Toward single molecule detection through tip-enhanced near-field Raman spectroscopy

When Raman scattering is excited from the evanescent light field created by illuminating the apex of a sharp metallic nano-tip, it achieves new aspects with strong enhancement of scattering efficiencies and super resolving capabilities. The primary mechanism of tip-sample interaction is electromagnetic, which is based on the excitation of localized surface plasmon polaritons. However, when the tip is close enough to the sample, typically at molecular distances, the chemical interactions between the tip and the sample become important. Strong temporal fluctuations of Raman scattering, including fluctuations of peak frequencies and peak intensities, together with extraordinary enhancement of several peaks, were observed. These temporal fluctuations, which are typical signature of single molecule detection, were attributed to the changes of molecular orientations of the sample molecules in the upper layer of the nanocluster, which got chemically adsorbed at the tip molecules.