Journal of Luminescence 132 (2012) 1683–1687

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Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Plasmon-enhanced luminescence of Sm complex using silver nanoparticlesin Polyvinyl Alcohol

Gagandeep Kaur, R.K. Verma, D.K. Rai, S.B. Rai n

Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, India 221005

a r t i c l e i n f o

Article history:

Received 11 November 2011

Received in revised form

31 January 2012

Accepted 7 February 2012Available online 15 February 2012

Keywords:

Nanoparticles

Surface plasmon resonance

Optical properties

Lifetime

Rare earth complexes

13/$ - see front matter & 2012 Elsevier B.V. A

016/j.jlumin.2012.02.014

esponding author. Tel.: þ91 542 2307308; fa

ail address: sbrai49@yahoo.co.in (S.B. Rai).

a b s t r a c t

Silver (Ag) nanoparticles (NPs) were prepared by laser ablation in water with an aim to enhance the

luminescence of rare earth coordinated complex in polymer host. A fixed concentration of the complex

containing Samarium (Sm), Salicylic acid (Sal) and 1, 10-phenanthroline (Phen) were combined with different

concentrations of silver NPs in PolyVinyl Alcohol at room temperature. Absorption spectrum and XRD

patterns of the sample show that the Sm(Sal)3Phen complex is accompanied by Ag NPs. The luminescence

from the complex was recorded in the presence and absence of Ag NPs using two different excitation

wavelengths viz. 400 and 355 nm. Of these, 400 nm radiation falls in the surface plasmon resonance of Ag

NPs. It was found that the Ag NPs led to a significant enhancement in luminescence of the complex.

Surprisingly, a high concentration of Ag NPs tends to quench the luminescence.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Rare Earth (RE) ions with their unusual luminescent propertieshave been shown to have varied applications. Polymers doped REwith high luminescence efficiency have many applications as ampli-fiers, fibers and waveguides [1]. RE ions before being dispersed inpolymeric hosts are combined with an organic ligand which absorbsthe incident light and transfers it to the ion. This increases the energyabsorbed by the RE ion and hence, the observed luminescence. ThusRE coordinated complexes in polymer hosts are used for numerouspractical applications [2,3]. The fluorescence efficiency of RE com-plexes which is an important parameter for their use as a photonicdevice may be improved by a variety of methods [1,4–6]. Any effortto increase the efficiency by increasing the RE concentration, does notsucceed as at higher concentration aggregation of ions takes place andthe aggregates act as quenching centers.

Therefore another strategy to enhance the luminescence efficiencyof RE coordinated complex through the use of nanoparticles (NPs) isbeing tried. The surface plasmonic field of the metallic nanostructuresoften increases the fluorescence of the RE ions. Intensification ofluminescence of RE complexes by Ag NPs has also been taken up byother workers [7–9]. A study showing the effect of Ag colloidalnanoparticles on the luminescent properties of Eu b-diketonatecomplex has been demonstrated by Sun et al. [8]. In a recent studyby Karthikeyan et al. [10], Samarium doped Au nano-compositepolymer film in polyvinyl alcohol was synthesized which acts as a

ll rights reserved.

x: þ91 542 2368390.

reducing as well as capping agent and has shown that incorporationof nanoparticles quenches the luminescence from the Sm3þ ions.

PolyVinyl Alcohol (PVA) due to its unique characteristics hasalready been used as host for RE ions [3]. In the present study,spherical Ag NPs in water were prepared by laser ablation andthen dispersed with Sm(Sal)3Phen complex in PVA. The fluores-cence spectra of the prepared samples have been monitored inpresence and absence of Ag NPs using 355 and 400 nm radiationsfor excitation. The motive for selecting these wavelengths is thatone is away from the SPR of Ag NPs whereas the other one isresonant with SPR band. We report an increase in luminescence ofthe RE complex in the presence of Ag NPs in both the cases.

2. Experimental details

2.1. Materials

The chemicals used (Samarium (99.9%), Salicylic acid (99.9%),1, 10-Phenanthroline (99.9%) and Polyvinyl Alcohol (mw 14000))were obtained from Sigma Aldrich and were used without furtherpurification. A silver plate (99.9%) purchased from the same firmwas used in the preparation of Ag nanoparticles.

2.2. Methodology

2.2.1. Preparation of Ag nanoparticles

Spherical Ag NPs were prepared using laser ablation from a silverplate (diameter�20 mm and thickness�2.0 mm) immersed in water

Fig. 1. The process of laser ablation to obtain silver nanoparticles using 355 nm

excitation.

Fig. 2. X-ray diffraction patterns of pure PVA and Sm complex with Ag NPs in PVA.

G. Kaur et al. / Journal of Luminescence 132 (2012) 1683–16871684

(3 mm below the air–water interface) kept in a glass beaker accordingto literature [11]. The silver target was irradiated by 355 nm radiationfrom a pulsed Nd:YAG laser (repetition rate 10 Hz, pulse width 7 nsand pulse energy�80 mJ). The laser beam was focused on the metaltarget to within 1–4 mm diameter spot with a 15 cm focal length lenswhile the target plate was kept rotating during the ablation. As thelaser pulse strikes the silver target, a characteristic cracking soundemanated. A figure showing the process of laser ablation is givenas Fig. 1.

2.2.2. Preparation of Sm(Sal)3Phen complex

SmCl3 (2.0 mol%) was obtained by dissolving Sm2O3 in con-centrated hydrochloric acid (HCl). Salicylic acid (0.6 mol%) and 1,10-phenanthroline (0.2 mol%) were separately dissolved in etha-nol (2.0 ml). This ethanolic solution was added drop wise toSmCl3 and stirred for half an hour to get Sm(Sal)3Phen complex(as described in Ref. [12]).

2.2.3. Preparation of PVA sample with Sm complex and Ag

nanoparticles

PVA (1.5 g) was dissolved in distilled water (50 ml) to obtaintransparent homogeneous solution.

The previously prepared Sm(Sal)3Phen complex (2.0 mol%)was mixed with known amounts of the as-prepared Ag nanopar-ticles (concentration varying from 0.5–X3 mol%, where X denotestimes the initial concentration). The mixture was homogenized atroom temperature using a magnetic stirrer for two hours andthen poured into aqueous solution of PVA. The resulting solutionwas stirred rigorously for 4–5 h and then poured into a Petri dishand allowed to dry overnight. This process was repeated fordifferent concentrations of Ag NPs to get different samples forour study.

2.3. Characterization

The absorption spectra were recorded using Cary 2390 doublebeam UV–vis–NIR spectrophotometer. XRD patterns wererecorded using 18 kW rotating (Cu) anode Rigaku diffractometerfitted with a graphite monochromator. 355 and 400 nm radiationfrom Nd:YAG laser was used for recording the fluorescence. Thetime domain measurements were also recorded using 355 nmradiation.

Fig. 3. UV–vis absorption spectra of Ag NPs in PVA with concentration of NPs

varying from X0.5–3 mol% (where X denotes times the initial concentration).

3. Results and discussions

3.1. Structural characterization using XRD

X-ray diffraction patterns from a pure PVA film and a PVA filmcontaining Sm(Sal)3Phen complex with Ag NPs are shown

together in Fig. 2. The XRD pattern of the pure PVA film clearlyshows the characteristic known pattern with relatively broadpeaks at 2y�201 and 2y�421, corresponding to the crystallinephase of PVA. The smaller diffraction peaks at 2y �38.6, 43.6 and64.71 seen in the film containing Sm(Sal)3Phen and Ag NPs areattributed to crystalline Ag (111, 200 and 220) planes. Thisconfirms that Ag NPs are well distributed in the polymer film.

3.2. Optical absorption spectra

Fig. 3 shows the absorption spectra of Ag NPs in PVA withconcentration of NPs varying from 0.5—X3 mol% (where Xdenotes times the initial concentration). For all the samples, onesees the surface plasmon resonance absorption of Ag at 402 nm.The absorption intensity of the plasmon band gradually increaseswith the increasing concentration of nanoparticles. The averageradius of Ag NPs could be estimated using the absorption ofplasmon band as

r¼ AVF=2pcðDl=l2PÞ ð1Þ

where A is the line broadening constant (�1.2), VF is the Fermivelocity of electrons and its value is 1.39�106 m/s for silvernanoparticles, c is the velocity of light, lP is the plasmonresonance wavelength, and Dl is the full width at half maxima

G. Kaur et al. / Journal of Luminescence 132 (2012) 1683–1687 1685

of Ag NPs plasmon resonance peak. The calculated average radiusof Ag nanoparticles is �7.4 nm. The composition and someproperties of various samples of Ag NPs in PVA are listed inTable 1. The inset to Fig. 3 shows the dependence of absorbanceon Ag concentration at 402 nm which corresponds to the absorp-tion peak of Ag NPs. It is clearly visible that the absorbance of AgNPs increases almost linearly with concentration. This definitelyimplies that the silver nanoparticles do not aggregate together ordeposit [13].

The absorption spectra of Ag NPs in PVA, of the Sm(Sal)3Phencomplex in PVA and of Sm(Sal)3Phen complex together with AgNPs in PVA are shown in Fig. 4. The Ag NPs in the PVA film showthe typical surface plasmon resonance absorption at 402 nm andalso an absorption band at 304 nm due to p-p* transition of PVA.The Sm(Sal)3Phen complex in PVA film shows a band centered at305 nm due to the S0-S1 absorption of Salicylic acid (Sal) over-lapping the absorption due to PVA. This absorption peak due to

Table 1Composition and some properties of various samples of Ag NPs in PVA.

Sampleno.

Composition Smcomplex (mol%)

Ag NPs(mol%)

SPR band,

kmax (nm)

Particle Radius (r)from Eq. 1 (nm)

1 2.00 0.5 402.0 7.1

2 2.00 1.0 402.3 7.3

3 2.00 1.5 403.0 7.4

4 2.00 2.0 404.1 7.4

5 2.00 2.5 404.8 7.7

6 2.00 3.0 405.9 8.1

Fig. 4. Absorption spectra of Ag NPs in PVA, Sm complex in PVA and Sm complex

with Ag NPs in PVA.

Fig. 5. Luminescence spectra of Sm complex in PVA in presenc

Sal shows a red shift of �10 nm when Ag NPs are also embeddedin the PVA film along with Sm(Sal)3Phen complex. This shift isattributed to an aggregation of the complex through NPs [14]. Theabsorption due to Sm3þ ions at 400 nm is also observed in caseof Sm(Sal)3Phen complex which gets covered with the intensesurface plasmon band of Ag NPs in case of the PVA film dopedwith Sm(Sal)3Phen complex together with Ag NPs.

3.3. Photoluminescence spectra

The dependence of emission enhancement of Sm complex inPVA, with the concentration of Sm complex fixed at 2 mol%and varying the concentration of silver nanoparticles, has beenrecorded using excitation wavelengths (a) 355 nm i.e. the off-resonance condition with Ag NPs and (b) 400 nm i.e. resonantexcitation condition with Ag NPs, respectively. The resultantspectra for both the cases are shown in Fig. 5.

3.3.1. Off-resonance excitation

The luminescence spectra of Sm complex in PVA in presenceand absence of Ag NPs in the range of 500–750 nm (where Sm3þ

has most intense emission bands) on excitation with 355 nm areshown in Fig. 5(A). Distinct emission bands at 560, 595, 642 and

e and absence of Ag NPs using 355 and 400 nm radiation.

Fig. 6. Partial energy level diagram of Sm complex in PVA showing ground state

absorption, local field enhancement (LFE) and energy transfer by surface plasmon

resonance (SPR) of Ag nanoparticles (bold arrows and dotted arrows represent the

radiative and non-radiative transitions, respectively).

Fig. 7. Luminescence decay curves for 4G5/2 level using 355 nm radiation in presence and absence of Ag NPs in PVA.

G. Kaur et al. / Journal of Luminescence 132 (2012) 1683–16871686

705 nm attributed to 4G5/2-[6H5/2, 6H7/2,6H9/2 and 6H11/2]

transitions of Sm3þ are seen. An enhancement in the emissionintensity of Sm3þ bands is clearly seen on adding NPs. However,when the concentration of NPs is above X2.5 mol% the emissionintensity starts to decrease.

The enhancement in the RE luminescence intensity in thepresence of Ag NPs is greatly attributed to two different mechan-isms. These are local field enhancement (LFE) effect and an energytransfer from the irradiated NPs to the RE ion [15]. In LFE, it is theenhanced interaction between the very high field gradients ofelectromagnetic field near the magnetic particle which cause anenhanced emission through energy transfer to the RE ions [16,17].In addition, a variation in the distance between the different REions from the NPs, inhomogeneous broadening of the lines willtake place. Also the energy transfer process in this case is twofold:they may enhance the luminescence by energy transfer fromparticles to RE ion and/or may quench the luminescence byenergy transfer from ions to particles [15–23].

The enhancement in the emission intensity observed in thepresent case may be understood as follows: The 355 nm radiationis absorbed by the Sm3þ ions as well as Sal/Phen molecules to butAg NPs do not absorb at this wavelength. The optical energyabsorbed by the Sal/Phen molecule is transferred to the Sm3þ

ions thereby increasing the population in the emitting 4G5/2 level[12]. The additional channel of excitation of 4D3/2 level of Sm3þ

ions is also the cause of enhancement in the emission intensity.The plasmonic field created around the Sm complex by the Ag NPsincreases the lifetime of the emitting level of Sm3þ ions alsopermitting a buildup of population in 4G5/2 level adding to theluminescent intensity [16,17,24] (see Fig. 6). The increase in thelifetime of 4G5/2 level of Sm3þ is clearly seen in the decay curve of4G5/2-

6H7/2 transition with and without Ag NPs (Fig. 7).At large concentration of NPs, an aggregation of NPs reduces

the plasmon field thus reducing the coupling causing theenhancement of intensity. It is interesting to note that thedifferent transitions of Sm3þ are affected differently by the NPs.Among the bands observed in the emission spectrum 4G5/2-

6H5/2

transition is of magnetic dipole hypersensitive character and itwill be affected more compared to other transitions. The coordi-nation number of Sm ion increases in presence of silver nano-particles. Due to this reason the field around the rare earth ischanged. The enhancement corresponding to this transition ismore than the other transitions because the magnetic dipoleallowed transition differs in interaction with surface plasmonfield as compared to the electric dipole allowed transitions.

3.3.2. Resonant excitation

Fig. 5(B) shows the effect on the luminescence intensity of Smcomplex in the presence of NPs using 400 nm radiation. Theoverall behavior is similar to the case of 355 nm excitation exceptthat the luminescence intensity in this case is smaller.

In this case, the Sal/Phen ligands do not absorb at 400 nm butSm3þ and Ag NPs both absorb this radiation. The excitation energy istransferred from Ag NPs to Sm3þ ion which tends to enhance theluminescence intensity. The large absorption cross section of thesilver plasmon band causes increased excitation of 4G5/2 level of Smion via energy transfer from the excited SPR level of Ag NPs(see Fig. 6). Among the bands observed in the emission spectrum4G5/2-

6H5/2 transition is of magnetic dipole hypersensitive characterand it will be affected more compared to other transitions.

However the intensity of Sm3þ bands using 355 nm excitationis much larger than using 400 nm excitation. This clearly demon-strates that the energy transfer through Sal is much efficient thanthrough other channels.

3.4. Time resolved photoluminescence

The time resolved photoluminescence of 4G5/2-6H7/2 transi-

tion at 595 nm of Sm3þ ions using 355 nm excitation with andwithout Ag NPs in PVA yield the decay curves shown in Fig. 7.A first order exponential decay fit for the curves leads to a largerlifetime (of 4G5/2 level) in presence of NPs. This is explained asdue to the surface fields of Ag NPs. This shows that the lumines-cence properties of polymer doped RE complexes can beimproved by adding Ag NPs [8].

4. Conclusions

To summarize, an organic complex containing Sm ions hasbeen dispersed with Ag NPs in PVA polymer film at roomtemperature. Optical absorption spectrum and XRD patternsconfirm the presence of Sm3þ ions and Ag NPs in the film. Theemission efficiency of the Samarium complex is seen to beenhanced in the presence of Ag NPs on pumping with 355 and400 nm radiations. The enhancement is larger for 355 nm excita-tion. This is attributed to the coupling of plasmonic field of Ag NPsto Sm3þ ions which affect the lifetime of the radiative level of Smas well as energy transfer from Sal to Sm3þ .

Acknowledgments

G. Kaur is grateful to CSIR, India, for financial assistance in theform of SRF.

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