Raman spectra collected from all three substrates reveal the presence of CNTs (Figure ). The spectra plotted in the figure are averages of maps collected over large areas (500 µm 2 ), where the spacing between each point was 20 µm to avoid significant overlaps. The RBM frequencies are highest in the 750 ºC growth, corresponding to smaller diameter CNTs compared to the other two spectra. Notably, the Si substrate Raman peak is completely absent in the spectrum from the non-isothermal growth (bottom red trace in Figure ), showing the high density of CNTs. The broad G band in this spectrum is typical of bundled nanotubes and could also be from the presence of few-walled CNTs in the aligned array.