Contribution Of Hot Electrons Research Articles

Thermal-effect dominated plasmonic catalysis on silver nanoislands.

Plasmonic metal nanostructures with the intrinsic property of localized surface plasmon resonance can effectively promote energy conversion in many applications such as photocatalysis, photothermal therapy, seawater desalinization, etc. It is known that not only are plasmonically excited hot electrons generated from metal nanostructures under light irradiation, which can effectively trigger chemical reactions, but also plasmonically induced heating simultaneously occurs. Although plasmonic catalysis has been widely explored in recent years, the underlying mechanisms for distinguishing the contribution of hot electrons from thermal effects are not fully understood. Here, a simple and efficient self-assembly system using silver nanoislands as plasmonic substrates is designed to investigate the photo-induced azo coupling reaction of nitro- and amino-groups at various temperatures. In the experiments, surface-enhanced Raman spectroscopy is employed to monitor the time and temperature dependence of plasmon-induced catalytic reactions. It was found that a combination of hot electrons and thermal effects contribute to the reactivity. The thermal effects play the dominant role in the plasmon-induced azo coupling reaction of nitro-groups, which suggests that the localized temperature must be considered in the development of photonic applications based on plasmonic nanomaterials.

Maxwellian and non-Maxwellian rate coefficients of the tungsten ions: W46+–W55+

This study focuses on the significance of suprathermal (‘hot’) electrons in the tokamak device. Hot electrons, which follow a non-Maxwellian energy distribution, are high-energy electrons that exert a substantial influence on various processes taking place within the plasma. Our aim was to investigate the influence of non-Maxwellian distribution on the rate coefficients of highly charged tungsten ions. This paper presents Maxwellian and non-Maxwellian electron impact ionization rate coefficients for W46+ to W55+ ions. The cross sections were calculated using the fully relativistic flexible atomic code with level-to-level distorted-wave method. We found that even for a small fraction of hot electrons, the contribution of hot electrons to the rate coefficients is still dominant at low bulk temperature.

Quantifying the Contribution of Hot Electrons in Photothermal Catalysis: A Case Study of Ammonia Synthesis over Carbon-supported Ru Catalyst.

Photothermal catalysis is one of the most promising green catalytic technologies, while distinguishing the effects of hot electrons and local heating remains challenging. Herein, we reported that the actual reaction temperature of photothermal ammonia synthesis over carbon-supported Ru catalyst can be measured based on Le Chatelier's principle, enabling the hot-electron contribution to be quantified. By excluding local heating effects, we established that the activation energy via photothermal catalysis was much lower than that of thermocatalysis (54.9 vs. 126.0 kJ mol-1), stemming from hot-electron injection lowering the energy barriers for both N2 dissociation and intermediates hydrogenation. Furthermore, hot-electron injection acted to suppress carbon support methanation, giving the catalyst outstanding operational stability over 1000 h. This work provides new insights into the hot-electron effects in ammonia synthesis, guiding the design of high-performance photothermal catalysts.

Quantifying the Contribution of Hot Electrons in Photothermal Catalysis: A Case Study of Ammonia Synthesis over Carbon‐supported Ru Catalyst

AbstractPhotothermal catalysis is one of the most promising green catalytic technologies, while distinguishing the effects of hot electrons and local heating remains challenging. Herein, we reported that the actual reaction temperature of photothermal ammonia synthesis over carbon‐supported Ru catalyst can be measured based on Le Chatelier′s principle, enabling the hot‐electron contribution to be quantified. By excluding local heating effects, we established that the activation energy via photothermal catalysis was much lower than that of thermocatalysis (54.9 vs. 126.0 kJ mol−1), stemming from hot‐electron injection lowering the energy barriers for both N2 dissociation and intermediates hydrogenation. Furthermore, hot‐electron injection acted to suppress carbon support methanation, giving the catalyst outstanding operational stability over 1000 h. This work provides new insights into the hot‐electron effects in ammonia synthesis, guiding the design of high‐performance photothermal catalysts.

Consideration of contribution of hot-electron injection to the resistive switching of sputter-deposited silicon oxide film

<p>This paper considers the contribution of hot electrons to the resistive switching of sputter-deposited silicon oxide films based on experiments together with semi-2D Monte Carlo simulations. Using various device stack structures, this paper examines the impact of hot-electron injection on resistive switching, where conduction-band offset and fermi-level difference are utilized. Support is found for the predictions that hot-electron injection reduces the switching voltage and this should reduce the dissipation energy of switching. It is predicted that two-layer metal stacks can significantly reduce the number of oxygen vacancies in the sputter-deposited silicon oxide film after the reset process. It is also demonstrated that, in unipolar switching, the number of E’ or E” centers of the sputter-deposited silicon oxide film is relatively large.</p>

Harmonic generation from gold nanolayers: bound and hot electron contributions to nonlinear dispersion.

Understanding how light interacts with matter at the nanoscale is pivotal if one is to properly engineer nano-antennas, filters and other devices whose geometrical features approach atomic size. We report experimental results on second and third harmonic generation from 20 nm- and 70 nm-thick gold layers, for TE- and TM-polarized incident light pulses. We discuss the relative roles that bound electrons and an intensity dependent free electron density (hot electrons) play in third harmonic generation. While planar structures are generally the simplest to fabricate, metal layers that are only a few nanometers thick and partially transparent are almost never studied. Yet, transmission offers an additional reference point to compare experimental measurements with theoretical models. Our experimental results are explained well within the context of the microscopic hydrodynamic model that we employ to simulate second and third harmonic conversion efficiencies. Using our experimental observations we estimate ∣χ1064nm(3)∣≈10-18(m/V)2, triggered mostly by hot electrons.

The role of hot electrons on ultrahigh pressure generation relevant to shock ignition conditions

On shock ignition (SI) scheme of inertial confinement fusion (ICF), a spherical shell target is compressed by relatively low intensity laser pulse irradiation, and hot spot is ignited by converging shock waves by high intensity laser pulse called “spike pulse”. The SI scheme is considered as a promising scheme to reduce the required drive laser energy for ignition. On the SI scheme, the strong shock by the spike pulse should be over 300 Mbar (=30 TPa). The key physics to produce such ultrahigh pressure is the contribution of hot electrons generated by parametric instabilities. The hot electron generation, transport, and their effects on shock wave parameters in various experimental conditions were explored. This paper focuses on experimental parameter of 0.527 μm (2ω) irradiation as spike pulse with/without pre-pulse irradiation conditions, and the experimental data was compared with one-dimensional hydrodynamic simulation. Experiments with pre-pulse irradiation that pre-compresses a target before the main pulse with producing more hot electrons generates a strong shock and hot electrons lead to an enhanced shock velocity. This fact indicates that the hot electron absorption per unit length in the target is increased leading to an enhanced shock strength.

Plasmonic Nanoparticle Film for Low-Power NIR-Enhanced Photocatalytic Reaction.

Plasmonic metal nanostructures offer the unique ability to effectively enhance sunlight harvesting by localized surface plasmon resonance (LSPR), which can induce direct photocatalytic reactions. However, only metal nanoparticles with a relatively low magnitude of electromagnetic field enhancement usually require a high illumination intensity to ensure the catalytic performance, which greatly limits the solar photocatalytic efficiency. Herein, we designed plasmonic Au nanoparticle film with high electromagnetic field enhancement to achieve high-efficiency catalytic activity under low-power NIR light illumination. This work minimized the influence of the photothermal effect on the reaction by using a low illumination intensity and further revealed the main contribution of plasmon-excited hot electrons to the photochemical reaction. This study provides important insights into the study of the mechanism of LSPR in photocatalytic reactions and further improves the efficiency of solar energy utilization.

Nonlinear optical properties of Rh–Pd and Rh–Pt solid-solution alloy nanoparticles prepared by a laser-induced nucleation method in aqueous solution

The nonlinear optical properties of Rh–Pd and Rh–Pt solid-solution alloy nanoparticles (NPs) were experimentally investigated by means of the z-scan technique. The open aperture (OA) measurement showed a reverse saturable behavior, whereas the closed aperture (CA) measurement showed a peak- and valley-shape. Both the Rh–Pd and Rh–Pt NPs exhibited a positive nonlinear optical index of refraction at 800 nm relating to the self-focusing phenomenon. In addition, the nonlinear absorption of the Rh–Pt NPs (4.39 × 10−12 cm/W) was higher than that of the Rh–Pd NPs (1.63 × 10−12 cm/W) due to the small interval between the occupied and unoccupied density of states (DOS) of Rh–Pt than Rh–Pd. The nonlinear responses of the Rh–Pd and Rh–Pt NPs was attributed to the hot electron contribution and the reverse saturation of intraband and interband transitions.

(Invited) Tuning Thermal– and Hot Electron–Driven H2 Generation with Plasmonic TiN/Pt Antenna–Reactor Nanohybrids

Plasmonic nanostructures have recently shown the capability to greatly enhance photocatalytic processes through the generation of highly energetic hot carriers.[1–3] These species, generated upon nonradiative decay of surface plasmons, enable to reach high quantum yield in chemical conversion with increasing photon flux. Photocatalysts composed by plasmonic metals or, more recently, by antenna–reactor systems [4,5] have shown their potential mainly for gas phase reactions (e.g. CO, NH3, and propylene oxidation, CO2 reduction), while detailed investigations of liquid phase reactions have been lacking. [6] Furthermore, the catalytic contribution related to thermoplasmonic heat generation and hot electron creation is still under lively debate.[7] In this invited talk, I will start discussing the challenges that limit the present understanding of hot carrier–driven chemical transformations. Thereafter, I will show our recent efforts to understand thermal and electronic contributions in hydrogen generation with plasmonic TiN/Pt antenna–reactor nanohybrids, correlating catalytic data with Electron Energy Loss Spectroscopy (EELS) spectral images. Uniform and highly dispersed Pt nanocrystals onto the TiN nanocrystals were prepared by reduction of appropriate amount of potassium tetrachloroplatinate (K2PtCl4) in the presence of formaldehyde (HCHO). The obtained plasmonic catalysts include TiN nanocubes with size of 50 nm homogeneously decorated with Pt nanocrystals (Figure 1a). Each plasmonic antenna (TiN nanocubes) is decorated with reactive catalytic center (Pt nanocrystals) to optimize the electromagnetic interactions and boost the photocatalytic efficiency. Once dispersed in water solution, the TiN/Pt nanohybrids show a strong and broad localized plasmonic resonance (LSPR) around 700 nm. In this talk, I will also discuss our Electron Energy Loss Spectroscopy (EELS) results showing the nanometric spatial distribution of different plasmonic modes of TiN/Pt catalysts excited through a highly focused, accelerated electron beam (Figure 1b). The hydrogen evolution experiments from ammonia borane (NH3BH3) hydrolysis were carried out by adding NH3BH3 solution to an aqueous dispersion of TiN/Pt hybrid nanocatalyst in a rounded glass flask under different experimental condition. The maximum temperature achieved by illuminating an aqueous dispersion of TiN/Pt hybrid nanocrystals under a particular light intensity is termed as plasmonic temperature. Different sets of experiments were conducted at plasmonic temperature in the dark with external thermal heating or under that particular light irradiation to distinguish the thermal and electronic contributions, respectively, in our plasmonic catalysis. All photocatalytic reactions were carried out under AM1.5G solar illumination at different light intensity. Figure 1c shows the catalytic hydrogen production rate expressed as turnover frequency (TOF) values for TiN/Pt in the dark and under solar light illumination. The TOF values for TiN/Pt nanocrystals in the dark at RT (22 °C) and under 10 Suns illumination are 32.1 and 346.1 mol(H2) molPt -1 min-1 respectively. Photocatalytic hydrogen evolution rates obtained under solar light show a drastic enhancement, reaching about 11 times TOF value in comparison to dark reaction when using 10 Suns illumination. The plasmonic nature of photocatalytic activity is demonstrated by a set of experiments carried out at different wavelength and revealing a TOF maximum at 700 nm, in well agreement with absorption spectrum of TiN/Pt hybrid nanocatalyst. Interestingly, Figure 1c clearly demonstrates that hot electron contributions in photocatalytic hydrogen production is enhanced under solar illumination, especially at high concentrated power (i.e. 10 Suns), where TOF value reaches 2.3 times the TOF obtained under plasmonic temperature in the dark. To summarize, in this invited talk I will show our effort in plasmonic photocatalysis with TiN/Pt antenna–reactor systems for liquid phase hydrogen generation from ammonium borane. Photocatalytic data will be discussed in terms of thermal and hot electron contribution, electronic vs. multielectronic mechanism, and activation of rate determining step. Finally, these photocatalytic data will be related to nanometer EELS plasmon maps and hot electron generation capability of TiN/Pt hybrids. References P. Christopher, H. Xin and S. Linic, Nat. Chem. 3, 467–472 (2011).P. Christopher, H. Xin, A. Marimuthu and S. Linic, Nat. Mater. 11, 1044–1050 (2012)A. Marimuthu, J. Zhang and S. Linic, Science 339, 1590–1593 (2013).U. Aslam, S. Chavez and S. Linic, Nat. Nanotech. 12, 1000–1005 (2017).D. F. Swearer, H. Zhao, L. Zhou, C. Zhang, H. Robatjazi, J. M. P. Martirez, C. M. Krauter, S. Yazdi, M. J. McClain, E. Ringe, E. A. Carter, P. Nordlander and N. J. Halas, Proc. Natl Acad. Sci. USA 113, 8916–8920 (2016).Y. Kim, J. G. Smith and P. K. Jain, Nat. Chem. 10, 763–769 (2018).L. Zhou, D. F. Swearer, C. Zhang, H. Robatjazi, H. Zhao, L. Henderson, L. Dong, P. Christopher, E. A. Carter, P. Nordlander and N. J. Halas, Science 362, 69–72 (2018). Figure 1

Engineering hot electrons of localized surface plasmon on InGaN photoanode for solar-powered water splitting

We demonstrate an effective approach to enhancing the performance of III-nitride-based photoelectrode by utilization of hot electrons generated from localized surface plasmons (LSPs). The device designed as a TiO2 / Au / InGaN sandwich structure is fabricated by cladding Au nanodroplets with a thin TiO2 layer to provide a large interfacial layer, which is beneficial for the hot electron collection efficiency by electron reflection via an additional energy barrier. The applied bias photo-to-current efficiency is enhanced by up to four times due to the contribution of hot electrons generated from the absorbed light with wavelengths >470 nm, corresponding to photon energies below the band gap of InGaN (2.67 eV) recognized from incident photon-to-current efficiency spectrum. The LSPs of our photoelectrodes also exhibit a suppressed size-dependence on the metal nanodroplets, feasible for mass production.

Correlation of the plasmon-enhanced photoconductance and photovoltaic properties of core-shell Au@TiO2 network

This study reveals the contribution of hot electrons from the excited plasmonic nanoparticles in dye sensitized solar cells (DSSCs) by correlating the photoconductance of a core-shell Au@TiO2 network on a micro-gap electrode and the photovolatic properties of this material as photoanodes in DSSCs. The distinct wavelength dependence of these two devices reveals that the plasmon-excited hot electrons can easily overcome the Schottky barrier at Au/TiO2 interface in the whole visible wavelength range and transfer from Au nanoparticles into the TiO2 network. The enhanced charge carrier density leads to higher photoconductance and facilitates more efficient charge separation and photoelectron collection in the DSSCs.

Highly Enhanced Photoelectrochemical Water Oxidation Efficiency Based on Triadic Quantum Dot/Layered Double Hydroxide/BiVO4 Photoanodes.

The water oxidation half-reaction is considered to be a bottleneck for achieving highly efficient solar-driven water splitting due to its multiproton-coupled four-electron process and sluggish kinetics. Herein, a triadic photoanode consisting of dual-sized CdTe quantum dots (QDs), Co-based layered double hydroxide (LDH) nanosheets, and BiVO4 particles, that is, QD@LDH@BiVO4, was designed. Two sets of consecutive Type-II band alignments were constructed to improve photogenerated electron-hole separation in the triadic structure. The efficient charge separation resulted in a 2-fold enhancement of the photocurrent of the QD@LDH@BiVO4 photoanode. A significantly enhanced oxidation efficiency reaching above 90% in the low bias region (i.e., E < 0.8 V vs RHE) could be critical in determining the overall performance of a complete photoelectrochemical cell. The faradaic efficiency for water oxidation was almost 90%. The conduction band energy of QDs is ∼1.0 V more negative than that of LDH, favorable for the electron injection to LDH and enabling a more efficient hole separation. The enhanced photon-to-current conversion efficiency and improved water oxidation efficiency of the triadic structure may result from the non-negligible contribution of hot electrons or holes generated in QDs. Such a band-matching and multidimensional triadic architecture could be a promising strategy for achieving high-efficiency photoanodes by sufficiently utilizing and maximizing the functionalities of QDs.

Ultrafast and Efficient Transport of Hot Plasmonic Electrons by Graphene for Pt Free, Highly Efficient Visible-Light Responsive Photocatalyst.

We report that reduced graphene-coated gold nanoparticles (r-GO-AuNPs) are excellent visible-light-responsive photocatalysts for the photoconversion of CO2 into formic acid (HCOOH). The wavelength-dependent quantum and chemical yields of HCOOH shows a significant contribution of plasmon-induced hot electrons for CO2 photoconversion. Furthermore, the presence and reduced state of the graphene layers are critical parameters for the efficient CO2 photoconversion because of the electron mobility of graphene. With an excellent selectivity toward HCOOH (>90%), the quantum yield of HCOOH using r-GO-AuNPs is 1.52%, superior to that of Pt-coated AuNPs (quantum yield: 1.14%). This indicates that r-GO is a viable alternative to platinum metal. The excellent colloidal stability and photocatalytic stability of r-GO-AuNPs enables CO2 photoconversion under more desirable reaction conditions. These results highlight the role of reduced graphene layers as highly efficient electron acceptors and transporters to facilitate the use of hot electrons for plasmonic photocatalysts. The femtosecond transient spectroscopic analysis also shows 8.7 times higher transport efficiency of hot plasmonic electrons in r-GO-AuNPs compared with AuNPs.

A hybrid model describing ion induced kinetic electron emission

We present a model to describe the kinetic internal and external electron emission from an ion bombarded metal target. The model is based upon a molecular dynamics treatment of the nuclear degree of freedom, the electronic system is assumed as a quasi-free electron gas characterized by its Fermi energy, electron temperature and a characteristic attenuation length.In a series of previous works we have employed this model, which includes the local kinetic excitation as well as the rapid spread of the generated excitation energy, in order to calculate internal and external electron emission yields within the framework of a Richardson–Dushman-like thermionic emission model. However, this kind of treatment turned out to fail in the realistic prediction of experimentally measured internal electron yields mainly due to the restriction of the treatment of electronic transport to a diffusive manner. Here, we propose a slightly modified approach additionally incorporating the contribution of hot electrons which are generated in the bulk material and undergo ballistic transport towards the emitting interface.

The third-order nonlinear optical susceptibility of gold

We critically analyze reported measured values of the third-order nonlinear optical susceptibility χ(3) of bulk gold. Reported values of this quantity span a range of more than three orders of magnitude. Much of this variation results from the use of different measurement procedures which are sensitive to different contributions to the nonlinear optical response. For example, values measured through use of third-harmonic generation or non-degenerate four-wave mixing tend to be significantly lower than those obtained from measurements of the intensity-dependent refractive index. We ascribe this behavior to the fact that the first two processes respond only to “instantaneous” nonlinearities, whereas the nonlinear refractive index has a contribution from the much stronger but much slower “hot electron,” or “Fermi-smearing” mechanism, which has a response time of the order of picoseconds. The data also reveal that the hot-electron contribution has a strong dependence on laser wavelength, because of the turn-on of the 5d to 6sp transition at about 550nm. It is hoped that the compilation presented here will prove useful in establishing what value of χ(3) is most appropriate for adoption under various laboratory conditions.

Separation of hot electron current component induced by hydrogen oxidation on resistively heated Pt/n-GaP Schottky nanostructures

Studies of chemically induced hot electron flow over Schottky barriers in catalytic planar nanostructures provide a direct insight into underlying charge transfer processes involved in chemical energy dissipation at solid surfaces. A systematic approach is described here to separate the hot electron and thermal current contributions to the total generated current based on in-situ resistive heating of cathode nanolayer of the Schottky structure. The method is applicable at high pressures in the gas phase. Analysis of the current induced by H2 oxidation to H2O on Pt/n-GaP nanostructure is performed for surface temperatures in the range of 453–513 K, and 120 Torr oxyhydrogen environment with 15 Torr H2. All the current components grow monotonously with temperature, while relative fraction of the hot electron current decreases with temperature from 85 to 52%.

Third-order nonlinearity of metal nanoparticles: Isolation of instantaneous and delayed contributions

Measurement of transient absorption of a sample allows estimation of the evolution of the nonlinear response of materials after excitation by a short pump pulse. In this work, the time dependent nonlinear response of silver nanoplatelets in water was measured using transient absorption technique for various volume fractions of silver in water. These measurements were carried out in subpicosecond time scales and a suitable theoretical model was developed. It is been shown that the hot-electron contribution to the third-order nonlinearity of metal colloids is much higher than its instantaneous third-order nonlinearity. At low volume fractions, the delayed hot-electron contribution to third-order nonlinearity increases linearly with the volume fraction.

Origin and time dependence of higher-order nonlinearities in metal nanocomposites

The higher-order nonlinear optical response of a composite medium having metal nanoparticles is usually attributed to that originating from the third-order nonlinearity of the metal. In this article, the time dependence of hot-electron contribution to the third-, fifth-, and seventh-order nonlinear absorption coefficients of the composite medium has been studied. By comparing the results of the calculation with that of the experiments, it has been shown that the higher-order nonlinearities originating from the hot electrons of metal do contribute to the measured higher-order nonlinear absorption coefficients of the composite material.

Influence of laser pulse characteristics on the hot electron contribution to the third-order nonlinear optical response of gold nanoparticles

The third-order nonlinear optical response of noble-metal nanoparticles embedded in a dielectric matrix depends on the particle intrinsic third-order susceptibility ${\ensuremath{\chi}}_{m}^{(3)}$. We propose a model which allows one to calculate the hot electron contribution ${\ensuremath{\chi}}_{\text{he}}^{(3)}$ to ${\ensuremath{\chi}}_{m}^{(3)}$ in the case of gold. This phenomenon stems from the modification of the conduction-electron distribution induced by an optical excitation, and is significant when picosecond or subpicosecond laser pulses are considered. We show, in the case of a weak perturbation, the importance of the athermal regime for pulse widths lower than about 1 ps. Applying this model to two different samples, we then highlight the strong influence of the linear optical properties of the material on the spectral dispersion of ${\ensuremath{\chi}}_{\text{he}}^{(3)}$. Finally, the variation in ${\ensuremath{\chi}}_{\text{he}}^{(3)}$ with intensity in the high excitation regime is discussed for picosecond pulses.