Identification of Individual Carbon Nanotubes by Near-Field IR/NIR Microscopy

Scattering scanning near-field optical microscopy (s-SNOM), based on the combination of atomic force microscopy and frequency-dependent light scattering, is an emerging method that combines high spatial resolution with high sensitivity even at long illuminating wavelengths. Its length scale is appropriate to observe nanoscale objects, and carbon nanotubes of 1-2 nm diameter are ideal candidates for such observation.<br/>I will present results in the infrared and near-infrared frequency range obtained on individual carbon nanotubes. Due to the weak scattering cross section, direct observation of localized vibrations is rarely possible, one has to exploit the coupling to collective excitations. In the lowest frequency region, metallic tubes can be detected and distinguished from semiconducting ones due to their free (Drude) electrons. The next step towards determining their chirality is near-infrared excitation where the illuminating laser is resonant with one of the transitions between Van Hove singularities. As the AFM image yields the diameter and the s-SNOM the metallicity (IR) and the electronic transition energy (NIR), the complete information resulting in the chirality is obtained.<br/>The intense field under the tip can also be used to launch and detect the charge distribution inside nanotubes caused by interference of quasiparticles: plasmon-polaritons or phonon-polaritons. I will show examples of such interference effects in both carbon and boron nitride nanotubes.