Combinatorial method to prepare metal nanoparticles that catalyze the growth of single-walled carbon nanotubes

Enhanced surface diffusion at the growth temperature of single-walled carbon nanotubes (SWNTs) can cause coarsening of metal catalysts. By balancing the nominal thickness and surface diffusion length of metals, metal nanoparticles of desirable size are expected to form spontaneously under the SWNTs growth conditions. Our combinatorial method, using a library of nominally 0.001to 1-nm-thick sputter-deposited cobalt patterns, successfully identified in a single experimental run that cobalt nanoparticles from submonolayers can catalyze the growth of high quality SWNTs. Since the discovery of carbon nanotubes (CNTs), they are attracting much attention as promising materials for application in nanodevices due to their excellent mechanical, electrical, and chemical properties. Many of the applications such as electron field emission sources, single electron transistors, field effect transistors, molecular wires, and chemical sensors require controlled growth of CNTs on a variety of substrates. CNTs are grown mainly by chemical vapor deposition (CVD) on substrates with transition metal catalysts, and extensive efforts have been applied to develop the preparation methods of catalysts by both wet and dry processes. The catalyst conditions, however, depend on the processing conditions, such as the type of substrate, types of reactant gases in CVD, and CVD temperature. Such dependence makes conventional trial-and-error approaches to find preparation conditions of active catalysts time-consuming. Aiming at efficient screening of catalyst preparation conditions, several studies have used combinatorial approaches. Cassell et al. successfully grew multi-walled CNTs (MWNTs) and single-walled CNTs (SWNTs) by using catalyst libraries produced from a solution containing three components, namely, metal salts as catalyst, SiCl4 or AlCl3 as substrate-forming components, and triblock copolymers as “structure-directing agents”. Ng et al. applied a dry process to prepare catalyst libraries of 5to 10-nm-thick metals, which yielded MWNTs. These studies confirm that combinatorial approaches are effective in the screening of catalyst preparation conditions, although the underlying mechanism that determines catalytic activity remains unclear. Kinloch et al. recently studied catalyst libraries similar to those studied by Cassell et al., and found that “the structuring agent was not controlling the size of the catalyst particle by encapsulating the nitrate salt in micelles but rather the agent improved the wetting and hence uniformity of the catalyst precursor spot.” Catalyst nanoparticles appear to have structures controlled by some spontaneous processes rather than by artificial efforts applied in those studies. Metal nanoparticles are generally believed to catalyze growth of CNTs of diameter similar to theirs. To catalyze the growth of SWNTs, catalyst nanoparticles should have diameters as small as 0.4−3 nm. Such tiny nanoparticles often suffer from their aggregation and coarsening at the elevated temperatures required for CVD. The structural evolution of a) Electronic mail: [email protected]