NSF Proposal Number: CBET-0651687 Principal Investigator: Wilson K. S. Chiu Affiliation: University of Connecticut Proposal Title: Carbon Nanotube Synthesis by Open-Air Laser-Induced Chemical Vapor Deposition
Carbon nanotubes have remarkable mechanical, electronic and electrochemical properties, but the full potential for application will be realized only if the growth of high quantity and quality carbon nanotubes can be optimized and well controlled. This study proposes a new synthesis technique capable of creating carbon nanotubes in open air (no reactor enclosure) at very high deposition rates, minimal contamination, and low defect densities. The open-air feature allows for continuous deposition which is favorable for scale-up, and has the capability to make carbon nanotube networks and patterns by selective area deposition and direct laser writing. However, this new process is limited by non-uniform temperature distribution which causes discontinuous deposition, large nanotube diameter variation and the lack of knowledge necessary to control nanotube quality. This project will develop the fundamental knowledge necessary to understand this chemical vapor deposition (CVD) process and provide necessary fundamental insight to pursue this project's long term goals, which are to control process conditions to obtain carbon nanotubes of desired quality. In this study, we will: (1) Perform extensive carbon nanotube synthesis and characterization experiments to provide the chemical kinetics and nanotube structure information necessary for model development and validation; (2) Develop a model capable of predicting fluid flow, heat and mass transfer during growth; and (3) Establish relationships between process parameters and carbon nanotube growth rate, properties and structure. Findings from this project can be directly relevant and applicable to other carbon nanotube synthesis methods. Intellectual merit for this study include (1) understanding of heat and mass transfer and the resultant temperature and chemical species in the deposition region during carbon nanotube growth; (2) thermal transport and interaction of the laser beam with catalyst nanoparticles and the subsequent deposition temperature field; and (3) modeling concepts integrated with experimental data to predict carbon nanotube structure and properties at prescribed processing parameters. The integration of knowledge will allow us to understand the complex heat and mass transfer during growth, and the subsequent nanotube structure and properties that evolve. Broader impacts of this study include unique collaborations to explore new nanotechnology applications for our society. We will work with universities, industries and national labs to produce high quality nanotubes for biomedical imaging, and to create high surface area electrodes to enhance the performance of energy storage and energy conversion devices. In terms of education, several top high school students will be identified annually, especially from underrepresented groups, from local area high schools for internships in the PI's lab. Knowledge obtained from this study will be applicable to CVD and CVD-related carbon nanotube synthesis methods. Characterization techniques, software tools and technology know-how can be readily transferred to industry, government labs and other users of carbon nanotubes.
This research has developed an enclosureless chemical vapor deposition technique to manufacture carbon nanotubes directly on substrates has been demonstrated. This method does not require the use of expensive vacuum equipment and the associated time-consuming processing steps, while still being able to synthesize carbon nanotubes at high purity and high growth rates. This research, being multidisciplinary in nature, promotes transformational research to further U.S. economic competitiveness in nanotechnology. Interactions with students and industry will prepare an engaging STEM workforce and build strong scientific and mathematic foundations and foster innovation. Results show carbon nanotubes growing perpendicular to the substrate achieving high deposition rates. Simulations have identified the detailed role of gas and surface species on carbon nanotube deposition regimes. It was observed that small changes in the number of active sites can have large impact on predicted deposition rates. Carbon nanotube deposition using methane was found to be rate limited by surface kinetics, due to atomic hydrogen availability at low temperatures and hydrocarbon adsorption at higher temperatures. Changes in reactor wall temperature therefore dictate different feed-stream compositions in order to achieve optimal deposition. Feed-stream composition should be tailored such that, at the defined reactor wall temperature, volumetric production of atomic hydrogen is capable of supporting complete hydrogen abstraction from surface bound hydrocarbons and enough methane is present to assure steady reaction site saturation with carbon nanotube forming methyl radicals. Single-wall and multi-wall carbon nanotubes exhibit some similar growth characteristics. A detailed investigation into the effect of iron catalyst nanoparticles on carbon nanotube growth was performed, indicating that Fe oxidation state may influence the type of carbon nanotube grown. A collaboration with industry successfully developed a non-contact FTIR-based technique to measure the thickness of very thin (nanometers) carbon-based films grown by CVD. This project will provide intensive training in research and teaching skills for the PI and others working on the project. The PI is developing an integrated teaching and research program where undergraduate and graduate students are involved in the research, while the students (graduate and undergraduate) have benefited from working in a research environment, integrated their research into their education curriculum, collaboration with industry, and developed a more comprehensive education upon entering the workforce. The PI and his students have collaborated with industry. They have visited industrial collaborators to present their latest research results, and collaborated on a project which resulted in a joint journal publication.
