Sep 17, 2009
Graphene buckles under stress
Graphene, a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice, has shown remarkable physical properties such as room temperature quantum Hall effect and high electron mobility. These discoveries were initially made on flakes of graphene sheets that are readily obtainable through mechanical exfoliation of a graphite crystal. Alternatively, wafer-sized graphene can be grown epitaxially on SiC, by the sublimation of silicon atoms from the SiC substrate at high temperatures (~1400 °C). However, ridges and wrinkles are often observed on these epitaxial graphene films.
Structural secrets uncovered
Reporting their results in the journal Nanotechnology, researchers from the University of Wisconsin, Milwaukee, US, have elucidated the origin of these ridges by imaging them at the atomic scale using scanning tunneling microscopy (STM).
The scientists found that these ridges, or wrinkles, are in fact bulged regions of the graphene layer, which is compressively strained because of the mismatch of its lattice and thermal coefficient with the SiC substrate. While bulk materials typically relieve strain through the formation of dislocations, a truly two-dimensional material such as graphene, which possesses strong in-plane covalent bonding and weak interlayer interaction, is free to buckle and bend away from the surface to relieve the strain.
The researchers also found that it is possible to manipulate and create these ridges and wrinkles using the STM tip during imaging. The team then posed the question – can ridge-free epitaxial graphene be grown by minimizing compressive strain? The answer is yes.
Using vicinal SiC substrates, wafers with a few degrees miscut, which creates smaller terrace sizes than those of nominally flat substrates, the group has successfully demonstrated growth of nearly ridge-free graphene films.
The team is currently investigating how these ridges and wrinkles affect electron mobility in epitaxial graphene.
About the author
The work was carried out at the physics department, University of Wisconsin, Milwaukee (UWM), and was supported by the US Department of Energy (DOE). G F Sun is a PhD candidate at the Institute of Physics, Chinese Academy of Sciences, Beijing, China, doing his thesis research at UWM. Dr J F Jia and Dr Q K Xue are physics professors at Tsinghua University, Beijing, China. Dr Lian Li is a physics professor at UWM.