Tm3+-activated transparent oxy-fluoride glass–ceramics: structural and spectroscopic properties

Glass Physics and Chemistry, Vol. 31, No. 4, 2005, pp. 519–524. Original English Text Copyright © 2005 by Fizika i Khimiya Stekla, Mattarelli, Montagna, Rossi, Ferrari, Tikhomirov, Seddon.

PROCEEDINGS OF THE TOPICAL MEETING OF THE EUROPEAN CERAMIC SOCIETY “NANOPARTICLES, NANOSTRUCTURES, AND NANOCOMPOSITES” (St. Petersburg, Russia, July 5–7, 2004)

Tm3+-Activated Transparent Oxyfluoride Glass Ceramics: A Study by Raman Scattering of the Nanocrystal Size Distribution1 M. Mattarelli*, M. Montagna*, F. Rossi*, M. Ferrari**, V. K. Tikhomirov***, and A. B. Seddon*** *Dipartimento di Fisica, Universitá di Trento and INFM, CSMFO Group, I-38050 Trento, Italy ** CNR-IFN, Institute of Photonics and Nanotechnologies, CSMFO Group, I-38050 Trento, Italy *** Centre for Advanced Materials, Wolfson Building, University of Nottingham, Nottingham, NG7 2RD, UK Abstract—Absorption and emission spectroscopy of Tm3+ dopants in ultratransparent oxyfluoride glass ceramics indicates that most of the active dopants have been incorporated into the nanocrystals. The size of the nanocrystals has been estimated based on experimental studies of polarized and depolarized low-frequency Raman scattering of the Tm3+-doped glass ceramics in the range 0.5–100 cm–1. Symmetric and quadrupolar acoustic vibrations have been observed and are ascribed to the respective modes of the nanoparticles, which are argued to be β-PbF2 nanocrystals. The size distribution of the nanoparticles has been deduced from the shape of the acoustic bands. The Raman data show that the mean size of the PbF2 nanocrystals increases and that the width of the size distribution decreases with the time and temperature of the heat treatment. 1

INTRODUCTION

Rare-earth doped glass–ceramic materials with active ions embedded in the crystalline phase combine the mechanical and optical properties of the glass with a crystallike environment for the rare-earth ions [1]. The cross sections of rare-earth optical transitions in crystals are usually higher than those in glasses [2–10]. The transparency of oxyfluoride glass ceramics is comparable to that of the precursor glass at all wavelengths [4, 6]. Furthermore, oxyfluoride glass ceramic, where the nucleated crystalline phase is fluoride, can provide a low-phonon-energy host. This is important for the quantum efficiency of the emitting levels of the rareearth ions that suffer a competitive nonradiative relaxation. In particular, ions such as Tm3+ and Pr3+, which are of high interest for wavelength division multiplexing because their emissions are complementary to the emission of Er3+ at 1.55 µm, would greatly benefit by being in a glass–ceramic host [11, 12]. Recently, we have shown that Er3+ and Tm3+ ions are nucleating agents of PbF2 crystals in oxyfluoride glasses [6–8]. Differential thermal analysis (DTA) measurements have shown that the crystallization peak shifts to lower temperatures with the rare-earth content and approaches the glass transition temperature, Tg, for high doping levels [6]. Erbium-activated glass ceramics 1 This

article was submitted by the authors in English.

with PbF2 nanocrystals with controlled sizes between about 2 and 12 nm were obtained by tailoring the temperature and time of the thermal annealing of the parent glass. For the 12-nm glass ceramics, the cubic β-PbF2 phase was detected, with most Er3+ ions in the crystalline phase [6–8, 13, 14]. Also, in the case of Tm3+-doped oxyfluoride glass, an important part of Tm3+ ions enter the crystalline phase obtained after creaming by thermal annealing, as indicated by the appearance of sharp peaks, typical of a crystalline environment, in the optical spectra [8]. In this paper, we use a simple characterization technique for controlling the nanocrystal size of Tm3+-activated oxyfluoride glass ceramics, i.e., the low-frequency Raman scattering from the acoustic vibrations of the nanoparticles. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE Precursor Tm3+-doped glasses 32SiO2 · 9AlO1.5 · 31.5CdF2 · 18.5PbF2 · 5.5ZnF2 · 3.5TmF3 mol %, as well as the resulting glass ceramics, were prepared using the procedures described in [6, 7]. The precursor glass (PG) was subjected to different heat-treatment schedules: 360°C for 5 h (GCA), 400°C for 5 h (GCB), 440°C for 15 min (GCC), and 440°C for 5 h (GCD) (table).

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Parameters of the heat treatments, time (tt) and temperature (Tt), carried out on the samples, peak frequencies and FWHM of the l = 0 and 2 modes, mean size (d0) and width (σ) of the size distribution Sample tt at Tt ω2 (cm–1) FWHM (cm–1) FWHM/ω2 ω0 (cm–1) from l = 2 band peak from l = 0 band peak from the model σ

d0 , nm

GCA

GCB

GCC

GCD

5 h at 360°C 9.5 13 1.37

5 h at 400°C 7.7 8 1.04 14 6.3 7.2 4±1 0.20 ± 0.07

15 min at 440°C 4.5 4.3 0.95 8 10.8 12.6 9±2 0.12 ± 0.04

5 h at 440°C 3 2.8 0.93 5.9 16.2 17.1 14 ± 3 0.05 ± 0.05

5.1 3±1 0.3 ± 0.1

The polarized VV and depolarized HV Raman spectra were obtained by exciting the samples with the 514.5-nm line of an Ar+-ion laser. The signal was selected by a double monochromator and analyzed by a photon-counting system. Two different equipments were employed. The maximum resolution was about 0.5 cm–1 for the double monochromator with a 1-m focal length and 0.03 cm–1 for that with a 2-m focal length, working at the eleventh order. Brillouin spectra were measured in a standard 90° geometrical configuration. RESULTS AND DISCUSSION The DTA spectrum for the Tm3+-doped precursor glass is very similar to that of the Er3+-doped precursor glass [6, 8]. The glass transition temperature Tg is at Counts 50000

6000

VV

40000

4000

30000

2000

20000

0

10000

500

750 1000 Raman shift, cm–1

HV 0

250

500 750 Raman shift, cm–1

1000

Fig. 1. VV and HV polarized Raman spectra of the oxyfluoride parent glass. The inset shows the same spectra, in the high-frequency range of optical phonons, on an expanded scale. Integration time is 2 s.

about 380°C. The crystallization peak at about 437°C is attributed to the growth of the nanocrystalline PbF2 : Tm3+ phase, nucleated by Tm3+ dopants. The basis for this attribution is the similarity of the analogous case of the nanocrystalline PbF2 : Er3+ phase grown in the same precursor glass doped with Er3+ [6]. The temperatures of the thermal treatment (table) were chosen on the basis of the DTA data. Figure 1 shows the VV and HV polarized Raman spectra of the parent glass. The boson peak (BP), the depolarized low-frequency feature that is typical of the glass structure, dominates the spectra [15]. The partially polarized peaks at frequencies higher than about 800 cm–1 (see the inset) are due to Si–O stretching vibrations of different Si(O,F)4 structural units [13, 14]. The many partially resolved structures in the 160– 500-cm–1 frequency range are attributed to bending and stretching vibrations of the complex oxyfluoride structure, involving bonds of Pb, Zn, Cd, Al, and Tm with O and F ions [13, 14]. Figure 2a shows the VV spectrum of the GCD sample. The spectrum is very similar to that of Fig. 1 for the parent glass. The best way to show the small differences among the spectra of the GCs and the glass is probably to take the spectra ratios, as shown in Fig. 2b. These spectra were smoothed and normalized to their maximum. This occurs at the BP, whose intensity, however, will change from sample to sample. Therefore, having no references for absolute intensity, the intensity changes shown by the ratios have to be analyzed in relation to the overall mean intensity of the spectra. We can observe a broad peak extending from about 150 to 350 cm–1, whose intensity increases with the temperature of the thermal treatment. It is probably due at least partly to the vibrations of PbF2 nanocrystals. In fact, the Raman spectrum of pure β-PbF2 consists of two peaks at 190 and 240 cm–1 [16]. In the high-frequency region, new, relatively sharp structures appear due to the changes in the structure of the glass network that are caused by the precipitation of the PbF2 component. As

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(b)

1.2

521

Intensity ratio

D/G 1.1

VV B/G 1000

1.0 × 0.005

A/G 0.9

Intensity, arb. units

5

(a)

0 1000

4

HV

3 2 500 1 0

200

400 600 800 Raman shift, cm–1

× 0.2

1000 0

3

6 9 Frequency shift, cm–1

12

15

Fig. 2. (a) VV polarized Raman spectrum of the GCD sample. (b) Intensity ratio of the VV spectra of the (A) GCA, (B) GCB, and (D) GCD samples to (G) the VV spectrum of the parent glass.

Fig. 3. VV and HV polarized Brillouin and Raman spectra of the GCD sample. Integration time is 6 s. The spectra are shown in two intensity scales with the indicated ratio.

already observed for the erbium-doped glass ceramic, the BP slightly shifts to higher frequencies and decreases in intensity as the creaming proceeds [13]. Both effects are a consequence of the depletion of the PbF2 component in the glass. In fact, the high polarizability of lead ions gives an important contribution to the BP intensity by the mechanism of electrical-disorder-induced light scattering [17, 18]. Furthermore, with the depletion of the PbF2 component, the glass becomes harder. More dramatic changes are observed in the very low-frequency part of the Raman spectra of the GCs with the appearance of new bands due to the acoustic vibration of the nanoparticles. Figure 3 shows the VV and HV spectra of the GCD sample, taken in a 90° scattering geometry. These measurements allow for the observation of Brillouin and Raman scattering in the same spectrum. The sharp peak at 0.7 cm–1, with a linewidth given by the resolution of about 0.13 cm–1, is the Brillouin peak due to the longitudinal phonons of the glass ceramic. Its intensity has been reduced by a factor of 200 in the VV spectrum and by a factor of five in the HV spectrum. Higher-resolution measurements also show the peak of the transverse phonons at about

0.35 cm–1, which is here covered by the tail of the elastic scattering. The depolarized (i.e., present both in VV and HV spectra) band centered at about 2.8 cm–1 and the broader polarized (present almost exclusively in the VV spectrum) band, appearing as a shoulder at about 6 cm–1, are the acoustic vibration bands. Their intensity is comparable to that of the BP, whose low-frequency tail appears in the spectra of Fig. 3 as a nearly constant background. The integrated intensity of the low-frequency band in the VV spectrum is about one-tenth of the integrated intensity of the Brillouin peak. The low-frequency vibrational dynamics of a crystalline cluster embedded in a glass can be described by a simple model that considers a homogeneous elastic sphere [19]. The vibrational frequencies and eigenvectors of a homogeneous elastic sphere with a free surface were studied in [20]. The vibrations are grouped into two categories, namely, torsional and spheroidal modes. Torsional and spheroidal modes are classified according to the symmetry groups of the sphere by the labels (l, m), as for the spherical harmonic functions Ylm. A third index (p = 1, 2, … n) labels the sequence of eigenmodes in increasing order of frequency and radial wavevector at a fixed angular shape (l, m). The funda-

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Intensity, arb. units GCD 2

1.20 1.15

GCC

1.10

GCB

1.05

1

1.00 PG 0

4

GCA 0.95 8 12 16 Frequency shift, cm–1

20

–40

Fig. 4. Smoothed, HV polarized, Raman spectra of the GCD, GCC, GCB, GCA, and PG samples. The tail of the Brillouin peak was subtracted, and the spectra were normalized in the frequency range of the BP.

0 20 Raman shift, cm–1

40

Fig. 5. Ratio of the HV polarized Stokes and anti-Stokes Raman spectra of the GCA and PG samples.

mental p = 1 mode is called the surface mode; its overtones (p > 1) are called inner modes. Only the l = 0 and 2 spheroidal modes are Raman-active [21, 22]. The former produce polarized, and the latter, depolarized spectra. Therefore, on the basis of the depolarization ratio IVH/IVV , the Raman peaks can be assigned to symmetrical or quadrupolar vibrations. The frequencies of the l = 0, 2 surface modes ω0, ω2 (in wavenumbers) are given by Si v i -, ω i = --------dc

–20

(1)

where S0 and S2 are constants of the order of unity that depend on the ratio of the longitudinal and transverse sound velocities, v0 = vL and v2 = vT are the longitudinal and transverse sound velocities, d = 2R is the diameter of the nanoparticle, and c is the velocity of light. Both types of modes appear in the spectra of Fig. 3. The l = 2 mode, which is active both in VV and VH polarizations, produces the lowest frequency peak, which is centered at about 3.0 cm–1. The second, weaker peak, at about 6 cm–1, which is present only in the VV spectrum, is attributed to the l = 0 surface mode. The contributions to the VV and HV Raman spectra of the l = 0 and 2 vibrations can be isolated from the other contributions: the BP of the glass and the tail of the Brillouin peak. This can be done by subtracting the VV and HV spectra of the parent glass, respectively. The procedure of subtraction works well for the BP, whose shape does not change appreciably with creaming, but needs some parameter adjustment for the tail of the Brillouin peak, whose frequency position changes with creaming. Figure 4 shows the HV polarized spectra of the four glass–ceramic (GC) samples and of the

parent glass, after smoothing for graphical purposes and after the subtraction of the tail of the Brillouin peak. The glass ceramics GCB, GCC, and GCD show a well-shaped l = 2 band, whose peak shifts towards a low frequency with the temperature (and time) of thermal annealing, indicating a progressive growth of particle size. At first view, the spectrum of the sample GCA does not show any particle peak, since it is very close to that of the parent glass. However, by taking the ratio of the spectra, as shown in Fig. 5, a weak l = 2 vibration peak centered at about 10 cm–1 emerges from the noise. The peak frequency and the full widths at half-maximum (FWHM) of the l = 0 and 2 bands are reported in the table. A first rough estimate of the particle size is obtained by (1), which is valid for a free sphere. Taking the values of β-PbF2 for the sound velocities of the nanocrystals averaged over the different crystalline directions, vL = 3470 m/s and vT = 1730 m/s [22], we obtain S0 = 0.87, S2 = 0.84, and the particle sizes reported in the table. A more detailed analysis considers the interaction of the particles with the surrounding glass. This interaction causes the broadening and shift of the free particle modes [23]. Another source of line broadening is the size distribution, due to the inverse proportionality of the frequencies to the size. The observed line shape can be fitted by taking into account the two different sources of line broadening [24]. If a modified lognormal size distribution is assumed, 2

[ ln ( d/d 0 ) ] - , G ( d, d 0 ) ∝ exp – --------------------------2 2σ

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the fit provides the maximum, d0, and the width measured by σ [24]. The lineshape of the l = 2 peak is obtained from the HV spectrum, since the l = 0 vibrations are Ramanactive only in VV polarization. The results of the fit are reported in the table. For the GCA sample, the weak Raman signal does not allow the use of the model, but a mean particle size of about 3 nm can be determined by an extrapolation of the peak frequency versus mean size curve obtained from the other three samples. The mean sizes obtained by this model are smaller than those estimated from the peak positions by using (1). Transmission electron microscopy experiments are needed for a direct measurement of the size distribution. This will allow one to check the model that calculates the size distribution from the Raman data also in cases, such as the present one, when the crystal is softer (with lower sound velocities) than the surrounding glass and an important shift of the Raman peak from the free sphere frequency is expected. However, the Raman data clearly indicate that the relative width of the size distribution, as measured by FWHM/ω2 or by σ, decreases as the mean size increases. CONCLUSIONS Low-frequency Raman scattering from the symmetric and quadrupolar acoustic vibrations of PbF2 nanocrystals present in annealed 32SiO2 · 9AlO1.5 · 31.5CdF2 · 18.5PbF2 · 5.5ZnF2 · 3.5TmF3 glasses was observed. The size distribution of the nanocrystals was obtained from the lineshape of the l = 2 spheroidal vibrations by taking into account the homogeneous broadening due to the interaction of the crystals with the surrounding glass. Nanocrystals with a mean size in the range 3– 14 nm are produced by tailoring the temperature and time of annealing. The width of the size distribution decreases with an increase in the mean size. ACKNOWLEDGMENTS We would like to acknowledge the financial support of ICCTI/CNR through a collaborative grant on “Optical Amplification,” and of CNR/CNRS through a collaborative grant on “Improvement of the Multitarget RF Sputtering and Sol–Gel Techniques for Silica-Based Photonic Components” project, as well as that of FIRB “Nanotecnologie, Microtecnologie, Sviluppo Integrato di Materiali” and MIUR-PRIN projects. REFERENCES 1. Goncalves, M.C., Santos, L.F., and Almeida, R.M., Rare-Earth–Doped Transparent Glass-Ceramics, Comp. Rend. Chim., 2002, vol. 5, no. 12, pp. 845–854. 2. Mortier, M. and Auzel, F., Rare-Earth–Doped Transparent Glass-Ceramics with High Cross-Sections, J. NonCryst. Solids, 1999, vol. 257, pp. 361–365. GLASS PHYSICS AND CHEMISTRY

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