Optical imaging at the nanometer scale through tip-enhanced Raman spectroscopy

Abstract

Nature has many interesting things to offer - one of them is the fact that visible light (from near-UV to near-IR) contains an energy that is comparable to the electronic or vibrational energies of most of the naturally existing materials that we interact with in our day-to-day lives. Visible light can therefore interact directly with the electronic or vibronic system of a sample, and can extract rich information related to the intrinsic properties of the sample. This is probably one of the most important reasons why optical techniques, such as Raman spectroscopy, have always been convenient tools for analyzing and imaging various materials. However, Raman microscopy in its conventional form is not suitable for analyzing and imaging nanomaterials due to two major reasons. First, the poor spatial resolution restricted by the diffraction limits of the probing light, makes it impossible to analyze materials smaller than about half of the wavelength. And second, due to the extremely small volume of nanomaterials, Raman scattering intensity is extremely weak for such samples. However, when conventional Raman microscopy is combined with the near-field techniques, it achieves new and exciting features as it goes beyond the conventional limits of optical microscopy, in terms of both the spatial resolution and scattering intensity. This can be done by utilizing the technique of tip-enhanced Raman spectroscopy (TERS), which is based on plasmonic enhancement and confinement of light field near the apex of a sharp metallic nanotip for characterizing and imaging samples at nanoscale. This plasmonics-based technique allows us to have a spatial resolution down to about 10 nm in optical nanoimaging.