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Plasmonic Tunnel Junctions for Single-Molecule Redox Chemistry

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Plasmonic Tunnel Junctions for Single-Molecule Redox Chemistry ARTICLE DOI: 10.1038/s41467-017-00819-7 OPEN Plasmonic tunnel junctions for single-molecule redox chemistry Bart de Nijs1, Felix Benz1, Steven J. Barrow2, Daniel O. Sigle1, Rohit Chikkaraddy1, Aniello Palma2, Cloudy Carnegie1, Marlous Kamp2, Ravishankar Sundararaman3, Prineha Narang4, Oren A. Scherman2 & Jeremy J. Baumberg 1 Nanoparticles attached just above a flat metallic surface can trap optical fields in 1234567890 the nanoscale gap. This enables local spectroscopy of a few molecules within each coupled plasmonic hotspot, with near thousand-fold enhancement of the incident fields. As a result of non-radiative relaxation pathways, the plasmons in such sub-nanometre cavities generate hot charge carriers, which can catalyse chemical reactions or induce redox processes in molecules located within the plasmonic hotspots. Here, surface-enhanced Raman spectroscopy allows us to track these hot-electron-induced chemical reduction processes in a series of different aromatic molecules. We demonstrate that by increasing the tunnelling barrier height and the dephasing strength, a transition from coherent to hopping electron transport occurs, enabling observation of redox processes in real time at the single-molecule level. 1 NanoPhotonics Centre, Cavendish Laboratory, Department of Physics, JJ Thompson Ave, University of Cambridge, Cambridge CB3 0HE, UK. 2 Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK. 3 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, 12180 NY, USA. 4 John A. Paulson School of Engineering and Applied Sciences, Faculty of Arts and Sciences, Harvard University, Cambridge, 02138 MA, USA. Bart de Nijs and Felix Benz contributed equally to this work. Correspondence and requests for materials should be addressed to J.J.B. (email: [email protected]) NATURE COMMUNICATIONS | 8: 994 | DOI: 10.1038/s41467-017-00819-7 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-00819-7 oble metal nanostructures support enhanced optical nanoscale18. However, using a single intense plasmonic hotspot Nproperties due to light-induced collective electron for both SERS enhancement and the generation of hot charge oscillations (plasmons), concentrating incident light carriers allows probing of the resulting hot-electron reduction at metal-dielectric interfaces. The self-assembly of complex and oxidation behaviours dynamically, with single-molecule nano-architectures allows for near-field coupling in closely sensitivity, through tracking changes in the SERS spectra of spaced plasmonic components forming highly localised intense adsorbates over time5. Here we focus on the generation and plasmonic hotspots. Carefully tuning the distance between transport of hot charge carriers and show the crucial role of metal surfaces can yield nanostructures with plasmonic field molecular binding to the plasmonic substrate as well as the effect enhancements approaching three orders of magnitude, enabling of the interaction strength between the charge carriers and the strong surface-enhanced Raman spectroscopy (SERS)1. One molecules. promising nano-architecture is the nanoparticle-on-mirror (NPoM) geometry where individual metal nanoparticles are placed on a flat metal mirror separated by a thin molecular Results – spacer2 5. This system functions similarly to a nanoparticle dimer Hot charge carrier generation in nanoparticle-on-mirror geo- due to the induced image charges of the nanoparticle within fi – metry. The NPoM geometry is used to provide optical eld the underlying mirror2 4, 6. The resulting highly localised enhancements sufficient for single-molecule sensitivity (Fig. 1a). plasmonic hotspot gives field enhancements >600 allowing for – Separating the gold surfaces by a self-assembled monolayer single-molecule sensitivities1, 4, 5, 7 12. Besides their SERS (SAM) of molecules results in a controllable separation here from enhancements, plasmonic structures have gained considerable 0.9 ± 0.05 nm22, to 1.1 ± 0.1 nm27, determined by molecular attention for their ability to generate hot charge carriers as a size28. The small gap separation results in a tightly confined result of plasmonic non-radiative relaxation pathways, which in plasmon coupled mode tuned to yield optimal SERS with the 633 turn can be used to induce or aid chemical reactions on or fi – nm excitation laser, giving eld enhancements exceeding 600 at near the surface of a plasmonic structure13 18. While hot carrier 760 nm (Fig. 1b)2. The calculated near-field as a function of harvesting into semiconductors is well understood, their ability wavelength shown in Supplementary Note 1 (Supplementary to perform aqueous-phase chemistry remains unclear. Fig. 1e) indicates the out-going SERS emission is resonant with In recent years, several devices have been proposed to utilise the NPoM-coupled plasmonic resonance. Atomic-scale mor- such plasmonic hot charge carrier generation, for photocatalytic – phological features on the gold interfaces further localise the devices or as a means of harvesting light16, 17, 19 21. Despite plasmonic hotspot resulting in roughly five-fold higher field demonstrated functionality, fundamental understanding of the enhancements29, 30, but while most molecules in the gap hot charge carrier generation and subsequent transfer process to contribute very weakly to the final SERS signal, emission is adjacent molecular orbitals is yet to be developed. This is in dominated by the individual molecules closest to the centre part due to the difficulty of measuring such processes on of these hotspots due to the highly nonlinear Raman enhance- the nanoscale since they are fast and hard to detect, resulting in ment31. At nanometre length-scales the molecular spacers act as most measurements being performed on bulk scales16, 17, 22 or molecular tunnelling junctions (MTJs), providing low-energy in ultra-high vacuum23. Difficulties also arise when combining tunnelling pathways for excited charge carriers travelling between plasmonic interfaces with semiconductors such as ZnO, TiO2 or the metal surfaces. In contrast to electrochemically driven Ag2O, since these are photocatalytic on their own, and for this tunnelling, electron transport here occurs back and forth in both reason are avoided here. directions with no net charge build-up on either metallic surface Further combining SERS or TERS (Tip-Enhanced Raman) with around the gap. electrochemistry allows redox to be driven by applying a bias to The initial spatial distribution of excited hot carriers in the the system, injecting or extracting charge carriers from the ana- material follows the optical field, while their energy distribution lytes. It has already been shown that some spectral changes from is a property of the plasmonic metal. Calculations using such bias-induced redox processes can be observed with single- – – the steady-state linearised Boltzmann equation with ab-initio molecule sensitivity8 10, 24 26. Redox processes are also observed energy-dependent electron–electron and electron–phonon for photocatalytically induced chemical reactions on the collision integrals31, 32 allow the initial spatial- and ab c 6 E 2 V LUMO 5 red 1 Field enh. (| 40 nm LUMO Elastic 0 4 – hν e Relaxation Energy (eV) –1 3 E Inelastic / E –2 0 E |×100) F 2 –7 –10 0 HOMOred + P (E) 1 h HOMO 0 Reduced molecule Metal Molecule + e– Metal Fig. 1 Plasmon generated hot charge carrier transport in a nanoparticle-on-mirror geometry. a Self-assembled 80 nm nanoparticle-on-mirror geometry used to elicit field enhancements required for single-molecule spectroscopy. b FDTD-calculated electric field enhancement (Emax/E0 > 600) for structure in a at 760 nm. Inset: plasmon-induced hot carrier population for different points around the NPoM geometry (marked with coloured arrows, calculated at 633 nm). c Schematic representation which depicts the plasmon-induced hot electron transfer between Au nanoparticles and unoccupied molecular orbitals of adsorbed molecules and the consequent reduction process 2 NATURE COMMUNICATIONS | 8: 994 | DOI: 10.1038/s41467-017-00819-7 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-00819-7 ARTICLE energy-resolved carrier distributions at the metal surface to be with peak intensities normalised to highlight the spectral changes. predicted (Fig. 1b). With increasing distance from the hotspot in The observed spectral fluctuations are a direct indication that the the gap, the contribution of the initial high-energy carriers signals mostly come from a single (or a few) molecules within reduces, and that of scattered carriers increases. The carriers that these nano-constructs46. reach the top of the nanoparticle are mostly thermalized. As a From I-VI an increase in fluctuations of the Raman shift of the result, the spatial region accessible to non-thermal carriers for main peak is observed, with a gradual increase from I-III and a driving the redox reaction is only slightly larger than the dramatic increase for IV-VI (Fig. 2b). This shows that plasmonic hotspot (by ~10 nm, the characteristic mean free path elimination of the covalent bond has a drastic effect on the for 1–2 eV carriers). Therefore, most of the hot carriers capable of electron transport properties of the MTJ, which agrees with driving the redox reaction are expected to reach the surface in the large increase in energy calculated for the conductive precisely the region detectable in the experiment,
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