Amplified Plasmonic Forces from DNA Origami-Scaffolded Single Dyes in Nanogaps

Developing highly enhanced plasmonic nanocavities allows direct observation of light–matter interactions at the nanoscale. With DNA origami, the ability to precisely nanoposition single-quantum emitters in ultranarrow plasmonic gaps enables detailed study of their modified light emission. By developing protocols for creating nanoparticle-on-mirror constructs in which DNA nanostructures act as reliable and customizable spacers for nanoparticle binding, we reveal that the simple picture of Purcell-enhanced molecular dye emission is misleading. Instead, we show that the enhanced dipolar dye polarizability greatly amplifies optical forces acting on the facet Au atoms, leading to their rapid destabilization. Using different dyes, we find that emission spectra are dominated by inelastic (Raman) scattering from molecules and metals, instead of fluorescence, with molecular bleaching also not evident despite the large structural rearrangements. This implies that the competition between recombination pathways demands a rethink of routes to quantum optics using plasmonics

Substituent effects on energetics and crystal morphology modulate singlet fission in 9,10-bis(phenylethynyl)anthracenes

Singlet fission (SF) converts a singlet exciton into two triplet excitons in two or more electronically coupled organic chromophores, which may then be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability or low triplet state energies. The results described here show that efficient SF occurs in derivatives of 9,10-bis(phenylethynyl)anthracene (BPEA), which is a highly robust and tunable chromophore. Fluoro and methoxy substituents at the 4- and 4′-positions of the BPEA phenyl groups control the intermolecular packing in the crystal structure, which alters the interchromophore electronic coupling, while also changing the SF energetics. The lowest excited singlet state (S1) energy of 4,4′-difluoro-BPEA is higher than that of BPEA so that the increased thermodynamic favorability of SF results in a (16 ± 2 ps)−1 SF rate and a 180% ± 16% triplet yield, which is about an order of magnitude faster than BPEA with a comparable triplet yield. By contrast, 4-fluoro-4′-methoxy-BPEA and 4,4′-dimethoxy-BPEA have slower SF rates, (90 ± 20 ps)−1 and (120 ± 10 ps)−1, and lower triplet yields, (110 ± 4)% and (168 ± 7)%, respectively, than 4,4′-difluoro-BPEA. These differences are attributed to changes in the crystal structure controlling interchromophore electronic coupling as well as SF energetics in these polycrystalline solids

Photoluminescence upconversion in monolayer WSe2 activated by plasmonic cavities through resonant excitation of dark excitons

Anti-Stokes photoluminescence (PL) is light emission at a higher photon energy than the excitation, with applications in optical cooling, bioimaging, lasing, and quantum optics. Here, we show how plasmonic nano-cavities activate anti-Stokes PL in WSe2 monolayers through resonant excitation of a dark exciton at room temperature. The optical near-fields of the plasmonic cavities excite the out-of-plane transition dipole of the dark exciton, leading to light emission from the bright exciton at higher energy. Through statistical measurements on hundreds of plasmonic cavities, we show that coupling to the dark exciton leads to a near hundred-fold enhancement of the upconverted PL intensity. This is further corroborated by experiments in which the laser excitation wavelength is tuned across the dark exciton. We show that a precise nanoparticle geometry is key for a consistent enhancement, with decahedral nanoparticle shapes providing an efficient PL upconversion. Finally, we demonstrate a selective and reversible switching of the upconverted PL via electrochemical gating. Our work introduces the dark exciton as an excitation channel for anti-Stokes PL in WSe2 and paves the way for large-area substrates providing nanoscale optical cooling, anti-Stokes lasing, and radiative engineering of excitons.The authors acknowledge funding from the EPSRC (EP/L027151/1 and EP/R013012/1), and the EU (883703 PICOFORCE, 861950 POSEIDON). B.d.N. acknowledges support from the Winton Program for the Physics of Sustainability and from Royal Society University Research Fellowship URF∖R1∖211162. L.M.L.-M. acknowledges funding from the Spanish Ministerio de Ciencia e Innovacion, MCIN/AEI/10.13039/501100011033 (Grant PID2020-117779RB-100). N.S.M. acknowledges support from the German National Academy of Sciences Leopoldina. R.A. acknowledges support from the Rutherford Foundation of the Royal Society Te Apārangi of New Zealand, the Winton Program for the Physics of Sustainability, and Trinity College Cambridge. L.A.J. acknowledges support from the Cambridge Commonwealth, European & International Trust, and EPSRC award 2275079. J.B.D. acknowledges support from the Blavatnik fellowship. F.L. acknowledges support from the Terman Fellowship and startup funds from the Department of Chemistry at Stanford University.Peer reviewe

Design principles for efficient singlet fission in anthracene-based organic semiconductors

Singlet fission (SF) in two or more electronically coupled organic chromophores converts a high-energy singlet exciton into two low-energy triplet excitons, which can be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability and low triplet state energies. This work summarizes structurally dependent SF dynamics for 9,10-bis(phenylethynyl)anthracene (BPEA) and its derivatives in the solid-state using time-resolved optical spectroscopies, and electronic structure calculations. By modulating the packing structure in thin films, we can effectively tune electronic energy and coupling. The systematic study in BPEA organic semiconductors shows that maximizing the thermodynamic driving force can achieve the highest SF rate and efficiency

Atomic-Level Insights into Defect-Driven Nitrogen Doping of Reduced Graphene Oxide

Nitrogen-doped graphene has been increasingly utilized in a variety of energy-related applications, serving as a catalyst or support material for fuel cells, and as an anode material for lithium-ion batteries, among others. The thermal reduction of graphene oxide (GO) in nitrogenous sources to incorporate nitrogen, producing nitrogen-doped reduced graphene oxide (NRGO), is the most favored method. Controlling atomic configurations of nitrogen-doped sites is the key factor for tailoring the physico-chemical properties of NRGO, but major challenges remain in identifying detailed atomic arrangements at nitrogen binding sites on highly defective and chemically functionalized GO surfaces. In this paper, we present atomistic-scale modeling of the nitrogen doping process of GO with different types of vacancy defects. Molecular dynamics simulations using a reactive force field indicate that the edge carbon atoms on defect sites are the dominant initiation location for nitrogen doping. Further, first-principles calculations using density functional theory present energetically favorable chemical transition pathways for nitrogen doping. The significance of this work lies in providing important chemical insights for the effective control of the desired properties of NRGO by suggesting a detailed mechanism of the nitrogen doping process of GO

Research data supporting "Amplified plasmonic forces from DNA-origami scaffolded single dyes in nanogaps"

This is the source data for Amplified plasmonic forces from DNA-origami scaffolded single dyes in nanogaps in Nano Letters. The data was generated by measuring light emitted from plasmonic cavities created with gold nanoparticles and dye-labelled DNA nanostructures. Collection of the data was performed using a home-built dark-field and Raman microscope. The microscope was operated with python scripts available on https://github.com/nanophotonics/nplab, in particular using a particle tracking code. For each nanoparticle, a dark field spectrum was recorded, then a Raman illumination was performed to obtained surface-enhanced Raman spectroscopy (SERS) spectra. All data were saved in hdf5 format, and analyses with the scripts avaialble in https://github.com/nanophotonics/nplab. In all cases, microsoft excel can be used as well to open and process the data. In all cases, analysis was performed using the python scripts from https://github.com/nanophotonics/nplab, which allowed the production of the paper figures

Amplified Plasmonic Forces from DNA Origami-Scaffolded Single Dyes in Nanogaps.

Developing highly enhanced plasmonic nanocavities allows direct observation of light-matter interactions at the nanoscale. With DNA origami, the ability to precisely nanoposition single-quantum emitters in ultranarrow plasmonic gaps enables detailed study of their modified light emission. By developing protocols for creating nanoparticle-on-mirror constructs in which DNA nanostructures act as reliable and customizable spacers for nanoparticle binding, we reveal that the simple picture of Purcell-enhanced molecular dye emission is misleading. Instead, we show that the enhanced dipolar dye polarizability greatly amplifies optical forces acting on the facet Au atoms, leading to their rapid destabilization. Using different dyes, we find that emission spectra are dominated by inelastic (Raman) scattering from molecules and metals, instead of fluorescence, with molecular bleaching also not evident despite the large structural rearrangements. This implies that the competition between recombination pathways demands a rethink of routes to quantum optics using plasmonics

Design rules for catalysis in single-particle plasmonic nanogap reactors with precisely aligned molecular monolayers.

Plasmonic nanostructures can both drive and interrogate light-driven catalytic reactions. Sensitive detection of reaction pathways is achieved by confining optical fields near the active surface. However, effective control of the reaction kinetics remains a challenge to utilize nanostructure constructs as efficient chemical reactors. Here we present a nanoreactor construct exhibiting high catalytic and optical efficiencies, based on a nanoparticle-on-mirror (NPoM) platform. We observe and track pathways of the Pd-catalysed C-C coupling reaction of molecules within a set of nanogaps presenting different chemical surfaces. Atomic monolayer coatings of Pd on the different Au facets enable tuning of the reaction kinetics of surface-bound molecules. Systematic analysis shows the catalytic efficiency of NPoM-based nanoreactors greatly improves on platforms based on aggregated nanoparticles. More importantly, we show Pd monolayers on the nanoparticle or on the mirror play significantly different roles in the surface reaction kinetics. Our data provides clear evidence for catalytic dependencies on molecular configuration in well-defined nanostructures. Such nanoreactor constructs therefore yield clearer design rules for plasmonic catalysis

In situ electrochemical regeneration of nanogap hotspots for continuously reusable ultrathin SERS sensors.

Surface-enhanced Raman spectroscopy (SERS) harnesses the confinement of light into metallic nanoscale hotspots to achieve highly sensitive label-free molecular detection that can be applied for a broad range of sensing applications. However, challenges related to irreversible analyte binding, substrate reproducibility, fouling, and degradation hinder its widespread adoption. Here we show how in-situ electrochemical regeneration can rapidly and precisely reform the nanogap hotspots to enable the continuous reuse of gold nanoparticle monolayers for SERS. Applying an oxidising potential of +1.5 V (vs Ag/AgCl) for 10 s strips a broad range of adsorbates from the nanogaps and forms a metastable oxide layer of few-monolayer thickness. Subsequent application of a reducing potential of -0.80 V for 5 s in the presence of a nanogap-stabilising molecular scaffold, cucurbit[5]uril, reproducibly regenerates the optimal plasmonic properties with SERS enhancement factors ≈106. The regeneration of the nanogap hotspots allows these SERS substrates to be reused over multiple cycles, demonstrating ≈5% relative standard deviation over at least 30 cycles of analyte detection and regeneration. Such continuous and reliable SERS-based flow analysis accesses diverse applications from environmental monitoring to medical diagnostics