Fresh focus: The microscope opens a new range of studies, says Ramasse
The Engineering and Physical Sciences Research Council has spent £3.7 million on a new microscope. One might say that is a somewhat hefty investment for such a humble piece of scientific equipment, yet this is no typical piece of kit.
The new Scanning Transmission Electron Microscope, installed at the SuperStem laboratory at Daresbury in Cheshire, is not only able to pinpoint the position of single atoms but to analyse the bonds between atoms. Scientists will now be able to examine how materials behave at a level a million times smaller than a human hair.
One of only a handful of similar devices that exist around the world, the SuperStem microscope provides new capabilities for UK research and will ultimately lead to a greater understanding of materials’ electronic properties when in bulk and how they may perform when used. This knowledge should see the development of new materials for many sectors, including aerospace, as well as innovations such as the miniaturisation of microchips.
Professor Quentin Ramasse, scientific director at SuperStem, explains that electron microscopes, which use high-voltage electron beams to create an image, have been around since the 1930s, with the lenses used to focus electrons – “essentially a big cylindrical magnet”. However, for years these devices suffered from aberrations. Ramasse says: “It is not like a nice piece of polished glass that you can shape and repolish to correct any issue on the surface to make sure it focuses perfectly. With magnetic lenses you can’t do that, which has meant that for decades it was impossible to focus the electrons to the point where you could see atoms or atomic columns.”
One of the problems experienced by those developing instrumentation to correct these electron optical aberrations between the 1930s and 1950s related to limitations of mechanical engineering. It is a very precise piece of machinery, explains Ramasse, and requires tolerances that are extremely tight – and getting pieces of metal machined down to better than micrometre precision was particularly difficult to do without computer-assisted design.
“It is also extremely complex because you are powering dozens of little individual lenses (or magnets) and each of those really needs to have its own power supply,” he says. “It requires high precision of hundredths of parts per million or better for stable current and voltage supplies. This is technology that has only become available of late.”
Furthermore, the microscope is incredibly complex, with hundreds of controls that could not be operated by hand. A computer can operate all the controls simultaneously and detect the lens aberrations through a number of diagnostic images present in the electron beam, which are then fed back to a corrector to reshape the beam and ensure the lenses are operating at the highest levels of accuracy.
The breakthrough in this aberration correction technology came in the late 1990s from two research groups, one in Germany led by Professors Rose and Haider and another group in Cambridge led by Ondrej Krivanek, who later formed the company Nion, where the SuperStem microscope was developed. The aberration correction technology that allows researchers to pinpoint single atoms has been used by the Engineering and Physical Sciences Research Council for the past five years. But the new SuperStem microscope has one special additional instrument, a monochromator for high-energy-resolution electron energy loss spectroscopy.
Professor Rik Brydson, from the school of chemical and process engineering at the University of Leeds and chairman of the SuperStem advisory consortium, explains that this increase in energy resolution for spatially resolved electron spectroscopy in the microscope produces an order of magnitude of 10 times improvement.
“This means it is now possible to access energy loss processes in the milli-electronvolt regime associated with very low-energy electronic excitations and also those associated with chemical bond vibrations in the sample, both induced by the incident electron probe,” he says. “In principle, this allows you to do spectroscopy at the sub-nanometre atomic level.” In other words, this advance means researchers can now analyse the bonds in between atoms.
This is achieved as the SuperStem microscope produces an electron beam with a very narrow electron energy distribution. As the electron beam interacts with and travels through the sample material being analysed, it will lose a minute amount of speed or energy and “change colour”, similar to light changing colour when it travels through a prism. “When the electrons go close to a certain bond, that bond then starts vibrating,” explains Ramasse. “To excite this vibration the beam loses a little bit of energy and the beam changes colour ever so slightly and you can measure that change if you have the precision, which we do now.”
This means researchers are now able to look at a material and detect minute changes, tell different elements apart and see how they are bonded together, playing an important role in the investigation of surface structure, catalysis and dispersion of surface photons.
Ramasse says: “It is going to open up a whole new range of studies.” Areas of potential research are as diverse as alloys, composites, drug delivery and developing materials for nuclear fusion.
Brydson explains that gaining access to vibrational signatures associated with chemical bonds can help with the understanding of structure-property relationships in thermoelectric materials for conversion of waste heat into electricity, ferroelectric materials which can convert between mechanical energy and electrical energy, such as piezomaterials and sonar, as well as surface reactions on catalytic materials.
He adds: “Vibrations associated with bonds involving light elements should be particularly easy to detect which may allow the identification of previously invisible elements like hydrogen in material structures such as hydrogen storage materials and pharmaceuticals.”
Furthermore, access to low-energy electronic excitations means it is possible to map band gaps in semiconductors. This could lead to a greater understanding of how these may change at interfaces between materials as well as revealing defects. Brydson explains: “This is of great benefit for functional materials engineering and electronic device materials.” His own research has been looking into nuclear graphite, to understand why cracks and defects may occur. The work could help to forecast the life of nuclear reactors and design materials for power stations.
Ramasse says that large companies are already making use of the SuperStem microscope’s enhanced research capabilities, and he believes that more engineering firms would benefit from incorporating this kind of scientific research into their businesses. For example, the second-largest chemical catalysis firm in the world, Haldor Topsøe, has used SuperStem to look at molybdenum disulphide, a catalyst used in oil refineries to remove harmful sulphur impurities from fossil fuels.
The firm has imaged its constituent atoms one by one to gain a detailed understanding of its structure, in particular a single atom rearranged at the edge of the nanoscale MoS2 catalysts. With this information the researchers have been able to see how additives, such as cobalt, alter its structure and boost its properties.
The SuperStem microscope is also expected to push forward advances in technologies that rely on semiconductors and transistors. The microscope will be able to measure the electronic response and understand the atomic electronic structure of materials used. This is expected to have an impact on the commercially desirable attempts to miniaturise microchips.
Ramasse says: “Intel are talking about 14 nanometre processors these days. They really want to get the smallest dimension of their device down to a few nanometers and then you’re entering a realm where you are able to add a few atoms of this and a few atoms of that. It is almost atomic-level cookery – you are trying to completely change the property of a material to make it behave as a proper semiconductor.”
Another promising material that may be pushed forward by research groups using the SuperStem microscope is the much discussed wonder substance graphene.
Ramasse says: “The reason why graphene is so popular and being talked about so much is because there is a real push towards single-atom materials design and engineering. This is what this microscope will enable – this ability to probe changes in those materials that you design at such a tiny level and then being able to understand it and scale it back up again for real-world applications. I hope that is what it is going to contribute.”