Excited Carriers Research Articles

Dipole-Modulated Carrier Dynamics and Charge Transport Behavior of Ligand-Protected Metal Chalcogenide Nanoclusters.

Atomically precise nanoclusters, distinguished by their unique nuclearity- and structure-dependent properties, hold great promise for applications of energy conversion and electronic transport. However, the relationship between ligands and their properties remains a mystery yet to be unrevealed. Here, the influence of ligands on the electronic structures, optical properties, excited-state dynamics, and transport behavior of Re12S16 dimer clusters with different ligands is explored using density functional theory combined with time-domain nonadiabatic molecular dynamic simulations. The correlation between ligands and the excited-state dynamics of nanoclusters is elucidated. The ligand replacement introduces a built-in electric field at the dimer interface, inhibiting the recombination of excited carriers and increasing the voltage threshold. This study paints a physical picture of the ligand effect on nanoclusters in terms of geometric configuration, electronic structure, optical properties, carrier dynamics, and transport behavior, paving a pathway toward their applications in optoelectronic materials.

Enhanced Carrier Dynamics and Excitation of Optically Stimulated Artificial Synapse Using van der Waals Passivation Layers.

Advances in the semiconductor industry have been limited owing to the constraints imposed by silicon-based CMOS technology; hence, innovative device design approaches are necessary. This study focuses on "more than Moore" approaches, specifically in neuromorphic computing. Although MoS2 devices have attracted attention as neuromorphic computing candidates, their performances have been limited due to environment-induced perturbations to carrier dynamics and the formation of defect states. This study explores the integration of hydrocarbon (HC) layers onto active MoS2 channels to enhance neuromorphic computing characteristics. HC layers were employed in the proposed MoS2 field-effect transistor to facilitate stable optoelectrical control over the MoS2 channel under high-power stimulation. The improved electrical performance, stability, and synaptic behaviors of the HC-capped MoS2 devices compared to uncapped counterparts were experimentally demonstrated. The combination of optical and electrical tuning allowed for in-sensor computing applications that mimic human sensory behaviors. The impact of HC passivation on device performance was evaluated, and its potential for applications in neuromorphic computing with high stability was demonstrated across wide-ranging environmental conditions. The unique capabilities of HC-capped MoS2 devices were demonstrated by examining the spike duration-dependent plasticity and spiking timing-dependent plasticity. Thus, the proposed approach offers a promising avenue for advancing neuromorphic computing technologies.

Ultrafast Thermal Switching Enabled by Transient Polaritons.

Ultrafast thermal switches are pivotal for managing heat generated in advanced solid-state applications, including high-speed chiplets, thermo-optical modulators, and on-chip lasers. However, conventional phonon-based switches cannot meet the demand for picosecond-level response times, and existing near-field radiative thermal switches face challenges in efficiently modulating heat transfer across vacuum gaps. To overcome these limitations, we propose an ultrafast thermal switch design based on pump-driven transient polaritons in asymmetric terminals. Demonstrated with WSe2 and graphene, this approach achieves an impressive thermal switching ratio exceeding 10,000 with response times on the picosecond scale, outperforming current designs by at least 2 orders of magnitude. This exceptional performance is driven by dynamic polaritonic coupling between terminals, activated by ultrafast photoexcitation. Additionally, the WSe2 monolayer-based switch exhibits a laser cooling effect, enabled by enhanced carrier excitation efficiency and prolonged carrier lifetimes, introducing a disruptive mechanism for laser cooling. Our findings highlight the strong potential of photodriven transient polaritons in advancing ultrafast thermal switches and nanoscale cooling technologies.

Intrinsically Slow Cooling of Hot Electrons in CdSe Nanocrystals Compared to CdS.

The utilization of excited charge carriers in semiconductor nanocrystals (NCs) for optoelectronic technologies has been a long-standing goal in the field of nanoscience. Experimental efforts to extend the lifetime of excited carriers have therefore been a principal focus. To understand the limits of these lifetimes, in this work, we theoretically study the time scales of pure electron relaxation in negatively charged NCs composed of two prototypical materials: CdSe and CdS. We find that hot electrons in CdSe have lifetimes that are 5 to 6 orders of magnitude longer than in CdS when the relaxation is governed only by the intrinsic properties of the materials. Although these two materials are known to have somewhat different electronic structure, we elucidate how this enormous difference in lifetimes arises from relatively small quantitative differences in electronic energy gaps and phonon frequencies, as well as the crucial role of Fröhlich-type electron-phonon couplings.

Interfacial Electronic Charge Trapping and Photonic Carrier Excitation Coupling in Solution-Processed Zinc-Tin Oxide Thin-Film Transistors Applied for Logic Gate Design and Quantized Neural Network.

Components needed in Artificial Intelligence with a higher information capacity are critically needed and have garnered significant attention at the forefront of information technology. This study utilizes solution-processed zinc-tin oxide (ZTO) thin-film phototransistors and modulates the values of VG, which allows for the regulation of electron trapping/detrapping at the ZTO/SiO2 interface. By coupling the excited photonic carrier and electronic trapping, logic gates such as "AND," "OR," "NAND," and "NOR" can be achieved. With the exponential growth in data generation, efficient processing and storage solutions are imperative. However, extensive data transfer between computing units and storage limits the level of artificial neural networks (ANNs). Consequently, quantized neural networks (QNNs) have gained interest for their reduced computational resource requirements and lower consumption. In this context, we introduce an optimized ternary logic circuit based on ZTO devices. By utilizing optical modulation to adjust the turn-on voltage of the single device, we demonstrate the achievement of ternary current states, thereby providing three distinct discrete states. This configuration can be extended to QNN computing, demonstrating multilevel quantized current values for in-memory computation. We achieved a handwriting digit recognition rate of 91.6%, thereby demonstrating reliable QNN hardware performance. This robust QNN performance indicates that the metal oxide phototransistor shows significant potential for future ternary computing systems.

Thermal excitation of carriers at tail states and recombination rate in hydrogenated amorphous silicon

Measurements of low-energy photoluminescence (PL) in hydrogenated amorphous silicon (a-Si:H) have been performed by means of frequency-resolved spectroscopy. Temperature variation of radiative recombination rate of electron–hole pairs and thermal excitation of carriers at the tail states are discussed. The results suggest that the tail electrons do not contribute to the PL of the energy lower than 1.0 eV. Experimental results are quantitatively explained by considering a two-level system instead of the tail states.

Scalable Hybrid Films of Polyimide‐Animated Quantum Dots for High‐Temperature Capacitive Energy Storage Utilizing Quantum Confinement Effect

AbstractThe miniaturization trend of future electrical and electronic systems tender higher demands for high‐temperature resistant dielectric capacitors. The exponential increase in leakage current of dielectric polymers under high temperatures and strong electric fields is a pivotal bottleneck constraining the application of capacitors. Herein, a design is described where various animated quantum dots (A‐QDs), serving as crosslinkers, are introduced into the polyimide (PI) matrix to regulate the excitation and transport of carriers. Using fluorescence lifetime imaging (FLIM) and Kelvin probe force microscopy (KPFM), it is demonstrated that the intrinsic quantum confinement effect of quantum dots (QDs) significantly enhances carrier immobilization. Moreover, the dynamic stability of the covalently bonded network at elevated temperatures has proven to be effective in reducing dielectric loss, while enhancing the electrical insulation properties of the hybrid films. Therefore, at 200 °C, the optimal hybrid film exhibits an exceptional discharged energy density of 5.8 J cm−3 with an efficiency exceeding 90%, surpassing the performance observed in the majority of other polymer nanocomposites. Collectively, these characteristics highlight the promising potential of incorporating A‐QDs as crosslinked points within the polymer matrix, thereby enhancing the applicability of dielectric polymers for high‐temperature applications.

Control of excited state carrier lifetime of ZnV2O6 photocatalyst by g-C3N4 non-adiabatic molecular dynamics analysis

Control of excited state carrier lifetime of ZnV2O6 photocatalyst by g-C3N4 non-adiabatic molecular dynamics analysis

Strategies for the alignment of electronic states in quantum-dot tunnel-injection lasers

In quantum-dot tunnel-injection lasers, the excited charge carriers are efficiently captured from the bulk states via an injector quantum well and then transferred into the quantum dots via a tunnel barrier. The alignment of the electronic levels is crucial for the high efficiency of these processes and especially for the fast modulation dynamics of these lasers. In particular, the quantum mechanical nature of the tunneling process must be taken into account in the transition from two-dimensional quantum well states to zero-dimensional quantum-dot states. This results in hybrid states, from which the scattering into the quantum-dot ground states takes place. We combine electronic state calculations of the tunnel-injection structures with many-body calculations of the scattering processes and insert this into a complete laser simulator. This allows us to study the influence of the structural design and the resulting electronic states as well as limitations due to inhomogeneous quantum-dot distributions. We find that the optimal electronic state alignment deviates from a simple picture in which the quantum-dot ground state energies are one LO-phonon energy below the injector quantum well ground state.

Ultra-sensitive broadband photoresponse realized in epitaxial SnSe/InSe/GaN heterojunction for light adaptive artificial optoelectronic synapses

By integrating sensing, memory, and computing functions within a single unit, the neuromorphic device enables brain-like computing that could facilitate artificial intelligence applications due to its superior scalability and efficiency. As an emerging form of neuromorphic devices, optoelectronic synapses for artificial visual perception with high optical sensing performance and tunable memory time are highly desired. Herein, we report a new design of ultra-sensitive broadband artificial optoelectronic synapses based on a tailored hybrid heterostructure of epitaxial SnSe/InSe/GaN multilayers, in which the interlayer InSe has been employed as the functional layer. Specifically, InSe plays the crucial role of charge trapping layer by blocking the thermally excited carriers in the potential well, thus an ultra-low dark current in 10−10 A level has been realized under the bias of −1.5 V. Moreover, the InSe layer could significantly extend the photocurrent decay time, which enabled the synaptic functions in the devices. Such multilayer heterojunctions exhibit a broadband photoresponse spanning from the near-infrared to the ultraviolet, with a specific detectivity up to 1.07×1011 Jones under 365 nm excitation. Combining all these merits, light adaptive artificial optoelectronic synapses with multi-color stimuli perception capability have been demonstrated. These results provide a novel and promising strategy for future applications in artificial vision systems.

(Invited) Semiconductor-Sensitized Thermal Cell: Difference of Photoexcitation and Thermal Excitation

A new thermal energy conversion approach called a semiconductor-sensitized thermal cell (STC) was recently reported by the presenters' group.[1,2] STC was inspired by a dye-sensitized solar cell (DSSC), i.e., a photoelectrochemical solar cell. The redox reactions of photoexcited carriers in dyes generate electric power in DSSC. In STC, the redox reactions of thermally-excited carriers in semiconductors generate electric power. It has a simple structure: semiconductor-electrode, counter-electrode, and electrolyte. The power generation was terminated once the chemical equilibrium reached the set temperature. Flipping the switch, the cell moved to a different equilibrium state, restoring its power generation characteristics.Over the past few years, STC research has progressed. The redox ability of thermally excited carriers in semiconductors has been demonstrated using FeSi2[1] and Ge.[2] The analogy of STC and DSSC has been shown by organic-perovskite-[3] and Ag2S-[4] sensitized cells since those semiconductor-sensitized cells could generate electricity from light and heat, respectively. Inductively coupled plasma (ICP) measurements showed that electric power generation was not accomplished through the elution of semiconductor electrodes.[5]In this presentation, we will carefully explain the STC process, starting with thermally excited charge generation in semiconductors, which is unfamiliar to chemists, and introduce the academic and social implementation directions of STC.[1] S. Matsushita, A. Tsuruoka, E. Kobayashi, T. Isobe, A. Nakajima, "Redox reactions by thermally excited charge carriers: towards sensitized thermal cells," Mater. Horiz., 4 649–656 (2017).[2] K. Tamaki, S. Matsushita, "Counter electrode dependence of germanium-sensitized thermal cells," Jpn. J. Appl. Phys. 63 01SP22 (2024).[3] S. Matsushita, S. Sugawara, T. Isobe, A. Nakajima, "Temperature Dependence of a Perovskite-Sensitized Solar Cell: A Sensitized “Thermal” Cell," ACS Appl. Energy Mater., 2 13-18 (2019).[4] Y. Inagawa, T. Isobe, A. Nakajima, S. Matsushita, "Ag2S-Sensitized Thermal Cell," J. Phys. Chem. C, 123 12135-12141 (2019).[5] S. Matsushita, T. Araki, B. Mei, S. Sugawara, Y. Inagawa, J. Nishiyama, T. Isobe, A. Nakajima, "A sensitized thermal cell recovered using heat," J. Mater. Chem. A, 7 18249-18256 (2019).

(Invited) Low-Temperature Processing of Mixed Conducting Oxides: Control of Defects, Transport, and Reactions Away from Equilibrium

Low-to-intermediate temperature processing and use of mixed conducting oxides targets the following potential benefits: ultra-fine nanostructuring with avoidance of coarsening, control of surface chemistry, lower energy consumption, integration with thermally sensitive components, and slower degradation vs. conventional high temperature synthesis and use. In this lower temperature regime, we have pursued two routes for non-equilibrium control of defect chemistry and therefore properties of mixed ionic/electronic conductors (MIECs) in the ferrite perovskite family: crystallization and illumination.Room-temperature growth followed by mild anneals to induce crystallization has proven to yield superior oxygen surface exchange kinetics, attributed to development of facile charge transfer and pristine surface chemistry in our prior work. The close link between crystallinity and oxygen stoichiometry appears to play an important role in the structural evolution and electrical response. However the effect of evolving crystallinity on the ionic conductivity has not been studied in-depth. Therefore we recently pursued an approach combining in-situ synchrotron grazing-incidence X-ray diffraction with ac and dc electrical methods on thin-film blocking electrode cells to evaluate evolution of transference numbers during crystallization. In certain ferrite films we observe a ~2 orders-of-magnitude increase in ionic conductivity during crystallization, while the electronic conductivity increases much more. Through the process, we demonstrate that it is possible to turn a predominantly ionic conductor into a predominantly electronic conductor - and to access almost any ionic transference number in between - simply by controlling the degree of crystallinity. [1]Oxygen stoichiometry can alternatively be controlled at low-to-intermediate temperatures through application of low fluence above-gap illumination. We have shown in certain non-dilute ferrite films that UV light causes oxygen to enter the films from the gas phase with kinetics limited by the oxygen surface-exchange reaction, which is largely unchanged by the presence of excited carriers. We explain the response through simulations of excited state defect equilibria and transient absorption spectroscopy measurements. [2]Together these processing strategies indicate that low-temperature control of oxygen stoichiometry is feasible in mixed conductors, enabling wide tailoring of properties and potential for use in low-to-intermediate temperature solid-state ionic devices.[1] H.B. Buckner, J. Simpson-Gomez, A. Bonkowski, K. Rubartsch, H. Zhou, R.A. De Souza, N.H. Perry (in review)[2] E. Skiba, H.B. Buckner, G. McKnight, C. Lee, R. Wallick, R. van der Veen, E. Ertekin, N.H. Perry (in review)We gratefully acknowledge funding from the US Department of Energy, Basic Energy Sciences, under DOE Early Career grant DE-SC-0018963 and from the US National Science Foundation, through the University of Illinois at Urbana-Champaign Materials Research Science and Engineering Center DMR-2309037.

Highly Efficient Hydrogen Production By Photoetched CdS-Pt

Photocatalytic hydrogen production is a promising method for solar energy harvesting. To achieve highly efficient photocatalytic reactions, it is necessary to utilize visible light effectively, which constitutes a significant portion of the sunlight spectrum. However, there are only few reports of photocatalytic materials that exceed an external quantum efficiency (EQE) of 50% in photocatalytic hydrogen evolution under visible light. Therefore, to achieve a higher efficiency, here we combined CdS, one of representative visible-light-responsive photocatalysts, with Pt nanoparticles as co-catalysts suitable for hydrogen production, and controlled the morphology of CdS through photoelectrochemical etching. We prepared the CdS-Pt photocatalysts through three-steps: (1) synthesis of highly crystalline CdS in a molten salt, (2) loading Pt co-catalysts onto CdS by photoelectrochemical deposition utilizing surface defects, and (3) partial etching of CdS during photocatalytic hydrogen evolution by using excessive holes.(1) Synthesis of highly crystalline CdS via molten salt treatment: Zincblende CdS particles were prepared from cadmium nitrate and treated further in molten NaCl and CaCl2 to obtain highly crystalline wurtzite CdS (Figure 1a).1 (2) Loading of Pt co-catalysts utilizing surface defects: The prepared wurtzite CdS was irradiated with light (430 nm) in an aqueous solution of lactic acid under a nitrogen atmosphere. In this process, photo-excited electrons are trapped in sulfide ion defects at the CdS surface, while holes are consumed by oxidation of lactic acid. Then, [PtCl6]2- ions were added to the solution in the dark and Pt co-catalysts were deposited onto CdS through reduction reaction of the ions by the trapped electrons (Figure 1b).2 (3) Morphology control of CdS through photoelectrochemical etching: The resulting CdS-Pt was dispersed again in the lactic acid solution and irradiated with light (430 nm) under nitrogen atmosphere. Photo-excited electrons are used for hydrogen evolution through reduction of protons, while holes are consumed by the oxidation of lactic acid and self-oxidation of CdS. In particular, when the light intensity is high, the generation rate of excited carriers as well as the reduction rate of protons exceed the oxidation rate of lactic acid, and partial photoetching of CdS via the self-oxidation occurs significantly. As a result of the photoetching, the surface area of the photocatalyst was increased (Figure 1c). After the photoetching process, we evaluated the photocatalytic activity of the photoetched CdS-Pt for a hydrogen production reaction in a NaOH solution containing 20 vol% 2-propanol, in which high pH suppresses dissolution of cadmium ions and thereby further photoetching. We found that the hydrogen generation rate was improved by ~8 times. The EQE of the hydrogen production reaction using the photoetched CdS-Pt was calculated to be 91%. The specific surface area of CdS-Pt increases and the distance of photogenerated carriers to reach the surface decreases due to the photoetching. These changes are favorable for the photocatalytic reaction and may contribute to the increased activity. If the rate of carrier excitation is too high in comparison with the rate of carrier consumption by redox reactions, EQE should be low. However, the photoetching process decreases both the particle volume and the generation rate of excited carriers, while the reaction sites are increased. These should be responsible for the improved EQE. The present technique, which was developed to optimize the morphology of CdS-Pt photocatalyst under given conditions, would lead to highly efficient photocatalytic water splitting and photoreforming reactions, as well as to mechanism analysis of the efficient photocatalytic reactions.References H. Nagakawa, T. Tatsuma, ACS Appl. Energy Mater., 2022, 5, 1465–14657.H. Nagakawa, T. Tatsuma, J. Phys. Chem. C, 2023, 127, 20337–20343. Figure 1

Comparative study of electronic, thermoelectric, and transport properties in germanene and carbon nanotubes

This study investigates the electronic, thermoelectric, and transport properties of Germanene nanotubes (GeNTs) using a tight-binding calculations and explores their potential advantages over carbon nanotubes (CNTs) in thermoelectric applications. The effect of external electric and magnetic fields, nanotube radius, and temperature on heat capacity, electrical conductivity, power factor, and Seebeck coefficient are systematically investigated. The heat capacity C(T) of GeNTs shows a more pronounced increase with the application of external fields, especially electric field and exceeds that of CNTs. GeNTs also exhibit a lower threshold temperature for non-zero electrical conductivity attributed to their smaller energy gap, which results from easier thermal excitation of charge carriers. This property becomes more pronounced as the electrical conductivity of GeNTs increases more rapidly with temperature than that of CNTs, particularly under stronger external fields. Additionally, the thermoelectric properties of GeNTs can be further enhanced under external fields. While CNTs possess higher Seebeck coefficients and Lorenz numbers, GeNTs demonstrate distinct advantages in tunable heat capacity and conductivity, making them promising candidates for efficient thermoelectric and nanoelectronic devices. These results underscore the potential of GeNTs as an effective alternative to CNTs for advanced energy conversion applications, demonstrating their promise as high-performance thermoelectric materials for energy conversion applications.

Hiding in plain sight: The prevalence and impact of trions and Fermi polarons in transient absorption spectroscopy experiments of 2D semiconductors.

Transient absorption (TA) spectroscopy is one of the most popular experimental methods to measure the excited state lifetimes and charge carrier recombination mechanisms in two dimensional (2D) semiconductors. This fundamental information is essential for designing and optimizing the next generation of ultrathin and lightweight 2D semiconductor-based optoelectronic devices. However, the interpretation of TA spectroscopy data varies across the community. The community lacks a unifying physical explanation for how and why experimental variables such as incident light intensity, sample-substrate interactions, and/or applied bias affect TA spectral data. This Perspective (1) compares the physical chemistry TA literature to nanomaterial physics literature from a historical perspective, (2) reviews multiple physical explanations that the TA community developed to explain spectral features and experimental trends, (3) provides a unifying explanation for how and why trions-and, more generally, Fermi polarons-contribute to TA spectra, and (4) quantifies the extent to which various physical interpretations and data analysis procedures yield different timescales and mechanisms for the same set of experimental results. We highlight the importance of considering trions/Fermi polarons in TA measurements and their implications for advancing our understanding of 2D material properties.

Exploring Ultrafast Carrier Dynamics in Photoelectrochemical Water Splitting Using Transient Absorption Spectroscopy

AbstractHydrogen fuel produced from water splitting using sunlight remains one of the cleanest and most sustainable energy sources. However, photoelectrochemical hydrogen production faces several challenges, including poor sunlight absorption, deficient charge carrier lifetime and transfer rates, and very sluggish kinetics of the water oxidation half‐reaction. To address these challenges, researchers have concentrated on enhancing the efficiency of photoelectrochemical water splitting. A critical factor in this enhancement is a deeper understanding of the fundamental processes involved in photoabsorption and the subsequent oxidation evolution reaction. The advent of ultrafast lasers has enabled the detailed tracking of charge carriers within materials after sunlight absorption, including their separation and migration to the surface for water oxidation. Ultrafast transient absorption spectroscopy is a powerful technique that allows real‐time observation of the behavior of excited states and charge carriers on femtosecond to nanosecond timescales. Recent studies utilizing ultrafast transient absorption spectroscopy are explored to investigate the dynamics of charge carriers in photoelectrochemical water splitting, providing insights into the mechanisms that lead to the design of enhanced photoanodes with improved efficiency.

Optical control of multiple resistance levels in graphene for memristic applications

Neuromorphic computing has emphasized the need for memristors with non-volatile, multiple conductance levels. This paper demonstrates the potential of hexagonal boron nitride (hBN)/graphene heterostructures to act as memristors with multiple resistance states that can be optically tuned using visible light. The number of resistance levels in graphene can be controlled by modulating doping levels, achieved by varying the electric field strength or adjusting the duration of optical illumination. Our measurements show that this photodoping of graphene results from the optical excitation of charge carriers from the nitrogen-vacancy levels of hBN to its conduction band, with these carriers then being transferred to graphene by the gate-induced electric field. We develop a qualitative model to describe our observations. Additionally, utilizing our device architecture, we propose a memristive crossbar array for vector-matrix multiplications.

Unveiling multimodal hot carrier excitation in plasmonic bimetallic Au@Ag nanostars for photochemistry and SERS sensing

Plasmonic nanostructures stand at the forefront of nanophotonics research, particularly in sensing and energy conversion applications. Their unique ability to confine light energy at the nanoscale makes them indispensable for a wide array of technological advancements. The study of these structures often makes use of different materials and, even more extensively, explores new shapes and configurations to extend our common repertoire of useful nanophotonics tools. Exploring the creation of bimetallic plasmonic nanostructures combines these two dimensions determining the space of possible plasmonic resonators and opens the possibility of tailoring systems with behavior unavailable to single-metal plasmonic structures. In this paper, we delve into the exploration of bimetallic systems employing plasmonic nanostars. These structures have demonstrated remarkable capabilities for surface-enhanced Raman scattering (SERS) spectroscopy and photochemistry, due to the strong plasmonic response of their peaks, whose disposition following a spherical symmetry makes them largely polarization- and orientation-insensitive. Herein, we report the colloidal synthesis of two different water-stable Au@Ag nanostars, explore their performance as photocatalysts and SERS substrates, and provide an in-depth account of their non-trivial physical response.

Directing Charge Carriers and Ferroelectric Domains at Lateral Interfaces in van der Waals Heterostructures.

Emergent phenomena in traditional ferroelectrics are frequently observed at heterointerfaces. Accessing such functionalities in van der Waals ferroelectrics requires the formation of layered heterostructures, either vertically stacked (similar to oxide ferroelectrics) or laterally stitched (without equivalent in 3D-crystals). Here, we investigate lateral heterostructures of the ferroelectric van der Waals semiconductors SnSe and SnS. A two-step process produces ultrathin crystals comprising an SnSe core laterally joined to an SnS edge-band, as confirmed by Raman spectroscopy, transmission electron microscopy (TEM) imaging, and electron diffraction. TEM shows a moiré pattern across the SnSe core due to coverage by an ultrathin SnS layer. The ability of the lateral interface (IF) to direct excited carriers, probed by cathodoluminescence, shows electron transfer over 560 nm diffusion length from the SnS edge-band. Large, thin flakes supporting ferroelectricity allow investigating domains and domain wall interactions in uniform crystals and lateral heterostructures. Polarized optical microscopy of sub-20 nm flakes consistently shows ⟨110⟩ oriented stripe domains with mirror-twin domain walls. Heterostructures adopt two domain configurations, with domains either constrained to the SnSe core or propagating across the entire SnSe-SnS flakes. The combined results demonstrate multifunctional van der Waals heterostructures with high-quality IFs presenting extraordinary opportunities for manipulating carrier flows and ferroelectric domain patterns.

Quantifying relaxation time constants in MoS2xSe2(1-x) alloys: Impact of Stoichiometry and Si/SiO2 interference

Despite a decade of extensive research on two-dimensional transition metal dichalcogenides, there are still some gaps in our understanding of their remarkable properties, limiting their potential applications. One such gap relates to the properties of these alloys, which offer tunable band gaps and can serve as the basis for various optoelectronic and nonlinear optical devices. A comprehensive understanding of the ultrafast charge carrier excitation and relaxation dynamics as well as their characteristic relaxation time constants, is crucial for the further use of these materials in specific applications. In this work, we experimentally investigate the dynamics of ultrafast relaxation processes in monolayer MoS2xSe2(1-x) alloys on a Si/SiO2 substrate using time-resolved spectroscopy. We determine the characteristic relaxation time constants for structures with different stoichiometric compositions and refine these constants by accounting for interference effects arising from the substrate.