Raman spectroscopy

About this technique

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The Raman effect is a process where light interacts with an atom or molecule and a vibrational quantum is excited or annihilated. When light interacts with a molecule, most photons are elastically scattered and therefore have the same energy as the incident photons. However, a very small fraction (1 in 1x 10⁷ photons) are inelastically scattered, which means that the energy of the scattered photon is different (usually lower) than the energy of the incident photon. The energy shift between the incident photon and the Raman scattered photon is caused by the excitation (or annihilation) of a molecular vibration. This energy shift is unique for the type of molecules involved in the scattering process. Therefore Raman spectra can be used as a chemical fingerprint.
The Raman effect is dependant on the polarisability of the molecule being studied. This differs from infra-red (IR) spectroscopy, which is dependant on the dipole moment of the molecule. Indeed IR and Raman are often seen as complementary techniques and are often used in conjunction when studying materials.
A great deal of information can be generated by Raman microscopes. Some of this information includes:

• identification of organic molecules, biomolecules, polymers, inorganic compounds
• detection of different types of carbon, which can include diamond, graphite, amorphous carbon, carbon nanotubes, graphene etc. The technique is particularly suited to these types of materials and can yield not just chemical information e.g. how disordered the carbon material is, but also structural information such as the diameter of carbon nanotubes from the position of the radial breathing mode (RBM) peak
• measurement of the crystalline structure and stress in semiconductors such as silicon.
• analysis of inorganic oxides e.g. the different forms of iron oxide has been studied extensively with Raman
• mapping the distribution of components in mixtures such as drugs in tablets

Depending on the type and configuration of the Raman instrument data can be collected on gases, liquids, solids or surfaces. It is not only possible to obtain a single Raman spectrum of a sample at a single location but also to scan the Raman laser in the XY and Z axes and obtain chemical maps of surfaces with sub-micrometre lateral resolution. An example of a Raman image can be seen below. If they are available on the instrument, different laser wavelengths can be used as the excitation source. This can assist in reducing competing scattering processes such as fluorescence at the cost of reduced signal.
Raman microscopy is considered to be a non-destructive technique although some caution should be used with the laser power. Surfaces can be damaged given that the excitation laser beam is often focussed to a small spot size on the order of 1 micrometre. Samples don’t require fixing or sectioning for Raman spectroscopy and the standard set-up for many Raman instruments is very similar to a standard optical microscope except that an additional excitation source (laser) is required and a detector to measure the energy shift of the sample molecules due to the Raman effect.
There now exist a number of variants of this technique, including surface-enhanced Raman (SERS), which produce large amplifications (up to 10¹¹ times) of the Raman signal by attaching molecules to roughened silver of gold surfaces. Another Raman technique gaining in popularity is tip-enhanced Raman spectroscopy (TERS), which combines Raman spectral imaging with atomic force microscope image resolution (50 nm lateral resolution).