Reversible Transition of Semiconducting PtSe2 and Metallic PtTe2 for Scalable All-2D Edge-Contacted FETs.

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers are highly promising as field-effect transistor (FET) channels in the atomic-scale limit. However, accomplishing this superiority in scaled-up FETs remains challenging due to their van der Waals (vdW) bonding nature with respect to conventional metal electrodes. Herein, we report a scalable approach to fabricate centimeter-scale all-2D FET arrays of platinum diselenide (PtSe2) with in-plane platinum ditelluride (PtTe2) edge contacts, mitigating the aforementioned challenges. We realized a reversible transition between semiconducting PtSe2 and metallic PtTe2 via a low-temperature anion exchange reaction compatible with the back-end-of-line (BEOL) processes. All-2D PtSe2 FETs seamlessly edge-contacted with transited metallic PtTe2 exhibited significant performance improvements compared to those with surface-contacted gold electrodes, e.g., an increase of carrier mobility and on/off ratio by over an order of magnitude, achieving a maximum hole mobility of ∼50.30 cm2 V-1 s-1 at room temperature. This study opens up new opportunities toward atomically thin 2D-TMD-based circuitries with extraordinary functionalities.

Thermally induced atomic reconstruction into fully commensurate structures of transition metal dichalcogenide layers.

Twist angle between two-dimensional layers is a critical parameter that determines their interfacial properties, such as moiré excitons and interfacial ferro-electricity. To achieve better control over these properties for fundamental studies and various applications, considerable efforts have been made to manipulate twist angle. However, due to mechanical limitations and the inevitable formation of incommensurate regions, there remains a challenge in attaining perfect alignment of crystalline orientation. Here we report a thermally induced atomic reconstruction of randomly stacked transition metal dichalcogenide multilayers into fully commensurate heterostructures with zero twist angle by encapsulation annealing, regardless of twist angles of as-stacked samples and lattice mismatches. We also demonstrate the selective formation of R- and H-type fully commensurate phases with a seamless lateral junction using chemical vapour-deposited transition metal dichalcogenides. The resulting fully commensurate phases exhibit strong photoluminescence enhancement of the interlayer excitons, even at room temperature, due to their commensurate structure with aligned momentum coordinates. Our work not only demonstrates a way to fabricate zero-twisted, two-dimensional bilayers with R- and H-type configurations, but also provides a platform for studying their unexplored properties.

In-plane anisotropic two-dimensional materials for twistronics.

In-plane anisotropic two-dimensional (2D) materials exhibit in-plane orientation-dependent properties. The anisotropic unit cell causes these materials to show lower symmetry but more diverse physical properties than in-plane isotropic 2D materials. In addition, the artificial stacking of in-plane anisotropic 2D materials can generate new phenomena that cannot be achieved in in-plane isotropic 2D materials. In this perspective we provide an overview of representative in-plane anisotropic 2D materials and their properties, such as black phosphorus, group IV monochalcogenides, group VI transition metal dichalcogenides with 1T' and Tdphases, and rhenium dichalcogenides. In addition, we discuss recent theoretical and experimental investigations of twistronics using in-plane anisotropic 2D materials. Both in-plane anisotropic 2D materials and their twistronics hold considerable potential for advancing the field of 2D materials, particularly in the context of orientation-dependent optoelectronic devices.

Gate-Tunable Electrostatic Friction of Grain Boundary in Chemical-Vapor-Deposited MoS2.

Two-dimensional (2D) semiconducting materials, such as MoS2, are widely studied owing to their great potential in advanced electronic devices. However, MoS2 films grown using chemical vapor deposition (CVD) exhibit lower-than-expected properties owing to numerous defects. Among them, grain boundary (GB) is a critical parameter that determines electrical and mechanical properties of MoS2. Herein, we report the gate-tunable electrostatic friction of GBs in CVD-grown MoS2. Using atomic force microscopy (AFM), we found that electrostatic friction of MoS2 is generated by the Coulomb interaction between tip and carriers of MoS2, which is associated with the local band structure of GBs. Therefore, electrostatic friction is enhanced by localized charge carrier distribution at GB, which is linearly related to the loading force of the tip. Our study shows a strong correlation between electrostatic friction and localized band structure in MoS2 GB, providing a novel method for identifying and characterizing GBs of polycrystalline 2D materials.

In Situ Imaging of an Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe2.

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 °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 heterophase devices with atomically sharp and coherent interfaces.

Laser-Induced Phase Transition and Patterning of hBN-Encapsulated MoTe2.

Transition metal dichalcogenides exhibit phase transitions through atomic migration when triggered by various stimuli, such as strain, doping, and annealing. However, since atomically thin 2D materials are easily damaged and evaporated from these strategies, studies on the crystal structure and composition of transformed 2D phases are limited. Here, the phase and composition change behavior of laser-irradiated molybdenum ditelluride (MoTe2 ) in various stacked geometry are investigated, and the stable laser-induced phase patterning in hexagonal boron nitride (hBN)-encapsulated MoTe2 is demonstrated. When air-exposed or single-side passivated 2H-MoTe2 are irradiated by a laser, MoTe2 is transformed into Te or Mo3 Te4 due to the highly accumulated heat and atomic evaporation. Conversely, hBN-encapsulated 2H-MoTe2 transformed into a 1T' phase without evaporation or structural degradation, enabling stable phase transitions in desired regions. The laser-induced phase transition shows layer number dependence; thinner MoTe2 has a higher phase transition temperature. From the stable phase patterning method, the low contact resistivity of 1.13 kΩ µm in 2H-MoTe2 field-effect transistors with 1T' contacts from the seamless heterophase junction geometry is achieved. This study paves an effective way to fabricate monolithic 2D electronic devices with laterally stitched phases and provides insights into phase and compositional changes in 2D materials.

STEM Image Analysis Based on Deep Learning: Identification of Vacancy Defects and Polymorphs of MoS2.

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 MoS2 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.

Improved Crystallinity of Graphene Grown on Cu/Ni (111) through Sequential Mobile Hot-Wire Heat Treatment.

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) cm2 V-1 s-1 for electrons (holes), respectively, which indicates that the crystallinity is high due to suppression of HATB domains.

Graphene Via Contact Architecture for Vertical Integration of vdW Heterostructure Devices.

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.

Recent trends in covalent functionalization of 2D materials.

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.

Tailoring Single- and Double-Sided Fluorination of Bilayer Graphene via Substrate Interactions.

While many technologies rely on multilayer heterostructures, most of the studies on chemical functionalization have been limited to monolayer graphene. In order to use functionalization in multilayer systems, we must first understand the interlayer interactions between functionalized and nonfunctionalized (intact) layers and how to selectively functionalize one layer at a time. Here, we demonstrate a method to fabricate single- or double-sided fluorinated bilayer graphene (FBG) by tailoring substrate interactions. Both the top and bottom surfaces of bilayer graphene on the rough silicon dioxide (SiO2) are fluorinated; meanwhile, only the top surface of graphene on hexagonal boron nitride (hBN) is fluorinated. The functionalization type affects electronic properties; double-sided FBG on SiO2 is insulating, whereas single-sided FBG on hBN maintains conducting, showing that the intact bottom layer becomes electrically decoupled from the fluorinated top insulating layer. Our results define a straightforward method to selectively functionalize the top and bottom surfaces of bilayer graphene.

Artificial Neuron and Synapse Devices Based on 2D Materials.

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.

Monolithic Interface Contact Engineering to Boost Optoelectronic Performances of 2D Semiconductor Photovoltaic Heterojunctions.

In optoelectronic devices based on two-dimensional (2D) semiconductor heterojunctions, the efficient charge transport of photogenerated carriers across the interface is a critical factor to determine the device performances. Here, we report an unexplored approach to boost the optoelectronic device performances of the WSe2-MoS2 p-n heterojunctions via the monolithic-oxidation-induced doping and resultant modulation of the interface band alignment. In the proposed device, the atomically thin WOx layer, which is directly formed by layer-by-layer oxidation of WSe2, is used as a charge transport layer for promoting hole extraction. The use of the ultrathin oxide layer significantly enhanced the photoresponsivity of the WSe2-MoS2 p-n junction devices, and the power conversion efficiency increased from 0.7 to 5.0%, maintaining the response time. The enhanced characteristics can be understood by the formation of the low Schottky barrier and favorable interface band alignment, as confirmed by band alignment analyses and first-principle calculations. Our work suggests a new route to achieve interface contact engineering in the heterostructures toward realizing high-performance 2D optoelectronics.

Tunable Wettability of Graphene through Nondestructive Hydrogenation and Wettability-Based Patterning for Bioapplications.

The wettability of graphene has been extensively studied and successfully modified by chemical functionalization. Nevertheless, the unavoidable introduction of undesired defects and the absence of systematic and local control over wettability by previous methods have limited the use of graphene in applications. In addition, microscale patterning, according to wettability, has not been attempted. Here, we demonstrate that the wettability of graphene can be systematically controlled and surface patterned into microscale sections based on wettability without creating significant defects, possible by nondestructive hydrogen plasma. Hydrophobic graphene is progressively converted to hydrophilic hydrogenated graphene (H-Gr) that reaches superhydrophilicity. The great contrast in wettability between graphene and H-Gr makes it possible to selectively position and isolate human breast cancer cells on arrays of micropatterns since strong hydrophilicity facilitates the adsorption of the cells. We believe that our method will provide an essential technique for enabling surface and biological applications requiring microscale patterns with different wettability.

Thickness-Insensitive Properties of α-MoO3 Nanosheets by Weak Interlayer Coupling.

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 contrast, the study of electrical and tribological properties and their thickness-insensitivity of van der Waals oxides are lacking due to difficulties in the fabrication of high quality two-dimensional oxides and the investigation of nanoscale properties. Here we investigated various tribological and electrical properties, such as, friction, adhesion, work function, tunnel current, and dielectric constant, of the single-crystal α-MoO3 nanosheets epitaxially grown on graphite by using atomic force microscopy. The friction of atomically smooth MoO3 is rapidly saturated within a few layers. The thickness insensitivity of friction is due to very weak mechanical interlayer interaction. Similarly, work function (4.73 eV for 2 layers (hereafter denoted as L)) and dielectric constant (6 for 2L and 10.5-11 for >3L) of MoO3 in MoO3 showed thickness insensitivity due to weak interlayer coupling. Tunnel current measurements by conductive atomic force microscopy showed that even 2L MoO3 of 1.4 nm is resistant to tunneling with a high dielectric strength of 14 MV/cm. The thickness-indifferent electrical properties of high dielectric constant and tunnel resistance by weak interlayer coupling and high crystallinity show a promise in the use of MoO3 nanosheets for nanodevice 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.

Tunable Electrical and Optical Characteristics in Monolayer Graphene and Few-Layer MoS2 Heterostructure Devices.

Lateral and vertical two-dimensional heterostructure devices, in particular graphene-MoS2, have attracted profound interest as they offer additional functionalities over normal two-dimensional devices. Here, we have carried out electrical and optical characterization of graphene-MoS2 heterostructure. The few-layer MoS2 devices with metal electrode at one end and monolayer graphene electrode at the other end show nonlinearity in drain current with drain voltage sweep due to asymmetrical Schottky barrier height at the contacts and can be modulated with an external gate field. The doping effect of MoS2 on graphene was observed as double Dirac points in the transfer characteristics of the graphene field-effect transistor (FET) with a few-layer MoS2 overlapping the middle part of the channel, whereas the underlapping of graphene have negligible effect on MoS2 FET characteristics, which showed typical n-type behavior. The heterostructure also exhibits a strongest optical response for 520 nm wavelength, which decreases with higher wavelengths. Another distinct feature observed in the heterostructure is the peak in the photocurrent around zero gate voltage. This peak is distinguished from conventional MoS2 FETs, which show a continuous increase in photocurrent with back-gate voltage. These results offer significant insight and further enhance the understanding of the graphene-MoS2 heterostructure.

Tightly bound trions in monolayer MoS2.

Two-dimensional (2D) atomic crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable physical properties. In contrast to graphene, monolayer MoS(2) is a non-centrosymmetric material with a direct energy gap. Strong photoluminescence, a current on/off ratio exceeding 10(8) in field-effect transistors, and efficient valley and spin control by optical helicity have recently been demonstrated in this material. Here we report the spectroscopic identification in a monolayer MoS(2) field-effect transistor of tightly bound negative trions, a quasiparticle composed of two electrons and a hole. These quasiparticles, which can be optically created with valley and spin polarized holes, have no analogue in conventional semiconductors. They also possess a large binding energy (~ 20 meV), rendering them significant even at room temperature. Our results open up possibilities both for fundamental studies of many-body interactions and for optoelectronic and valleytronic applications in 2D atomic crystals.

Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide.

Recent progress in large-area synthesis of monolayer molybdenum disulphide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chemical vapour deposition synthesis to grow highly crystalline islands of monolayer molybdenum disulphide up to 120 µm in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror twin boundaries that are stitched together by lines of 8- and 4-membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. We find that mirror twin boundaries cause strong photoluminescence quenching whereas tilt boundaries cause strong enhancement. Meanwhile, mirror twin boundaries slightly increase the measured in-plane electrical conductivity, whereas tilt boundaries slightly decrease the conductivity.

Graphene metallization of high-stress silicon nitride resonators for electrical integration.

High stress stoichiometric silicon nitride resonators, whose quality factors exceed one million, have shown promise for applications in sensing, signal processing, and optomechanics. Yet, electrical integration of the insulating silicon nitride resonators has been challenging, as depositing even a thin layer of metal degrades the quality factor significantly. In this work, we show that graphene used as a conductive coating for Si3N4 membranes reduces the quality factor by less than 30% on average, which is minimal when compared to the effect of conventional metallization layers such as chromium or aluminum. The electrical integration of Si3N4-Graphene (SiNG) heterostructure resonators is demonstrated with electrical readout and electrostatic tuning of the frequency by up to 0.3% per volt. These studies demonstrate the feasibility of hybrid graphene/nitride mechanical resonators in which the electrical properties of graphene are combined with the superior mechanical performance of silicon nitride.