Wafer-Scale Synthesis of Highly Oriented 2D Topological Semimetal PtTe2 via Tellurization.

Platinum ditelluride (1T-PtTe2) is a two-dimensional (2D) topological semimetal with a distinctive band structure and flexibility of van der Waals integration as a promising candidate for future electronics and spintronics. Although the synthesis of large-scale, uniform, and highly crystalline films of 2D semimetals system is a prerequisite for device application, the synthetic methods meeting these criteria are still lacking. Here, we introduce an approach to synthesize highly oriented 2D topological semimetal PtTe2 using a thermally assisted conversion called tellurization, which is a cost-efficient method compared to the other epitaxial deposition methods. We demonstrate that achieving highly crystalline 1T-PtTe2 using tellurization is not dependent on epitaxy but rather relies on two critical factors: (i) the crystallinity of the predeposited platinum (Pt) film and (ii) the surface coverage ratio of the Pt film considering lateral lattice expansion during transformation. By optimizing the surface coverage ratio of the epitaxial Pt film, we successfully obtained 2 in. wafer-scale uniformity without in-plane misalignment between antiparallelly oriented domains. The electronic band structure of 2D topological PtTe2 is clearly resolved in momentum space, and we observed an interesting 6-fold gapped Dirac cone at the Fermi surface. Furthermore, ultrahigh electrical conductivity down to ∼3.8 nm, which is consistent with that of single crystal PtTe2, was observed, proving its ultralow defect density.

Ion transport induced room-temperature insulator-metal transition in single-crystalline Cu2Se.

Cu2Se is a superionic conductor above 414 K, with ionic conductivities reaching that of molten salts. The superionic behavior results from hopping Cu ions between different crystallographic sites within the Se scaffold. However, the properties of Cu2Se below 414 K are far less known due to experimental limitations imposed by the bulk or polycrystalline samples that have been available so far. Here, we report the synthesis of ultra-thin, large-area single crystalline Cu2Se samples using a chemical vapor deposition method. The as-synthesized Cu2Se crystals exhibit optically and electrically detectable and controllable robust phases at room temperature and above. We demonstrate that Cu ion vacancies can be manipulated to induce an insulator-metal transition, which exhibits 6 orders of magnitude change in the electrical resistance of two terminal devices, accompanied by an optical change in the phase configuration. Our experiments show that the high mobility of the liquid-like Cu ion vacancies in Cu2Se causes macroscopic ordering in the Cu vacancies. Consequently, phase distribution over the crystals is not dictated by the diffusive motion of the ions but by the local energy minima formed due to the phase transition. As a result, long-range vacancy ordering of the crystal below 414 K becomes optically observable at a micrometer scale. This work demonstrates that Cu2Se could be a prototypical system where long-range ordering properties can be studied via electrical and optical methods.

Structural alignment of ZnO columns across multiple monolayer MoS2 layers as compliant substrates.

Understanding the behavior of materials in multi-dimensional architectures composed of atomically thin two-dimensional (2D) materials and three-dimensional (3D) materials has become mandatory for progress in materials preparation via various epitaxy techniques, such as van der Waals and remote epitaxy methods. We investigated the growth behavior of ZnO on monolayer MoS2 as a model system to study the growth of a 3D material on a 2D material, which is beyond the scope of remote and van der Waals epitaxy. The study revealed column-to-column alignment and inversion of crystallinity, which can be explained by combinatorial epitaxy, grain alignment across an atomically sharp interface, and a compliant substrate. The growth study enabled the formation of a ZnO/MoS2 heterostructure with type-I band alignment. Our findings will have a scientific impact on realizing 2D/3D heterostructures for practical device applications.

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.

Area-selective atomic layer deposition on 2D monolayer lateral superlattices.

The advanced patterning process is the basis of integration technology to realize the development of next-generation high-speed, low-power consumption devices. Recently, area-selective atomic layer deposition (AS-ALD), which allows the direct deposition of target materials on the desired area using a deposition barrier, has emerged as an alternative patterning process. However, the AS-ALD process remains challenging to use for the improvement of patterning resolution and selectivity. In this study, we report a superlattice-based AS-ALD (SAS-ALD) process using a two-dimensional (2D) MoS2-MoSe2 lateral superlattice as a pre-defining template. We achieved a minimum half pitch size of a sub-10 nm scale for the resulting AS-ALD on the 2D superlattice template by controlling the duration time of chemical vapor deposition (CVD) precursors. SAS-ALD introduces a mechanism that enables selectivity through the adsorption and diffusion processes of ALD precursors, distinctly different from conventional AS-ALD method. This technique facilitates selective deposition even on small pattern sizes and is compatible with the use of highly reactive precursors like trimethyl aluminum. Moreover, it allows for the selective deposition of a variety of materials, including Al2O3, HfO2, Ru, Te, and Sb2Se3.

Junctionless Negative-Differential-Resistance Device Using 2D Van-Der-Waals Layered Materials for Ternary Parallel Computing.

Negative-differential-resistance (NDR) devices offer a promising pathway for developing future computing technologies characterized by exceptionally low energy consumption, especially multivalued logic computing. Nevertheless, conventional approaches aimed at attaining the NDR phenomenon involve intricate junction configurations and/or external doping processes in the channel region, impeding the progress of NDR devices to the circuit and system levels. Here, an NDR device is presented that incorporates a channel without junctions. The NDR phenomenon is achieved by introducing a metal-insulator-semiconductor capacitor to a portion of the channel area. This approach establishes partial potential barrier and well that effectively restrict the movement of hole and electron carriers within specific voltage ranges. Consequently, this facilitates the implementation of both a ternary inverter and a ternary static-random-access-memory, which are essential components in the development of multivalued logic computing technology.

Layer-Controlled Growth of Single-Crystalline 2D Bi2O2Se Film Driven by Interfacial Reconstruction.

As semiconductor scaling continues to reach sub-nanometer levels, two-dimensional (2D) semiconductors are emerging as a promising candidate for the post-silicon material. Among these alternatives, Bi2O2Se has risen as an exceptionally promising 2D semiconductor thanks to its excellent electrical properties, attributed to its appropriate bandgap and small effective mass. However, unlike other 2D materials, growth of large-scale Bi2O2Se films with precise layer control is still challenging due to its large surface energy caused by relatively strong interlayer electrostatic interactions. Here, we present the successful growth of a wafer-scale (∼3 cm) Bi2O2Se film with precise thickness control down to the monolayer level on TiO2-terminated SrTiO3 using metal-organic chemical vapor deposition (MOCVD). Scanning transmission electron microscopy (STEM) analysis confirmed the formation of a [BiTiO4]1- interfacial structure, and density functional theory (DFT) calculations revealed that the formation of [BiTiO4]1- significantly reduced the interfacial energy between Bi2O2Se and SrTiO3, thereby promoting 2D growth. Additionally, spectral responsivity measurements of two-terminal devices confirmed a bandgap increase of up to 1.9 eV in monolayer Bi2O2Se, which is consistent with our DFT calculations. Finally, we demonstrated high-performance Bi2O2Se field-effect transistor (FET) arrays, exhibiting an excellent average electron mobility of 56.29 cm2/(V·s). This process is anticipated to enable wafer-scale applications of 2D Bi2O2Se and facilitate exploration of intriguing physical phenomena in confined 2D systems.

Evaluation of tin nitride (Sn3N4) via atomic layer deposition using novel volatile Sn precursors.

Novel Sn precursors, Sn(tbip)2, Sn(tbtp)2, and Sn(tbta)2, were synthesized and characterized using various analytical techniques and density functional theory calculations. These precursors contained cyclic amine ligands derived from iminopyrrolidine. X-ray crystallography revealed the formation of monomeric SnL2 with distorted seesaw geometry. Thermogravimetric analysis demonstrated the exceptional volatility of all complexes. Sn(tbtp)2 showed the lowest residual weight of 2.7% at 265 °C. Sn3N4 thin films were successfully synthesized using Sn(tbtp)2 as the Sn precursor and NH3 plasma. The precursor exhibited ideal characteristics for atomic layer deposition, with a saturated growth per cycle value of 1.9 Å cy-1 and no need for incubation when the film was deposited at 150-225 °C. The indirect optical bandgap of the Sn3N4 film was approximately 1-1.2 eV, as determined through ultraviolet-visible spectroscopy. Therefore, this study suggests that the Sn3N4 thin films prepared using the newly synthesized Sn precursor are suitable for application in thin-film photovoltaic devices.

Large Memory Window of van der Waals Heterostructure Devices Based on MOCVD-Grown 2D Layered Ge4 Se9.

Van der Waals (vdW) heterostructures have drawn much interest over the last decade owing to their absence of dangling bonds and their intriguing low-dimensional properties. The emergence of 2D materials has enabled the achievement of significant progress in both the discovery of physical phenomena and the realization of superior devices. In this work, the group IV metal chalcogenide 2D-layered Ge4 Se9 is introduced as a new selection of insulating vdW material. 2D-layered Ge4 Se9 is synthesized with a rectangular shape using the metalcorganic chemical vapor deposition system using a liquid germanium precursor at 240 °C. By stacking the Ge4 Se9 and MoS2 , vdW heterostructure devices are fabricated with a giant memory window of 129 V by sweeping back gate range of ±80 V. The gate-independent decay time reveals that the large hysteresis is induced by the interfacial charge transfer, which originates from the low band offset. Moreover, repeatable conductance changes are observed over the 2250 pulses with low non-linearity values of 0.26 and 0.95 for potentiation and depression curves, respectively. The energy consumption of the MoS2 /Ge4 Se9 device is about 15 fJ for operating energy and the learning accuracy of image classification reaches 88.3%, which further proves the great potential of artificial synapses.

Ultraefficient Electrocatalytic Hydrogen Evolution from Strain-Engineered, Multilayer MoS2.

This paper reports an approach to repurpose low-cost, bulk multilayer MoS2 for development of ultraefficient hydrogen evolution reaction (HER) catalysts over large areas (>cm2). We create working electrodes for use in HER by dry transfer of MoS2 nano- and microflakes to gold thin films deposited on prestrained thermoplastic substrates. By relieving the prestrain at a macroscopic scale, a tunable level of tensile strain is developed in the MoS2 and consequently results in a local phase transition as a result of spontaneously formed surface wrinkles. Using electrochemical impedance spectroscopy, we verified that electrochemical activation of the strained MoS2 lowered the charge transfer resistance within the materials system, achieving HER activity comparable to platinum (Pt). Raman and X-ray photoelectron spectroscopy show that desulfurization in the multilayer MoS2 was promoted by the phase transition; the combined effect of desulfurization and the lower charge resistance induced superior HER performance.

Fabrication of heterogeneous chemical patterns on stretchable hydrogels using single-photon lithography.

Curved hydrogel surfaces bearing chemical patterns are highly desirable in various applications, including artificial blood vessels, wearable electronics, and soft robotics. However, previous studies on the fabrication of chemical patterns on hydrogels employed two-photon lithography, which is still not widely accessible to most laboratories. This work demonstrates a new patterning technique for fabricating curved hydrogels with chemical patterns on their surfaces without two-photon microscopy. In this work, we show that exposing hydrogels in fluorophore solutions to single photons via confocal microscopy enables the patterning of fluorophores on hydrogels. By applying this technique to highly stretchable hydrogels, we demonstrate three applications: (1) improving pattern resolution by fabricating patterns on stretched hydrogels and then returning the hydrogels to their initial, unstretched length; (2) modifying the local stretchability of hydrogels at a microscale resolution; and (3) fabricating perfusable microchannels with chemical patterns by winding chemically patterned hydrogels around a template, embedding the hydrogels in a second hydrogel, and then removing the template. The patterning method demonstrated in this work may facilitate a better mimicking of the physicochemical properties of organs in tissue engineering and may be used to make hydrogel robots with specific chemical functionalities.

Gas-Phase Alkali Metal-Assisted MOCVD Growth of 2D Transition Metal Dichalcogenides for Large-Scale Precise Nucleation Control.

Advances in large-area and high-quality 2D transition metal dichalcogenides (TMDCs) growth are essential for semiconductor applications. Here, the gas-phase alkali metal-assisted metal-organic chemical vapor deposition (GAA-MOCVD) of 2D TMDCs is reported. It is determined that sodium propionate (SP) is an ideal gas-phase alkali-metal additive for nucleation control in the MOCVD of 2D TMDCs. The grain size of MoS2 in the GAA-MOCVD process is larger than that in the conventional MOCVD process. This method can be applied to the growth of various TMDCs (MoS2 , MoSe2 , WSe2 , and WSe2 ) and the generation of large-scale continuous films. Furthermore, the growth behaviors of MoS2 under different SP and oxygen injection time conditions are systematically investigated to determine the effects of SP and oxygen on nucleation control in the GAA-MOCVD process. It is found that the combination of SP and oxygen increases the grain size and nucleation suppression of MoS2 . Thus, the GAA-MOCVD with a precise and controllable supply of a gas-phase alkali metal and oxygen allows achievement of optimum growth conditions that maximizes the grain size of MoS2 . It is expected that GAA-MOCVD can provide a way for batch fabrication of large-scale atomically thin electronic devices based on 2D semiconductors.

Non-invasive digital etching of van der Waals semiconductors.

The capability to finely tailor material thickness with simultaneous atomic precision and non-invasivity would be useful for constructing quantum platforms and post-Moore microelectronics. However, it remains challenging to attain synchronized controls over tailoring selectivity and precision. Here we report a protocol that allows for non-invasive and atomically digital etching of van der Waals transition-metal dichalcogenides through selective alloying via low-temperature thermal diffusion and subsequent wet etching. The mechanism of selective alloying between sacrifice metal atoms and defective or pristine dichalcogenides is analyzed with high-resolution scanning transmission electron microscopy. Also, the non-invasive nature and atomic level precision of our etching technique are corroborated by consistent spectral, crystallographic, and electrical characterization measurements. The low-temperature charge mobility of as-etched MoS2 reaches up to 1200 cm2 V-1s-1, comparable to that of exfoliated pristine counterparts. The entire protocol represents a highly precise and non-invasive tailoring route for material manipulation.

Fabrication of a Microcavity Prepared by Remote Epitaxy over Monolayer Molybdenum Disulfide.

Advances in epitaxy have enabled the preparation of high-quality material architectures consisting of incommensurate components. Remote epitaxy based on lattice transparency of atomically thin graphene has been intensively studied for cost-effective advanced device manufacturing and heterostructure formation. However, remote epitaxy on nongraphene two-dimensional (2D) materials has rarely been studied even though it has a broad and immediate impact on various disciplines, such as many-body physics and the design of advanced devices. Herein, we report remote epitaxy of ZnO on monolayer MoS2 and the realization of a whispering-gallery-mode (WGM) cavity composed of a single crystalline ZnO nanorod and monolayer MoS2. Cross-sectional transmission electron microscopy and first-principles calculations revealed that the nongraphene 2D material interacted with overgrown and substrate layers and also exhibited lattice transparency. The WGM cavity embedding monolayer MoS2 showed enhanced luminescence of MoS2 and multimodal emission.

Strategies for chemical vapor deposition of two-dimensional organic-inorganic halide perovskites.

Two-dimensional (2D) organic-inorganic halide perovskites (OIHPs) with an alternating stacked structure of an organic layer and an inorganic layer draw significant attention for photovoltaics, multiple quantum-well, and passivation of three-dimensional perovskites. Although the low-cost and simple spin-coating process of these materials offers a vast platform to study fundamental properties and help them achieve rapid progress in electronics and optoelectronics, chemical vapor deposition (CVD) growth is also necessary for large-area, epitaxial, selective, and conformal growth. Here, one-step CVD strategies for 2D OIHP growth are proposed, and the growth trends depending on the precursor and substrate conditions are discussed. We report a CVD-grown nontoxic, lead-free 2D tin-OIHP flake to show the system offering a universal route to synthesize perovskite crystals based on arbitrary organic and inorganic components.

Enhancement of Photoresponse on Narrow-Bandgap Mott Insulator α-RuCl3 via Intercalation.

Charge doping to Mott insulators is critical to realize high-temperature superconductivity, quantum spin liquid state, and Majorana fermion, which would contribute to quantum computation. Mott insulators also have a great potential for optoelectronic applications; however, they showed insufficient photoresponse in previous reports. To enhance the photoresponse of Mott insulators, charge doping is a promising strategy since it leads to effective modification of electronic structure near the Fermi level. Intercalation, which is the ion insertion into the van der Waals gap of layered materials, is an effective charge-doping method without defect generation. Herein, we showed significant enhancement of optoelectronic properties of a layered Mott insulator, α-RuCl3, through electron doping by organic cation intercalation. The electron-doping results in substantial electronic structure change, leading to the bandgap shrinkage from 1.2 eV to 0.7 eV. Due to localized excessive electrons in RuCl3, distinct density of states is generated in the valence band, leading to the optical absorption change rather than metallic transition even in substantial doping concentration. The stable near-infrared photodetector using electronic modulated RuCl3 showed 50 times higher photoresponsivity and 3 times faster response time compared to those of pristine RuCl3, which contributes to overcoming the disadvantage of a Mott insulator as a promising optoelectronic device and expanding the material libraries.

Low-Temperature and High-Quality Growth of Bi2O2Se Layered Semiconductors via Cracking Metal-Organic Chemical Vapor Deposition.

Ternary metal-oxy-chalcogenides are emerging as next-generation layered semiconductors beyond binary metal-chalcogenides (i.e., MoS2). Among ternary metal-oxy-chalcogenides, especially Bi2O2Se has been demonstrated in field-effect transistors and photodetectors, exhibiting ultrahigh performance with robust air stability. The growth method for Bi2O2Se that has been reported so far is a powder sublimation based chemical vapor deposition. The first step for pursuing the practical application of Bi2O2Se as a semiconductor material is developing a gas-phase growth process. Here, we report a cracking metal-organic chemical vapor deposition (c-MOCVD) for the gas-phase growth of Bi2O2Se. The resulting Bi2O2Se films at very low growth temperature (∼300 °C) show single-crystalline quality. By taking advantage of the gas-phase growth, the precise phase control was demonstrated by modulating the partial pressure of each precursor. In addition, c-MOCVD-grown Bi2O2Se exhibits outstanding electrical and optoelectronic performance at room temperature without passivation, including maximum electron mobility of 127 cm2/(V·s) and photoresponsivity of 45134 A/W.

Electric-Field-Induced Reversible Phase Transitions in a Spontaneously Ion-Intercalated 2D Metal Oxide.

Electric field driven reversible phase transitions in two-dimensional (2D) materials are appealing for their potential in switching applications. Here, we introduce potassium intercalated MnO2 as an exemplary case. We demonstrate the synthesis of large-area single-crystal layered MnO2 via chemical vapor deposition as thin as 5 nm. These crystals are spontaneously intercalated by potassium ions during the synthesis. We showed that the charge transport in 2D K-MnO2 is dominated by motion of hydrated potassium ions in the interlayer space. Under a few volts bias, separation of potassium and the structural water leads to formation of different phases at the opposite terminals, and at larger biases K-MnO2 crystals exhibit reversible layered-to-spinel phase transition. These phase transitions are accompanied by electrical and optical changes in the material. We used the electric field driven ionic motion in K-MnO2 based devices to demonstrate the memristive capabilities of two terminal devices.

Arrayed MoS2 -In0.53 Ga0.47 As van der Waals Heterostructure for High-Speed and Broadband Detection from Visible to Shortwave-Infrared Light.

A high-speed and broadband 5 × 5 photodetector array based on MoS2 /In0.53 Ga0.47 As heterojunction is successfully demonstrated to take full advantage of the type-II band-aligned multilayer MoS2 /In0.53 Ga0.47 As. The fabricated devices exhibit good uniformity in the Raman spectrum and clear rectifying characteristics. The fabricated MoS2 /In0.53 Ga0.47 As photodetectors show good optical performances at a broad wavelength range showing high responsivities corresponding to the detectivity of ≈1010 Jones at -3 V for the incident broadband light from 400 to 1550 nm. A very fast photoresponse is also obtained with a small rise/fall time in the order of microseconds both for visible (638 nm) and shortwave infrared (1310 nm). Finally, the image scanning properties of MoS2 /In0.53 Ga0.47 As devices are demonstrated for visible and infrared light, indicating that the suggested device is one of the promising options for future broadband imager, which can be integrated on the focal plane arrays (FPAs).