A non-self-consistent range-separated time-dependent density functional approach for large-scale simulations

Non-adiabatic Exciton Dynamics in van der Waals Heterostructures

In this Perspective, we introduce a first-principles method that combines time-dependent density functional theory with non-adiabatic molecular dynamics (NAMD) to explore exciton dynamics in two-dimensional (2D) van der Waals (vdW) heterostructures. The theoretical foundation and computational efficiency of the method are discussed and compared with those of related methods (e.g., GW-BSE). Using three 2D vdW heterostructures as examples, we demonstrate that the proposed method can provide a reliable description of many-body electron-hole interactions crucial to exciton dynamics. With much lower computational costs than the GW-BSE method, the proposed method represents a particularly promising theoretical tool to probe exciton dynamics in solids. Moreover, we find that the NAMD simulations widely used in the literature cannot capture the excitonic effect in 2D materials and often yield incorrect results because they are formulated in a single-particle picture. The instances where the single-particle picture fails are pointed out and contrasted with the many-body simulation results.

Accelerating Linear-Response Time-Dependent Hybrid Density Functional Theory with Low-Rank Decomposition Techniques in the Plane-Wave Basis

We present an efficient low-rank implementation of linear-response time-dependent density functional theory for hybrid functionals (hybrid-LR-TDDFT) within the plane-wave pseudopotential framework. The adaptively compressed exchange (ACE) operator and the natural transition orbitals (NTOs) are introduced to build the low-rank representation of the nonlocal exchange operator in the hybrid-LR-TDDFT Hamiltonian. Numerical tests demonstrate that the ACE approximation significantly reduces the computational cost of applying the nonlocal exchange operator without loss of accuracy, and the NTO approximation can further accelerate the hybrid-LR-TDDFT calculations by introducing an NTO cutoff parameter. This new method enables us to effectively study the excitonic properties of two-dimensional MoS₂ consisting of 216 atoms and ∼1900 electrons with range-separated hybrid functionals on a single graphics processing unit.

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.

Shedding light on moiré excitons: A first-principles perspective

Moiré superlattices in van der Waals (vdW) heterostructures could trap long-lived interlayer excitons. These moiré excitons could form ordered quantum dot arrays, paving the way for unprecedented optoelectronic and quantum information applications. Here, we perform first-principles simulations to shed light on moiré excitons in twisted MoS₂/WS₂ heterostructures. We provide direct evidence of localized interlayer moiré excitons in vdW heterostructures. The interlayer and intralayer moiré potentials are mapped out based on spatial modulations of energy gaps. Nearly flat valence bands are observed in the heterostructures. The dependence of spatial localization and binding energy of the moiré excitons on the twist angle of the heterostructures is examined. We explore how vertical electric field can be tuned to control the position, polarity, emission energy, and hybridization strength of the moiré excitons. We predict that alternating electric fields could modulate the dipole moments of hybridized moiré excitons and suppress their diffusion in moiré lattices.

Excitonic Effect Drives Ultrafast Dynamics in van der Waals Heterostructures

Recent experiments revealed stacking-configuration-independent and ultrafast charge transfer in transition metal dichalcogenides van der Waals (vdW) heterostructures, which is surprising given strong exciton binding energies and large momentum mismatch across the heterojunctions. Previous theories failed to provide a comprehensive physical picture for the charge transfer mechanisms. To address this challenge, we developed a first-principles framework which can capture exciton-phonon interaction in extended systems. We find that excitonic effect does not impede, but actually drives ultrafast charge transfer in vdW heterostructures. The many-body electron-hole interaction affords cooperation among the electrons, which relaxes the constraint on momentum conservation and reduces energy gaps for charge transfer. We uncover a two-step process in exciton dynamics: ultrafast hole transfer followed by much longer relaxation of intermediate "hot" excitons. This work establishes that many-body excitonic effect is crucial to the ultrafast dynamics and provides a basis to understand relevant phenomena in vdW heterostructures.

Suppression of Electron-Hole Recombination by Intrinsic Defects in 2D Monoelemental Material

The Shockley-Read-Hall (SRH) model, in which the deep trap defect states in the band gap are proposed as nonradiative electron-hole (e-h) recombination centers, has been widely used to describe the nonradiative e-h recombination through the defects in semiconductor. By using the ab initio nonadiabatic molecular dynamics method, we find that the SRH model fails to describe the e-h recombination behavior for defects in 2D monoelemental material such as monolayer black phosphorus (BP). Through the investigation of three intrinsic defects with shallow and deep defect states in monolayer BP, it is found that, surprisingly, none of these defects significantly accelerates the e-h recombination. Further analysis shows that because monolayer BP is a monoelemental material, the distinct impurity phonon, which often induces fast e-h recombination, is not formed. Moreover, because of the flexibility of 2D material, the defects scatter the phonons present in pristine BP, generating multiple modes with lower frequencies compared with the pristine BP, which further suppresses the e-h recombination. We propose that the conclusion can be extended to other monoelemental 2D materials, which is important guidance for the future design of functional semiconductors.

Enhanced charge transfer by phenyl groups at a rubrene/C60 interface

Exciton dynamics at an interface between an electron donor, rubrene, and a C(60) acceptor is studied by nonadiabatic quantum molecular dynamics simulation. Simulation results reveal an essential role of the phenyl groups in rubrene in increasing the charge-transfer rate by an order-of-magnitude. The atomistic mechanism of the enhanced charge transfer is found to be the amplification of aromatic breathing modes by the phenyl groups, which causes large fluctuations of electronic excitation energies. These findings provide insight into molecular structure design for efficient solar cells, while explaining recent experimental observations.