Synthesis, alignment, growth mechanism and functional properties of carbon nanotubes and their hybrid materials with inorganic and biomaterials

The present work comprises a novel method for selective growth of carbon nanotubes, study of their growth mechanism as well as synthesis and application of their various hybrid materials. An experimental setup is established to grow carbon nanotubes using water assisted chemical vapor deposition method. Various growth parameters were scrutinized carefully and a growth mechanism is put forth for the same method. A new methodology to prepare different hybrid materials of aligned carbon nanotubes together with nano sized metal oxides particles, biological nerve cell, noble metal nano particles is demonstrated. Growth of carbon nanotubes and study of its growth mechanism: Carbon nanotubes are synthesized using water assisted chemical vapor deposition method, in which water acts as a weak oxidant. CNTs are crystalline, aligned, double walled and impurity free. The standard growth rate obtained was 30 to 40 µm per minute and the growth rate, quality and typology of the CNTs can be fine tuned by controlling hydrogen flow rate, quantity of iron and aluminum deposited and their deposition method. The present study confirms the precise role of hydrogen and aluminum in WCVD growth mechanism. The process follows the root growth mechanism with bimetallic catalyst composed of iron and aluminum. The small quantity of water acts as weak oxidant, held responsible for keeping catalyst active for longer time. By employing advanced techniques such as EELS, tomography in transmission electron microscopy, catalyst nano particles were analyzed for their chemical composition. Each catalyst nano particle is made up of two phases; the central core made up of pure iron surrounded by intermixed phase of iron and aluminum. The two-phase catalyst was continuously hydrolyzed by supplied water vapor; confirmed by XRD and XPS measurements. Hydroxyl groups produced on surface of the catalyst utilized to selectively etch the amorphous carbon from the surface to keep the catalyst clean. Therefore, catalyst stays active for longer time leading the CNT growth up to millimeter long length. Solid state nuclear magnetic resonance spectroscopy of pristine CNTs show majority of CNTs´are double walled along with a small percentage of single walled carbon nanotubes. The shift to lower energy indicates most of the CNTs possess semiconducting characteristics. A split of 10 ppm was recorded for DWNTs consisting one metallic and one semi conducting tube. Smaller tube diameter or metallic nature of CNTs shift the peak position to higher ppm. Electronic applications of carbon nanotubes: Carbon nanotubes can be grown vertically or even horizontally as required. The horizontally grown carbon nanotubes were used to fabricate field effect transistors. CNTs are single walled, semiconducting, p-type in nature delivering a device with high (2000 to 5000) on off ratio. Transistor is sensitive enough to depict good modulation of current for small value of gate voltage; ideal to realize various sensors. Also, using alcohol-CVD method, growth of single CNT bridging two silicon pillars is demonstrated. The CNT bridging two Si-pillars will be studied to gas sensor applications. On the other hand, vertically grown CNT blocks have already been tested for room temperature gas sensing. Vertically grown structured blocks of aligned carbon nanotubes exhibit excellent field emission characteristics. Current emission in milliampere range, stable emission and constant enhancement factor for up and down cycle are salient features. CNT blocks with appropriate size and intermediate distance are proposed for cold cathode applications. Synthesis of metal oxide - CNT hybrid materials: A new methodology is developed to prepare hybrid materials of metal oxides and carbon nanotubes. Using various techniques, metal oxide nano particles or film are deposited on previously grown carbon nanotubes blocks or films. In the present study, the ordered structure of carbon nanotubes is modified with nano particles of titanium dioxide, zinc oxide and iron oxide. The average distribution of particle size is of order 5 to 10 nm, covering the side walls of carbon nanotubes. CNT blocks deposited with ZnO and TiO2 particles show excellent field emission properties. Zinc oxide - CNT bilayer film is demonstrated as a light sensor. The bi-layer film is sensitive to light of different wavelength. A single step process to form magnetic composite of CNT and iron oxide is introduced. The particle size of iron oxide is small enough (5 to 15 nm) that the composite exhibits super paramagnetic properties. As the CNT and iron oxide both are bio compatible, composite shows potential in bio medical applications. Ordered CNT blocks deposited using ZnO and TiO2 can serve as a potential material for solar energy harvesting or hydrogen generation by water splitting. The electrode membrane assembly of as grown film of carbon nanotubes decorated using platinum nano particles is fabricated and being tested for fuel cell applications. The average size of the platinum nano particle is of order 2 to 4nm, obtained by self reduction technique. Growth of Neuron cells on CNT blocks: Carbon nanotube blocks are used as substrate to grow varieties of nerve cells. The growth takes place selectively on carbon nanotubes demonstrating affinity of cells towards CNTs. Unlike previous reports, growth is demonstrated on pristine CNTs. Somas found to have numerous neurites with several sub branching i.e. axons forming interconnects. Not only this, cells interconnect each other forming a cell bridge between two CNT blocks. The investigation using spheroids made up of neurons affirms the affinity of cells towards CNTs as many of the somas migrate to CNT blocks leaving spheroids. The complete study corroborates the potential of CNT structure as a electrode for the growth of neuron cell. CNTs are biocompatible, nano-sized and electrically conducting; in short, an ideal electrode for studying cell transport mechanism, neuron cell signaling and can even be a promising material for developing neuronal prosthesis. As a center point of this work, water assisted chemical vapor deposition method is proved to be an optimal growth technique for multi-dimensional field of carbon nanotubes. This method not only offers the possibility to grow good quality CNTs of different typology or morphology, but also ease of maneuvering the experimental parameter to visualize different forms or architectures serving devices with high order sensitivity at sub micron scale. The study on composite of metal oxides and CNTs demonstrates the diversity of this field. Few potential applications were successfully demonstrated while many more can be thought of. The introductory work on growth of nerve cells shows exquisite potential of CNTs in the field of bio-nano-technology. By combining two fields; CNT based FET devices and growth of biological cells on CNTs; together, biosensors can be visualized. Nonetheless, CNTs put forth a wide range of properties addressing the constant demand of new or better materials to sustain the technological development.