Direct Observation of a Plasmon-Induced Hot Electron Flow in a Multimetallic Nanostructure

High-Performance Planar Broadband Hot-Electron Photodetection through Platinum-Dielectric Triple Junctions

Recently, planar and broadband hot-electron photodetectors (HE PDs) were established but exhibited degraded performances due to the adoptions of the single-junction configurations and the utilizations of absorbable films with thicknesses larger than the electronic mean free path. In this work, we present a five-layer design for planar HE PDs assisted by triple junctions in which an ultrathin Pt layer dominates the broadband and displays strong optical absorption (>0.9 from 900 nm to 1700 nm). Optical studies reveal that the optical admittance matching between optical admittances of designed device and air at all interested wavelengths is responsible for broadband light-trapping that induces prominent energy depositions in Pt layers. Electrical investigations show that, benefitting from suppressed hot-electron transport losses and increased hot-electron harvesting junctions, the predicted responsivity of the designed HE PD is up to 8.51 mA/W at 900 nm. Moreover, the high average absorption (responsivity) of 0.96 (3.66 mA/W) is substantially sustained over a broad incidence angle regardless of the polarizations of incident light. The comparison studies between five-layer and three-layer devices emphasize the superiority of five-layer design in strong optical absorption in Pt layers and efficient hot-electron extraction.

Decoupling Plasmonic Hot Carrier from Thermal Catalysis via Electrode Engineering

Increased attention has been directed toward generating nonequilibrium hot carriers resulting from the decay of collective electronic oscillations on metal known as surface plasmons. Despite numerous experimental endeavors, demonstrating hot carrier-mediated photocatalysis without a heating contribution has proven challenging, particularly for single electron transfer reactions where the thermal contribution is generally detrimental. An innovative engineering solution is proposed to enable single electron transfer reactions with plasmonics. It consists of a photoelectrode designed as an energy filter and photocatalysis performed with light function modulation instead of continuously. The photoelectrode, consisting of FTO/TiO2 amorphous (10 nm)/Au nanoparticles, with TiO2 acting as a step-shape energy filter to enhance hot electron extraction and charge-separated state lifetime. The extracted hot electrons were directed toward the counter electrode, while the hot holes performed a single electron transfer oxidation reaction. Light modulation prevented local heat accumulation, effectively decoupling hot carrier catalysis from the thermal contribution.

Grating-assisted hot-electron photodetectors for S- and C-band telecommunication

Although outstanding detectivities, InGaAs photodetectors for optic fiber communication are often costly due to the need for cooling. Therefore, cryogen-free and cost-effective alternatives working in telecommunication bands are highly desired. Here, we present a design of hot-electron photodetectors (HE PDs) with attributes of room-temperature operation and strong optical absorption over S and C bands (from 1460 to 1565 nm). The designed HE PD consists of a metal-semiconductor-metal hot-electron stack integrated with a front grating. Optical simulations reveal that mode hybridizations between Fabry-Pérot resonance and grating-induced surface plasmon excitation lead to high absorption efficiencies (≥0.9) covering S and C bands. Probability-based electrical calculations clarify that device responsivity is mainly determined by working wavelength on the premise of broadband strong absorption. Moreover, through comparison studies between the grating-assisted HE PD and purely planar microcavity system that serves as a reference, we highlight the design superiorities in average absorption and average responsivity with optimized values of 0.97 and 0.73 mA W-1, respectively. The upgraded peformances of the designed device are promising for efficient photoelectric conversion in optic fiber communication systems.

Hydrogen evolution with hot electrons on a plasmonic-molecular catalyst hybrid system

Plasmonic systems convert light into electrical charges and heat, mediating catalytic transformations. However, there is ongoing controversy regarding the involvement of hot carriers in the catalytic process. In this study, we demonstrate the direct utilisation of plasmon hot electrons in the hydrogen evolution reaction with visible light. We intentionally assemble a plasmonic nanohybrid system comprising NiO/Au/[Co(1,10-Phenanthrolin-5-amine)2(H2O)2], which is unstable at water thermolysis temperatures. This assembly limits the plasmon thermal contribution while ensuring that hot carriers are the primary contributors to the catalytic process. By combining photoelectrocatalysis with advanced in situ spectroscopies, we can substantiate a reaction mechanism in which plasmon-induced hot electrons play a crucial role. These plasmonic hot electrons are directed into phenanthroline ligands, facilitating the rapid, concerted proton-electron transfer steps essential for hydrogen generation. The catalytic response to light modulation aligns with the distinctive profile of a hot carrier-mediated process, featuring a positive, though non-essential, heat contribution.

Exploring the potential of broadband Tamm plasmon resonance for enhanced photodetection

Tamm plasmon polaritons (TPPs) have emerged as a promising platform for photodetector applications due to their strong light-matter interaction and potential for efficient light absorption. In this work, a design for a broadband photodetector (PD) based on the optical Tamm plasmon (OTS) state generated in a periodic metal-semiconductor-distributed Bragg reflector (DBR) geometry is proposed. The transfer matrix method (TMM) was used to study the propagation of electromagnetic waves through the proposed structure. By exciting the structure with incident light and analyzing the electric field profile within the multilayer structure at the resonant wavelength, we observe a distinctive electric field distribution that indicates the presence of Tamm plasmon modes. A comparative study was conducted to investigate the optical properties of a photodetector in the near-infrared (NIR) range by varying parameters such as thickness. By optimizing the thickness, we successfully achieved a broadband photoresponse in the photodetector, with a maximum responsivity of 21.8 mA/W at a wavelength of 1354 nm, which falls within the photonic bandgap region. FWHM was found to be 590 nm for the responsivity spectrum. The geometry also presents maximum absorption with FWHM calculated to be about 871.5 nm. The proposed geometry offers a broadband photoresponse, which is advantageous for the advancement of Tamm-based detector technologies. The ability to detect light over a wide operation range makes this mechanism highly beneficial for various applications.

Planar hot-electron photodetection with polarity-switchable photocurrents controlled by the working wavelength

Hot-electron photodetection is attracting increasing interests. Based on internal photoemission mechanism, hot-electron photodetectors (HE PDs) convert incident photon energy into measurable photocurrent. To obtain polarity-switchable photocurrent, one often applies electric bias to reverse the hot-electron flow. However, the employment of bias reduces the device flexibility and increasing the bias voltage degrades the detectivity of the device. Herein, we design a planar HE PD with the polarity-switchable photocurrent controlled by the working wavelength. Optical simulations show that the device exhibits two absorption peaks due to the resonances of two Tamm plasmons (TPs). Electrical calculations predict two corresponding TP-assisted responsivity peaks, but with opposite photocurrent polarities, which are determined by the hot-electron flows with opposite directions. We find that the hot-electron flows are closely related with the population differences of TP-induced hot electrons in two electrodes. We further demonstrate that the photocurrent polarity of the HE PD can be switched by altering working wavelength from one TP wavelength to the other. We believe that this approach paves a route to achieve flexible hot-electron photodetection for extensive applications.

Ultralong Lifetime of Plasmon-Excited Electrons Realized in Nonepitaxial/Epitaxial Au@CdS/CsPbBr3 Triple-Heteronanocrystals

Combination of the strong light-absorbing power of plasmonic metals with the superior charge carrier dynamics of halide perovskites is appealing for bio-inspired solar-energy conversion due to the potential to acquire long-lived plasmon-induced hot electrons. However, the direct coupling of these two materials, with Au/CsPbBr₃ heteronanocrystals (HNCs) as a prototype, results in severe suppression of plasmon resonances. The present work shows that interfacial engineering is a key knob for overcoming this impediment, based on the creation of a CdS mediate layer between Au and CsPbBr₃ forming atomically organized Au-CdS and CdS-CsPbBr₃ interfaces by nonepitaxial/epitaxial combined strategy. Transient spectroscopy studies demonstrate that the resulting Au@CdS/CsPbBr₃ HNCs generate remarkably long-lived plasmon-induced charge carriers with lifetime up to nanosecond timescale, which is several orders of magnitude longer than those reported for colloidal plasmonic metal-semiconductor systems. Such long-lived carriers extracted from plasmonic antennas enable to drive CO₂ photoreduction with efficiency outperforming previously reported CsPbBr₃ -based photocatalysts. The findings disclose a new paradigm for achieving much elongated time windows to harness the substantial energy of transient plasmons through realization of synergistic coupling of plasmonic metals and halide perovskites.

Synergistic Combination of Charge Carriers and Energy-Transfer Processes in Plasmonic Photocatalysis

Important efforts are currently under way in order to develop further the nascent field of plasmonic photocatalysis, striving for improved efficiencies and selectivities. A significant fraction of such efforts has been focused on distinguishing, understanding, and enhancing specific energy-transfer mechanisms from plasmonic nanostructures to their environment. Herein, we report a synthetic strategy that combines two of the main physical mechanisms driving plasmonic photocatalysis into an engineered system by rationally combining the photochemical features of energetic charge carriers and the electromagnetic field enhancement inherent to the plasmonic excitation. We do so by creating hybrid photocatalysts that integrate multiple plasmonic resonators in a single entity, controlling their joint contribution through spectral separation and differential surface functionalization. This strategy allows us to create complex hybrids with improved photosensitization capabilities, thanks to the synergistic combination of two photosensitization mechanisms. Our results show that the hot electron injection can be combined with an energy-transfer process mediated by the near-field interaction, leading to a significant increase in the final photocatalytic response of the material and moving the field of plasmonic photocatalysis closer to energy-efficient applications. Furthermore, our multimodal hybrids offer a test system to probe the properties of the two targeted mechanisms in energy-related applications such as the photocatalytic generation of hydrogen and open the door to wavelength-selective photocatalysis and novel tandem reactions.

Wavelength-Dependent Bifunctional Plasmonic Photocatalysis in Au/Chalcopyrite Hybrid Nanostructures

Excited, or "hot" charge carrier generation and transfer driven by the decay of localized surface plasmon resonances (LSPRs) are key steps in plasmonic photocatalysis. Hybrid structures that contain both metal and semiconductor building blocks facilitate the extraction of reactive charge carriers and their utilization for photoelectrocatalysis. Additional functionality arises from hybrid structures that combine noble metal nanostructures with semiconductor components, such as chalcopyrite (CuFeS2) nanocrystals (NCs), which by themselves support quasistatic resonances. In this work, we use a hybrid membrane to integrate Au nanorods (NRs) with a longitudinal LSPR at 745 nm and CuFeS2 NCs with a resonance peak at 490 nm into water-stable nanocomposites for robust and bifunctional photocatalysis of oxygen and hydrogen evolution reactions in a wavelength-dependent manner. Excitation of NRs or NCs in the nanocomposite correlates with increased hydrogen or oxygen evolution, respectively, consistent with a light-driven electron transfer between the metal and semiconductor building blocks, the direction of which depends on the wavelength. The bifunctional photoreactivity of the nanocomposite is enhanced by Cu(I)/Cu(II)-assisted catalysis on the surface of the NCs.

Experimental characterization techniques for plasmon-assisted chemistry

Plasmon-assisted chemistry is the result of a complex interplay between electromagnetic near fields, heat and charge transfer on the nanoscale. The disentanglement of their roles is non-trivial. Therefore, a thorough knowledge of the chemical, structural and spectral properties of the plasmonic/molecular system being used is required. Specific techniques are needed to fully characterize optical near fields, temperature and hot carriers with spatial, energetic and/or temporal resolution. The timescales for all relevant physical and chemical processes can range from a few femtoseconds to milliseconds, which necessitates the use of time-resolved techniques for monitoring the underlying dynamics. In this Review, we focus on experimental techniques to tackle these challenges. We further outline the difficulties when going from the ensemble level to single-particle measurements. Finally, a thorough understanding of plasmon-assisted chemistry also requires a substantial joint experimental and theoretical effort.

Kinetics-Controlled Synthesis of Gold-Silver Nanosheets with Abundant in-Plane Cracking and Their Trimetallic Derivatives for Plasmon-Enhanced Catalysis

Controlled synthesis of two-dimensional noble metal nanomaterials with in-plane branching morphology has been of great research interest recently, which yet achieves limited success for AuAg-based nanocrystals. Herein, we report the...

Seed surface doping-mediated seeded growth of Au–Ag Janus nanoparticles with tunable sizes and multiple plasmonic absorption modes

Gold–silver Janus nanoparticles with tunable sizes are successfully prepared, where the anisotropic deposition is induced by seed surface doping.

State-of-the-art progress in tracking plasmon-mediated photoredox catalysis

Metal nanocrystals (NCs), particularly for plasmonic metal NCs with specific morphology and size, can strongly interact with ultraviolet-visible or even near-infrared photons to generate energetic charge carriers, localized heating, and electric field enhancement. These unique properties offer a promising opportunity for maneuvering solar-to-chemical energy conversion through different mechanisms. As distinct from previous works, in this review, recent advances of various characterization techniques in probing and monitoring the photophysical/photochemical processes, as well as the reaction mechanisms of plasmon-mediated photoredox catalysis are thoroughly summarized. Understanding how to distinguish and track these reaction mechanisms would furnish basic guidelines to design next-generation photocatalysts for plasmon-enhanced catalysis.