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Plasmonic phenomena in molecular junctions: principles and applications

Nature Reviews Chemistry volume 6, pages 681–704 (2022)Cite this article

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Abstract

Molecular junctions are building blocks for constructing future nanoelectronic devices that enable the investigation of a broad range of electronic transport properties within nanoscale regions. Crossing both the nanoscopic and mesoscopic length scales, plasmonics lies at the intersection of the macroscopic photonics and nanoelectronics, owing to their capability of confining light to dimensions far below the diffraction limit. Research activities on plasmonic phenomena in molecular electronics started around 2010, and feedback between plasmons and molecular junctions has increased over the past years. These efforts can provide new insights into the near-field interaction and the corresponding tunability in properties, as well as resultant plasmon-based molecular devices. This Review presents the latest advancements of plasmonic resonances in molecular junctions and details the progress in plasmon excitation and plasmon coupling. We also highlight emerging experimental approaches to unravel the mechanisms behind the various types of light–matter interactions at molecular length scales, where quantum effects come into play. Finally, we discuss the potential of these plasmonic–electronic hybrid systems across various future applications, including sensing, photocatalysis, molecular trapping and active control of molecular switches.

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Fig. 1: Interplay between plasmonics and molecular junctions.
Fig. 2: Different types of plasmon-induced charge transport processes across metal–molecule structures.
Fig. 3: Different coupling regimes in plasmon–exciton junctions.
Fig. 4: Illustration of plasmonic modes within point–point-shaped molecular junctions.
Fig. 5: Illustration of plasmonic modes and scattering spectra within molecular junctions in point–plane junctions.
Fig. 6: Typical plasmon excitation or detection by tunnelling electrons in STM-BJs.
Fig. 7: Measurements of different molecular–plasmon coupling schemes in plasmonic nanocavities.
Fig. 8: Typical structures and plasmon excitation in STJs.
Fig. 9: Selected examples of applications of plasmonic devices.

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Acknowledgements

The authors thank J. J. Baumberg, J. Aizpurua, P. Leiderer, J. Boneberg, J. C. Cuevas, F. Pauly, H. Liu and T. Huhn for fruitful discussions. They acknowledge financial support from the National Key R&D Program of China (2021YFA1200103), National Natural Science Foundation of China (91950116, 61571242, 62071318), Natural Science Foundation of Tianjin (19JCZDJC31000, 19JCYBJC16500) and National Research Foundation of Korea (NRF) grants (No. 2021R1A2C3004783 and NRF-2021R1C1C1010266). The authors acknowledge the National Research Foundation (NRF) for supporting this research under the Prime Minister’s Office, Singapore, under its Medium-Sized Centre Programme and the Competitive Research Programme (NRF-CRP17-2017-08), as well as the Deutsche Forschungsgemeinschaft (DFG) through SFB 767 (project number 32152442).

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Authors and Affiliations

  1. Institute of Modern Optics and Center of Single-Molecule Sciences, Nankai University, Tianjin Key Laboratory of Micro-Scale Optical Information Science and Technology, Tianjin, China

    Maoning Wang & Dong Xiang

  2. Department of Physics, University of Konstanz, Konstanz, Germany

    Maoning Wang & Elke Scheer

  3. Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu, China

    Tao Wang

  4. Department of Chemistry, National University of Singapore, Singapore, Singapore

    Tao Wang, Thorin Jake Duffin & Christian A. Nijhuis

  5. Dynamic Nanophotonics, MESA+ Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, Enschede, The Netherlands

    Oluwafemi S. Ojambati

  6. National University of Singapore Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore, Singapore

    Thorin Jake Duffin & Christian A. Nijhuis

  7. Department of Materials Science and Engineering, Research Institute of Advanced Materials, and Institute of Applied Physics, Seoul National University, Seoul, Korea

    Keehoon Kang

  8. Department of Physics and Astronomy, and Institute of Applied Physics, Seoul National University, Seoul, Korea

    Takhee Lee

  9. Department of Molecules and Materials, MESA+ Institute for Nanotechnology and Center for Brain-Inspired Nano Systems, Faculty of Science and Technology, University of Twente, Enschede, The Netherlands

    Christian A. Nijhuis

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Contributions

T.L., D.X. and C.A.N conceived the outline. M.W., T.W., O.S.O., T.J.D., E.S. and K.K. wrote the manuscript. All authors contributed to discussions, editing and corrections. E.S., D.X. and C.A.N revised the manuscript before the final submission.

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Correspondence to Takhee Lee, Elke Scheer, Dong Xiang or Christian A. Nijhuis.

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Nature Reviews Chemistry thanks J. Baumberg, A. Dhawan, Z.-C. Dong, and J.-C. Lacroix for their contribution to the peer review of this work.

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Glossary

Molecular junctions

Molecular monolayers or single molecules sandwiched between two (metal) electrodes.

Mechanically controllable break junctions

(MCBJs). Adjustable nanogap widths with sub-angstrom precision can be obtained in MCBJs by mechanically controlled bending of the substrate, which effectively stretches the suspended nanowire on top of the substrate until the nanowire breaks.

Electromigration break junctions

(EBJs). Stable nanogaps as small as 1–3 nm in EBJs are made by increasing the current density in a thin metal wire, leading to precisely controlled electromigration of metal atoms and eventual breakage of the wire.

Surface plasmons

Coherent, collective oscillations of free electrons at dielectric–metal interfaces.

Floquet states

Eigenstates of electrons in time-periodic fields, equivalent to Bloch states in spatially periodic fields.

Excitons

Excitons can form when a molecule emitter absorbs photons where the frequency is in or close to resonance with the HOMO–LUMO gap. This process excites an electron from the HOMO into the LUMO, consequently resulting in the Coulomb attraction between the electron in the LUMO and the positive hole left behind. An exciton is such a bound state. When an exciton recombines radiatively, a photon is emitted.

Fano interference

Generally speaking, Fano resonances occur when a discrete state and a continuous quantum mechanical state interfere. As a result, the line shape of their coupled resonance is altered: depending on the relative amplitude and phase of the two interfering states, the line shape may split up or change its shape to an anti-resonance. In the special case here, the two interfering states are the emission of the molecule in the junction and the continuum-like state of the plasmonic nanocavity.

Rabi splitting

When the coupling strength exceeds the dissipation rates of the coupled system of the emitter and cavity plasmons, new hybrid polariton modes can form. The new hybrid modes can appear as two distinct peaks in either scattering or emission spectra and the separation between the two peaks is proportional to the coupling strength g. This characteristic feature is called Rabi splitting.

Electron energy-loss spectroscopy

(EELS). An analysis tool to measure and map the energy of the inelastically scattered electrons in the low-loss region of 0.5–3.5 eV.

Internal photoemission

Unlike external photoemission, the photoenergy of the internal photoemission process is lower than the electrodes’ work function but high enough to lead to a measurable current across the junction.

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Wang, M., Wang, T., Ojambati, O.S. et al. Plasmonic phenomena in molecular junctions: principles and applications. Nat Rev Chem 6, 681–704 (2022). https://doi.org/10.1038/s41570-022-00423-4

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