Mechanistic Insights into Nonadiabatic Interband Transitions on a Semiconductor Surface Induced by Hydrogen Atom Collisions.

To understand the recently observed enigmatic nonadiabatic energy transfer for hyperthermal H atom scattering from a semiconductor surface, Ge(111)(2 × 8), we present a mixed quantum-classical nonadiabatic molecular dynamics model based on the time-dependent evolution of Kohn-Sham orbitals and a classical path approximation. Our results suggest that facile nonadiabatic electronic transitions from the valence band to the conduction band occur selectively at the rest atom site, where surface states are doubly occupied, but not at the adatom site, where empty surface states are localized. This drastic site specificity can be attributed to the changes of the local band structure upon energetic H collisions at different surface sites, leading to transient near degeneracies and significant couplings between occupied and unoccupied orbitals at the rest atom but not at the adatom.

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.

Real-Time Investigation on Hot Electron Relaxation Dynamics in Silicon.

The relaxation of hot electrons in semiconductors is pivotal for both energy harvesting processes and optoelectronics. Utilizing a self-developed non-adiabatic molecular dynamics simulation approach in the momentum space (NAMD_), we have examined the dynamics of hot electrons in silicon. Whether excited from the Γ or L point, the relaxation dynamics exhibit two distinct stages.

Ab initio real-time quantum dynamics of charge carriers in momentum space.

Application of the non-adiabatic molecular dynamics (NAMD) approach is limited to studying carrier dynamics in the momentum space, as a supercell is required to sample the phonon excitation and electron-phonon (e-ph) interaction at different momenta in a molecular dynamics simulation. Here we develop an ab initio approach for the real-time charge carrier quantum dynamics in the momentum space (NAMD_k) by directly introducing e-ph coupling into the Hamiltonian based on the harmonic approximation. The NAMD_k approach maintains the zero-point energy and includes memory effects of carrier dynamics.

Ultrafast many-body bright-dark exciton transition in anatase TiO.

The momentum-forbidden dark excitons can have a pivotal role in quantum information processing, Bose-Einstein condensation, and light-energy harvesting. Anatase TiO with an indirect band gap is a prototypical platform to study bright to momentum-forbidden dark exciton transition. Here, we examine, by GW plus the real-time Bethe-Salpeter equation combined with the nonadiabatic molecular dynamics (GW + rtBSE-NAMD), the many-body transition that occurs within 100 fs from the optically excited bright to the strongly bound momentum-forbidden dark excitons in anatase TiO.

High Photoreactivity on a Reconstructed Anatase TiO(001) Surface Predicted by Nonadiabatic Molecular Dynamics.

Anatase TiO(001) surface with (4 × 1) reconstruction is proposed to be a highly active catalytic surface. In this work, using time-domain nonadiabatic molecular dynamics, we reveal that the ridge structure formed by anatase(001) surface reconstruction is the photoreactive site for hole migration and trapping. Moreover, the ridge structure is destroyed by low-coverage CHOH adsorption, leading to the suppression of its high photoreactivity.

Ultrafast charge transfer coupled to quantum proton motion at molecule/metal oxide interface.

Understanding how the nuclear quantum effects (NQEs) in the hydrogen bond (H-bond) network influence the photoexcited charge transfer at semiconductor/molecule interface is a challenging problem. By combining two kinds of emerging molecular dynamics methods at the ab initio level, the path integral-based molecular dynamics and time-dependent nonadiabatic molecular dynamics, and choosing CHOH/TiO as a prototypical system to study, we find that the quantum proton motion in the H-bond network is strongly coupled with the ultrafast photoexcited charge dynamics at the interface. The hole trapping ability of the adsorbed methanol molecule is notably enhanced by the NQEs, and thus, it behaves as a hole scavenger on titanium dioxide.

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

We perform an non-adiabatic molecular dynamics simulation to investigate the non-equilibrium spin and electron dynamics in a prototypical topological insulator (TI) BiSe. 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.

Effects of oxygen vacancies on the photoexcited carrier lifetime in rutile TiO.

The photoexcited carrier lifetime in semiconductors plays a crucial role in solar energy conversion processes. The defects or impurities in semiconductors are usually proposed to introduce electron-hole (e-h) recombination centers and consequently reduce the photoexcited carrier lifetime. In this report, we investigate the effects of oxygen vacancies (O) on the carrier lifetime in rutile TiO, which has important applications in photocatalysis and photovoltaics.

Improved phase retrieval in holographic data storage based on a designed iterative embedded data.

Embedded data are used to retrieve phases quicker with high accuracy in phase-modulated holographic data storage (HDS). We propose a method to design an embedded data distribution using iterations to enhance the intensity of the high-frequency signal in the Fourier spectrum. The proposed method increases the antinoise performance and signal-to-noise ratio (SNR) of the Fourier spectrum distribution, realizing a more efficient phase retrieval.

Ultrafast Ferroelectric Ordering on the Surface of a Topological Semimetal MoTe.

Transient tuning of material properties by light usually requires intense laser fields in the nonlinear excitation regime. Here, we report ultrafast ferroelectric ordering on the surface of a paraelectric topological semimetal 1T'-MoTe in the linear excitation regime, with the order parameter directly proportional to the excitation intensity. The ferroelectric ordering, driven by a transient electric field created by electrons trapped ångstroms away from the surface in the image potential state (IPS), is evidenced in two-photon photoemission spectroscopy showing the energy relaxation rate proportional to IPS electron density, but with negligible change in the free-electron-like parallel dispersion.

Time- and momentum-resolved image-potential states of 2H-MoS surface.

Rydberg-like image potential states (IPSs) form special series surface states on metal and semiconducting surfaces. Here, using time-resolved and momentum-resolved multi-photon photoemission (PPE), we measured the energy positions, band dispersion, and carrier lifetimes of IPSs at the 2H-MoS surface. The energy minima of the IPSs ( = 1 and 2) were located at 0.

Bidirectional and reversible tuning of the interlayer spacing of two-dimensional materials.

Interlayer spacing is expected to influence the properties of multilayer two-dimensional (2D) materials. However, the ability to non-destructively regulate the interlayer spacing bidirectionally and reversibly is challenging. Here we report the preparation of 2D materials with tunable interlayer spacing by introducing active sites (Ce ions) in 2D materials to capture and immobilize Pt single atoms.

Strong Modulation of Band Gap, Carrier Mobility and Lifetime in Two-Dimensional Black Phosphorene through Acoustic Phonon Excitation.

Black phosphorene (BP) has been attracting intense attention due to its high charge mobility and potential applications in electronic, optical and optoelectronic devices. We demonstrate by molecular dynamics and nonadiabatic quantum dynamics simulations that the excitation of out-of-plane acoustic phonon (ZA) provides strong modulation of the band gap, carrier lifetime and carrier mobility in BP. A 1% tensile strain can significantly enhance ZA mode excitation at room temperature, distinctly reducing the band gap, carrier mobility, and lifetime.

Dynamic sampling iterative phase retrieval for holographic data storage.

A dynamic sampling iterative phase retrieval method, which dynamically samples the Fourier intensity distribution of the reconstruction beam captured by the detector, is proposed to shorten the iterative number and decrease the phase error rate of phase retrieval in the phase-modulated holographic data storage. By the dynamic sampling method, that keeping relatively low frequency component of Fourier intensity spectrum at the beginning of iteration and gradually releasing more high frequency component at the subsequent iterations, we shortened the iterative number by 2 times and decreased the phase error rate to some extent because our method provided a better convergent path to the phase retrieval. We also believe the thought of our method can be used in more image retrieval fields.

Real-time -BSE investigations on spin-valley exciton dynamics in monolayer transition metal dichalcogenide.

We develop an ab initio nonadiabatic molecular dynamics (NAMD) method based on plus real-time Bethe-Salpeter equation ( + rtBSE-NAMD) for the spin-resolved exciton dynamics. From investigations on MoS, we provide a comprehensive picture of spin-valley exciton dynamics where the electron-phonon (e-ph) scattering, spin-orbit interaction (SOI), and electron-hole (e-h) interactions come into play collectively. In particular, we provide a direct evidence that e-h exchange interaction plays a dominant role in the fast valley depolarization within a few picoseconds, which is in excellent agreement with experiments.

Dynamics of Photoexcited Small Polarons in Transition-Metal Oxides.

The dynamics of photoexcited polarons in transition-metal oxides (TMOs), including their formation, migration, and quenching, plays an important role in photocatalysis and photovoltaics. Taking rutile TiO as a prototypical system, we use nonadiabatic molecular dynamics simulation to investigate the dynamics of small polarons induced by photoexcitation at different temperatures. The photoexcited electron is trapped by the distortion of the surrounding lattice and forms a small polaron within tens of femtoseconds.

Accurate Computation of Nonadiabatic Coupling with Projector Augmented-Wave Pseudopotentials.

Synergy of nonadiabatic molecular dynamics with real-time time-dependent density functional theory has led to significant progress in modeling excited-state dynamics in nanoscale and condensed matter systems over the past decade. Nonadiabatic coupling (NAC) is the central quantity in such simulations, and its accurate and efficient evaluation is an enduring challenge in time-dependent Kohn-Sham theory, particularly in conjunction with planewave basis sets and projector augmented-wave (PAW) pseudopotentials because of the complexity of the PAW "all-electron" wave function. We report a method for rigorous evaluation of the NAC with PAW wave functions and demonstrate an efficient approximation to the rigorous NAC that gives comparable accuracy.

Interlayer Polarization Explains Slow Charge Recombination in Two-Dimensional Halide Perovskites by Nonadiabatic Molecular Dynamics Simulation.

Two-dimensional (2D) perovskites for applications in photovoltaics and optoelectronics are attracting a great deal of research interest. The nonradiative electron-hole (e-h) recombination is the major efficiency loss channel. Herein, we report a study of the thickness dependence of the e-h recombination dynamics in diamine-based 2D perovskite via NAMD.

Tuning the Carrier Lifetime in Black Phosphorene through Family Atom Doping.

It is highly desirable to control the carrier lifetime in two-dimensional (2D) materials to suit the needs of various device functionalities. In this work, by nonadiabatic molecular dynamics simulation, we find the single atom doping from phosphorus family elements can sufficiently tune the carrier lifetime in black phosphorene (BP). Instead of forming electron-hole (e-h) recombination centers, the e-h recombination is suppressed by doping compared with the pristine BP.

Low-frequency lattice phonons in halide perovskites explain high defect tolerance toward electron-hole recombination.

Low-cost solution-based synthesis of metal halide perovskites (MHPs) invariably introduces defects in the system, which could form Shockley-Read-Hall (SRH) electron-hole recombination centers detrimental to solar conversion efficiency. Here, we investigate the nonradiative recombination processes due to native point defects in methylammonium lead halide (MAPbI) perovskites using ab initio nonadiabatic molecular dynamics within surface-hopping framework. Regardless of whether the defects introduce a shallow or deep band state, we find that charge recombination in MAPbI is not enhanced, contrary to predictions from SRH theory.

CO Photoreduction on Metal Oxide Surface Is Driven by Transient Capture of Hot Electrons: Quantum Dynamics Simulation.

The most critical bottleneck in CO photoreduction lies in the activation of CO to form an anion radical, CO, or other intermediates by the photoexcited electrons, because CO has a high-energy lowest unoccupied molecular orbital (LUMO). Taking rutile TiO(110) as a prototypical surface, we use time-dependent nonadiabatic molecular dynamics simulations to reveal that the excitation of bending and antisymmetric stretching vibrations of CO can sufficiently stabilize the CO LUMO below the conduction band minimum, allowing it to trap photoexcited hot electrons and get reduced. Such vibrational excitations occur by formation of a transient CO adsorbed in an oxygen vacancy.

Dynamics of Single-Molecule Dissociation by Selective Excitation of Molecular Phonons.

Breaking bonds selectively in molecules is vital in many chemistry reactions and custom nanoscale device fabrications. The scanning tunneling microscope (STM) has proved to be an ideal tool to initiate and view bond-selective chemistry at the single-molecule level, offering opportunities for the further study of the dynamics in single molecules on metal surfaces. We demonstrate H─HS and H─S bond breaking on Au(111) induced by tunneling electrons using low-temperature STM.

Tensile Strain-Controlled Photogenerated Carrier Dynamics at the van der Waals Heterostructure Interface.

Customizing the photogenerated carrier dynamics would make the two-dimensional (2D) materials highly adaptable to various application scenarios. On the basis of time-domain ab initio nonadiabatic molecular dynamics simulation, we find that 4% tensile strain can suppress the electron transfer at the van der Waals heterostructure MoS/WS interface. Our analysis shows that after the electron-hole pair is excited in the K valley in WS direct electron transfer from WS@K to MoS@K is very difficult because of the weak interlayer coupling in the K valley, and thus, it happens through the T valley as WS@K-MoS@T-MoS@K.