Synthesis, Characterization and Electrical Transport In Carbon Nanotubes

Abstract

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.