Journal ofMaterials Chemistry C

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aDepartment of Physics, University

anthapuram-695581, Kerala, India.

drcimuneera@gmail.com; Fax: +91-471-230bSchool of Chemical Sciences, M. G. UniverscLight & Matter Physics Group, Raman Resea

560080, India

Cite this: J. Mater. Chem. C, 2013, 1,3851

Received 6th March 2013Accepted 17th April 2013

DOI: 10.1039/c3tc30427b

www.rsc.org/MaterialsC

This journal is ª The Royal Society of

Organic dye impregnated poly(vinyl alcohol)nanocomposite as an efficient optical limiter: structure,morphology and photophysical properties

S. Sreeja,*a S. Sreedhanya,b N. Smijesh,c Reji Philipc and C. I. Muneera*a

A new polymer based nanocomposite system comprising the organic chromophore Light Green (LG) has

been fabricated using poly(vinyl alcohol) (PVA) as the host template, and its structure, morphology, and

linear and nonlinear optical properties have been investigated. Microstructural analysis reveals a

semicrystalline nature of the nanocomposites, with a uniform dispersion of nanoclusters of dye

molecules encapsulated between the molecular chains of the host polymer PVA. XRD, FTIR and electrical

conductivity studies indicate considerable interactions between the dye molecules and the polymer

chains. LG–PVA nanocomposite films exhibit PL emission in the red region of the visible spectrum, when

excited in the vicinity of their absorption maximum (645 nm). The composite films also display nonlinear

absorption and optical limiting behavior under nanosecond (5 ns) Nd:YAG laser light excitation at

532 nm. The LG–PVA nanocomposite films are found to possess a high laser damage threshold,

improved thermal and photostability, and excellent durability, which signify the scope of utilizing them

as smart materials for applications in optoelectronic nanodevices.

1 Introduction

The rapid technological advancements in nanophotonicsduring the last few years have driven the need for novel opticallyfunctional materials for future applications in optical commu-nication, information storage, switching, computing, signalprocessing, optical limiting, etc.1–7 In this regard, a variety ofnovel molecular architectures including low dimensionalnanostructures, inorganic–organic hybrid materials, polymer-based nanocomposites, quantum dots, nanocrystals, etc. havebeen developed.8–12 Recent advances in polymer science havedemonstrated the capability to realize a vast array of nanolledpolymer composite materials exhibiting tailored mechanical,thermal, electro-active, linear and nonlinear optical proper-ties.12–17 In addition, these polymeric systems possess severaladvantages like high optical damage threshold, gas-barrierproperties, ease of integration into photonic devices, improvedtransparency and low processing costs.18–21 The possibility ofimmobilizing functional nanoparticles as llers into polymersprovides an efficient way to develop composite materialsexhibiting unique combinations of functionalities superior totheir constituents.13,22

of Kerala, Kariavattom, Thiruvan-

E-mail: sreejasreedhar83@gmail.com;

5632; Tel: +91-471-2308920

ity, Kottayam, Kerala-686560, India

rch Institute, Sadashivanagar, Bangalore-

Chemistry 2013

Various organic/inorganic materials, metal nanoparticlesand carbon-basedmaterials are commonly used as llers for thefabrication of polymer matrix nanocomposites.12,14,15 Organicdye-impregnated polymer nanocomposites, in particular, havebeen proven to be ideal candidates for a variety of applicationsranging from optical limiters to polymer based nanodevices,electroluminescent devices, photovoltaic solar cells, storagedevices and organic light emitting diodes (OLEDs).16,23–33

Organic chromophores with delocalized p-electrons are widelyinvestigated as nonlinear optical (NLO) materials, as they canexhibit large and fast optical nonlinearities resulting from theease of polarization of the extended mobile p-electron cloudsover large molecular distances.16,34 However, the majorconcerns, which limit their device applications, include lack ofstability against solvents, poor thermal and photostability, andtoxicity of organic solvents evaporated by high power laserbeams.35,36 The incorporation of ‘functional’ organic chromo-phores into suitable host polymers leads to elimination of theirparticular limitations, thereby providing improved function-ality. Poly(vinyl alcohol) (PVA) has been widely used as a hostmatrix for the fabrication of polymer matrix nanocompositesdue to its unique physical and chemical properties.37–40

However, it has been noticed that, compared to inorganicsystems, organic–polymer nanocomposites have not yetreceived much attention owing to the lower melting tempera-tures, thermal instability and poor mechanical properties oforganic materials. Moreover, it is difficult to control the particlesize, morphology and uniformity of their assembled products,particularly in the nanoscale regime.41 Furthermore, there are

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only a few reliable routes demonstrated for fabricating organicnanostructures.

The present study aims at developing a new PVA-basedorganic dye (Light Green) nanocomposite system, by a simpleprocessing technique, with special emphasis on exploring theeffect of incorporation of the organic ller on the structure,morphology, linear and nonlinear optical properties of the hostpolymer. To the best of our knowledge, this article features therst report of the fabrication as well as the photophysical andoptical limiting properties of the LG–PVA nanocomposite system.

Fig. 2 Photographs of LG–PVA nanocomposite films deposited onto glasssubstrates (7.5 � 2.5 cm2 micro-slides) at dye concentrations (a) 3 � 10�3 M, (b)5 � 10�3 M and (c) 7 � 10�3 M.

2 Experimental2.1 Materials

Light Green dye. Light Green (LG) or Light Green SFYellowish is a triarylmethane dye with the molecular formulaC37H34N2O9S3Na3, extensively used in ophthalmology, planthistology and for staining collagen. It is a syntheticallyproduced organic molecule containing triphenylmethanebackbones and is usually available as sodium salt.42 Themolecular structure of the Light Green dye molecule is illus-trated in Fig. 1(a).

Poly(vinyl alcohol) (PVA). PVA is a non-toxic, biodegradableand biocompatible polyhydroxy polymer, used in a broad rangeof industrial, medical, and bio-medical applications due to itssignicant physical and chemical properties.40,43–45 The chem-ical structure of the vinyl polymer PVA is given in Fig. 1(b). Theunique properties of PVA arise from the presence of OH groupsand its capacity to form inter- and intra-molecular hydrogenbonds even at room temperature.40 It has a two-dimensionalhydrogen bonded network sheet structure. The physical char-acteristics of PVA can vary depending on the degree of poly-merization, hydrolysis and distribution of hydroxyl groups. PVAhaving a molecular weight of 125 000 g mol�1 and a degree ofhydrolysis of 86–89% (partially hydrolyzed grade) was used asthe host template for the present work.

2.2 Fabrication of LG–PVA nanocomposite lms

The LG–PVA nanocomposite lms were prepared by the solu-tion-cast method,46–52 which is considered to be the mostversatile and easiest method for the synthesis of polymernanocomposites.21 Weighed amounts (low molar concentra-tions) of the organic dye LG (3.1 � 10�4 M, 5.2 � 10�4 M, 7 �10�4 M, 8.9 � 10�4 M, 1.0 � 10�3 M, 3.1 � 10�3 M, 5 � 10�3 M,and 7 � 10�3 M) were incorporated into the host polymer PVA,which serves as the template. A known amount (5 wt%) of PVAwas dissolved into distilled water at 90 �C, with magnetic stir-ring, until the polymer was completely dissolved (�2 hours).The clear solution was then cooled to room temperature under

Fig. 1 Molecular structures of (a) LG dye molecule and (b) poly(vinyl alcohol).

3852 | J. Mater. Chem. C, 2013, 1, 3851–3861

continuous stirring. Different concentrations of the organic dyewere then dissolved into the solution with continuous stirring(�1 hour), and the resulting mixture was ultrasonically agitatedfor 30–45 minutes to get a perfectly homogeneous solution. Themixture containing different concentrations of the organic dyemolecules was then casted onto perfectly leveled glass micro-slides. The fabrication method was improved by allowing lmformation in a dark, enclosed environment at room tempera-ture.9,37,53,54 Good quality transparent lms of uniform surfacenish, and thickness in the range of 30–35 mm were thusobtained within ve to six days. Fig. 2 shows the photographs ofthe LG–PVA nanocomposite lms deposited onto 7.5 � 2.5 cm2

micro-slides. These composite lms (Fig. 2(a)–(c)) are easy tohandle as they can be simply peeled off intact from the micro-glass slides on which they are deposited.

2.3 Structure, morphology and linear opticalcharacterization methods

The composite lms were characterized for their structure,microstructure and linear optical properties employing varioustechniques. X-ray diffraction (XRD) analysis (Bruker AXIS-D8Advance diffractometer) was performed to study the effect ofincorporation of the LG dye on the structure and crystallinity ofthe host polymer PVA. The X-ray diffractograms were recordedusing Ni-ltered Cu Ka radiation of wavelength 1.54 A with agraphite monochromator. The scan was taken in the 2q range of10–70� with a scanning step size of 0.14�. The characteristicfundamental vibrations of the functional groups as well as thepresence of specic interactions between PVA and LG dyemolecules were obtained from Fourier transform infrared (FTIR)spectroscopy. The FTIR spectra were recorded using a Perkin-Elmer infrared spectrometer with an Attenuated Total Reec-tance (ATR) device of resolution 4 cm�1 in the wavenumberrange of 4000–400 cm�1 at transmittance mode. Themorphology andmicrostructural analysis of the composite lmswere carried out employing atomic force microscopy (AFM) atcontact mode (Digital Instruments Nanoscope E, with Si3N4

100 mm cantilever of force constant 0.58 N m�1). The linearabsorption spectra of the samples were recorded using a UV-visspectrophotometer (SHIMADZU UV-2450). The fundamentaloptical constants required for the analysis of the Z-scan data of

This journal is ª The Royal Society of Chemistry 2013

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the samples were obtained from these optical absorptionmeasurements. A spectrouorometer (Perkin Elmer, LS 55) wasused to record the PL emission spectra of the samples.

Fig. 4 Reaction scheme illustrating the possible interactions of LG dye moleculesand PVA. The molecular structure of the LG dye molecule, which occupies the freevolumes available between the PVA chains, favors hydrogen bonding/hydro-phobic interactions with PVA.

2.4 Nonlinear optical studies: Z-scan technique

Nonlinear absorption in LG–PVA nanocomposite lms wasinvestigated using the standard Z-scan technique in the openaperture (OA) mode.55 This technique, which has sensitivitycomparable to that of interferometric methods, makes use ofthe distortions induced in the spatial and temporal proles ofthe input beam passing through the sample to give a qualitativeas well as quantitative measure of the nonlinearities occurringin the material medium. A frequency doubled, Q-switchedNd:YAG laser (Quanta Ray, Spectra Physics) operating at 532 nm(pulse width 5 ns) served as the excitation source. The Gaussianbeam was focused to a spot size of 17 mm, using a lens of focallength 10 cm. This yielded a Rayleigh range of 1.705 mm, whichwas greater than the sample thickness, satisfying the thinsample approximation condition required for the Z-scantheory.55,56 Using two pyroelectric energy probes (RjP 735, Laserprobe Inc.), the incident laser energy and the energy of thetransmitted beam were monitored during regular intervals, asthe sample was translated along the propagation direction ofthe focused Gaussian beam.

3 Result and discussion3.1 X-ray diffraction

XRD patterns of the LG–PVA nanocomposite lms at differentdye concentrations, and those of LG dye molecules in powderform and unlled PVA (UP) lm, are shown in Fig. 3. The LG dyemolecules in powder form are crystalline (Fig. 3(d)) as indicatedby the sharp XRD peaks at 2q � 31.7� and 45.9�. The UP-lm(Fig. 3(a)) displays a relatively sharp and broad peak centered at2q � 19.6� (corresponding to 101 spacing), characteristic of thesemi-crystalline nature of PVA.40 It is reported that the semi-crystalline nature of PVA arises from the inter-molecular

Fig. 3 XRD patterns of (a) unfilled PVA (UP) film, (b and c) LG–PVA nano-composite films of dye concentrations 5.1� 10�4 M and 5� 10�3 M, respectively.The inset shows the XRD pattern of the LG dye in powder form.

This journal is ª The Royal Society of Chemistry 2013

hydrogen bonding between the PVA chains, which can cause a‘partial ordering’ of the polymer chains.46 Moreover, the crys-talline phase of PVA is treated as an imperfect crystalline lattice,in which free volumes are lled with the amorphous phase.40 Onincorporation of LG dyemolecules into PVA, the crystalline peakcharacteristic of PVA gets broadened and enhanced in intensity,indicating an improvement in the degree of crystallinity ofPVA.40,57 The absence of additional peaks corresponding to theLG dye molecules suggests that the dye molecules are welldispersed in PVA, and does not change the structural congu-ration of PVA. The increase in crystallinity of the compositelms compared to that of the UP-lm may be due to the inter-actions of LG dye molecules with PVA, probably with thehydroxyl groups of PVA. The dye molecules, when introducedinto the polymer PVA, may get trapped or distributed inside thefree volumes (interstitial sites/voids) available between thepolymeric chains.58 The interactions between the dye moleculesand polymeric chains may result in an improved ordering of thepolymer chains, thereby enhancing the degree of crystallinity ofPVA with the addition of LG dye molecules. A reaction schemeshowing possible weak bonding interactions (hydrogenbonding/hydrophobic interactions) of LG dye molecules withPVA to form complexes is given in Fig. 4. Such weak bondinginteractions are reported for Congo red organic dye doped PVAlm,59 BaCl2 doped PVA,40 TiCl3 doped PVA,52 etc.

3.2 FTIR spectroscopy

The IR spectral analysis shows good agreement with the XRDresults by indicating the presence of specic interactionsbetween PVA and LG dyemolecules. The ATR-FTIR spectra of LGdye powder, UP-lm and LG–PVA nanocomposite lms atdifferent dye concentrations (5.1 � 10�4 M and 5 � 10�3 M) arepresented in Fig. 5. The UP-lm (Fig. 5(b)) indicates majorbands corresponding to hydroxyl and acetate groups. The LG–PVA nanocomposite lms (Fig. 5(c) and (d)) display the char-acteristic vibrational bands of the host polymer PVA, and no

J. Mater. Chem. C, 2013, 1, 3851–3861 | 3853

Fig. 5 FTIR spectra of (a) LG dye in powder form, (b) PVA film, (c and d) LG–PVAnanocomposite films of dye concentrations 5.1 � 10�4 M and 5 � 10�3 M,respectively.

Fig. 6 Schematic illustration indicating the dispersion of organic dye moleculesin the free volumes (interstitial sites) between the PVA chains.

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additional vibration bands correspond to the LG dye (Fig. 5(a)).The major IR bands can be assigned to the characteristicfundamental vibrations of the functional groups, and the bandassignments are given in Table 1. The broad and strong band ofPVA at 3271 cm�1, corresponding to the stretching vibrations ofthe hydroxyl group,60,61 shis to 3288 cm�1 and 3292 cm�1

respectively for LG–PVA composite lms of dye concentrations5.1 � 10�4 M and 5 � 10�3 M, which suggests the possibility ofinteractions of LG dye molecules with the hydroxyl groups ofPVA.60 The band due to C]C stretching vibrations of PVAoccurring at 1655 cm�1 shis to 1714 cm�1 for the dye incor-porated PVA lms.61 The very weak IR absorption bands of PVAat 1377 cm�1 and 1236 cm�1 become prominent with a corre-sponding shi to 1375 cm�1 and 1247 cm�1, respectively onincorporation of LG dye molecules. A similar shi in thevibrational band of PVA at 1327 cm�1 was also observed with theaddition of LG dye molecules. The shis in the major band

Table 1 FTIR peak assignments for PVA and LG–PVA composite films

Peak positions (cm�1)

Peak assignmentsPVALG–PVA(5.1 � 10�4 M)

LG–PVA(5 � 10�3 M)

3271 3288 3292 O–H stretching60,61

2941 2941 2941 Alkyl C–H stretching(asymmetric)63

2907 2910 2910 Alkyl C–H stretching(symmetric)64

1655 1714 1714 C]C stretching62

1414 1415 1415 CH2 bending61

1377(s) 1375 1375 CH2 wagging40,61

1327 1329 1329 O–H inplane deformationwith C–H wagging61

1236(s) 1247 1258 C–H wagging65

1087 1087 1088 C–O stretching of acetyl groupsin the PVA back bone45,61

833 834 834 PVA skeletal band61

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positions together with the signicant changes in the intensi-ties of the respective bands imply the presence of weak inter-actions of LG dye molecules with PVA (Fig. 4).40 Furthermore,the absence of any additional bands in the FTIR spectrum ofLG–PVA nanocomposite lms compared to that of the UP-lmindicates that no new covalent bonds are formed between thedye molecule and PVA, and the structural conguration of PVAis not perturbed by the addition of LG dye molecules, which arealso obvious from the XRD analysis.

In semicrystalline polymers, crystalline regions are con-nected to amorphous regions by polymer chains, and the spacebetween the molecules or molecular chains of the polymerconstitutes the free volume or voids.58 Several studies havepointed out that, when a composite lm is formed by castingfrom an aqueous solution containing a ller and the hostpolymer, the llers may either get covalently linked to the mainchain or side chains of the host polymer, or may get dispersedas an isolated/clustered phase with only weak bonding inter-actions acting between them.66 In the present case, it may beinferred from the above observations that, when LG dye mole-cules are introduced into the host polymer, the dye moleculesmay get dispersed in the polymer matrix and get trapped in thefree volume/voids present in the host matrix. These dye mole-cules can then interact with the polymer chains by means ofpossible weak-type interactions (Fig. 4), so that the structuralconguration of PVA is not affected due to the addition of dyemolecules. A schematic diagram illustrating the dispersion ofLG dye molecules in PVA is shown in Fig. 6. Encapsulation ofthe organic dye molecules in the interstitial sites between thepolymer chains, and their interactions with the polymer, mayhinder the free motion of the dye molecules. Therefore theircharacteristic vibration modes are either not prominent, or theyare not excited to give sufficient intensities, which is evidentfrom IR measurements. Moreover, it is also reported that thepresence of polymer chains and the interactions betweenorganic dyes and polymer molecules can limit the growth ofhigher aggregates of dye molecules in the polymer matrix.67,68

3.3 Electrical conductivity

The electrical conductivity of the UP-lm as well as LG–PVAnanocomposite lms were measured using a Keithley model

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Fig. 7 Variation of the electrical conductivity with the concentration of the dyecontent in LG–PVA nanocomposite films.

Fig. 8 2D and 3D AFM images of the (a) UP-film, and LG–PVA nanocompositefilms with dye concentrations (b) 5.1 � 10�4 M and (c) 5 � 10�3 M.

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617 electrometer. The LG–PVA nanocomposite lms exhibitedan electrical conductivity in the range of 1.5 � 10�2 S m�1 to1.8 � 10�2 S m�1, while the UP-lm showed very poor conduc-tivity. The variation of electrical conductivity as a function ofdye concentration is given in Fig. 7. PVA is reported to be a poorconductor of electricity and it can become semiconductive ondoping with suitable dopants.69,70 PVA being an insulator, theelectrical conductivity exhibited by these LG–PVA compositelms can be ascribed to the possible weak bonding interactionsof LG dye molecules with the hydroxyl groups belonging todifferent chains of PVA (Fig. 4). In the case of semi-crystallinepolymers, ionic transport occurs in the amorphous region andis governed by the segmental motions of polymer chains.71 Theaddition of llers to PVA and its interaction with the polymerchains increase the volume required for ionic carriers to dri inthe polymer matrix.72 This enhances the ionic mobility andhence increases the conductivity.

3.4 Atomic force microscopy (AFM)

The surface morphology of the unlled as well as LG dyeincorporated PVA lms was studied using AFM. In contactmode AFM, color differences in the micrographs can be used todistinguish crystalline and amorphous regions of polymersurfaces: light-colored regions may correspond to higherapparent topography (crystalline regions) and dark-coloredregions may correspond to amorphous regions.73 The AFMimage of the UP-lm (Fig. 8(a)) signies the semicrystallinenature, which may arise from the random distribution of poly-meric chains in an amorphous matrix. On introduction ofLG dye molecules (dye concentrations: 5.1 � 10�4 M and 5 �10�3 M) into PVA, the AFM images (Fig. 8(b) and (c)) indicate animproved crystallinity and the presence of uniformly distrib-uted nanocrystallites. Moreover, as the concentration of the dyecontent in the LG–PVA lm increases, the grain size increases(Fig. 8(b) and (c)) (with the appearance of spherical grains/clusters). This evolution of surface morphology may be due tothe separation of crystalline and amorphous phases (enhance-ment in crystallinity) resulting from the encapsulation oforganic dye molecules in the free volumes/interstitial sites

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between PVA chains. Dispersion of the dye molecules in thesesites modies the pore structure of the polymer and reduces themicro-porosity. The enhancement in the degree of crystallinityof PVA with the addition of LG dyemolecules may arise from theimproved ordering of PVA chains, probably due to the weakbonding interactions (Fig. 4) between the dye molecules andPVA chains, as evident from the XRD and FTIR analyses.Moreover, the presence of polymeric chains and interactionsbetween dye molecules and PVA can limit the growth of aggre-gates of dye molecules in the polymer matrix.67,68 The compositelms revealed smooth surface topography, marked by very lowaverage roughness of 1.29 nm and 1.64 nm for the lm surfaceswith dye concentrations of 5.1 � 10�4 M and 5 � 10�3 M,respectively. The average roughness of the nanocomposite lmsare found to be much less than those reported for manynanostructured materials74,75 and nanocomposite lms.76,77

3.5 Optical absorption spectroscopy

The linear absorption spectra of LG–PVA nanocomposite lmsat different dye concentrations along with that of an aqueoussolution of the LG dye (dotted lines) and the UP-lm (inset) areillustrated in Fig. 9. The spectrum of the UP-lm shows negli-gible absorption in the higher wavelength region (visible region)of the spectrum, and presents an absorption band centeredaround 245 nm, which can be ascribed to the semi-crystallinenature of PVA.52 An aqueous solution of the LG dye(dotted lines) possesses three broad absorption bands centered

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Fig. 9 UV-vis absorption spectra of LG–PVA nanocomposite films at different dyeconcentrations ((a) 3.1 � 10�4 M, (b) 5.2 � 10�4 M, (c) 7.0 � 10�4 M, (d) 8.9 �10�4 M, (e) 1.0 � 10�3 M, (f) 3.1 � 10�3 M, (g) 5.0 � 10�3 M, (h) 7.0 � 10�3 Mand (i) LG solution (conc. 0.7 � 10�4 M)). The inset shows the absorption spec-trum of the UP-film.

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at �305 nm, 429 nm and 638 nm. The LG–PVA lms display allmajor bands characteristic of LG dye molecules, and aretherefore attributed to the monomeric form of the dye mole-cules introduced in PVA.67,68,78 However, the absorption bandsare slightly red shied (with the absorption maximum showinga red shi of�11 nm) compared to that of the aqueous solutionof the dye. This may be attributed to the change in the electronicenvironment of the dye molecules and possible interactions ofthe dye molecules encapsulated between the PVA chains.78,79 Itcan therefore be inferred that the optical absorption propertiesof the host matrix PVA can be drastically varied by incorporatingLG dye molecules.

In order to understand the possibility of interactions of LGdye molecules with PVA in detail, the difference spectrum(Fig. 10(a)) of aqueous solutions of LG–PVA and PVA, having the

Fig. 10 (a) Difference spectrum of aqueous solutions of PVA and LG dye addedPVA solution (taken before film casting) and (b) absorption spectrum of anaqueous solution of LG dye.

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same concentration of PVA (taken before lm casting), wasmeasured. The difference spectrum shows all the characteristicbands of the dye, however it is not superimposing with that ofthe LG dye solution having the same concentration. Theabsence of additional absorption bands in the difference spec-trum as compared to that in the spectrum of aqueous solutionof the organic dye signies that no new covalent bonds orcharge transfer states are present in the ground state. Theresults of UV-vis analysis are consistent with the structural andmicrostructural analyses and the observed changes in theabsorption spectra (Fig. 10) therefore conrm the possibility ofweak interactions of the dye molecule with PVA (Fig. 4).

3.6 Photoluminescence emission

The photoluminescence (PL) emission spectra of LG–PVAnanocomposite lms measured at different dye concentrationsare given in Fig. 11. The lms exhibit PL emission in the redregion (centered at�699 to 713 nm), when excited in the vicinityof their absorption maxima (645 nm). However, the host matrixPVA (unlled) lm does not show any emission in this wave-length range. Because absorption of PVA is negligible near theexcitation wavelength, the observed red emission can beattributed to the monomeric dye molecules which are encap-sulated between the PVA chains as isolated entities. As theconcentration of the dye content in the nanocomposite lmsincreases, the spectra reveal luminescence quenching, with aprogressive red-shi in the emission maxima. As the absorptionspectrum gives no indication of the formation of aggregates ofthe dye molecules, the observed luminescence quenching mayarise due to some competent non-radiative processes in thenanocomposite system.80–83 An electron in the excited energylevel undergoes an energy exchange/transfer with anotherelectron in the ground state, which can severely suppress theuorescence at higher concentrations.84,85 In addition, theuorescence quenching effect can occur as a result of trivialnon-molecular mechanisms like attenuation of the lightintensity by the uorophore itself or by other absorbing

Fig. 11 PL emission spectra of LG–PVA nanocomposite films at different dyeconcentrations (a) 7.0 � 10�4 M, (b) 1.0 � 10�3 M, (c) 3.1 � 10�3 M and (d) 5.0 �10�3 M.

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species.86 Here, when LG dye molecules are introduced intoPVA, the lack of mobility of the dye molecules in the rigidenvironment can limit the intra-molecular rotation modes withthe subsequent reduction of the deactivation processes and canlead to uorescence quenching (S1 / S0 internal conversionprocesses).81 The connement of organic dye molecules in thenanopores of the host template PVA, and its interactions withthe polymer chains, may contribute to the red-shi in theemission maximum with the addition of LG dye molecules.

3.7 Z-Scan studies

To investigate the nonlinear absorption behavior, open apertureZ-scan experiments were conducted for two different peak inci-dent intensities (2.2 GW cm�2 and 4.4 GW cm�2) of nanosecondNd:YAG laser excitation at 532 nm. It may be noted that the UP-lm (Fig. 12(a)) as well as LG–PVA nanocomposite lms havinglower dye concentrations (<3.1� 10�3 M) did not give any Z-scansignals at both laser peak intensities. At an incident intensity of2.2 GW cm�2, the nanocomposite lms (Fig. 12(b)) with dyeconcentrations of 5 � 10�3 M and 7 � 10�3 M display anormalized transmittance valley indicating the occurrence of anenhanced absorption process like reverse saturable absorption(RSA, positive nonlinear absorption).87 However, at a laserintensity of 4.4 GW cm�2, nanocomposite lms with three dyeconcentrations (3.1� 10�3 M, 5� 10�3 M and 7� 10�3 M) showreverse saturable absorption behavior (Fig. 12(c)). The inuence

Fig. 12 OA Z-scan profiles of: (a) UP-film and (b and c) LG–PVA nanocompositefilms at different dye concentrations for peak incident intensities of 2.2 GW cm�2

and 4.4 GW cm�2 respectively. (d) OA Z-scan profile of a LG–PVA nanocompositefilm with a dye concentration of 5 � 10�3 M, at two different peak incidentintensities. (e) and (f) represent the OA Z-scan profiles of the LG dye in water andethanol solutions (concentrations: �4 � 10�3 M), respectively. Solid curvesrepresent the theoretical fits to the experimental data, generated using eqn (1).

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of the laser peak intensity on the RSA behavior of composite lmsat a dye concentration of 5� 10�3 M is shown in Fig. 12(d), whichindicates that NLO properties are enhanced at higher laserintensity. Moreover, the optical nonlinearity of the samples wasfound to increase with dye concentration. Under similar experi-mental conditions, the LG dye in water and ethanol solutionsalso displayed reverse saturable absorption behavior (Fig. 12(e)and (f)), while the pure solvents (water and ethanol) did not showany Z-scan signals. As given in Fig. 12(a), the UP-lm did not giveany Z-scan signals under the same experimental conditions. Thisconrms that the hostmatrix has no contribution to the observednonlinear absorption in LG–PVA nanocomposite lms, and it istherefore attributed to the contribution from the LG dye mole-cules (ller). Moreover, the samples did not show any sign oflaser damage under the experimental conditions. The improvedlaser damage threshold may also arise from the entanglement ofthe organic dye molecule (ller) in the structural pores/intersti-tials/voids available between the polymeric chains of the hostmatrix, as mentioned earlier.

The observed intensity dependent nonlinear absorption canhave contributions from various mechanisms such as excitedstate absorption (ESA), multiphoton absorption (two-photon(2PA) or three-photon absorption (3PA)), nonlinear scattering,etc.7,9,10,16 To evaluate the probable mechanism of the nonline-arity and to estimate the nonlinear absorption parameters, theOA Z-scan data were analyzed using the standard Z-scantheory.88–90 The effective coefficients of nonlinear absorption(beff) were evaluated from the theoretical simulations to theexperimental data. The normalized transmittance for the stan-dard open aperture Z-scan is expressed by the relationship,55

Tðz; s ¼ 1Þ ¼XN

m¼0

½�q0ðzÞ�m½mþ 1�3=2

; for jq0ð0Þj\1 (1)

where; q0ðzÞ ¼ beffI0Leff

1þ z2=z02(2)

Here, Leff is the effective length of the sample given by Leff¼ (1�exp(�aL))/a, with L being the sample length and a being thelinear absorption coefficient; I0 is the on-axis intensity at thefocus and z0 is the diffraction length of the beam. The theo-retical curves generated using the above equations (shown assolid lines in Fig. 12) were tted to the experimental data, usingb as the tting parameter.88–92 The nonlinear absorption coef-cient b obtained from the best theoretical t was found to be ofthe order of 10�7 to 10�8 cm W�1 (Table 1). From repeatedmeasurements, these estimated values were found to lie withinan error limit of �10%. The measured values of the effectivenonlinear absorption coefficient (beff) are one to two ordersgreater than those reported for Rhodamine B (10�9 cm W�1),93

Ag2Te nanowires (10�11 m W�1),10 bis-phthalocyanine (Nd(Pc)2solution in DMF),90 Pt–PVP lms (10�10 m W�1),94 silver-coatedpolydiacetylene nanocomposite (10�11 mW�1),95 etc., under 532nm nanosecond laser light excitation. The values of beff

obtained for water and ethanol solutions of LG dye are found tobe �10�10 and 10�9 cm W�1, respectively, under the sameexperimental conditions. These observations suggest a ller-induced property (optical nonlinearity) enhancement with the

J. Mater. Chem. C, 2013, 1, 3851–3861 | 3857

Fig. 13 Optical limiting curves of LG–PVA nanocomposite films, for differentconcentrations of the dye content (a) 3.1 � 10�3 M, (b) 5.1 � 10�3 M and (c) 7 �10�3 M, when excited with 532 nm, 5 ns laser pulses. Filled squares show theexperimental data and the solid curves represent the best fit to the experimentaldata.

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addition of LG dye molecules (ller) into PVA (host matrix).Moreover, the useful range of concentrations of the organic dyemolecules for performing NLO experiments was much larger indye encapsulated nanocomposites than in dye solutions. Thesenanocomposites therefore offer an easier way of tailoring theNLO properties by the suitable choice of the concentrations ofthe matrix and ller materials.

The imaginary part of the third-order nonlinear opticalsusceptibility Imc(3) is directly related to the intensity depen-dent absorption coefficient b, which is expressed as,55

Imcð3Þ ¼ 30c2n0

2

ubeff (3)

where 30 is the permittivity of free space, n0 is the linearrefractive index, c is the speed of light, u is the opticalfrequency, and beff is the effective coefficient of nonlinearabsorption. The calculated values of the imaginary part of thethird-order nonlinear susceptibility (in esu) are given inTable 2.

Depending on the wavelength of the excitation beam, RSA inorganic dyes is generally explained in terms of ESA or 2PAmechanisms. In organic dyes, the occurrence of nonlinearoptical effects even at low excitation power is possible since theradiative life times of the lowest triplet states are quite long as aresult of which there can be a population build up in the tripletstates.96,97 Under such conditions, non-radiative processes likeintersystem crossing to the lower-lying triplet states can takeplace, whereby molecules can further be excited to the excitedtriplet state, thereby giving way to a RSA process.5,16,53,96 Onexcitation with nanosecond laser pulses, the intersystemcrossing rate and triplet state lifetime of organic dyes becomesignicant. Moreover, organic dyes embedded in a rigid envi-ronment exhibit a prolonged life time of the triplet states andtherefore ESA assisted RSA processes can occur in dye–polymersystems. In the present case, as the excitation wavelength(532 nm) does not fulll the 2PA requirement (labs < lexc < 2labs),the role of 2PA in the observed nonlinear absorption behavior isless predominant and can therefore be neglected.88,98 The beff

values obtained for LG–PVA nanocomposite lms (Table 2) isfound to decrease with the increase in incident intensity for allthe concentrations studied, which also suggest the contributionfrom a possible ESA process.99 The observed nonlinearabsorption under nanosecond laser excitation at 532 nm can

Table 2 Estimated values of beff and Im c(3) for different dye concentrations of LG

Sample ConcentrationLineartransmission (%)

LG–PVA 3.1 � 10�3 M 68

LG–PVA 5.1 � 10�3 M 62

LG–PVA 7.0 � 10�3 M 50

LG–watersolution

4.0 � 10�3 M 30

LG–ethanolsolution

4.1 � 10�3 M 34

3858 | J. Mater. Chem. C, 2013, 1, 3851–3861

therefore be ascribed to an ESA process actuated by the pop-ulation buildup in the triplet state.16,97,100

3.8 Optical limiting studies

In an open aperture Z-scan scheme with a Gaussian beam as theexcitation source, the sample experiences a varying laser uenceat each position z, which will be maximum at the focal point. Soto further investigate the optical limiting responses, thenormalized transmittance is plotted against the incident laserpulse energy density (J m�2). Such plots represent a bettercomparison of the nonlinear absorption or transmission inthese samples and are generated from Z-scan traces. Fig. 13shows the optical limiting curves of the LG–PVA nanocompositelms having a linear transmission in the range of�68% to 50%,extracted from the Z-scan data. From Fig. 13, it is clear that forhigh input irradiances, there is marked deviation from linearity,indicating the optical limiting effect occurring in LG–PVAnanocomposite lms. The uence value corresponding to theonset of optical limiting (optical limiting threshold) decreases

–PVA composite films and LG dye solutions

Incidentintensity, I0 (W cm�2)

beff(cm GW�1)

Im c(3)

(10�11 esu)

2.2 � 109 — —4.4 � 109 30 0.912.2 � 109 74 2.174.4 � 109 73 2.152.2 � 109 185 5.194.4 � 109 145 4.074.4 � 109 1.4 —

4.4 � 109 1.8 —

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with the increase in concentration of the dye content in thecomposite lms, indicating an enhancement in the opticallimiting performance. In general, the optical limiting behaviorof a medium can have contributions from effects such as ESA(including excited singlet or triplet absorption, free-carrierabsorption, etc.), two- or three-photon absorption (2PA, 3PA),self-focusing/defocusing, thermal blooming, and nonlinearscattering, etc.10 In the present case, the optical limitingbehavior exhibited by LG–PVA nanocomposite lms can beattributed to an excited state absorption process, as discussedearlier. Solid lines are the theoretical ts generated using eqn(1). The large values of the effective nonlinear absorption coef-cient beff and Im c(3) indicate the possibility of utilizing thisnew nanocomposite material for potential applications inoptical limiting devices.

4 Concluding remarks

A new NLO material, an organic dye (Light Green)–PVA nano-composite system, has been fabricated employing the solutioncast method. The samples are characterized as nanoclustersconsisting of organic dye molecules, dispersed uniformly in thefree volumes/interstitial sites available between the polymericchains. The microstructure of the nanocomposite lms displayscharacteristic features of the host polymer PVA. However, thelinear and nonlinear optical properties are contributed by theorganic ller molecules. The addition of LG dye molecules intoPVA enhances the degree of crystallinity of the semicrystallinehost polymer. FTIR studies indicate considerable interactionsbetween the dye molecules and PVA. The dispersion of the dyemolecules in the polymer matrix and their interactions result inan improved electrical conductivity of the LG–PVA lms. Thelms display red emission when excited in the vicinity of theirabsorption maxima (645 nm). When excited with 532 nm, 5 nsNd:YAG laser light, the samples exhibit strong positivenonlinear absorption and optical limiting based on reversesaturable absorption. The measured values of the nonlinearabsorption coefficients are found to be comparable to thosereported for many organic materials in the nanosecond excita-tion regime. The present study establishes the role of thesenanocomposites as a promising NLO medium for applicationsin optical limiting devices.

Acknowledgements

S. Sreeja would like to thank Dr N. Illyaskutty, ISIS, KarlsruheUniversity of Applied Sciences, Germany for his valuablesupport throughout the course of this work. The authors alsothank Dr C. T. Aravindakumar, School of EnvironmentalSciences, Mahatma Gandhi University, Kerala, India for hisvaluable support in providing PL measurements.

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