New technique can build and break individual atomic bonds with electric fields

June 5, 2021

A single bond between a gold-tipped atomic force microscope and a layer of graphene was controlled by changing the direction of electric voltage: a positive voltage from carbon to gold strengthened the bond, while a negative voltage weakened it. Above, a rendering of a graphene surface. (Shutterstock)

German and Danish physicists used an electric field to control the single atomic bond between a specialized microscope and a one-atom-thick layer of graphene. The newly realized approach, accomplished by changing the voltage across the bond, allowed the researchers to pick up and drop the graphene with the microscope like a crane.

Such precise control over polar bonds between atoms could allow for new applications in nanotechnology and the meticulous studying and control of chemical reactions, according to the developers of the technique. They reported their findings May 25 in Physical Review Letters.

"The presented method represents a controlled approach to the manipulation of matter and now belongs to the tool box of nano-engineers," said Jörg Kröger, a senior author of the study and a professor of experimental physics at Technische Universität Ilmenau in Germany. 

Scientists have been able to target and manipulate specific chemical bonds at an atomic level for decades. Bonds between atoms have been broken using lasers and scanning tunneling microscopes, while a beam of electrons has been employed to quickly reposition atoms.

This latest paper takes a new approach by changing the strength of a polar covalent bond, in which atoms share electrons unevenly and create a positively charged and a negatively charged end — as seen in the bond between oxygen and hydrogen in water molecules. 

"While the manipulation of matter atom by atom has become ubiquitous," Kröger said, "the results presented in our work rely on exploiting the short-range bond force in a single-atom chemical bond whose strength depends on the polarity of this bond."

Using electric fields to control the strength of bonds would be particularly useful in testing nanoscale materials, according to Mads Brandbyge, another senior author and a physics professor at the Technical University of Denmark.

"Theoretical ideas of influencing chemical bonds by an external electric field are longstanding," Brandbyge said. "The experimental realization was therefore a strong motivation to simulate the findings."

In this case, the physicists focused on the covalent bond between carbon and gold. They adorned a single gold atom at the tip of an atomic force microscope, which can be used to scan the shape of extremely small objects by physically "touching" them. The gold tip was moved very close to a layer of graphene, a versatile and widely used material composed of a single layer of carbon atoms in a honeycomb structure. 

After a bond formed between the microscope's gold tip and the graphene's carbon, the researchers ran an electric voltage across the bond to generate an electric field that would interact with the atoms' electrons. By applying a positive voltage in the direction of carbon to gold, the physicists strengthened the bond enough to be able to lift the graphene with the microscope.

A voltage in the opposite direction weakened the bond, to the point where simply retracting the microscope was enough to break the connection.

"In principle, the microscope tip acts as a nano-crane that picks up and transports material at positive voltage and releases it at negative voltage," Kröger said.

Using these experimental findings as well as theoretical calculations, Kröger and his co-authors concluded that the electric field from the positive voltage strengthened the carbon-gold bond by boosting the transfer of the gold electrons to the carbon atom. Conversely, the negative voltage diminished the electron transfer and subsequently weakened the bond.

According to Kröger, the study was made possible by advanced experimental design that used physical probing approaches such as atomic force microscopy. He said the technology allowed for the construction of atomic systems that can be effectively recreated with computer simulations; Brandbyge said there was "astounding agreement" between his team's experiments and theory.

Matteo Rini, editor at the American Physical Society's Physics Magazine, said the method developed by the German and Danish physicists could be applied in nanomachines to apply mechanical loads or to study the role of bond strength in chemical reactions.

Brandbyge said there are plans to use the technique and accompanying simulations to study how nanomaterials deform, such as by energetically exciting graphene to make it quickly vibrate before suddenly releasing it from the microscope.

It could also be used to explore whether bonds between other elements or with other materials can be similarly controlled, according to Kröger.

"The experimental setup is flexible in the choice of tip and substrate materials, which opens the path to explore bonding characteristics and their control by external parameters for a variety of atom combinations," he said.

The study, "Electric-field control of a single-atom polar bond," published May 25 in Physical Review Letters, was authored by Maryam Omidian, Nicolas Néel and Jörg Kröger, Technische Universität Ilmenau; and Susanne Leitherer and Mads Brandbyge, Technical University of Denmark.

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