Tailoring the aluminum nanocrystal surface oxide for all-aluminum-based antenna-reactor plasmonic photocatalysts

Aluminum nanocrystals (AlNCs) are of increasing interest as sustainable, earth-abundant nanoparticles for visible wavelength plasmonics and as versatile nanoantennas for energy-efficient plasmonic photocatalysis. Here, we show that annealing AlNCs under various gases and thermal conditions induces substantial, systematic changes in their surface oxide, modifying crystalline phase, surface morphology, density, and defect type and concentration. Tailoring the surface oxide properties enables AlNCs to function as all-aluminum-based antenna-reactor plasmonic photocatalysts, with the modified surface oxides providing varying reactivities and selectivities for several chemical reactions.

A Quasi-Bound States in the Continuum Dielectric Metasurface-Based Antenna-Reactor Photocatalyst

Metasurfaces are a class of two-dimensional artificial resonators, creating new opportunities for strong light-matter interactions. One type of nonradiative optical metasurface that enables substantial light concentration is based on quasi-Bound States in the Continuum (quasi-BIC). Here we report the design and fabrication of a quasi-BIC dielectric metasurface that serves as an optical frequency antenna for photocatalysis. By depositing Ni nanoparticle reactors onto the metasurface, we create an antenna-reactor photocatalyst, where the virtually lossless metasurface funnels light to drive a chemical reaction. This quasi-BIC-Ni antenna-reactor drives H2 dissociation under resonant illumination, showing strong polarization, wavelength, and optical power dependencies. Both E-field-induced electronic and photothermal heating effects drive the reaction, supported by load-dependent reactivity studies and our theoretical model. This study unlocks new opportunities for photocatalysis that employ dielectric metasurfaces for light harvesting in an antenna-reactor format.

Combined Surface-Enhanced Raman and Infrared Absorption Spectroscopies for Streamlined Chemical Detection of Polycyclic Aromatic Hydrocarbon-Derived Compounds

Polycyclic aromatic hydrocarbons (PAHs) constitute a class of universally prevalent carcinogenic environmental contaminants. It is increasingly recognized, however, that PAHs derivatized with oxygen, sulfur, or nitrogen functional groups are frequently more dangerous than their unfunctionalized counterparts. This much larger family of chemicals─polycyclic aromatic compounds─PACs─is far less well characterized than PAHs. Using surface-enhanced Raman and IR Absorption spectroscopies (SERS + SEIRA) combined on a single substrate, along with density functional theoretical (DFT) calculations, we show that direct chemical detection and identification of PACs at sub-parts-per-billion concentration can be achieved. Focusing our studies on 9,10-anthraquinone, 5,12-tetracenequinone, 9-nitroanthracene, and 1-nitropyrene as model PAC contaminants, detection is made possible by incorporating a hydroxy-functionalized self-assembled monolayer that facilitates hydrogen bonding between analytes and the SERS + SEIRA substrate. 5,12-Tetracenequinone was detected at 0.3 ppb, and the limit of detection was determined to be 0.1 ppb using SEIRA alone. This approach is straightforwardly extendable to other families of analytes and will ultimately facilitate fieldable chemical detection of these dangerous yet largely overlooked environmental contaminants.

Identifying Surface-Enhanced Raman Spectra with a Raman Library Using Machine Learning

Since its discovery, surface-enhanced Raman spectroscopy (SERS) has shown outstanding promise of identifying trace amounts of unknown molecules in rapid, portable formats. However, the many different types of nanoparticles or nanostructured metallic SERS substrates created over the past few decades show substantial variability in the SERS spectra they provide. These inconsistencies have even raised speculation that substrate-specific SERS spectral libraries must be compiled for practical use of this type of spectroscopy. Here, we report a machine learning (ML) algorithm that can identify chemicals by matching their SERS spectra to those of a standard Raman spectral library. We use an approach analogous to facial recognition that utilizes feature extraction in the presence of multiple nuisance variables for spectral recognition. The key element is a metric we call "Characteristic Peak Similarity" (CaPSim) that focuses on the characteristic peaks in the SERS spectra. It has the flexibility to accommodate substrate-specific variability when quantifying the degree of similarity to a Raman spectrum. Analysis shows that CaPSim substantially outperforms existing spectral matching algorithms in terms of accuracy. This ML-based approach could greatly facilitate the spectroscopic identification of molecules in fieldable SERS applications.

Chiral Plasmonic Pinwheels Exhibit Orientation-Independent Linear Differential Scattering under Asymmetric Illumination

Plasmonic nanoantennas have considerably stronger polarization-dependent optical properties than their molecular counterparts, inspiring photonic platforms for enhancing molecular dichroism and providing fundamental insight into light-matter interactions. One such insight is that even achiral nanoparticles can yield strong optical activity when they are asymmetrically illuminated from a single oblique angle instead of evenly illuminated. This effect, called extrinsic chirality, results from the overall chirality of the experimental geometry and strongly depends on the orientation of the incident light. Although extrinsic chirality has been well-characterized, an analogous effect involving linear polarization sensitivity has not yet been discussed. In this study, we investigate the differential scattering of rotationally symmetric chiral plasmonic pinwheels when asymmetrically irradiated with linearly polarized light. Despite their high rotational symmetry, we observe substantial linear differential scattering that is maintained over all pinwheel orientations. We demonstrate that this orientation-independent linear differential scattering arises from the broken mirror and rotational symmetries of our overall experimental geometry. Our results underscore the necessity of considering both the rotational symmetry of the nanoantenna and the experimental setup, including illumination direction and angle, when performing plasmon-enhanced chiroptical characterizations. Our results demonstrate spectroscopic signatures of an effect analogous to extrinsic chirality for linear polarizations.

Energy Transfer to Molecular Adsorbates by Transient Hot Electron Spillover

Hot electron (HE) photocatalysis is one of the most intriguing fields of nanoscience, with a clear potential for technological impact. Despite much effort, the mechanisms of HE photocatalysis are not fully understood. Here we investigate a mechanism based on transient electron spillover on a molecule and subsequent energy release into vibrational modes. We use state-of-the-art real-time Time Dependent Density Functional Theory (rt-TDDFT), simulating the dynamics of a HE moving within linear chains of Ag or Au atoms, on which CO, N₂, or H₂O are adsorbed. We estimate the energy a HE can release into adsorbate vibrational modes and show that certain modes are selectively activated. The energy transfer strongly depends on the adsorbate, the metal, and the HE energy. Considering a cumulative effect from multiple HEs, we estimate this mechanism can transfer tenths of an eV to molecular vibrations and could play an important role in HE photocatalysis.

Facet Tunability of Aluminum Nanocrystals

Aluminum nanocrystals (Al NCs) with a well-defined size and shape combine unique plasmonic properties with high earth abundance, potentially ideal for applications where sustainability and cost are important factors. It has recently been shown that single-crystal Al {100} nanocubes can be synthesized by the decomposition of AlH₃ with Tebbe's reagent, a titanium(IV) catalyst with two cyclopentadienyl ligands. By systematically modifying the catalyst molecular structure, control of the NC growth morphology is observed spectroscopically, as the catalyst stabilizes the {100} NC facets. By varying the catalyst concentration, Al NC faceted growth is tunable from {100} faceted nanocubes to {111} faceted octahedra. This study provides direct insight into the role of catalyst molecular structure in controlling Al NC morphology.

Earth-abundant photocatalyst for H 2 generation from NH 3 with light-emitting diode illumination

Catalysts based on platinum group metals have been a major focus of the chemical industry for decades. We show that plasmonic photocatalysis can transform a thermally unreactive, earth-abundant transition metal into a catalytically active site under illumination. Fe active sites in a Cu-Fe antenna-reactor complex achieve efficiencies very similar to Ru for the photocatalytic decomposition of ammonia under ultrafast pulsed illumination. When illuminated with light-emitting diodes rather than lasers, the photocatalytic efficiencies remain comparable, even when the scale of reaction increases by nearly three orders of magnitude. This result demonstrates the potential for highly efficient, electrically driven production of hydrogen from an ammonia carrier with earth-abundant transition metals.

Fast Topology Optimization for Near-Field Focusing All-Dielectric Metasurfaces Using the Discrete Dipole Approximation

Using an efficient implementation of the discrete dipole approximation and topology optimization, we design all-dielectric metasurfaces capable of focusing light into intense deep subwavelength hotspots. The light focusing of these metasurfaces far outweighs conventional lenses and can provide dramatic enhancements of processes that depend superlinearly on light intensity, such as light-powered membrane distillation and photocatalysis. Our approach can easily be generalized to optimize metasurfaces for other functionalities, such as nonlinear optics or photothermal conversion.

Plasmonic Photocatalysis with Chemically and Spatially Specific Antenna-Dual Reactor Complexes

Plasmonic antenna-reactor photocatalysts have been shown to convert light efficiently to chemical energy. Virtually all chemical reactions mediated by such complexes to date, however, have involved relatively simple reactions that require only a single type of reaction site. Here, we investigate a planar Al nanodisk antenna with two chemically distinct and spatially separated active sites in the form of Pd and Fe nanodisks, fabricated in 90° and 180° trimer configurations. The photocatalytic reactions H₂ + D₂ → 2HD and NH₃ + D₂ → NH₂D + HD were both investigated on these nanostructured complexes. While the H₂-D₂ exchange reaction showed an additive behavior for the linear (180°) nanodisk complex, the NH₃ + D₂ reaction shows a clear synergistic effect of the position of the reactor nanodisks relative to the central Al nanodisk antenna. This study shows that light-driven chemical reactions can be performed with both chemical and spatial control of the specific reaction steps, demonstrating precisely designed antennas with multiple reactors for tailored control of chemical reactions of increasing complexity.

Plasmonic gadolinium oxide nanomatryoshkas: bifunctional magnetic resonance imaging enhancers for photothermal cancer therapy

Nanoparticle-assisted laser-induced photothermal therapy (PTT) is a promising method for cancer treatment; yet, visualization of nanoparticle uptake and photothermal response remain a critical challenge. Here, we report a magnetic resonance imaging-active nanomatryoshka (Gd₂O₃-NM), a multilayered (Au core/Gd₂O₃ shell/Au shell) sub-100 nm nanoparticle capable of combining T₁ MRI contrast with PTT. This bifunctional nanoparticle demonstrates an r₁ of 1.28 × 10⁸ mM^-1 s^-1, an MRI contrast enhancement per nanoparticle sufficient for T₁ imaging in addition to tumor ablation. Gd₂O₃-NM also shows excellent stability in an acidic environment, retaining 99% of the internal Gd(3). This report details the synthesis and characterization of a promising system for combined theranostic nanoparticle tracking and PTT.

A Dual Catalyst Strategy for Controlling Aluminum Nanocrystal Growth

The synthesis of Al nanocrystals (Al NCs) is a rapidly expanding field, but there are few strategies for size and morphology control. Here we introduce a dual catalyst approach for the synthesis of Al NCs to control both NC size and shape. By using one catalyst that nucleates growth more rapidly than a second catalyst whose ligands affect NC morphology during growth, one can obtain both size and shape control of the resulting Al NCs. The combination of the two catalysts (1) titanium isopropoxide (TIP), for rapid nucleation, and (2) Tebbe's reagent, for specific facet-promoting growth, yields {100}-faceted Al NCs with tunable diameters between 35 and 65 nm. This dual-catalyst strategy could dramatically expand the possible outcomes for Al NC growth, opening the door to new controlled morphologies and a deeper understanding of earth-abundant plasmonic nanocrystal synthesis.

Towards scalable plasmonic Fano-resonant metasurfaces for colorimetric sensing

Transitioning plasmonic metasurfaces into practical, low-cost applications requires meta-atom designs that focus on ease of manufacturability and a robustness with respect to structural imperfections and nonideal substrates. It also requires the use of inexpensive, earth-abundant metals such as Al for plasmonic properties. In this study, we focus on combining two aspects of plasmonic metasurfaces-visible coloration and Fano resonances-in a morphology amenable to scalable manufacturing. The resulting plasmonic metasurface is a candidate for reflective colorimetric sensing. We examine the potential of this metasurface for reflective strain sensing, where the periodicity of the meta-atoms could ultimately be modified by a potential flexion, and for localized surface plasmon resonance refractive index sensing. This study evaluates the potential of streamlined meta-atom design combined with low-cost metallization for inexpensive sensor readout based on human optical perception.

Al@TiO2 Core-Shell Nanoparticles for Plasmonic Photocatalysis

Plasmon-induced photocatalysis is a topic of rapidly increasing interest, due to its potential for substantially lowering reaction barriers and temperatures and for increasing the selectivity of chemical reactions. Of particular interest for plasmonic photocatalysis are antenna-reactor nanoparticles and nanostructures, which combine the strong light-coupling of plasmonic nanostructures with reactors that enhance chemical specificity. Here, we introduce Al@TiO₂ core-shell nanoparticles, combining earth-abundant Al nanocrystalline cores with TiO₂ layers of tunable thickness. We show that these nanoparticles are active photocatalysts for the hot electron-mediated H₂ dissociation reaction as well as for hot hole-mediated methanol dehydration. The wavelength dependence of the reaction rates suggests that the photocatalytic mechanism is plasmonic hot carrier generation with subsequent transfer of the hot carriers into the TiO₂ layer. The Al@TiO₂ antenna-reactor provides an earth-abundant solution for the future design of visible-light-driven plasmonic photocatalysts.

Vacuum ultraviolet nonlinear metalens

Vacuum ultraviolet (VUV) light plays an essential role across science and technology, from molecular spectroscopy to nanolithography and biomedical procedures. Realizing nanoscale devices for VUV light generation and control is critical for next-generation VUV sources and systems, but the scarcity of low-loss VUV materials creates a substantial challenge. We demonstrate a metalens that both generates-by second-harmonic generation-and simultaneously focuses the generated VUV light. The metalens consists of 150-nm-thick zinc oxide (ZnO) nanoresonators that convert 394 nm (~3.15 eV) light into focused 197-nm (~6.29 eV) radiation, producing a spot 1.7 μm in diameter with a 21-fold power density enhancement as compared to the wavefront at the metalens surface. The reported metalens is ultracompact and phase-matching free, allowing substantial streamlining of VUV system design and facilitating more advanced applications. This work provides a useful platform for developing low-loss VUV components and increasing the accessibility of the VUV regime.

Plasmon-induced trap filling at grain boundaries in perovskite solar cells

The deep-level traps induced by charged defects at the grain boundaries (GBs) of polycrystalline organic-inorganic halide perovskite (OIHP) films serve as major recombination centres, which limit the device performance. Herein, we incorporate specially designed poly(3-aminothiophenol)-coated gold (Au@PAT) nanoparticles into the perovskite absorber, in order to examine the influence of plasmonic resonance on carrier dynamics in perovskite solar cells. Local changes in the photophysical properties of the OIHP films reveal that plasmon excitation could fill trap sites at the GB region through photo-brightening, whereas transient absorption spectroscopy and density functional theory calculations correlate this photo-brightening of trap states with plasmon-induced interfacial processes. As a result, the device achieved the best efficiency of 22.0% with robust operational stability. Our work provides unambiguous evidence for plasmon-induced trap occupation in OIHP and reveals that plasmonic nanostructures may be one type of efficient additives to overcome the recombination losses in perovskite solar cells and thin-film solar cells in general.

Plasmon Energy Transfer in Hybrid Nanoantennas

Plasmonic metal nanoparticles exhibit large dipole moments upon photoexcitation and have the potential to induce electronic transitions in nearby materials, but fast internal relaxation has to date limited the spatial range and efficiency of plasmonic mediated processes. In this work, we use photo-electrochemistry to synthesize hybrid nanoantennas comprised of plasmonic nanoparticles with photoconductive polymer coatings. We demonstrate that the formation of the conductive polymer is selective to the nanoparticles and that polymerization is enhanced by photoexcitation. In situ spectroscopy and simulations support a mechanism in which up to 50% efficiency of nonradiative energy transfer is achieved. These hybrid nanoantennas combine the unmatched light-harvesting properties of a plasmonic antenna with the similarly unmatched device processability of a polymer shell.

A 3D Plasmonic Antenna-Reactor for Nanoscale Thermal Hotspots and Gradients

Plasmonic nanoantennas focus light below the diffraction limit, creating strong field enhancements, typically within a nanoscale junction. Placing a nanostructure within the junction can greatly enhance the nanostructure's innate optical absorption, resulting in intense photothermal heating that could ultimately compromise both the nanostructure and the nanoantenna. Here, we demonstrate a three-dimensional "antenna-reactor" geometry that results in large nanoscale thermal gradients, inducing large local temperature increases in the confined nanostructure reactor while minimizing the temperature increase of the surrounding antenna. The nanostructure is supported on an insulating substrate within the antenna gap, while the antenna maintains direct contact with an underlying thermal conductor. Elevated local temperatures are quantified, and high local temperature gradients that thermally reshape only the internal reactor element within each antenna-reactor structure are observed. We also show that high local temperature increases of nominally 200 °C are achievable within antenna-reactors patterned into large extended arrays. This simple strategy can facilitate standoff optical generation of high-temperature hotspots, which may be useful in applications such as small-volume, high-throughput chemical processes, where reaction efficiencies depend exponentially on local temperature.

Hot carrier multiplication in plasmonic photocatalysis

Light-induced hot carriers derived from the surface plasmons of metal nanostructures have been shown to be highly promising agents for photocatalysis. While both nonthermal and thermalized hot carriers can potentially contribute to this process, their specific role in any given chemical reaction has generally not been identified. Here, we report the observation that the H2-D2 exchange reaction photocatalyzed by Cu nanoparticles is driven primarily by thermalized hot carriers. The external quantum yield shows an intriguing S-shaped intensity dependence and exceeds 100% for high light intensities, suggesting that hot carrier multiplication plays a role. A simplified model for the quantum yield of thermalized hot carriers reproduces the observed kinetic features of the reaction, validating our hypothesis of a thermalized hot carrier mechanism. A quantum mechanical study reveals that vibrational excitations of the surface Cu-H bond is the likely activation mechanism, further supporting the effectiveness of low-energy thermalized hot carriers in photocatalyzing this reaction.

Thousand-fold Increase in Plasmonic Light Emission via Combined Electronic and Optical Excitations

Surface plasmon enhanced processes and hot-carrier dynamics in plasmonic nanostructures are of great fundamental interest to reveal light-matter interactions at the nanoscale. Using plasmonic tunnel junctions as a platform supporting both electrically and optically excited localized surface plasmons, we report a much greater (over 1000× ) plasmonic light emission at upconverted photon energies under combined electro-optical excitation, compared with electrical or optical excitation separately. Two mechanisms compatible with the form of the observed spectra are interactions of plasmon-induced hot carriers and electronic anti-Stokes Raman scattering. Our measurement results are in excellent agreement with a theoretical model combining electro-optical generation of hot carriers through nonradiative plasmon excitation and hot-carrier relaxation. We also discuss the challenge of distinguishing relative contributions of hot carrier emission and the anti-Stokes electronic Raman process. This observed increase in above-threshold emission in plasmonic systems may open avenues in on-chip nanophotonic switching and hot-carrier photocatalysis.

All-Optically Reconfigurable Plasmonic Metagrating for Ultrafast Diffraction Management

Hot-electron dynamics taking place in nanostructured materials upon irradiation with fs-laser pulses has been the subject of intensive research, leading to the emerging field of ultrafast nanophotonics. However, the most common description of nonlinear interaction with ultrashort laser pulses assumes a homogeneous spatial distribution for the photogenerated carriers. Here we theoretically show that the inhomogeneous evolution of the hot carriers at the nanoscale can disclose unprecedented opportunities for ultrafast diffraction management. In particular, we design a highly symmetric plasmonic metagrating capable of a transient symmetry breaking driven by hot electrons. The subsequent power imbalance between symmetrical diffraction orders is calculated to exceed 20% under moderate (∼2 mJ/cm²) laser fluence. Our theoretical investigation also indicates that the recovery time of the symmetric configuration can be controlled by tuning the geometry of the metaatom, and can be as fast as 2 ps for electrically connected configurations.

Phonon-Assisted Hot Carrier Generation in Plasmonic Semiconductor Systems

Plasmonic materials have optical cross sections that exceed by 10-fold their geometric sizes, making them uniquely suitable to convert light into electrical charges. Harvesting plasmon-generated hot carriers is of interest for the broad fields of photovoltaics and photocatalysis; however, their direct utilization is limited by their ultrafast thermalization in metals. To prolong the lifetime of hot carriers, one can place acceptor materials, such as semiconductors, in direct contact with the plasmonic system. Herein, we report the effect of operating temperature on hot electron generation and transfer to a suitable semiconductor. We found that an increase in the operation temperature improves hot electron harvesting in a plasmonic semiconductor hybrid system, contrasting what is observed on photodriven processes in nonplasmonic systems. The effect appears to be related to an enhancement in hot carrier generation due to phonon coupling. This discovery provides a new strategy for optimization of photodriven energy production and chemical synthesis.

UV-Resonant Al Nanocrystals: Synthesis, Silica Coating, and Broadband Photothermal Response

The field of plasmonics has largely been inspired by the properties of Au and Ag nanoparticles, leading to applications in sensing, photocatalysis, nanomedicine, and solar water treatment. Recently the quest for new plasmonic materials has focused on earth-abundant elements, where aluminum is a sustainable, low-cost potential alternative. Here we report the chemical synthesis of sub-50 nm diameter Al nanocrystals with a plasmon-resonant absorption in the UV region of the spectrum. We observe a transition from a UV-resonant response, that is, a colorless solution, to a broadband absorptive response, that is, a completely black solution, as the nanocrystal concentration is increased. The strong absorptive interband transition in Al provides the dominant mechanism responsible for this effect. We developed a robust method to functionalize Al nanocrystals with silica to increase their stability in H₂O from hours to weeks enabling us to observe efficient broadband photothermal heating with these nanoparticles.

Increased Intraband Transitions in Smaller Gold Nanorods Enhance Light Emission

Photoinduced light emission from plasmonic nanoparticles has attracted considerable interest within the scientific community because of its potential applications in sensing, imaging, and nanothermometry. One of the suggested mechanisms for the light emission from plasmonic nanoparticles is the plasmon-enhanced radiative recombination of hot carriers through inter- and intraband transitions. Here, we investigate the nanoparticle size dependence on the photoluminescence through a systematic analysis of gold nanorods with similar aspect ratios. Using single-particle emission and scattering spectroscopy along with correlated scanning electron microscopy and electromagnetic simulations, we calculate the emission quantum yields and Purcell enhancement factors for individual gold nanorods. Our results show strong size-dependent quantum yields in gold nanorods, with higher quantum yields for smaller gold nanorods. Furthermore, by determining the relative contributions to the photoluminescence from inter- and intraband transitions, we deduce that the observed size dependence predominantly originates from the size dependence of intraband transitions. Specifically, within the framework of Fermi's golden rule for radiative recombination of excited charge carriers, we demonstrate that the Purcell factor enhancement alone cannot explain the emission size dependence and that changes in the transition matrix elements must also occur. Those changes are due to electric field confinement enhancing intraband transitions. These results provide vital insight into the intraband relaxation in metallic nanoconfined systems and therefore are of direct importance to the rapidly developing field of plasmonic photocatalysis.

Morphology-Dependent Reactivity of a Plasmonic Photocatalyst

The shape of a plasmonic nanoparticle strongly controls its light-matter interaction, which in turn affects how specific morphologies may be used in applications such as sensing, photodetection, and active pixel displays. Here, we show that particle shape also controls plasmonic photocatalytic activity. Three different Al nanocrystal morphologies, octopods, nanocubes, and nanocrystals, all with very similar plasmon resonance frequencies, were used as photocatalysts for the H₂ dissociation reaction. We observe widely varying reaction rates for the three different morphologies. Octopods show a 10 times higher reaction rate than nanocrystals and a 5 times higher rate than nanocubes, with lower apparent activation energies than either nanocubes or nanocrystals by 45% and 49%, respectively. A theoretical model of hot electron direct transfer from photoexcited Al nanoparticles to H₂ molecules is consistent with this observed morphological dependence. This research strongly suggests that nanoparticle geometry, in addition to plasmon resonance energy, is a critical factor in plasmonic photocatalyst design.

Aluminum Nanocrystals Grow into Distinct Branched Aluminum Nanowire Morphologies

Plasmonic nanowires (NWs) have generated great interest in their applications in nanophotonics and nanotechnology. Here we report the synthesis of Al nanocrystals (NCs) with controlled morphologies that range from nanospheres to branched NW and NW bundles. This is accomplished by catalyzing the pyrolysis of triisobutyl aluminum (TIBA) with Tebbe's reagent, a titanium(III) catalyst with two cyclopentadienyl ligands. The ratio of TIBA to Tebbe's reagent is critical in determining the morphology of the resulting Al NC. The branched Al NWs grow in their ⟨100⟩ directions and are formed by oriented attachment of isotropic Al NCs on their {100} facets. Branched NWs are strongly absorptive from the UV to the mid-IR, with longitudinal dipolar, higher-order, and transverse plasmons, all contributing to their broadband response. This rapid Al NW synthesis enables the expanded use of Al for plasmonic and nanophotonic applications in the ultraviolet, visible, and infrared regions of the spectrum.

Electrically Driven Hot-Carrier Generation and Above-Threshold Light Emission in Plasmonic Tunnel Junctions

Above-threshold light emission from plasmonic tunnel junctions, when emitted photons have energies significantly higher than the energy scale of incident electrons, has attracted much recent interest in nano-optics, while the underlying physics remains elusive. We examine above-threshold light emission in electromigrated tunnel junctions. Our measurements over a large ensemble of devices demonstrate a giant (∼10⁴) material-dependent photon yield (emitted photons per incident electrons). This dramatic effect cannot be explained only by the radiative field enhancement due to localized plasmons in the tunneling gap. Emission is well described by a Boltzmann spectrum with an effective temperature exceeding 2000 K, coupled to a plasmon-modified photonic density of states. The effective temperature is approximately linear in the applied bias, consistent with a suggested theoretical model describing hot-carrier dynamics driven by nonradiative decay of electrically excited localized plasmons. Electrically generated hot carriers and nontraditional light emission could open avenues for active photochemistry, optoelectronics, and quantum optics.

Giant photothermoelectric effect in silicon nanoribbon photodetectors

The photothermoelectric (PTE) effect enables efficient harvesting of the energy of photogenerated hot carriers and is a promising choice for high-efficiency photoelectric energy conversion and photodetection. Recently, the PTE effect was reported in low-dimensional nanomaterials, suggesting the possibility of optimizing their energy conversion efficiency. Unfortunately, the PTE effect becomes extremely inefficient in low-dimensional nanomaterials, owing to intrinsic disadvantages, such as low optical absorption and immature fabrication methods. In this study, a giant PTE effect was observed in lightly doped p-type silicon nanoribbons caused by photogenerated hot carriers. The open-circuit photovoltage responsivity of the device was 3-4 orders of magnitude higher than those of previously reported PTE devices. The measured photovoltage responses fit very well with the proposed photothermoelectric multiphysics models. This research proposes an application of the PTE effect and a possible method for utilizing hot carriers in semiconductors to significantly improve their photoelectric conversion efficiency.

Site-Selective Nanoreactor Deposition on Photocatalytic Al Nanocubes

Photoactivation of catalytic materials through plasmon-coupled energy transfer has created new possibilities for expanding the scope of light-driven heterogeneous catalysis. Here we present a nanoengineered plasmonic photocatalyst consisting of catalytic Pd islands preferentially grown on vertices of Al nanocubes. The regioselective Pd deposition on Al nanocubes does not rely on complex surface ligands, in contrast to site-specific transition-metal deposition on gold nanoparticles. We show that the strong local field enhancement on the sharp nanocube vertices provides a mechanism for efficient coupling of the plasmonic Al antenna to adjacent Pd nanoparticles. A substantial increase in photocatalytic H₂ dissociation on Pd-bound Al nanocubes relative to pristine Al nanocubes can be observed, incentivizing further engineering of heterometallic antenna-reactor photocatalysts. Controlled growth of catalytic materials on plasmonic hot spots can result in more efficient use of the localized surface plasmon energy for photocatalysis, while minimizing the amount and cost of precious transition-metal catalysts.

Duplicating Plasmonic Hotspots by Matched Nanoantenna Pairs for Remote Nanogap Enhanced Spectroscopy

Plasmonic nanoantennas are capable of reversibly interconverting free-space radiation with localized modes at the nanoscale. However, optical access to a single nanoantenna, through a laser beam, is always accompanied by disruptive background perturbations and heating effects. Remote spectroscopy is one promising route to overcome these effects. Here, we demonstrate excitation-collection-separated enhanced spectroscopy using a matched nanoantenna pair. The receiving and transmitting antennas are geometrically separated but bridged by the propagating surface plasmon polaritons (SPPs) on the metal film. The receiving antenna, consisting of a silver nanowire on a mirror, ensures a high light-to-plasmon conversion efficiency. The transmitting antenna consists of a silver nanocube over a mirror and is impedance matched to free space photons and the propagating SPPs. As a proof-of-principle, we demonstrate remote surface-enhanced Raman scattering with a high signal-to-noise ratio. This matched nanoantenna pair may have applications for remote entanglement of quantum emitters, biochemistry detection, or optical interconnects.

Monolithic Metal Dimer-on-Film Structure: New Plasmonic Properties Introduced by the Underlying Metal

Dimers, two closely spaced metallic nanostructures, are one of the primary nanoscale geometries in plasmonics, supporting high local field enhancements in their interparticle junction under excitation of their hybridized "bonding" plasmon. However, when a dimer is fabricated on a metallic substrate, its characteristics are changed profoundly. Here we examine the properties of a Au dimer on a Au substrate. This structure supports a bright "bonding" dimer plasmon, screened by the metal, and a lower energy magnetic charge transfer plasmon. Changing the dielectric environment of the dimer-on-film structure reveals a broad family of higher-order hybrid plasmons in the visible region of the spectrum. Both of the localized surface plasmons resonances (LSPR) of the individual dimer-on-film structures as well as their collective surface lattice resonances (SLR) show a highly sensitive refractive index sensing response. Implementation of such all-metal magnetic-resonant nanostructures offers a promising route to achieve higher-performance LSPR- and SLR-based plasmonic sensors.

Generating Third Harmonic Vacuum Ultraviolet Light with a TiO2 Metasurface

Dielectric metasurfaces have recently been shown to provide an excellent platform for the harmonic generation of light due to their low optical absorption and to the strong electromagnetic field enhancement that can be designed into their constituent meta-atoms. Here, we demonstrate vacuum ultraviolet (VUV) third harmonic generation from a specially designed dielectric metasurface consisting of a titanium dioxide (TiO₂) nanostructure array. The metasurface was designed to enhance the generation of VUV light at a wavelength of 185 nm by tailoring its geometric design parameters to achieve an optical resonance at the fundamental laser wavelength of 555 nm. The metasurface exhibits an enhancement factor of nominally 180 compared to an unpatterned TiO₂ thin film of the same thickness, evidence of strong field enhancement at the fundamental wavelength. Mode analysis reveals that the origin of the enhancement is an anapole resonance near the pump wavelength. This work demonstrates an effective strategy for the compact generation of VUV light that could enable expanded access to this useful region of the electromagnetic spectrum.

Aluminum Nanocubes Have Sharp Corners

Of the many plasmonic nanoparticle geometries that have been synthesized, nanocubes have been of particular interest for creating nanocavities, facilitating plasmon coupling, and enhancing phenomena dependent upon local electromagnetic fields. Here we report the straightforward colloidal synthesis of single-crystalline {100} terminated Al nanocubes by decomposing AlH₃ with Tebbe's reagent in tetrahydrofuran. The size and shape of the Al nanocubes is controlled by the reaction time and the ratio of AlH₃ to Tebbe's reagent, which, together with reaction temperature, establish kinetic control over Al nanocube growth. Al nanocubes possess strong localized field enhancements at their sharp corners and resonances highly amenable to coupling with metallic substrates. Their native oxide surface renders them extremely air stable. Chemically synthesized Al nanocubes provide an earth-abundant alternative to noble metal nanocubes for plasmonics and nanophotonics applications.

Plasmonic Photocatalysis of Nitrous Oxide into N2 and O2 Using Aluminum-Iridium Antenna-Reactor Nanoparticles

Photocatalysis with optically active "plasmonic" nanoparticles is a growing field in heterogeneous catalysis, with the potential for substantially increasing efficiencies and selectivities of chemical reactions. Here, the decomposition of nitrous oxide (N₂O), a potent anthropogenic greenhouse gas, on illuminated aluminum-iridium (Al-Ir) antenna-reactor plasmonic photocatalysts is reported. Under resonant illumination conditions, N₂ and O₂ are the only observable decomposition products, avoiding the problematic generation of NO_x species observed using other approaches. Because no appreciable change to the apparent activation energy was observed under illumination, the primary reaction enhancement mechanism for Al-Ir is likely due to photothermal heating rather than plasmon-induced hot-carrier contributions. This light-based approach can induce autocatalysis for rapid N₂O conversion, a process with highly promising potential for applications in N₂O abatement technologies, satellite propulsion, or emergency life-support systems in space stations and submarines.

Photocatalytic Hydrogenation of Graphene Using Pd Nanocones

Plasmonic photocatalytic processes typically use the interaction of light with metallic nanoparticles to drive chemical reactions on their surfaces. Here we show that a plasmonic photocatalyst can also induce a reaction on an adjacent material. A combination of spontaneous H₂ dissociation and plasmon-induced H desorption from tilted palladium (Pd) nanocones yields reactive H atoms which, in the direct vicinity of a graphene monolayer, results in its local hydrogenation. The conversion of pristine to hydrogenated graphene, a semiconductor, is detectable by visible local fluorescence of the hydrogenated regions of the graphene sheet, as well as by Raman spectroscopic analysis. These results may lead to new approaches for local, light-driven functionalization of graphene and other 2D materials and for precision patterning of functional devices.

Solar thermal desalination as a nonlinear optical process

The ever-increasing global need for potable water requires practical, sustainable approaches for purifying abundant alternative sources such as seawater, high-salinity processed water, or underground reservoirs. Evaporation-based solutions are of particular interest for treating high salinity water, since conventional methods such as reverse osmosis have increasing energy requirements for higher concentrations of dissolved minerals. Demonstration of efficient water evaporation with heat localization in nanoparticle solutions under solar illumination has led to the recent rapid development of sustainable, solar-driven distillation methods. Given the amount of solar energy available per square meter at the Earth's surface, however, it is important to utilize these incident photons as efficiently as possible to maximize clean water output. Here we show that merely focusing incident sunlight into small "hot spots" on a photothermally active desalination membrane dramatically increases--by more than 50%--the flux of distilled water. This large boost in efficiency results from the nearly exponential dependence of water vapor saturation pressure on temperature, and therefore on incident light intensity. Exploiting this inherent but previously unrecognized optical nonlinearity should enable the design of substantially higher-throughput solar thermal desalination methods. This property provides a mechanism capable of enhancing a far wider range of photothermally driven processes with supralinear intensity dependence, such as light-driven chemical reactions and separation methods.

Efficient Second Harmonic Generation in a Hybrid Plasmonic Waveguide by Mode Interactions

Developing highly efficient nanoscale coherent light sources is essential for advances in technological applications such as integrated photonic circuits, bioimaging, and sensing. An on-chip wavelength convertor based on second harmonic generation (SHG) would be a crucial step toward this goal, but the light-conversion efficiency would be low for small device dimensions. Here we demonstrate strongly enhanced SHG with a high conversion efficiency of 4 × 10^-5 W^-1 from a hybrid plasmonic waveguide consisting of a CdSe nanowire coupled with a Au film. The strong spatial overlap of the waveguide mode with the nonlinear material and momentum conservation between the incident and reflected modes are the key factors resulting in such high efficiency. The SHG emission angles vary linearly with excitation wavelength, indicating a nonlinear steering of coherent light emission at the subwavelength scale. Our work is promising for the realization of efficient and tunable nonlinear coherent sources and opens new approaches for efficient integrated nonlinear nanophotonic devices.

Ultrafast Electron Dynamics in Single Aluminum Nanostructures

Aluminum nanostructures are a promising alternative material to noble metal nanostructures for several photonic and catalytic applications, but their ultrafast electron dynamics remain elusive. Here, we combine single-particle transient extinction spectroscopy and parameter-free first-principles calculations to investigate the non-equilibrium carrier dynamics in aluminum nanostructures. Unlike gold nanostructures, we find the sub-picosecond optical response of lithographically fabricated aluminum nanodisks to be more sensitive to the lattice temperature than the electron temperature. We assign the rise in the transient transmission to electron-phonon coupling with a pump-power-independent lifetime of 500 ± 100 fs and theoretically confirm this strong electron-phonon coupling behavior. We also measure electron-phonon lifetimes in chemically synthesized aluminum nanocrystals and find them to be even longer (1.0 ± 0.1 ps) than for the nanodisks. We also observe a rise and decay in the transient transmissions with amplitudes that scale with the surface-to-volume ratio of the aluminum nanodisks, implying a possible hot carrier trapping and detrapping at the native oxide shell-metal core interface.

Response to Comment on “Quantifying hot carrier and thermal contributions in plasmonic photocatalysis”

Sivan et al. claim that the methods used to distinguish thermal from hot carrier effects in our recent report are inaccurate and that our data can be explained by a purely thermal mechanism with a fixed activation energy. This conclusion is invalid, because they substantially misinterpret the emissivity of the photocatalyst and assume a linear intensity–dependent temperature in their model that is unrealistic.

Hydrated Electron Generation by Excitation of Copper Localized Surface Plasmon Resonance

Hydrated electrons are important in radiation chemistry and charge-transfer reactions, with applications that include chemical damage of DNA, catalysis, and signaling. Conventionally, hydrated electrons are produced by pulsed radiolysis, sonolysis, two-ultraviolet-photon laser excitation of liquid water, or photodetachment of suitable electron donors. Here we report a method for the generation of hydrated electrons via single-visible-photon excitation of localized surface plasmon resonances (LSPRs) of supported sub-3 nm copper nanoparticles in contact with water. Only excitations at the LSPR maximum resulted in the formation of hydrated electrons, suggesting that plasmon excitation plays a crucial role in promoting electron transfer from the nanoparticle into the solution. The reactivity of the hydrated electrons was confirmed via proton reduction and concomitant H₂ evolution in the presence of a Ru/TiO₂ catalyst.

Polydopamine-Stabilized Aluminum Nanocrystals: Aqueous Stability and Benzo[a]pyrene Detection

Aluminum nanocrystals have emerged as an earth-abundant material for plasmonics applications. Al nanocrystals readily oxidize in aqueous-based solutions, however, transforming into highly stratified γ-AlOOH nanoparticles with a 700% increase in surface area in a matter of minutes. Here we show that by functionalizing Al nanocrystals with the bioinspired polymer polydopamine, their stability in aqueous media is dramatically increased, maintaining their integrity in aqueous solution for over 2 weeks with no discernible structural changes. Polydopamine functionalization also provides a molecular capture layer that enables the capture of polycyclic aromatic hydrocarbon pollutants in H₂O samples and their detection by surface-enhanced Raman spectroscopy, when polydopamine-stabilized Al nanocrystal aggregates are used as substrates. This approach was used to detect a prime carcinogenic H₂O pollutant, benzo[a]pyrene with a sensitivity in the sub part-per-billion range.

Anti-Stokes Emission from Hot Carriers in Gold Nanorods

The origin of light emission from plasmonic nanoparticles has been strongly debated lately. It is present as the background of surface-enhanced Raman scattering and, despite the low yield, has been used for novel sensing and imaging applications because of its photostability. Although the role of surface plasmons as an enhancing antenna is widely accepted, the main controversy regarding the mechanism of the emission is its assignment to either radiative recombination of hot carriers (photoluminescence) or electronic Raman scattering (inelastic light scattering). We have previously interpreted the Stokes-shifted emission from gold nanorods as the Purcell effect enhanced radiative recombination of hot carriers. Here we specifically focused on the anti-Stokes emission from single gold nanorods of varying aspect ratios with excitation wavelengths below and above the interband transition threshold while still employing continuous wave lasers. Analysis of the intensity ratios between Stokes and anti-Stokes emission yields temperatures that can only be interpreted as originating from the excited electron distribution and not a thermally equilibrated phonon population despite not using pulsed laser excitation. Consistent with this result as well as previous emission studies using ultrafast lasers, the power-dependence of the upconverted emission is nonlinear and gives the average number of participating photons as a function of emission wavelength. Our findings thus show that hot carriers and photoluminescence play a major role in the upconverted emission.

Metal-organic frameworks tailor the properties of aluminum nanocrystals

Metal-organic frameworks (MOFs) and metal nanoparticles are two classes of materials that have received considerable recent attention, each for controlling chemical reactivities, albeit in very different ways. Here, we report the growth of MOF shell layers surrounding aluminum nanocrystals (Al NCs), an Earth-abundant metal with energetic, plasmonic, and photocatalytic properties. The MOF shell growth proceeds by means of dissolution-and-growth chemistry that uses the intrinsic surface oxide of the NC to obtain the Al3+ ions accommodated into the MOF nodes. Changes in the Al NC plasmon resonance provide an intrinsic optical probe of its dissolution and growth kinetics. This same chemistry enables a highly controlled oxidation of the Al NCs, providing a precise method for reducing NC size in a shape-preserving manner. The MOF shell encapsulation of the Al NCs results in increased efficiencies for plasmon-enhanced photocatalysis, which is observed for the hydrogen-deuterium exchange and reverse water-gas shift reactions.

Ligand-Dependent Colloidal Stability Controls the Growth of Aluminum Nanocrystals

The precise size- and shape-controlled synthesis of monodisperse Al nanocrystals remains an open challenge, limiting their utility for numerous applications that would take advantage of their size and shape-dependent optical properties. Here we pursue a molecular-level understanding of the formation of Al nanocrystals by titanium(IV) isopropoxide-catalyzed decomposition of AlH₃ in Lewis base solvents. As determined by electron paramagnetic resonance spectroscopy of intermediates, the reaction begins with the formation of Ti³⁺-AlH₃ complexes. Proton nuclear magnetic resonance spectroscopy indicates isopropoxy ligands are removed from Ti by Al, producing aluminum(III) isopropoxide and low-valent Ti³⁺ catalysts. These Ti³⁺ species catalyze elimination of H₂ from AlH₃ inducing the polymerization of AlH₃ into colloidally unstable low-valent aluminum hydride clusters. These clusters coalesce and grow while expelling H₂ to form colloidally stable Al nanocrystals. The colloidal stability of the Al nanocrystals and their size is determined by the molecular structure and density of coordinating atoms in the reaction, which is controlled by choice of solvent composition.