Unraveling the Mechanism of Photoinduced Charge-Transfer Process in Bilayer Heterojunction

Electronic and Transport Properties of InSe/PtTe2 van der Waals Heterostructure

Two-dimensional (2D) InSe and PtTe2 have drawn extensive attention due to their intriguing properties. However, the InSe monolayer is an indirect bandgap semiconductor with a low hole mobility. van der Waals (vdW) heterostructures produce interesting electronic and optoelectronic properties beyond the existing 2D materials and endow totally new device functions. Herein, we theoretically investigated the electronic structures, transport behaviors, and electric field tuning effects of the InSe/PtTe2 vdW heterostructures. The calculated results show that the direct bandgap type-II vdW heterostructures can be realized by regulating the stacking configurations of heterostructures. By applying an external electric field, the band alignment and bandgap of the heterostructures can also be flexibly modulated. Particularly, the hole mobility of the heterostructures is improved by 2 orders of magnitude to ∼103 cm2 V-1 s-1, which overcomes the intrinsic disadvantage of the InSe monolayer. The InSe/PtTe2 vdW heterostructures have great potential applications in developing novel optoelectronic devices.

A resilient type-III broken gap Ga2O3/SiC van der Waals heterogeneous bilayer with band-to-band tunneling effect and tunable electronic property

The potential of van der Waals (vdW) heterostructure to incorporate the outstanding features of stacked materials to meet a variety of application requirements has drawn considerable attention. Due to the unique quantum tunneling mechanisms, a type-III broken-gap obtained from vdW heterostructure is a promising design strategy for tunneling field-effect transistors. Herein, a unique Ga2O3/SiC vdW bilayer heterostructure with inherent type-III broken gap band alignment has been revealed through first-principles calculation. The underlying physical mechanism to form the broken gap band alignment is thoroughly studied. Due to the overlapping band structures, a tunneling window of 0.609 eV has been created, which enables the charges to tunnel from the VBM of the SiC layer to the CBM of the Ga2O3 layer and fulfills the required condition for band-to-band tunneling. External electric field and strain can be applied to tailor the electronic behavior of the bilayer heterostructure. Positive external electric field and compressive vertical strain enlarge the tunneling window and enhance the band-to-band tunneling (BTBT) scheme while negative electric field and tensile vertical strain shorten the BTBT window. Under external electric field as well as vertical and biaxial strain, the Ga2O3/SiC vdW hetero-bilayer maintains the type-III band alignment, revealing its capability to tolerate the external electric field and strain with resilience. All these results provide a compelling platform of the Ga2O3/SiC vdW bilayer to design high performance tunneling field effect transistor.

Mo2CF2/WS2: Two-Dimensional Van Der Waals Heterostructure for Overall Water Splitting Photocatalyst from Five-Step Screening

With the increasing demand for renewable energy and clean energy, photocatalysis is considered an economical and feasible source of energy. In this work, we select two-dimensional (2D) materials of X₂CT₂ (X = Cr, Hf, Mo, Sc, Ti, Zr; T = Cl, F, O, OH), Mxene, and MS₂ (M = Mo, W) to form 20 systems of 2D van der Waals (vdW) heterostructures. We establish five screening steps, and the 2D Mo₂CF₂/WS₂ vdW heterostructures meet all the screening conditions. Mo₂CF₂/WS₂ is a type II semiconductor with a band gap of 1.58 eV, proper band edge position and high solar-to-hydrogen efficiency (17.15%) and power conversion efficiency (23.4%). An excellent electron-hole recombination time of 21.2 ps and electron (hole) migration time of 149 (265) fs are obtained in the 2D Mo₂CF₂/WS₂ vdW heterostructure. In addition, the calculation results of Gibbs free energy show that a hydrogen reduction reaction and water oxidation reaction can proceed smoothly under the driving of photogenerated holes.

Superior Limit of Light-Absorption Improvement in Two-Dimensional Haeckelite GaN-ZnO by Nonadiabatic Molecular Dynamics Simulation

A weak internal electrostatic field is usually required to improve optical performance; however, this is not the case in two-dimensional haeckelite (8|4) GaN-ZnO that has physical properties that are better than those of their binary counterparts. By performing nonadiabatic molecular dynamics simulations, we ascribe the superior limit of improvement of light absorption to the convergence of the electron-hole recombination time when the thickness of the 8|4 phase exceeds a critical value, which arises from the competition between nonadiabatic coupling and quantum decoherence. We show that nonadiabatic coupling continuously becomes weaker because of the reduced nucleus velocity with an increase in thickness. We further demonstrate that the quantum decoherence is first accelerated and then decelerated because of the thickness-dependent electron-phonon coupling controlled by the peculiar in-plane A' and A″ phonon modes. Our study clarifies the issue with regard to light absorption, which provides useful guidance for improving our understanding of the optical properties in two-dimensional polar semiconductors.

Two-dimensional van der Waals heterostructures (vdWHs) with band alignment transformation in multi-functional devices

Two-dimensional van der Waals heterostructures (vdWHs) with tunable band alignment have the potential to be benignant in the development of minimal multi-functional and controllable electronics, but they have received little attention thus far. It is crucial to characterize and control the band alignment in semiconducting vdWHs, which determines the electronic and optoelectronic properties. The future success of optoelectronic devices will require improved electronic property control techniques, such as using an external electric field or strain engineering, to change the electronic structures directly. Herein, we review heterostructures fabricated as transition metal dichalcogenides (TMDCs) as one of their constituent monolayers with other notable 2D materials that can transfer from type-II to type-III (type-III > type-II) band alignment when a biaxial strain or electric field is applied.

Mechanism of the two-dimensional WSeTe/Zr2CO2 direct Z-scheme van der Waals heterojunction as a photocatalyst for water splitting

The direct Z-scheme van der Waals (vdW) heterojunctions based on biomimetic artificial photosynthesis are a promising strategy for enhancing photocatalytic activity. Therefore, the search for superior direct Z-scheme photocatalysts is an urgent task. Herein, we predicted the WSeTe/Zr₂CO₂ heterostructure as a potential direct Z-scheme photocatalyst based on density functional theory (DFT). The bands of the WSeTe/Zr₂CO₂ heterojunction follow a typical Type-II arrangement, where the interlayer band gap is smaller than that of the individual molecular layers, and staggered alignment of the large band-edge creates conditions that allow for a direct Z-scheme. The position of the Fermi energy levels of the two monolayers determines the formation of the built-in electric field pointing from WSeTe to Zr₂CO₂, promoting the desired interlayer electron-hole (e^--h⁺) recombination and suppressing the undesired carrier recombination. Finally, in-plane biaxial strain can effectively modulate the optoelectronic properties of the catalyst, while compressive strain has a more pronounced effect on the overpotential driving force of the material. Therefore, the WSeTe/Zr₂CO₂ heterojunction is an effective new photocatalyst satisfying the direct Z-scheme charge transfer mechanism with its specific application.

Manipulation of Band Alignment in Two-Dimensional Vertical WSe2/BA2PbI4 Ruddlesden-Popper Perovskite Heterojunctions via Defect Engineering

Transition metal dichalcogenides (TMDs), two-dimensional (2D) layered Ruddlesden-Popper perovskite material, and their heterojunctions have attracted a great deal of interest in optoelectronic applications. Although various approaches for modulating their properties and applications have been demonstrated, knowledge of the interface band alignment and defect engineering on the TMD/2D perovskite heterojunction is still lacking. Herein, the optoelectronic properties and defect engineering of the WSe₂/BA₂PbI₄ heterojunction have been investigated with density functional theory simulations. We find that the WSe₂/BA₂PbI₄ van der Waals heterojunction maintains an indirect bandgap and S-scheme alignment, facilitating the efficient splitting of light excited carriers across the interface. Importantly, we find that defect engineering could manipulate the band alignment. The introduction of the BA vacancies could switch the interface from the S-scheme to the typical type II interface, whereas Se vacancies would facilitate recombination at the S-scheme interface. Our work proves that the interfacial properties of heterojunctions can be regulated by defect modulation to address different optoelectronic applications.

Unveiling the layer-dependent electronic properties in transition-metal dichalcogenide heterostructures assisted by machine learning

The electronic properties of layered two-dimensional (2D) transition-metal dichalcogenide (TMD) van der Waals (vdW) heterostructures are strongly dependent on their layer number (N). However, extremely large computational resources are required to investigate the layer-dependent TMD vdW heterostructures for every possible combination if N varies in a large range. Fortunately, the machine learning (ML) technique provides a feasible way to probe this problem. In this work, based on the density functional theory (DFT) calculations combined with the ML technique, we effectively predict the layer-dependent electronic properties of TMD vdW heterostructures composed of MoS₂, WS₂, MoSe₂, WSe₂, MoTe₂, or WTe₂, in which the layer number varies from 2-10. The cross-validation scores of our trained ML models in predicting the bandgaps as well as the band edge positions exceed 90%, suggesting excellent performance. The predicted results show that in the case of a few-layer system, the number of layers has a significant effect on the electronic properties. The bandgap and band alignment could be dramatically changed from bilayer to triple-layer heterostructures. However, with the increase of the number of layers, the electronic properties change, and some general trends can be summarized. When the layer number is larger than 8, the properties of the TMD heterostructures tend to be stable, and the influence of the layer number decreases. Based on these results, our work not only sheds light on the understanding of the layer-dependent electronic properties of multi-layer TMD vdW heterostructures, but also provides an efficient way to accelerate the discovery of functional materials.

Direct Z-scheme WTe2/InSe van der Waals heterostructure for overall water splitting

WTe2/InSe is a direct Z-scheme vdW heterostructure for water splitting. The Te-vacancy can effectively lower the energy of the HER, and the overall water splitting can proceed spontaneously on the surface of the WTe2/InSe heterostructure when pH > 7.

Probing interfacial charge transfer in the heterojunctions for photocatalysis

Photocatalytic reactions can sustainably employ inexhaustible solar energy for environmental remediation and conversion of photon energy into chemical energy, and thereby show great potential in alleviating the environmental stress and...

Highly Efficient Photocatalytic CO2 Reduction in Two-Dimensional Ferroelectric CuInP2S6 Bilayers

Photocatalytic CO₂ conversion into reproducible chemical fuels (e.g., CO, CH₃OH, or CH₄) provides a promising scheme to solve the increasing environmental problems and energy demands simultaneously. However, the efficiency is severely restricted by the high overpotential of the CO₂ reduction reaction (CO₂RR) and rapid recombination of photoexcited carriers. Here, we propose that a novel type-II photocatalytic mechanism based on two-dimensional (2D) ferroelectric multilayers would be ideal for addressing these issues. Using density-functional theory and nonadiabatic molecular dynamics calculations, we find that the ferroelectric CuInP₂S₆ bilayers exhibit a staggered band structure induced by the vertical intrinsic electric fields. Different from the traditional type-II band alignment, the unique structure of the CuInP₂S₆ bilayer not only effectively suppresses the recombination of photogenerated electron-hole (e-h) pairs but also produces a sufficient photovoltage to drive the CO₂RR. The predicted recombination time of photogenerated e-h pairs, 1.03 ns, is much longer than the transferring times of photoinduced electrons and holes, 5.45 and 0.27 ps, respectively. Moreover, the overpotential of the CO₂RR will decrease by substituting an S atom with a Cu atom, making the redox reaction proceed spontaneously under solar radiation. The solar-to-fuel efficiency with an upper limit of 8.40% is achieved in the CuInP₂S₆ bilayer and can be further improved to 32.57% for the CuInP₂S₆ five-layer. Our results indicate that this novel type-II photocatalytic mechanism would be a promising way to achieve highly efficient photocatalytic CO₂ conversion based on the 2D ferroelectric multilayers.

Tunable Schottky barrier in InTe/graphene van der Waals heterostructure

The structures and electronic properties of InTe/graphene van der Waals heterostructures are systematically investigated using the first-principles calculations. The electronic properties of InTe monolayer and graphene are well preserved respectively and the bandgap energy of graphene is opened to 36.5 meV in the InTe/graphene heterostructure. An n-type Schottky contact is formed in InTe/graphene heterostructure at the equilibrium state. There is a transformation between n-type and p-type Schottky contact when the interlayer distance is smaller than 3.56 Å or the applied electric field is larger than -0.06 V Å^-1. In addition, the Schottky contact converts to Ohmic contact when the applied vertical electric field is larger than 0.11 V Å^-1 or smaller than -0.13 V Å^-1.

Monolayer HfTeSe4: A Promising Two-Dimensional Photovoltaic Material for Solar Cells with High Efficiency

Currently, atomically thin materials with high photovoltaic performance are urgently needed for applications in solar cells. Herein, by using first-principles calculations, we propose an excellent two-dimensional photovoltaic material, monolayer HfTeSe₄, which can be exfoliated feasibly from its layered bulk. It behaves in the semiconductor character with a moderate direct gap of 1.48 eV and exhibits remarkable absorbance coefficient of ∼10⁵ cm^-1 in the visible-light region. Meanwhile, monolayer HfTeSe₄ shows ultrahigh photocurrent and a long carrier recombination lifetime. Also, strain engineering can further modulate the recombination time of carriers. Moreover, the heterostructure between HfTeSe₄ and Bi₂WO₆ is proposed as potential solar cells with the solar conversion efficiency up to ∼20.8%. These extraordinary properties combined with its experimental feasibility makes monolayer HfTeSe₄ particularly promising for photovoltaic device applications.

Growth of vertical heterostructures based on orthorhombic SnSe/hexagonal In2Se3 for high-performance photodetectors

Vertical heterostructures based on two-dimensional (2D) layered materials are ideal platforms for electronic structure engineering and novel device applications. However, most of the current heterostructures focus on layered crystals with a similar lattice. In addition, the heterostructures made by 2D materials with different structures are rarely investigated. In this study, we successfully fabricated vertical heterostructures by combining orthorhombic SnSe/hexagonal In₂Se₃ vertical heterostructures using a two-step physical vapor deposition (PVD) method. Structural characterization reveals that the heterostructures are formed of vertically stacked SnSe on the top of the In₂Se₃ film, and vertical heterostructures possess high quality, where In₂Se₃ exposed surface is the (0001) plane and SnSe prefers growing along the [100] direction. Raman maps confirm the precise spatial modulation of the as-grown SnSe/In₂Se₃ heterostructures. In addition, high-performance photodetectors based on the vertical heterostructures were fabricated directly on the substrate, which showed a broadband response, reversibility and stability. Compared with the dark current, the device demonstrated one order magnification of photocurrent, about 186 nA, under 405 nm laser illumination and power of 1.5 mW. Moreover, the device shows an obvious increase in the photocurrent intensity with the changing incident laser power, where I _ph ∝ P ^0.7. Also, the device demonstrated a high responsivity of up to 350 mA W^-1 and a fast response time of about 139 ms. This study broadens the horizon for the synthesis and application of vertical heterostructures based on 2D layered materials with different structures and further develops exciting technologies beyond the reach of the existing materials.

Non-adiabatic molecular dynamics with ΔSCF excited states

Accurate modelling of nonadiabatic transitions and electron-phonon interactions in extended systems is essential for understanding the charge and energy transfer in photovoltaic and photocatalytic materials. The extensive computational costs of the advanced excited state methods have stimulated the development of many approximations to study the nonadiabatic molecular dynamics (NA-MD) in solid-state and molecular materials. In this work, we present a novel ▵SCF-NA-MD methodology that aims to account for electron-hole interactions and electron-phonon back-reaction critical in modelling photoinduced nuclear dynamics. The excited states dynamics is described using the delta self-consistent field (▵SCF) technique within the density functional formalism and the trajectory surface hopping. The technique is implemented in the open-source Libra-X package freely available on the Internet (https://github.com/Quantum-Dynamics-Hub/Libra-X). This work illustrates the general utility of the developed ▵SCF-NA-MD methodology by characterizing the excited state energies and lifetimes, reorganization energies, photoisomerization quantum yields, and by providing the mechanistic details of reactive processes in a number of organic molecules.