Change in the Electrical Conductivity of N+ Ion Implanted Polycarbonate

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Change in the Electrical Conductivity of N+ Ion Implanted Polycarbonate
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   Available online a t www.pelagiaresearchlibrary.com   Pelagia Research Library Advances in Applied Science Research, 2011, 2 (3): 227-231 ISSN: 0976-8610 CODEN (USA): AASRFC   227  Pelagia Research Library   Change in the Electrical Conductivity of N +  Ion Implanted Polycarbonate P. K. Goyal * , V. Kumar, Renu Gupta, S. Mahendia, T. Sharma and S. Kumar    Department of Physics, Kurukshetra University, Kurukshetra, India ______________________________________________________________________________ ABSTRACT The electrical conductivity of 100 keV N  +  ion implanted polycarbonate (PC) to the doses 1x10 15  , 1x10 16   and 5x10 16   ions/cm 2  has been studied. The electrical conductivity of ion beam modified  polycarbonate has been found to be an increasing function of the implanted dose. Its value changes from ~10 -14  S, for virgin PC sample to ~10 -8 S for the sample implanted to the dose 5x10 16   ions/cm 2 . By Raman spectroscopic analysis, the difference in the electrical conductivity has been explained in terms of the difference in carbon structure of the implanted polycarbonate. Keywords:  Polycarbonate, Ion-Implantation, Electrical Conductivity, Raman Spectroscopy. ______________________________________________________________________________ INTRODUCTION In polymers, the electrical conductivity is characteristically very low and in most of the electrical applications, they are essentially used as insulators [1-6]. However, there are requirements for increasing their conductivity in a controlled manner. Therefore, the polymer substrates are frequently modified to meet these needs. In the recent years, the ion implantation technique has been effectively used to modify the electrical properties of polymeric materials [7-11]. Ion implantation in polymers is accompanied with various effects like free radical formation, chain-scissioning, cross-linking, carbonization etc. [10-15] which results in the change in structural and electrical properties of these materials. In this study, the effect of N +  ion implantation on the properties of polycarbonate (PC), an optically transparent polymer with chemical structure of the repeating unit shown in Figure1,   has been investigated. OOO CH 3 CH 3 n   Figure 1: Monomer structure of Polycarbonate polymer  P. K. Goyal  et al Adv. Appl. Sci. Res., 2011, 2 (3): 227-231 _____________________________________________________________________________ 228  Pelagia Research Library  To study the N +  ion implantation induced effects in PC, the surface conductivity of this polymer has been predicted through V-I measurements using two point probe method. Further, the ion implanted polycarbonate has been characterised through Raman spectroscopy to correlate the observed conductivity behaviour with the induced structural changes. MATERIALS AND METHODS The bulk sheet of polycarbonate (monomer composition C 16 H 14 O 3 )   of thickness 250 micron was obtained from Goodfellow, United Kingdom and was cut into samples of area 15x15 mm 2 . These samples were irradiated with N +  ions having the energy 100 keV to the doses 1x10 15 , 1x10 16  and 5x10 16  ions/cm 2  under vacuum (~10 -6  torr) at room temperature utilizing the Low Energy Ion Beam Facility (LEIBF) at Inter-University Accelerator Centre (IUAC), New Delhi, India. The beam was electrostatically scanned over the entire area of the sample keeping the beam current density below 1.0 µA/cm 2 . The surface conductivity measurements, at room temperature, were then carried out with two point probe method using Keithley 6517 Digital Electrometer, interfaced with computer. RESULTS AND DISCUSSION  3.1 Electrical conductivity Figure 2 presents the current versus voltage plots measured on the surface of virgin and N +  ion implanted PC samples in the voltage range 0-100V. It is clearly observed from this figure that the current in the virgin sample which is of the order of ~10 -12 A increases to ~10 -6  A at a maximum dose of 5x10 16  ions/cm 2 . Figure 2: Current versus voltage plots for Virgin and N +  implanted PC samples From this data, the surface conductivity of virgin and implanted polycarbonate samples has been calculated using the relation [6]  Rr d  s π  σ   )2(cosh 01 − =  where, σ s  = surface conductivity, d = separation between electrodes, r 0  = radius of the circular electrode, R = resistance measured on the conductive surface.  P. K. Goyal  et al Adv. Appl. Sci. Res., 2011, 2 (3): 227-231 _____________________________________________________________________________ 229  Pelagia Research Library  Figure 3 shows the variation in the conductivity as a function of ion fluence and the values of conductivity at different fluences are presented in Table 1. Figure 3: Plot for log ( σ ) versus the N +  ion fluence Table 1: Electrical conductivity of virgin and N +  implanted Polycarbonate Dose (ions/cm 2 )   σ s  (S)  Virgin 3.64E-14 1.0x10 15  2.22E-11 1.0x10 4.38E-10 5.0x10 16  6.40E-09 Figure 4: Raman spectra of virgin polycarbonate From table 1, it is clear that the value of electrical conductivity changes from 3.64x10 -14  S for virgin PC sample to 6.40x10 -9  S for the sample implanted to the dose 5x10 16  ions/cm 2 . In order to understand such an increase in the conductivity of polycarbonate with increasing implantation  P. K. Goyal  et al Adv. Appl. Sci. Res., 2011, 2 (3): 227-231 _____________________________________________________________________________ 230  Pelagia Research Library  dose, in terms of the change in structural behaviour of this polymer, these samples were subjected to Raman analysis.  3.2 Raman Analysis Figure 4 and 5 report the Raman spectra taken at an excitation wavelength of 632.8 nm of He-Ne laser for virgin and N +  ion implanted PC samples respectively. In the Raman signal of non implanted PC sample (Figure 4), the various peaks observed correspond to the characteristic peaks of polycarbonate [16-19] and thus confirming the monomer structure of this polymer. 8001000120014001600180020002200 Wavenumber (cm -1 )    I  n   t  e  n  s   i   t  y   (  a .  u .   )   a   10 16 N + /cm 2 b 5x10 16 N + /cm 2 ab   Figure 5: Raman spectra of N +  implanted polycarbonate. At an implantation dose of 1x10 16  ions/cm 2 , as is evident from Figure 5 (curve a), a large band at around 1550 cm -1  appears, with the elimination of the characteristic peaks of polycarbonate. The spectral position and the shape of this band in Raman spectra are consistent with the characteristic G band of hydrogenated amorphous carbon [9, 14, 20]. With the increase in the concentration of nitrogen ions within polycarbonate specimen at a dose of 5x10 16  ions/cm 2 (Figure 5, curve b), the intensity of this band reduces indicating further reduction of hydrogen content. Thus, it can be inferred that as a result of ion implantation, a three dimensional carbonaceous network, made up of a system of distorted bonds emerges. These carbonaceous clusters on the implanted surface which are rich in charge carriers [8-14] provide a continuous path for the charge transfer within the insulating polymer chain and thus influence the hopping mechanism within the chain of polymers and results the boosted current through the surface of polymer as an outcome of implantation.  P. K. Goyal  et al Adv. Appl. Sci. Res., 2011, 2 (3): 227-231 _____________________________________________________________________________ 231  Pelagia Research Library   CONCLUSION The surface electrical conductivity of polycarbonate was found to be increased by approximately six orders of magnitude after N +  ion implantation at the fluence of 5x10 16  ions/cm 2 . The formation of a three dimensional carbonaceous network, consisting of disordered bonds, emerges in the implanted regions of the polycarbonate, as confirmed through Raman spectroscopy, is considered responsible for increased surface conductivity as a result of implantation. Acknowledgements   The authors are thankful to Dr. D. Kanjilal, IUAC, New Delhi, for valuable discussions. Thanks are due to Dr. P. Kumar, IUAC for helping during irradiation. Two of the authors (P.K. Goyal & V. Kumar) are thankful to Council of Scientific & Industrial Research (CSIR) New Delhi, India for financial assistance in form of Senior Research Fellowship (SRF). REFERENCES [1]   M. Teruyoshi,  J. Soc. Rubber Industry   2003 , 76,   114. [2]   J. J.   Shea,  Electrical Insulation Magazine, IEEE    1998 , 14,   41. [3]   A. M. Brown  , Electrical Insulation Magazine, IEEE    1994 , 10,   16. [4]   J. V. Masi,  Electrical Insulation Conference and Electrical Manufacturing & Coil Winding Technology  (Conference Proceedings) 2003 , 199. [5]   H. Hiroya,  Journal of the Institute of Electrical Engineers of Japan  2000 , 120 ,  152. [6]   A. R. Blythe, Electrical Properties of Polymers (Cambridge University Press, New York, USA) 1979 . [7]   V. 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