Near-field Raman-spectroscopy of carbon nanotubes and fullerenes

M.Kh.Salakhov, S.S.Kharintsev

Russia

 

The paper demonstrates the effect of the giant electric field enhancement close to a nano-sized metallic probe. Based on this effect, the method of near-field Raman spectroscopy with ultrahigh spatial resolution is developed. The interpretation of Raman spectra of carbon single-walled nanotubes and fullerens is given.

Optical spectroscopy methods are of fundamental importance in studying the structure and properties of matter. However, their applications are restricted because of the diffraction Abbe limit ~/(2n) ( is a wavelength, n is a refractive index). This means that objects with linear sizes less than this limit cannot be studied with the help of optical spectroscopy methods. Nowadays, there are several ways to overcome the diffraction limit and, therefore, to improve the spatial resolution. Among them the near-field Raman spectroscopy method with ultrahigh resolution has been fast developing. This method is based on the giant electromagnetic field enhancement near the nanoscale probe. In contrast to widely used optical spectroscopy methods the resolution capability is determined by the probe geometry rather not an aperture of optical elements. From the physical viewpoint the probe placed at a highly focused laser beam, locally enhances the electromagnetic field in the proximity of the tip apex due to the resonance excitation of localized surface plasmons. An additional contribution to the field enhancement is made by a geometric singularity of the tip apex and by the chemical effect of adsorbed molecules.

In contrast to widely used methods of electron microscopy, the near-field Raman spectroscopy method allows one to not only visualize the object of interest but also to in situ obtain information on its chemical structure, composition, conformations on the nanometric scale. Besides, this method has high sensitivity (scattering cross-section is equal to 10^-18¸10^-14cm^-2ster^-1), selectivity and can be used at a wide range of temperatures and pressures.

The purpose of the paper is to demonstrate a giant enhancement of the electromagnetic field using carbon single-walled nanotubes and fullerenes. The interpretation of Raman spectra of carbon single walled nanotubes and fullerenes, and results of experiment are discussed.

In the paper possibilities of near-field Raman-spectroscopy based on the effect of the giant electric field in the proximity of nanoscale probe are shown. The field-enhancement of an optical signal is ~10³. Thus, this method might be used as an effective tool for performing a local non-destructive chemical analysis of samples on nano- and/or sub-micrometer scale at normal conditions.