Carbon-containing compounds exhibit an impressive range of physical and chemical properties. Many of these compounds – biological materials, polymers, petrochemicals, etc. – either play a pivotal role in our daily lives or are tremendously useful to science and industry. Forms of pure carbon can also have a wide range of useful characteristics. Two examples that are notable for their seemingly disparate properties are diamond (the hardest known material and optically transparent) and graphite (a dark, soft material useful as a lubricant).
In recent years, new pure-carbon compounds have been discovered in the form of the two-dimensional nanomaterial graphene. Graphene is attracting intense study due to its own interesting and highly desirable properties, including its extraordinarily high electrical and thermal conductivity, high mechanical strength, and optical transparency. With the explosion in carbon-based research in the last two decades, fueled by the interest in carbon nanotubes and graphene, it has become more important to characterize these kinds of materials in fundamental research and for quality control purposes in both research and industry. But, in contrast to graphene’s macroscopic counterparts diamond and graphite, the material exists solely in the nano-scale, meaning that specialized analytical characterization methods are needed. Raman spectroscopy is one such tool for optically investigating the chemical properties of graphene samples.
Graphene and Raman Spectroscopy
The study of graphene represents a uniquely well-suited application of this analytical technique. Raman spectroscopy is inherently very sensitive to symmetric covalent bonds – such as those found in graphene2. Moreover, Raman is sensitive to the local chemical environment of these covalent carbon bonds, providing additional insight. Aspects of graphene and local composition that can be determined with Raman include the number of layers, how those layers are oriented, how much strain those layers are experiencing, the degree of doping, and lattice disorder3.
Many of these properties are directly tied to the two-dimensional nature of graphene, and so understanding these aspects about a graphene sample is important because the properties of these nanomaterials can change dramatically with even small structural differences. Graphene primarily exists as a flat sheet of carbon atoms, but if these layers begin to stack, they begin to form graphite, which does not have the same desirable properties for the intended applications. This makes it crucial to be able to determine the exact number and form of the layers in a high throughput and reproducible manner.
A similar situation presents itself during the analysis of carbon nanotubes4. These can be thought of as nano-scale straws made of graphene, and like the sheet form can also exist as multi-layered variants. Multi-walled carbon nanotubes also have differing properties to those possessing a single wall and will exhibit different properties. When designing a product or process that utilizes these types of materials, it is crucial that the exact number of layers or walls is known, otherwise unintended behaviors of the material could ensue.
Principles of Raman Spectroscopy
Raman spectroscopy is an optical technique that can gather information about how atoms and functional chemical groups are bonded within a compound. When monochromatic light (such as a laser) is introduced to a specimen, inelastic scattering occurs, causing a wavelength (color) shift related to the fundamental frequencies of the molecular vibrations within that specimen1. If this scattered light is collected, carefully separated by wavelength, and recorded with a scientific camera, a Raman spectrum is obtained which provides a wealth of chemical insights. Not only can the identity of different molecules be ascertained, but also information about the local chemical environment, degree of order (or disorder), and other properties.
This technique has extra utility when applied in a microscopy format, providing the ability to characterize the chemical composition of specific microscopic volumes of a specimen. In addition, sampling numerous regions of a specimen can create maps or images which are sensitive to chemical structure, elucidating how various chemical or physical properties vary spatially, or even observing how these properties evolve over time for dynamic systems.
For more information about analyzing graphene and other carbon materials visit our nanomaterials page.
For Raman spectroscopy basics and theories visit our Raman Academy page.
To learn about Raman microscopes visit our Raman instruments page.
- Bumbrah, G. S. & Sharma, R. M. Raman spectroscopy – Basic principle, instrumentation and selected applications for the characterization of drugs of abuse. Egypt. J. Forensic Sci. 6, 209–215 (2016).
- Wang, Y., Alsmeyer, D. C. & McCreery, R. L. Raman Spectroscopy of Carbon Materials: Structural Basis of Observed Spectra. Chem. Mater. 2, 557–563 (1990).
- Childres, Isaac; Jauregui, Luis A; Park, Wonjun; Cao, Helin; Chen, Yong P (2013). ‘Raman spectroscopy of graphene and related materials’, in Jang, Joon I (ed.) New developments in photon and materials research. Hauppauge, NY: Nova Science Publishers.
- Brady, G., Jacobberger, R., Roy, S. S., Wall, M. & Arnold, M. Raman imaging as a tool for characterizing carbon nanomaterials. Am. Lab. 46, 1–4 (2014).
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