Hot Electrons Research Articles

Tandem Electric-Fields Prolong Energetic Hot Electrons Lifetime for Ultra-Fast and Stable NO2 Detection.

Prolonging energetic hot electrons lifetimes and surface activity in the reactive site can overcome the slow kinetics and unfavorable thermodynamics of photo-activated gas sensors. However, bulk and surface recombination limit simultaneous optimization of both kinetics and thermodynamic. Here we deploy tandem electric-fields at (111)/(100)Au-CeO2 to ensure a sufficient driving force for carriers transfer and elucidate the mechanism of the relationship between charges transport and gas-sensing performance. The asymmetric structure of (111)/(100)CeO2 facet junction provide interior electric-fields, which facilitates electron transfer from (100)face to (111)face. This separation of reduction and oxidation reaction sites across different crystal faces helps inhibit surface recombination. The increased electron concentration at the (111)face intensifies the interface electric field, which promotes electron transfer to Au site. The local electric-field generated by the surface plasmon resonance effect promotes the generation of high energy hot-electrons, which maintains charge concentration in interface field by injecting into (111)/(100)CeO2, thereby provide thermodynamic contributions and inhibit bulk recombination. The tandem electric fields enable the (111)/(100)Au-CeO2 to rapidly detect 5ppm of NO2 at room temperature with stability maintained within 20 s. This article is protected by copyright. All rights reserved.

Plasmon-Induced Ultrafast Hot Hole Transfer in Nonstoichiometric CuxInyS/CdS Heteronanocrystals.

Plasmonic semiconductors are promising candidates for developing energy conversion devices due to their tunable band gap, cost-effectiveness, and nontoxicity. Such materials exhibit remarkable capabilities for harvesting infrared photons, which constitute half of the solar energy spectrum. Herein, we have synthesized near-infrared (NIR) active CuxInyS nanocrystals and CuxInyS/CdS heterostructure nanocrystals (HNCs) to investigate plasmon-induced charge transfer dynamics on an ultrafast time scale. Employing femtosecond transient absorption spectroscopy, we demonstrate that upon exciting the HNCs with sub-band gap NIR photons (λ = 840 nm), the hot holes are generated in the valence band of plasmonic CuxInyS and transferred to the adjacent semiconductor. The decreased signal intensity and accelerated hole phonon relaxation dynamics for HNCs reveal efficient transfer of plasmon-induced hot carriers from CuxInyS to CdS under both 840 and 350 nm laser excitations, providing a pathway for enhanced carrier utilization. These findings shed light on the potential of ternary chalcogenides in plasmonic applications, highlighting efficient hot carrier extraction to adjacent semiconductors.

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

The 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.

Hot Carrier Cooling Mediated Efficiency Enhancement in Diamine Passivated Perovskite Solar Cells

Hot Carrier Cooling Mediated Efficiency Enhancement in Diamine Passivated Perovskite Solar Cells

Perovskite quantum laser with enhanced population inversion driven by plasmon-induced hot electron transfer under potential shift polarization conditions

The hot electron transfer resulting in fluorescence enhancement is significantly meaningful for theory and experiment of the study on photoelectric devices. However, the laser emission based on direct hot electron transfer is difficult to realize because of the low transfer efficiency. To achieve a laser with a new-generation mechanism based on hot electron transfer, the photoelectric co-excitation is proposed for improving the efficiency of hot electron transfer. The lasing behavior at 532 nm is realized with a threshold of 5  kw cm−2 and 1 μA, which can be considered as the hot electron transfer resulting in population inversion enhancement. Meanwhile, the lasing output power is 0.3 mW. The hot electrons transfer process was described via the transient absorption spectrum according to the improved ground-state bleaching and excited-state absorption signal in device ON. Through comparison with the optical pump only, the quantum efficiencies of hot electron generation (HEG) and hot electron transfer (HET) were increased ∼31% and 31%, respectively. Most importantly, a triple gain mode coupling device including local surface plasmon, hot electron transfer, and array oscillation was presented. Two modes of population inversion enhancement are proposed. This study can provide theoretical and experimental reference for the research of hot electron lasers and devices.

Providing a Review of HC Relaxation Dynamics and Strategies of HC Extraction in Perovskites Nanocrystals

Metal halide perovskite (MHPs) nanocrystals (NCs) have recently emerged as promising materials for optoelectronic applications due to their exceptional photophysical properties. A comprehensive understanding of the underlying principles and key challenges associated with controlling the hot carrier (HCs) cooling dynamics in MHPs is pivotal for the design of efficient perovskite‐based optoelectronic applications. This review highlights the general carrier cooling mechanism and identifies crucial controlling parameters influencing carrier dynamics. The discussion includes strategies for manipulating HC cooling dynamics through carrier‐phonon and carrier‐carrier interactions. Notably, recent advancements in HC extraction with suitable acceptors are explored, as efficient HC extraction is imperative for the advancement of optoelectronic applications. The objective of this review is to outline potential pathways for strategically modifying carrier cooling processes to align with specific optoelectronic applications. A profound comprehension of charge carrier dynamics in MHPs NCs will pave the way for the development of highly efficient optoelectronic applications.

Circular Differential Photocurrent Mapping of Hot Electron Response from Individual Plasmonic Nanohelicoids.

Chiral plasmonic nanocrystals have recently attracted increasing attention in circular polarization-dependent photocatalysis driven by hot carriers. While being concealed in traditional ensemble measurements, the individual chiral photocatalytic activity of nanocrystals can exclusively be revealed by directly correlating the circular differential photocurrent response to helical morphologies using single-particle techniques. Herein, we develop a method named circular differential photocurrent mapping (CDPM) and demonstrate that CDPM can be used to characterize the circular differential hot electron (CDHE) response from individual Au nanohelicoids (AuNHs) on a TiO2 photoanode in a photoelectrochemical cell. The single-particle circular differential scattering and CDHE measurements were interpreted with calculations performed on a model in direct correlation to the helical morphologies of the nanocrystal. While CDHE response was found inactive at a dipolar resonance of 750 nm, helicity-convoluted sites of HE generation were identified on the AuNH at a specific higher-order mode of 550 nm, resulting in a significant response of CDHE in association with the handedness of the AuNH. Details of circular differential contributions were further resolved by examining the efficiencies of individual AuNHs in terms of g-factors. Our study provides a powerful microscopic method at the single-particle level for the photocatalytic characterization of chiral nanocrystals, gaining fundamental insights into the photocatalysis of chirality, especially toward plasmon-induced asymmetrical photochemistry or photoelectrochemistry.

Perovskite Crystallization and Hot Carrier Dynamics Manipulation Enables Efficient and Stable Perovskite Solar Cells with 25.32% Efficiency

AbstractModulating perovskite crystallization and understanding hot carriers (HCs) dynamics in perovskite films are very critical to achieving high‐performance perovskite solar cells (PSCs). Herein, a small organic molecule (6BAS) with multisite anchors (C═O) as an efficient additive is introduced into PbI2 precursors to modulate perovskite crystallization during two‐step sequential deposition. The chemical interaction between 6BAS and PbI2 enables more preferential PbI2 crystal with enlarged interplanar spacing of PbI2 lattice, which is beneficial to the penetration of organic ammonium salts into PbI2 layer and the complete conversion to perovskite, consequently promoting the preferential crystallization of perovskite to realize high‐quality perovskite films with larger grain size and reduced defect state. By ultrafast spectroscopy, it is found that the incorporation of 6BAS can efficiently prolong HCs cooling, which helps to enhance HCs transfer and retard the charge carrier recombination in device. As a result, 6BAS doped‐PSCs efficiency significantly enhances to 25.32% from 22.91%. The target device achieves the enhanced long‐term stability. Only a 6% efficiency degradation is realized for un‐encapsulated device with 6BAS after 70 days under N2. Meanwhile, the 6BAS‐treated device retains 95% of its initial PCE after 1160 h of operation at the maximum power point under continuous AM 1.5 G illumination.

Photogenerated carrier dynamics of Mn2+ doped CsPbBr3 assembled with TiO2 systems: Effect of Mn doping content.

In recent years, all-inorganic perovskite materials have become an ideal choice for new thin film solar cells due to their excellent photophysical properties and have become a research hotspot. Studying the ultrafast dynamics of photo-generated carriers is of great significance for further improving the performance of such devices. In this work, we focus on the transient dynamic process of CsPbBr3/TiO2 composite systems with different Mn2+ doping contents using femtosecond transient absorption spectroscopy technology. We used singular value decomposition and global fitting to analyze the transient absorption spectra and obtained three components, which are classified as hot carrier cooling, charge transfer, and charge recombination processes, respectively. We found that the doping concentration of Mn2+ has an impact on all three processes. We think that the following two factors are responsible: one is the density of defect states and the other is the bandgap width of perovskite. As the concentration of doped Mn2+ increases, the charge transfer time constant shows a trend of initially increasing, followed by a subsequent decrease, reaching a turning point. This indicates that an appropriate amount of Mn2+ doping can effectively improve the photoelectric performance of solar cell systems. We proposed a possible charge transfer mechanism model and further elucidated the microscopic mechanism of the effect of Mn2+ doping on the interface charge transfer process of the CsPbBr3/TiO2 solar cell system.

Sensitivity improvement in gate engineered technique dielectric modulated GaN MOSHEMT with InGaN notch for label-free biosensing

This paper focuses on the impact of gate-engineered dielectric-modulated GaN MOSHEMT with InGaN notch on sensitivity enhancement for label-free biosensing. The novelty of this study utilizes the charge-plasma effect induced by the dual metal gate (DMG) technology adopted to realize the effect of sensitivity on different biomolecules. Moreover, the presence of an InGaN notch enhances carrier confinement in the 2DEG, subsequently improving the threshold voltage and device sensitivity at the AlGaN/GaN interface. The maximum drain current, IDS of 4.602 A mm−1, transconductance, gm of 18 mS/mm, and sensitivity has been improved by around 61% for the Uricase biomolecule by introducing the dual metal gate technology. The work function difference of the two metal gates suppresses the Short Channel Effects (SCEs) and hot carrier effects in DMG MOSHEMT, thereby screening the drain potential variations by the gate near the drain. In addition, increased carrier transport efficiency results from a more consistent electric field along the channel. All the simulations are carried out using the Sentaurus TCAD simulator, and the results imply the feasibility of gate-engineered GaN MOSHEMT for label-free biosensing.

Hot carrier relaxation dynamics of an aza-covalent organic framework during photoexcitation: An insight from abinitio quantum dynamics.

In order to develop an efficient metal-free solar energy harvester, we herein performed the electronic structure calculation, followed by the hot carrier relaxation dynamics of two dimensional (2D) aza-covalent organic framework by time domain density functional calculations in conjunction with non-adiabatic molecular dynamics (NAMD) simulation. The electronic structure calculation shows that the aza-covalent organic framework (COF) is a direct bandgap semiconductor with acute charge separation and effective optical absorption in the UV-visible region. Our study of non-adiabatic molecular dynamics simulation predicts the sufficiently prolonged electron-hole recombination process (6.8 nanoseconds) and the comparatively faster electron (22.48ps) and hole relaxation (0.51ps) dynamics in this two-dimensional aza-COF. According to our theoretical analysis, strong electron-phonon coupling is responsible for the rapid charge relaxation, whereas the electron-hole recombination process is slowed down by relatively weak electron-phonon coupling, relatively lower non-adiabatic coupling, and quick decoherence time. We do hope that our results of NAMD simulation on exciton relaxation dynamics will be helpful for designing photovoltaic devices based on this two dimensional aza-COF.

Quantum-mechanical effects in photoluminescence from thin crystalline gold films.

Luminescence constitutes a unique source of insight into hot carrier processes in metals, including those in plasmonic nanostructures used for sensing and energy applications. However, being weak in nature, metal luminescence remains poorly understood, its microscopic origin strongly debated, and its potential for unraveling nanoscale carrier dynamics largely unexploited. Here, we reveal quantum-mechanical effects in the luminescence emanating from thin monocrystalline gold flakes. Specifically, we present experimental evidence, supported by first-principles simulations, to demonstrate its photoluminescence origin (i.e., radiative emission from electron/hole recombination) when exciting in the interband regime. Our model allows us to identify changes to the measured gold luminescence due to quantum-mechanical effects as the gold film thickness is reduced. Excitingly, such effects are observable in the luminescence signal from flakes up to 40 nm in thickness, associated with the out-of-plane discreteness of the electronic band structure near the Fermi level. We qualitatively reproduce the observations with first-principles modeling, thus establishing a unified description of luminescence in gold monocrystalline flakes and enabling its widespread application as a probe of carrier dynamics and light-matter interactions in this material. Our study paves the way for future explorations of hot carriers and charge-transfer dynamics in a multitude of material systems.

Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions.

Plasmonic tunnel junctions are a unique electroluminescent system in which light emission occurs via an interplay between tunnelling electrons and plasmonic fields instead of electron-hole recombination as in conventional light-emitting diodes. It was previously shown that placing luminescent molecules in the tunneling pathway of nanoscopic tunnel junctions results in peculiar upconversion electroluminescence where the energy of emitted photons exceeds that of excitation electrons. Here we report the observation of upconversion electroluminescence in macroscopic van der Waals plasmonic tunnel junctions comprising gold and few-layer graphene electrodes separated by a ~2-nm-thick hexagonal boron nitride tunnel barrier and a monolayer semiconductor. We find that the semiconductor ground exciton emission is triggered at excitation electron energies lower than the semiconductor optical gap. Interestingly, this upconversion is reached in devices operating at a low conductance (<10-6 S) and low power density regime (<102 W cm-2), defying explanation through existing proposed mechanisms. By examining the scaling relationship between plasmonic and excitonic emission intensities, we elucidate the role of inelastic electron tunnelling dipoles that induce optically forbidden transitions in the few-layer graphene electrode and ultrafast hot carrier transfer across the van der Waals interface.

A novel MOSFET with lateral–vertical charge coupling for extremely low C gd

A novel MOSFET with lateral–vertical charge coupling (LVCC-MOSFET) is proposed in this paper. Lateral charge coupling is enabled by a metal field plate, lightly doped drain and P-Epi layer to reduce the C gd. Vertical charge coupling is enabled by a shield gate, sinker and P-Epi layer to support a high breakdown voltage (BV) and further reduce the C gd. By combining both lateral charge coupling and vertical charge coupling, which is first proposed in low-voltage power MOSFETs, the trade-off relationship between BV, R on and C gd can be significantly improved. It is verified by both small-signal analysis and transient capacitance simulation that the gate-to-drain capacitance of the proposed LVCC-MOSFET can be reduced by more than 99% without deterioration of BV and R on. The LVCC-MOSFET achieves 93.7% reduction in Q gd, and R on × Q gd is only 0.81 mΩ·nC, which is reduced by 93.9%. Furthermore, E on and E off can be reduced by 73.6% and 53.8%, respectively. The hot carrier injection reliability can be enhanced by reducing the electric field and the impact ionization generation rate near the drain-side gate oxide. Moreover, the LVCC-MOSFET can be feasibly manufactured with compatible fabrication process flow, and only three extra steps are needed.

Organic-Inorganic Heterointerface-Expediting Electron Transfer Realizes Efficient Plasmonic Catalytic Sterilization via a Carbon-Dot Nanozyme.

Plasmonic nanozymes bring enticing prospects for catalytic sterilization by leveraging plasmon-engendered hot electrons. However, the interface between plasmons and nanozymes as the mandatory path of hot electrons receives little attention, and the mechanisms of plasmonic nanozymes still remain to be elucidated. Herein, a plasmonic carbon-dot nanozyme (FeCG) is developed by electrostatically assembling catalytic iron-doped carbon dots (Fe-CDs) with plasmonic gold nanorods. The energy harvesting and hot-electron migration are remarkably expedited by a spontaneous organic-inorganic heterointerface holding a Fermi level-induced interfacial electric field. The accumulated hot electrons are then fully utilized by conductive Fe-CDs to boost enzymatic catalysis toward overproduced reactive oxygen species. By synergizing with localized heating from hot-electron decay, FeCG achieves rapid and potent disinfection with an antibacterial efficiency of 99.6% on Escherichia coli within 5 min and is also effective (94.2%) against Staphylococcus aureus. Our work presents crucial insights into the organic-inorganic heterointerface in advanced plasmonic biocidal nanozymes.

Infrared light driven overall water vapor splitting on a plasmonic semiconductor p-n heterojunction photocatalyst

Photocatalytic overall water splitting (OWS) has yet to be realized on plasmonic semiconductor photocatalysts. In this work, we reported a plasmonic heterojunction of p-Cu1.81S/n-CdS with homogeneously distributed p-type and n-type domains in a single nanoparticle. The plasmonic heterojunction displays a strong localized surface plasmon resonance (LSPR) absorption peak at 1480 nm and can drive the OWS into hydrogen and oxygen under infrared light (IR) irradiation. The IR light-induced hot electrons and holes in Cu1.81S nanodisks alone can split water vapor into hydrogen and oxygen, but with poor efficiency. Surprisingly, the plasmonic p-Cu1.81S/n-CdS heterojunction shows significantly improved IR light-driven OWS performance. Under the irradiation of IR light (λ > 800 nm), the hydrogen and oxygen production rates were 0.73 mmol gcat−1 h−1 and 0.33 mmol gcat−1 h−1, respectively. The heterojunction photocatalyst delivered a solar-to-hydrogen conversion efficiency (STH) of 0.02 % under AM 1.5G irradiation. The present study has demonstrated that plasmonic semiconductors are promising photocatalysts for harvesting IR light for green hydrogen generation.

Enhancing perovskite solar cell performance: The role of polymer-assisted hole transport layers in hot carrier dynamics

Enhancing the efficiency and stability is very critical for perovskite solar cells (PSCs) commercialization. Herein, we report an effective approach to enable highly efficient and stable PSCs by incorporating a polymer material, poly[N, N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) (named P(NDI2OD-T2)), into hole transport layer (HTL). Our research results exhibit that the addition of P(NDI2OD-T2) not only effectively improves the hole mobility and conductivity of HTL, but also makes its energy level more alignment with perovskite. More importantly, ultrafast carrier results demonstrate that the introduction of P(NDI2OD-T2) into Spiro-OMeTAD HTL can effectively improve the extraction and transfer of hot carriers from perovskite layer to HTL, and consequently reduce the recombination of charge carriers in the device. As a result, PSCs based on Spiro + P(NDI2OD-T2) as a HTL shows an enhanced power conversion efficiency (PCE) of 24.20 %, which is much higher than that of control device without P(NDI2OD-T2) (21.94 %). Furthermore, the un-encapsulated P(NDI2OD-T2)-treated PSCs exhibit excellent humidity, thermal and light stabilities, with the devices maintaining 94 % of its initial efficiency after 3000 h at ≈40 % humidity, 95 % of its initial efficiency after storage at 65 °C and 85 °C for 600 h, and 90 % of its initial efficiency after 600 h under one sun light irradiation, respectively. This work provides an effective strategy for improving the device efficiency and long-term stability of PSCs.

Ab initio investigation of hot electron transfer in CO2 plasmonic photocatalysis in the presence of hydroxyl adsorbate.

Photoreduction of carbon dioxide (CO2) on plasmonic structures is of great interest in photocatalysis to aid selectivity. While species commonly found in reaction environments and associated intermediates can steer the reaction down different pathways by altering the potential energy landscape of the system, they are often not addressed when designing efficient plasmonic catalysts. Here, we perform an atomistic study of the effect of the hydroxyl group (OH) on CO2 activation and hot electron generation and transfer using first-principles calculations. We show that the presence of OH is essential in breaking the linear symmetry of CO2, which leads to a charge redistribution and a decrease in the OCO angle to 134°, thereby activating CO2. Analysis of the partial density of states (pDOS) demonstrates that the OH group mediates the orbital hybridization between Au and CO2 resulting in more accessible states, thus facilitating charge transfer. By employing time-dependent density functional theory (TDDFT), we quantify the fraction of hot electrons directly generated into hybridized molecular states at resonance, demonstrating a broader energy distribution and an 11% increase in charge-transfer in the presence of OH groups. We further show that the spectral overlap between excitation energy and plasmon resonance plays a critical role in efficiently modulating electron transfer processes. These findings contribute to the mechanistic understanding of plasmon-mediated reactions and demonstrate the importance of co-adsorbed species in tailoring the electron transfer processes, opening new avenues for enhancing selectivity.

Ab initio-simulated optical response of hot electrons in gold and ruthenium

Ab initio-simulated optical response of hot electrons in gold and ruthenium

Influence of Hot Carrier Cooling and Band‐Filling Effect on Linewidth of Perovskite Microlasers

AbstractIn semiconductor‐based microlasers, the generated excited carriers can affect both the optical absorption coefficient and refractive index, influencing the lasing behavior and device performance. In this study, the lasing behavior of individual MAPbBr3 microrods pumped by femtosecond laser pulses is investigated using an ultrafast optical Kerr gating technique. By analyzing the spectral and temporal evolution of the lasing behavior, it is found that the band‐filling (BF) effect can cause a significant change in the refractive index of the material, which results in an unfavorable red‐shift and broadening of the resonant modes, deteriorating the laser linewidth and quality factor. The hot carrier cooling process can provide a buffer for alteration of the energy level occupation state, resulting in a small transient refractive index change and slight red‐shift. These results offer insights into the lasing behavior driven by photogenerated carrier dynamics and provide an optimization strategy for semiconductor‐based microlasers.