Article updated on 25 November 2020
Credit: Scientific Reports
Tip-Enhanced Raman Spectroscopy – or TERS – is a technique uniting the spatial resolution of atomic force microscopy (AFM) with the chemical information garnered from Raman Spectroscopy.
TERS was first proposed in 1985, but it wasn’t until 15 years later that it was experimentally realised. Since then, the technique has been rapidly progressing to become a non-destructive scanning probe microscopy tool for surface chemical characterisation. It has found uses in biology, catalysis and single molecule detection amongst many others.
What is TERS?
This super-resolution chemical imaging technique is an enhancement of Raman Spectroscopy, a method often employed in chemistry to determine which molecules are present in a sample and the bonds between them. The result is a structural fingerprint with which specific molecules can be identified. It employs light from a laser in either the visible, near infrared or near ultraviolet range to observe vibrational, rotational and other low-frequency modes in a system, to understand which functional groups might be in a sample.
Raman spectroscopy studies the scattering of photons resulting from a light source’s interaction with molecules in a sample, specifically those with are inelastically scattered. Most photons will share the wavelength of the incident light (the laser) and when interacting with molecules scatter elastically – this is known as Rayleigh scattering. However, approximately one in a million interactions cause the photon to be scattered inelastically, causing a shift in wavelength, known as the Raman effect. Here, the photon interacts with the electron cloud of a functional group on the molecule, causing the electron to move to a virtual state and the photon to lose energy. The loss is directly related to the functional group and the structure of the molecule.
How does TERS work?
Atomic force microscopy (AFM) has three uses; to measure force, to manipulate or change the characteristics of sample in a controlled manner, or in imaging to explore the three-dimensional shape of a sample surface at high resolution. AFM utilizes a cantilever with a sharp-tipped probe; when the probe is brought close to a sample surface, the forces between the tip and the sample lead to a deflection of the cantilever, which can be measured and recorded to yield a topographic image.
When combined with Raman spectroscopy, the technique can provide much more information about a sample surface than just its geography. The precisely controlled AFM tip is coated in gold or silver and placed at the centre of a laser focus to act as an antenna. The electromagnetic field at the apex of the tip is confined and heightened causing a very localised enhancement of the Raman signal from molecules at the tip-apex.
The technique offers a greater spatial resolution of 10nm, allowing for the nanoscale chemical imaging of the surface – something not possible with traditional Raman spectroscopy.
An example of TERS imaging of a mixed molecular chain. a) and b) show SPM images, c) shows a dendogram of the data. d) and e), and f) and g), show Raman data overlayed on AFM images with the corresponding spectra. Credit: Light
Applications of TERS Spectroscopy
TERS takes AFM beyond the realms of topographical imaging down to the nanoscale. The technique can be used in an ambient environment and is ideal for probing samples in aqueous medium. What’s more, it doesn’t require the use of fluorescent labels, so it can be employed to study chemical composition and molecular dynamics in biological samples, such as pathogens, lipid and cell membranes and nucleic acids, peptides and proteins, directly.
The technique also has uses in catalysis to monitor a single catalytic site for molecular dynamics and chemical reactions. In organic solar cells it has been employed for nanoscale chemical mapping of the constituents in the photovoltaic polymer blends. Other uses also include studying layer structure and defectivity in carbon allotropes such as graphene and carbon nanotubes, nanochemical imaging to investigate the structure of polymers, nanomaterials and pharmaceuticals and probing nanostructure and strain detection in semiconductors.
References & Further Reading