ISSN 10876596, Glass Physics and Chemistry, 2012, Vol. 38, No. 4, pp. 366–372. © Pleiades Publishing, Ltd., 2012. Original Russian Text © V.A. Aseev, P.A. Burdaev, E.V. Kolobkova, N.V. Nikonorov, 2012, published in Fizika i Khimiya Stekla.
Fluorophosphate Glasses Activated by RareEarth Ions and AgBr V. A. Aseeva, P. A. Burdaevb, E. V. Kolobkovaa, b, and N. V. Nikonorova a
St. Petersburg State University of Information Technologies, Mechanics, and Optics, Birzhevaya liniya 4, St. Petersburg, 199034 Russia bSt. Petersburg State Institute of Technology (Technical University), Moskovskii pr. 26, St. Petersburg, 190013 Russia email:
[email protected] Received April 11, 2011
Abstract—Analysis of the absorption band position for colloidal silver in fluorophosphate glasses of the com position (0.95 – x – y)(MgCaBaSrAl2F14)–0.05Ba(PO3)2–xPbF2–yLnF3 (where Ln = Eu, Ce, Sm, Tb; 0 ≤ x ≤ 0.2; y = 0.01, 0.02) with small additives of AgBr (0.04 wt % above 100%) has been performed. It has been shown that joint introduction of AgBr and EuF3 with additional heat treatment of the glasses below the temperature of the onset of crystallization leads to a rise of the plasmon absorption band of silver nanoparti cles. It has been assumed that the reason for the observed shift of the plasmon absorption band is the forma 0
tion of a ~0.5nmthick shell of AgBr on the surface of a metallic Ag n cluster. Keywords: silver nanocluster, surface plasmon resonance, nanoparticle, electron donor, rareearth ion, nano composite particle, metallic cluster shell DOI: 10.1134/S1087659612040037
INTRODUCTION The optical properties of nanoparticles have been under investigation for many years, and nanocompos ite materials containing assemblies of nanoparticles are increasingly finding practical applications [1, 2]. A distinctive feature of metallic particles is the occur rence in them of collective excitations of conduction electrons, called also surface plasmons. Surface plas mons may be excited by electromagnetic radiation, leading to the appearance of resonances in the optical spectrum of an assembly of metallic particles (so called Mie resonances), which are also called surface plasmon resonances. Optical phenomena in the vicin ity of nanoparticles are determined not only by the electromagnetic radiation wavelength, but also by the size and shape of nanoparticles and specific features of their environment. The most important feature of metallic nanoparticles is the presence of intense elec tromagnetic fields close to their surface. The electro magnetic field strength near metallic nanoparticles may be high enough to essentially affect the excited state lifetimes of atoms and molecules surrounding a nanoparticle, the optical nonlinearity of the material containing nanoparticles, and plasmon–phonon interactions [3]. It should also be noted that, lately, even more complex structures of the core/shell type with various combinations of materials, being poten tially applicable not only in physics but also in biology and medicine, have been obtained by the method of chemical precipitation from colloidal solutions [4].
The synthesis and behavior of metallic nanoclus ters in a glassy matrix are of special interest. Compos ite optical materials containing metallic nanoparticles are promising materials for nonlinear optics and pho tonics owing to the strong electric fields arising near the surface of nanoparticles upon excitations of sur face plasmons, as well as due to possibilities of mutual photon–plasmon transformations. In addition, these materials are promising for laser patterning of microoptical structures. The problem of the behavior of various silver forms in oxide silicate glasses was solved long ago and quite successfully. Examples of these composite materials are represented by photosensitive glass ceramics (pho tositalles)—photochromic, photorefractive, and mul tichromic glasses [5–9]. Currently, no analogs of these materials have been discovered among phosphate and fluorophosphate glassy systems. However, individual publications have appeared describing some specific features of the formation and behavior of silver nano clusters upon introduction of Ag+ directly into the batch [10] or via ion exchange [11]. Metal nanoparticles are formed in glasses as a result of reduction by hydrogen of ions, usually Ag+, intro duced into a glass by its ionexchange treatment in melts containing these salt ions. The absorption spec tra of Agcontaining silicate and phosphate glasses are presented in Fig. 1 [11].
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0.28
367
(b) 0.28
0.24 0.24
Absorbance
Absorbance
0.20 0.16
0.12
0.20 0.16 0.12
0.08
0.08
0.04
0.04 400
600 800 1000 Wavelength, nm
1200
400
600 800 1000 Wavelength, nm
1200
Fig. 1. Absorption spectra of Agcontaining (a) silicate and (b) phosphate glasses obtained via ion exchange. Narrow bands cor respond to the erbium ion absorption [11].
In [10], glasses of the system P2O5–Al2O3–CaO– SrO–BaO, Ag2O (4–8 mol %), jointly with SnO in the same amount, were synthesized and used for creation of Ag0particles. During heat treatment at 750–870 K, silver is trans formed by Sn2+ into the neutral state via the reaction 2Ag+ + Sn2+ → 2Ag0 + Sn4+, leading to an increase in the quantity of Ag0particles in a glassy matrix. As a result, nanoclusters with dimensions of 13–26 nm (Fig. 2) were grown and changes in the plasmon resonance frequency, depend ing on the size of nanoclusters, were studied by the methods of optic spectroscopy. In the present paper, the methods of optic spectros copy were used to study the diffusion phase separation process in fluorophosphate glasses of the composition (0.95 – x – y)(MgCaBaSrAl2F14)–0.05Ba(PO3)2– xPbF2–yLnF3 (where 0 ≤ x ≤ 0.2, y = 0.01 and 0.02) with small additives of AgBr (0.04 wt % above 100%). As Ln, Eu, Ce, Sm, and Tb were used. The choice of ion activators is determined by the ability of some ions to be present in glasses in two oxidation states. The aim of this work is to elucidate specific features of the for mation of metallic nanoparticles in a glassy matrix and to study the role of the activator in this process. EXPERIMENTAL TECHNIQUE Glasses of the system (0.95 – x – y) (MgCaBaSrAl 2F 14)–0.05Ba(PO 3) 2–xPbF2–yLnF3 GLASS PHYSICS AND CHEMISTRY
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with AgBr additives, activated by rareearth element (REE) fluorides (EuF3, SmF3, TbF3, and CeF3) and SbF2, were synthesized. The glass synthesis was per formed in an electric laboratory furnace with silite heaters at the temperature 950°C in a glass–carbon melting set using the scheme “crucible in crucible.” Samples were formed upon glassmass processing between two cooled glass–carbon slabs. The obtained flat ~2mmthick samples were annealed at the tem perature just below Tg to release thermal stresses. The annealing temperature and the secondary heattreat ment temperature were determined on the basis of data of differential scanning calorimetry (DSC) and differential thermal analysis (DTA). Measurements and mathematical processing of the data were per formed on a Netzsch STA 449F1 Jupiter differential scanning calorimeter. The glasses were heat treated in muffle furnaces at temperatures close to Tg but below the temperature of the onset of crystallization Ton. cr. The choice of this regime of the second phase growth is brought about, on one hand, by an increase in the mass transfer rate, leading to crystalline phase formation, and, on the other hand, by the necessity to provide a minimal deformation of the sample shape. Heat treat ment was carried out for 5, 10, 15, 20, 30, and 60 min. Heat treatment of the glasses was aimed at the forma tion of silver nanoclusters in the glass bulk. In order to detect a change of the environment of ion activators during the heattreatment process, luminescence spectra were measured, which are sensitive to the tran 2012
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4 2
20
laser excitation
40 laser excitation
Absorption factor, cm–1
60
1
Phosphate glass matrix without silver doping
0 2.5
3.0
3.5 hν, eV
4
5
6
Fig. 2. Absorption spectra of metal–dielectric nanocomposite materials containing spherical silver nanoparticles in phosphate glass [10]. Radii of Ag nanoparticles, nm: (1), 13, (2) 17, (3) 20, and (4) 26.
electrondonor ions, no coloring was observed under the same heattreatment conditions when reactions of the type
sition of REE from a glassy structure into a crystalline one. Luminescence spectra were excited by the emis sion line 488 nm of an ILA120 argon laser. A Jobin Yvon U1000 double monochromator was used for recording the luminescence spectra. Absorption spec tra were measured on a Varian Cary 500 spectropho tometer in the wavelength range 250–700 nm with the step 0.1 nm. In glasses containing EuF3, a red coloring of a glass was observed upon heat treatment at temperatures within the temperature interval Tg–Ton. cr, the color becoming more intense with increasing temperature. In glasses containing Sm3+, Tb3+, Ce3+, and Sb3+ as
Ag+ + Ce3+ → Ag0 + Ce4+ occur. For the experiment, two series of glasses differing in concentration of europium fluoride (Table 1) were synthesized. Series 1 is represented by glasses 2–4 with concentrations of 1 mol % EuF3 and 0, 10, and 20 mol % PbF2, whereas in series 2, (glasses 1, 5, and 6), the concentration of EuF3 is 2 mol % and the concentra tion of PbF2 is 0, 10, and 20 mol %.
Table 1. Compositions of fluorophosphate glasses (mol %) with additive of 0.04 wt % AgBr (above 100%) Glass number 1 Ba(PO3)2 CaF2
2
3
4
5
6
5
5
5
5
5
5
18.5
18.5
18.5
18.5
18.5
18.5
BaF2
10
MgF2
10
10
10 10
10
AlF3
35
35
35
35
35
35
YF3
3
4
4
4
3
3
SrF2
18.5
18.5
18.5
18.5
18.5
18.5
PbF2
20
10
20
10
EuF3
2
1
1
2
1
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Thermophysical Characteristics Characteristic temperatures were determined on the basis of the analysis of DSC and DTA data (Table 2). The measurements were performed for annealed heat untreated glasses. The difference in temperatures obtained by these two methods is caused by the fact that powders of the investigated glasses were used for DTA measurements, while bulk samples were mea sured by DSC. No peaks related to the formation of metallic clusters were observed. It turned out that introduction of 10 mol % PbF2 lowers all characteris tic temperatures by 20°C on average. An increase in the concentration of an REE fluoride by 1 mol % changes the temperatures insignificantly. 0
Formation of Ag n Nanoclusters in Glasses A distinctive feature of metallic particles is the occurrence in them of collective excitations of con ductivity electrons, called also surface plasmons, which are revealed in a certain spectral range as an intense absorption band. As seen from the abovepre sented data on the growth of Ag clusters in phosphate glasses, the band maximum may be positioned within a rather wide wavelength range (500 nm upon an ion exchange for a glass with a high refraction index and 400–430 nm for glasses with a high concentration of Agions added into the original batch). According to theoretical concepts, the position of the plasmon res onance band depends on two parameters: the nano cluster size and the refraction index of a dielectric matrix surrounding it. Figures 3–5 show the absorption spectra of the original and heattreated glasses of various composi
Table 2. Change of characteristic temperatures of fluoro phosphate glasses on the basis of DTA data Glasses
Tg
Ton. cr
Tk1*
1 2 3 4 3 5
390 450 410 380 410 420
430 500 450 420 450 460
465 540 485 455 485 500
* Tk1 is the temperature of the first exoeffect
tions differing in content of PbF2. It is evident that the bands arising during the heattreatment process corre 0 spond to surface plasmons characteristic of Ag n nano particles in the glass under investigation. It should be noted that the observed band differs noticeably in the maximum frequency (464 nm) from the plasmon band, occurring in silicate systems with close refrac tion index and AgBr concentration (~410 nm) [13]. Two explanations for the position of the plasmon absorption band may be suggested. According to [12], a 35nmsize silver nanoparticle has an intense band maximum at 465 nm (Fig. 6). One possible explana tion of this position of the plasmon resonance band 0 may be the assumption of the formation of Ag n nano clusters of predominantly the same size in the glasses under investigation. However, clusters of such a size should cause opalescence. It should also be noted that changes in the content of lead fluoride in all investigated glasses cause no
2.0
2.0 1
2
1.6
3 1.2 0.8 0.4
350
450 550 Wavelength, nm
650
750
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0.8
0.4
0 250
350
450 550 Wavelength, nm
650
750
Fig. 4. Absorption spectra of glass 4: (1) original sample and (2, 3) samples heat treated at T = 360°C for (2) 15 and (3) 20 min.
Fig. 3. Absorption spectra of glass 4 heat treated for 15 min at T = (1) 340, (2) 360, and (3) 380°C. GLASS PHYSICS AND CHEMISTRY
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1.6 Absorption, optical density
Absorption, optical density
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1.0 1
Absorption, optical density
2 Intensity, rel. units
1.2
0.8
0.4
1 0.5
400
0 250
350
450 550 650 Wavelength, nm
4
3
2
500
600 λ, nm
750
Fig. 7. Spectral dependences of the absorption cross sec 0
tion for an Ag n nanoparticle with a shell of silver bromide.
Fig. 5. Absorption spectra of glass 6: (1) original sample and (2) sample heat treated at T = 420°C for 30 min.
Shell thickness, nm: (1) 0, (2) 0.5, (3) 1.5, and (4) 3 [13].
10 100
Optical extinction/Ag concentration, arb. units
9 8
140
7 6
40
5 250 4
25
3 350
10 2
450
1
340
380
420
460 500 λ, nm
540
580
620
effect on the plasmon resonance position for silver nanoclusters, despite a noticeable change in the refraction index of the matrix glass, which contradicts the conception of the environment refraction index effect on the nanocluster absorption band position. For glasses containing 0 and 20 mol % PbF2, only the char acteristic temperatures and, consequently, the tempera ture–time heattreatment regime are changed, while the position and shape of the plasmon resonance band remain unchanged (Figs. 3–5). In the context of this controversy, another interpreta tion of the longwave shift of the plasmon resonance can be proposed. It is obvious that the obtained result may be explained by assuming the formation of similar sur roundings of metallic nanoclusters in the investigated glasses with different refraction indexes; i.e., a shell with a chemical composition differing from that of the matrix glass. It was shown in [13] that the formation of a shell with a high refraction index on the surface of a silver nanoparticle as a result of the heat treatment should give rise to a longwave shift of the plasmon absorption band. In the glasses under consideration, such a shell may form of AgBr (n = 2.2). Figure 7 shows theoretically calculated spectral dependences of the absorption cross section for a nanoparticle consisting of silver and a shell of silver bro mide [13]. Comparing these results with our data, it may be assumed that there is a possibility of formation in flu 0 orophosphate glasses of Ag n clusters with an ~0.5nm thick surface shell of AgBr. SpectralLuminescent Investigations
Fig. 6. Wavelength dependence of the specific optical extinction coefficient per silver concentration for particles of different radii [12]. Numbers near the curves are the radii of particles in Å.
Luminescence spectra of the original and heat treated glasses were measured. The analysis of these spectra provides information on the change of the ion GLASS PHYSICS AND CHEMISTRY
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371
(a) 5D 4 5
25
5L
G2–6
6
20
(b)
5D 2 5D 1
369 nm
5D 0
5
D0 7F 0
15
10 7
F0
5
0
5 4 3 2 1 0
Intensity, arb. units
Energy, 103 cm–1
5D 3
7
F1
7F
7
F3
2
7
F4
D1→7F0–4
5
D2→ F0–3
5
7
1000°C
800°C 450
500
550
600 Wavelength, nm
650
700
Fig. 8. (a) Energy level scheme for transitions of Eu3+ ions and (b) luminescence spectra of Eu3+ : LaF3 nanocrystals in SiO2– glassceramics in the range 450–700 nm upon excitation at 396 nm [14].
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activator surrounding in the material and on the effect of the field of forming metallic clusters. The scheme of transitions of Eu3+ ions and a view of their lumines cence spectrum in fluoride nanocrystals formed in the glass are presented in Figs. 8a and 8b. It is known that europium ions may be present in glasses in two oxidation states, Eu3+ and Eu2+. In fluo rophosphate glasses with a high fluorine content, europium ions are simultaneously in two charged forms, with Eu2+ prevailing at low europium fluoride concentrations (lower than 0.1 mol %), which is evi denced by an intense blue emission. Summing up all the presented observations, it may with rather high confidence be concluded that Eu2+ is an electron donor promoting the reaction Ag+ + Eu2+ → Ag0 + Eu3+. With an increase in the concentration of europium fluoride in the matrix, a rise of the relative content of Eu3+ with respect to Eu2+ takes place, being exhibited as a characteristic Eu3+ spectrum (Fig. 9). The lumi nescence band characteristic of twocharged europium vanishes upon an increase in the concentra tion of europium fluoride due to the occurrence of nonradiative energy transfer from Eu2+ to Eu3+ ion.
14000 12000 10000 8000 6000 4000 2000 0
556
Wavelength, nm 588 625
667
714
4.1 4.2 4.3 1
19000 18000 17000 10000 15000 14000 Wave number, cm–1 Fig. 9. Luminescence spectra of heattreated glasses 4: (4.1) 370°C, 20 min; (4.2) 370°C, 40 min; (4.3) 420°C, 20 min; and glass 1: (1) 460°C, 10 min upon excitation at 488 nm.
In Fig. 9, changes of luminescence spectra of Eu3+ ions during heat treatment of glasses 1 and 4 contain ing 20 mol % PbF2 and 1 and 2 mol % EuF3 are pre sented. For samples 4.1, 4.2, and 4.3 of glass 4, heat 2012
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treatments slightly above Tg cause no visible changes of the spectrum. It should be noted that a rise of the plas mon band occurs already in this temperature range. Upon increasing the heattreatment temperature up to Ton. cr, the intensity of Eu3+ ion emission caused by electron transitions from the upper excited level 5D1 (520–580 nm) decreases and the intensity of the 5 D0 ⎯ 7F2 band increases relative to the 5D0–7F1 band (the intensity ratio changes from 1.14 for glass 4.1 to 1.2 for glass 4.3). The position and shape of the 5D0–7F0 band, which are typical of changes in the environment of Eu3+ ions, testify to a practically unchanged fluoride surrounding in the original and heattreated glasses. Glass 1 was heat treated at a temperature considerably exceeding the glass transition temperature and virtu ally coinciding with the temperature of the first crys tallization peak. As a result of this heat treatment, noticeable growth of the intensity of the electric dipole 5D –7F transition occurs relative to the magnetic 0 2 dipole 5D0–7F1 one (1.54 instead of 1.14), which testi fies to a reduction of the symmetry of the europium 0 ion surrounding in the process of Ag n formation and may be caused by both formation of a new Eucon taining crystalline phase and the electromagnetic field effect arising near metallic clusters. Changes in the band intensities in the range of transitions from the 5D level may be explained by the effect of the increas 1 ing concentration of silver nanoclusters, the absorp tion band of which falls in the blue spectral range. With increasing europium concentration, the quantity of 0 Ag n nanoclusters grows and the bands in the range of transitions from 5D1 practically disappear. CONCLUSIONS Analysis of the absorption band position for colloi dal silver in fluorophosphate glasses of the composi tion (0.95 – x – y)(MgCaBaSrAl2F14)–0.05Ba(PO3)2– xPbF2–yLnF3 (where Ln = Eu, Ce, Sm, Tb; 0 ≤ x ≤ 0.2, y = 0.01 and 0.02) with small additives of AgBr (0.04 wt % above 100%) showed that a rise of the plas mon absorption band of silver nanoparticles occurs in spectra of glasses containing AgBr and EuF3 upon additional heat treatment below the temperature of the onset of crystallization. It is assumed that the rea son for a longwave shift of the plasmon absorption band and for the absence of the influence of the matrix glass refraction index on its position is the formation of an ~0.5nmthick shell of AgBr on the surface of metallic Agclusters. A redistribution of the band intensities in luminescence spectra testifies to interac 0 tion of Eu3+ with Ag n and, perhaps, to the formation
of a Eucontaining nanocrystalline phase near metal lic nanoclusters. REFERENCES 1. Kreibig, U. and Vollmer, M., Optical Properties of Metal Clusters, Berlin: SpringerVerlag, 1995. 2. Optical Properties of Semiconductor Nanostructures, Klingshirn, C., Ed., New York: SpringerVerlag, 2004. 3. Moskovits, M., SurfaceEnhanced Spectroscopy, Rev. Mod. Phys., 1985, vol. 57, no. 3, part 1, pp. 783–826. 4. Kalele, S., Gosavi, S.W., Urban, J., and Kulkarni, S.K., Nanoshell Particles: Synthesis, Properties, and Appli cations, Curr. Sci., 2006, vol. 91, no. 8, pp. 1038–1052. 5. Stookey, S.D., Beall, G.H., and Pierson, J.S., Full Color Photosensitive Glass, J. Appl. Phys., 1978, vol. 49, no. 10, pp. 5114–5120. 6. Berezhnoi, A.I., Sitally i fotositally, Moscow: Mashi nostroenie, 1966. Translated under the title Glass Ceramics and PhotoSitals, New York: Plenum, 1970. 7. Glebova, L., Lumeau, J., Klimov, M., Zanotto, E.D., and Glebov, L.B., Role of Bromine in the Thermal and Optical Properties of Photo–Thermo–Refractive Glass, NonCryst. Solids, 2008, vol. 354, nos. 2–9, pp. 456–461. 8. Dotsenko, A.V., Glebov, L.B., and Tsekhomsky, V.A., Physics and Chemistry of Photochromic Glasses, New York: CRC Press, 1998. 9. Glebov, L.B., Nikonorov, N.V., Panysheva, E.I., Petrovckii, G.T., Savvin, V.V., and Tunimanova, I.V., Multichromic Glasses—New Materials for Recording Volume Phase Holograms, Dokl. Akad. Nauk SSSR, 1990, vol. 314, no. 4, pp. 849–853. 10. Lysenko, S., Jimernez, J., Vikhnin, V., and Liu, H., Excited State Dynamics in Silver Nanoparticles Embedded in Phosphate Glass, J. Lumin., 2008, vol. 128, pp. 821–823. 11. Portales, H., Mattarelli, M., and Montayna, M., Inves tigation of the Role of Silver on Spectroscopic Features of Er3+Activated AgExchanged Silicate and Phos phate Glasses, J. NonCryst. Solids, 2005, vol. 35, no. 11, pp. 1738–1742. 12. Arnold, G.W. and Borders, J.A., Aggregation and Migration of IonImplanted Silver in Lithia–Alu mina–Silica Glass, J. Appl. Phys., 1977, vol. 48, no. 4, pp. 1494–1499. 13. Nikonorov, N.V., Sidorov, A.I., Tsekhomskii, V.A., and Lazareva, K.E., Effect of a Dielectric Shell of a Silver Nanoparticle on the Spectral Position of the Plasmon Resonance of the Nanoparticle in Photochromic Glass, Opt. Spectrosc., 2009, vol. 107, no. 5, pp. 705– 707. 14. Yanes, A.C., DelCastillo, J., MendezRamos, J., Rodriguez, V.D., Torres, M.E., and Arbiol, J., Lumi nescence and Structural Characterization of Transpar ent Nanostructured Eu3+Doped LaF3–SiO2 Glass Ceramics Prepared by Sol–Gel Method, Opt. Mater., 2007, vol. 298, no. 11, pp. 999–1003.
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