Graphene, a single two-dimensional layer of carbon atoms, has captured broad interest  because of its exceptional optical, mechanical and electrical properties. The synthesis of graphene by chemical vapor deposition (CVD) is currently being studied extensively by many groups of research, in particular large-scale growth on copper surfaces. Despite much research in this field, CVD graphene in different growth conditions exhibits various deformities, including the presence of hillocks, defects, grain boundaries and multilayers island formation. In order to obtain large-scale homogeneous graphene, Prof. Martel’s group from Université de Montreal rigorously studied the role of hydrogen and oxidizing impurities present during graphene CVD growth. A series of growths were conducted using H2/CH4, purified H2/unpurified CH4, and purified Ar/CH4 gas mixtures, as well as unpurified CH4 and purified CH4 gas. For more details on their fabrication methods see [1].

As a first step in characterizing the uniformity of the deposit, SEM and LEEM images were collected. This helped determine that hydrogen is not required for complete growth of graphene layers in the absence of oxidizing impurities. To further investigate the number of layers and the quality of the graphene produced, Raman microspectrometry measurements were acquired. This Raman analysis revealed strong 2D intensity and low D band intensity, a sign of high-quality graphene layers. In addition to the typical G (1590 cm-1) and D (1350 cm-1) bands, two modes, R (~1455 cm-1) and D’ (~1625 cm-1), were also present. These bands mark the presence of bi-, tri- and higher order multilayer graphene grown with random angles between the layers.

To thoroughly evaluate the structural properties of graphene grown in this manner, hyperspectral Raman imaging was carried out on the two opposite runs: a deposit achieved by using the original recipe of unpurified CH4 and H2 (FIG. 1a), and one using only purified CH4 (FIG. 1 b). Raman imaging was performed with the hyperspectral Raman imaging platform RIMA™ based on Bragg tunable filters. In these measurements, a CW laser at λ = 532
 nm illuminated a 100 × 100 μm2 sample surface area through a 100X microscope objective. The sample was excited with a fluence of 150 μW/μm2, and the resolution was diffraction limited.

FIG. 1 presents Raman images of the graphene G band (a-b) with associated Raman spectra (c-d) taken at different locations on the sample. The intensity variations of the G band reveals information on the stacking of layers. The most significant changes in intensity observed in FIG. 1b can be explained by resonance resulting from the modified twisted angle (13.5° at λexc = 532
 nm [2]) of the bilayer graphene. The enhancement of the G band intensity and the appearance of the R and D’ bands indicate that twisted graphene layers are mainly present in purified conditions.

The intrinsic specificity of Raman scattering combined with global imaging capabilities allows users to assess large maps (hundreds of microns and more) of defects, number of layers and stacking order.

[1] Choubak S., Levesque P. L., Gaufres E., Biron M., Desjardins P., Martel R., Graphene CVD : Interplay Between Growth and Etching on Morphology and Stacking by Hydrogen and Oxidizing Impurities, The Journal of Physical Chemistry C, 2014, 18.

[2] Havener R. W., Zhuang H. L., Brown, L., Hennig, R. G., Park, J., Angle-Resolved Raman Imaging of Interlayer Rotations and Interactions in Twisted Bilayer Graphene. Nano Letters. 2012, 12.



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