Molecular dynamics theory and second reactive empirical bond order (REBO) potential are employed to determine the mechanical and dynamic properties of (10,10) and (17,0) single-wall carbon nanotubes (SWNT). According to the different simulated environment, the article can be divided into two parts and discussed. The mechanical properties of SWNT in vacuum environment are investigated by tensile process. The physical parameters can be obtained during the tensile process, for example, the yield stress and the Young¡¦s modulus. In addition, the slip vector can be used to investigate the dynamic behaviors of carbon nanotubes during the tensile process and the variation of microstructure after carbon nanotubes broken. Moreover, the mechanical properties of SWNT in the bulk water are also taken into account. In this section, we mainly investigate the effect of the structure of water molecules in the SWNT with different diameters of SWNT. Finally, the mechanical properties of SWNT influenced by water molecules inside the carbon nanotubes are investigated, and compare the results with those in vacuum environment.

This thesis is focused on the high-frequency performance of carbon nanotube field-effect transistors (CNFETs). Such transistors show their promising performance in the nanoscale regime where quantum mechanics dominates. The short-circuit, common-source, unity-current-gain frequency ft is analyzed through regional signal-delay theory. An energy-dependent effective-mass feature has been added to an existing SP solver and used to compare with results from a constant-effective-mass SP solver. At high drain bias, where electron energies considerably higher than the edge of the first conduction sub-band may be encountered, ft for CNFETs is significantly reduced with respect to predictions using a constant effective mass. The opinion that the band-structure-determined velocity limits the high-frequency performance has been reinforced by performing simulations for p-i-n and n-i-n CNFETs. This necessitated incorporating band-to-band tunneling into the SP solver. Finally, to help put the results from different CNFETs into perspective, a meaningful comparison between CNFETs with doped-contacts and metallic contacts has been made. Band-to-band tunneling, which is a characteristic feature of p-i-n CNFETs, can also occur in n-i-n CNFETs, and it reduces the ft dramatically.

Ion irradiation has been known to be an effective tool for structure modification with micro/nano-scale precision. Recently, demonstrations have been made for nano-machining, such as the cutting and welding of carbon nanotubes. Understanding the fundamental effects of ion irradiation on carbon nanotubes is critical for advancing this technique as well as for scientific curiosity. Molecular dynamics modeling was performed to study irradiation stability, structural changes, and corresponding thermal properties. In our study, Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) was used to perform atomic scale simulation. In order to understand size and geometry effects on carbon damage creation, the threshold energy of displacement was calculated as a function of recoiling angles for both single-walled and multi-walled nanotubes. A strong directional dependence was found to exist in different shells of multi-walled carbon nanotubes. We found that carbon atoms on the innermost tube were more susceptible to be displaced toward the center of axis. The calculation matrix was further extended to nanotubes having different diameters for a full-scale understanding of the creation of defects. Besides studies on defects creation, thermal properties of carbon nanotubes were studied via a simplified model of the carbon nanotube network. Thermal conductivity, were found to be increased nearly one order of magnitude in carbon nanotube networks after irradiation and subsequent annealing. All the modeling results were compared with experimental observations either obtained from this project as a parallel study or from previous works, for the purpose of verification and validation. For experimental works, atomic scale characterization was performed by using transmission electron microscopy and the thermal conductivity measurement was characterized by using laser flash technique. Through a combination of modeling and experimentation, we proved that ion beam techniques can be used to enhance thermal conductivity in carbon nanotube bundles by inter-tube defects mediated phonon transport.

The mass production of carbon nanotubes (CNTs) by a cost effective process is still a challenge for further research and application of CNTs. This research focussed on the deposition of CNTs on a continuously-fed carbon substrate via arc discharge at atmospheric pressure. In this work, modifications, control and optimization of the available arc-discharge reactor were conducted. New reactor support and new tape feeding mechanisms were added to the reactor for better temperature assessment, longer operating period and better control of the speed of the tape. The influence of inter-electrode gap, substrate velocity and arc current on the surface temperature were investigated. Multiwalled carbon nanotubes (MWNTs) were produced at lower currents (< 20 A) and at larger inter-electrode gaps. Further investigation shows that inter-electrode gap influenced both the arc characteristic and the anode surface temperature (Ts). Here, Ts was measured by an optical pyrometer. The inter-electrode gap was found to indirectly affect the formation of NTs. Anode surface temperature (Ts) varied with gap, reaching a minimum at an intermediate gap. Higher CNTs yield was found at this lowest Ts. This minimum Ts is consistent with the presence of a cloud of nanoparticles ejected by the heated graphite/carbon surfaces. These graphene fragments are thought to later fold and form nanotube “seeds” and then develop into multiwall nanotubes. This cloud of nanoparticles also may affect the electrical conductivity at the front of the anode. Simulation of the arc behaviour, i.e. temperature distributions and flow properties of the plasma, using a computer package Comsol Multiphysics 3.2, was stable only when the electrical conductivity of a dusty plasma near to the electrodes was included. Our experiments show that carbon nanotubes grew better at a Ts range of ~ 3650 K - 3700 K and at the tape speed of 3 mm/s. The results from our work also strongly suggested that tiny carbon crystallites are the main intermediates for CNT growth in an electric arc. The limiting factor for a solid state growth mechanism, therefore, is high temperature annealing of carbon or graphene fragments. Further work should aim to understand the growth mechanism of CNTs, produce comprehensive analysis on the arc plasma composition and also explore the possibility of producing CNTs at higher rates.

The ability to bypass most cellular barriers to gain access to intracellular compartments has great potential in cell biology. The possibilities range from efficient delivery of macromolecules such as plasmids to small proteins and oligonucleotides that are sensitive to degradation. In biomedicine, easy access means enhanced cellular imaging and delivery of many therapeutics currently hampered by poor stability and cellular uptake. Carbon Nanotubes (CNTs) are attractive in these applications due to their efficient cellular uptake. While mode of entry of CNTs into cells is debatable, possibly their natural shape allows for their selective penetration across biological barriers in a non-destructive way, making them versatile as membrane permeating particles. The present study explores the diverse functionalities of CNTs including: 1) Efficient delivery of DNA into HeLa cells using vertically aligned MWNT arrays, 2) The use of Single Walled Carbon Nanotubes (SWNTs) as nano detoxifiers and 3) the design of SWNTs for efficient cellular uptake. Generally, vertically aligned nanoneedles have been used to influence the behavior and differentiation of various cell types. In the first work described in chapter 2, periodic high-density array MWNT nanoneedles is shown to support cell growth and penetrate into HeLa cells, making it ideal for use in cellular imaging and the efficient delivery of plasmid DNA into cells. Most importantly, we show that transfection with the MWNT substrate exhibited more uniformity in comparison to the commercially available lipofection procedure. Lipofection involves the formation of a complex of DNA and cationic lipids that interact with the cell via electrostatic interactions, leading to internalization, DNA escape into the cytosol, and the eventual transport into the nucleus. Functionalized CNTS have demonstrated great biocompatibility and potential for drug delivery in vitro. In the work described in chapter 3, we synthesized acid-oxidized and non-covalently PEGlyated SWNTs, which were reported previously for drug delivery purposes, and explored their potential for detoxification in the bloodstream. We investigated the binding of SWNTs to a pore-forming toxin pyolysin. The SWNTs were found to prevent toxin-induced pore formation in the cell membrane of human red blood cells. Quantitative hemolysis assay and scanning electron microscopy were used to evaluate the inhibition of hemolytic activity of pyolysin. Unlike HeLa cells, human red blood cells did not internalize oxidized SWNTs according to Raman spectroscopy data. Molecular modeling and circular dichroism measurements were used to predict the 3D structure of pyolysin (domain 4) and its interaction with SWNTs. The Tryptophan-rich hydrophobic motif in the membrane-binding domain of pyolysin, a common construct in a large family of cholesterol-dependent cytolysins (CDCs), showed high affinity for SWNTs. In the final two chapters, chapters 4 and 5, we focused on shorter CNTs (<70 nm) that have less length variations. This enabled the determination of several length related characteristics such as cellular uptake and distribution of SWNTs within between cells. Here, cellular uptake of two water-soluble SWNTs, Short SWNTs (S_SWNTs) and Ultra-Short SWNTs (US_SWNTs), was evaluated against various mammalian cells. Cellular entry of S_SWNTs (chapter 4), similar in dimensions to those reported in the literature, is shown to be affected by their hydrophilic corona and exhibit time-dependent nuclear accumulation. In contrast, US_SWNTs show no dependence of cellular entry on their hydrophilic exterior (chapter 5). Furthermore, intracellular localization and excretion of the US_SWNTs is observed to be cell type-dependent. Results presented in this work show the potential of CNTs as nano detoxifiers. We also use CNTs as vertically aligned nanoneedles and as colloids to efficiently traverse the plasma membrane. While CNTs as nanoneedles show the potential as an efficient means of transfecting mammalian cells, the use of S_SWNTs and US_SWNTs highlight some key observations including the physical and chemical properties (size, surface functionality) and cell type influence on cellular uptake and intracellular trafficking. These findings contribute to the interpretation of SWNT-cell interactions by providing a correlation between CNT length and cellular uptake and also cell type on trafficking of internalized SWNTs. With the realization of the enhanced permeability and retention effects, tumor vascular leakiness resulting from increased angiogenesis and vasoactive factors enhancing permeability at the diseased site, nanoparticles that have long circulation time have higher chance of accumulating at the diseased sites.

This thesis is focused on the high-frequency performance of carbon nanotube field-effect transistors (CNFETs). Such transistors show their promising performance in the nanoscale regime where quantum mechanics dominates. The short-circuit, common-source, unity-current-gain frequency ft is analyzed through regional signal-delay theory. An energy-dependent effective-mass feature has been added to an existing SP solver and used to compare with results from a constant-effective-mass SP solver. At high drain bias, where electron energies considerably higher than the edge of the first conduction sub-band may be encountered, ft for CNFETs is significantly reduced with respect to predictions using a constant effective mass. The opinion that the band-structure-determined velocity limits the high-frequency performance has been reinforced by performing simulations for p-i-n and n-i-n CNFETs. This necessitated incorporating band-to-band tunneling into the SP solver. Finally, to help put the results from different CNFETs into perspective, a meaningful comparison between CNFETs with doped-contacts and metallic contacts has been made. Band-to-band tunneling, which is a characteristic feature of p-i-n CNFETs, can also occur in n-i-n CNFETs, and it reduces the ft dramatically.

This thesis is focused on the high-frequency performance of carbon nanotube field-effect transistors (CNFETs). Such transistors show their promising performance in the nanoscale regime where quantum mechanics dominates. The short-circuit, common-source, unity-current-gain frequency ft is analyzed through regional signal-delay theory. An energy-dependent effective-mass feature has been added to an existing SP solver and used to compare with results from a constant-effective-mass SP solver. At high drain bias, where electron energies considerably higher than the edge of the first conduction sub-band may be encountered, ft for CNFETs is significantly reduced with respect to predictions using a constant effective mass. The opinion that the band-structure-determined velocity limits the high-frequency performance has been reinforced by performing simulations for p-i-n and n-i-n CNFETs. This necessitated incorporating band-to-band tunneling into the SP solver. Finally, to help put the results from different CNFETs into perspective, a meaningful comparison between CNFETs with doped-contacts and metallic contacts has been made. Band-to-band tunneling, which is a characteristic feature of p-i-n CNFETs, can also occur in n-i-n CNFETs, and it reduces the ft dramatically. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Different carbon nanostructures have different properties and different applications. It is needed to synthesize good quality and also on large scale. From the point of industrial applications, highly productive and low cost synthesis method is very essential. Research has been done extensively on the intrinsic and individual properties of both single walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWC-NTs) in the range of nanometer to micrometer length scale. The important question is how the properties change beyond this length scale and if they are used in group in the form of an array instead of the individual carbon nanotubes (CNTs). Some applications require large current output, large energy production etc. For such kind of applications, it becomes essential to use CNTs in large number in the form of arrays or array, instead of using large numbers of CNTs in individual level. Future nanotechnology scope requires large scale application using the very rich intrinsic properties of the CNTs and nanomaterials. Keeping these problems and challenges in front, this thesis work is devoted to the research of the large scale synthesis of mm long MWCNTs, having different morphology and studies on various physical properties of MWCNTs in the form of arrays. Synthesis of mm long aligned and buckled MWCNTs have been reported for the first time. Generally buckled CNTs were obtained by compressing the straight CNTs. Apart from this, different morphologies like, aligned straight, helical or coiled CNTs are also synthesized. Resistance of the individual CNT increases with the increase in length. Resistance versus length of an array of CNT also shows similar behaviour. The thermal conductivity of CNT array is observed to decrease with the increase of array diameter (diameter �100 µm). There are few reports of the similar behaviour with the experiments done on small diameter CNT arrays (diameter �100 nm). From these observations, it seems that in the arrays of CNT, their intrinsic individual property is preserved though the magnitudes are different. The conductance measurements done on buckled CNT array by compressing it to apply uniaxial strain, shows the conductance oscillation. This conductance oscillation seems to be originating from the band gap change due to strain when the CNTs bend during compression. Recent research focuses on the arrays of CNT as they can carry large current of the order of several milliamperes that make the arrays suitable in nanoscale electronics and in controlling macroscopic devices such as light emitting diodes and electromotors. Regarding this aspect, a part of this thesis work is devoted on the application of CNT array to field effect transistor (FET) and study of thermoelectric power generation using CNT arrays. The entire thesis is based on the works discussed above. It has been organized as follows: Chapter 1 deals with introduction about the different carbon nanostructures and different synthesis methods. A brief introduction about the different current-voltage (IV) characteristics of SWCNTs and MWCNTs, length and diameter dependence and effect of the mode of contacts, are given. Some applications of the array of CNTs like buckling effect on compression, stretching of CNT into the form of rope, and conduction change on compression are discussed. Application of CNT as FET, as a thermometer, and thermoelectric effect of CNT are discussed. The electromechanical effect of CNT is also discussed briefly. Chapter 2 deals with experimental setup for synthesis of different morphologies of carbon nanostructures. The samples are characterized using common characterization techniques like, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. A brief introduction about Raman Spectroscopy of CNT is given. Chapter 3 reports the unusual IV characteristics and breakdown of long CNT arrays. The current carrying ability and the threshold voltage as a function of array diameter are reported. The effect of the ambient like temperature and pressure are discussed. Chapter 4 deals with theoretical models to analyze the IV characteristics reported in Chapter 3. It has been shown that a set of classical equations are applicable to quantum structures and the band gap can be evaluated. Chapter 5 describes with application of CNT arrays as temperature sensors. It has been shown that CNT arrays of suitable diameters are used as temperature sensors after calibration. Chapter 6 reports the high current FET application of CNT arrays. Effects of temperature and ambient pressure are discussed. The type of the majority charge carrier is determined. Chapter 7 deals with application of CNT arrays as thermoelectric power generator to get large thermoelectric current. Effects of different array diameter are discussed. Modulation of thermoemf with gate voltage is discussed. The type of the majority charge carrier is determined. Chapter 8 reports the effect of compressive strain on buckled MWCNT arrays. Conductance is measured during the compression of the array. Quantum electromechanical conductance oscillation is observed. The structural changes are observed with SEM. Raman spectroscopic study supports the explanation of the effect. Chapter 9 provides the conclusion and overall summary of the thesis.

In this research work, the relationships between structure and properties in micro-composites and nano-composites of polyurethane (PU) and conductive carbon particles have been studied. PU is a class of block copolymers containing the urethane linkage (-NHCO-O-) within its structure. Most PU block copolymers consist of alternating ‘soft’ and ‘hard’ segments. The hard segment used in this study was based on 4,4’-methylenebisphenylisocyanate (MDI) and 2-methyl 1,3 propanediol (MP-Diol) which produced a stiff aromatic polyurethane. Two soft segments; poly(tetrahydofuran) (PTHF) and poly(propylene oxide) based polyol end-capped with ethylene oxide (PPO-EO) were used to study the effects of soft segment structure on PU properties. DMTA, DSC and modulated-DSC indicated that PU-PTHF had higher microphase separation due to greater immiscibility between PTHF and the MDI/MP-Diol hard segments. In order to improve the electrical and mechanical properties of PU, conductive carbon particles were incorporated. The critical factor was the dispersion of these conductive fillers in the PU matrix to obtain optimum properties. The first carbon filler studied was carbon black (CB). PU composites prepared by the adding of MP-Diol plus ultrasonication (MU) gave the best dispersion of CB aggregates resulting in higher thermal decomposition temperature and good conductivity. However, the mechanical toughness was reduced. In subsequent studies, PU composites incorporating three different treated multiwalled carbon nanotubes (MWCNT) were investigated. MWCNT were disentangled and shortened by ultrasonication and acid cutting treatments. The ultrasonicated MWCNT (MWCNT_U) had longer length than the acid-cut MWCNT (MWCNT_AC). Ultrasonication was the best technique for dispersing MWCNT since the storage modulus was increased by ~200% at low MWCNT_U loading and the toughness remained the same as unfilled PU. PU/MWCNT_AC nanocomposites at 1 – 3 wt% of MWCNT_AC exhibited similar electrical conductivities to unfilled PU at an order of 10-8 S/cm, implying that the acid cutting treatment might disturb the inherent conductivity in MWCNT. The conductive percolation thresholds of composites were determined following the percolation theory. It was found that the percolation thresholds for MWCNT-filled composites were significantly lower than that of CB-filled composites. The lowest percolation threshold was observed in MWCNT_U-filled composite at 0.31 wt%.

Polymer nanocomposites have been proposed as lightweight replacements for traditional composite materials in various applications. Montmorillonite (MMT) and carbon nanotubes (CNTs) are two nanofillers which have accrued significant interest in the past 20 years due to their superior mechanical and electrical properties, respectively. However, efficient dispersion of the nanofiller and damage to CNTs prevent widespread utilization of these materials in polymer nanocomposites. Novel methods of nanocomposite generation combining the use of supercritical carbon dioxide (scCO2) with melt compounding have been investigated to overcome these issues. The focus of this work is on developing the scCO2 treatment of nanomaterial for thermoplastic nanocomposite generation. First, the supercritical carbon dioxide aided melt blending method was applied to nanoclay nanocomposites of Nylon 6/ and organoclay where the polymer may interact with the nanoclay surface. Second, the effect of scCO2 processing of CNTs was investigated with special consideration to the processing variables. Finally, a study was carried out to analyze the electrical conductivity of polycarbonate nanocomposites generated using CNTs deagglomerated by scCO2 processing. The initial focus of this dissertation is the use of supercritical carbon dioxide (scCO2) as a processing aid in the generation of nylon 6 nanocomposites in which the nylon 6 may interact with the nanoclay surface. Wide-angle X-ray diffraction, transmission electron microscopy, rheology, and tensile tests were carried out to investigate the effect of processing with scCO2 on the final composite morphology and properties. It was observed that mechanical properties of composites prepared with the scCO2 aided melt blending method were similar to or higher than those reported in the literature for samples prepared with twin screw compounding. At 7.6 wt% nanoclay the modulus value reaches 4.75 +/- 0.194 GPa which is one of the highest increases (1.7 GPa) reported for these materials processed at intermediate concentrations. Beyond 7.6 wt% the improvement due to scCO2 processing matched that of direct blending. The next objective of this work is to develop a method for the deagglomeration of commercially available multi-walled carbon nanotubes (MWCNTs) by manipulating processing variables and observing carbon nanotube aspect ratios after deagglomeration. High levels of deagglomeration of Baytubes C 150 P and Nanocyl NC-7000 MWCNT agglomerates were observed, resulting in 30 fold and 50 fold decreases in bulk density, respectively, with median agglomerate sizes < 8 um in diameter. These results were obtained while retaining the aspect ratio of the as-received nanomaterial, irrespective of the MWCNT agglomerate morphology. It was found that the supercritical temperature and pressure of 40 deg C and 7.86 MPa were the optimal temperature and pressure for maximum deagglomeration without damaging the MWCNTs. The final goal of this work is to apply the scCO2 aided melt blending process to generate polycarbonate/ carbon nanotube (CNT) nanocomposites with enhanced electrical conductivity and improved dispersion while maintaining the aspect ratio of the as-received CNTs. Different degrees of scCO2 processed Baytubes C 150 P CNT were benignly deagglomerated with scCO2 resulting in 5 fold (5X), 10X, and 15X decreases in bulk density from the as-received CNTs. The CNT were melt compounded with polycarbonate using single screw melt extrusion and compression molded into plaques. A surface conductivity of 4.8 x 10-8 +/- 2.0 x 10-9 S was observed for samples prepared with the scCO2 aided melt blending at 15X scCO2 processing. Electrical percolation thresholds as low as 0.83 wt% were observed for composites prepared with 15X CNTs using the scCO2 aided melt blending method, while concentrations as high as 1.5% are required without scCO2 processing. The percolation concentration in nanocomposites prepared with 15X scCO2 processing and single screw extrusion is competitive with values reported for similar nanocomposites generated using twin screw melt compounding in the literature. Optical microscopy, transmission electron microscopy, and rheology were used to investigate the dispersion and mechanical network of CNTs in the nanocomposites. The dispersion of CNTs generally improved with scCO2 processing compared to direct melt blending but was found to be significantly worse than that of twin screw melt compounded nanocomposites from the literature. Because the percolation thresholds are similar with substantially different extents of dispersion, the importance of maintaining longer CNTs during nanocomposite generation is emphasized. / Ph. D.