The characterization of graphene with combined Raman-AFM imaging provides a nondestructive method to extensively investigate
Graphene consists of carbon atoms which form angstrom-thick two dimensional sheets. These sheets occur as multilayers in graphene
flakes. The characterization of graphene requires nondestructive measuring tools like combined Raman-AFM imaging to determine
its properties. While AFM can provide information about the physical dimensions of nano-materials, Raman imaging gives insights
into the molecular composition of a material. Using the combination of confocal Raman microscopy with AFM, the high spatial
and topographical resolution obtained with an AFM can be directly linked to the molecular information provided by confocal
AFM Imaging of Multilayered Graphene
The graphene flake was deposited on the oxide top layer of a Si-wafer by exfoliation. The AFM topography image reveals the
3D-physical dimensions of the graphene flake (Figure 1a). The black line in Figure 1a indicates the cross section of a height
profile. This height profile shows the topography variation over a bi-, mono-, and no-graphene layer. The height difference
between the Si-substrate and the first grapheme layer is 1 ± 0.1 nm, whereas the double layer of graphene is 1.36 ± 0.1 nm
Figure 1: (a) AFM topography image (AC-mode) of graphene, (b) Height profile along the cross section indicated in Figure 1a
Chirality Investigations of Graphene with Confocal Raman Microscopy
The Raman image (Figure 2a) was evaluated from a 2D spectral array, consisting of 150 × 150 Raman spectra (22,500 spectra
in total). The integration time for each Raman spectrum was 0.05 s per spectrum, leading to a total acquisition time of less
than 20 min.
Figure 2: (a) Color coded Raman image and comparison of edge chirality (zigzag or armchair) with the angles of the different
graphene layers. (b) Raman spectra measured at different positions of the graphene flake (colors correspond to Figure 2a).
For detailed information please refer to (1).
The characteristic Raman spectra for a mono-, bi-, and multilayer of graphene are presented in Figure 2b and show the unique
D, G, and G' Raman bands of carbon nano-materials. The colors of the spectra match the colors in the Raman image. Different
layers of a graphene flake can appear in different chiralities (zigzag or armchair). Thereby the chirality of graphene correlates
with the angles between the edges of the graphene flake (1). By analyzing the differences in the intensity of the D-band,
Raman spectroscopy is a fast and nondestructive method to determine the chirality of edges and the crystal orientation in
graphene (Figure 2a).
These findings highlight the potential of confocal Raman-AFM imaging in the design and characterization of new optoelectronic
devices, based on the unique magnetic, optical, and superconductive properties of graphene edges.
(1) Y. You, Z. Ni, T. Yu, and Z. Shen, Appl. Phys. Lett.
93, 163112 (2008).
Samples: Courtesy of N. Manyala, David Dodoo Arhin, Fabiane Mopeli, University of Pretoria, South Africa.
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