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Breaking the single-molecule limit with terahertz spectroscopy

An antenna helps researchers measure vibrations in a buckyball

by Sam Lemonick
September 9, 2018 | A version of this story appeared in Volume 96, Issue 36


Drawing of a buckyball trapped between the points of two gold electrodes.
Credit: Kazuhiko Hirakawa and Shaoqing Du
Triangular gold electrodes concentrate terahertz radiation on a single buckyball molecule (center).

Terahertz spectroscopy has become a useful tool for studying materials because the energy and timescale of terahertz radiation match those of electron transitions, molecular vibrations, and other molecular excitations. But the radiation’s long wavelengths have prevented researchers from using the technique to study single molecules. One team of researchers now reports they’ve used an antenna to observe vibrations in a single 60-carbon buckyball (Nat. Photonics 2018, DOI: 10.1038/s41566-018-0241-1).

Terahertz wavelengths are about 100 µm, 100,000 to 1 million times as large as a typical molecule, even a large one like a buckyball, which is about 1 nm across. To overcome that size discrepancy, Kazuhiko Hirakawa and postdoctoral researcher Shaoqing Du of the University of Tokyo and colleagues trapped a buckyball at the tips of two triangular gold electrodes, which acted as an antenna to focus the radiation.

The researchers applied a voltage across the electrodes, allowing them to measure changes in current through the molecule as it absorbed terahertz radiation. That allowed them to observe vibrations in several buckyball samples. The researchers say the technique could clarify the structure and function of biological molecules like DNA, though they acknowledge they still struggle to reliably trap molecules between the electrodes.

Charles A. Schmuttenmaer of Yale University, a terahertz spectroscopy pioneer, calls it “a really nice experimental result” but wants to see follow-up studies to verify that these signals definitively correspond to vibrations.


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