Physicists have always wanted to reach the quantum tunnelling limit (~0.5 nm) to exploit novel quantum mechanics and quantum devices, which is the future of IT industry. However, dynamic modulation of such a small cavity is extremely challenging due to the lack of proper spacers that can reversibly actuate the sub-1-nm gaps (one hundred thousandth of the width of human hair).
Recently, researchers at Wuhan University, collaborating with chemists of Huazhong University of Science and Technology (HUST), have utilized synthesized oligoamide sequence (OS) to switch the gap plasmons (a collective oscillation of free electron gas with phase of light) between the classical and quantum regimes (see the image below).
A self-assembled monolayer of OS double helix in the metallic nanogap.
Credit：Lubin Liu from Wuhan University
The uniqueness of this OS system lies in that they can form rigid helical superstructures via H-bonding and π-π interactions (similar to DNA but smaller than 1 nm), which exactly fits into the quantum tunnelling regime with excellent reconfigurability to solvents and temperature. The experimental observation verifies both the circuit model and the quantum-corrected model. Moreover, they observe plasmon-driven hot electron tunnelling effect, showing reversible blue shifts of the quantum plasmons due to the modulation of gap conductance with light, which lays the foundation for ultrafast quantum optoelectronic devices. These results are published in Science’s sister journal, Science Advances (https://www.science.org/doi/10.1126/sciadv.abj9752).
“Our specialty is the chemical engineering of OS molecules so that the supramolecular structure can fit just below the quantum limit (~0.5 nm) of the nanogaps,” said Dr. Quan Gan, a chemistry professor at HUST.
This plasmonic “ruler” also helps chemists to monitor the supramolecular (dis)assembly with angstrom resolution. These supramolecules can also be viewed as molecular machines (nanojacks) which elevate the gold nanoparticle up and down.
“This type of OS molecules is very useful for the reversible switching of quantum plasmonics, which we didn’t realize at first,” said Prof. Tao Ding,, a chemical physicist and nanoscientist from School of Physics and Technology. “This is an excellent collaboration between chemists and physicists where we can see how one discipline (supramolecule chemistry) that seems totally irrelevant can be a neat solution to a critical problem of another field (quantum plasmonics).”
He further commented, “This supramolecular plasmonic system is intriguing and fantastic, and we are collaborating further on many new possibilities of extreme nanophotonics, such as single chiral molecule sensing and strong coupling modulation.”
The research is funded by the National Key Research and Development Program of China (2020YFA0211300 and 2017YFA0303402) and National Natural Science Foundation of China (NSFC 11974265, 91850207 and 21703160).
(Edited by Zhou Chuangyu, Sun Xiaojie & Hu Sijia)
