Excited Hot Electrons Research Articles

Investigation of Plasmon-Mediated Oxygen Evolution Reaction

To achieve a carbon-neutral future, sustainable energy is needed through storing energy through its chemical bond. Water electrolysis, an exemplary form of electrocatalysis, presents an example of storing energy within chemical bonds of the high-energy hydrogen gas. Nevertheless, the anodic reaction involved in the oxygen evolution reaction (OER) poses limitations on the overall rate of the process. Plasmonic gold nanoparticles (Au NPs) have been added to enhance the charge transfer at the interface of the OER electrocatalysts and electrolyte under light illumination.1-3 However, the mechanistic understanding of how the Au NPs on the photo-assisted electrochemical process is still lacking. We applied a model system of plasmonic Au electrode with nickel (Ni) and cobalt (Co)-based OER electrocatalyst to investigate the plasmon-mediated OER process. The composite system has been characterized with XPS, AFM, (photo)electrochemical and spectroscopic characterizations. Our preliminary data shows that the electrodeposited plasmonic Au electrode is free of surfactant and the resonant light illumination could modulate the electrochemical properties of the Au electrode and the Ni- and Co-based electrocatalysts in the alkaline electrolytes. Liu, G.; Li, P.; Zhao, G.; Wang, X.; Kong, J.; Liu, H.; Zhang, H.; Chang, K.; Meng, X.; Kako, T.; Ye, J. Promoting Active Species Generation by Plasmon-Induced Hot-Electron Excitation for Efficient Electrocatalytic Oxygen Evolution. Am. Chem. Soc. 2016, 138(29), 9128–9136.Wang, M.; Wang, P.; Li, C.; Li, H.; Jin, Y. Boosting Electrocatalytic Oxygen Evolution Performance of Ultrathin Co/Ni-MOF Nanosheets via Plasmon-Induced Hot Carriers. ACS Appl. Mater. Interfaces 2018, 10(43), 37095–37102.Zeng, X.; Choi, S. M.; Bai, Y.; Jang, M. J.; Yu, R.; Cho, H.-S.; Kim, C.-H.; Myung, N. V.; Yin, Y. Plasmon-Enhanced Oxygen Evolution Catalyzed by Fe2N-Embedded TiOxNy ACS Appl. Energy Mater. 2020, 3(1), 146–151.

Surface plasmon decorated InGaO deep-UV photodetector array for image sensing and water quality monitoring via highly effective hot electron excitation and interfacial injection

Surface plasmon decorated InGaO deep-UV photodetector array for image sensing and water quality monitoring via highly effective hot electron excitation and interfacial injection

Near-infrared detection based on the excitation of hot electrons in Au/Si microcone array

Although the photoresponse cut-off wavelength of Si is about 1100 nm due to the Si bandgap energy, the internal photoemission effect (IPE) of the Au/Si junction in Schottky detector can extend the absorption wavelength, which makes it a promising candidate for the Si-based infrared detector. However, due to low light absorption, low photon–electron interaction, and poor electron injection efficiency, the near-infrared light detection efficiency of the Schottky detector is still insufficient. The synergistic effect of Si nano/microstructures with a strong light trapping effect and nanoscale Au films with surface plasmon enhanced absorption may provide an effective solution for improving the detection efficiency. In this paper, a large-area periodic Si microcone array covered by an Au film has successfully been fabricated by one-time dry etching based on the mature polystyrene microspheres lithography technique and vacuum thermal deposition, and its properties for hot electron-based near infrared photodetection are investigated. Optical measurements show that the 20 nm-thick Au covered Si microcone array exhibits a low reflectance and a strong absorption (about 85%) in wide wavelength range (900–2500 nm), and the detection responsivity can reach a value as high as 17.1 and 7.0 mA W−1 at 1200 and 1310 nm under the front illumination, and 35.9 mA W−1 at 1310 nm under the back illumination respectively. Three-dimensional finite difference time domain (3D-FDTD) simulation results show that the enhanced local electric field in the Au layer distributes near the air/Au interface under the front illumination and close to the Au/Si interface under the back illumination. The back illumination favors the injection of photo-generated hot electrons in Au layer into Si, which can explain the higher responsivity under the back illumination. Our research is expected to promote the practical application of Schottky photodetectors to Si-compatible near infrared photodetectors.

Construction of Ultrathin BiVO4‐Au‐Cu2O Nanosheets with Multiple Charge Transfer Paths for Effective Visible‐Light‐Driven Photocatalytic Degradation of Tetracycline

AbstractIn this study, unique BiVO4‐Au‐Cu2O nanosheets (NSs) are well designed and multiple charge transfer paths are consequently constructed. The X‐ray photoelectron spectroscopy measurement during a light off‐on‐off cycle and redox capability tests of the photo‐generated charge carriers confirmed the formation of Z‐scheme heterojunction, which can facilitate the charge carrier separation and transfer and maintain the original strong redox potentials of the respective component in the heterojunction. The ultrathin 2D structure of the BiVO4 NSs provided sufficient surface area for the photocatalytic reaction. The local surface plasmon resonance (LSPR) effect of the electron mediator, Au NPs, enhanced the light absorption and promoted the excitation of hot electrons. The multiple charge transfer paths effectively promoted the separation and transfer of the charge carrier. The synergism of the abovementioned properties endowed the BiVO4‐Au‐Cu2O NSs with satisfactory photocatalytic activity in the degradation of tetracycline (Tc) with a removal rate of ≈80% within 30 min under visible light irradiation. The degradation products during the photocatalysis are confirmed by using ultra‐high performance liquid chromatography‐mass spectrometry and the plausible degradation pathways of Tc are consequently proposed. This work paves a strategy for developing highly efficient visible‐light‐driven photocatalysts with multiple charge transfer paths for removing organic contaminants in water.

Insight into the Synergistic Effect of the Oxide–Metal Interface on Hot Electron Excitation

Insight into the Synergistic Effect of the Oxide–Metal Interface on Hot Electron Excitation

Time-Dependent Ultrafast Quadratic Nonlinearity in an Epsilon-Near-Zero Platform.

Ultrafast nonlinearity, which results in modulation of the linear optical response, is a basis for the development of time-varying media, in particular those operating in the epsilon-near-zero (ENZ) regime. Here, we demonstrate that the intraband excitation of hot electrons in the ENZ film results in a second-harmonic resonance shift of ∼10 THz (40 nm) and second-harmonic generation (SHG) intensity changes of >100% with only minor (<1%) changes in linear transmission. The modulation is 10-fold enhanced by a plasmonic metasurface coupled to a film, allowing for ultrafast modulation of circularly polarized SHG. The effect is described by the plasma frequency renormalization in the ENZ material and the modification of the electron damping, with a possible influence of the hot-electron dynamics on the quadratic susceptibility. The results elucidate the nature of the second-order nonlinearity in ENZ materials and pave the way to the rational engineering of active nonlinear metamaterials and metasurfaces for time-varying applications.

Gigahertz electromagnetic pulse emission from femtosecond relativistic laser-irradiated solid targets.

The interactions between high-intensity laser and matter produce particle flux and electromagnetic radiation over a wide energy range. The generation of extremely intense transient fields in the radio frequency-microwave regime has been observed in femtosecond-to-nanosecond laser pulses with 1011-1020-W/cm2 intensity on both conductive and dielectric targets. These fields typically cause saturation and damage to electronic equipment inside and near an experimental chamber; nevertheless, they can also be effectively used as diagnostic tools. Accordingly, the characterization of electromagnetic pulses (EMPs) is extremely important and currently a popular topic for present and future laser facilities intended for laser-matter interaction. The picosecond and sub-picosecond laser pulses are considerably shorter than the characteristic electron discharge time (∼0.1 ns) and can be efficient in generating GHz EMPs. The EMP characterization study of femtosecond laser-driven solid targets is currently mainly in the order of 100 mJ laser energy, in this study, the EMP generated by intense (Joule class) femtosecond laser irradiation of solid targets has been measured as a function of laser energy, laser pulse duration, focal spot size, and target materials. And a maximum electric field of the EMP reaching up to 105 V/m was measured. Analyses of experimental results confirm a direct correlation between measured EMP energy and laser parameters in the ultrashort pulse duration regime. The EMP signals generated by femtosecond laser irradiation of solid targets mainly originate from the return current inside the target after hot electron excitation. Numerical simulations of EMP are performed according to the target charging model, which agree well with the experimental results.

Robust Bi-anchoring carbon dot/BiOCl sheet heterojunction photocatalysts toward superior photocatalytic activity.

BiOCl has attracted much attention due to its robust layered structure, excellent photocatalytic activity and nontoxicity. However, its practical application is hindered by its narrowband UV photoresponse and rapid recombination of photocarriers. Herein, zero-dimensional Bi-anchoring carbon quantum dot (Bi-CD)/two-dimensional BiOCl heterojunction (Bi-CD/BiOCl) photocatalysts are designed and synthesized by a facile hydrothermal method. Under 190-1100 nm broadband light irradiation, the optimized Bi-CD/BiOCl sample exhibits a superb rhodamine B (RhB) degradation rate of nearly 100%, which is 2.3 (1.7) times that of pristine BiOCl (CD/BiOCl). Additionally, the optimized sample exhibits an RhB degradation rate of up to 88.1% even under direct outdoor light and robust durability in water solution. Experimental results combined with DFT calculations reveal that the superior photocatalytic activity arises from the synergetic effects of broader light absorption due to the incorporation of CD, extra hot electron excitation by the localized surface plasmon resonance (LSPR) effect of metallic Bi, and enhanced electron transfer across the heterojunction interface as well as the existence of more oxygen vacancy traps in BiOCl. This work gives insights into the structure and photocatalytic properties of Bi-CD/BiOCl and provides a new strategy for the design and fabrication of robust high-performance photocatalysts under wide spectrum light irradiation.

Hot‐Electron Driven Ultrafast Optical Polarization Conversion with Graphene‐Loaded Metasurface

AbstractThe switching speed of light polarization plays a crucial role in determining the upper‐limit bandwidth of applications like optical communications and laser microscopy. However, conventional polarization elements based on macroscopic anisotropic crystals like birefringent crystals and chalcogenide glasses are either static or restricted by the low switching time of about hundreds of picoseconds. Here, a femtosecond‐scale all‐optical polarization controlling method is proposed through engineering the excited hot electrons in a graphene‐loaded metasurface. Remarkably, a giant polarization orientation from left‐handed polarization (LCP) to right‐handed polarization (RCP) in the Poincaré sphere (≈80° rotation) is realized within only 200 fs in the mid‐infrared. With pumping, the dedicated polarization‐sensitive design allows the metasurface to exhibit a consistent resonance blueshift as the transient increase of the hot‐electron temperature in graphene for y‐polarized incidence. This polarization conversion approach features a giant modulation range and enables the reflected light to be dynamically and arbitrarily modulated into RCP, LCP, and linear states at femtosecond timescale. A few logic operations “AND”, “OR”, “NAND”, and “XOR” based on this method are also demonstrated by monitoring the normalized Stokes parameters. It is believed that this work may find practical application in next‐generation signal‐processing systems with large capacity.

Optoelectronic tuning of plasmon resonances via optically modulated hot electrons.

Fast optical modulation of nanoplasmonics is fundamental for on-chip integration of all-optical devices. Although various strategies have been proposed for dynamic modulation of surface plasmons, critical issues of device compatibility and extremely low efficiency in the visible spectrum hamper the application of optoplasmonic nanochips. Here we establish an optoplasmonic system based on Au@Cu2-xS hybrid core-shell nanoparticles. The optical excitation of hot electrons and their charge transfer to the semiconductor coating (Cu2-xS) lead to lowered electron density of Au, which results in the red shift of the localized surface plasmon resonance. The hot electrons can also transport through the Cu2-xS layer to the metal substrate, which increases the conductance of the nanogap. As such, the coupled gap plasmon blue-shifts with a magnitude of up to ∼15nm, depending on the excitation power and the thickness of the coatings, which agrees with numerical simulations. All of this optoelectronic tuning process is highly reversible, controllable and fast with a modulated laser beam, which is highly compatible and sufficiently useful for on-chip integration of nanophotonic devices.

Efficient electroluminescence in doped-GaAs via terahertz metamaterials

We investigate the highly efficient terahertz nonlinearity exhibited by n-type GaAs crystals under metallic metamaterials. An intense THz field applied to the metamaterials leads to impact ionization in the GaAs substrate, which emits electroluminescence in the near-infrared region. Even for a similar THz field strength, n-type GaAs emits near-infrared photons more efficiently than semi-insulating GaAs. We analyzed the luminescence lineshapes and intensity as a function of the excitation field strength, using Fermi–Dirac statistics and the density of states in the conduction band to quantify electron density and locate the Fermi level after the relaxation of excited hot electrons.

Spiky surface topography of heterostructured nanoparticles for programmable acceleration of multistage wound healing

Wound healing is a complex and orchestrated physiological process that involves multiple stages, including hemostasis, inflammation, proliferation, and remodeling stages. Various nanomaterials have been developed to improve the wound healing; however, most of them can only promote an individual stage of this intricate process, lacking hierarchical acceleration for multiple stages. In the present study, spiky gold-palladium heterostructured nanoparticles (AuPd SHs) were designed to possess topographical architectures on the surface of nanoparticles, capable of promoting multistage wound healing in a programmable manner. First, the spiky surface topography of AuPd SHs exhibited the potent nanobridge effect for rapid wound closure, promoting the hemostasis stage. Second, the heterostructures of AuPd SHs realized visible-light-mediated hot electron excitation and electron–hole spatial separation between Au cores and Pd spikes to efficiently generate reactive oxygen species, beneficial for the inflammation stage. Third, the spiky surface topography of AuPd SHs triggered macrophage polarization from M1 to M2 phenotype, promoting the proliferation and maturation stages. Spiky AuPd SHs with simple composition and compact structures exhibit hierarchical acceleration on multiple stages of wound healing.

Coupling doping and localized surface plasmon resonance toward acidic pH-preferential catalase-like nanozyme for oxygen-dominated synergistic cancer therapy

Hypoxia increases patient treatment resistance and favors cancer progression. Nanozymes have received unprecedented attention in alleviating hypoxia via catalase (CAT)-mimicking catalytic reaction. However, the oxygen production capability of CAT-like nanozymes undergoes the deficiency and even absolute loss in acidic tumor microenvironment. Herein, we propose a doping-localized surface plasmon resonance (LSPR) coupling strategy to shatter the acidic pH limitation and reinforce the catalytic activity of CAT-like nanozyme for oxygen-dominated synergistic cancer therapy. Platinum-doped plasmonic gold nanostar modified with glucose oxidase (Pt-AuNS-GOx) is constructed as a proof-of-concept. Density functional theory calculations reveal the *O-assisted and *OH-assisted CAT-like reaction paths in Pt-AuNS-GOx, and demonstrate that Pt doping-induced energy barrier reduction accounts for the oxygen generation in acidity. Furthermore, the CAT-like reaction kinetics of Pt-AuNS-GOx can be significantly enhanced upon plasmon irradiation resulted from the excited hot electrons and local heating accompanied with LSPR decay. Such outstanding acidic pH-preferential oxygen generation capability of Pt-AuNS-GOx significantly boosts the efficiency of synergistic tumor aerobic therapeutics of glutathione oxidation-medicated oxidative stress and GOx-activated starvation in vitro. Remarkably, Pt-AuNS-GOx substantially reverses the hypoxic microenvironment of tumor tissue, and enables greatly accelerated apoptosis and suppressed metastasis of cancer in vivo, possessing conspicuous therapeutic efficiency. The proposed doping-LSPR coupling strategy offers a powerful modality to modulate the restricted reaction pH of CAT-like nanozymes, and provides a paradigm shift for hypoxia alleviation-boosted cancer synergistic treatment.

Nanowatt-Level Photoactivated Gas Sensor Based on Fully-Integrated Visible MicroLED and Plasmonic Nanomaterials.

Photoactivated gas sensors that are fully integrated with micro light-emitting diodes (µLED) have shown great potential to substitute conventional micro/nano-electromechanical (M/NEMS) gas sensors owing to their low power consumption, high mechanical stability, and mass-producibility. Previous photoactivated gas sensors mostly have utilized ultra-violet (UV) light (250-400nm) for activating high-bandgap metal oxides, although energy conversion efficiencies of gallium nitride (GaN)LEDs are maximized in the blue range (430-470nm). This study presents a more advanced monolithic photoactivated gas sensor based on a nanowatt-level, ultra-low-power blue (λpeak = 435 nm) µLED platform (µLP). To promote the blue light absorbance of the sensing material, plasmonic silver (Ag) nanoparticles (NPs) are uniformly coated on porous indium oxide (In2 O3 ) thin films. By the plasmonic effect, Ag NPs absorb the blue light and spontaneously transfer excited hot electrons to the surface of In2 O3 . Consequently, high external quantum efficiency (EQE, ≈17.3%) and sensor response (ΔR/R0 (%) = 1319%) to 1ppm NO2 gas can be achieved with a small power consumption of 63nW. Therefore, it is highly expected to realize various practical applications of mobile gas sensors such as personal environmental monitoring devices, smart factories, farms, and home appliances.

How Hot Electron Generation at the Solid-Liquid Interface Is Different from the Solid-Gas Interface.

Excitation of hot electrons by energy dissipation under exothermic chemical reactions on metal catalyst surfaces occurs at both solid-gas and solid-liquid interfaces. Despite extensive studies, a comparative operando study directly comparing electronic excitation by electronically nonadiabatic interactions at solid-gas and solid-liquid interfaces has not been reported. Herein, on the basis of our in situ techniques for monitoring energy dissipation as a chemicurrent using a Pt/n-Si nanodiode sensor, we observed the generation of hot electrons in both gas and liquid phases during H2O2 decomposition. As a result of comparing the current signal and oxygen evolution rate in the two phases, surprisingly, the efficiency of reaction-induced excitation of hot electrons increased by ∼100 times at the solid-liquid interface compared to the solid-gas interface. The boost of hot electron excitation in the liquid phase is due to the presence of an ionic layer lowering the potential barrier at the junction for transferring hot electrons.

Basic aspects of gold nanoparticle photo-functionalization using oxides and 2D materials: Control of light confinement, heat-generation, and charge separation in nanospace.

Although the optical properties of localized surface plasmon resonance and the relaxation processes of excited hot electrons in gold nanoparticles (AuNPs) have been well understood, the phenomena that occur when AuNPs relax on solid surfaces of semiconductors or insulators remain largely unknown. Thermal energy diffusion and electron transfer are relatively simple physical processes, but the phenomena they induce are interesting because of a variety of new application developments. In this Perspective, we introduce the fundamental aspects as well as advanced applications of several new physical phenomena induced by AuNPs-based hybrid materials with oxides or 2D materials. Localized heat can induce a great force on the surrounding medium to control mass transport, and plasmon-induced charge transfer reactions are expected to have applications in photocatalysis and solar cells. We also review increasing reports on the development of nano-optical sensors, transistors, and nano-light sources based on precisely controlled device structures utilizing AuNPs.

Hot Electron Phenomena at Solid-Liquid Interfaces.

Understanding the role of energy dissipation and charge transfer under exothermic chemical reactions on metal catalyst surfaces is important for elucidating the fundamental phenomena at solid-gas and solid-liquid interfaces. Recently, many surface chemistry studies have been conducted on the solid-liquid interface, so correlating electronic excitation in the liquid-phase with the reaction mechanism plays a crucial role in heterogeneous catalysis. In this review, we introduce the detection principle of electron transfer at the solid-liquid interface by developing cutting-edge technologies with metal-semiconductor Schottky nanodiodes. The kinetics of hot electron excitation are well correlated with the reaction rates, demonstrating that the operando method for understanding nonadiabatic interactions is helpful in studying the reaction mechanism of surface molecular processes. In addition to the detection of hot electrons excited by a catalytic reaction, we highlight recent results on how the transfer of the hot electrons influences surface chemical and photoelectrochemical reactions.

Oxygen vacancy-regulated metallic semiconductor MoO2 nanobelt photoelectron and hot electron self-coupling for photocatalytic CO2 reduction in pure water

This communication describes metallic MoO2−x nanobelts intrinsic photocatalytic CO2 reduction performance in pure water. The intrinsic wide-bandgap endowed MoO2 nanobelts great potential for CO2 reduction in ultraviolet, while localized surface plasmonic resonance (LSPR) and the metallic feature induced by oxygen vacancies ensured energetic electrons transform and fast kinetic transfer. Credit to the unique degenerate doping properties of MoO2−x nanobelts, the absorbed wavelength of intrinsic semiconductor bandgap overlapped with the LSPR absorption. Therefore, the high performance was ascribed to the self-coupling of intrinsic excited photoelectrons and plasmonic hot electrons. The surface abundant oxygen vacancies also facilitated preferable adsorption of CO2 and hence promoted the chemical activation. The according intrinsic CO production rate was 62.75 µmol/g/h in pure water under ultraviolet light, which was 6 times higher than that of commercial P25 photocatalyst (11.13 µmol/g/h). Our findings provide new insights and inspirations for parsing the mechanism and developing new-type MoO2-based photocatalysts.

Non-Adiabatic Dynamics Simulation of Plasmon-Mediated Chemistry

Localized surface plasmon resonances (LSPRs) have attracted much recent attention for their potential in promoting chemical reactions with light. However, the mechanism of LSPR-induced chemical reactions is still not clear and suffers from many controversies. This presentation will discuss the atomic-scale mechanism of plasmonic hot-carrier mediated chemical reaction exampled by H2 dissociation by employing time-dependent density functional calculations theory and non-adiabatic molecular dynamics. The key observation is that there are nested excited states corresponding to both hot-electron excitation and charge transfer [1]. These nested states cross, facilitating the transitions depicted in the desorption induced by the electronic transitions model and the surface hopping model. I believe this is the first time that such a connection has been made based on a first-principles calculation. Diabatization of these states shows that the charge transfer states are responsible for H2 dissociation, while the hot electron states do not. Previous works only identified the hot electron states, thus were unable to explain the dissociation in a convincing way. Moreover, we also found chemical reaction is tunable if the molecule is placed in the center of the plasmonic dimer [2]. The reaction rate can be either suppressed or enhanced depending on the geometry.[1] Q. Wu, L. Zhou, G. C. Schatz, Y. Zhang*, H. Guo*, J. Am. Chem. Soc. 2020, 142, 13090–13101.[2] Y. Zhang*, T. Nelson, S. Tretiak, H. Guo, G. C. Schatz. ACS Nano, 2018, 12, 8415-8422.

Bifacial omnidirectional and band-tunable light absorption in free-standing core–shell resonators

Effective optical absorption is highly desirable for numerous applications in energy harvesting and optoelectronics. Bifacial absorbers can significantly enhance light absorption by capturing albedo light from the environment. Here, we experimentally demonstrate that free-standing silica-silver core–shell nano-resonator arrays allow bifacial and omnidirectional optical absorption across the visible spectrum. Specifically, resonator arrays can highly absorb light (&amp;gt;80%) with all polarizations from a directional range (−40° to 40°) on both front and rear sides of a surface. Numerical simulations reveal that such bifacial and omnidirectional light absorption results from hybridized excitation of surface plasmons and whispering gallery modes in a symmetrical configuration. The absorption band can be flexibly adjusted by changing the silica core size. In addition, the absorbed optical energy quickly decays as the excitation of plasmonic hot electrons as observed using transient absorption spectroscopy. Our work provides a bifacial absorber for many optoelectronic applications in photodetection, photovoltaics, and photocatalysis.