Rare Earth-Activated Silica-Based Nanocomposites

1 CSMFO group, CNR-IFN, Istituto di Fotonica e Nanotecnologie, via Sommarive, 14-38050 Povo, Trento, Italy 2 Laboratoire de Chimie Appliquéede de l’Etat Solide, Ecole Nationale Supérieure de Chimie de Paris (ENSCP), UMR-CNRS 7574, 11 rue Pierre et Marie Curie, 75231 Paris, France 3 Dipartimento di Fisica, Università degli Studi di Trento, Gruppo CSMFO, Via Sommarive, 14-38100 Povo, Trento, Italy 4 Laboratoire CRISMAT, UMR 6508, ENSICAEN, 6 Boulevard du Maréchal Juin, 14050 Caen Cedex, France 5 CNR-IFAC, Istituto di Fisica Applicata, Nello Carrara, via Madonna del Piano, 10-50019 Sesto Fiorentino, Firenze, Italy 6 CNR, Dipartimento Materiali e Dispositivi, via dei Taurini 19, 00185 Roma, Italy


INTRODUCTION
The recent developments of nanocomposite materials activated by rare-earth ions have opened new possibilities in the field of both basic and applied physics, in a large area covering information communication technologies, health and biology, structural engineering, and environment monitoring systems.As far as luminescence properties are concerned, Er 3+ -activated nanocomposite glasses have become one of the key materials because of their relevance for to the development of optical amplifiers.The short-term goal is to develop appropriate material systems and devices to exploit, at the best, the luminescence properties of erbium with optimal optical properties of the host.The last decade has seen a remarkable increase in the experimental efforts to control and enhance emission properties of luminescent ions by tailoring the dielectric surrounding of the source [1][2][3][4].With this aim, several approaches, using nanocomposite materials and/or specific geometries, such as planar interfaces, photonic crystals, solid state planar microcavities, dielectric nanospheres, and spherical microresonators, have been proposed.
The aim of this paper is to give an overview of the advances in glass-based photonic systems, where the nanoscale structures or the presence of nanostructured hosts induces an enhancement of optical and spectroscopic properties of the rare-earth ions.In particular, the following topics will be highlighted: (i) rare earth-activated glass-ceramics planar waveguides, where the active ions are embedded in the crystalline phase, combining the mechanical and optical properties of the glass with a crystal-like environment for the rare-earth ions, and (ii) a procedure for the synthesizing of monosize silica spheres and the fabrication of core-shelllike structures of Er 3+ -activated silica spheres obtained by a seed growth method.Optical and spectroscopic assessment, as well as morphological and structural characterization of these systems, is reported.

TRANSPARENT GLASS-CERAMICS
Since the pioneer work in 1993, when Wang and Ohwaki discovered a novel glass-ceramic system characterized by a transparency comparable to glass [5], considerable efforts have been made in order to fabricate rare earth-activated glass-ceramic materials with active ions embedded in the crystalline phase [6].The motivation for this research is combining the mechanical and optical properties of the glass with a crystal-like environment for the rare-earth ions, where their higher cross-sections can be exploited in order to fabricate more compact devices [4,7].Moreover, glass-ceramic materials may be a valid alternative method to control chemical parameters of the rare-earth ions, and thus may avoid undesirable effect like clustering as proposed by Auzel and Goldner [8].Thanks to the low phonon environment favourable to enhance the radiative rate and quantum efficiency, significant results have been achieved using oxyfluoride and fluoride transparent glass-ceramics activated by rare-earth ions, such as Er 3+ , Eu 3+ , Tm 3+ , Nd 3+ , and Pr 3+ incorporated in fluoride nanocrystalline phases [9][10][11][12][13][14].Using a top-down technique based on an appropriate thermal process of the glasses containing rare-earth fluorides, nanocrystals of β-PbF 2 were nucleated in the glassy matrix.Concerning telecom application, the paper by Hayashi et al. concerning Tm 3+ ion-doped transparent oxyfluoride glass-ceramics containing PbF 2 nanocrystals around 20 nm in size, is of particular interest [15].From spectroscopic measurements performed at low temperature, authors show the possibility of optical amplification in the S telecommunication band.Tikhomirov et al. have measured a bandwidth of about 90 nm for the 4 I 13/2 → 4 I 15/2 transition when Er 3+activated oxyfluoride glass-ceramics were prepared with β-PbF 2 nanocrystals of about 2.5 nm in diameter [16].
Oxide-based glass-ceramics have been obtained both by conventional melting and by sol-gel route.Oishi et al. reported highly transparent glass ceramics obtained by heating, at 410-460 • C, the K 2 O-MgO-Nb 2 O 5 -TeO 2 glass precursor activated by Er 3+ and Eu 3+ ions [17].It was demonstrated that the addition of Er 3+ and Eu 3+ ions was effective for the formation of the crystalline phase showing second harmonic generation, as well as enhancing the NIR-to-green upconversion, in the case of Er 3+ -activated glass ceramics [17].Attractive results have been obtained by sol-gel route.Ke ¸pi ński and Wołcyrz report on formation of nanocrystalline rareearth silicates inside or at the surface of amorphous SiO 2 matrix upon heat treatment in air [18].An intense room temperature photoluminescence at 1.531 μm is reported by Que et al. for erbium oxide nanocrystals dispersed in TiO 2 /γglycidoxypropyltrimethoxysilane (GLYMO) composite solgel thin films [19].
It should be mentioned that these nanocomposite systems are of particular interest for photonic application when the glass-ceramics can be prepared in waveguiding configuration.Concerning this, Strohhöfer et al., ten years ago, reported on active optical properties of Er-containing crystallites in sol-gel derived glass films [20].Authors employed controlled heat treatment procedures to obtain a certain fraction of the rare-earth ions in crystallites.The active phases precipitated were Er 2 Ti 2 O 7 and ErPO 4 in SiO 2 -TiO 2based sol-gel films.Doping of the glass matrix with Er 3+containing crystallites improved the Er 3+ fluorescence lifetime of the 1.55 μm transition, in some cases by more than 200%, unfortunately, no satisfactory waveguiding properties were achieved [20].A pure Er 2 Ti 2 O 7 pyrochlore waveguiding structure was proposed by Langlet et al. with interesting spectroscopic properties because this compound has a much lower phonon energy than silica, which would permit to minimize nonradiative absorption mechanisms [21].Recently, Jestin et al. have shown that SiO 2 -HfO 2 : Er 3+ glassceramic planar waveguides prepared by sol-gel route present valuable optical, spectroscopic, and structural features for successful applications in the telecommunication area [22][23][24].

Glass-ceramics fabrication and characterization
(100-x)SiO 2 -xHfO 2 (x = 10, 20, 30 mol) planar waveguides, activated by 0.3 mol % Er 3+ ions, were prepared by solgel route, using dip-coating deposition on v-SiO 2 substrates cleaned by ultrasound and alcohol [24].Depending on their HfO 2 molar content (x = 10, 20, 30 mol), the waveguides discussed in this section are labeled W10, W20, and W30, respectively.The starting solution obtained by mixing tetraethylorthosilicate (TEOS), ethanol, deionized water, and hydrochloric acid as a catalyst was prehydrolyzed for one hour at 65 • C. The molar ratio of TEOS : HCl : EtOH : H 2 O was 1 : 0.01 : 37.9 : 2.An ethanolic colloidal suspension was prepared using a precursor HfOCl 2 , and then added to the TEOS solution with an Si/Hf molar ratio of 90/10, 80/20, and 70/30.Erbium was added as Er(NO 3 ) 3 •5 H 2 O with an Er/(Si + Hf) molar concentration of 0.3 mol %.Erbium-activated silica-hafnia films were deposited on v-SiO 2 substrates by dip-coating with a dipping rate of 40 mm/min.Before further coating, each layer was annealed in air for 50 seconds at 900 • C. Finally, the films, resulting of 30 coatings, were stabilized by a treatment in air and introduced directly in the furnace at 900 • C (optimized temperature to fully densify the waveguides) for 5, 210 minutes, and 30 hours for the waveguides W30, W20, and W10, respectively.As a result of this procedure, transparent and crack-free waveguides were obtained.In order to nucleate nanocrystals inside the planar waveguide, an additional heat treatment was performed in air at a temperature of 1000 • C for 30 minutes [22].
Specimens for high resolution transmission electron microscopy (HRTEM) observations were prepared by scraping off the thin films in ethanol using a diamond knife.A drop of the suspension is deposited and dried onto a carbon coated copper grid.A HRTEM study of the scraped samples was performed in a 200 kV side entry JEOL 2010 FEG by transmission electron microscope (TEM) fitted with a double-tilt sample holder (tilt ±30 • ).
The losses at 1542 nm, for the TE 0 mode, were evaluated using the moving fiber method in which the exponential decay of light is measured by a fiber probe scanning down the length of the propagating steak [25].The TE 0 mode waveguiding excitation was used for photoluminescence (PL) measurements.PL measurements in the region of the 4 I 13/2 → 4 I 15/2 transitionand the decay curves from the 4 I 13/2 level were obtained using the 514 nm line of a CW argon laser as an excitation source, and dispersing the luminescence light with a 320 mm single-grating monochromator with a resolution of 2 nm, with the experimental setup described elsewhere [22,23,26].

Glass-ceramics results and discussion
Nanostructured morphology of the sample W30, annealed at 1000 • C for 30 minutes, was analyzed by means of HRTEM.The HRTEM images of a scrapped part of the film presented in Figures 1(a the nanocrystals and the surrounding amorphous matrix are composed of Hf oxide and Si oxide, respectively.
The glass-ceramic waveguides present a thickness of 1.3, 1.15, and 1 μm, and a refractive index close to 1.485, 1.538, and 1.597 measured at 1532 nm in the TE polarization for W10, W20, and W30, respectively.They are all a single mode at 1.5 μm, and the refractive index is highly dependent on the hafnium concentration.The losses of about 1 dB cm −1 measured at 1542 nm, for this kind of glass-ceramics, make them suitable for operation in the C telecommunication band.
Figure 2 shows the normalized PL spectra for the glassceramic W10, W20, and W30 waveguides in the region of the 4 I 13/2 → 4 I 15/2 transition of Er 3+ ions.The changes in the shape and bandwidth, with respect to the shape usually observed in the amorphous silica hafnia system (Figure 2(d)) [22,26], indicate the significant modification of erbium ions environment in glass-ceramic systems.PL spectra show that Stark components are better resolved in the glass-ceramic, indicating that the Er 3+ environment is more crystalline like.As the Er 3+ local environment becomes ordered, it limits the inhomogeneous broadening typical of glassy environment, and therefore, the bandwidth narrows.The spectral width measured at 3 dB from the maximum intensity is 28, 19, 17 ± 1 nm for W10, W20, and W30 glass ceramic waveguides, respectively.Note that the bandwidth of the planar waveguide annealed at 900 • C for 5 minutes, which is amorphous, is 51 ± 1 nm (see Figure 2(d)).
The decay curves of the luminescence from the 4 I 13/2 metastable state of Er 3+ ions exhibit a single exponential profile for W20 and W30 glass-ceramics, and a double exponential profile for W10 sample.This nonsingle exponential profile is explained by the fact that not all erbium ions are feeling the same local environment.By fitting the decay curves with two different single exponential profiles, we can notice two different contributions: one corresponding to the erbium ions in the glass matrix leading to a lifetime close to 7.0 milliseconds, and the second part corresponding to erbium ions embedded in the crystalline environment leading to a lifetime close to 9 milliseconds.In the case of W20 and W30, as the decay profiles are single exponential, we can assume that all erbium ions are embedded in the crystalline environment.The decrease of the lifetime of Er in crystals from 9 milliseconds to 8.7 and 7.2 milliseconds, as the HfO 2 content increases from 10% to 20% and to 30%, is attributed to an increase of the radiative electric dipole transition rate in a hafnia-dominated local field [24,27].The shortening of the lifetime with the increasing of HfO 2 content was already observed also for amorphous waveguides.Zampedri et al. have shown that in the case of (100-x)SiO 2 -xHfO 2 waveguides(x = 10, 20, 30, 40 mol), the 4 I 13/2 lifetime decreases from 7.1 milliseconds to 5.5 milliseconds when the HfO 2 content increases from 10 mol % to 40 mol % [28].They invoke the role of hafnia in increasing both distortion and crystal field strength around the Er 3+ ion.However, this effect surely deserves further experimental investigation.Besides, computer simulation could be useful to take into account possible differences in the chemical environment of Er 3+ ion for waveguides of different composition.

CORE-SHELL-LIKE STRUCTURES BY SOL-GEL-DERIVED ER 3+ -ACTIVATED SILICA NANOSPHERES
Monodisperse colloidal spheres in solution can be selforganized into an ordered structure if their size is adequate and their size polydispersivity is low enough, yielding a periodic photonic bandgap structure or photonic crystal [29].Furthermore, monodisperse colloidal spheres of predictable size and shape, activated with a controllable concentration of rare-earth ions like Er 3+ , have significant potential for use in optical devices such as microlasers, integrated optics structures, luminescent markers or nanosensors, and active photonic bandgap materials.In particular, in order to use rareearth-activated colloids in photonic crystals, it is well known that the size polydispersivity of the particles needs to be low and controllable.

Core-shell-like Er 3+ -activated silica spheres fabrication and characterization
The core-shell-like Er 3+ -activated silica spheres, where the core is the silica sphere and the shell is an Er 2 O 3 -SiO 2 coating, were prepared using the following protocol: (i) the core was realized using the Stober method, discussed in detail in [30].and (ii) the shell was obtained by a seeded growth method [31].Briefly, 150 mg of silica spheres of 270 nm diameter, obtained by the Stober method, were added to a solution with the molar ratio TEOS : CH 3 COOH : H 2 O of 1 : 8 : 8, plus 0.2 wt% of ErCl 3 .The mixture was stirred for 45 minutes with a magnetic stirrer.After synthesis, the SiO 2 particles were separated from the solution by centrifuging at 1000 rpm and washed at least twice with pure ethanol.Subsequently, the core-shell-like Er 3+ -activated silica spheres were heat treated at 950 • C for 30 minutes [32].The particle sizes were determined from electron micrographs taken with a scanning electron microscope, (SEM: JEOL-JSM 6300).The diameters of over a hundred particles were used in calculations of the average size and standard deviation δ of each sample.For photoluminescence spectroscopy on core-shell-like structures, the experimental setup described above for glass-ceramics was employed.The photoluminescence measurements were performed on pressed KBr pellets, containing 5% of doped silica spheres.

Core-shell-like Er 3+ -activated silica spheres results and discussion
Figure 3 shows an SEM image of the core-shell-like Er 3+activated silica spheres, obtained using a seeded growth method on the 270 nm monosize silica spheres.Figure 4 presents the PL spectra for the core-shell-like structure in the region of the 4 I 13/2 → 4 I 15/2 transition of Er 3+ ions, obtained upon excitation at 980 nm.
The SEM image of Figure 3 demonstrates that seeded growth in the Er solution occurs on individual particles that do not collapse in higher-size clusters.PL measurements confirm the incorporation of Er 3+ ions in the silica shell .In fact, the shape of the emission band in Figure 4 is typical of Er 3+ -activated silica glasses with a main emission peak at 1533 nm [33] and a spectral bandwidth, measured at 3 dB from the maximum intensity of 27 ± 2 nm.The luminescence decay curve from the 4 I 13/2 state of Er 3+ ions for the silica core-shell-like structure, upon excitation at 514.5 nm, is reported in Figure 5.The decay profile exhibits a single exponential behavior with a lifetime of 12.8 ± 0.1 milliseconds.In order to give an estimation of the quantum efficiency η, defined by the ratio η = τ meas /τ rad , the measured lifetime (τ meas ) must be compared with the radiative lifetime τ rad .Recently, many investigations were performed on SiO 2 spherical colloids doped with Er 3+ ions in order to study the spontaneous emission and determine the local optical density of states in these systems [34,35].In these papers, Dood et al. determined a radiative lifetime of 18 ± 3 milliseconds for the 4 I 13/2 state of Er 3+ ions in pure silica.Note that this value is larger than the radiative lifetime reported for the 4 I 13/2 state of Er 3+ ions in silicate glasses [33].Assuming a τ rad of 18 milliseconds, a quantum efficiency η of 71% can be estimated for our core-shell spheres.

CONCLUSIONS
The paper presents some recent results in specific dielectricbased nanostructured and nanocomposite systems useful for the enhancement of Er 3+ spectroscopic properties.
SiO 2 -HfO 2 : Er 3+ glass-ceramic planar waveguides were prepared by sol-gel route, with nanocrystals of about 4 nm to 6 nm in size, homogeneously dispersed in the amorphous matrix.These waveguides exhibit a single mode at 1.5 μm, their refractive index is highly dependent on the hafnium concentration, and the attenuation coefficient is about 1 dB cm −1 at 1542 nm.The spectroscopic properties of Er 3+ ions are determined by the crystalline local environment.
Seeded growth was successfully applied to synthesize core-shell-like Er 3+ -activated silica spheres.Typical photoluminescence spectrum of erbium, with a lifetime of 12.8 milliseconds, was observed for the core-shell-like structure after annealing for 30 minutes at 950 • C leading to a quantum efficiency of 71%.

Figure 1 :
Figure 1: HRTEM image of the silica-hafnia W30 sample, annealed at 1000 • C for 30 minutes, showing HfO 2 nanocrystals homogeneously dispersed in the amorphous matrix (a).Single domain nanocrystals are clearly evidenced in (b).

Figure 2 :
Figure 2: Room temperature luminescence spectra, obtained by exciting the TE 0 mode at 514.5 nm, of the 4 I 13/2 → 4 I 15/2 transition of Er 3+ ion for the (a) W10, (b) W20, and (c) W30 planar waveguides annealed at 1000 • C for 30 minutes.The emission spectrum of the W30 amorphous waveguide annealed at 900 • C for 5 minutes is also reported for comparison (d).

Figure 3 :
Figure 3: SEM images of the core-shell-like particles after seeded growth using the acid-based reaction.