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Understanding the phase transition mechanisms in two-dimensional (2D) materials is a key to precisely tailor their properties at the nanoscale. Molybdenum ditelluride (MoTe2) exhibits multiple phases at room temperature, making it a promising candidate for phase-change applications. Here, we fabricate lateral 2H-Td interfaces with laser irradiation and probe their phase transitions from micro- to atomic scales with in situ heating in the transmission electron microscope (TEM). By encapsulating the MoTe2 with graphene protection layers, we create an in situ reaction cell compatible with atomic resolution imaging. We find that the Td-to-2H phase transition initiates at phase boundaries at low temperatures (200-225 degree C) and propagates anisotropically along the b-axis in a layer-by-layer fashion. We also demonstrate a fully reversible 2H-Td-2H phase transition cycle, which generates a coherent 2H lattice containing inversion domain boundaries. Our results provide insights on fabricating 2D hetero-phase devices with atomically sharp and coherent interfaces., Comment: 38 pages, 5 figures, 9 supplementary figures, and supplementary information

Heterointerfaces between two-dimensional (2D) materials and bulk metals determine the electrical and optical properties of their heterostructures. Although deposition of various metals on 2D materials has been studied, there is still a lack of studies on the interaction at the van der Waals (vdW) heterointerface between 2D materials and metals. Here, we report quasi-van der Waals (qvdW) epitaxial recrystallization of a gold thin film into crystallographically aligned single crystals by encapsulation annealing of a gold thin film with hexagonal boron nitride (hBN). When a polycrystalline gold thin film passivated with hBN was annealed, it was recrystallized into single gold crystals with a planar shape and crystallographic alignment with hBN due to a strong interaction between the gold film and hBN at the heterointerface. This reflects that a weak vdW force at the heterointerface is sufficiently strong to induce epitaxial recrystallization. Using this method, we fabricated a gold nanocrystal array with the same crystalline orientation and smooth top surface. Our work demonstrates a new method for epitaxial recrystallization of bulk crystals and provides a deep understanding of the interaction at the vdW heterointerface of 2D materials and metals.

Over the past few years, many efforts have been devoted to growing single-crystal graphene due to its great potential in future applications. However, a number of issues remain for single-crystal graphene growth, such as control of nanoscale defects and the substrate-dependent nonuniformity of graphene quality. In this work, we demonstrate a possible route toward single-crystal graphene by combining aligned nucleation of graphene nanograins on Cu/Ni (111) and sequential heat treatment over pregrown graphene grains. By use of a mobile hot-wire CVD system, prealigned grains were stitched into one continuous film with up to ∼97% single-crystal domains, compared to graphene grown on polycrystalline Cu, which was predominantly high-angle tilt boundary (HATB) domains. The single-crystal-like graphene showed remarkably high thermal conductivity and carrier mobility of ∼1349 W/mK at 350 K and ∼33 600 (38 400) cm

Transition metal dichalcogenides (TMDs) are promising candidates for the semiconductor industry owing to their superior electrical properties.

A photoluminescence mapping image of monolayer (1L) MoS2 clearly shows the difference in PL intensity at the boundary between bare SiO2 and plasma-treated SiO2 (3 min).

Materials in 2-dimensional (2D) form show remarkable properties and unexplored scientific phenomena compared to their bulk form. Layered, van der Waals (vdW) materials have an obvious 2D structure, whereas non-vdW materials have no preference to obtain 2D form. This limits the number of currently available 2D non-vDW materials. Here, we introduce a straightforward electrochemical method utilizing the angstrom-confinement of laminar reduced graphene oxide (rGO) nanochannels to obtain a porous polycrystalline network of 2D transition metal oxides (2D-TMO), a class of non-vdW materials. During synthesis the angstrom-confinement provides a thickness limitation, forcing sub-unit cell growth of 2D-TMO with oxygen and metal vacancies. We showcase that Cr2O3, a material without significant catalytic activity for OER in bulk form, can be activated as a high-performing catalyst if synthesized in the 2D sub-unit cell form. Our approach creates a strategy for combining the high activity of 2D form and high stability and robustness of bulk form. Our accessible method for obtaining 2D-TMO holds high promise to yield exciting properties for fundamental science and applications.

Previous studies indicate that the properties of graphene oxide (GO) can be significantly improved by enhancing its graphitic domain size through thermal diffusion and clustering of functional groups. Remarkably, this transition takes place below the decomposition temperature of the functional groups and thus allows fine tuning of graphitic domains without compromising with the functionality of GO. By studying the transformation of GO under mild thermal treatment, we directly observe this size enhancement of graphitic domains from originally ≤40 nm2 to >200 nm2 through an extensive transmission electron microscopy (TEM) study. Additionally, we confirm the integrity of the functional groups during this process by a comprehensive chemical analysis. A closer look into the process confirms the theoretical predicted relevance for the room temperature stability of GO and the development of the composition of functional groups is explained with reaction pathways from theoretical calculations. We further investigate the influence of enlarged graphitic domains on the hydration behaviour of GO and the catalytic performance of single atom catalysts supported by GO. Additionally, we show that the sheet resistance of GO is reduced by several orders of magnitude during the mild thermal annealing process.

Graphene is influenced by its surrounding environment, such as adsorbates, charged impurities, and interface traps, owing to its large surface area and ultra-thin thickness. Herein, the effect of substrate conditions on the doping and degradation of graphene is investigated. The hydroxyl (-OH) groups on the silicon dioxide (SiO2) substrate formed by oxygen plasma treatment altered the characteristics of the overlying graphene. On exposure to ultraviolet (UV) light, the p-doping level of graphene on oxygen-plasma-treated SiO2 (P-SiO2) increased and degradation occurred, while graphene on bare SiO2 showed no change. The graphene on P-SiO2 had higher reactivity due to doping induced by –OH groups on the SiO2 surface. The graphene field-effect transistors (G-FETs) on the P-SiO2 also showed the reduced carrier mobility and larger shift of charge neutral point. However, during UV exposure, the device showed sever degradation in electrical conductivity and failure after 60 min. Meanwhile, the device on the bare SiO2 showed negligible changes even after UV exposure. Our results unveil the origin of degradation in the graphene and show a way to prevent the unwanted changes or degradation of graphene, which is highly important for the practical application of graphene.

Materials in 2-dimensional (2D) form often show remarkable properties and unexplored scientific phenomena compared to their bulk form. Layered, van der Waals (vdW) materials have an obvious 2D structure, whereas non-vdW materials have no preference to obtain 2D form. This severely limits the number of currently available 2D non-vDW materials. Here, we introduce a straightforward electrochemical method utilizing the angstrom-confinement of laminar reduced graphene oxide (rGO) nanochannels to obtain 2D transition metal oxides (2D-TMO), a class of non-vdW materials. During synthesis the angstrom-confinement provides a thickness limitation, forcing a sub-unit cell growth of 2D-TMO with oxygen and metal vacancies. The resulting flexible sandwich structure of rGO sheets inserted by a porous polycrystalline network of 2D-TMO is created in centimetre scale. Our accessible method for obtaining 2D-TMO holds high promise to yield exciting properties for fundamental science and applications.

Metallic glass (MG), an intrinsic heterogeneous structure at the atomic scale, is one of the promising engineering materials with intriguing physical properties. MG often suffers from the fatigue issue caused by the repetitive mechanical loading, but it is still elusive how the local heterogeneity evolves and affects the macroscale fatigue and deformation against bulky external stress. In this study, we investigate the fatigue effect in Zr-Cu-Al ribbon using a bending fatigue method. We used scanning probe microscopy (SPM) in parallel with X-ray diffraction and X-ray photoelectron spectroscopy to figure out the loading effect on the local heterogeneities. The spatially resolved SPM images show that there is a local fluctuation of mechanical and electrical properties on the fatigued side along with morphological deformation compared to the unloaded side. Approaching the broken edge where the fatigue failure occurs, the decaying tendency is not only more dominant but also accelerated by surface oxidation of the fatigued regions. Our study provides a useful guideline on how to monitor structural changes of MGs under fatigue conditions in service and will open a door toward commercialization of high-performance structural engineering materials.

Carbon configuration is a critical factor in determining the electrical, tribological, and mechanical properties of carbon films. In this work, we synthesized carbon films on a silicate glass by sp...

Scanning transmission electron microscopy (STEM) is an indispensable tool for atomic-resolution structural analysis for a wide range of materials. The conventional analysis of STEM images is an extensive hands-on process, which limits efficient handling of high-throughput data. Here, we apply a fully convolutional network (FCN) for identification of important structural features of two-dimensional crystals. ResUNet, a type of FCN, is utilized in identifying sulfur vacancies and polymorph types of MoS

Scanning transmission electron microscopy (STEM) is an indispensable tool for atomic-resolution structural analysis for a wide range of materials. The conventional analysis of STEM images is an extensive hands-on process, which limits efficient handling of high-throughput data. Here we apply a fully convolutional network (FCN) for identification of important structural features of two-dimensional crystals. ResUNet, a type of FCN, is utilized in identifying sulfur vacancies and polymorph types of ${MoS_2}$ from atomic resolution STEM images. Efficient models are achieved based on training with simulated images in the presence of different levels of noise, aberrations, and carbon contamination. The accuracy of the FCN models toward extensive experimental STEM images is comparable to that of careful hands-on analysis. Our work provides a guideline on best practices to train a deep learning model for STEM image analysis and demonstrates FCN's application for efficient processing of a large volume of STEM data., 24 pages, 5 figures

Two-dimensional (2D) devices and their van der Waals (vdW) heterostructures attract considerable attention owing to their potential for next-generation logic and memory applications. In addition, 2D devices are projected to have high integration capabilities, while maintaining nanoscale thickness. However, the fabrication of 2D devices and their circuits is challenging because of the high precision required to etch and pattern ultrathin 2D materials for integration. Here, the fabrication of a graphene via contact architecture to electrically connect graphene electrodes (or leads) embedded in vdW heterostructures is demonstrated. Graphene via contacts comprising of edge and fluorinated graphene (FG) electrodes are fabricated by successive fluorination and plasma etching processes. A one-step fabrication process that utilizes the graphene contacts is developed for a vertically integrated complementary inverter based on n- and p-type 2D field-effect transistors (FETs). This study provides a promising method to fabricate vertically integrated 2D devices, which are essential in 2D material-based devices and circuits.

Covalent functionalization of the surface is more crucial in 2D materials than in conventional bulk materials because of their atomic thinness, large surface-to-volume ratio, and uniform surface chemical potential. Because 2D materials are composed of two surfaces with no dangling bond, covalent functionalization enables us to improve or precisely modify the electrical, mechanical, and chemical properties. In this review, we summarize the covalent functionalization methods and related changes in properties. First, we discuss possible sites for functionalization. Consequently, functionalization techniques are introduced, followed by the direct synthesis of functionalized 2D materials and characterization methods of functionalized 2D materials. Finally, we suggest how the issues may be solved to enlarge the research area and understanding of the chemistry of 2D materials. This review will help in understanding the functionalization of 2D materials.

Monolayer transition metal dichalcogenides (TMDs) are promising for optoelectronics because of their high optical quantum yield and strong light-matter interaction. In particular, the van der Waals (vdW) heterostructures consisting of monolayer TMDs sandwiched by large gap hexagonal boron nitride have shown great potential for novel optoelectronic devices. However, a complicated stacking process limits scalability and practical applications. Furthermore, even though lots of efforts, such as fabrication of vdW heterointerfaces, modification of the surface, and structural phase transition, have been devoted to preserve or modulate the properties of TMDs, high environmental sensitivity and damage-prone characteristics of TMDs make it difficult to achieve a controllable technique for surface/interface engineering. Here, we demonstrate a novel way to fabricate multiple two-dimensional (2D) vdW heterostructures consisting of alternately stacked MoS2 and MoOx with enhanced photoluminescence (PL). We directly oxidized multilayer MoS2 to a MoOx/1 L-MoS2 heterostructure with atomic layer precision through a customized oxygen plasma system. The monolayer MoS2 covered by MoOx showed an enhanced PL intensity 3.2 and 6.5 times higher in average than the as-exfoliated 1 L- and 2 L-MoS2 because of preserved crystallinity and compensated dedoping by MoOx. By using layer-by-layer oxidation and transfer processes, we fabricated the heterostructures of MoOx/MoS2/MoOx/MoS2, where the MoS2 monolayers are separated by MoOx. The heterostructures showed the multiplied PL intensity as the number of embedded MoS2 layers increases because of suppression of the nonradiative trion formation and interlayer decoupling between stacked MoS2 layers. Our work shows a novel way toward the fabrication of 2D material-based multiple vdW heterostructures and our layer-by-layer oxidation process is beneficial for the fabrication of high performance 2D optoelectronic devices.

The large-scale synthesis of two-dimensional transition metal dichalcogenides has been actively investigated in recent years. Here, we introduce a nonconventional synthesis process of 2-inch-scale ...

Platinum diselenide (PtSe2) is an emerging class of two-dimensional (2D) transition metal dichalcogenide (TMD) crystals recently gaining substantial interests owing to its extraordinary properties ...

Transition metal dichalcogenides (TMDs) are promising candidates for the semiconductor industry owing to their superior electrical properties. Their surface oxidation is of interest because their electrical properties can be easily modulated by an oxidized layer on top of them. Here, we demonstrate the XeF

Structural symmetry-breaking is a key strategy to modify the physical and chemical properties of two-dimensional transition metal dichalcogenides. However, little is known about defect formation during this process. Here, with atomic-scale microscopy, we investigate the evolution of defect formation in monolayer MoS2 exposed indirectly to hydrogen plasma. At the beginning of the treatment only top-layer sulfur atoms are removed, while vacancies and the molybdenum atomic layer are maintained. As processing continues, hexagonal-shaped nanocracks are generated along the zigzag edge during relaxation of defect-induced strain. As defect density increases, both photoluminescence and conductivity of MoS2 gradually decreases. Furthermore, MoS2 showed increased friction by 50% due to defect-induced contact stiffness. Our study reveals the details of defect formation during the desulfurization of MoS2 and helps to design the symmetry-breaking transition metal dichalcogenides, which is of relevance for applications including photocatalyst for water splitting, and Janus heterostructures. Desulfurization of MoS2 alters its chemical and physical properties by breaking structural symmetry. Here, the atomic-scale mechanistic pathway by which this occurs is investigated during plasma etching, and changes in chemical structure and physical properties are revealed.

van der Waals (vdW) materials have shown unique electrical and optical properties depending on the thickness due to strong interlayer interaction and symmetry breaking at the monolayer level. In co...

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.

Two-dimensional (2D) transition-metal dichalcogenides (2D TMDs) in the form of MX2 (M: transition metal, X: chalcogen) exhibit intrinsically anisotropic layered crystallinity wherein their material properties are determined by constituting M and X elements. 2D platinum diselenide (2D PtSe2) is a relatively unexplored class of 2D TMDs with noble-metal Pt as M, offering distinct advantages over conventional 2D TMDs such as higher carrier mobility and lower growth temperatures. Despite the projected promise, much of its fundamental structural and electrical properties and their interrelation have not been clarified, and so its full technological potential remains mostly unexplored. In this work, we investigate the structural evolution of large-area chemical vapor deposition (CVD)-grown 2D PtSe2 layers of tailored morphology and clarify its influence on resulting electrical properties. Specifically, we unveil the coupled transition of structural-electrical properties in 2D PtSe2 layers grown at a low temperature (i.e., 400 °C). The layer orientation of 2D PtSe2 grown by the CVD selenization of seed Pt films exhibits horizontal-to-vertical transition with increasing Pt thickness. While vertically aligned 2D PtSe2 layers present metallic transports, field-effect-transistor gate responses were observed with thin horizontally aligned 2D PtSe2 layers prepared with Pt of small thickness. Density functional theory calculation identifies the electronic structures of 2D PtSe2 layers undergoing the transition of horizontal-to-vertical layer orientation, further confirming the presence of this uniquely coupled structural-electrical transition. The advantage of low-temperature growth was further demonstrated by directly growing 2D PtSe2 layers of controlled orientation on polyimide polymeric substrates and fabricating their Kirigami structures, further strengthening the application potential of this material. Discussions on the growth mechanism behind the horizontal-to-vertical 2D layer transition are also presented.

The advent of 3D printing has had a disruptive impact in manufacturing and can potentially revolutionize industrial fields. Thermoplastic materials printable into complex structures are widely employed for 3D printing. Polylactic acid (PLA) is among the most promising polymers used for 3D printing, owing to its low cost, biodegradability, and nontoxicity. However, PLA is electrically insulating and mechanically weak; this limits its use in a variety of 3D printing applications. This study demonstrates a straightforward and environment-friendly method to fabricate conductive and mechanically reinforced PLA composites by incorporating graphene nanoplatelets (GNPs). To fully utilize the superior electrical and mechanical properties of graphene, liquid-exfoliated GNPs are dispersed in isopropyl alcohol without the addition of any surfactant and combined with PLA dissolved in chloroform. The GNP-PLA composites exhibit improved mechanical properties (improvement in tensile strength by 44% and maximum strain by 57%) even at a low GNP threshold concentration of 2 wt %. The GNP-PLA composites also exhibit an electrical conductivity of over 1 mS/cm at >1.2 wt %. The GNP-PLA composites can be 3D-printed into various features with electrical conductivity and mechanical flexibility. This work presents a new direction toward advanced 3D printing technology by providing higher flexibility in designing multifunctional 3D printed features.

Biomaterials and bioproducts have unique characteristics of being renewable, abundant, biodegradable, and having rough surfaces. In order to implement them into highly efficient triboelectric nanogenerator (TENG) applications, the contact electrode should be cheap, flexible, able to withstand outdoor environments, and have a rough surface. Here, microwave-welded single-walled carbon nanotubes (SWCNTs) are shown to effectively harvest the mechanical vibrational energy from biomaterials and bioproducts. Selective and flash microwave heating provides firm welding of SWCNTs to a polycarbonate substrate without significant losses in flexibility, transparency, and electrical conductivity. Microwave-welded SWCNT electrodes were successfully deployed as single-electrode TENGs to harvest energy from cellulose film, hanji paper, and cherry leaf. The cellulose- and paper-based TENGs showed the quite stable triboelectric outputs even after excessive contacts and a long period of time. The leaf-based TENG showed the significantly modified triboelectric outputs due to the moisture evaporation induced shrinkage and roughness of the surface. The SWCNT electrode generated ca. ten- and two-fold larger voltage and current, respectively, than those obtained using an indium-tin oxide (ITO) electrode. Using a fan-shaped leaf-based TENG, multiple light emitting diodes and a cellular phone were successfully powered without a battery. This work implies that the microwave-welded SWCNT electrode with rough pored surface and strong resistance against environmental shocks could be a good candidate for the outdoor biomaterials and indoor bioproducts implemented TENGs to harness random- and low-frequency vibrational energy.

Epitaxial growth, a crystallographically oriented growth induced by the chemical bonding between crystalline substrate and atomic building blocks, has been a key technique in the thin-film and heterostructure applications of semiconductors. However, the epitaxial growth technique is limited by different lattice mismatch and thermal expansion coefficients of dissimilar crystals. Two-dimensional (2D) materials with dangling bond-free van der Waals surfaces have been used as growth templates for the hetero-integration of highly mismatched materials. Moreover, the ultrathin nature of 2D materials also allows for remote epitaxial growth and confinement growth of quasi-2D materials via intercalation. Here, we review the hetero-dimensional growth on 2D substrates: van der Waals epitaxy (vdWE), quasi vdWE, and intercalation growth. We discuss the growth mechanism and fundamental challenges for vdWE on 2D substrates. We also examine emerging vdWE techniques that use epitaxial liftoff and confinement epitaxial growth in detail. Finally, we give a brief review of radiation effects in 2D materials and contrast the damage induced with their 3D counterparts.

The properties of metal-semiconductor junctions are often unpredictable because of non-ideal interfacial structures, such as interfacial defects or chemical reactions introduced at junctions. Black phosphorus (BP), an elemental two-dimensional (2D) semiconducting crystal, possesses the puckered atomic structure with high chemical reactivity, and the establishment of a realistic atomic-scale picture of BP's interface toward metallic contact has remained elusive. Here we examine the interfacial structures and properties of physically-deposited metals of various kinds on BP. We find that Au, Ag, and Bi form single-crystalline films with (110) orientation through guided van der Waals epitaxy. Transmission electron microscopy and X-ray photoelectron spectroscopy confirm that atomically sharp van der Waals metal-BP interfaces forms with exceptional rotational alignment. Under a weak metal-BP interaction regime, the BP's puckered structure play an essential role in the adatom assembly process and can lead to the formation of a single crystal, which is supported by our theoretical analysis and calculations. The experimental survey also demonstrates that the BP-metal junctions can exhibit various types of interfacial structures depending on metals, such as the formation of polycrystalline microstructure or metal phosphides. This study provides a guideline for obtaining a realistic view on metal-2D semiconductor interfacial structures, especially for atomically puckered 2D crystals., 27 pages, 5 figures

Neuromorphic systems, which emulate neural functionalities of a human brain, are considered to be an attractive next-generation computing approach, with advantages of high energy efficiency and fast computing speed. After these neuromorphic systems are proposed, it is demonstrated that artificial synapses and neurons can mimic neural functions of biological synapses and neurons. However, since the neuromorphic functionalities are highly related to the surface properties of materials, bulk material-based neuromorphic devices suffer from uncontrollable defects at surfaces and strong scattering caused by dangling bonds. Therefore, 2D materials which have dangling-bond-free surfaces and excellent crystallinity have emerged as promising candidates for neuromorphic computing hardware. First, the fundamental synaptic behavior is reviewed, such as synaptic plasticity and learning rule, and requirements of artificial synapses to emulate biological synapses. In addition, an overview of recent advances on 2D materials-based synaptic devices is summarized by categorizing these into various working principles of artificial synapses. Second, the compulsory behavior and requirements of artificial neurons such as the all-or-nothing law and refractory periods to simulate a spike neural network are described, and the implementation of 2D materials-based artificial neurons to date is reviewed. Finally, future challenges and outlooks of 2D materials-based neuromorphic devices are discussed.