Jeremy Baumberg1
University of Cambridge1
Our ability to trap light into extreme nanoscale gaps between coinage metals using plasmonics has enabled routine vibrational measurements of molecular monolayers, even within active molecular electronics devices [1]. We recently showed that single metal atoms can be pulled out of the interface by light [2] (or electric fields), and that coordination bonds with individual molecules can be tracked in real time [3-6], or their redox state observed [7]. We discuss new observations for the thousand-fold enhanced optical forces in such nanogaps.<br/>Such spectroscopy reveals a wealth of information: from how applying voltages can twist the conducting molecules in a gap [8], to how solvated charge is able to penetrate self-assembled monolayers [9]. The robust and precise self-assembly of the nanoparticle-on-mirror geometry is ideal for tunnelling photocurrent devices [10], and shows that asymmetry is created in the junction, likely by such picocavities. We also discuss the dynamics of the picocavity formation [11], and how this may be now controlled.<br/><br/>[1] Extreme nanophotonics from ultrathin metallic gaps, Nature Materials <b>18</b>, 668 (2019); DOI: 10.1038/s41563-019-0290-y<br/>[2] Optical suppression of energy barriers in single molecule-metal binding, Science Advances 8: eabp9285 (2022); DOI: 10.1126/sciadv.abp9285<br/>[3] Picocavities: a primer, Nano Letters <b>22</b>, 5859 (2022); DOI: 10.1021/acs.nanolett.2c01695<br/>[4] Single-molecule optomechanics in picocavities, Science <b>354</b>, 726 (2016); DOI: 10.1126/science.aah5243<br/>[5] Locating Single-Atom Optical Picocavities Using Wavelength Multiplexed Raman Scattering, ACS Photonics (2021); DOI: 10.1021/acsphotonics.1c01100<br/>[6] Resolving Sub-Å Ambient Motion through Reconstructions from Vibrational Spectra, Nature Comm (2021); DOI: 10.1038/s41467-021-26898-1<br/>[7] Tracking interfacial single-molecule pH and binding dynamics..., Science Advances 7:eabg1790 (2021); DOI: 10.1126/sciadv.abg1790<br/>[8] Optical probes of molecules as nano-mechanical switches, Nature Comm <b>11</b>:5905 (2020); DOI: 10.1038/s41467-020-19703-y<br/>[9] Vibrational Stark Effects: Ionic Influence on Local Fields, JPCL <b>13</b>, 4905 (2022); DOI: 10.1021/acs.jpclett.2c01048<br/>[10] Quantum Tunneling Induced Optical Rectification and Plasmon-Enhanced Photocurrent in Nanocavity Molecular Junctions, ACS Nano (2021); DOI: 10.1021/acsnano.1c04100<br/>[11] Cascaded Nano-Optics to Probe Microsecond Atomic Scale Phenomena, PNAS (2020); DOI 10.1073/pnas.1920091117