Nanomechanical resonators are small vibrating beams that oscillate at very high resonant frequencies, which means they might be employed in radio communications applications and to amplify small signals. They can also be used to detect and weigh tiny objects, like single DNA molecules or viruses. When a small particle is absorbed onto the beam, it alters the frequency at which the beam vibrates and this change can be monitored and used to calculate the mass of the particle.

A team led by Stefan Müllegger at the Johannes Kepler University has now made the tiniest such resonator ever from four to 12 molecules of α,γ-bisdiphenylene-β-phenylallyl (or BDPA). In previous work, the researchers showed that when BDPA molecules are deposited on the (111) crystallographic surface of gold, they segregate into triangular clusters. Some of these clusters then act as nucleation sites and allow a chain of molecules to grow in one direction and form tiny resonating structures. The molecules in these chains are separated by around 0.7 nm.

Vibrating chain

Now, the same team has succeeded in imaging these molecules by displacing a scanning tunnelling microscope (STM) tip across the chain and measuring the small electrical current between the tip and Au(111) surface. The researchers found that at temperatures of 5K, the chain appears as a fine line of molecules. However, when the temperature is increased to 20K or higher, the molecules in the chain look wider the further the tip is raised from the pinned end of sample.

These observations indicate that the chain is vibrating, say Müllegger and colleagues, and it is unique in that it comprises individual molecules linked to each other by simple attractive interactions. In theory, it should not resonate because the molecules in such structures (made only from carbon nanotubes, nanowires or graphene sheets until now) are usually held together by stronger chemical bonds. This property means that the new resonator will also be useful for fundamental physics studies.

Radio-frequency STM

And that is not all: the team has also come up with a way to measure the frequency of the structure by modifying its STM so that it can detect tunnelling currents in the radio-frequency range. The researchers found that a five-molecule chain resonates at 98 MHz and, just like in musical instrument strings, these frequencies decrease as the chain length increases. For example, a four-molecule chain was found to resonate at around 127 MHz while a seven-molecule one resonated at around 51 MHz.

“Our study shows that radio-frequency scanning tunnelling microscopy is a complementary new experimental tool for characterizing dynamic processes at the scale of single molecules in nanoscience and technology,” said Müllegger. “We now hope to study the mechanisms behind a molecular chain’s vibrations and further adapt our modified STM for single-molecule magnetic resonance spectroscopy,” he told

The current work is reported in Physical Review Letters.