The Applications of Raman Spectroscopy in Nanotechnology

Modern Raman spectroscopy is a very actively developing area in physics, chemical sciences, and biological and biomedical applications. This type of spectroscopy can be successfully used for studying nanostructures and its applications.

In the early twentieth century, while swimming in the Mediterranean Sea aboard S.S. Narkunda from England to India, C.V. Raman pondered: “Why is the sea blue?”. Thinking about this issue, Raman came to the conclusion that the “blue” color of the sea is due to molecular scattering, just as the blue of the sky is. This way Raman became interested in the mechanism of light scattering. This was the beginning for the discovery of the Raman Effect: Raman and his team observed a change in frequency of the scattered light and a change in polarization. After years of research, Raman called these changes “modified scattering”, or a “new type of secondary radiation”, when he realized what he was observing as the visible light analog of Compton Effect in X-ray photons. In other words, the Raman Effect has become the practical proof of the theoretical Kramers-Heisenberg effect (Quantum theory of Dispersion). The Raman Effect was hailed by an American physicist R.W. Wood as “…one of the best convincing proofs of the quantum theory”.

Speaking specifically, Raman spectroscopy is the study of the spectrum of the monochromatic light (due to Raman effect) from substances giving information about the molecular structure. If a molecule has a center of symmetry then Raman active vibrations are infrared inactive. If there is no center of symmetry then some (but not necessarily all) vibrations may be both Raman and infrared active. Thus the study of Raman spectroscopy reveals the interplay between atomic positions, electron distributors, and intermolecular forces. Each type of bond has characteristic modes of vibrations, therefore, each type of molecule has its own spectral “fingerprint”. Thus the Raman spectrum is a chemical fingerprint that brings in a wide range of application tools and study.

Raman spectroscopy is a giant tool for nano-scientists. Nanostructures are defined as structures which have at least one or all of the dimensions in the range of 1-100 nm. The nanostructures can be engineered to different properties, and have a wide range of applications in any field today: nanotubes, nanospheres, nanopores, nanosurfaces, nanocubes, nanowires are a few nanostructures to name. Optical study of nanostructures is an interesting field, and Raman spectroscopy adds an altogether different perspective. Firstly, Raman spectroscopy can be used for biophysical structure analysis of molecules present on the nanostructures. Polymeric nanostructures, silica nanoparticles, metal nanoparticles such as gold or silver nanoparticles, and suchlike can be detected by normal or resonant Raman scattering. These nanoparticles are used as drug vehicles or in phototherapy in biomedical applications. Secondly, an interesting example of the application of Raman spectroscopy is one study in which this spectroscopy is integrated with optical tweezers. There Raman tweezers are used to monitor molecules of single living cells, to identify microorganisms and surface molecules on different nanostructures.

In addition to this, fiber-laser-based stimulated Raman scattering microscopy is employed to detect “essential diagnostic features” such as information on specific protein nanostructures/nanopores, structural changes leading to aggregation of cells help in intraoperative histology. Raman spectroscopy is also used in the study of the toxicity of different nanostructures, in living systems.

The highly specific and sensitive spectrum, that is a Raman fingerprint of each structure/molecule makes Raman spectroscopy an actively developing technique in all facets of science.

Summing up the aforesaid, areas which benefit from Raman spectroscopy include:

  • identification of materials constituting nanostructures
  • number of layers for graphene and TMDC layers
  • the thickness of heterostructure layers
  • diameter and chirality of carbon nanotubes
  • stress/strain characterization
  • electronic properties of materials (metallic/semiconductive)
  • localization of separated carbon nanotubes or quantum dots

Optromix Raman fiber optic probes are miniaturized without compromising its performance, which is enabled by the technology of direct deposition of the dielectric filters at the fiber end faces. It results in a small, cost-effective Raman probe for endoscopy and other applications.

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