The unique electronic, mechanical and physical properties led Carbon nanotubes (CNTs) to be potential candidate for field emitter, hydrogen storage, sensors, nano electronic devices, nano electromechanical systems, polymer composites. In order to make them in the industrial scale we need large quantity production of CNTs with low cost. The present thesis work deals with the preparation of CNTs by pyrolysis method from xylene and further studies on the grown CNTs. Magnetic characterization of CNTs has been done using SQUID. The interaction of CNTs with the microwave irradiation is studied and it was found for the first time that there is light emission from the CNTs apart from direct electric field. In this process we also observed that the static charge develops on the CNTs. A composite of CNTs/DNA has been prepared with varying CNT content and the electrical conductivity measurements have been done. The first chapter of the thesis provides an introduction to carbon family. Carbon nanotubes, which are potential candidates from carbon family, is a growing field for research in science and technology. A glimpse of various methods of preparation of CNTs like arc-discharge, laser ablation, chemical vapour deposition (CVD), hot-filament CVD, plasma enhanced chemical vapor deposition (PECVD), electron cyclotron resonance (ECR PECVD), high-pressure catalytic decomposition of carbon monoxide (HiPCO), pyrolysis are discussed. Some applications of CNTs are also included in this chapter. The second chapter deals with the experimental techniques employed for the preparation of CNTs and their characteristics studied by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and Raman Spectroscopy (RS). The preparation of CNTs from xylene as carbon source and ferrocene as catalyst in pyrolysis method is described in detail. Well aligned CNTs with a length of several tens of micrometers and diameter of 40 to 80 nanometers were obtained as confirmed by SEM. TEM and XRD confirms the graphitic crystal structure of the CNTs. RS also confirms the information about the crystal structure. The third chapter discusses the magnetic studies on CNTs using Superconducting Quantum Interference Device (SQUID) as a function of magnetic field and temperature. In the random mixture of parallel, perpendicular and oblique nanotubes, the applied field produces diamagnetic behavior, although the sample possess different kinds of tubes with various chirality and radii. Paramagnetic deviation was observed on the diamagnetic susceptibility at weak fields and low temperature, confirming qualitatively with the Aharonov-Bhom effect on the energy gap for the magnetic field parallel to the tube axis Chapter four presents the light emission from the CNTs. It describes the light emission from different processes reported in the literature. Here we have observed a new process to generate light from CNTs through microwave irradiation. Along with the light emission some of the tubes get charged and some tubes are physically broken. We provide a simple approach as to why the tubes break and the nature of the breakage is also discussed. The fifth chapter discusses the preparation of CNTs/DNA composites. The conductivity increases with increasing carbon nanotube weight percentage. The increase in conductivity as a function of the CNTs weight percent is attributed to the introduction of conducting CNTs path in the DNA matrix. A summary of the results obtained and the scope for future work are included in the chapter six of the thesis.

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.

Carbon nanotubes (CNTs) have been recognized as potential candidates for mainstream device fabrication and technologies. CNTs have become a topic of interest worldwide due to their unique mechanical and electrical properties. In addition, CNTs possess high aspect ratio and low density that make them an important material for various technological applications. The field of carbon nanotube devices is rapidly evolving and attempts have been made to use CNTs in the fabrication of devices like field emitters, gas sensors, flow meters, batteries, CNT-field effect transistors etc. These molecular nanostructures are proposed to be an efficient hydrogen storage material. CNT cylindrical membranes are reported to be used as filters for the elimination of multiple components of heavy hydrocarbons from petroleum and for the filtration of bacterial contaminants of size less than 25 nm from water. Recently, CNT bundles have been proposed to be a good material for low-temperature sensing. CNTs have also been considered as promising filler materials due to extraordinary characteristics mentioned above. Fabrication of nanocomposites using CNTs as reinforcing material has completely renewed the research interest in polymer composites. The conductive and absorptive properties of insulating polymer doped with conducting filler are sensitive to the exposure to gas vapors and hence they can be used in monitoring various gases. The application of fiibre reinforced polymer composites in aeronautic industry are well known due to their high mechanical strength and light weight. Also, the conductive composite materials can be used for electromagnetic shielding. Desired properties in CNT-composites can be attained by adding small amount of CNTs in comparison to traditional filler materials. Due to high aspect ratio and low density of CNTs, percolation threshold in CNT-polymer composites can be achieved at 0.1 vol % as compared to ~16 vol. % in case of carbon particles. The research work ׽0.1 vol. %, as compared to reported in this thesis includes the preparation of multiwall carbon nanotube (MWNTs) and MWNT-polystyrene composites, experimental investigations on low temperature charge transport, and magnetic properties in these systems. This thesis contains 7 chapters. Chapter 1 provides an overview of CNTs and CNT-polymer composites. This chapter briefly describes the methods for synthesizing CNTs and fabricating CNT-polymer composites, charge transport mechanisms in CNTs and composites, and their magnetic properties as well. Chapter 2 deals with the concise introduction of various structural characterization tools and experimental techniques employed in the present work. An adequate knowledge of the strengths and limitations of experimental equipment can help in gathering necessary information about the sample, which helps in studying and interpreting its physical properties correctly. Chapter 3 describes the synthesis of MWNTs and their use as filler material for the fabrication of composites with polystyrene (PS). The characterization results of as-prepared MWNT and composites show that MWNTs possess high aspect ratio (~4000), and are well dispersed in the composite samples (thickness ~50-70 µm). The composite samples are prepared by varying the MWNT concentration from 0.1 to 15 wt %. The as¬fabricated composites are electrically conductive and expected to display novel magnetic properties since MWNTs are embedded with iron (Fe) nanoparticles. Chapter 4 presents the study of charge transport properties of aligned and random MWNTs in the temperature range 300-1.4 K. The low temperature electrical conductivity follows the weak localization (WL) and electron-electron (e-e) interaction model in both samples. The dominance of WL and e-e interaction is further verified by magneto-conductance (MC) measurements in the perpendicular magnetic field up to 11 T at low temperatures. The MC data of these samples consists of both positive and negative contributions, which originates from WL (at lower fields and higher temperatures) and e-e interaction (at higher fields and lower temperatures). Chapter 5 contains the results of charge transport studies in MWNT-PS composite near the percolation threshold (~0.4 wt %) at low temperatures down to 1.4 K. Metallic-like transport behavior is observed in composite sample of 0.4 wt %, which is quite unusual. In general, the usual activated transport is observed for systems near the percolation threshold. The unusual weak temperature dependence of conductivity in MWNT-PS sample at percolation threshold is further verified from the negligible frequency dependence of conductivity, in the temperature range from 300 to 5 K. Chapter 6 accounts on the experimental results of magnetization studies of MWNTs and MWNT-PS composites. The observation of maxima in coercivity and squareness ratio at 1 wt % of Fe-MWNT in a polymer matrix show the dominance of dipolar interactions among the encapsulated Fe-nanorods within MWNTs. The hysteresis loop of 0.1 wt % sample shows anomalous narrowing at low temperatures, which is due to significant contribution from shape anisotropy of Fe-nanorods. Chapter 7 presents brief summary and future perspectives of the research work reported in the thesis.