Article updated on 9 December 2020.
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Raman spectroscopy is an analytical method based on measuring the scattering of infrared and visible light. This technique can be used to investigate the structure and various properties of the material.
In a typical Raman spectroscopy process, a test sample scatters high-intensity laser light based on vibrations of the sample’s molecules. Almost all the scattered light is at the same wavelength as the laser light and it does not offer useful data. However, a small quantity of scattered light is at different wavelengths. Representative of the chemical makeup of the sample, this is known as Raman Scatter.
Nearly all materials display Raman scattering, with the main exclusion being pure metals, which reflect laser light.
A Raman spectrum is a graph that shows the intensity (y-axis) and wavelengths (x-axis) of all the Raman scattered light from a test sample. Typically, a Raman spectrum will have several "peaks" at various wavelengths, with each peak being associated with a molecular bond, group of bonds, polymer chain vibration or crystal lattice mode.
Raman peaks and bands come from the vibrations of a sample’s molecules. These vibrations are extremely responsive to modifications in structure and chemistry, which allows for the recognition of small shifts in molecular structure. The direct connection between Raman bands and vibrations also makes for easy analysis.
Raman spectroscopy can perform highly local analyses on structure, phase, the strain on material and crystallinity. Normally, a Raman spectrum contains one or more definitive chemical fingerprints and spectral libraries to identify materials based on their Raman spectrum. Computer algorithms can search thousands of spectra quickly to match a spectrum and a material.
Identifying Materials Using Raman Spectroscopy
Identifying an analyte based on its Raman spectra is possible for a few layers of materials. In some cases, amorphous materials can be identified based on bands, rather than peaks, in a spectrum.
Often the spectrum produced by a Raman spectroscopy test is highly complex, so extensive Raman spectral libraries are usually needed to locate a match, and therefore identify an analyte.
There is a direct relationship between the intensity of a signal and the concentration of an analyte. A calibration process must be used to establish the precise relationship between intensity and concentration. After calibration is complete, analyses can be performed to investigate the analyte concentrations in test samples.
Relative peak intensities can offer information on the relative composition of a test sample. If the general composition of a sample is already known, the relative peak intensities can provide quantitative data on the sample’s makeup.
There is a significant amount of scientific research currently being conducted on layered materials, such as certain forms of graphene and gallium selenide. Raman spectroscopy is an appropriate method to investigate layered materials and differentiate between the various layers.
Raman Combination Techniques
To create a more comprehensive analytical system, a Raman spectrometer can be combined with a range of other tools, including an atomic force microscope (AFM), scanning electron microscope (SEM) and confocal laser scanning microscope (CLSM).
Raman spectroscopy is often used in conjunction with microscopic analysis. A Raman microscope combines a Raman spectrometer and a conventional optical microscope, enabling both the magnification of a sample and a Raman analysis. To perform a Raman microanalysis, a sample is placed under the microscope for observation, and a Raman spectrometer performs a laser-spot analysis.
A confocal Raman microscope can analyze multi-layered micron-sized samples, while motorized stages enable the creation of Raman spectral images. These images comprise thousands of Raman spectra from various locations on the sample. Images can be colorized using Raman spectral information to reveal the distribution of various components, different phases, effects of stress and crystallinity.
References and Further Reading
https://core.ac.uk/download/pdf/81108376.pdf
https://www.horiba.com/en_en/raman-imaging-and-spectroscopy/
https://www.renishaw.com/en/why-we-use-raman-spectroscopy--25803
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