Centimeter-scale Green Integration of Layer-by-Layer 2D TMD vdW Heterostructures on Arbitrary Substrates by Water-Assisted Layer Transfer

Van der Waals Layer Transfer of 2D Materials for Monolithic 3D Electronic System Integration: Review and Outlook

Two-dimensional materials (2DMs) have attracted a great deal of interest due to their immense potential for scientific breakthroughs and technological innovations. While some 2D transition metal dichalcogenides (TMDC) such as MoS₂ and WS₂ are considered as the ultimate channel materials in unltrascaled transistors as replacements for Si, there has also been increasing interest in the monolithic 3D integration of 2DMs on the Si CMOS platform or in flexible electronics as back-end-of-line transistors, memory devices/selectors, and sensors, taking advantage of 2DM properties such as a high current driving capability with low leakage current, nonvolatile switching characteristics, a large surface-to-volume ratio, and a tunable bandgap. However, the realization of both of these scenarios critically depends on the development of manufacturing-viable high-yield 2DM layers transfer from the growth substrate to the Si, since the growth of high-quality 2DM layers often requires a high-temperature growth process on template substrates. Motivated by this, extensive efforts have been made by the 2DM research community to develop various 2DM layer transfer methods, leveraging the van der Waals transfer capability of the layer-structured 2DMs. These efforts have led to a number of successful demonstrations of wafer-scale 2D TMDC layer transfer, while 2DM-enabled template growth/transfer of some functional bulk materials such as III-V, Ge, and AlN has also been demonstrated. This review surveys and compares different 2DM transfer methods developed recently, with a focus on large-area 2D TMDC film transfer along with an introduction of 2DM template-assisted van der Waals growth/transfer of non-2D thin films. We will also briefly present an outlook of our envisioned multifunctionalities in 3D integrated electronic systems enabled by monolithic 3D integration of 2DMs and III-V via van der Waals transfer and discuss possible technology options for overcoming remaining challenges.

Nanomembrane-assembled nanophotonics and optoelectronics: from materials to applications

Nanophotonics and optoelectronics are the keys to the information transmission technology field. The performance of the devices crucially depends on the light-matter interaction, and it is found that three-dimensional (3D) structures may be associated with strong light field regulation for advantageous application. Recently, 3D assembly of flexible nanomembranes has attracted increasing attention in optical field, and novel optoelectronic device applications have been demonstrated with fantastic 3D design. In this review, we first introduce the fabrication of various materials in the form of nanomembranes. On the basis of the deformability of nanomembranes, 3D structures can be built by patterning and release steps. Specifically, assembly methods to build 3D nanomembrane are summarized as rolling, folding, buckling and pick-place methods. Incorporating functional materials and constructing fine structures are two important development directions in 3D nanophotonics and optoelectronics, and we settle previous researches on these two aspects. The extraordinary performance and applicability of 3D devices show the potential of nanomembrane assembly for future optoelectronic applications in multiple areas.

In situ synthesis of hierarchically-assembled three-dimensional ZnS nanostructures and 3D printed visualization

Nanomaterials have gained enormous interest in improving the performance of energy harvest systems, biomedical devices, and high-strength composites. Many studies were performed fabricating more elaborate and heterogeneous nanostructures then the structures were characterized using TEM tomographic images, upgrading the fabrication technique. Despite the effort, intricate fabrication process, agglomeration characteristic, and non-uniform output were still limited to presenting the 3D panoramic views straightforwardly. Here we suggested in situ synthesis method to prepare complex and hierarchically-assembled nanostructures that consisted of ZnS nanowire core and nanoparticles under Ag₂S catalyst. We demonstrated that the vaporized Zn and S were solidified in different shapes of nanostructures with the temperatures solely. To our knowledge, this is the first demonstration of synthesizing heterogeneous nanostructures, consisting of a nanowire from the vapor-liquid-solid and then nanoparticles from the vapor-solid grown mechanism by in situ temperature control. The obtained hierarchically-assembled ZnS nanostructures were characterized by various TEM technologies, verifying the crystal growth mechanism. Lastly, electron tomography and 3D printing enabled the nanoscale structures to visualize with centimeter scales. The 3D printing from randomly fabricated nanomaterials is rarely performed to date. The collaborating work could offer a better opportunity to fabricate advanced and sophisticated nanostructures.

Temperature dependent studies on centimeter-scale MoS2and vdW heterostructures

Transition metal dichalcogenides is an emerging 2D semiconducting material group which has excellent physical properties in the ultimately scaled thickness dimension. Specifically, van der Waals heterostructures hold the great promise in further advancing both the fundamental scientific knowledge and practical technological applications of 2D materials. Although 2D materials have been extensively studied for various sensing applications, temperature sensing still remains relatively unexplored. In this work, we experimentally study the temperature-dependent Raman spectroscopy and electrical conductivity of molybdenum disulfide (MoS₂) and its heterostructures with platinum dichalcogenides (PtSe₂and PtTe₂) to explore their potential to become the next-generation temperature sensor. It is found that the MoS₂-PtX₂heterostructure shows the great promise as the high-sensitivity temperature sensor.

Large-area vertically aligned 2D MoS2layers on TEMPO-cellulose nanofibers for biodegradable transient gas sensors

Crystallographically anisotropic two-dimensional (2D) molybdenum disulfide (MoS₂) with vertically aligned (VA) layers is attractive for electrochemical sensing owing to its surface-enriched dangling bonds coupled with extremely large mechanical deformability. In this study, we explored VA-2D MoS₂layers integrated on cellulose nanofibers (CNFs) for detecting various volatile organic compound gases. Sensor devices employing VA-2D MoS₂/CNFs exhibited excellent sensitivities for the tested gases of ethanol, methanol, ammonia, and acetone; e.g. a high response rate up to 83.39% for 100 ppm ethanol, significantly outperforming previously reported sensors employing horizontally aligned 2D MoS₂layers. Furthermore, VA-2D MoS₂/CNFs were identified to be completely dissolvable in buffer solutions such as phosphate-buffered saline solution and baking soda buffer solution without releasing toxic chemicals. This unusual combination of high sensitivity and excellent biodegradability inherent to VA-2D MoS₂/CNFs offers unprecedented opportunities for exploring mechanically reconfigurable sensor technologies with bio-compatible transient characteristics.

Peel-and-Stick Integration of Atomically Thin Nonlayered PtS Semiconductors for Multidimensionally Stretchable Electronic Devices

Various near-atom-thickness two-dimensional (2D) van der Waals (vdW) crystals with unparalleled electromechanical properties have been explored for transformative devices. Currently, the availability of 2D vdW crystals is rather limited in nature as they are only obtained from certain mother crystals with intrinsically possessed layered crystallinity and anisotropic molecular bonding. Recent efforts to transform conventionally non-vdW three-dimensional (3D) crystals into ultrathin 2D-like structures have seen rapid developments to explore device building blocks of unique form factors. Herein, we explore a "peel-and-stick" approach, where a nonlayered 3D platinum sulfide (PtS) crystal, traditionally known as a cooperate mineral material, is transformed into a freestanding 2D-like membrane for electromechanical applications. The ultrathin (∼10 nm) 3D PtS films grown on large-area (>cm²) silicon dioxide/silicon (SiO₂/Si) wafers are precisely "peeled" inside water retaining desired geometries via a capillary-force-driven surface wettability control. Subsequently, they are "sticked" on strain-engineered patterned substrates presenting prominent semiconducting properties, i.e., p-type transport with an optical band gap of ∼1.24 eV. A variety of mechanically deformable strain-invariant electronic devices have been demonstrated by this peel-and-stick method, including biaxially stretchable photodetectors and respiratory sensing face masks. This study offers new opportunities of 2D-like nonlayered semiconducting crystals for emerging mechanically reconfigurable and stretchable device technologies.

Low-power-consumption CMOS inverter array based on CVD-grown p-MoTe2 and n-MoS2

Two-dimensional (2D) semi-conductive transition metal dichalcogenides (TMDCs) have shown advantages for logic application. Complementary metal-oxide-semiconductor (CMOS) inverter is an important component in integrated circuits in view of low power consumption. So far, the performance of the reported TMDCs-based CMOS inverters is not satisfactory. Besides, most of the inverters were made of mechanically exfoliated materials, which hinders their reproducible production and large-scale integration in practical application. In this study, we demonstrate a practical approach to fabricate CMOS inverter arrays using large-area p-MoTe₂ and n-MoS₂, which are grown via chemical vapor deposition method. The current characteristics of the channel materials are balanced by atomic layer depositing Al₂O₃. Complete logic swing and clear dynamic switching behavior are observed in the inverters. Especially, ultra-low power consumption of ∼0.37 nW is achieved. Our work paves the way for the application of 2D TMDCs materials in large-scale low-power-consumption logic circuits.

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A practical approach to fabricate large-scale CMOS inverter arrays is demonstrated

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A method to balance the current characteristics of the channel materials is developed

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Complete logic swing and clear dynamic switching behavior are observed

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Ultra-low power consumption of ∼0.37 nW is achieved

Enhancing and quantifying spatial homogeneity in monolayer WS2

Controlling the radiative properties of monolayer transition metal dichalcogenides is key to the development of atomically thin optoelectronic devices applicable to a wide range of industries. A common problem for exfoliated materials is the inherent disorder causing spatially varying nonradiative losses and therefore inhomogeneity. Here we demonstrate a five-fold reduction in the spatial inhomogeneity in monolayer WS2, resulting in enhanced overall photoluminescence emission and quality of WS2 flakes, by using an ambient-compatible laser illumination process. We propose a method to quantify spatial uniformity using statistics of spectral photoluminescence mapping. Analysis of the dynamic spectral changes shows that the enhancement is due to a spatially sensitive reduction of the charged exciton spectral weighting. The methods presented here are based on widely adopted instrumentation. They can be easily automated, making them ideal candidates for quality assessment of transition metal dichalcogenide materials, both in the laboratory and industrial environments.

Unprecedentedly Uniform, Reliable, and Centimeter-Scale Molybdenum Disulfide Negative Differential Resistance Photodetectors

Negative differential resistance (NDR) can be applied to various devices such as reflection amplifiers, relaxation oscillators, and neuromorphic devices. However, the development of NDR photodetectors with uniformity, stability, and reproducibility for use in practical applications is still lacking. Herein, we demonstrate highly reliable NDR photodetectors by constructing a MoS₂/p-Si heterostructure. Owing to the formation of a MoS₂ layer with uniform thickness by the plasma-enhanced sulfurization process, a 100% yield with high uniformity (peak-to-valley ratio = 1.195 ± 0.065) was achieved for 120 devices. Furthermore, the proposed NDR photodetectors exhibit unprecedented high cycle-to-cycle endurance, which maintains their NDR characteristics through 100 000 consecutive sweeps without operational failure. This work paves the way for the development of a reliable NDR device and reports unprecedented results of high uniformity, reproducibility, and robustness for practical applications.

Mechanically rollable photodetectors enabled by centimetre-scale 2D MoS2 layer/TOCN composites

Two-dimensional (2D) molybdenum disulfide (MoS2) layers are suitable for visible-to-near infrared photodetection owing to their tunable optical bandgaps. Also, their superior mechanical deformability enabled by an extremely small thickness and van der Waals (vdW) assembly allows them to be structured into unconventional physical forms, unattainable with any other materials. Herein, we demonstrate a new type of 2D MoS2 layer-based rollable photodetector that can be mechanically reconfigured while maintaining excellent geometry-invariant photo-responsiveness. Large-area (>a few cm2) 2D MoS2 layers grown by chemical vapor deposition (CVD) were integrated on transparent and flexible substrates composed of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOCNs) by a direct solution casting method. These composite materials in three-dimensionally rollable forms exhibited a large set of intriguing photo-responsiveness, well preserving intrinsic opto-electrical characteristics of the integrated 2D MoS2 layers; i.e., light intensity-dependent photocurrents insensitive to illumination angles as well as highly tunable photocurrents varying with the rolling number of 2D MoS2 layers, which were impossible to achieve with conventional photodetectors. This study provides a new design principle for converting 2D materials to three-dimensional (3D) objects of tailored functionalities and structures, significantly broadening their potential and versatility in futuristic devices.

A wafer-scale synthesis of monolayer MoS2 and their field-effect transistors toward practical applications

Molybdenum disulfide (MoS2) has attracted considerable research interest as a promising candidate for downscaling integrated electronics due to the special two-dimensional structure and unique physicochemical properties. However, it is still challenging to achieve large-area MoS2 monolayers with desired material quality and electrical properties to fulfill the requirement for practical applications. Recently, a variety of investigations have focused on wafer-scale monolayer MoS2 synthesis with high-quality. The 2D MoS2 field-effect transistor (MoS2-FET) array with different configurations utilizes the high-quality MoS2 film as channels and exhibits favorable performance. In this review, we illustrated the latest research advances in wafer-scale monolayer MoS2 synthesis by different methods, including Au-assisted exfoliation, CVD, thin film sulfurization, MOCVD, ALD, VLS method, and the thermolysis of thiosalts. Then, an overview of MoS2-FET developments was provided based on large-area MoS2 film with different device configurations and performances. The different applications of MoS2-FET in logic circuits, basic memory devices, and integrated photodetectors were also summarized. Lastly, we considered the perspective and challenges based on wafer-scale monolayer MoS2 synthesis and MoS2-FET for developing practical applications in next-generation integrated electronics and flexible optoelectronics.

Temperature-dependent phonon dynamics and anharmonicity of suspended and supported few-layer gallium sulfide

Phonons play a fundamental role in the electronic and thermal transport of 2D materials which is crucial for device applications. In this work, we investigate the temperature-dependence of A[Formula: see text] and A[Formula: see text] Raman modes of suspended and supported mechanically exfoliated few-layer gallium sulfide (GaS), accessing their relevant thermodynamic Grüneisen parameters and anharmonicity. The Raman frequencies of these two phonons soften with increasing temperature with different [Formula: see text] temperature coefficients. The first-order temperature coefficients θ of A[Formula: see text] mode is ∼ -0.016 cm^-1/K, independent of the number of layers and the support. In contrast, the θ of A[Formula: see text] mode is smaller for two-layer GaS and constant for thicker samples (∼ -0.006 2 cm^-1 K^-1). Furthermore, for two-layer GaS, the θ value is ∼ -0.004 4 cm^-1 K^-1 for the supported sample, while it is even smaller for the suspended one (∼ -0.002 9 cm^-1 K^-1). The higher θ value for supported and thicker samples was attributed to the increase in phonon anharmonicity induced by the substrate surface roughness and Umklapp phonon scattering. Our results shed new light on the influence of the substrate and number of layers on the thermal properties of few-layer GaS, which are fundamental for developing atomically-thin GaS electronic devices.

Large-area 2D PtTe2/silicon vertical-junction devices with ultrafast and high-sensitivity photodetection and photovoltaic enhancement by integrating water droplets

2D PtTe2 layers, a relatively new class of 2D crystals, have unique band structure and remarkably high electrical conductivity promising for emergent opto-electronics. This intrinsic superiority can be further leveraged toward practical device applications by merging them with mature 3D semiconductors, which has remained largely unexplored. Herein, we explored 2D/3D heterojunction devices by directly growing large-area (>cm2) 2D PtTe2 layers on Si wafers using a low-temperature CVD method and unveiled their superior opto-electrical characteristics. The devices exhibited excellent Schottky transport characteristics essential for high-performance photovoltaics and photodetection, i.e., well-balanced combination of high photodetectivity (>1013 Jones), small photo-responsiveness time (∼1 μs), high current rectification ratio (>105), and water super-hydrophobicity driven photovoltaic improvement (>300%). These performances were identified to be superior to those of previously explored 2D/3D or 2D layer-based devices with much smaller junction areas, and their underlying principles were confirmed by DFT calculations.

Vertically Aligned 2D MoS2 Layers with Strain-Engineered Serpentine Patterns for High-Performance Stretchable Gas Sensors: Experimental and Theoretical Demonstration

Two-dimensional (2D) molybdenum disulfide (MoS₂) with vertically aligned (VA) layers exhibits significantly enriched surface-exposed edge sites with an abundance of dangling bonds owing to its intrinsic crystallographic anisotropy. Such structural variation renders the material with exceptionally high chemical reactivity and chemisorption ability, making it particularly attractive for high-performance electrochemical sensing. This superior property can be further promoted as far as it is integrated on mechanically stretchable substrates well retaining its surface-exposed defective edges, projecting opportunities for a wide range of applications utilizing its structural uniqueness and mechanical flexibility. In this work, we explored VA-2D MoS₂ layers configured in laterally stretchable forms for multifunctional nitrogen dioxide (NO₂) gas sensors. Large-area (>cm²) VA-2D MoS₂ layers grown by a chemical vapor deposition (CVD) method were directly integrated onto a variety of flexible substrates with serpentine patterns judiciously designed to accommodate a large degree of tensile strain. These uniquely structured VA-2D MoS₂ layers were demonstrated to be highly sensitive to NO₂ gas of controlled concentration preserving their intrinsic structural and chemical integrity, e.g., significant current response ratios of ∼160-380% upon the introduction of NO₂ at a level of 5-30 ppm. Remarkably, they exhibited such a high sensitivity even under lateral stretching up to 40% strain, significantly outperforming previously reported 2D MoS₂ layer-based NO₂ gas sensors of any structural forms. Underlying principles for the experimentally observed superiority were theoretically unveiled by density functional theory (DFT) calculation and finite element method (FEM) analysis. The intrinsic high sensitivity and large stretchability of VA-2D MoS₂ layers confirmed in this study are believed to be applicable in sensing diverse gas species, greatly broadening their versatility in stretchable and wearable technologies.

Two-Dimensional Near-Atom-Thickness Materials for Emerging Neuromorphic Devices and Applications

Two-dimensional (2D) layered materials and their heterostructures have recently been recognized as promising building blocks for futuristic brain-like neuromorphic computing devices. They exhibit unique properties such as near-atomic thickness, dangling-bond-free surfaces, high mechanical robustness, and electrical/optical tunability. Such attributes unattainable with traditional electronic materials are particularly promising for high-performance artificial neurons and synapses, enabling energy-efficient operation, high integration density, and excellent scalability. In this review, diverse 2D materials explored for neuromorphic applications, including graphene, transition metal dichalcogenides, hexagonal boron nitride, and black phosphorous, are comprehensively overviewed. Their promise for neuromorphic applications are fully discussed in terms of material property suitability and device operation principles. Furthermore, up-to-date demonstrations of neuromorphic devices based on 2D materials or their heterostructures are presented. Lastly, the challenges associated with the successful implementation of 2D materials into large-scale devices and their material quality control will be outlined along with the future prospect of these emergent materials.

Wafer-Scale Two-Dimensional MoS2 Layers Integrated on Cellulose Substrates Toward Environmentally Friendly Transient Electronic Devices

We explored the feasibility of wafer-scale two-dimensional (2D) molybdenum disulfide (MoS₂) layers toward futuristic environmentally friendly electronics that adopt biodegradable substrates. Large-area (> a few cm²) 2D MoS₂ layers grown on silicon dioxide/silicon (SiO₂/Si) wafers were delaminated and integrated onto a variety of cellulose-based substrates of various components and shapes in a controlled manner; examples of the substrates include planar papers, cylindrical natural rubbers, and 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose nanofibers. The integrated 2D layers were confirmed to well preserve their intrinsic structural and chemical integrity even on such exotic substrates. Proof-of-concept devices employing large-area 2D MoS₂ layers/cellulose substrates were demonstrated for a variety of applications, including photodetectors, pressure sensors, and field-effect transistors. Furthermore, we demonstrated the complete "dissolution" of the integrated 2D MoS₂ layers in a buffer solution composed of baking soda and deionized water, confirming their environmentally friendly transient characteristics. Moreover, the approaches to delaminate and integrate them do not demand any chemicals except for water, and their original substrates can be recycled for subsequent growths, ensuring excellent chemical benignity and process sustainability.

An Overview and Future Perspectives of Rechargeable Zinc Batteries

Aqueous rechargeable zinc-based batteries have sparked a lot of enthusiasm in the energy storage field recently due to their inherent safety, low cost, and environmental friendliness. Although remarkable progress has been made in the exploration of performance so far, there are still many challenges such as low working voltage and dissolution of electrode materials at the material and system level. Herein, the central tenet is to establish a systematic summary for the construction and mechanism of different aqueous zinc-based batteries. Details for three major zinc-based battery systems, including alkaline rechargeable Zn-based batteries (ARZBs), aqueous Zn ion batteries (AZIBs), and dual-ion hybrid Zn batteries (DHZBs) are given. First, the electrode materials and energy storage mechanism of the three types of zinc-based batteries are discussed to provide universal guidance for these batteries. Then, the electrode behavior of zinc anodes and strategies to deal with problems such as dendrite and passivation are recommended. Finally, some challenge-oriented solutions are provided to facilitate the next development of zinc-based batteries. Combining the characteristics of zinc-based batteries with good use of concepts and ideas from other disciplines will surely pave the way for its commercialization.

Wafer-scale 2D PtTe2 layers for high-efficiency mechanically flexible electro-thermal smart window applications

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers have gained increasing attention for a variety of emerging electrical, thermal, and optical applications. Recently developed metallic 2D TMD layers have been projected to exhibit unique attributes unattainable in their semiconducting counterparts; e.g., much higher electrical and thermal conductivities coupled with mechanical flexibility. In this work, we explored 2D platinum ditelluride (2D PtTe2) layers - a relatively new class of metallic 2D TMDs - by studying their previously unexplored electro-thermal properties for unconventional window applications. We prepared wafer-scale 2D PtTe2 layer-coated optically transparent and mechanically flexible willow glasses via a thermally-assisted tellurization of Pt films at a low temperature of 400 °C. The 2D PtTe2 layer-coated windows exhibited a thickness-dependent optical transparency and electrical conductivity of >106 S m-1 - higher than most of the previously explored 2D TMDs. Upon the application of electrical bias, these windows displayed a significant increase in temperature driven by Joule heating as confirmed by the infrared (IR) imaging characterization. Such superior electro-thermal conversion efficiencies inherent to 2D PtTe2 layers were utilized to demonstrate various applications, including thermochromic displays and electrically-driven defogging windows accompanying mechanical flexibility. Comparisons of these performances confirm the superiority of the wafer-scale 2D PtTe2 layers over other nanomaterials explored for such applications.

Automated Assembly of Wafer-Scale 2D TMD Heterostructures of Arbitrary Layer Orientation and Stacking Sequence Using Water Dissoluble Salt Substrates

We report a novel strategy to assemble wafer-scale two-dimensional (2D) transition metal dichalcogenide (TMD) layers of well-defined components and orientations. We directly grew a variety of 2D TMD layers on "water-dissoluble" single-crystalline salt wafers and precisely delaminated them inside water in a chemically benign manner. This manufacturing strategy enables the automated integration of vertically aligned 2D TMD layers as well as 2D/2D heterolayers of arbitrary stacking orders on exotic substrates insensitive to their kind and shape. Furthermore, the original salt wafers can be recycled for additional growths, confirming high process sustainability and scalability. The generality and versatility of this approach have been demonstrated by developing proof-of-concept "all 2D" devices for diverse yet unconventional applications. This study is believed to shed a light on leveraging opportunities of 2D TMD layers toward achieving large-area mechanically reconfigurable devices of various form factors at the industrially demanded scale.

Thickness-Independent Semiconducting-to-Metallic Conversion in Wafer-Scale Two-Dimensional PtSe2 Layers by Plasma-Driven Chalcogen Defect Engineering

Platinum diselenide (PtSe₂) is an emerging class of two-dimensional (2D) transition-metal dichalcogenide (TMD) crystals recently gaining substantial interest, owing to its extraordinary properties absent in conventional 2D TMD layers. Most interestingly, it exhibits a thickness-dependent semiconducting-to-metallic transition, i.e., thick 2D PtSe₂ layers, which are intrinsically metallic, become semiconducting with their thickness reduced below a certain point. Realizing both semiconducting and metallic phases within identical 2D PtSe₂ layers in a spatially well-controlled manner offers unprecedented opportunities toward atomically thin tailored electronic junctions, unattainable with conventional materials. In this study, beyond this thickness-dependent intrinsic semiconducting-to-metallic transition of 2D PtSe₂ layers, we demonstrate that controlled plasma irradiation can "externally" achieve such tunable carrier transports. We grew wafer-scale very thin (a few nm) 2D PtSe₂ layers by a chemical vapor deposition (CVD) method and confirmed their intrinsic semiconducting properties. We then irradiated the material with argon (Ar) plasma, which was intended to make it more semiconducting by thickness reduction. Surprisingly, we discovered a reversed transition of semiconducting to metallic, which is opposite to the prediction concerning their intrinsic thickness-dependent carrier transports. Through extensive structural and chemical characterization, we identified that the plasma irradiation introduces a large concentration of near-atomic defects and selenium (Se) vacancies in initially stoichiometric 2D PtSe₂ layers. Furthermore, we performed density functional theory (DFT) calculations and clarified that the band-gap energy of such defective 2D PtSe₂ layers gradually decreases with increasing defect concentration and dimensions, accompanying a large number of midgap energy states. This corroborative experimental and theoretical study decisively verifies the fundamental mechanism for this externally controlled semiconducting-to-metallic transition in large-area CVD-grown 2D PtSe₂ layers, greatly broadening their versatility for futuristic electronics.

Probing the Origin and Suppression of Vertically Oriented Nanostructures of 2D WS2 Layers

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) such as WS2 are promising materials for nanoelectronic applications. However, growth of the desired horizontal basal-plane oriented 2D TMD layers is often accompanied by the growth of vertical nanostructures that can hinder charge transport and, consequently, hamper device application. In this work, we discuss both the formation and suppression of vertical nanostructures during plasma-enhanced atomic layer deposition (PEALD) of WS2. Using scanning transmission electron microscopy studies, formation pathways of vertical nanostructures are established for a two-step (AB-type) PEALD process. Grain boundaries are identified as the principal formation centers of vertical nanostructures. Based on the obtained insights, we introduce an approach to suppress the growth of vertical nanostructures, wherein an additional step (C)-a chemically inert Ar plasma or a reactive H2 plasma-is added to the original two-step (AB-type) PEALD process. This approach reduces the vertical nanostructure density by 80%. It was confirmed that suppression of vertical nanostructures goes hand in hand with grain size enhancement. The vertical nanostructure density reduction consequently lowers film resistivity by an order of magnitude. Insights obtained in this work can contribute toward devising additional pathways, besides plasma treatments, for suppressing the growth of vertical nanostructures and improving the material properties of 2D TMDs that are relevant for nanoelectronic device applications.

Multifunctional Two-Dimensional PtSe2-Layer Kirigami Conductors with 2000% Stretchability and Metallic-to-Semiconducting Tunability

Two-dimensional transition-metal dichalcogenide (2D TMD) layers are highly attractive for emerging stretchable and foldable electronics owing to their extremely small thickness coupled with extraordinary electrical and optical properties. Although intrinsically large strain limits are projected in them (i.e., several times greater than silicon), integrating 2D TMDs in their pristine forms does not realize superior mechanical tolerance greatly demanded in high-end stretchable and foldable devices of unconventional form factors. In this article, we report a versatile and rational strategy to convert 2D TMDs of limited mechanical tolerance to tailored 3D structures with extremely large mechanical stretchability accompanying well-preserved electrical integrity and modulated transport properties. We employed a concept of strain engineering inspired by an ancient paper-cutting art, known as kirigami patterning, and developed 2D TMD-based kirigami electrical conductors. Specifically, we directly integrated 2D platinum diselenide (2D PtSe₂) layers of controlled carrier transport characteristics on mechanically flexible polyimide (PI) substrates by taking advantage of their low synthesis temperature. The metallic 2D PtSe₂/PI kirigami patterns of optimized dimensions exhibit an extremely large stretchability of ∼2000% without compromising their intrinsic electrical conductance. They also present strain-tunable and reversible photoresponsiveness when interfaced with semiconducting carbon nanotubes (CNTs), benefiting from the formation of 2D PtSe₂/CNT Schottky junctions. Moreover, kirigami field-effect transistors (FETs) employing semiconducting 2D PtSe₂ layers exhibit tunable gate responses coupled with mechanical stretching upon electrolyte gating. The exclusive role of the kirigami pattern parameters in the resulting mechanoelectrical responses was also verified by a finite-element modeling (FEM) simulation. These multifunctional 2D materials in unconventional yet tailored 3D forms are believed to offer vast opportunities for emerging electronics and optoelectronics.

Experimental Realization of Few Layer Two-Dimensional MoS2 Membranes of Near Atomic Thickness for High Efficiency Water Desalination

A globally imminent shortage of freshwater has been demanding viable strategies for improving desalination efficiencies with the adoption of cost- and energy-efficient membrane materials. The recently explored 2D transition metal dichalcogenides (2D TMDs) of near atomic thickness have been envisioned to offer notable advantages as high-efficiency membranes owing to their structural uniqueness; that is, extremely small thickness and intrinsic atomic porosity. Despite theoretically projected advantages, experimental realization of near atom-thickness 2D TMD-based membranes and their desalination efficiency assessments have remained largely unexplored mainly due to the technical difficulty associated with their seamless large-scale integration. Herein, we report the experimental demonstration of high-efficiency water desalination membranes based on few-layer 2D molybdenum disulfide (MoS₂) of only ∼7 nm thickness. Chemical vapor deposition (CVD)-grown centimeter-scale 2D MoS₂ layers were integrated onto porous polymeric supports with well-preserved structural integrity enabled by a water-assisted 2D layer transfer method. These 2D MoS₂ membranes of near atomic thickness exhibit an excellent combination of high water permeability (>322 L m^-2 h^-1 bar^-1) and high ionic sieving capability (>99%) for various seawater salts including Na⁺, K⁺, Ca²⁺, and Mg²⁺ with a range of concentrations. Moreover, they present near 100% salt ion rejection rates for actual seawater obtained from the Atlantic coast, significantly outperforming the previously developed 2D MoS₂ layer membranes of micrometer thickness as well as conventional reverse osmosis (RO) membranes. Underlying principles behind such remarkably excellent desalination performances are attributed to the intrinsic atomic vacancies inherent to the CVD-grown 2D MoS₂ layers as verified by aberration-corrected electron microscopy characterization.