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.