Seeing is believing - closely observed atoms
The column of the abberation corrected STEM instrument. The stainless steel on the right half of the picture is mainly concerned with the vacuum. The electron beam travels from the bottom to the top of the column on the left half of the image with the sample and the corrector roughly in the middle of the column.
Atoms are small. So small that if you scaled up a football to the size of the earth the atoms would still only be approximately ten millimetres in size. Yet scientists at the SuperSTEM laboratory in Daresbury have developed a microscope so powerful that it can make atoms visible. 'The major breakthrough at Daresbury is imaging atoms within structures, so we can see how atoms interact at the interface between different substances,' explains Alan Craven. 'Previously we could only see atoms at the surface.'
The SuperSTEM (Scanning Transmission Electron Microscope) uses high-energy electrons to image atoms. Electrons are used as their wavelength is about 100 times smaller than the size of an atom, light has a wavelength about 1000 times larger than an atom limiting the smallest detail visible in ordinary light microscopes. But defects in electron lenses meant that until recently the smallest visible detail was still bigger than an atom. 'The challenge for scientists was to develop and install a corrector to overcome the defect known as ""spherical aberration"", a defect common to all lenses. It is similar to the human visual problem of astigmatism where focus can be achieved in one plane but not in the plane perpendicular to this giving a distorted and blurry image,' explains Alan.
In 1997, Ondrej Krivanek and Nicolas Delby designed and constructed the first corrector, supported by Mick Brown leader of the Microstructural Physics Group within the Cavendish Laboratory, and a generous grant from the Royal Society Paul Instrument Fund. Thus the world's most powerful electron microscope was created. 'You could describe it as an electron microscope with spectacles,' says Mick.
'SuperSTEM is one of only four such microscopes in the world and its key advantage is its incredible stability. If the system is unstable the image changes before all the necessary parameters for the microscope can be set', explains Alan. Our system is so stable that any sample in the microscope would move no more than half a millimeter in 100 years. That's 2000 times slower than continental drift.'
'Imaging how atoms interact at interfaces is key to the development of the next generation of computer chips. Computing power continues to increase as transistor size decreases but we are now reaching our technical limits. The key insulating layer of silica in these transistors has just five silicon atoms across it,' explains Alan. 'Any thinner and the current leaking across this insulating layer will increase rapidly because of an effect known as quantum mechanical tunnelling, making the transistor unusable.' Alternatives to silica are currently being sought. 'With SuperSTEM we can see how the atoms in these alternatives behave at interfaces which determines their suitability as the next generation insulators,' says Alan.
SuperSTEM also has applications in medicine and is being used to aid understanding of diseases such as haemochromatosis, where the liver becomes overloaded with iron. The tiny nanocrystals that hold iron within the body are being examined as their structure will shed light on how iron is transported, stored and released in the body.