Ultrafast Laser Pulse Induced Transient Ferrimagnetic State and Spin Relaxation Dynamics in Two-Dimensional Antiferromagnets.

We employ real-time time-dependent density functional theory (rt-TDDFT) and ab initio nonadiabatic molecular dynamics (NAMD) to systematically investigate the ultrafast laser pulses induced spin transfer and relaxation dynamics of two-dimensional (2D) antiferromagnetic-ferromagnetic (AFM/FM) MnPS3/MnSe2 van der Waals heterostructures. We demonstrate that laser pulses can induce a ferrimagnetic (FiM) state in the AFM MnPS3 layer within tens of femtoseconds and maintain it for subpicosecond time scale before reverting to the AFM state. We identify the mechanism in which the asymmetric optical intersite spin transfer (OISTR) effect occurring within the sublattices of the AFM and FM layers drives the interlayer spin-selective charge transfer, leading to the transition from AFM to FiM state. Furthermore, the unequal electron-phonon coupling of spin-up and spin-down channels of AFM spin sublattice causes an inequivalent spin relaxation, in turn extending the time scale of the FiM state. These findings are essential for designing novel optical-driven ultrafast 2D magnetic switches.

Ultrafast Laser Control of Antiferromagnetic-Ferrimagnetic Switching in Two-Dimensional Ferromagnetic Semiconductor Heterostructures.

Realizing ultrafast control of magnetization switching is of crucial importance for information processing and recording technology. Here, we explore the laser-induced spin electron excitation and relaxation dynamics processes of CrCl3/CrBr3 heterostructures with antiparallel (AP) and parallel (P) systems. Although an ultrafast demagnetization of CrCl3 and CrBr3 layers occurs in both AP and P systems, the overall magnetic order of the heterostructure remains unchanged due to the laser-induced equivalent interlayer spin electron excitation. More crucially, the interlayer magnetic order switches from antiferromagnetic (AFM) to ferrimagnetic (FiM) in the AP system once the laser pulse disappears. The microscopic mechanism underpinning this magnetization switching is dominated by the asymmetrical interlayer charge transfer combined with a spin-flip, which breaks the interlayer AFM symmetry and ultimately results in an inequivalent shift in the moment between two FM layers. Our study opens up a new idea for ultrafast laser control of magnetization switching in two-dimensional opto-spintronic devices.

Glass-like Transport Dominates Ultralow Lattice Thermal Conductivity in Modular Crystalline Bi4O4SeCl2.

Crystalline Bi4O4SeCl2 exhibits record-low 0.1 W/mK lattice thermal conductivity (κL), but the underlying transport mechanism is not yet understood. Using a theoretical framework which incorporates first-principles anharmonic lattice dynamics into a unified heat transport theory, we compute both the particle-like and glass-like components of κL in crystalline and pellet Bi4O4SeCl2 forms. The model includes intrinsic three- and four-phonon scattering processes and extrinsic defect and extended defect scattering contributing to the phonon lifetime, as well as temperature-dependent interatomic force constants linked to phonon frequency shifts and anharmonicity. Bi4O4SeCl2 displays strongly anisotropic complex crystal behavior with dominant glass-like transport along the cross-plane direction. The uncovered origin of κL underscores an intrinsic approach for designing extremely low κL materials.

Photoresponse of Solution-Synthesized Graphene Nanoribbon Heterojunctions on Diamond Indicating Phototunable Photodiode Polarity.

Graphene nanoribbon heterostructures and heterojunctions have attracted interest as next-generation molecular diodes with atomic precision. Their mass production via solution methods and prototypical device integration remains to be explored. Here, the bottom-up solution synthesis and characterization of liquid-phase-processable graphene nanoribbon heterostructures (GNRHs) are demonstrated. Joint photoresponsivity measurements and simulations provide evidence of the structurally defined heterostructure motif acting as a type-I heterojunction. Real-time, time-dependent density functional tight-binding simulations further reveal that the photocurrent polarity can be tuned at different excitation wavelengths. Our results introduce liquid-phase-processable, self-assembled heterojunctions for the development of nanoscale diode circuitry and adaptive hardware.

Wavy Two-Dimensional Conjugated Metal-Organic Framework with Metallic Charge Transport.

Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) have emerged as a new class of crystalline layered conducting materials that hold significant promise for applications in electronics and spintronics. However, current 2D c-MOFs are mainly made from organic planar ligands, whereas layered 2D c-MOFs constructed by curved or twisted ligands featuring novel orbital structures and electronic states remain less developed. Herein, we report a Cu-catecholate wavy 2D c-MOF (Cu3(HFcHBC)2) based on a fluorinated core-twisted contorted hexahydroxy-hexa-cata-hexabenzocoronene (HFcHBC) ligand. We show that the resulting film is composed of rod-like single crystals with lengths up to ∼4 µm. The crystal structure is resolved by high-resolution transmission electron microscopy (HRTEM) and continuous rotation electron diffraction (cRED), indicating a wavy honeycomb lattice with AA-eclipsed stacking. Cu3(HFcHBC)2 is predicted to be metallic based on theoretical calculation, while the crystalline film sample with numerous grain boundaries apparently exhibits semiconducting behavior at the macroscopic scale, characterized by obvious thermally activated conductivity. Temperature-dependent electrical conductivity measurements on the isolated single-crystal devices indeed demonstrate the metallic nature of Cu3(HFcHBC)2, with a very weak thermally activated transport behavior and a room-temperature conductivity of 5.2 S cm-1. Furthermore, the 2D c-MOFs can be utilized as potential electrode materials for energy storage, which display decent capacity (163.3 F g-1) and excellent cyclability in an aqueous 5 M LiCl electrolyte. Our work demonstrates that wavy 2D c-MOF using contorted ligands are capable of intrinsic metallic transport, marking the emergence of new conductive MOFs for electronic and energy applications.

Carrier Relaxation and Multiplication in Bi Doped Graphene.

By introducing different contents of Bi adatoms to the surface of monolayer graphene, the carrier concentration and their dynamics have been effectively modulated as probed directly by the time- and angle-resolved photoemission spectroscopy technique. The Bi adatoms are found to assist acoustic phonon scattering events mediated by supercollisions as the disorder effectively relaxes the momentum conservation constraint. A reduced carrier multiplication has been observed, which is related to the shrinking Fermi sea for scattering, as confirmed by time-dependent density functional theory simulation. This work gives insight into hot carrier dynamics in graphene, which is crucial for promoting the application of photoelectric devices.

Plasmonic Resonant Intercluster Coulombic Decay.

Light-induced energy confinement in nanoclusters via plasmon excitations influences applications in nanophotonics, photocatalysis, and the design of controlled slow electron sources. The resonant decay of these excitations through the cluster's ionization continuum provides a unique probe of the collective electronic behavior. However, the transfer of a part of this decay amplitude to the continuum of a second conjugated cluster may offer control and efficacy in sharing the energy nonlocally to instigate remote collective events. With the example of a spherically nested dimer Na_{20}@C_{240} of two plasmonic systems we find that such a transfer is possible through the resonant intercluster Coulombic decay (RICD) as a fundamental process. This plasmonic RICD signal can be experimentally detected by the photoelectron velocity map imaging technique.

Photoinduced Dynamics at the Water/TiO_{2}(101) Interface.

We present a femtosecond time-resolved optical pump-soft x-ray probe photoemission study in which we follow the dynamics of charge transfer at the interface of water and anatase TiO_{2}(101). By combining our observation of transient oxygen O 1s core level peak shifts at submonolayer water coverages with Ehrenfest molecular dynamics simulations we find that ultrafast interfacial hole transfer from TiO_{2} to molecularly adsorbed water is completed within the 285 fs time resolution of the experiment. This is facilitated by the formation of a new hydrogen bond between an O_{2c} site at the surface and a physisorbed water molecule. The calculations fully corroborate our experimental observations and further suggest that this process is preceded by the efficient trapping of the hole at the surface of TiO_{2} by hydroxyl species (-OH), that form following the dissociative adsorption of water. At a water coverage exceeding a monolayer, interfacial charge transfer is suppressed. Our findings are directly applicable to a wide range of photocatalytic systems in which water plays a critical role.

Fano Resonance and Incoherent Interlayer Excitons in Molecular van der Waals Heterostructures.

Complex van der Waals heterostructures from layered molecular stacks are promising optoelectronic materials offering the means to efficient, modular charge separation and collection layers. The effect of stacking in the electrodynamics of such hybrid organic-inorganic two-dimensional materials remains largely unexplored, whereby molecular scale engineering could lead to advanced optical phenomena. For instance, tunable Fano engineering could make possible on-demand transparent conducting layers or photoactive elements, and passive cooling. We employ an adapted Gersten-Nitzan model and real time time-dependent density functional tight-binding to study the optoelectronics of self-assembled monolayers on graphene nanoribbons. We find Fano resonances that cause electromagnetic induced opacity and transparency and reveal an additional incoherent process leading to interlayer exciton formation with a characteristic charge transfer rate. These results showcase hybrid van der Waals heterostructures as paradigmatic 2D optoelectronic stacks, featuring tunable Fano optics and unconventional charge transfer channels.

Improved Electrocatalytic Activity and Stability by Single Iridium Atoms on Iron-based Layered Double Hydroxides for Oxygen Evolution.

Full understanding to the origin of the catalytic performance of a supported nanocatalyst from the points of view of both the active component and support is significant for the achievement of high performance. Herein, based on a model electrocatalyst of single-iridium-atom-doped iron (Fe)-based layered double hydroxides (LDH) for oxygen evolution reaction (OER), we reveal the first completed origin of the catalytic performance of such supported nanocatalysts. Specially, besides the activity enhancement of Ir sites by LDH support, the stability of surface Fe sites is enhanced by doped Ir sites: DFT calculation shows that the Ir sites can reduce the activity and enhance the stability of the nearby Fe sites; while further finite element simulations indicate, the stability enhancement of distant Fe sites could be attributed to the much low concentration of OER reactant (hydroxyl ions, OH- ) around them induced by the much fast consumption of OH- on highly active Ir sites. These new findings about the interaction between the main active components and supports are applicable in principle to other heterogeneous nanocatalysts and provide a completed understanding to the catalytic performance of heterogeneous nanocatalysts.

Insight into the Molecular Mechanism for Enhanced Longevity of Supramolecular Vesicular Photocatalysts.

Supramolecular self-assembly is a promising strategy for stabilizing the photo-sensitive components in photocatalysis. However, the underlying correlation between the enhanced photostability and supramolecular structure at the molecular level has not yet been fully understood. Herein, we develop a biomimetic vesicular membrane-based polyporphyrin photocatalyst exhibiting excellent photocatalytic stability with at least activity time of 240 h in hydrogen generation. Time-domain ab initio modelling together with transient absorption spectroscopy, visual frontier orbitals and Gibbs free energy calculation disclose that the ordered aggregation of porphyrin units in the vesicle membrane facilitates "hot" electron relaxation and the rapid dissipation of photo-generated charges, thereby contributing to the longevity. This work deepens the molecular-level understanding on photostability and photocatalytic mechanism of supramolecular photocatalysts.

Control of Hot Carrier Cooling in Lead Halide Perovskites by Point Defects.

Hot carriers (HCs) in lead halide perovskites are prone to rapidly relax at the band edge and waste plentiful photon energy, severely limiting their conversion efficiency as HC photovoltaic devices. Here, the HC cooling dynamics of MAPbI3 perovskite with common vacancy point defects (e.g., MAv+ and Iv-) and an interstitial point defect (e.g., Ii-) is elucidated, and the underlying physics is explicated using ab initio nonadiabatic molecular dynamics. Contrary to vacancy point defects, the interstitial point defect reduces the band degeneracy, decreases the HC -phonon interaction, weakens the nonadiabatic coupling, and ultimately slows down hot electron cooling by a factor of 1.5-2. Furthermore, the band-by-band relaxation pathway and direct relaxation pathway are uncovered for hot electron cooling and hot hole cooling, respectively, explaining why hot electrons can store more energy than hot holes during the cooling process. Besides, oxygen molecules interacting with Ii- sharply accelerate the hot electron cooling, making it even faster than that of the pristine system and revealing the detrimental effect of oxygen on HC cooling. This work provides significant insights into the defect-dependent HC cooling dynamics and suggests a new strategy to design high-efficiency HC photovoltaic devices.

Charge Delocalization and Vibronic Couplings in Quadrupolar Squaraine Dyes.

Squaraines are prototypical quadrupolar charge-transfer chromophores that have recently attracted much attention as building blocks for solution-processed photovoltaics, fluorescent probes with large two-photon absorption cross sections, and aggregates with large circular dichroism. Their optical properties are often rationalized in terms of phenomenological essential state models, considering the coupling of two zwitterionic excited states to a neutral ground state. As a result, optical transitions to the lowest S1 excited state are one-photon allowed, whereas the next higher S2 state can only be accessed by two-photon transitions. A further implication of these models is a substantial reduction of vibronic coupling to the ubiquitous high-frequency vinyl-stretching modes of organic materials. Here, we combine time-resolved vibrational spectroscopy, two-dimensional electronic spectroscopy, and quantum-chemical simulations to test and rationalize these predictions for nonaggregated molecules. We find small Huang-Rhys factors below 0.01 for the high-frequency, 1500 cm-1 modes in particular, as well as a noticeable reduction for those of lower frequency modes in general for the electronic S0 → S1 transition. The two-photon allowed state S2 is well separated energetically from S1 and has weak vibronic signatures as well. Thus, the resulting pronounced concentration of the oscillator strength in a narrow region relevant to the lowest electronic transition makes squaraines and their aggregates exceptionally interesting for strong and ultrastrong coupling of excitons to localized light modes in external resonators with chiral properties that can largely be controlled by the molecular architecture.

DFT exchange: sharing perspectives on the workhorse of quantum chemistry and materials science.

In this paper, the history, present status, and future of density-functional theory (DFT) is informally reviewed and discussed by 70 workers in the field, including molecular scientists, materials scientists, method developers and practitioners. The format of the paper is that of a roundtable discussion, in which the participants express and exchange views on DFT in the form of 302 individual contributions, formulated as responses to a preset list of 26 questions. Supported by a bibliography of 777 entries, the paper represents a broad snapshot of DFT, anno 2022.

Investigation of plasmon relaxation mechanisms using nonadiabatic molecular dynamics.

Hot carriers generated from the decay of plasmon excitation can be harvested to drive a wide range of physical or chemical processes. However, their generation efficiency is limited by the concomitant phonon-induced relaxation processes by which the energy in excited carriers is transformed into heat. However, simulations of dynamics of nanoscale clusters are challenging due to the computational complexity involved. Here, we adopt our newly developed Trajectory Surface Hopping (TSH) nonadiabatic molecular dynamics algorithm to simulate plasmon relaxation in Au20 clusters, taking the atomistic details into account. The electronic properties are treated within the Linear Response Time-Dependent Tight-binding Density Functional Theory (LR-TDDFTB) framework. The relaxation of plasmon due to coupling to phonon modes in Au20 beyond the Born-Oppenheimer approximation is described by the TSH algorithm. The numerically efficient LR-TDDFTB method allows us to address a dense manifold of excited states to ensure the inclusion of plasmon excitation. Starting from the photoexcited plasmon states in Au20 cluster, we find that the time constant for relaxation from plasmon excited states to the lowest excited states is about 2.7 ps, mainly resulting from a stepwise decay process caused by low-frequency phonons of the Au20 cluster. Furthermore, our simulations show that the lifetime of the phonon-induced plasmon dephasing process is ∼10.4 fs and that such a swift process can be attributed to the strong nonadiabatic effect in small clusters. Our simulations demonstrate a detailed description of the dynamic processes in nanoclusters, including plasmon excitation, hot carrier generation from plasmon excitation dephasing, and the subsequent phonon-induced relaxation process.

Nonadiabatic molecular dynamics simulations based on time-dependent density functional tight-binding method.

Nonadiabatic excited state molecular dynamics underpin many photophysical and photochemical phenomena, such as exciton dynamics, and charge separation and transport. In this work, we present an efficient nonadiabatic molecular dynamics (NAMD) simulation method based on time-dependent density functional tight-binding (TDDFTB) theory. Specifically, the adiabatic electronic structure, an essential NAMD input, is described at the TDDFTB level. The nonadiabatic effects originating from the coupled motions of electrons and nuclei are treated by the trajectory surface hopping algorithm. To improve the computational efficiency, nonadiabatic couplings between excited states within the TDDFTB method are derived and implemented using an analytical approach. Furthermore, the time-dependent nonadiabatic coupling scalars are calculated based on the overlap between molecular orbitals rather than the Slater determinants to speed up the simulations. In addition, the electronic decoherence scheme and a state reassigned unavoided crossings algorithm, which has been implemented in the NEXMD software, are used to improve the accuracy of the simulated dynamics and handle trivial unavoided crossings. Finally, the photoinduced nonadiabatic dynamics of a benzene molecule are simulated to demonstrate our implementation. The results for excited state NAMD simulations of benzene molecule based on TDDFTB method compare well to those obtained with numerically expensive time-dependent density functional theory. The proposed methodology provides an attractive theoretical simulation tool for predicting the photophysical and photochemical properties of complex materials.

Unravelling Photoinduced Interlayer Spin Transfer Dynamics in Two-Dimensional Nonmagnetic-Ferromagnetic van der Waals Heterostructures.

Although light is the fastest means to manipulate the interfacial spin injection and magnetic proximity related quantum properties of two-dimensional (2D) magnetic van der Waals (vdW) heterostructures, its potential remains mostly untapped. Here, inspired by the recent discovery of 2D ferromagnets Fe3GeTe2 (FGT), we applied the real-time density functional theory (rt-TDDFT) to study photoinduced interlayer spin transfer dynamics in 2D nonmagnetic-ferromagnetic (NM-FM) vdW heterostructures, including graphene-FGT, silicene-FGT, germanene-FGT, antimonene-FGT and h-BN-FGT interfaces. We observed that laser pulses induce significant large spin injection from FGT to nonmagnetic (NM) layers within a few femtoseconds. In addition, we identified an interfacial atom-mediated spin transfer pathway in heterostructures in which the photoexcited spin of Fe first transfers to intralayered Te atoms and then hops to interlayered NM layers. Interlayer hopping is approximately two times slower than intralayer spin transfer. Our results provide the microscopic understanding for optically control interlayer spin dynamics in 2D magnetic heterostructures.

Ultralow Thermal Conductivity in Two-Dimensional MoO3.

Monolayer molybdenum trioxide (MoO3) is an emerging two-dimensional (2D) material with high electrical conductivity but unexplored thermal conductivity. Using first-principles calculations and a Boltzmann transport theoretical framework, we predict a record low room-temperature phonon thermal conductivity (κp) of 1.57 and 1.26 W/mK along the principal in-plane directions of the MoO3 monolayer. The behavior is attributed to the combination of soft flexural and in-plane acoustic modes, which are coupled through the finite layer thickness, and to the strong bonding anharmonicity, which gives rise to significant 3- and 4-phonon scattering. These insights suggest new indicators for guiding the search of 2D materials with low κp and motivates κp measurements in MoO3 and its applications as a thermoelectric and thermally protective material.

Electric Field Tunable Ultrafast Interlayer Charge Transfer in Graphene/WS2 Heterostructure.

Van der Waals heterostructures composed of two-dimensional materials offer an unprecedented control over their properties and have attracted tremendous research interest in various optoelectronic applications. Here, we study the photoinduced charge transfer in graphene/WS2 heterostructure by time-dependent density functional theory molecular dynamics. Our results show that holes transfer from graphene to WS2 two times faster than electrons, and the occurrence of interlayer charge transfer is found correlated with vibrational modes of graphene and WS2. It is further demonstrated that the carrier dynamics can be efficiently modulated by external electric fields. Detailed analysis confirms that the carrier transfer rate at heterointerface is governed by the coupling between donor and acceptor states, which is the result of the competition between interlayer and intralayer relaxation processes. Our study provides insights into the understanding of ultrafast interlayer charge transfer processes in heterostructures and broadens their future applications in photovoltaic devices.

High-Throughput Screening of Synergistic Transition Metal Dual-Atom Catalysts for Efficient Nitrogen Fixation.

Great enthusiasm in single-atom catalysts (SACs) for the nitrogen reduction reaction (NRR) has been aroused by the discovery of metal-Nx as a promising catalytic center. However, the poor activity and low selectivity of available SACs are far away from the industrial requirement. Through the first-principles high-throughput screening, we find that Fe-Fe distributed on graphite carbon nitride (Fe2/g-CN) can manipulate the binding strength of the target reaction species (compromises the ability to adsorb N2H and NH2), therefore achieving the best NRR performance among 23 transition metal (TM) centers. Our results show that Fe2/g-CN achieves a high theoretical Faradaic efficiency of 100% and, impressively, the lowest limiting potential of -0.13 V. Particularly, multiple-level descriptors shed light on the origin of NRR activity, achieving a fast prescreening among various candidates. Our predictions not only accelerate discovery of catalysts for ammonia synthesis but also contribute to further elucidate the structure-performance correlations.