To truly understand the essence of something, pelt it with projectiles. That has long been the preferred approach of some physicists, anyway. These scientists routinely study the subtle properties of solids by bombarding them with charged particles and watching for those that bounce off, get stuck or pass through to emerge, somehow changed. The specifics of what happens to such particles while they are inside some materials have remained elusive, however. Recently physicists at the Technical University of Vienna (TU Wien) and their colleagues uncovered some of those details by shooting a charged particle called an ion through a solid they were peeling like a banana, one layer of atoms at a time. Their work, published in Communications Physics in August, could make several techniques for analyzing and fabricating materials more accurate and precise.
Modern efforts to study matter using charged particle interactions trace all the way to the work of physicist Niels Bohr in the 1940s. Bohr studied how an ion’s charge changes as it travels through a solid. A positively charged ion, for example, can lower its charge by stealing some negatively charged electrons from atoms in the solid. Bohr noted that physicists could capture and examine such an ion after it tunneled through a target and then use his theory to infer the electronic structure the ion had encountered 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 have not been able to experimentally examine the details of how quickly electrons jump into an ion or how close an ion must be to a solid’s atoms for such jumps to happen. The new study adds detail to Bohr’s work by being the first to experimentally observe precisely how these leaps occur.
“We wanted to understand what processes happen when the ion hits the material,” says Anna Niggas, a TU Wien physicist and the study’s first author. These processes can involve distinct interactions with so many electrons that it is nearly impossible to keep track of all of their permutations. Even more troublesome, they happen extremely quickly—too fast to be directly imaged or recorded, explains Daniel Primetzhofer, a physicist at Uppsala University in Sweden, who was not part of the experiment. He notes that incoming ions and the material’s electrons interact for a quadrillionth of a second, but current technology only allows physicists to examine the ion after a microsecond—a billion times longer. It is as if physicists are trying to deduce the fine details of a brief conversation between a bus driver (an ion) and a large number of passengers (many electrons interacting with the ion) by looking at the driver’s facial expressions at the trip’s end. In this analogy, to parse the “conversation” between an ion and its surrounding electrons, Niggas and her collaborators had to disassemble the “bus” (that is, the solid) piece by piece.
They started by knocking electrons out of xenon atoms to transform the atoms into a highly charged ions. Then the researchers shot the ions through atom-thin stacks of carbon, where they interacted with and captured electrons. By progressively peeling layers of carbon off the stack, the team was able to examine how ions behave when they pass through one, two or three layers in total. When an ion passed through a single layer of carbon atoms, known as graphene, its journey was analogous to a collision with just the surface of some three-dimensional solid. For two stacked graphene sheets, it was as if the ion was passing through an extremely thin solid. With each graphene layer they added, the researchers could determine what happens to the ion at different positions in a regular solid. Each layer of carbon atoms is like a row of seats on the metaphorical bus: if the driver’s face changes after only one row has been added, scientists know that this is where the most important interactions happen. Primetzhofer notes that pinpointing exactly where the ion interacts with most of the electrons in the carbon solid is a big advantage of the new approach. “The specific interaction point is something [that] is super difficult to assess in all ion beam experiments,” he says. “That may be the holy grail of ion-matter interaction research.”
The team in Vienna pioneered this technique and used it to determine that a single graphene layer usually supplies sufficient electrons to neutralize an incoming ion. “When the first [ion] experiments with graphene were done years ago, nobody would have expected that so many electrons can be captured just by going through one material layer,” Niggas notes. This suggests that layers of graphene could be used to shield semiconductors in delicate electronics devices from highly charged ions. She also says that her team’s studies revealed some surprisingly simple relationships concerning how fast an ion must travel to pick up a certain number of electrons from a certain number of graphene layers—“need to know” information for incorporating ion beams into practical applications with increasing precision. The researchers expected some surprises, however: they know how much is missing in theoretical models of an ion’s journey.
“There isn’t really a comprehensive theory that describes all the ion-matter interactions and can predict their outcomes very accurately,” says Svenja Lohmann, a physicist at the German research institution Helmholtz-Zentrum Dresden-Rossendorf, who was uninvolved with the study, about the kind of ions studied by Niggas and her colleagues. In their experiment, an ion captured dozens of electrons from the graphene’s carbon atoms. Those electrons interacted with the ones already in the ion—as well as with one another and all the other electrons inside the graphene. A mathematical model that could predict the speeds and proximities for electrons jumping into the ion would thus have to keep track of all those interactions simultaneously. On the metaphorical bus, physicists would have to try to listen to a cacophony of myriad overlapping conversations to decide which of them were most important.
“Making a really good quantum mechanical theory of all these interacting electrons is extremely challenging,” says Michael Bonitz, a theoretical physicist at Kiel University in Germany, who was not part of the new experiment. He believes those theories can be improved because of the study. “This work is not only experimentally interesting and relevant for applications, but it could also stimulate theory,” he says.
Advanced mathematical and computational models would be important for improving the use of ions in fabrication and the analysis of materials. For instance, to manufacture semiconductor devices, engineers sometimes change the electron structure of materials by bombarding them with ions. Detailed knowledge of those interactions could lead to more precise manufacturing.
For materials analysis, scientists follow Bohr’s old idea: they want to use measurements of an ion’s properties after it interacts with a material to reveal the details of the material’s electronic structure. “Highly charged ions can act as magnifying glasses,” Primetzhofer says. More precise theoretical models would mean higher magnification. Bonitz takes the idea further. “The question is: Can you now use ions to study unknown materials and maybe get something out that you cannot with other tools?” he says.
As a next step, the TU Wien researchers are planning on probing a new artificial solid of their own design: this time, they want to see how highly charged ions interact with two substances rather than one by sending them through stacks of graphene interlayered with another material. “The cool thing is that this does not only work for graphene,” Niggas says.