A Maki, a Sensor and an XPS Machine

In Peru, I am part of a research project focused on developing sensors that can detect acetone. The samples were fabricated by members of our research group using different thin layers and thermal treatments. My task is to understand what happened to their surface during this process and how these changes may affect their performance as sensors.

Although we know how the samples were made, we do not know their exact surface composition after they were annealed. Well, you know, not everything remains the same after you “cook” it—anneal it—at 500 °C and trigger chemical reactions, diffusion, and who knows what else.

What sauce on top of a maki ?

Normally, I would simply use an electron microscope to measure the composition using EDS and be done with it… but I cannot. The samples are too thin. The sensor has two layers—three if we count the substrate—and with EDS, I measure everything at the same time. I tried, believe me. It is like wanting to know the flavour of the sauce on top of a maki, but instead of taking a small taste, I put the whole maki in my mouth and try to identify the ingredients of only the sauce.

XPS helps

That is why I brought these samples with me to Berlin. At HZB, I am using surface-analysis techniques to investigate what is actually present in the outermost layer of the sensors. And that is where XPS saved me. XPS can analyze only the first few nanometers of a material, which makes it exactly the kind of technique I need. So, when I arrived at HZB, I thought: “great, problem solved!” Well… not quite.

Safety first

Before I could obtain my first useful measurement, I had to overcome several smaller challenges. The first was the safety system. I quickly realized that people here take safety very seriously. To be honest, one part of me feels protected and happy about that, while the other part thinks, “Oh, really? Was it necessary to complete one safety course for HZB in general, another for the building where I work, another for the specific laboratory, and then one more for that particular piece of equipment?” I feel that some of them could perhaps have been combined, but anyway… 

Tricky transfer

Then there was the question of how to introduce my samples into the equipment. The laboratory works extensively with perovskites, which often contain lead. Since I do not want to contaminate my samples with lead, I must follow a different procedure from the one normally used. My samples must be carefully transferred without coming into contact with potentially contaminated parts of the system.

Of course, I must also be extremely careful with the equipment itself. However, learning all these procedures has also given me the opportunity to understand how the XPS works. And it’s quite awesome.

And how does XPS work?

Basically, XPS illuminates the sample with photons of a fixed energy—around 1400 eV in our case—and activates the photoelectric effect. These photons transfer their energy to electrons inside the material, causing some of them to escape from the surface.

The emitted electrons are then slowed down and focused using electric fields before entering an analyzer, where they are separated according to their kinetic energy. Finally, a detector records how many electrons arrive and the energy with which they arrive. Using this information, we can calculate their binding energy and identify the elements and chemical states present in the sample.

Just the surface

Like EDS, XPS allows us to determine which elements are present. However, there is one very important difference: the electrons detected by XPS can escape only from approximately the first 10 nanometers of the material. For this reason, XPS is considered a surface-sensitive technique. And that is exactly what I needed.

Thanks!