Controlled synthesis, characterization and applications of carbon nanotubes

2014-09-22Single-walled carbon nanotubes (SWCNTs) possess superior electrical and optical properties and hold great promise for electronic and biomedical applications. The properties of a SWCNT strongly depend on its structure, therefore, a great effort of this dissertation has been devoted to control the structure of single-walled carbon nanotubes (SWCNTs) during the chemical vapor deposition (CVD) growth process. Despite the enormous investigation efforts and the tremendous achievements made in the nanotube growth field over the past several decades, controlled synthesis of nanotubes with predefined and precise structure still remain a huge challenge. We have developed a new approach to the synthesis of SWCNTs with predefined chirality by combining nanotube separation with growth. This novel approach can also be used with organic molecules. ❧ The thesis will start with a brief introduction of the carbon nanotube, then the synthesis and characterization methods as well as the applications of SWCNTs is described in detail to explain the importance of controlling the structure of the carbon nanotube. ❧ In chapter 2, a novel carbon feedstock is introduced to selectively synthesize the predominantly semiconducting nanotube by using the chemical vapor deposition (CVD) method. The development of guided CVD growth of single-walled carbon nanotubes provides a great platform for wafer-scale integration of aligned nanotubes into circuits and functional electronic systems. However, the coexistence of metallic and semiconducting nanotubes is still a major obstacle for the development of the carbon nanotube based on nanoelectronics. To address this problem, we have developed a method to obtain predominantly semiconducting nanotubes from direct CVD growth. By using isopropanol as the carbon feedstock, a semiconducting nanotube purity of above 90% is achieved, which is unambiguously confirmed by both electrical and micro-Raman measurements. A mass spectrometric study was performed to elucidate the underlying chemical mechanism. Furthermore, high performance thin-film transistors with an on/off ratio above 10⁴ and mobility of up to 116 cm²/V∙s have been achieved using the isopropanol-synthesized nanotube networks grown on silicon substrate. The method reported in this contribution is easy to operate and the results are highly reproducible. Therefore, such semiconducting predominantly single-walled carbon nanotubes could serve as an important building block for future practical and scalable carbon nanotube electronics. ❧ Chapter 3 presents our new approach to the selective synthesis of nanotube with predefined chirality via the Vapor phase epitaxy (VPE) growth method. The lack of synthetic control in chirality has long been recognized as a fundamental impediment in the science and application of SWCNTs. Since the electronic property of a SWCNT strongly depends on its chirality,¹ previous efforts to address this issue have resulted in significant progress in the separation of synthetic mixtures, which has yielded predominantly single-chirality nanotube species.¹¹⁻¹⁵ However, separation processes are limited by their small scale, their high cost, and the short length (< 500 nm) of the resulting chirality-pure nanotubes. They are, therefore, not viable for many, especially electronic device applications.¹¹ Here we demonstrate a general strategy for producing both metallic and semiconducting SWCNTs with predefined chiralities by using purified single-chirality nanotubes of (7, 6), (6, 5) and (7, 7) as seeds for subsequent metal-catalyst-free growth, resembling the vapor phase epitaxy (VPE) commonly used for semiconductor films. ❧ To shed light on the growth mechanism of our VPE process, we carefully studied the chirality-dependent growth kinetics and termination mechanism for seven single-chirality nanotubes of (9, 1), (6, 5), (8, 3), (7, 6), (10, 2), (6, 6), and (7, 7); covering near zigzag, medium chiral angle, and near armchair semiconductors, as well as armchair metallic nanotubes. Details of the experiment are presented in chapter 4. Our results reveal that the growth rates of nanotubes increase with their chiral angles while the active lifetimes of the growth hold an opposite trend. Consequently, the chirality distribution of a nanotube ensemble is jointly determined by both growth rates and lifetimes. These results correlate nanotube structures and properties with their growth behaviors and deepen our understanding of chirality-controlled growth of nanotubes. ❧ The VPE process is found to be highly robust and can also be used to synthesize SWCNTs from various organic molecules, such as the (5, 5) end-cap. Details of the experiment results can be found in chapter 5. Finally, a brief review and various future research directions are proposed with some preliminary results