Tip-enhanced Raman scattering (TERS) is a powerful technique that combines Raman spectroscopy with the enhanced spatial resolution provided by a sharp metallic or dielectric tip. TERS allows researchers to achieve nanoscale spatial resolution in Raman spectroscopy, overcoming the diffraction limit of traditional Raman microscopy.
Key components and principles of tip-enhanced Raman scattering include:
Raman scattering: Raman spectroscopy is a non-destructive technique that provides information about the vibrational modes of molecules. When a sample is illuminated with laser light, some of the scattered light undergoes a shift in frequency due to interactions with molecular vibrations. The resulting Raman spectrum reveals details about the chemical composition and molecular structure of the sample.
Enhanced spatial resolution: The spatial resolution of traditional Raman microscopy is limited by the diffraction of light, typically to a few hundred nanometers. TERS overcomes this limitation by utilizing a sharp metallic or dielectric tip to focus the incident light to a much smaller spot size, enabling spatial resolutions on the order of a few nanometers.
Near-field enhancement: The enhancement in TERS primarily arises from the near-field interaction between the sample and the tip. When the tip is brought close to the sample, a strong near-field enhancement occurs in the immediate vicinity of the tip. This enhanced electric field enhances the Raman scattering signals from molecules located in this region.
Tip-substrate junction: The most significant enhancement in TERS occurs at the junction between the tip and the substrate. This is often referred to as the "hot spot." The localized enhancement in the near field at the hot spot allows for extremely high spatial resolution, making it possible to probe individual molecules.
Metallic or dielectric tips: The choice of tip material is crucial in TERS. Metallic tips, such as gold or silver, are commonly used due to their strong plasmonic properties. However, dielectric tips can also be employed, and they offer unique advantages in certain situations.
Applications: TERS has applications in various fields, including surface science, catalysis, materials science, and biology. It allows researchers to study the chemical composition and structural details of materials at the nanoscale, providing insights into molecular processes and interactions.