Type-I van der Waals heterostructure formed by MoS2 and ReS2 monolayers

Theoretical prediction of the electronic structure, optical properties and contact characteristics of a type-I MoS2/MoGe2N4 heterostructure towards optoelectronic devices

Recently, the combination of two different two-dimensional (2D) semiconductors to generate van der Waals (vdW) heterostructures has emerged as an effective strategy to tailor their physical properties, paving the way for the development of next-generation devices with improved performance and functionality. In this work, we designed an MoS2/MoGe2N4 heterostructure and explored its electronic structures, optical properties and contact characteristics using first-principles calculations. The MoS2/MoGe2N4 heterostructure is predicted to be energetically, thermally and dynamically stable, indicating its feasibility for experimental synthesis in the future. The MoS2/MoGe2N4 heterostructure forms type-I band alignment, suggesting that it can be considered as a promising material for optoelectronic devices, such as light-emitting diodes, and in laser applications. Furthermore, the type-I MoS2/MoGe2N4 heterostructure has enhanced optical absorption in both the visible and ultraviolet regions. More interestingly, the electronic properties and contact characteristics of the MoS2/MoGe2N4 heterostructure can be tailored by applying in-plane biaxial strain. Under the application of compressive and tensile strains, transformations between type-I and type-II band alignments and between semiconductor and metal can be achieved in the MoS2/MoGe2N4 heterostructure. Our findings could provide useful guidance for experimental synthesis of materials based on the MoS2/MoGe2N4 heterostructure for electronic and optoelectronic applications.

Transient Absorption Spectroscopy of Films: Impact of Refractive Index

Transient absorption spectroscopy is a powerful technique to study the photoinduced phenomena in a wide range of states from solutions to solid film samples. It was designed and developed based on photoinduced absorption changes or that photoexcitation triggers a chain of reactions with intermediate states or reaction steps with presumably different absorption spectra. However, according to general electromagnetic theory, any change in the absorption properties of a medium is accompanied by a change in the refractive properties. Although this photoinduced change in refractive index has a negligible effect on solution measurements, it may significantly affect the measured response of thin films. In this Perspective paper, we examine why and how the measured responses of films differ from their expected "pure" absorption responses. The effect of photoinduced refractive index change can be concluded and studied by comparing the transmitted and reflected probe light responses. Another discussed aspect is the effect of light interference on thin films. Finally, new opportunities of monitoring the photocarrier migration in films and studying nontransparent samples using the reflected probe light response are discussed. Most of the examples provided in this article focus on studies involving perovskite, TiO2, and graphene-based films, but the general discussion and conclusions can be applicable to a wide range of semiconductor and thin metallic films.

Crucial role of interfacial interaction in 2D polar SiGe/GeC heterostructures

The planar charge transfer is a distinctive characteristic of the two-dimensional (2D) polar materials. When such 2D polar materials are involved in vertical heterostructures (VHs), in addition to the van der Waals (vdW) interlayer interaction, the interfacial interaction triggered by the in-plane charge transfer will play a crucial role. To deeply understand such mechanism, we conducted a comprehensive theoretical study focusing on the structural stability and electronic properties of 2D polar VHs built by commensurate SiGe/GeC bilayers with four species ordering patterns (classified as a C-group with patterns I and II and a Ge-group with patterns III and IV, respectively). It was found that the commensurate SiGe/GeC VHs are mainly stabilized by interfacial interactions (including the electrostatic interlayer bonding, the vdW force, as well as thesp2/sp3orbital hybridization), with the Ge-group being the most energetically favorable than the C-group. A net charge redistribution occurs between adjacent layers, which is significant (∼0.23-0.25 e cell-1) in patterns II and IV, but slightly small (∼0.05-0.09 e cell-1) in patterns I and III, respectively, forming spontaneousp-nheterojunctions. Such interlayer charge transfer could also lead to a polarization in the interfacial region, with the electron depletion (accumulation) close to the GeC layer and the electron accumulation (depletion) close to the SiGe layer in the C-group (the Ge-group). This type of interface dipoles could induce a built-in electric field and help to promote photogenerated electrons (holes) migration. Furthermore, a semi-metal nature with a tiny direct band gap at the SiGe layer and a semiconducting nature at the GeC layer indicate that the commensurate SiG/GeC VHs possess a type-I band alignment of heterojunction and have a wide spectrum of light absorption capabilities, indicating its promising applications for enhancing light-matter interaction and interfacial engineering.

Transition Metal Dichalcogenides: Making Atomic-Level Magnetism Tunable with Light at Room Temperature

The capacity to manipulate magnetization in 2D dilute magnetic semiconductors (2D-DMSs) using light, specifically in magnetically doped transition metal dichalcogenide (TMD) monolayers (M-doped TX2 , where M = V, Fe, and Cr; T = W, Mo; X = S, Se, and Te), may lead to innovative applications in spintronics, spin-caloritronics, valleytronics, and quantum computation. This Perspective paper explores the mediation of magnetization by light under ambient conditions in 2D-TMD DMSs and heterostructures. By combining magneto-LC resonance (MLCR) experiments with density functional theory (DFT) calculations, we show that the magnetization can be enhanced using light in V-doped TMD monolayers (e.g., V-WS2 , V-WSe2 ). This phenomenon is attributed to excess holes in the conduction and valence bands, and carriers trapped in magnetic doping states, mediating the magnetization of the semiconducting layer. In 2D-TMD heterostructures (VSe2 /WS2 , VSe2 /MoS2 ), the significance of proximity, charge-transfer, and confinement effects in amplifying light-mediated magnetism is demonstrated. We attributed this to photon absorption at the TMD layer that generates electron-hole pairs mediating the magnetization of the heterostructure. These findings will encourage further research in the field of 2D magnetism and establish a novel design of 2D-TMDs and heterostructures with optically tunable magnetic functionalities, paving the way for next-generation magneto-optic nanodevices.

Study on interface engineering and chemical bonding of the ReS2@ZnO heterointerface for efficient charge transfer and nonlinear optical conversion efficiency

Rhenium sulfide (ReS2) has emerged as a promising two-dimensional material, demonstrating broad-spectrum visible absorption properties that make it highly relevant for diverse optoelectronic applications. Manipulating and optimizing the pathway of photogenerated carriers play a pivotal role in enhancing the efficiency of charge separation and transfer in novel semiconductor composites. This study focuses on the strategic construction of a semiconductor heterostructure by synthesizing ZnO on vacancy-containing ReS2 (VRe-ReS2) through chemical bonding processes. The ingeniously engineered built-in electric field within the heterostructure effectively suppresses the recombination of photogenerated electron-hole pairs. A direct and well-established interfacial connection between VRe-ReS2 and ZnO is achieved through a robust Zn-S bond. This distinctive bond configuration leads to enhanced nonlinear optical conversion efficiency, attributed to shortened carrier migration distances and accelerated charge transfer rates. Furthermore, theoretical calculations unveil the superior chemical interactions between Re vacancies and sulfide moieties, facilitating the formation of Zn-S bonds. The photoluminescence (PL) intensity is increased by the formation of VRe-ReS2 and ZnO heterostructure and the PL quantum yield of VRe-ReS2 is improved. The intricate impact of the Zn-S bond on the nonlinear absorption behavior of the VRe-ReS2@ZnO heterostructure is systematically investigated using femtosecond Z-scan techniques. The charge transfer from ZnO to ReS2 defect levels induces a transition from saturable absorption to reverse saturable absorption in the VRe-ReS2@ZnO heterostructure. Transient absorption measurements further confirm the presence of the Zn-S bond between the interfaces, as evidenced by the prolonged relaxation time (τ3) in the VRe-ReS2@ZnO heterostructure. This study offers valuable insights into the rational construction of heterojunctions through tailored interfacial bonding and surface/interface charge transfer pathways. These endeavors facilitate the modulation of electron transfer dynamics, ultimately yielding superior nonlinear optical conversion efficiency and effective charge regulation in optoelectronic functional materials.

Ultrafast Charge Transfer and Recombination Dynamics in Monolayer-Multilayer WSe2 Junctions Revealed by Time-Resolved Photoemission Electron Microscopy

The ultrafast carrier dynamics of junctions between two chemically identical, but electronically distinct, transition metal dichalcogenides (TMDs) remains largely unknown. Here, we employ time-resolved photoemission electron microscopy (TR-PEEM) to probe the ultrafast carrier dynamics of a monolayer-to-multilayer (1L-ML) WSe2 junction. The TR-PEEM signals recorded for the individual components of the junction reveal the sub-ps carrier cooling dynamics of 1L- and 7L-WSe2, as well as few-ps exciton-exciton annihilation occurring on 1L-WSe2. We observe ultrafast interfacial hole (h) transfer from 1L- to 7L-WSe2 on an ∼0.2 ps time scale. The resultant excess h density in 7L-WSe2 decays by carrier recombination across the junction interface on an ∼100 ps time scale. Reminiscent of the behavior at a depletion region, the TR-PEEM image reveals the h density accumulation on the 7L-WSe2 interface, with a decay length ∼0.60 ± 0.17 μm. These charge transfer and recombination dynamics are in agreement with ab initio quantum dynamics. The computed orbital densities reveal that charge transfer occurs from the basal plane, which extends over both 1L and ML regions, to the upper plane localized on the ML region. This mode of charge transfer is distinctive to chemically homogeneous junctions of layered materials and constitutes an additional carrier deactivation pathway that should be considered in studies of 1L-TMDs found alongside their ML, a common occurrence in exfoliated samples.

Interlayer Coupling in Anisotropic/Isotropic Van der Waals Heterostructures of ReS2 and WS2

Van der Waals (vdW) heterostructures are composed of atomically thin layers assembled through weak (vdW) force, which have opened a new era for integrating materials with distinct properties and specific applications. However, few studies have focused on whether and how anisotropic materials affect heterostructure system. The study introduces anisotropic and isotropic materials in a heterojunction system to change the in-plane symmetry, offering a new degree of freedom for modulating its properties. The sample is fabricated by manually stacking ReS2 and WS2 flakes prepared by mechanical exfoliation. Raman spectra and photoluminescence measurements confirm the formation of an effective heterojunction, indicating interlayer coupling of the system. The anisotropy and asymmetry of the WS2 -ReS2 heterostructure system can be adjusted by the introduction of isotropic WS2 and anisotropic ReS2 , which can be proved by the change of the polarized Raman pattern. In the transient absorption measurement, the transient absorption spectra of WS2 -ReS2 heterostructure are red-shifted compared to those of WS2 monolayer, and the charge transfer is observed in the heterostructure. These results show the potential of anisotropic 2D materials in anisotropy modulation of heterostructures, which may promote future electronic or photonic application.

Observation of Strong Anisotropic Interlayer Excitons

Anisotropic interlayer excitons had been theoretically predicted to exist in two-dimensional (2D) anisotropy/isotropy van der Waals heterojunctions. However, experimental results consolidating the theoretical prediction and exploring the related anisotropic optoelectronic response have not been reported so far. Herein, strong photoluminescence (PL) of anisotropic interlayer excitons is observed in a symmetric anisotropy/isotropy/anisotropy heterojunction exemplified by 3L-ReS2/1L-MoS2/3L-ReS2 using monolayer (1L) MoS2 and trilayer (3L) ReS2 as components. Sharp interlayer exciton PL peaks centered at ∼1.64, ∼1.61, and ∼1.57 eV are only observed at low temperatures of ≤120 K and become more pronounced as the temperature decreases. These interlayer excitons exhibit strong anisotropic PL intensity variations with periodicities of 180° as functions of the incident laser polarization angles. The polarization ratios of these interlayer excitons are calculated to be 1.33-1.45. Our study gives new insight into the manipulation of excitons in 2D materials and paves a new way for a rational design of novel anisotropic optoelectronic devices.

Tuning the Electronic Properties of Two-Dimensional Lepidocrocite Titanium Dioxide-Based Heterojunctions

Two-dimensional (2D) heterostructures reveal novel physicochemical phenomena at different length scales that are highly desirable for technological applications. We present a comprehensive density functional theory study of van der Waals (vdW) heterostructures constructed by stacking 2D TiO2 and 2D MoSSe monolayers to form the TiO2-MoSSe heterojunction. The heterostructure formation is found to be exothermic, indicating stability. We find that by varying the atomic species at the interfaces, the electronic structure can be considerably altered due to the differences in charge transfer arising from the inherent electronegativity of the atoms. We demonstrate that the heterostructures possess a type II or type III band alignment, depending on the atomic termination of MoSSe at the interface. The observed charge transfer occurs from MoSSe to TiO2. Our results suggest that the Janus interface enables the tuning of electronic properties, providing an understanding of the possible applications of the TiO2-MoSSe heterostructure.

First-principles studies on the structural, electronic, and optical properties of 2D transition metal dichalcogenide (TMDC) and Janus TMDCs heterobilayers

Van der Waals heterobilayers formed by vertically stacked two-dimensional materials could be a viable candidate for optoelectronics. This study carried out first-principles calculations to study the geometrical, electronic and optical properties of heterobilayers consisting transition metal dichalcogenide (TMDC) SnSe2and Janus TMDCs ZrSSe and SnSSe. Eight possible configurations SeSnSe-SSnSe (I), SeSnSe-SeSnS (II), SeSnSe-SZrSe (III), SeSnSe-SeZrS (IV), SSnSe-SZrSe (V), SSnSe-SeZrS (VI), SeSnS-SZrSe (VII) and SeSnS-SeZrS (VIII) are dynamically, thermally, energetically and mechanical stable. Six configurations, (I, II, III, IV, V and VI) have indirect band gaps with type-II band alignments, enhancing carrier lifetime an essential feature for potential applications in photovoltaic and nanoelectronics devices. In contrast, VII and VIII have indirect band gap with a type-I band alignment, facilitating efficient recombination of electron-hole pairs under high irradiation. All heterobilayers demonstrated significant optical absorption in the visible region. These findings highlight the potential utilization of heterobilayers in electronic and optoelectronic devices.

Electrostatic gating dependent multiple band alignments in ferroelectric VS2/Ga2O3 van der Waals heterostructures

Two-dimensional (2D) van der Waals (vdW) heterostructures with spontaneous intrinsic ferroelectrics play an essential role in ferroelectric memories. Also, the reversal of polarized directions induces band alignment transitions among different types to provide a new path for multifunctional devices. In this work, the structural and electronic properties of 2D VS2/Ga2O3 vdW heterostructures under different polarizations were investigated using first-principles calculations with the vdW correction of the DFT-D2 method. The results reveal that the polarized direction of a 2D Ga2O3 monolayer can cause a distinct band structure reversion from a metal to a semiconductor due to the shift of band alignment induced by the interlayer charge transfer. Moreover, the VS2/P↑ Ga2O3 heterostructures retain type-I and type-II band alignments in the majority and minority channel, respectively, under an external electric field. Interestingly, applying the external electric field for VS2/P↓ Ga2O3 heterostructures can lead to a transition from type-II to type-I in the majority channel, and from type-II to type-III in the minority channel. Our work provides a feasible way to realize 2D VS2/Ga2O3 vdW heterostructures for potential applications in ferroelectric memories and electrostatic gating dependent multiple band alignment devices.

First-principles study of BX-SiS (X = As, P) van der Waals heterostructures for enhanced photocatalytic performance

The vertical integration of two-dimensional (2D) materials through weak van der Waals (vdW) interactions is gaining tremendous attention for application in nanotechnology and photovoltaics. Here, we performed first-principles study of the electronic band structure, optical and photocatalytic properties of vertically stacked heterostructures based on boron pnictides BX (X = As, P) and SiS monolayers. Both heterobilayers possess a stable geometry and reveal type I band alignment with a direct band gap, indicating substantial transfer of charge across the junction of the same layer. Interestingly, a redshift is found in the visible light region of the optical absorption spectra of BX-SiS heterobilayers. The comparatively larger hole mobility (14 000 cm2 V-1 s-1) of BP-SiS preferably allows hole conduction in the zigzag-direction. More importantly, the excellent band edge values of the standard redox potential and smaller Gibbs free energy for the adsorption of hydrogen (ΔGH*) make them ideal for performing the hydrogen evolution reaction (HER) mechanism under solar irradiation. These findings offer exciting opportunities for developing next-generation devices based on BX-SiS heterobilayers for promising applications in nanoelectronics, optoelectronic devices and photocatalysts for water dissociation into hydrogen to produce renewable clean energy.

Energy transfer and charge transfer between semiconducting nanocrystals and transition metal dichalcogenide monolayers

Nowadays, as a result of the emergence of low-dimensional hybrid structures, the scientific community is interested in their interfacial carrier dynamics, including charge transfer and energy transfer. By combining the potential of transition metal dichalcogenides (TMDs) and nanocrystals (NCs) with low-dimensional extension, hybrid structures of semiconducting nanoscale matter can lead to fascinating new technological scenarios. Their characteristics make them intriguing candidates for electronic and optoelectronic devices, like transistors or photodetectors, bringing with them challenges but also opportunities. Here, we will review recent research on the combined TMD/NC hybrid system with an emphasis on two major interaction mechanisms: energy transfer and charge transfer. With a focus on the quantum well nature in these hybrid semiconductors, we will briefly highlight state-of-the-art protocols for their structure formation and discuss the interaction mechanisms of energy versus charge transfer, before concluding with a perspective section that highlights novel types of interactions between NCs and TMDs.

Photocatalytic Applications of ReS2-Based Heterostructures

ReS2-based heterostructures, which involve the coupling of a narrow band-gap semiconductor ReS2 with other wide band-gap semiconductors, have shown promising performance in energy conversion and environmental pollution protection in recent years. This review focuses on the preparation methods, encompassing hydrothermal, chemical vapor deposition, and exfoliation techniques, as well as achievements in correlated applications of ReS2-based heterostructures, including type-I, type-II heterostructures, and Z-scheme heterostructures for hydrogen evolution, reduction of CO2, and degradation of pollutants. We believe that this review provides an overview of the most recent advances to guide further research and development of ReS2-based heterostructures for photocatalysis.

Revealing the electronic, optical and photocatalytic properties of PN-M2CO2 (P = Al, Ga; M = Ti, Zr, Hf) heterostructures

Using DFT, the electronic structure, optical, and photocatalytic properties of PN (P = Ga, Al) and M₂CO₂ (M = Ti, Zr, Hf) monolayers and their PN-M₂CO₂ van der Waals heterostructures (vdWHs) are investigated. Optimized lattice parameters, bond length, bandgap, conduction and valence band edges show the potential of PN (P = Ga, Al) and M₂CO₂ (M = Ti, Zr, Hf) monolayers in photocatalytic applications, and the application of the present approach to combine these monolayers and form vdWHs for efficient electronic, optoelectronic and photocatalytic applications is shown. Based on the same hexagonal symmetry and experimentally achievable lattice mismatch of PN (P = Ga, Al) with M₂CO₂ (M = Ti, Zr, Hf) monolayers, we have fabricated PN-M₂CO₂ vdWHs. Binding energies, interlayer distance and AIMD calculations show the stability of PN-M₂CO₂ vdWHs and demonstrate that these materials can be easily fabricated experimentally. The calculated electronic band structures show that all the PN-M₂CO₂ vdWHs are indirect bandgap semiconductors. Type-II[-I] band alignment is obtained for GaN(AlN)-Ti₂CO₂[GaN(AlN)-Zr₂CO₂ and GaN(AlN)-Hf₂CO₂] vdWHs. PN-Ti₂CO₂ (PN-Zr₂CO₂) vdWHs with a PN(Zr₂CO₂) monolayer have greater potential than a Ti₂CO₂(PN) monolayer, indicating that charge is transfer from the Ti₂CO₂(PN) to PN(Zr₂CO₂) monolayer, while the potential drop separates charge carriers (electron and holes) at the interface. The work function and effective mass of the carriers of PN-M₂CO₂ vdWHs are also calculated and presented. A red (blue) shift is observed in the position of excitonic peaks from AlN to GaN in PN-Ti₂CO₂ and PN-Hf₂CO₂ (PN-Zr₂CO₂) vdWHs, while significant absorption for photon energies above 2 eV for AlN-Zr₂CO₂, GaN-Ti₂CO₂ and PN-Hf₂CO₂, give them good optical profiles. The calculated photocatalytic properties demonstrate that PN-M₂CO₂ (P = Al, Ga; M = Ti, Zr, Hf) vdWHs are the best candidates for photocatalytic water splitting.

Multi-functional application potential of Ruddlesden-Popper perovskite-based heterostructure PtSe2/Cs2PbI4with tunable electronic properties

Heterogeneous stacking based on two-dimensional Ruddlesden-Popper (RP) perovskite is a desired strategy for the reasonable combination of stability and efficiency. Constructing heterostructures with tunable optoelectronic properties further provide opportunities to design multi-functional devices. Herein, we present a first-principle research to investigate the geometric and electronic structures of RP perovskite heterostructure PtSe₂/Cs₂PbI₄and its tunable electronic properties induced by thickness modulation and external strains. The results indicate that the heterostructure based on Cs₂PbI₄monolayer and PtSe₂monolayer has a type-II band alignment, which is suitable for the photovoltaic applications. With the layer number of PtSe₂in heterostructure increases from monolayer to bilayer, the band alignment of PtSe₂/Cs₂PbI₄heterostructure can switch from type-II to type-I, which is beneficial for the luminescence device applications. However, when the thickness of PtSe₂in heterostructure further increases to trilayer, the heterostructure exhibits metallic characteristic with a p-type Schottky barrier. In addition, we find the strain engineering is an effective knob in tuning the electronic properties of PtSe₂/Cs₂PbI₄heterostructures with different thickness. These findings reveal the potential of PtSe₂/Cs₂PbI₄heterostructure as a tunable hybrid material with substantial prospect in multi-functional applications.

Optoelectronic properties and applications of two-dimensional layered semiconductor van der Waals heterostructures: perspective from theory

Van der Waals heterostructures (vdWHs) which combine two different materials together have attracted extensive research attentions due to the promising applications in optoelectronic and electronic devices, the investigations from theoretical simulations can not only predict the novel properties and the interfacial coupling, but also provide essential guidance for experimental verification and fabrications. This review summarizes the recent theoretical studies on electronic and optical properties of two-dimensional semiconducting vdWHs. The characteristics of different band alignments are discussed, together with the optoelectronic modulations from external fields and the promising applications in solar cells, tunneling field-effect transistors and photodetectors. At the end of the review, the further perspective and possible research problems of the vdWHs are also presented.

Type-I Heterostructure Based on WS2/PtS2 for High-Performance Photodetectors

van der Waals (vdW) heterodiodes composed of two-dimensional (2D) layered materials led to a new prospect in photoelectron diodes and photovoltaic devices. Existing studies have shown that Type-I heterostructures have great potential to be used as photodetectors; however, the tunneling phenomena in Type-I heterostructures have not been fully revealed. Herein, a highly efficient nn⁺ WS₂/PtS₂ Type-I vdW heterostructure photodiode is constructed. The device shows an ultrahigh reverse rectification ratio of 10⁵ owing to the transmission barrier-induced low reverse current. A unilateral depletion region is formed on WS₂, which inhibits the recombination of carriers at the interface and makes the external quantum efficiency (EQE) of the device reach 67%. Due to the tunneling mechanism of the device, which allows the co-existence of a large photocurrent and a low dark current, this device achieves a light on/off ratio of over 10⁵. In addition, this band design allows the device to maintain a high detectivity of 4.53 × 10¹⁰ Jones. Our work provides some new ideas for exploring new high-efficiency photodiodes.

Electronic property modulation in two-dimensional lateral superlattices of monolayer transition metal dichalcogenides

Fabricating lateral heterostructures (HSs) and superlattices (SLs) provides a unique degree of freedom for modulating the physical properties of two-dimensional (2D) materials by varying the chemical component, geometric size and interface structure in the ultra-thin atomic thickness limit. While a variety of 2D lateral HSs/SLs have been synthesized, especially for transition metal dichalcogenides (TMDs), how such structures affect quantitatively the physical properties of 2D materials has not yet been established. We herein explore electronic property modulation in 2D lateral SLs of monolayer TMDs through first-principles high-throughput calculations. The dependence of the electronic structure, bandgap, carrier effective masses, charge density overlap on chemical components, interface type, and sub-lattice size of lateral TMD-SLs are investigated. We find that by comparison with their random alloy counterparts, the lateral TMD-SLs exhibit generally type-II band alignment, a wider range of bandgap tunability, larger carrier effective masses, and stronger electron-hole charge separation tendency. The bandgap variation with a sub-lattice size shows larger bowing parameters for the SLs with heterogeneous anions, by comparison with the homogeneous anion cases. A similar behavior is observed for the SLs with an armchair-type interface, by comparison with the zigzag-type interface cases. Further analyses reveal that the underlying physical mechanism can be attributed to the synergistic interplay among the band offset of sub-lattices, quantum confinement effect, and existing internal strain.

2D multifunctional SiAs2/GeAs2van der Waals heterostructure

The structural and electronic properties of two-dimensional (2D) SiAs₂/GeAs₂van der Waals heterostructure (vdWH) and its applications are investigated by combing first-principles calculations and Silvaco Atlas simulations. The stable SiAs₂/GeAs₂vdWH exhibits an indirect bandgap of 0.99 eV in type II band alignment for light detection and energy harvesting. The vdWH can exhibit a direct bandgap up to 0.66 eV by applying an appropriate electric field (E_ext). Due to theE_extinduced charge redistribution, its band alignment can be transformed from type II to type I for light-emitting. Further simulation shows that the band alignment of SiAs₂/GeAs₂vdWH can be tuned back and forth between type II and type I by gate voltage in a single field-effect transistor for multiple functional applications. These results may be useful for applications of the SiAs₂/GeAs₂heterostructure in future electronic and optoelectronic devices.

Prediction of Effective Photoelectron and Hole Separation in Type-I MoS2/PtSe2 van der Waals Junction

Understanding the dynamics of charge transfer at vertical heterostructures of transition metal dichalcogenide monolayers is fundamentally important for future technological applications, given the unique feature of van der Waals interactions at the interface. Here, we employ time-dependent density functional theory formalism combined with molecular dynamics to investigate photoexcited electrons and holes in the type-I MoS₂/PtSe₂ van der Waals heterobilayer. While type-I junctions have been traditionally viewed as being ineffective in photocarrier separation, we show that here a different mechanism from type-II is at play, which effectively separates photoelectrons from photoholes. The key is the phonon bottleneck, arising from the characteristically different dynamic band alignments in the valence and conduction bands, respectively, which only affects the transfer of holes but not electrons. The disparity between electron and hole transfer rates offers a new direction for effective control of charge separation at interfaces.

Band Alignment Engineering by Twist Angle and Composition Modulation for Heterobilayer

Atomically thin monolayer semiconducting transition metal dichalcogenides (TMDs), exhibiting direct band gap and strong light-matter interaction, are promising for optoelectronic devices. However, an efficient band alignment engineering method is required to further broaden their practical applications as versatile optoelectronics. In this work, the band alignment of two vertically stacked monolayer TMDs using the chemical vapor deposition (CVD) method is effectively tuned by two strategies: 1) formulating the compositions of MoS_2(1-x) Se_2x alloys, and 2) varying the twist angles of the stacked heterobilayer structures. Photoluminescence (PL) results combined with density functional theory (DFT) calculation show that by changing the alloy composition, a continuously tunable band alignment and a transition of type II-type I-type II band alignment of TMD heterobilayer is achieved. Moreover, only at moderate (10°-50°) twist angles, a PL enhancement of 28%-110% caused by the type I alignment is observed, indicating that the twist angle is coupled with the global band structure of heterobilayer. A heterojunction device made with MoS_0.76 Se_1.24 /WS₂ of 14° displays a significantly high photoresponsivity (55.9 A W^-1 ), large detectivity (1.07 × 10¹⁰ Jones), and high external quantum efficiency (135%). These findings provide engineering tools for heterostructure design for their application in optoelectronic devices.

Novel Van Der Waals Heterostructures Based on Borophene, Graphene-like GaN and ZnO for Nanoelectronics: A First Principles Study

At present, the combination of 2D materials of different types of conductivity in the form of van der Waals heterostructures is an effective approach to designing electronic devices with desired characteristics. In this paper, we design novel van der Waals heterostructures by combing buckled triangular borophene (tr-B) and graphene-like gallium nitride (GaN) monolayers, and tr-B and zinc oxide (ZnO) monolayers together. Using ab initio methods, we theoretically predict the structural, electronic, and electrically conductive properties of tr-B/GaN and tr-B/ZnO van der Waals heterostructures. It is shown that the proposed atomic configurations of tr-B/GaN and tr-B/ZnO heterostructures are energetically stable and are characterized by a gapless band structure in contrast to the semiconductor character of GaN and ZnO monolayers. We find the phenomenon of charge transfer from tr-B to GaN and ZnO monolayers, which predetermines the key role of borophene in the formation of the features of the electronic structure of tr-B/GaN and tr-B/ZnO van der Waals heterostructures. The results of the calculation of the current–voltage (I–V) curves reveal that tr-B/GaN and tr-B/ZnO van der Waals heterostructures are characterized by the phenomenon of current anisotropy: the current along the zigzag edge of the ZnO/GaN monolayers is five times greater than along the armchair edge of these monolayers. Moreover, the heterostructures show good stability of current to temperature change at small voltage. These findings demonstrate that r-B/GaN and tr-B/ZnO vdW heterostructures are promising candidates for creating the element base of nanoelectronic devices, in particular, a conducting channel in field-effect transistors.

First-Principles Study of Electronic and Optical Properties of Tri-Layered van der Waals Heterostructures Based on Blue Phosphorus and Zinc Oxide

The creation of van der Waals heterostructures with tunable properties from various combinations of modern 2D materials is one of the promising tasks of nanoelectronics, focused on improving the parameters of electronic nanodevices. In this paper, using ab initio methods, we theoretically predict the existence of new three-layer van der Waals zinc oxide/blue phosphorus/zinc oxide (ZnO/BlueP/ZnO) heterostructure with AAA, ABA, ABC layer packing types. It is found that AAA-, ABA-, and ABC-stacked ZnO/BlueP/ZnO heterostructures are semiconductors with a gap of about 0.7 eV. The dynamic conductivity and absorption spectra are calculated in the wavelength range of 200–2000 nm. It is revealed that the BlueP monolayer makes the greatest contribution to the formation of the profiles the dynamic conductivity and absorption coefficient spectrums of the ZnO/BlueP/ZnO heterostructure. This is indicated by the fact that, for the ZnO/BlueP/ZnO heterostructure, conductivity anisotropy is observed at different directions of wave polarization, as for blue phosphorus. It has been established that the absorption maximum of the heterostructure falls in the middle ultraviolet range, and, starting from a wavelength of 700 nm, there is a complete absence of absorption. The type of layer packing has practically no effect on the regularities in the formation of the spectra of dynamic conductivity and the absorption coefficient, which is important from the point of view of their application in optoelectronics.

Engineering Relaxation-Paths of C-Exciton for Constructing Band Nesting Bypass in WS2 Monolayer

Transition-metal dichalcogenides exhibit strong photon absorption characteristics in the band nesting region (denoted as C-exciton) due to intrinsic van Hove singularities despite being atomically thin. However, because of unique parallel band structure and ineluctably unfavorable recombination process, only a small fraction of the hot carriers from C-excitons are converted into optically active band-edge excitons via inherent relaxation-paths. The resultant photoluminescence quantum yield (PLQY) is severely suppressed for the resonant excitation of C-exciton. To overcome this limitation, we have designed double type-I band alignments to construct a band nesting bypass in a monolayer WS₂/CdS quantum dot heterostructure for cooling the C-excitons. Transient optical measurements confirmed that the hot carriers from the C-excitons were effectively transferred from WS₂ to CdS with an efficiency of 50% and subsequently back to the WS₂ band-edge to form A-excitons over an ultrafast subpicosecond time scale, accompanied by a record high PLQY of ∼11.1% for near-resonance C-exciton excitation.

2D Molybdenum Sulfide-Based Materials for Photo-Excited Antibacterial Application

Bacterial infections have seriously threatened human health and the abuse of natural or artificial antibiotics leads to bacterial resistance, so development of a new generation of antibacterial agents and treatment methods is urgent. 2D molybdenum sulfide (MoS₂ ) has good biocompatibility, high specific surface area to facilitate surface modification and drug loading, adjustable energy bandgap, and high near-infrared photothermal conversion efficiency (PCE), so it is often used for antibacterial application through its photothermal or photodynamic effects. This review comprehensively summarizes and discusses the fabrication processes, structural characteristics, antibacterial performance, and the corresponding mechanisms of MoS₂ -based materials as well as their representative antibacterial applications. In addition, the outlooks on the remaining challenges that should be addressed in the field of MoS₂ are also proposed.

Spatiotemporally Coupled Electron-Hole Dynamics in Two Dimensional Heterostructures

Coulomb interactions play a crucial role in low-dimensional semiconductor materials, e.g., 2D layered semiconductors, dictating their electronic and optical properties. However, fundamental questions remain as to whether and how Coulomb interactions affect the charge or energy flow in 2D heterostructures, which is essential for their light-electricity conversions. Herein, using ultrafast spectroscopy, we report real space coupled electron-hole dynamics in 2D heterostructures. We show in (WSe₂/)WS₂/MoTe₂ with a controlled energy gradient for the hole and a near flat band for electron transfer, the fate of the electron is controlled by the hole in coupled dynamics. The interfacial electron transfer from WS₂ to MoTe₂ follows the hole closely and can be facilitated or suppressed by dynamic Coulomb interaction. In parallel to the band alignment, this study reveals the critical role of Coulomb interactions on the fate of photogenerated charges in 2D heterostructures, providing experimental evidence for coupled electron-hole dynamics and a new knob for steering nanoscale charge or energy transfer process.

Dominating Interlayer Resonant Energy Transfer in Type-II 2D Heterostructure

Type-II heterostructures (HSs) are essential components of modern electronic and optoelectronic devices. Earlier studies have found that in type-II transition metal dichalcogenide (TMD) HSs, the dominating carrier relaxation pathway is the interlayer charge transfer (CT) mechanism. Here, this report shows that, in a type-II HS formed between monolayers of MoSe₂ and ReS₂, nonradiative energy transfer (ET) from higher to lower work function material (ReS₂ to MoSe₂) dominates over the traditional CT process with and without a charge-blocking interlayer. Without a charge-blocking interlayer, the HS area shows 3.6 times MoSe₂ photoluminescence (PL) enhancement as compared to the MoSe₂ area alone. In a completely encapsulated sample, the HS PL emission further increases by a factor of 6.4. After completely blocking the CT process, more than 1 order of magnitude higher MoSe₂ PL emission was achieved from the HS area. This work reveals that the nature of this ET is truly a resonant effect by showing that in a similar type-II HS formed by ReS₂ and WSe₂, CT dominates over ET, resulting in a severely quenched WSe₂ PL. This study not only provides significant insight into the competing interlayer processes but also shows an innovative way to increase the PL emission intensity of the desired TMD material using the ET process by carefully choosing the right material combination for HS.

Optical Inspection of 2D Materials: From Mechanical Exfoliation to Wafer-Scale Growth and Beyond

Optical inspection is a rapid and non-destructive method for characterizing the properties of two-dimensional (2D) materials. With the aid of optical inspection, in situ and scalable monitoring of the properties of 2D materials can be implemented industrially to advance the development and progress of 2D material-based devices toward mass production. This review discusses the optical inspection techniques that are available to characterize various 2D materials, including graphene, transition metal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), group-III monochalcogenides, black phosphorus (BP), and group-IV monochalcogenides. First, the authors provide an introduction to these 2D materials and the processes commonly used for their fabrication. Then they review several of the important structural properties of 2D materials, and discuss how to characterize them using appropriate optical inspection tools. The authors also describe the challenges and opportunities faced when applying optical inspection to recently developed 2D materials, from mechanically exfoliated to wafer-scale-grown 2D materials. Most importantly, the authors summarize the techniques available for largely and precisely enhancing the optical signals from 2D materials. This comprehensive review of the current status and perspective of future trends for optical inspection of the structural properties of 2D materials will facilitate the development of next-generation 2D material-based devices.

Intriguing electronic, optical and photocatalytic performance of BSe, M2CO2 monolayers and BSe-M2CO2 (M = Ti, Zr, Hf) van der Waals heterostructures

Using density functional (DFT) theory calculations, we have investigated the electronic band structure, optical and photocatalytic response of BSe, M₂CO₂ (M = Ti, Zr, Hf) monolayers and their corresponding BSe–M₂CO₂ (M = Ti, Zr, Hf) van der Waals (vdW) heterostructures. Optimized lattice constant, bond length, band structure and bandgap values, effective mass of electrons and holes, work function and conduction and valence band edge potentials of BSe and M₂CO₂ (M = Ti, Zr, Hf) monolayers are in agreement with previously available data. Binding energies, interlayer distance and Ab initio molecular dynamic simulations (AIMD) calculations show that BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures are stable with specific stacking and demonstrate that these heterostructures might be synthesized in the laboratory. The electronic band structure shows that all the studied vdW heterostructures have indirect bandgap nature – with the CBM and VBM at the Γ–K and Γ-point of BZ for BSe–Ti₂CO₂, respectively; while for BSe–Zr₂CO₂ and BSe–Hf₂CO₂ vdW heterostructures the CBM and VBM lie at the K-point and Γ-point of BZ, respectively. Type-II band alignment in BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures prevent the recombination of electron–hole pairs, and hence are crucial for light harvesting and detection. Absorption spectra are investigated to understand the optical behavior of BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures, where the lowest energy transitions are dominated by excitons. Furthermore, BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures are found to be potential photocatalysts for water splitting at pH = 0, and exhibit enhanced optical properties in the visible light zones.

Using density functional theory calculations, we have investigated the electronic band structure, optical and photocatalytic response of BSe, M₂CO₂ (M = Ti, Zr, Hf) monolayers and their corresponding BSe–M₂CO₂ (M = Ti, Zr, Hf) van der Waals heterostructures.

Ab Initio Calculations for the Electronic, Interfacial and Optical Properties of Two-Dimensional AlN/Zr2CO2 Heterostructure

Recently, expanding the applications of two-dimensional (2D) materials by constructing van der Waals (vdW) heterostructures has become very popular. In this work, the structural, electronic and optical absorption performances of the heterostructure based on AlN and Zr2CO2 monolayers are studied by first-principles simulation. It is found that AlN/Zr2CO2 heterostructure is a semiconductor with a band gap of 1.790 eV. In the meanwhile, a type-I band structure is constructed in AlN/Zr2CO2 heterostructure, which can provide a potential application of light emitting devices. The electron transfer between AlN and Zr2CO2 monolayer is calculated as 0.1603 |e| in the heterostructure, and the potential of AlN/Zr2CO2 heterostructure decreased by 0.663 eV from AlN layer to Zr2CO2 layer. Beisdes, the AlN/Zr2CO2 vdW heterostructure possesses excellent light absorption ability of in visible light region. Our research provides a theoretical guidance for the designing of advanced functional heterostructures.

Mechanical, electronic and optical properties of a novel B2P6 monolayer: ultrahigh carrier mobility and strong optical absorption

Two-dimensional (2D) materials with a moderate bandgap and high carrier mobility are useful for applications in optoelectronics. In this work, we present a systematic investigation of the mechanical, electronic and optical properties of a B₂P₆ monolayer using first-principles calculations. Monolayer B₂P₆ was estimated to be an anisotropic material from direction-dependent in-plane Young's moduli and Poisson's ratios. Also, B₂P₆ exhibits an ultrahigh electron mobility of ∼5888 cm² V^-1 s^-1, showing advantages for application in high-speed optoelectronic devices. More importantly, for the B₂P₆ monolayer, a desirable transformation from an indirect to direct band gap was observed at a biaxial tensile strain of ∼4%. Increasing the biaxial strain reduces the gap and preserves the suitable band edge positions for photocatalytic water splitting in the observed strain range of 1-8%. The decreased gap also enhances the visible light absorption of the B₂P₆ monolayer. These findings indicate that the B₂P₆ monolayer has promising applications in photocatalytic and photovoltaic devices.

Electronic and optical properties of two-dimensional GaN/ZnO heterojunction tuned by different stacking configurations

Two-dimensional (2D) semiconductors show novel electronic and optoelectronic applications due to their excellent performance. The van der Waals (vdW) heterostructures are also a new method for the design of low dimensional optoelectronic devices. However, their fundamental electronic structure and optical properties are sensitive to stacking configurations. Herein, we perform systematic first-principle calculations for monolayer GaN and ZnO by six different stacking styles. The results suggest that the bonding type and stability vary with the stacking method. Chemical bonding and vdW interaction are respectively observed in different models. However, the carrier mobilities for different models are all enhanced after integration. Both type I and II band alignment can be generated from different stacking models. The optical properties suggest high absorptivity in the solar-blind region. This study is an early stage for the design and synthesis of photodetectors or solar cells based on 2D GaN/ZnO heterojunctions and also opens a far-ranging research interest in optoelectronic materials and devices with more advanced semiconductor materials.

Understanding the interfacial charge transfer in the CVD grown Bi2O2Se/CsPbBr3 nanocrystal heterostructure and its exploitation in superior photodetection: experiment vs. theory

Efficient charge transfer in a 2D semiconductor heterostructure plays a crucial role in high-performance photodetectors and energy harvesting devices. Non-van der Waals 2D Bi₂O₂Se has enormous potential for high-performance optoelectronics, though very little is known about the interfacial charge transport at the corresponding 2D heterojunction. Herein, we report a combined experimental and theoretical investigation of interfacial charge transfer in the Bi₂O₂Se/CsPbBr₃ heterostructure through various microscopic and spectroscopic tools corroborated with density functional theory calculations. The CVD-grown few-layer Bi₂O₂Se nanosheet possesses high crystallinity and a high absorption coefficient in the visible-near IR region. We integrated the few-layer Bi₂O₂Se nanosheet possessing superior electron mobility and CsPbBr₃ nanocrystals with high light-harvesting capability for efficient broadband photodetection. The band alignment reveals a type-I heterojunction, and the device under reverse bias reveals a fast response time of 12 μs/24 μs (rise time/fall time) and an improved responsivity in the 390 to 840 nm range due to the effective interfacial charge transfer and efficient interlayer coupling at the Bi₂O₂Se/CsPbBr₃ interface. Notably, a photodetector with a better light on/off ratio and a peak responsivity of ∼10³ A W^-1 was achieved in the Bi₂O₂Se/CsPbBr₃ heterostructure due to the synergistic effects in the heterostructure under ambient conditions. The DFT analysis of the density of states and charge density plots in the heterostructure revealed a net transfer of electrons/holes from perovskite nanocrystals to Bi₂O₂Se layers and additional density of states in Bi₂O₂Se. These results are significant for the development of non-van der Waals heterostructure based high-performance low-powered photodetectors.

Electronic and optical properties of two-dimensional heterostructures based on Janus XSSe (X = Mo, W) and Mg(OH)2: a first principles investigation

Two-dimensional (2D) materials have attracted numerous investigations after the discovery of graphene. 2D van der Waals (vdW) heterostructures are a new generation of layered materials, which can provide more desirable applications. In this study, the first principles calculation was implemented to study the heterostructures based on Janus TMDs (MoSSe and WSSe) and Mg(OH)₂ monolayers, which were constructed by vdW interactions. Both MoSSe/Mg(OH)₂ and WSSe/Mg(OH)₂ vdW heterostructures have thermal and dynamic stability. Besides, XSSe/Mg(OH)₂ (X = Mo, W) possesses a direct bandgap with a type-I band alignment, which provides promising applications for light-emitting devices. The charge density difference was investigated, and 0.003 (or 0.0042) |e| were transferred from MoSSe (or WSSe) layer to Mg(OH)₂ layer, and the potential drops were calculated to be 11.59 and 11.44 eV across the interface of the MoSSe/Mg(OH)₂ and WSSe/Mg(OH)₂ vdW heterostructures, respectively. Furthermore, the MoSSe/Mg(OH)₂ and WSSe/Mg(OH)₂ vdW heterostructures have excellent optical absorption wave. Our studies exhibit an effective method to construct new heterostructures based on Janus TMDs and develop their applications for future light emitting devices.

Two-dimensional (2D) materials have attracted numerous investigations after the discovery of graphene.

Electronic and Optical Properties of Atomic-Scale Heterostructure Based on MXene and MN (M = Al, Ga): A DFT Investigation

After the discovery of graphene, a lot of research has been conducted on two-dimensional (2D) materials. In order to increase the performance of 2D materials and expand their applications, two different layered materials are usually combined by van der Waals (vdW) interactions to form a heterostructure. In this work, based on first-principles calculation, some charming properties of the heterostructure constructed by Hf2CO2, AlN and GaN are addressed. The results show that Hf2CO2/AlN and Hf2CO2/GaN vdW heterostructures can keep their original band structure shape and have strong thermal stability at 300 K. In addition, the Hf2CO2/MN heterostructure has I-type band alignment structure, which can be used as a promising light-emitting device material. The charge transfer between the Hf2CO2 and AlN (or GaN) monolayers is 0.1513 (or 0.0414) |e|. The potential of Hf2CO2/AlN and Hf2CO2/GaN vdW heterostructures decreases by 6.445 eV and 3.752 eV, respectively, across the interface. Furthermore, both Hf2CO2/AlN and Hf2CO2/GaN heterostructures have remarkable optical absorption capacity, which further shows the application prospect of the Hf2CO2/MN heterostructure. The study of this work provides theoretical guidance for the design of heterostructures for use as photocatalytic and photovoltaic devices.

Multimodal Nanoscopic Study of Atomic Diffusion and Related Localized Optoelectronic Response of WS2/MoS2 Lateral Heterojunctions

The atomic diffusion in transition metal dichalcogenides (TMDs) van der Waals heterojunctions (HJs) strongly modifies their optoelectronic properties in the nanoscale. However, probing such localized properties challenges the spatial resolution and the sensitivity of a variety of analytic tools. Herein, a multimodal nanoscopy (based on tip enhanced Raman spectroscopy (TERS) and photoluminescence (TEPL)) combined with the Kelvin probe force microscopy (KPFM) method was used to probe such nanoscale localized optoelectronic properties induced by atomic diffusion. Chemical vapor deposition (CVD)-grown lateral bilayer (2L) WS₂/MoS₂ HJs were imaged with a spatial resolution better than 40 nm via TERS and TEPL mapping by using intrinsic Raman and photoluminescence (PL) peaks. The contact potential difference (CPD), capacitance, and PL variation in a nanoscale vicinity of the HJ interface can be correlated to the local stoichiometry variation determined by TERS. The diffusion coefficients of W and Mo were obtained to be ∼0.5 × 10^-12 and ∼1 × 10^-12 cm²/s, respectively, by using Fick's second law. The obtained results would be useful to further understand the localized optoelectronic response of the TMDs HJs.

Stacking effects in van der Waals heterostructures of blueP and Janus XYO (X = Ti, Zr, Hf: Y = S, Se) monolayers

Using first-principles calculations, the geometry, electronic structure, optical and photocatalytic performance of blueP and XYO (X = Ti, Zr, Hf; Y = S, Se) monolayers and their corresponding van der Waal heterostructures in three possible stacking patterns, are investigated. BlueP and XYO (X = Ti, Zr, Hf; Y = S, Se) monolayers are indirect bandgap semiconductors. A tensile strain of 8(10)% leads to TiSeO(ZrSeO) monolayers transitioning to a direct bandgap of 1.30(1.61) eV. The calculated binding energy and AIMD simulation show that unstrained(strained) blueP and XYO (X = Ti, Zr, Hf; Y = S, Se) monolayers and their heterostructures are thermodynamically stable. Similar to the corresponding monolayers, blueP-XYO (X = Ti, Zr, Hf: Y = S, Se) vdW heterostructures in three possible stacking patterns are indirect bandgap semiconductors with staggered band alignment, except blueP-TiSeO vdW heterostructure, which signifies straddling band alignment. Absorption spectra show that optical transitions are dominated by excitons for blueP and XYO (X = Ti, Zr, Hf; Y = S, Se) monolayers and the corresponding vdW heterostructures. Both E VB and E CB in TiSO, ZrSO, ZrSeO and HfSO monolayers achieve energetically favorable positions, and therefore, are suitable for water splitting at pH = 0, while TiSeO and HfSeO monolayers showed good response for reduction and fail to oxidise water. All studied vdW heterostructures also show good response to any produced O2, while specific stacking reduces H+ to H2.

Strain-controlled electronic and magnetic properties of tVS2/hVS2 van der Waals heterostructures

The structural, electronic and magnetic properties of the T-phase and H-phase of the VS₂ monolayer and their heterobilayers are studied by means of first-principles calculations. We find that the two phases of the VS₂ monolayer are both ferromagnetic (FM) semiconductors and that these two monolayers form a typical van der Waals (vdW) heterostructure with a weak interlayer interaction. By comparing the energy of different magnetic configurations, the FM state of the tVS₂/hVS₂ heterostructure is found to be in the ground state under normal conditions or biaxial strains. Under compressive strains, the anti-FM (AFM) and FM states degenerate. Based on the band structure obtained and the work function, it is found that the T-phase and H-phase are capable of forming an efficient p-n heterostructure. Due to spontaneous charge transfer at the interface, a gapless semiconductor is formed in our HSE06 calculations. We also find that the twist angle between the monolayers has a negligible impact on the band structure of the heterostructure in its spin-down channel. Moreover, the tVS₂/hVS₂ heterostructure is found to switch from a gapless semiconductor to a metal or a half-metal under some given biaxial or uniaxial strains. Therefore, the heterostructure could be a half-metallic property with strains, realizing 100% polarization at the Fermi level. Our study provides the possibility of realizing 100% spin-polarization at the Fermi level in these FM vdW heterostructures, which is significant for further spin transport exploration.

Contact Engineering of Vertically Grown ReS2 with Schottky Barrier Modulation

Forming metal contact with low contact resistance is essential for the development of electronics based on layered van der Waals materials. ReS₂ is a semiconducting transition metal dichalcogenide (TMD) with an MX₂ structure similar to that of MoS₂. While most TMDs grow parallel to the substrate when synthesized using chemical vapor deposition (CVD), ReS₂ tends to orient itself vertically during growth. Such a feature drastically increases the surface area and exposes chemically active edges, making ReS₂ an attractive layered material for energy and sensor applications. However, the contact resistances of vertically grown materials are known to be relatively high, compared to those of common 2H-phase TMDs, such as MoS₂. Most reported methods for lowering the contact resistance have been focused on exfoliated 2H-phase materials with only a few devices tested, and few works on distorted T-phase materials exist. Moreover, nearly all reported studies have been conducted on only a few devices with mechanically exfoliated fl Most reported methods for lowering the contact resistance have been ₂ contacts was modulated by conformally coating a thin tunneling interlayer between the metal and the dendritic ReS₂ film. Over a hundred devices were tested, and contact resistances were extracted for large-scale statistical analysis. Importantly, we compared various known materials and techniques for lowering contact resistance and found an optimized method. Finally, the reductions in barrier height were directly correlated with exponential reductions in contact resistance and increases in drive-current by almost 2 orders of magnitude.

Seamlessly Splicing Metallic Sn x Mo1- x S2 at MoS2 Edge for Enhanced Photoelectrocatalytic Performance in Microreactor

Accurate design of the 2D metal-semiconductor (M-S) heterostructure via the covalent combination of appropriate metallic and semiconducting materials is urgently needed for fabricating high-performance nanodevices and enhancing catalytic performance. Hence, the lateral epitaxial growth of M-S Sn x Mo1- x S2/MoS2 heterostructure is precisely prepared with in situ growth of metallic Sn x Mo1- x S2 by doping Sn atoms at semiconductor MoS2 edge via one-step chemical vapor deposition. The atomically sharp interface of this heterostructure exhibits clearly distinguished performance based on a series of characterizations. The oxygen evolution photoelectrocatalytic performance of the epitaxial M-S heterostructure is 2.5 times higher than that of pure MoS2 in microreactor, attributed to the efficient electron-hole separation and rapid charge transfer. This growth method provides a general strategy for fabricating seamless M-S lateral heterostructures by controllable doping heteroatoms. The M-S heterostructures show increased carrier migration rate and eliminated Fermi level pinning effect, contributing to their potential in devices and catalytic system.

2D Re-Based Transition Metal Chalcogenides: Progress, Challenges, and Opportunities

The rise of 2D transition-metal dichalcogenides (TMDs) materials has enormous implications for the scientific community and beyond. Among TMDs, ReX2 (X = S, Se) has attracted significant interest regarding its unusual 1T' structure and extraordinary properties in various fields during the past 7 years. For instance, ReX2 possesses large bandgaps (ReSe2: 1.3 eV, ReS2: 1.6 eV), distinctive interlayer decoupling, and strong anisotropic properties, which endow more degree of freedom for constructing novel optoelectronic, logic circuit, and sensor devices. Moreover, facile ion intercalation, abundant active sites, together with stable 1T' structure enable them great perspective to fabricate high-performance catalysts and advanced energy storage devices. In this review, the structural features, fundamental physicochemical properties, as well as all existing applications of Re-based TMDs materials are comprehensively introduced. Especially, the emerging synthesis strategies are critically analyzed and pay particular attention is paid to its growth mechanism with probing the assembly process of domain architectures. Finally, current challenges and future opportunities regarding the controlled preparation methods, property, and application exploration of Re-based TMDs are discussed.

Tunable band alignment in boron carbon nitride and blue phosphorene van der Waals heterostructure

The hybrid monolayer of boron nitride and graphene, namely the BC x N monolayer, has been recently revealed as a direct bandgap semiconductor with exceptional thermal, mechanical and optical properties. The integration of such monolayer with other 2D materials into a van der Waals heterostructure (VDWH), however, remains largely unexplored thus far. In this work, we investigate the electronic and structural properties of a new class of VDWH obtained via the vertical stacking of BC x N (x = 2, 6) and blue phosphorene monolayers. By using first-principle density functional theory (DFT) simulation, we show that BC x N couples to the blue phosphorene layer via weak van der Waals interactions and exhibits a type-II band alignment which is beneficial for electron-hole pair separation in photodetection and solar cell applications. Intriguingly, changing the interlayer separation induces a indirect-to-direct band gap transition which changes the band alignment types of the VDWH. The interlayer separation, which can be readily tuned via a vertical strain, thus provides a useful tuning knob for switching the heterostructures between type-I and type-II VDWHs. Our findings reveals the BC x N-based VDWH as a versatile material platform with tunable band alignments, thus opening a route towards novel VDWH-based optoelectronic devices.

In Situ Growth of 3D/2D (CsPbBr3/CsPb2Br5) Perovskite Heterojunctions toward Optoelectronic Devices

Two-dimensional (2D) CsPb₂Br₅ exhibits intriguing functions in enhancing the performance of optoelectronic devices in terms of environmental stability and luminescence properties when composited with other perovskites in different dimensionalities. We built a type I three-dimensional (3D) CsPbBr₃/2D CsPb₂Br₅ heterojunction through phase transition where CsPbBr₃ quantum dots in situ grew into 2D CsPb₂Br₅. A thorough growth mechanism study in combination with excited state dynamic investigations via femtosecond spectroscopy and first-principles calculations revealed that the type I hierarchy enhanced the stability of the heterojunction and spurred its luminous quantum yield by prolonging the lifetime of photogenerated carriers. Mixing the heterojunction with other phosphors yielded white-light-emitting diodes with a color rendering index of 94%. The work thus not only offered one new avenue for building heterojunctions by using the "soft crystal" nature of perovskites but also disentangled the enhanced luminescence mechanism of the heterojunction that can be harnessed for promising applications in the luminescence and display fields.

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.

Electrical switching between exciton dissociation to exciton funneling in MoSe2/WS2 heterostructure

The heterostructure of monolayer transition metal dichalcogenides (TMDCs) provides a unique platform to manipulate exciton dynamics. The ultrafast carrier transfer across the van der Waals interface of the TMDC hetero-bilayer can efficiently separate electrons and holes in the intralayer excitons with a type II alignment, but it will funnel excitons into one layer with a type I alignment. In this work, we demonstrate the reversible switch from exciton dissociation to exciton funneling in a MoSe₂/WS₂ heterostructure, which manifests itself as the photoluminescence (PL) quenching to PL enhancement transition. This transition was realized through effectively controlling the quantum capacitance of both MoSe₂ and WS₂ layers with gating. PL excitation spectroscopy study unveils that PL enhancement arises from the blockage of the optically excited electron transfer from MoSe₂ to WS₂. Our work demonstrates electrical control of photoexcited carrier transfer across the van der Waals interface, the understanding of which promises applications in quantum optoelectronics.

The ultrafast carrier dynamics across the van der Waals interface of transition metal dichalcogenide heterostructures govern the formation and funnelling of excitons. Here, the authors demonstrate a reversible switch from exciton dissociation to exciton funnelling in a MoSe₂/WS₂ heterostructure, which manifests itself as a photoluminescence quenching-to-enhancement transition.

Point defect induced intervalley scattering for the enhancement of interlayer electron transport in bilayer MoS2 homojunctions

Since the emergence of transition metal dichalcogenide (TMDC) based van der Waals (vdW) structures, interlayer charge transport has become an important issue towards the application of these novel materials. Due to the unique layered structure of these materials, charge transport across the vdW gaps via tunneling is governed by individual valleys with different interlayer coupling strengths. On the other hand, the omnipresent point defects in TMDCs could possibly cause intervalley scattering between these valleys. In this article, we investigate the influence of point defect induced intervalley scattering on the interlayer charge transport of the MoS2 homojunction by first principles calculation. We find that S vacancies and Mo-S antisite defects enhance the electron interlayer transport by intervalley scattering that divert the electrons from the non-interlayer coupling K valley to the strong interlayer coupling Q valley. The interlayer charge transport enhancement caused by such an intervalley scattering mechanism could pave the way towards understanding the interlayer charge transport in TMDC based vdW structures.