Ambiguity in the Characterization of Chemically Modified Single-Walled Carbon Nanotubes: A Raman and Ultraviolet−Visible−Near-Infrared Study

Single-walled carbon nanotubes (SWCNTs) sonicated in o-dichlorobenzene and benzyl chloride show anomalous behavior when characterized with a Raman microscope and ultraviolet−visible−near-infrared spectroscopy. SWCNTs treated with the aforementioned solvents lead to a small but distinct increase in the Raman D peak, when irradiated with laser power higher than 0.12 mW/μm2. This can be mistakenly interpreted as covalent functionalization, but we have correlated this increase in the D peak to the charring of polymeric material, which is formed during sonication of the aforementioned solvents. At a temperature estimated to be 280 °C, corresponding to a laser power of 0.31 mW/μm2, the polymers are charred, resulting in an increase in amorphous material. This behavior is in contrast to that of the covalently functionalized SWCNTs, which show a decrease in the D peak as the laser power is increased. These samples also show a depletion in the spectral intensity of the optical absorption spectra of the SWCNTs, which is again a result commonly associated with covalent functionalization. However, by using a washing protocol, we find the Raman and optical spectra of the resulting SWCNTs no longer show features associated with functionalization. Species formed during sonication can drastically affect data interpretation. Our results provide an unambiguous assessment of the cause and effect of wet chemical processing and its impact on characterization.