Near-field scanning optical microscopy (NSOM) was described in 1984, and used in many applications since then. The combination of Raman scattering and NSOM techniques was first realized in 1995, when it was used for imaging a Rb-doped KTP crystal at a spatial resolution of 250 nm.
NSOM employs two different methods for data collection and analysis: the fiber tip aperture approach and the apertureless metal tip approach. NSOM with aperture probes has a smaller aperture that can increase the spatial resolution of NSOM; however, the transmission of light to the sample and the collection efficiency of the scattered/emitted light is also diminished. The apertureless near-field scanning microscopy (ANSOM) was developed in the 1990s. ANSOM employs a metalized tip instead of an optical fiber probe. The performance of the ANSOM strongly depends on the electric field enhancement factor of the metalized tip. This technique is based on surface plasmon resonance (SPR) which is the precursor of tip-enhanced Raman scattering (TERS) and surface-enhanced Raman scattering (SERS).
In 1997, Martin and Girard demonstrated theoretically that electric field under a metallic or dielectric tip (belonging to NSOM apertureless technique) can be strongly enhanced if the incident field is along the tip axis. Since then a few groups have reported Raman or fluorescence enhancement in near field optical spectroscopy by apertureless microscopy. In 2000, T. Kalkbrenner et al. used a single gold particle as a probe for apertureless scanning and presented images of an aluminium film with 3 μm holes on a glass substrate. The resolution of this apertureless method was 100 nm, that is comparable to that of fiber-based systems Recently, a carbon nanotube (CNT) having a conical end, tagged with gold nanoparticles, was applied as a nanometer-resolution optical probe tip for NSOM. NSOM images were obtained with a spatial resolution of ~5 nm, demonstrating the potential of a composite CNT probe tip for nanoscale-resolution optical imaging.