Excited Carriers Research Articles

Differentiating Plasmon-Enhanced Chemical Reactions on AgPd Hollow Nanoplates through Surface-Enhanced Raman Spectroscopy.

Plasmonic photocatalysis demonstrates great potential for efficiently harnessing light energy. However, the underlying mechanisms remain enigmatic due to the transient nature of the reaction processes. Typically, plasmonic photocatalysis relies on the excitation of surface plasmon resonance (SPR) in plasmonic materials, such as metal nanoparticles, leading to the generation of high-energy or "hot electrons", albeit accompanied by photothermal heating or Joule effect. The ability of hot electrons to participate in chemical reactions is one of the key mechanisms, underlying the enhanced photocatalytic activity observed in plasmonic photocatalysis. Interestingly, surface-enhanced Raman scattering (SERS) spectroscopy allows the analysis of chemical reactions driven by hot electrons, as both SERS and hot electrons stem from the decay of SPR and occur at the hot spots. Herein, we propose a highly efficient SERS substrate based on cellulose paper loaded with either Ag nanoplates (Ag NPs) or AgPd hollow nanoplates (AgPd HNPs) for the in situ monitoring of C-C homocoupling reactions. The data analysis allowed us to disentangle the impact of hot electrons and the Joule effect on plasmon-enhanced photocatalysis. Computational simulations revealed an increase in the rate of excitation of hot carriers from single/isolated AgPd HNPs to an in-plane with a vertical stacking assembly, suggesting its promise as a photocatalyst under broadband light. In addition, the results suggest that the incorporation of Pd into an alloy with plasmonic properties may enhance its catalytic performance under light irradiation due to the collection of plasmon-excitation-induced hot electrons. This work has demonstrated the performance-oriented synthesis of hybrid nanostructures, providing a unique route to uncover the mechanism of plasmon-enhanced photocatalysis.

Boosting photocatalytic overall water splitting performance by dual-metallic single Ni and Pd atoms decoration of MoS2: A DFT study

Herein, we propose a novel MoS2 supported single Ni and Pb atoms catalyst (Ni/Pd@MoS2), on which the enhanced efficiency of water splitting is confirmed by quantitative density functional theory (DFT) calculations. The Bader charge, electron density difference (EDD) and projected density of state (PDOS) analysis demonstrates that the moderate electron transfer is reached by the synergy of the Ni atom and the Pd atom. As extra channel for excited carrier migration, the present of defect states obviously prolongs the carrier lifetime (143.45 ps). The unique electronic environment contributes to the electronic synergy and spatially separated of the effective active sites, leading the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) happen smoothly on the Pd and S site, respectively. Our research presents a novel idea for the development of highly efficient diatomic catalysts towards overall water splitting.

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.

Lasing emission from ZnO hierarchical spherical microcavity

We investigated lasing characteristics of ZnO hierarchical micro-spherical particles with diameters of 1–5 μm . The lasing emission consists of a small number of discrete laser peaks unlike conventional random lasing from ZnO nanopowder-assembly. Theoretical calculations based on a many-body theory reveal that the optical gain is achieved at the observed lowest lasing threshold value, 4 mJ⋅cm−2⋅pulse−1 , which corresponds to the excited carrier density ∼2.4×1025 m−3 . Because the carrier density is much higher than the Mott density, the gain origin for lasing is electron-hole plasma recombination. The lasing frequency mode shift ( ∼1.2 meV ) is due to the refractive index change induced by exciting high carrier density up to 6.1×1025 m−3 . By changing hierarchical structures via controlling growth condition and performing the annealing treatment and performing the calculation of scattering efficiency of ZnO particles, we found that the hierarchy of the micro-spherical particle plays a crucial role of the lasing feedback: the strong light scattering at the interface between outer nanoparticles with the sizes of 100–200 nm and smaller ones with the sizes of 10–60 nm consisting in the micro-spherical particle results in a light confinement. Furthermore, it has been confirmed that each discrete lasing mode shows strong gain competition with each other probably due to spatial overlap between the modes. These results suggest that both random scattering and microcavity modes contribute to the lasing oscillation.

A-Cation-Dependent Excited State Charge Carrier Dynamics in Vacancy-Ordered Halide Perovskites: Insights from Computational and Machine Learning Models

A-Cation-Dependent Excited State Charge Carrier Dynamics in Vacancy-Ordered Halide Perovskites: Insights from Computational and Machine Learning Models

Size‐Dependent Multi‐Electron Donation in Metal‐Complex Quantum Dots Hybrid Catalyst for Photocatalytic Carbon Dioxide Reduction

AbstractThe effective conversion of carbon dioxide (CO2) into valuable chemical fuels relies significantly on the donation of multiple electrons. Its efficiency is closely linked to both the density and lifetime of excited charge carriers. In this study, a hybrid catalyst system comprising covalently bonded InP/ZnS quantum dots (QDs) and Re‐complexes is showcased. The electronic band alignment between the QDs and the Re‐complexes is revealed to dominate the multi‐electron transfer process for photocatalytic conversion to methane (CH4). Notably, the size of the QDs is found to be a determining parameter. Among the three QD sizes investigated, transient absorption spectroscopy studies unveil that rapid multi‐electron transfer from the QDs to the Re‐catalyst occurs in smaller QDs (2.3 nm) due to the substantial driving force. Consequently, the photocatalytic conversion of CO2 to CH4 is significantly enhanced with a turnover number of 6, corresponding to the overall apparent quantum yield of ≈1%. This research underscores the possibilities of engineering multi‐electron transfer by manipulating the electronic band alignment within a catalytic system. This can serve as a guide for optimizing photocatalytic CO2 reduction.

On thermal stability of Fröhlich polarons in bulk and nano-structured polar solids I: Klemens channel formalism and two-phonon DOS

The “Klemens channel” formalism is commonly used in interpretations of experimental results on the lifetime of longitudinal optic (LO) phonons and Fröhlich polarons (FPs) in polar bulk, polycrystalline, and nanostructured solids. Here, in Paper I of this bipartite article, the two-particle vibrational DOS function(s) for spatially confined longitudinal acoustic and LO phonons is implemented at evaluation on FP decay probability via the “Klemens channel” and its lifetime for the case of FPs confined within InAs quantum dots. This approach yields a convincing elucidation of the earlier experimental data. An alternative model based on the idea of diabatic thermally activated excitation of FP charge carriers and consecutive lattice “depolarization” will be discussed in detail and implemented at simulations on FP lifetime in Paper II [V. Ligatchev, “On thermal stability of Fröhlich polarons in bulk and nano-structured polar solids II: Diabatic ionization and depolarization,” AIP Adv. 14, (2024)].

Reaction amplification with a gain: Triplet exciton–mediated quantum chain using mixed crystals with a tailor-made triplet sensitizer

Photochemical valence bond isomerization of a crystalline Dewar benzene (DB) diacid monoanion salt with an acetophenone-linked piperazinium cation that serves as an intramolecular triplet energy sensitizer (DB-AcPh-Pz) exhibits a quantum chain reaction with as many as 450 product molecules per photon absorbed (Φ ≈ 450). By contrast, isomorphous crystals of the DB diacid monosalt of an ethylbenzene-linked piperazinium (DB-EtPh-Pz) lacking a triplet sensitizer showed a less impressive quantum yield of ca. Φ ≈ 22. To establish the critical importance of a triplet excited state carrier in the adiabatic photochemical reaction we prepared mixed crystals with DB-AcPh-Pz as a dilute triplet sensitizer guest in crystals of DB-EtPh-Pz. As expected from their high structural similarities, solid solutions were easily formed with the triplet sensitizer salt in the range of 0.1 to 10%. Experiments carried out under conditions where light is absorbed by the triplet sensitizer-linked DB-AcPh-Pz can be used to initiate a triplet state adiabatic reaction from 3DB-AcPh-Pz to 3HB*-AcPh-Pz, which can serve as a chain carrier and transfer energy to an unreacted DB-EtPh-Pz where exciton delocalization in the crystalline solid solution can help carry out an efficient energy transfer and enable a quantum chain employing the photoproduct as a triplet chain carrier. Excitation of mixed crystals with as little as 0.1% triplet sensitizer resulted in an extraordinarily high quantum yield Φ ≈ 517.

Frictional heat-assisted performance enhancement in dynamic Schottky contact of Al/Ag2Se-based tribovoltaic nanogenerator

The tribovoltaic nanogenerator (TVNG) has evolved in recent years as a novel type of nanogenerator designed to address the limitations of the standard triboelectric nanogenerator in terms of output signal and charge generation. Besides the outstanding characteristics, the tribovoltaic effect can also well be coupled with another effect to further boost the output performance. In this work, we proposed firstly a frictional heat-assisted performance enhancement in dynamic Schottky contact from the rubbing between n-type silver selenide (Ag2Se) and aluminum. The chemical composition and physical characteristics of the Ag2Se ceramic were analyzed using X-ray diffraction, scanning electron microscopy, and Synchrotron X-ray tomography techniques. UV–Vis spectroscopy and UPS were also utilized in order to validate the semiconducting property of the n-type Ag2Se ceramic. Moreover, the presence of the Schottky junction was demonstrated through the analysis of the current-bias voltage characteristic curve of the Ag2Se/aluminum (Al) contact under varying stress and temperature conditions. The built-in electric field plays a crucial part in the tribovoltaic effect by efficiently transferring the excited carriers to an external load through sliding contact between Ag2Se and Al. Demonstrating the synergy between tribovoltaic and thermoelectric effects becomes achievable through the excellent thermoelectric property of Ag2Se. Herein, the proposed TVNG generated a peak output voltage and current of around 0.7 V and 24.8 nA, respectively, achieving a maximum output power of 12.6 nW at a load resistance of 10 kΩ. The influence of frictional heat on the output performance of the proposed TVNG was well demonstrated by the thermal-induced voltage and enhanced electrical output from continuous sliding. The concepts given in this study establish the basis for the progress of effective energy collection employing semiconducting materials and the advancement of flexible harvesting and sensing device development in the future.

Diffusion-Dominated Luminescence Dynamics of CsPbBr3 Studied Using Cathodoluminescence and Microphotoluminescence Spectroscopy.

Time-resolved or time-correlation measurements using cathodoluminescence (CL) reveal the electronic and optical properties of semiconductors, such as their carrier lifetimes, at the nanoscale. However, halide perovskites, which are promising optoelectronic materials, exhibit significantly different decay dynamics in their CL and photoluminescence (PL). We conducted time-correlation CL measurements of CsPbBr3 using Hanbury Brown-Twiss interferometry and compared them with time-resolved PL. The measured CL decay time was on the order of subnanoseconds and was faster than PL decay at an excited carrier density of 2.1 × 1018 cm-3. Our experiment and analytical model revealed the CL dynamics induced by individual electron incidences, which are characterized by highly localized carrier generation followed by a rapid decrease in carrier density due to diffusion. This carrier diffusion can play a dominant role in the CL decay time for undoped semiconductors, in general, when the diffusion dynamics are faster than the carrier recombination.

Evidence for an Excitonic Insulator State in Ta2Pd3Te5

The excitonic insulator (EI) is an exotic ground state of narrow-gap semiconductors and semimetals arising from spontaneous condensation of electron-hole pairs bound by attractive Coulomb interaction. Despite research on EIs dating back to half a century ago, their existence in real materials remains a subject of ongoing debate. In this study, through systematic experimental and theoretical investigations, we provide evidence for the existence of an EI ground state in a van der Waals compound Ta2Pd3Te5. Density-functional-theory calculations suggest that it is a semimetal with a small band overlap, whereas various experiments exhibit an insulating ground state with a clear band gap. Upon incorporating electron-hole Coulomb interaction into our calculations, we obtain an EI phase where the electronic symmetry breaking opens a many-body gap. Angle-resolved photoemission spectroscopy measurements exhibit that the band gap is closed with a significant change in the dispersions as the number of thermally excited charge carriers becomes sufficiently large in both equilibrium and nonequilibrium states. Structural measurements reveal a slight breaking of crystal symmetry with exceptionally small lattice distortion in the insulating state, which cannot account for the significant gap opening. Therefore, we attribute the insulating ground state with a gap opening in Ta2Pd3Te5 to exciton condensation, where the coupling to the symmetry-breaking electronic state induces a subtle change in the crystal structure. Published by the American Physical Society 2024

Ag-CsPbI3 nanocomposite structure in glasses boosts photoluminescence for efficient light emitting diodes

Nanocomposite structures based on perovskite nanocrystals (NCs) and metal nanostructures within transparent monolithic media hold promise for a wide range of applications in optoelectronics. However, the microstructure of such nanocomposites and the reason for enhanced emissions remain incompletely understood. In this work, we present the successful creation of Ag-CsPbI3 nanocomposite within a highly transparent borosilicate glass (Ag-CsPbI3 NCs@glass). We reveal that Ag and CsPbI3 NCs form after thermal treatment of the glass, either in the form of Ag-CsPbI3 heterostructures or of separate Ag and CsPbI3 NCs. Based on combined characterization of Mott-Schottky and steady-state photoluminescence (PL) of the products, we attribute the enhanced PL in the nanocomposite of Ag-CsPbI3 to the regulation of CsPbI3 lattice by Ag ions and/or the plasmonic effect of Ag NCs. Temperature-dependent time-resolved PL decay measurements suggest that Ag NCs could play an important role in the relaxation process of excited carriers by increasing the radiative recombination. Moreover, the Ag-CsPbI3 nanocomposite structure within the glass matrix curtails PL quenching caused by water molecules and high temperature. Using the Ag-CsPbI3 nanocomposite, we achieve high-performance white light-emitting diodes with a color rendering index of 83.9 and luminous efficiency of 58.07 lmW−1. This work elucidates the underlying mechanism on the enhanced PL in nanocomposites containing perovskite NCs and metal nanostructures, and aids in the rational design of glass-based perovskite nanocomposites for a diversity of optoelectronic applications.

Establishing carrier transport channels based on TiS bonds and enhancing the photocatalytic performance of MXene quantum dots–ZnIn2S4 for ammonia synthesis

AbstractIn the process of photocatalytic synthesis of ammonia, the kinetics of carrier separation and transport, adsorption of nitrogen, and activation of the NN triple bond are key factors that directly affect the efficiency of converting nitrogen to ammonia. Here, we report a new strategy for anchoring MXene quantum dots (MXene QDs) onto the surface of ZnIn2S4 by forming TiS bonds, which provide a channel for the rapid separation and transport of charge carriers and effectively extend the lifespan of photogenerated carriers. The unique charge distribution caused by the sulfurization of the MXene QDs further enhances the performance of the photocatalysts for the adsorption and activation of nitrogen. The photocatalytic ammonia synthesis efficiency of MXene QDs–ZnIn2S4 can reach up to 360.5 μmol g−1 h−1. Density functional theory calculations, various in situ techniques, and ultrafast spectroscopy are used to characterize the successful construction of TiS bonds and the dynamic nature of excited state charge carriers in MXene QDs–ZnIn2S4, as well as their impact on nitrogen adsorption activation and photocatalytic ammonia synthesis efficiency. This study provides a new example of how to improve nitrogen adsorption and activation in photocatalytic material systems and enhance charge carrier dynamics to achieve efficient photocatalytic nitrogen conversion.image

Ligand Desorption and Fragmentation in Oleate-Capped CdSe Nanocrystals under High-Intensity Photoexcitation.

Semiconductor nanocrystals (NCs) offer prospective use as active optical elements in photovoltaics, light-emitting diodes, lasers, and photocatalysts due to their tunable optical absorption and emission properties, high stability, and scalable solution processing, as well as compatibility with additive manufacturing routes. Over the course of experiments, during device fabrication, or while in use commercially, these materials are often subjected to intense or prolonged electronic excitation and high carrier densities. The influence of such conditions on ligand integrity and binding remains underexplored. Here, we expose CdSe NCs to laser excitation and monitor changes in oleate that is covalently attached to the NC surface using nuclear magnetic resonance as a function of time and laser intensity. Higher photon doses cause increased rates of ligand loss from the particles, with upward of 50% total ligand desorption measured for the longest, most intense excitation. Surprisingly, for a range of excitation intensities, fragmentation of the oleate is detected and occurs concomitantly with formation of aldehydes, terminal alkenes, H2, and water. After illumination, NC size, shape, and bandgap remain constant although low-energy absorption features (Urbach tails) develop in some samples, indicating formation of substantial trap states. The observed reaction chemistry, which here occurs with low photon to chemical conversion efficiency, suggests that ligand reactivity may require examination for improved NC dispersion stability but can also be manipulated to yield desired photocatalytically accessed chemical species.

Self-absorption and investigation of excited carrier dynamics in two-dimensional perovskite scintillator

The PEA2PbBr4 scintillator exhibits varied performance in photoluminescence (PL) and radioluminescence (RL) profiles. PL in the reflectivity mode shows emission peaks at 414 and 434 nm, whereas PL and RL in the transmission mode exhibit only one peak at 434 nm. Temperature-dependent PL spectra measurements confirm that self-absorption contributes to this difference. PEA2PbBr4 displays decay times of 1.3 ns (90%) and 4.4 ns (10%) for PL and 11.5 ns (89%) and 47 ns (11%) for RL. To explain the distinct decay times, we construct two models for simulating laser and radiation excitation and investigate carrier dynamics using non-adiabatic molecular dynamics. Our findings suggest that the relatively long radiative recombination process of carriers contributes to the slow RL decay time.

Carrier Transport in 2D Hybrid Organic-Inorganic Perovskites: The Role of Spacer Molecules.

Two-dimensional organic-inorganic hybrid perovskites (2D HOIPs) have been widely used for various optoelectronics applications owing to their excellent photoelectric properties. However, the selection of organic spacer cations is mostly qualitative without quantitative guidance. Meanwhile, the fundamental mechanism of the carrier transport across the organic spacer layer is still unclear. Here, by using the first-principles nonadiabatic molecular dynamics (NAMD) method, we have studied the transport process of excited carriers between 2D HOIPs separated by a spacer cation layer in real time at atomic levels. We find that the excited electrons and holes can transfer from single-inorganic-layer 2D HOIP to bi-inorganic-layer 2D HOIP on a subpicosecond to picosecond scale. Moreover, we have developed a new method to capture the electron-hole interaction in the frame of NAMD. This work provides a promising direction to design new materials toward high-performance optoelectronics.

Unveiling the Electronic Band Structure and Temporal Dynamics of Excited Carriers in Formamidinium Lead Bromide Perovskite

AbstractIn this study, the electronic band structure and the dynamics of the excited carriers in the formamidinium lead bromide (FAPbBr3) perovskite are investigated by combining the information obtained from steady‐state absorption, photoluminescence, femtosecond transient absorption spectroscopy, photoelectron spectroscopy, and density functional theory calculations. A detailed description of the electronic transitions in the UV–vis energy range has been provided, giving an estimation of the exciton binding energy (40 ± 5 meV), the transition energy from the first valence band (VB1) to the conduction band (CB1) (electronic bandgap at 2.37 ± 0.01 eV) and assigning the broad peak at ≈3.4 eV to the transition from the second innermost valence band (VB2) to CB1. The temporal dynamics of the excited carriers involved in these transitions are investigated using different excitation energies and carrier densities. Different trends are observed in the dynamics of the transient signals associated with the VB1→CB1 (PB1) and VB2→CB1 (PB2) transitions. As the carrier density increased, PB1 exhibited a slowing down of its rise time, while PB2 showed an acceleration attributed to the thermalization of the excited holes in VB2. These valuable findings have the potential to unlock new strategies aimed at maximizing the efficiencies and performance of perovskite‐based solar cell devices.

Hematite (α-Fe2O3) with Oxygen Defects: The Effect of Heating Rate for Photocatalytic Performance.

Hematite (α-Fe2O3) emerges as an enticing material for visible-light-driven photocatalysis owing to its remarkable stability, low toxicity, and abundance. However, its inherent shortcomings, such as a short hole diffusion length and high recombination rate, hinder its practical application. Recently, oxygen vacancies (Vo) within hematite have been demonstrated to modulate its photocatalytic attributes. The effects of Vo can be broadly categorized into two opposing aspects: (1) acting as electron donors, enhancing carrier conductivity, and improving photocatalytic performance and (2) acting as surface carrier traps, accelerating excited carrier recombination, and deteriorating performance. Critically, the generation rate, distribution, role, and behavior of Vo significantly differ for synthesis methods due to differences in formation mechanisms and oxygen diffusion. This complexity hampers simplified discussions of Vo, necessitating careful investigation and nuanced discussion tailored to the specific method and conditions employed. Among various approaches, hydrothermal synthesis offers a simple and cost-effective route. Here, we demonstrate a hydrothermal synthesis method for Vo introduction to hematite using a carbon source, where variations in the heating rate have not been previously explored in terms of their influence on Vo generation. The analyses revealed that the concentration of Vo was maximized at a heating rate of 16 °C/min, indicative of a high density of surface defects. With regard to photocatalytic performance, elevated heating rates (16 °C/min) fostered the formation of Vo primarily on the hematite surface. The photocatalytic activity was 7.1 times greater than that of the sample prepared at a low heating rate (2 °C/min). These findings highlight the crucial role of surface defects, as opposed to bulk defects, in promoting hematite photocatalysis. Furthermore, the facile control over Vo concentration achievable via manipulating the heating rate underscores the promising potential of this approach for optimizing hematite photocatalysts.

Size onset of metallic behavior in neutral aluminum clusters.

The excited state lifetimes of neutral (Al)n clusters up to ∼1 nm in diameter in size, where n ≤ 43, are systematically measured with femtosecond time-resolved mass spectrometry. The onset of metallic behavior is identified as a distinct change in the relaxation behavior initiated with single ultraviolet (400 nm) photon excitation. The experimentally measured excited state lifetimes gradually decrease with size for small molecular scale clusters (n < 10) before becoming indistinguishable for larger clusters (n > 9), where the measurements are comparable to electron-lattice relaxation time of bulk Al (∼300 fs). Particularly intense, or magic, Aln clusters do not exhibit any significant excited state lifetime behavior. Time-dependent density functional theory quantify the excited state properties and are presented to show that dynamics are strongly tied to the excited state charge carrier distributions and overlap, rather than detailed changes related to changes in the cluster's electronic and geometric structure. The consistency in excited state lifetimes for clusters larger than n = 9 is attributed to the hybridization of the s- and p-orbitals as well as increasing delocalization. Al3 exhibits unique temporal delay in its transient behavior that is attributed to a transition from triangular ground state to linear structure upon excitation.

Hot carrier dynamics in metalated porphyrin-naphthalimide thin films.

This study employs femtosecond transient absorption spectroscopy to investigate the rapid dynamics of excited state carriers in three metalated porphyrin-naphthalimide (PN) molecules and one free-base molecule. The dynamics of electron injection, from PN to mesoporous titania (TiO2), in PN adsorbed TiO2 films (Ti-PN), were carefully investigated and compared to PN adsorbed ZrO2 films (Zr-PN). In addition, we examined the self-assembled PN films and found that, in their self-assembled state, these molecules exhibited a longer relaxation time than Zr-PN monomeric films, where the charge injection channel was insignificant. The ground-state bleach band in the Ti-PN films gradually shifted to longer wavelengths, indicating the occurrence of the Stark effect. Faster electron injection was observed for the metalated PN systems and the electron injection times from the various excited states to the conduction band of TiO2 (CB-TiO2) were obtained from the target model analysis of the transient absorption spectra data matrix. In these metal-organic complexes, hot electron injection from PN to CB-TiO2 occurred on a time scale of <360 fs. Importantly, Cu(II)-based PN complexes exhibited faster injection and longer recombination times. The injection times have been estimated to result from a locally excited state at ≈280 fs, a hot singlet excited state at 4.95 ps, and a vibrationally relaxed singlet excited state at 97.88 ps. The critical photophysical and charge injection processes seen here provide the potential for exploring the underlying factors involved and how they correlate with photocatalytic performance.