Electron irradiation manipulates single atoms


Rebecca Pool

Tuesday, May 21, 2019 - 15:15
Image: Controlled movement of atoms within a graphite lattice [Ju Li, Toma Susi and colleagues]
A team of researchers from MIT, the University of Vienna and other institutions, has used electron irradiation to reposition atoms, controlling the exact location and bonding orientation.
Atom manipulation is not new with several researchers having already used a STM tip to pick up and re-position single atoms.
However, this time, Professor Ju Li from MIT and colleagues have moved atoms using a relativistic electron beam in a scanning tunnelling electron microscopy, opening the door to faster atom manipulation and more practical applications.
“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,” says Li. “[Our process] could ultimately lead to new ways of making quantum computing devices or sensors, and usher in a new age of 'atomic engineering'.”
To manipulate the atoms, the researchers used an aberration-corrected Nion UltraSTEM 100 at the Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences User Facility as well as the same instrument at the University of Vienna.
By carefully controlling the electron beam angle and energy, the researchers were able to use the STEMs to both drive and identify the atomic motion of individual phosphorus dopants in graphene.
Schematic of the controlled switching of positions of a phosphorus atom within a layer of graphite by using an electron beam [Li, Toma and colleagues]
During these experiments, the researchers observed phosphorous atoms substituting carbon atoms within graphene. 
As Li's colleague, Professor Toma Susi from the University of Vienna points out, this is the first time electronically distinct dopant atoms have been manipulated in graphene. 
“Although we’ve worked with silicon impurities before, phosphorus is both potentially more interesting for its electrical and magnetic properties,” says Susi. “But as we’ve now discovered, [it] also behaves in surprisingly different ways.”
With atom manipulation demonstrated, the researchers went on to construct a theoretical scheme for predicting the effects of different beam angles and positions on atom manipulation, calling their theory, 'primary knock-on space formalism'.
According to Li, the cascade of effects that results from the initial beam takes place over multiple time scales, making observations and analysis tricky to carry out.
For example, the actual initial collision of the relativistic electron with an atom takes place on a scale of zeptoseconds but the resulting movement and collisions of atoms in the lattice unfolds over time scales of picoseconds or longer.
The researchers' ultimate goal is to use the electron beam to move multiple atoms in more complex ways.
“We could make a pyramid or some defect complex, where we can state precisely where each atom sits,” says Li. “[Using electronic controls and artificial intelligence], we also think we can eventually manipulate atoms at microsecond timescales.”
“That’s many orders of magnitude faster than we can manipulate them now with mechanical probes,” he adds. “And it should be possible to have many electron beams working simultaneously on the same piece of material.”
Research is published in Science Advances.
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