Photovoltaic effect in an electrically tunable van der Waals heterojunction

A Type-II BiTeCl/SnSe2 Heterostructure with High Photoelectric Conversion Efficiency and Tunable Optoelectronic Properties for Photovoltaic Applications

In this study, a Janus BiTeCl/SnSe2 van der Waals (vdW) heterostructure is constructed and systematically investigated for its potential in solar cell applications using first-principles calculations. The heterostructure introduces distinct contact interfaces (Cl-Se and Te-Se), both exhibiting a type-II band alignment. However, the conduction band minimum (CBM) and valence band maximum (VBM) contributions vary, depending on the interface. The Cl-Se interface demonstrates a significantly higher power conversion efficiency (PCE) of 20.11%, attributed to the suitable bandgap of the SnSe2 donor material and a smaller conduction band offset. Both interfaces exhibit enhanced optical properties compared to those of isolated BiTeCl and SnSe2 monolayers. Additionally, the electronic structure of the heterostructure is tunable via biaxial strain and electric fields, enabling further optimization of the PCE. Moreover, optical absorption can be adjusted by biaxial strain and electric fields. These findings position the Janus BiTeCl/SnSe2 heterostructure, particularly the Cl-Se interface, as a promising candidate for next-generation photovoltaic devices, offering both high efficiency and an external tunability.

Interlayer Exciton-Phonon Coupling in MoSe2/WSe2 Heterostructures

Transition metal dichalcogenide heterostructures have garnered strong interest for their robust excitonic properties, mixed light-matter states such as exciton-polaritons, and tailored properties, vital for advanced device engineering. Two-dimensional heterostructures inherit their physics from monolayers with the addition of interlayer processes that have been particularly emphasized for their electronic and optical properties. Here, we demonstrate the interlayer coupling of the MoSe2 phonons to WSe2 excitons in a WSe2/MoSe2 heterostructure using resonant Raman scattering. The WSe2 monolayer induces an interlayer resonance in the Raman cross-section of the MoSe2 A1g phonons. Frozen-phonon calculations within density functional theory reveal a strong deformation-potential coupling between the A1g MoSe2 phonon and the electronic states of the close-by WSe2 layer approaching 20% of the intralayer coupling to the MoSe2 electrons. Understanding the vibrational properties of van der Waals heterostructures requires going beyond the sum of their constituents and considering cross-material coupling.

Compositionally Graded MoS2xTe2(1-x)/MoS2 van der Waals Heterostructures for Ultrathin Photovoltaic Applications

van der Waals heterojunctions utilizing two-dimensional (2D) transition-metal dichalcogenide (TMD) materials have emerged as focal points in the field of optoelectronic devices, encompassing applications in light-emitting devices, photodetectors, solar cells, and beyond. In this study, we transferred few-atomic-layer films of compositionally graded ternary MoS2xTe2(1-x) alloys onto metal-organic chemical vapor deposition-grown molybdenum disulfide (MoS2) as p- and n-type structures, leading to the creation of a van der Waals vertical heterostructure. The characteristics of the fabricated MoS2xTe2(1-x)/MoS2 vertical-stacked heterojunction were investigated considering the influence of tellurium (Te) incorporation. The systematic variation of parameter x (i.e., 0.8, 0.6, 0.5, 0.3, and 0) allowed for an exploration of the impact of Te incorporation on the photovoltaic performance of these heterojunctions. As a result, the power conversion efficiency was enhanced by approximately 6 orders of magnitude with increasing Te concentration; notably, photoresponsivities as high as ∼6.4 A/W were achieved. These findings emphasize the potential for enhancing ultrathin solar energy conversion in heterojunctions based on 2D TMDs.

Surface Doping and Dual Nature of the Band Gap in Excitonic Insulator Ta2NiSe5

Excitons in semiconductors and molecules are widely utilized in photovoltaics and optoelectronics, and high-temperature coherent quantum states of excitons can be realized in artificial electron-hole bilayers and an exotic material of an excitonic insulator (EI). Here, we investigate the band gap evolution of a putative high-temperature EI Ta2NiSe5 upon surface doing by alkali adsorbates with angle-resolved photoemission and density functional theory (DFT) calculations. The conduction band of Ta2NiSe5 is filled by the charge transfer from alkali adsorbates, and the band gap decreases drastically upon the increase of metallic electron density. Our DFT calculation, however, reveals that there exist both structural and excitonic contributions to the band gap tuned. While electron doping reduces the band gap substantially, it alone is not enough to close the band gap. In contrast, the structural distortion induced by the alkali adsorbate plays a critical role in the gap closure. This work indicates a combined electronic and structural nature for the EI phase of the present system and the complexity of surface doping beyond charge transfer.

Efficient Charge Transfer in Graphene/CrOCl Heterostructures by van der Waals Interfacial Coupling

Due to the large volume of exposed atoms and electrons at the surface of two-dimensional materials, interfacial charge coupling has been proven as an efficient strategy to engineer the electronic structures of two-dimensional materials assembled in van der Waals heterostructures. Recently, heterostructures formed by graphene stacked with CrOCl have demonstrated intriguing quantum states, including a distorted quantum Hall phase in the monolayer graphene and the unconventional correlated insulator in the bilayer graphene. Yet, the understanding of the interlayer charge coupling in the heterostructure remains challenging. Here, we demonstrate clear evidences of efficient hole doping in the interfacial-coupled graphene/CrOCl heterostructure by detailed Raman spectroscopy and electrical transport measurements. The observation of significant blue shifts and stiffness of graphene Raman modes quantitatively determines the concentration of hole injection of about 1.2 × 1013 cm-2 from CrOCl to graphene, which is highly consistent with the enhanced conductivity of graphene. First-principles calculations based on density functional theory reveal that due to the large work function difference and the electronegativity of Cl atoms in CrOCl, the electrons are efficiently transferred from graphene to CrOCl, leading to hole doping in graphene. Our findings provide clues for understanding the exotic physical properties of graphene/CrOCl heterostructures, paving the way for further engineering of quantum electronic states by efficient interfacial charge coupling in van der Waals heterostructures.

Enhanced Optoelectronic Performance of p-WSe2/Re0.12W0.42Mo0.46S2 Heterojunction

Stacking of van der Waals (vdW) heterostructures and chemical element doping have emerged as crucial methods for enhancing the performance of semiconductors. This study proposes a novel strategy for modifying heterostructures by codoping MoS2 with two elements, Re and W, resulting in the construction of a RexWyMo1-x-yS2/WSe2 heterostructure for the preparation of photodetectors. This approach incorporates multiple strategies to enhance the performance, including hybrid stacking of materials, type-II band alignment, and regulation of element doping. As a result, the RexWyMo1-x-yS2/WSe2 devices demonstrate exceptional performance, including high photoresponsivity (1550.22 A/W), high detectivity (8.17 × 1013 Jones), and fast response speed (rise/fall time, 190 ms/1.42 s). Moreover, the ability to tune the band gap through element doping enables spectral response in the ultraviolet (UV), visible light, and near-infrared (NIR) regions. This heterostructure fabrication scheme highlights the high sensitivity and potential applications of vdW heterostructure (vdWH) in optoelectronic devices.

Photodetectors Based on MASnI3/MoS2 Hybrid-Dimensional Heterojunction Transistors: Breaking the Responsivity-Speed Trade-Off

Hybrid-dimensional heterojunction transistor (HDHT) photodetectors (PDs) have achieved high responsivities but unfortunately are still with unacceptably slow response speeds. Here, we propose a MASnI3/MoS2 HDHT PD, which exhibits the possibility to obtain high responsivity and fast response simultaneously. By exploring the detailed photoelectric responses utilizing a precise optoelectronic coupling simulation, the electrical performance of the device is optimally manipulated and the underlying physical mechanisms are carefully clarified. Particularly, the influence and modulation characteristics of the trap effects on the carrier dynamics of the PDs are investigated. We find that the localized trap effect in perovskite, especially at its top surface, is primarily responsible for the high responsivity and long response time; moreover, it is normally hard to break such a responsivity-speed trade-off due to the inherent limitation of the trap effect. By synergistically coupling the photogating effect, trap effect, and gate regulation, we indicate that it is possible to achieve an enhancement of the responsivity-bandwidth product by about 3 orders of magnitude. This study facilitates a fine modulation of the responsivity-speed relationship of hybrid-dimensional PDs, enabling breaking the traditional responsivity-speed trade-off of many PDs.

High Efficiency of Exciton-Polariton Lasing in a 2D Multilayer Structure

We have placed a van der Waals homostructure, formed by stacking three two-dimensional layers of WS2 separated by insulating hBN, similar to a multiple-quantum well structure, inside a microcavity, which facilitates the formation of quasiparticles known as exciton-polaritons. The polaritons are a combination of light and matter, allowing laser emission to be enhanced by nonlinear scattering, as seen in prior polariton lasers. In the experiments reported here, we have observed laser emission with an ultralow threshold. The threshold was approximately 59 nW/μm2, with a lasing efficiency of 3.82%. These findings suggest a potential for efficient laser operations using such homostructures.

CVD-Grown Monolayer MoS2 and GaN Thin Film Heterostructure for a Self-Powered and Bidirectional Photodetector with an Extended Active Spectrum

Photodetector technology has evolved significantly over the years with the emergence of new active materials. However, there remain trade-offs between spectral sensitivity, operating energy, and, more recently, an ability to harbor additional features such as persistent photoconductivity and bidirectional photocurrents for new emerging application areas such as switchable light imaging and filter-less color discrimination. Here, we demonstrate a self-powered bidirectional photodetector based on molybdenum disulfide/gallium nitride (MoS2/GaN) epitaxial heterostructure. This fabricated detector exhibits self-powered functionality and achieves detection in two discrete wavelength bands: ultraviolet and visible. Notably, it attains a peak responsivity of 631 mAW-1 at a bias of 0V. The device's response to illumination at these two wavelengths is governed by distinct mechanisms, activated under applied bias conditions, thereby inducing a reversal in the polarity of the photocurrent. This work underscores the feasibility of self-powered and bidirectional photocurrent detection but also opens new vistas for technological advancements for future optoelectronic, neuromorphic, and sensing applications.

Thermally Activated Photoluminescence Induced by Tunable Interlayer Interactions in Naturally Occurring van der Waals Superlattice SnS/TiS2

van der Waals (vdW) superlattices, comprising different 2D materials aligned alternately by weak interlayer interactions, offer versatile structures for the fabrication of novel semiconductor devices. Despite their potential, the precise control of optoelectronic properties with interlayer interactions remains challenging. Here, we investigate the discrepancies between the SnS/TiS2 superlattice (SnTiS3) and its subsystems by comprehensive characterization and DFT calculations. The disappearance of certain Raman modes suggests that the interactions alter the SnS subsystem structure. Specifically, such structural changes transform the band structure from indirect to direct band gap, causing a strong PL emission (∼2.18 eV) in SnTiS3. In addition, the modulation of the optoelectronic properties ultimately leads to the unique phenomenon of thermally activated photoluminescence. This phenomenon is attributed to the inhibition of charge transfer induced by tunable intralayer strains. Our findings extend the understanding of the mechanism of interlayer interactions in van der Waals superlattices and provide insights into the design of high-temperature optoelectronic devices.

Colloidal Quantum Dots for Nanophotonic Devices

Colloidal quantum dots (CQDs) have unique advantages in the wide tunability of visible-to-infrared emission wavelength and low-cost solution processibility [...].

A MoS2 nanosheet-based CRISPR/Cas12a biosensor for efficient miRNA quantification for acute myocardial infarction

Acute myocardial infarction (AMI) represents the leading cause of cardiovascular death worldwide, and it is thus pivotal to develop effective approaches for the timely detection of AMI markers, especially possessing the characteristics of antibody-free, signal amplification, and manipulation convenience. We herein construct a MoS2 nanosheet-powered CRISPR/Cas12a sensing strategy for sensitive determination of miR-499, a superior AMI biomarker to protein markers. The presence of miR-499 at a trace level is able to induce a significantly enhanced fluorescence signal in a DNA-based molecular engineering platform, which consists of CRISPR/Cas12a enzymatic reactions and MoS2 nanosheet-controllable signal reporting components. The MoS2 nanosheets were characterized by using atomic force microscopy (AFM) and transmission electron microscope (TEM). The detection feasibility was verified by using polyacrylamide gel electrophoresis (PAGE) analysis and fluorescence measurements. The detection limit is determined as 381.78 pM with the linear range from 0.1 ⅹ 10-9 to 13.33 ⅹ 10-9 M in a fast manner (about 30 min). Furthermore, miRNA detection in real human serum is also conducted with desirable recovery rates (89.5 %-97.6 %), which may find potential application for the clinic diagnosis. We describe herein the first example of MoS2 nanosheet-based signal amplified fluorescence sensor for effective detection of AMI-related miRNA.

Piezo-photovoltaic Effect in Monolayer 2H-MoS2

Noncentrosymmetric bulk materials effectively convert light energy into electricity by making use of the bulk photovoltaic effect (BPVE). However, whether such an effect persists when reducing the thickness of materials down to atomic-scale remains to be revealed. Here, we show the piezo-photovoltaic effect in atomically thin two-dimensional materials, where the strain-induced polarization can generate photovoltaic outputs in the noncentrosymmetric mono- and few-layer 2H-MoS2 crystals. The photocurrent is enhanced by orders of magnitude when the MoS2 crystals experience an in-plane strain of about 0.2%, with photopower-dependent responsivity up to 0.1 A/W that rivals other state-of-the-art BPVE materials. In addition, studies on the spatial distributions of photocurrents on MoS2 with a controlled number of layers also allow us to disentangle various factors that couple the piezoelectricity and photovoltaics. Therefore, our results also provide insights into the mechanisms of the piezo-photovoltaic effect in two-dimensional materials with thicknesses at the atomic-scale limit.

Recent progress in emerging two-dimensional organic-inorganic van der Waals heterojunctions

Two-dimensional (2D) materials have attracted significant attention in recent decades due to their exceptional optoelectronic properties. Among them, to meet the growing demand for multifunctional applications, 2D organic-inorganic van der Waals (vdW) heterojunctions have become increasingly popular in the development of optoelectronic devices. These heterojunctions demonstrate impressive capability to synergistically combine the favourable characteristics of organic and inorganic materials, thereby offering a wide range of advantages. Also, they enable the creation of innovative device structures and introduce novel functionalities in existing 2D materials, avoiding the need for lattice matching in different material systems. Presently, researchers are actively working on improving the performance of devices based on 2D organic-inorganic vdW heterojunctions by focusing on enhancing the quality of 2D materials, precise stacking methods, energy band regulation, and material selection. Therefore, this review presents a thorough examination of the emerging 2D organic-inorganic vdW heterojunctions, including their classification, fabrication, and corresponding devices. Additionally, this review offers profound and comprehensive insight into the challenges in this field to inspire future research directions. It is expected to propel researchers to harness the extraordinary capabilities of 2D organic-inorganic vdW heterojunctions for a wider range of applications by further advancing the understanding of their fundamental properties, expanding the range of available materials, and exploring novel device architectures. The ongoing research and development in this field hold potential to unlock captivating advancements and foster practical applications across diverse industries.

Optical spectroscopic detection of Schottky barrier height at a two-dimensional transition-metal dichalcogenide/metal interface

Atomically thin two-dimensional transition-metal dichalcogenides (2D-TMDs) have emerged as semiconductors for next-generation nanoelectronics. As 2D-TMD-based devices typically utilize metals as the contacts, it is crucial to understand the properties of the 2D-TMD/metal interface, including the characteristics of the Schottky barriers formed at the semiconductor-metal junction. Conventional methods for investigating the Schottky barrier height (SBH) at these interfaces predominantly rely on contact-based electrical measurements with complex gating structures. In this study, we introduce an all-optical approach for non-contact measurement of the SBH, utilizing high-quality WS2/Au heterostructures as a model system. Our approach employs a below-bandgap pump to excite hot carriers from the gold into WS2 with varying thicknesses. By monitoring the resultant carrier density changes within the WS2 layers with a broadband probe, we traced the dynamics and magnitude of charge transfer across the interface. A systematic sweep of the pump wavelength enables us to determine the SBH values and unveil an inverse relationship between the SBH and the thickness of the WS2 layers. First-principles calculations reveal the correlation between the probability of injection and the density of states near the conduction band minimum of WS2. The versatile optical methodology for probing TMD/metal interfaces can shed light on the intricate charge transfer characteristics within various 2D heterostructures, facilitating the development of more efficient and scalable nano-electronic and optoelectronic technologies.

Optical Modulation of MoTe2/Ferroelectric Heterostructure via Interface Doping

Optical modulation through interface doping offers a convenient and efficient way to control ferroelectric polarization, thereby advancing the utilization of ferroelectric heterostructures in nanoelectronic and optoelectronic devices. In this work, we fabricated heterostructures of MoTe2/BaTiO3/La0.7Sr0.3MnO3 (MoTe2/BTO/LSMO) and demonstrated opposite ultraviolet (UV) light-induced polarization switching behaviors depending on the varied thicknesses of MoTe2. The thickness-dependent band structure of MoTe2 film results in interface doping with opposite polarity in the respective heterostructures. The polarization field of BTO interacts with the interface charges, and an enhanced effective built-in field (Ebi) can trigger the transfer of massive UV light-induced carriers in both MoTe2 and BTO films. As a result, the interplay among the contact field of MoTe2/BTO, the polarization field, and the optically excited carriers determines the UV light-induced polarization switching behavior of the heterostructures. In addition, the electric transport characteristics of MoTe2/BTO/LSMO heterostructures reveal the interface barrier height and Ebi under opposite polarization states, as well as the presence of inherent in-gap trap states in MoTe2 and BTO films. These findings represent a further step toward achieving multifield modulation of the ferroelectric polarization and promote the potential applications in optoelectronic, logic, memory, and synaptic ferroelectric devices.

Lateral junctions of transition metal dichalcogenides as ballistic channels for straintronic applications

In the context of advanced nanoelectronics, two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are gaining considerable interest due to their ultimate thinness, clean surface and high carrier mobility. The engineering prospects offered by those materials are further enlarged by the recent realization of atomically sharp TMD-based lateral junctions, whose electronic properties are governed by strain effects arising from the constituents lattice mismatch. Although most theoretical studies considered only misfit strain, first-principles simulations are employed here to investigate the transport properties under external deformation of a three-terminal device constructed from a MoS2/WSe2/MoS2junction. Large modulation of the current is reported owing to the change in band offset, illustrating the importance of strain on the p-n junction characteristics. The device operation is demonstrated for both local and global deformations, even for ultra-short channels, suggesting potential applications for ultra-thin body straintronics.

Spontaneous exciton dissociation in transition metal dichalcogenide monolayers

Since the seminal work on MoS2, photoexcitation in atomically thin transition metal dichalcogenides (TMDCs) has been assumed to result in excitons, with binding energies order of magnitude larger than thermal energy at room temperature. Here, we reexamine this foundational assumption and show that photoexcitation of TMDC monolayers can result in a substantial population of free charges. Performing ultrafast terahertz spectroscopy on large-area, single-crystal TMDC monolayers, we find that up to ~10% of excitons spontaneously dissociate into charge carriers with lifetimes exceeding 0.2 ns. Scanning tunneling microscopy reveals that photocarrier generation is intimately related to mid-gap defects, likely via trap-mediated Auger scattering. Only in state-of-the-art quality monolayers, with mid-gap trap densities as low as 109 cm-2, does intrinsic exciton physics start to dominate the terahertz response. Our findings reveal the necessity of knowing the defect density in understanding photophysics of TMDCs.

Advances in Heterostructures for Optoelectronic Devices: Materials, Properties, Conduction Mechanisms, Device Applications

Atomically thin 2D transition metal dichalcogenides (TMDs) have recently been spotlighted for next-generation electronic and photoelectric device applications. TMD materials with high carrier mobility have superior electronic properties different from bulk semiconductor materials. 0D quantum dots (QDs) possess the ability to tune their bandgap by composition, diameter, and morphology, which allows for a control of their light absorbance and emission wavelength. However, QDs exhibit a low charge carrier mobility and the presence of surface trap states, making it difficult to apply them to electronic and optoelectronic devices. Accordingly, 0D/2D hybrid structures are considered as functional materials with complementary advantages that may not be realized with a single component. Such advantages allow them to be used as both transport and active layers in next-generation optoelectronic applications such as photodetectors, image sensors, solar cells, and light-emitting diodes. Here, recent discoveries related to multicomponent hybrid materials are highlighted. Research trends in electronic and optoelectronic devices based on hybrid heterogeneous materials are also introduced and the issues to be solved from the perspective of the materials and devices are discussed.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

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.

Self-Powered Programmable van der Waals Photodetectors with Nonvolatile Semifloating Gate

Tunable photovoltaic photodetectors are of significant relevance in the fields of programmable and neuromorphic optoelectronics. However, their widespread adoption is hindered by intricate architectural design and energy consumption challenges. This study employs a nonvolatile MoTe2/hexagonal boron nitride/graphene semifloating photodetector to address these issues. Programed with pulsed gate voltage, the MoTe2 channel can be reconfigured from an n+-n to a p-n homojunction and the photocurrent transition changes from negative to positive values. Scanning photocurrent mapping reveals that the negative and positive photocurrents are attributed to Schottky junction and p-n homojunction, respectively. In the p-n configuration, the device demonstrates self-driven, linear, rapid response (∼3 ms), and broadband sensitivity (from 405 to 1500 nm) for photodetection, with typical performances of responsivity at ∼0.5 A/W and detectivity ∼1.6 × 1012 Jones under 635 nm illumination. These outstanding photodetection capabilities emphasize the potential of the semifloating photodetector as a pioneering approach for advancing logical and nonvolatile optoelectronics.

Contemporary innovations in two-dimensional transition metal dichalcogenide-based P-N junctions for optoelectronics

Two-dimensional transition metal dichalcogenides (2D-TMDCs) with various physical characteristics have attracted significant interest from the scientific and industrial worlds in the years following Moore's law. The p-n junction is one of the earliest electrical components to be utilized in electronics and optoelectronics, and modern research on 2D materials has renewed interest in it. In this regard, device preparation and application have evolved substantially in this decade. 2D TMDCs provide unprecedented flexibility in the construction of innovative p-n junction device designs, which is not achievable with traditional bulk semiconductors. It has been investigated using 2D TMDCs for various junctions, including homojunctions, heterojunctions, P-I-N junctions, and broken gap junctions. To achieve high-performance p-n junctions, several issues still need to be resolved, such as developing 2D TMDCs of superior quality, raising the rectification ratio and quantum efficiency, and successfully separating the photogenerated electron-hole pairs, among other things. This review comprehensively details the various 2D-based p-n junction geometries investigated with an emphasis on 2D junctions. We investigated the 2D p-n junctions utilized in current rectifiers and photodetectors. To make a comparison of various devices easier, important optoelectronic and electronic features are presented. We thoroughly assessed the review's prospects and challenges for this emerging field of study. This study will serve as a roadmap for more real-world photodetection technology applications.

van der Waals 2D transition metal dichalcogenide/organic hybridized heterostructures: recent breakthroughs and emerging prospects of the device

The near-atomic thickness and organic molecular systems, including organic semiconductors and polymer-enabled hybrid heterostructures, of two-dimensional transition metal dichalcogenides (2D-TMDs) can modulate their optoelectronic and transport properties outstandingly. In this review, the current understanding and mechanism of the most recent and significant breakthrough of novel interlayer exciton emission and its modulation by harnessing the band energy alignment between TMDs and organic semiconductors in a TMD/organic (TMDO) hybrid heterostructure are demonstrated. The review encompasses up-to-date device demonstrations, including field-effect transistors, detectors, phototransistors, and photo-switchable superlattices. An exploration of distinct traits in 2D-TMDs and organic semiconductors delves into the applications of TMDO hybrid heterostructures. This review provides insights into the synthesis of 2D-TMDs and organic layers, covering fabrication techniques and challenges. Band bending and charge transfer via band energy alignment are explored from both structural and molecular orbital perspectives. The progress in emission modulation, including charge transfer, energy transfer, doping, defect healing, and phase engineering, is presented. The recent advancements in 2D-TMDO-based optoelectronic synaptic devices, including various 2D-TMDs and organic materials for neuromorphic applications are discussed. The section assesses their compatibility for synaptic devices, revisits the operating principles, and highlights the recent device demonstrations. Existing challenges and potential solutions are discussed. Finally, the review concludes by outlining the current challenges that span from synthesis intricacies to device applications, and by offering an outlook on the evolving field of emerging TMDO heterostructures.

In Situ Defect Engineering of Controllable Carrier Types in WSe2 for Homomaterial Inverters and Self-Powered Photodetectors

WSe2 has a high mobility of electrons and holes, which is an ideal choice as active channels of electronics in extensive fields. However, carrier-type tunability of WSe2 still has enormous challenges, which are essential to overcome for practical applications. In this work, the direct growth of n-doped few-layer WSe2 is realized via in situ defect engineering. The n-doping of WSe2 is attributed to Se vacancies induced by the H2 flow purged in the cooling process. The electrical measurements based on field effect transistors demonstrate that the carrier type of WSe2 synthesized is successfully transferred from the conventional p-type to the rarely reported n-type. The electron carrier concentration is efficiently modulated by the concentration of H2 during the cooling process. Furthermore, homomaterial inverters and self-powered photodetectors are fabricated based on the doping-type-tunable WSe2. This work reveals a significant way to realize the controllable carrier type of two-dimensional (2D) materials, exhibiting great potential in future 2D electronics engineering.

Polymer-Waveguide-Integrated 2D Semiconductor Heterostructures for Optical Communications

The demand for high-speed and low-loss interconnects in modern computer architectures is difficult to satisfy by using traditional Si-based electronics. Although optical interconnects offer a promising solution owing to their high bandwidth, low energy dissipation, and high-speed processing, integrating elements such as a light source, detector, and modulator, comprising different materials with optical waveguides, presents many challenges in an integrated platform. Two-dimensional (2D) van der Waals (vdW) semiconductors have attracted considerable attention in vertically stackable optoelectronics and advanced flexible photonics. In this study, optoelectronic components for exciton-based photonic circuits are demonstrated by integrating lithographically patterned poly(methyl methacrylate) (PMMA) waveguides on 2D vdW devices. The excitonic signals generated from the 2D materials by using laser excitation were transmitted through patterned PMMA waveguides. By introducing an external electric field and combining vdW heterostructures, an excitonic switch, phototransistor, and guided-light photovoltaic device on SiO2/Si substrates were demonstrated.

Nonadiabatic Dynamics Simulations for Photoinduced Processes in Molecules and Semiconductors: Methodologies and Applications

Nonadiabatic dynamics (NAMD) simulations have become powerful tools for elucidating complicated photoinduced processes in various systems from molecules to semiconductor materials. In this review, we present an overview of our recent research on photophysics of molecular systems and periodic semiconductor materials with the aid of ab initio NAMD simulation methods implemented in the generalized trajectory surface-hopping (GTSH) package. Both theoretical backgrounds and applications of the developed NAMD methods are presented in detail. For molecular systems, the linear-response time-dependent density functional theory (LR-TDDFT) method is primarily used to model electronic structures in NAMD simulations owing to its balanced efficiency and accuracy. Moreover, the efficient algorithms for calculating nonadiabatic coupling terms (NACTs) and spin-orbit couplings (SOCs) have been coded into the package to increase the simulation efficiency. In combination with various analysis techniques, we can explore the mechanistic details of the photoinduced dynamics of a range of molecular systems, including charge separation and energy transfer processes in organic donor-acceptor structures, ultrafast intersystem crossing (ISC) processes in transition metal complexes (TMCs), and exciton dynamics in molecular aggregates. For semiconductor materials, we developed the NAMD methods for simulating the photoinduced carrier dynamics within the framework of the Kohn-Sham density functional theory (KS-DFT), in which SOC effects are explicitly accounted for using the two-component, noncollinear DFT method. Using this method, we have investigated the photoinduced carrier dynamics at the interface of a variety of van der Waals (vdW) heterojunctions, such as two-dimensional transition metal dichalcogenides (TMDs), carbon nanotubes (CNTs), and perovskites-related systems. Recently, we extended the LR-TDDFT-based NAMD method for semiconductor materials, allowing us to study the excitonic effects in the photoinduced energy transfer process. These results demonstrate that the NAMD simulations are powerful tools for exploring the photodynamics of molecular systems and semiconductor materials. In future studies, the NAMD simulation methods can be employed to elucidate experimental phenomena and reveal microscopic details as well as rationally design novel photofunctional materials with desired properties.

Time-domain observation of interlayer exciton formation and thermalization in a MoSe2/WSe2 heterostructure

Vertical heterostructures of transition metal dichalcogenides (TMDs) host interlayer excitons with electrons and holes residing in different layers. With respect to their intralayer counterparts, interlayer excitons feature longer lifetimes and diffusion lengths, paving the way for room temperature excitonic optoelectronic devices. The interlayer exciton formation process and its underlying physical mechanisms are largely unexplored. Here we use ultrafast transient absorption spectroscopy with a broadband white-light probe to simultaneously resolve interlayer charge transfer and interlayer exciton formation dynamics in a MoSe2/WSe2 heterostructure. We observe an interlayer exciton formation timescale nearly an order of magnitude (~1 ps) longer than the interlayer charge transfer time (~100 fs). Microscopic calculations attribute this relative delay to an interplay of a phonon-assisted interlayer exciton cascade and thermalization, and excitonic wave-function overlap. Our results may explain the efficient photocurrent generation observed in optoelectronic devices based on TMD heterostructures, as the interlayer excitons are able to dissociate during thermalization.

Optical Enhancement of Indirect Bandgap 2D Transition Metal Dichalcogenides for Multi-Functional Optoelectronic Sensors

The unique electrical and optical properties of transition metal dichalcogenides (TMDs) make them attractive nanomaterials for optoelectronic applications, especially optical sensors. However, the optical characteristics of these materials are dependent on the number of layers. Monolayer TMDs have a direct bandgap that provides higher photoresponsivity compared to multilayer TMDs with an indirect bandgap. Nevertheless, multilayer TMDs are more appropriate for various photodetection applications due to their high carrier density, broad spectral response from UV to near-infrared, and ease of large-scale synthesis. Therefore, this review focuses on the modification of the optical properties of devices based on indirect bandgap TMDs and their emerging applications. Several successful developments in optical devices are examined, including band structure engineering, device structure optimization, and heterostructures. Furthermore, it introduces cutting-edge techniques and future directions for optoelectronic devices based on multilayer TMDs.

The influence of twist angle on the electronic and phononic band of 2D twisted bilayer SiC

Silicon carbide has a planar two-dimensional structure; therefore it is a potential material for constructing twisted bilayer systems for applications. In this study, DFT calculations were performed on four models with different twist angles. We chose angles of 21.8°, 17.9°, 13.2°, and 5.1° to estimate the dependence of the electronic and phononic properties on the twist angle. The results show that the band gap of bilayer SiC can be changed proportionally by changing the twist angle. However, there are only small variations in the band gaps, with an increment of 0.24 eV by changing the twist angle from 5.1° to 21.8°. At four considered twist angles, the band gaps decrease significantly when fixing the structure of each layer and pressing the separation distance down to 3.5 Å, 3.0 Å, 2.7 Å, and 2.5 Å. A noteworthy point is that the pressing also makes the band linearly smaller at a certain rate regardless of the twist angles. Meanwhile, the phonon bands are not affected by the value of the twist angle. The optical bands are between 900 cm-1 and 1100 cm-1 and the acoustic bands are between 0 cm-1 and 650 cm-1 at four twist angles.

High-performance multilayer WSe2/SnS2p-n heterojunction photodetectors by two step confined space chemical vapor deposition

Two-dimensional (2D) p-n heterojunctions have attracted great attention due to their outstanding properties in electronic and optoelectronic devices, especially in photodetectors. Various types of heterojunctions have been constituted by mechanical exfoliation and stacking. However, achieving controlled growth of heterojunction structures remains a tremendous challenge. Here, we employed a two-step KI-assisted confined-space chemical vapor deposition method to prepare multilayer WSe2/SnS2p-n heterojunctions. Optical characterization results revealed that the prepared WSe2/SnS2vertical heterostructures have clear interfaces as well as vertical heterostructures. The electrical and optoelectronic properties were investigated by constructing the corresponding heterojunction devices, which exhibited good rectification characteristics and obtained a high detectivity of 7.85 × 1012Jones and a photoresponse of 227.3 A W-1under visible light irradiation, as well as a fast rise/fall time of 166/440μs. These remarkable performances are likely attributed to the ultra-low dark current generated in the depletion region at the junction and the high direct tunneling current during illumination. This work demonstrates the value of multilayer WSe2/SnS2heterojunctions for applications in high-performance photodetectors.

Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2

In future solar cell technologies, the thermodynamic Shockley-Queisser limit for solar-to-current conversion in traditional p-n junctions could potentially be overcome with a bulk photovoltaic effect by creating an inversion broken symmetry in piezoelectric or ferroelectric materials. Here, we unveiled mechanical distortion-induced bulk photovoltaic behavior in a two-dimensional (2D) material, MoTe2, caused by the phase transition and broken inversion symmetry in MoTe2. The phase transition from single-crystalline semiconducting 2H-MoTe2 to semimetallic 1T'-MoTe2 was confirmed using X-ray photoelectron spectroscopy (XPS). We used a micrometer-scale system to measure the absorption of energy, which reduced from 800 to 63 meV during phase transformation from hexagonal to distorted octahedral and revealed a smaller bandgap semimetallic behavior. Experimentally, a large bulk photovoltaic response is anticipated with the maximum photovoltage VOC = 16 mV and a positive signal of the ISC = 60 μA (400 nm, 90.4 Wcm-2) in the absence of an external electric field. The maximum values of both R and EQE were found to be 98 mAW-1 and 30%, respectively. Our findings are distinctive features of the photocurrent responses caused by in-plane polarity and its potential from a wide pool of established TMD-based nanomaterials and a cutting-edge approach to optimize the efficiency in converting photons-to-electricity for power harvesting optoelectronics devices.

Recent Advances in Moiré Superlattice Systems by Angle-Resolved Photoemission Spectroscopy

The last decade has witnessed a flourish in 2D materials including graphene and transition metal dichalcogenides (TMDs) as atomic-scale Legos. Artificial moiré superlattices via stacking 2D materials with a twist angle and/or a lattice mismatch have recently become a fertile playground exhibiting a plethora of emergent properties beyond their building blocks. These rich quantum phenomena stem from their nontrivial electronic structures that are effectively tuned by the moiré periodicity. Modern angle-resolved photoemission spectroscopy (ARPES) can directly visualize electronic structures with decent momentum, energy, and spatial resolution, thus can provide enlightening insights into fundamental physics in moiré superlattice systems and guides for designing novel devices. In this review, first, a brief introduction is given on advanced ARPES techniques and basic ideas of band structures in a moiré superlattice system. Then ARPES research results of various moiré superlattice systems are highlighted, including graphene on substrates with small lattice mismatches, twisted graphene/TMD moiré systems, and high-order moiré superlattice systems. Finally, it discusses important questions that remain open, challenges in current experimental investigations, and presents an outlook on this field of research.

Doping-Free High-Performance Photovoltaic Effect in a WSe2 Lateral p-n Homojunction Formed by Contact Engineering

Two-dimensional transition metal dichalcogenides (TMDs) are promising materials for semiconductor nanodevices owing to their flexibility, transparency, and appropriate band gaps. A variety of optoelectronic and electronic devices based on TMDs p-n diodes have been extensively investigated due to their unique advantages. However, improving their performance is challenging for commercial applications. In this study, we propose a facile and doping-free approach based on the contact engineering of a few-layer tungsten di-selenide to form a lateral p-n homojunction photovoltaic. By combining surface and edge contacts for p-n diode fabrication, the photovoltaic effect is achieved with a high fill factor of ≈0.64, a power conversion efficiency of up to ≈4.5%, and the highest external quantum efficiency with a value of ≈67.6%. The photoresponsivity reaches 283 mA/W, indicating excellent photodiode performance. These results demonstrate that our technique has great potential for application in next-generation optoelectronic devices.

Dual transmission channels at metal-MoS2/WSe2 hetero-bilayer interfaces

van der Waals heterostructures (vdWHs) open the possibility of creating novel semiconductor materials at the atomic scale that demonstrate totally new physics and enable unique functionalities, and have therefore attracted great interest in the fields of advanced electronic and optoelectronic devices. However, the interactions between metals and vdWHs semiconductors require further investigation as they directly affect or limit the advancement of high-performance electronic devices. Here we study the contact behavior of MoS₂/WSe₂ vdWHs in contact with a series of bulk metals using ab initio electronic structure calculations and quantum transport simulations. Our study shows that dual transmission paths for electrons and holes exist at the metal-MoS₂/WSe₂ hetero-bilayer interfaces. In addition, the metal-induced bandgap state (MIGS) of the original monolayer disappears due to the creation of the heterolayer, which weakens the Fermi level pinning (FLP) effect. We also find that the creation of the heterolayer causes a change in the Schottky barrier height (SBH) of the non-ohmic contact systems, whilst this does not occur so easily in the ohmic contact systems. In addition, our results indicate that when Al, Ag and Au are in contact with a MoS₂/WSe₂ hetero-bilayer semiconductor, a low contact barrier exists throughout the whole transmission process causing the charge to tunnel to the MoS₂ layer, irrespective of whether the MoS₂ is in contact with the metals as the nearest layer or as the next-nearest layer. Our work not only offers new insights into electrical contact issues between metals and hetero-bilayer semiconductors, but also provides guidance for the design of high-performance vdWHs semiconductor devices.

Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structures

Hybrid structures with an interface between two different materials with properly aligned energy levels facilitate photo-induced charge separation to be exploited in optoelectronic applications. Particularly, the combination of 2D transition metal dichalcogenides (TMDCs) and dye molecules offers strong light-matter interaction, tailorable band level alignments, and high fluorescence quantum yields. In this work, we aim at the charge or energy transfer-related quenching of the fluorescence of the dye perylene orange (PO) when isolated molecules are brought onto monolayer TMDCs via thermal vapor deposition. Here, micro-photoluminescence spectroscopy revealed a strong intensity drop of the PO fluorescence. For the TMDC emission, in contrast, we observed a relative growth of the trion versus exciton contribution. In addition, fluorescence imaging lifetime microscopy quantified the intensity quenching to a factor of about 103 and demonstrated a drastic lifetime reduction from 3 ns to values much shorter than the 100 ps width of the instrument response function. From the ratio of the intensity quenching that is attributed to hole or energy transfer from dye to semiconductor, we deduce a time constant of several picoseconds at most, pointing to an efficient charge separation suitable for optoelectronic devices.

Self-Powered Photodetector with High Efficiency and Polarization Sensitivity Enabled by WSe2/Ta2NiSe5/WSe2 van der Waals Dual Heterojunction

Self-powered photodetectors have triggered widespread attention because of the requirement of Internet of Things (IoT) application and low power consumption. However, it is challenging to simultaneously implement miniaturization, high quantum efficiency, and multifunctionalization. Here, we report a high-efficiency and polarization-sensitive photodetector enabled by two-dimensional (2D) WSe₂/Ta₂NiSe₅/WSe₂ van der Waals (vdW) dual heterojunctions (DHJ) along with a sandwich-like electrode pair. On account of enhanced light collection efficiency and two opposite built-in electric fields at the hetero-interfaces, the DHJ device achieves not only a broadband spectral response of 400-1550 nm but outstanding performance under 635 nm light illumination including an ultrahigh external quantum efficiency (EQE) of 85.5%, a pronounced power conversion efficiency (PCE) of 1.9%, and a fast response speed of 420/640 μs, which is much better than that of the WSe₂/Ta₂NiSe₅ single heterojunction (SHJ). Significantly, based on the strong in-plane anisotropy of 2D Ta₂NiSe₅ nanosheets, the DHJ device shows competitive polarization sensitivities of 13.9 and 14.8 under 635 and 808 nm light, respectively. Furthermore, an excellent self-powered visible imaging capability based on the DHJ device is demonstrated. These results pave a promising platform for realizing self-powered photodetectors with high performance and multifunctionality.

Dielectrics for Two-Dimensional Transition-Metal Dichalcogenide Applications

Despite over a decade of intense research efforts, the full potential of two-dimensional transition-metal dichalcogenides continues to be limited by major challenges. The lack of compatible and scalable dielectric materials and integration techniques restrict device performances and their commercial applications. Conventional dielectric integration techniques for bulk semiconductors are difficult to adapt for atomically thin two-dimensional materials. This review provides a brief introduction into various common and emerging dielectric synthesis and integration techniques and discusses their applicability for 2D transition metal dichalcogenides. Dielectric integration for various applications is reviewed in subsequent sections including nanoelectronics, optoelectronics, flexible electronics, valleytronics, biosensing, quantum information processing, and quantum sensing. For each application, we introduce basic device working principles, discuss the specific dielectric requirements, review current progress, present key challenges, and offer insights into future prospects and opportunities.

Facile construction of a robust CuS@NaNbO3 nanorod composite: A unique p-n heterojunction structure with superior performance in photocatalytic hydrogen evolution

The construction of heterojunctions is commonly regarded as an efficient way to promote the production of hydrogen via photocatalytic water splitting through the enhancement of interfacial interactions. The p-n heterojunction is an important kind of heterojunction with an inner electric field based on the different properties of semiconductors. In this work, we reported the synthesis of a novel CuS/NaNbO₃ p-n heterojunction by depositing CuS nanoparticles on the external surface of NaNbO₃ nanorods, using a facile calcination and hydrothermal method. Through the screening of different ratios, the optimum hydrogen production activity reached 1603 μmol·g^-1·h^-1, which is much higher than that of NaNbO₃ (3.6 times) and CuS (2.7 times). Subsequent characterizations proved semiconductor properties and the existence of p-n heterojunction interactions between the two materials, which inhibited the recombination of photogenerated carriers and improved the efficiency of electron transfer. This work provides a meaningful strategy to utilize the p-n heterojunction structure for the promotion of photocatalytic hydrogen production.

The effects of intercalated environmental gas molecules on carrier dynamics in WSe2/WS2 heterostructures

Effective tuning of carrier dynamics in two-dimensional (2D) materials is significant for multi-scene device applications. Using first-principles and ab initio nonadiabatic molecular dynamics calculations, the kinetics of O₂, H₂O, and N₂ intercalation into 2D WSe₂/WS₂ van der Waals heterostructures and its effect on carrier dynamics have been comprehensively explored. It is found that the O₂ molecule prefers to dissociate into atomic O atoms spontaneously after intercalation of WSe₂/WS₂ heterostructures, whereas H₂O and N₂ molecules remain intact. O₂ intercalation significantly speeds up the electron separation process, while H₂O intercalation largely speeds up the hole separation process. The lifetime of excited carriers can be prolonged by O₂ or H₂O or N₂ intercalations. These intriguing phenomena can be attributed to the effect of interlayer coupling, and the underlying physical mechanism for tuning the carrier dynamics is fully discussed. Our results provide useful guidance for the experimental design of 2D heterostructures for optoelectronic applications in photocatalysts and solar energy cells.

Gate-Tunable van der Waals Photodiodes with an Ultrahigh Peak-to-Valley Current Ratio

Photodetectors and imagers based on 2D layered materials are currently subject to a rapidly expanding application space, with an increasing demand for cost-effective and lightweight devices. However, the underlying carrier transport across the 2D homo- or heterojunction channel driven by the external electric field, like a gate or drain bias, is still unclear. Here, a visible-near infrared photodetector based on van der Waals stacked molybdenum telluride (MoTe₂ ) and black phosphorus (BP) is reported. The type-I and type-II band alignment can be tuned by the gate and drain voltage combined showing a dynamic modulation of the conduction polarity and negative differential transconductance. The heterojunction devices show a good photoresponse to light illumination ranging from 520-2000 nm. The built-in potential at the MoTe₂ /BP interface can efficiently separate photoexcited electron-hole pairs with a high responsivity of 290 mA W^-1 , an external quantum efficiency of 70%, and a fast photoresponse of 78 µs under zero bias.

Solar cells based on 2D Janus group-III chalcogenide van der Waals heterostructures

Janus monolayers, realized by breaking the vertical structural symmetry of two-dimensional (2D) materials, pave the way for a new era of high-quality and high-performance atomically-thin vertical p-n heterojunction solar cells. Herein, employing first-principles computations, Janus group-III chalcogenide monolayers, MX, M₂XY, MM'X₂ and MM'XY (M, M' = Ga, In; X, Y = S, Se, Te), are deeply investigated in view of their implementation in 2D photovoltaic systems. Their stability analysis reveals that the 21 investigated monolayers are energetically, thermodynamically, mechanically, dynamically, and thermally stable, confirming their growth feasibility under ambient conditions. Furthermore, owing to their optimal band gap, high charge carrier mobilities, and strong light absorption, 2D Janus group-III monolayers are predicted as promising candidates for 2D excitonic solar cell applications. In fact, 46 type-II van der Waals (vdW) heterostructures with a lattice mismatch of less than 5% are identified by analyzing the band alignments of the investigated monolayers obtained through the HSE + SOC approach. In particular, 7 vertical vdW heterojunctions with a power conversion efficiency (PCE) higher than 20% are predicted and might be the focus of future experimental and theoretical studies. To further confirm the type II band alignment, the Ga₂STe-GaInS₂ vdW heterostructure, which reveals the highest PCE of 23.69%, is thoroughly investigated. Our results not only predict and evaluate stable 2D Janus group-III chalcogenide monolayers and vdW heterostructures, but also suggest that they could be used as materials for next-generation optoelectronic and photovoltaic devices.

Type-II Bi2O2Se/MoTe2 van der Waals Heterostructure Photodetectors with High Gate-Modulation Photovoltaic Performance

In recent years, two-dimensional (2D) nonlayered Bi₂O₂Se-based electronics and optoelectronics have drawn enormous attention owing to their high electron mobility, facile synthetic process, stability to the atmosphere, and moderate narrow band gaps. However, 2D Bi₂O₂Se-based photodetectors typically present large dark current, relatively slow response speed, and persistent photoconductivity effect, limiting further improvement in fast-response imaging sensors and low-consumption broadband detection. Herein, a Bi₂O₂Se/2H-MoTe₂ van der Waals (vdWs) heterostructure obtained from the chemical vapor deposition (CVD) approach and vertical stacking is reported. The proposed type-II staggered band alignment desirable for suppression of dark current and separation of photoinduced carriers is confirmed by density functional theory (DFT) calculations, accompanied by strong interlayer coupling and efficient built-in potential at the junction. Consequently, a stable visible (405 nm) to near-infrared (1310 nm) response capability, a self-driven prominent responsivity (R) of 1.24 A·W^-1, and a high specific detectivity (D*) of 3.73 × 10¹¹ Jones under 405 nm are achieved. In particular, R, D*, fill factor, and photoelectrical conversion efficiency (PCE) can be enhanced to 4.96 A·W^-1, 3.84 × 10¹² Jones, 0.52, and 7.21% at V_g = -60 V through a large band offset originated from the n⁺-p junction. It is suggested that the present vdWs heterostructure is a promising candidate for logical integrated circuits, image sensors, and low-power consumption detection.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Chemical Dopant-Free Controlled MoTe2/MoSe2 Heterostructure toward a Self-Driven Photodetector and Complementary Logic Circuits

Two-dimensional (2D) van der Waals heterostructures based on transition metal dichalcogenides are expected to be unique building blocks for next-generation nanoscale electronics and optoelectronics. The ability to control the properties of 2D heterostructures is the key for practical applications. Here, we report a simple way to fabricate a high-performance self-driven photodetector based on the MoTe₂/MoSe₂ p-n heterojunction, in which the hole-dominated transport polarity of MoTe₂ is easily achieved via a straightforward thermal annealing treatment in air without any chemical dopants or special gases needed. A high photoresponsivity of 0.72 A W^-1, an external quantum efficiency up to 41.3%, a detectivity of 7 × 10¹¹ Jones, and a response speed of 120 μs are obtained at zero bias voltage. Additionally, this doping method is also utilized to realize a complementary inverter with a voltage gain of 24. By configuring 2D p-MoTe₂ and n-MoSe₂ on demand, logic functions of NAND and NOR gates are also accomplished successfully. These results present a significant potential toward future larger-scale heterogeneously integrated 2D electronics and optoelectronics.

Research Process on Photodetectors based on Group-10 Transition Metal Dichalcogenides

Rapidly evolving group-10 transition metal dichalcogenides (TMDCs) offer remarkable electronic, optical, and mechanical properties, making them promising candidates for advanced optoelectronic applications. Compared to most TMDCs semiconductors, group-10-TMDCs possess unique structures, narrow bandgap, and influential physical properties that motivate the development of broadband photodetectors, specifically infrared photodetectors. This review presents the latest developments in the fabrication of broadband photodetectors based on conventional 2D TMDCs. It mainly focuses on the recent developments in group-10 TMDCs from the perspective of the lattice structure and synthesis techniques. Recent progress in group-10 TMDCs and their heterostructures with different dimensionality of materials-based broadband photodetectors is provided. Moreover, this review accounts for the latest applications of group-10 TMDCs in the fields of nanoelectronics and optoelectronics. Finally, conclusions and outlooks are summarized to provide perspectives for next-generation broadband photodetectors based on group-10 TMDCs.

Fabrication of MoS2/C60 Nanolayer Field-Effect Transistor for Ultrasensitive Detection of miRNA-155

As a major public health issue, early cancer detection is of great significance. A field-effect transistor (FET) based on an MoS₂/C₆₀ composite nanolayer as the channel material enhances device performance by adding a light source, allowing the ultrasensitive detection of cancer-related miRNA. In this work, atomic layer deposition (ALD) was used to deposit MoS₂ layer by layer, and C₆₀ was deposited by an evaporation coater to obtain a composite nanolayer with good surface morphology as the channel material of the FET. Based on the good absorption of C₆₀ by blue-violet light, a 405 nm laser was selected to irradiate the channel material, improving the function of FET biosensors. A linear detection window from 10 pM to 1 fM with an ultralow detection limit of 5.16 aM for miRNA-155 was achieved.

Two Dimensional Heterostructures for Optoelectronics: Current Status and Future Perspective

Researchers have found various families of two-dimensional (2D) materials and associated heterostructures through detailed theoretical work and experimental efforts. Such primitive studies provide a framework to investigate novel physical/chemical characteristics and technological aspects from micro to nano and pico scale. Two-dimensional van der Waals (vdW) materials and their heterostructures can be obtained to enable high-frequency broadband through a sophisticated combination of stacking order, orientation, and interlayer interactions. These heterostructures have been the focus of much recent research due to their potential applications in optoelectronics. Growing the layers of one kind of 2D material over the other, controlling absorption spectra via external bias, and external doping proposes an additional degree of freedom to modulate the properties of such materials. This mini review focuses on current state-of-the-art material design, manufacturing techniques, and strategies to design novel heterostructures. In addition to a discussion of fabrication techniques, it includes a comprehensive analysis of the electrical and optical properties of vdW heterostructures (vdWHs), particularly emphasizing the energy-band alignment. In the following sections, we discuss specific optoelectronic devices, such as light-emitting diodes (LEDs), photovoltaics, acoustic cavities, and biomedical photodetectors. Furthermore, this also includes a discussion of four different 2D-based photodetector configurations according to their stacking order. Moreover, we discuss the challenges that remain to be addressed in order to realize the full potential of these materials for optoelectronics applications. Finally, as future perspectives, we present some key directions and express our subjective assessment of upcoming trends in the field.

Model for Nanopore Formation in Two-Dimensional Materials by Impact of Highly Charged Ions

We present a first qualitative description of the atomic dynamics in two-dimensional (2D) materials induced by the impact of slow, highly charged ions. We employ a classical molecular dynamics simulation for the motion of the target atoms combined with a Monte Carlo model for the diffusive charge transport within the layer. Depending on the velocity of charge transfer (hopping time or hole mobility) and the number of extracted electrons which, in turn, depends on the charge state of the impinging ions, we find regions of stability of the 2D structure as well as parameter combinations for which nanopore formation due to Coulomb repulsion is predicted.

Enhanced photoelectric performance of MoSSe/MoS2 van der Waals heterostructures with tunable multiple band alignment

Janus MoSSe with mirror asymmetry has recently emerged as a new two-dimensional (2D) material with a sizeable out-of-plane dipole moment. Here, based on first-principles calculations, we theoretically investigate the electronic properties of two patterns of 2D MoSSe/MoS₂ van der Waals heterostructures (vdWHs). The electronic properties of MoSSe can be tuned by the intrinsic out-of-plane dipole field. When the Se side of the Janus layer faces the MoS₂ layer, the dipole field points from the MoSSe layer towards the MoS₂ layer, and the vdWH possesses a type-I band alignment which is desirable for light emission applications. With a reversal of the Janus layer, the intrinsic field inverts accordingly, and the band alignment becomes a typical type-II band alignment, which benefits carrier separation. Moreover, it possesses superior optical absorption (∼10⁵ cm^-1), and the calculated photocurrent density under visible-light radiation is up to 0.9 mA cm^-2 in the MoSSe/MoS₂ vdWH. Meanwhile, an external electric field and vertical strain can remarkably modulate the band alignment to switch it between type-I and type-II. Thus, MoSSe/MoS₂ vdWHs have promising applications in next-generation photovoltaic devices.