Breaking the resolution barrier | University of Oxford
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Breaking the resolution barrier

Pete Wilton

When Robert Hooke first published his book Micrographia in 1665 he demonstrated the wonders of what could be seen through a microscope.

Ever since then scientists have been attempting to image smaller and smaller objects using first light and then electrons: but these efforts to achieve ever-higher resolution run up against fundamental physical laws.

Today many scientists are developing techniques to sidestep these 'resolution limits': Angus Kirkland of Oxford's Department of Materials is one such scientist and his Oxford group, working with JEOL in Japan, recently reported how they have been able to achieve a resolution of 78 picometres at an electron energy of 200kV in Physical Review Letters.

I asked him about resolution limits and how they can be overcome:

OxSciBlog: What determines the conventional resolution limit?
Angus Kirkland: In a light microscope the resolution limit is determined by the wavelength of light. In an electron microscope the wavelength of the electrons is far smaller but the resolution is limited by the quality of the electromagnetic lenses used to form the image. These impose a resolution limit that is far worse than that to be expected from the electron wavelength. It is convenient to think of the resolution limit as an effective aperture; the larger this aperture, the better the resolution.

OSB: How has your work overcome this limit?
AK: Tilting the direction of the electron beam by around one degree has the effect of shifting the position of the resolution-limiting effective aperture. By computationally combining images acquired with several different incident beam directions we can create a synthetic aperture, which is larger than in any single image, hence increasing the resolution. In our paper we use this 'aperture synthesis' approach to improve the resolution by up to 40 per cent compared to a single image.

OSB: What is the significance of attaining a resolution of 78 picometres?
AK: Improved resolution increases the range of materials and structural orientations that can be imaged  in the electron microscope. The particular value of 78pm has enabled us to image silicon in a specific orientation as a test case for the method.

OSB: What might it mean if it was possible to break the '30pm barrier'?
AK: With an electron microscope capable of 30pm resolution it is believed that the scattering cross-section of the atoms themselves will limit the resolution attainable and as such this is the ultimate resolution target.

Professor Angus Kirkland is based at Oxford's Department of Materials