Charge transport in functionalized carbon nanotube and composite with polyaniline at low temperature

Abstract

Carbon nanotube (CNT) is a very well-known material with very high thermal conductivity, high Young modulus and high electrical conductivity. The sp2 bonded C=C π-electrons delocalize in the whole lattice give rise to high electrical conductivity. So, the defect, disorder and disturbance of this π-electrons in CNT affects the charge transport significantly at low temperature. Chemical functionalization breaks sp2 bonds and creates sp3 bonds and enhances the disorder in the system. On the other hand, polyaniline (PANI) is a conducting polymer, known for high conductivity, easy to synthesize and low-cost monomer. The composite with CNT enhances the electrical conductivity and environmental stability of the PANI composite. All these good properties emerge in the system due to Van der Waals interaction between CNT and PANI. The chemical functionalization of CNT attaches different functional groups on the surface, edges and enhances the electrostatic polar interaction between CNT and PANI. Due to high conductivity, stability they are used in many applications like flexible technology, the electrode of a supercapacitor, gas sensing and so on. In this thesis, I have studied the charge transport of CNT and composite with PANI at low temperature to understand the effect of chemical functionalization of CNT. The electrical charge transport and magnetoresistance (MR) of functionalized CNT of different degree of chemical functionalization are measured at low temperature and high magnetic field. It is observed that resistivity increases with decreasing temperature and the variation of resistivity increases with increasing degree of functionalization. Functionalization of CNT increases the disorders and electron-electron interaction in the system. The charge transport is at the boundary of strong and weak localization regime not in the deep of either. To understand the effect of enhanced polar interaction between fCNT and PANI in charge transport, the charge transport and MR of (f)CNT/PANI composites by varying weight percentage of filler and degree of functionalization of CNT in the composite are measured at low temperature. It is observed that for fCNT/PANI composite resistivity increases almost four orders of magnitude higher than CNT/PANI composite at 4.2 K. Interestingly, MR transition from negative to positive takes place above 10 wt% of fCNT/PANI composites due to chemical functionalization of CNT. Also, for lower degree of chemical functionalization, MR crossover from positive to negative is observed by increasing temperature (T > 10 K). To understand the charge transport mechanism in the composites, the impedance spectroscopy, dielectric measurement, modulus spectroscopy and ac conductivity measurements of (f)CNT/PANI composites are performed at low temperature. It is observed that negative dielectric constant appears in CNT/PANI composite below 10 K and it goes to positive value by increasing temperature and chemical functionalization of CNT. The ac conductivity reveals that the charge transport is dominated by single electron tunneling in CNT/PANI composite and due to chemical functionalization of CNT the charge transport is manifested by small polaron tunneling in the fCNT/PANI composites.