Grain selective Cu oxidation and anomalous shift of graphene 2D Raman peak in the graphene–Cu system
Javier Bartolomé, Leo Álvarez-Fraga, Montserrat X Aguilar-Pujol, Sandra Cortijo, Ana Cremades, Carlos Prieto, and Alicia de Andrés
Understanding the interaction between graphene and its supporting substrate is of paramount importance for the development of graphene based applications. In this work the interplay of the technologically relevant graphene–Cu system is investigated in detail as a function of substrate grain orientation in Cu polycrystalline foils. While (1 0 0) and (1 1 1) Cu grains show the well-known graphene-enhanced oxidation, (1 1 0) grains present a superior oxidation resistance compared to uncovered Cu and an anomalous shift of its graphene 2D Raman band which cannot be explained by the known effects of strain and doping. These results are interpreted in terms of a weak graphene–Cu coupling at the (1 1 0) grains, and show that graphene can actually be used as anticorrosion coating, contrary to previously reported. The anomalous shift is suggested to be the result of an enhanced outer Raman scattering process which surpasses the usually dominant inner process. Since Raman spectroscopy is widely used as first and main characterization tool of graphene, the existence of an anomalous shift on its 2D band not only challenges the current theory of Raman scattering in graphene, but also has profound implications from an experimental point of view.
(a) and (b) Optical micrographs of the control sample and the Gr/Cu foil after several months of air exposure. Some low index grains are indicated. Insets show the corresponding orientation maps as determined by EBSD. The orientation color code is displayed at the bottom left. (c) and (d) Raman spectra acquired at different grains of the control sample and the Gr/Cu foil, respectively. Due to the inhomogeneous distribution of the oxide layer on the Gr/Cu foil, two spectra, representative of the highly and lowly oxidized areas, are shown in (d). The two peaks at 1350–1600 cm−1 on (c) correspond to carbonaceous species deposited by the SEM during the EBSD measurements.