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Physica B 406 (2011) 1919–1925 Contents lists available at ScienceDirect Physica B journal homepage: www.elsevier.com/locate/physb A study on optical absorption and constants of doped poly(ethylene oxide) R.S. Al-Faleh, A.M. Zihlif n Physics Department, The University of Jordan, Amman, Jordan a r t i c l e i n f o abstract Article history: Received 12 October 2010 Received in revised form 25 January 2011 Accepted 26 January 2011 Available online 24 February 2011 Thin films of polymer electrolyte based on poly(ethylene oxide) doped with sodium iodide (NaI) were prepared using the solution cast method. The films obtained have average thickness of 70 mm and different NaI concentrations. Absorption and reflectance spectra of UV-radiation were studied in the wavelength range 300–800 nm. The optical results were analyzed in terms of absorption formula for non-crystalline materials. The optical energy gap and the basic optical constants, refractive index, and dielectric constants of the prepared films have been investigated and showed a clear dependence on the NaI concentration. The interpreted absorption mechanism is a direct electron transition. The observed optical energy gap for neat poly(ethylene oxide) is about 2.6 eV, and decreases to a value 2.36 eV for the film of 15 wt% NaI content. It was found that the calculated refractive index and the dielectric constants of the polymer electrolyte thin films increase with NaI content. Models were used to describe the dependences of the dielectric constant on the NaI concentration, and the refractive index on the incident photon energy. & 2011 Published by Elsevier B.V. Keywords: Polymer electrolyte Thin films Absorption Energy gap Direct transition Dielectric constant Refractive index 1. Introduction Polymer composites and polymer electrolytes are the most widely used materials in the past few decades. Understanding their properties is extremely important not only for scientific knowledge but also for modern and advanced technological applications. Most polymeric materials are poor conductors of electricity because of the unavailability of large number of free electrons or ions to participate in the conduction process, so a great attention has been focused on enhancing their electrical conduction and improving their properties. In order to promote optical, thermal, and electrical properties to polymeric systems, many different materials have been developed as conducting polymers electrolytes and polymer composites [1–6]. One important aspect of these characteristics is the transition from insulating to conductive behavior by adding some conducting or electrolytic substances. Many studies have shown that the physical behavior of polymer/electrolytes depends on the filler type or structure and the method of composite preparation. Understanding the chemical and physical nature of the composites structure helps greatly to evaluate and interpret the conduction mechanism taking place in the composites bulk. Space charge limited current can be suggested to explain the charge transfer in polymers in addition to ionic conduction dominated by the amorphous phase in the polymer matrix [7–10]. Solid polymer electrolytes (SPE) are promising materials for electrochemical device applications. Nowadays they are successfully used in high energy density rechargeable batteries, full cells integrated optical devices, and electrochromic displays [5,6]. Sodium iodide is a white, crystalline salt with good properties and high luminescence efficiency. It is used in radiation detection, treatment of iodine deficiency, nuclear medicine, and in many other applications, such as X-ray detectors with high spectrometric quality. The physical characterization of solid films of polymer electrolyte can be carried out using a variety of analytical techniques such as impedance, X-ray diffraction, and UV-spectroscopy. Polymer electrolytes with low NaI concentration were prepared for the purpose of percolation detection or anomalous behavior in the electrical conductivity as we have recently reported in PEO/I2 polymer electrolyte with very low I2 content less than 0.5 wt% [14]. In the present study, thin films of PEO doped with sodium iodide were prepared with different concentrations: 0%, 1%, 2%, 4%, 6%, 8%, 10%, and 15% by weight. Optical and dielectric constants of those films will be determined under different UV-radiation wavelengths and sodium iodide concentrations. 2. Experimental work 2.1. Composites films preparation n Corresponding author. E-mail address: [email protected] (A.M. Zihlif). 0921-4526/$ - see front matter & 2011 Published by Elsevier B.V. doi:10.1016/j.physb.2011.01.076 Poly(ethylene oxide) with average molecular weight of 300,000 g/mol was used to prepare electrolyte films by casting 1920 R.S. Al-Faleh, A.M. Zihlif / Physica B 406 (2011) 1919–1925 from solution. Poly(ethylene oxide) powder and solid sodium iodine were mixed together and dissolved in methanol as a suitable solvent. The solution was then stirred continuously by a rotary magnet at room temperature for a few hours. The stirring process lasted until the mixture reached a homogeneous viscous molten state. The mixture was immediately cast to thin films on a glass plate, and the methanol was allowed to evaporate completely at room temperature and under atmospheric pressure for a few days. The composite films were dried in an oven at 40 1C for 2 days. The films obtained have thicknesses of about 70 mm and sodium iodide concentrations of 0%, 1%, 2%, 4%, 6%, 8%, 10%, and 15% by weight. 2.2. Optical measurements where k is the extinction coefficient, which is related to the absorption coefficient and wavelength by k¼ al ð4Þ 4p The dielectric constant (e0 ) and dielectric loss (e00 ) are calculated from the relations [13]: e0 ¼ n2 k2 , e00 ¼ 2nk One of the most direct and simplest methods for probing the band structure of materials is studying their absorption spectra. In the absorption process, a photon of known energy excites an electron from a lower energy state to a higher one. By analyzing the radiation spectra, we can understand the transition mechanism. The fundamental absorption manifests itself by a rapid rise in absorption, known as the absorption edge. This can be used to determine the optical energy gap and the type of transition [13,15]. Absorption is expressed in terms of the coefficient a(o), which is defined as the relative decrease rate in light intensity. Using a UV-spectrophotometer, the optical absorbance and reflectance spectra of the PEO/NaI composites were collected at room temperature in the wavelength range 300–800 nm. 3. Results and discussion The absorption coefficient a(o) was obtained from the absorbance A. After correction for reflection, a(o) can be calculated using the Beer Lambert’s formula [12,13] I ¼ I0 expðaxÞ ð1Þ Hence 2:303 I0 2:303 log AðoÞ ¼ x x I ð2Þ ð5Þ Solids absorb an amount of the incident light of intensity I0, and consequently optical transitions start when the energy of photon absorbed is higher than or equal to the forbidden energy gap. If the required energy is almost equal to the difference between the lowest level of conduction band and the highest level of valence band, electrons will be transferred from the valence band to conduction band. At high absorption coefficient levels, where a(o)4104 cm 1, the absorption coefficient a for noncrystalline materials can be related to the energy of incident photon energy (_o) according to the formula: aðoÞ:o ¼ Bð:oEopt Þr ð6Þ where B is a constant, Eopt is the optical energy gap, and the exponent r is an index determined by the type of electronic transition causing the optical absorption and can take values 1/2, 3/2 for direct and 2, 3 for indirect transitions [15–17]. Figs. 1 and 2 illustrate the absorption and reflectance spectra of PEO composites with NaI content of 1, 2, 4, 6, 8, 10, and 15 wt%. It is clearly shown that absorption decreases rapidly with increasing wavelength up to 400 nm. Fig. 3 shows graphs of the product of absorption coefficient (a) and photon energy (a _o) versus photon energy (_o) at room temperature. The drawn straight lines obtained with r ¼1/2 indicate that the electron transition is direct in k-space. Extrapolation of the linear portion of these curves gives the value of optical energy gap (Eopt). 8.0 7.5 pure PEO 1% NaI 2% NaI 4% NaI 6% NaI 8% NaI 10% NaI 15% NaI 7.0 6.5 6.0 5.5 Absorption aðwÞ ¼ where x is the sample thickness; I0 and I are the incident and transmitted intensities, respectively [12–15]. The refractive index can be obtained from 8" 9 #1=2 < 4R R þ1= 2 n¼ k ð3Þ : ðR1Þ2 R1 ; 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 300 400 500 600 Wavelength (nm) Fig. 1. Absorption spectra for PEO/NaI composites. 700 800 R.S. Al-Faleh, A.M. Zihlif / Physica B 406 (2011) 1919–1925 1921 70 Pure PEO 1% NaI 2% NaI 4% NaI 6% NaI 8% NaI 10% NaI 15% NaI 65 60 Reflectance% 55 50 45 40 35 30 25 20 15 10 5 300 400 500 600 700 800 Wavelength (nm) Fig. 2. Reflectance spectra for PEO/NaI composites. 12 Pure PEO 1% NaI 2% NaI 4% NaI 6% NaI 8% NaI 10% NaI 15% NaI 11 10 (αhω)2 (eV/cm)2 ∗ 106 9 8 7 6 5 4 3 2 1 0 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 hω (eV) Fig. 3. (a_o)2 versus photon energy for PEO/NaI composites. Table 1 includes determined values of (Eopt), which decrease with increasing NaI content from 2.6 eV for pure PEO to 2.36 eV for 15 wt% composite. At lower absorption coefficient level, in the range of 1–104 cm 1, a(o) is described by the Urbach formula [16] aðoÞ ¼ a0 expð:o=DEÞ ð7Þ where a0 is a constant and DE is the energy gap tail interpreted as the width of the tails of localized states in the forbidden band gap [13,18]. Fig. 4 presents the Urbach plots for the composite films. The extrapolated (DE) values listed in Table 1 were determined from the slope reciprocal of the linear part of each curve and Eq. (7) as lnðaÞ ¼ :=DE þlnða0 Þ ð8Þ Table 1 Optical energy results for (NaI) composite films. Composite Eopt (eV) DE (eV) Eg + DE (eV) Pure PEO 1 wt% NaI 2 wt% NaI 4 wt% NaI 6 wt% NaI 8 wt% NaI 10 wt% NaI 15 wt% NaI 2.60 2.56 2.54 2.50 2.48 2.46 2.44 2.36 0.34 0.36 0.37 0.40 0.41 0.42 0.43 0.50 4.94 2.92 2.91 2.90 2.89 2.88 2.87 2.86 The exponential dependence of a(o) on photon energy (_o) indicates that the absorption processes taking place in the studied films obey the Urbach rule. These energy tails become smaller as 1922 R.S. Al-Faleh, A.M. Zihlif / Physica B 406 (2011) 1919–1925 7.00 Pure PEO 1% NaI 2% NaI 4% NaI 6% NaI 8% NaI 10% NaI 15% NaI 6.95 6.90 6.85 6.80 6.75 ln(α) 6.70 6.65 6.60 6.55 6.50 6.45 6.40 6.35 6.30 6.25 6.20 6.15 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 hω (eV) Refractive index Fig. 4. Urbach plot of ln(a) versus photon energy for PEO/NaI composites. 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 Pure PEO 1wt.% NaI 2wt.% NaI 4wt.% NaI 6wt.% NaI 8wt.% NaI 10wt.% NaI 15wt.% NaI 300 400 500 600 700 800 Wavelength (nm) Fig. 5. Refractive index versus UV-radiation wavelength. the concentration of NaI decreases, which is consistent with the variation of the optical energy gap (Eopt). In general, the sum Eopt + DE represents the mobility energy gap; Table 1 includes values of the mobility gap for different PEO electrolyte composites. The increments of the energy tail widths can be explained by the fact that increasing the NaI content may lead to the creation of ionic complexes, disorder, and imperfections in the structure of the composites, a case that may increase localized states within the forbidden energy gap. The energy tails have the smallest values due to neatness of the PEO structure and scanty impurities, which lead to decreased localized states within the forbidden band gap. This usually contributes to the increase in the optical energy gap [13,18]. Fig. 5 shows the variation of the refractive index of the prepared PEO/NaI composites with UV-wavelength, calculated from Eqs. (3) and (4), which relate the refractive index (n) with the absorption coefficient (a) and the extinction coefficient (k). The figure shows that the refractive index decreases rapidly at lower wavelengths ( o500 nm). The dielectric constant (e0 ) and 00 dielectric loss (e ) of the PEO/NaI composites were calculated from Eq. (6). Figs. 6 and 7 show the variations of e0 and e00 as a function of radiation wavelength. Both constants decrease with increasing wavelength. Fig. 8 shows the effect of NaI content on the refractive index and the dielectric constant, where both of them increase with increasing concentration of the electrolyte NaI in the PEO matrix. The refractive index values ranged from 1.46 for pure PEO to 3.8 for 15 wt% NaI composite, and the dielectric constant (e0 ) ranged from 4.5 to 12 measured at 660 nm wavelength. The observed increase in the dielectric constant, refractive index, and electrical conductivity due to incorporation of concentration of NaI salt particles in the PEO matrix was reported R.S. Al-Faleh, A.M. Zihlif / Physica B 406 (2011) 1919–1925 1923 45 Pure PEO 1wt.% NaI 2wt.% NaI 4wt.% NaI 6wt.% NaI 8wt.% NaI 10wt.% NaI 15wt.% NaI 40 Dielectric Constant (ε′) 35 30 25 20 15 10 5 300 400 500 600 700 800 Wavelength (nm) Fig. 6. Dielectric constant (e0 ) versus wavelength. 0.07 Pure PEO 1wt.% NaI 2wt.% NaI 4wt.% NaI 6wt.% NaI 8wt.% NaI 10wt.% NaI 15wt.% NaI Dielectric Loss (ε″) 0.06 0.05 0.04 0.03 0.02 0.01 0.00 300 400 500 600 700 800 Wavelength (nm) Fig. 7. Dielectric loss (e00 ) versus wavelength. 4.0 Refractive index 3.5 3.0 2.5 2.0 1.5 -2 0 2 4 6 8 10 12 NaI filler concentrations (wt.%) Fig. 8. Dependence of refractive index on NaI concentrations. 14 16 1924 R.S. Al-Faleh, A.M. Zihlif / Physica B 406 (2011) 1919–1925 Dielectric Constant (ε′ ) at (λ= 660 nm) 14 Experimental Data 13 EMT Model (Rao et al.) 12 11 10 9 8 7 6 5 4 -2 0 2 4 6 8 10 12 14 16 Volume fraction of NaI (%) Fig. 9. Fit of EMT model: dielectric constant as a function of NaI concentration. 0.15 pure PEO 1 wt%. NaI 2 wt%. NaI 4 wt%. NaI 6 wt%. NaI 8 wt%. NaI 10 wt%. NaI 15 wt%. NaI 0.14 0.13 0.12 (1/n2-1) 0.11 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 8.0 8.2 8.4 8.6 8.8 9.0 9.2 9.4 9.6 9.8 (hν)2 (eV)2 Fig. 10. Plots of (n2–1) 1 versus (hn)2 of the composites. Table 2 Excitation and dispersion energies for composites. Composite E0 (eV) Ed (eV) Pure PEO 1 wt% NaI 2 wt% NaI 4 wt% NaI 6 wt% NaI 8 wt% NaI 10 wt% NaI 15 wt% NaI 4.03 3.77 3.71 3.69 3.68 3.65 3.64 3.63 8.7 10.05 11.42 13.37 15.59 17.89 18.67 38.21 in many researches [7,10,11]. Liu et al. [19] fitted a modeling relation between the dielectric constant and filler content based on the effective-medium theory (EMT), which relates the inter-phase between particles, polymer matrix, and composite morphology. The EMT model is expressed in the form of Rao equation [5,8] as e ¼ e1 ½1 þv2 ðe2 e1 Þ=ðe1 þ nð1v2 Þðe2 e1 Þ ð9Þ where e, e1, and e2 are the dielectric of composite, PEO matrix, and discrete fillers, respectively, v1 and v2 are the volume fraction of continuous matrix and discrete fillers, and n is a factor related to the morphology of filled particles. Fig. 9 shows the fit of the EMT model of Eq. (9) to our PEO/NaI composites prepared via the casting method. The model fit predicts properly the increase of the dielectric constant (e0 ) of PEO/NaI composites with increasing filler concentrations, especially at lower NaI filler concentrations. The refractive index, n, of the PEO/NaI thin films is found to decrease with increase in the wavelength of incident photon, and tends to be constant at high wavelengths as shown in Fig. 5. R.S. Al-Faleh, A.M. Zihlif / Physica B 406 (2011) 1919–1925 The dispersion of refractive index below the inter-band absorption edge according to the Wemple–DiDomenico single oscillator model [20–23] is given as n2 ¼ 1þ E0 Ed E20 ðhvÞ2 ð10Þ where E0 is the average excitation energy for electronic transitions, Ed is the dispersion energy, which is a measure of the strength of inter-band optical transitions, h is Planck’s constant, n is the frequency, and hn is the photon energy. E0 and Ed values were calculated from the slope and intercept (E0/Ed) on the vertical axis of plot of 1/(n2–1) versus (hn)2 (Fig. 10). The values of E0 and Ed for the PEO/NaI composites are given in Table 2. Although Ed values increase with increasing NaI concentration, E0 values tend to decrease. 4. Conclusion In this work, polymer electrolyte thin films of PEO/NaI have been prepared by the casting method. The films contain NaI electrolyte component as a filler at different concentrations of 0%, 1%, 2%, 4%, 6%, 8%, 10%, and 15% by weight. Optical quantities such as absorption coefficient, optical energy gap, energy tails, refractive index, and dielectric constants were determined from analysis of the absorbance and reflectance of UV-radiation spectra. From the optical results obtained it was found that the electron transition is direct in k-space and the optical energy gap decreases with increasing sodium iodide concentration. The observed dielectric constant and the refractive index increase with NaI content. The fit of the EMT model on the variation of the 1925 dielectric constant with the electrolyte content is, in general, acceptable. Acknowledgment The authors thank the Chemistry Department of Jordan University for providing the sodium iodide powder. References [1] F. 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