On the 22nd February 2016, the Royal Microscopical Society hosted a workshop at Queen Mary University of London under the title ‘Electron Microscopy Characterisation of Organic-Inorganic Interfaces‘. The meeting addressed the progress in using electron microscopy to study organic materials. Traditionally this is challenging because the energy and number of electrons passing through organic material can very quickly destroy its structure. However, with advancements in electron detection and microscope automation, images can now be acquired before the damage is done by the beam.
There were many interesting talks covering a range of samples, from museum specimens to magnetic organisms. Probably the closest related work was in studying defects in graphene, presented by Professor Angus Kirkland from University of Oxford. They had used the electron beam as an intense spot to create defects, followed by using it in a more-spread, less-intense beam to study what had been created. They could even watch how these defects move through the graphene lattice, nudged along by the energy of the electron beam.
I presented our work on vanadyl phthalocyanine (VOPc) on graphene as a poster during the meeting (a write-up of this work can be found here). VOPc, like many other organic materials, is very difficult to study with electron microscopy as it damages so quickly. By carefully controlling the microscope, we could take images of the VOPc molecules just at the point they are exposed to the electron beam, and get meaningful information about them before they are destroyed shortly after.
An essential area of materials research is the electronic properties of a solid. Fundamentally, it is the answer to the question: how do electrons (and so electricity) behave in that solid? You can measure these properties by making a device and testing it, but this can be hard to interpret.
An alternative is to measure the energy and momentum of electrons in the solid, called the electronic band structure. The most powerful tool for this is angle-resolved photoelectron spectroscopy (ARPES). With ARPES, you shine a light (a photon source) on the surface of the material and measure the electrons that are emitted. The electron’s energy after leaving the surface can be related directly back their energy in the material. Further, the ‘angle-resolved’ part of ARPES means you collect the number of electrons at different angles off the surface, which tells you what momentum the electrons had in the solid. With both the electron’s energy and momentum, you get the electronic band structure.
A futher improvement to ARPES is to focus the light to a microscopic point, called scanning photoemission microscopy (SPEM). While ARPES is now a relatively common laboratory technique, SPEM is not. This is because you need a lot of photons to be able to collect meaningful spectra at each microscopic point, and even more so because a lot are lost during focusing. This is where a synchrotron comes in. They can produce the many photons needed for SPEM.
We have used SPEM for two main purposes. The first is to make a map of the property that you are interested in. For example, we can use this to see how the orientation of graphene differs across a surface and see if its electronic properties change with this orientation.
The other way we have used the microscope is to find a small (1 µm) area of interest that can then have its band structure mapped. This has prooved quite successful in our investigations of small flakes of exfoliated materials where the sample is only 1 µm across.
Spectromicroscopy is a beamline at Elettra synchrotron near Trieste in Italy, run by Alexei Barinov and his PhD student, Victor. At Spectromicroscopy we have used SPEM to measure and map the band structures of many different 2D materials.
Me, Neil, Alexei, Victor and Natalie in front of Spectromicroscopy at Elletra.