Scientists from the LOEWE research-initiative PriOSS (Principles of On-Surface Synthesis) and the University of Newcastle (UON) in Australia have jointly developed a new measurement method to visualize the chemical structure of single molecules. A low-temperature atomic force microscope (AFM) was used for the measurements, which scans the sample with an extremely sharp tip. The principle is thus similar to sensing bumps on a surface with the fingers, but the AFM tip consists of only a single carbon monoxide (CO) molecule, which enables the visualization of individual atoms and chemical bonds. To do this, the researchers are using tuning fork sensors (qPlus sensors), which can be excited to very small vibrations in the range of just a few picometers. The study was published in the journal Nanoscale.
Usually, tuning fork sensors are operated so that the CO tip oscillates perpendicular to the sample surface, similar to a diving board above the water surface. Here, the scientists have succeeded in using a torsional oscillation mode (i.e. a twist along the longitudinal axis, see image) of the sensors for these high-resolution measurements of individual molecules. This causes the CO tip to oscillate almost parallel to the surface. This provides an impressive image contrast that is based on lateral forces (forces acting from the side) with a particularly high distance dependence.
Until now, such measurements could only be performed with special lateral force sensors. By using the new method and by exciting the sensors at a different resonance frequency, it is possible to switch easily between the conventional bond imaging method and the lateral force method. This eliminates the need to replace the entire sensor, which is time consuming in case of low-temperature atomic force microscopes.
In the future, the scientists would like to use the new technique to investigate molecular reaction processes on surfaces as part of the LOEWE research-initiative PriOSS.
Daniel Martin-Jimenez, Michael G. Ruppert, Alexander Ihle, Sebastian Ahles, Hermann A. Wegner, André Schirmeisen and Daniel Ebeling. "Chemical bond imaging using torsional and flexural higher eigenmodes of qPlus sensors." Nanoscale 14, 5329, 2022 (Cover Article). https://doi.org/10.1039/D2NR01062C