Determining exact molar absorbance coefficients of single-wall carbon nanotubes.

A new spray technique coupled with atomic force microscopy is developed to obtain the absolute number of carbon nanotubes in a unit volume. By using the present technique, the absolute molar absorbance coefficient of single-wall carbon nanotubes (SWNTs) has been determined and found to be ca. 2-5 x 10(7) L mol(-1) cm(-1). This molar absorbance coefficient enables us to deal with carbon nanotubes as individual "molecules" and to adequately compare them with other related compounds such as fullerenes. The absorbance coefficient of SWNTs is found to be only 100 times as large as those of fullerenes. This indicates that the large aspect ratio, random orientation in dispersion solution and anisotropic absorption properties of SWNTs substantially reduce their absorption probability.