Carbon nanotubes : their synthesis and integration into nanofabricated structures

Abstract

The field of nanotechnology has experienced constantly increasing interest over the past decades both from industry and academy. Commonly used nanomaterials include: nanoparticles, nanowires, quantum dots, fullerenes, and carbon nanotubes. Carbon nanotubes, in particular, are promising building blocks for a large variety of potential applications. Because of their structure and high aspect ratio, nanotubes have unique electronic, chemical and mechanical properties. These properties attract much interest to the investigation of carbon nanotubes for potential applications in electronics devices, batteries, solar cells, gas storage technologies, and other fields. Topics addressed in this dissertation relate to the synthesis of carbon nanotubes and their integration into different structures, with particular focus on the basic problems of nanofabrication. Chapter 1 discusses the recent developments of the research activity in the field of post-synthesis placement of carbon nanotubes (CNTs) on substrates. This includes alignment guided by physical forces, external fields and chemical interactions. The usefulness of any given technique strongly depends on the desired application, while additional innovations for the further expansion of the post-synthesis alignment field need to be introduced. Chapter 2 introduces the microwave-assisted synthesis of one-dimensional carbon nanostructures. Selective heating of small iron nanoparticles, often used as a catalyst to initiate the growth of CNTs, was investigated under microwave irradiation. An important advantage of this approach is the fact that the heat development is limited to the close vicinity of the nanoparticles, while the average overall temperature in the reaction vial remains low, allowing the utilization of a diverse range of substrate materials. The approach to synthesize carbon nanofibers (CNFs) and nanotubes was adapted to the special requirements of the microwave apparatus and had to be optimized for safety. By using ethanol as a carbon source, provided by a liquid reservoir located beneath the sample, a flux of highly flammable and explosive gas mixtures was avoided. The reaction conditions for the microwave-assisted synthesis of carbon nanotubes and nanofibers were investigated in detail. These were observed to have a strong influence on the CNT/CNF formation and on the quality of the obtained materials. Further improvement of the quality and size of the synthesized materials was obtained by variation of the catalyst material. Nickel was identified as the most favorable catalyst material to obtain small nanotube diameters down to 15 nm using very short irradiation times of two minutes. Compensation of the heat dissipation, for substrates showing a low absorption of microwave irradiation (mica and quartz glass), resulted in reliable processes that enable the microwave-assisted growth of CNTs on a variety of substrates. It was demonstrated that the growth of individual CNTs can be achieved. In particular, the relatively low experimental effort, as well as the fast fabrication times, are general advantages of this method and provide a promising, cheap technique to fabricate CNT-modified AFM tips. The deposition of the catalyst material can be further improved by, e.g., utilizing particle picking approaches or by force versus distance curve recording, to further increase the controllability of the presented approach. Results show that small areas can be covered with a suitable catalyst layer with this method. This permits the growth of individual CNTs, as opposed to bundles, and has important implications for the effective integration of carbon nanomaterials into the framework of devices. Carbon nanotubes were successfully grown on micro- and nanoscale patterned areas. These findings are expected to have an additional impact on the use of the selective heating mechanism, as it provides advantages over conventional methods, i.e., the reduced reaction time, the lower overall exposure temperature to the substrates and for the integration of CNTs/CNFs into predefined device frameworks consisting of different materials. Chapter 3 gives a comprehensive overview of the electro-oxidation lithography on chemically active surfaces. This powerful technique can be used to organize nanomaterials into defined structures. The main advantage of this technique is the fact that it can manipulate and guide the position of catalyst particles, nanowires or other nanometer-scale objects that are required for the desired structures. Due to the fact that addressable functional groups are created during the electrochemical oxidation, it is possible to utilize the entire range of intermolecular interactions to modify the structures. These include electrostatic and van der Waals interactions, hydrogen bonding, covalent bonding and complexation reactions to selectively bind suitable building blocks. This approach can also be used for the post-synthesis organization of carbon nanotubes and, moreover, provides unique possibilities for the fabrication of nanomaterials. Chapter 4 discusses the post-synthesis assembly of carbon nanotubes of the pre-patterned structures. Stable suspensions of carbon nanotubes were prepared via several different approaches, including agitation in organic solvents or the use of surfactants. The latter yielded in stable suspensions of carbon nanotubes. Electro-chemical oxidation lithography was utilized for the placement of individual carbon nanotubes. The structuring of the n-octadecyltrichlorosilane (OTS) monolayer was repeated and, in a second oxidation process, new active binding sites were generated. This was followed by the sequential placement of CNTs onto chemically active surface templates created in the vicinity of the existing tube. Major advantages of this approach include good control over the lateral placement of the CNTs and the availability of addressable chemical functional surface templates. Furthermore, the possibility to preselect the self-assembling building blocks as well as the sequential nature of the patterning process are discussed, which are not easily accessible by conventional lithographic tools, i.e., photo- and e-beam lithography. This process provides the possibility to carefully select the tube material and to combine pre-defined building blocks, e.g. in transistor layouts. Thus, a powerful approach has been developed that allows control over the device layout at several length scales. Chapter 5 demonstrates two new concepts for the use of electro-oxidative lithography for the formation of nanoscale building blocks, e.g., nanometer gaps and metallic circles as shown in this work. The electro-chemical oxidation of monolayers and bilayers consisting of OTS was investigated in detail, including the different oxidation times required to perform the electrochemical oxidation on monolayer and bilayer systems. Thus, a new rational design to generate well-defined gap-structures was established. In particular, the fabrication of a nanometric gap structure and an approach to assemble a nanoelectronic-based device layout was developed. The second concept introduces a new fabrication method to obtain ring structures with nanometer dimensions. This method combines the silicon growth mode and the monolayer oxidation mode from the available electro-oxidation lithography techniques. The oxidation conditions, as well as the scaling options of this lithographic process were investigated and revealed good controllability of the feature dimensions. These structures were further functionalized with silver particles, thus, converting the structure into mesoscopic ring structures with sufficiently high uniformity and reproducible quality. These concepts can be used for the formation of nano-scale functional devices. In conclusion, new concepts have been developed to target different, challenging aspects of nanofabrication. This combines alternative synthesis strategies for carbon nanotubes and the implementation of these nanotubes into nanostructures. Electro-oxidative lithography was utilized as a chemical structuring tool to guide self-assembly processes of nanotubes and nanoparticles. Fundamental investigations on the oxidation conditions allowed a significant expansion of the applicability of this structuring technique and demonstrated the possibility to target different aspects of modern nanofabrication.