Hot Electrons Research Articles

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.

Ultrafast Interfacial Charge Transfer Initiates Mechanical Stress and Heat Transport at the Au-TiO2 Interface.

Metal-semiconductor interfaces are crucial components of optoelectronic and electrical devices, the performance of which hinges on intricate dynamics involving charge transport and mechanical interaction at the interface. Nevertheless, structural changes upon photoexcitation and subsequent carrier transportation at the interface, which crucially impact hot carrier stability and lifetime, remain elusive. To address this long-standing problem, they investigated the electron dynamics and resulting structural changes at the Au/TiO2 interface using ultrafast electron diffraction (UED). The analysis of the UED data reveals that interlayer electron transfer from metal to semiconductor generates a strong coupling between the two layers, offering a new way for ultrafast heat transfer through the interface and leading to a coherent structural vibration that plays a critical role in propagating mechanical stress. These findings provide insights into the relationship between electron transfer and interfacial mechanical and thermal properties.

The role of the plasmon in interfacial charge transfer.

The lack of a detailed mechanistic understanding for plasmon-mediated charge transfer at metal-semiconductor interfaces severely limits the design of efficient photovoltaic and photocatalytic devices. A major remaining question is the relative contribution from indirect transfer of hot electrons generated by plasmon decay in the metal to the semiconductor compared to direct metal-to-semiconductor interfacial charge transfer. Here, we demonstrate an overall electron transfer efficiency of 44 ± 3% from gold nanorods to titanium oxide shells when excited on resonance. We prove that half of it originates from direct interfacial charge transfer mediated specifically by exciting the plasmon. We are able to distinguish between direct and indirect pathways through multimodal frequency-resolved approach measuring the homogeneous plasmon linewidth by single-particle scattering spectroscopy and time-resolved transient absorption spectroscopy with variable pump wavelengths. Our results signify that the direct plasmon-induced charge transfer pathway is a promising way to improve hot carrier extraction efficiency by circumventing metal intrinsic decay that results mainly in nonspecific heating.

Magnesium Nanoparticles for Surface-Enhanced Raman Scattering and Plasmon-Driven Catalysis.

Nanostructures of some metals can sustain localized surface plasmon resonances, collective oscillations of free electrons excited by incident light. This effect results in wavelength-dependent absorption and scattering, enhancement of the incident electric field at the metal surface, and generation of hot carriers as a decay product. The enhanced electric field can be utilized to amplify the spectroscopic signal in surface-enhanced Raman scattering (SERS), while hot carriers can be exploited for catalytic applications. In recent years, cheaper and more earth abundant alternatives to traditional plasmonic Au and Ag have gained growing attention. Here, we demonstrate the ability of plasmonic Mg nanoparticles to enhance Raman scattering and drive chemical transformations upon laser irradiation. The plasmonic properties of Mg nanoparticles are characterized at the bulk and single particle level by optical spectroscopy and scanning transmission electron microscopy coupled with electron energy-loss spectroscopy and supported by numerical simulations. SERS enhancement factors of ∼102 at 532 and 633 nm are obtained using 4-mercaptobenzoic acid and 4-nitrobenzenethiol. Furthermore, the reductive coupling of 4-nitrobenzenethiol to 4,4'-dimercaptoazobenzene is observed on the surface of Mg nanoparticles under 532 nm excitation in the absence of reducing agents, indicating a plasmon-driven catalytic process. Once decorated with Pd, Mg nanostructures display an enhancement factor of 103 along with an increase in the rate of catalytic coupling. The results of this study demonstrate the successful application of plasmonic Mg nanoparticles in sensing and plasmon-enhanced catalysis.

Study on a p-GaN HEMT with composite passivation and composite barrier layers

A novel structure of p-GaN high-electron-mobility transistor (HEMT) is proposed and studied. It features two composite layers. One is the composite passivation (CP) layer consisting of Si3N4 and high-permittivity (HK) film. The other is a composite barrier (CB) layer consisting of Al x Ga1−x N/AlN/Al0.23Ga0.77N. Due to the coordinated effect of CP and CB, the specific on-resistance (R ON, SP) can be reduced under the premise of ensuring breakdown voltage (BV). Meanwhile, since the HK film in CP introduces a mechanism to automatically compensate the hot electrons trapped by surface states, the current collapse effect could be suppressed. According to the simulation results, in comparison with the conventional p-GaN HEMT, the proposed one using TiO2 as the HK material and using Al-component of 0.35 for Al x Ga1−x N gains a 29.5% reduction in R ON, SP while getting a 9.8% increase in BV, which contributes to a 50.5% decrease in the energy loss during one cycle at 200 kHz. It is also demonstrated by the simulation results that the current collapse in the proposed device is reduced by 28.6%. Thereby, a promising p-GaN HEMT with improved performance and reliability is invented.

Efficient photoanode with a MoS2/TiO2/Au nanoparticle heterostructure for ultraviolet-visible photoelectrocatalysis

Green energy technology is generally becoming one of hot issues that need to be solved due to the adverse effects on the environment of fossil fuels. One of the strategies being studied and developed by theorists and experimentalists is the use of photoelectrochemical (PEC) cells, which are emerging as a candidate to produce hydrogen from water splitting. However, creating photoelectrodes that meet the requirements for PEC water splitting has emerged as the primary obstacle in bringing this technology to commercial fruition. Here, we construct a heterostructure, which consists of MoS2/TiO2/Au nanoparticles (NPs) to overcome the drawbacks of the photoanode. Owing to the dependence on charge transfer, the bandgap of MoS2/TiO2 and the utilization the Au NPs as a stimulant for charges separation of TiO2 by localized surface plasmon resonances effect as well as the increase of hot electron injection to cathode, leading to photocatalytic activities are improved. The results have recorded a significant increase in the photocurrent density from 2.3 μAcm−2 of TiO2 to approximately 16.3 μAcm−2 of MoS2/TiO2/Au NPs. This work unveils a promising route to enhance the visible light adsorption and charge transfer in photo-electrode of the PEC cells by combining two-dimensional materials with metal NPs.

Plasmonic Hot Carrier‐Driven Photoelectrochemical Processes: Principle, Detection and Application

AbstractUtilizing hot carriers induced by surface plasmon resonance in solar energy conversion to chemical fuels is a crucial issue in the field of photocatalysis. To achieve practical efficiency in plasmonic hot carrier application to photocatalysts, it is essential to theoretically and experimentally understand the generation, relaxation, and transfer of hot carriers. Photoelectrochemical (PEC) processes can offer a pathway to investigate the transfer dynamics of hot electrons (or hot holes) and enhance the efficiency of both hot carrier transfer and catalytic activity. This work summarizes a theoretical and experimental understanding of the hot carrier effect, emphasizing the hot carrier transfer dynamics to investigate the exact role of hot carriers in PEC reactions. The principles of hot carrier detection, diverse applications of hot carrier‐based photocatalysis, and perspectives for possible future progress are discussed.

Hot electron effect in high-order harmonic generation from graphene driven by elliptically polarized light

We studied high-order harmonic generation (HHG) in graphene driven by either linearly or elliptically polarized mid-infrared (MIR) light, and we additionally applied terahertz (THz) pulses to modulate the electron distribution in graphene. The high-harmonic spectrum obtained using linearly polarized MIR light contains only odd-order harmonics. We found that the intensities of the fifth- and seventh-order harmonics are reduced by the modulation with the THz pulses. In addition, we found that the THz-induced reduction of the seventh-order harmonic driven by elliptically polarized MIR light (at ellipticity ε = 0.3) is larger than that of seventh-order harmonic driven by linearly polarized MIR light (ε = 0). The observed behavior can be reproduced by theoretical calculations that consider different electron temperatures (caused by the THz pulses). Furthermore, the observed stronger suppression of HHG driven by elliptically polarized light reveals the following: in the case of elliptically polarized light, the generation of harmonics via interband transitions to conduction-band states that are closer to the Dirac point is more important than in the case of linearly polarized light. In other words, the quantum pathways via interband transitions to low-energy states are the origin of the enhancement of HHG that can be achieved in graphene by using elliptically polarized light.

Degradation analysis of GaN-based high–electron-mobility transistors under different stresses in semi-on state conditions

The large numbers of high-energy carriers that occur in semiconductor devices under semi-on state conditions can cause significant device degradation. The effects of different stresses on the electrical and trapping characteristics of GaN-based high-electron-mobility transistors (HEMTs) are investigated. Test results for GaN HEMTs under semi-on state conditions show that the electrical characteristics of these devices degrade to a certain extent after they are subjected to electrical pulse stress cycles with different drain voltage, frequencies, and duty cycles; a degree of degradation also occurs in the electrical characteristics of the devices when they are subjected to direct current electrical stresses. After electrical stress is applied, the absolute amplitude of the traps in the device increases, thus indicating an increase in the trap density. The results show that voltage is the main driver for device damage, with the current playing an accelerating role through its effects on device temperature or by supplying hot electrons; therefore, the drain voltage has the most significant effect on device degradation, which is mainly due to channel high-energy hot electron injection.

How the secrets behind photocurrents are revealed in Ag-TiO2 heterostructures-based plasmonic photoelectrochemical systems: A collaborative approach of EC-SERS and photoelectrochemical methods

Plasmon-mediated chemical reactions (PMCR) have garnered growing interest as a promising concept for photocatalysis. However, in electrochemical systems at solid–liquid interfaces, the photo-induced charge transfer on the surface of metal–semiconductor heterostructures involves complex processes and mechanisms, which are still poorly understood. We explore the plasmon-mediated carrier transfer mechanism and the synergistic effect of light and electric fields on Ag-TiO2 heterostructures, through a combination of electrochemical surface-enhanced Raman spectroscopy and photoelectrochemical methods, with para-aminothiophenol (PATP) serving as a probe molecule. The results show that photocurrent responses are dependent on not only excitation wavelengths and applied potentials, but also the irreversibility of redox. The relationship between photocurrent responses and the chemical transformation between PATP and 4,4′-dimercaptoazobenzene is established, reflecting the photo-induced charge transfer of the heterostructures. The collaboration of spectroscopic and photoelectrochemical methods provide valuable insights into the chemical transformation and kinetic information of adsorbed molecules on the heterostructure during PMCR, offering opportunities for modulating of photocatalytic activities of hot carriers.

Ultrafast Hot‐Electron Transfer and Plasmonic Coupling Enable Au/MXene Nanohybrids with Broadband Excellent Nonlinear Absorption

AbstractAchieving coupling between plasma oscillators can significantly modulate their intrinsic optical characteristics. In this paper, strong coupling generated by the surface plasma of MXene and Au nanobipyramids (NBPs) for broadband ultrafast nonlinear optical modulation is reported. The femtosecond transient absorption spectra reveal that the ultrafast hot electron transfer occurred in the Au/MXene nanohybrids at <200 fs. Au/Ta4C3Tx nanohybrids outperform the outstanding broadband saturable absorption (SA), with a saturation intensity 106 times less than that of the initial MXene. Laser excitation at 1100 nm results in reverse saturable absorption for Ta4C3Tx nanosheets and a strong SA for Au/Ta4C3Tx nanohybrids, which extends the range of SA. Theoretical calculations indicate that excellent nonlinearity is synergistically achieved via hot electron transfer and plasmon coupling. Femtosecond transient absorption spectra reveal that Au NBPs offer the additional occupied state for carriers through the transfer process and extend the carrier lifetime considerably. This study provides theoretical guidance for the exploration of remarkable novel nonlinear‐active 2D layer‐based materials and candidates for the practical applications of saturable absorbers.

Energy-Conserving Surface Hopping for Auger Processes.

Auger-type processes are ubiquitous in nanoscale materials because quantum confinement enhances Coulomb interactions, and there exist large densities of states. Modeling Auger processes requires the modification of nonadiabatic (NA) molecular dynamics algorithms to include transitions caused by both NA and Coulomb couplings. The system is split into quantum and classical subsystems, e.g., electrons and vibrations, and as a result, energy conservation becomes nontrivial. In surface hopping, an electronic transition induced by NA coupling is accompanied by a classical velocity readjustment to ensure conservation of the total quantum-classical energy. A different treatment is needed for Auger transitions driven by Coulomb interactions. We develop a nonadiabatic molecular dynamics methodology that meticulously differentiates the energy redistribution accompanying hops induced by the NA coupling and the Coulomb interaction and correctly conserves the total energy at each transition. If the transition is driven by a Coulomb interaction, the hop energy is redistributed within the quantum electronic subsystem only. If the transition is NA, the energy is redistributed between the quantum and classical subsystems. Properly maintaining energy conservation for both types of transitions is crucial to generate a correct order of events, obtain accurate transition times, maintain a proper statistical distribution of state populations, and reach thermodynamic equilibrium. We test the method with biexciton annihilation and Auger-assisted hot electron relaxation in a CdSe quantum dot. The sequence of Auger and phonon-driven processes and the calculated time scales are in excellent agreement with the experimental results. The developed approach can be coupled with any surface-hopping method and provides a crucial practical advance to study charge-carrier dynamics in the nanoscale and condensed matter systems.

Intraband Cooling and Auger Recombination in Weakly to Strongly Quantum-Confined CsPbBr3 Perovskite Nanocrystals.

Semiconductor nanocrystals (NCs) with size-tuned energy gaps present unique and desirable properties for optoelectronic applications. Recent synthetic advancements offer routes to spheroidal CsPbBr3 perovskite NCs in the strong quantum confinement regime with narrow size dispersion. Using tunable femtosecond laser pulses, we examine intraband carrier relaxation using transient absorption spectroscopy and show that, across the transition from weak to strong confinement, hot carrier lifetime increases compared to larger bulk-like particles. However, further increases of confinement subsequently lead to a reduction of the hot carrier lifetime and increase of the non-radiative Auger recombination rate. Finally, we show that hot carrier lifetimes increase as a function of excess energy above the band gap less sensitively under high confinement in comparison to the bulk. Understanding such unique trends is important for maximizing hot carrier lifetimes for use in next-generation hot carrier devices as well as evaluating the transition from weak to strong confinement.

Elucidating Facet-Dependent Photocatalytic Activities of Metastable CdS and Au@CdS Core-Shell Nanocrystals.

Controlling the crystal facets of semiconductor nanocrystals (NCs) has been proven as an effective approach to tune their physicochemical properties. However, the study on facet-engineering of metastable zinc blende CdS (zb-CdS) and its heterostructures is still not fully explored. In this study, the zb-CdS and Au@zb-CdS core-shell NCs with tunable terminating facets are controllably synthesized, and their photocatalytic performance for water splitting are evaluated. It is found that the {111} facets of the zb-CdS NCs display higher intrinsic activity than the {100} counterparts, which originates from these surfaces being much more efficient, facilitating electron transition to enhance the adsorption ability and the dissociation of the adsorbed water, as revealed by theoretical calculations. Moreover, the Au@zb-CdS core-shell NCs exhibit better photocatalytic performance than the zb-CdS NCs terminated with the same facets under visible light irradiation (≥400 nm), which is mainly ascribed to the accelerated electron separation at the interface, as demonstrated by femtosecond transient absorption (fs-TA) spectroscopy. Importantly, the quantum yield of plasmon-induced hot electron transfer quantified by fs-TA in the Au@zb-CdS core-shell octahedrons can be reached as high as 1.2% under 615 nm excitation, which is higher than that of the Au@zb-CdS core-shell cubes. This work unravels the face-dependent photocatalytic performance of the metastable semiconductor NCs via a combination of experiments and theoretical calculations, providing the understanding of the underlying mechanism of these photocatalysts.

Achieving Visible Light Triggered Overall Water Splitting over Plasmonic Au/SrTiO3:Al Photocatalyst

AbstractThe surface plasmon resonance (SPR) effect has garnered extensive attention in semiconductor photocatalysis for solar energy conversion, thanks to its remarkable optical properties. However, the majority SPR‐induced photocatalytic systems have been limited to achieving hydrogen evolution or oxygen evolution half reactions, and attaining overall water splitting on a SPR‐induced photocatalyst under visible light remains a formidable challenging. In this study, we employed a plasmonic photocatalyst Au/SrTiO3, and further enhanced its performance by doping aluminum (Al) into the SrTiO3 lattice (denoted as Au/SrTiO3:Al). By constructing reduction cocatalyst (RhCrOx) and oxidation cocatalyst (CoOOH), the Au/SrTiO3:Al photocatalyst successfully realizes photocatalytic overall water splitting with a stoichiometric ratio of H2 and O2 under visible light (λ≥440 nm). We revealed that the introduction of Al species effectively modified the electronic structure of SrTiO3, thereby enhancing the hydrogen evolution reaction in Au/SrTiO3:Al. Simultaneously, the RhCrOx and CoOOH cocatalysts synergistically capitalized on the short‐lived hot electrons and holes generated by the plasmonic Au/SrTiO3:Al photocatalyst, enabling to realize photocatalytic overall water splitting. This work offers a promising avenue for the rational design of plasmon‐induced overall water splitting photocatalysts through the integration of suitable cocatalysts and surface/interface engineering strategies.

Photon-carrier-spin coupling in One-dimensional Ni(II) doped ZnTe Nanostructure.

The transition metal ions doping in II-VI semiconductor can produce exciton magnetic polaron (EMP) and localized EMP, in which contain longitudinal optical (LO) phonon coupling will be discussed in this paper. Transition metal ion doping in II-VI semiconductor for dilute magnetic semiconductor (DMS) show emission by magnetic polarons together with hot carrier effect that need to be understand by its optical properties. The high excitation power responsible for hot carrier effect that suppressed the charge trapping effect for low exciton binding energy (8.12 meV) semiconductor even at room temperature. The large polaron radius exhibits strong interaction between carrier and magnetic polaron results anharmonicty effect in which side-band energy overtone to LO phonon. The photon-like polariton show the polarized spin interaction with LO that show strong spin-phonon polariton at room temperature. The temperature-dependent photoluminescence spectra of Ni-doped ZnTe show free exciton (FX), FX interact with 2LO phonon-spin interaction corresponding to 3T1(3F) → 1T1(1G) and EMP peaks with ferromagnetically coupled Ni ions at 3T1(3F) → 1E(1G). In addition, other d-d transition of single Ni ions (600-900 nm) appears at low energy side. Room temperature energy shift of 14-38 meV due to localized states with density of states tails extending far into bandgap related spin induced localization at valance band. These results show spin-spin magnetic coupling and spin-phonon interaction at room temperature that open a new horizon of optically controlled dilute magnetic semiconductor applications more realistic.

Ultrafast Surface Plasmon Probing of Interband and Intraband Hot Electron Excitations.

Upon the interaction of light with metals, nonthermal electrons are generated with intriguing transient behavior. Here, we present femtosecond hot electron probing in a noveloptical pump/plasmon probe scheme. With this, we probed ultrafast interband and intraband dynamics with 15 nm interface selectivity, observing a two-component-decay of hot electron populations. Results are in good agreement with a three-temperature model of the metal; thus, we could attribute the fast (∼100 fs) decay to the thermalization of hot electrons and the slow (picosecond) decay to electron-lattice thermalization. Moreover, we could modulate the transmission of our plasmonic channel with ∼40% depth, hinting at the possibility of ultrafast information processing applications with plasmonic signals.

Plasmon-Induced Hot Electrons in Nanostructured Materials: Generation, Collection, and Application to Photochemistry.

Plasmon refers to the coherent oscillation of all conduction-band electrons in a nanostructure made of a metal or a heavily doped semiconductor. Upon excitation, the plasmon can decay through different channels, including nonradiative Landau damping for the generation of plasmon-induced energetic carriers, the so-called hot electrons and holes. The energetic carriers can be collected by transferring to a functional material situated next to the plasmonic component in a hybrid configuration to facilitate a range of photochemical processes for energy or chemical conversion. This article centers on the recent advancement in generating and utilizing plasmon-induced hot electrons in a rich variety of hybrid nanostructures. After a brief introduction to the fundamentals of hot-electron generation and decay in plasmonic nanocrystals, we extensively discuss how to collect the hot electrons with various types of functional materials. With a focus on plasmonic nanocrystals made of metals, we also briefly examine those based upon heavily doped semiconductors. Finally, we illustrate how site-selected growth can be leveraged for the rational fabrication of different types of hybrid nanostructures, with an emphasis on the parameters that can be experimentally controlled to tailor the properties for various applications.

Accretion flow dynamics and characteristics of MAXI J153–571-spectral analysis using combination of XSPEC and TCAF models

MAXI J153–571 accretion flow consists of Keplerian (optically thick) and sub-Keplerian (optically thin) flow, and their mass accretion rates seem to regulate other accretion flow characteristics. Hard X-rays are produced when the Keplerian seed soft photons are thermally or inverse comptonized in the Compton cloud/post-shock region by hot electrons. The variations/fluctuations of components of the accretion flow during the hard states create propagating Quasi-periodic oscillation (QPO) when their timescales are roughly matched and resonance phenomena occur. The QPO and its frequency are timing properties and the accretion flow spectra-temporal characteristics can be determined via spectral analysis. In this study, we looked into the accretion flow characteristics of MAXI J153–571 during the hard-intermediate state. Spectral analysis of MAXI J153–571 observed by MAXI/GSC, Swift/BAT, and NuSTAR on the same or close-in epochs was carried out. XSPEC and TCAF models were used in fitting/modeling the data. A robust and statistically acceptable fit spectra with a reduced Chi-squared value of ∼ 0.84 – 1.20 and best-fit photon index of 2.0–2.29 was obtained. The track of the accretion flow characteristics was obtained using models’–fitted parameters and MATLAB written codes of physical equations. Some accretion flow characteristics are positively correlated while others are anti-correlated at different phases and their correlation are statistically significant. The correlation of accretion flow characteristics with one another suggests that saturation effects, variation/fluctuations in the accretion flow, and intermittent/flickering behavior of MAXI J153–571 are tied to the variations/fluctuations of the intrinsic properties; mass accretion rates. Moreover, a resonance condition of 0.70 to 0.83 indicates that the cooling and infall timescales are roughly matched and affirms the presence of QPO in the accretion flow. This suggests that the origin of the photon index–QPO frequency (Γ–vQPO) relation is strongly linked to the variation/fluctuations in mass accretion flow rates. Hence, the accretion flow is dynamic, and independent variations/fluctuations of mass accretion rates could regulate the variation/fluctuations of other accretion flow parameters and perhaps, spectral evolution.

Simultaneous NICER and NuSTAR observations of the ultraluminous source NGC 4190 ULX-1

Aims. We present an X-ray analysis of three different XMM-Newton observations together with simultaneous NICER and NuSTAR observations of the ultraluminous X-ray source NGC 4190 ULX-1. Our goal is to constrain the structure of the accretion disk and the geometrical properties of the source. Methods. We performed temporal and spectral analyses in the 0.4−30 keV energy range in which the source is significantly detected in dedicated XMM-Newton, NICER, and NuSTAR observations. Results. The temporal analysis shows no flaring activity in the light curves. No pulsation is detected throughout. The source exhibits a typical ULX spectrum, which can be fitted with two thermal blackbody components plus a Comptonization tail at high energies. The luminosity–temperature (L − T) relation of each thermal spectral component is consistent with the L ∝ T2 relation expected from an advection-dominated supercritical disk. Conclusions. We interpret these results as a super-Eddington accreting black hole seen almost face-on. A dense wind ejected from the disk obscures the central source, and a hot electron plasma is evacuated through the funnel formed above the hole. Geometric beaming is responsible for the ULX soft emission, whereas the hard tail is the result of the Comptonization of soft photons by the electrons ejected through the funnel.