Unravelling the mystery of slow electrons

Electrons behave very differently depending on how much time they have; whether an electron is shot with high or low energy determines which effects can be triggered. Electrons with low energy can be responsible for the development of cancer, but conversely they can also be used to destroy tumours. They are also important in technology, for the production of tiny structures in microelectronics.

These slow electrons are difficult to measure and knowledge about their behaviour in solid materials is limited, with scientists often relying on trial and error. Now, researchers from TU Wien have obtained new information about the behaviour of these electrons, by using fast electrons to generate slow electrons directly in the material. This allows details to be deciphered that were previously indecipherable.

Professor Wolfgang Werner from the Institute of Applied Physics at TU Wien said the researchers wanted to determine what the slow electrons do inside a material, such as a crystal or a living cell. “To find out, you would actually have to build a mini-laboratory directly in the material to be able to measure directly on site. But that’s not possible, of course,” Werner said.

Werner explained that researchers can only measure electrons that come out of the material, but that does not explain where in the material they were released and what has happened to them since then. The team at TU Wien solved this problem by using fast electrons to penetrate the material and stimulate various processes. For example, the fast electrons can disturb the balance between the positive and negative electrical charges of the material, which can then lead to another electron detaching itself from its place, travelling at a relatively low speed and in some cases escaping from the material.

The crucial step is to measure these different electrons simultaneously. “On the one hand, we shoot an electron into the material and measure its energy when it leaves again. On the other hand, we also measure which slow electrons come out of the material at the same time,” Werner said. By combining this data, it is possible to obtain information that was previously inaccessible.

The amount of energy that the fast electron loses on its journey through the material provides information on how deeply it has penetrated the material. This in turn provides information about the depth at which the slower electrons were released from their place. This data can be used to calculate to what extent and in what way the slow electrons in the material release their energy. Numerical theories on this can be validated for the first time using the researchers’ data.

It was previously thought that the release of electrons in the material took place in a cascade: a fast electron enters the material and hits another electron, which is then ripped away from its place, causing two electrons to move. These two electrons would then move two more electrons from their place, and so on. The new data shows that the fast electron undergoes a series of collisions but always retains a large part of its energy and only one comparatively slow electron is detached from its place in each of these interactions.

“Our new method offers opportunities in very different areas. We can now finally investigate how the electrons release energy in their interaction with the material. It is precisely this energy that determines whether tumour cells can be destroyed in cancer therapy, for example, or whether the finest details of a semiconductor structure can be correctly formed in electron beam lithography,” Werner said.

The research findings have been published in the journal Physical Review Letters.

Image credit: TU Wien