Considering that CNT diameters are proportional to catalyst particle sizes, our observations suggest that lowering the temperature continuously during growth has an effect not only on CNT yield but also on particle sizes. This inference is substantiated by the higher proportion of smaller diameter CNTs shown in Figure , which raises the question as to what is the underlying mechanism driving the higher yields in the non-isothermal growth. Given that both isothermal and non-isothermal growths always started with the same catalyst film thickness and composition, is the higher yield in non-isothermal growth a result of more CNTs nucleating from a fixed particle density (i.e. higher nucleation efficiency), or a result of an increase in particle density?