Dr. Saw-Wai Hla
By Dr. Saw Wai Hla, Department of Physics and Astronomy, Ohio University
Viewpoint Published in Physics
Guitar strings, pendulums in clocks, and quartz crystals in watches are all examples of resonators. As their name suggests, resonators oscillate most strongly at specific frequencies (resonant frequencies), which depend on properties such as the resonator’s dimensions and mass. Musical instruments, for instance, produce sound waves of specific tones at their resonant frequencies.
Researchers in nanoscience have been pushing to make instruments of a different variety by fabricating truly minute resonators for both scientific and technological applications. Because of their small sizes, nanoresonators can oscillate at very high resonant frequencies, which makes them potentially useful for radio communication, small signal amplification, and sensing minute masses, forces, and quantum mechanical spin [1, 2, 3, 4]. Writing in Physical Review Letters, Stefan Müllegger and his colleagues at the Johannes Kepler University in Austria report they can image and effectively “listen” to the oscillations of cantileverlike resonators made of just seven to twelve individual molecules—the smallest artificially fabricated resonators detected so far [5]. These nanoscale resonators are small enough that their resonant frequency varies with the addition of one extra molecule, a sensitivity that could be put to use to detect single molecules in certain experiments. They are also of fundamental interest because, unlike carbon nanotubes and other common nanoscale resonators, the molecules aren’t held together by strong chemical bonds.
Müllegger and his colleagues used a type of molecule called α,γ-bisdiphenylene-β-phenylallyl, or BDPA for short. In earlier work, they showed that when the BDPA molecules were deposited on the (111) crystallographic surface of gold [Au(111)], they accumulated in triangular clusters, some of which served as nucleation sites for a chain of molecules to grow in one direction and form tiny, cantileverlike structures [6]. Molecules in these structures are separated by about 0.73 nanometers and are more strongly attracted to each other than to the surface.
In their new work, the researchers imaged the molecules by running a scanning tunneling microscope (STM) tip back and forth across the chain and measuring the small electrical current between the tip and the gold surface. (At a fixed tip height, the amount of current depends on the presence of a molecule between the tip and the gold.) When the gold was held at a temperature of 5 kelvin (K), the imaged BDPA chains looked like a line of molecules. But when the substrate temperature was raised to 20K or higher, the researchers observed that the molecules in the chain appeared wider or blurrier the further the tip was from the pinned end.
These images indicate the molecular chain is behaving like a vibrating string that is pinned down on one end (the node) and oscillating with a large amplitude (the antinode) at the free end….
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