In this study, a novel polyimide (PI) film, consisting of multiwall carbon nanotubes (MWCNTs) dispersed in an Ionic Liquid (IL), were demonstrated to be high shielding effectiveness (SE). The film was potentially useful for screening electromagnetic interference(EMI) in an optical transceiver module. The experimental results showed MWCNT-PI composite dispersed well in IL exhibits a high far-field SE of 38 ~ 45 dB within the frequency range of 1 ~ 3 GHz. It was also demonstrated the MWCNT-PI composite prepared with IL dispersed process have higher SE and lower weight percentage of MWCNTs than those with non-IL-dispersed process. Their intermolecular forces were carefully examined in order to understand dispersion mechanisms among MWCNTs. The aggregation phenomenon of MWCNTs was known, resulting from van der Waals forces. In our study, IL was employed to disperse MWCNTs. A proposal reason was that the attractive force between cation of the IL and £k electrons of MWCNTs is greater than the van der Waals forces among MWCNTs. From conductivity measurement, percolation threshold of the IL-dispersed MWCNT-PI composite was 5.2 wt%; percolation threshold of the non-IL-dispersed MWCNT-PI composite was 11.5 wt%. Given the lower percolation threshold ,we demonstrated the successful dispersion of MWCNT by adding IL. From the results of Raman spectrometer analyses, the IL dispersion was proved to be a physical interaction. Furthermore, the IL-dispersed MWCNT-PI composite was used as package material in monopole antenna and got a near-field SE of 37dB within the frequency of 2.8 GHz. It implied that the IL-dispersed MWCNT-PI composite has an excellent EMI performance.The IL-dispersed MWCNT-PI composite is suitable for packaging low-cost and high-performance optical transceiver modules in the application of the fiber-to-the-home (FTTH) lightwave transmission systems.

Vertically aligned Multiwalled Carbon Nanotubes (MWCNTs) were grown in the pores of Anodized Aluminum Oxide (AAO) templates and investigated for resistive sensor applications. High Sensitivity of 23% to low concentration (100 ppm) of ammonia was observed. An equivalent circuit model was developed to understand the current flow path in the resistive sensor. This helped us in achieving high sensitivities through amorphous carbon (a-C) layer thickness tailoring by employing post-growth processing techniques like plasma etching. A simulation model in MATLAB was developed to calculate the device resistance and the change in the sensitivity as a function of device parameters. The steady state response and transient response of the model to the number of ammonia molecules and its adsorption rate were studied. Effects of oxygen plasma, argon plasma and water plasma etch on thinning of the a-C layer were studied. In order to enhance the sensitivity, the top and bottom a-C layers were replaced by a more conductive metal layer. This also helped in understanding the current flow in the device and in the estimation of the resistivity of the a-C layer.

Electron Beam Induced Deposition (EBID) is an emerging additive nanomanufacturing tool which enables growth of complex 3-D parts from a variety of materials with nanoscale resolution. Fundamentals of EBID and its application to making a robust, low-contact-resistance electromechanical junction between a Multiwall Carbon Nanotube (MWNT) and a metal electrode are investigated in this thesis research. MWNTs are promising candidates for next generation electrical and electronic devices, and one of the main challenges in MWNT utilization is a high intrinsic contact resistance of the MWNT-metal electrode junction interface. EBID of an amorphous carbon interface has previously been demonstrated to simultaneously lower the electrical contact resistance and to improve mechanical characteristics of the MWNT-electrode junction. In this work, factors contributing to the EBID formation of the carbon joint between a MWNT and an electrode are systematically explored via complimentary experimental and theoretical investigations. A comprehensive dynamic model of EBID using residual hydrocarbons as a precursor molecule is developed by coupling the precursor mass transport, electron transport and scattering, and surface deposition reaction. The model is validated by comparison with experiments and is used to identify different EBID growth regimes and the growth rates and shapes of EBID deposits for each regime. In addition, the impact of MWNT properties, the electron beam impingement location and energy on the EBID-made carbon joint between the MWNT and the metal electrode is critically evaluated. Lastly, the dominant factors contributing to the overall electrical resistance of the MWNT-based electrical interconnect and relative importance of the mechanical contact area of the EBID-made carbon joint to MWNT vs. that to the metal electrode are determined using carefully designed experiments.

Multifunctional materials both with damping properties and strain sensing properties are very important. They promise to be more weight-efficient, and provide volume-efficient performance, flexibility and potentially, less maintenance than traditional multi-component brass-board systems. The goal of this dissertation work was to design, synthesize, investigate and apply polyaniline/Multiwall carbon nanotube (PANI/MWCNT) and polyurethane (PU) /MWCNT composites films for structural vibration control and strain sensors using free layer damping methods and static and dynamic strain sensing test methods. The PANI/MWCNT was made by in situ polymerization of PANI in the presence of MWCNT, then frit compression was used to make circular and rectangular PANI/MWCNT composite films. PU/MWCNT composites were made by the layer-by-layer method. Free end vibration test results showed both of PANI/MWCNT and PU/MWCNT have better damping ratios than each of their components. Static sensing test indicated that though there appears to be residual strain in both composite sensors after the load is removed, both the sensor and the foil strain gage react linearly when re-engaged. A drift test of the sensor showed that it is stable. The dynamic sensing test results showed that over the range of 10-1000 Hz, the PANI/MWCNT composite sensor was consistently superior to foil strain gage for sensing purposes since the highest peak consistently corresponded to the input frequency and was much higher, for example, at 20 Hz, 820 times higher than those of the strain gage. Using the same criterion, the PU/Buckypaper composite sensor was comparable to or superior to the foil strain gage for sensing purposes over the range of 10 Hz to 200 Hz. The relationship of loss factor, η, and beam coverage length, L1/L, is discussed for single sided and double sided attachment. For both PANI/MWCNT and PU/MWCNT, the loss factor, η, was found to increase as coverage length, L1/L, increases. The loss factor, η, was found to have a maximum as with coverage length, L1/L, as the coverage length continues to increase. The trend for double sided attachment was found to follow the trends discussed by Rao (2004) and Levy and Chen (1994) for viscoelastic material constrained damping.

Field emission is the emission of electrons from a solid under an intense electric field, of the order of 109 V/m. Emission occurs by the quantum mechanical tunneling of electrons through a potential barrier to vacuum. Field emission sources offer several attractive features such as instantaneous response to field variation, resistance to temperature fluctuation and radiation, a high degree of focusing ability in electron optics, good on/off ratio, ballistic transport, and a nonlinear current-voltage relationship. Carbon nanotubes (CNTs) are potential candidates as field emitters since they possess high aspect ratio and are chemically inert to poisoning, and physically inert to sputtering during field emission. They can carry a very high current density and do not suffer field-induced tip sharpening like metallic tips. In addition, the CNT field emitters have the advantage of charge transport through 1D channels and electron emission at the sharp tips due to large enhancement. But the injection of electrons from the back contact remains a technical challenge which requires binding of CNT emitters to metallic substrate. Also, detachment of the CNT from the substrate tends to occur with time. The electrically conducting mixtures of CNTs and polymer can provide an alternative route to address these issues in the field emission of CNTs. The composites can be casted on any substrate in desired shape and the polymer matrix provides necessary support. The research work reported in this thesis includes the preparation of high quality multiwall carbon nanotubes (MWCNTs), MWCNT-polystyrene (PS) composites, and experimental investigation on field emission properties of MWCNT¬PS composites in two different configurations. Electrical conductivity and percolation threshold of the MWCNT-PS composites are also investigated to ensure their high quality prior to the field emission studies. The study has been further extended to reduced graphene oxide (rGO) coated on polymer substrate. The main results obtained in present work are briefly summarized below. This thesis contains eight chapters. Chapter 1 provides an overview of basics of field emission, and the potential of CNT and CNT-polymer composites as field emitters. Chapter 2 deals with the concise introduction of various structural characterization tools and experimental techniques employed in this study. Chapter 3 describes the synthesis of MWCNTs and characterization by using electron microscopy and Raman spectroscopy. MWCNTs are synthesized by chemical vapor deposition (CVD) of toluene [(C6H5) CH3] and ferrocene [(C5H5)2 Fe] mixture at 980 °C. Here toluene acts as carbon source material and ferrocene provides catalytic iron (Fe) particles. The MWCNT formation is based on the thermal decomposition of the precursor mixture. Scanning electron microscopy (SEM) characterization shows that the MWCNTs are closely packed and quite aligned in one direction. The average length of MWCNTs is about 200 μm and outer diameter lies in the range of 50-80 nm. The high quality of as-prepared MWCNT sample is confirmed by Raman spectroscopy. The as-grown MWCNTs are encapsulated with catalytic Fe nanoparticles, revealed by transmission electron microscopy. The Fe nanoparticles trapped within the MWCNT serve as fantastic system for studying the magnetic properties. Three types of MWCNT samples filled with Fe nanoparticles of different aspect ratio (~10, 5 and 2) are synthesized by varying the amount of ferrocene in the precursor material, and their magnetic properties are investigated. Enhanced values of coercivity (Hc) are observed for all samples, Hc being maximum (~2.6 kOe) at 10 K. The enhancement in Hc values is attributed to the strong shape anisotropy of Fe nanoparticles and significant dipolar interactions between Fe nanoparticles. Chapter 4 deals with the field emission studies of MWCNT-PS composites in the parallel configuration. By incorporating as-prepared MWCNTs in PS matrix in a specific ratio, composites with varying loading from 0.01-0.45 weight (wt.) fraction are prepared using solution mixing and casting. High degree of dispersion of MWCNTs in PS matrix without employing any surfactant is achieved by ultrasonication. Low percolation threshold (~0.0025 wt. fraction) in the MWCNT-PS composites ensures the good connectivity of filler in the fabricated samples. Field emission of MWCNT¬PS composites is studied in two different configurations: along the top surface of the film (parallel configuration) and along the cross section of the sample (perpendicular configuration). In this chapter field emission results of the MWCNT-PS composites in parallel configuration are presented. The effect of charge transport in limiting the field emission of MWCNT-PS composite is discussed. Field emission results of MWCNT-PS composites in parallel configuration indicate that the emission performance can be maximized at moderate wt. fraction of MWCNT (0.15). The obtained current densities are ~10 µA/cm2 in the parallel configuration. Chapter 5 presents the study of field emission characteristics of MWCNT¬PS composites of various wt. fractions in the perpendicular configuration. Till date most studies using nanotube composites tend to have the nanotubes lying in two dimensional plane, perpendicular to the applied electric field. In the perpendicular configuration, the nanotubes are nearly aligned parallel to the direction of the applied electric field which results in high field enhancement, and electron emission at lower applied fields. SEM micrographs in cross-sectional view reveal that MWCNTs are homogeneously distributed across the thickness and the density of protruding tubes can be scaled with wt. fraction of the composite film. Field emission from composites has been observed to vary considerably with density of MWCNTs in the polymer matrix. High emission current density of 100 mA/cm2 is achieved at a field of 2.2 V/µm for 0.15 wt. fraction. The field emission is observed to follow the Fowler– Nordheim tunneling mechanism, however, electrostatic screening plays a role in limiting the current density at higher wt. fractions. Chapter 6 highlights the field emission response of rGO coated on a flexible PS film. Field emission of rGO coated PS film along the cross section of the sample is studied in addition to the top film surface of the film. The effect of geometry on the improved field emission efficiency of rGO coated polymer film is demonstrated. The emission characteristics are analyzed by Fowler–Nordheim tunneling for field emission. Low turn-on field (~0.6 V/µm) and high emission current (~200 mA/cm2) in the perpendicular configuration ensure that rGO can be a potential field emitter. Furthermore, stability and repeatability of the field emission characteristics are also presented. Chapter 7 deals with the synthesis, characterization, and field emission of two different kinds of hybrid materials: (1) MWCNT coated with zinc oxide (ZnO) nanoparticles (2) ZnO/graphitic carbon (g-C) core-shell nanowires. The field emission from the bucky paper is improved by anchoring ZnO nanoparticles on the surface of MWCNT. A shift in turn on field from 3.5 V/µm (bucky paper) to 1.0 V/µm is observed by increasing the ZnO nanoparticle loading on the surface of MWCNT with an increase in enhancement factor from 1921 to 4894. Field emission properties of a new type of field emitter ZnO/g-C core-shell nanowires are also presented in this chapter. ZnO/g-C core/shell nanowires are synthesized by CVD of zinc acetate at 1300 °C. Overcoming the problems of ZnO nanowire field emitters, which in general possess high turn on fields and low current densities, the core-shell nanowires exhibit excellent field emission performance with low turn on field of 2.75 V/µm and high current density of 1 mA/cm2. Chapter 8 presents a brief summary of the important results and future perspectives of the work reported in the thesis.

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.

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.