AFM tip reveals bonds as never seen before


Rebecca Pool

Thursday, April 12, 2018 - 11:30
Hydrogen-bonded assembly imaged with a CuOx tip, showing the intermolecular hydrogen bond structure. [2018 Macmillan Publishers Limited, part of Springer Nature.]
Germany-based researchers have developed a novel probe tip with extraordinary stability to image molecular structures using atomic force microscopy.
The copper oxide tip can determine bond lengths, access bond order effects and reliably characterise intermolecular bonds.
"Our special technique involves a copper-based probe tip which is passivated by a single oxygen atom at the tip termination," says Dr Harry Mönig from the Center for Nanotechnology (CeNTech) at the University Münster.
AFM methods usually involve chemical passivation of the probe-tip termination by attaching single molecules or atoms such as CO or Xe.
However, these probe particles are only weakly connected to the apex of the tip, resulting in considerable dynamic deflection. 
Right: Enlarged image extracted from the left image. It shows a single molecule (centre) surrounded by six partly visible ones. The faint lines between the molecules indicate dominant sites of the molecule-molecule interactions. [2018 Macmillan Publishers Limited, part of Springer Nature.]
According to Mönig, the oxygen atom reduces undesired interactions between the atoms of the probe tip and the atoms in the molecules under investigation, boosting imaging resolution.
Crucially, the bond between the oxygen atom at the tip and copper base is particularly strong, thereby reducing imaging artefacts.
Researchers performed AFM experiments using a low-temperature scanning probe microscopy system - Scienta Omicron, LT-STM/AFM - under ultrahigh-vacuum conditions and at liquid-helium temperatures.
As lead researcher, Professor Harald Fuchs, highlights: "The potential of the new method is considerable as it allows us to investigate bonding structures of molecular networks with exceptional accuracy."
Using the probe, the researchers provided conclusive experimental evidence for an unusual intermolecular N–Au–N three-centre bond.
What's more, they demonstrated that the CuOx tips allow the characterization of the strength and configuration of individual hydrogen bonds within a molecular assembly.
Research is published in Nature Nanotechnology.
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