Tip-enhanced resonance Raman spectroscopy and imaging of a single molecule

Abstract

In optical spectroscopy experiments, the diffraction limit restricts the spatial resolution of measurements to half the wavelength of light used, making optical based techniques particularly challenging for nanoscale materials. For Raman spectroscopy in particular, an additional challenge is the inherently low conversion efficiency of the Raman scattering effect. Tip-enhanced Raman spectroscopy (TERS) is one of the techniques that can break both the diffraction limit and the low signal intensity in Raman spectroscopy. In TERS, the tip of a scanning probe microscope such as a scanning tunnelling microscope (STM) is brought close to the target sample. The tip boosts both the incident and Raman scattered radiation leading to several orders of magnitude increase in the signal allowing the spatial resolution to be well below the diffraction limit. In previous works, STM-TERS was shown to have both vibrational mode selectivity and less than 1 nm spatial resolution. This achievement however, was obtained by severely perturbing the molecule’s electronic structure. In this talk, I will present our recent works on tip-enhanced Raman spectroscopy in ultrahigh vacuum and low temperature environments. We have achieved single molecule Resonance spectroscopy by utilizing an ultrathin sodium chloride film. The sodium chloride film allows us to perform TERS while keeping the original electronic structure of the molecule. The Raman maps show sensitivity to vibrational mode symmetry with 1 nm spatial resolution. Such a technique can be useful in the discovery of new materials and new material functions.