Plasmon-induced hot carriers in metallic nanoparticles

Direct observation of ultrafast plasmonic hot electron transfer in the strong coupling regime

Achieving strong coupling between plasmonic oscillators can significantly modulate their intrinsic optical properties. Here, we report the direct observation of ultrafast plasmonic hot electron transfer from an Au grating array to an MoS₂ monolayer in the strong coupling regime between localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs). By means of femtosecond pump-probe spectroscopy, the measured hot electron transfer time is approximately 40 fs with a maximum external quantum yield of 1.65%. Our results suggest that strong coupling between LSPs and SPPs has synergetic effects on the generation of plasmonic hot carriers, where SPPs with a unique nonradiative feature can act as an 'energy recycle bin' to reuse the radiative energy of LSPs and contribute to hot carrier generation. Coherent energy exchange between plasmonic modes in the strong coupling regime can further enhance the vertical electric field and promote the transfer of hot electrons between the Au grating and the MoS₂ monolayer. Our proposed plasmonic strong coupling configuration overcomes the challenge associated with utilizing hot carriers and is instructive in terms of improving the performance of plasmonic opto-electronic devices.

Differential photothermal and photodynamic performance behaviors of gold nanorods, nanoshells and nanocages under identical energy conditions

The corner angle structure of Au nanostructures could more efficiently convert the photon energy into the photodynamic performance.

Spiers Memorial Lecture : Introductory lecture: Hot-electron science and microscopic processes in plasmonics and catalysis

In these introductory remarks we discuss the generation of nonequilibrium electrons in metals, their properties, and how they can be utilized in two emerging applications: for extending the capabilities of photodetection (left), and for photocatalysis (right), lowering the barriers of chemical reactions.

Plasmonic photocatalysis applied to solar fuels

We show the impact of structural, chemical and interfacial features of gold–titania composites on solar and visible photocatalytic gas phase reduction of CO₂ and the specificities of the hot electron-based process.

"Hot" electrons in metallic nanostructures-non-thermal carriers or heating?

Understanding the interplay between illumination and the electron distribution in metallic nanostructures is a crucial step towards developing applications such as plasmonic photocatalysis for green fuels, nanoscale photodetection and more. Elucidating this interplay is challenging, as it requires taking into account all channels of energy flow in the electronic system. Here, we develop such a theory, which is based on a coupled Boltzmann-heat equations and requires only energy conservation and basic thermodynamics, where the electron distribution, and the electron and phonon (lattice) temperatures are determined uniquely. Applying this theory to realistic illuminated nanoparticle systems, we find that the electron and phonon temperatures are similar, thus justifying the (classical) single-temperature models. We show that while the fraction of high-energy "hot" carriers compared to thermalized carriers grows substantially with illumination intensity, it remains extremely small (on the order of 10^-8). Importantly, most of the absorbed illumination power goes into heating rather than generating hot carriers, thus rendering plasmonic hot carrier generation extremely inefficient. Our formulation allows for the first time a unique quantitative comparison of theory and measurements of steady-state electron distributions in metallic nanostructures.

3D characterization of heat-induced morphological changes of Au nanostars by fast in situ electron tomography

A thorough understanding of the thermal stability and potential reshaping of anisotropic gold nanostars is required for various potential applications. Combination of a tomographic heating holder with fast tilt series acquisition has been used to monitor temperature-induced morphological changes of Au nanostars. The outcome of our 3D investigations can be used as an input for boundary element method simulations, enabling us to investigate the influence of reshaping on the nanostars' plasmonic properties. Our work leads to a better understanding of the mechanism behind thermal reshaping. In addition, the approach presented here is generic and can hence be applied to a wide variety of nanoparticles made of different materials and with arbitrary morphology.

Optical properties of pyridine adsorbed polycyclic aromatic hydrocarbons using quantum chemical calculations

Polycyclic aromatic hydrocarbons (PAHs), the molecular version of graphene, having edges saturated with hydrogen atoms, have recently emerged as a novel nanoplasmonic material. In this work, we investigate the optical properties of pristine and pyridine adsorbed circular and triangular PAHs. We base our calculations on computationally efficient first-principles time-dependent density-functional theory (TD-DFT) calculations. We find substantial changes in the optical absorption spectra induced by the presence of the pyridine molecule. In addition, with the help of electron difference density (EDD) maps, we demonstrate a strong optical interaction between PAHs and pyridine molecules. The main effect of pyridine adsorption is to split the plasmon band and to redistribute the optical absorption in a wider energy range. We believe that our findings can help in the design of novel plasmonic devices having PAHs as basic building blocks.

Effect of Silica Supports on Plasmonic Heating of Molecular Adsorbates as Measured by Ultrafast Surface-Enhanced Raman Thermometry

Plasmonic materials show great potential for selective photocatalysis under relatively mild reaction conditions. However, the catalytic activity of these plasmonic catalysts can also depend upon the support material that stabilizes the catalysts, where the composition of the catalytic support may change the overall photocatalytic efficiency and yield. It is unknown how changes in the support material may change the plasmon-driven photocatalysis, which may be initiated by plasmon-derived hot carriers, localized heating, or enhanced electromagnetic fields. Herein, we probe the effects of catalytic supports on heating in plasmon-driven catalysis by examining various gold nanoparticle oxide systems. We use ultrafast surface-enhanced Raman thermometry to measure the effective temperature, equivalent to the vibrational kinetic energy, of reporter molecules located between plasmonic gold nanostructures and local environments ranging from ligands to mesoporous silica shells to silica shells. Upon photoexcitation, the transient effective temperature, equivalent to the energy deposited into a vibrational mode, of adsorbed molecules on the silica-coated samples increases, and the energy quickly dissipates within 3 ps. However, the baseline effective temperature that arises from the surface-enhanced Raman spectroscopy probing process depends upon the encapsulant, where the energy deposition differs by 200-300 K between the ligand-coated (citrate or CTAB) and the silica-coated samples. Adsorbates surrounded by a silica shell experience significantly higher effective temperatures than the adsorbates surrounded by ligands or solvent, likely because of the differing effective heat capacities of these media. Taken together, this work shows that a silica support impacts the localized heating of molecular adsorbates on the gold surface and may play a role in enhanced plasmonic photocatalysis because of increased thermal contributions.

Plasmon-Driven Photocatalysis Leads to Products Known from E-beam and X-ray-Induced Surface Chemistry

Plasmonic metal nanostructures can concentrate incident optical fields in nanometer-sized volumes, called hot spots. This leads to enhanced optical responses of molecules in such a hot spot but also to chemical transformations, driven by plasmon-induced hot carriers. Here, we employ tip-enhanced Raman spectroscopy (TERS) to study the mechanism of these reactions in situ at the level of a single hot spot. Direct spectroscopic measurements reveal the energy distribution of hot electrons, as well as the temperature changes due to plasmonic heating. Therefore, charge-driven reactions can be distinguished from thermal reaction pathways. The products of the hot-carrier-driven reactions are strikingly similar to the ones known from X-ray or e-beam-induced surface chemistry despite the >100-fold energy difference between visible and X-ray photons. Understanding the analogies between those two scenarios implies new strategies for rational design of plasmonic photocatalytic reactions and for the elimination of photoinduced damage in plasmon-enhanced spectroscopy.

Controlling Reaction Selectivity over Hybrid Plasmonic Nanocatalysts

The localized surface plasmon resonance (LSPR) excitation in plasmonic nanoparticles has been used to accelerate several catalytic transformations under visible-light irradiation. In order to fully harness the potential of plasmonic catalysis, multimetallic nanoparticles containing a plasmonic and a catalytic component, where LSPR-excited energetic charge carriers and the intrinsic catalytic active sites work synergistically, have raised increased attention. Despite several exciting studies observing rate enhancements, controlling reaction selectivity remains very challenging. Here, by employing multimetallic nanoparticles combining Au, Ag, and Pt in an Au@Ag@Pt core-shell and an Au@AgPt nanorattle architectures, we demonstrate that reaction selectivity of a sequential reaction can be controlled under visible light illumination. The control of the reaction selectivity in plasmonic catalysis was demonstrated for the hydrogenation of phenylacetylene as a model transformation. We have found that the localized interaction between the triple bond in phenylacetylene and the Pt nanoparticle surface enables selective hydrogenation of the triple bond (relative to the double bond in styrene) under visible light illumination. Atomistic calculations show that the enhanced selectivity toward the partial hydrogenation product is driven by distinct adsorption configurations and charge delocalization of the reactant and the reaction intermediate at the catalyst surface. We believe these results will contribute to the use of plasmonic catalysis to drive and control a wealth of selective molecular transformations under ecofriendly conditions and visible light illumination.

Unified theory of plasmon-induced resonance energy transfer and hot electron injection processes for enhanced photocurrent efficiency

Plasmons in metal nanoparticles (MNPs) promise to enhance solar energy conversion in semiconductors. Two essential mechanisms of enhancement in the near-field regime are hot electron injection (HEI) and plasmon-induced resonance energy transfer (PIRET). Individual studies of both mechanisms indicate that the PIRET efficiency is limited by the short lifetime of the plasmon, whereas the hot electrons result from the plasmon decay. The development of a unified theory of the coupled HEI and PIRET processes is fundamentally interesting and necessary for making reliable predictions but is complicated by the multiple interactions between various components that participate in the enhancement process. In this paper, we use the model-Hamiltonian approach to develop a combined theoretical framework including both PIRET and HEI. The coupled dynamics as well as the time evolution of hot electron energy distribution are studied. The theory further predicts an interference-induced asymmetry in the spectral dependence of PIRET, which can be used to distinguish it from HEI. As the relative contributions of PIRET and HEI strongly depend on the size of the MNPs, this presents itself as a simple route to control the strength of their contributions. The results presented here can further guide future applications of plasmonic solar energy harvesting.

Boosting Electrocatalytic Oxygen Evolution Performance of Ultrathin Co/Ni-MOF Nanosheets via Plasmon-Induced Hot Carriers

Ultrathin metal-organic framework (MOF) nanosheets with large active sites and superior catalytic properties have attracted extensive interests and are promising for oxygen evolution reaction (OER) for water splitting. Herein, we report a novel and highly efficient hetero-nanostructured OER system based on plasmonic Au nanoparticles (NPs) and ultrathin semiconductor-like Co/Ni-MOF nanosheets. The OER performance of the hybrid system can be tuned (by varying the AuNP sizes) and the oxidation current significantly enhanced to ∼10-fold with incorporated AuNPs of ∼20 nm. An onset overpotential (η) of only 0.33 V was achieved under light illumination, which was much lower than the pure Ni/Co-MOF (0.48 V). Further analysis revealed the key role of the plasmonically induced hot holes (via electric- and combined photoexcitation) in boosting the OER performance of the resulting system. The finding and the proposed concept provide a new insight for understanding the plasmon enhancements in catalysis and may open a new avenue to design MOF hetero-nanostructures with high performance for photoelectrocatalysis.

Momentum-resolved TDDFT algorithm in atomic basis for real time tracking of electronic excitation

Ultrafast electronic dynamics in solids lies at the core of modern condensed matter and materials physics. To build up a practical ab initio method for studying solids under photoexcitation, we develop a momentum-resolved real-time time dependent density functional theory (rt-TDDFT) algorithm using numerical atomic basis, together with the implementation of both the length and vector gauge of the electromagnetic field. When applied to simulate elementary excitations in two-dimensional materials such as graphene, different excitation modes, only distinguishable in momentum space, are observed. The momentum-resolved rt-TDDFT is important and computationally efficient for the study of ultrafast dynamics in extended systems.

Molecular fingerprinting of nanoparticles in complex media with non-contact photoacoustics: beyond the light scattering limit

Optical instruments can probe physical systems even to the level of individual molecules. In particular, every molecule, solution, and structure such as a living cell has a unique absorption spectrum representing a molecular fingerprint. This spectrum can help identify a particular molecule from others or quantify its concentration; however, scattering limits molecular fingerprinting within a complex compound and must be overcome. Here, we present a new, non-contact photoacoustic (PA)-based method that can almost completely remove the influence of background light scattering on absorption measurements in heterogeneous highly scattering solutions and, furthermore, separate the intrinsic absorption of nanoscale objects from their scattering. In particular, we measure pure absorption spectra for solutions of gold nanorods (GNRs) as an example of a plasmonic agent and show that these spectra differ from the extinction measured with conventional UV-VIS spectrophotometry. Finally, we show how the original GNR absorption changes when nanoparticles are internalized by cells.

Plasmon-induced hot electron transfer in AgNW@TiO2@AuNPs nanostructures

Compared to the limited absorption cross-section of conventional photoactive TiO₂ nanoparticles (NPs), plasmonic metallic nanoparticles can efficiently convert photons from an extended spectrum range into energetic carriers because of the localized surface plasmon resonance (LSPR). Using these metal oxide semiconductors as shells for plasmonic nanoparticles (PNPs) that absorb visible light could extend their applications. The photophysics of such systems is performed using transient absorption measurements and steady extinction simulations and shows that the plasmonic energy transfer from the AgNWs core to the TiO₂ shell results from a hot carrier injection process. Lifetimes obtained from photobleaching decay dynamics suggest that (i) the presence of gold nanoparticles (AuNPs) in AgNWs@TiO₂@AuNPs systems can further promote the hot carrier transfer process via plasmonic coupling effects and (ii) the carrier dynamics is greatly affected by the shell thickness of TiO₂. This result points out a definite direction to design appropriate nanostructures with tunable charge transfer processes toward photo-induced energy conversion applications.

Quantifying Photothermal and Hot Charge Carrier Effects in Plasmon-Driven Nanoparticle Syntheses

The excitation of localized surface plasmon resonances in Au and Ag colloids can be used to drive the synthesis of complex nanostructures, such as anisotropic prisms, bipyramids, and core@shell nanoparticles. Yet, after two decades of research, it is challenging to paint a complete picture of the mechanisms driving such light-induced chemical transformations. In particular, whereas the injection of hot charge carriers from the metal nanoparticles is usually proposed as the dominant mechanism, the contribution of plasmon-induced heating can often not be neglected. Here, we tackle this uncertainty and quantify the contribution of different activation mechanisms using a temperature-sensitive synthesis of Au@Ag core@shell nanoparticles. We compare the rate of Ag shell growth in the dark at different temperatures with the one under plasmon excitation with varying laser intensities. Our controlled illumination geometry, coupled to numerical modeling of light propagation and heat diffusion in the reaction volume, allows us to quantify both localized and collective heating effects and determine their contribution to the total growth rate of the nanoparticles. We find that nonthermal effects can be dominant, and their relative contribution depends on the fraction of nanoparticle suspension under irradiation. Understanding the mechanism of plasmon-activated chemistry at the surface of metal nanoparticles is of paramount importance for a wide range of applications, from the rational design of novel light-assisted nanoparticle syntheses to the development of plasmonic nanostructures for catalytic and therapeutic purposes.

Quantifying the role of surface plasmon excitation and hot carrier transport in plasmonic devices

Harnessing photoexcited “hot” carriers in metallic nanostructures could define a new phase of non-equilibrium optoelectronics for photodetection and photocatalysis. Surface plasmons are considered pivotal for enabling efficient operation of hot carrier devices. Clarifying the fundamental role of plasmon excitation is therefore critical for exploiting their full potential. Here, we measure the internal quantum efficiency in photoexcited gold (Au)–gallium nitride (GaN) Schottky diodes to elucidate and quantify the distinct roles of surface plasmon excitation, hot carrier transport, and carrier injection in device performance. We show that plasmon excitation does not influence the electronic processes occurring within the hot carrier device. Instead, the metal band structure and carrier transport processes dictate the observed hot carrier photocurrent distribution. The excellent agreement with parameter-free calculations indicates that photoexcited electrons generated in ultra-thin Au nanostructures impinge ballistically on the Au–GaN interface, suggesting the possibility for hot carrier collection without substantial energy losses via thermalization.

Understanding the role of plasmon excitation is crucial for the realization of hot carrier devices. Here, the authors report internal quantum efficiency measurements in photoexcited gold gallium nitride Schottky diodes and elucidate the roles of surface plasmon excitation, hot carrier transport, and carrier injection in device performance.

Plasmon-Mediated Drilling in Thin Metallic Nanostructures

Thin and ultraflat conductive surfaces are of particular interest to use as substrates for tip-enhanced spectroscopy applications. Tip-enhanced spectroscopy exploits the excitation of a localized surface plasmon resonance mode at the apex of a metallized atomic force microscope tip, confining and enhancing the local electromagnetic field by several orders of magnitude. This allows for nanoscale mapping of the surface with high spatial resolution and surface sensitivity, as demonstrated when coupled to local Raman measurements. In gap-mode tip-enhanced spectroscopy, the specimen of interest is deposited onto a flat metallic surface and probed by a metallic tip, allowing for further electromagnetic confinement and subsequent enhancement. We investigate here a geometry where a gold tip is used in conjunction with a silver nanoplate, thus forming a heterometallic platform for local enhancement. When irradiated, a plasmon-mediated reaction is triggered at the tip-substrate junction due to the enhanced electric field and the transfer of hot electrons from the tip to the nanoplate. This resulting nanoscale reaction appears to be sufficient to ablate the thin silver plates even under weak laser intensity. Such an approach may be further exploited for patterning metallic nanostructures or photoinduced chemical reactions at metal surfaces.

Plasmon-enhanced photocatalytic activity of Na0.9Mg0.45Ti3.55O8 loaded with noble metals directly observed with scanning Kelvin probe microscopy

The noble metals Au, Ag and Pt were loaded onto Na_0.9Mg_0.45Ti_3.55O₈ (NMTO) using a chemical bath deposition method devised in our recent work for the first time. The composite photocatalysts exhibit more effective photodegradation of methylene blue, due to the Schottky barrier built between NMTO and noble metal. Hot electrons generated during localized surface plasmon processes in metal nanoparticles transfer to the semiconductor, manifesting as a depression of surface potential directly detectable by scanning Kelvin probe microscopy. The key factor responsible for the improved ability of semiconductor-based photocatalysts is charge separation. The most effective weight concentrations of Au, Ag and Pt loaded onto NMTO were found to be 5.00%, 12.6% and 5.55% respectively. NMTO loaded with noble metals shows good photostability and recyclability for the degradation of methylene blue. A possible mechanism for the photodegradation of methylene blue over NMTO loaded with noble metals is proposed. This work highlights the potential application of NMTO-based photocatalysts, and provides an effective method to detect localized surface plasmons.

Tunable Nonthermal Distribution of Hot Electrons in a Semiconductor Injected from a Plasmonic Gold Nanostructure

For semiconductors photosensitized with organic dyes or quantum dots, transferred electrons are usually considered thermalized at the conduction band edge. This study suggests that the electrons injected from a plasmonic metal into a thin semiconductor shell can be nonthermal with energy up to the plasmon frequency. In other words, the electrons injected into the semiconductor are still hot carriers. Photomodulated X-ray absorption measurements of the Ti L_2,3 edge are compared before and after excitation of the plasmon in Au@TiO₂ core-shell nanoparticles. Comparison with theoretical predictions of the X-ray absorption, which include the heating and state-filling effects from injected hot carriers, suggests that the electrons transferred from the plasmon remain nonthermal in the ∼10 nm TiO₂ shell, due in part to a slow trapping in defect states. By repeating the measurements for spherical, rod-like, and star-like metal nanoparticles, the magnitude of the nonthermal distribution, peak energy, and number of injected hot electrons are confirmed to be tuned by the plasmon frequency and the sharp corners of the plasmonic nanostructure. The results suggest that plasmonic photosensitizers can not only extend the sunlight absorption spectral range of semiconductor-based devices but could also result in increased open circuit voltages and elevated thermodynamic driving forces for solar fuel generation in photoelectrochemical cells.

Synergetic photocatalytic effect between 1 T@2H-MoS2 and plasmon resonance induced by Ag quantum dots

Semiconductor phase transitions and plasma noble metal quantum dots (QDs) for visible-light-driven photocatalysts have attracted significant research interest. In this study, novel microwave hydrothermal and photo-reduction methods are proposed to synthesise a visible-light-driven plasma photocatalytic 1T@2H-MoS₂/Ag composite. Photoelectrochemical results show that the introduction of the 1T phase and Ag significantly enhances the light response range and charge separation. The 1T phase can act as a co-catalyst to provide a high electron concentration. Ag QDs can effectively improve the light absorption and catalytic effect. The synergistic effect between the 1T@2H-MoS₂ microspheres and localised surface plasmon resonance of the Ag QDs can effectively enhance the photocatalytic activity of 1T@2H-MoS₂/Ag. The developed 1T@2H-MoS₂/Ag composite is superior, not only with respect to a visible-light photocatalytic degradation of conventional dyes, but also in the photocatalytic reduction of Cr(VI). Compared with 2H-MoS₂, the catalytic efficiency of 1T@2H-MoS₂/Ag for Cr(VI) and MB is increased by 81% and 41%, respectively. This study demonstrates that the introduction of 1T-MoS₂ and Ag QDs can significantly enhance the catalytic properties of 2H-MoS₂. The microwave and photo-reduction technologies can be employed as green, safe, simple, and rapid methods for the synthesis of noble metal plasma composites.

Silica-Coated Plasmonic Metal Nanoparticles in Action

Hybrid colloids consisting of noble metal cores and metal oxide shells have been under intense investigation for over two decades and have driven progress in diverse research lines including sensing, medicine, catalysis, and photovoltaics. Consequently, plasmonic core-shell particles have come to play a vital role in a plethora of applications. Here, an overview is provided of recent developments in the design and utilization of the most successful class of such hybrid materials, silica-coated plasmonic metal nanoparticles. Besides summarizing common simple approaches to silica shell growth, special emphasis is put on advanced synthesis routes that either overcome typical limitations of classical methods, such as stability issues and undefined silica porosity, or grant access to particularly sophisticated nanostructures. Hereby, a description is given, how different types of silica can be used to provide noble metal particles with specific functionalities. Finally, applications of such nanocomposites in ultrasensitive analyte detection, theranostics, catalysts, and thin-film solar cells are reviewed.

Ultrafast Nanoscale Raman Thermometry Proves Heating Is Not a Primary Mechanism for Plasmon-Driven Photocatalysis

Plasmonic materials efficiently convert light to various forms of energies for many applications, including photocatalysis, photovoltaics, and photothermal therapies. In particular, plasmonic photocatalysts hold incredible promise for highly selective sunlight-driven catalysis through the generation of highly energetic holes and electrons used to drive chemical reactions. However, plasmons are also known to generate heat, and the partitioning of photoexcitation energy into hot carriers and heat on molecularly relevant time scales is not well understood, yet plays a crucial role in designing and understanding these photocatalysts. Using an ultrafast surface-enhanced Raman thermometry technique, we probe the effective temperature, equivalent to the mode-specific increase of vibrational kinetic energy, of molecules adsorbed to gold nanoparticle aggregates in the most active hot spots on the picosecond time scale of chemical reactivity. This represents the first measurement of vibrational energy deposition for coupled molecular-plasmonic systems on the picosecond time scale of molecular motion. We find that upon plasmon excitation, the adsorbates in the hot spots undergo an initial energy transfer within several picoseconds that changes the effective temperature of the system by less than 100 K, even at peak flux values 10⁸ times stronger than focused sunlight. The energy quickly dissipates from the adsorbates into the surroundings in less than 5 ps, even at the highest values of photoexcitation. This surprisingly modest energy transfer of the most active regions of the plasmonic materials on the ultrafast time scale decisively proves that most plasmonic photocatalysis is not primarily thermally driven.

Near infrared light induced plasmonic hot hole transfer at a nano-heterointerface

Localized surface plasmon resonance (LSPR)-induced hot-carrier transfer is a key mechanism for achieving artificial photosynthesis using the whole solar spectrum, even including the infrared (IR) region. In contrast to the explosive development of photocatalysts based on the plasmon-induced hot electron transfer, the hole transfer system is still quite immature regardless of its importance, because the mechanism of plasmon-induced hole transfer has remained unclear. Herein, we elucidate LSPR-induced hot hole transfer in CdS/CuS heterostructured nanocrystals (HNCs) using time-resolved IR (TR-IR) spectroscopy. TR-IR spectroscopy enables the direct observation of carrier in a LSPR-excited CdS/CuS HNC. The spectroscopic results provide insight into the novel hole transfer mechanism, named plasmon-induced transit carrier transfer (PITCT), with high quantum yields (19%) and long-lived charge separations (9.2 μs). As an ultrafast charge recombination is a major drawback of all plasmonic energy conversion systems, we anticipate that PITCT will break the limit of conventional plasmon-induced energy conversion.

Hot hole transfer has applications in plasmonics, photocatalysis, and light harvesting, but is often limited by low quantum yields and short-lived charge separation times. Here, Lian et al. overcome these limitations in heterostructured nanocrystals and proposed a new hot hole transfer mechanism.

Comparison of thermally actuated retro-diels-alder release groups for nanoparticle based nucleic acid delivery

The present study explores alternate pericyclic chemistries for tethering amine-terminal biomolecules onto silver nanoparticles. Employing the versatile tool of the retro-Diels-Alder (rDA) reaction, three thermally-labile cycloadducts are constructed that cleave at variable temperature ranges. While the reaction between furan and maleimide has widely been reported, the current study also evaluates the reverse reaction kinetics between thiophene-maleimide, and pyrrole-maleimide cycloadducts. Density Functional Theorem (DFT) calculations used to model and plan the experiments, predict energy barriers for the thiophene-maleimide reverse reaction to be greatest, and the pyrrole-maleimide barriers the lowest. Based on the computational analyses, it is projected that the cycloreversion rate would occur slowest with the thiophene, followed by furan, and finally pyrrole would yield the promptest release. These thermally-responsive linkers, characterized by Electrospray Ionization Mass Spectrometry, ¹H and ¹³C NMR, are thiol-linked to silver nanoparticles and conjugate single stranded siRNA mimics with 5' fluorescein tag. Second harmonic generation spectroscopy (SHG) and fluorescence spectroscopy are used to measure release and rate of release. The SHG decay constants and fluorescence release profiles obtained for the three rDA reactions confirm the trends obtained from the DFT computations.

Hot Hole Collection and Photoelectrochemical CO2 Reduction with Plasmonic Au/p-GaN Photocathodes

Harvesting nonequilibrium hot carriers from plasmonic-metal nanostructures offers unique opportunities for driving photochemical reactions at the nanoscale. Despite numerous examples of hot electron-driven processes, the realization of plasmonic systems capable of harvesting hot holes from metal nanostructures has eluded the nascent field of plasmonic photocatalysis. Here, we fabricate gold/p-type gallium nitride (Au/p-GaN) Schottky junctions tailored for photoelectrochemical studies of plasmon-induced hot-hole capture and conversion. Despite the presence of an interfacial Schottky barrier to hot-hole injection of more than 1 eV across the Au/p-GaN heterojunction, plasmonic Au/p-GaN photocathodes exhibit photoelectrochemical properties consistent with the injection of hot holes from Au nanoparticles into p-GaN upon plasmon excitation. The photocurrent action spectrum of the plasmonic photocathodes faithfully follows the surface plasmon resonance absorption spectrum of the Au nanoparticles and open-circuit voltage studies demonstrate a sustained photovoltage during plasmon excitation. Comparison with Ohmic Au/p-NiO heterojunctions confirms that the vast majority of hot holes generated via interband transitions in Au are sufficiently hot to inject above the 1.1 eV interfacial Schottky barrier at the Au/p-GaN heterojunction. We further investigated plasmon-driven photoelectrochemical CO₂ reduction with the Au/p-GaN photocathodes and observed improved selectivity for CO production over H₂ evolution in aqueous electrolytes. Taken together, our results offer experimental validation of photoexcited hot holes more than 1 eV below the Au Fermi level and demonstrate a photoelectrochemical platform for harvesting hot carriers to drive solar-to-fuel energy conversion.

Nanoscale Control of Molecular Self-Assembly Induced by Plasmonic Hot-Electron Dynamics

Self-assembly processes allow designing and creating complex nanostructures using molecules as building blocks and surfaces as scaffolds. This autonomous driven construction is possible due to a complex thermodynamic balance of molecule-surface interactions. As such, nanoscale guidance and control over this process is hard to achieve. Here we use the highly localized light-to-chemical-energy conversion of plasmonic materials to spatially cleave Au-S bonds on predetermined locations within a single nanoparticle, enabling a high degree of control over this archetypal system for molecular self-assembly. Our method offers nanoscale precision and high-throughput light-induced tailoring of the surface chemistry of individual and packed nanosized metallic structures by simply varying wavelength and polarization of the incident light. Assisted by single-molecule super-resolution fluorescence microscopy, we image, quantify, and shed light onto the plasmon-induced desorption mechanism. Our results point toward localized distribution of hot electrons, contrary to uniformly distributed lattice heating, as the mechanism inducing Au-S bond breaking. We demonstrate that plasmon-induced photodesorption enables subdiffraction and even subparticle multiplexing. Finally, we explore possible routes to further exploit these concepts for the selective positioning of nanomaterials and the sorting and purification of colloidal nanoparticles.

Plasmonic Hot Carriers-Controlled Second Harmonic Generation in WSe2 Bilayers

Modulating second harmonic generation (SHG) by a static electric field through either electric-field-induced SHG or charge-induced SHG has been well documented. Nonetheless, it is essential to develop the ability to dynamically control and manipulate the nonlinear properties, preferably at high speed. Plasmonic hot carriers can be resonantly excited in metal nanoparticles and then injected into semiconductors within 10-100 fs, where they eventually decay on a comparable time scale. This allows one to rapidly manipulate all kinds of characteristics of semiconductors, including their nonlinear properties. Here we demonstrate that plasmonically generated hot electrons can be injected from plasmonic nanostructure into bilayer (2L) tungsten diselenide (WSe₂), breaking the material inversion symmetry and thus inducing an SHG. With a set of pump-probe experiments we confirm that it is the dynamic separation electric field resulting from the hot carrier injection (rather than a simple optical field enhancement) that is the cause of SHG. Transient absorption measurement further substantiate the plasmonic hot electrons injection and allow us to measure a rise time of ∼120 fs and a fall time of 1.9 ps. Our study creates opportunity for the ultrafast all-optical control of SHG in an all-optical manner that may enable a variety of applications.

Surface-enhanced Raman spectroscopic detection of molecular chemo- and plasmo-catalysis on noble metal nanoparticles

The in situ detection of reactions catalyzed by metal NPs is challenging because the underlying chemical transformations occur at interfaces. Surface-enhanced Raman scattering (SERS), a surface-selective, sensitive and label-free vibrational spectroscopic technique, is ideally suited for monitoring of heterogeneous catalysis with high chemical specificity. A major limitation in the past, however, was that small, catalytically active metal NPs do not exhibit the high plasmonic activity required for SERS. This feature article focuses on the design, synthesis and use of bifunctional NPs with both catalytic and plasmonic activity for in situ SERS detection of reactions catalyzed by metal NPs. We focus on model reactions induced by chemical reducing agents such as hydride or molecular hydrogen as well as on plasmon-induced photo-catalysis including both photo-oxidation and photo-reduction. Finally, we highlight the concept of photo-recycling on halide-containing silver surfaces for unprecedented multi-electron reduction chemistry.

Hot-Electron-Assisted Femtosecond All-Optical Modulation in Plasmonics

The optical Kerr nonlinearity of plasmonic metals provides enticing prospects for developing reconfigurable and ultracompact all-optical modulators. In nanostructured metals, the coherent coupling of light energy to plasmon resonances creates a nonequilibrium electron distribution at an elevated electron temperature that gives rise to significant Kerr optical nonlinearities. Although enhanced nonlinear responses of metals facilitate the realization of efficient modulation devices, the intrinsically slow relaxation dynamics of the photoexcited carriers, primarily governed by electron-phonon interactions, impedes ultrafast all-optical modulation. Here, femtosecond (≈190 fs) all-optical modulation in plasmonic systems via the activation of relaxation pathways for hot electrons at the interface of metals and electron acceptor materials, following an on-resonance excitation of subradiant lattice plasmon modes, is demonstrated. Both the relaxation kinetics and the optical nonlinearity can be actively tuned by leveraging the spectral response of the plasmonic design in the linear regime. The findings offer an opportunity to exploit hot-electron-induced nonlinearities for design of self-contained, ultrafast, and low-power all-optical modulators based on plasmonic platforms.

Metabolic Fingerprinting on a Plasmonic Gold Chip for Mass Spectrometry Based in Vitro Diagnostics

Current metabolic analysis is far from ideal to engage clinics and needs rationally designed materials and device. Here we developed a novel plasmonic chip for clinical metabolic fingerprinting. We first constructed a series of chips with gold nanoshells on the surface through controlled particle synthesis, dip-coating, and gold sputtering for mass production. We integrated the optimized chip with microarrays for laboratory automation and micro-/nanoscaled experiments, which afforded direct high-performance metabolic fingerprinting by laser desorption/ionization mass spectrometry using 500 nL of various biofluids and exosomes. Further we for the first time demonstrated on-chip in vitro metabolic diagnosis of early stage lung cancer patients using serum and exosomes. This work initiates a new bionanotechnology based platform for advanced metabolic analysis toward large-scale diagnostic use.

Photoluminescence of Gold Nanorods: Purcell Effect Enhanced Emission from Hot Carriers

We demonstrate, experimentally and theoretically, that the photon emission from gold nanorods can be viewed as a Purcell effect enhanced radiative recombination of hot carriers. By correlating the single-particle photoluminescence spectra and quantum yields of gold nanorods measured for five different excitation wavelengths and varied excitation powers, we illustrate the effects of hot carrier distributions evolving through interband and intraband transitions and the photonic density of states on the nanorod photoluminescence. Our model, using only one fixed input parameter, describes quantitatively both emission from interband recombination and the main photoluminescence peak coinciding with the longitudinal surface plasmon resonance.

Gold Nanoparticles as Absolute Nanothermometers

Nanothermometry is a challenging field that can open the door to intriguing questions ranging from biology and medicine to material sciences. Gold nanorods are excellent candidates to act as nanoprobes because they are reasonably bright emitters upon excitation with a monochromatic source. Gold nanoparticles are commonly used in photothermal therapy as efficient transducers of electromagnetic radiation into heat. In this work we use the spectrum of the anti-Stokes emission from gold nanorods irradiated in resonance to measure the absolute temperature of the nanoparticles and their surrounding medium without the need for a previous calibration. We show a 4 K accuracy in the determination of the temperature of the medium with spectral measurements of 180 s integration time. This procedure can be easily implemented in any microscope capable of acquiring emission spectra, and it is not limited to any specific shape of nanoparticles.

Reactive tunnel junctions in electrically driven plasmonic nanorod metamaterials

Non-equilibrium hot carriers formed near the interfaces of semiconductors or metals play a crucial role in chemical catalysis and optoelectronic processes. In addition to optical illumination, an efficient way to generate hot carriers is by excitation with tunnelling electrons. Here, we show that the generation of hot electrons makes the nanoscale tunnel junctions highly reactive and facilitates strongly confined chemical reactions that can, in turn, modulate the tunnelling processes. We designed a device containing an array of electrically driven plasmonic nanorods with up to 10¹¹ tunnel junctions per square centimetre, which demonstrates hot-electron activation of oxidation and reduction reactions in the junctions, induced by the presence of O₂ and H₂ molecules, respectively. The kinetics of the reactions can be monitored in situ following the radiative decay of tunnelling-induced surface plasmons. This electrically driven plasmonic nanorod metamaterial platform can be useful for the development of nanoscale chemical and optoelectronic devices based on electron tunnelling.

Origin of Plasmon Lineshape and Enhanced Hot Electron Generation in Metal Nanoparticles

Plasmon-generated hot carriers are currently being studied intensively for their role in enhancing the efficiency of photovoltaic and photocatalytic processes. Theoretical studies of the hot electrons subsystem have generated insight, but we show that a unified quantum-mechanical treatment of the plasmon and hot electrons reveals new physical phenomena. Instead of a unidirectional energy transfer process in Landau damping, back energy transfer is predicted in small metal nanoparticles (MNPs) within a model-Hamiltonian approach. As a result, the single Lorentzian plasmonic line shape is modulated by a multipeak structure, whose individual line width provides a direct way to probe the electronic dephasing. More importantly, the hot electron generation can be enhanced greatly by matching the incident energy to the peaks of the modulated line shape.

Light-assisted surface reactions on metal nanoparticles

Light-assisted surface reaction can lower reaction temperature, potentially reducing the energy use by providing light together with heat.

Quantifying photothermal heating at plasmonic nanoparticles by scanning electrochemical microscopy

Photothermal heating at metal nanoparticles results from the non-radiative decay of localized surface plasmons. The local heat generation enhances the mass transport rate of redox molecules and causes a shift in their formal potential, both of which can impact an electrochemical process at the nanoparticle interface. Here we present a methodology for probing the surface temperature at a plasmonic nanoparticle substrate using scanning electrochemical microscopy (SECM). Light is used to excite a plasmonic substrate electrode, while an ultramicroelectrode tip is positioned close to the substrate to read out both the mass transfer rate and concentration profile of the redox molecules. The measured mass transfer rate and the shift in the equilibrium potential provide a quantitative value of the temperature increase at the substrate surface, which is verified by simulations using a mass transfer model coupled with heat dissipation. The developed SECM approach is suitable for probing heat generation at a variety of both plasmonic and non-plasmonic nanostructures.

Plasmonic Janus hybrids for the detection of small metabolites

Janus hybrids with amphiphilic structures were used for the sensitive detection of small metabolites.

Ultrathin Polydiacetylene-Based Synergetic Composites with Plasmon-Enhanced Photoelectric Properties

Fabricating plasmon-enhanced organic nanomaterials with technologically relevant supporting architectures on planar solids remains a challenging task in the chemistry of thin films and interfaces. In this work, we report a bottom-up assembly of ultrathin layered composites of conductive polymers with photophysical properties enhanced by gold nanoparticles. The polydiacetylene component was formed by photopolymerization of a catanionic mixture of pentacosadiynoic surfactants on a surface of citrate-stabilized gold hydrosol monitored by a fiber optic spectrometer. Microscopic examination of the 3 nm thick solid-immobilized film showed that gold nanoparticles (AuNPs) do not aggregate within the monolayer upon polymerization. This polydiacetylene/AuNPs monolayer was coupled with 60 nm thick polyaniline-based layer deposited atop. The resulting polymer composite with an integrated 4-stripe electric cell showed nonadditive electric behavior due to the formation of electron-hole pairs with increased charge carrier mobility at the interface between the polymer layers. Under visible light irradiation of the composite film, a plasmonic effect of the gold nanoparticles was observed at the onset of photoconductivity, although neither polydiacetylene nor the polyaniline component alone are photoconductive polymers. The results indicate that our bottom-up strategy can be expanded to design other plasmon-enhanced ultrathin polymer composites with potential applications in optoelectronics and photovoltaics.

Photovoltaic Properties and Ultrafast Plasmon Relaxation Dynamics of Diamond-Like Carbon Nanocomposite Films with Embedded Ag Nanoparticles

Ultrafast relaxation dynamics of diamond-like carbon (DLC) films with embedded Ag nanoparticles (DLC:Ag) and photovoltaic properties of heterojunctions consisting of DLC:Ag and crystalline silicon (DLC:Ag/Si) were investigated by means of transient absorption (TAS) spectroscopy and photovoltaic measurements. The heterojunctions using both p type and n type silicon were studied. It was found that TAS spectra of DLC:Ag films were dependent on the used excitation wavelength. At wavelengths where Ag nanoparticles absorbed light most intensively, only DLC signal was registered. This result is in good accordance with an increase of the DLC:Ag/Si heterojunction short circuit current and open circuit voltage with the excitation wavelength in the photovoltaic measurements. The dependence of the TAS spectra of DLC:Ag films and photovoltaic properties of DLC:Ag/Si heterostructures on the excitation wavelength was explained as a result of trapping of the photoexcited hot charge carriers in DLC matrix. The negative photovoltaic effect was observed for DLC:Ag/p-Si heterostructures and positive ("conventional") for DLC:Ag/n-Si ones. It was explained by the excitation of hot plasmonic holes in the Ag nanoparticles embedded into DLC matrix. Some decrease of DLC:Ag/Si heterostructures photovoltage as well as photocurrent with DLC:Ag film thickness was observed, indicating role of the interface in the charge transfer process of photocarriers excited in Ag nanoparticles.

Boosting Hot Electrons in Hetero-superstructures for Plasmon-Enhanced Catalysis

Hetero-nanostructures featured with both strong plasmon absorption and high catalytic activity are believed to be ideal platforms to realize efficient light-driven catalysis. However, in reality, it remains a great challenge to acquire high-performance catalysis in such hetero-nanostructures due to poor generation and transfer of plamson-induced hot electrons. In this report, we demonstrate that Au nanorod@Pd superstructures (Au@Pd SSs), where the ordered Pd nanoarrays are precisely grown on Au nanorod surfaces via solution-based seed-mediated approach, would be an excellent solution for this challenge. Both experiment and theory disclose that the ordered arrangement of Pd on Au nanorod surfaces largely promotes hot electron generation and transfer via amplified local electromagnetic field and decreased electron-phonon coupling, respectively. Each effect is separately highlighted in experiments by the significant plasmon-enhanced catalytic activity of Au@Pd SSs in two types of important reactions with a distinct time scale of bond-dissociation event: molecular oxygen activation and carbon-carbon coupling reaction. This work opens the door to design and application of new generation photocatalysts.

Increased rise time of electron temperature during adiabatic plasmon focusing

Decay of plasmons to hot carriers has recently attracted considerable interest for fundamental studies and applications in quantum plasmonics. Although plasmon-assisted hot carriers in metals have already enabled remarkable physical and chemical phenomena, much remains to be understood to engineer devices. Here, we present an analysis of the spatio-temporal dynamics of hot electrons in an emblematic plasmonic device, the adiabatic nanofocusing surface-plasmon taper. With femtosecond-resolution measurements, we confirm the extraordinary capability of plasmonic tapers to generate hot carriers by slowing down plasmons at the taper apex. The measurements also evidence a substantial increase of the “lifetime” of the electron gas temperature at the apex. This interesting effect is interpreted as resulting from an intricate heat flow at the apex. The ability to harness the “lifetime” of hot-carrier gases with nanoscale circuits may provide a multitude of applications, such as hot-spot management, nonequilibrium hot-carrier generation, sensing, and photovoltaics.

Knowledge of the electron-gas dynamics in nanometric hot spots is of importance for hot-carrier technologies. Here Lozan et al. present a theoretical and experimental analysis of the spatio-temporal dynamics of hot electrons in a nano-focusing surface-plasmon polariton taper.