by Tim Batten
Chemistry
Rapid Characterization of Large Areas of Graphene Using Raman Spectroscopy
aman spectroscopy probes the vibra- tions (phonons) in a material. These phonons are characteristic of a given material and allow its chemical and structural properties to be investigated. Raman spec- troscopy can be employed in a wide range of application areas, including semiconduc- tors, pharmaceuticals, and living cells. It can be used to investigate any substance (whether it be solid, liquid, or gas), except pure metals.
R
A technique has been developed for the rapid characterization of large regions of gra- phene—as big as cm2
—enabling the number
of graphene layers, along with strain and de- fects, to be investigated. The mass production of graphene is still in its infancy, and significant research is being conducted into growing large regions of pristine single-layer material. A contribution of Raman spectroscopy to this research will be discussed here.
Properties of graphene and
graphene research Graphene is a revolutionary two-dimensional material consisting of a single layer of carbon atoms. Electrons in graphene are able to travel ballistically, making it the most conductive material known to science. As a result, it can be foreseen that graphene will be the building block of the next generation of electronic devices.
Graphene is still a newly discovered mate- rial and there are many challenges that must be overcome before it can be used commercially. Its high conductivity relies on the material con- sisting of just a single layer of carbon atoms. As the number of layers increases, the properties of the graphene degrade until the point at which they match those of graphite. The production of large-area, single-layer graphene still requires significant research and development. Recently, the European Union set up a one billion euro research initiative focusing on graphene.1
This is clearly an important area of research and investigation. An effective tool for studying
AMERICAN LABORATORY • 15 • SEPTEMBER 2013 Figure 1 – inVia Raman microscope. (Figures 1–4 courtesy of Renishaw plc.)
the properties of graphene is the inVia Raman microscope from Renishaw plc (Old Town, Wotton-under-Edge, Gloucestershire, U.K.). See Figure 1.
The first Raman measurements on graphene were conducted by Andrea Ferrari et al. at the University of Cambridge in 2006.2
Using
a Raman spectrometer from Renishaw, they were able to show that the Raman spectrum changes depending on the number of graphene layers present. This makes Raman an ideal tool for graphene research. Since then it has been found that Raman spectroscopy can be used to characterize a wide range of material proper- ties in graphene, including stress and strain, doping, edges, electron mobility, disorder, defects, and thermal conductivity. An excellent review that discusses the capabilities of Raman on graphene can be found elsewhere.3
Raman spectroscopy as a tool for identifying the number of
graphene layers This article looks at how Raman spectroscopy can be used to examine the number of graphene
layers present in flakes and films. Knowledge of this information is vital because the electrical properties of graphene decrease as the number of layers increases. Figure 2 illustrates Raman spectra collected from mechanically exfoliated flakes consisting of a single-layer, a bi-layer, and multiple layers of graphene. The number of layers was confirmed using atomic force microscopy (AFM) measurements. Here it can be seen that the 2-D Raman band undergoes a significant change in shape and also broadens with increasing number of graphene layers. This makes it an excellent indicator of the number of layers present in a flake.
Raman imaging of graphene Renishaw’s patented StreamLine Plus fast im-
aging technique was used to collect a Raman image from mechanically exfoliated graphene flakes deposited on a SiO2
/Si substrate.
StreamLine differs from conventional Raman mapping in that it uses a laser line instead of a laser spot. This spreads the laser power over the sample, reducing the power density on the sample, and thus allowing ~20× more laser
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