A micellar approach for diameter- and positon- controlled growth of carbon nanotubes

Abstract

Since the discovery of carbon nanotubes (CNTs), tremendous progress has been made during the past couple of decades on optimizing synthesis and understanding their unique properties. The synthesis of CNTs remains a key in the field of CNT science and technology, and plays a crucial role for their wide range of applications. For most of the applications, growth of nanotubes at predefined locations with tunable properties is required. The requirement for synthesizing CNTs with predefined, uniform and reproducible properties is to synthesize catalyst systems with controllable and adjustable properties which, however, continue to be a very challenging task. Among the different strategies for synthesizing catalyst nanoparticles for CNT growth, the micellar method is one of the most promising approaches which can satisfy most of the requirements. Chemical vapor deposition (CVD) on the other hand, is the most promising growth technique for the technological integration of CNTs because of the ability to control location, direction, and diameter. This work first focuses on the preparation of ordered arrays of size-selected nanoparticle catalysts using the micellar approach. A rapid and facile route which allows the self-assembly of precursor micelles loaded with metal salts on an unprecedented short timescale is demonstrated. Later, we report a low pressure CVD synthesis of CNTs grown in-situ from acetylene on ordered arrays of sizeselected mono-metallic (iron, iron oxide, cobalt and cobalt oxide) and bi-metallic (Fe-Mo oxide) catalyst nanoparticles pre-synthesized on top of Si/SiOX substrate and Al2O3 buffer layer substrate. An optimization control has been demonstrated to obtain CNTs having specific diameter distribution, structure, growth direction and density during its synthesis. The critical role of different growth parameters for growing vertically aligned carbon nanotubes is investigated. The chemical transformations undergone by the catalyst during the growth process were monitored in-situ by X-ray photoelectron spectroscopy (XPS). We have also investigated ways to control the initial chemical state of the catalyst independently of the employed substrate by tuning the oxidation state of the catalyst with different plasma conditions. As determined by in-situ XPS, NPCs supported on Si/SiOX substrate and Al2O3 substrate experience different chemical changes during the growth process. Notable differences were also observed both in growth and nanotube characteristics, as determined by ex-situ SEM, AFM and Raman spectroscopy, depending on the choice of the substrate.