Van der Waals Epitaxially Grown Molecular Crystal Dielectric Sb2O3 for 2D Electronics.

Two-dimensional (2D) semiconducting materials have attracted significant interest as promising candidates for channel materials owing to their high mobility and gate tunability at atomic-layer thickness. However, the development of 2D electronics is impeded due to the difficulty in formation of high-quality dielectrics with a clean and nondestructive interface. Here, we report the direct van der Waals epitaxial growth of a molecular crystal dielectric, Sb2O3, on 2D materials by physical vapor deposition. The grown Sb2O3 nanosheets showed epitaxial relations of 0 and 180° with the 2D template, maintaining high crystallinity and an ultrasharp vdW interface with the 2D materials. As a result, the Sb2O3 nanosheets exhibited a high breakdown field of 18.6 MV/cm for 2L Sb2O3 with a thickness of 1.3 nm and a very low leakage current of 2.47 × 10-7 A/cm2 for 3L Sb2O3 with a thickness of 1.96 nm. We also observed two types of grain boundaries (GBs) with misorientation angles of 0 and 60°. The 0°-GB with a well-stitched boundary showed higher electrical and thermal stabilities than those of the 60°-GB with a disordered boundary. Our work demonstrates a method to epitaxially grow molecular crystal dielectrics on 2D materials without causing any damage, a requirement for high-performance 2D electronics.

Boltzmann Switching MoS2 Metal-Semiconductor Field-Effect Transistors Enabled by Monolithic-Oxide-Gapped Metal Gates at the Schottky-Mott Limit.

A gate stack that facilitates a high-quality interface and tight electrostatic control is crucial for realizing high-performance and low-power field-effect transistors (FETs). However, when constructing conventional metal-oxide-semiconductor structures with two-dimensional (2D) transition metal dichalcogenide channels, achieving these requirements becomes challenging due to inherent difficulties in obtaining high-quality gate dielectrics through native oxidation or film deposition. Here, a gate-dielectric-less device architecture of van der Waals Schottky gated metal-semiconductor FETs (vdW-SG MESFETs) using a molybdenum disulfide (MoS2) channel and surface-oxidized metal gates such as nickel and copper is reported. Benefiting from the strong SG coupling, these MESFETs operate at remarkably low gate voltages, <0.5 V. Notably, they also exhibit Boltzmann-limited switching behavior featured by a subthreshold swing of ≈60 mV dec-1 and negligible hysteresis. These ideal FET characteristics are attributed to the formation of a Fermi-level (EF) pinning-free gate stack at the Schottky-Mott limit. Furthermore, authors experimentally and theoretically confirm that EF depinning can be achieved by suppressing both metal-induced and disorder-induced gap states at the interface between the monolithic-oxide-gapped metal gate and the MoS2 channel. This work paves a new route for designing high-performance and energy-efficient 2D electronics.

Ultrathin and Deformable Graphene Etch Mask for Fabrication of 3D Microstructures.

Three-dimensional (3D) microfabrication techniques play a crucial role across various research fields. These techniques enable the creation of functional 3D structures on the microscale, unlocking possibilities for diverse applications. However, conventional fabrication methods have limits in producing complex 3D structures, which require numerous fabrication steps that increase the costs. Graphene is an atomically thin material known for its deformability and impermeability to small gases and molecules, including reactive gases like XeF2. These features make graphene a potential candidate as an etch mask for 3D microfabrication. Here, we report the fabrication of various 3D microstructures using graphene etch masks directly grown and patterned on a Si substrate. The patterned graphene deforms and wraps the etched structures, allowing for the fabrication of complicated 3D microstructures, such as mushroom-like and step-like microstructures. As a practical demonstration of the graphene etch mask, we fabricate an omniphobic surface of reentrant 3D structures on a Si substrate. Our work provides a method for fabricating complex 3D microstructures using a graphene etch mask, contributing to advancements in etching and fabrication processes.

Optical grade transformation of monolayer transition metal dichalcogenides via encapsulation annealing.

Monolayer transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for optoelectronic applications due to their direct band gap and strong light-matter interactions. However, exfoliated TMDs have demonstrated optical characteristics that fall short of expectations, primarily because of significant defects and associated doping in the synthesized TMD crystals. Here, we report the improvement of optical properties in monolayer TMDs of MoS2, MoSe2, WS2, and WSe2, by hBN-encapsulation annealing. Monolayer WSe2 showed 2000% enhanced photoluminescence quantum yield (PLQY) and 1000% increased lifetime after encapsulation annealing at 1000 °C, which are attributed to dominant radiative recombination of excitons through dedoping of monolayer TMDs. Furthermore, after encapsulation annealing, the transport characteristics of monolayer WS2 changed from n-type to ambipolar, along with an enhanced hole transport, which also support dedoping of annealed TMDs. This work provides an innovative approach to elevate the optical grade of monolayer TMDs, enabling the fabrication of high-performance optoelectronic devices.

Atom-by-atom imaging of moiré transformations in 2D transition metal dichalcogenides.

Understanding the atomic-scale mechanisms that govern the structure of interfaces is critical across materials systems but particularly so for two-dimensional (2D) moiré materials. Here, we image, atom-by-atom, the thermally induced structural evolution of twisted bilayer transition metal dichalcogenides using in situ transmission electron microscopy. We observe low-temperature, local conversion of moiré superlattice into nanoscale aligned domains. Unexpectedly, this process occurs by nucleating a new grain within one monolayer, whose crystal orientation is templated by the other. The aligned domains grow through collective rotation of moiré supercells and hopping of 5|7 defect pairs at moiré boundaries. This provides mechanistic insight into the atomic-scale interactions controlling moiré structures and illustrates the potential to pattern interfacial structure and properties of 2D materials at the nanoscale.

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.

Electrically Confined Electroluminescence of Neutral Excitons in WSe2 Light-Emitting Transistors.

Monolayer transition metal dichalcogenides (TMDs) have drawn significant attention for their potential in optoelectronic applications due to their direct band gap and exceptional quantum yield. However, TMD-based light-emitting devices have shown low external quantum efficiencies as imbalanced free carrier injection often leads to the formation of non-radiative charged excitons, limiting practical applications. Here, electrically confined electroluminescence (EL) of neutral excitons in tungsten diselenide (WSe2) light-emitting transistors (LETs) based on the van der Waals heterostructure is demonstrated. The WSe2 channel is locally doped to simultaneously inject electrons and holes to the 1D region by a local graphene gate. At balanced concentrations of injected electrons and holes, the WSe2 LETs exhibit strong EL with a high external quantum efficiency (EQE) of ≈8.2 % at room temperature. These experimental and theoretical results consistently show that the enhanced EQE could be attributed to dominant exciton emission confined at the 1D region while expelling charged excitons from the active area by precise control of external electric fields. This work shows a promising approach to enhancing the EQE of 2D light-emitting transistors and modulating the recombination of exciton complexes for excitonic devices.

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.

Low-Temperature Centimeter-Scale Growth of Layered 2D SnS for Piezoelectric Kirigami Devices.

Tin monosulfide (SnS) is a promising piezoelectric material with an intrinsically layered structure, making it attractive for self-powered wearable and stretchable devices. However, for practical application purposes, it is essential to improve the output and manufacturing compatibility of SnS-based piezoelectric devices by exploring their large-area synthesis principle. In this study, we report the chemical vapor deposition (CVD) growth of centimeter-scale two-dimensional (2D) SnS layers at temperatures as low as 200 °C, allowing compatibility with processing a range of polymeric substrates. The intrinsic piezoelectricity of 2D SnS layers directly grown on polyamides (PIs) was confirmed by piezoelectric force microscopy (PFM) phase maps and force-current corroborative measurements. Furthermore, the structural robustness of the centimeter-scale 2D SnS layers/PIs allowed for engraving complicated kirigami patterns on them. The kirigami-patterned 2D SnS layer devices exhibited intriguing strain-tolerant piezoelectricity, which was employed in detecting human body motions and generating photocurrents irrespective of strain rate variations. These results establish the great promise of 2D SnS layers for practically relevant large-scale device technologies with coupled electrical and mechanical properties.

In-plane anisotropy of graphene by strong interlayer interactions with van der Waals epitaxially grown MoO3.

van der Waals (vdW) epitaxy can be used to grow epilayers with different symmetries on graphene, thereby imparting unprecedented properties in graphene owing to formation of anisotropic superlattices and strong interlayer interactions. Here, we report in-plane anisotropy in graphene by vdW epitaxially grown molybdenum trioxide layers with an elongated superlattice. The grown molybdenum trioxide layers led to high p-doping of the underlying graphene up to p = 1.94 × 1013 cm-2 regardless of the thickness of molybdenum trioxide, maintaining a high carrier mobility of 8155 cm2 V-1 s-1. Molybdenum trioxide-induced compressive strain in graphene increased up to -0.6% with increasing molybdenum trioxide thickness. The asymmetrical band distortion of molybdenum trioxide-deposited graphene at the Fermi level led to in-plane electrical anisotropy with a high conductance ratio of 1.43 owing to the strong interlayer interaction of molybdenum trioxide-graphene. Our study presents a symmetry engineering method to induce anisotropy in symmetric two-dimensional (2D) materials via the formation of asymmetric superlattices with epitaxially grown 2D layers.

Angstrom-Confined Electrochemical Synthesis of Sub-Unit-Cell Non-Van Der Waals 2D Metal Oxides.

Bottom-up electrochemical synthesis of atomically thin materials is desirable yet challenging, especially for non-van der Waals (non-vdW) materials. Thicknesses below a few nanometers have not been reported yet, posing the question how thin can non-vdW materials be electrochemically synthesized. This is important as materials with (sub-)unit-cell thickness often show remarkably different properties compared to their bulk form or thin films of several nanometers thickness. Here, a straightforward electrochemical method utilizing the angstrom-confinement of laminar reduced graphene oxide (rGO) nanochannels is introduced to obtain a centimeter-scale network of atomically thin (<4.3 Å) 2D-transition metal oxides (2D-TMO). The angstrom-confinement provides a thickness limitation, forcing sub-unit-cell growth of 2D-TMO with oxygen and metal vacancies. It is showcased that Cr2 O3 , a material without significant catalytic activity for the oxygen evolution reaction (OER) in bulk form, can be activated as a high-performing catalyst if synthesized in the 2D sub-unit-cell form. This method displays the high activity of sub-unit-cell form while retaining the stability of bulk form, promising to yield unexplored fundamental science and applications. It is shown that while retaining the advantages of bottom-up electrochemical synthesis, like simplicity, high yield, and mild conditions, the thickness of TMO can be limited to sub-unit-cell dimensions.

Nucleation and Growth of Monolayer MoS2 at Multisteps of MoO2 Crystals by Sulfurization.

Two-dimensional (2D) materials and their heterostructures are promising for next-generation optoelectronics, spintronics, valleytronics, and electronics. Despite recent progress in various growth studies of 2D materials, mechanical exfoliation of flakes is still the most common method to obtain high-quality 2D materials because precisely controlling material growth and synthesizing a single domain during the growth process of 2D materials, for the desired shape and quality, is challenging. Here, we report the nucleation and growth behaviors of monolayer MoS2 by sulfurizing a faceted monoclinic MoO2 crystal. The MoS2 layers nucleated at the thickness steps of the MoO2 crystal and grew epitaxially with crystalline correlation to the MoO2 surface. The epitaxially grown MoS2 layer expands outwardly on the SiO2 substrate, resulting in a monolayer single-crystal film, despite multiple nucleations of MoS2 layers on the MoO2 surface owing to several thickness steps. Although the photoluminescence of MoS2 is quenched owing to efficient charge transfer between MoS2 and metallic MoO2, the MoS2 stretched out to the SiO2 substrate shows a high carrier mobility of (15 cm2 V-1 s-1), indicating that a high-quality monolayer MoS2 film can be grown using the MoO2 crystal as a seed and precursor. Our work shows a method to grow high-quality MoS2 using a faceted MoO2 crystal and provides a deeper understanding of the nucleation and growth of 2D materials on a step-like surface.

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.

Quasi-van der Waals Epitaxial Recrystallization of a Gold Thin Film into Crystallographically Aligned Single Crystals.

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.

Fatigue-Induced Surface Modification of Zr-Based Metallic Glass under Environmental Conditions.

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.

Exciton-dominant photoluminescence of MoS2 by a functionalized substrate.

Transition metal dichalcogenides (TMDs) have been considered as promising candidates for transparent and flexible optoelectronic devices owing to their large exciton binding energy and strong light-matter interaction. However, monolayer (1L) TMDs exhibited different intensities and spectra of photoluminescence (PL), and the characteristics of their electronic devices also differed in each study. This has been explained in terms of various defects in TMDs, such as vacancies and grain boundaries, and their surroundings, such as dielectric screening and charged impurities, which lead to non-radiative recombination of trions, low quantum yield (QY), and unexpected doping. However, it should be noted that the surface conditions of the substrate are also a critical factor in determining the properties of TMDs located on the substrate. Here, we demonstrate that the optical and electrical properties of 1L MoS2 are strongly influenced by the functionalized substrate. The PL of 1L MoS2 placed on the oxygen plasma-treated SiO2 substrate was highly p-doped owing to the functional groups of -OH on SiO2, resulting in a strong enhancement of PL by approximately 20 times. The PL QY of 1L MoS2 on plasma-treated SiO2 substrate increased by one order of magnitude. Surprisingly, the observed PL spectra show the suppression of non-radiative recombination by trions, thus the exciton-dominant PL led to a prolonged lifetime of MoS2 on the plasma-treated substrate. The MoS2 field-effect transistors fabricated on plasma-treated SiO2 also exhibited a large hysteresis in the transfer curve owing to charge trapping of the functional groups. Our study demonstrates that the functional groups on the substrate strongly affect the characteristics of 1L MoS2, which provides clues as to why MoS2 exfoliated on various substrates always exhibited different properties in previous studies.

Modulation of electrical properties in MoTe2 by XeF2-mediated surface oxidation.

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 XeF2-mediated surface oxidation of 2H-MoTe2 (alpha phase MoTe2). MoTe2 exposed to XeF2 gas forms a thin and uniform oxidized layer (∼2.5 nm-thick MoO x ) on MoTe2 regardless of the exposure time (within ∼120 s) due to the passivation effect and simultaneous etching. We used the oxidized layer for contacts between the metal and MoTe2, which help reduce the contact resistance by overcoming the Fermi level pinning effect by the direct metal deposition process. The MoTe2 field-effect transistors (FETs) with a MoO x interlayer exhibited two orders of magnitude higher field-effect hole mobility of 6.31 cm2 V-1 s-1 with a high on/off current ratio of ∼105 than that of the MoTe2 device with conventional metal contacts (0.07 cm2 V-1 s-1). Our work shows a straightforward and effective method for forming a thin oxide layer for MoTe2 devices, applicable for 2D electronics.