13500 Permatang Pauh, Pulau Pinang, Malaysia. Kem Sg. Besi, Kuala Lumpur, Malaysia **

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1 Advanced Materials Research Vols (2013) pp (2013) Trans Tech Publications, Switzerland doi: / Optical Studies of poly (N-carbazole) (PVK) Blending With Polyvinylpyrrolidone (PVP) Using Tauc/Davis-Mott Model A. N. Alias 1,2*, T.I.T Kudin 2, Z. M. Zabidi 1, M. K. Harun 2, A. M. M. Ali 2, M.Z.A. Yahya 3** 1 Department of Applied Sciences, Universiti Teknologi MARA Permatang Pauh, Pulau Pinang, Malaysia 2 Faculty of Applied Sciences, Universiti Teknologi MARA, Shah Alam, Malaysia 3 Faculty of Science & Defence Technology, National Defence University of Malaysia Kem Sg. Besi, Kuala Lumpur, Malaysia *ahmadnazib111@ppinang.uitm.edu.my, ** mzay@upnm.edu.my Keywords: Polymer Blend, Tauc/Davis-Mott Model, Abstract. The optical absorption spectra of blended poly (N-carbazole) (PVK) with polyvinylpyrrolidone (PVP) in various compositions are investigated. A doctor blade technique was used to coat the blended polymer on a quartz substrate. The electronic parameters such as absorption edge (E e ), allowed direct band gap (E d ), allowed indirect band gap (E i ), Urbach edge (E u ) and steepness parameter (γ) were calculated using Tauc/Davis-Mott Model. The results reveal that the E e, E d and E i increase with increasing of PVP ratio. There also have variation changing in Urbach energy and steepness parameter. Introduction Increased pursuit of electronic and opto-electronic polymer based devices has led to seek new polymeric materials which fulfill specific requirement on electronic applications. It is well known that polymer blending or polymer mixture is the simplest technique in polymer engineering of creating new solid materials and composites with more properties enhancement than that of homopolymer. Polymer blending can exhibit either heterogeneous polymer blends (immiscible) or homogeneous polymer blends (miscible) [1]. In semiconductor application, polymer blends is one of the technique that has widely been used to optimize the performance of opto-electronic devices. For example, in polymer light emitting diode, by changing the polymer composition colour can be tuned resulting from energy transfer of the polymer-polymer system [2]. Therefore the composition ratio plays an importance rule in optical properties in thin solid polymer film. UV/Vis absorption spectra are one of the tools to understand the electronic structure of polymeric materials. At the visible absorption region of this spectra, it presents information of molecular dipole moment, meanwhile at the UV absorption region, it will be able to provide essential information of electronic states of molecule. In term of solid state, the optical absorption provides the information of band structure which is related to the electronic structure of polymer blending system. The band structure of amorphous solid polymer (viz. lack of long range order) can be explained by using Cohen-Fritzsche-Ovshinsky (CFO) model [3]. However, Davis and Mott has improved CFO model by introducing a band of localized state near the center gap [4]. The changing of composition will be able to create new polymer-polymer arrangement in lattice of blending cells (compatibility) which will affect the electronic structure of the polymer blending system [1]. Most of papers [5,6] report on the effect of polymer composition using thermal decomposition and surface topology. However, the main purpose of this paper is to investigate the effect of composition on optical absorption of polymer blending system. In this study, blended PVK and PVP thin films in different compositions ratio were prepared. By investigating optical properties of these polymer blends, it is anticipated to provide vital information for any opto-electronic devices application. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /01/13,02:14:44)

2 528 Advances in Materials and Materials Processing Experimental PVK and PVP were purchased from Sigma-Aldrich. The polymer blending systems consist of blended PVK with PVP (PVK:PVP) in different weight ratios namely S1 9:1, S2 8:2, S3 7:3, S4 6:4, S5 5:5, and S6 4:6. All the samples were dissolved in 10 ml dimethylformamide (DMF) common solvent and stirred for 6 hours at room temperature (30ºC). Then, each solution was coated on a spectrosil quartz substrate using doctor blade technique to form a thin film. Prior to that, the spectrosil quartz substrate substrate was clean using Decon 90 and rinsed by deionized water followed by ultrasonic treatment using acetone. The film was then dried in vacuum oven at 50 o C for an hour. The thickness of those thin films was measured by filmetrics F20 Series. Optical absorption was measured using Varian 5000 uv-vis spectrophotometer. The optical absorption for non-crystilline materials is given by Tauc & Davis-Mott Model [7] n E (1) opt where β is constant and n is the exponential constant index. There are four types of transition occur in amorphous material which can be represented with value of n. The value of n normally 1/3,1/2, 2/3 and 2 for indirect forbidden, indirect allowed, direct forbidden and direct allowed transition respectively [7,8]. Urbach energy can be calculated as follow, 0 exp E U (2) where α 0 is a constant and E U is Urbach edge. The steepness parameter γ also can be calculated using Urbach plot as [9]. γ =kt/ E U (3) Results and Discussion The physical properties of solid thin films polymer depend upon the historical preparation. Therefore, it is very importance to initially understand the electronic structure of the polymeric material [10]. The fundamental absorption edge was calculated using optical absorption method as shown in Figure 1. The absorption edge E e can be determined by extrapolating a straight line at the exponential region to the value of α (viz. equal to zero) and the value of E e (given in Table 1). In amorphous semiconductor materials, the value of k-space for absorption transition is not conserved. Besides that, the samples thickness are strongly depend upon optical absorption edge as well as Urbach edge [11]. Figure 1: The absorption coefficient vs ħω for S1, S2, S3, S4 and S5

3 Advanced Materials Research Vols Table 1. Value of absorption edge (E e ), allowed direct band gap (E d ), allowed indirect band gap (E i ), Urbach edge (E u ) and steepness paramater (γ) Composition E e E d E i E u S S S S S S γ The allowed direct energy gap E d and the allowed indirect energy gap E i was determined by extrapolating straight line of the plot of 2 (see Figure 2a) and the plot of 1/ 2 (see Figure 2b) versus photon energy respectively. The value of E d and E i is tabulated in Table 1. It is noticed that, the value of E e, E d and E i increases with the increase of PVP composition as shown in Figure 3. The variation changes in E e, E d and E i may be due to the increasing ratio of PVP, and thus changes the polymer-polymer arrangement in the lattice of blending cells by reducing long range orders. The result is in agreement with [12,13] that reported on the occurrence of the optical energy gap changing in poly (vinyl alcohol)/hydroxypropyl methylcellulose and poly(vinyl chloride)/ poly(ethylene oxide) blends. They also reported that the different compositions in polymer blends have induced the structure changing in such polymer blending system. Besides that, the plausible explanation on the changing in E e, E d and E i are due to the energy charge transfer between polymer-polymer blending system [14]. (a) Figure 2: (a) The (αħω) 2 vs ħω (b) The (αħω) 1/2 vs ħω for different compositions of PVP (b)

4 530 Advances in Materials and Materials Processing Figure 3: Variation of fundamental absorption edge, E e, allowed direct transition direct and allowed indirect transition energy with different compositions of PVP The Urbach energy was determined using equation (2). The Urbach energy represents how much the valence band tail or conduction band tail extends into the forbidden energy gap (localized state) and known as Urbach edge. Figure 4 (a) shows the Urbach plot lnα vs ћω in the range of ev. The Urbach energy can be determined by reciprocal of a straight line slope of Urbach plot as listed in Table 1. The steepness parameter at room temperature (30 o C) is calculated using equation (3) and the results are given in Table 1. The steepness parameter describes the broadening of the absorption edge which due to interaction of electron either with exciton or phonon [15]. As we can see in Figure 4(b), the Urbach energy starts to increase from S1 to S2 and then decreases from S2 to S4. However, it starts to increase again and reach the maximum value at S6. This variation in Urbach energy are possibly due to the fluctuation in the internal potential at atomic scale which associated with structure disorder in the polymer blending system [3]. The fluctuation of the internal potential arise from deformation (elastic) to electrostatic changing in solids [16]. (a) Figure 4 (a) The Urbach plot ln (α) vs the photon energy (ħω) (b) Variation of Urbach energy, E u, with different compositions of PVP (b)

5 Advanced Materials Research Vols Conclusion We have analyzed the absorption spectra of PVK:PVP at various compositions in polymer blends. The values of optical transition parameter, absorption edge, E e, allowed direct band gap, E d, and allowed indirect band gap, E i, Urbach edge, E u, and steepness parameter, γ, have been calculated using tauc/davis-mott model. The values of E e, E d and E i increase with the increase of composition in PVP. The Urbach energy starts to increase from S1 to S2 and then decreases from S2 to S4, however it starts to increase again and reach the maximum value at S6. The changing in the optical energy gap as well as the Urbach energy shows that the ratio volume of composition plays important roles in the electronic structure of the polymer blending system. Acknowledgements This work was funded by The Fundamental Grant Scheme (FRGS): 600-RMI/ST/FRGS 5/3/Fst(198/2010). The authors would like to thank Fakulti Sains Gunaan, UiTM and NanoScience Tech, Institutes of Science (IOS) for giving permission to carry out the measurements. References [1] L.M. Robeson: Polymer Blends, A Comprehensive Review (Carl Hanser Verlag, Munich, Switzerland 2007). [2] H.L. Chou, K.F. Lin, D.C. Wang: J. Polym. Res. Vol 13 (2005) p.79. [3] M.H. Cohen, H. Fritzsche, S. R. Ovshinsky: Phys. Rev. Lett. Vol 22, (1969) p [4] E.A. Davis, N.F. Mott: Phil. Mag. Vol 22 (1970) p. 903 [5] H. Younes, D. Cohn: European Polym. J. Vol. 24 (1988) p. 765 [6] A. Karim, T. M. Slawecki, S. K. Kumar, J. F. Douglas, S. K. Satija, C. C. Han, T. P. Russell, Y. Liu, R. Overney, J. Sokolov, M. H. Rafailovich: Macromolecules Vol 31(1998) p 857 [7] A. N. Alias, T.I.T Kudin, Z. M. Zabidi, M. K. Harun, M.Z.A. Yahya, Advanced Materials Research (2012) Vol pp [8] Z. M Elimat, A. M. Zihlif, G. Ragosta: Physica B. Vol 405(2010) p [9] Y. Calgar, S. Ilican, M. Caglar: Eur. Phys. J. B Vol 58 (2007) p. 251 [10] N. A. Bakr, A.M. Funde, V.S.Waman, M.M Kamble, R.R. Hawaldar, D.P. Amalnerkar, S.W. Gosavi, S.R. Jadki: Pramana J. Phys. Vol 76 (2011) p 519 [11] F. Yakuphanoglu, M. Sekerci, A. Balaban: Optical Materials, Vol 27 (2005) p [12] O. W. Guirguis, M.T. Moselhey: J. Mater Sci. Vol 46 (2011) p [13] Y. A. Ramadin: Optical Materials Vol 14. (2000) p 287 [14] F. Yakuphanoglu, M. Arslan: Optical Materials Vol 27 (2004) p 29 [15] Calgar Y., Ilican S., Caglar M.: Eur. Phys. J. B Vol 58 (2007) p. 251 [16] J Tauc, A. Menth: Journal of Non-crystalline Solid Vol 8 (1972) p 569