Scientists in Germany have found that quantum corals – synthetic circular nanostructures that act as artificial atoms – can form very weak chemical bonds with an atom at the tip of an atomic force microscope (AFM). “For the first time, we were able to demonstrate the chemical bonds of such artificial atoms experimentally,” says study director Franz Giessibl from the University of Regensburg.

Like natural atoms, artificial atoms have a fixed number of electrons with discrete electronic states. Quantum corals – in the form of a circle of 48 iron atoms on a copper surface – were first introduced by a US team in 1993,2 offers fascinating insights into quantum mechanics and opens up new possibilities in materials science. “One of the attractive properties of such artificial atoms is that they control properties that cannot be controlled in real atoms,” says Ingmar Swart from the University of Utrecht in the Netherlands, who was not involved in the study. “They can also be coupled together in grids that can be used to create and study materials with potentially exciting and useful properties that don’t exist in nature.”

With the tip of a scanning probe microscope, the researchers built two artificial atoms of different sizes: a classic 48-atom quantum corral and a smaller version with only 24 iron atoms. “The quantum corral forms a cage for electrons on the copper surface,” explains Klaus Richter, one of the theorists in the team. “The ring of natural atoms provides a mechanism for trapping electrons and forms an artificial atom – it’s like the attractive Coulomb forces between atomic nuclei and electrons in natural atoms!”

The microscope used in the experiments was equipped with a qPlus sensor, which is based on quartz clock technology and can measure very small forces.3 The scientists examined the interaction between the electrons of the corals and the front atom of the AFM tip and found a bond with an energy of about 5 meV for the larger system. A similar binding force was also found for the smaller corral. “Covalent bonds of natural atoms correspond to binding energies of a few electron volts,” says Giessibl.

An image that shows a ring of red dots on a blue surface.  Inside the ring there are several concentric rings of darker dots and a single red dot off the center

“Natural covalent bonds are established between atomic orbitals of the same microscopic size,” explains Richter. “In our case, a natural atom bonds with an object – the artificial atom – about 100 times as much. Microscopic and mesoscopic length scales meet. ‘

Swart says this is an amazing feat. “The force between the quantum corral and the AFM tip is on the order of a thousandth of the typical value used in atomically resolved AFM studies, which makes these experiments a real showdown. This work pushes the limits of what is currently technically feasible in terms of measuring small forces. “

“The authors were able to measure very weak interactions within a certain distance range and thus demonstrate the attraction for a metal-terminated tip and the repulsion for a CO-terminated tip,” comments Willi Auwärter from the Technical University of Munich in Germany. “This work gives an unprecedented insight into the bonding properties of an artificial atom on the nano-scale that is” large and electronically thinned “compared to natural atoms. ‘

Giessibl points out that although the interactions are weak, they still change the occupation of quantum mechanical states in the artificial atoms. “Our theory colleagues helped us understand the patterns that were created when perturbations were added to the quantum corral,” he says. Thanks to the 2D nature of the systems, the team was able to make targeted changes to the structures in order to study certain properties. “We built an obstacle into the atom and observed what happened – an achievement that natural atoms cannot,” says Giessibl.

He believes the results could be useful in a number of areas. “This first measurement of the chemical bond between artificial atoms reveals the character of elusive quantum mechanics. Applications in quantum information, materials science, and chemistry come with just a little imagination and thinking. ‘

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