Locating Single-Atom Optical Picocavities Using Wavelength-Multiplexed Raman Scattering.

Funder: Leverhulme TrustFunder: Trinity College, University of CambridgeFunder: Isaac Newton TrustTransient atomic protrusions in plasmonic nanocavities confine optical fields to sub-1-nm3 picocavities, allowing the optical interrogation of single molecules at room temperature. While picocavity formation is linked to both the local chemical environment and optical irradiation, the role of light in localizing the picocavity formation is unclear. Here, we combine information from thousands of picocavity events and simultaneously compare the transient Raman scattering arising from two incident pump wavelengths. Full analysis of the data set suggests that light suppresses the local effective barrier height for adatom formation and that the initial barrier height is decreased by reduced atomic coordination numbers near facet edges. Modeling the system also resolves the frequency-dependent picocavity field enhancements supported by these atomic scale features.European Research Council (ERC) under Horizon 2020 research and innovation programme PICOFORCE (Grant Agreement No. 883703) and POSEIDON (Grant Agreement No. 861950). We acknowledge funding from the EPSRC (Cambridge NanoDTC EP/L015978/1, EP/L027151/1, EP/S022953/1, EP/P029426/1, and EP/R020965/1

Self-aligned colloidal lithography for controllable and tuneable plasmonic nanogaps.

Au nanoparticles (NPs) deposited on a substrate function as ring shaped colloidal shadow masks. Using e-beam evaporation of gold, nanometer sized gaps are formed as a result. The size of these gaps can be accurately tuned by controlling the thickness of the gold deposition, thereby tuning the plasmonic coupling of the NPs with the substrate. The clean cavity produced between the Au NPs and the Au film provides an excellent SERS platform for trace molecule detection.The authors acknowledge the fi nancial support of this research from EPSRC grant EP/G060649/1, EP/I012060/1, and ERC grant LINASS 320503. FB acknowledges support from the Winton Pro-gramme for the Physics of Sustainability.This article was originally published in Small, 2015, DOI: 10.1002/smll.201402639

Controllable Tuning Plasmonic Coupling with Nanoscale Oxidation.

The nanoparticle on mirror (NPoM) construct is ideal for the strong coupling of localized plasmons because of its simple fabrication and the nanometer-scale gaps it offers. Both of these are much harder to control in nanoparticle dimers. Even so, realizing controllable gap sizes in a NPoM remains difficult and continuous tunability is limited. Here, we use reactive metals as the mirror so that the spacing layer of resulting metal oxide can be easily and controllably created with specific thicknesses resulting in continuous tuning of the plasmonic coupling. Using Al as a case study, we contrast different approaches for oxidation including electrochemical oxidation, thermal annealing, oxygen plasma treatments, and photo-oxidation by laser irradiation. The thickness of the oxidation layer is calibrated with depth-mode X-ray photoemission spectroscopy (XPS). These all consistently show that increasing the thickness of the oxidation layer blue-shifts the plasmonic resonance peak while the transverse mode remains constant, which is well matched by simulations. Our approach provides a facile and reproducible method for scalable, local and controllable fabrication of NPoMs with tailored plasmonic coupling, suited for many applications of sensing, photochemistry, photoemission, and photovoltaics.EPSRC grant EP/G060649/1, EP/I012060/1, ERC grant LINASS 320503.This is the final version of the article. It first appeared from the American Chemical Society via http://dx.doi.org/10.1021/acsnano.5b0128

Tracking interfacial single-molecule pH and binding dynamics via vibrational spectroscopy.

Understanding single-molecule chemical dynamics of surface ligands is of critical importance to reveal their individual pathways and, hence, roles in catalysis, which ensemble measurements cannot see. Here, we use a cascaded nano-optics approach that provides sufficient enhancement to enable direct tracking of chemical trajectories of single surface-bound molecules via vibrational spectroscopy. Atomic protrusions are laser-induced within plasmonic nanojunctions to concentrate light to atomic length scales, optically isolating individual molecules. By stabilizing these atomic sites, we unveil single-molecule deprotonation and binding dynamics under ambient conditions. High-speed field-enhanced spectroscopy allows us to monitor chemical switching of a single carboxylic group between three discrete states. Combining this with theoretical calculation identifies reversible proton transfer dynamics (yielding effective single-molecule pH) and switching between molecule-metal coordination states, where the exact chemical pathway depends on the intitial protonation state. These findings open new domains to explore interfacial single-molecule mechanisms and optical manipulation of their reaction pathways

Accelerated Molecular Vibrational Decay and Suppressed Electronic Nonlinearities in Plasmonic Cavities through Coherent Raman Scattering

Molecular vibrations and their dynamics are of outstanding importance for electronic and thermal transport in nanoscale devices as well as for molecular catalysis. The vibrational dynamics of <100 molecules are studied through three-colour time-resolved coherent anti-Stokes Raman spectroscopy (trCARS) using plasmonic nanoantennas. This isolates molecular signals from four-wave mixing (FWM), while using exceptionally low nanowatt powers to avoid molecular damage via single-photon lock-in detection. FWM is found to be strongly suppressed in nm-wide plasmonic gaps compared to plasmonic nanoparticles. The ultrafast vibrational decay rates of biphenyl-4-thiol molecules are accelerated ten-fold by a transient rise in local non-equilibrium temperature excited by enhanced, pulsed optical fields within these plasmonic nanocavities. Separating the contributions of vibrational population decay and dephasing carefully explores the vibrational decay channels of these tightly confined molecules. Such extreme plasmonic enhancement within nanogaps opens up prospects for measuring single-molecule vibrationally-coupled dynamics and diverse molecular optomechanics phenomena

Contact angle as a powerful tool in anisotropic colloid synthesis

Nucleation and growth is a technique widely used to prepare colloids, in which droplets are adsorbed onto substrate particles. Changing the contact angle of the substrates can greatly alter the morphology of the product particles. Here, we investigate the nucleation and growth of 3-methacryloxypropyltrimethoxysilane (MPTMS) both onto Stöber spheres and onto (cross-linked) MPTMS* spheres. The former results in ‘snowman’ particles with a cap-shaped MPTMS* compartment, and we show that their morphology is highly controllable via the MPTMS content in the reaction mixture. The contact angle of the MPTMS* compartment decreases with droplet diameter, suggesting that this wetting process is affected not only by surface tension but also by line tension. In contrast to Stöber spheres, MPTMS* substrate particles yield highly reproducible and tuneable ‘engulfed-sphere’ colloids with an internal reference axis (but a homogeneous mass distribution). These engulfed-sphere particles can be fully index-matched for confocal microscopy on account of their homogeneous refractive index. Suitable index-matching mixtures of polar and of low-polar media are presented, where cyclohexyl iodide (CHI) is introduced as a new medium for colloids of high refractive index. Finally, the index-matched engulfed-sphere colloids are self-assembled into (close-packed and long-range) plastic phases, and the particles’ rotational diffusion inside the crystal phases is tracked via confocal microscopy

Nanometer control in plasmonic systems through discrete layer-by-layer macrocycle-cation deposition.

In this work, we demonstrate that coordination interactions between Fe3+ and cucurbit[7]uril (CB[7]) can be utilised to build up defined nanoscale spacing layers in metallic nanosystems. We begin by characterising the layer-by-layer deposition of CB[7] and FeCl3·6H2O coordination layers through the use of a Quartz-Crystal Microbalance (QCM) and contact angle measurements. We then apply this layered structure to accurately control the spacing, and thus optical properties, of gold nanoparticles in a Nanoparticle-on-Mirror (NPoM) structure, which is demonstrated via normalising plasmon resonance spectroscopy.European Commission for a Marie Curie Fellowship (NANOSPHERE, 658360) ERC starting investigator grant (ASPiRe 240629) RC acknowledges support from the Dr. Manmohan Singh scholarship from St. John’s College BdN acknowledges support from the Leverhulme Trust and the Isaac Newton trust ECF

SERS Sensing of Dopamine with Fe(III)-Sensitized Nanogaps in Recleanable AuNP Monolayer Films

Sensing of neurotransmitters (NTs) down to nm concentrations is demonstrated by utilizing self-assembled monolayers of plasmonic 60 nm Au nanoparticles in close-packed arrays immobilized onto glass substrates. Multiplicative surface-enhanced Raman spectroscopy enhancements are achieved by integrating Fe(III) sensitizers into the precisely-defined <1 nm nanogaps, to target dopamine (DA) sensing. The transparent glass substrates allow for efficient access from both sides of the monolayer aggregate films by fluid and light, allowing repeated sensing in different analytes. Repeated reusability after analyte sensing is shown through oxygen plasma cleaning protocols, which restore pristine conditions for the nanogaps. Examining binding competition in multiplexed sensing of two catecholamine NTs, DA and epinephrine, reveals their bidentate binding and their interactions. These systems are promising for widespread microfluidic integration enabling a wide range of continuous biofluid monitoring for applications in precision health

Single-molecule strong coupling at room temperature in plasmonic nanocavities

Emitters placed in an optical cavity experience an environment that changes their coupling to light. In the weak-coupling regime light extraction is enhanced, but more profound effects emerge in the single-molecule strong-coupling regime where mixed light-matter states form [1,2] . Individual two-level emitters in such cavities become non-linear for single photons, forming key building blocks for quantum information systems as well as ultra-low power switches and lasers [3–6] . Such cavity quantum electrodynamics has until now been the preserve of low temperatures and complex fabrication, severely compromising their use [5,7,8] . Here, by scaling the cavity volume below 40 nm^3 and using host-guest chemistry to align 1-10 protectively-isolated methylene-blue molecules, we reach the strong-coupling regime at room temperature and in ambient conditions. Dispersion curves from >50 plasmonic nanocavities display characteristic anticrossings, with Rabi frequencies of 300 meV for 10 molecules decreasing to 90 meV for single molecules, matching quantitative models. Statistical analysis of vibrational spectroscopy time-series and dark-field scattering spectra provide evidence of single-molecule strong coupling. This dressing of molecules with light can modify photochemistry, opening up the exploration of complex natural processes such as photosynthesis [9] and pathways towards manipulation of chemical bonds [10].We acknowledge financial support from EPSRC grants EP/G060649/1 and EP/I012060/1, and ERC grant LINASS 320503. RC acknowledges support from the Dr. Manmohan Singh scholarship from St. John’s College. FB acknowledges support from the Winton Programme for the Physics of Sustainability. SJB acknowledges support from the European Commission for a Marie Curie Fellowship (NANOSPHERE, 658360).This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/nature17974

SERS of individual nanoparticles on a mirror : size does matter, but so does shape

The authors thank Javier Aizpurua (CSIC − UPV/EHU/DIPC) for helpful discussions. We acknowledge financial support from EPSRC Grants EP/G060649/1, EP/K028510/1, EP/L027151/1, ERC Grant LINASS 320503. F.B. acknowledges support from the Winton Programme for the Physics of Sustainability. R.C. acknowledges support from the Dr. Manmohan Singh scholarship from St. John’s College.Coupling noble metal nanoparticles by a 1 nm gap to an underlying gold mirror confines light to extremely small volumes, useful for sensing on the nanoscale. Individually measuring 10 000 of such gold nanoparticles of increasing size dramatically shows the different scaling of their optical scattering (far-field) and surface-enhanced Raman emission (SERS, near-field). Linear red-shifts of the coupled plasmon modes are seen with increasing size, matching theory. The total SERS from the few hundred molecules under each nanoparticle dramatically increases with increasing size. This scaling shows that maximum SERS emission is always produced from the largest nanoparticles, irrespective of tuning to any plasmonic resonances. Changes of particle facet with nanoparticle size result in vastly weaker scaling of the near-field SERS, without much modifying the far-field, and allows simple approaches for optimizing practical sensing.Publisher PDFPeer reviewe