To truly understand the nature of something, hit it with a projectile. In any case, this has always been the method of choice for some physicists. These scientists usually study the subtle properties of solids by bombarding them with charged particles, and observe those that bounce, get stuck, or pass through and change in some way. However, when these particles are inside certain materials, the details of what happens to them are still elusive.Recently, physicists from the Technical University of Vienna (TU Wien) and their colleagues Found some of the details By shooting charged particles called ions into the solid, they peel off like bananas, one layer of atoms at a time.Their work was published in Communication Physics In August, several techniques for analyzing and manufacturing materials can be made more accurate and precise.
Modern efforts to study matter using the interaction of charged particles can be traced back to the work of physicist Niels Bohr in the 1940s. Bohr studied how the charge of ions changes as they pass through a solid. For example, positively charged ions can reduce their charge by stealing some negatively charged electrons from atoms in a solid. Bohr points out that physicists can capture and examine this ion after it passes through a target, and then use his theory to infer the electronic structure that the ion encounters during its journey. Ions have since become a key tool for probing the structure and composition of materials-an activity called materials analysis-but physicists cannot experimentally study the speed at which electrons jump into ions or the details of how close the ions must be. This kind of jump occurs in solid atoms. This new study is the first to accurately observe how these leaps occur through experiments, thus adding details to Bohr’s work.
“We want to understand what happens when ions strike a material,” said Anna Niggas, a physicist at the Vienna University of Technology and the first author of the study. These processes may involve different interactions with so many electrons that it is almost impossible to track all their arrangements. What’s more troublesome is that they happen very fast—too fast to be directly imaged or recorded. Daniel Primetzhofer, a physicist at Uppsala University in Sweden, explained that he was not involved in the experiment. He pointed out that the incoming ions interact with the electrons of the material for one quadrillionth of a second, but current technology only allows physicists to examine the ions after one microsecond-a billion times longer. It’s as if physicists are trying to infer the details of a brief conversation between the bus driver (an ion) and a large number of passengers (many electrons interact with ions) by observing the facial expressions of the driver at the end of the trip. In this analogy, in order to resolve the “dialogue” between the ion and its surrounding electrons, Niggas and her collaborators had to disassemble the “bus” (ie, solid) piece by piece.
They first knocked out electrons from xenon atoms, turning the atoms into highly charged ions. The researchers then emit ions through the atom-thin carbon stack, where they interact with and trap electrons. By gradually peeling the carbon layer from the stack, the team was able to examine the behavior of ions as they passed through one, two, or three layers in total. When an ion passes through a single layer of carbon atoms (called graphene), its journey is similar to a collision with a three-dimensional solid surface. For two stacked graphene sheets, it is as if ions have passed through an extremely thin solid. With each layer of graphene they add, researchers can determine what happens to ions at different locations in conventional solids. Each layer of carbon atoms is like a row of seats on a bus: if the driver’s face changes after only one row is added, scientists know that this is where the most important interaction takes place. Primetzhofer pointed out that the precise location of the interaction between ions and most of the electrons in the carbon solid is a major advantage of the new method. “The specific point of interaction is [that] It is very difficult to evaluate in all ion beam experiments,” he said. “This may be the holy grail of ion-matter interaction research. “
The Vienna team pioneered this technique and used it to determine that a single graphene layer usually provides enough electrons to neutralize incoming ions. “Be the first [ion] The graphene experiment was completed a few years ago. No one would have thought that so many electrons could be captured by just one layer of material,” Niggas pointed out. This shows that graphene layers can be used to protect semiconductors in precision electronic devices from highly charged ions. She also said that her team’s research revealed some surprisingly simple relationships, namely, how fast ions must obtain a certain number of electrons from a certain number of graphene layers—a “need to know” will The fact that the ion beam is incorporated into practical applications is accurate. However, the researchers expected some surprises: they knew how much the theoretical model of ion journey was missing.
Svenja Lohmann, a physicist at the German research institute Helmholtz-Zentrum Dresden-Rossendorf, was not involved in this research. He said: “In fact, there is no comprehensive theory to describe all ion-matter interactions and predict their behavior very accurately. Result.” About the ion species studied by Niggas and her colleagues. In their experiment, an ion trapped dozens of electrons from the carbon atoms of graphene. These electrons interact with the existing electrons in the ion, as well as with each other and all other electrons in the graphene. Therefore, a mathematical model that can predict the speed and distance at which electrons jump into ions must track all these interactions at the same time. On the metaphorical bus, physicists will have to try to listen to the noise of countless overlapping conversations to decide which of them is the most important.
“It is extremely challenging to establish a really good quantum mechanics theory for all these interacting electrons,” said Michael Bonitz, a theoretical physicist at Kiel University in Germany, who was not involved in the new experiment. He believes that these theories can be improved through this research. “This work is not only experimentally interesting and application-related, but it can also inspire theory,” he said.
Advanced mathematics and calculation models are very important to improve the use of ions in manufacturing and material analysis. For example, in order to manufacture semiconductor devices, engineers sometimes bombard the material with ions to change the electronic structure of the material. Detailed knowledge of these interactions can lead to more precise manufacturing.
For material analysis, scientists follow Bohr’s old idea: They hope to use the measurement of ion properties to reveal the details of the material’s electronic structure after the ion interacts with the material. “Highly charged ions can act as a magnifying glass,” Pritzhof said. A more accurate theoretical model means a higher magnification. Bonitz took this idea one step further. “The question is: can you now use ions to study unknown materials, maybe you can get things that other tools can’t get?” he said.
As a next step, the TU Wien researchers are planning to explore new man-made solids of their own design: this time, they hope to see how highly charged ions interact with two substances by sending them to a graphene stack interlayered with another kind , Rather than interacting with a substance. “The cool thing is that this doesn’t just apply to graphene,” Niggas said.