Raman spectroscopy has been extensively used to assess SWNT quality. Among all the methods that involve Raman spectroscopy to determine the quality of SWNT, one of the most used are the full width at half maximum (FWHM) of the D-band, the absolute intensity of the G and D-bands; and the G/D and D/G’ ratios. Although Raman has been used for several years as a technique to evaluate the purity of bulk SWNT, its reproducibility has been widely criticized.
We have developed two improved techniques that show that Raman can be used as a quantitative and qualitative technique. One of them is based on the G-band intensity profile obtained for a SWNT sample suspension diluted at different concentrations and the second one is based on the application of an statistical method to determine the margin of error of the different measurements (G/D, D/G’, etc.) obtained from solid samples.
The figure on the left shows that if only few spots are used to quantify the sample, the G/D value obtained might not be reliable.On the other hand, the figure on the right shows that the intensity of the G band goes through a maximum at certain relative concentration independently of the sample quality.The model developed gives a purity sample factor related to the maximum G band intensity.
The role of water in carbon feed on the surface-guided growth of horizontally aligned single-walled carbon
nanotubes (HA-SWCNTs) was investigated. It is shown that the amount of water can be optimized to favor HA-SWCNT growth, which is proposed to be due to selective etching of carbon deposits at carbon–metal interface. Without water, nanotube–nanotube interaction and carbon accumulation at the interface are disproportionately large compared to the rate of nanotube growth, leading to catalyst deactivation. With excess water, suppression of nanotube growth occurs, resulting in reduced carbon yield on the surface.
Intermediate carbon/water feed ratios achieve cleaner growth with high efficiency.