The use of carbon nanotubes has exploded in recent years. The unique properties of this one-dimensional, nanosized material mean that carbon nanotubes have a great potential for use in numerous applications in a range of industries. However, our understanding of the properties of carbon nanotubes is still developing, and scientists are relying heavily on techniques such as Raman spectroscopy to continue developing this vital knowledge.
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The Growing Use of Carbon Nanotubes Across Scientific Sectors
Carbon nanotubes (CNTs) are allotropes of carbon, formed by constructing cylindrical tubes of graphite that are just nanometers in diameter. As with nanoparticles, the tiny size of nanotubes awards them unique properties, such as a robust mechanical potency, large surface area, and high electrical and thermal conductivity, making them valuable to a wide range of industries for a vast number of purposes.
While the use of carbon nanotubes is relatively new, having been discovered in 1991, it is already well-established in a myriad of applications. This includes pharmaceuticals and medicine (e.g. drugs, gene therapies, vaccines, antibodies, biosensors), composites for electrical, electronics, field emissions displays, renewable energy (e.g. solar panels), flexible electronics (e.g. wearables), optics, and more.
Realizing the Full Potential of Carbon Nanotubes
To continue developing new and vital applications of carbon nanotubes, scientists must explore and deepen knowledge of their electronic, mechanical, optical, and thermal properties. Although properties such as these are similar to closely related materials like amorphous carbon, graphite, and graphene, those possessed by carbon nanotubes are unique. Furthermore, these properties can differ between materials constructed from CNTs.
Raman Spectroscopy as a Key Method for Investigation
In recent years, Raman spectroscopy has emerged as a standard tool for establishing the various unique properties of carbon nanotubes, allowing scientists to develop their use in a growing number of applications.
Raman spectroscopy is a non-invasive technique whereby incident light provided by a high-intensity laser is scattered by a molecule. While most of the light scattered is the same wavelength as the original source and thus provides no useful information, a tiny proportion is scattered at different wavelengths. This scattered light, known as Raman scatter, is of interest because it depends on the analyte's chemical structure.
In the case of analyzing carbon nanotubes, Raman spectroscopy can also measure a single isolated carbon nanotube and gather information from the vibrational properties of the material. Also, this technique can go further than alternative measuring tools when it comes to examining carbon nanotubes; it can characterize strain, doping, defects, and interactions with the surrounding materials.
Applications of Raman Spectroscopy in Carbon Nanotubes
Raman spectroscopy is enlisted to characterize and control for many different aspects of carbon nanotubes. The first and arguably most common application is that of classifying CNTs as being either small diameter few-walls carbon nanotubes (single-wall nanotubes [SWNT]), double-wall nanotubes (DWNT) with smaller diameter tubes below 2 nm, or larger diameter tubes (greater than 2mn) with many walls (multi-wall nanotubes [MWNT]). Unique Raman spectral signatures allow SWNTs, DWNTs, and MWNTs to be classified.
Defects can also be characterized with this method. Given the importance of CNT properties for their functioning, such as within pharmaceuticals, it is vital to ensure their efficacy.
Structural changes can occur in carbon nanotubes, such as dislocations, grain boundaries, fictionalizing structures, substitutional atoms, and tube ends. New Raman modes are activated by such changes, allowing Raman spectra the opportunity to detect these defects.
The classification and dispersion of CNT within composites is another common application of Raman spectroscopy. Assessing dispersion enables the interactions between matrices and carbon nanotubes to be evaluated, as well as detecting polymer phase transitions.
All of these factors are vital to understanding the properties of carbon nanotubes, how they function in different scenarios, and how they interact when introduced to different molecules. The future use of carbon nanotubes significantly depends on the current work conducted with Raman spectroscopy to expand our understanding of this relatively new class of material. Thanks to this technique, we can expect many more applications of carbon nanotubes to emerge in the coming years, which will likely benefit the fields of pharmaceuticals and medicine, alongside many more.
References and Further Reading
He, H., Pham-Huy, L., Dramou, P., Xiao, D., Zuo, P. and Pham-Huy, C., (2013) Carbon Nanotubes: Applications in Pharmacy and Medicine. BioMed Research International, 2013, pp.1-12. Available at: https://www.hindawi.com/journals/bmri/2013/578290/
Jorio, A. and Saito, R., (2021) Raman spectroscopy for carbon nanotube applications. Journal of Applied Physics, 129(2), p.021102. Available at: https://aip.scitation.org/doi/10.1063/5.0030809
Mulvaney, S. and Keating, C., (2000) Raman Spectroscopy. Analytical Chemistry, 72(12), pp.145-158. Available at: https://pubs.acs.org/doi/full/10.1021/a10000155