Ab Initio Nonadiabatic Molecular Dynamics Research Articles

Self-passivation of Halide Interstitial Defects by Organic Cations in Hybrid Lead-Halide Perovskites: Ab Initio Quantum Dynamics.

Halide interstitial defects severely hinder the optoelectronic performance of metal halide perovskites, making research on their passivation crucial. We demonstrate, using ab initio nonadiabatic molecular dynamics simulations, that hydrogen vacancies (Hv) at both N and C atoms of the methylammonium (MA) cation in MAPbI3 efficiently passivate iodine interstitials (Ii), providing a self-passivation strategy for dealing with the Hv and Ii defects simultaneously. Hv at the N site (Hv-N) introduces a defect state into the valence band, while the state contributed by Hv at the C site (Hv-C) evolves from a shallow level at 0 K to a deep midgap state at ambient temperature, exhibiting a high environmental activity. Both Hv-N and Hv-C are strong Lewis bases, capable of capturing and passivating Ii defects. Hv-C is a stronger Lewis base, bonds with Ii better, and exhibits a more pronounced passivation effect. The charge carrier lifetimes in the passivated systems are significantly longer than in those containing either Hv or Ii, and even in pristine MAPbI3. Our demonstration of the Hv and Ii defect self-passivation in MAPbI3 suggests that systematic control of the relative concentrations of Hv and Ii can simultaneously eliminate both types of defects, thereby minimizing charge and energy losses. The demonstrated defect self-passivation strategy provides a promising means for defect control in organic-inorganic halide perovskites and related materials and deepens our atomistic understanding of defect chemistry and charge carrier dynamics in solar energy and optoelectronic materials.

Tunable Ultrafast Charge Transfer across Homojunction Interface.

Charge transfer at heterojunction interfaces is a fundamental process that plays a crucial role in modern electronic and photonic devices. The essence of such charge transfer lies in the band offset, making charge transfer uncommon in a homojunction. Recently, sliding ferroelectricity has been proposed and confirmed in two-dimensional van der Waals stacked materials such as bilayer boron nitride. During the sliding of these layers, the band alignment shifts, creating conditions for charge separation at the interface. We employ ab initio nonadiabatic molecular dynamics simulations to elucidate the excited state carrier dynamics in bilayer boron pnictides. We propose that, akin to ferroelectric polarization flipping, the precise modulation of the distribution of excited state carriers can also be reached by sliding. Our results demonstrate that sliding induces a reversal of the frontier orbital distribution on the upper and lower layers, facilitating a robust interlayer carrier transfer. Notably, the interlayer carrier transfer is more pronounced in boron phosphide than in boron nitride, attributed to strong electron scattering in momentum space in boron nitride. We propose this novel method to manipulate carrier distribution and dynamics in a homojunction exhibiting sliding ferroelectricity, in general, paving a new way for developing advanced electronic and photonic devices.

Simulating ultrafast transient absorption spectra from first principles using a time-dependent configuration interaction probe.

Transient absorption spectroscopy (TAS) is among the most common ultrafast photochemical experiments, but its interpretation remains challenging. In this work, we present an efficient and robust method for simulating TAS signals from first principles. Excited-state absorption and stimulated emission (SE) signals are computed using time-dependent complete active space configuration interaction (TD-CASCI) simulations, leveraging the robustness of time-domain simulation to minimize electronic structure failure. We demonstrate our approach by simulating the TAS signal of 1'-hydroxy-2'-acetonapthone (HAN) from ab initio multiple spawning nonadiabatic molecular dynamics simulations. Our results are compared to gas-phase TAS data recorded from both jet-cooled (T ∼ 40 K) and hot (∼403 K) molecules via cavity-enhanced TAS (CE-TAS). Decomposition of the computed spectrum allows us to assign a rise in the SEsignal to excited-state proton transfer and the ultimate decay of the signal to relaxation through a twisted conical intersection. The total cost of computing the observable signal (∼1700 graphics processing unit hours for ∼4ns of electron dynamics) was markedly less than that of performing the abinitio multiple spawning calculations used to compute the underlying nonadiabatic dynamics.

First-Principles Computational Screening of Two-Dimensional Polar Materials for Photocatalytic Water Splitting.

The band gap constraint of the photocatalyst for overall water splitting limits the utilization of solar energy. A strategy to broaden the range of light absorption is employing a two-dimensional (2D) polar material as photocatalyst, benefiting from the deflection of the energy level due to their intrinsic internal electric field. Here, by using first-principles computational screening, we search for 2D polar semiconductors for photocatalytic water splitting from both ground- and excited-state perspectives. Applying a unique electronic structure model of polar materials, there are 13 photocatalyst candidates for the hydrogen evolution reaction (HER) and 8 candidates for the oxygen evolution reaction (OER) without barrier energies from the perspective of the ground-state free energy variation calculation. In particular, Cu2As4Cl2S3 and Cu2As4Br2S3 can catalyze HER and OER simultaneously, becoming promising photocatalysts for overall water splitting. Furthermore, by combining ground-state band structure calculations with excited-state charge distribution and transfer calculated by linear-response time-dependent density functional theory (LR-TDDFT) and time-dependent ab initio nonadiabatic molecular dynamics (NAMD), respectively, the rationality of the 2D polar material model has been manifested. The intrinsic built-in electric field promotes the separation of charge carriers while suppressing their recombination. Therefore, our computational work provides a high-throughput method to design high-performance photocatalysts for water splitting.

Extending the Charge Carrier Recombination Lifetime by Octahedral Rotations in Ruddlesden-Popper Ba3Zr2S7 Perovskites.

Intentional distortions of [BX6] octahedra within perovskite structures have been recognized as a potent strategy for precise band gap adjustments and optimization of their photovoltaic properties, yet information regarding charge carrier dynamics linked to octahedral distortion under ambient conditions for chalcogenide perovskites remains limited. In this study, we utilize ab initio nonadiabatic molecular dynamics to explore the dynamics of photogenerated carriers in a representative two-dimensional Ba3Zr2S7 material in the Ruddlesden-Popper phase. The theoretical results highlight the influence of octahedral rotation on the materials' stability and carrier recombination lifetime of the system. Specifically, the octahedrally rotating P42/mnm phase exhibits a prolonged nonradiative carrier recombination lifetime attributed to the stabilized electron-phonon coupling. These findings offer valuable insights into the fundamental physical characteristics of imposed octahedral distortion and its potential for optimizing the optoelectronic performance of 2D Ruddlesden-Popper Ba-Zr-S chalcogenide materials.

Tin doping modulates electron-hole recombination in Dion-Jacobson phase 2D hybrid perovskite

To obtain less-toxic and optimal-band-gap perovskite materials, tin (Sn)-lead (Pb) hybrid technology comes into the sight of researchers. The Sn–Pb ratio primarily affects the electronic band structure, conductivity, and carrier transport, which are essential for optoelectronic performance. While most reports are about 3D materials, there has been little coverage of 2D Sn–Pb perovskites which exhibiting improved stability. In this paper, the e-h recombination dynamics in Dion-Jacobson Phase 2D Sn–Pb perovskites are studied using time-dependent ab initio nonadiabatic molecular dynamics (NAMD) simulation. It is found that Sn doping plays a significant role in narrowing the band-gap of 2D perovskites and localizing the charge distribution. Only a small amount of Sn doping can minimize NA coupling and shorten decoherence time of frontier orbitals simultaneously, then leading to longer charge recombination time. NAMD simulations reveal the optimal Sn ratio (25 mol%) and provide an effective way to design high-performance 2D perovskite materials.

Compression of Organic Molecules Coupled with Hydrogen Bonding Extends the Charge Carrier Lifetime in BA2SnI4.

Two-dimensional (2D) metal halide perovskites, such as BA2SnI4 (BA═CH3(CH2)3NH3), exhibit an enhanced charge carrier lifetime in experiments under strain. Experiments suggest that significant compression of the BA molecule, rather than of the inorganic lattice, contributes to this enhancement. To elucidate the underlying physical mechanism, we apply a moderate compressive strain to the entire system and subsequently introduce significant compression to the BA molecules. We then perform ab initio nonadiabatic molecular dynamics simulations of nonradiative electron-hole recombination. We observe that the overall lattice compression reduces atomic motions and decreases nonadiabatic coupling, thereby delaying electron-hole recombination. Additionally, compression of the BA molecules enhances hydrogen bonding between the BA molecules and iodine atoms, which lengthens the Sn-I bonds, distorts the [SnI6]4- octahedra, and suppresses atomic motions further, thus reducing nonadiabatic coupling. Also, the elongated Sn-I bonds and weakened antibonding interactions increase the band gap. Altogether, the compression delays the nonradiative electron-hole recombination by more than a factor of 3. Our simulations provide new and valuable physical insights into how compressive strain, accommodated primarily by the organic ligands, positively influences the optoelectronic properties of 2D layered halide perovskites, offering a promising pathway for further performance improvements.

Chloride Ion-Induced Spatial Separation and Long Recombination Time of Photogenerated Electrons and Holes in Crystalline Carbon Nitride.

Poly(triazine imide)·Li+Cl- (PTI/Li+Cl-) as one of the most reported crystalline carbon nitrides has shown exciting potential for photocatalysis. However, understanding the role of Li+/Cl- in the photoexcited charge transfer in the time and space of PTI is a challenging problem. Here, we have investigated the nonradiative charge recombination of series ion intercalated PTI systems (PTI/Li+X-, where X = F, Cl, Br, and I) by performing the ab initio nonadiabatic molecular dynamics simulations. The results indicate that the intercalated anions in PTI/Li+Cl- and PTI/Li+Br- have the potential to trap holes, separate the electrons and holes, and prolong the nonradiative electron-hole recombination. In particular, ∼70% of holes in PTI/Li+Cl- can transport among interlayers toward the {0001} planes, while most of the electrons are transferred to the {101̅0} planes, exhibiting different transport pathways and directions. Furthermore, PTI/Li+Cl- has an electron-hole recombination time as long as 136 ns, which explains its excellent optoelectronic properties. This work provides a theoretical baseline for the reported facet engineering improvement of crystalline carbon nitride for overall water splitting.

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.

Impact of large A-site cations on electron-vibrational interactions in 2D halide perovskites: Ab initio quantum dynamics.

Using abinitio nonadiabatic molecular dynamics, we study the effect of large A-site cations on nonradiative electron-hole recombination in two-dimensional Ruddlesden-Popper perovskites HA2APb2I7, HA = n-hexylammonium, A = methylammonium (MA), or guanidinium (GA). The steric hindrance created by large GA cations distorts and stiffens the inorganic Pb-I lattice, reduces thermal structural fluctuations, and maintains the delocalization of electrons and holes at ambient and elevated temperatures. The delocalized charges interact more strongly in the GA system than in the MA system, and the charge recombination is accelerated. In contrast, replacement of only some MA cations with GA enhances disorder and increases charge lifetime, as seen in three-dimensional perovskites. This study highlights the key influence of structural fluctuations and disorder on the properties of charge carriers in metal halide perovskites, providing guidance for tuning materials' optoelectronic performance.

Photoexcitation-induced spin dynamics in 1T-VSe2 investigated by ab initio nonadiabatic molecular dynamics

Photoexcitation-induced spin dynamics in 1T-VSe2 investigated by ab initio nonadiabatic molecular dynamics

Enhanced Charge Separation in Single Atom Cobalt Based Graphitic Carbon Nitride: Time Domain Ab Initio Analysis.

In recent years, single atom catalysts have been at the forefront of energy conversion research, particularly in the field of catalysis. Carbon nitrides offer great potential as hosts for stabilizing metal atoms due to their unique electronic structure. We use ab initio nonadiabatic molecular dynamics to study photoexcitation dynamics in single atom cobalt based graphitic carbon nitride. The results elucidate the positive effect of the doped cobalt atom on the electronic structure of GCN. Cobalt doping produces filled midgap states that serve as oxidation centers, advantageous for various redox reactions. The presence of midgap states enables the harvesting of longer wavelength photons, thereby extending the absorption range of solar light. Although doping accelerates charge relaxation overall, charge recombination is significantly slower than charge separation, creating beneficial conditions for catalysis applications. The simulations reveal the detailed microscopic mechanism underlying the improved performance of the doped system due to atomic defects and demonstrate an effective charge separation strategy to construct highly efficient and stable photocatalytic two-dimensional materials.

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.

Lattice Distortion and Low-Frequency Anharmonic Phonons Suppress Charge Recombination in Lead Halide Perovskites upon Pseudohalide Doping: Time-Domain Ab Initio Analysis.

Perovskite solar cells have witnessed a surge in interest as a promising technology for low-cost, high-efficiency photovoltaics with certified power conversion efficiencies beyond 25%. However, their commercial development is hindered by poor stability and nonradiative losses that restrict their approach to the theoretical efficiency limit. Using ab initio nonadiabatic molecular dynamics, we demonstrate that nonradiative charge recombination is suppressed when the iodide in formamidinium lead iodide (FAPbI3) is partially replaced with pseudohalide anions (SCN-, BF4-, and PF6-). The replacement breaks the symmetry of the system and creates local structural distortion and dynamic disorder, decreasing electron-hole overlap and nonadiabatic electron-vibrational coupling. The charge carrier lifetime is found to increase with increased structural distortion and is the longest for PF6-. This work is fundamentally relevant to the design of high-performance perovskite materials for optoelectronic applications.

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.

The carbonyl-lock mechanism underlying non-aromatic fluorescence in biological matter

Challenging the basis of our chemical intuition, recent experimental evidence reveals the presence of a new type of intrinsic fluorescence in biomolecules that exists even in the absence of aromatic or electronically conjugated chemical compounds. The origin of this phenomenon has remained elusive so far. In the present study, we identify a mechanism underlying this new type of fluorescence in different biological aggregates. By employing non-adiabatic ab initio molecular dynamics simulations combined with a data-driven approach, we characterize the typical ultrafast non-radiative relaxation pathways active in non-fluorescent peptides. We show that the key vibrational mode for the non-radiative decay towards the ground state is the carbonyl elongation. Non-aromatic fluorescence appears to emerge from blocking this mode with strong local interactions such as hydrogen bonds. While we cannot rule out the existence of alternative non-aromatic fluorescence mechanisms in other systems, we demonstrate that this carbonyl-lock mechanism for trapping the excited state leads to the fluorescence yield increase observed experimentally, and set the stage for design principles to realize novel non-invasive biocompatible probes with applications in bioimaging, sensing, and biophotonics.