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When a chemist wants to know what molecules are present in a sample and the bonds between them they employ a technique called Raman spectroscopy. Tip-Enhanced Raman Spectroscopy – also known as TERS or nano-Raman – is an enhancement of this technique and is a direct, label-free, non-destructive means of analysing chemical matter at the nanoscale.
TERS was experimentally realised in 2000, 15 years after its initial proposal, and has since rapidly advanced to become a non-destructive scanning probe microscopy tool for surface chemical characterisation, finding uses in biology, catalysis and single molecule detection amongst many others.
Raman spectroscopy uses laser light in either the visible, near infrared (IR) or near ultraviolet (UV) range to observe vibrational, rotational and other low-frequency modes in a system, to ascertain which functional groups are present in a sample. It studies the scattering of photons which have interacted with the molecules in a sample, specifically those which are inelastically scattered.
Most photons interacting with a sample scatter elastically – known as Rayleigh scattering – because they share the same wavelength as the laser light. However, in around one in a million interactions, the photon is inelastically scattered. The photon interacts with the sample molecules, and its wavelength is shifted higher or lower; this is known as the Raman effect and is used to chemically identify the molecules via vibrational spectroscopy. The photon interacts with the electron cloud of a molecule’s functional group which causes the excitation of electrons to a virtual state and the photon to lose energy. This loss is directly related to the functional group, the chemical structure of the molecule it is attached to and the types of atoms in the molecule and environment.
TERS unites the spatial resolution of atomic force microscopy (AFM) with the chemical information gathered from Raman Spectroscopy to give a super-resolution chemical imaging technique which provides much more information about a sample surface than just its geography. It is one of only a few ways to obtain chemical and structural information under ambient conditions at high resolution.
TERS takes AFM beyond topographical imaging down to the nanoscale offering a greater spatial resolution of 10nm – something not possible with traditional Raman spectroscopy. AFM can be used in high resolution imaging to determine the three-dimensional shape of a sample surface. It employs a cantilever with a sharp-tipped probe coated in gold or silver which is placed at the centre of a laser focus and electromagnetic field to act as an antenna. When in close proximity to a sample surface, the cantilever is deflected due to the forces between the tip and the sample. This movement can be measured and recorded to yield a topographic image. The gold or silver coating on the AFM tip serves to generate a very localised enhancement of the Raman signal. An increase in the electromagnetic field also enhances the Raman signal from molecules at the tip-apex, enabling nanoscale chemical imaging of the surface.
TERS can be used to investigate a wide range of materials or chemicals including organic and inorganic nanostructures, thin films and biological materials. The technique is ideal for probing samples in aqueous medium as can be used in an ambient environment. It can also be useful for investigating the chemical composition and molecular dynamics in biological samples - pathogens, lipid and cell membranes and nucleic acids, peptides and proteins – directly because it doesn’t require the use of fluorescent labels.
It can be used in nanoscale characterisation of the layer structure and defectivity in carbon allotropes, graphene and carbon nanotubes for example, nanochemical imaging in polymers, nanomaterials and pharmaceuticals and in nanostructure and strain detection in semiconductors. The method can be employed in catalysis to monitor a single catalytic site for chemical reactions and molecular dynamics and in organic solar cells for nanoscale chemical mapping of the elements in the photovoltaic polymer blends.
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