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Raman spectroscopy is an analytical characterization technique which has been around for many decades. Since it’s first inception to build a database of molecular vibrational frequencies, it has become a technique that is now widely used within the chemical and physical sciences and has a found a lot of use of late for characterizing nanomaterials and nanoscale surfaces.
Overview of Raman Spectroscopy
Raman spectroscopy is a type of vibrational spectroscopy, and it is a method which is used to identify the vibrational, rotational, and other low-frequency modes of a molecule/molecules in a more complex system. The identification of these modes then enables the chemical structure, among other physical and structural properties of a molecule/molecules to be determined.
Raman spectroscopy uses a phenomenon known as the Raman effect which occurs when the electric dipole of a molecule interacts with photons of light. This interaction utilizes the inelastic scattering of light, so when an incident photon of light from the spectrometer hits the molecule(s), the scattered frequency is different to the original frequency due to the way in which the molecule(s) become excited (and subsequently behave). These unique scattered frequencies can then be used to determine the composition and the properties of the sample under analysis.
For many nanomaterials, Raman spectroscopy has become one of the go-to characterization methods. The main reason is that Raman spectroscopy can not only determine the composition of each nanomaterial, allowing it to be identified, but it can often determine the structural arrangement that distinguishes two different forms of the same type of nanomaterial—one example being the ability to distinguish between single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs).
Nanomaterials come in many forms—some all contain the same element (such as graphene), some have different elements, some have many layers, some are spherical, while others have complex architectures and/or compositions.
Raman spectroscopy can be used as a tool to identify the phases and phase transitions of various nanoparticles and other nanostructured materials, determine which regions of a nanomaterial are amorphous or crystalline, whether there are any defects present in the nanomaterial, determine the size (diameter, lateral dimensions, etc) of various types of nanomaterials, whether the nanomaterial is homogenous or whether a dispersion of nanoparticles are uniform in size, determine the shape of nanomaterials (rod, spherical, etc) and to differentiate between different allotropes of the same material (such as the carbon nanotube example).
These are just a few examples of how manipulating the excited bonds (causing them to stretch or bend) of a nanomaterial can be used to identify many of their structural and compositional features.
Some nanomaterials are even doped by other elements to induce certain properties, and the properties of this doping process can also be determined. Aside from the structural aspects mentioned above, by determining how the molecular bonds stretch, bend and move, the conductive properties and or conductive domains within a nanomaterial can be determined, alongside the other electric/dielectric properties, chemical reactivity, and the mechanical properties of the nanomaterial, to name a few of the main examples.
Raman spectroscopy can even be used to determine the processes which are happening on the surface of a material. Surfaces at this scale rely on nanoscale interactions and Raman spectroscopy has been known to determine if a substance on a surface will degrade in the presence of light, i.e. photodegradation. Another example is to see how surface defects absorb certain gas molecules by measuring the gas concentration change over time in the presence of the surface. Again, these are just a couple of examples and there are many more.
Surface-Enhanced Raman Spectroscopy (SERS)
Aside from conventional Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS) has also emerged. SERS uses metallic nanoparticles deposited on the surface of a nanomaterial to enhance the Raman signal of the bonds/atoms in the nanomaterial by a significant amount.
This is a particularly useful technique for ultra-thin nanomaterials which don’t exhibit the highest signal intensities. SERS has become an advancing technique in its own right and is often used with biological-based materials/bionanomaterials, as it is generally less destructive than conventional Raman spectroscopy (even though conventional Raman spectroscopy is not technically a destructive technique, it can induce some molecular instabilities).
In short, Raman spectroscopy and the various other methods (such as SERS) offer a way for scientists to characterize various types of nanomaterials, their chemical and structural compositions, many of their properties, and how they behave in certain situations/how other molecules behave in the presence of nanomaterials. So, it is a useful technique for many reasons, and this is why it has become a go-to technique for many scientists who work with nanomaterials.
Sources and Further Reading