Vertical Dipole Dominates Charge Carrier Lifetime in Monolayer Janus MoSSe.

Two-dimensional semiconductor materials with vertical dipoles are promising photocatalysts as vertical dipoles not only promote the electron-hole separation but also enhance the carrier redox ability. However, the influence of vertical dipoles on carrier recombination in such materials, especially the competing relationship between vertical dipoles and band gaps, is not yet clear. Herein, first-principles calculations and nonadiabatic molecular dynamics simulations were combined to clarify the influence of band gap and vertical dipole on the carrier lifetime in Janus MoSSe monolayer. By comparing with the results of MoS2 and MoSe2 as well as exploring the carrier lifetime of MoSSe under strain regulation, it has been demonstrated that the vertical dipole, rather than the band gap, is the dominant factor affecting the carrier lifetime. Strikingly, a linear relationship between the carrier lifetime and vertical dipole is revealed. These findings have important implications for the design of high-performance photocatalysts and optoelectronic devices.

Direct Hydrogen Production Promoted by Laser-Induced Water Plasma.

Hydrogen, as a clean energy carrier, plays an important role in addressing the current energy and environmental crisis. However, conventional hydrogen production technologies require extreme reaction conditions, such as high temperature, high pressure, and catalysts. Herein, we study the microscopic mechanism of laser-induced water plasma and subsequent H2 production with real-time time-dependent density functional theory simulations and ab initio molecular dynamics simulations. The results demonstrate that intense laser excites liquid water to generate nonequilibrium plasma in a warm-dense state, which constitutes a superior reaction environment. Subsequent annealing leads to the recombination of energetic reactive particles to generate H2, O2, and H2O2 molecules. Annealing rate and laser wavelength are shown to modulate the product ratio, and the energy conversion efficiency can reach ∼9.2% with an annealing rate of 1.0 K/fs. This work reveals the nonequilibrium atomistic mechanisms of hydrogen production from laser-induced water plasma and shows far-reaching implications for the design of optically controllable hydrogen technology.

Interlayer Carrier Dynamics in Two-Dimensional Perovskites Determined by the Length of Conjugated Organic Cations.

Unlocking the restricted interlayer carrier transfer in a two-dimensional perovskite is a crucial means to achieve the harmonization of efficiency and stability in perovskite solar cells. In this work, the effects of conjugated organic molecules on the interlayer carrier dynamics of 2D perovskites were investigated through nonadiabatic molecular dynamics simulations. We found that elongated conjugated organic cations contributed significantly to the accelerated interlayer carrier dynamics, originating from lowered transport barrier and boosted π-p coupling between organic and inorganic layers. Utilizing conjugated molecules of moderate length as spacer cations can yield both superior efficiency and exceptional stability simultaneously. However, conjugated chains that are too long lead to structural instability and stronger carrier recombination. The potential of conjugated chain-like molecules as spacer cations in 2D perovskites has been demonstrated in our work, offering valuable insights for the development of high-performance perovskite solar cells.

Magnetic Structure-Dependent Ultrafast Spin Relaxation in Magnet CrI3: A Time-Domain ab Initio Study.

Two-dimensional magnet CrI3 is a promising candidate for spintronic devices. Using nonadiabatic molecular dynamics and noncollinear spin time-dependent density functional theory, we investigated hole spin relaxation in two-dimensional CrI3 and its dependence on magnetic configurations, impacted by spin-orbit and electron-phonon interactions. Driven by in-plane and out-of-plane iodine motions, the relaxation rates vary, extending from over half a picosecond in ferromagnetic systems to tens of femtoseconds in certain antiferromagnetic states due to significant spin fluctuations, associated with the nonadiabatic spin-flip in tuning to the adiabatic flip. Antiferromagnetic CrI3 with staggered layer magnetic order notably accelerates adiabatic spin-flip due to enhanced state degeneracy and additional phonon modes. Ferrimagnetic CrI3 shows a transitional behavior between ferromagnetic and antiferromagnetic types as the magnetic moment changes. These insights into the spin dynamics of CrI3 underscore its potential for rapid-response spintronic applications and advance our understanding of two-dimensional materials for spintronics.

Substrate Ferroelectric Proximity Effects Have a Strong Influence on Charge Carrier Lifetime in Black Phosphorus.

By stacking monolayer black phosphorus (MBP) with nonpolarized and ferroelectric polarized bilayer hexagonal boron nitride (h-BN), we demonstrate that ferroelectric proximity effects have a strong influence on the charge carrier lifetime of MBP using nonadiabatic (NA) molecular dynamics simulations. Through enhancing the motion of phosphorus atoms, ferroelectric polarization enhances the overlap of electron-hole wave functions that improves NA coupling and decreases the bandgap, resulting in a rapid electron-hole recombination completing within a quarter of nanoseconds, which is two times shorter than that in nonpolarized stackings. In addition to the dominant in-plane Ag2 mode in free-standing MBP, the out-of-plane high-frequency Ag1 and low-frequency interlayer breathing modes presented in the heterojunctions drive the recombination. Notably, the resonance between the breathing mode within bilayer h-BN and the B1u mode of MBP provides an additional nonradiative channel in ferroelectric stackings, further accelerating charge recombination. These findings are crucial for charge dynamics manipulation in two-dimensional materials via substrate ferroelectric proximity effects.

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.

Charge Carrier Dynamics at the Interface of 2D Metal-Organic Frameworks and Hybrid Perovskites for Solar Energy Harvesting.

Interfacing perovskites with two-dimensional materials such as metal-organic frameworks (MOFs) for improved stability and electron or hole extraction has emerged as a promising path forward for the generation of highly efficient and stable solar cells. In this work, we examine the structural properties and excitation dynamics of two MOF-perovskite systems: UMCM309-a@MAPbI3 and ZrL3@MAPbI3. We find that precise band alignment and electronegativity of the MOF-linkers are necessary to facilitate the capture of excited charge carriers. Furthermore, we demonstrate that intraband relaxation of hot electrons to the MOF subsystem results in optically disallowed transitions across the band gap, suppressing radiative recombination. Furthermore, we elucidate the key mechanisms associated with improved structural stability afforded to the perovskites by the two-dimensional MOFs, highlighting the necessity of broad surface coverage and strong MOF-perovskite interaction.

Strongly anisotropic ultrafast dynamic behavior of GaTe dominated by the tilted and flat bands.

The anisotropic transport properties of gallium telluride (GaTe) have been reported by several experiments, giving rise to many debates recently. The anisotropic electronic band structure of GaTe shows the extreme difference between the flat band and tilted band in two distinct directions,Γ¯-X¯andΓ¯-Y¯, and which we called as the mixed flat-tilted band (MFTB). Focusing on such two directions, the relaxation of photo-generated carriers has been studied using the non-adiabatic molecular dynamics (NAMD) method to investigate the anisotropic behavior of ultrafast dynamics. The results show that the relaxation lifetime is different in flat band direction and tilted band direction, which is evidence for the existence of anisotropic behavior of the ultrafast dynamic, and such anisotropic behavior comes from the different intensities of electron-phonon coupling of the flat band and tilted band. Furthermore, the ultrafast dynamic behavior is found to be affected strongly by spin-orbit coupling (SOC) and such anisotropic behavior of the ultrafast dynamic can be reversed by SOC. The tunable anisotropic ultrafast dynamic behavior of GaTe is expected to be detected in ultrafast spectroscopy experiments and it may provide a tunable application in nanodevice design. The results may also provide a reference for the investigation of MFTB semiconductors.

Two Dimensional MOene: From Superconductors to Direct Semiconductors and Weyl Fermions.

The number of semiconducting MXenes with direct band gaps is extremely low; thus, it is highly desirable to broaden the MXene family beyond carbides and nitrides to expand the palette of desired chemical and physical properties. Here, we theoretically report the existence of the single-layer (SL) dititanium oxide 2H-Ti2O MOene (MXene-like 2D transition oxides), showing an Ising superconducting feature. Moreover, SL halogenated 2H- and 1T-Ti2O monolayers display tunable semiconducting features and strong light-harvesting ability. In addition, the external strains can induce Weyl fermions via quantum phase transition in 2H-Ti2OF2 and Ti2OCl2 monolayers. Specifically, 2H- and 1T-Ti2OF2 are direct semiconductors with band gaps of 0.82 and 1.18 eV, respectively. Furthermore, the carrier lifetimes of SL 2H- and 1T-Ti2OF2 are evaluated to be 0.39 and 2.8 ns, respectively. This study extends emerging phenomena in a rich family of 2D MXene-like MOene materials, which provides a novel platform for next-generation optoelectronic and photovoltaic fields.

Excited-State Distortions Promote the Photochemical 4π-Electrocyclizations of Fluorobenzenes via Machine Learning Accelerated Photodynamics Simulations.

Benzene fluorination increases chemoselectivities for Dewar-benzenes via 4π-disrotatory electrocyclization. However, the origin of the chemo- and regioselectivities of fluorobenzenes remains unexplained because of the experimental limitations in resolving the excited-state structures on ultrafast timescales. The computational cost of multiconfigurational nonadiabatic molecular dynamics simulations is also currently cost-prohibitive. We now provide high-fidelity structural information and reaction outcome predictions with machine-learning-accelerated photodynamics simulations of a series of fluorobenzenes, C6 F6-n Hn , n=0-3, to study their S1 →S0 decay in 4 ns. We trained neural networks with XMS-CASPT2(6,7)/aug-cc-pVDZ calculations, which reproduced the S1 absorption features with mean absolute errors of 0.04 eV (<2 nm). The predicted nonradiative decay constants for C6 F4 H2 , C6 F6 , C6 F3 H3 , and C6 F5 H are 116, 60, 28, and 12 ps, respectively, in broad qualitative agreement with the experiments. Our calculations show that a pseudo Jahn-Teller distortion of fluorinated benzenes leads to an S1 local-minimum region that extends the excited-state lifetimes of fluorobenzenes. The pseudo Jahn-Teller distortions reduce when fluorination decreases. Our analysis of the S1 dynamics shows that the pseudo-Jahn-Teller distortions promote an excited-state cis-trans isomerization of a πC-C bond. We characterized the surface hopping points from our NAMD simulations and identified instantaneous nuclear momentum as a factor that promotes the electrocyclizations.

Modeling Spin-Crossover Dynamics.

In this article, we review nonadiabatic molecular dynamics (NAMD) methods for modeling spin-crossover transitions. First, we discuss different representations of electronic states employed in the grid-based and direct NAMD simulations. The nature of interstate couplings in different representations is highlighted, with the main focus on nonadiabatic and spin-orbit couplings. Second, we describe three NAMD methods that have been used to simulate spin-crossover dynamics, including trajectory surface hopping, ab initio multiple spawning, and multiconfiguration time-dependent Hartree. Some aspects of employing different electronic structure methods to obtain information about potential energy surfaces and interstate couplings for NAMD simulations are also discussed. Third, representative applications of NAMD to spin crossovers in molecular systems of different sizes and complexities are highlighted. Finally, we pose several fundamental questions related to spin-dependent processes. These questions should be possible to address with future methodological developments in NAMD.

Modeling Auger Processes with Nonadiabatic Molecular Dynamics.

Auger-type energy exchange plays key roles in the carrier dynamics in nanomaterials due to strong carrier-carrier interactions. However, theoretical descriptions are limited to perturbative calculations of scattering rates on static structures. We develop an accurate and efficient ab initio technique to model Auger scattering with nonadiabatic molecular dynamics. We incorporate the many-body Coulomb matrix into several surface hopping methods and describe simultaneously charge-charge and charge-phonon scattering in the time-domain and in a nonperturbative, configuration-dependent manner. The approach is illustrated with a CdSe quantum dot. Auger scattering between electrons and holes breaks the phonon bottleneck to electron relaxation. The bottleneck is recovered when electrons and holes are decoupled. The simulations correctly reproduce all experimental processes and time scales, including Auger- and phonon-assisted cooling of hot electrons, intraband carrier relaxation, and carrier recombination. Providing detailed insights into the energy flow, the developed method allows studies of carrier dynamics in nanomaterials with strong carrier-carrier interactions.

PyUNIxMD: A Python-based excited state molecular dynamics package.

Theoretical/computational description of excited state molecular dynamics is nowadays a crucial tool for understanding light-matter interactions in many materials. Here we present an open-source Python-based nonadiabatic molecular dynamics program package, namely PyUNIxMD, to deal with mixed quantum-classical dynamics for correlated electron-nuclear propagation. The PyUNIxMD provides many interfaces for quantum chemical calculation methods with commercial and noncommercial ab initio and semiempirical quantum chemistry programs. In addition, the PyUNIxMD offers many nonadiabatic molecular dynamics algorithms such as fewest-switch surface hopping and its derivatives as well as decoherence-induced surface hopping based on the exact factorization (DISH-XF) and coupled-trajectory mixed quantum-classical dynamics (CTMQC) for general purposes. Detailed structures and flows of PyUNIxMD are explained for the further implementations by developers. We perform a nonadiabatic molecular dynamics simulation for a molecular motor system as a simple demonstration.

Phonon-Mediated and Weakly Size-Dependent Electron and Hole Cooling in CsPbBr3 Nanocrystals Revealed by Atomistic Simulations and Ultrafast Spectroscopy.

We combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over a very broad size regime, from 0.8 to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites, no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling (over ∼10 ps) of "warm" electrons in the vicinity (within ∼0.1 eV) of the conduction band edge. At higher excess energies, electrons and holes cool with similar rates, on the order of 1 eV ps-1 carrier-1, increasing weakly with size. Our ab initio simulations suggest that cooling proceeds via fast phonon-mediated intraband transitions driven by strong and size-dependent electron-phonon coupling. The presented experimental and computational methods yield the spectrum of involved phonons and may guide the development of devices utilizing hot charge carriers.

Effect of Zinc-Doping on the Reduction of the Hot-Carrier Cooling Rate in Halide Perovskites.

The fast hot-carrier cooling process in the solar-absorbers fundamentally limits their photon-conversion efficiencies. It is highly desirable to develop a solar absorber with long-lived hot-carriers at sun-illumination intensity, which can be used to develop the hot-carrier solar cells with enhanced efficiency. Herein, we reveal that zinc-doped (0.34 %) halide perovskites have the slower hot-carrier cooling compared with the pristine sample through the transient absorption spectroscopy measurements and theoretical calculations. The hot-carrier energy loss rate at the low photoexcitation level of 1017  cm-3 is found to be ≈3 times smaller than that of un-doped perovskites for T=500 K hot carriers, and up to ten times when the hot-carrier temperature approaches the lattice temperature. The incorporation of zinc-dopant into perovskites can reduce the nonadiabatic couplings between conduction bands, which retards the photogenerated hot-carriers relaxation processes. Our findings present a practical strategy to slow down the hot-carrier cooling in perovskites at low carrier densities, which would be invaluable for the further development of practical hot-carrier photovoltaics based on perovskites.

Conical Intersections at the Nanoscale: Molecular Ideas for Materials.

The ability to predict and describe nonradiative processes in molecules via the identification and characterization of conical intersections is one of the greatest recent successes of theoretical chemistry. Only recently, however, has this concept been extended to materials science, where nonradiative recombination limits the efficiencies of materials for various optoelectronic applications. In this review, we present recent advances in the theoretical study of conical intersections in semiconductor nanomaterials. After briefly introducing conical intersections, we argue that specific defects in materials can induce conical intersections between the ground and first excited electronic states, thus introducing pathways for nonradiative recombination. We present recent developments in theoretical methods, computational tools, and chemical intuition for the prediction of such defect-induced conical intersections. Through examples in various nanomaterials, we illustrate the significance of conical intersections for nanoscience. We also discuss challenges facing research in this area and opportunities for progress.

The Interplay Between Lead Vacancy and Water Rationalizes the Puzzle of Charge Carrier Lifetimes in CH3 NH3 PbI3 : Time-Domain Ab Initio Analysis.

The perovskite CH3 NH3 PbI3 excited-state lifetimes exhibit conflicting experimental results under humid environments. Using ab initio nonadiabatic (NA) molecular dynamics, we demonstrate that the interplay between lead vacancy and water can rationalize the puzzle. The lead vacancy reduces NA coupling by localizing holes, slowing electron-hole recombination. By creating a deep electron trap state, the coexistence of a neutral lead vacancy and water molecules enhances NA coupling, accelerating charge recombination by a factor of over 3. By eliminating the mid-gap state by accepting two photoexcited electrons, the negatively charged lead vacancy interacting with water molecules increases the carrier lifetime over 2 times longer than in the pristine system. The simulations rationalize the positive and negative effects of water on the solar cell performance exposure to humidity.

Effect of Aspect Ratio on Multiparticle Auger Recombination in Single-Walled Carbon Nanotubes: Time Domain Atomistic Simulation.

Many-particle Auger-type processes are common in nanoscale materials due to a combination of high densities of states that can support multiple excitations and substantial Coulomb coupling between charges enhanced by quantum confinement. Auger decay dynamics in (10,5) semiconductor carbon nanotubes (CNT) with different aspect ratios and particle densities are simulated in time domain using global flux surface hopping, recently developed and implemented within Kohn-Sham tight-binding density functional theory. Despite an increasing density of states, the multiparticle Auger recombination rate decreases in longer CNTs. The atomistic simulation shows that the effect is directly related to the coupling between electronic states, which decreases as the aspect ratio becomes larger. The dependence on tube length is stronger for three-exciton than two-exciton recombination and the calculated time scale ratio approaches the experimental value measured for long CNTs. Phonon-assisted transitions play a particularly important role during Auger recombination. Electron-phonon relaxation is faster than the recombination, and Auger transitions are assisted by phonons over a range of frequencies up to the G-mode. The involvement of phonons strongly enhances the probability of transitions involving asymmetric electron-hole pairs. The time-domain atomistic simulation mimics directly time-resolved optical experiments and provides a detailed, systematic analysis of the phonon-assisted Auger dynamics.

Why Chemical Vapor Deposition Grown MoS2 Samples Outperform Physical Vapor Deposition Samples: Time-Domain ab Initio Analysis.

Two-dimensional transition metal dichalcogenides (TMDs) have drawn strong attention due to their unique properties and diverse applications. However, TMD performance depends strongly on material quality and defect morphology. Experiments show that samples grown by chemical vapor deposition (CVD) outperform those obtained by physical vapor deposition (PVD). Experiments also show that CVD samples exhibit vacancy defects, while antisite defects are frequently observed in PVD samples. Our time-domain ab initio study demonstrates that both antisites and vacancies accelerate trapping and nonradiative recombination of charge carriers, but antisites are much more detrimental than vacancies. Antisites create deep traps for both electrons and holes, reducing energy gaps for recombination, while vacancies trap primarily holes. Antisites also perturb band-edge states, creating significant overlap with the trap states. In comparison, vacancy defects overlap much less with the band-edge states. Finally, antisites can create pairs of electron and hole traps close to the Fermi energy, allowing trapping by thermal activation from the ground state and strongly contributing to charge scattering. As a result, antisites accelerate charge recombination by more than a factor of 8, while vacancies enhance the recombination by less than a factor of 2. Our simulations demonstrate a general principle that missing atoms are significantly more benign than misplaced atoms, such as antisites and adatoms. The study rationalizes the existing experimental data, provides theoretical insights into the diverse behavior of different classes of defects, and generates guidelines for defect engineering to achieve high-performance electronic, optoelectronic, and solar-cell devices.

Rapid Decoherence Suppresses Charge Recombination in Multi-Layer 2D Halide Perovskites: Time-Domain Ab Initio Analysis.

Two-dimensional (2D) Ruddlesden-Popper halide perovskites are appealing candidates for optoelectronics and photovoltaics. Nonradiative electron-hole recombination constitutes a major pathway for charge and energy losses in these materials. Surprisingly, experimental recombination is slower in multilayers than a monolayer, even though multilayer systems have smaller energy gaps and higher frequency phonons that should accelerate the recombination. Focusing on (BA)2(MA) n-1Pb nI3 n+1 with n = 1 and 3, BA = CH3(CH2)3NH3, and MA = CH3NH3, we show that it is the enhancement of elastic electron-phonon scattering that suppresses charge recombination for n = 3, by causing rapid loss of electronic coherence. The scattering is enhanced in the multilayer 2D perovskites because, in contrast to the monolayer, they contain MA cations embedded into the inorganic Pb-I lattice. Although MAs do not contribute directly to electron and hole wave functions, they perturb the Pb-I lattice and create strong electric fields that interact with the charges. The rapid loss of coherence explains long excited state lifetimes that extend into nanoseconds. Both electron-hole recombination and coherence times show excellent agreement with the corresponding lifetime and line width measurements. The simulations rationalize the observed dependence of excited state lifetime in 2D layered halide perovskites on layer thickness and advance our understanding of the atomistic mechanisms underlying charge-phonon dynamics in nanoscale materials.