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R&D Profile: Reinventing optical microscopy with nanotubes, W. Bacsa

Nanotubes are of much interest for more than 15 years for applications in molecular electronics, transparent electrodes or functional composites. While working on the spectroscopy with individual carbon nanotubes we realized that nanotubes can be used to learn about optics at scales below the diffraction limit.

Research Overview Courtesy of W.S. Bacsa, CEMES/CNRS, University of Toulouse

Nanotubes are of much interest for more than 15 years for applications in molecular electronics, transparent electrodes or functional composites. A range of measurements have been performed on individual carbon nanotubes. The one dimensional structure results in singularities in the density of electronic states which results in an enormous enhancement of the optical response. This made it possible to study individual carbon nanotubes using optical spectroscopy. Much has been learned in recent years about the interaction of carbon nanotubes with their in environment.

While working on the spectroscopy with individual carbon nanotubes we realized that nanotubes can be used to learn about optics at scales below the diffraction limit. Single wall carbon nanotubes are only one nanometer wide and several micrometers long. That means nanotube have a diameter two orders of magnitude smaller and about one order of magnitude longer than the size of the focal spot of the microscope. The diameter is atomically precise and this makes nanotubes an ideal nanoscale system to probe the focal spot.

When observing the radial breathing mode of an individual nanotube we noticed that the recorded spectra depend on the location of the carbon nanotube within the focal spot of the optical microscope. By changing the orientation of the tube with respect to the grating groves of the spectrometer it became clear that the dispersion of the recorded spectra is directly correlated with the grating orientation. In fact the diffraction angle depends on the angle of the incident rays and since the rays emerging from the nano sized object came only from a particular region of the focal spot, the rays did not incident at exactly the same angle resulting in measurable spectral shifts. Furthermore the measured spectral bands of nanotubes are narrower due to the narrower beam emerging from the nano sized object and are shifted by 3cm-1/micrometer resulting in a lateral resolution of wavelength/12 in the direction perpendicular to the grating grooves (1). Interestingly we learned that the spectral resolution now depended on the width of the carbon nanotube and not on the size of the spectrometer slits.

This demonstrates that by combining an optical spectrometer with an optical microscope it is possible to locate a single nanoscale object well below the diffraction limit. The main message of our work is that optical spectra recorded from nanosized objects depend on their location with respect to the optical axis of the system.

(1) AG Walsh, WS Bacsa, AN Vamivakas, AK Swan, Nano Letters 8 (2008), in print

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