Atomic Soccer. Manipulating individual atoms with a beam of electrons

The ability to manipulate individual atoms with a beam of electrons has been cracked by scientists at MIT and the University of Vienna — opening up the potential for an age of atomic engineering.

This diagram illustrates the controlled switching of positions of a phosphorus atom within a layer of graphite by using an electron beam, as was demonstrated by the research team. (Courtesy of the Researchers)

The ultimate degree of control for engineering would be the ability to create and manipulate materials at the most basic level. Thus enabling us to create devices atom by atom with precise control.

Researchers at MIT, the University of Vienna, and several other institutions have taken a step in that direction, by developing a method that can reposition atoms with a highly focused electron beam. In addition to controlling their exact location, the beam can also control how the bonds between the atoms are orientated.

This research could eventually lead to new ways of making quantum computing devices or sensors, and usher in a new age of “atomic engineering,” the team believe.

Ju Li, MIT professor of nuclear science and engineering, explains: “We’re using a lot of the tools of nanotechnology.”

The new research, published in the journal Science Advances, utilises those tools to control processes that are yet an order of magnitude smaller.

Li continues: “The goal is to control one to a few hundred atoms, to control their positions, control their charge state, and control their electronic and nuclear spin states.”

Scientists have previously successfully manipulated the positions of individual atoms — even creating a neat circle of atoms on a surface. That process involved picking up individual atoms on the needle-like tip of a scanning tunnelling microscope and then dropping them in position — a slow mechanical process.

A scanning transmission electron microscope (STEM) similar to that used by the team to manipulate individual atoms (University of Waterloo)

This new process manipulates atoms with a relativistic electron beam in a scanning transmission electron microscope (STEM). The beam requires no mechanical moving parts as the beam is controlled by magnetic lenses. This makes the process potentially much faster and thus, could lead to greater practical applications.

Li says that by using electronic controls and artificial intelligence, they should eventually be able to manipulate atoms at microsecond timescales. He adds: “That’s many orders of magnitude faster than we can manipulate them now with mechanical probes. Also, it should be possible to have many electron beams working simultaneously on the same piece of material.”

Professor Toma Susi of the University of Vienna adds: “This is an exciting new paradigm for atom manipulation.”

Computer chips are typically made by “doping” a silicon crystal with other atoms needed to confer specific electrical properties, thus creating “defects’ in the material — regions that do not preserve the perfectly orderly crystalline structure of the silicon.

Li explains that process is scattershot, thus meaning there’s no way of controlling with atomic precision where those dopant atoms go — a level of control the team’s new system provides.

Football with atoms

The research shows that a single electron beam — very narrowly focused to roughly as wide as an atom — can be used to knock an atom out of one position and into another. Then by measuring the exact angle of the beam, the researchers can “read” the new position to verify that the atom ended up where it was meant to.

While the positioning is essentially determined by probabilities and is not 100% accurate, the ability to determine the actual position makes it possible to select out only those that ended up in the right configuration.

Microscope images are paired with diagrams illustrating the controlled movement of atoms within a graphite lattice, using an electron beam to manipulate the positions of atoms one a time. (Courtesy of the Researchers)

Li says: “We want to use the beam to knock out atoms and essentially to play atomic soccer, dribbling the atoms across the graphene field to their intended “goal” position.

“Like soccer, it’s not deterministic, but you can control the probabilities. Like soccer, you’re always trying to move toward the goal.”

In the team’s experiments, they primarily used phosphorus atoms — a commonly used dopant — in a sheet of graphene — a two-dimensional sheet of carbon atoms arranged in a honeycomb pattern. The phosphorus atoms act as substitutes for carbon atoms in parts of that pattern — altering the material’s electronic, optical, and other properties in ways that can be predicted if the positions of those atoms are known.

The aim is is to move multiple atoms in complex ways. Li adds: “We hope to use the electron beam to basically move these dopants so we could make a pyramid, or some defect complex, where we can state precisely where each atom sits.”

This marks the first time electronically distinct dopant atoms have been manipulated in graphene. As Prof Susi points out: “Although we’ve worked with silicon impurities before, phosphorus is both potentially more interesting for its electrical and magnetic properties, but as we’ve now discovered, also behaves in surprisingly different ways. Each element may hold new surprises and possibilities.”

But there are potential pitfalls. For example, the system requires precise control of the beam angle and energy.

Susi says: “Sometimes we have unwanted outcomes if we’re not careful. For example, sometimes a carbon atom that was intended to stay in position ‘just leaves,’ and sometimes the phosphorus atom gets locked into position in the lattice, and then no matter how we change the beam angle, we cannot affect its position.”

“We have to find another ball.”

Follow the ball — a theoretical framework

In addition to detailed experimental testing and observation of the effects of different angles and positions of the beams and graphene, the team also devised a theoretical basis to predict the effects. In other words, it tracks the momentum of the “soccer ball.” They call this ‘primary knock-on space formalism’.

The cascade of effects that results from the initial beam takes place over multiple time scales, which made the observations and analysis tricky to carry out. The actual initial collision of the relativistic electron, which is moving at about 45% of the speed of light, with an atom takes place on a scale of zeptoseconds — trillionths of a billionth of a second — but the resulting movement and collisions of atoms in the lattice unfolds over time scales of picoseconds or longer — billions of times longer.

Dopant atoms such as phosphorus have a nonzero nuclear spin, which is a key property needed for quantum-based devices because that spin state is easily affected by elements of its environment such as magnetic fields. So the ability to place these atoms precisely, in terms of both position and bonding, could be a key step toward developing quantum information processing or sensing devices, Li concludes.