A Confocal Raman-AFM Study of Graphene
Table 1: Summary of measured data obtained along cross section 2 from the AFM topography image and various Raman images recorded from the same sample area shown in Figure 1.
Number of
graphene layers 1
1 or 2?
Step height measured with AFM [nm]
0.9 ± 0.2 2.00 ± 0.2
Integrated intensity of G-band [CCD cts]
524 ± 10 1072 ± 10
position and D′-band width with increasing number of graphene layers is due to strong electron-phonon coupling in single and bi-layers of graphene [2, 10–12]. Based on these results, one can assign the number of layers that form the studied graphene flake. Numbers in the Raman images in Figure 4 indicate the number of graphene layers. An inconsistency however can be seen in the area marked with a question mark. In this area the position of the G-band shows the presence of one graphene layer, however the Raman image obtained from the Lorenz fit of the position of the D´-band (Figure 4 middle), the integrated intensity of the G-band (Figure 1b), and the AFM image (Figure 1a) show the presence of at least two graphene layers. Te data measured from cross sections along section 2 and 2′ in the AFM topography image (Figure 1a), the Raman images of the intensity of G-band (Figure 1b), position of G-band (Figure 4 top right), center of D′-band (Figure 4 middle right) are summarized in Table 1. Te inconsistency described above may arise from a flipped-over, decoupled part of the graphene layer. One can clearly see that the topography of this double layer is in good agreement with the double height of a single layer of graphene deposited on a SiO2/Si substrate. A similar result is obtained from the integrated intensity of the G-band, showing clearly the presence of two graphene layers. Nevertheless, the center position of the G and D′-bands clearly show the properties of a single, loose graphene layer.
Summary Te combination of a confocal Raman microscope with
an AFM was used for the characterization of a graphene flake. Te AFM image reveals the topographic structure of the graphene flake with a sub-nanometer z-resolution. From this image it is possible to determine the number of graphene layers in the graphene flake. Raman spectroscopy allows the identification of chemically
different materials or different properties within a material. Graphene flakes consisting of single or bi-layers of graphene of sub-nanometer height provide a strong Raman signal because
Center of G-band [rel. 1/cm]
1583 ± 1 1583 ± 1
Center of D´-band [rel. 1/cm]
2672 ± 1 2679 ± 1
of their special electronic and photonic properties. Te Raman signal is very sensitive to the number of graphene layers, thus Raman imaging of graphene flakes provides a fast and detailed description of the structure of graphene.
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