Initio Non-adiabatic Molecular Dynamics Simulations Research Articles

Hot Carrier Controlled Nitrogen Fixation Reaction in Metal-Free Boron-Anchored Aza-COF: Insight from Nonadiabatic Molecular Dynamics Simulation.

Designing highly efficient photocatalysts for the production of renewable energy is a challenging task that necessitates simultaneous control of chemical activity and photocarrier dynamics for a particular reaction. To this end, we have investigated the catalytic mechanism and real-time photocarrier dynamics of the nitrogen reduction reaction (NRR) at the metal-free boron-functionalized 2D aza-COF (B-aza-COF), an inexpensive and environmentally friendly semiconductor. By employing density functional theory (DFT) and time-dependent ab initio nonadiabatic molecular dynamics simulation, we have investigated the electronic structure, light harvesting ability, free energy change, and dynamics of photoexcited carriers. Our calculated results reveal that the gas phase N2 molecule can be effectively reduced into NH3 on B-aza-COF under UV-visible light. Therefore, our investigation on the design of efficient photocatalysts for the nitrogen reduction reaction (NRR) provides a cost-effective opportunity for the sustainable production of NH3.

Unveiling the Dual Role of Humidity: The Interplay with Defects Manipulating the Charge Carrier Lifetime in Metal Halide Perovskites.

Humidity has exhibited experimentally either beneficial or detrimental effects on the charge carrier lifetime of CH3NH3PbI3 perovskites, leaving the mechanism unresolved. By using ab initio nonadiabatic molecular dynamics simulations, we unveil the dual role of humidity stemming from the complex interplay between water and defects. Beneficially, water passivates iodine vacancies (VI) or grain boundaries (GBs), mitigating electron trapping by reducing nonadiabatic coupling and delaying charge recombination. However, when VI and GBs coexist, water molecules make the two unsaturated lead atoms approach closer and exacerbate electron trapping by deepening the Pb-dimer electron trap that was created by the VI defect, shortening the carrier lifetime to half of pristine CH3NH3PbI3. The study uncovers the origin of the positive and negative effects of humidity on the charge carrier lifetime of perovskites and offers strategies for improving perovskite devices, particularly by avoiding simultaneous point defects and GBs.

Investigating the photo-relaxation mechanism of 6-azauracil through ab initio nonadiabatic molecular dynamics simulations

On-the-fly ab initio molecular dynamics simulations were performed to study the decay dynamics of photo-excited 6-azauracil. The Zhu–Nakamura trajectory surface hopping method was used to determine the nonadiabatic transitions across different electronic states. The lifetime of the photoexcited S2 state was estimated to be 201 fs, which is in line with the experimentally measured lifetime (Hua et al., 2015). The results of the molecular dynamics simulations revealed that the internal conversion from S2 to S1 is facilitated by the destabilization of the ground state caused by molecular ring puckering.

Efficient photoreduction of carbon dioxide to ethanol using diatomic nitrogen-doped black phosphorus.

Successful conversion of CO2 into C2 products requires the development of new catalysts that overcome the difficulties in efficient light harvesting and CO-CO coupling. Herein, density functional theory (DFT) is used to assess the photoreduction properties of nitrogen-doped black phosphorus. The geometric structure, redox potential, first step of hydrogenation activation, CO desorption, and CO-CO coupling are systematically calculated, based on which the diatomic nitrogen-doped black phosphorus (N2@BPV) stands out. The calculated results of the CO2RR pathway demonstrate that N2@BPV has excellent selectivity and high activity for CH3CH2OH production. The results of the time-dependent ab initio nonadiabatic molecular dynamics simulation show that the diatomic N active sites of N2@BPV facilitate charge separation and inhibit electron-hole recombination. In addition, the activation mechanism of CO2 is studied. The main reason for CO2 activation is attributed to the imbalance in electron transfer that destroys the symmetry of CO2. We expect that our study will offer some theoretical guidance in CO2 conversion.

Vitality of Intralayer Vibration in hBN for Effective Long-Range Interlayer Hole Transfer across High Barriers in MoSe2/hBN/WSe2 Heterostructures.

Introducing the two-dimensional (2D) hexagonal boron nitride (hBN) between 2D transition metal dichalcogenide (TMD) layers promises convenient manipulation of the interlayer exciton (IX) and interlayer charge transfer in TMD/hBN/TMD heterostructures, while the role of inserted hBN layers during IX formation is controversial. Employing ab initio nonadiabatic molecular dynamics (NAMD) simulations and the electron-phonon coupling model, we systematically investigate interlayer hole transfer in MoSe2/WSe2 bilayers intercalated by hBN layers with various thicknesses. The conventional direct hole transfer from MoSe2 to WSe2 is decelerated by 2-3 orders of magnitude after the hBN insertion. Meanwhile, a novel channel intermediated by a deeper hole of WSe2 becomes dominant, where the intralayer shear mode of hBN plays a crucial role by reducing the energy barriers for this new channel. The unique role of hBN layers is revealed for the first time, enriching the knowledge of the underlying microscopic mechanisms and providing instructive guidance to practical van der Waals optoelectronic devices.

Tensile Strain-Dependent Ultrafast Electron Transfer and Relaxation Dynamics in Flexible WSe2/MoS2 Heterostructures.

Understanding and controlling carrier dynamics in two-dimensional (2D) van der Waals heterostructures through strain are crucial for their flexible applications. Here, femtosecond transient absorption spectroscopy is employed to elucidate the interlayer electron transfer and relaxation dynamics under external tensile strains in a WSe2/MoS2 heterostructure. The results show that a modest ∼1% tensile strain can significantly alter the lifetimes of electron transfer and nonradiative electron-hole recombination by >30%. Ab initio non-adiabatic molecular dynamics simulations suggest that tensile strain weakens the electron-phonon coupling, thereby suppressing the transfer and recombination dynamics. Theoretical predictions indicate that strain-induced energy difference increases along the electron transfer path could contribute to the prolongation of the transfer lifetime. A subpicosecond decay process, related to hot-electron cooling, remains almost unaffected by strain. This study demonstrates the potential of tuning interlayer carrier dynamics through external strains, offering insights into flexible optoelectronic device design with 2D materials.

Influence of S vacancy and O doping in MoS2/GaN heterostructure on charge carrier dynamics: A time-domain ab initio study

Van der Waals heterostructure MoS2/GaN is proposed as a promising candidate material for optoelectronic and photovoltaic applications, and we have studied the influence of S vacancy and O doping on charge carrier dynamics in MoS2/GaN, by means of ab initio nonadiabatic molecular dynamics simulations. It is found that these defects have negligible effect on charge carrier separation, and electron transfer is faster than hole transfer, owing to the stronger nonadiabatic coupling, originating from the more intermediate states and out-of-plane high frequency A1g mode. The presence of S vacancy in MoS2/GaN significantly accelerates the electron-hole recombination by introducing additional electronic states in band gap. It is notable that doping O atom in S vacancy in MoS2 layer induces stronger out-of-plane N atom vibration mode at 250 cm−1, resulting in shorter coherence between CBM and VBM states, which prolongs the carrier lifetime compared to that of the pristine MoS2/GaN.

Resonant Charge Transport Assisted by the Molecular Vibration in Single-Molecule Junction from Time-Domain ab initio Nonadiabatic Molecular Dynamics Simulations

Using ab initio nonadiabatic molecular dynamics simulation, we study the time-dependent charge transport dynamics in a single-molecule junction formed by gold (Au) electrodes and a single benzene-1,4-dithiol (BDT) molecule. Two different types of charge transport channels are found in the simulation. One is the routine non-resonant charge transfer path, which occurs in several picoseconds. The other is activated when the electronic state of the electrodes and that of the molecule get close in energy, which is referred to as the resonant charge transport. More strikingly, the resonant charge transfer occurs in an ultrafast manner within 100 fs, which notably increases the conductance of the device. Further analysis shows that the resonant charge transport is directly assisted by the B 2 and A 1 molecular vibration modes. Our study provides atomic insights into the time-dependent charge transport dynamics in single-molecule junctions, which is important for designing highly efficient single-molecule devices.

Surface Hydroxylation during Water Splitting Promotes the Photoactivity of BiVO4(010) Surface by Suppressing Polaron-Mediated Charge Recombination.

Polaron-based electron transport restricts the photoelectrochemical (PEC) water splitting efficiency of BiVO4. However, the location and dynamics of polarons are significantly dependent on the surface hydroxylation. By performing ab initio nonadiabatic molecular dynamics simulations, we demonstrated that hydroxylation of BiVO4(010) surface greatly alleviates the detrimental effect of oxygen-vacancy-induced electron polaron (EP). Surface hydroxylation stabilizes the EP at the surface to facilitate water splitting, makes the polaron a shallow localized state, and reduces the intensity of high-frequency V-O bond stretching vibrations. By decreasing the nonadiabatic coupling and decoherence time, the charge carrier lifetimes are extended by 1-3 orders of magnitude depending on the hydroxylation coverage. Our study not only reveals that the surface hydroxylation mitigated detrimental impacts of polarons in metal oxides but also provided valuable insights into the benign effect of intermediate species on the photocatalytic reactivity.

Ab initio nonadiabatic molecular dynamics study on spin–orbit coupling induced spin dynamics in ferromagnetic metals

Understanding the photoexcitation induced spin dynamics in ferromagnetic metals is important for the design of photo-controlled ultrafast spintronic device. In this work, by the ab initio nonadiabatic molecular dynamics simulation, we have studied the spin dynamics induced by spin–orbit coupling (SOC) in Co and Fe using both spin-diabatic and spin-adiabatic representations. In Co system, it is found that the Fermi surface (E F) is predominantly contributed by the spin-minority states. The SOC induced spin flip will occur for the photo-excited spin-majority electrons as they relax to the E F, and the spin-minority electrons tend to relax to the E F with the same spin through the electron–phonon coupling (EPC). The reduction of spin-majority electrons and the increase of spin-minority electrons lead to demagnetization of Co within 100 fs. By contrast, in Fe system, the E F is dominated by the spin-majority states. In this case, the SOC induced spin flip occurs for the photo-excited spin-minority electrons, which leads to a magnetization enhancement. If we move the E F of Fe to higher energy by 0.6 eV, the E F will be contributed by the spin-minority states and the demagnetization will be observed again. This work provides a new perspective for understanding the SOC induced spin dynamics mechanism in magnetic metal systems.

In-Situ-Grown Cu Dendrites Plasmonically Enhance Electrocatalytic Hydrogen Evolution on Facet-Engineered Cu2 O.

Herein, facet-engineered Cu2 O nanostructures are synthesized by wet chemical methods for electrocatalytic HER, and it is found that the octahedral Cu2 O nanostructures with exposed crystal planes of (111) (O-Cu2 O) has the best hydrogen evolution performance. Operando Raman spectroscopy and ex-situ characterization techniques showed that Cu2 O is reduced during HER, in which Cu dendrites are grown on the surface of the Cu2 O nanostructures, resulting in the better HER performance of O-Cu2 O after HER (O-Cu2 O-A) compared with that of the as-prepared O-Cu2 O. Under illumination, the onset potential of O-Cu2 O-A is ca. 52 mV positive than that of O-Cu2 O, which is induced by the plasmon-activated electrochemical system consisting of Cu2 O and the in-situ generated Cu dendrites. Incident photon-to-current efficiency (IPCE) measurements and the simulated UV-Vis spectrum demonstrate the hot electron injection (HEI) from Cu dendrites to Cu2 O. Ab initio nonadiabatic molecular dynamics (NAMD) simulations revealed the transfer of photogenerated electrons (27 fs) from Cu dendrites to Cu2 O nanostructures is faster than electron relaxation (170 fs), enhancing its surface plasmons activity, and the HEI of Cu dendrites increases the charge density of Cu2 O. These make the energy level of the catalyst be closer to that of H+ /H2 , evidenced by the plasmon-enhanced HER electrocatalytic activity.

Observation of a transient intermediate in the ultrafast relaxation dynamics of the excess electron in strong-field-ionized liquid water

A unified picture of the electronic relaxation dynamics of ionized liquid water has remained elusive despite decades of study. Here, we employ sub-two-cycle visible to short-wave infrared pump-probe spectroscopy and ab initio nonadiabatic molecular dynamics simulations to reveal that the excess electron injected into the conduction band (CB) of ionized liquid water undergoes sequential relaxation to the hydrated electron s ground state via an intermediate state, identified as the elusive p excited state. The measured CB and p-electron lifetimes are 0.26 ± 0.02 ps and 62 ± 10 fs, respectively. Ab initio quantum dynamics yield similar lifetimes and furthermore reveal vibrational modes that participate in the different stages of electronic relaxation, with initial relaxation within the dense CB manifold coupled to hindered translational motions whereas subsequent p-to-s relaxation facilitated by librational and even intramolecular bending modes of water. Finally, energetic considerations suggest that a hitherto unobserved trap state resides ~0.3-eV below the CB edge of liquid water. Our results provide a detailed atomistic picture of the electronic relaxation dynamics of ionized liquid water with unprecedented time resolution.

Thickness Dependent Ultrafast Charge Transfer in BP/MoS2 Heterostructure

AbstractConstructing high‐performance‐2D heterostructures and deciphering the underlying microscopic mechanism of carrier dynamics are crucial in optoelectronic and photovoltaic applications. Here, taking black phosphorus (BP)/MoS2 heterostructure with type‐II band alignment as a prototypical example, the ab initio nonadiabatic molecular dynamics simulations demonstrate that the interlayer carrier dynamics are thickness dependent. Specifically, the electron transfer from a monolayer (1L)‐BP to MoS2 occurs quickly within 54 fs. In contrast, hole transfer can only be observed within 1 ps with BP's layer number N ≥ 2, triggered by the excitation of low‐frequency acoustic phonon and interlayer shear and breathing phonon modes within 100 cm–1 that enhances the interlayer coupling. Particularly, the electron and hole transfer time exhibits respectively linear and exponential dependence on the layer number N of BP component. The present findings shed new light on improving the process of ultrafast carrier dynamics of 2D heterostructures for photoconversion.

First-principles studies of monolayers MoSi2N4 decorated with transition metal single-atom for visible light-driven high-efficient CO2 reduction by strain and electronic engineering

Solar-driven CO2 reduction into value-added hydrocarbons could promote CO2 utilization and solve related environmental issues, while the key lies in the modulation of the active site’s orbital electronic state. By performing first-principles calculations, we systematically explore MoSi2N4 decorated with 3d and 4d period transition metal single atoms (TMSAs) as efficient CO2 reduction photocatalysts and find that product selectivity is highly correlated with the d-band center of TMSAs. Compressive and tensile strains in mutually perpendicular directions are introduced in the horizontal plane to improve the catalytic performance. After filtering, V, Mn, and Ni were found to be the best TMSAs for CO, HCOOH, and CH4 production, respectively. Ni/MoSi2N4 under −6% tensile strain exhibited the lowest CO2 reduction limiting potential (UL) of 0.24 V. Notably, the time-dependent ab initio nonadiabatic molecular dynamics simulations were carried out to elucidate the photogenerated electron transport properties, where the photo-generated electrons can be efficiently transferred from MoSi2N4 to TMSAs active sites. We also identify that the applied strains affect the CO2 reduction performance by modulating the d-band center and valence electrons. These findings provide pioneering design schemes of precisely tailored photocatalytic systems for CO2 reduction under visible light.

Excited electron and spin dynamics in topological insulator: A perspective from ab initio non-adiabatic molecular dynamics

We perform an ab initio non-adiabatic molecular dynamics simulation to investigate the non-equilibrium spin and electron dynamics in a prototypical topological insulator (TI) Bi2Se3. Different from the ground state, we reveal that backscattering can happen in an oscillating manner between time-reversal pair topological surface states (TSSs) in the non-equilibrium dynamics. Analysis shows the phonon excitation induces orbital composition change by electron-phonon interaction, which further stimulates spin canting through spin-orbit coupling. The spin canting of time-reversal pair TSSs leads to the non-zero non-adiabatic coupling between them and then issues in backscattering. Both the spin canting and backscattering result in ultrafast spin relaxation with a timescale around 100 fs. This study provides critical insights into the non-equilibrium electron and spin dynamics in TI at the ab initio level and paves a way for the design of ultrafast spintronic materials.

Two-Dimensional WO3-Transition-Metal Dichalcogenide Vertical Heterostructures for Nitrogen Fixation: A Photo(Electro) Catalysis Theoretical Strategy

Nitrogen fixation is one of the most critical issues in chemical science and technology. However, the exploitation of efficient and stable photo(electro) catalysts remains a full challenge. Herein, we systematically investigate the general strategy for constructing a novel vertical heterostructure constructed using two-dimensional (2D) WO3 in contact with transition-metal dichalcogenides (TMDCs), including MoS2, MoSe2, WS2, and WSe2, and all heterostructures are theoretically used as efficient photo(electro) catalysts for the nitrogen reduction reaction (NRR). As expected, all the heterostructures are thermodynamically stable by computational results. Notably, the photocatalytic mechanism is elucidated based on time-dependent ab initio nonadiabatic molecular dynamics simulations. The N2 molecules can be efficiently reduced into NH3 through a distal mechanism with onset potentials lower than 0.50 V in the four counterparts. In particular, the optimized WO3-MoS2 possesses the lowest onset potential (0.25 V) with respect to free energy. The catalytic activity can be enhanced by repositioning the d-band center, accurately controlled by the TMDC, to a higher energy level. The synergistic effect of the two components and the regulation of the d-band are critical factors to improve the heterostructure catalytic efficiency. Foreseeably, these findings provide a new avenue for developing cost-effective alternatives for nitrogen fixation.

Photoinduced Spin Injection and Ferromagnetism in 2D Group III Monochalcogenides.

Strong light-matter interactions in low-dimensional materials offer an opportunity for flexible property-tuning by optical switching. Herein, we exploit photoexcitation for spin injection into semiconductors by rationally designing heterojunctions having distinct dynamic behavior for photocarriers in two spin channels. As a proof-of-concept, we trigger homogeneous magnetism in a group III monochalcogenide monolayer (MX with M = In, Ga; X = S, Se) by placing it on a ferromagnetic CrI3 substrate under light illumination. Our time-dependent ab initio nonadiabatic molecular dynamics simulations reveal fast electron-hole separation for the majority spin channel but rapid recombination for the minority spin channel at this heterostructure. The majority carriers cause hole doping and strong ferromagnetic ordering in the MX sheet, with magnetic moment tunable by the injected carriers' concentration. The interplay between photoexcited hole carriers, the Van Hove singularity of MX monolayers, and interfacial charge transfer provides essential physical insights for nondestructively manipulating charge and spin in two-dimensional semiconductors via light switching.

Exploring the photocatalytic properties and carrier dynamics of 2D Janus XMMX' (X = S, Se; M = Ga, In; and X' = Te) materials.

Recently, two-dimensional (2D) Janus structures have been extensively explored because of their robust electron mobility and unique photocatalytic properties. In spite of the increasing interest, the origin of high photocatalytic activities and the behaviors of photoinduced carriers in this kind of materials have not been well understood. Herein, we present a step-by-step protocol based on the first-principles calculations combined with the ab initio non-adiabatic molecular dynamics (NAMD) simulations to unveil the origin of high photocatalytic activity of highly stable typical 2D Janus XMMX' structures (X = S, Se; M = Ga, In; and X' = Te). Their band structures, optical properties, exciton binding energies, carrier effective masses, solar-to-hydrogen efficiency, hot carrier relaxation and recombination times, etc. have been calculated. We find that the difference between X and X' atoms on the two surfaces of the XMMX' monolayer not only builds an out-of-plane electric field, which significantly affects the charge distributions on the valence band maxima (VBM) and the conduction band minima (CBM) and subsequently decreases the exciton binding energy, but also transforms the indirect band structures of XM into the direct ones with well suitable energy gaps for visible-light absorption as well as endows the XMMX' structures with unequal electron and hole mobility, rapid hot carrier relaxation and slow electron-hole recombination processes on a timescale of tens of nanoseconds. The current work suggests that Janus XMMX' monolayers are good photocatalytic materials for overall water splitting and provides a guide to regulate the materials' properties for efficient energy harvesting and optoelectronic applications.

Designing Direct Z-Scheme Heterojunctions Enabled by Edge-Modified Phosphorene Nanoribbons for Photocatalytic Overall Water Splitting.

Direct Z-scheme photocatalyst possess promising potential to utilize solar radiation for photocatalytic overall water splitting; however, the design and characterization remain challenging. Here, we construct and verify a direct Z-scheme heterojunction using edge-modified phosphorene-nanoribbons (X-PNRs, where X = OH and OCN) with first-principles ground-state and excited-state density functional theory (DFT) calculations. The ground-state calculations provide fundamental properties such as geometric structure and band alignment. The linear-response time-dependent DFT (LR-TDDFT) calculations exhibit the photogenerated charge distribution and demonstrate the generation of interlayer excitons in heterojunctions, which are advantageous to the electron-hole recombination in Z-scheme heterojunctions. The ultrafast charge transfer at the interface studied by time-dependent ab initio nonadiabatic molecular dynamics (NAMD) simulations indicates that interlayer electron-hole recombination is prior to intralayer recombination for the OH/OCN-PNRs heterojunction, showing the characteristics of a Z-scheme heterojunction. Therefore, our computational work provides a universal strategy to design direct Z-scheme heterojunction photocatalysts for overall water splitting.

Tuning the Lifetime of Photoexcited Small Polarons on Rutile TiO2 Surface via Molecular Adsorption

TiO2 is a famous wide-bandgap semiconductor which has important applications in photocatalysis and photovoltaics. In the solar energy conversion process, photoexcited electrons can be trapped by the lattice distortion and can form a small polaron, which can quench through electron–hole recombination. On the TiO2 surface, the molecule adsorption is proposed to play an important role to influence the dynamics of a photoexcited polaron. In this study, by ab initio nonadiabatic molecular dynamics simulation, we study the dynamics of a photoexcited small polaron in the rutile TiO2(110) surface system and the effects of H2O adsorption. It is found that the photoexcited electron can be trapped by the lattice distortion in the subsurface layer within 25 fs. The H2O adsorption stabilizes the polaron by further stretching the Ti–O bonds around it. In a comparison with the photoexcited free electrons, the formation of the polaron shortens the carrier lifetime distinctly due to the reduction of the energy difference with the photoexcited hole at the valence band maximum, and reinforcement of electron–phonon (e–ph) coupling. Moreover, the H2O adsorption is found to further reduce the photoexcited polaron lifetime by enhancing the e–ph coupling with high-frequency phonons. This work suggests that molecule adsorption provides a new stratagem to tune the lifetime of a photoexcited small polaron.