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Raman as a tool for Carbon allotropes identification: from coal to diamond

08 Nov

Raman as a tool for Carbon allotropes identification: from coal to diamond

Carbon is a very versatile atom in terms of formation of different allotropes (the forms that an element can exist). this post will only refer to pure materials with C-C Bonds, as enlarging to other elements will extend too much the length of the presentation.

Some of most communes’ forms to find carbon in the nature are - in terms of crystalline-: amorphous carbon (no crystalline structure), graphite (parallel planes of C atoms) and diamond (perfect clear and hard mineral with perfect disposition of the C atoms in a tetrahedral lattice). These forms are found regularly in the nature and we all know their broad uses, from fuel source to jewellery. The chemical and physical properties of these materials are very broad and of great importance in our economy. In the past decades, the research of the carbon allotropes has confirmed the presence of more exotic forms of carbon and one of the most commonly used  tools for its identification is Raman spectroscopy. The discovery of the fullerenes (buckminsterfullerenes, Nobel price in Chemistry in 1996), graphene (Nobel prize in chemistry in 2010) evidence the strong interest these materials do present in the scientific world. The strong development of the nano-materials in the recent past, has enhanced even more the research of these new forms of the Carbon and the research has produce a very good number of new allotropes of carbon with significant and important properties.

As mentioned, Raman spectroscopy is one of the most commonly used tools for the identification and characterisation of the carbon allotropes. This is due to the clean and distinctive peaks (sometimes called bands) that the different materials do generated. Each peak contains important information on the characteristic of the functional groups such as orientation, close environment, hybridisation, etc. the Raman bands for the different forms of carbon are very well known and they are rich in information on the crystalline and properaties of the materials. the peaks are normally labelled as: RBM (radial breathing modes for nanotubes), D, G, 2D (or G'), etc bands. figure 1 shows some of the bands displayed in the application note from Bwtek> Graphene Raman Analyzer: Carbon Nanomaterials Characterization

 

The analysis of carbon related materials is normally reported with high confocal Raman system. this is due to the very thin nature of the samples and the need to focus in the nm scale to obtain a good representation of the local environment of analysis. The laser source used in the majority of the publications 514 or 532nm laser (green lasers are excellent to obtain good Raman spectra of inorganic materials or resonance Raman experiments), in our case, these election of these lasers this is due to the weak Raman signal that is obtained from the different carbon materials; so we are required to use wavelengths (λ) that maximise the Raman spectrum. in this case, it is necessary remember that the intensity of the Raman signature is proportional to 1/λ4, been λ the laser excitation wavenumber, i.e. lower the frequency, higher the signal. For research purposes and due to the nature and size of the samples, the Raman analysis of the carbon allotropes has been mostly completed with high end confocal microscopes, or even SEM systems.

That need for high end confocal devices for description of the production of carbon allotropes is less and less acute as the research and understanding on the production and monitoring of the carbon forms are improving significantly. The tendency in the market today is to star transferring the know-how from research laboratories to production sites. Under these conditions, the need of top range Raman systems is lower and in fact is not necessarily if good practice for production are in place. The need of accurate analytical control techniques is nevertheless there, but industrialise conditions requires simple of use, reliable devices and tools that can read much larger areas and provide a good average understanding of the product average quality and properties. The reason behind is that the presence of a potential local maximum of a given signal are not necessary representative enough of the quality of a large batch. The i-Raman Prime system from Bwtek has proved to be a very cost-effective tool for routine analysis in the incipient “new carbon” industry, provided state-of-the-art results with an sensible cost and very low to none cost of maintenance.

Graphene, carbon nanotubes, fullerenes, etc do present an extremely high level of potential reactive points, the extreme been the graphene, were every atom is available for chemical reaction. The carbon allotropes are commonly modified as derivates of the oxide, nitrite or sulphur derivatives i.e. oxide-graphene, oxide-nanotubes, etc. after activation with strong acids such nitric of sulfuric acid. This step is a critical step for activation, but with the use of such strong oxidants, the carbon structure can be damaged and/or lost. Raman spectroscopy has proved again to be an ideal tool for the monitoring of this activation process. These materials have as well extraordinary properties by themselves.

Oxidised carbon allotropes are commonly modified with functional groups to introduce or to modify properties. Some of the modifications are but not limited to the use of organic ligands, metals, metals ions, organometallic compounds, etc. these modifications require the characterisation with products like the OSTEC confocal Raman system RAMOS S120, this is more a research type of user, where the identification of the fine details of the synthesis is critical for a good understanding and characterisation of the quality and properties of the final product.

Elodiz supports these customers with 2 different types of products:

Confocal systems, like OSTEC RAMOS S120 for state-of-the-art confocal microscope entry level or BWTek iRaman Prime 532H for routine analysis!

For further details: info@elodiz.com

 

 

 

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