Molybdenum carbide (MoC) is a promising candidate for substituting expensive platinum-group metals in many applications owing to its low cost and excellent properties. A comprehensive understanding of the carrier dynamics in MoC facilitates its implementations and helps designing synthesis strategies. In this work, the carrier relaxation in MoC nanosheets is investigated by combining femtosecond transient reflection spectroscopy with first-principles calculations. The observed processes of electron-electron, electron-phonon, and phonon-phonon scattering show longer lifetimes compared to those of other transition metal carbides. The nanosecond carrier lifetime is explained by the restricted phonon decay pathways induced by the large mass difference between C and Mo atoms, which is revealed through the analysis of calculated phonon dispersion. The slow cooling of hot carriers in MoC nanosheets offers a simple approach for designing devices that effectively utilize hot carriers, which are expected to improve photothermal and photovoltaic performances.

Semimetal molybdenum ditelluride (1T'-MoTe_2) possess diverse phase transitions enriching its application prospects. The structural response during these transitions is crucial to understanding the underlying mechanisms, but the desired details of pathway and time span are still insufficient. Here, we investigate the lattice evolution in few-layer 1T'-MoTe_2 after photoexcitation, using ultrafast electron diffraction and density functional theory (DFT) calculations. The observed complex lattice responses with unintuitively evolving Bragg peak intensity and interplanar spacing, are best interpreted as the combination of shear displacement and Mo-Mo bond shortening in a few picoseconds, and a metastable structure in nanoseconds, basing on the analyses of structure factor and pair distribution function. The DFT calculations reveal that, the photodoped electrons induced population change of the antibonding states close to Fermi level, lead to the shear displacement and the dimerization of Mo pairs. Our findings present new insights for elucidating the picture of Peierls distortion in 1T'-MoTe_2.

Surface effects and quantum confinement render nanomaterials' optoelectronic properties more susceptible to nonradiative processes than their bulk counterparts. These nonradiative processes usually contain a series of interwoven and competing subprocesses, which are challenging to disentangle. Here, we investigate the structural origin of Auger recombination in ZnO nanoparticles using transient absorption spectroscopy and ultrafast electron diffraction. The photogenerated hot holes are captured by oxygen vacancies via an excitonic Auger process, inducing significant local polaronic distortions around the oxygen vacancy and its neighboring zinc tetrahedron on a subpicosecond time scale. The recombination of trapped holes accelerates lattice thermalization and stabilizes the formed small hole polarons. Subsequently, the distorted lattice captures additional electrons in a conduction band, forming a long-lived (>6 ns) exciton-polaron complex that may account for the visible luminescence. Our findings are potentially applicable to other transition metal oxide nanomaterials, bringing insights for optimizing their functional properties.

ABSTRACT Metal halide perovskites derive their remarkable optoelectronic properties from the combined effects of strong electron‐phonon coupling, exciton localization, and the dynamic stereochemical activity of n s 2 lone pairs. However, Ge (II)‐based perovskites typically exhibit static lone‐pair expression (LPE) and pronounced octahedral distortion, attributed to the second‐order Jahn‐Teller effect, which leads to severe nonradiative recombination. Here, we demonstrate that incorporating 1‐methylpiperazium (MPZ 2+ ), with bidentate coordination sites and balanced polarity, steric effect, and conformational flexibility, can effectively tune the organic‐inorganic interactions in a newly synthesized Ge(II)‐based zero‐dimensional perovskite, (MPZ) 2 GeI 6 , with suppressed LPE in Ge(II) ground states and minimized octahedral distortion. The structural symmetry facilitates strong orbital overlap between Ge 2+ and I − , leading to increased electron‐phonon coupling and high defect tolerance. Coupled with the enhanced quantum confinement of isolated 0D [GeI 6 ] octahedra, (MPZ) 2 GeI 6 exhibits a large Stokes shift of 216 nm and a record‐high photoluminescence quantum yield of 19.1% among reported Ge 2+ ‐based perovskites. Additionally, it demonstrates remarkable resistance to moisture, maintaining structural integrity and photoluminescence even after absorbing 0.5 g of water per gram of material, attributable to the polarity of MPZ molecules that help stabilize water molecules. These findings offer new insights into the structural and optical modulation of Ge‐based perovskites.

Wide bandgap perovskite could pave the way for the development of ultraviolet photodetectors for smart devices. As demonstrated in the extant literature, wide bandgap methylammonium lead chloride (MAPbCl3) photodetectors have been shown to exhibit excellent stability, a fast response time, and high sensitivity. However, ultraviolet photodetectors based on MAPbCl3 perovskite thin polycrystalline films exhibit deficiencies in light utilization within the UV region and inadequate carrier extraction. In this study, nanocrystals-in-perovskite thin films are proposed for use as UV photodetectors, featuring exceptional light-harvesting capabilities and strong dielectric response. The proof-of-concept experiment incorporates metal oxide nanocrystals of tin (SnO2) into the MAPbCl3 host material as guest components, with the objective of modulating the optical and electrical properties of the resultant hybrid thin films. This is the first report to demonstrate that the nanocrystal embedding strategy can enhance electrical and optical performance in a simultaneous manner. Nanocrystal-inperovskite thin films exhibit over 20 % absorption enhancement and can prolong charge carrier lifetime via modulated dielectric response. The resulting ultraviolet photodetectors that incorporate a thin layer of nanocrystals-in-perovskite in a glass/ITO/SnO2/photoactive layer/PTAA/poly-TPD/Au device structure demonstrate a high linear dynamic range of over 110 dB under a 375 nm LED from 0.05 up to 105 mu W cm-2. This feature is of particular interest for applications in night vision imaging and intelligent driving. The device manifests a large D* of 1.56 x 1012 Jones with a suppressed arms of 0.09 nA cm- 2 and long stability.

The two/three-dimensional (2D/3D) heterojunctions hold promising prospects for improving the performance and stability of perovskite optoelectronic devices. However, the ultrathin thickness of 2D perovskite capping layer makes it difficult to directly obtain the characteristics of 2D/3D heterojunctions, leading to insufficient understanding of its compositional and configurational details. Here, we systematically investigated the heterojunction composition by precisely controlling the thickness of 3D perovskite layer using vacuum deposition techniques. Surprisingly, contrary to the traditional view that the 2D perovskite layer contains only 2D or quasi- 2D phases, we found that zero-dimensional (0D) Cs4Pb(Br/Cl)6 and 3D CsPb(Br/Cl)3 grains exist alongside quasi- 2D perovskite grain in the quasi-2D perovskite capping layer. By carrier dynamics analysis, we proposed a 0D-3D cascade model to elucidate the anomalous electrical performance enhancement. By introducing a multifunctional additive PEATFA to manipulate the phase distribution, the feasibility of the model was verified and the cascade effect was further enhanced, resulting in a maximum external quantum efficiency of 8.92 %, representing the optimal performance of blue PeLEDs fabricated by vacuum deposition. This work deepens the understanding of 2D/3D heterojunctions and provides a new research path to study 2D/3D heterojunctions.

Efficient and stable pure-red perovskite light-emitting diodes (PeLEDs) show great potential for high-definition displays. Despite significant advancements in device performance over recent years, achieving both high external quantum efficiency (EQE) and long-term stability remains a significant challenge. In this study, we employed 5amino valeric acid (5-AVA) as a crystallization agent and utilized in situ analytical techniques, including X-ray diffraction, UV-visible spectroscopy, and photoluminescence, to explore its role in the crystallization kinetics of quasi-2D perovskite (POEA0.7NMA0.3)2CsPb2I7 films. Our results reveal that the strong coordination capability of 5-AVA with Pb2+ ions promotes direct nucleation of perovskite phases and slows down crystal growth following a "rapid nucleation, slow growth" mechanism. This well-controlled crystallization kinetics leads to enhanced crystallinity, reduced defect density, and a narrower cascade phase distribution for quasi-2D perovskites. Ultimately, these improvements enable pure-red PeLEDs to achieve higher EQEs, increasing from 15.68 % to 21.23 %, and extending the operational lifetime, from 242 min to 506 min. Our research highlights the potential of precisely regulating crystallization kinetics to address the key challenges of phase impurity and non-radiative energy losses in quasi-2D PeLEDs, paving the way toward reliable and efficient PeLEDs.

We report the observation of exciton formation from the cooled band-edge carriers in mixed two-dimensional hybrid organic-inorganic perovskites using femtosecond transient absorption spectroscopy. By monitoring the changes of bleach signal upon excitations with various photon energy, we are able to extract the values of exciton binding energy and the occupancies of carriers of free and bound states for each two-dimensional phase. We also confirm the existence of Mahan exciton when injected carrier density is above the Mott criterion.

Antimony selenide (Sb 2 Se 3 ) is an attractive candidate for photovoltaics with not yet satisfying efficiency. Beside defects, the polaron formation originated from lattice distortion was proposed to account for the free‐carrier trapping and the subsequent carrier dynamics and optoelectronic properties. However, such a mechanism is still lack of the information of structural responses. Here, the evolution pathways of carrier and lattice after photoexcitation are tracked through the pump‐probe methods of optical absorption and electron diffraction, which reveal the temporal correlations between the dynamics of both degrees of freedom. Opposite separation changes of atom pairs in the (Sb 4 Se 6 ) n ribbons are observed within a few picoseconds then followed by the structural distortions induced intermediate state, which last several tens of picoseconds, coinciding with the optical phonon population and the trapping process of carriers, respectively. Together with the analysis of electron diffuse scattering modulated by the Gaussian atomic displacement fields of polaron model, the picture of formation of anisotropic polarons with large size is constructed. The findings reported in this work provide the direct information of carrier and structure for elucidating the polaron scenario in Sb 2 Se 3 , and probably in other novel optoelectronic materials with anisotropic structure and soft lattice.

Upscaling perovskite solar cells to the module level while ensuring long-term stability is crucial for their commercialization. Here, we report a bottom-up crosslinking strategy utilizing 4-(aminomethyl)benzoic acid as a dual-anchor linker integrated into quasi-two-dimensional (2D) perovskite to reduce the weak van der Waals gap between individual 3D perovskite layers and to functionalize the NiOx/perovskite interface. This approach not only enhances the coupling of the perovskite slabs within the quasi-2D structure, leading to enhanced stability, but it also promotes the vertical growth of highly ordered, phase-pure low-dimensional perovskite films with improved carrier transport. The quasi-2D perovskite solar modules (PSMs) fabricated using this method have demonstrated an optimal efficiency of 16.05% over an aperture area of 9.66 cm2, utilizing a blade-coating technique in ambient air. Meanwhile, the PSMs have passed the standard damp- heat and operational stability tests for 1,000 h with negligible degradation, as verified by the VDE Prufund Zertifizierungsinstitut GmbH (VDE Institute).

Semiconductor molybdenum ditelluride (2H-MoTe2) possess multiple valleys in the band structure, enriching its physical properties and potentials in applications. However, the effect of multiple valleys on the mechanisms of population and relaxation of carriers and phonons remains limited, particularly due to the inadequacy of current optical probes that lack momentum sensitivity. Here, we rely on time resolved measurements of optical absorption and electron diffraction to investigate the carrier intra- and intervalley scattering and the phonon dynamics in different valleys in photoexcited few-layer 2H-MoTe2. Our experimental results are complemented by density functional theory calculations and molecular dynamics simulations. We reveal the pathways and timescales of carrier relaxation, accompanied with the emissions of optical phonons at the Brillouin zone center and acoustic phonons at the zone border. We estimate the population of different phonon modes based on the measured results, identifying quantitatively the occurrences of phonon bottleneck located in different valleys. Our technique allows constructing a comprehensive picture of the complex interactions between carriers and phonons in 2H-MoTe2 with the valley degree of freedom resolved. The effect of multiple energy valleys in the conduction band of semiconductor molybdenum ditelluride on energy dissipation remains unclear. Here, the authors employ time resolved measurements to reveal the pathways and timescales of carrier relaxation and phonon dynamics in different valleys, identifying the phonon bottlenecks of such relaxation.

Abstract Rare earth ions with d-f transitions (Ce3+, Eu2+) have emerged as promising candidates for electroluminescence applications due to their abundant emission spectra, high light conversion efficiency, and excellent stability. However, directly injecting charge into 4f orbitals remains a significant challenge, resulting in unsatisfied external quantum efficiency and high operating voltage in rare earth light-emitting diodes. Herein, we propose a scheme to solve the difficulty by utilizing the energy transfer process. X-ray photoelectron spectroscopy and transient absorption spectra suggest that the Cs3CeI6 luminescence process is primarily driven by the energy transfer from the I2-based self-trapped exciton to the Ce-based Frenkel exciton. Furthermore, energy transfer efficiency is largely improved by enhancing the spectra overlap between the self-trapped exciton emission and the Ce-based Frenkel exciton excitation. When implemented as an active layer in light-emitting diodes, they show the maximum brightness and external quantum efficiency of 1073 cd m−2 and 7.9%, respectively.

The fabrication of perovskite light-emitting diodes (PeLEDs) with vacuum deposition shows great potential and commercial value in realizing large-area display panel manufacturing. However, the electroluminescence (EL) performance of vacuum-deposited PeLEDs still lags behind the counterparts fabricated by solution process, especially in the field of blue PeLEDs. Here, the fabrication of high-quality CsPbBr3- x Clx film through tri-source co-evaporation is reported to achieve high photoluminescence quantum yield (PLQY). Compared with the conventional traditional dual-source co-evaporation, the tri-source co-evaporation method allows for freely adjustable elemental ratios, enabling the introduction of the lattice-matched Cs4 Pb(Br/Cl)6 phase with the quantum-limited effect into the inorganic CsPb(Br/Cl)3 emitter. By adjusting the phase distribution, the surface defects of the emitter can be effectively reduced, leading to better blue emission and film quality. Further, the effects of Cs/Pb ratio and Br/Cl ratio on the PLQY and carrier recombination dynamics of perovskite films are investigated. By optimizing the deposition rate of each precursor source, spectrally stable blue PeLEDs are achieved with tunable emission ranging from 468 to 488 nm. Particularly, the PeLEDs with an EL peak at 488 nm show an external quantum efficiency (EQE) of 4.56%, which is the highest EQE value for mixed-halide PeLEDs fabricated by vacuum deposition.

Ultraviolet photodetectors (UPDs) based on low-dimensional halide perovskites have undergone rapid development. Here, regulation of the electronic configuration of low-dimensional hybrid perovskites are reported via organic cations for self-powered UPDs. For the first time, it is determine that the rational design of organic cation phenyl alkylammonium can effectively prevent phonon scattering thus increasing charge carrier extraction in low dimensional lead chlorine perovskite thin-films. As a result, the exciton-binding energy can be reduced to 62.91 meV in (PMA)2PbCl4 perovskite films with a charge-carrier mobility of 0.335 cm2 V-1 s-1. The fabricated (PMA)2PbCl4-based self-powered UPDs has achieved a high detectivity of 6.32 × 1013 jones with a low noise current of 0.35 pA Hz-1/2 under zero bias. A further demonstration of images with high UV to visible light rejection ratio under weak-light illumination of 70 nW cm-2 highlights the feasible potential application of low-dimensional perovskite.

Two-dimensional (2D) and quasi-2D modifications of three-dimensional (3D) perovskite active layers have contributed to advances in the performance of perovskite solar cells (PSCs). However, the ionic diffusion between the surface 2D and bulk 3D perovskites leads to the degradation of the 3D/2D perovskite heterostructures and limits the long-term stability of PSCs. Here we incorporate a cross-linked polymer (CLP) on the top of a 3D perovskite layer and then deposit a 2D perovskite layer via a vapour-assisted two-step process to form a 3D/CLP/2D perovskite heterostructure. Photoluminescence spectra and thickness-profiled elemental analysis indicate that the CLP stabilizes the heterostructure by inhibiting the diffusion of cations (formamidinium, FA(+) and 4-fluorophenylethylammonium, 4F-PEA(+)) between the 2D and 3D perovskites. For devices based on carbon electrodes, we report small-area devices with an efficiency of 21.2% and mini-modules with an efficiency of 19.6%. Devices retain 90% of initial performance after 4,390 hours operation under maximum power point tracking and one-sun illumination at elevated temperatures. Solar cells based on 3D/2D perovskite heterostructures show promising performance, but ion diffusion limits the device stability. Now Luo et al. suppress ion diffusion by inserting a cross-linked polymer between the 2D and 3D layers, improving the operational stability.

Heterostructures constructed by noble metals and two-dimensional (2D) semiconductors offer a unique charge transport path to collect hot carriers from plasmonic nanostructures and thus are promising for various plasmonic and optoelectronic devices. However, the desired charge transfer speed and efficiency of the conventional heterostructures are usually restricted by the limited interface area and inevitable interface distortion and contamination. Herein, we report the ultrafast and high-efficiency hot electron transfer by creating a novel Au@MoS2 core-shell heterostructure with atomically sharp and dramatically enlarged interface. Our femtosecond transient absorption spectroscopy study indicates the hot-electron injection from Au nanoparticles to MoS2 in Au@MoS2 is within 244 fs, compared with the 493 fs of the mechanically-transferred Au/MoS2 control sample. And meanwhile, the injection efficiency is improved from 3.33% of Au/MoS2 to 25.3% of our Au@MoS2. The results are further proved by Kelvin probe force microscopy and discrete dipolar approximation studies, which provide strong evidences that the improved charge transfer is attributed to the atomic-level clean and fully-encapsulated interface of the product. This study provides fundamental understanding of the intrinsic charge transfer within Au@MoS2 heterostructures and thus demonstrates an intriguing material geometry for future plasmonic and optoelectronic devices.

Mixed dimensional van der Waals heterostructure based on layered two-dimensional molybdenum disulfide (MoS2) interfaced to gallium nitride (GaN) has attracted tremendous attention due to its unique properties and application in novel electronic, optoelectronic, and quantum devices. However, developing facile synthesis methods and insights into the exciton dynamics for this system still remains a major challenge. Here, a simple and cost-effective method is demonstrated for large-scale synthesis of monolayer MoS2 on differently doped GaN substrates. A mixed aqueous solution of Na2MoO4 and NaOH is spin-coated on GaN and sulfurated in one step by chemical vapor deposition (CVD). High quality monolayer MoS2 nanosheets with side length over 400 μm and surface coverage ratio of more than 90 % are achieved on GaN. Furthermore, the PL intensity, excitonic transition ratios and ultrafast exciton dynamics of MoS2 are observed to be largely modulated by the doping type of GaN, owing to substrate-induced doping, which is proved by Raman, PL and transient absorption spectroscopy. Notably, p-GaN can attract electrons from monolayer MoS2 and weaken its intrinsic n-doping, thereby facilitating higher PL intensity as well as longer exciton lifetime, while n-GaN provides strong n-doping and generates opposite effect. This work hereby presents a pathway for large-scale synthesis of MoS2/GaN heterostructures and further understanding of their charge transfer properties and exciton dynamics, which should inspire their applications for optoelectronic devices.

Many perovskite oxides (ABO 3 ) are considered the most promising alternatives to noble metal catalysts for oxygen reduction reaction (ORR) due to their high intrinsic activities. However, their electrocatalytic performance is often limited by poor electrical conductivity and low specific surface area. Here an electrochemically induced calcium-leaching process is reported to greatly increase the electrochemical surface area (ECSA) of La 0.6 Ca 0.4 MnO 3 (LCMO64). The ECSA of the activated, Ca-deficient LCMO64 is ≈33.84% higher than that of the unactivated materials, demonstrating superior electrocatalytic ORR performance to the benchmark commercial Pt/C catalyst in an alkaline solution. Theoretical analysis coupled with electrochemical surface state probing and pH-dependent microkinetic modeling suggests that this catalyst with the identified most favorable state under ORR operating conditions reaches the Sabatier optimum of alkaline ORR. This reconstructed LCMO64 is among the best-performing ORR catalysts ever reported, providing new insights into the design of advanced perovskite materials with optimal surface chemistry.

Power conversion efficiencies of inverted perovskite solar cells (PSCs) based on methylammonium- and bromide-free formamidinium lead triiodide (FAPbI 3 ) perovskites still lag behind PSCs with a regular configuration. Here we improve the quality of both the bulk and surface of FA 0.98 Cs 0.02 PbI 3 perovskite films to reduce the efficiency gap. First, we use dibutyl sulfoxide, a Lewis base additive, to improve the crystallinity and reduce the defect density and internal residual stress of the perovskite bulk. Then, we treat the surface of the perovskite film with trifluorocarbon-modified phenethylammonium iodide to optimize the energy levels, passivate defects and protect the film against moisture. The inverted PSCs simultaneously achieve 25.1% efficiency (24.5% from the reverse current–voltage scan measured by a third-party institution) and improved stability. The devices maintained 97.4% and 98.2% of their initial power conversion efficiencies after operating under continuous 1-sun air mass 1.5 G illumination for 1,800 h and under damp heat conditions (85 °C and 85% relative humidity) for 1,000 h, respectively.

Perovskite nanocrystals (PNCs) have recently become promising optoelectronic materials due to their excellent photophysical properties. However, the highly dynamic binding state between ligands and the surface of PNCs has severely restricted their luminescent properties and stabilities. In this work, 1,3-bisbenzyl-2-oxoimidazolidine-4,5-dicarboxylic acid (cycle acid, CA) is introduced as both an etchant and a ligand upon post-synthetic surface treatment of PNCs. By removing the imperfect octahedrons [PbX6]4-and passivating the surface defects synergistically, this treatment improves photoluminescence quantum yields from 76% to 95% and enhances the stability of PNCs against polar solvent, moisture, heat, and illumination. Meanwhile, CA can effectively and instantly recover the luminescence emission for aged PNCs. As a result, the CA-CsPbBr3 PNCs and CA-CsPbIxBr3_x PNCs are applied as color-converting lay-ers on a blue LED chip for warm white light-emitting diodes (WLEDs) with a color coordinate of (0.41, 0.40). Importantly, the CA-based WLED device exhibits superior stability in operational conditions. (c) 2022 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.