New Pathway for Hot Electron Relaxation in Two-Dimensional Heterostructures

Electronic and optical properties of CrI3/Nb3Cl8heterojunction: a first principles investigation

Constructing heterostructures has been used as an effective way to circumvent the shortcomings of composite layers since the interactions and charge transfer between individual layers can thus change the properties in forming heterostructure. In this work, the stability and physical properties of two-dimensional van der Waals CrI3/Nb3Cl8heterojunction in different stacking modes have been investigated using the first principles calculations based on density functional theory. The results demonstrate that the most stable CrI3/Nb3Cl8heterojunction possesses a typical type-II band alignment with a 0.753 eV indirect band gap. The electrons moves from the Nb3Cl8layer to the CrI3layer due to the former one has a higher energy level for valence band maximum, resulting in a built-in electric field. Comparing to CrI3and Nb3Cl8monolayers, the light absorption is enhanced in the infrared, visible and ultraviolet regions, and may hence improve the efficiency in energy conversion or optoelectronics. The rather narrow band gap hinders its application in water splitting, but may have potential applications related with infrared lights. Thus, the investigation provides theoretical insights for CrI3/Nb3Cl8heterojunction and may promote its applications.

Machine-Learned Kohn-Sham Hamiltonian Mapping for Nonadiabatic Molecular Dynamics

In this work, we report a simple, efficient, and scalable machine-learning (ML) approach for mapping non-self-consistent Kohn-Sham Hamiltonians constructed with one kind of density functional to the nearly self-consistent Hamiltonians constructed with another kind of density functional. This approach is designed as a fast surrogate Hamiltonian calculator for use in long nonadiabatic dynamics simulations of large atomistic systems. In this approach, the input and output features are Hamiltonian matrices computed from different levels of theory. We demonstrate that the developed ML-based Hamiltonian mapping method (1) speeds up the calculations by several orders of magnitude, (2) is conceptually simpler than alternative ML approaches, (3) is applicable to different systems and sizes and can be used for mapping Hamiltonians constructed with arbitrary density functionals, (4) requires a modest training data, learns fast, and generates molecular orbitals and their energies with the accuracy nearly matching that of conventional calculations, and (5) when applied to nonadiabatic dynamics simulation of excitation energy relaxation in large systems yields the corresponding time scales within the margin of error of the conventional calculations. Using this approach, we explore the excitation energy relaxation in C60 fullerene and Si75H64 quantum dot structures and derive qualitative and quantitative insights into dynamics in these systems.

Photothermionic Effect-Assisted Ultrafast Charge Transfer in NbS2/MoS2 Heterostructure

Two-dimensional (2D) van der Waals heterostructures (vdW HSs) composed of transition metal dichalcogenides (TMDCs) have emerged as frontrunners in the optoelectronics field, owing to their exceptional optical and electrical properties. Recent research on the intrinsic interlayer charge transfer mechanism has been primarily focused on the Type II HSs, while metal-semiconductor (MS) vertical HSs, promising for advancing photodetector technology, have received comparatively less attention. Here, we reveal the first experimental observation of photothermionic effect-assisted ultrafast interlayer charge transfer in the NbS2/MoS2 heterostructure using femtosecond transient absorption technology and first-principles calculations, effectively ignoring the Schottky barrier height. We demonstrate that within 500 fs, charge transfer occurs from NbS2 to MoS2 in the heterostructure, resulting in supplementary carrier generation in the visible spectrum when excited with infrared light below the MoS2 bandgap, at wavelengths of 1030 and 1500 nm. Such promising characteristics of 2D NbS2-semiconductor heterostructures offer a potential platform for synergistically combining low contact resistance with broadband photocarrier generation, marking a significant advancement in optoelectronics and light harvesting.

Selective Interfacial Excited-State Carrier Dynamics and Efficient Charge Separation in Borophene-Based Heterostructures

Borophene-based van der Waals heterostructures have demonstrated enormous potential in the realm of optoelectronic and photovoltaic devices, which has sparked a wide range of interest. However, a thorough understanding of the microscopic excited-state electronic dynamics at interfaces is lacking, which is essential for determining the macroscopic optoelectronic and photovoltaic performance of borophene-based devices. In this study, photoexcited carrier dynamics of β12 , χ3 , and α΄ borophene/MoS2 heterostructures are systematically studied based on time-domain nonadiabatic molecular dynamics simulations. Different Schottky contacts are found in borophene/semiconductor heterostructures. The interplay between Schottky barriers, electronic coupling, and the involvement of different phonon modes collectively contribute to the unique carrier dynamics in borophene-based heterostructures. The diverse borophene allotropes within the heterostructures exhibit distinct and selective carrier transfer behaviors on an ultrafast timescale: electrons tunnel into α΄ borophene with an ultrafast transfer rate (≈29 fs) in α΄/MoS2 heterostructures, whereas β12 borophene only allows holes to migrate with a lifetime of 176 fs. The feature enables efficient charge separation and offers promising avenues for applications in optoelectronic and photovoltaic devices. This study provides insight into the interfacial carrier dynamics in borophene-based heterostructures, which is helpful in further design of advanced 2D boron-based optoelectronic and photovoltaic devices.

Ab Initio Quantum Dynamics Simulation of the Impact of Graphene on the Carrier Lifetime of the ZnV2O6 Photocatalyst

We used a nonadiabatic molecular dynamics simulation to determine the carrier dynamics of a graphene/ZnV2O6 heterostructure in the search for an effective photocatalyst material. The C-2p orbital promotes the wave function overlap, guiding electrons to move between graphene and ZnV2O6, successfully achieving good mixing with the valence and conduction bands in ZnV2O6 materials, which is conducive to supporting carrier migration. The overlap between graphene/ZnV2O6 electrons and hole wave functions is less than that of ZnV2O6, and there is small absolute nonadiabatic coupling. The charge separation caused by graphene increases the carrier lifetime and prevents nonradiative electron-hole recombination. This study reveals the microscopic mechanism of extending the carrier lifetime of ZnV2O6 by introducing graphene, providing useful insights for regulating the electronic structure, promoting electron transfer and ultrafast electron and hole transfer. This strategy provides design considerations for advanced photocatalytic materials.

Ultrafast Interlayer Charge Transfer Outcompeting Intralayer Valley Relaxation in Few-Layer 2D Heterostructures

While 2D transition metal dichalcogenides (TMDs) feature interesting layer-tunable multivalley band structures, their preeminent role in determining the photoexcitation charge transfer dynamics in 2D heterostructures (HSs) is yet to be unraveled, as previous charge transfer studies on TMD HSs have been mostly focused on monolayers with a direct bandgap at the K valley. By ultrafast transient absorption spectroscopy and deliberately designed few-layer WSe2/WS2 HSs, we have observed an ultrafast interlayer electron transfer from photoexcited few-layer WSe2 to WS2, prior to intralayer relaxation to lower lying dark valleys. More interestingly, we have identified an unconventional ∼0.5 ps electron back-transfer process after the initial interlayer electron transfer in HSs with WSe2 layers ≥ 3, regenerating indirect intralayer excitons. The result reveals an ielectron and valley relaxation pathway mediated by interlayer charge transfer in 2D HSs, faster than intralayer relaxation. It also sheds light on the origin of generally observed robust ultrafast interlayer charge transfer in TMD HSs and provides guidance toward optoelectronic and valleytronic devices using few-layer TMDs.

Probing the Formation of Dark Interlayer Excitons via Ultrafast Photocurrent

Optically dark excitons determine a wide range of properties of photoexcited semiconductors yet are hard to access via conventional time-resolved spectroscopies. Here, we develop a time-resolved ultrafast photocurrent technique (trPC) to probe the formation dynamics of optically dark excitons. The nonlinear nature of the trPC makes it particularly sensitive to the formation of excitons occurring at the femtosecond time scale after the excitation. As a proof of principle, we extract the interlayer exciton formation time of 0.4 ps at 160 μJ/cm2 fluence in a MoS2/MoSe2 heterostructure and show that this time decreases with fluence. In addition, our approach provides access to the dynamics of carriers and their interlayer transport. Overall, our work establishes trPC as a technique to study dark excitons in various systems that are hard to probe by other approaches.

Electronic structures and photovoltaic applications of vdW heterostructures based on Janus group-IV monochalcogenides: insights from first-principles calculations

The van der Waals integration can help 2D materials modulate their properties and provide more opportunities for 2D materials in the next-generation high-performance optoelectronic devices. Using first-principles calculations, we explored the atomic and electronic structures of 2D pristine and Janus group-IV monochalcogenides and found the internal vertical electric field at Janus group-IV monochalcogenides. Then, we constructed vdW heterostructures with pristine and Janus group-IV monochalcogenides monolayers as building blocks and explored their atomic structures and band alignments. Our results demonstrate that these vdW heterostructures can be synthesized experimentally, and the surface termination of the Janus monolayer at the interface can significantly help the heterostructure realize the transition from type I to type II due to the intrinsic electric field. Moreover, we found eight vdW heterostructures with a mismatch of less than 5% exhibiting type II band alignment with charge densities of VBM and CBM mainly localized at different domains of heterostructures, and excellent power conversion efficiency (∼19%) in photovoltaics are also predicted for these heterostructures with type II band alignment. Our results not only give an idea to use the Janus monolayer as building blocks to construct vdW heterostructures and modulate their band alignment but also provide a guide to the experimental researcher to design more efficient photovoltaic devices with Janus group-IV monochalcogenides.

Efficient Hot Electron Capture in CuPc/MoSe2 Heterostructure Assisted by Intersystem Crossing

Efficient hot electron extraction is a promising approach to develop photovoltaic devices that exceed the Shockley-Queisser limit. However, experimental evidence of hot electron harvesting employing an organic-inorganic interface is still elusive. Here, we reveal the hot electron dynamics at a CuPc/MoSe₂ interface using steady-state spectroscopy and transient absorption spectroscopy. A hot electron transfer efficiency of greater than 78% from MoSe₂ to CuPc is observed, comparable to that achieved in quantum dot hybrid systems. The mechanism is proposed as follows: the photogenerated hot electrons in MoSe₂ transfer to CuPc and form singlet charge transfer states, which subsequently transform into triplet charge transfer states assisted by the rapid intersystem crossing, inhibiting back-donation of electrons and facilitating exciton dissociation into CuPc polarons with a nanosecond lifetime. Our results demonstrate that the intersystem crossing of the hybrid electronic state at organic-inorganic interfaces may serve as a scheme to enable efficient hot electron extraction in photovoltaic devices.

Vertical Barrier Heterostructures for Reliable, Robust, and High-Performance Ultraviolet Detection

Photodetectors based on low-dimensional materials usually suffer from serious optical power-dependent photoresponse and low reliability, particularly in the ultraviolet regime. The barrier photodetector is an effective and reliable strategy where the barrier layer can block the low-energy charge carriers while allowing for a flow of the high-energy photocarriers. Here, vertical barrier heterostructure photodetectors (VBHPs), consisting of a graphene bottom electrode, a MoS₂ light absorber, and an h-BN energy barrier, for reliable, robust, and high-performance ultraviolet detection are reported. The asymmetric barrier distribution in the conduction/valence band at the MoS₂ /h-BN interface results in an ultralow noise current of 1.69 × 10^-15 A Hz^-1/2 at room temperature, stable photo on/off states exceeding 10⁴ cycles at 300 K and 400 K, a light power-independent high responsivity of 416.2 mA W^-1 at 360 nm, a high photo on-off ratio of 1.2 × 10⁵ at 360 nm, high measured detectivities (3.2 × 10¹⁰ Jones at 266 nm and 9.9 × 10¹⁰ Jones at 360 nm), and wide linear dynamic ranges. The VBHPs show a high potential for new-type reliable ultraviolet detection.

Interfacial Coupling and Modulation of van der Waals Heterostructures for Nanodevices

In recent years, van der Waals heterostructures (vdWHs) of two-dimensional (2D) materials have attracted extensive research interest. By stacking various 2D materials together to form vdWHs, it is interesting to see that new and fascinating properties are formed beyond single 2D materials; thus, 2D heterostructures-based nanodevices, especially for potential optoelectronic applications, were successfully constructed in the past few decades. With the dramatically increased demand for well-controlled heterostructures for nanodevices with desired performance in recent years, various interfacial modulation methods have been carried out to regulate the interfacial coupling of such heterostructures. Here, the research progress in the study of interfacial coupling of vdWHs (investigated by Photoluminescence, Raman, and Pump-probe spectroscopies as well as other techniques), the modulation of interfacial coupling by applying various external fields (including electrical, optical, mechanical fields), as well as the related applications for future electrics and optoelectronics, have been briefly reviewed. By summarizing the recent progress, discussing the recent advances, and looking forward to future trends and existing challenges, this review is aimed at providing an overall picture of the importance of interfacial modulation in vdWHs for possible strategies to optimize the device's performance.

Phonon-drag thermopower and thermoelectric performance of MoS2monolayer in quantizing magnetic field

We present a theory of phonon-drag thermopower,Sxxg, in MoS₂monolayer at a low-temperature regime in the presence of a quantizing magnetic fieldB. Our calculations forSxxgconsider the electron-acoustic phonon interaction via deformation potential (DP) and piezoelectric (PE) couplings for longitudinal (LA) and transverse (TA) phonon modes. The unscreened TA-DP is found to dominateSxxgover other mechanisms. TheSxxgis found to oscillate with the magnetic field where the lifting effect of the valley and spin degeneracies in MoS₂monolayer has been clearly observed. An enhancedSxxgwith a peak value of∼1mV K^-1at aboutT = 10 K is predicted, which is closer to the zero field experimental observation. In the Bloch-Grüneisen regime the temperature dependence ofSxxggives the power-lawSxxg∝Tδe, whereδ_evaries marginally around 3 and 5 for unscreened and screened couplings, respectively. In addition,Sxxgis smaller for larger electron densityn_e. The power factor PF is found to increase with temperatureT, decrease withn_e, and oscillate withB. The prediction of an increase of thermal conductivity with temperature and the magnetic field is responsible for the limit of the figure of merit (ZT). At a particular magnetic field and temperature,ZTcan be maximized by optimizing electron density. By fixingne=1012cm^-2, the highestZTis found to be 0.57 atT = 5.8 K andB = 12.1 T. Our findings are compared with those in graphene and MoS₂for the zero-magnetic field.

Rational Unraveling of Alkali Metal Concentration-Dependent Photovoltaic Performance of Halide Perovskites: Octahedron Distortion vs Surface Reconstruction

Adding alkali metal in organic-inorganic halide perovskites effectively improves its photovoltaic performance, while excessive alkali metal incorporation would produce a detrimental effect. Through density functional theory and nonadiabatic molecular dynamics simulations, we demonstrate how and why the photogenerated carrier lifetime mutates with the increase of alkali metal concentration. A small amount of Rb doping in the lattice introduces a slight distortion of the octahedron, reducing the overlap of frontier orbitals and decreasing the nonadiabatic coupling, effectively enhancing the photogenerated carrier lifetime. In contrast, excessive Rb will introduce defect states, resulting in the low carrier lifetime by a factor of 2-3 orders of magnitude. Strikingly, the surface formamidinium (FA) cations exhibit unexpected responsibility for the carrier dynamics since its high-frequency thermal vibration strongly leads to the ultrafast hole trapping and carrier recombination. Our results provide new insight into the concentration-dependent photovoltaic performance of alkali metal cations in organic-inorganic halide perovskites.

Identifying the Intermediate Free-Carrier Dynamics Across the Charge Separation in Monolayer MoS2/ReSe2 Heterostructures

Van der Waals heterostructures composed of different two-dimensional films offer a unique platform for engineering and promoting photoelectric performances, which highly demands the understanding of photocarrier dynamics. Herein, large-scale vertically stacked heterostructures with MoS₂ and ReSe₂ monolayers are fabricated. Correspondingly, the carrier dynamics have been thoroughly investigated using different ultrafast spectroscopies, including Terahertz (THz) emission spectroscopy, time-resolved THz spectroscopy (TRTS), and near-infrared optical pump-probe spectroscopy (OPPS), providing complementary dynamic information for the out-of-plane charge separation and in-plane charge transport at different stages. The initial charge transfer (CT) within the first 170 fs, generating a transient directional current, is directly demonstrated by the THz emissions. Furthermore, the TRTS explicitly unveils an intermediate free-carrier relaxation pathway, featuring a pronounced augmentation of THz photoconductivity compared to the isolated ReSe₂ layer, which likely contains the evolution from immigrant hot charged free carriers to bounded interlayer excitons (∼0.7 ps) and the surface defect trapping (∼13 ps). In addition, the OPPS reveals a distinct enhancement in the saturable absorption along with long-lived dynamics (∼365 ps), which originated from the CT and interlayer exciton recombination. Our work provides comprehensive insight into the photocarrier dynamics across the charge separation and will help with the development of optoelectronic devices based on ReSe₂-MoS₂ heterostructures.

Energy-Level Alignment and Orbital-Selective Femtosecond Charge Transfer Dynamics of Redox-Active Molecules on Au, Ag, and Pt Metal Surfaces

Charge transfer (CT) dynamics across metal-molecule interfaces has important implications for performance and function of molecular electronic devices. CT times, on the order of femtoseconds, can be precisely measured using synchrotron-based core-hole clock (CHC) spectroscopy, but little is known about the impact on CT times of the metal work function and the bond dipole created by metals and the anchoring group. To address this, here we measure CT dynamics across self-assembled monolayers bound by thiolate anchoring groups to Ag, Au, and Pt. The molecules have a terminal ferrocene (Fc) group connected by varying numbers of methylene units to a diphenylacetylene (DPA) wire. CT times measured using CHC with resonant photoemission spectroscopy (RPES) show that conjugated DPA wires conduct electricity faster than aliphatic carbon wires of a similar length. Shorter methylene connectors exhibit increased conjugation between Fc and DPA, facilitating CT by providing greater orbital mixing. We find nearly 10-fold increase in the CT time on Pt compared to Ag due to a larger bond dipole generated by partial electron transfer from the metal-sulfur bond to the carbon-sulfur bond, which creates an electrostatic field that impedes CT from the molecules. By fitting the RPES signal, we distinguish electrons coming from the Fe center and from cyclopentadienyl (Cp) rings. The latter shows faster CT rates because of the delocalized Cp orbitals. Our study demonstrates the fine tuning of CT rates across junctions by careful engineering of several parts of the molecule and the molecule-metal interface.

Thermal-Driven Dynamic Shape Change of Bimetallic Nanoparticles Extends Hot Electron Lifetime of Pt/MoS2 Catalysts

Using a combination of time-domain density functional theory and nonadiabatic (NA) molecular dynamics, we demonstrate that the replacement of noble Pt with cheap Sn in the Pt nanoparticles sensitized MoS₂ greatly retards the photoexcited "hot" electron relaxation. The simulations show that Sn substitution causes significant geometry distortion associated with the Sn dopant detaching from the Pt nanoparticle base, which decreases the NA coupling and creates an isolated trap state distant from the electron donor state. Generally, smaller NA coupling delays "hot" electron relaxation. At the same time, the photoexcited electron on MoS₂ first populates the nanoparticles state and then slowly goes to the trap state, following relaxation to the nanoparticle acceptor state over 1 ps. As a result, the "hot" electron lives over 3.5 times longer than that in pristine Pt/MoS₂ system. The long-lived "hot" electron associated with the reduced cost establishes a novel concept for developing high-efficient and cost-effective photocatalysts and photovoltaics.

Unravelling a Zigzag Pathway for Hot Carrier Collection with Graphene Electrode

The capture of photoexcited deep-band hot carriers, excited by photons with energies far above the bandgap, is of significant importance for photovoltaic and photoelectronic applications because it is directly related to the quantum efficiency of photon-to-electron conversion. By employing time-resolved photoluminescence and state-of-the-art time-domain density functional theory, we reveal that photoexcited hot carriers in organic-inorganic hybrid perovskites prefer a zigzag interfacial charge-transfer pathway, i.e., the hot carriers transfer back and forth between CH₃NH₃PbI₃ and graphene electrode, before they reach a charge-separated state. Driven by quantum coherence and interlayer vibrational modes, this pathway at the semiconductor-graphene interface takes about 400 fs, much faster than the relaxation process within CH₃NH₃PbI₃ (several picoseconds). Our work provides new insight into the fundamental understanding and precise manipulation of hot carrier dynamics at the complex interfaces, paving the way for highly efficient photovoltaic and photoelectric device optimization.

Crystal Symmetry and Static Electron Correlation Greatly Accelerate Nonradiative Dynamics in Lead Halide Perovskites

Using a recently developed many-body nonadiabatic molecular dynamics (NA-MD) framework for large condensed matter systems, we study the phonon-driven nonradiative relaxation of excess electronic excitation energy in cubic and tetragonal phases of the lead halide perovskite CsPbI₃. We find that the many-body treatment of the electronic excited states significantly changes the structure of the excited states' coupling, promotes a stronger nonadiabatic coupling of states, and ultimately accelerates the relaxation dynamics relative to the single-particle description of excited states. The acceleration of the nonadiabatic dynamics correlates with the degree of configurational mixing, which is controlled by the crystal symmetry. The higher-symmetry cubic phase of CsPbI₃ exhibits stronger configuration mixing than does the tetragonal phase and subsequently yields faster nonradiative dynamics. Overall, using a many-body treatment of excited states and accounting for decoherence dynamics are important for closing the gap between the computationally derived and experimentally measured nonradiative excitation energy relaxation rates.

Giant Photoluminescence Enhancement and Resonant Charge Transfer in Atomically Thin Two-Dimensional Cr2Ge2Te6/WS2 Heterostructures

Hybridization of two-dimensional (2D) magnetic semiconductors with transition-metal dichalcogenides (TMDC) monolayers can significantly engineer the light-matter interactions and provide a promising platform for enhanced excitonic systems with artificially tailored band alignments. Here, we report the fabrication of heterostructures with monolayer WS₂ on 2D Cr₂Ge₂Te₆ (CGT), which displayed giant photoluminescence enhancement at specific CGT layer numbers. The highly enhanced quantum yield obtained can be explained by novel photoexcited carrier dynamics, facilitated by alternate relaxation channels, resulting in resonance charge transfer at the heterointerface. 2D CGT revealed a strongly layer-dependent work function (up to ∼750 meV), which greatly modulates the band positioning in the heterostructure. These heterostructures conceived both type I and type II band alignments, which are verified by Kelvin probe force microscopy and PL measurements. In addition to layer modulation, we uncover temperature and power dependence of the resonance charge transfer in the multilayer heterostructure. Our findings provide further insights into the ultrafast charge dynamics occurring at the atomic interfaces. The results may pave the way for novel optoelectronics based on van der Waals heterostructures.

New theoretical insights into the photoinduced carrier transfer dynamics in WS2/WSe2 van der Waals heterostructures

Shedding light on the dynamics of charge transfer is fundamental and important to understand the light-photocurrent power conversion in transition-metal dichalcogenide (TMD) heterostructures. Herein, based on time-dependent ab initio nonadiabatic molecular dynamics simulation, we studied the photoinduced carrier transfer dynamics in the WS2/WSe2 heterostructure and further analyzed the effects of stacking configuration and temperature. Our calculations show that the time scales of ultrafast hole transfer in the C7 and T stacking configurations are 35 fs and 30 fs, respectively, which are mainly caused by the adiabatic charge transfer mechanism. Meanwhile, the time scales of ultrafast electron transfer in the C7 and T stacking configurations are 12 fs and 40 fs, respectively, which are in good agreement with the experimental result. We also investigated in detail the photoinduced carrier transfer pathways of C7 and T stacking configurations, which appear to have some significant differences. In addition, we found that the temperature basically has no effect on the electron transfer dynamics of the WS2/WSe2 heterostructure; this is in excellent agreement with the experimental observation. In short, the reported findings can provide more in-depth insights into the photoinduced carrier transfer dynamics of TMD-based van der Waals heterostructures.

Efficient hot-electron extraction in two-dimensional semiconductor heterostructures by ultrafast resonant transfer

Energy loss from hot-carrier cooling sets the thermodynamic limit for the photon-to-power conversion efficiency in optoelectronic applications. Efficient hot-electron extraction before cooling could reduce the energy loss and leads to efficient next generation devices, which, unfortunately, is challenging to achieve in conventional semiconductors. In this work, we explore hot-electron transfer in two-dimensional (2D) layered semiconductor heterostructures, which have shown great potential for exploring new physics and optoelectronic applications. Using broadband micro-area ultrafast spectroscopy, we firmly established a type I band alignment in the WS₂-MoTe₂ heterostructure and ultrafast (∼60 fs) hot-electron transfer from photoexcited MoTe₂ to WS₂. The hot-electron transfer efficiency increases with excitation energy or excess energy as a result of a more favorable continuous competition between resonant electron transfer and cooling, reaching 90% for hot electrons with 0.3 eV excess energy. This study reveals exciting opportunities of designing extremely thin absorber and hot-carrier devices using 2D semiconductors and also sheds important light on the photoinduced interfacial process including charge transfer and generation in 2D heterostructures and optoelectronic devices.

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.

Probing Nonequilibrium Dynamics of Photoexcited Polarons on a Metal-Oxide Surface with Atomic Precision

Understanding the nonequilibrium dynamics of photoexcited polarons at the atomic scale is of great importance for improving the performance of photocatalytic and solar-energy materials. Using a pulsed-laser-combined scanning tunneling microscopy and spectroscopy, here we succeeded in resolving the relaxation dynamics of single polarons bound to oxygen vacancies on the surface of a prototypical photocatalyst, rutile TiO_{2}(110). The visible-light excitation of the defect-derived polarons depletes the polaron states and leads to delocalized free electrons in the conduction band, which is further corroborated by ab initio calculations. We found that the trapping time of polarons becomes considerably shorter when the polaron is bound to two surface oxygen vacancies than that to one. In contrast, the lifetime of photogenerated free electrons is insensitive to the atomic-scale distribution of the defects but correlated with the averaged defect density within a nanometer-sized area. Those results shed new light on the photocatalytically active sites at the metal-oxide surface.

Modeling nonadiabatic dynamics in condensed matter materials: some recent advances and applications

This review focuses on recent developments in the field of nonadiabatic molecular dynamics (NA-MD), with particular attention given to condensed-matter systems. NA-MD simulations for small molecular systems can be performed using high-level electronic structure (ES) calculations, methods accounting for the quantization of nuclear motion, and using fewer approximations in the dynamical methodology itself. Modeling condensed-matter systems imposes many limitations on various aspects of NA-MD computations, requiring approximations at various levels of theory-from the ES, to the ways in which the coupling of electrons and nuclei are accounted for. Nonetheless, the approximate treatment of NA-MD in condensed-phase materials has gained a spin lately in many applied studies. A number of advancements of the methodology and computational tools have been undertaken, including general-purpose methods, as well as those tailored to nanoscale and condensed matter systems. This review summarizes such methodological and software developments, puts them into the broader context of existing approaches, and highlights some of the challenges that remain to be solved.

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 MoS₂/WS₂ interface. Our analysis shows that after the electron-hole pair is excited in the K valley in WS₂ direct electron transfer from WS₂@K to MoS₂@K is very difficult because of the weak interlayer coupling in the K valley, and thus, it happens through the T valley as WS₂@K-MoS₂@T-MoS₂@K. When the tensile strain is applied, the energy of WS₂@K is decreased, resulting in the suppression of electron transfer. Our study suggests that tuning of the interlayer charge-transfer dynamics by external strain is possible, which provides valuable insights into the functional design of photonic devices based on 2D materials.

Ultrafast Hot Carrier Injection in Au/GaN: The Role of Band Bending and the Interface Band Structure

Plasmon photochemistry can potentially play a significant role in photocatalysis. To realize this potential, it is critical to enhance the plasmon excited hot carrier transfer and collection. However, the lack of atomistic understanding of the carrier transfer across the interface, especially when the carrier is still "hot", makes it challenging to design a more efficient system. In this work, we apply the nonadiabatic molecular dynamics simulation to study hot carrier dynamics in the system of a Au nanocluster on top of a GaN surface. By setting up the initial excited hole in Au, the carrier transfer from Au to GaN is found to be on a subpicosecond time scale. The hot hole first cools to the band edge of Au d-states while it transfers to GaN. After the hole has cooled down to the band edge of GaN, we find that some of the charges can return back to Au. By applying different external potentials to mimic the Schottky barrier band bending, the returning charge can be reduced, demonstrating the importance of the internal electric field. Finally, with the understanding of the carrier transfer's pathway, we suggest that a ZnO layer between GaN and Au can effectively block the "cold" carrier from returning back to Au but still allow the hot carrier to transfer from Au to GaN.

Coexistence of Different Charge-Transfer Mechanisms in the Hot-Carrier Dynamics of Hybrid Plasmonic Nanomaterials

Hot-carrier dynamics at the interfaces of semiconductors and nanoclusters is of significant importance for photovoltaic and photocatalytic applications. Plasmon-driven charge separation processes are considered to be only dependent on the type of donor-acceptor interactions, that is, the conventional hot-electron-transfer mechanism for van der Waals interactions and the plasmon-induced interfacial charge-transfer transition mechanism for chemical bonds. Here, we demonstrate that the two mechanisms can coexist in a nanoparticle-semiconductor hybrid nanomaterial, both leading to faster transfer than carrier relaxation. The origin of the two mechanisms is attributed to the spatial polarization of the excited hot carriers, where the longitudinal state couples to semiconductors more strongly than the transverse state. Our findings provide a new insight into the photoinduced carrier dynamics, which is relevant for many applications in solar energy conversion, including efficient water splitting, photocatalysis, and photovoltaics.

Carrier dynamics and spin-valley-layer effects in bilayer transition metal dichalcogenides

Transition metal dichalcogenides are an interesting class of low dimensional materials in mono- and few-layer form with diverse applications in valleytronic, optoelectronic and quantum devices. Therefore, the general nature of the band-edges and the interplay with valley dynamics is important from a fundamental and technological standpoint. Bilayers introduce interlayer coupling effects which can have a significant impact on the valley polarization. The combined effect of spin-orbit and interlayer coupling can strongly modify the band structure, phonon interactions and overall carrier dynamics in the material. Here we use first-principles calculations of electron-electron and electron-phonon interactions to investigate bilayer MoS₂ and WSe₂ in both the AA' and AB stacking configurations. We find that in addition to spin-orbit coupling, interlayer interactions present in the two configurations significantly alter the near-band-edge dynamics. Scattering lifetimes and dynamic behavior are highly material-dependent, despite the similarities and typical trends in TMDCs. Additionally, we capture significant differences in dynamics for the AA' and AB stacking configurations, with lifetime values differing by up to an order of magnitude between them for MoS₂. Further, we evaluate the valley polarization times and find that maximum lifetimes at room temperature are of the scale of 1 picosecond for WSe₂ in the AB orientation. These results present a pathway to understanding complex heterostructure configurations and 'magic angle' physics in TMDCs.

The mirror asymmetry induced nontrivial properties of polar WSSe/MoSSe heterostructures

Janus MoSSe and WSSe as new members to the family of transitional metal dichalcogenides (TMDCs) present intriguing properties that absent in its parent MX ₂ (M  =  Mo, W; X  =  S, Se, Te) monolayers due to the out-of-plane mirror asymmetry. For WSSe/MoSSe van der Waals (vdW) heterostructures, intralayer/interlayer potential drops lead to significantly larger band offset than MX ₂ heterobilayers, ensuring the long lifetimes for valley polarized interlayer excitons. Regard to the spin-valley-layer locking effects in WSSe/MoSSe vdW heterostructures, the band offset larger than the Zeeman-type spin splitting guarantees effective interlayer hopping and, therefore, large degree of valley polarization. Rashba-type spin splitting can coexist with the valley spin splitting and thus add the carrier transport paths, and intralayer/interlayer potential drops show obvious effects on the Rashba parameter. According to these results, WSSe/MoSSe vdW heterostructures manifest themselves the most promising candidates for spintronics and valleytronics with superiorities to the MX₂ counterparts.

Recording interfacial currents on the subnanometer length and femtosecond time scale by terahertz emission

Electron dynamics at interfaces is a subject of great scientific interest and technological importance. Detailed understanding of such dynamics requires access to the angstrom length scale defining interfaces and the femtosecond time scale characterizing interfacial motion of electrons. In this context, the most precise and general way to remotely measure charge dynamics is through the transient current flow and the associated electromagnetic radiation. Here, we present quantitative measurements of interfacial currents on the subnanometer length and femtosecond time scale by recording the emitted terahertz radiation following ultrafast laser excitation. We apply this method to interlayer charge transfer in heterostructures of two transition metal dichalcogenide monolayers less than 0.7 nm apart. We find that charge relaxation and separation occur in less than 100 fs. This approach allows us to unambiguously determine the direction of current flow, to demonstrate a charge transfer efficiency of order unity, and to characterize saturation effects.

Dielectric Environment-Robust Ultrafast Charge Transfer Between Two Atomic Layers

Understanding electron transfer across two-dimensional (2D) van der Waals (vdW) interfaces especially the effect of dielectric environment not only contributes to the rational design of high performance optoelectronic and photo/electrocatalytic devices but also unravels the nature of charge motion. Herein, we investigated the electron transfer process between two atomic thin layered materials coupled by vdW force at ultimate proximity. Despite their susceptible electronic properties, we show electron transfer at 2D vdW interface is robust and ultrafast (∼30 fs), regardless of the surrounding dielectrics and solvents. Considering the static energy landscape and dynamic nuclear rearrangements, our result suggests the electronic coupling at 2D vdW heterointerfaces is sufficiently strong such that electron transfers adiabatically in a barrierless and ultrafast manner where energetics and solvent relaxation are not that relevant. The robust ultrafast electron transfer against the variation of dielectric environment is highly encouraging for 2D optoelectronic and photo/electrocatalytic devices.

High-Performance Photoinduced Memory with Ultrafast Charge Transfer Based on MoS2 /SWCNTs Network Van Der Waals Heterostructure

Photoinduced memory devices with fast program/erase operations are crucial for modern communication technology, especially for high-throughput data storage and transfer. Although some photoinduced memories based on 2D materials have already demonstrated desirable performance, the program/erase speed is still limited to hundreds of micro-seconds. A high-speed photoinduced memory based on MoS₂ /single-walled carbon nanotubes (SWCNTs) network mixed-dimensional van der Waals heterostructure is demonstrated here. An intrinsic ultrafast charge transfer occurs at the heterostructure interface between MoS₂ and SWCNTs (below 50 fs), therefore enabling a record program/erase speed of ≈32/0.4 ms, which is faster than that of the previous reports. Furthermore, benefiting from the unique device structure and material properties, while achieving high-speed program/erase operation, the device can simultaneously obtain high program/erase ratio (≈10⁶ ), appropriate storage time (≈10³  s), record-breaking detectivity (≈10¹⁶  Jones) and multibit storage capacity with a simple program/erase operation. It even has a potential application as a flexible optoelectronic device. Therefore, the designed concept here opens an avenue for high-throughput fast data communications.