We calculated the electric field enhancement factor of the apertureless near-field scanning optical microscope (ANSOM) tip using the finite-differential time-domain method. The results revealed that the enhancement factor of the silver tip remarkably decreased as the rounded tip radius increased. On the other hand, aggregated silver nanoparticles resulted in a strong field enhancement. Therefore, we developed a silver nanoparticles immobilized tip for the ANSOM probe and attempted to measure the surface enhanced Raman scattering spectrum of carbon onion molecules using it. We successfully detected the Raman spectra of individual carbon onions with a high sensitivity. (c) 2006 Elsevier B.V. All rights reserved.