Improvements in Field Emission Properties of Carbon Nanotube Field Emitters

Abstract

With an increasing demand for better medical diagnostic systems, improving the efficiency and effectiveness of medical diagnostics has never been more crucial. Field electron emission (FE) based X-ray source which is a more responsive and controllable alternative to traditional thermionic-emission-based X-ray sources has stimulated much research interest. Among the materials for FE, carbon nanotube (CNT) is one of the most promising candidates due to exceptional properties such as a high aspect ratio and good electrical and thermal conductivities. CNT-based FE has been studied for many years, however, a significant challenge in the use of CNT has been the difficulty in achieving a uniform emission current over extended periods of time. Notably, rapid degradation of CNT emitters under high current load is one of the major concerns. Two main failure mechanisms proposed in the literature are melting and catastrophic explosion of CNTs due to the generation of Joule heat and the excessive Coulomb energy. Thus, to improve the reliability of CNT emitters in high-current applications, CNT’s failure mechanisms need to be investigated and methods to improve CNT-based FE need to be developed. The most destructive failure mechanism in FE is Coulomb explosion. To study this phenomenon, the underlying causes were investigated through experimental and theoretical analyses. Then, the explosion process was modeled and simulated from the perspective of computational chemistry based on the experimental results. In addition, to slow down melting of CNTs and stabilize emission current during FE, two methods were studied including chemical treatments and coating a metallic protective layer on CNTs. For the chemical treatment method, dimethyl sulfoxide (DMSO) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) solutions were used after CNT synthesis. Both treatments force CNTs to be bundled into clusters and reduce the threshold electric field and significantly improve the lifetime of CNT emitters. On the other hand, the effects of three different metal shells (Au, W, and Ti) on the FE performance are investigated through long-term FE testing and characterization to gain insight into the underlying causes. Therefore, the findings in this thesis play an important role in the development of efficient, stable, and robust field emitters. As such, CNT-based FE technology can be reliably adopted in multiple fields.