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For practical applications, carbon nanotubes (CNTs) have to be assembled into macroscopic architectures, and CNT fiber presents itself as a promising candidate. However, CNTs inside a CNT fiber slide considerably during static and cyclic tensile tests, which is unfavorable for long-term performances. In this study, a second phase has been introduced into CNT fiber in order to enhance its reliability. ASTAR (Agency for Sci., Tech. and Research, S’pore)

Carbon nanotubes (CNTs) have excellent mechanical, chemical, and electronic properties, but realizing these excellences in practical applications needs to assemble individual CNTs into larger-scale products. Recently, CNT fibers demonstrate the potential of retaining CNT's superior properties at macroscale level. High-performance CNT fibers have been widely obtained by several fabrication approaches. Here in this paper, we review several key spinning techniques including surfactant-based coagulation spinning, liquid-crystal-based solution spinning, spinning from vertical-aligned CNT arrays, and spinning from CNT aerogel. The method, principle, limitations, and recent progress of each technique have been addressed, and the fiber properties and their dependences on spinning parameters are also discussed.

Polarization behaviors of carbon nanotube (CNT) fibers with different twisting were reported. Scanning electron microscope and polarized Raman spectroscopy were used to investigate the prepared samples. Results indicate that surface twisting angle affects greatly the polarization angle and I///I⊥ ratio of twisted CNT fibers. Raman depth profile measurements imply that the twisted fibers consist of non-uniform CNT alignments. A simplified two-CNT-alignment geometric model was proposed to illuminate the experimental observations. The results suggest that polarized Raman depth profile measurement would be a very useful approach for determining the distribution of CNT alignments in CNT fibers.

We present a strategically smart method to controllably synthesize and rapidly identify carbon nanotube (CNT) intramolecular junctions (IMJs). Ultralong, horizontally aligned, and defect-free single-walled carbon nanotubes (SWNTs) grown in a chemical vapor deposition system were used as structural carriers of IMJs. The trenched structures were employed into the synthesis processes to induce strains onto SWNTs that lie suspended across them. As a consequence, the nanotube lattice and electronic structures were altered and topological defects were generated, leading to the formation of IMJs in the vicinity of the edge of the trenches. Characterizations using in situ Raman spectroscopy verified the presence of IMJs and classified them as S-S junctions. Electrical transport measurements reviewed the rectifying behavior of the as-obtained IMJs. Other types of IMJs induced by tube-substrate and tube-tube interactions were also characterized by taking the advantages of trenched structure. DOCTOR OF PHILOSOPHY (MAE)

Carbon nanomaterials (including carbon nanotube and graphene) and semiconductor nanoparticles have attracted considerable research effort in past two decades in terms of both fundamental and application research. Semiconductor nanoparticles have large optical absorption cross-sections and tenable electronic properties. Carbon nanomaterials are unique in terms of structure, carbon nanotube (CNT) bears a hollow one-dimensional structure; and graphene is an one-atom-thick sheet and has extremely high specific surface area; both of them are presenting excellent charge transport capability. The combination of them will provide an ideal model for nanoscale optoelectronic devices. The key step in the operation of this kind of hybrid optoelectronic devices is the interfacial charge transfer between carbon nanomaterials and semiconductors nanoparticles. The fast transfer and transport of charge will help to separate bound excitons (electron-hole pairs generated by light excitation), and suppress the recombination. Basing on the understanding on excitonic solar cells, the diffusion length of exciton is only in a range less than 100 nm before energy quenching; thus it is very important to dissociate the exciton effectively before recombination.' A hybrid nanostructure is extremely beneficial for exciton dissociation for following reasons. First, there is a builtin electric field at the interfacial area, which helps to separate the bound exciton; then, the hybrid nanostructure bears an extremely high interfacial area which provides countless sites for exciton dlssoclation." According to the understanding on electrons transport in nanoparticle structures, under illumination, the photogenerated electrons repeatedly interact with a distribution of traps as they undertake a random hopping through the nanoparticles. A photogenerated electron may experience a million trapping events before being collected by electrode." The longer transporting time in nanoparticles will increase the possibility of recombination. Thus, if the exciton is separated at the interfacial area of the hybrid nanostructure, with one kind of charge{electron or hole) injected into a better transport pathway, say, carbon nanomaterial, it will transport to the collecting electrode very fast, lowering the possibility of recombination. Thus the recombination is not a dominant problem in hybrid nanostructures. In this thesis, we utilized facile methods to synthesize several typical carbon-semiconductor hybrid nanostructures. We will investigate the charge transfer mechanisms in the hybrid materials by studying their photoresponse behaviors. We first studied the photoresponse behavior of multiwalled carbon nanotube-Zinc Oxide (MWNT-ZnO) hybrid nanostructures and charge transfer mechanism. Based on the mechanism, conversion of conductance from n-typed to p-typed is realized. The conductance of the hybrids could be strongly tuned by the amount of ZnO loading: in the case of shorter deposition time or fewer ZnO nanoparticles (NPs), the photogenerated electrons in ZnO transfer into the MWNT, resulting in a decrease in conductance of MWNT-ZnO hybrids; While in the case of longer deposition time or more ZnO NPs, the photogenerated electrons will not only decrease the conductance of holes in the MWNT matrix, but also convert the conductance of hybrids from p-typed to n-typed. A device structure based on another hybrid system, MWNT-CuS, was then designed to study the charge transfer behaviors. By using an asymmetrical contact structure, the hybrid devices present significant rectification in I-V curves, and show predominant photoresponse to light illumination. The strong photoresponse to light illumination is a result of dissociation of photogenerated excitons by the built-in electric field across the Schottky junctions at the MWNT/CuS interfaces due to the intimate contact resulted from in situ synthesis. The asymmetrical contacts provide transport loops for both electrons and holes. Finally the charge transfer and photoresponse behaviors in reduced graphene oxide-ZnO (rGO-ZnO) hybrid nanostructures were investigated in detail. Under visible light illumination, a significant zero-bias photocurrent was observed in rGO-ZnO hybrids device. This photocurrent could be attributed to the fast charge transfer at rGO-ZnO interfaces, effective charge transport in rGO sheets, and continuous adsorption and desorption of ionized surface oxygen. The work function difference (Fermi energy) between the ZnO and rGO provides the driving force for charge separation. The charge-transfer phenomenon in these hybrid nanostructures demonstrates the carrier separation capability for optoelectronic devices. This investigation of charge transfer behavior in carbon nanomaterials based hybrid nanostructures will provide knowledge for the design of high performance optoelectronics devices, such as solar cells, photodetectors and so on. Doctor of Philosophy (MAE)

Due to their unique structure, carbon nanotubes (CNTs) have excellent properties including high thermal conductivity, high current capacity, and superior mechanical and chemical properties, which make them excellent one-dimensional materials with potentially wide applications. In order to utilize these properties, nano-scale CNTs must be prepared into macro-scale assemblies. CNT fibers, notably the fibers spun from vertically aligned CNT arrays, have therefore attracted great attention in recent years. Although these fibers have the inherent potential of leveraging on the excellence of CNTs, the individual fiber’s properties strongly depend on their structures and morphologies. The strength of the best fibers is still much lower than the theoretical and experimental results from individual CNTs or small CNT bundles. Moreover, up to now, no attention has been paid to the influence of test condition, specifically strain rate effects on mechanical performances of CNT fibers. The goal of this research is to investigate the structural properties of CNT fibers, and study the strain rate dependent failure mechanisms and its relation to the fibers’ structure and morphology of failure. CNT fibers were directly spun from vertically aligned CNT arrays. The mechanical behaviors of CNT fibers were investigated for a wide range of tensile strain rates employing several characterization techniques, including static/dynamic tensile tests, polarized Raman spectroscopy, and fiber fracture surface observations. From these systematic studies, the failure mechanism of CNT fibers was analyzed and their relation to fibers’ structures was deduced. The results could be divided into the following sections: Firstly, the general properties of CNT fibers that were spun from CNT arrays were provided. The mechanical strength and electrical conductivity of CNT fibers were measured statistically. The structural properties of CNT fibers were characterized by Raman spectroscopy and non-uniform twisting was found inside the fiber. It was found that once the spinning process was fixed, the uniformity of fibers’ mechanical strength and electrical conductivity was fixed. Based on these systematic experiments, a fixed fiber spinning process was determined for the follow-on studies. Then, CNT fibers were tensile tested in a wide range of strain rates. It was found that the mechanical response of CNT fibers exhibited a strain rate strengthening effect, and two different failure mechanisms dominated at high and low tensile strain rates, respectively. The key factors, inter-tube slippage and CNT alignment, that limited the mechanical properties of current CNT fibers were then discussed and possible failure mechanisms were proposed based on fibers’ mechanical behavior and the observations of fracture surfaces by scanning electron microscopy (SEM). Mechanical behavior and reliability of CNT fibers were then studied in detail under low strain rates. Static mechanical test and electrical measurement, combined with an in situ Raman spectroscopy, were used to monitor the load transfer and failure process. The morphology of CNTs in the fibers was characterized by transmission electron microscopy (TEM). The results further confirmed that the performance of CNT fibers at low strain rates was determined by slippage and breakage between nanotubes or small tube bundles. An improvement approach was proposed and studied by introducing a second element with a 3-D network structured polymer to improve the load transfer efficiency between CNTs (or small CNT bundles) inside the fibers. It was demonstrated that the CNTs in the fiber could be effectively constrained, resulting in more effective load transfer. Comparatively, at high strain rates, the structure dependent mechanical performance of the CNT fibers was investigated. A facile approach was applied to improve the alignment of CNTs by re-wetting, swelling, and re-drying the CNT fibers in a dilute HCl solution. The alignment of CNTs and its influence on fibers were examined. The mechanical behaviors of CNT fibers before and after post-treatment were compared as well. It was found that the alignment of CNTs showed significant influence on fibers’ mechanical strength. Finally, statistical methods employing modified Weibull models was developed to analyze the distribution of mechanical strength and failure mechanisms of fibril materials, was introduced to examine the breaking mechanisms under different strain rates. Variation of fibers’ strength and its dependence on fibers’ diameters and strain rates were investigated. Based on statistical analysis of our experimental data, further mathematical evidence could be deduced to support the failure mechanisms proposed for low and high strain rates. DOCTOR OF PHILOSOPHY (MAE)

In this work, Raman spectroscope is used to characterize the Fe/Al2O3 catalysts to find the structure differences between different thicknesses of catalysts. These catalysts are used for carbon nanotube (CNT) growth in thermal chemical vapor deposition (CVD) method. In the previous CNT growth experiments, it is found that only certain thickness of catalyst has the best catalytic effect, but the reason is not very clear, so the objective of this dissertation is to characterize the structure differences between catalysts with different thickness to find the mechanism of catalytic effect in CNT growth. Master of Science (Precision Engineering)

70 p. Since the discovery in 1991 by Sumio Iijima, carbon nanotubes (CNTs) have attracted much research attention because of their mechanical, thermal and electrical properties. CNTs can be produced by methods of arc discharge, laser ablation, thermal synthesis and chemical vapor deposition (CVD). CVD is probably the most versatile production method of CNTs as well as the most promising method for producing Single-walled CNTs (SWCNTs) with high quality and yield. Master of Science (Precision Engineering)