Conformation And Charge Transpsort In Conducting Polymers, Carbon Nanotubes And Their Nanocomposites

Abstract

The main motivation in this thesis is to compare the conformation and charge transport in conducting polymers and carbon nanotubes (CNTs) and to investigate those physical properties in their combined form of nanocomposites. It is known that both conducting polymers and carbon nanotubes are intrinsically 1-dimensional systems which consist of delocalized π-electrons. However, the main difference between these is the fact that flexibility of conducting polymers can be varied depending on the extent of conjugation while CNTs are rigid. Hence a comparison of electronic properties as correlated to their morphology has been carried out and their individual role in nanocomposites is further studied. The thesis consists of 6 chapters and appendix. Chapter 1 consists of brief introduction of general properties of both conducting polymers, CNTs and their nanocomposites. Chapter 2 deals with the sample preparation and experimental techniques used for the work. Chapter 3 elaborates on the conformational / structural studies on the systems. Chapter 4 focuses on the transport measurements to study the electronic properties of the samples. Chapter 5 reveals the magnetic properties of these systems which can be applied in technological devices. And chapter 6 gives the conclusion and future directions of the work being done. Chapter 1: Nanocomposites represent a guest-host matrix consisting of easily processible functionalized conjugated polymer as host, incorporating carbon nanotubes as fillers with versatile electronic and magnetic properties, which provide a wide range of technological applications. The conformation, charge dynamics as well as magnetic properties of these conducting polymers and carbon nanotubes, and various aspects of transport mechanism and spin dynamics present in the nanocomposite matrix are studied and presented in a consistent framework. Chapter 2: The multiwall carbon nanotubes (MWNTs) are grown by thermal chemical vapor deposition (CVD). The MWNTs are dispersed in solution of conducting polymers by ultrasonication and then the suspension is cast on glass substrate and slowly dried by moderate heating. Once dried completely, the free-standing films of thickness 15-25 μm are peeled off the substrate for measurements. The MWNTs, above a certain concentration, form an interconnected network in the 3-dimensional polymer matrix, following percolation mechanism. The disorder is brought into the system mainly by bundling of tubes and bundle intersections. The morphology and conformation of the samples are studied by SEM, TEM and small angle X-ray scattering (SAXS) techniques. Chapter 3: Small angle X-ray scattering (SAXS) studies in polymeric systems are carried out to probe local nanoscale morphology at various length scales to show the correlation among conformation and assembly of chains. Small angle X-ray scattering (SAXS) studies are carried out in poly [2-methoxy5-(2’–ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) solution of varying conjugation lengths as well as different solvents. By increasing the extent of πconjugation from 30 to 100 %, the persistence length increases by a factor of three. Moreover, a pronounced second peak in the pair distribution function is observed in fully conjugated chain, at larger length scales which indicates that the chain segments tend to self-assemble as the conjugation along the chain increases. The chain assembly and aggregation are further studied for suspensions of MWNTs in polyethylene dioxythiophene-polystyrene (PEDOT-PSS) with aqueous medium and DMSO (dimethyl sulphoxide). The SAXS profile of MWNT dispersion in aqueous PEDOT-PSS clearly show rigid-rod feature of the individual nanotubes evident by the q-1 behavior at short ranges. The crossover from q-1 to q-2 in the longer range further suggest that the suspension consists of individual nanotubes, nanotubes bundles and aggregates that give rise to a 3-dimensonal meshwork of intersecting tubes and ropes. For the MWNT dispersion in PEDOT-PSS with DMSO, however, such q-1 behavior is absent; which evidently shows that the rods are not isolated in the solution and are rather agglomerated. How these conformations affect the electrical and magnetic properties of these systems are studied further in Chapter 4. Chapter 4: Transport mechanism in single wall carbon nanotubes (SWNT), MWNT pellets, and nanocomposite films of MWNT and PEDOT-PSS is studied. The positive and negative magnetoresistance (MR) data in various SWNT samples are analyzed by taking into account the electron-electron interaction (EEI) contribution, in addition to the weak localization (WL) regime. The contribution from EEI to the total MR is confirmed from the universal scaling of MC relation showing that EEI plays a significant role at higher fields and lower temperatures. Intrinsic parameters like inelastic scattering length extracted for barely metallic sample follows the T-3/4 dependence due to inelastic electron-electron scattering in the dirty limit. Conductivity and magnetoresistance (MR) measurements on nanocomposite films with varying MWNT content (0.03 - 3 %) are performed at a field range 0-11 Tesla, and temperature range 1.3–300 K. The temperature dependence of resistance above 4 K suggests a Coulomb-gap variable range hopping (CG-VRH) transport in the network. Alhough solely negative MR (~ 5-6 %) is observed for pristine MWNT pellets; the nanocomposite films show a combination of large negative MR (~ 80 %) at T < 4 K, and a comparatively weaker positive MR (~ 30 %) for T > 4 K. This suggest that there are two mechanism interplaying and dominant at different temperature regimes which can be explained by the mechanism of transport of the charge carriers of MWNT intervened by that of the polymer matrix. In conclusion how the individual properties of conducting polymer and carbon nanotubes contribute to the unique electronic and conformational properties in their nanocomposites is framed in this investigation. Chapter 5: Magnetic properties of the pristine MWNTs as well as metal nanowires of nickel, nickel-iron (NiFe), nickel-iron-cobalt (NiFeCo) encapsulated in the MWNTs are studied using superconducting quantum interference device (SQUID) magnetometer. A typical example of Ni nanowires encapsulated in MWNT (Ni-MWNT) is taken and the results are compared to other forms of nickel (bulk, nanorod cluster, nanowire) to see the effect of size, shape and environment on the magnetic kproperties. The saturation magnetization and coercivity for Ni-MWNTs are 1.0 emu/gm and 230 Oe. The temperature dependence of magnetization indicates superparamagnetic which is supported by the field-cooled and zero-field-cooled plots determining a blocking temperature ~ 300 K. These altered magnetic properties of Ni-MWNTs are mainly due to the contribution from carbon nanotube encapsulation. Both the shape and environment enhance the total magnetic anisotropy of encapsulated nanowires at least by a factor of four. The encapsulation of metal nanowires in MWNTs tunes the magnetic properties of the system widely, e.g. from diamagnetic (pristine MWNTs) to paramagnetic (Ni-MWNT) to ferromagnetic (NiFe-MWNT) and a combination of para and ferro (NiFeCo-MWNT). Chapter 6: The conclusions of the different works presented in the thesis are coherently summarized in this chapter. Thoughts for future directions are also summed up. Appendix A: Spin dynamics in conducting polymer PEDOT-PSS in its pristine, processed with DMSO and nanocomposite form (with carbon nanotubes) is studied using solid state nuclear magnetic resonance (NMR). Plots of proton spin lattice relaxation times vs. temperature at a fixed frequency 23.4 MHz are compared to study the effect of the external agents on the polymer dynamics.