Slow electrons yield an unexpected picture of graphene
‘People expected that the material or the energy wouldn’t matter much, but this is not the case at all’, says Johannes Jobst. He and his colleagues recently (DATUM VAN MAKEN) published in Physical Review Letters. The physicists found out that when you aim a beam of slow electrons at several layers of graphene, the material either acts as a mirror or would not reflect at all, depending on the electron energy.
‘People said: there is nothing that can be learned there’, says Sense Jan van der Molen, ‘but we really learned a lot that we didn’t have a clue about five years ago.’
He is talking about their (HET IS HIER NIET HELEMAAL DUIDELIJK WIE "THEIR" ZIJN want ze zijn nog niet geïntroduceerd) findings with a new measurement technique, eV-TEM (electron volt-Transmission Electron Microscopy) in the top physics journal Physical Review Letters.
Electrons as light
PhD student Daniël Geelen developed this new technique as an upgrade to the LEEM (Low Energy Electron Microscope) machine that has been a centrepiece of Van der Molens group since 2010. Today, the group publishes the first paper about their eV-TEM and LEEM findings on graphene, which challenges the earlier understanding of how materials interact with electrons.
Electron microscopes use beams of electrons instead of light -as regular microscopes do- to image samples. Quantum mechanics ordains that electrons behave as waves, just like light, but with much shorter wavelengths. This means that they can resolve nanometer-scale details, unlike light microscopes. There is one catch, however: intense beams of fast electrons tend to blow samples to bits, especially soft tissues.
Gentle electrons
The LEEM offers one solution to that: it fires electrons softly, with energies of a few electron volts (eV) instead of thousands. While the imaging resolution of the reflected electrons is lower, the sample survives longer or indefinitely. Also, Van der Molen’s group hoped that the gentle nature of the impinging electrons would allow measuring characteristics of the sample material.
To really home in on this last aspect, Daniël Geelen built a second electron cannon, which shoots electron volt electrons through the sample. On top of reflections, this allowed the group to do transmission microscopy (eV-TEM), a wholly new microscopy technique.
Swiss watchmaker
‘To make use of the existing setup, the second electron source had to be really small, so Daniël needed the skills of a Swiss watchmaker’, says Johannes Jobst, while showing the cannon plus sample holder, all the size of an AAA-battery.
One of the first samples to be tested was a sheet of graphene, the flat sheet of carbon atoms arranged in a tight pattern of hexagons. ‘There has been much research on how electrons move within sheets of graphene, but not on how they move across the sheets’, says Van der Molen.
No more 'universal curve'
A theoretical model from the seventies predicts that low energy electrons will penetrate thin layers easily, since there are no modes of interaction with the electrons in the material. The higher the electron energy, the more interactions, the less of them pass through. ‘This is called ‘the universal curve’, says Jobst.
Contrary to what this theory predicts, the researchers found something entirely different and far from universal: a heavy dependence of the penetration depth on the electronic structure of the sample. For instance, the number of layers of graphene stacked on each other carried a heavy influence.
'Scientist should stop talking about a universal curve’
In the eV-TEM images, parts of the sample that consist of two or three layers have very different transmissions, and therefore grey shadings, than single sheet graphene. On top of that, the exact energy of the electrons matters a great deal. Instead of a steadily decaying a ‘universal’ transmission curve, the team found strong reflection peaks for certain electron energies, again depending on the number of layers of graphene. ‘I think scientists should stop talking about a universal curve’, says Jobst, since transmission depends heavily on the material.’
‘For electrons with specific energies multilayer graphene acts as a mirror’, Van der Molen adds. This, they describe in the paper, makes perfect sense. Electrons are waves, and the waves reflected off each single stacked layer can interfere: for some wavelengths, the tops of all reflected waves coincide, and the reflection is strong. For other wavelengths, the interference is destructive, and there is little or no reflection.
Sunglasses
This is the same effect that gives anti-reflective coatings on sunglasses their greenish or purplish hue: most wavelengths of light are suppressed by the interference of light from different stacked layers, except for one wavelength or colour. This opens the possibility of using graphene as a semitransparent mirror for electron beams. ‘You could maybe even build an interferometer’, says Van der Molen.
Interferometers are used in extreme precision measurements, for instance in gravitational wave detectors which use intense laser light. Instead, electron beam interferometry would open completely new modes of measurement.
‘There are many other ways to go from here, including imaging molecules like DNA’, says Van der Molen. The main message, however, is that this paper presents eV-TEM an adult microscopy technique for the first time, showing unexpected results and new avenues of research.