Hot Electron Generation Research Articles

Promoting reactivity of photoexcited hot electrons in small-sized plasmonic metal nanoparticles that are supported on dielectric nanospheres.

Plasmonic metal nanoparticles (NPs) represent a promising class of photocatalysts to drive chemical transformations by the photoexcited hot electrons in the NPs. In this work, the dependence of photon-to-chemical conversion efficiency on the size of plasmonic silver nanoparticles (Ag NPs) has been comprehensively studied with the use of the photocatalytic degradation of methylene blue as a probe reaction. Comparison of Ag NPs with two different sizes (6 nm and 13 nm in diameter) highlights that the smaller sized Ag NPs favor the photocatalytic activity by positively translating the high efficiency of hot electron generation to the hot-electron-driven chemical reaction on the surface of the Ag NPs. Loading the small Ag NPs to the dielectric silica nanospheres (SiOX NSs, average diameter of 400 nm) with high surface coverage increases the light absorption power in the Ag NPs due to the surface light scattering resonances of the SiOX NSs and interparticle plasmon coupling of the adjacent Ag NPs. The enhanced light absorption can also be rendered to the improved photocatalytic activity. This design principle of plasmonic photocatalysts provides a promise of utilizing solar energy to drive desirable chemical reactions with high photon-to-chemical conversion efficiency.

Synthesis and characterization of Bi-BiPO4 nanocomposites as plasmonic photocatalysts for oxidative NO removal

Bi metal–BiPO4 (Bi-BPO) nanocomposites formed by in situ solvothermal reduction were employed as plasmonic photocatalysts for oxidative NO removal, achieving a removal efficiency of 32.8% in a continuous NO flow (400 ppb) under illumination with visible light. This high performance was ascribed to the generation of energetic hot electrons (and their subsequent surface chemical reactions) due to the surface plasmon resonance (SPR) of Bi metal, as validated by numerical simulations. The combined results of density functional theory (DFT) calculations and electrochemical analysis revealed that hot electrons are transferred from Bi metal to BPO via the Bi-BPO interface. DFT calculations further showed that enhanced O2 activation on the Bi-BPO interface facilitates the generation of both superoxide (O2−) and hydroxyl (OH) radicals, as confirmed by electron spin resonance, while in situ DRIFTS analysis demonstrated that NO is activated on the Bi-BPO interface and then oxidized to nitrates. Thus, this work highlights the SPR effects of Bi metal and promoted O2 and NO activation in plasmonic photocatalysis, showing that the adopted approach can be generalized to design efficient and cost-effective photocatalytic systems for the removal of other gaseous pollutants.

Dark plasmon modes for efficient hot electron generation in multilayers of gold nanoparticles.

The excitation of dark plasmons, i.e., coupled plasmon modes with a vanishing net dipole, is expected to favor Landau damping over radiative damping. Dark plasmon excitation might, therefore, lead to an increased absorption of energy within gold nanoparticles, resulting in a strong generation of hot electrons compared to the generation via bright plasmons. We performed transient-absorption spectroscopy on gold nanoparticle films to assess the initial electronic temperature before thermalization. We observe a significant increase in the electron-phonon coupling time when dark plasmon modes are excited in these films. The results indicate an efficient energy absorption due to the suppressed radiative decay of dark plasmon modes and a subsequent energy transformation into hot electrons.

Hot Electrons Generated in Chiral Plasmonic Nanocrystals as a Mechanism for Surface Photochemistry and Chiral Growth.

The realization of chiral photochemical reactions at the molecular level has proven to be a challenging task, with invariably low efficiencies originating from very small optical circular dichroism signals. On the contrary, colloidal nanocrystals offer a very large differential response to circularly polarized light when designed with chiral geometries. We propose taking advantage of this capability, introducing a novel mechanism driving surface photochemistry in a chiral nanocrystal. Plasmonic nanocrystals exhibit anomalously large asymmetry factors in optical circular dichroism (CD), and the related hot-electron generation shows in turn a very strong asymmetry, serving as a mechanism for chiral growth. Through theoretical modeling, we show that chiral plasmonic nanocrystals can enable chiral surface growth based on the generation of energetic (hot) electrons. Using simple and realistic phenomenological models, we illustrate how this kind of surface photochemistry can be observed experimentally. The proposed mechanism is efficient if it operates on an already strongly chiral nanocrystal, whereas our proposed mechanism does not show chiral growth for initially nonchiral structures in a solution. The asymmetry factors for the chiral effects, driven by hot electrons, exceed the values observed in chiral molecular photophysics at least 10-fold. The proposed chiral-growth mechanism for the transformation of plasmonic colloids is fundamentally different to the traditional schemes of chiral photochemistry at the molecular level.

Experimental study of hot electron generation in shock ignition relevant high-intensity regime with large scale hot plasmas

In the shock ignition (SI) laser fusion scheme, hot electrons generated by the laser spike pulse can either preheat the fuel or strengthen the ignition shock, depending on the hot electron characteristics. We conducted a planar target experiment on the OMEGA-EP laser facility and characterized the temperature and total energy of hot electrons generated from a kilojoule-class 100-ps infrared (IR) or a 1-ns ultraviolet (UV) laser interacting with a large (Ln∼330−450 μm) and hot (Te∼1−2 keV) coronal plasma at the SI-relevant intensities (∼1016 W/cm2). The IR laser converts ∼2.5% energy into hot electrons with Thot∼ 60–90 keV, while the UV laser couples 0.8% ± 0.7% energy into Thot=27 ± 9 keV hot electrons. The IR-produced hot electrons yield five times higher Cu Kα emission than the UV case, confirming the higher electron conversion efficiency with the IR laser. The low energy conversion from the UV laser to hot electrons may be due to the refraction of the off-normal incident laser in the large coronal plasma. These findings are the first comparisons of hot electron generation between the IR and UV pulses at kilojoule scales in SI-relevant large-scale plasmas. The findings may expand the SI design space to include IR lasers as the possible spike lasers.

Direct Evidence of an Efficient Plasmon-Induced Hot-Electron Transfer at an in Situ Grown Ag/TiO2 Interface for Highly Enhanced Solar H2 Generation

Plasmon-induced hot-electron generation and its efficient transfer to the conduction band (CB) of neighboring metal oxides is an effective route to solar energy harvesting. However, until now, this...

Enhancement of laser-driven ion acceleration in non-periodic nanostructured targets

Using particle-in-cell simulations, we demonstrate an improvement of the target-normal-sheath acceleration (TNSA) of protons in non-periodically nanostructured targets with micron-scale thickness. Compared to standard flat foils, an increase in the proton cutoff energy by up to a factor of two is observed in foils coated with nanocones or perforated with nanoholes. The latter nano-perforated foils yield the highest enhancement, which we show to be robust over a broad range of foil thicknesses and hole diameters. The improvement of TNSA performance results from more efficient hot-electron generation, caused by a more complex laser–electron interaction geometry and increased effective interaction area and duration. We show that TNSA is optimized for a nanohole distribution of relatively low areal density and that is not required to be periodic, thus relaxing the manufacturing constraints.

Au–Ag alloy nanoparticles with tunable cavity for plasmon-enhanced photocatalytic H2 evolution

Au–Ag alloy nanoparticles with different cavity sizes have great potential for improving photocatalytic performance due to their tunable plasmon effect. In this study, galvanic replacement was combined with co-reduction with the reaction kinetics processes regulated to rapidly synthesize Au–Ag hollow alloy nanoparticles with tunable cavity sizes. The position of the localized surface plasmon resonance (LSPR) peak could be effectively adjusted between 490 nm and 713 nm by decreasing the cavity size of the Au–Ag hollow nanoparticles from 35 nm to 20 nm. The plasmon-enhanced photocatalytic H 2 evolution of alloy nanoparticles with different cavity sizes was investigated. Compared with pure P25 (TiO 2 ), intact and thin-shelled Au–Ag hollow nanoparticles (HNPs)-supported photocatalyst exhibited an increase in the photocatalytic H 2 evolution rate from 0.48 µmol h −1 to 4 µmol h −1 under full-spectrum irradiation. This improved photocatalytic performance was likely due to the plasmon-induced electromagnetic field effect, which caused strong photogenerated charge separation, rather than the generation of hot electrons. Au–Ag hollow alloy nanoparticles with tunable cavity sizes were constructed by regulating reaction kinetics processes and by combining galvanic replacement with co-reduction. This hollow alloy nanoparticle dramatically enhance the photocatalytic performance.

Quantitative Understanding of Charge-Transfer-Mediated Fe3+ Sensing and Fast Photoresponse by N-Doped Graphene Quantum Dots Decorated on Plasmonic Au Nanoparticles.

The formation of a heterostructure with plasmonic nanoparticles drastically alters the optoelectronic properties of graphene quantum dots (GQDs), resulting in exceptional properties. In the present work, we prepare nitrogen-doped GQDs decorated on gold nanoparticles (Au@N-GQDs) by a one-step green reduction method and study its extraordinary fluorescence and photoresponse characteristics. The as-prepared Au@N-GQDs show more than one order of magnitude enhancement in the fluorescence intensity as compared to the bare N-GQDs, which is attributed to hot electron generation and improved absorption in N-GQDs by local field enhancement and the modification of the edge functional groups. Because of the selective coordination to Fe3+ ions, the Au@N-GQDs exhibit extraordinary quenching of fluorescence, with ultrahigh sensitivity for the detection of Fe3+ (<1 nM). A new model for the charge-transfer dynamics is developed involving the Langmuir's law of adsorption to explain the unusual quenching, which strongly deviates from the known models of static/dynamic quenching. The proposed sensor is successfully implemented for the ultrasensitive detection of Fe3+ ions in human serum and Brahmaputra river water samples, representing its high potential applications in clinical as well as environmental diagnosis. Additionally, because of its high absorption in the UV-vis-NIR region and high charge density with long life excitons, the Au@N-GQDs are utilized as photodetectors with ∼104 times faster response than that of bare N-GQDs. The Au@N-GQD-based photodetector possesses a high responsivity of ∼1.36 A/W and a remarkably high external quantum efficiency of ∼292.2%, which is much superior to the GQD-based photodetectors reported till date. The underlying mechanism of ultrafast photoresponse is ascribed to the transfer of hot electrons along with the tunneling of the electrons from Au NPs to N-GQDs as well as the defect reduction of N-GQDs by the incorporation of Au NPs. Without the use of any charge transporting layer, the outstanding performance of N-GQD-based plasmonic photodetector opens up unique opportunities for future high-speed optoelectronic devices.

Hot Electron Generation in Silicon Micropyramids Covered with Nanometer-Thick Gold Films for Near-Infrared Photodetectors

Plasmonic metallic nanostructure/silicon composites are gaining increasing attention due to their NIR photodetection capability and potential compatibility with CMOS technology. However, those kind...

Nitrogen vacancy mediated exciton dissociation in carbon nitride nanosheets: Enhanced hydroxyl radicals generation for efficient photocatalytic degradation of organic pollutants

Polymeric materials are promising candidates as photocatalysts for environmental purification, however their catalytic performance are still unsatisfactory mainly due to the strong Coulomb interactions between electron and hole that leads to fast charge recombination. Herein, taking graphitic carbon nitride as an example, we verify that installing carbon nitride nanosheets with nitrogen vacancy could break the intrinsic electronic state distribution, forming energy disordered interfaces around the vacancies with the energy difference as large as 0.35 eV. Such a large energy difference is found energetic enough to overcome the strong Coulomb interactions between electron and hole for hot electron and hole generation, as a result showing high electron-hole separation efficiency. Benefited from these advantages, the as prepared material shows remarkable photocatalytic performance toward organic pollutants degradation. The improved catalytic performance is originated from the promoted exciton dissociation that leads to ultra high hydroxyl radical generation. This study offers a new understanding of the excitonic effects for designing advanced polymeric photocatalyst for energy and environment related applications.

Photocatalytic hot-carrier chemistry

Abstract

Analysis of electromagnetic pulses generation from laser coupling with polymer targets: Effect of metal content in target

Powerful lasers interacting with solid targets can generate intense electromagnetic pulses (EMPs). In this study, EMPs produced by a pulsed laser (1 ps, 100 J) shooting at CH targets doped with different titanium (Ti) contents at the XG-III laser facility are measured and analyzed. The results demonstrate that the intensity of EMPs first increases with Ti doping content from 1% to 7% and then decreases. The electron spectra show that EMP emission is closely related to the hot electrons ejected from the target surface, which is confirmed by an analysis based on the target–holder–ground equivalent antenna model. The conclusions of this study provide a new approach to achieve tunable EMP radiation by adjusting the metal content of solid targets, and will also help in understanding the mechanism of EMP generation and ejection of hot electrons during laser coupling with targets.

Nonequilibrium hot-electron-induced wavelength-tunable incandescent-type light sources

The collective oscillation of electrons located in the conduction band of metal nanostructures being still energized, with the energy up to the bulk plasmon frequency, are called nonequilibrium hot electrons. It can lead to the state-filling effect in the energy band of the neighboring semiconductor. Here, we report on the incandescent-type light source composed of Au nanorods decorated with single Ga-doped ZnO microwire (AuNRs@ZnO:Ga MW). Benefiting from Au nanorods with controlled aspect ratio, wavelength-tunable incandescent-type lighting was achieved, with the dominating emission peaks tuning from visible to near-infrared spectral regions. The intrinsic mechanism was found that tunable nonequilibrium distribution of hot electrons in ZnO:Ga MW, injected from Au nanorods, can be responsible for the tuning emission features. Apart from the modification over the composition, bandgap engineering, doping level, etc., the realization of electrically driving the generation and injection of nonequilibrium hot electrons from single ZnO:Ga MW with Au nanostructure coating may provide a promising platform to construct electronics and optoelectronics devices, such as electric spasers and hot-carrier-induced tunneling diodes.

Plasmon‐Enhanced Photoluminescence and Photocatalysis Reactions in Metal‐Semiconductor Nanomaterials: UV‐Generated Hot Electron in Gold‐Zinc Oxide

AbstractHerein, we introduce a mechanistic study to design a hybrid junction in metallic‐semiconductor (M/SC) nanostructures. UV‐light‐induced hot electron generation in ZnO nanostructures is used to precisely tune the photoluminescence (PL) and photocatalytic (PC) properties in hybrid Au/ZnO nanomaterials. Both enhancement and quenching of the PL and PC functionalities are observed, depending on the properties of the Au nanoparticles (AuNPs) and the Au/ZnO molecular distance. Under UV irradiation free‐ligand AuNPs quench the luminescence of ZnONPs through direct charge transfer (CT) from ZnO to the AuNPs. In contrast, capped AuNPs enhance the ZnO emission through indirect CT from AuNPs to ZnONPs facilitated by the distance created by the CTAB ligand between both constituents of the hybrid systems. It is necessary to optimise the Au/ZnO molecular distance to achieve an enhancement of both the plasmonic photocatalysis reaction and photoelectric properties of M/SC nanostructures. This phenomena is mediated by the energy transfer (ET) from ZnONPs to AuNPs. The resulting PL enhancement is described by the plasmon‐induced resonance energy transfer effect (PIRET effect).

Low-voltage DC electroluminescent device using semiconductive band barriers with Cu2O

To improve the efficiency of a DC Electroluminescent (DC-EL) device, we investigated the reduction of the driving voltage while maintaining the stability of the DC-EL device in this study. Mechanisms for both the current control and hot-electron generation under low voltage were embedded in the device. Usage of a p-Cu2O thick film with high-performance electrical properties for the formation of two semiconductive band barriers in series achieved these functions. The n-ZnO/p-Cu2O controlled current, and the p-Cu2O/n-ZnS:Mn generated hot electrons. The electrical characteristics of each junction demonstrated those of the p–n junction, and the driving voltage was divided into both the junctions. 98% of the driving voltage was applied to the hot-electron generation junction. Electroluminescence was obtained at approximately 20 V, because the local electric field at the junction accelerated electrons. Although the luminance was insufficient, it has been inferred that such a device structure is effective in decreasing driving voltage.

Characterization of laser-driven proton acceleration from water microdroplets

We report on a proton acceleration experiment in which high-intensity laser pulses with a wavelength of 0.4 μm and with varying temporal intensity contrast have been used to irradiate water droplets of 20 μm diameter. Such droplets are a reliable and easy-to-implement type of target for proton acceleration experiments with the potential to be used at very high repetition rates. We have investigated the influence of the laser’s angle of incidence by moving the droplet along the laser polarization axis. This position, which is coupled with the angle of incidence, has a crucial impact on the maximum proton energy. Central irradiation leads to an inefficient coupling of the laser energy into hot electrons, resulting in a low maximum proton energy. The introduction of a controlled pre-pulse produces an enhancement of hot electron generation in this geometry and therefore higher proton energies. However, two-dimensional particle-in-cell simulations support our experimental results confirming, that even slightly higher proton energies are achieved under grazing laser incidence when no additional pre-plasma is present. Illuminating a droplet under grazing incidence generates a stream of hot electrons that flows along the droplet’s surface due to self-generated electric and magnetic fields and ultimately generates a strong electric field responsible for proton acceleration. The interaction conditions were monitored with the help of an ultra-short optical probe laser, with which the plasma expansion could be observed.

Plasmonic Metamaterials for Nanochemistry and Sensing.

Plasmonic nanostructures were initially developed for sensing and nanophotonic applications but, recently, have shown great promise in chemistry, optoelectronics, and nonlinear optics. While smooth plasmonic films, supporting surface plasmon polaritons, and individual nanostructures, featuring localized surface plasmons, are easy to fabricate and use, the assemblies of nanostructures in optical antennas and metamaterials provide many additional advantages related to the engineering of the mode structure (and thus, optical resonances in the given spectral range), field enhancement, and local density of optical states required to control electronic and photonic interactions. Focusing on two of the many applications of plasmonic metamaterials, in this Account, we review our work on the sensing and nanochemistry applications of metamaterials based on the assemblies of plasmonic nanorods under optical, as well as electronic interrogation. Sensors are widely employed in modern technology for the detection of events or changes in their local environment. Compared to their electronic counterparts, optical sensors offer a combination of high sensitivity, fast response, immunity to electromagnetic interference, and provide additional options for signal retrieval, such as optical intensity, spectrum, phase, and polarization. Owing to the ability to confine and enhance electromagnetic fields on subwavelength scales, plasmonics has been attracting increasing attention for the development of optical sensors with advantages including both nanometer-scale spatial resolution and single-molecule sensitivity. Inherent hot-electron generation in plasmonic nanostructures under illumination or during electron tunneling in the electrically biased nanostructures provides further opportunities for sensing and stimulation of chemical reactions, which would otherwise not be energetically possible. We first provide a brief introduction to a metamaterial sensing platform based on arrays of strongly coupled plasmonic nanorods. Several prototypical sensing examples based on this versatile metamaterial platform are presented. Record-high refractive index sensitivity of gold nanorod arrays in biosensing based on the functionalization of the nanorod surface for selective absorption arises because of the modification of the electromagnetic coupling between the nanorods in the array. The capabilities of nanorod metamaterials for ultrasound and hydrogen sensing were demonstrated by precision coating of the nanorods with functional materials to create core-shell nanostructures. The extension of this metamaterial platform to nanotube and nanocavity arrays, and metaparticles provides additional flexibility and removes restrictions on the illumination configurations for the optical interrogation. We then discuss a nanochemical platform based on the electrically driven metamaterials to stimulate and detect chemical reactions in the tunnel junctions constructed with the nanorods by exploiting elastic tunneling for the activation of chemical reactions via generated hot-electrons and inelastic tunneling for the excitation of plasmons facilitating optical monitoring of the process. This represents a new paradigm merging electronics, plasmonics, photonics and chemistry at the nanoscale, and creates opportunities for a variety of practical applications, such as hot-electron-driven nanoreactors and high-sensitivity sensors, as well as nanoscale light sources and modulators. With a combination of merits, such as the ability to simultaneously support both localized and propagating modes, nanoporous texture, rapid and facile functionalization, and low cost and scalability, plasmonic nanorod metamaterials provide an attractive and versatile platform for the development of optical sensors and nanochemical platforms using hot-electrons with high performance for applications in fundamental research and chemical and pharmaceutical industries.

Generation of Hot Electrons with Chiral Metamaterial Perfect Absorbers: Giant Optical Chirality for Polarization-Sensitive Photochemistry

Chiral plasmonic metamaterials have shown very interesting possibilities as chiral optical absorbers for circularly polarized light detection, as their optical response can be manipulated through t...

Synthesis of plasmonic gold nanoparticles supported on morphology-controlled TiO2 for aerobic alcohol oxidation

Abstract In the present research study, we report the synthesis of nano-sized gold (Au) particles immobilized on TiO2 support material. The morphologies of TiO2 support material was tailored and tuned to be rod, tube and microporous materials. The effect of plasmonic gold nanoparticles (NPs) deposited on different morphologies of TiO2 was studied. The prepared catalysts were characterized by various characterization techniques in order to have a detailed insight on the textural properties, morphology and electronic state of the material via UV–vis spectroscopy, N2 physisorption analysis, Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). We tested the controlled oxidation of benzyl alcohol to benzaldehyde using molecular oxygen at 373 K under atmospheric pressure. The results obtained from the thermal reaction conditions were compared with those performed under visible light irradiation (λ > 420 nm) at 373 K. Among all, Au/TiO2 nanorod catalyst displayed the superior performance of 33% yield in the absence of light and 55% yield of benzaldehyde with an external quantum efficiency of 3.4% under visible light irradiation (λ ≥ 420 nm) at 373 K for 4 h. The enhanced catalytic performances were attributed to the Au-LSPR phenomenon and a plausible reaction pathway has been discussed involving the generation of hot electrons during the course of reaction.