Playing with single electrons in designer atomic structures

29 Oct 2018 Belle Dumé

AFM mechanically lifts silicon atoms

Being able to manipulate and monitor single electrons in predesigned atomically-defined structures has been a long-standing goal for condensed matter physicists. A team led by Robert Wolkow of the University of Alberta in Edmonton, Canada, has now succeeded in doing just this. As well as being useful for fundamental studies, such designer lattices could also be used to make atomic circuits in the future.

“We are now able to play with single charges (electrons) in atomically-defined structures of our own design for the first time,” explains team member Mohammad Rashidi, who is also at the Nanotechnology Research Centre and Quantum Silicon, both in Edmonton. “Previously, this had only been done on isolated atoms and molecules on insulating substrates. In our work, we can place atoms at will and compose structures of dangling bonds on a hydrogen-terminated silicon surface. We can then follow how electrons ‘jump’ between the atoms in the artificial atom structures.

“We create and erase the dangling bonds (which are silicon atoms missing a hydrogen atom to bond to, unlike their closest neighbours) with atomic precision using the scanning tip of an atomic force microscope (AFM),” he says. “This technique allows us to design specific arrangements of dangling bonds with precisely tuned interactions between them and allows for a huge range of possibilities for targeted experiments in engineered designer structures beyond those that chemists can synthesize.”

AFM tip mechanically lifts a silicon atom

AFM is a widely-used ultrahigh-resolution technique that allows researchers to observe extremely small objects, even down to single atoms. It works by sensing the topography of a sample as it scans across it thanks to an extremely sharp tip mounted on a resonating cantilever.

The Edmonton researchers have used the AFM for something other than just imaging. As well as creating and erasing dangling bonds in artificial atomic structures, they have also used it to mechanically lift silicon atoms in the structures. This process, which makes the silicon atoms want to become negatively charged, is quite different to conventional techniques that use the bias voltage on the tip to electrically control the charge state of individual atoms.

AFM tip can “see” which atoms have one extra electron

“When the tip interacts with the atoms on the material’s surface, the resonance frequency of the tip shifts and you can use this frequency shift signal to map out the interactions between the tip and the material surface,” explains team member Thomas Dienel. “In our experiments, we exploited the atomic precision of the AFM to image a structure containing several silicon atoms and unambiguously say which ones have one extra electron and are thus negatively charged,” he tells Physics World. “We can record a large number of these images and create a time-dependent movie of our structure that ultimately allows us to visualize electron movement.”

There are two main types of forces that affect the frequency at which the AFM tip resonates: Coulomb interactions from the charges present on the atoms in the sample and van der Waals forces. “These two forces have slightly different distance-dependent decay rates – that is, they contribute in different amounts depending on the tip-sample distance,” says team member Wyatt Vine. “We made precise measurements of these decay rates to distinguish between the two types of forces at various surface sites. We were thus able to calculate that the force required to lift a silicon atom so that it becomes negatively charged is 75 pN.”

Important first step to making atomic circuits

The work is an important first step to making atomic circuits, adds Vine. “It also opens the way to investigating more complex fundamental physics that has only been studied theoretically until now, such as testing and quantifying quantum interactions in predesigned artificial atomic lattices (like those that have recently been made from defects). Our technique could be used to realize simple ‘toy’ equations that physicists often make use of to explain various phenomena, but which are often too simple to be found directly in nature.”

“Another idea of ours is to fabricate larger assemblies of dangling bonds with slightly different coupling between them and see how complicated many-body electron interactions evolve,” reveals Dienel. “Classically, these would be difficult to compute but if we can set up the correct initial electron populations and then ‘let them go’ to see how they evolved, we could obtain interesting information.”

The research is detailed in Physical Review Letters 10.1103/PhysRevLett.121.166801.

29/10/2018 from physicsworld.com