Harnessing Persistent Photocurrent in a 2D Semiconductor-Polymer Hybrid Structure: Electron Trapping and Fermi Level Modulation for Optoelectronic Memory.

Recently, 2D semiconductor-based optoelectronic memory has been explored to overcome the limitations of conventional von Neumann architectures by integrating optical sensing and data storage into one device. Persistent photocurrent (PPC), essential for optoelectronic memory, originates from charge carrier trapping according to the Shockley-Read-Hall (SRH) model in 2D semiconductors. The quasi-Fermi level position influences the activation of charge-trapping sites. However, the correlation between quasi-Fermi level modulations and PPC in 2D semiconductors has not been extensively studied. In this study, we demonstrate optoelectronic memory based on a 2D semiconductor-polymer hybrid structure and confirm that the underlying mechanism is charge trapping, as the SRH model explains. Under light illumination, electrons transfer from polyvinylpyrrolidone to p-type tungsten diselenide, resulting in high-level injection and majority carrier-type transitions. The quasi-Fermi level shifts upward with increasing temperature, improving PPC and enabling optoelectronic memory at 433 K. Our findings offer valuable insights into optimizing 2D semiconductor-based optoelectronic memory.

Universal Platform for Robust Dual-Atom Doped 2D Catalysts with Superior Hydrogen Evolution in Wide pH Media.

Layered 2D transition metal dichalcogenides (TMDs) have been suggested as efficient substitutes for Pt-group metal electrocatalysts in the hydrogen evolution reaction (HER). However, poor catalytic activities in neutral and alkaline electrolytes considerably hinder their practical applications. Furthermore, the weak adhesion between TMDs and electrodes often impedes long-term durability and thus requires a binder. Here, a universal platform is reported for robust dual-atom doped 2D electrocatalysts with superior HER performance over a wide pH range media. V:Co-ReS2 on a wafer scale is directly grown on oxidized Ti foil by a liquid-phase precursor-assisted approach and subsequently used as highly efficient electrocatalysts. The catalytic performance surpasses that of Pt group metals in a high current regime (≥ 100 mA cm-2) at pH ≥ 7, with a high durability of more than 70 h in all media at 200 mA cm-2. First-principles calculations reveal that V:Co dual doping in ReS2 significantly reduces the water dissociation barrier and simultaneously enables the material to achieve the thermoneutral Gibbs free energy for hydrogen adsorption.

Wafer-Scale Epitaxial Growth of an Atomically Thin Single-Crystal Insulator as a Substrate of Two-Dimensional Material Field-Effect Transistors.

As the electron mobility of two-dimensional (2D) materials is dependent on an insulating substrate, the nonuniform surface charge and morphology of silicon dioxide (SiO2) layers degrade the electron mobility of 2D materials. Here, we demonstrate that an atomically thin single-crystal insulating layer of silicon oxynitride (SiON) can be grown epitaxially on a SiC wafer at a wafer scale and find that the electron mobility of graphene field-effect transistors on the SiON layer is 1.5 times higher than that of graphene field-effect transistors on typical SiO2 films. Microscale and nanoscale void defects caused by heterostructure growth were eliminated for the wafer-scale growth of the single-crystal SiON layer. The single-crystal SiON layer can be grown on a SiC wafer with a single thermal process. This simple fabrication process, compatible with commercial semiconductor fabrication processes, makes the layer an excellent replacement for the SiO2/Si wafer.

Synthesis of hexagonal boron nitride heterostructures for 2D van der Waals electronics.

Among two dimensional (2D) van der Waals (vdW) layered materials such as graphene, which is used like a metal, and transition metal chalcogenides (TMdCs), which are used as semiconductors and metals, hexagonal boron nitride (hBN), which is used as an insulator, is ubiquitous as a building block to construct 2D vdW electronics for versatile tunneling devices. Monolayer and few-layer hBN films have been prepared with flake sizes of a few hundred micrometer via mechanical exfoliation and transfer methods. Another approach used to synthesize hBN films on a large scale is chemical vapor deposition (CVD). Although the single-crystal film growth of hBN on the wafer scale is the key to realizing realistic electronic applications, the various functionalities of hBN for 2D electronics are mostly limited to the microscale. Here, we review the recent progress for the large-area synthesis of hBN and other related vdW heterostructures via CVD, and the artificial construction of vdW heterostructures and 2D vdW electronics based on hBN, in terms of charge fluctuations, passivation, gate dielectrics, tunneling, Coulombic interactions, and contact resistances. The challenges and future perspectives for practical applications are also addressed.

A Novel and Facile Route to Synthesize Atomic-Layered MoS2 Film for Large-Area Electronics.

High-quality and large-area molybdenum disulfide (MoS2 ) thin film is highly desirable for applications in large-area electronics. However, there remains a challenge in attaining MoS2 film of reasonable crystallinity due to the absence of appropriate choice and control of precursors, as well as choice of suitable growth substrates. Herein, a novel and facile route is reported for synthesizing few-layered MoS2 film with new precursors via chemical vapor deposition. Prior to growth, an aqueous solution of sodium molybdate as the molybdenum precursor is spun onto the growth substrate and dimethyl disulfide as the liquid sulfur precursor is supplied with a bubbling system during growth. To supplement the limiting effect of Mo (sodium molybdate), a supplementary Mo is supplied by dissolving molybdenum hexacarbonyl (Mo(CO)6 ) in the liquid sulfur precursor delivered by the bubbler. By precisely controlling the amounts of precursors and hydrogen flow, full coverage of MoS2 film is readily achievable in 20 min. Large-area MoS2 field effect transistors (FETs) fabricated with a conventional photolithography have a carrier mobility as high as 18.9 cm2 V-1 s-1 , which is the highest reported for bottom-gated MoS2 -FETs fabricated via photolithography with an on/off ratio of ≈105 at room temperature.

Chemically Conjugated Carbon Nanotubes and Graphene for Carrier Modulation.

Nanocarbons such as fullerene and carbon nanotubes (CNT) in late 20th century have blossomed nanoscience and nanotechnology in 21st century, which have been further proliferated by the new finding of graphene and have indeed opened a new carbon era. Several new branches of research, for example, zero-dimensional nanoparticles, one-dimensional nanowires, and two-dimensional insulating hexagonal boron nitride, and semiconducting and metallic transition metal dichalcogenides including the recently emerging black phosphorus, have been explored and numerous unprecedented quantum mechanical features have been revealed, that have been hardly accessible otherwise. Extensive research has been done on devices and applications related to such materials. Many experimental instruments have been developed with high sensitivity and improved spatial and temporal resolution to detect such tiny objects. The need for multidisciplinary research has been growing stronger than ever, which will be the tradition in the next few decades. In this Account, we will demonstrate an example of multidisciplinary effort of utilizing CNTs and graphene for electronics by modulating electronic structures. While there are several methods of modifying electronic structures of nanocarbons such as gate bias, contact metal, and conventional substitutional doping, we focus on chemical doping approaches here. We first introduce the concept of chemical doping on CNTs and graphene in terms of electronegativity of molecules and electrochemical potential of CNTs and graphene. To understand the relationship of electrochemical potential of CNTs and graphene to electronegativity of molecules, we propose a simple water bucket model: how to fill or drain water (electrons in CNTs or graphene) in the bucket (density of states) by the chemical dopants. The doping concept is then demonstrated experimentally by tracking the absorption spectroscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Raman spectroscopy, transmittance, and transport measurements and by relating them to the reduction potential of molecules relative to that of CNTs or graphene. Two effects of chemical doping in electronics, transparent conducting films, and field effect transistors are extensively discussed. One critical issue, the stability of chemical dopants under ambient conditions, is further discussed. We believe that the presented doping concept will be useful tools for other low dimensional materials such as recently emerging transition metal dichalcogenides and black phosphorus.

Metal-Insulator-Semiconductor Diode Consisting of Two-Dimensional Nanomaterials.

We present a novel metal-insulator-semiconductor (MIS) diode consisting of graphene, hexagonal BN, and monolayer MoS2 for application in ultrathin nanoelectronics. The MIS heterojunction structure was fabricated by vertically stacking layered materials using a simple wet chemical transfer method. The stacking of each layer was confirmed by confocal scanning Raman spectroscopy and device performance was evaluated using current versus voltage (I-V) and photocurrent measurements. We clearly observed better current rectification and much higher current flow in the MIS diode than in the p-n junction and the metal-semiconductor diodes made of layered materials. The I-V characteristic curve of the MIS diode indicates that current flows mainly across interfaces as a result of carrier tunneling. Moreover, we observed considerably high photocurrent from the MIS diode under visible light illumination.

Seed growth of tungsten diselenide nanotubes from tungsten oxides.

We report growth of tungsten diselenide (WSe2) nanotubes by chemical vapor deposition with a two-zone furnace. WO3 nanowires were first grown by annealing tungsten thin films under argon ambient. WSe2 nanotubes were then grown at the tips of WO3 nanowires through selenization via two steps: (i) formation of tubular WSe2 structures on the outside of WO3 nanowires, resulting in core (WO3)-shell (WSe2) and (ii) growth of WSe2 nanotubes at the tips of WO3 nanowires. The observed seed growth is markedly different from existing substitutional growth of WSe2 nanotubes, where oxygen atoms are replaced by selenium atoms in WO3 nanowires to form WSe2 nanotubes. Another advantage of our growth is that WSe2 film was grown by simply supplying hydrogen gas, where the native oxides were reduced to thin film instead of forming oxide nanowires. Our findings will contribute to engineer other transition metal dichacogenide growth such as MoS2, WS2, and MoSe2.

Effective characterization of polymer residues on two-dimensional materials by Raman spectroscopy.

Large-area two-dimensional (2D) materials grown by chemical vapor deposition need to be transferred onto a target substrate for real applications. Poly(methyl methacrylate) as a supporting layer is widely used during the transfer process and removed after finishing it. However, it is a challenge to diminish the polymer layer completely. It is necessary to readily characterize the polymer residues on 2D materials to facilitate the removal process. Here, we report a method that characterizes the polymer residues on 2D materials by tracking the presence of G-band of amorphous carbons (a-Cs) in the Raman spectrum after forming carbonized a-Cs through thermal annealing. The (13)C-graphene is employed to separate the Raman signal G-band between (12)C-a-Cs and (13)C-graphene in the Raman spectrum. The residence of the polymer residues is clearly confirmed by the different Raman signals of two different isotopes ((12)C and (13)C) due to differences in mass. Our effective method recognizes that while the polymer residue is not easily removed on graphene, those on hexagonal boron nitride and molybdenum disulfide are almost diminished under optimum thermal annealing conditions. Our method will not only contribute to the development of a new transfer process, but also help to achieve a clean surface of 2D materials.

Synthesis of patched or stacked graphene and hBN flakes: a route to hybrid structure discovery.

Two-dimensional (2D) materials such as graphene and hexagonal boron nitride (hBN) have attracted significant attention due to their remarkable properties. Numerous interesting graphene/hBN hybrid structures have been proposed but their implementation has been very limited. In this work, the synthesis of patched structures through consecutive chemical vapor deposition (CVD) on the same substrate was investigated. Both in-plane junctions and stacked layers were obtained. For stacked layers, depending on the synthesis sequence, in one case turbostratic stacking with random rotations were obtained. In another, "AA-like", slightly twisted stacking between graphene and hBN was observed with lattice orientation misalignment consistently to be <1°. Raman characterizations not only confirmed that hBN is a superior substrate but also revealed for the first time that a graphene edge with hBN passivation displays reduced D band intensity compared to an open edge. These studies pave the way for the proposed well-ordered graphene/hBN structures and outline exciting future directions for hybrid 2D materials.

Low Ohmic contact resistance and high on/off ratio in transition metal dichalcogenides field-effect transistors via residue-free transfer.

Beyond-silicon technology demands ultrahigh performance field-effect transistors. Transition metal dichalcogenides provide an ideal material platform, but the device performances such as the contact resistance, on/off ratio and mobility are often limited by the presence of interfacial residues caused by transfer procedures. Here, we show an ideal residue-free transfer approach using polypropylene carbonate with a negligible residue coverage of ~0.08% for monolayer MoS2 at the centimetre scale. By incorporating a bismuth semimetal contact with an atomically clean monolayer MoS2 field-effect transistor on hexagonal boron nitride substrate, we obtain an ultralow Ohmic contact resistance of ~78 Ω µm, approaching the quantum limit, and a record-high on/off ratio of ~1011 at 15 K. Such an ultra-clean fabrication approach could be the ideal platform for high-performance electrical devices using large-area semiconducting transition metal dichalcogenides.

Mitigating substrate effects of van der Waals semiconductors using perfluoropolyether self-assembled monolayers.

The properties of transition metal dichalcogenides (TMDCs) are critically dependent on the dielectric constant of substrates, which significantly limits their application. To address this issue, we used a perfluorinated polyether (PFPE) self-assembled monolayer (SAM) with low surface energy to increase the van der Waals (vdW) gap between TMDCs and the substrate, thereby reducing the interaction between them. This resulted in a reduction in the subthreshold swing value, an increase in the photoluminescence intensity of excitons, and a decrease in the doping effect by the substrate. This work will provide a new way to control the TMDC/dielectric interface and contribute to expanding the applicability of TMDCs.

Atomic sawtooth-like metal films for vdW-layered single-crystal growth.

Atomic sawtooth surfaces have emerged as a versatile platform for growth of single-crystal van der Waals layered materials. However, the mechanism governing the formation of single-crystal atomic sawtooth metal (copper or gold) films on hard substrates (tungsten or molybdenum) remains a puzzle. In this study, we aim to elucidate the formation mechanism of atomic sawtooth metal films during melting-solidification process. Utilizing molecular dynamics, we unveil that the solidification of the liquid copper initiates at a high-index tungsten facet with higher interfacial energy. Subsequent tungsten facets follow energetically favourable pathways of forming single-crystal atomic sawtooth copper film during the solidification process near melting temperature. Formation of atomic sawtooth copper film is guaranteed with a film thickness exceeding the grain size of polycrystalline tungsten substrate. We further demonstrate the successful growth of centimeter-scale single-crystal monolayer hexagonal boron nitride films on atomic sawtooth copper films and explore their potential as efficient oxygen barrier.

Photo-oxidative Crack Propagation in Transition Metal Dichalcogenides.

Monolayered transition-metal dichalcogenides (TMDs) are easily exposed to air, and their crystal quality can often be degraded via oxidation, leading to poor electronic and optical device performance. The degradation becomes more severe in the presence of defects, grain boundaries, and residues. Here, we report crack propagation in pristine TMD monolayers grown by chemical vapor deposition under ambient conditions and light illumination. Under a high relative humidity (RH) of ∼60% and white light illumination, the cracks appear randomly. Photo-oxidative cracks gradually propagated along the grain boundaries of the TMD monolayers. In contrast, under low RH conditions of ∼2%, cracks were scarcely observed. Crack propagation is predominantly attributed to the accumulation of water underneath the TMD monolayers, which is preferentially absorbed by hygroscopic alkali metal-based precursor residues. Crack propagation is further accelerated by the cyclic process of photo-oxidation in a basic medium, leading to localized tensile strain. We also found that such crack propagation is prevented after the removal of alkali metals via the transfer of the sample to other substrates.

Continuous Template Growth of Large-Scale Tellurene Films on 1T'-MoTe2.

Use of a template triggers an epitaxial interaction with the depositing material during synthesis. Recent studies have demonstrated that two-dimensional tellurium (tellurene) can be directionally oriented when grown on transition metal dichalcogenide (TMD) templates. Specifically, employing a T-phase TMD, such as WTe2, restricts the growth direction even further due to its anisotropic nature, which allows for the synthesis of well-oriented tellurene films. Despite this, producing large-area epitaxial films still remains a significant challenge. Here, we report the continuous synthesis of a 1T'-MoTe2 template via chemical vapor deposition and tellurene via vapor transport. The interaction between helical Te and the 1T'-MoTe2 template facilitates the Te chains to collapse into ribbon shapes, enhancing lateral growth at a rate approximately 6 times higher than in the vertical direction, as confirmed by scanning electron microscopy and atomic force microscopy. Interestingly, despite the predominance of the lateral growth, cross-sectional transmission electron microscopy analysis of the tellurene ribbons revealed a consistent 60-degree incline at the edges. This suggests that the edges of the tellurene ribbons, where they contact the template surface, are favorable sites for additional Te absorption, which then stacks along the incline angle to expand. Furthermore, controlling the synthesis temperature, duration, and preheating time has facilitated the successful synthesis of tellurene films. The resultant tellurene exhibited hole mobility as high as ∼400 cm2/V s. After removing the underlying metallic template with plasma treatment, the film showed a current on/off ratio of ∼103. This ratio was confirmed by two-terminal field-effect transistor measurements and supported by near-field terahertz (THz) spectroscopy mapping.

The effect of copper pre-cleaning on graphene synthesis.

Copper foil is the most common substrate to synthesize monolayer graphene by chemical vapor deposition (CVD). The surface morphology and conditions of the copper foil can be very different depending on the various suppliers or different batches. These surface properties of copper strongly affect the growth behavior of graphene, thus rendering the growth conditions irreproducible when different batches of Cu foil are used. Furthermore, the quality of the graphene is severely affected as well. In this work, we report a facile method of copper pre-cleaning to improve the graphene quality and the reproducibility of the growth process. We found that the commercial Ni etchant (based on nitric acid) or nitric acid is the most effective cleaning agent among various acidic or basic solutions. The graphene grown on thus-treated copper surfaces is very clean and mostly monolayer when observed under scanning electron microscopy (SEM) and optical imaging, as compared to the graphene grown on untreated copper foil. Different batches (but with the same catalog number) of copper foil from Alfa Aesar Company were examined to explore the effect of copper pre-cleaning; consistent growth results were obtained when pre-cleaning was used. This method overcomes a commonly encountered problem in graphene growth and could become one of the standard protocols for preparing the copper foil substrate for growing graphene or other 2D materials.

Fabrication of Al2O3 coated 2D TiO2 nanoparticle photonic crystal layers by reverse nano-imprint lithography and plasma enhanced atomic layer deposition.

This paper reports simple process to enhance the extraction efficiency of photoluminescence (PL) from Eu-doped yttrium oxide (Y2O3:Eu3+) thin-film phosphor (TFP). Two-dimensional (2D) photonic crystal layer (PCL) was fabricated on Y2O3:Eu3+ phosphor films by reverse nano-imprint method using TiO2 nanoparticle solution as a nano-imprint resin and a 2D hole-patterned PDMS stamp. Atomic scale controlled Al2O3 deposition was performed onto this 2D nanoparticle PCL for the optimization of the photonic crystal pattern size and stabilization of TiO2 nanoparticle column structure. As a result, the light extraction efficiency of the Y2O3:Eu3+ phosphor film was improved by 2.0 times compared to the conventional Y2O3:Eu3+ phosphor film.

Synthesis of monolayer hexagonal boron nitride on Cu foil using chemical vapor deposition.

Hexagonal boron nitride (h-BN) is very attractive for many applications, particularly, as protective coating, dielectric layer/substrate, transparent membrane, or deep ultraviolet emitter. In this work, we carried out a detailed investigation of h-BN synthesis on Cu substrate using chemical vapor deposition (CVD) with two heating zones under low pressure (LP). Previous atmospheric pressure (AP) CVD syntheses were only able to obtain few layer h-BN without a good control on the number of layers. In contrast, under LPCVD growth, monolayer h-BN was synthesized and time-dependent growth was investigated. It was also observed that the morphology of the Cu surface affects the location and density of the h-BN nucleation. Ammonia borane is used as a BN precursor, which is easily accessible and more stable under ambient conditions than borazine. The h-BN films are characterized by atomic force microscopy, transmission electron microscopy, and electron energy loss spectroscopy analyses. Our results suggest that the growth here occurs via surface-mediated growth, which is similar to graphene growth on Cu under low pressure. These atomically thin layers are particularly attractive for use as atomic membranes or dielectric layers/substrates for graphene devices.

van der Waals epitaxy of MoS2 layers using graphene as growth templates.

We present a method for synthesizing MoS(2)/Graphene hybrid heterostructures with a growth template of graphene-covered Cu foil. Compared to other recent reports, (1, 2) a much lower growth temperature of 400 °C is required for this procedure. The chemical vapor deposition of MoS(2) on the graphene surface gives rise to single crystalline hexagonal flakes with a typical lateral size ranging from several hundred nanometers to several micrometers. The precursor (ammonium thiomolybdate) together with solvent was transported to graphene surface by a carrier gas at room temperature, which was then followed by post annealing. At an elevated temperature, the precursor self-assembles to form MoS(2) flakes epitaxially on the graphene surface via thermal decomposition. With higher amount of precursor delivered onto the graphene surface, a continuous MoS(2) film on graphene can be obtained. This simple chemical vapor deposition method provides a unique approach for the synthesis of graphene heterostructures and surface functionalization of graphene. The synthesized two-dimensional MoS(2)/Graphene hybrids possess great potential toward the development of new optical and electronic devices as well as a wide variety of newly synthesizable compounds for catalysts.

Multi-order graph attention network for water solubility prediction and interpretation.

The water solubility of molecules is one of the most important properties in various chemical and medical research fields. Recently, machine learning-based methods for predicting molecular properties, including water solubility, have been extensively studied due to the advantage of effectively reducing computational costs. Although machine learning-based methods have made significant advances in predictive performance, the existing methods were still lacking in interpreting the predicted results. Therefore, we propose a novel multi-order graph attention network (MoGAT) for water solubility prediction to improve the predictive performance and interpret the predicted results. We extracted graph embeddings in every node embedding layer to consider the information of diverse neighboring orders and merged them by attention mechanism to generate a final graph embedding. MoGAT can provide the atomic-specific importance scores of a molecule that indicate which atoms significantly influence the prediction so that it can interpret the predicted results chemically. It also improves prediction performance because the graph representations of all neighboring orders, which contain diverse range of information, are employed for the final prediction. Through extensive experiments, we demonstrated that MoGAT showed better performance than the state-of-the-art methods, and the predicted results were consistent with well-known chemical knowledge.