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.

Studies on charge transport properties in carbon nanotubes (CNTs) have been a subject of great interest for a long time not only as an important topic in fundamental science, but also as a basic requirement for the application of CNTs for nanoelectronics. CNTs show a wide range of transport behavior that varies from ballistic to hopping regime, depending on the dimensionality and nature of disorder in the system. Minute variations in disorder can lead from weak to strong localization, and this yields complex and intriguing features in the analysis of transport data. It is particularly important to carry out such a study for multiwall carbon nanotubes (MWCNTs), in which both dimensionality and disorder play an important role and the nature of localization is non-trivial as wave functions are extended along the tube or bundle of tubes. A proper understanding of the mechanisms of charge transport and their quantitative knowledge is an essential requirement for any possible application of CNTs in nanodevices. Such studies not only yield information on the transport parameters crucial for applications but can also provide a test for any possible microscopic theories of transport. Main focus of the current thesis is to understand the mechanism of charge transport in iron-embedded MWCNTs and to gain more knowledge on the transport behavior. Magnetically functionalized CNTs, in particular the CNTs filled with ferromagnetic materials are of profound interest for the basic scientific research as well as for technological application. Iron-embedded MWCNTs are synthesized by one step pyrolysis method. This method gives a proper route to synthesize the magnetic particles encapsulated CNTs. Beyond the geometrical advantage of a cylinder-shaped nanostructure design, the carbon shells provide an effective protection against oxidation of magnetic nanoparticles. The iron-embedded MWCNTs exhibit excellent magnetic properties like the uniaxial magnetic anisotropy, and the high coercivity, which is larger than the coercivity of bulk iron. Thus, they have significant potential for data storage devices and biomedical applications. Vertical alignment of CNTs is an important issue for device applications such as field electron emitters and flat-panel displays. Vertically aligned MWCNTs are grown on various substrates in the present work and the role of catalyst particles in vertical alignment is discussed. This thesis also reports the investigations on the magnetic properties including magnetotransport studies. The thesis is organized in seven chapters and a brief summary of each chapter is given below. Chapter 1 presents an introduction of the CNTs and its structural and electronic properties. Charge transport in CNTs is then discussed in terms of the fundamental aspects of conduction regimes and transport length scales. The synthesis and characterization of iron-embedded MWCNTs is described in chapter 2. It is important to get good quality CNTs in a scalable way. The various methods available for CNT synthesis are arc discharge, laser ablation, chemical vapor deposition etc. A one-step thermally assisted pyrolysis method employed for synthesizing MWCNTs is a simple and cost-effective method. Benzene is used as a precursor and ferrocene as a catalyst in the present case. Good quality CNTs are obtained from this method, which are of multiwall in nature (outer diameter in the range of 10-25 nm). Vertically aligned mats of MWCNTs are also obtained on the quartz substrate. The thickness of the mats is several tens of microns. The prepared MWCNTs are characterized by electron microscopic studies for its structure and surface morphology. Many iron particles are seen inside the tubes. Energy dispersive x-ray (EDX) spectra taken from the small region of the sample under TEM show the presence of iron. Raman spectra of the sample suggest good quality of the tubes. Prominent G-peak in this spectrum shows that the sample is of well-graphitic nature. X-ray diffraction pattern of MWCNT material shows the presence of -Fe and Fe3C apart from the graphitic peak. Chapter 3 describes the growth of vertically aligned MWCNTs (v-MWCNTs) on various substrates and role of catalyst particles in the alignment. The v-MWCNTs are grown on sapphire, quartz and thermally oxidized silicon substrates without pre-deposition of any catalyst. The grown MWCNT mats had a thickness of several tens of microns. Surface elemental analysis shows the presence of catalyst particles on the substrate which is essential for vertical alignment of the tubes. It is found that the order in which the precursor and the catalyst were introduced during chemical vapor deposition determines the orientation of the nanotubes. When there were no catalyst particles on the substrate in the beginning, random alignment of CNTs took place instead of vertical alignment. Base growth mode of CNTs is proposed in the present case from the results obtained. Chapter 4 deals with the magnetic properties of the as-synthesized MWCNTs. The CNTs in pristine form are of diamagnetic in nature. The ferromagnetic-like behavior arises from the iron particles embedded in MWCNTs. These ferromagnetic particles are retained in the MWCNTs automatically, as the catalyst in this case contains iron. MWCNTs of different iron weight percentage are prepared by taking different amount of ferrocene as a precursor. These particles exhibit a magnetic moment up to 98 emu/g and coercivity in the range of 500–2000 Oe. Reduced magnetization is attributed to the formation of surface shell with spin disorder and to the presence of Fe3C phase. Large coercivity compared to the bulk vale of few orested is due to the complex state of interactions, which can create strong pinning centers for the core moments during the demagnetization. In addition the observed dependence of the magnetoresistance on the direction of applied field, is correlated with the shape anisotropy of the Fe particles. The trend of saturation of magnetization at higher fields suggests that exchange coupling in the present case is one-dimensional. The charge transport properties of MWCNT mats are discussed in chapter 5. Many of the transport parameters are often affected by the presence of magnetic field. In order to gain a deeper insight into the conduction mechanism, the study of the electrical transport in presence of magnetic field is highly useful. The temperature and magnetic field dependence of the conductivity of MWCNT mat is studied in the temperature range of 1.4-150 K in the magnetic field up to 10 T. The charge transport in the system is governed by Mott’s variable-range hopping (VRH) of three-dimensional type in the higher temperature range and two-dimensional type in the lower temperature range. Mott’s various parameters like localization length, hopping length, hopping energy, and density of states at the Fermi level are deduced from the VRH fit. The hopping length decreases from 13.2 to 12.2 nm, as temperature increases from 110 to 150 K. The obtained value of hopping length around ~13 nm is within the range of nanotube diameters of 10 to 25 nm. This is the main component of the hopping length, which indicates that VRH takes place on the tube scale. The localization lengths observed in the case of 3D VRH and 2D VRH conduction are well within the range of outer diameter of MWCNTs, which indicates that the localization takes place at the tube scale along the boundaries of the tubes. If the charges are localized at the tube boundaries, then the localization length gives an average diameter of the tubes and the results obtained supports this argument. It is also important to note that the defects present in the nanotubes in the form of structural defects and bad matching of chirality gives rise to localization. There are not many reports on the effect of a magnetic field on the VRH process for MWCNT systems. The resistance of the sample decreases with the magnetic field in the direction of tube axis of the nanotubes. The magnetic field gives rise to delocalization of states as evident from the values of localization lengths at different fields. The application of magnetic field lowers the crossover temperature, at which three-dimensional VRH turns to two-dimensional VRH. The conductivity at the lower temperature side is governed by the weak localization (WL) give rise to positive magnetoconductance (MC). Here a phase diagram with temperature and magnetic field is proposed, showing different regions for different kind of transport mechanisms. This may be applicable for other class of disordered material as well. Chapter 6 deals with the magnetotransport studies on disordered MWCNT mat. The electrical conductivity and MC data are analyzed in the temperature range of 1.4-150 K and in the magnetic fields up to 11 T. The system is in the critical regime obeying conductivity of metallic systems as suggested in weak localization-electron electron interaction model. The MC is positive for the whole temperature range except at temperature below 4.2 K. Results are analyzed in the terms of weak localization, electron-electron interaction and VRH. The H 2 dependence at lower magnetic fields and H dependence at higher magnetic fields is found supporting weak localization. Inelastic scattering lengths are also deduced from the low temperature MC data and its temperature dependence shows that the dominant dephasing mechanism in the present case is inelastic electron-electron scattering in the dirty limit. Chapter 7 describes measurements on individual MWCNTs and subsequent charge transport studies. After many trials a suitable method was devised to isolate single tubes and to put contacts on it for the four probe measurement. For electrical measurements on isolated single tube, it is found that the joule heating due to excess current is an important issue. A current of the order of few µA burns the sample immediately. I-V characteristics of the MWCNTs show that the electrical contacts are ohmic and the resistance is few k. Initial electrical measurements show that there is slight decrease in resistance with increase of temperature and MR is approximately negative. This behavior suggests that signature of weak localization is present in the sample. Further studies are required in order to gain the insight into the transport mechanism for individual MWCNT. Finally, the thesis concludes with a general conclusion and future directions for this work.