Microwave Enabled Dispersion Of Highly Conductive Carbon Nanomaterials And Their Interfacial Assemblies

Due to its phenomenal mechanical characteristics and remarkable electrical properties, graphene, a perfect single-atomic thick two-dimensional lattice carbon layer, has attracted extensive attention in nanoscience and condensed matter physics. With all the similarities, it is believed that graphene can compete with or even surpass carbon nanotubes in many fields, and it is expected to replace silicon in many electronic applications and in other advanced technologies. A single layer of graphene sheet was first isolated in 2004 from highly oriented pyrolysis graphite with Scotch tape. The invention of "The Scotch-tape" method seems very simple, and it has enabled a whole new path in many graphene-based research areas. It also resulted in Andre Geim and Konstantin Novoselov's winning the 2010 Nobel Prize in physics. This solvent-free method however suffers from low yields, low repeatability, and being extremely labor intensive. Solution-based fabrications have shown to be able to overcome these problems. However, the next challenge in the graphene research field and applications is the tedious chemical path that is required to convert oxidized graphene using toxic chemicals, such as hydrazine. In this thesis, we first developed a novel and an unprecedentedly fast and simple approach to directly exfoliate graphite flakes with the aid of both nitronium ion and microwave irradiation with the aim of solving the main research problems in the field. To utilize the produced graphene in practical applications, our knowledge of interfacial science was exploited to controllably self-assemble these wonderful materials into desired structures. The research results combined with an introduction of the development and future aspects of these fields will be presented in the five chapters of this thesis. Chapter 1 will include a general overview of basic but important information concerning the two main carbon-based materials, carbon nanotubes and graphene. Their structures, physical properties, methods of fabrications and applications will be discussed in depth. In addition, interfacial science for self-assembly of nanomaterials will be summarized. In Chapter 2, an efficient, simple and promising way to prepare graphene sheets directly from graphite flakes with the aid of nitronium ions and microwave irradiation will be presented. Knowledge of the chemistries related to nitronium ions and microwave has enabled us to purposely omit strong oxidants, such as KMnO4, with an aim not to heavily oxidize the materials, as many methods are based on, thus reduction reactions can be completely avoided. Experimental results demonstrate that this non-destructive method resulted in concentrated stable dispersions of flat, high-quality, conductive graphene sheets in both aqueous and organic solvents. This mildly oxidized material was extensively characterized by atomic force microscope (AFM), Infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy, thermo-gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM). In chapter 3, we extended the nitronium ions and microwave enabled dispersed approach to carbon nanotubes. Different sources of both single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) were tested and the results showed that all the CNTs from different sources can be quickly dispersed into aqueous solutions with remarkably high concentrations compared to those of graphene solutions even though the same parameters were applied during dispersion. We found that depending on the existence of a small amount of defects from the original CNT sources, the yield, and quality of the dispersed CNTs are varied. With a long term aim of fabricating highly transparent and conductive films to replace Indium tin oxide (ITO) in a wide variety of optoelectronic devices, in Chapter 4, a new method referred to as an interfacial self-assembly approach is developed to assemble the microwave dispersed graphene and CNTs into highly conductive films. The self-assembly behavior of graphene, CNT, and a mixture of graphene and CNT with different ratios were studied separately, and the knowledge obtained was used to fabricate graphene, CNT, and a hybrid of graphene-CNT thin films at an oil/water interface, respectively. Compared to the generally used vacuum filtration method, this new approach does not need any membrane, thus theoretically any size film can be easily fabricated. To transfer the formed films to substrates for practical applications, a simple film-transferring method was also developed. The films fabricated with different film fabrication methods will also be compared and a systematic study on how the compositions of these two materials affect the performance of the final films will be summarized. The dispersed graphene sheets are often composed with graphene sheets of different sizes, to separate them for different applications. In Chapter 5, interfacial self-assembly reactions were also applied to separate the graphene sheets based on their size-and electronic-dependent surface energies Chapter 6 will then focus on fine-tuning the surface chemistry of the graphene sheets and the oil/water ratio to efficiently emulsify the graphene sheets into core-shell capsules for drug delivery applications. Poly(N-isopropylacrylamide) (PNIPAA), a thermally sensitive polymer is introduced to form a temperature-sensitive and stable oil-in-water microemulsion with the ability to release the encapsulated materials in a graphene/PNIPAA shell above its transition temperature. Experimental observations show that the emulsion with graphene has a slightly increased transitional temperature from 34 °C to 38 °C.

Carbon nanotubes (CNTs) and graphene are the two most conductive members among carbon nanomaterials. For industrial applications, these nanomaterials are attracting great attention for fabrication of flexible conducting films. However, the electronic performance of either CNT or graphene film has yet to reach their theoretical expectations due to high resistance and tunneling/Schottky barriers at the junctions between nanotubes or between graphene sheets. One of the important observations was that CNTs and graphene sheets can be crosslinked during and/or after film fabrication, which largely decrease inter-tube or inter-sheet resistance. However, the current solution-processing techniques for the film fabrication, such as spin coating, layer-by-layer assembly, and vacuum filtration have disadvantages and limitations. In this thesis, we developed an efficient film assembly approach as well as a facile transfer process. The first chapter of this thesis provides an overview on structure and properties of CNTs and graphene. In the second chapter, we used our newly developed microwave-enabled dispersion technique to synthesize highly conductive dispersible CNTs and graphene with low-density of oxygen-containing groups, without a need of surfactant/stabilizer. As we fabricated Microwave-enabled low-oxygen multi-walled nanotube only (ME-LOMWNT-only), Microwave-enabled low-oxygen graphene only (ME-LOGr-only), and ME-LOMWNT/ME-LOGr hybrid films using vacuum filtration, we found that the hybrid films are highly conductive relative to either the ME-LOMWNT-only or ME-LOGr-only film. The conductivity of the hybrid films depends on their composition, where a weight ratio of 97/3 between MWNTs and graphene reached the highest conductivity of 247,812 S m-1, which is two times higher than those of SWNT/graphene hybrid films reported by Coleman et al. 8 In this work, we found crosslinks between MWNTs and graphene, which could be further promoted in acidic environment. These crosslinks between MWCNT and graphene enhanced the film conductivity. The aim of the third chapter was to fabricate high quality graphene films and MWNT/graphene hybrid films using interfacial self-assembly approach. We observed the different assembly behavior of ME-LOMWNT and ME-LOGr due to their different shape and surface energy. Then, we optimized the parameters to fabricate high quality of ME-LOMWNT/ME-LOGr hybrid films. Moreover, we developed an efficient approach to transfer the self-assembled film at this water/oil interface onto substrates for future electrical characterization and device fabrications.

Carbon is the most abundant material next to oxygen in terms of sustainability. The potential of carbon based materials has been recognized in recent decades by the discovery of fullerene (1996 Nobel prize in chemistry), carbon nanotubes (2008 Kavli prize in nanoscience) and graphene (2010 Nobel prize in physics). The synthesis of carbon materials with well controlled morphologies lead to their exploration in both fundamental research and industrial applications. Graphene also commonly referred to as a wonder material has been under extensive research for more than a decade, due to its excellent electronic, optical, thermal and mechanical properties. However, the realization of these applications for practical purposes require its large scale synthesis. The common method of graphene synthesis involves reduction of graphene oxide. Nevertheless, complete restoration of intact graphene basal plane destroyed by oxidation cannot be achieved, limiting the application of as synthesized graphene in flexible macro electronics, mechanically and electronically reinforced composites etc. Hence, research was pursued in regards to achieve controlled oxidation, sufficient enough to overcome the Vander-Waals forces and preserving the graphene domains. One such approach reported by our group is the solution processable graphene achieved via controlled oxidation, by the use of nitronium oxidation approach. However, toxic NOx gases and byproducts generated during the synthesis, limits the scalability of this approach. In this thesis, for the first time, we reported the synergy of piranha etching solution with intercalated graphite for the controlled oxidation of graphite particles via microwave heating in chapter 2. The controlled oxidation leads to rapid (60 seconds) and direct generation of highly conductive, clean low oxygen containing graphene sheets without releasing any detectable toxic gases or aromatic by-products as demonstrated by gas chromatography-mass spectrometry. These highly conductive graphene sheets have unique molecular structures, different from both graphene oxide and pristine graphene sheets. They can be dispersed in both aqueous and common organic solvents without surfactants/stabilizers producing "clean" graphene sheets in solution phase. "Paper-like" graphene films are generated via simple filtration resulting in films with a conductivity of 2.26 × 104 S m-1, the highest conductivity observed for graphene films assembled via vacuum filtration from solution processable graphene sheets to date. After 2-hour low temperature annealing at 300 C, the conductivity further increased to 7.44 × 104 S m-1. This eco-friendly and rapid approach for scalable production of highly conductive and "clean" solution-phase graphene sheets would enable a broad spectrum of applications at low cost. Irrespective of the vast applications of highly conductive graphene, it exhibits limited catalytic centers, is impervious, and limits the diffusion of ions. This inadequacy can be overcome by the hole generation on highly conductive graphene. Current approaches for large scale production of holey graphene require graphene oxide (GO) or reduced GO (rGO) as starting materials. Thus generated holey graphene derivatives still contain a large number of defects on their basal planes, which not only complicates fundamental studies, but also influences certain practical applications due to their largely decreased conductivity, thermal and chemical stability. This work reports a novel scalable approach exploiting the wireless joule heating mechanism provided by microwave irradiation of partially oxidized graphite intercalation compounds in chapter 3. The wireless joule heating mechanism affords region-selective heating, which not only enable fabrication of holey graphene materials with their basal plane nearly intact, but also engineers the edges associated with holes to be rich in zigzag geometry. The term pristine holey graphene was given, to differentiate from the holey graphene derivatives with basal plane defects, as reported in the literature. The pristine holey graphene with zigzag edges were studied and explored as a metal free catalyst for reduction reactions via hydrogen atom transfer mechanism. The pristine holey graphene nanoplatelets not only exhibited high catalytic activity and desired selectivity, but also provided excellent chemical stability for recyclability, which is very different from its counterpart holey graphene derivatives with basal plane defects. It was also reported that the reduction of nitrobenzene occurs via condensation pathway with this catalyst. To further provide insight into combustion of graphite in air with microwave irradiation, the stabilized intercalated graphene without point defects was used to generate holes in chapter 4. The co-intercalated O2 into graphite intercalated compound act as the internal oxidant, to oxidize the carbon, along with the surrounding air. High local temperatures were achieved via joule heating mechanism, hence promoting combustion of graphene to generate holes and edges. We observed that in combination to hole generation, higher conductivity was also observed in comparison to the holey graphene synthesized in chapter 3. The highly conductive holey graphene was tested for their electro-catalytic activity in the reduction of oxygen. The reduction of oxygen occurs via 2e- pathway, where peroxide with 90% yield was recorded. This opens path for onsite peroxide production in alkaline media, and therefore allowing its use in bleaching industries. In concern of carbon based materials being explored for catalysis, their high amount to facilitate the reaction, limits practicality of the catalyst for industrial applications. However, the immobilization of metal nanoparticles onto porous carbon supports, synthesized from sustainable and cheap biomass was widely pursued. It was widely reported that the doping of carbon support with N further improved their interaction with the metal and promoted higher catalytic activity. In chapter 5, for the first time, the influence of P doped carbon support on catalytic activity of Pd was reported. A single step microwave assisted fabrication of Pd embedded into porous phosphorous doped graphene like carbon was demonstrated. Structural characterization revealed that, the metal nanoparticles are in the range of 10nm with a surface area of 1133m2/g. The developed method is not only sustainable as it is synthesized from biomass and anti-nutrient molecule (phytic acid), but also energy efficient as microwave irradiation (50sec) is used for the catalyst synthesis. The as synthesized catalyst recorded 90% conversion with a TOF of 23000h-1 for benzyl alcohol oxidation, which remained constant even after 8 recycles indicating the stability of catalyst. Different wt% of Pd onto PGC was tested for their alcohol oxidation capacity and found that the 3% Pd-PGc which activates O2 more towards 4e- in ORR has the best conversion and selectivity. The biomass molecule phytic acid used for the synthesis of phosphorous doped carbon support was also used as a phosphorous source in the synthesis of tin phosphides in chapter 6. Current studies have shown that sodium, a low cost and naturally abundant metal, can act as a substituent for lithium in lithium ion batteries (LIB), hence, allowing their applications in real world. This transition towards the use of sodium ion batteries (SIB) has entailed research to improve the cycle stability and energy density of battery by introducing tin phosphides as anodes for batteries. Tin phosphides exhibit a self-healing mechanism, hence decreases the capacity decay as observed in the case of Sn metal. However, it was reported that the self-healing mechanism is not completely reversible with partial pulverization observed. Therefore, we pursued a time efficient method to synthesize tin phosphide in a phosphorous doped carbon matrix (SnP@PGc) via microwave irradiation. The SnP@PGc formed when tested as anode for SIBs, demonstrated superior capacity of 515 mAh/g after 750 cycles at a charge and discharge current of 0.2 C. The superior cycle stability can be attributed to the protection against volume expansion by phosphorous doped porous carbon shell during battery charge and discharge process and hence mitigating the pulverization of tin phosphides.

As the integration scale of transistors/devices in a chip/system keeps increasing, effective cooling has become more and more important in microelectronics. To address the thermal dissipation issue, one important solution is to develop thermal interface materials with higher performance. Carbon nanotubes, given their high intrinsic thermal and mechanical properties, and their high thermal and chemical stabilities, have received extensive attention from both academia and industry as a candidate for high-performance thermal interface materials.\r : The thesis is devoted to addressing some challenges related to the potential application of carbon nanotubes as thermal interface materials in microelectronics. These challenges include: 1) controlled synthesis of vertically aligned carbon nanotubes on various bulk substrates via chemical vapor deposition and the fundamental understanding involved; 2) development of a scalable annealing process to improve the intrinsic properties of synthesized carbon nanotubes; 3) development of a state-of-art assembling process to effectively implement high-quality vertically aligned carbon nanotubes into a flip-chip assembly; 4) a reliable thermal measurement of intrinsic thermal transport property of vertically aligned carbon nanotube films; 5) improvement of interfacial thermal transport between carbon nanotubes and other materials.\r : The major achievements are summarized.\r : 1. Based on the fundamental understanding of catalytic chemical vapor deposition processes and the growth mechanism of carbon nanotube, fast synthesis of high-quality vertically aligned carbon nanotubes on various bulk substrates (e.g., copper, quartz, silicon, aluminum oxide, etc.) has been successfully achieved. The synthesis of vertically aligned carbon nanotubes on the bulk copper substrate by the thermal chemical vapor deposition process has set a world record. In order to functionalize the synthesized carbon nanotubes while maintaining their good vertical alignment, an in situ functionalization process has for the first time been demonstrated. The in situ functionalization renders the vertically aligned carbon nanotubes a proper chemical reactivity for forming chemical bonding with other substrate materials such as gold and silicon.\r : 2. An ultrafast microwave annealing process has been developed to reduce the defect density in vertically aligned carbon nanotubes. Raman and thermogravimetric analyses have shown a distinct defect reduction in the CNTs annealed in microwave for 3 min. Fibers spun from the as-annealed CNTs, in comparison with those from the pristine CNTs, show increases of ~35% and ~65%, respectively, in tensile strength (~0.8 GPa) and modulus (~90 GPa) during tensile testing; an ~20% improvement in electrical conductivity (~80000 S m−1) was also reported. The mechanism of the microwave response of CNTs was discussed. Such an microwave annealing process has been extended to the preparation of reduced graphene oxide.\r : 3. Based on the fundamental understanding of interfacial thermal transport and surface chemistry of metals and carbon nanotubes, two major transfer/assembling processes have been developed: molecular bonding and metal bonding. Effective improvement of the interfacial thermal transport has been achieved by the interfacial bonding.\r : 4. The thermal diffusivity of vertically aligned carbon nanotube (VACNT, multi-walled) films was measured by a laser flash technique, and shown to be ~30 mm2 s−1 along the tube-alignment direction. The calculated thermal conductivities of the VACNT film and the individual CNTs are ~27 and ~540 W m−1 K−1, respectively. The technique was verified to be reliable although a proper sampling procedure is critical. A systematic parametric study of the effects of defects, buckling, tip-to-tip contacts, packing density, and tube-tube interaction on the thermal diffusivity was carried out. Defects and buckling decreased the thermal diffusivity dramatically. An increased packing density was beneficial in increasing the collective thermal conductivity of the VACNT film; however, the increased tube-tube interaction in dense VACNT films decreased the thermal conductivity of the individual CNTs. The tip-to-tip contact resistance was shown to be ~1×10−7 m2 K W−1. The study will shed light on the potential application of VACNTs as thermal interface materials in microelectronic packaging.\r : 5. A combined process of in situ functionalization and microwave curing has been developed to effective enhance the interface between carbon nanotubes and the epoxy matrix. Effective medium theory has been used to analyze the interfacial thermal resistance between carbon nanotubes and polymer matrix, and that between graphite nanoplatlets and polymer matrix.

Chemically-modified carbon nanotubes (CNTs) exhibit a wide range of physical and chemical properties which makes them an attractive starting material for the preparation of super-strong and highly-conductive fibres and films. Much information is available across the primary literature, making it difficult to obtain an overall picture of the state-of-the-art. This volume brings together some of the leading researchers in the field from across the globe to present the potential these materials have, not only in developing and characterising novel materials but also the devices which can be fabricated from them. Topics featured in the book include Raman characterisation, industrial polymer materials, actuators and sensors and polymer reinforcement, with chapters prepared by highly-cited authors from across the globe. A valuable handbook for any academic or industrial laboratory, this book will appeal to newcomers to the field and established researchers alike.

Beginning with a general overview of nanocomposites, Bionanocomposites: Integrating Biological Processes for Bio-inspired Nanotechnologies details the systems available in nature (nucleic acids, proteins, carbohydrates, lipids) that can be integrated within suitable inorganic matrices for specific applications. Describing the relationship between architecture, hierarchy and function, this book aims at pointing out how bio-systems can be key components of nanocomposites. The text then reviews the design principles, structures, functions and applications of bionanocomposites. It also includes a section presenting related technical methods to help readers identify and understand the most widely used analytical tools such as mass spectrometry, calorimetry, and impedance spectroscopy, among others.

Carbon Nanotube-Reinforced Polymers: From Nanoscale to Macroscale addresses the advances in nanotechnology that have led to the development of a new class of composite materials known as CNT-reinforced polymers. The low density and high aspect ratio, together with their exceptional mechanical, electrical and thermal properties, render carbon nanotubes as a good reinforcing agent for composites. In addition, these simulation and modeling techniques play a significant role in characterizing their properties and understanding their mechanical behavior, and are thus discussed and demonstrated in this comprehensive book that presents the state-of-the-art research in the field of modeling, characterization and processing. The book separates the theoretical studies on the mechanical properties of CNTs and their composites into atomistic modeling and continuum mechanics-based approaches, including both analytical and numerical ones, along with multi-scale modeling techniques. Different efforts have been done in this field to address the mechanical behavior of isolated CNTs and their composites by numerous researchers, signaling that this area of study is ongoing. Explains modeling approaches to carbon nanotubes, together with their application, strengths and limitations Outlines the properties of different carbon nanotube-based composites, exploring how they are used in the mechanical and structural components Analyzes the behavior of carbon nanotube-based composites in different conditions

Since their discovery in 1977, the evolution of conducting polymers has revolutionized modern science and technology. These polymers enjoy a special status in the area of materials science yet they are not as popular among young readers or common people when compared to other materials like metals, paper, plastics, rubber, textiles, ceramics and composites like concrete. Most importantly, much of the available literature in the form of papers, specific review articles and books is targeted either at advanced readers (scientists / technologists / engineers / senior academicians) or for those who are already familiar with the topic (doctoral / postdoctoral scholars). For a beginner or even school / college students, such compilations are bit difficult to access / digest. In fact, they need proper introduction to the topic of conducting polymers including their discovery, preparation, properties, applications and societal impact, using suitable examples and already known principles/knowledge/phenomenon. Further, active participation of readers in terms of "question & answers", "fill-in-the-blanks", "numerical" along with suitable answer key is necessary to maintain the interest and to initiate the "thought process". The readers also need to know about the drawbacks and any hazards of such materials. Therefore, I believe that a comprehensive source on the science / technology of conducting polymers which maintains a link between grass root fundamentals and state-of-the-art R&D is still missing from the open literature.

With the phenomenal development of electromagnetic wave communication devices and stealth technology, electromagnetic wave absorbing materials have been attracting attention as antielectromagnetic interference slabs, stealth materials, self-concealing technology, and microwave darkrooms. This book starts with the fundamental theory of electromagnetic wave absorption in loss medium space, followed by a discussion of different microwave absorbents, such as manganese dioxide, iron-based composite powder, conductive polyaniline, barium titanate powder, and manganese nitride. Then, structural absorbing materials are explored, including multilayer materials, new discrete absorbers, microwave absorption coatings, cement-based materials, and structural pyramid materials. Many of the graphics demonstrate not only the principles of physics and experimental results but also the methodology of computing. The book will be useful for graduate students of materials science and engineering, physics, chemistry, and electrical and electronic engineering; researchers in the fields of electromagnetic functional materials and nanoscience; and engineers in the fields of electromagnetic compatibility and stealth design.

Since their discovery more than a decade ago, carbon nanotubes (CNTs) have held scientists and engineers in captive fascination, seated on the verge of enormous breakthroughs in areas such as medicine, electronics, and materials science, to name but a few. Taking a broad look at CNTs and the tools used to study them, Carbon Nanotubes: Properties and Applications comprises the efforts of leading nanotube researchers led by Michael O’Connell, protégé of the late father of nanotechnology, Richard Smalley. Each chapter is a self-contained treatise on various aspects of CNT synthesis, characterization, modification, and applications. The book opens with a general introduction to the basic characteristics and the history of CNTs, followed by discussions on synthesis methods and the growth of “peapod” structures. Coverage then moves to electronic properties and band structures of single-wall nanotubes (SWNTs), magnetic properties, Raman spectroscopy of electronic and chemical behavior, and electromechanical properties and applications in NEMS (nanoelectromechanical systems). Turning to applications, the final sections of the book explore mechanical properties of SWNTs spun into fibers, sidewall functionalization in composites, and using SWNTs as tips for scanning probe microscopes. Taking a fresh look at this burgeoning field, Carbon Nanotubes: Properties and Applications points the way toward making CNTs commercially viable.