Terahertz Spectroscopy for Single Molecules

A single molecule transistor with a bowtie antenna structure with source, drain, and gate electrodes. A single fullerene is captured in the created nanogap. (Source: K. Hirakawa, IIS, Univ. of Tokyo)

For spectro­scopy, a broad sweep of wavelengths is used to stimu­late vibra­tions, electron tran­sitions, and other processes, thus probing the world of atoms and molecules. However, one lesser-used form of light is the terahertz region. Lying on the electro­magnetic spectrum between infrared and micro­waves, this radiation does have the right frequency to excite molecular vibrations. Unfor­tunately, its long wavelength makes it impos­sible to focus beams onto a single molecule by conven­tional optics. Only large ensembles of molecules can be studied.

Recently, a team led by The Uni­versity of Tokyo’s Institute of Industrial Science (IIS) found a way around this problem. They showed that tera­hertz radiation can indeed detect the motion of individual molecules, over­coming the classical diffrac­tion limit for focusing light beams. In fact, the method was sensitive enough to measure the tunneling of a single electron. The team show­cased a single-molecule tran­sistor: two adjacent metal electrodes, the source and the drain of the tran­sistor, are placed on a thin silicon wafer in a bowtie shape. Then, single molecules – in this case C60 – are deposited in the sub-nano­meter gaps between the source and drain. The electrodes act as antennas to tightly focus the tera­hertz beam onto the isolated ful­lerenes.

“The ful­lerenes absorb the focused tera­hertz radiation, making them oscillate around their center-of-mass,” explains Shaoqing Du. “The ultrafast mole­cular oscil­lation raises the electric current in the tran­sistor, on top of its inherent conduc­tivity.” Although this current change is minuscule on the order of femto-amps it can be precisely measured with the same elec­trodes used to trap the molecules. In this way, two vibra­tional peaks at around 0.5 and 1 THz were plotted.

In fact, the measure­ment is sensitive enough to measure a slight splitting of the absorp­tion peaks, caused by adding or subtracting only one electron. When C60 oscil­lates on a metal surface, its vibra­tional quantum can be absorbed by an electron in the metal electrode. Thus stimu­lated, the electron tunnels into the C60 molecule. The resulting negatively charged C60 molecule vibrates at a slightly lower frequency than neutral C60, thus absorbing a different frequency of tera­hertz radiation.

Apart from providing a glimpse of tunneling, the study demon­strates a practical method to obtain elec­tronic and vibronic infor­mation on molecules that only weakly absorb tera­hertz photons. This could open up the wider use of terahertz spectro­scopy, an under-developed method that is comple­mentary to visible-light and X-ray spectro­scopy, and highly relevant to nano­electronics and quantum computing. (Source: U. Tokyo)

Reference: S. Du et al.: Terahertz dynamics of electron–vibron coupling in single molecules with tunable electrostatic potential, Nat. Phot., online 3 September 2018; DOI: 10.1038/s41566-018-0241-1

Link: Institute for Nano Quantum Information Electronics, University of Tokyo, Tokyo, Japan

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