+ clustering occurs in the 0.1−3 mol % Eu doping range,34 as also expected from molecular dynamics simulations29 and proved by the small nanoparticles evidenced by the TEM image in Figure 8a for a 3 mol % Eudoped silica sample. The narrow line spectra monitored in the 10 mol % Eu-doped silica (Figure 1, curve e and Figure 2A, curve c) suggest that likely a separate Eu-containing phase is present, as also supported by the HAADF image in Figure 8c. Possible candidates are Eu oxyorthosilicate (Eu2SiO5), Eu disilicate or pyrosilicate (Eu2Si2O7), and Eu oxide (Eu2O3). Thus, the spectra of CsI pellets containing powdered polycrystalline Eu2SiO5, Eu2Si2O7, and Eu2O3 (curves e, f,

and g, respectively) are collected in Figure 2B for comparison. The spectra of the three polycrystalline compounds show narrow lines, as does the spectrum of 10 mol % Eu3+-doped silica (Figure 2A, curve c). The Eu3+ line positions in silica show a good correspondence with those of Eu2Si2O7, and a scarce one with either Eu2SiO5 or Eu2O3 (see Table S2 in the Supporting Information). The root-mean-square (rms) deviation of the Eu3+ line positions in silica from the corresponding ones in Eu2Si2O7 pyrosilicate is only 2.3 cm−1 for the set of 27 lines detected in the 2500−5100 cm−1 range. The similarity of the related spectra is impressive: an example is supplied in Figure 6B, where the spectra of the SiO2:10 mol % Eu3+ sample (curve b) and the Eu2Si2O7 pellet (curve c) are compared in the region of the 7F0 → 7F4 Eu3+ transition. In addition, the two sets of lines show the same blue shift as a function of temperature. An example is portrayed in the inset of Figure 10 for three lines, while the main frame provides a magnification for the 3044.5 cm−1 one.

Figure 10. The inset shows the line positions of three absorption peaks common to 10 mol % Eu-doped silica (solid symbols) and the CsI/Eu2Si2O7 pellet (open symbols; see Table 1 for composition) as functions of temperature. The solid lines in the inset show the average peak positions at 9 K. In the main frame, a magnification of the curve for the 3044.4 cm−1 peak position ω is shown. The solid line is the fit of ω(T) data according to the single-phonon coupling model (see the formula provided, in which δω is the coupling constant and ωph is the coupled phonon).

Moreover, the separation between the two lowest 7F0 and 7F1 levels can be estimated as ΔE1−0 = 250.5 cm−1 for the Eu pyrosilicate by comparing the RT and 9 K spectra of a Eu2Si2O7/CsI pellet (Figure 9, top left, curves a and b), and following the procedure described above for Eu-doped silica samples. This value practically coincides with the value of 251 cm−1 obtained for the 10 mol % Eu3+-doped silica sample (Figure 9, bottom left). Furthermore, the 4579 cm−1 peak intensity at a given temperature T, normalized to that measured at RT, shows the same temperature behavior for both the Eu pyrosilicate pellet and the 10 mol % Eu3+-doped silica sample, as displayed by open squares and black stars, respectively, in the top right panel in Figure 9. The ratio of the populations of the 7F1 and 7F0 levels, n1/n0, as a function of temperature T can be written as 26838


The Journal of Physical Chemistry C n1(T ) = g1 exp( −ΔE1 − 0 /kBT ) n 0 (T )

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Eu2Si2O7, ρ = 6.23 g/cm3) with respect to that of SiO2 (ρ = 2.2 g/cm3). This leads to stronger absorption of X-rays within the nanoparticles than in the matrix, leading to a higher probability of excitation of the incorporated Eu3+ ions. The number of CF lines detected in the heavily Eu-doped silica is, however, larger than that monitored in the pyrosilicate pellet (Table S2 in the Supporting Information), which means that an additional minor amount of Eu3+-related species may contribute to the CF spectra. An example is the set of lines displayed in the dashed frame for curve a in Figure 6B, which were exhibited by a selected area of the 10% mol Eu3+-doped silica but not by the pyrosilicate pellet (curve c). In addition to the Eu pyrosilicate phase, electron diffraction analysis identified the presence of hexagonal Eu5(SiO4)3O, which might be responsible for minor contributions to the CF spectrum (see section 4.3). According to section 4.1.3, in the 10 mol % Eu3+-doped sample the phase separation does not involve only Eu2Si2O7 but also α-quartz inclusions; thus, some of the supplementary CF lines might be originated by Eu in quartz. However, this remains a hypothesis since no reference data are available on Eu absorption lines in quartz in the present considered spectral range. 4.1.2. Role of Eu3+ Doping Level on OH Concentration. As the Eu3+ concentration increases from 0.1 to 10 mol %, the Si− OH peak at 3670 cm−1 decreases, becoming no longer detectable in the 10 mol % sample (Figure 1). The OH concentration (COH) was evaluated from the amplitude and/or the subtended area of the 4521 cm−1 peak,45 and the analysis was extended to still lower Eu3+ doping levels (down to 10−3 mol %). An initial increase in the OH content with respect to that monitored in the undoped sample (C0,OH ≈ 0.4 mol %; curve a), is followed by a decrease that becomes more evident for Eu3+ concentrations ranging between 1 and 10 mol % (Figure 1). The maximum OH content (Cmax,OH ≈ 1.4 mol %) is found in the 0.03 mol % Eu3+-doped sample. Similar trends have been detected for sol−gel silica samples doped with increasing concentrations of Ce3+ and Yb3+.26 However, even heavy Ce3+ doping (10 mol %) did not cause the complete vanishing of the Si−OH band, as in the present case (Figure 1, curve e), confirming the peculiar features of the 10 mol % Eu3+ sample. A decrease in the Si−OH and H2O content as a function of Pr3+ doping level (in the 2 × 10−4 to 1 mol % range) has also been reported for silica xerogel pellets annealed at 900 °C,36 but such a temperature was apparently too low to remove water completely (section 3.1). The trend of COH versus Eu3+ concentration confirms once more the interpretation already supplied for other RE3+ ions.26 At low doping levels, Eu3+ is present as an isolated defect in the glassy matrix: it substitutes for Si4+ and thus requires charge compensation, which may be easily offered by substitution of OH− for O2−. At higher doping levels, Eu clustering starts until phase separation may occur, as clearly supported by the CF spectra (section 4.1.1), vibrational spectra (section 4.1.3), and TEM images (Figure 8). Under these circumstances, charge compensation provided by OH is no longer necessary, and thus, the Si−OH content decreases again because OH groups can be desorbed during the annealing at 1050 °C. Recent molecular dynamics modeling of Eu3+ in an ideal OH-free silica showed that a few Eu3+ pairs are already present at Eu3+ contents as low as 1 mol % in addition to dominating isolated Eu3+; increasing the doping level results in the formation of

(1)

where g1 is the degeneracy of the 7F1 level (the 7F0 is nondegenerate), kB is the Boltzmann constant, and n0(T) + n1(T) = N is a constant under the assumption that only the 7F0 and 7F1 levels are populated over the temperature range considered (the next level, 7F2, falls ∼1000 cm−1 above 7F0 according to Carnall58 for Eu3+(aq) ions and to the present RL spectra, e.g., curve c in the Figure 5 inset). Thus, eq 1 becomes n1(T ) = N

g1 exp( −ΔE1 − 0 /kBT ) 1 + g1 exp( −ΔE1 − 0 /kBT )

(2)

The red solid curve in the top right panel of Figure 9 provides a satisfactory fit of the experimental data according to eq 2 under the hypothesis that the normalized 4579 cm−1 peak intensity at given temperature T is proportional to n1(T). The parameters ΔE1−0 = (240 ± 30) cm−1 and g1 = 11.6 ± 6.1 can be extracted. The ΔE1−0 value of (240 ± 30) cm−1 is in reasonable agreement with those directly obtained from the spectra (i.e., 251 and 250.5 cm−1 for the 10 mol % Eu-doped sample and the Eu pyrosilicate pellet, respectively). The g1 value requires a more detailed analysis: (1) both of the natural Eu isotopes, 151 Eu and 153Eu, whose natural abundances are 47.8 and 52.2%, respectively, are endowed with a nuclear spin I = 5/2, which contributes a factor of 2I + 1 = 6 to the degeneracy; (2) J = 1 for the 7F1 level, which contributes a factor of 3 to the degeneracy. Thus, g1 = 18 is expected. In view of (1) the rather large error (±6.1) in the g1 value extracted from the fit, (2) the approximations introduced, (3) the rather weak signals, mainly at low T, and (4) the limited number of experimental data, the agreement is not too bad. The green dashed curve in the top right panel of Figure 9, which simulates the trend of eq 2 by keeping ΔE1−0 fixed at 251 cm−1 (the value deduced from the spectra; see Figure 9, top left panel) and g1 fixed at 18, does not deviate significantly from the experimental data. These results strongly suggest that phase separation occurs in the 10% mol Eu3+-doped silica: Eu2Si2O7 crystalline inclusions rather than generic Eu3+-rich clusters are present in the glass. The finding agrees also with the molecular dynamics simulations,29 which envisaged in 4−5 mol % Eu-doped silica the onset of Eu clusters where the number and the distance between Eu3+−Eu3+ neighbors have the typical values exhibited by crystalline Eu2Si2O7. However, at those relatively dilute Eu concentrations, such clusters remain only precursors of small crystals, which form in the present 10 mol % Eu-doped sample and have been identified by CF spectra. Electron diffraction analysis (see section 4.3) proves that the big nanoparticle in the 10% mol Eu3+-doped silica (indicated by the arrow in Figure 8c) is indeed a Eu2Si2O7 single crystal. Further support comes from the RL efficiency: in amorphous silica, the maximum is reached within a rather low Eu concentration range (between 0.01 and 0.1 mol %) and is followed by a monotonic decrease, possibly due to concentration quenching. The crystallization occurring in the 10 mol % Eu-doped silica causes an increase of nearly 2 orders of magnitude in the RL efficiency (section 3.3). Such an increase may be attributed to the higher crystalline order experienced by Eu3+ ions. In fact, point defects present in the amorphous silica can favor energy transfer processes from Eu3+ excited levels and, as a consequence, nonradiative deexcitations. The much higher RL efficiency could also be related to the remarkably higher density (ρ) of the Eu-rich phase (for 26839



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large Eu3+ clusters at the expense of both Eu3+ pairs and isolated Eu3+.29 The bump at 3600 cm−1 overlapping the Si−OH peak in the RT spectra of the 1 and 3 mol % Eu3+-doped samples and being more marked in the latter (Figure 1, curves c and d) is absent in undoped silica (curve a). This suggests that Eu3+ may be responsible for it. Two hypotheses may be put forth to explain its origin. The former attributes the bump to a Eu3+ CF transition, while the latter associates it to an OH (or Si−OH) group perturbed by a neighboring Eu3+. If the bump were due to a Eu3+ CF transition, it should be present as a well-isolated, rather strong peak in the 10 mol % Eu3+-doped silica spectrum, in which no disturbing overlap with the Si−OH band occurs; however, this is not the case, as proved by curve e in Figure 1 and curve c in Figure 2A. Moreover, no Eu3+ CF line was monitored around 3600 cm−1 in any of the Eu3+ compounds investigated in the present work (Table S2 in the Supporting Information). The latter hypothesis requires the simultaneous presence of Eu3+ and OH (or Si−OH) groups: this is the case for the 1 and 3 mol % Eu3+-doped samples (Figure 1, curves c and d), but not for the 10 mol % Eu3+-doped silica, where OH is absent (Figure 1, curve e). An interaction between OH groups and Eu3+ is well-known to take place in sol−gel silica: it causes Eu3+ fluorescence quenching and favors hole burning within the Eu3+ 5D0 → 7F0 emission line.59 Thus, the 3600 cm−1 bump monitored in the 1 and 3 mol % Eu3+-doped samples can be reasonably attributed to a Si−OH stretching mode perturbed by one or more neighboring Eu3+ ions. Further support for this interpretation is supplied by the inset in Figure 1. The bump present in the 3 mol % Eu3+-doped sample (curve a) vanishes as a consequence of an RTT (curve b): the hightemperature treatment followed by fast quenching (section 2) breaks the Eu−Si−OH complexes responsible for the bump at 3600 cm−1 and causes a moderate increase in the Si−OH absorption at 3670 cm−1 (compare curves a and b in the inset of Figure 1). 4.1.3. IR Absorption Spectra: Fundamental Vibrations. The fundamental vibrational modes of O−Si−O groups are responsible for the “reststrahlen” monitored by absorption and microreflectance spectra of silica samples, as displayed in Figures 3, 4, and 6A. The modes and the related transverse optical (TO) and longitudinal optical (LO) frequencies in undoped silica are the rocking modes at ∼460 cm−1 (TO1) and ∼505 cm−1 (LO1), the bending modes at ∼810 cm−1 (TO2) and ∼820 cm−1 (LO2), and the asymmetric stretching (AS) modes at ∼1075 cm−1 (TO3) and ∼1255 cm−1 (LO3).60 Two different AS modes have been distinguished according to the two Si-adjacent oxygens oscillating either in phase (AS1) or out of phase (AS2), respectively. The disorder typical of a glass matrix causes a mechanical coupling of the two AS modes, thus introducing an additional pair of LO and TO modes at ∼1165 cm−1 (LO4) and ∼1200 cm−1 (TO4) with an inversion with respect to the ordinary sequence of the LO and TO frequencies.61 No meaningful differences in the peak positions and attributions were found for densified sol−gel silica with respect to fused silica.62 In the present work, to get a more comprehensive picture of the vibrational modes in undoped and Eu3+-doped silica, the spectra were taken even in the regions where overtones and combinations of the fundamental modes are expected to originate absorptions (Figure 11). Figure 3 summarizes the changes induced by the higher Eu3+ doping levels (i.e., 3 and 10 mol %) in the region of fundamental vibration modes of O−Si−O groups, as monitored

Figure 11. Optical absorption spectra measured at 9 K in the regions of the vibrational overtones and combinations. Curves a, SiO2:10 mol % Eu3+; curves b, Eu2Si2O7 pellet; curves c, smoky quartz pellet (see Table 1 for the pellet composition). Curves have been vertically shifted for clarity. (A) Δn = 2 region; (B) Δn = 3 region. The inset in (A) shows the absorption spectra of the SiO2:10 mol % Eu3+ sample in the region of fundamental vibrational transitions (Δn = 1) measured at different temperatures (from bottom to top: 9, 80, 150, 220, and 300 K). For clarity, some spectra have been vertically shifted.

by measuring the absorption spectra of CsI/sample pellets. With respect to the undoped sample (curve a), the 3 mol % Eu3+ doping causes a moderate red shift of the main peak at ∼1102 cm−1 and the appearance of a shoulder peaking around 920 cm−1 (curve b). Such a structureless band closely recalls those reported for 10 mol % Tb3+-doped and 8 mol % Gd3+doped sol−gel silicas at ∼940 and 880 cm−1, respectively.25,26 Quite different patterns are exhibited by the spectra measured on pellets containing chips of 10 mol % Eu3+-doped silica, related to two different portions of the same sample, the former being partially transparent and the latter being completely milky (curves c and d, respectively). In addition to a more marked red shift of the main peak from ∼1102 cm−1 in the undoped sample (curve a) to ∼1096 cm−1 in the transparent sample (curve c) and ∼1077 cm−1 in the milky sample (curve d), the main changes are represented by the appearance of new, rather narrow peaks. Lowering the temperature of the milky sample caused such peaks to sharpen gradually and look more resolved, as usually occurs in the absorption spectra of ordered structures (inset in Figure 11A for the 750−950 cm−1 range). The result agrees with that coming from the analysis of the CF and overtone spectra (sections 4.1.1 and 4.1.4, respectively) and emphasizes once more that at the 10 mol % Eu3+ doping level, segregation of a nearly ordered phase takes place within the silica matrix, as supported by TEM analysis (section 4.3). As for the CF spectra, the vibrational ones were compared with those of CsI pellets containing powdered polycrystalline Eu2SiO5, Eu2O3, and Eu2Si2O7 (Figure 3, curves e, f, and g, respectively). The similarity of the additional peaks displayed in the 290−1130 cm−1 range by 10 mol % Eu3+-doped samples (curves c and d) to those of the Eu2Si2O7 pellet (curve g) is evident, as also supported by considering the correspondences indicated in Table S3 in the Supporting Information, which extends to the overtone and combination-mode ranges. The rms deviation of the line positions of 10 mol % Eu3+-doped silica from those of Eu pyrosilicate is only 2.9 cm−1 for the set of 26 lines detected in the 290−2500 cm−1 range. Fewer correspondences or none at all can be found with the Eu2SiO5 or Eu2O3 pellets (curves e and f; see Table S3). 26840



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Figure 10). A similar feature (at ∼554 cm−1) was reported for 10 mol % Tb3+-doped silica and attributed, together with the ∼940 cm−1 shoulder, to amorphous Tb clusters with a composition similar to that of Tb oxyorthosilicate.26 4.1.4. NIR Absorption Spectra: Anharmonicity. Further insight into the stretching modes of SiO4 tetrahedra within the separate phase inclusions in the milky sample can be derived by inspecting the related spectra at higher wavenumbers, in the spectral regions where the weaker absorptions due to combination and overtone modes are expected. Figure 11A,B summarizes the results in the 1400−2400 and 2400−3000 cm−1 (Δn = 2, 3) ranges, respectively, where n is the quantum number labeling the energy levels of an anharmonic oscillator. The anharmonicity of high-energy stretching modes of SiO4 tetrahedra has been demonstrated, for example, in sillenite single crystals.67 As shown in Figure 11A, at the 9 K the milky 10 mol % Eu3+-doped silica displays a well-structured, peak-rich spectrum extending below 2300 cm−1 (curve a), which closely resembles that exhibited by α-quartz (curve c), confirming once more the formation of α-quartz inclusions in the 10 mol % Eu3+-doped silica. The peaks below 2300 cm−1 (curve a) are too strong to be attributed to the partially forbidden Eu3+ CF 7 F 0 → 7 F 3 transition (falling in the 1750−1900 cm −1 range);51,58 more likely they are associated with the first overtones (Δn = 2) of modes whose fundamental absorptions fall in the 820−980 cm−1 range (Δn = 1) for the CsI/10 mol % Eu3+-doped silica and α-quartz pellets (Figure 3, curves d and h and Figure 11A inset). Also, combinations of two different modes may contribute to the 1500−1900 cm−1 spectrum if at least one of them is IR-active.67 Curve b in Figure 11A, which is related to a CsI/Eu2Si2O7 pellet, shows that overtone or combination modes due to Eu pyrosilicate may also contribute to the spectrum of the milky silica sample (curve a); however, the role of α-quartz modes seems dominant. For clarity and simplicity, the spectra of CsI/Eu2SiO5 and CsI/Eu2O3 pellets, which were measured as well, are not reported in Figure 11A, as they do not provide any useful additional information. Higher-order overtones (Δn = 3) and combinations of three modes (at least one of which is IR-active)67 may be responsible for the much weaker absorptions in the 2100−3000 cm−1 region. These partially overlap the peaks originating from the Eu3+ CF 7F0 → 7F4 transition, which appear in the 2600−3200 cm−1 range according to ref 51 and Table S2 in the Supporting Information (see curves a, b, and c for massive 10 mol % Eu3+doped silica, Eu2Si2O7, and smoky quartz pellets, respectively, in Figure 11). The peak at ∼2138 cm−1, which is common between the spectra of the milky silica and the α-quartz pellet (compare curves a and c in Figure 11A), is an example and can be attributed to a second overtone (Δn = 3) of a quartz mode at ∼780 cm−1 (also see Figure 12A). The set of peaks appearing between 2220 and 2700 cm−1 in the milky silica sample cannot be ascribed to CF transitions but rather are due to Δn = 3 vibrational transitions. In fact, the literature data available for Eu3+ in a variety of materials (e.g., LiNbO3, YAG, LaCl3, KY3F10, and YVO4,)68−72 show that no Eu3+ transition occurs in that range. Furthermore, spectra taken on an undoped pyrosilicate (YAlSi2O7) sample show similar peaks, which obviously cannot be attributed to Eu3+ CF transitions but rather are due to pyrosilicate vibrational modes. Curve a in Figure 11B, related to the milky silica, resembles quite closely that of the CsI/Eu2Si2O7 pellet (curve b), although several minor contributions due to overtone or combination vibrational modes of α-quartz cannot be excluded (curve c). For example

It should be noted that there is a strong and a priori unexpected similarity between many peaks displayed in Figure 3 in the 265−2000 cm−1 range by pellets of 10 mol % Eu3+doped silica (curve d) and those of crystalline α-quartz (curve h), as also supported by the correspondences indicated in Table S3 and by TEM analysis (section 4.3). Unequivocal signatures of the presence of α-quartz are two doublets at 367−394 and 780−800 cm−1 and the peak at ∼700 cm−1 (compare curves d and h in Figure 3): such features are absent in IR spectra of another form of crystalline SiO2 (i.e., α-cristobalite).63,64 The tight correspondence extends to the overtone and combination modes, as monitored in massive 10 mol % Eu3+-doped silica and α-quartz samples (Table S3). The rms deviation of the line positions in 10 mol % Eu3+-doped silica from the corresponding ones in α-quartz is only 2.1 cm−1 for the set of 30 lines detected in the 265−2450 cm−1 range. The lines attributed to α-quartz show a weak (nearly negligible) red shift with increasing temperature, at variance with the blue shift recorded for the CF lines attributed to Eu pyrosilicate (Figure 10). These results suggest that at high (10 mol %) Eu3+ doping levels, phase separation occurs with the formation of nanocrystals of pyrosilicate and of α-quartz: it is possible to account for the main features of absorption spectrum of the milky 10 mol % Eu3+-doped silica (curve d) by superimposing the spectra due to Eu pyrosilicate (curve g) and α-quartz (curve h). The Raman results (section 4.2) portray the same picture, as shown by a comparison of both curves d and e with curves h and f in Figure 7. The spectrum related to partially transparent 10 mol % Eu3+doped silica (Figure 3, curve c) looks to be intermediate between those of 3 mol % Eu3+-doped and milky 10 mol % Eu3+-doped silica (curves b and d, respectively): the onset of phase separation is monitored by a weak α-quartz doublet at 394−460 cm−1 and by Eu silicate absorptions peaking at ∼570 cm−1 and in the 750−950 cm−1 range, which overlap the dominant spectrum of pristine silica. The structureless, weak shoulder around 920 cm−1 (curve b) falls in the same range as the high-energy vibrational modes of Eu2SiO5 and Eu2Si2O7 (curves e and g). It may be attributed to generic Eu silicatebased nanoclusters/nanocrystals whose phase could not be determined because of their small dimensions (see section 3.4 for 3 mol % Eu3+-doped silica), as for heavily Tb- and Gddoped silica.25,26 In the partially transparent 10 mol % Eu3+doped silica (curve c), some peaks resembling those of curve d but less defined are superimposed on a broad shoulder in the 750−1000 cm−1 range, recalling that displayed by curve b, as expected for a sample where the Eu3+ distribution is apparently inhomogeneous and where amorphous and nearly ordered nanoclusters/nanocrystals of different sizes may coexist. On the low-energy side, a broad peak at ∼567 cm−1 appears in the spectra of both the 3 mol % Eu3+-doped silica and the partially transparent 10 mol % Eu3+-doped one (curves b and c, respectively) and is more marked in the latter. It recalls peaks displayed by Eu2SiO5 and Eu2Si2O7 in the 520−600 and 500− 620 cm−1 ranges (curves e and g), respectively. Peaks at 530 and 551 cm−1 monitored in Eu pyrosilicate are attributed to terminal bending modes [ν5(A1)].56 It should be remarked that just a (554.5 ± 20.2) cm−1 phonon is responsible for the thermally induced shift of the CF line peaking at 3044.5 cm−1, as determined by the fit (Figure 10, black line) to the experimental data (Figure 10, solid and open triangles for 10 mol % Eu-doped silica and Eu pyrosilicate, respectively) according to the single-phonon coupling model65,66 (formula in 26841



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where De is a parameter appearing in the expression of the Morse potential as a function of distance r: U (r − re) = De{1 − exp[−β(r − re)]}2

in which re is the equilibrium distance and β is the width parameter.73 The De values so obtained (listed in Table S4 in the Supporting Information) range between 0.6 and 0.9 eV; as a consequence of eq 4 and the larger anharmonicities, they are much lower than both the value of 4.69 eV found for the stretching modes of the well-isolated SiO4 groups in Bi12SiO20 sillenite67 and the value of 5.89 eV estimated for the Si−OH groups in synthetic silica. In molecular dynamics simulations of the structural properties of oxides (vitreous silica included) and silicates, the Morse function (eq 5) has also been used to model the short-range contribution of a pairwise interatomic potential using the parameter Dij (replacing the above-mentioned De) to indicate the interaction between two atoms i and j. Different authors have employed for the Si−O pair Dij values of 0.3429,75 and ∼2 eV,76 respectively. Such values average all possible Si− O vibrational modes, while in the present case specific modes are selected, for example, the α-quartz bending mode at 780 cm−1, for which De = 0.66 eV (Table S4). 4.1.5. IR Microreflectivity Spectra: Fundamental Vibrations. Reflectivity spectra have in the recent past been exploited to monitor the effects produced by increasing doses of implanted ions (e.g., Ar, N, Cu) into silica films. They consist of a shift of the vibrational frequencies to lower values and a decrease and a broadening of the peaks; subsequent sample annealing at high temperature causes a partial recovery of the spectra.60 The shift and the shoulder appearing at ∼1040 cm−1 are attributed to Si−O−Si bond breaking.77 The microreflectivity results, summarized in the top panel of Figure 4 for samples doped with increasing amounts of Eu3+, suggest that doping up to 3 mol % Eu3+ causes only gradual changes in the shape of the spectrum, mainly in the region of AS stretching modes. The difference spectra for 0.01, 1, and 3 mol % Eu3+doped silica with respect to that of the undoped sample (curve a), displayed in the bottom panel of Figure 4, show that with increasing Eu3+ amount the normalized reflectivity decreases mainly in two spectral regions: that of the pure silica LO3 frequency and that of the TO4 and LO4 frequencies (curves h− l). The LO modes are sensitive to long-range electrostatic interactions: the presence of the Eu3+ ions (which may substitute for Si4+)29 modify such an interaction, decreasing the number of pristine O−Si−O active modes. The LO3 mode contribution starts to be weakened at Eu3+ concentrations as low as 0.01 mol % (curve h). In the case of the LO4 and TO4 modes, which arise from the mechanical coupling induced by disorder,61 Eu3+ may play a similar role by interrupting the interaction sequence, thus decreasing their statistical weight. Similar results were obtained for Tb3+-, Ce3+-, Gd3+-, and Yb3+doped silica.26 The smooth increase in the normalized microreflectivity in the 700−1000 cm−1 range induced by increasing the Eu3+ doping level up to 3 mol % (curves b, c, i, and l in Figure 4) recalls the broad shoulder peaking around 920 cm−1 exhibited by the absorption spectrum of the CsI/3 mol % Eu3+-doped silica pellet (curve b in Figure 3). It may be attributed to vibrational modes of Eu-related clusters with a composition similar to that of Eu2Si2O7 (the diffuse reflectance spectrum of Eu2Si2O7 powders is portrayed as curve f in the top panel of Figure 4 for comparison).

Figure 12. Plots of ωn/n vs n for sets of fundamental (n = 1), first overtone (n = 2), and second overtone (n = 3) frequencies of a few vibrational modes monitored at 9 K for the 10 mol % Eu3+-doped silica sample (solid black squares, panels A−C), compared to those of quartz (solid red circles, panel A) and of Eu2Si2O7 pellet (open blue circles, panels B and C). For pellet compositions, see Table 1. The green open stars in panel A are related to quartz modes measured at RT. The full, dashed, dotted, and dot-dashed lines are the linear fits of the solid squares, solid circles, open stars, and open circles data according to eq 3.

the peak at ∼2595 cm−1, common between milky silica and Eu pyrosilicate, can be regarded as a Δn = 3 vibrational transition of the pyrosilicate mode at ∼935 cm−1 (Figure 11; also see Figure 12C). Possible associations among the fundamental frequencies (ω1, Δn = 1, measured on pellets; see Figure 3 and the Figure 11 inset) and the related overtones (ω2, Δn = 2; ω3, Δn = 3, measured on the massive sample; see Figure 11) for a set of selected modes in 10 mol % Eu3+-doped silica, α-quartz, and Eu pyrosilicate have been found (Table S4 in the Supporting Information). If the associations are correct, in the framework of the Morse anharmonic oscillator model, a plot of the ratio ωn/n versus n should give a straight line67,73 according to the equation ωn = ω0 − ω0xe(n + 1) (3) n where ω0 is the natural oscillation frequency and xe is the anharmonicity parameter. The linear dependence was found indeed for a few vibrational modes (Figure 12). Very good overlap was found between the data related to α-quartz (at 9 K and RT) and the milky silica sample (at 9 K) in the case of the ∼780 cm−1 mode for Δn = 1, 2, and 3 (Figure 12A). The anharmonicity parameters are in the (3.5−4) × 10−2 range, which is higher than the value found for SiO4 stretching modes in Bi12SiO20 sillenite (5.9 × 10−3).67 Such a difference can be understood as follows: In sillenite, the SiO4 groups behave practically as noninteracting Einstein oscillators (and thus are less anharmonic) because they are isolated from each other by the “wall” represented by the heavy Bi network. This is at variance with silica, where SiO4 groups constitute the backbone of the material. The anharmonicity is also little higher than that reported for silanol stretching mode in synthetic silica (xe = 2.1 × 10−2);74 this is a terminal Si−OH group, thus enabling the light H atom to vibrate nearly decoupled from the silica network. From the anharmonicity parameter xe, the binding energy De can be evaluated as De = ω0 ·(4xe)−1

(5)

(4) 26842



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red shift should depend mainly on the local Eu concentration: it accounts once more for the inhomogeneous dopant distribution in the sample, as proved by IR absorption (compare curves c and d in Figure 3), microreflectivity (compare curves d, e, and f in Figure 6A), and Raman spectra (compare curves c, d, and e in Figure 7). Furthermore, curve e in Figure 6A shows that in the relatively small area investigated, crystalline phases coexist also with a variety of amorphous silica networks as a result of different local Eu concentrations, as proved, for example, by the three peaks shifted to 1115, 1091, and 1076 cm−1 with respect to the 1123 cm−1 peak monitored in the undoped silica sample (curve a in Figure 6A). All of the analyses performed (CF, RL, and vibrational spectra) emphasize the peculiar effects produced by 10 mol % Eu3+ doping on silica, namely, the growth of ordered phases (αquartz and Eu2Si2O7), while at lower concentrations (e.g., 3 mol %) the largest part of network and of the Eu-induced clusters remains amorphous (even if some nanocrystalline Eu-related clusters are detected by TEM; see Figure 8a and section 4.3). The Eu specificity can also be seen by comparing the effects produced by heavy doping (e.g., 10 mol %) with other RE3+ ions, such as Ce3+ and Tb3+.26 The spectra related to 10 mol % Ce3+ and Tb3+ with respect to the undoped sample show a moderate red shift of the AS peak at ∼1123 cm−1 (open star and diamond in Figure 5), a more or less marked dip around ∼1200 cm−1, and the growth of a smooth shoulder in the 800− 1000 cm−1 range; these features are reminiscent of those detected in the spectrum of 3 mol % Eu3+-doped silica (curve c in Figure 4) rather than the rich structure displayed by the 10 mol % Eu3+-doped silica (curve d in Figure 4 and curve e in Figure 6A). A tentative explanation may be found by considering that both Ce and Tb can also be tetravalent, in which case the silicon−oxygen network is not disrupted as a consequence of Ce (or Tb) substitution for Si4+; in contrast, Eu can also be divalent (although this hypothesis is excluded in the present case by RL measurements; see section 3.3), thus increasing the number of broken Si−O bonds. 4.2. Raman Spectra. Evidence of Eu clustering is clearly demonstrated also by the evolution of Raman spectra as a function of Eu concentration (Figure 7). While the spectrum related to the 3 mol % Eu3+-doped sample features some modifications of the silica pattern indicating the onset of clustering, the spectra collected on 10 mol % Eu3+-doped SiO2 evidence clear fingerprints of RE aggregates. The comparison with spectra of Eu oxide (Eu2O3), Eu silicate (Eu2SiO5), and Eu pyrosilicate (Eu2Si2O7) powders rules out the formation of Eu2O3 (Table S5 in the Supporting Information). In turn, similarities with the spectra of Eu2SiO5 and Eu2Si2O7 are evidenced. The presence in curve d of the main peaks at 740 and 998 cm−1 (and the correspondences indicated in Table S5) suggests a more pronounced agreement with the latter, that is, the silicate structure with the highest number of SiO 4 tetrahedra in its stoichiometry. Moreover, the Raman spectra of 10 mol % Eu3+-doped SiO2 also display very clearly the presence of crystalline α-quartz (curve f for comparison and correspondences listed in Table S5), as demonstrated by the strong peak at around 465 cm−1 that is unequivocally associated with this allotropic form of crystalline SiO2,79 in agreement with the IR−NIR absorption and microreflectivity results (sections 4.1.3−4.1.5). 4.3. TEM Analysis. As mentioned in section 3.4, the poor quality of the diffraction patterns recorded on a small area of the 3 mol % Eu-doped silica prevented the determination of the

As for the absorption, RL, and Raman spectra (Figures 1−3, the Figure 5 inset, and Figure 7, respectively), the microreflectivity spectra undergo amazing changes as the silica samples are doped with a Eu3+ concentration as high as 10 mol % (curve d in Figure 4 and curves d and e in Figure 6A). A set of well-structured peaks appears between 820 and 1500 cm−1 (i.e., in the region of Eu2Si2O7 vibrational modes; compare curves d and f in Figure 4). The detailed pattern of these additional peaks depends on the specific area of the milky sample investigated (compare curves d and e in Figure 6A); such differences can be caught also by examining the CF absorption spectra in the 3200−3400 cm−1 region (compare curves a and b in Figure 6B). Although the above-mentioned inhomogeneous dopant distribution in a heavily doped sample can be easily justified in terms of exceeding the solubility limit, it should be noticed that some departure from uniformity was occasionally detected for Eu3+ concentrations as low as 0.001 mol % (e.g., compare curves b and c in Figure 6A). As supported by the absorption spectra (Figures 3 and 11) and Raman spectra (Figure 7), some peaks are common to 10 mol % Eu-doped silica and α-quartz (see the correspondences indicated in Tables S3 and S4 in the Supporting Information). Such coincidences are confirmed by the microreflectance spectra: the dip at ∼1160 cm−1 and the two well-resolved peaks at 780 and 796 cm−1 are features common to microreflectance spectra of an α-quartz single crystal and 10 mol % Eu-doped silica (compare curves d and e in the top panel of Figure 4). At variance, only a single peak at 790 cm−1 appears in the α-cristobalite reflectance spectrum.78 Since the α-quartz inclusions in 10 mol % Eu-doped silica are expected to be randomly oriented, the microreflectance spectrum of 10 mol % Eu-doped silica (Figure 4, top panel, curve d) was compared to the weak diffuse reflectance spectrum of α-quartz powders (Figure 4, top panel, curve g): they look very similar mainly in the 1130−1220 cm−1 range, where the diffuse reflectance accessory exhibits better performance. The results emphasize once more that phase separation occurs within the doped silica with the formation of α-quartz and Eu2Si2O7 nanocrystals. As in the IR absorption spectrum (Figure 3, curve d), the intrinsic silica AS modes are heavily modified by the 10 mol % Eu3+ doping: the reststrahlen peak at 1123 cm−1 (undoped sample, Figure 6A, curve a), whose position remains nearly constant for Eu concentrations up to 1 mol % (Figure 5), shifts to ∼1115 and 1116 cm−1 for the smooth face of the milky piece and the partially transparent chip (Figure 6A, curves e and f, respectively), while two additional side peaks (a doublet) appear at ∼1076 and 1091 cm−1 (curve e). The microreflectivity spectrum of the rough face of the milky piece displays a unique broad peak at ∼1078 cm−1 (Figure 6A, curve d). In all cases, the high-energy contribution to the pristine silica reststrahlen appears to be strongly reduced. In addition to the presence of Eu2Si2O7 inclusions, it seems that even the residual silica network has been heavily modified. The red shifts in both the main IR absorption at ∼1100 cm−1 and the microreflectance peak at ∼1123 cm−1 induced by Eu doping of ≥3 mol % may be attributed to both the larger atomic mass of dispersed Eu3+ ions, which substitute for some of the lighter Si4+ in the Si−O−Si chains, and the more loosely bound oxygens. The role played by the nonbridging oxygens, whose concentration may be enhanced by an RTT (section 2), is proved by the microreflectance peak red shift monitored even in undoped and 1 mol % Eu-doped silica samples (red line and red symbol in Figure 5). In as-prepared samples, the size of the 26843



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Figure 13. (a) Electron diffraction pattern of a 10 mol % Eu3+-doped silica sample taken on a 25 μm2 selected area, similar to that in Figure 8b. (b) Plot of the radial integration of the pattern in (a). The ticks at the bottom correspond to the positions of the reflections of the Eu5(SiO4)3O hexagonal phase.

Figure 14. (a) Bright-field TEM image of a small area of a 10 mol % Eu3+-doped silica sample near the border of the grain displayed in Figure 8b. (b−d) Diffraction patterns (gray/black) collected in ASTAR mode on three different nanoparticles. The best matching pattern, generated according to the hexagonal structure of Eu5(SiO4)3O, is displayed in red and superimposed. (e) Pattern collected in ASTAR mode in between the nanoparticles in an area of image (a) where the contrast is lighter. The absence of any diffraction spot indicates that the embedding matrix is amorphous.

cannot be indexed with the hexagonal Eu5(SiO4)3O phase. These Eu nanoparticles were large enough to be investigated with standard electron diffraction. The smallest condenser aperture of the Libra microscope allowed an area with a diameter of 150 nm to be illuminated, avoiding collection of signal from the surrounding matrix in the case of larger crystals. The 400 nm crystal displayed in Figure 8c was analyzed using the ADT technique. Data collection over 100° gave a triclinic unit cell with a = 8.50(7) Å, b = 12.9(1) Å, c = 5.32(5) Å, α = 91.9(5)°, β = 92.1(5)°, and γ = 89.9(5)° (Figure 16) which is compatible to the triclinic high-temperature phase of Eu2Si2O7.81 Thus, the presence of α-quartz and Eu nanocrystals in the 10 mol % Eu-doped silica predicted by the spectroscopic measurements is nicely supported by direct evidence from the TEM analyses. 4.4. Devitrification Process. Devitrification of silica glasses as a consequence of prolonged (20 h) heat treatment at 1300 °C was reported by Crookes already in the early 20th century.82 As a “supercooled liquid”, silica glass is characterized by high viscosity, which prevents the structural rearrangement needed during a fast cooling to promote the formation of crystalline quartz, which is the thermodynamically stable state. Crystallization may occur at elevated temperatures in the presence of contaminants that lower the viscosity by breaking up the highly connected silicon−oxygen network and/or acting as nucleation centers. Examples of impurities that catalyze devitrification are alkalis,83,84 OH,85 chlorine, and fluorine.86 In sol−gel silica doped with some REs (Er, Ce), the fluorine codopant initiates the crystallization either by inducing local order around the RE

crystal phase of the nanocrystals displayed in Figure 8a. On the other hand, the broad and structureless bands displayed by the CF (Figure 2A, curve b), RL (Figure 5, inset, curve c), and vibrational spectra (Figure 3, curve b; Figure 4, curve c; Figure 7, curve b) suggest that such nanocrystals represent a very minor fraction of the nanoclusters. The TEM results do not contradict the optical spectroscopy ones because the former give local information related to a very small area while the latter gather information from a much larger sample area and thickness. In the case of Figure 8b, related to the 10 mol % Eu-doped silica, the high density of nanoparticles allowed the collection of powder electron diffraction patterns with almost complete rings (Figure 13a). The integrated pattern shows peak positions that can be indexed with a hexagonal cell of a = 9.58(5) Å and c = 6.96(5) Å and extinction conditions compatible with the P63/m space group (Figure 13b). This unit cell and symmetry are compatible with a Sm5(SiO4)3O structure,80 where Eu replaces Sm. To confirm this interpretation, a 1 μm × 1 μm area in the thinnest part of a nanocrystal-rich glass particle was scanned using the ASTAR technique. All of the nanocrystals close to the edge and not overlapping one another could be indexed with the hexagonal phase, while the matrix between them did not show any diffraction spots, indicating that it is amorphous (Figure 14). In the case of the bigger nanoparticles embedded in a crystalline matrix (Figure 8c) the ASTAR technique shows that the matrix is formed by α-quartz nanocrystals (Figure 15), while the Eu-rich nanoparticles exhibit diffraction patterns that 26844



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(section 4.1.2). (2) Eu3+ clusters, which are already present in the 0.1−3 mol % Eu doping range,29,34 may act as nucleation centers. Eu3+ catalyzes silica dissolution, as proved by fluorescence analysis of Eu3+ sorption−desorption processes on nonporous silica in aqueous solutions.88 In the initial step, a complex of Eu3+ with Si−OH groups forms at the silica surface (at the expense of water molecules). The further decrease in the number of water molecules, the concomitant silica dissolution, and the irreversibility of the Eu3+ sorption suggest that a new phase grows at the silica surface. The increased Eu3+ fluorescence lifetime during the Eu3+ sorption process rules out the involvement of Eu3+ hydroxide. Eu3+ silicate has been proposed as a candidate, but no hint of its specific composition and structure has been supplied. The above picture, obtained in a completely different framework (i.e., that of geochemistry and radioactive waste management88), can be applied to the steps that bring the initial sol to the present 10 mol % Eu3+-doped silica and account for the OH removal and the separation of phases, which CF and vibrational spectroscopies, supported by TEM images, were able to identify as Eu pyrosilicate and αquartz. A process that occurs during the rather slow cooling from the densification temperature (1050 °C; see section 2) may be envisaged: in the early step, nanocrystals of Eu pyrosilicate grow, and the two SiO4 tetrahedra may act as crystallization seeds for the distorted ones belonging to the surrounding silica, with the final formation of a matrix including α-quartz, as clearly supported by Figure 8c and section 4.3. Crystallization of undoped sol−gel silica has been already mentioned as a consequence of heat treatment for 3 h at 1000 °C62 and for 2 h at 1300 °C.89 In both cases, the crystalline phase was identified as α-cristobalite and not α-quartz; on the contrary, the occurrence of α-quartz in the present 10 mol % Eu-doped silica heat-treated at 1050 °C is clearly proved by the TEM and spectroscopic analyses. According to XRD measurements performed on sol−gel silica, undoped or doped with RE (Pr, Eu, Ho, Er) amounts increasing from 0.5 to 6 mol % and heat-treated at 1200 °C for 2 h, crystallization takes place in form of α-cristobalite.90 Thus, prolonged thermal treatments at rather high temperatures seem to favor the onset of the silica to α-cristobalite transformation. The rate of subsequent cooling may also play an important role because at ordinary pressures α-cristobalite is metastable at temperatures lower than ∼270 °C, while α-quartz is the stable form of silica below 573 °C.91

Figure 15. (a) Virtual bright-field images obtained by integrating the central spot of all the patterns recorded in ASTAR mode (see ref 44 for details) on the left part of the grain displayed in Figure 8c. (b) Display of the index value for α-quartz. The index value was calculated through a cross-correlation between the collected patterns in ASTAR mode and the patterns generated according to the α-quartz structure (see ref 44 for details). An intense green color means a good match with some α-quartz crystal orientation. The matching is good in the matrix and bad in the heavy-element-containing nanoparticles. (c) Diffraction patterns (in gray/black) collected in ASTAR mode on three nanoparticles of the matrix where the index with α-quartz is high. The best matching pattern, generated according to the crystal structure of α-quartz, is displayed in red and superimposed.

5. CONCLUSION Complementary spectroscopic techniques have been applied to thoroughly study Eu incorporation into silica samples prepared via a sol−gel route. The crystal field absorption, radioluminescence, and vibrational (absorption, Raman, and microreflectance) spectra have converged to prove unequivocally (1) the growth of amorphous Eu3+-rich clusters and Eu−OH complexes as the dopant concentration increases in the 0.001− 3 mol % range and (2) the devitrification of silica, which occurs at a Eu concentration of 10 mol %, due to the formation of nanocrystals of Eu pyrosilicate and (unpredictably in principle) α-quartz. The onset of the ordered nanocrystalline phase is proved by the following observations: (1) narrowing of the Eu spectral lines with increasing Eu concentration from 3 to 10 mol % and decreasing temperature over the 300−9 K range; (2) the tight correspondence of the line positions in the 10 mol % Eu-doped silica with those of (poly)crystalline Eu pyrosilicate and αquartz as well as the common thermally induced line shift (well-

Figure 16. (a−c) 3D reconstruction of the reciprocal space of Eu2Si2O7 viewed along the three main zone axis directions. For clarity, 16 unit cells of the reciprocal lattice are superimposed in the images. (d) 3D reconstruction viewed along the tilt axis to point out the 100° reciprocal space coverage. The reconstruction was obtained from electron diffraction tomography data collected on the crystal indicated by the arrow in Figure 8c.

ion or by its partial decoupling from the disordered host matrix due to the creation of network terminating sites.52,87 The present Eu-doped sol−gel silica samples sintered at 1050 °C exhibit the preconditions for the onset of crystallization: (1) Eu3+, by substituting for Si4+, favors the formation of nonbonding oxygen, thus interrupting the continuity of the silicon−oxygen network. Increasing the Eu concentration increases the silica depolymerization as well and is further emphasized by the simultaneous presence of Si−OH groups 26845



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(4) Pazik, R.; Gluchowski, P.; Hreniak, D.; Strek, W.; Ros, M.; Fedyk, R.; Lojkowski, W. Fabrication and Luminescence Studies of Ce:Y3Al5O12 Transparent Nanoceramics. Opt. Mater. 2008, 30, 714− 718. (5) Lempicki, A.; Brecher, C.; Szupryczynski, P.; Lingertat, H.; Nagarkar, V. V.; Tipnis, S. V.; Miller, S. R. A New Lutetia-Based Ceramic Scintillator for X-ray Imaging. Nucl. Instrum. Methods Phys. Res., Sect. A 2002, 488, 579−590. (6) Li, H.-L.; Liu, X.-J.; Xie, R.-J.; Zeng, Y.; Huang, L.-P. Fabrication of Transparent Cerium-Doped Lutetium Aluminum Garnet Ceramics by Co-Precipitation Routes. J. Am. Ceram. Soc. 2006, 89, 2356−2358. (7) Lempicki, A.; Brecher, C.; Lingertat, H.; Miller, S. R.; Glodo, J.; Sarin, V. K. A Ceramic Version of the LSO Scintillator. IEEE Trans. Nucl. Sci. 2008, 55, 1148−1151. (8) Brecher, C.; Bartram, R. H.; Lempicki, A. Hole Trap in Lu2O3:Eu Ceramic Scintillators I. Persistent Afterglow. J. Lumin. 2004, 106, 159−168. (9) Bartram, R. H.; Lempicki, A.; Kappers, L. A.; Hamilton, D. S. Hole Traps in Lu2O3:Eu Ceramic Scintillators II. Radioluminecence and Thermoluminescence. J. Lumin. 2004, 106, 169−176. (10) Mihóková, E.; Nikl, M.; Mareš, J. A.; Beitlerová, A.; Vedda, A.; Nejezchleb, K.; Blažek, K.; D’Ambrosio, C. Luminescence and Scintillation Properties of YAG:Ce Single Crystal and Optical Ceramics. J. Lumin. 2006, 126, 77−80. (11) Juzenas, P.; Chen, W.; Sun, Y.-P.; Coelho, M. A. N.; Generalov, R.; Generalova, N.; Christensen, I. L. Quantum Dots and Nanoparticles for Photodynamic and Radiation Therapy of Cancer. Adv. Drug. Delivery Rev. 2008, 60, 1600−1614. (12) Liu, Y.; Chen, W.; Wang, S.; Joly, A. G. Investigation of WaterSoluble X-ray Luminescence Nanoparticles for Photodynamic Activation. Appl. Phys. Lett. 2008, 92, No. 043901. (13) Bihari, B.; Eilers, H.; Tissue, B. M. Spectra and Dynamics of Monoclinic Eu2O3 and Eu3+:Y2O3 Nanocrystals. J. Lumin. 1997, 75, 1− 10. (14) Meltzer, R. S.; Feofilov, S. P.; Tissue, B.; Yuan, H. B. Dependence of Fluorescence Lifetimes of Y2O3:Eu3+ Nanoparticles on the Surrounding Medium. Phys. Rev. B 1999, 60, R14012−R14015. (15) Goldburt, E. T.; Kulkarni, B.; Bhargava, R. N.; Taylor, J.; Libera, M. Size Dependent Efficiency in Tb-Doped Y2O3 Nanocrystalline Phosphor. J. Lumin. 1997, 72−74, 190−192. (16) Muenchausen, R. E.; Jacobsohn, L. G.; Bennet, B. L.; McKigney, E. A.; Smith, J. F.; Valdez, J. A.; Cooke, D. W. Effects of Tb Doping on the Photoluminescence of Y2O3:Tb Nanophosphors. J. Lumin. 2007, 126, 838−842. (17) Mercier, B.; Dujardin, C.; Ledoux, G.; Luis, C.; Tillement, O.; Perriat, P. Observation of the Gap Blueshift on Gd 2 O3 :Eu 3+ Nanoparticles. J. Appl. Phys. 2004, 96, 650−653. (18) Mercier, B.; Ledoux, G.; Dujardin, C.; Nicolas, N.; Masenelli, B.; Melinon, P.; Bergeret, G. Quantum Confinement Effect on Gd2O3 Clusters. J. Chem. Phys. 2007, 126, No. 044507. (19) Cooke, D. W.; Lee, J.-K.; Bennett, B. L.; Grooves, J. R.; Jacobson, L. G.; McKigney, E. A.; Muenchausen, R. E.; Nastasi, M.; Sickafus, K. E.; Tang, M.; Valdez, J. A. Luminescent Properties and Reduced Dimensional Behaviour of Hydrothermally Prepared Y2SiO5:Ce Nanophosphors. Appl. Phys. Lett. 2006, 88, No. 103108. (20) Muenchausen, R. E.; Jacobson, L. G.; Bennett, B. L.; McKigney, E. A.; Smith, J. F.; Cooke, D. W. A Novel Method for Extracting Oscillator Strength of Select Rare-Earth Ion Optical Transitions in Nanostructured Dielectric Materials. Solid State Commun. 2006, 139, 497−500. (21) Chen, W.; Zhang, J.; Wescott, S. L.; Joly, A. G.; Malm, J.-O.; Bovin, J.-O. The Origin of X-ray Luminescence from CdTe Nanoparticles in CdTe/BaFBr:Eu2+ Nanocomposite Scintillators. J. Appl. Phys. 2006, 99, No. 034302. (22) Létant, S. E.; Wang, T.-F. Semiconductor Quantum Dot Scintillation under γ-Ray Irradiation. Nano Lett. 2006, 6, 2877−2880. (23) Kesanli, B.; Hong, K.; Meyer, K.; Im, H.-J.; Dai, S. Highly Efficient Solid-State Neutron Scintillators Based on Hybrid Sol−Gel Nanocomposite Materials. Appl. Phys. Lett. 2006, 89, No. 214104.

described by the single-phonon coupling model), the common sequences of fundamental vibrations and related overtones, and the comparable anharmonicity parameters (in the framework of Morse model); (3) the sharp increase of radioluminescence intensity in 10 mol % Eu samples and the simultaneous suppression of OH, a widespread impurity often responsible for luminescence quenching; and (4) further direct evidence by TEM and electron diffraction analysis. The peculiar behavior of Eu in comparison with other REs (e.g., Ce, Gd, Tb, and Yb), to develop nanocrystal growth at 10 mol % doping accompanied by OH removal opens some prospects in the field of lasers and scintillators: the present approach may be extended to silica doped with Eu 3+ concentrations in the 3−10% range (not yet investigated) to identify the proper doping level and thermal treatments capable of producing size-controlled, Eu-rich luminescent nanocrystals embedded in a transparent host matrix.

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ASSOCIATED CONTENT

* Supporting Information S

Tables with wave numbers of the main absorption lines, fit results, and Raman shifts. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses ∥

E.B.: IMEM-CNR Institute, Parco Area delle Scienze 37/A, 43124 Parma, Italy. ⊥ M.M.: ICFO-The Institute of Photonic Sciences, Mediterranean Technology Park, Av. Carl Friedrich Gauss, num. 3, 08860 Castelldefels, Barcelona, Spain. # A.L.: Laboratory for Multifunctional Materials, Department of Materials, ETH Zürich, Wolfgang-Pauli-Strasse 10, 8093 Zürich, Switzerland. ∇ F.M.: ILM UMR5306 Université Claude Bernard Lyon 1CNRS, Bâtiment Alfred Kastler, 10 rue Ada Byron, 69622 Villeurbanne cedex, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are indebted to Prof. Roberto Boscaino (University of Palermo, Italy) for supplying samples of pure fused silica, to Dr. Pavel Bohacek (Academy of Sciences, Prague, Czech Republic) for supplying some reference compounds, and to Dr. Norberto Chiodini and Dr. Elena Rizzelli (University of Milano-Bicocca, Italy) for help in sol−gel sample preparation and in RL measurements.

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Eu Incorporation into Sol–Gel Silica for Photonic Applications

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