Cascaded nanooptics to probe microsecond atomic-scale phenomena

Spearheading a new era in complex colloid synthesis with TPM and other silanes

Colloid science has recently grown substantially owing to the innovative use of silane coupling agents (SCAs), especially 3-trimethoxysilylpropyl methacrylate (TPM). SCAs were previously used mainly as modifying agents, but their ability to form droplets and condense onto pre-existing structures has enabled their use as a versatile and powerful tool to create novel anisotropic colloids with increasing complexity. In this Review, we highlight the advances in complex colloid synthesis facilitated by the use of TPM and show how this has driven remarkable new applications. The focus is on TPM as the current state-of-the-art in colloid science, but we also discuss other silanes and their potential to make an impact. We outline the remarkable properties of TPM colloids and their synthesis strategies, and discuss areas of soft matter science that have benefited from TPM and other SCAs.

Manipulating Coupled Field Enhancement in Slot-under-Groove Nanoarrays for Universal Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) is an ultrasensitive spectroscopic technique that can identify materials and chemicals based on their inelastic light-scattering properties. In general, SERS relies on sub-10 nm nanogaps to amplify the Raman signals and achieve ultralow-concentration identification of analytes. However, large-sized analytes, such as proteins and viruses, usually cannot enter these tiny nanogaps, limiting the practical applications of SERS. Herein, we demonstrate a universal SERS platform for the reliable and sensitive identification of a wide range of analytes. The key to this success is the prepared "slot-under-groove" nanoarchitecture arrays, which could realize a strongly coupled field enhancement with a large spatial mode distribution via the hybridization of gap-surface plasmons in the upper V-groove and localized surface plasmon resonance in the lower slot. Therefore, our slot-under-groove platform can simultaneously deliver high sensitivity for small-sized analytes and the identification of large-sized analytes with a large Raman gain.

The Effect of Nanoparticle Composition on the Surface-Enhanced Raman Scattering Performance of Plasmonic DNA Origami Nanoantennas

A versatile generation of plasmonic nanoparticle dimers for surface-enhanced Raman scattering (SERS) is presented by combining a DNA origami nanofork and spherical and nonspherical Au or Ag nanoparticles. Combining different nanoparticle species with a DNA origami nanofork to form DNA origami nanoantennas (DONAs), the plasmonic nanoparticle dimers can be optimized for a specific excitation wavelength in SERS. The preparation of such nanoparticle dimers is robust enough to enable the characterization of SERS intensities and SERS enhancement factors of dye-modified DONAs on a single dimer level by measuring in total several thousands of dimers from five different dimer designs, each functionalized with three different Raman reporter molecules and measured at four different excitation wavelengths. Based on these data, SERS enhancement factor (EF) distributions have been determined for each dimer design and excitation wavelengths. The structures and measurement conditions with the highest EFs are suitable for single-molecule SERS (SM-SERS), which is realized by placing single dye molecules into hot spots. We demonstrate that the probability of placing single molecules in a strongly enhancing hot spot for SM-SERS can be increased by using anisotropic nanoparticles with several sharp edges, such as nanoflowers. Combining a Ag nanoparticle with a Au particle in one dimer structure allows for a broadband excitation covering almost the whole visible range. The most versatile plasmonic dimer structure for SERS combines a spherical Ag nanoparticle with a Au nanoflower. Employing the discontinuous Galerkin time domain method, we numerically investigate the bare, symmetric dimers with respect to spectral and near-field properties, showing that, indeed, the nanoflowers induce multiple hot spots located at the edges which surpass the intensity of the spherical dimers, indicating the possibility for SM-SERS. The presented DONA structures and SERS data provide a robust basis for applying such designs as versatile SERS tags and as substrates for SM-SERS measurements.

Optoplasmonic Effects in Highly Curved Surfaces for Catalysis, Photothermal Heating, and SERS

Surface curvature can be used to focus light and alter optical processes. Here, we show that curved surfaces (spheres, cylinders, and cones) with a radius of around 5 μm lead to maximal optoplasmonic properties including surface-enhanced Raman scattering (SERS), photocatalysis, and photothermal processes. Glass microspheres, microfibers, pulled fibers, and control flat substrates were functionalized with well-dispersed and dense arrays of 45 nm Au NP using polystyrene-block-poly-4-vinylpyridine (PS-b-P4VP) and chemically modified with 4-mercaptobenzoic acid (4-MBA, SERS reporter), 4-nitrobenzenethiol (4-NBT, reactive to plasmonic catalysis), or 4-fluorophenyl isocyanide (FPIC, photothermal reporter). The various curved substrates enhanced the plasmonic properties by focusing the light in a photonic nanojet and providing a directional antenna to increase the collection efficacy of SERS photons. The optoplasmonic effects led to an increase of up to 1 order of magnitude of the SERS response, up to 5 times the photocatalytic conversion of 4-NBT to 4,4'-dimercaptoazobenzene when the diameter of the curved surfaces was about 5 μm and a small increase in photothermal effects. Taken together, the results provide evidence that curvature enhances plasmonic properties and that its effect is maximal for spherical objects around a few micrometers in diameter, in agreement with a theoretical framework based on geometrical optics. These enhanced plasmonic effects and the stationary-phase-like plasmonic substrates pave the way to the next generation of sensors, plasmonic photocatalysts, and photothermal devices.

Giant optomechanical spring effect in plasmonic nano- and picocavities probed by surface-enhanced Raman scattering

Molecular vibrations couple to visible light only weakly, have small mutual interactions, and hence are often ignored for non-linear optics. Here we show the extreme confinement provided by plasmonic nano- and pico-cavities can sufficiently enhance optomechanical coupling so that intense laser illumination drastically softens the molecular bonds. This optomechanical pumping regime produces strong distortions of the Raman vibrational spectrum related to giant vibrational frequency shifts from an optical spring effect which is hundred-fold larger than in traditional cavities. The theoretical simulations accounting for the multimodal nanocavity response and near-field-induced collective phonon interactions are consistent with the experimentally-observed non-linear behavior exhibited in the Raman spectra of nanoparticle-on-mirror constructs illuminated by ultrafast laser pulses. Further, we show indications that plasmonic picocavities allow us to access the optical spring effect in single molecules with continuous illumination. Driving the collective phonon in the nanocavity paves the way to control reversible bond softening, as well as irreversible chemistry.

Full Control of Plasmonic Nanocavities Using Gold Decahedra-on-Mirror Constructs with Monodisperse Facets

Bottom-up assembly of nanoparticle-on-mirror (NPoM) nanocavities enables precise inter-metal gap control down to ≈ 0.4 nm for confining light to sub-nanometer scales, thereby opening opportunities for developing innovative nanophotonic devices. However limited understanding, prediction, and optimization of light coupling and the difficulty of controlling nanoparticle facet shapes restricts the use of such building blocks. Here, an ultraprecise symmetry-breaking plasmonic nanocavity based on gold nanodecahedra is presented, to form the nanodecahedron-on-mirror (NDoM) which shows highly consistent cavity modes and fields. By characterizing > 20 000 individual NDoMs, the variability of light in/output coupling is thoroughly explored and a set of robust higher-order plasmonic whispering gallery modes uniquely localized at the edges of the triangular facet in contact with the metallic substrate is found. Assisted by quasinormal mode simulations, systematic elaboration of NDoMs is proposed to give nanocavities with near hundred-fold enhanced radiative efficiencies. Such systematically designed and precisely-assembled metallic nanocavities will find broad application in nanophotonic devices, optomechanics, and surface science.

Shape-Tunable Biconcave Disc-Like Polymer Particles by Swelling-Induced Phase Separation of Seeded Particles with Hydrophilic Shells

Anisotropic shape-tunable polymer particles have gained significant attention for their wide applications, and their performances are usually strongly correlated to their shapes. In contrast to convex particles, the synthesis of highly uniform concave polymer particles remains a great challenge. Here, we present a facile and effective route to synthesize biconcave polystyrene (PS) discs by swelling-induced phase separation of hydrophilically modified PS microspheres and report an unexpected finding that even a tiny amount of hydrophilic units that were incorporated into PS microspheres can significantly change the shape of phase interfaces, resulting in the transformation of disc shapes from convex to flat to concave. This is realized by several typical hydrophilic monomers, such as sodium styrene sulfonate (NaSS), acrylic acid (AA), or (2-(methacryloyloxy)ethyl)trimethylammonium chloride (METAC). The effect of the distribution of hydrophilic units in microspheres was investigated, and the mechanism of shape tuning has been discussed. The curvatures of the bottom surfaces of discs show a strong correlation to the content of hydrophilic units. In particular, we emphasize that the shape control method is general since it does not depend on specific hydrophilic units. This research paves the way for precisely structuring polymer particle shapes, which is important for polymer particles to be used for self-assembly, diffusion, rheology, transport, filler, and many other applications.

Plasmonic phenomena in molecular junctions: principles and applications

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.

Fingerprinting the Hidden Facets of Plasmonic Nanocavities

The optical properties of nanogap plasmonic cavities formed by a NanoParticle-on-Mirror (NPoM, or patch antenna) are determined here, across a wide range of geometric parameters including the nanoparticle diameter, gap refractive index, gap thickness, facet size and shape. Full understanding of the confined optical modes allows these nanocavities to be utilized in a wide range of experiments across many fields. We show that the gap thickness t and refractive index n are spectroscopically indistinguishable, accounted for by a single gap parameter G = n/t ^0.47. Simple tuning of mode resonant frequencies and strength is found for each quasi-normal mode, revealing a spectroscopic "fingerprint" for each facet shape, on both truncated spherical and rhombicuboctahedral nanoparticles. This is applied to determine the most likely nanoscale morphology of facets hidden below each NPoM in experiment, as well as to optimize the constructs for different applications. Simple scaling relations are demonstrated, and an online tool for general use is provided.

Molecular Optomechanics Approach to Surface-Enhanced Raman Scattering

Molecular vibrations constitute one of the smallest mechanical oscillators available for micro-/nanoengineering. The energy and strength of molecular oscillations depend delicately on the attached specific functional groups as well as on the chemical and physical environments. By exploiting the inelastic interaction of molecules with optical photons, Raman scattering can access the information contained in molecular vibrations. However, the low efficiency of the Raman process typically allows only for characterizing large numbers of molecules. To circumvent this limitation, plasmonic resonances supported by metallic nanostructures and nanocavities can be used because they localize and enhance light at optical frequencies, enabling surface-enhanced Raman scattering (SERS), where the Raman signal is increased by many orders of magnitude. This enhancement enables few- or even single-molecule characterization. The coupling between a single molecular vibration and a plasmonic mode constitutes an example of an optomechanical interaction, analogous to that existing between cavity photons and mechanical vibrations. Optomechanical systems have been intensely studied because of their fundamental interest as well as their application in practical implementations of quantum technology and sensing. In this context, SERS brings cavity optomechanics down to the molecular scale and gives access to larger vibrational frequencies associated with molecular motion, offering new possibilities for novel optomechanical nanodevices.

The molecular optomechanics description of SERS is recent, and its implications have only started to be explored. In this Account, we describe the current understanding and progress of this new description of SERS, focusing on our own contributions to the field. We first show that the quantum description of molecular optomechanics is fully consistent with standard classical and semiclassical models often used to describe SERS. Furthermore, we note that the molecular optomechanics framework naturally accounts for a rich variety of nonlinear effects in the SERS signal with increasing laser intensity.

Furthermore, the molecular optomechanics framework provides a tool particularly suited to addressing novel effects of fundamental and practical interest in SERS, such as the emergence of collective phenomena involving many molecules or the modification of the effective losses and energy of the molecular vibrations due to the plasmon–vibration interaction. As compared to standard optomechanics, the plasmonic resonance often differs from a single Lorentzian mode and thus requires a more detailed description of its optical response. This quantum description of SERS also allows us to address the statistics of the Raman photons emitted, enabling the interpretation of two-color correlations of the emerging photons, with potential use in the generation of nonclassical states of light. Current SERS experimental implementations in organic molecules and two-dimensional layers suggest the interest in further exploring intense pulsed illumination, situations of strong coupling, resonant-SERS, and atomic-scale field confinement.

Vibrational Stark Effects: Ionic Influence on Local Fields

Molecules containing vibrational Stark shift reporters provide a useful tool for measuring DC electric fields in situ. To quantify this effect theoretically, density functional theory (DFT) calculations are usually utilized in a uniform electric field. However, using a combined theoretical and experimental study, we demonstrate here that uniform field DFT cannot simultaneously model the behavior of the three strongest vibrational modes in molecules forming a monolayer on an electrode. We show, by directly modeling ionic movement, that the measured Stark shifts are explained by partial electrical double-layer penetration into the molecular layer. This effect is sensitive to the local environment, and the Stark shifts can be fully suppressed experimentally by introducing a mixed molecular layer that prevents ionic double-layer penetration.

Asymmetrical ring-shaped colloidal particles for self-assembly and superhydrophobic coatings

A frame-guided wetting strategy is reported to synthesize highly uniform but asymmetrical colloidal particles from rings to oblate ellipsoids through symmetrical discs, which can self-assemble into diversified highly open 2D superstructures. In particular, ring-shaped particle monolayers have a higher contact angle of water than similar spherical ones, suggesting an attractive particle material for self-cleaning superhydrophobic coatings.

Coupling, lifetimes, and "strong coupling" maps for single molecules at plasmonic interfaces

The interaction between excited states of a molecule and excited states of a metal nanostructure (e.g., plasmons) leads to hybrid states with modified optical properties. When plasmon resonance is swept through molecular transition frequency, an avoided crossing may be observed, which is often regarded as a signature of strong coupling between plasmons and molecules. Such strong coupling is expected to be realized when 2|⟨U⟩|/ℏΓ > 1, where ⟨U⟩ and Γ are the molecule-plasmon coupling and the spectral width of the optical transition, respectively. Because both ⟨U⟩ and Γ strongly increase with decreasing distance between a molecule and a plasmonic structure, it is not obvious that this condition can be satisfied for any molecule-metal surface distance. In this work, we investigate the behavior of ⟨U⟩ and Γ for several geometries. Surprisingly, we find that if the only contributions to Γ are lifetime broadenings associated with the radiative and nonradiative relaxation of a single molecular vibronic transition, including effects on molecular radiative and nonradiative lifetimes induced by the metal, the criterion 2|⟨U⟩|/ℏΓ > 1 is easily satisfied by many configurations irrespective of the metal-molecule distance. This implies that the Rabi splitting can be observed in such structures if other sources of broadening are suppressed. Additionally, when the molecule-metal surface distance is varied keeping all other molecular and metal parameters constant, this behavior is mitigated due to the spectral shift associated with the same molecule-plasmon interaction, making the observation of Rabi splitting more challenging.

Resolving sub-angstrom ambient motion through reconstruction from vibrational spectra

Metal/organic-molecule interactions underpin many key chemistries but occur on sub-nm scales where nanoscale visualisation techniques tend to average over heterogeneous distributions. Single molecule imaging techniques at the atomic scale have found it challenging to track chemical behaviour under ambient conditions. Surface-enhanced Raman spectroscopy can optically monitor the vibrations of single molecules but understanding is limited by the complexity of spectra and mismatch between theory and experiment. We demonstrate that spectra from an optically generated metallic adatom near a molecule of interest can be inverted into dynamic sub-Å metal-molecule interactions using a comprehensive model, revealing anomalous diffusion of a single atom. Transient metal-organic coordination bonds chemically perturb molecular functional groups > 10 bonds away. With continuous improvements in computational methods for modelling large and complex molecular systems, this technique will become increasingly applicable to accurately tracking more complex chemistries.

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

Transient 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.

Advances of surface-enhanced Raman and IR spectroscopies: from nano/microstructures to macro-optical design

Raman and infrared (IR) spectroscopy are powerful analytical techniques, but have intrinsically low detection sensitivity. There have been three major steps (i) to advance the optical system of the light excitation, collection, and detection since 1920s, (ii) to utilize nanostructure-based surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA) since 1990s, and (iii) to rationally couple (i) and (ii) for maximizing the total detection sensitivity since 2010s. After surveying the history of SERS and SEIRA, we outline the principle of plasmonics and the different mechanisms of SERS and SEIRA. We describe various interactions of light with nano/microstructures, localized surface plasmon, surface plasmon polariton, and lightning-rod effect. Their coupling effects can significantly increase the surface sensitivity by designing nanoparticle-nanoparticle and nanoparticle-substrate configuration. As the nano/microstructures have specific optical near-field and far-field behaviors, we focus on how to systematically design the macro-optical systems to maximize the excitation efficiency and detection sensitivity. We enumerate the key optical designs in particular ATR-based operation modes of directional excitation and emission from visible to IR spectral region. We also present some latest advancements on scanning-probe microscopy-based nanoscale spectroscopy. Finally, prospects and further developments of this field are given with emphasis on emerging techniques and methodologies.

Bright Optical Eigenmode of 1 nm^{3} Mode Volume

We report on the discovery and rationale to devise bright single optical eigenmodes that feature quantum-optical mode volumes of about 1  nm^{3}. Our findings rely on the development and application of a quasinormal mode theory that self-consistently treats fields and electron nonlocality, spill-out, and Landau damping around atomistic protrusions on a metallic nanoantenna. By outpacing Landau damping with radiation via properly designed antenna modes, the extremely localized modes become bright with radiation efficiencies reaching 30% and could provide up to 4×10^{7} times intensity enhancement.

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.

Directional Modulation of Exciton Emission Using Single Dielectric Nanospheres

Coupling emitters with nanoresonators is an effective strategy to control light emission at the subwavelength scale with high efficiency. Low-loss dielectric nanoantennas hold particular promise for this purpose, owing to their strong Mie resonances. Herein, a highly miniaturized platform is explored for the control of emission based on individual subwavelength Si nanospheres (SiNSs) to modulate the directional excitation and exciton emission of 2D transition metal dichalcogenides (2D TMDs). A modified Mie theory for dipole-sphere hybrid systems is derived to instruct the optimal design for desirable modulation performance. Controllable forward-to-backward intensity ratios are experimentally validated in 532 nm laser excitation and 635 nm exciton emission from a monolayer WS2 . Versatile light emission control is achieved for different emitters and excitation wavelengths, benefiting from the facile size control and isotropic shape of SiNSs. Simultaneous modulation of excitation and emission via a single SiNS at visible wavelengths significantly improves the efficiency and directionality of TMD exciton emission and leads to the potential of multifunctional integrated photonics. Overall, the work opens promising opportunities for nanophotonics and polaritonic systems, enabling efficient manipulation, enhancement, and reconfigurability of light-matter interactions.

Integrating Sphere Fourier Microscopy of Highly Directional Emission

Accurately controlling light emission using nano- and microstructured lenses and antennas is an active field of research. Dielectrics are especially attractive lens materials due to their low optical losses over a broad bandwidth. In this work we measure highly directional light emission from patterned quantum dots (QDs) aligned underneath all-dielectric nanostructured microlenses. The lenses are designed with an evolutionary algorithm and have a theoretical directivity of 160. The fabricated structures demonstrate an experimental full directivity of 61 ± 3, three times higher than what has been estimated before, with a beaming half-angle of 2.6°. This high value compared to previous works is achieved via three mechanisms. First, direct electron beam patterning of QD emitters and alignment markers allowed for more localized emission and better emitter-lens alignment. Second, the lens fabrication was refined to minimize distortions between the designed shape and the final structure. Finally, a new measurement technique was developed that combines integrating sphere microscopy with Fourier microscopy. This enables complete directivity measurements, contrary to other reported values, which are typically only partial directivities or estimates of the full directivity that rely partly on simulations. The experimentally measured values of the complete directivity were higher than predicted by combining simulations with partial directivity measurements. High directivity was obtained from three different materials (cadmium-selenide-based QDs and two lead halide perovskite materials), emitting at 520, 620, and 700 nm, by scaling the lens size according to the emission wavelength.

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.