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Van der Waals heterostructures are set as strong contenders for post‐CMOS quantum materials engineering. A major step for their systematic exploration and exploitation of technological component demonstrators resides in their eased large‐scale design. In this direction, the growth of artificial van der Waals 2D superlattices is presented here such as (MoS2/WS2)n, (WS2/WSe2)n, and (MoS2/WSe2)n with unit cells repetitions reaching n > 10. The fabrication of these materials is enabled by a fully automated in‐situ pulsed laser deposition (PLD) tool. This approach provides cm2 scale homogeneous superlattices with on‐demand material parameters tailoring (layer number, order, and composition). The process is rapid and simple compared to manual pickup exfoliation methods or to sequential transfers of single layers grown by techniques such as chemical vapor deposition, allowing a large repetition of the unit cells in a "mille‐feuille" cake configuration. The computational exploration of this family of superlattice materials sheds light on the potential for optoelectronic property design by shaping the band‐structure landscape while taking into account the influential effects induced by proximity. Overall, this large‐area approach is proposed as an entry point for the systematic design of complex van der Waals heterostructures. [ABSTRACT FROM AUTHOR]

Electrochemical water splitting has received tremendous attention as an eco‐friendly approach to produce hydrogen. Noble metals and their oxides are commonly used as electrocatalysts to reduce activation energy barriers for hydrogen and oxygen evolution reactions in high‐performance electrodes, but their cost, scarcity, and limited stability hinder widespread adoption of electrochemical water splitting. Further advancements are therefore needed to reduce reliance on noble metals and improve the long‐term stability. Herein, solution‐processed 2D van der Waals (vdW) p–n heterostructures as an interfacial layer between catalysts and the electrode are introduced to enhance the catalytic performance. These heterostructures are formed by sequentially assembling electrochemically exfoliated black phosphorus and molybdenum disulfide nanosheets into electronic‐grade p‐ and n‐type semiconductor thin films, with the scalability extending across tens‐of‐centimeter scale areas. Benefiting from the charge distribution and built‐in electric field developed upon heterojunction formation, the vdW heterostructure interfacial layer increases both the catalytic activity and stability of commercial Pt/C and Ir/C catalysts compared to when these catalysts are directly loaded onto electrodes. Additionally, the vdW heterostructure also serves as a template for synthesizing nanostructured Pt and Ir catalysts through electrodeposition, further enhancing the catalytic performance in terms of mass activity and stability. [ABSTRACT FROM AUTHOR]

2D layered semiconductors have attracted considerable attention for beyond‐Si complementary metal‐oxide‐semiconductor (CMOS) technologies. They can be prepared into ultrathin channel materials toward ultrascaled device architectures, including double‐gate field‐effect‐transistors (DGFETs). This work presents an experimental analysis of DGFETs constructed from chemical vapor deposition (CVD)‐grown monolayer (1L) molybdenum disulfide (MoS2) with atomic layer deposition (ALD) of hafnium oxide (HfO2) high‐k gate dielectrics (top and bottom). This extends beyond previous studies of DGFETs based mostly on exfoliated (few‐nm thick) MoS2 flakes, and advances toward large‐area wafer‐scale processing. Here, significant improvements in performance are obtained with DGFETs (i.e., improvements in ON/OFF ratio, ON‐state current, sub‐threshold swing, etc.) compared to single top‐gate FETs. In addition to multi‐gate device architectures (e.g., DGFETs), the scaling of the equivalent oxide thickness (EOT) is crucial toward improved electrostatics required for next‐generation transistors. However, the impact of EOT scaling on the characteristics of CVD‐grown MoS2 DGFETs remains largely unexplored. Thus, this work studies the impact of EOT scaling on subthreshold swing (SS) and gate hysteresis using current–voltage (I–V) measurements with varying sweep rates. The experimental analysis and results elucidate the basic mechanisms responsible for improvements in CVD‐grown 1L‐MoS2 DGFETs compared to standard top‐gate FETs. [ABSTRACT FROM AUTHOR]

The use of solar light to accelerate chemical processes (photocatalysis) has the potential to alleviate the pollution and energy crises. Thanks to their large surface area, unusual electronic structure, and abundance of low‐coordinate surface atoms, 2D semiconductors have shown enormous promise in photocatalytic applications. The synthesis, photoexcitation processes, design, and development of 2D semiconductor photocatalysts are thoroughly examined in this perspective, as well as their possible applications in air purification, solar energy conversion, organic synthesis, carbon capture and storage, and water treatment. This work highlights ongoing research efforts focused on improving the selectivity and efficiency of photocatalytic applications based on 2D semiconductors by means of hybrid systems, heterostructures, doping, and computational methodologies, together with open challenges. Finally, the integration of 2D semiconductor photocatalysts into indoor and outdoor environments is discussed, thereby facilitating the purification of air and water and generating clean energy, which assists in the pursuit of sustainable development objectives. [ABSTRACT FROM AUTHOR]

Previous work that studied hexagonal boron nitride (h‐BN) memristor DC resistive‐switching characteristics is extended to include an experimental understanding of their dynamic behavior upon programming or synaptic weight update. The focus is on the temporal resistive switching response to driving stimulus (programming voltage pulses) effecting conductance updates during training in neural network crossbar implementations. Test arrays are fabricated at the wafer level, enabled by the transfer of CVD‐grown few‐layer (8 layer) or multi‐layer (18 layer) h‐BN films. A comprehensive study of their temporal response under various conditions–voltage pulse amplitude, edge rate (pulse rise/fall times), and temperature–provides new insights into the resistive switching process toward optimized devices and improvements in their implementation of artificial neural networks. The h‐BN memristors can achieve multi‐state operation through ultrafast pulsed switching (< 25 ns) with high energy efficiency (≈10 pJ pulse−1). [ABSTRACT FROM AUTHOR]

Ferroelectric tunnel junctions (FTJs) have gained substantial attention as emerging electronic devices such as nonvolatile memory and artificial synapse, owing to their low power consumption and nonvolatile properties. In this work, a 2D semiconductor (2DS)/α‐In2Se3/metal FTJ structure is proposed that combines a semiconductor ferroelectric material and a semiconducting electrode. The incorporation of 2DS not only enhances the barrier height modulation but also provides an effective approach to mitigate the thermionic current leakage. Notably, the proposed MoS2/α‐In2Se3/Ti FTJs exhibit both room‐temperature negative differential resistance (NDR) effect and high tunnel electroresistance (TER) exceeding 104 simultaneously. Furthermore, the versatility of this structure extends to several 2DS (including MoS2, PdSe2, and SnSe2) and graphene electrodes to rationalize both tunneling and thermionic current transport mechanisms. The proposed 2DS/α‐In2Se3/metal FTJs present great superiority over existing structures in terms of robustness, temperature independence, high TER, and versatility for various potential application scenarios. [ABSTRACT FROM AUTHOR]

Two-dimensional materials give access to the ultimate physical limits of photonics with appealing properties for ultracompact optical components such as waveguides and modulators. Specifically, in monolayer semiconductors, a strong excitonic resonance leads to a sharp oscillation in permittivity from positive to even negative values. This extreme optical response enables surface exciton–polaritons to guide visible light bound to an atomically thin layer. However, such ultrathin waveguides support a transverse electric (TE) mode with low confinement and a transverse magnetic (TM) mode with short propagation. Here, we propose that realistic semiconductor–insulator–semiconductor superlattices comprising monolayer WS2 and hexagonal boron nitride (hBN) can improve the properties of both TE and TM modes. Compared to a single monolayer, a heterostructure with a 1-nm hBN spacer separating two monolayers enhances the confinement of the TE mode from 1.2 to around 0.5 μm, while the out-of-plane extension of the TM mode increases from 25 to 50 nm. We propose two simple additivity rules for mode confinement valid in the ultrathin film approximation for heterostructures with increasing spacer thickness. Stacking additional WS2 monolayers into superlattices further enhances the waveguiding properties. Our results underscore the potential of monolayer-based superlattices as a platform for visible-range nanophotonics with promising optical, electrical, and magnetic tunability. [ABSTRACT FROM AUTHOR]

Abstract Van der Waals heterostructures are set as strong contenders for post‐CMOS quantum materials engineering. A major step for their systematic exploration and exploitation of technological component demonstrators resides in their eased large‐scale design. In this direction, the growth of artificial van der Waals 2D superlattices is presented here such as (MoS2/WS2)n, (WS2/WSe2)n, and (MoS2/WSe2)n with unit cells repetitions reaching n > 10. The fabrication of these materials is enabled by a fully automated in‐situ pulsed laser deposition (PLD) tool. This approach provides cm2 scale homogeneous superlattices with on‐demand material parameters tailoring (layer number, order, and composition). The process is rapid and simple compared to manual pickup exfoliation methods or to sequential transfers of single layers grown by techniques such as chemical vapor deposition, allowing a large repetition of the unit cells in a “mille‐feuille” cake configuration. The computational exploration of this family of superlattice materials sheds light on the potential for optoelectronic property design by shaping the band‐structure landscape while taking into account the influential effects induced by proximity. Overall, this large‐area approach is proposed as an entry point for the systematic design of complex van der Waals heterostructures.

Abstract 2D layered semiconductors have attracted considerable attention for beyond‐Si complementary metal‐oxide‐semiconductor (CMOS) technologies. They can be prepared into ultrathin channel materials toward ultrascaled device architectures, including double‐gate field‐effect‐transistors (DGFETs). This work presents an experimental analysis of DGFETs constructed from chemical vapor deposition (CVD)‐grown monolayer (1L) molybdenum disulfide (MoS2) with atomic layer deposition (ALD) of hafnium oxide (HfO2) high‐k gate dielectrics (top and bottom). This extends beyond previous studies of DGFETs based mostly on exfoliated (few‐nm thick) MoS2 flakes, and advances toward large‐area wafer‐scale processing. Here, significant improvements in performance are obtained with DGFETs (i.e., improvements in ON/OFF ratio, ON‐state current, sub‐threshold swing, etc.) compared to single top‐gate FETs. In addition to multi‐gate device architectures (e.g., DGFETs), the scaling of the equivalent oxide thickness (EOT) is crucial toward improved electrostatics required for next‐generation transistors. However, the impact of EOT scaling on the characteristics of CVD‐grown MoS2 DGFETs remains largely unexplored. Thus, this work studies the impact of EOT scaling on subthreshold swing (SS) and gate hysteresis using current–voltage (I–V) measurements with varying sweep rates. The experimental analysis and results elucidate the basic mechanisms responsible for improvements in CVD‐grown 1L‐MoS2 DGFETs compared to standard top‐gate FETs.

Electrochemical water splitting has received tremendous attention as an eco‐friendly approach to produce hydrogen. Noble metals and their oxides are commonly used as electrocatalysts to reduce activation energy barriers for hydrogen and oxygen evolution reactions in high‐performance electrodes, but their cost, scarcity, and limited stability hinder widespread adoption of electrochemical water splitting. Further advancements are therefore needed to reduce reliance on noble metals and improve the long‐term stability. Herein, solution‐processed 2D van der Waals (vdW) p–n heterostructures as an interfacial layer between catalysts and the electrode are introduced to enhance the catalytic performance. These heterostructures are formed by sequentially assembling electrochemically exfoliated black phosphorus and molybdenum disulfide nanosheets into electronic‐grade p‐ and n‐type semiconductor thin films, with the scalability extending across tens‐of‐centimeter scale areas. Benefiting from the charge distribution and built‐in electric field developed upon heterojunction formation, the vdW heterostructure interfacial layer increases both the catalytic activity and stability of commercial Pt/C and Ir/C catalysts compared to when these catalysts are directly loaded onto electrodes. Additionally, the vdW heterostructure also serves as a template for synthesizing nanostructured Pt and Ir catalysts through electrodeposition, further enhancing the catalytic performance in terms of mass activity and stability.

2D transition metal dichalcogenides (TMDs) have emerged as a novel class of semiconductors with promising applications in optoelectronics, owing to their rich and tunable valley fine structure, known as valleytronics. Strain engineering in TMDs presents opportunities to tailor their valley fine structures and band alignment, which greatly expands the potential to investigate their intrinsic properties and improve device performance, thus opening a new field of straintronics. In this review, recent advances in strain‐engineered 2D TMDs are summarized, with a focus on new phenomena and applications enabled by precision tuning of valley physics. The underlying mechanisms and connections are delineated between strain‐induced modifications to the valley fine structures based on intravalley, intervalley, and interlayer band alignment in single and heterostructure TMDs. These insights allow targeted strain control strategies to be devised for optimizing optoelectronic characteristics. This review provides perspectives and guidance on the future directions of valley‐straintronics and flexible 2D optoelectronics using TMDs, highlighting the substantial promise of valley‐strain engineering in TMDs for fundamental valley physics studies as well as practical device applications. [ABSTRACT FROM AUTHOR]

Abstract Previous work that studied hexagonal boron nitride (h‐BN) memristor DC resistive‐switching characteristics is extended to include an experimental understanding of their dynamic behavior upon programming or synaptic weight update. The focus is on the temporal resistive switching response to driving stimulus (programming voltage pulses) effecting conductance updates during training in neural network crossbar implementations. Test arrays are fabricated at the wafer level, enabled by the transfer of CVD‐grown few‐layer (8 layer) or multi‐layer (18 layer) h‐BN films. A comprehensive study of their temporal response under various conditions–voltage pulse amplitude, edge rate (pulse rise/fall times), and temperature–provides new insights into the resistive switching process toward optimized devices and improvements in their implementation of artificial neural networks. The h‐BN memristors can achieve multi‐state operation through ultrafast pulsed switching (< 25 ns) with high energy efficiency (≈10 pJ pulse−1).

Chiral semiconductor nanostructures and nanoparticles are promising materials for applications in biological sensing, enantioselective separation, photonics, and spin-polarized devices. Here, we studied the induction of chirality in atomically thin only two-monolayer-thick CdSe nanoplatelets (NPLs) grown using a colloidal method and exchanged with L-alanine and L-phenylalanine as model thiol-free chiral ligands. We have developed a novel two-step approach to completely exchange the native oleic acid ligands for chiral amino acids at the basal planes of NPLs. We performed an analysis of the optical and chiroptical properties of the chiral CdSe nanoplatelets with amino acids, which was supplemented by an analysis of the composition and coordination of ligands. After the exchange, the nanoplatelets retained heavy-hole, light-hole, and spin-orbit split-off exciton absorbance and bright heavy-hole exciton luminescence. Capping with thiol-free enantiomer amino acid ligands induced the pronounced chirality of excitons in the nanoplatelets, as proven by circular dichroism spectroscopy, with a high dissymmetry g-factor of up to 3.4 × 10−3 achieved for heavy-hole excitons in the case of L-phenylalanine. [ABSTRACT FROM AUTHOR]

Artificial retina technologies aim to restore visual function by mimicking the natural processes of the eye. These biomimetic devices can convert light into electrical signals that the brain can interpret as visual information, bypassing damaged or non‐functional cells of the eye. To be effective, these devices should possess high sensitivity to light, high spatial resolution, biocompatibility, power efficiency, and so on. Recently, 2D semiconductor materials have appeared as a promising candidate for artificial retinal devices, thanks to their excellent optoelectronic properties, ultrathin body, flexible nature, and biocompatibility. Here the recent developments in the field of 2D semiconductors‐based optoelectronics for visual function recovery are reviewed and their potential applications are discussed. The photodetector, optoelectronic memory, and artificial synapse mechanisms and devices utilized in artificial systems that are based on 2D semiconductor materials are summarized. Additionally, a range of application scenarios for devices that are inspired by retinal cells and vision systems is explored. Finally, it is concluded with an overview of the critical technical challenges and strategies that must be addressed for the successful development of artificial retina technologies. It also highlights the potential for new applications in other fields, such as robotics and artificial intelligence. [ABSTRACT FROM AUTHOR]

The synergetic combination of transition metal dichalcogenides (TMDCs) with organic dye molecules in functional heterostructures is promising for various optoelectronic applications. Here resonance energy transfer (RET) from a red‐emitting WS2 monolayer (1L‐WS2) to a layer of near‐infrared (NIR) emitting organic dye molecules is demonstrated. It is found that the total photoluminescence (PL) yield of the heterostructures is up to a factor of eight higher as compared to the PL yield of pristine 1L‐WS2. This is attributed to the efficient conversion of the mostly non‐radiative excitons in 1L‐WS2 into radiative excitons in the dye layer. A type‐I energy level alignment of the 1L‐WS2/dye interface assures the emission of bright PL. From excitation density‐dependent PL experiments, it is concluded that RET prevails against defect‐assisted non‐radiative recombination as well as Auger‐type exciton‐exciton annihilation in 1L‐WS2. The work paves the way for employing organic dye molecules in heterostructures with TMDCs in nanoscale light‐emitting devices with improved efficiency and tunable color. [ABSTRACT FROM AUTHOR]

Near‐band‐edge emissions ranging from red (≈630 to 602 nm) to green (≈550 to 534 nm) and blue (≈494 to 480 nm) colors have been detected in full‐series GaSe1‐xSx (0≤x≤1) multilayers using micro‐photoluminescence (µPL) measurements from 4 to 300 K. The multilayered chalcogenides crystallize in a hexagonal structure, with observed mixed stacking phases of ε and β polymorphs through X‐ray diffraction and Raman measurements. The µPL results, along with experimental band‐edge characterization through thermoreflectance, identify the ε‐stacked phase as the crucial phase responsible for direct recombination and the emission of free excitons within the visible range. A mixed‐color white light is created by PL, which is emitted from the layered GaSe1‐xSx series of different colors, and is positioned at the center of the CIE coordination plot (i.e., white color). To assess the emission properties of both band edges and defects in the layered compounds, time‐resolved photoluminescence (TRPL) is employed, using an area mapping function. The photoluminescence decay lifetime increases as the sulfur content is increased, owing to the greater occurrence of mixed‐phase stacking faults near the β‐GaS end within the GaSe1‐xSx (0≤x≤1) series. The multilayer GaSe1‐xSx (0≤x≤1) represents a distinct 2D chalcogenide series well‐suited for emitting full‐color visible light. [ABSTRACT FROM AUTHOR]

At present, the available experimental data on the optical properties of layered III–VI monochalcogenide compounds are scattered and somewhat contradictory, although they are the parent materials for promising two-dimensional (2D) structures. This work is devoted to optical studies of bulk GaSe crystals, whose perfect structural properties are confirmed by Raman studies, observation of singlet-triplet splitting of 1.5 meV, and the polarized photoluminescence measurements from the sample edge. We analyze the band structure of GaSe, namely the sequence and energies of direct and indirect exciton transitions, using cw and time-resolved micro-photoluminescence measurements with variation of temperature. It turns out that the direct band gap in bulk GaSe is at 2.13 eV, close to calculated values. The indirect exciton transition is located ∼15 meV below the direct exciton (2.11 eV). Its intensity quickly quenches and characteristic decay time strongly shortens with increasing temperature, while the contribution of the direct exciton is relatively enhanced. [ABSTRACT FROM AUTHOR]

The exclusive features of two‐dimensional (2D) semiconductors, such as high surface‐to‐volume ratios, tunable electronic properties, and biocompatibility, provide promising opportunities for developing highly sensitive biosensors. However, developing practical biosensors that can promptly detect low concentrations of target analytes remains a challenging task. Here, a field‐effect‐transistor comprising n‐type transition metal dichalcogenide tin disulfide (SnS2) is developed over the hexagonal boron nitride (h‐BN) for the detection of streptavidin protein (Strep.) as a target analyte. A self‐designed receptor based on the pyrene‐lysine conjugated with biotin (PLCB) is utilized to maintain the sensitivity of the SnS2/h‐BN FET because of the π–π stacking. The detection capabilities of SnS2/h‐BN FET are investigated using both Raman spectroscopy and electrical characterizations. The real‐time electrical measurements exhibit that the SnS2/h‐BN FET is capable of detecting streptavidin at a remarkably low concentration of 0.5 pm, within 13.2 s. Additionally, the selectivity of the device is investigated by measuring its response against a Cow‐like serum egg white protein (BSA), having a comparative molecular weight to that of the streptavidin. These results indicate a high sensitivity and rapid response of SnS2/h‐BN biosensor against the selective proteins, which can have significant implications in several fields including point‐of‐care diagnostics, drug discovery, and environmental monitoring. [ABSTRACT FROM AUTHOR]

Advancements in display technology have primarily focused on discovering new materials to develop thin‐film transistors (TFTs) that complement mainstream technologies. The emerging 2D semiconductors are one of the most promising candidates due to their ultra‐thin thickness, exceptional electrical qualities, and large‐scale availability. However, these atomically thin materials are delicate and typically prepared through standard gate‐first fabrication processes, necessitating their transfer onto specific substrates. In this study, a demonstration of an in situ gate‐last process for 2D semiconductor‐based TFTs technology is presented. This approach bypasses the yield‐limiting transfer process, enabling large‐scale display applications. The as‐fabricated MoS2 TFTs retains their intrinsic properties with a current density reaching ≈≈10 µA µm−1. Additionally, it is successfully showcased that the two transistor‐one capacitor active‐matrix display driving circuits with a high pixel yield. The patterned matrix exhibits no crosstalk and can be driven by either the pulse amplitude modulation or pulse width modulation scheme, offering flexible applications. [ABSTRACT FROM AUTHOR]

Over the last few years, great advances have been achieved in exploration of high-mobility two-dimensional (2D) semiconductors such as metal chalcogenide InSe and noble-transition-metal dichalcogenide PdSe2. These materials are competitive candidates for constructing next-generation optoelectronic devices owing to their unique crystalline and electronic structures. Moreover, the optical and electronic properties of 2D materials can be efficiently modified via precisely engineering their band structures, which is critical for widening specific applications ranging from high-performance optoelectronics to catalysis and energy harvesting. In this review, we focus on the progress in bandgaps engineering of newly emerging high-mobility 2D semiconductors and their applications in optoelectronic devices, incorporating our recent study in the InSe and PdSe2 systems. First of all, we discuss the structure-property relationship of typical high-mobility 2D semiconductors (InSe and PdSe2). Next, we analyze several viable strategies for bandgap engineering, including thickness, strain or pressure, alloying, heterostructure, surface modification, intercalation, and so on. Furthermore, we summarize the optoelectronic devices fabricated with such high-mobility 2D semiconductors. The conclusion and outlook in this topic are finally presented. This review aims to provide valuable insights in bandgap engineering of newly emerging 2D semiconductors and explore their potential in future optoelectronic applications.

Abstract The exclusive features of two‐dimensional (2D) semiconductors, such as high surface‐to‐volume ratios, tunable electronic properties, and biocompatibility, provide promising opportunities for developing highly sensitive biosensors. However, developing practical biosensors that can promptly detect low concentrations of target analytes remains a challenging task. Here, a field‐effect‐transistor comprising n‐type transition metal dichalcogenide tin disulfide (SnS2) is developed over the hexagonal boron nitride (h‐BN) for the detection of streptavidin protein (Strep.) as a target analyte. A self‐designed receptor based on the pyrene‐lysine conjugated with biotin (PLCB) is utilized to maintain the sensitivity of the SnS2/h‐BN FET because of the π–π stacking. The detection capabilities of SnS2/h‐BN FET are investigated using both Raman spectroscopy and electrical characterizations. The real‐time electrical measurements exhibit that the SnS2/h‐BN FET is capable of detecting streptavidin at a remarkably low concentration of 0.5 pm, within 13.2 s. Additionally, the selectivity of the device is investigated by measuring its response against a Cow‐like serum egg white protein (BSA), having a comparative molecular weight to that of the streptavidin. These results indicate a high sensitivity and rapid response of SnS2/h‐BN biosensor against the selective proteins, which can have significant implications in several fields including point‐of‐care diagnostics, drug discovery, and environmental monitoring.

Joule heating‐induced temperature distribution is a critical thermal issue in 2D electronics, with implications for their performance optimization and potential device failure. Herein, the feasibility of using van der Waals integrated HfSe2 to indicate surface temperature distribution in 2D electronics is demonstrated. The air oxidation of HfSe2 is found to be sensitive to its thickness and ambient temperatures, with the resulting oxidation products varying accordingly. Joule heating from the operation of 2D devices rises the local temperature, which accelerates the air oxidation of HfSe2 and changes the composition of the oxidized HfSe2 accordingly. By observing the surface conditions of HfSe2 with a Raman spectroscope or an optical microscope, the temperature distribution is semiquantitatively revealed for a graphene ribbon device, the accuracy of which is verified by the finite element simulations. Using the HfSe2, temperature indicator provides a simple and rapid method for investigating thermal issues and related device failure mechanisms in 2D electronics. [ABSTRACT FROM AUTHOR]

Achieving low contact resistance (RC) is one of the major challenges in producing 2D FETs for future CMOS technology applications. In this work, the electrical characteristics for semimetal (Sb) and normal metal (Ti) contacted MoS2 devices are systematically analyzed as a function of top and bottom gate‐voltages (VTG and VBG). The semimetal contacts not only significantly reduce RC but also induce a strong dependence of RC on VTG, in sharp contrast to Ti contacts that only modulate RC by varying VBG. The anomalous behavior is attributed to the strongly modulated pseudo‐junction resistance (Rjun) by VTG, resulting from weak Fermi level pinning (FLP) of Sb contacts. In contrast, the resistances under both metallic contacts remain unchanged by VTG as metal screens the electric field from the applied VTG. Technology computer aided design simulations further confirm the contribution of VTG to Rjun, which improves overall RC of Sb‐contacted MoS2 devices. Consequently, the Sb contact has a distinctive merit in dual‐gated (DG) device structure, as it greatly reduces RC and enables effective gate control by both VBG and VTG. The results offer new insight into the development of DG 2D FETs with enhanced contact properties realized by using semimetals. [ABSTRACT FROM AUTHOR]

Two‐dimensional van der Waals heterostructures (2D vdWHs) have recently gained widespread attention because of their abundant and exotic properties, which open up many new possibilities for next‐generation nanoelectronics. However, practical applications remain challenging due to the lack of high‐throughput techniques for fabricating high‐quality vdWHs. Here, we demonstrate a general electrochemical strategy to prepare solution‐processable high‐quality vdWHs, in which electrostatic forces drive the stacking of electrochemically exfoliated individual assemblies with intact structures and clean interfaces into vdWHs with strong interlayer interactions. Thanks to the excellent combination of strong light absorption, interfacial charge transfer, and decent charge transport properties in individual layers, thin‐film photodetectors based on graphene/In2Se3 vdWHs exhibit great promise for near‐infrared (NIR) photodetection, owing to a high responsivity (267 mA W−1), fast rise (72 ms) and decay (426 ms) times under NIR illumination. This approach enables various hybrid systems, including graphene/In2Se3, graphene/MoS2 and graphene/MoSe2 vdWHs, providing a broad avenue for exploring emerging electronic, photonic, and exotic quantum phenomena. [ABSTRACT FROM AUTHOR]

Weak interlayer couplings at 2D van der Waals (vdW) interfaces fundamentally distinguish out‐of‐plane charge flow, the information carrier in vdW‐assembled vertical electronic and optical devices, from the in‐plane band transport processes. Here, the out‐of‐plane charge transport behavior in 2D vdW semiconducting transition metal dichalcogenides (SCTMD) is reported. The measurements demonstrate that, in the high electric field regime, especially at low temperatures, either electron or hole carrier Fowler–Nordheim (FN) tunneling becomes the dominant quantum transport process in ultrathin SCTMDs, down to monolayers. For few‐layer SCTMDs, sequential layer‐by‐layer FN tunneling is observed to dominate the charge flow, thus serving as a material characterization probe for addressing the Fermi level positions and the layer numbers of the SCTMD films. Furthermore, it is shown that the physical confinement of the electron or hole carrier wave packets inside the sub‐nm thick semiconducting layers reduces the vertical quantum tunneling probability, leading to an enhanced effective mass of tunneling carriers. [ABSTRACT FROM AUTHOR]

Following logic in the silicon semiconductor industry, the existence of native oxide and suitable fabrication technology is essential for 2D semiconductors in planar integronics, which are surface‐sensitive to typical coating technologies. To date, very few types of integronics are found to possess this feature. Herein, the 2D Bi2O2Te developed recently is reported to possess large‐area synthesis and controllable thermal oxidation behavior toward single‐crystal native oxides. This shows that surface‐adsorbed oxygen atoms are inclined to penetrate across [Bi2O2]n2n+ layers and bond with the underlying [Te]n2n− at elevated temperatures, transforming directly into [TeO4]n2n− with the basic architecture remaining stable. The oxide can be adjusted to form in an accurate layer‐by‐layer manner with a low‐stress sharp interface. The native oxide Bi2TeO6 layer (bandgap of ≈2.9 eV) exhibits visible‐light transparency and is compatible with wet‐chemical selective etching technology. These advances demonstrate the potential of Bi2O2Te in planar‐integrated functional nanoelectronics such as tunnel junction devices, field‐effect transistors, and memristors. [ABSTRACT FROM AUTHOR]

2D semiconductors, especially 2D transition metal dichalcogenides (TMDCs), have attracted ever‐growing attention toward extending Moore's law beyond silicon. Metal–organic chemical vapor deposition (MOCVD) has been widely considered as a scalable technique to achieve wafer‐scale TMDC films for applications. However, current MOCVD process usually suffers from small domain size with only hundreds of nanometers, in which dense grain boundary defects degrade the crystalline quality of the films. Here, a periodical varying‐temperature ripening (PVTR) process is demonstrated to grow wafer‐scale high crystalline TMDC films by MOCVD. It is found that the high‐temperature ripening significantly reduces the nucleation density and therefore enables single‐crystal domain size over 20 µm. In this process, no additives or etchants are involved, which facilitates low impurity concentration in the grown films. Atom‐resolved electron microscopy imaging, variable temperature photoluminescence (PL) spectroscopy, and electrical transport results further confirm comparable crystalline quality to those observed in mechanically exfoliated TMDC films. The research provides a scalable route to produce high‐quality 2D semiconducting films for applications in electronics and optoelectronics. [ABSTRACT FROM AUTHOR]

Abstract Achieving low contact resistance (RC) is one of the major challenges in producing 2D FETs for future CMOS technology applications. In this work, the electrical characteristics for semimetal (Sb) and normal metal (Ti) contacted MoS2 devices are systematically analyzed as a function of top and bottom gate‐voltages (VTG and VBG). The semimetal contacts not only significantly reduce RC but also induce a strong dependence of RC on VTG, in sharp contrast to Ti contacts that only modulate RC by varying VBG. The anomalous behavior is attributed to the strongly modulated pseudo‐junction resistance (Rjun) by VTG, resulting from weak Fermi level pinning (FLP) of Sb contacts. In contrast, the resistances under both metallic contacts remain unchanged by VTG as metal screens the electric field from the applied VTG. Technology computer aided design simulations further confirm the contribution of VTG to Rjun, which improves overall RC of Sb‐contacted MoS2 devices. Consequently, the Sb contact has a distinctive merit in dual‐gated (DG) device structure, as it greatly reduces RC and enables effective gate control by both VBG and VTG. The results offer new insight into the development of DG 2D FETs with enhanced contact properties realized by using semimetals.

Wafer-scale, conformal, two-dimensional (2D) TiO2-Ga2O3 n-p heterostructures with a thickness of less than 10 nm were fabricated on the Si/SiO2 substrates by the atomic layer deposition (ALD) technique for the first time with subsequent post-deposition annealing at a temperature of 250 °C. The best deposition parameters were established. The structure and morphology of 2D TiO2-Ga2O3 n-p heterostructures were characterized by the scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), etc. 2D TiO2-Ga2O3 n-p heterostructures demonstrated efficient photocatalytic activity towards methyl orange (MO) degradation at the UV light (λ = 254 nm) irradiation. The improvement of TiO2-Ga2O3 n-p heterostructure capabilities is due to the development of the defects on Ga2O3-TiO2 interface, which were able to trap electrons faster.

Wafer-scale, conformal, two-dimensional (2D) TiO2-Ga2O3 n-p heterostructures with a thickness of less than 10 nm were fabricated on the Si/SiO2 substrates by the atomic layer deposition (ALD) technique for the first time with subsequent post-deposition annealing at a temperature of 250 °C. The best deposition parameters were established. The structure and morphology of 2D TiO2-Ga2O3 n-p heterostructures were characterized by the scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), etc. 2D TiO2-Ga2O3 n-p heterostructures demonstrated efficient photocatalytic activity towards methyl orange (MO) degradation at the UV light (λ = 254 nm) irradiation. The improvement of TiO2-Ga2O3 n-p heterostructure capabilities is due to the development of the defects on Ga2O3-TiO2 interface, which were able to trap electrons faster.

Abstract Weak interlayer couplings at 2D van der Waals (vdW) interfaces fundamentally distinguish out‐of‐plane charge flow, the information carrier in vdW‐assembled vertical electronic and optical devices, from the in‐plane band transport processes. Here, the out‐of‐plane charge transport behavior in 2D vdW semiconducting transition metal dichalcogenides (SCTMD) is reported. The measurements demonstrate that, in the high electric field regime, especially at low temperatures, either electron or hole carrier Fowler–Nordheim (FN) tunneling becomes the dominant quantum transport process in ultrathin SCTMDs, down to monolayers. For few‐layer SCTMDs, sequential layer‐by‐layer FN tunneling is observed to dominate the charge flow, thus serving as a material characterization probe for addressing the Fermi level positions and the layer numbers of the SCTMD films. Furthermore, it is shown that the physical confinement of the electron or hole carrier wave packets inside the sub‐nm thick semiconducting layers reduces the vertical quantum tunneling probability, leading to an enhanced effective mass of tunneling carriers.

Two-dimensional (2D) semiconductors have attracted great attention to extend Moore’s law, which motivates the quest for fast growth of high-quality materials. However, taking MoS2 as an example, current methods yield 2D MoS2 with a low growth rate and poor quality with vacancy concentrations three to five orders of magnitude higher than silicon and other commercial semiconductors. Here, we develop a strategy of using an intermediate product of iodine as a transport agent to carry metal precursors efficiently for ultrafast growth of high-quality MoS2. The grown MoS2 has the lowest density of sulfur vacancies (~1.41×1012 cm−2) reported so far and excellent electrical properties with high on/off current ratios of 108 and carrier mobility of 175 cm2 V−1 s−1. Theoretical calculations show that by incorporating iodine, the nucleation barrier of MoS2 growth with sulfur-terminated edges reduces dramatically. The sufficient supply of precursor and low nucleation energy together boost the ultrafast growth of sub-millimeter MoS2 domains within seconds. This work provides an effective method for the ultrafast growth of 2D semiconductors with high quality, which will promote their applications.

2D magnetic materials hold promise for quantum and spintronic applications. 2D antiferromagnetic materials are of particular interest due to their relative insensitivity to external magnetic fields and higher switching speeds compared to 2D ferromagnets. However, their lack of macroscopic magnetization impedes detection and control of antiferromagnetic order, thus motivating magneto-electrical measurements for these purposes. Additionally, many 2D magnetic materials are ambient-reactive and electrically insulating or highly resistive below their magnetic ordering temperatures, which imposes severe constraints on electronic device fabrication and characterization. Herein, these issues are overcome via a fabrication protocol that achieves electrically conductive devices from the ambient-reactive 2D antiferromagnetic semiconductor NiI2. The resulting gate-tunable transistors show band-like electronic transport below the antiferromagnetic and multiferroic transition temperatures of NiI2, revealing a Hall mobility of 15 cm2 V-1 s-1 at 1.7 K. These devices also allow direct electrical probing of the thickness-dependent multiferroic phase transition temperature of NiI2 from 59 K (bulk) to 28 K (monolayer). [ABSTRACT FROM AUTHOR]

2D magnetic materials hold promise for quantum and spintronic applications. 2D antiferromagnetic materials are of particular interest due to their relative insensitivity to external magnetic fields and higher switching speeds compared to 2D ferromagnets. However, their lack of macroscopic magnetization impedes detection and control of antiferromagnetic order, thus motivating magneto-electrical measurements for these purposes. Additionally, many 2D magnetic materials are ambient-reactive and electrically insulating or highly resistive below their magnetic ordering temperatures, which imposes severe constraints on electronic device fabrication and characterization. Herein, these issues are overcome via a fabrication protocol that achieves electrically conductive devices from the ambient-reactive 2D antiferromagnetic semiconductor NiI2. The resulting gate-tunable transistors show band-like electronic transport below the antiferromagnetic and multiferroic transition temperatures of NiI2, revealing a Hall mobility of 15 cm2 V-1 s-1 at 1.7 K. These devices also allow direct electrical probing of the thickness-dependent multiferroic phase transition temperature of NiI2 from 59 K (bulk) to 28 K (monolayer). [ABSTRACT FROM AUTHOR]

Chiral nanostructures exhibiting different absorption of right- and left-handed circularly polarized light are of rapidly growing interest due to their potential applications in various fields. Here, we have studied the induction of chirality in atomically thin (0.6–1.2 nm thick) ZnSe and CdSe nanoplatelets grown by a colloidal method and coated with L-cysteine and N-acetyl-L-cysteine ligands. We conducted an analysis of the optical and chiroptical properties of atomically thin ZnSe and CdSe nanoplatelets, which was supplemented by a detailed analysis of the composition and coordination of ligands. Different signs of circular dichroism were shown for L-cysteine and N-acetyl-L-cysteine ligands, confirmed by different coordination of these ligands on the basal planes of nanoplatelets. A maximum value of the dissymmetry factor of (2–3) × 10−3 was found for N-acetyl-L-cysteine ligand in the case of the thinnest nanoplatelets. [ABSTRACT FROM AUTHOR]

Nanosheet (NS) vertical‐stacked complementary field‐effect transistors (CFETs), where the NS n‐FET and NS p‐FET are vertically stacked and controlled using a common gate, would result in maximum device footprint reduction. However, silicon‐based transistor will become invalid due to mobility degradation and leakage current rising when scaling the thickness of channel and dielectric. Here, it is experimentally demonstrated that CFET can scaling down to 1 nm channel thickness with excellent performance, where chemical vapor deposition (CVD) one layer (1L) WSe2 p‐type NS FET is vertically stacked on top of CVD 1L MoS2 n‐type NS FET. Bottom MoS2 NS FET achieves high on‐state current of ION = 3.3 × 10−5 A µm µm−1 and low off‐state current of IOFF = 3.3 × 10−13 A µm µm−1 at VDS = 0.7 V, with the subthreshold swing reaching 80 mV dec−1. Top WSe2 NS FET achieves high on‐state current of ION = 1.2 × 10−5 A µm µm−1 and IOFF = 4 × 10−11 A µm µm−1 at VDS = −0.7 V, while the subthreshold swing reaching 150 mV dec−1. Statistical data of 22 CFET devices demonstrate excellent uniformity toward large‐area applications. The CFET based on large‐scale 2D materials breaks the limit of channel scaling and provides a technological base for future high‐performance and low‐power electronics. [ABSTRACT FROM AUTHOR]

Black phosphorus (BP) has quickly gained popularity in the scientific community owing to its interesting semiconducting properties, such as direct bandgap, high mobility, and intrinsic ambipolar behavior. However, its sensitivity to oxygen, moisture, and other air species has restricted its integration into active devices. Here, the lithography‐free via‐encapsulation scheme to fabricate fully‐encapsulated BP‐based field‐effect transistors (FETs) is employed. The full encapsulation is achieved by sandwiching the BP layers between the top and bottom hexagonal boron nitride (hBN) layers; top hBN passivating the BP layer from the environment and bottom hBN acting as a spacer and suppressing charge transfer to the BP layer from the SiO2 substrate. The embedded via‐metal electrodes allow the authors to perform reliable electrical measurements of the BP FETs. Based on these results, it is found that the electronic properties of the via‐encapsulated BP FETs are significantly improved compared to unencapsulated devices. This further establishes that the via‐contacting scheme leads to superior results compared to graphene‐hBN heterostructures and bare hBN layers combined with evaporated metal contacts (both use top and bottom hBN to encapsulate BP) by revealing higher mobility, lower hysteresis, and long‐term ambient‐stability in BP FETs. [ABSTRACT FROM AUTHOR]

Gallium monochalcogenides (GaX, where X represents Te, Se, or S) have attracted significant attention in the development of 2D semiconductor materials owing to their specific optical and electrical characteristics. Multilayered mixed GaX compounds, ternary alloys of gallium chalcogenides, are mostly direct semiconductors and are considered excellent candidates for wide energy-range light-emitting materials for application in future optoelectronic devices. This review provides a thorough investigation into ternary alloys of gallium monochalcogenides, focusing on the GaTe 1−x Se x , GaSe 1−x S x , and GaTe 1−x S x series of layered semiconductor compounds. We provide a comprehensive overview of the methods used to grow these materials, analyze their crystal structures, and characterize their properties. Various growth methods and conditions and their material yields are described. Structural characterization methods reveal detailed information on the composition-driven variations in crystal structure and phase. An optical property analysis reveals the remarkable tunability of their bandgaps and emission spectra, establishing their potential for optoelectronics applications. The light emission range of the GaTe 1−x Se x series is from near-infrared (NIR) to visible (620–780 nm), while the GaSe 1−x S x series emits from the visible to the blue region (478–620 nm) achieving white light. The GaTe 1−x S x exhibits the most extensive emission range, spanning from NIR to the blue region (478–780 nm). Furthermore, GaTe 1−x S x exhibit high photocatalytic degradation activity for water splitting and organic pollutant degradation. Overall, this review highlights the promising prospects of ternary gallium chalcogenides for advancing future optoelectronics technologies. [ABSTRACT FROM AUTHOR]

Abstract Nanosheet (NS) vertical‐stacked complementary field‐effect transistors (CFETs), where the NS n‐FET and NS p‐FET are vertically stacked and controlled using a common gate, would result in maximum device footprint reduction. However, silicon‐based transistor will become invalid due to mobility degradation and leakage current rising when scaling the thickness of channel and dielectric. Here, it is experimentally demonstrated that CFET can scaling down to 1 nm channel thickness with excellent performance, where chemical vapor deposition (CVD) one layer (1L) WSe2 p‐type NS FET is vertically stacked on top of CVD 1L MoS2 n‐type NS FET. Bottom MoS2 NS FET achieves high on‐state current of ION = 3.3 × 10−5 A µm µm−1 and low off‐state current of IOFF = 3.3 × 10−13 A µm µm−1 at VDS = 0.7 V, with the subthreshold swing reaching 80 mV dec−1. Top WSe2 NS FET achieves high on‐state current of ION = 1.2 × 10−5 A µm µm−1 and IOFF = 4 × 10−11 A µm µm−1 at VDS = −0.7 V, while the subthreshold swing reaching 150 mV dec−1. Statistical data of 22 CFET devices demonstrate excellent uniformity toward large‐area applications. The CFET based on large‐scale 2D materials breaks the limit of channel scaling and provides a technological base for future high‐performance and low‐power electronics.

Devices operating with excitons have promising prospects for overcoming the dilemma of response time and integration in current generation of electron‐ or/and photon‐based elements and devices. In combination with the advantages of emerging twistronics and valleytronics, the atomically thin transition metal dichalcogenide semiconductors open up new opportunities for pursuing practical excitonic devices, where the strong exciton binding energy enables operating exciton at room temperature. The essential and foremost step toward exciton devices is the control of spatiotemporal exciton flux, which is density‐dependent and affected by the complex many‐body interactions. It can be effectively controlled by the strain, electric field, electron‐doping, and local dielectric environment. Intriguingly, exotic phenomena such as exciton condensation, electron‐hole liquid, exciton Hall effects, and exciton halo effects can be occurred in 2D exciton system, providing new possibilities for excitonic devices. Up to now, the proof‐of‐principle of room temperature exciton devices, including excitonic switching and transistor, exciton guides, and excitonic nanolaser, have been realized. Here the authors review the recent advances in molding 2D exciton flux from basic principle, manipulation, exotic phenomena to promising applications and discuss the opportunities and challenges in pushing the frontiers of room temperature excitonic devices. [ABSTRACT FROM AUTHOR]

As traditional silicon-based materials almost reach their limits in the post-Moore era, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been regarded as next-generation semiconductors for high-performance electrical and optical devices. Chemical vapor deposition (CVD) is a widely used technique for preparing large-area and high-quality TMDCs. Yet, it suffers from the challenge of transfer due to the strong interaction between 2D materials and substrates. The traditional PMMA-assisted wet etching method tends to induce damage, wrinkles, and inevitable polymer residues. In this work, we propose an etch-free and clean transfer method via a water intercalation strategy for TMDCs, ensuring a high-fidelity, wrinkle-free, and crack-free transfer with negligible residues. Furthermore, metal electrodes can also be transferred via this method and back-gate field-effect transistors (FETs) based on CVD-grown monolayer WSe 2 with van der Waals (vdW) metal/semiconductor contacts are fabricated. Compared to the PMMA-assisted transfer method (∼1.2 cm 2 V -1 s -1 hole mobility with ∼2 × 10 6 ON/OFF ratio), our high-fidelity transfer method significantly enhances the electrical performance of WSe 2 FET over one order of magnitude, achieving a hole mobility of ∼43 cm 2 V -1 s -1 and a high ON/OFF ratio of ∼5 × 10 7 in air at room temperature.

Solar-powered water evaporation as a clean and abundant renewable energy-efficient desalination technology provides a promising strategy to solve the shortage of freshwater resources. However, the development and application of solar vapor technology are hindered by the relatively low near-infrared photothermal conversion efficiency of existing materials and the lack of effective improvement strategies. In this work, the conductivity characteristics of 2D semiconductors are capitalized on the high visible light absorption and ultra-low thermal. Specifically, rare-earth ion dopants into SnSe nanosheets, significantly boosting their near-infrared photothermal conversion efficiency and solar water evaporation performance are introduced. Remarkably, the photothermal conversion efficiency of the doped SnSe nanosheets surged from 51.56% to 82.11%, surpassing many previously reported photothermal materials. Furthermore, leveraging these nanosheets with enhanced photothermal conversion efficiency, a solar interfacial evaporation system is constructed. The evaporation rate of 2.17 kg m -2  h -1 and the efficiency of 96.5% can be achieved at one solar irradiance, and it also has good salt-resistance properties. The findings demonstrate the potential of rare earth ion-doped 2D semiconductor nanosheets in solar water evaporation, paving the way for future sustainable desalination solutions., (© 2024 Wiley‐VCH GmbH.)

Controlling excitons at the nanoscale in semiconductor materials represents a formidable challenge in the quantum photonics and optoelectronics fields. Monolayers of transition metal dichalcogenides (TMDs) offer inherent 2D confinement and possess significant exciton binding energies, making them promising candidates for achieving electric-field-based confinement of excitons without dissociation. Exploiting the valley degree of freedom associated with these confined states further broadens the prospects for exciton engineering. Here, we show electric control of light polarization emitted from one-dimensional (1D) quantum-confined states in MoSe 2 . Building on previous reports of tunable trapping potentials and linearly polarized emission, we extend this understanding by demonstrating how nonuniform in-plane electric fields enable in situ control of these effects and highlight the role of gate-tunable valley hybridization in these localized states. Their polarization is entirely engineered through either the 1D confinement potential's geometry or an out-of-plane magnetic field. Controlling nonuniform in-plane electric fields in TMDs enables control of the energy (up to five times its line width), polarization state (from circular to linear), and position of 1D confined excitonic states (5 nm V -1 ).

2D semiconductors can drive advances in quantum science and technologies. However, they should be free of any contamination; also, the crystallographic ordering and coupling of adjacent layers and their electronic properties should be well-controlled, tunable, and scalable. Here, these challenges are addressed by a new approach, which combines molecular beam epitaxy and in situ band engineering in ultra-high vacuum of semiconducting gallium selenide (GaSe) on graphene. In situ studies by electron diffraction, scanning probe microscopy, and angle-resolved photoelectron spectroscopy reveal that atomically-thin layers of GaSe align in the layer plane with the underlying lattice of graphene. The GaSe/graphene heterostructure, referred to as 2semgraphene, features a centrosymmetric (group symmetry D 3d ) polymorph of GaSe, a charge dipole at the GaSe/graphene interface, and a band structure tunable by the layer thickness. The newly-developed, scalable 2semgraphene is used in optical sensors that exploit the photoactive GaSe layer and the built-in potential at its interface with the graphene channel. This proof of concept has the potential for further advances and device architectures that exploit 2semgraphene as a functional building block., (© 2024 The Author(s). Small published by Wiley‐VCH GmbH.)

Wafer-scale two-dimensional (2D) semiconductors with atomically thin layers are promising materials for fabricating optic and photonic devices. Bright-field microscopy is a widely utilized method for large-area characterization, layer number identification, and quality assessment of 2D semiconductors based on optical contrast. Out-of-focus microscopic images caused by instrumental focus drifts contained blurred and degraded structural and color information, hindering the reliability of automated layer number identification of 2D nanosheets. To achieve automated restoration and accurate characterization, deep-learning-based microscopic imagery deblurring (MID) was developed. Specifically, a generative adversarial network with an improved loss function was employed to recover both the structural and color information of out-of-focus low-quality images. 2D MoS2 grown by the chemical vapor deposition on a SiO2/Si substrate was characterized. Quantitative indexes including structural similarity (SSIM), peak signal-to-noise ratio, and CIE 1931 color space were studied to evaluate the performance of MID for deblurring of out-of-focus images, with a minimum value of SSIM over 90% of deblurred images. Further, a pre-trained U-Net model with an average accuracy over 80% was implemented to segment and predict the layer number distribution of 2D nanosheet categories (monolayer, bilayer, trilayer, multi-layer, and bulk). The developed automated microscopic image deblurring using MID and the layer number identification by the U-Net model allow for on-site, accurate, and large-area characterization of 2D semiconductors for analyzing local optical properties. This method may be implemented in wafer-scale industrial manufacturing and quality monitoring of 2D photonic devices. [ABSTRACT FROM AUTHOR]

In this study, we present the investigation of optical properties on a series of HfS2−xSex crystals with different Se compositions x changing from 0 to 2. We used the chemical-vapor transport method to grow these layered ternary compound semiconductors in bulk form. Their lattice constants and crystal properties were characterized by X-ray diffraction, high-resolution transmission electron microscopy, and Raman spectroscopy. We have performed absorption spectroscopies to determine their optical band-gap energies, which started from 2.012 eV with x = 0, and gradually shifts to 1.219 eV for x = 2. Furthermore, we measured the absorption spectroscopies at different temperatures in the range of 20–300 K to identify the temperature dependence of band-gap energies. The band-gap energies of HfS2−xSex were determined from the linear extrapolation method. We have noticed that the band-gap energy may be continuously tuned to the required energy by manipulating the ratio of S and Se. The parameters that describe the temperature influence on the band-gap energy are evaluated and discussed. [ABSTRACT FROM AUTHOR]

Surface charge transfer doping (SCTD) using oxygen plasma to form a p‐type dopant oxide layer on transition metal dichalcogenide (TMDs) is a promising doping technique for 2D TMDs field‐effect transistors (FETs). However, patternability of SCTD is a key challenge to effectively switch FETs. Herein, a simple method to selectively pattern degenerately p‐type (p+)‐doped WSe2 FETs via electron beam (e‐beam) irradiation is reported. The effect of the selective e‐beam irradiation is confirmed by the gate‐tunable optical responses of seamless lateral p+–p diodes. The OFF state of the devices by inducing trapped charges via selective e‐beam irradiation onto a desired channel area in p+‐doped WSe2, which is in sharp contrast to globally p+‐doped WSe2 FETs, is realized. Selective e‐beam irradiation of the PMMA‐passivated p+‐WSe2 enables accurate control of the threshold voltage (Vth) of WSe2 devices by varying the pattern size and e‐beam dose, while preserving the low contact resistance. By utilizing hBN as the gate dielectric, high‐performance WSe2 p‐FETs with a saturation current of −280 µA µm−1 and on/off ratio of 109 are achieved. This study's technique demonstrates a facile approach to obtain high‐performance TMD p‐FETs by e‐beam irradiation, enabling efficient switching and patternability toward various junction devices. [ABSTRACT FROM AUTHOR]

2D materials and their heterostructures are prominent for fabricating next‐generation optical and photonic devices. The optical, electrical, and mechanical properties of 2D materials largely depend on atomic layer numbers. Although machine learning techniques are implemented to identify large‐area thickness distribution using microscopic images, the existing work mainly focuses on rough identification of thicknesses with in‐house datasets which limits fair and comprehensive comparisons of new machine learning approaches. Here, first a microscopic dataset is collected and released for three fundamental image processing tasks including multilabel classification, segmentation, and detection. Then three deep‐learning architectures DenseNet, U‐Net, and Mask‐region convolutional neural network (RCNN) are benchmarked on three tasks and their robustness is evaluated on the augmented 2D microscopic images with different optical contrast variations. Deep learning models are trained and evaluated to identify mono‐, bi‐, tri‐, multilayer and bulk flakes using microscopic images of MoS2 fabricated on the SiO2/Si substrate by chemical vapor deposition. The relation between model performances and statistics of datasets is studied based on the international commission on illumination (CIE) 1931 color space and red, green, blue (RGB) histograms of optical contrast differences. Finally, the robust pretrained models are integrated into a graphic user interface for the on‐site use of full field‐of‐view images captured by bright‐field microscopes. [ABSTRACT FROM AUTHOR]