The commercial exploitation of the remarkable properties of graphene relies on the development of efficient methods to fabricate the large quantities required for industrial applications. One of the most promising approaches is the epitaxial growth of graphene layers on metals, where chemical vapor deposition can produce large areas of graphene with uniform thickness. However, this technique relies on the metals’ catalytic activity and is expected to be less effective on low reactivity metals. Recently, however, A.J. Martínez-Galera and co-researchers at the Universidad Autónoma de Madrid, Spain, have devised a novel method for preparing graphene on the surface of relatively inert metals, and used this technique to grow high-quality, monolayer graphene films on Cu(111) and Au(111).
The researchers report in the September 14 issue of Nano Letters (DOI: 10.1021/nl201281m; p. 3576) that thermal decomposition of hydrocarbon fragments produced by irradiating a metal surface at high temperature with low-energy ethylene ions can result in the formation of graphene on the metal surface. The researchers placed clean metal single crystals as substrates in an ultrahigh vacuum (UHV) chamber at 800°C together with ethylene at high pressure. Irradiation with an ion gun results in low-energy ethylene ions that are accelerated against the metal surface, after which the sample is annealed at 900–950°C. The researchers observed negligible ethylene adsorption or decomposition when the same process was followed but without the ion gun treatment.
The researchers obtained monolayers of high-quality graphene on Cu(111) substrates and atomically characterized them in UHV using scanning tunneling microscopy (STM). As expected for graphene monolayers weakly coupled to substrates, several moiré patterns with different periodicities were observed due to small rotations of the graphene lattice with respect to the substrate. As shown in the figure, relatively large defect-free regions of graphene monolayer were grown on the Au(111) surface. These findings were supported by low-energy electron diffraction measurements and Auger spectroscopy and confirmed the macroscopic formation of graphene on the Au(111) surface.
The properties of the graphene/ metal contact were also investigated and were shown to be weaker than in any previously reported graphene/metal system. The Fermi wave vector, estimated from low-bias STM images, where standing waves coming from the Au(111) surface are observed through the graphene layer, is in perfect agreement with the value for pristine Au(111). The minimum around the Fermi level observed in differential conductance plots, as obtained with STM, is associated with the Dirac point of the graphene’s electronic structure—an indication of the lack of doping for this system. The researchers said, “Our new method paves the way to extend the range of possible substrates for the epitaxial growth of graphene to other low reactivity metals.”
(a) Three graphene flakes grown epitaxially on a Au(111) surface are shown in this scanning tunneling micrograph with area 53 nm × 41 nm. Reconstruction of the herringbone pattern characteristic of the Au(111) pattern is evident. (b) A scanning tunneling micrograph of a portion of defect-free graphene grown on a Au(111) surface displays a honeycomb pattern. Reprinted with permission from Nano Lett. 11 (9) (2011), DOI: 10.1021/nl201281m; p. 3576. © 2011 American Chemical Society.
