Energy-resolved plasmonic chemistry in individual nanoreactors

Local electric field in nanocavities dictates the vibrational relaxation dynamics of interfacial molecules

Plasmonic nanocavities enable the generation of strong light-matter coupling and exhibit great potential in plasmon-mediated chemical reactions (PMCRs). Although an electric field generated by nanocavities (E n) has recently been reported, its effect on the vibrational energy relaxation (VER) of the molecules in the nanocavities has not been explored. In this study, we reveal the impact of an electric field sensed by molecules (para-substituted thiophenol derivatives) in a nanocavity (E f) on VER processes by employing advanced time-resolved femtosecond sum frequency generation vibrational spectroscopy (SFG-VS) supplemented by electrochemical measurements. The magnitude of E n is almost identical (1.0 ± 0.2 V nm-1) beyond the experimental deviation while E f varies from 0.3 V nm-1 to 1.7 V nm-1 depending on the substituent. An exponential correlation between E f and the complete recovery time of the ground vibrational C[double bond, length as m-dash]C state (T 2) of the phenyl ring is observed. Substances with a smaller T 2 are strongly correlated with the reported macroscopic chemical reactivity. This finding may aid in enriching the current understanding of PMCRs and highlights the possibility of regulating vibrational energy flow into desired reaction coordinates by using a local electric field.

Versatile Nanoring Fabrication Assisted by Hole-mask Colloidal Lithography

Nanomaterials shaped as rings are interesting nanostructures with control of the materials properties at the nanoscale. Nanoring plasmonic resonators provide tunable optical resonances in the near-infrared with application in sensing. Fabrication of nanorings can be carried out via top-down approaches based on electron beam lithography with high control of the ring size parameters but at high cost. Alternatively, fabrication via self-assembly approaches has a higher speed/lower cost but at the cost of control of ring parameters. Current colloidal lithography approaches can provide nanoring fabrication over large areas but only of specific materials and a select set of rings (large ring diameters or small rings with ultrathin walls). We extend Hole-mask Colloidal Lithography to use ring shaped holes, allow the deposition of arbitrary materials, and allow the independent tuning of ring-wall thickness over a large range of values. We present a generic approach for the fabrication of nanorings formed from a range of materials including low cost (e.g., Cu, Al) and nonplasmonic (e.g., W) materials and with control of ring wall thickness and diameter allowing tuning of ring parameters and materials for applications in nanooptics and beyond.

Identifying high-order plasmon modes in silver nanoparticle-over-mirror configuration

Metallic nanoparticle-over-mirror (NPOM) represents as a versatile plasmonic configuration for surface enhanced spectroscopy, sensing and light-emitting metasurfaces. However, experimentally identifying the high-order localized surface plasmon modes in NPOM, especially for the best plasmonic material silver, is often hindered by the small scattering cross-section of high-order plasmon modes and the poor reproducibility of the spectra across different NPOMs, resulted from the polyhedral morphology of the colloidal nanoparticles or the rough surface of deposited polycrystalline metals. In this study, we identify the high-order localized surface plasmon modes in silver NPOM by using differential reflection spectroscopy. We achieved reproducible single-particle absorption spectra by constructing uniform NPOM consisting of silver nanospheres, single-crystallized silver microplates, and a self-assembled monolayer of 1,10-decanedithiol. For comparison, silver NPOM created from typical polycrystalline films exhibits significant spectral fluctuations, even when employing template stripping methods to minimize the film roughness. Identifying high-order plasmon modes in the NPOM configuration offers a pathway to construct high-quality plasmonic substrates for applications such as colloidal metasurface, surface-enhanced Raman spectroscopy, fluorescence, or infrared absorption.

Synergetic surface enhancement of quantum dots-based electrochemiluminescence with photonic crystal light scattering and metal surface plasmon resonance for sensitive bioanalysis

Noble metal nanostructures and photonic crystals (PhCs) have been widely investigated as substrates for constructing surface enhanced electrochemiluminescence (SE-ECL) biosensors. However, their applications are hindered by the limited enhancement intensity of surface plasmon resonance (SPR) and an incomplete mechanism for the photonic enhancement effect. Hence, developing a novel SE-ECL strategy with better signal enhanced capability and enriching our understanding of the intrinsic mechanisms for efficient bioanalysis is extremely urgent. Here, a synergistic SE-ECL strategy was developed for the sensitive determination of prostate specific antigen (PSA) protein. The randomly arranged polystyrene (r-PS) spheres and PS PhC arrays were applied to enhance the ECL emission of cadmium sulfide quantum dots (CdS QDs) and the results suggested that the PhC arrays displayed superior intensity (0.22) than the r-PS interface (0.10). Au nanoparticles (NPs) were introduced onto the two kinds of surfaces and further boosted the ECL intensity. According to the ECL measurements, Au NPs modified at the r-PS surface exhibited only a slight increase (0.13), while the PhC arrays showed approximately 5-fold enhancement (0.92), benefiting from the synergistic enhancement. The finite-difference time-domain (FDTD) simulation indicated that the ECL enhancement was ascribed to the coupled electromagnetic (EM) field at the surfaces of PS PhCs and Au NPs. The SE-ECL could achieve a detection range from 1 pg/mL to 1 μg/mL with a detection limit of 0.41 pg/mL (S/N = 3). This study provides the first combination of PhC arrays and metal surface plasmon nanostructure for the synergetic enhancement of SE-ECL systems. It opens a new avenue for the rational design of advanced ECL biosensors and shows great perspective for clinical diagnosis.

Exploiting hot electrons from a plasmon nanohybrid system for the photoelectroreduction of CO2

Plasmonic materials convert light into hot carriers and heat to mediate catalytic transformation. The participation of hot carriers (photocatalysis) remains a subject of vigorous debate, often argued on the basis that carriers have ultrashort lifetime incompatible with drive photochemical processes. This study utilises plasmon hot electrons directly in the photoelectrocatalytic reduction of CO2 to CO via a Ppasmonic nanohybrid. Through the deliberate construction of a plasmonic nanohybrid system comprising NiO/Au/ReI(phen-NH2)(CO)3Cl (phen-NH2 = 1,10-Phenanthrolin-5-amine) that is unstable above 580 K; it was possible to demonstrate hot electrons are the main culprit in CO2 reduction. The engagement of hot electrons in the catalytic process is derived from many approaches that cover the processes in real-time, from ultrafast charge generation and separation to catalysis occurring on the minute scale. Unbiased in situ FTIR spectroscopy confirmed the stepwise reduction of the catalytic system. This, coupled with the low thermal stability of the ReI(phen-NH2)(CO)3Cl complex, explicitly establishes plasmonic hot carriers as the primary contributors to the process. Therefore, mediating catalytic reactions by plasmon hot carriers is feasible and holds promise for further exploration. Plasmonic nanohybrid systems can leverage plasmon's unique photophysics and capabilities because they expedite the carrier's lifetime.

Construction of nanoparticle-on-mirror nanocavities and their applications in plasmon-enhanced spectroscopy

Plasmonic nanocavities exhibit exceptional capabilities in visualizing the internal structure of a single molecule at sub-nanometer resolution. Among these, an easily manufacturable nanoparticle-on-mirror (NPoM) nanocavity is a successful and powerful platform for demonstrating various optical phenomena. Exciting advances in surface-enhanced spectroscopy using NPoM nanocavities have been developed and explored, including enhanced Raman, fluorescence, phosphorescence, upconversion, etc. This perspective emphasizes the construction of NPoM nanocavities and their applications in achieving higher enhancement capabilities or spatial resolution in dark-field scattering spectroscopy and plasmon-enhanced spectroscopy. We describe a systematic framework that elucidates how to meet the requirements for studying light-matter interactions through the creation of well-designed NPoM nanocavities. Additionally, it provides an outlook on the challenges, future development directions, and practical applications in the field of plasmon-enhanced spectroscopy.

Key Factors for Controlling Plasmon-Induced Chemical Reactions on Metal Surfaces

Plasmon-induced chemical reactions based on direct interactions between the plasmons of metal nanostructures and molecules have attracted increasing attention as a means of efficiently utilizing sunlight. In recent years, achievements in complex synthetic reactions as well as simple dissociation reactions of gaseous molecules using plasmons have been reported. However, recent research progress has revealed that multiple factors govern plasmon-induced chemical reactions. This perspective provides an overview of the key factors that influence plasmon-induced chemical reactions on metal surfaces and discusses the difficulty of controlling the reactions, which is caused by the entanglement of the key factors. A strategy for designing plasmonic metal catalysts to achieve the desired reactions is also discussed based on the current understanding, and directions for further research are provided.

Spatial Distributions of Single-Molecule Reactivity in Plasmonic Catalysis

Plasmonic catalysts have the potential to accelerate and control chemical reactions with light by exploiting localized surface plasmon resonances. However, the mechanisms governing plasmonic catalysis are not simple to decouple. Several plasmon-derived phenomena, such as electromagnetic field enhancements, temperature, or the generation of charge carriers, can affect the reactivity of the system. These effects are convoluted with the inherent (nonplasmonic) catalytic properties of the metal surface. Disentangling these coexisting effects is challenging but is the key to rationally controlling reaction pathways and enhancing reaction rates. This study utilizes super-resolution fluorescence microscopy to examine the mechanisms of plasmonic catalysis at the single-particle level. The reduction reaction of resazurin to resorufin in the presence of Au nanorods coated with a porous silica shell is investigated in situ. This allows the determination of reaction rates with a single-molecule sensitivity and subparticle resolution. By variation of the irradiation wavelength, it is possible to examine two different regimes: photoexcitation of the reactant molecules and photoexcitation of the nanoparticle's plasmon resonance. In addition, the measured spatial distribution of reactivity allows differentiation between superficial and far-field effects. Our results indicate that the reduction of resazurin can occur through more than one reaction pathway, being most efficient when the reactant is photoexcited and is in contact with the Au surface. In addition, it was found that the spatial distribution of enhancements varies, depending on the underlying mechanism. These findings contribute to the fundamental understanding of plasmonic catalysis and the rational design of future plasmonic nanocatalysts.

Hybrid cavity-antenna architecture for strong and tunable sideband-selective molecular Raman scattering enhancement

Plasmon resonances at the surface of metallic antennas allow for extreme enhancement of Raman scattering. Intrinsic to plasmonics, however, is that extreme field confinement lacks precise spectral control, which would hold great promise in shaping the optomechanical interaction between light and molecular vibrations. We demonstrate an experimental platform composed of a plasmonic nanocube-on-mirror antenna coupled to an open, tunable Fabry-Perot microcavity for selective addressing of individual vibrational lines of molecules with strong Raman scattering enhancement. Multiple narrow and intense optical resonances arising from the hybridization of the cavity modes and the plasmonic broad resonance are used to simultaneously enhance the laser pump and the local density of optical states, and are characterized using rigorous modal analysis. The versatile bottom-up fabrication approach permits quantitative comparison with the bare nanocube-on-mirror system, both theoretically and experimentally. This shows that the hybrid system allows for similar SERS enhancement ratios with narrow optical modes, paving the way for dynamical backaction effects in molecular optomechanics.

Plasmon Energy Transfer Driven by Electrochemical Tuning of Methylene Blue on Single Gold Nanorods

Plasmonic photocatalysis has attracted interest for its potential to generate energy-efficient reactions, but ultrafast internal conversion limits efficient plasmon-based chemistry. Resonance energy transfer (RET) to surface adsorbates offers a way to outcompete internal conversion pathways and also eliminate the need for sacrificial counter-reactions. Herein, we demonstrate RET between methylene blue (MB) and gold nanorods (AuNRs) using in situ single-particle spectroelectrochemistry. During electrochemically driven reversible redox reactions between MB and leucomethylene blue (LMB), we show that the homogeneous line width is broadened when spectral overlap between AuNR scattering and absorption of MB is maximized, indicating RET. Additionally, electrochemical oxidative oligomerization of MB allowed additional dipole coupling to generate RET at lower energies. Time-dependent density functional theory-based simulated absorption provided theoretical insight into the optical properties, as MB molecules were electrochemically oligomerized. Our findings show a mechanism for driving efficient plasmon-assisted processes by RET through the change in the chemical states of surface adsorbates.

Honeycomb-like Ag Nanocavity Array for SERS Observations Using Plasmon-Mediated Chemical Reactions

Organized two-dimensional polystyrene bead arrays perform ion etching, and protruding nanostructures are created on polystyrene beads due to the shadow effects from the ring beads, leading to nucleus selection and growth in Au nanostructure deposition. Ag nanostructures are prepared via plasmon-mediated chemical reactions (PMCRs), leading to the Ag nanocavity geometry of the honeycomb pattern when the etching time and Ag growth time are tuned. Due to the strong electromagnetic coupling, the Ag honeycomb-shaped nanocavity array works as the SERS substrate with high sensitivity and good repeatability, which is used to detect thiram pesticide residues with a concentration down to 10-9 M.

Nanoparticle Imprint Lithography: From Nanoscale Metrology to Printable Metallic Grids

Large scale and low-cost nanopatterning of materials is of tremendous interest for optoelectronic devices. Nanoimprint lithography has emerged in recent years as a nanofabrication strategy that is high-throughput and has a resolution comparable to that of electron-beam lithography (EBL). It is enabled by pattern replication of an EBL master into polydimethylsiloxane (PDMS), that is then used to pattern a resist for further processing, or a sol-gel that could be calcinated into a solid material. Although the sol-gel chemistry offers a wide spectrum of material compositions, metals are still difficult to achieve. This gap could be bridged by using colloidal nanoparticles as resist, but deep understanding of the key parameters is still lacking. Here, we use supported metallic nanocubes as a model resist to gain fundamental insights into nanoparticle imprinting. We uncover the major role played by the surfactant layer trapped between nanocubes and substrate, and measure its thickness with subnanometer resolution by using gap plasmon spectroscopy as a metrology platform. This enables us to quantify the van der Waals (VDW) interactions responsible for the friction opposing the nanocube motion, and we find that these are almost in quantitative agreement with the Stokes drag acting on the nanocubes during nanoimprint, that is estimated with a simplified fluid mechanics model. These results reveal that a minimum thickness of surfactant is required, acting as a spacer layer mitigating van der Waals forces between nanocubes and the substrate. In the light of these findings we propose a general method for resist preparation to achieve optimal nanoparticle mobility and show the assembly of printable Ag and Au nanocube grids, that could enable the fabrication of low-cost transparent electrodes of high material quality upon nanocube epitaxy.

Surface plasmon-enhanced photo-driven CO2 hydrogenation by hydroxy-terminated nickel nitride nanosheets

The majority of visible light-active plasmonic catalysts are often limited to Au, Ag, Cu, Al, etc., which have considerations in terms of costs, accessibility, and instability. Here, we show hydroxy-terminated nickel nitride (Ni₃N) nanosheets as an alternative to these metals. The Ni₃N nanosheets catalyze CO₂ hydrogenation with a high CO production rate (1212 mmol g^-1 h^-1) and selectivity (99%) using visible light. Reaction rate shows super-linear power law dependence on the light intensity, while quantum efficiencies increase with an increase in light intensity and reaction temperature. The transient absorption experiments reveal that the hydroxyl groups increase the number of hot electrons available for photocatalysis. The in situ diffuse reflectance infrared Fourier transform spectroscopy shows that the CO₂ hydrogenation proceeds via the direct dissociation pathway. The excellent photocatalytic performance of these Ni₃N nanosheets (without co-catalysts or sacrificial agents) is suggestive of the use of metal nitrides instead of conventional plasmonic metal nanoparticles.

Observation of Charge Separation Enhancement in Plasmonic Photocatalysts under Coupling Conditions

Surface plasmon resonance-induced charge separation plays key roles in plasmon-related applications, especially in photocatalysis and photovoltaics. Plasmon coupling nanostructures exhibit extraordinary behaviors in hybrid states, phonon scattering, and ultrafast plasmon dephasing, but plasmon-induced charge separation in these materials remains unknown. Here, we design Schottky-free Au nanoparticle (NP)/NiO/Au nanoparticles-on-a-mirror plasmonic photocatalysts to support plasmon-induced interfacial hole transfer, evidenced by surface photovoltage microscopy at the single-particle level. In particular, we observe a nonlinear increase in charge density and photocatalytic performance with an increase in excitation intensity in plasmonic photocatalysts containing hot spots as a result of varying the geometry. Such charge separation increased the internal quantum efficiency by 14 times at 600 nm in catalytic reactions as compared to that of the Au NP/NiO without a coupling effect. These observations provide an improved understanding of charge transfer management and utilization by geometric engineering and interface electronic structure for plasmonic photocatalysis.

Interface-Dependent Selectivity in Plasmon-Driven Chemical Reactions

Plasmonic nanoparticles can drive chemical reactions powered by sunlight. These processes involve the excitation of surface plasmon resonances (SPR) and the subsequent charge transfer to adsorbed molecular orbitals. Nonetheless, controlling the flow of energy and charge from SPR to adsorbed molecules is still difficult to predict or tune. Here, we show the crucial role of halide ions in modifying the energy landscape of a plasmon-driven chemical reaction by carefully engineering the nanoparticle-molecule interface. By doing so, the selectivity of plasmon-driven chemical reactions can be controlled, either enhancing or inhibiting the metal-molecule charge and energy transfer or by regulating the vibrational pumping rate. These results provide an elegant method for controlling the energy flow from plasmonic nanoparticles to adsorbed molecules, in situ, and selectively targeting chemical bonds by changing the chemical nature of the metal-molecule interface.

Accurate Transfer of Individual Nanoparticles onto Single Photonic Nanostructures

Controlled integration of metallic nanoparticles (NPs) onto photonic nanostructures enables the realization of complex devices for extreme light confinement and enhanced light-matter interaction. For instance, such NPs could be massively integrated on metal plates to build nanoparticle-on-mirror (NPoM) nanocavities or photonic integrated waveguides (WGs) to build WG-driven nanoantennas. However, metallic NPs are usually deposited via drop-casting, which prevents their accurate positioning. Here, we present a methodology for precise transfer and positioning of individual NPs onto different photonic nanostructures. Our method is based on soft lithography printing that employs elastomeric stamp-assisted transfer of individual NPs onto a single nanostructure. It can also parallel imprint many individual NPs with high throughput and accuracy in a single step. Raman spectroscopy confirms enhanced light-matter interactions in the resulting NPoM-based nanophotonic devices. Our method mixes top-down and bottom-up nanofabrication techniques and shows the potential of building complex photonic nanodevices for multiple applications ranging from enhanced sensing and spectroscopy to signal processing.

Deep Learning Image Analysis of Nanoplasmonic Sensors: Toward Medical Breath Monitoring

Sensing biomarkers in exhaled breath offers a potentially portable, cost-effective, and noninvasive strategy for disease diagnosis screening and monitoring, while high sensitivity, wide sensing range, and target specificity are critical challenges. We demonstrate a deep learning-assisted plasmonic sensing platform that can detect and quantify gas-phase biomarkers in breath-related backgrounds of varying complexity. The sensing interface consisted of Au/SiO₂ nanopillars covered with a 15 nm metal-organic framework. A small camera was utilized to capture the plasmonic sensing responses as images, which were subjected to deep learning signal processing. The approach has been demonstrated at a classification accuracy of 95 to 98% for the diabetic ketosis marker acetone within a concentration range of 0.5-80 μmol/mol. The reported work provides a thorough exploration of single-sensor capabilities and sets the basis for more advanced utilization of artificial intelligence in sensing applications.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Nanocube Epitaxy for the Realization of Printable Monocrystalline Nanophotonic Surfaces

Plasmonic nanoparticles of the highest quality can be obtained via colloidal synthesis at low-cost. Despite the strong potential for integration in nanophotonic devices, the geometry of colloidal plasmonic nanoparticles is mostly limited to that of platonic solids. This is in stark contrast to nanostructures obtained by top-down methods that offer unlimited capability for plasmon resonance engineering, but present poor material quality and have doubtful perspectives for scalability. Here, an approach that combines the best of the two worlds by transforming assemblies of single-crystal gold nanocube building blocks into continuous monocrystalline plasmonic nanostructures with an arbitrary shape, via epitaxy in solution at near ambient temperature, is introduced. Nanocube dimers are used as a nanoreactor model system to investigate the mechanism in operando, revealing competitive redox processes of oxidative etching at the nanocube corners and simultaneous heterogeneous nucleation at their surface, that ensure filling of the sub-nanometer gap in a self-limited manner. Applying this procedure to nanocube arrays assembled in a patterned poly(dimethylsiloxane) (PDMS) substrate, it is able to obtain printable monocrystalline nanoantenna arrays that can be swiftly integrated in devices. This may lead to the implementation of low-cost nanophotonic surfaces of the highest quality in industrial products.

Correlating structure, morphology and properties of metal nanostructures by combining single-particle optical spectroscopy and electron microscopy

The nanoscale morphology of metal nanostructures directly defines their optical, catalytic and electronic properties and even small morphological changes can cause significant property variations. On the one hand, this dependence allows for precisely tuning and exploring properties by shape engineering; next to advanced synthesis protocols, post-synthesis modification through tailored laser modification has become an emerging tool to do so. On the other hand, with this interconnection also comes the quest for detailed structure-property correlation and understanding of laser-induced reshaping processes on the individual nanostructure level beyond ensemble averages. With the development of single-particle (ultrafast) optical spectroscopy techniques and advanced electron microscopy such understanding can in principle be gained at the femtosecond temporal and atomic spatial scale, respectively. However, accessing both on the same individual nanostructure is far from straightforward as it requires the combination of optical spectroscopy and electron microscopy. In this Minireview, we highlight key studies from recent years that performed such correlative measurements on the same individual metal nanostructure either in a consecutive ex situ manner or in situ inside the electron microscope. We demonstrate that such a detailed correlation is critical for revealing the full picture of the structure-property relationship and the physics behind light-induced nanostructure modifications. We put emphasis on the advantages and disadvantages of each methodology as well as on the unique information that one can gain only by correlative studies performed on the same individual nanostructure and end with an outlook on possible further development of this field in the near future.