Pressure effects on unoriented and oriented single-walled carbon nanotube films studied by infrared microscopy

We present the results of pressure-dependent infrared transmission measurements on films of oriented and unoriented single-walled carbon nanotubes. Up to the highest applied pressure (∼9 GPa), the optical response of the oriented single-walled carbon nanotube film is highly anisotropic, with strong absorption bands for the polarization of the radiation along the alignment direction due to optical transitions between the Van Hove singularities in the density of states. With increasing pressure, the optical transitions shift to smaller energies, with an anomaly in the pressure-induced shifts at 2–3 GPa related to the deformation of the nanotubes’ cross section. Weak signatures of a second anomaly are found at around 5–6 GPa, probably related to a more drastic deformation of the nanotubes. Different pressure transmitting media change the pressure-induced effects only quantitatively. The results for the oriented nanotube films are very similar to those for the unoriented ones.