Mar 7, 2016
Can atomic force microscopy really measure the thickness of graphene?
The major issue with imaging 1-atom thick materials is that there is rarely a perfect contact between the substrate and sample. This is often the case when investigating graphene, which is prepared by transfer onto a silicon wafer. This imperfect contact can be further exacerbated by the presence of a single layer of water atoms, often present on all surfaces under standard conditions. This issue is most commonly observed when imaging with an atomic force microscope (AFM), which directly images a sample in 3 dimensions using an atomically sharp tip. Reporting in Nanotechnology, researchers at Flinders University have optimized an AFM technique called PeakForce Tapping AFM to accurately measure graphene by imaging with high pressure.
The measured height of a graphene flake has been shown to vary with applied force from 1.7 nm to 0.4 nm, with a linear correlation. Since the thickness of a single graphene layer is expected to be 0.34 nm (atomic layer spacing in graphite), the error in measured thickness has decreased drastically by simply imaging with a higher applied force.
The key parameter to accurately measuring graphene is found to be the applied pressure. At low applied pressure the measured height is equivalent to the sum of the graphene layer thickness and the buffer layer thickness. As the pressure applied to the graphene by the AFM tip increases, the graphene is pushed into the buffer layer and a more accurate value is measured until finally the graphene is pushed through the buffer layer to the underlying substrate.
Figure (above) and video (below) demonstrating the mechanism for change in measured height with applied pressure.
More information about this research can be found in the journal Nanotechnology 27 125704.
About the author
Cameron Shearer is a Research Associate at the Centre for NanoScale Science and Technology at Flinders University, South Australia, who specialises in nanomaterial synthesis and characterisation. His current research interests include the application of carbon nanomaterials in energy conversion devices, particularly photovoltaics.