INSTITUTE OF PHYSICS PUBLISHING
JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS
J. Opt. A: Pure Appl. Opt. 8 (2006) S450–S454
doi:10.1088/1464-4258/8/7/S21
Nanocomposite Er–Ag silicate glasses G Speranza1 , S N B Bhaktha2,3 , A Chiappini2 , A Chiasera4 , M Ferrari4 , C Goyes4,5 , Y Jestin4 , M Mattarelli2 , L Minati1 , M Montagna2, G Nunzi Conti6 , S Pelli6 , G C Righini6 , C Tosello2 and K C Vishunubhatla2,3 1
IRST-Itc, via Sommarive 18, 38050, Povo-Trento, Italy Dipartimento di Fisica, CSMFO group, Universit`a di Trento, via Sommarive 14, 38050 Povo-Trento, Italy 3 School of Physics, University of Hyderabad, Gachibowli, Hyderabad-500 046, India 4 CNR-IFN, Istituto di Fotonica e Nanotecnologie, CSMFO group, via Sommarive 14, 38050 Povo-Trento, Italy 5 ´ Grupo de Optica Cu´antica, Universidad del Valle, Santiago de Cali, Colombia 6 CNR-IFAC, Institute of Applied Physics ‘Nello Carrara’, Laboratory of Optoelectronic Technologies, Via Panciatichi 64, 50127 Firenze, Italy 2
E-mail:
[email protected]
Received 31 October 2005, accepted for publication 23 December 2005 Published 7 June 2006 Online at stacks.iop.org/JOptA/8/S450 Abstract Particular attention has being given to metal–dielectric nanostructured materials, due to the well known surface plasmon resonance, described as the oscillation of the free electrons with respect to the ionic background of the nanoparticle when they are collectively excited by laser irradiation. It is claimed that metal nanoparticles can be used for increasing the intensity of the luminescence emitted by rare earth ions. This effect is attributed to the strong absorption cross section related to the surface plasmon excitation in noble-metal nanoparticles and/or to the large local field enhancement generated around the excited nanoparticles. In spite of the large amount of work published on this topic, the mechanism of optical amplification remains controversial. Here we present x-ray photoelectron spectra and transmission electron images together with photoluminescence absorption and emission measurements, with the aim of providing a better understanding of the effective role of silver as a sensitizer for erbium. Keywords: silver, erbium, silicate glasses, ion exchange, luminescence enhancement, x-ray photoelectron spectroscopy
1. Introduction In the last two decades several works have been published in the literature concerning nanometre-sized metal particles in glasses, due to their unusual properties including optical nonlinearity [1], magnetism [2], electrical properties [3, 4]. In addition to the basic interest, the presence of quantum-size behaviour has attracted much attention in view of the potential photonic applications [5–7]. In particular we have focused on the effect of Ag nanoparticles on the rates of emission of rare earth ions embedded in a glassy network. Despite the number of papers published, the role the Ag nanoparticles play in the non-linear optical behaviour of these systems is, in fact, not completely understood. Recent photoluminescence (PL) 1464-4258/06/070450+05$30.00
experiments performed on borosilicate Er3+ doped glasses raised some doubts concerning the effective role of Ag as an erbium sensitizer [8]. Other doubts are raised as regards the effects of silver on the spectroscopic properties of sol–gel derived silica–titania nanocomposite planar waveguides [9]. Understanding the interactions between the rare earth ions and metal nanoparticles is crucial for the development of photonic devices. There are several methods for producing dispersions of Ag nanoparticles in glassy networks doped with rare earth ions. Among them we mention: melt quenching [3], the sol–gel route combined with γ irradiation [10], ion implantation [8], the sol–gel route plus ion exchange [11]. In this work we have analysed the photoluminescence of
© 2006 IOP Publishing Ltd Printed in the UK
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ABSORBANCE
A 0.3 0.2 0.1 0.0 400
600 800 1000 WAVELENGTH (nm)
1200
1.0
B INTENSITY(ARB.UNITS)
EMISS. INTENS.(arb. units)
0.4
0.8 0.6 0.4 0.2
1.0 0.8 0.6 0.4 0.2 0.0 1400
1500 1600 WAVELENGTH(nm)
1700
0.0 400 500 600 700 800 EXCITATION WAVELENGTH (nm)
Figure 1. Panel (A): example of an absorption spectrum (sample SAgEr5). Note the plasmon peak at ∼420 nm. Panel (B): example of an excitation spectrum (same sample) detected at 1532 nm. In the inset are compared the emission of Er3+ measured in the absence (reference sample, broken line) and in the presence of Ag nanoparticles (SAgEr5 sample, solid line) obtained using 476.5 nm excitation.
Table 1. Intensities and durations of the thermal treatment applied to the five soda-lime glasses. Sample label Ref. SAgEr1 SAgEr2 SAgEr3 SAgEr4 SAgEr5
Temperature ( ◦ C)
Time of annealing (min)
Simple glass 500 500 500 500 500 600
— 30 30 + 30 30 + 70 30 + 30 + 60 30 + 70 60
Er3+ in ion exchanged soda-lime glasses. Thermal treatments were performed to study the effect of cluster dimension on the Er3+ absorption as well as emission in the 4 I13/2 → 4 I15/2 transition. Further analyses were performed by using transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) to shed light on the structure and the chemistry of Er3+ + Ag silicate glasses.
2. Experimental details Silicate glass samples of composition 66.9 SiO2 , 14.4 Na2 O, 9.1 CaO, 1.9 Al2 O3 , 0.9 P2 O5 , 0.9 K2 O, 1.9 Er2 O3 , 3.9 Yb2 O3 (wt%) were prepared as described elsewhere [12, 13]. They were cut into plates, thinned down to 200 µm and optically polished. One plate was kept as a reference while five others were Ag+ ion exchanged. The five samples were annealed in air at 500 ◦ C for different times, except the last sample which was annealed in two steps; first at 500 ◦ C and then at 600 ◦ C. The thermal treatments are summarized in table 1. Specimens for TEM observations were prepared by scraping off thin films in absolute ethanol using a diamond knife. A drop of the suspension was deposited onto and dried on a carbon coated copper grid. TEM study of the scraped samples was performed using a 200 kV side entry JEOL 2010 transmission electron microscope. XP spectra were acquired using a 200 mm Scienta (Gammadata—Uppsala) hemispherical analyser. The vacuum was in the region of 10−8 Pa. For each sample a wide scan, 0– 1200 eV, was acquired to detect the chemical elements together with the possible contaminations. Core lines were acquired using a pass energy of 150 eV. In this condition the energy
resolution of the analyser at the Fermi edge is about 0.4 eV. With this energy resolution, chemical shifts due to different bonds may be distinguished. Glasses being strong insulators need charge compensation during spectra acquisition. The value of 285 eV for the core line of the CHx components was chosen as a reference for determining the correct binding energy (BE) values of the other glass chemical elements. Atomic concentrations were obtained from the intensities of the spectral components. Linear background subtraction and Gaussian components were used to perform peak fitting. For each chemical element, the core line was deconvoluted into components pertinent to the chemical bonds formed.
3. Results Figure 1 collects the results of optical measurements. An example of absorption spectra obtained for Er3+ Ag exchanged soda-lime glass SAgEr5 is shown in figure 1(A). On the edge of the Urbach tail the well defined broad feature at ∼417 nm is associated with the surface plasmon excitation of the Ag nanoclusters. For samples which underwent a softer annealing, this feature is weaker. In figure 1(B) is shown an example of an excitation spectrum obtained from the same sample. Similar spectra were obtained for all the other samples irrespective of the duration and the strength of the annealing. In the inset is shown the Er3+ fluorescence, obtained using a 476.5 nm excitation wavelength. There is clearly visible an enhancement of the Er3+ emission. Further information about the spectroscopic features was previously reported [14]. In panels (A) and (B) of figure 2 we show the effect of annealing on the Ag dispersion in the glassy network. In particular in panel A is presented a TEM image obtained from sample SAgEr1 as an example of mild thermal treatment. The image shows silver condensed into a few clusters of dimension ∼10 nm and the arrows in the magnified portion provide evidence of the possible presence of smaller silver aggregates of dimensions around 1–3 nm. In panel B is reported the TEM image obtained from SAgEr5, the sample annealed for longer time and at higher temperature. Again silver is present in clusters having now a higher diameter of about 15 nm. Let us switch now to the XPS analyses. In table 2 are summarized the chemical elements revealed by our S451
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Components 3, 4, 6 at 532.3, 531.3 and 529.8 eV correspond to –C–O–C–, (C=O∗ )–C– and Na2 O + K2 O respectively. In figure 3(B) is presented the silver core line split in the two spin–orbit components 3d3/2 at higher BE and 3d5/2 . A pure Gaussian component was used to fit each peak. Broken lines indicate the position of silver oxide. Finally in figure 4 is presented the position of the Ag 3d5/2 as a function of the annealing time. The dashed line represents the BE relative to that of metallic silver. A small decrease of the Ag 3d5/2 BE value is observed, increasing the annealing time.
(A)
(B)
4. Discussion
Figure 2. Panel (A): TEM image obtained for an Ag exchanged soda-lime glass annealed at 500 ◦ C for 30 min. Silver aggregates into a few clusters of 10 nm size and probably into small clusters of ∼1–3 nm size. Panel (B): TEM image obtained from an Ag exchanged soda-lime glass annealed at 500 ◦ C for a total of 100 min plus a second treatment at 600 ◦ C for 60 min. Now the dimensions of the Ag cluster are ∼15 nm. Arrows indicate Ag clusters.
Table 2. Composition of the samples analysed given in atomic abundances (%). Atom Reference SAgEr1 SAgEr2 SAgEr3 SAgEr4 SAgEr5 Si Na Ca Al K O Ag
30.8 2.1 2.4 2.3 1.4 61.1 —
33.0 1.5 0.4 1.8 1.1 61.8 0.3
30.4 2.2 0.5 2.6 0.8 62.9 0.5
32.3 1.7 0.9 1.6 1.1 61.7 0.6
30.6 1.8 0.8 2.7 1.2 62.1 0.6
31.1 2.13 0.9 2.7 1.3 61.3 0.5
instrument and the relative total atomic abundance. It is worth remembering that XP spectra show the composition related to the material surface. For this reason some of the glass component elements are not detected because their concentration is below the instrumental sensitivity (preferential superficial arrangement of Si, Al, K and C due to contamination). The atomic concentration of Ag is almost constant among the samples and around 0.5%. In figures 3(A), (B) are shown examples of the core line spectra of oxygen and silver respectively. Oxygen is decomposed into six components. Each of them is associated with a specific chemical bond formed by oxygen with the other chemical elements of the soda-lime glass. Peak fittings are performed linking component integrals to the stoichiometry an optimizing the χ 2 . Component 1 at ∼534 eV is related to (C=O)–O∗ (∗ indicates the oxygen atom whose BE is referred to). Component 2 at ∼532.6 eV is related to the bridging oxygen in SiO2 while component 5 at ∼531 eV is related to the non-bridging oxygen atoms in agreement with [15]. S452
The inset of figure 1(B) clearly shows an enhancement of the Er3+ fluorescence when a laser excitation at 476.5 nm impinges on the Ag exchanged soda-lime glasses. Despite this effect being observed also by other authors [3, 8, 14, 16] the mechanism underlying the process of erbium excitation leading to the radiative 4 I13/2 → 4 I15/2 transition is not well established. In this respect we can give some hints. First of all Ag plays a fundamental role in this emission process. Some authors [3, 16] related the enhancement of the Er3+ fluorescence to the presence of the well known Ag surface plasmon. In brief, due to the high Ag cross section, a large portion of the exciting energy is converted into free electron plasma oscillations. The strong local field enhancement around the rare earth ion promotes energy transfer from the Ag nanoparticle to the Er3+ and then the emission at 1532 nm. To verify this model it is of paramount importance to determine the chemical state of the Ag atoms. Unfortunately the composition of the glassy network is too complex and the concentration of Ag too small for detecting the presence of silver oxide from the oxygen core line peak (see figure 3(A)). Nonetheless inspection of the 3d core lines of Ag gives us some clear indications. The dashed lines in figure 3(B) point to the BE of silver oxide. Ag enters in the glassy network via ion exchange as Ag+ which could form Ag2 O. Our spectra do not show any deformation at low BE: pure Gaussian lineshapes perfectly describe the peaks. We can conclude that silver is present in our glasses in a nonoxidized form. As a consequence the electrons on the surface of the Ag nanoparticles are not involved in strong chemical bonds with oxygen and free to oscillate when excited by laser radiation. Other information may be gained from the analysis of the Ag spectra. It is known that the XP lineshapes of metallic system are asymmetric. In metals the presence of the core hole screening introduces a tail on the high BE side of the core line [17]. The spectra from each of the SAgEr samples have symmetric shapes fitted with simple Gaussian components. This reflects the loss of the Ag metallic character due to condensation in nanoclusters. In fact, known from the literature is a tendency to electronic quantum confinement when the dimensions of metal clusters scale down to the nanometre size [18]. This effect is confirmed also by the energy shifts towards higher BE of the Ag 3d peaks. The results shown in figures 2(A), (B) and 4 tell us that the lower the Ag nanocluster size the higher the BE shift. The dependence of the BE on the metallic nanoparticle size was already observed by other authors [19] and explained theoretically [18]. So XPS enables us to relate electronic quantum confinement
Nanocomposite Er–Ag silicate glasses
0.9
A 8
INTENSITY (arb. units)
INTENSITY (arb. units)
10 2
6 4 2 1
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5 4
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Ag 3d5/2
0.6 0.5 0.4
536
534 532 530 BINDING ENERGY (eV)
528
380
375 370 BINDING ENERGY (eV)
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Figure 3. Panel (A): XP spectrum of the O 1s core line deconvolved into the Gaussian components. The numbers indicate different chemical bonds (see the text). Panel (B): the Ag 3d 3/2 and Ag 3d 5/2 core lines. Broken lines indicate the positions of silver oxide.
(iii) TEM analyses show that thermal treatments induce Ag atoms to condense into nanoparticles. The size of the nanoparticles depends on the annealing parameters. (iv) XP spectra show a tendency to electronic quantum confinement of the Ag nanoclusters with decreasing cluster size. (v) XP spectra show shifts in the BE of the Ag 3d which can be correlated with the Ag cluster dimensions, i.e. the strength of the thermal treatments. The longer the annealing, the higher the cluster size and the lower the BE shift.
369.0
BE(Ag3d5/2) (eV)
368.8
368.6
368.4
368.2
368.0 40
60 80 100 120 140 TOTAL ANNEALING TIME (min.)
160
Figure 4. Trend of the Ag binding energy as a function of the total annealing time. The dashed line indicates the BE relative to that of pure metallic Ag.
To conclude, XPS is a very powerful technique for obtaining both structural and chemical information on Ag nanoclusters embedded in glassy networks. The use of this technique is important for tailoring the experimental parameters used to produce doped soda-lime glasses, for optimizing their optical properties.
Acknowledgment to the strength of the thermal treatment. It is known that heating induces a condensation of Ag into clusters. Figure 4 indicates that there is a progressive increase of the average Ag nanoparticle size up to 100 min of annealing. In agreement with TEM images, further thermal treatments are likely to induce an increase of the nanoparticle dimensions with a loss of the electron localization and an increase of the metallic character of the Ag aggregates.
5. Conclusion In this work XPS analyses were performed on Ag exchanged Er doped soda-lime glasses to shed light on the process underlying the Er emission. Some conclusions may be drawn on the basis of the experimental results: (i) The presence of Ag nanoparticles induces an enhancement of Er emission under excitation at 476.5 nm. (ii) The Ag atoms inserted in the glassy network by ion exchange are in a non-oxidized form. The absence of chemical bonds, i.e. localization of the Ag electrons in molecular orbitals, allows laser excitation of free electron surface plasmons.
The authors acknowledge the financial support of MIUR-FIRB RBNE012N3X, PAT FAPVU 2004-2006, MIUR-PRIN 20042005, ITPAR (2003-2006).
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