Longer nanotubes at lower temperatures: The influence of effective activation energies on carbon nanotube growth by thermal chemical vapor deposition

Growth of carbon nanotubes (CNTs) by metal-catalyzed thermal chemical vapor deposition (CVD) upon flat silicon substrates is studied as a function of growth temperature. It is found that the CNT growth rate at a given temperature is constant for a certain amount of growth time, after which growth ceases; the product of the growth rate and the growth time gives the ultimate length of the CNTs. Both the growth rate and the growth time are found to depend on the CVD temperature, and this dependence is such that the ultimate CNT length increases as temperature decreases; that is, longer CNTs can be grown at lower temperatures than at higher temperatures. This surprising and counter-intuitive result reflects the interaction of competing factors affecting the CNT growth: the rate at which carbon is incorporated into growing CNTs versus the rate at which catalytic metal particles become inactive. Both of these rates are found to have an Arrhenius form of temperature dependence, with activation energies of 2.0 and 3.4 eV, respectively, when an Al2O3 diffusion barrier layer is used. These energies are interpreted as "effective" activation barriers arising from activation energy contributions from multiple chemical processes. CNT bundles as long as one millimeter have been grown at a temperature of 600 degrees C.