A study of the atomic structure of graphene supported on a silicon oxide substrate suggests that what is measured reflects not just the graphene itself, but the surface on which it sits
Graphene, a sheet comprising a single layer of carbon atoms, has unusual properties that may form the basis for electronic devices. For almost any device, however, graphene will need to rest on top of something, which raises the question of whether the measured electronic characteristics will correspond to the intrinsic properties of graphene, or the underlying substrate.
To explore this question, Masa Ishigami and co-workers1 from the University of Maryland in the US combine scanning tunnelling microscopy and atomic force microscopy in an ultrahigh vacuum to study the atomic structure of graphene on SiO2 — a material that is frequently used as an insulating substrate. SiO2 has a tendency to trap charges, however, which may affect electron transport in graphene. Moreover, lithographic processing of SiO2, which can leave behind a chemically damaging photoresist, can cause similar effects.
Ishigami and colleagues find that atomic resolution images of graphene taken before the resist is removed are blurry, indicating a perturbed graphene sheet, whereas images taken afterwards have well-defined features. They also show that graphene's characteristic 'waviness', known to affect its transport properties, is strongly correlated with the SiO2 surface roughness. The results may influence existing explanations for how structural defects affect spin transport in graphene, as well as how previous imaging studies have been interpreted.
References
Ishigami, M. Chen, J. H. Cullen, W. G. Fuhrer, M. S. & Williams, E. D. Atomic structure of graphene on SiO2 . Nano Lett. 10.1021/nl070613a (2007).
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Thomas, J. What lies beneath. Nature Nanotech (2007). https://doi.org/10.1038/nnano.2007.183
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DOI: https://doi.org/10.1038/nnano.2007.183