Low-threshold Saturable Absorption and

Photoluminescence Emission in Phenol Red Dye

Impregnated Poly(Vinyl Alcohol) Nanocomposite

S. Sreeja, I. Navas, S. Mayadevi, P.G. Louie Frobel, S.R. Suresh and C. I. Muneera Department of Physics, University of Kerala, Kariavattom, Thiruvananthapuram, Kerala -695581

Abstract— A new organic-polymer nanocomposite system

constituted of organic chromophore Phenol Red (PR) was

fabricated using poly(vinyl alcohol) (PVA) as host template. The

structure, morphology, linear and nonlinear optical absorption as

well as photoluminescence emission behaviour of the organic-

polymer composites were investigated. The composite films were

characterized as nanoclusters consisting of the dye molecules

encapsulated between the larger molecules/molecular chains of

the semicrystalline polymer and having an average roughness as

low as ≈0.62 nm for the surface. XRD studies indicated an

increase in the semicrystalline nature of PVA, with the addition

of Phenol Red (PR) dye molecules into the polymer. The

composite films exhibited PL emission in the orange-red region of

the visible spectrum, when excited in the vicinity of its absorption

maximum. An investigation into the absorptive nonlinearity

exhibited by the Phenol Red-PVA nanocomposite system under

low power continuous wave (cw) laser light illumination at 442

nm, performed using the Z-scan technique is presented. The

samples exhibited saturable absorption under the experimental

conditions. The estimated values of the saturation intensity were

found to be very low. The results indicate that the nanocomposite

films can be potential candidates for applications in light emitting

devices and also as low threshold saturable absorbers.

Keywords- PVA nanocomposites, organic-polymer

nanocomposites, saturable absorption, low-threshold nonlinear

absorption

I. INTRODUCTION

Future generation optoelectronic devices for telecommunications, information storage, optical switching, and signal processing are predicted to a large degree to development of materials with exceptional non-linear optical (NLO) responses. The rapid progress of nanotechnology promoted the fabrication of nano-scale optoelectronic and photonic devices and, has lead to the design and synthesis of new NLO molecular architectures and hybrid NLO materials, with various structures and multiple functions that can offer large nonlinear figure of merit, fast optical response time, etc [1-4]. In this respect, polymer-matrix nanocomposites are considered to be of great importance because of their light weight, unique optical, electric and magnetic properties, as well as their dimensional and thermal stability [5-7]. Polymer matrix reinforced with micro and nanoparticles possesses properties superior to those of the starting materials and exhibits good thermal stability, high optical damage threshold, micro-

hardness, etc. [8-11]. Among several choices of polymers, poly vinyl alcohol (PVA) has been frequently explored as an important non-toxic, biodegradable and biocompatible polymer, with large scale use in the medical, biomedical and industrial fields [12]; and, above all, a high performance matrix for polymer composites. The wider applicability of PVA is because of its several interesting physical properties which arise from the presence of OH group and the hydrogen bond formation in them [12]. Several recent reports show that PVA could function as a template for growing nanostructured materials [13, 14].

During the last couple of years, there has been a focus of attention in developing and fabricating polymeric materials doped with optically active organic molecules. Organic dye-doped polymer nanocomposites have emerged as prospective materials for applications such as electroluminescent devices, photo diodes, sensors, organic light emitting diodes (OLEDs), photovoltaic solar cells, polymer based nanodevices, optical limiters, display technology etc. and in recording and storage of information [14]. The high degree of compatibility of organic dyes with polymers, their architectural flexibility, and easy modes of preparation, together with the diverse functionality they exhibit, make them particularly suitable for these applications. Earlier reports in literature have proved that organic materials embedded in solid hosts can be used for many applications involving low intensity light sources [15-17]. The present work features the fabrication of a new, poly (vinyl alcohol) (PVA) based organic dye nanocomposite, and explores the structure, morphology, linear as well as nonlinear optical absorption, and photoluminescence emission behaviour of the nanocomposites.

II. EXPERIMENTAL DETAILS

The PVA based nanocomposite films for the present work

were prepared by the solution-cast method, incorporating very

small wt% of the organic dye Phenol Red (PR) (Loba Chemie,

Mumbai ) in the host polymer PVA (molecular weight =

125,000 g/mol s.d. Fine chemicals, Mumbai, India), as been

adopted by many researchers [12, 18, 19]. However, the

method was improved to allow film formation in a dark, dust

free environment, for NLO study [20, 21]. Good quality

transparent films with uniform surface finish and, thickness in

the range 30-35 μm were obtained after five to six days.

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The structure and crystallinity of the composite films were examined by X-ray diffraction (XRD) technique. A Bruker AXIS-D8 Advance diffractometer with Ni-filtered CuKα radiation of wavelength 1.54 Å was used to record the X-ray diffractograms of the samples. The composite films were characterized for details of their microstructure and surface homogeneity employing atomic force microscopy (AFM) (Digital Instruments Nanoscope E, with Si3N4 100 µm cantilever, 0.58 N/m force constant in the contact mode). The linear absorption spectra of the nanocomposite films were recorded using a UV-visible spectrophotometer (SHIMADZU UV-2450). A spectrofluorometer (Perkin Elmer, LS 55) recorded the PL emission spectra of the samples. The open aperture Z-scan technique [22] was employed to investigate the nonlinear absorption behaviour exhibited by the PR-PVA nanocomposite films. For this, a continuous wave (cw), He-Cd laser of maximum output power 135 mW, operating at 442 nm was used as the excitation source.

III. RESULTS AND DISCUSSION

Figure 1. XRD patterns of (a) PR dye in powder form (b) PVA film, and (c)

PR-PVA nanocomposite film.

The XRD patterns of the PR-PVA nanocomposite film along with that of an unfilled PVA film and PR dye molecules in the powder form are shown in figure 1. The observed XRD pattern of the PVA film (Fig. 1(b)) characterizes a semi-crystalline polymer may be treated as an imperfect crystalline lattice, in which free volumes/interstitials are filled with amorphous phase [12]. The dye molecules in the powder form (Fig. 1(a)) are crystalline in nature. However, the crystalline peaks characteristic of the PR dye molecules are absent in the XRD spectrum of PR-PVA nanocomposite (Figs. 1(c), which indicate that the dye molecule loses the crystallinity when incorporated in the polymer matrix. This suggests that, the dye molecules when incorporated into PVA may occupy the interstitial space between the polymer chains and lose the crystallinity of their powder form [23]. For the PR-PVA nanocomposite film, the intensity of the peak at 2θ=19.60, characteristic of the host matrix PVA increases, which indicate an enhancement in the degree of crystallinity of PVA. Such an increase in the crystallinity of the composite film with respect to that of clean (unfilled) PVA may be due to the interactions

of PR dye molecules probably with the hydroxyl groups of PVA.

Figure 2. 2D and 3D AFM (noncontact mode) images of 2x2 µm2 regions of

PR-PVA composite film surfaces (dye concentration 5 x 10-3M

The results of AFM analysis are also in agreement with the above observations, and revealed a smooth surface topography with uniform dispersion of nanoclusters marked by an average roughness as low as ≈ 0.62 nm for the nanocomposite film. Figure 2 shows the 2D and 3D AFM images of PR-PVA nanocomposite film with dye concentration 5×10-3 M. The low average roughness for the nanocomposite films resulted from the entanglement of the dye molecules in the microscopic/nanoscopic free volumes/interstitials available between the polymer chains. This reduces the porosity, which in turn would result in prolonged life, good photostability and resistance against environmental degradation. In previous articles, we have showed that the roughness and microporosity of the dye-polymer composite films were less than that of unfilled PVA films [20, 21]. It is to be noted that the surface roughness of the PR-PVA nanocomposite film was found to be much less than those reported for many nanostructured materials [24, 25] and nanocomposite films [26, 27].

Figure 3. UV-Vis absorption spectra of PR-PVA composite films for dye

concentrations 3.4×10-3M and 5×10-3M. The dotted line indicates the spectrum

of PR dye in aqueous solution. Inset shows the structure of PR dye molecule.

The linear absorption spectra of PR-PVA nanocomposite films for different concentrations of the dye content are illustrated in figure 3. The spectra indicate well structured and wide absorption band centered at 434 nm, red shifted by ~3 nm

632

compared to that of aqueous solution of PR dye, as has been observed for other solvatochromic dyes [28, 29]. This can be due to the possible interactions of the dye molecule with the polymeric host material.

The nanocomposite films exhibited PL emission in the visible region (~610 nm) of the spectrum, when excited in the vicinity of their absorption band (430nm). The photoluminescence emission spectra of PR-PVA nanocomposite films for two different concentrations of the dye content are illustrated in Fig. 4. When the molecules are excited to a singlet state, they eventually de-excite to the ground state either by fluorescence emission or by internal conversion process (i.e. dissipating the excess energy by friction or momentum transfer to the matrix). It is observed that for the PR-PVA nanocomposite film with higher dye content, the luminescence intensity decreases. According to literature, such decrease in luminescence intensity is ascribed to the formation of higher aggregates [9-11]. Radiative and nonradiative energy transfer between the neighbouring molecules can also contribute to the fluorescence quenching at higher concentrations. It has been reported that intermolecular energy transfer in clustered laser dye molecules severely suppresses the fluorescence at higher concentrations [30]. In the present case, the decrease in fluorescence emission intensity can be ascribed to the concentration quenching effect [31], which can occur as a result of trivial nonmolecular mechanisms like attenuation of light intensity by the fluorophore itself or by other absorbing species [32]. Organic dye-polymer composite films emitting in the visible region of the spectrum are considered as a medium of great potential, as they can provide stable sources of light for displays and illumination sources at a significantly lower cost than the semiconductors [33].

Figure 4. PL emission spectra of PR-PVA nanocomposite films for dye

concentrations (a) 5×10-3 M and (b) 3.4×10-3 M.

Figure 5 shows the typical open aperture (OA) Z scan profiles of the PR-PVA nanocomposite films for two different concentrations of the dye content, on irradiation with cw He-Cd laser light. The OA Z-scan traces exhibit a normalized transmission peak, indicating saturable absorption behavior. Here, since the excitation wavelength is in the near resonance region, where the absorption coefficient is comparatively larger, strong pumping can lead to SA rather than RSA [31].

Figure 5. OA Z-scan profile for PR-PVA films with dye concentrations (a)

3.4×10-3M and (b) 5×10-3M, for a peak incident intensity (I0) of

4.62×103W/cm2.

In general, for a thin absorber where the population relaxation rates are fast enough to warrant a steady-state saturation (i.e., the population relaxation time is far shorter than the duration of the laser pulses), the steady-state reduced absorption coefficient α (I) is related to the applied optical intensity I by the relation (1) [34].

)/(1)( 0

sIII

Where, α0 is the linear absorption coefficient under

low-intensity approximation, I is the incident intensity and Is, the saturation intensity. The case described by Eq. (1) is often referred to as homogeneous saturation and is often used when the saturation is considered in terms of depletion of the ground state concentration [35-37]. Solid curves of Fig. 5 represent the theoretical fit to the experimental data obtained using eqn. (1) with only one adjustable parameter, Is, and the model is in good agreement with the experimental data. The value of Is, thus evaluated from the theoretical fit (1.15 – 1.5 ×104 W/cm2) are found to be very low. The low values of Is indicate that the PR-PVA nanocomposite films are potential low-threshold saturable absorbers. Materials exhibiting this kind of nonlinear absorption behaviour are of interest for applications in Q switching and in optical bi-stability applications [37].

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IV. CONCLUSIONS

A new, low cost, nanocomposite NLO material comprising Phenol Red dye and PVA were prepared by a simple processing technique and their structure, morphology, linear as well as nonlinear optical absorption and PL emission behaviour were investigated. Employing the techniques of XRD and AFM, the samples were characterized as nanocomposites with dye molecules encapsulated between the larger molecules (molecular chains) of the polymer host PVA. The PL emission behaviour exhibited by the composite films highlights the scope for utilizing these nanocomposites as potential materials for applications in organic light emitting devices and in polymer based display devices. The nonlinear absorption behaviour exhibited by the PVA based nanocomposites on excitation with 442 nm, cw He-Cd laser light was investigated employing the Z-scan technique. The low-threshold saturable absorption behavior exhibited by these PR-PVA nanocomposite films together with their excellent surface features and structural properties indicate that these nanocomposite films are potential candidates for photonic device applications.

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