Solid State Communications 150 (2010) 1947–1950

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Solid State Communications

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Pulsed laser ablation synthesis of silver nanoparticles and their use influorescence enhancement of Tb3+-doped aluminosilicate glassR.K. Verma, K. Kumar, S.B. Rai ∗Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi-221005, India

a r t i c l e i n f o

Article history:Received 14 April 2010Received in revised form22 June 2010Accepted 7 July 2010by D.D. SarmaAvailable online 14 July 2010

Keywords:B. NanoparticlesD. Optical propertiesD. Energy transferD. Lifetime

a b s t r a c t

Spherical silver nanoparticles have been synthesized using laser ablation in distilled water. Thesenanoparticles are embedded in Tb3+-doped aluminosilicate glass through the sol–gel technique. Thepresence of these nanoparticles is seen to increase the emission intensity of the Tb3+ ions by more than100%. Energy transfer from the excited silver nanoparticles to Tb3+ ions is the probable cause for thisincrease in emission intensity.

© 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Nanotechnology deals with nanometer-size materials and hasseveral practical applications. Spherical nanoparticles are moreimportant anduseful as compared to nanoparticles of other shapes,as they have a large surface energy in comparison to volumeenergy [1]. Several methods such as chemical reaction, laserablation and ball milling [2,3] are used for the preparation ofspherical nanoparticles, of which the laser ablation method is theone which avoids any chances of contamination.The study of the interaction of rare-earth ions with metal

nanoparticles has received considerable attention in recent yearsbecause of the scientific interest in understanding the mechanisminvolved and also in searching for new applications in differentdevices. Silver nanoparticles are one of the most extensivelystudied systems and these are still attractive due to their potentialapplications [4–7]. Silver nanoparticles have surface plasmonresonance energy very close to the inter-band transition energy [8],which is 6–7 times larger than for bulk silver [1]. The interactionbetween silver nanoparticles and rare-earth/transition metal ionsboth dispersed in a glass or xerogel matrix can result in enhancedfluorescence from the metal ion [9,10].In the present study, spherical silver nanoparticles prepared

in water environment by laser ablation have been dispersed inan aluminosilicate glass along with terbium ions through the

∗ Corresponding author. Tel.: +91 542 230 7308; fax: +91 542 236 9889.E-mail address: sbrai49@yahoo.co.in (S.B. Rai).

0038-1098/$ – see front matter© 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2010.07.014

sol–gel method. The fluorescence spectra of the terbium ions havebeen recorded both in the presence and the absence of silvernanoparticles. It is observed that the fluorescence intensity of Tb3+is enhanced several times in presence of silver nanoparticles. Apossible interpretation has been attempted.

2. Experimental procedure

2.1. Preparation of silver nanoparticles

Silver nanoparticles were prepared using 355 nm radiationfrom a pulsed Nd:YAG laser (repetition rate 10 Hz, pulsewidth 7 nsandpulse energy∼80mJ). The laser beamwas focused on the silvermetal target with a 15 cm focal length convergent lens. The targetplate was kept immersed in water (3 mm below the air–waterinterface). The target plate was rotated during the ablation and thespot size of the focused laser beam was in the range 1–4 mm indiameter. The experimental set-up is similar to the one used byMafune et al. [11].

2.2. Dispersion of silver nanoparticles in glass

Terbium-doped aluminum silicate glass samples with andwithout silver nanoparticles were prepared by the sol–geltechnique described in the literature [12,13]. We used tetraethylorthosilicate (TEOS), ethanol and water with nanoparticles in theratio 1:2:4 with one mole percent of terbium; however, no acidwas used in the whole process. The mixture was magnetically

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Fig. 1. Absorption spectrum of silver nanoparticles produced by 355 nm laserablation in water.

stirred to obtain a clear homogeneous solution. The solutionwas transferred to a Petri dish and left for three days at roomtemperature. During this period the solution changes into a gel.The gel was dried at 40 °C for four weeks. During this period, threeimportant reactions, namely hydrolysis, water condensation andalcohol condensation, takes place. These reactions areM–O–R+ H2O→ M–OH+ R–OH (Hydrolysis)M–OH+ HO–M→ M–O–M+ H2O (Water condensation)M–O–R+ HO–M→ M–O–M+ R–OH

(Alcohol condensation).The samples were then heated for two hours at 700 °C. This ledto the densification of the samples and also the removal of theunwanted volatile impurities.The optical absorption of the silver nanoparticles in water was

recorded using a Perkin–Elmer Lamda-35 spectrophotometer. Thefluorescence spectra were recorded using 355 nm radiation froma Nd:YAG laser. A Horiba Jobin Yvon iHR320 spectrometer wasused to disperse and detect the fluorescence signal. The radiativelifetime of the 5D4 level of Tb3+ ion was also measured under thesame excitation conditions.

3. Results and discussion

3.1. Optical absorption spectra

The absorption spectrum of silver nanoparticles in water isshown in Fig. 1. The plasmon absorption peak is clearly seen at404 nm. The radius of the silver nanoparticles is estimated usingthe relation

r = AVF/2πc(∆λ/λ2P).

Here A is the line broadening constant, and its value is 1.2. VF is theFermi velocity of the electrons, and its value for silver nanoparticlesis 1.39 × 106 m/s. λP is the plasmon resonance wavelength, and∆λ is the full width at half maximum of the plasmon peak. Thecalculated average radius of silver nanoparticles is estimated tobe 6.48 nm. The TEM image and the corresponding histogram forparticle size are shown in Fig. 2. The size estimated from bothtechniques is in agreement.

3.2. Fourier transform infrared (FTIR) spectra

The Fourier transform infrared (FTIR) spectra of the twosamples with and without silver nanoparticles were recordedin the range 400–4000 cm−1. A broad peak in the region3400–3600 cm−1 shows the presence of residual water moleculeseven though the sample had been heated at 973 K for two hours.The band at 780 cm−1 is ascribed to Al–O stretching mode whilethe band at 545 cm−1 is related to the motion of Al–O–Al linkage.Also the Si–O–Si linkage gives peaks at 780 and 450 cm−1. Aweak band observed at 950 cm−1 is due to Si–O stretching. Thesepeaks superpose on each other in the spectrum. In the presenceof silver nanoparticles in silica glass a new strong band appears at1090 cm−1. This new band is attributed to the silver nanoparticles.

3.3. Luminescence

Two types of electronic transition in Tb3+ ions, namely 4fn–4fnand 4fn–4fn−15d, contribute bands in the visible and near-UVregions. The 4f–4f transitions are forbidden and result in weakbands due to perturbations; this partially changes the character ofthe 4f suborbitals. On the other hand,4f–5d transitions are allowedand important for energy transfer because these transitions areeasily perturbed by traditional interactions, i.e. the crystal field,spin–orbit interaction, electrostatic interaction, etc. In the present

Fig. 2. TEM image of silver nanoparticles and their size distribution in the form of a histogram.

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Fig. 3. Luminescence spectra of terbium ions with and without nanoparticles insilica glasses.

case we observe energy transfer from silver nanoparticles toterbium ions due to localized interactions. The luminescencespectra of terbium-doped silicate glass on excitation with 355 nmradiation in the presence and absence of silver nanoparticles areshown in Fig. 3. A substantial enhancement in the Tb3+ emissionintensity in the presence of silver nanoparticles is clearly seen. Twopossible mechanisms may be responsible for this enhancement.Either there is an energy transfer from silver nanoparticles to theTb3+ ions, increasing the population of excited ions, or theremightbe an increase in lifetime of the emitting level (5D4) of the terbiumions due to interaction with the silver nanoparticles.In the sample containing silver nanoparticles, irradiation with

a 355 nm laser pulse excites the silver plasmon band as well asthe band due to terbium ions. The absorption cross-section for thesilver plasmon band is very large andmany silver nanoparticles areexcited from the lower to the upper state. Further, the near equalityof the excitation energies of the plasmon level and the 5D4 levelof the terbium ions makes energy transfer from the nanoparticlesto the Tb3+ ions very likely. The increased number of Tb3+ ionsin the 5D4 level would naturally result in an enhancement of all5D4 → 7FJ (J = 0–6) transitions which lie between 480 and700 nm. The transitions are 5D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4 and5D4 → 7F3, and the correspondingwavelength values are 487, 542,585 and 620 nm, respectively. The relative increase in intensitiesof different bands will depend on the intrinsic probabilities for theindividual transitions. For example, the largest increase in intensitytakes place for the transition 5D4 → 7F5, which is a magneticdipole allowed hypersensitive transition. Its hypersensitive natureis responsible for the large increase in intensity seen in thepresence of silver nanoparticles. The energy transfer probabilityis proportional to the superposition integral of the two spectralshapes, namely donor (Ag nanoparticle) emission and acceptor(Tb3+ ion) absorption [14,15].

PAg−Tb = C∫σ(ν)Tbabs σ(ν)

Agemdν. (1)

Eq. (1) represents the probability for both radiative and non-radiative energy transfer. C is a constant which is a measure ofthe coupling strength and is determined by the nature of theinteraction.The fluorescence spectra of terbium ions in the presence

of silver nanoparticles have also been recorded at differenttemperatures, namely 973, 1073 and 1173 K (see Fig. 4). It

Fig. 4. Luminescent spectra of terbium ions with silver nanoparticles, annealed atdifferent temperatures.

Fig. 5. Lifetime of the 5D4 level of terbium ion in the presence and absence of silvernanoparticles.

is observed that the fluorescence intensity is maximum at973 K. The fluorescence intensity is almost the same at 1073 K.However, at 1173 K it is reduced drastically, but it is still largerthan the fluorescence intensity of the sample without silvernanoparticles heated at the same temperature. It seems that athigher temperatures (≥1073 K), there is a change in shape/or sizeof the silver nanoparticles which has a deleterious effect on thefluorescence.We have also measured the radiative lifetime of the 5D4 level

using 355 nm radiation from a pulsed Nd:YAG laser both in thepresence and absence of silver nanoparticles. Fig. 5 shows the twofluorescence decay curves for the 5D4 → 7F5 transition of Tb3+. Itis noted that the lifetime of the 5D4 level of terbium is increasedby nearly 30% (to 420 from 310 µs) in the presence of silvernanoparticles (see Fig. 5). The lifetime of the 5D4 level measuredon samples heated at different temperatures, i.e. 973, 1073 and1173 K, shows that heating has no effect on the lifetime of thelevel. In fact the fluorescence intensity decreases with heating. Ourresults (shown in Fig. 4) thus suggest that the energy transfer from

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silver nanoparticles to Tb3+ ions is the dominant mechanism forthe enhancement of the fluorescence intensity [9,10].

4. Conclusions

In this work we have prepared spherical silver nanoparticlesby laser ablation and added them to terbium-doped silica glass bythe sol–gel technique.We have demonstrated that, in the presenceof silver nanoparticles, the emission efficiency of terbium ionsin silica glass heated at 973 K is enhanced appreciably. This isexplained as being due to energy transfer from the plasmon levelof the silver nanoparticles to the Tb3+ ions. An increase in the 5D4lifetime is also noted, though its effect on the fluorescence intensityis shown to be insignificant.

Acknowledgements

The authors are grateful to the AvH Foundation, Germany, forproviding the Nd:YAG laser and to DST, New Delhi, India, forfinancial assistance.

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