No description available.

Single-walled carbon nanotubes (SWNTs) are envisioned as one of the most promising materials for next-generation electronic devices such as field-effect transistors, photovoltaics, new power sources and bio/chemical sensors. In particular, solution processable SWNT networks are of great interest for flexible and stretchable electronics. However, most of these applications specifically require pure semiconducting (sc-) or metallic (m-) SWNTs. However, large scale synthetic methods for SWNTs always produce a mixture of semiconducting and metallic carbon nanotubes. In recent years, several biochemical separation techniques such as DNA assisted separation, density gradient ultracentrifugation, and gel chromatography techniques have been utilized to separate semiconducting and metallic SWNTs. Although these methods can be used for sorting SWNTs according to their chiralities, they are either time-consuming or not easily scalable. In addition, the supramolecular functionalization of SWNTs with conjugated polymers has received a great deal of attention due to its capability to extract sc-SWNTs via simple sonication and centrifugation steps within a few hours. Furthermore, π-conjugated polymers can be modified by suitably changing monomers and/or comonomers, and it is also easy to control molecular weight and solubility of resulting polymers in organic solvents and aqueous media. There is also the possibility for selectively extracting specific chirality (n, m) nanotubes using specifically designed macromolecular structures. Except for its application to the separation of SWNTs, the supramolecular complexes of π-conjugated polymer and SWNTs have potential applications in many research areas such as new composite materials. After a brief overview of the current work related to the investigation of the supramolecular interaction between various conjugated polymers and SWNTs (chapter 1), synthesis of a series of different types of fluorene-based conjugated copolymers and their supramolecular complex formation properties with SWNTs are described (chapter 2, 3, 4, 5 and 6). In order to understand the effect of conjugated polymer backbone and side-chain structure on formation of supramolecular complexes with SWNTs, several crucial factors were investigated by: (1) altering the polymer backbone composition; (2) introducing different solubilizing (functional) groups while the polymer backbone remains the same; (3) changing the side-chain functional groups, and (4) introducing different polymer repeat units with varying physical and chemical properties. The experimental results indicated that all of the resulting polymer-SWNTs complexes possess excellent (or moderate) solution stability in organic solvents such as tetrahydrofuran (THF), toluene, and xylene. It was also demonstrated that the interaction between the conjugated polymers and SWNTs is strongly influenced by polymer structure; even minor changes on side-chains have a significant effect on the selectivity of the polymers in dispersing specific SWNT structures. This investigation highlights the potential importance of incorporating different types of heterocyclic aromatic rings (electron rich or electron poor), and introducing side chains with varying electronic and geometric structure on selective solubilization of SWNTs. Polymer molecular weight and solvent properties also strongly influence the π-conjugated polymer assisted dispersion of specific chirality SWNTs. Although some progress has been made, the search for a conjugated polymer that selectively solubilizes specific SWNT chiralities on large scale remains a challenge. / Thesis / Doctor of Philosophy (PhD)

No description available.

Control of single-walled carbon nanotube dispersion properties is of substantial interest to the scientific community. In this work, we sought to investigate the effect of a macrocycle, the pillar[5]arene, on the dispersion properties of a polymer-nanotube complex. Pillar[5]arenes are a class of electron-rich macrocyclic hosts capable of forming inclusion complexes with electron-poor guests, such as alkyl nitriles. A hydroxyl-functionalized pillar[5]arene derivative was coupled to the alkyl bromide side-chains of a polyfluorene, which was then used to coat the surface of single-walled carbon nanotubes. Differentiation of semiconducting and metallic SWNT species was analyzed by a combination of UV-Vis-NIR, Raman, and fluorescence spectroscopy. Raman spectroscopy confirmed that the concentrated nanotube dispersion produced by the macrocycle-containing polymer was due to well-exfoliated nanotubes, rather than bundle formation. The polymer-nanotube dispersion was investigated using 1H-NMR spectroscopy, and it was found that host-guest chemistry between pillar[5]arene and 1,6-dicyanohexane occurred in the presence of the polymer-nanotube complex. Utilizing the host-guest capability of pillar[5]arene, the polymer-nanotube complex was incorporated into a supramolecular organogel. / Thesis / Master of Science (MSc)

Ion irradiation of carbon nanotubes is a tool that can be used to achieve modification of the structure. Irradiation stability of carbon nanotubes was studied by ion and electron bombardment of the samples. Different ion species at various energies were used in experiments, and several defect characterization techniques were applied to characterize the damage. Development of dimensional changes of carbon nanotubes in microscopes operated at accelerating voltages of 30 keV revealed that binding energy of carbon atoms in CNs is much lower than in bulk materials. Resistivity measurements during irradiation demonstrated existence of a quasi state of defect creation. Linear relationship between ID/IG ratio and increasing irradiation fluence was revealed by Raman spectroscopy study of irradiated carbon buckypapers. The deviations from linear relationship were observed for the samples irradiated to very high fluence values. Annealing of irradiated samples was able to reduce the value of ID/IG ratio and remove defects. However, annealing could not affect ID/IG ratio and remove defects in amorphized samples. The extracted value of activation energy for irradiated sample was 0.36 ±0.05 eV. The value of activation energy was in good agreement with theoretical studies.

No description available.

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.

Carbon nanotubes (CNTs) are well known as excellent electrical conductors. However, their transport properties are limited by electrical breakdown in ambient. Moreover, the electronic properties can further be modulated by doping. Devices such as Schottky diodes, transistors and logic gates based on un-doped and doped CNT junctions have been realized. Recently, nitrogen doped CNTs show potential application in replacing platinum cathode catalyst in fuel cell technology. We synthesize pristine, nitrogen-doped and nitrogen-doped:pristine CNT intratubular junctions by one-step co-pyrolysis and explore them for different applications. We show that the position of electrical breakdown can be predicted which is essential to know for high current applications. Among other applications, we show that individual CNT intratubular junction exhibits rectifying characteristics. Further investigation indicates the intratubular junction behaves like Schottky diode. Lastly, the potential replacement of platinum by nitrogen doped CNTs in direct methanol fuel cell has been explored.

In this thesis, synthesis, characterization and electrical transport of Carbon nanotubes (CNTs) have been discussed. The first chapter contains a brief introduction of various forms of carbon including CNT. The CNTs are currently the materials of intense research interest due to their remarkable mechanical and electrical properties. CNTs can be visualized as a graphene sheet that has been rolled into a seamless tube. CNTs are either single-walled carbon nanotubes (SWCNT) or multi-walled carbon nanotubes (MWCNT). SWCNT is a tube with only one wall and MWCNT has many coaxial tubes and weak Van der Waal forces hold them together. The properties depend on chirality, diameter and length of the tubes. Chirality is defined by the symmetry and the chiral angle formed between the carbon bonds. The atomic structure of CNTs is described in terms of the tube chirality, which is defined by the chiral vector Ch and the chiral angle . The chiral vector is Ch = na1 + ma2, where the integers (n, m) are the number of steps along the zig-zag carbon. Depending on the tube chirality the electrical properties of the CNTs differ; they can be metallic or semiconducting. When n-m = 3p, where p is an integer, the CNTs are metallic and when n-m  3p, the CNTs are semiconducting. Due to the high anisotropy and high aspect ratio, CNTs have many potential applications with great technological importance such as functionalized molecules, conductive wires, bearings of rotational motors, field emitters, hydrogen storage, sensors, polymer composites, nanotube yarn and nanotube filters, X-ray generator, electron sources for microscopy and lithography, gas discharge tubes and vacuum microwave amplifiers, etc. The first chapter gives a brief introduction about various forms of carbon and their properties, particularly of CNTs. The nature of the CNTs depends on the method of production, which controls the degree of graphitization, the tube diameter and the chirality. Most synthesis methods originate from the idea of obtaining adequately active carbon atomic species or clusters from carbon sources and assembling them into CNTs without or with catalysts. The commonly used methods for the synthesis of carbon nanotubes are arc-discharge, Laser ablation, high-pressure catalytic decomposition of carbon monoxide (HiPCO), electrophoretic deposition (EPD), flame synthesis, pyrolysis, chemical vapour deposition (CVD), hot-filament CVD, plasma enhanced chemical vapour deposition (PECVD) using DC, RF, and micro wave power sources, hot-filament dc (HF-dc PECVD), inductively coupled plasma (ICPECVD) and electron cyclotron resonance (ECR PECVD). Although many efforts have been made to develop various synthesis methods, most of them require many steps. Moreover, the complicated and rigorous control of parameters and expensive materials are unavoidable that has put limitation in reproducing the same in large scale. In this chapter, a simple method for the synthesis of CNTs on a large scale that eliminates nearly the entire complex and expensive machinery associated with widely used growth techniques has been discussed. In Chapter 2, the synthesis and characterization of entangled CNTs are discussed. It is shown that entangled CNTs can be synthesized in one step by using double stage furnace. Tetrahydrofuran as carbon source material and nickelocene as catalyst source material have been used to synthesize CNTs. With this method CNTs can be synthesized at a temperature as low as at 600 0C. In this technique the self-developed pressure carries the vapours to the hot zone of the furnace. This has led to think in modifying the double stage furnace. A single stage furnace having temperature gradient is made to synthesize CNTs. The vapours are carried from low temperature zone to hot zone where the carbon species and catalysts react to form CNTs. The advantage of this furnace is that it is one-step process. Using another carbon source material such as Diethyl Ether and nickelocene as catalyst source material CNTs are synthesized. The as synthesized and purified CNTs are characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), transmission electron microscope (TEM), high resolution TEM (HRTEM) and Raman spectroscopy. The CNTs are multi-walled in nature as observed by HRTEM. In Chapter 3, the synthesis of aligned CNTs is discussed by using benzene as carbon source and ferrocene as catalyst source materials. Aligned MWCNTs were synthesized in the temperature range between 650 - 1100 0C in a single stage furnace without the need for carrier gas nor predeposited metal catalyst substrate. The essential need of CNTs are (1) to obtain aligned nanotubes with millimeter lengths to enable the formation of novel nanotube-polymer composites that incorporate continuous nanotubes throughout their thickness for highly anisotropic thermal and electrical conductivities; and (2) to provide samples for detailed physical characterization - tensile strength, thermal, electrical conductivity, field emission etc. SEM observation reveals the increase in length of nanotubes from 85 m to 1.4 mm with the increase of preparation temperature. The diameter as investigated by high-resolution transmission electron microscopy (HRTEM) remains almost constant 70-80 nm (75-85 layers). Once nanotube formation is established, the growth continues in the same direction and may well be reinforced by the presence of surrounding CNTs i.e. almost every particle produces a nanotube and bundling of neighboring tubes lead to collective vertical growth. The increase in length is due to the enhanced diffusion of active carbon with increasing preparation temperatures. The alignment of CNTs is also observed to the lateral side of the substrate. In Chapter 4, the synthesis and characterization of carbon nanoribbon and singled crystal iron filled CNTs is discussed. Particularly interesting are those CNTs filled with magnetic nanowires, which can provide an effective barrier against oxidation and consequently ensure a long-term stability in the core. The filling of metals within carbon nanotubes has extended the potential application base of these materials to quantum memory elements, high density magnetic storage media, semiconducting devices, field electron emitters, high resolution magnetic atomic force microscopy tips, magnetic field sensors and scanning probe microscopes etc. Tetrahydrofuran as carbon source material and ferrocene as catalyst materials has been used to synthesize mixture of carbon nanoribbons and iron filled CNTs. The techniques used to characterize the materials are XRD, SEM, HRTEM and superconducting quantum interference device (SQUID). The powder XRD pattern shows that the bcc -Fe phase of iron is present. HRTEM studies reveal the presence of multi-walled carbon nanotubes and well-crystallized -Fe phase filled inside the core region. Closer inspection of the HRTEM images indicated that the bcc structure -Fe nanowires are monocrystalline and Fe (110) plane is indeed perpendicular to the G (002) plane. Large coercivity (i.e. 1037 Oe at 300 K and 2023 Oe at 10 K) in the iron filled CNTs and carbon nanoribbons have been observed. The high coercivity is mainly attributed to the following two factors. Firstly, it is known that due to the uniaxial magnetic anisotropy of the nano size iron in the core region of the carbon nanotubes. Secondly, ferromagnetic behavior exhibited by the localized states at the edges of the carbon nanoribbons. The anisotropic electrical transport property of MWCNTs has been discussed in the chapter 5. The activated diffusive nature of transport along axial direction of CNT is explained. The transport perpendicular to the tube direction is explained in terms of a hopping mechanism. The anisotropic resistivity (N/P) value obtained is 3. The temperature dependent magnetoresistance (MR) is studied in magnetic fields up to 11 Tesla at low temperatures both in the parallel and perpendicular direction of an aligned MWCNT mat. In both cases a negative MR is observed. Chapter 6 discusses the preparation of CNT-polymer composites. The temperature dependence of the conductivity and magnetoresistance (MR) has been studied making four-point contact method on the carbon nanotubes polymer composites as result of increasing CNT content. The conductivity increases with increasing carbon nanotube weight percentage. The increase in conductivity as a function of the CNT weight percent is attributed to the introduction of conducting CNT paths in the polymer matrix. With the increasing CNT content the number of interconnections present in a random system is found to vary. Electrical conduction in nanotube mat or nanotube composites is explained by a variable range hopping (VRH) conduction mechanism. The negative magnetoresistance has been observed for the polymer composites. It is consistent with the report on CNTs bundles and polymer composites. Finally a brief summary of the work presented in this dissertation is discussed along with future directions in this research.

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.