Control of Multiwalled Carbon Nanotube Diameter by Selective Growth on the Exposed Edge of a Thin Film Multilayer Structure

Selective area growth of carbon nanotubes (CNT) has been used to control the diameter of CNTs. Narrow lines of SiO2 (12−60 nm) are formed at the cleaved face of a Si/SiO2/Si multilayer structure. CNTs are then grown by a chemical vapor deposition process with a ferrocene/xylene/ H2/Ar mixture at 700 °C. CNTs are observed to grow only on the exposed SiO2 surface at the edge of the “mesa” structure with a diameter equal to the thickness of the SiO2 layer. Carbon nanotubes (CNTs) have been demonstrated to be the basis of several key device applications in electronics,1-3 sensors,4 and nanoelectromechanical systems (NEMS).5 The advantages of incorporating CNTs include high functional density, low power consumption, and the potential for selective chemical modification. Particularly intriguing is that, due to the long length of tubes, it is possible to use commercially prevalent micron scale lithography to result in nm-scale line features if one can selectively incorporate CNTs. Thus the development of a method to control CNT diameter and its placement becomes critical for applications. A particularly promising study has shown that it is possible to bridge the tops of photolithographically defined “posts” with CNTs grown from nanoscale catalyst support particles.6,7 With this technique CNTs are grown in all directions, but those that do not bridge the gap between posts fall to the side. Further refinements of the concept of growing CNTs between photolithographically defined pillars could become the basis of nm-scale thin wiring. There are a variety of growth techniques for CNT synthesis such as arc discharge, CVD, laser ablation, and template assisted growth.8,9 The CVD method offers the most commercially viable technique due to a large uniform reaction area and high mass flux flows.10-13 In all growth techniques, the CNT diameter is determined largely by the size of the nm-scale catalyst particle.14-16 These catalysts can be metal particles17 or a coating of transition metal on catalyst support particles.18,19 Thus, to control CNT diameter there is a significant challenge to control colloid size dispersion, and there are promising reports to this end.20,21 However, it remains a challenge to place only a few nanoparticles in predefined locations without agglomeration of nanoparticles or coarsening.22 Catalytic nanoparticles can be supplied during growth in a xyleneand ferrocene-based CVD process. This process can grow CNTs at high densities with noteworthy vertical alignment.11 The diameter of resultant CNTs is a complicated system that involves the surface free energy of catalyst and substrate, surface migration, and ferrocene decomposition rates. Under a certain temperature and xylene and ferrocene feed rate it is possible determine CNT diameter in the range of 30100 nm with a dispersion approximately 30%.14 Importantly, CNT growth with the ferrocene-catalyzed CVD process can be selective to substrate composition.23-26 In particular, these surfaces can be easily patterned by conventional microprocessing, resulting in predetermined lines of vertically aligned dense arrays of CNT “forests”. The H-terminated Si surface will not grow CNTs, while a SiO2 surface will in this xylene/ ferrocene CVD process.24 Because of the limitation of conventional lithography in the near-micron regime, it is not known if the resultant CNT diameter could actually be determined by line width of an SiO2 surface. By using the exposed edged of a thin film multilayer structure with a 1060 nm thick SiO2 layer, we demonstrate that CNT diameter * Corresponding author. E-mail: [email protected] † Department of Chemical and Materials Engineering. ‡ Center for Applied Energy Research. NANO LETTERS 2002 Vol. 2, No. 1