Raman Imaging of Singlelayer and Multilayer Graphene
Raman imaging of singlelayer and multilayer graphene
Only 5 minutes shows the distribution image of singlelayer and multilayer graphene
The image above shows the Raman imaging of graphene thin film distributed on a heat oxidized silicon substrate. It takes only a few minutes to identify and show the distribution of singlelayer graphene which is one carbon atom sheet and multilayer graphene in double layers, triple layers and quadruple layers respectively with a high spatial resolution of 350nm.
About the sample used for measurement
The sample observed by this measurement is prepared by cleaving graphite with adhesive tape then transcribing it into silicon substrate. In this way, all graphene are formed on silicon substrate at random, and if the substrate has a certain amount of SiO2 film on it, they can be detected by optical microscopy. However, it is difficult to distinguish exactly how many layers each graphene consists of (the right hand side image).
*This sample is provided by Dr. Daiju Tsuya of National Institute for Materials Science.
(References) K.S. Novoselov et al., Science 306, 666 (2004).
K.S. Novoselov et al., Proc. Natl. Acad. Sci. U.S.A.102, 10451 (2005).
Optical microscopic image of graphene
Distinction of graphene layer by G/D’ ratio
In this measurement, the number of graphene layers can be distinguished by examining the intensity ratio of G-band and D'-band in the Raman spectra of graphene. In singlelayer graphene, a very sharp strong peak can be seen in D-band. Moreover, peak intensity in G-band strengthens as double layers, triple layers and the number of layers increases.
Averaged Ramnan spectra of 4 selected rectangular regions(red, blue, green and grey frame)
In conventional Raman microscope, distribution observation of graphene is done by the Raman mapping method that detects Raman light at every single point while moving the stage. But this method cannot catch the large-area Raman signal in a short time, so it is difficult to distinguish the graphene layer included singlelayer efficiently. Moreover, the excitation light intensity of every point is so strong that the sample might be burnt.
Using the line illumination, RAMAN-11 can illuminate a wide region at one time, distinguish graphene layer at overwhelming speed and minimize the damage to every point of sample. Moreover, RAMAN-11 offers a high definition Raman image brought about by its high spatial resolution of 350nm.
(Reference)Method of distinguishing graphene layer by Raman spectroscopy
In this measurement, the intensity ratio of G-band and D'-band (G/D') in Raman spectra was used to distinguish graphene layer. This G/D’ is about 0.3 in single layer, increased linearly until about quintuple layers and saturated in more than sextuple layers. In singlelayer graphene, the peak position of D’-band appears at about 2678.8±1.0cm-1, but when it becomes more than double layers, the peak position of D’-band shifts to the high wavenumber side and the FWHM broadens too. Moreover, when the graphene becomes more than double layers, D’-band is composed of two or more subpeaks, though D’-band of singlelayer graphene can be fitted with Single Lorentzian. That is why the wave forms on the low wavenumber side of D’-band in double-layer graphene have a little distortion of shape. Thus, a lot of useful information which suggests the layers is included in the Raman spectra of graphene.
(References) D. Graf et al., Nano Letters. 7, 238 (2007).
