Characterization of Graphene by Raman spectroscopy - NanoLabStaff/nanolab GitHub Wiki

by Junghwan Huh ([email protected])

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Raman spectroscopy is a powerful tool for characterizing both the number of graphene layers and to quantify defects in the graphene

1. Sample preparation

• Typically, no special sample preparation is required.
• Basically, a graphene on Cu-foil sample can be investigated by Raman spectroscopy. However, the Raman spectrum of the sample has a sloped background due to emission from the Cu. On the contrary, the Raman spectrum of the graphene on SiO2/Si can be enhanced due to interference by the dielectric substrate. Therefore, the transferred graphene on SiO2/Si is more suitable for Raman study.
• Put a graphene sample on the middle of slide glass. Make sure that the sample lies flat.

2. Load the sample

• Push "Door release" button on the front panel.
• Open main door and insert the slide glass with the graphene sample.
• Make sure that a spring-loaded stage clip is well fitted with an edge of the slide glass.

3. Raman spectroscopy settings

• A 532nm laser is used for characterization of graphene. Typically, for stabilization, the laser needs 20 ~ 30 min. to warm up.
• Make sure that the sample and the laser are well focused.
• Run a "quick calibration" before measurement.

Typical measurement condition of Raman spectroscopy for graphene are:

• Optical microscope magnification : x100
• Spectrum rage : from 1100 to 3200 cm-1
• Laser intensity : 5 %
• Time : 10 s
  
  The measurement conditions can be changed in the “Spectral acquisition setup” menu.

4. Raman spectrum of graphene

• A typical Raman spectrum of graphene consists of two main peaks and a few more very small peaks.
• The two major peaks are found around 1586 and 2686 cm−1, so-called G and 2D peaks, respectively.

G peak : The G peak is due to the Stokes Raman scattering with one phonon (E2g) emission at the Brillouin zone center. As a doping concentration (both an electron and a hole) increases, its peak frequency blue-shifts and its width becomes narrow.

2D peak : The 2D peak originates from the Stokes-Stokes double resonant Raman scattering with two-phonon (A1´) emissions. It is known that the peak blue-shifts with increasing hole doping and red-shifts with increasing electron doping. The shape of 2D peak is very sensitive to the number of graphene layers.

D peak : A third peak known as the D peak at ~1350 cm-1 is associated with disorder and defect states. The peak is typically very weak in a graphene. If the D peak is significant compared to other peaks, it means that there are a lot of defects in the graphene. It is known that the intensity of the D-peak is proportional to the level of defects in the graphene.

5. 2-dimensional (2-D) mapping of Raman spectra (optional)

• Select “Map image acquisition”. In “Map image area selection”, you can select various types of mapping, as seen in Figure 7. If you want to measure typical 2-D Raman mapping, select the “Rectangle filled”.

• Drag a mouse point over optic image window or insert location values into the “Map image acquisition” menu to select the mapping area

• Insert same values used in section 3 into “Map measurement setup” and run the measurement.

References [1] Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rep. 2009, 473 (5-6), 51–87.

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