Taken together, our CVD growth and STEM imaging results indicate that cooling at a specific rate results in a higher density of catalyst particles. This is the reason why we observed a vertically aligned array of CNTs in the non-isothermal growth from a 1 nm Co film (Figure and 4f). For the same starting amount of material (for example, a 1 nm film of Co or Fe), the appearance of a higher density of particles implies that film dewetting and particle formation occur differently in isothermal and non-isothermal annealing/growth conditions. Upon deposition and exposure to the ambient prior to CVD, the catalyst film is metastable and in the oxide state. When heated, the metal oxide film may dewet/agglomerate to form islands as a result of the increased mobility of its constituent components (i.e. diffusion of lattice atoms, vacancies, defects, impurities, etc.) and the consequential hole formation and rearrangement of the film. This process is driven by the minimization of the total energy of the free surfaces (e.g. of the film and substrate), and of the film-substrate interface. In general, heating a metal oxide film in the absence of a reducing environment results in little or partial dewetting and marginal particle formation, unless an external driving force aids the process. This is the first objective of exposing the films to a reducing environment; the second is to enable growth, which can only proceed from fully reduced particles . By reducing the metal oxide film, its constituent atoms (in this case iron/cobalt and oxygen) can diffuse and rearrange themselves more readily, therefore facilitating dewetting and particle formation. Note that reduction and diffusion can also act as a driving force for detrimental processes such as coarsening and sub-surface diffusion. Because all of the aforementioned processes rely on atomic transport, their rates are strongly dependent on temperature. Hence, we would expect them to be more efficient at 850 compared to 750 ºC.