Epitaxial single-crystal hexagonal boron nitride multilayers on Ni (111).

Large-area single-crystal monolayers of two-dimensional (2D) materials such as graphene1-3, hexagonal boron nitride (hBN)4-6 and transition metal dichalcogenides7,8 have been grown. hBN is considered to be the 'ideal' dielectric for 2D-materials-based field-effect transistors (FETs), offering the potential for extending Moore's law9,10. Although hBN thicker than a monolayer is more desirable as substrate for 2D semiconductors11,12, highly uniform and single-crystal multilayer hBN growth has yet to be demonstrated. Here we report the epitaxial growth of wafer-scale single-crystal trilayer hBN by a chemical vapour deposition (CVD) method. Uniformly aligned hBN islands are found to grow on single-crystal Ni (111) at early stage and finally to coalesce into a single-crystal film. Cross-sectional transmission electron microscopy (TEM) results show that a Ni23B6 interlayer is formed (during cooling) between the single-crystal hBN film and Ni substrate by boron dissolution in Ni. There are epitaxial relationships between hBN and Ni23B6 and between Ni23B6 and Ni. We also find that the hBN film acts as a protective layer that remains intact during catalytic evolution of hydrogen, suggesting continuous single-crystal hBN. This hBN transferred onto the SiO2 (300 nm)/Si wafer acts as a dielectric layer to reduce electron doping from the SiO2 substrate in MoS2 FETs. Our results demonstrate high-quality single-crystal  multilayered hBN over large areas, which should open up new pathways for making it a ubiquitous substrate for 2D semiconductors.

Unusual Raman Enhancement Effect of Ultrathin Copper Sulfide.

In surface-enhanced Raman spectroscopy (SERS), 2D materials are explored as substrates owing to their chemical stability and reproducibility. However, they exhibit lower enhancement factors (EFs) compared to noble metal-based SERS substrates. This study demonstrates the application of ultrathin covellite copper sulfide (CuS) as a cost-effective SERS substrate with a high EF value of 7.2 × 104 . The CuS substrate is readily synthesized by sulfurizing a Cu thin film at room temperature, exhibiting a Raman signal enhancement comparable to that of an Au noble metal substrate of similar thickness. Furthermore, computational simulations using the density functional theory are employed and time-resolved photoluminescence measurements are performed to investigate the enhancement mechanisms. The results indicate that polar covalent bonds (Cu─S) and strong interlayer interactions in the ultrathin CuS substrate increase the probability of charge transfer between the analyte molecules and the CuS surface, thereby producing enhanced SERS signals. The CuS SERS substrate demonstrates the selective detection of various dye molecules, including rhodamine 6G, methylene blue, and safranine O. Furthermore, the simplicity of CuS synthesis facilitates large-scale production of SERS substrates with high spatial uniformity, exhibiting a signal variation of less than 5% on a 4-inch wafer.

Multifunctional-high resolution imaging plate based on hydrophilic graphene for digital pathology.

In the present study, we showed that hydrophilic graphene can serve as an ideal imaging plate for biological specimens. Graphene being a single-atom-thick semi-metal with low secondary electron emission, array tomography analysis of serial sections of biological specimens on a graphene substrate showed excellent image quality with improvedz-axis resolution, without including any conductive surface coatings. However, the hydrophobic nature of graphene makes the placement of biological specimens difficult; graphene functionalized with polydimethylsiloxane oligomer was fabricated using a simple soft lithography technique and then processed with oxygen plasma to provide hydrophilic graphene with minimal damage to graphene. High-quality scanning electron microscopy images of biological specimens free from charging effects or distortion were obtained, and the optical transparency of graphene enabled fluorescence imaging of the specimen; high-resolution correlated electron and light microscopy analysis of the specimen became possible with the hydrophilic graphene plate.

Phase-dependent gas sensitivity of MoS2 chemical sensors investigated with phase-locked MoS2.

In the present study, phase-dependent gas sensitivities of MoS2 chemical sensors were examined. While 1T-phase MoS2 (1T-MoS2) has shown better chemical sensitivity than has 2H-phase MoS2 (2H-MoS2), the instability of the 1T phase has been hindering applications of 1T-MoS2 as chemical sensors. Here, the chemical sensitivity of MoS2 locked in its 1T phase by using a ZnO phase lock was investigated. To develop MoS2 chemical sensors locked in the 1T phase, we synthesized a multi-dimensional nanomaterial by growing ZnO nanorods onto MoS2 nanosheets (ZnO@1T-MoS2). Raman spectroscopy and x-ray photoelectron spectroscopy analyses of such phase-locked 1T-MoS2 subjected to flash light irradiation 100 times confirmed its robustness. ZnO nanomaterials hybridized on MoS2 nanosheets not only froze the MoS2 at its 1T phase, but also increased the active surface area for chemical sensing. The resulting hybridized material showed better response, namely better sensitivity, to NO2 gas exposure at room temperature than did 1T-MoS2 and 2H-MoS2. This result indicated that increased surface area and heterojunction formation between MoS2 and ZnO constitute a more promising route for improving sensitivity than using the 1T phase itself.

Large area patterning of residue-free metal oxide nanostructures by liquid transfer imprint lithography.

One-dimensional (1D) and three-dimensional (3D) residue-free metal oxide patterns are directly fabricated over large areas using liquid transfer imprint lithography (LTIL) with an ultraviolet-curable metal oxide precursor resist. A 1D line or pillar array of metal oxides nano-patterns without a residual layer is formed by LTIL and annealing processes. A 3D layer-by-layer nanomesh structure is successfully constructed by repeating the LTIL method without a complex etching process. In addition, it is possible to form a hierarchical structure in which zinc oxide nanowires are selectively grown on a desired zinc oxide (ZnO) seed pattern formed by LTIL via a hydrothermal method. Unlike the pattern fabricated by the conventional nanoimprint lithography method, in the case of the pattern formed by LTIL the residues accumulated between the patterns during the patterning procedure can be removed, and thus it is possible to easily form various types of nanostructures.

Strain-Mediated Interlayer Coupling Effects on the Excitonic Behaviors in an Epitaxially Grown MoS2/WS2 van der Waals Heterobilayer.

van der Waals heterostructures composed of two different monolayer crystals have recently attracted attention as a powerful and versatile platform for studying fundamental physics, as well as having great potential in future functional devices because of the diversity in the band alignments and the unique interlayer coupling that occurs at the heterojunction interface. However, despite these attractive features, a fundamental understanding of the underlying physics accounting for the effect of interlayer coupling on the interactions between electrons, photons, and phonons in the stacked heterobilayer is still lacking. Here, we demonstrate a detailed analysis of the strain-dependent excitonic behavior of an epitaxially grown MoS2/WS2 vertical heterostructure under uniaxial tensile and compressive strain that enables the interlayer interactions to be modulated along with the electronic band structure. We find that the strain-modulated interlayer coupling directly affects the characteristic combined vibrational and excitonic properties of each monolayer in the heterobilayer. It is further revealed that the relative photoluminescence intensity ratio of WS2 to MoS2 in our heterobilayer increases monotonically with tensile strain and decreases with compressive strain. We attribute the strain-dependent emission behavior of the heterobilayer to the modulation of the band structure for each monolayer, which is dictated by the alterations in the band gap transitions. These findings present an important pathway toward designing heterostructures and flexible devices.

Molecular Beam Epitaxy of Highly Crystalline Monolayer Molybdenum Disulfide on Hexagonal Boron Nitride.

Atomically thin molybdenum disulfide (MoS2), a direct-band-gap semiconductor, is promising for applications in electronics and optoelectronics, but the scalable synthesis of highly crystalline film remains challenging. Here we report the successful epitaxial growth of a continuous, uniform, highly crystalline monolayer MoS2 film on hexagonal boron nitride (h-BN) by molecular beam epitaxy. Atomic force microscopy and electron microscopy studies reveal that MoS2 grown on h-BN primarily consists of two types of nucleation grains (0° aligned and 60° antialigned domains). By adopting a high growth temperature and ultralow precursor flux, the formation of 60° antialigned grains is largely suppressed. The resulting perfectly aligned grains merge seamlessly into a highly crystalline film. Large-scale monolayer MoS2 film can be grown on a 2 in. h-BN/sapphire wafer, for which surface morphology and Raman mapping confirm good spatial uniformity. Our study represents a significant step in the scalable synthesis of highly crystalline MoS2 films on atomically flat surfaces and paves the way to large-scale applications.

Wafer-Scale and Wrinkle-Free Epitaxial Growth of Single-Orientated Multilayer Hexagonal Boron Nitride on Sapphire.

Large-scale growth of high-quality hexagonal boron nitride has been a challenge in two-dimensional-material-based electronics. Herein, we present wafer-scale and wrinkle-free epitaxial growth of multilayer hexagonal boron nitride on a sapphire substrate by using high-temperature and low-pressure chemical vapor deposition. Microscopic and spectroscopic investigations and theoretical calculations reveal that synthesized hexagonal boron nitride has a single rotational orientation with AA' stacking order. A facile method for transferring hexagonal boron nitride onto other target substrates was developed, which provides the opportunity for using hexagonal boron nitride as a substrate in practical electronic circuits. A graphene field effect transistor fabricated on our hexagonal boron nitride sheets shows clear quantum oscillation and highly improved carrier mobility because the ultraflatness of the hexagonal boron nitride surface can reduce the substrate-induced degradation of the carrier mobility of two-dimensional materials.

Catalytic Conversion of Hexagonal Boron Nitride to Graphene for In-Plane Heterostructures.

Heterostructures of hexagonal boron nitride (h-BN) and graphene have attracted a great deal of attention for potential applications in 2D materials. Although several methods have been developed to produce this material through the partial substitution reaction of graphene, the reverse reaction has not been reported. Though the endothermic nature of this reaction might account for the difficulty and previous absence of such a process, we report herein a new chemical route in which the Pt substrate plays a catalytic role. We propose that this reaction proceeds through h-BN hydrogenation; subsequent graphene growth quickly replaces the initially etched region. Importantly, this conversion reaction enables the controlled formation of patterned in-plane graphene/h-BN heterostructures, without needing the commonly employed protecting mask, simply by using a patterned Pt substrate.

Growth of high-crystalline, single-layer hexagonal boron nitride on recyclable platinum foil.

Hexagonal boron nitride (h-BN) is gaining significant attention as a two-dimensional dielectric material, along with graphene and other such materials. Herein, we demonstrate the growth of highly crystalline, single-layer h-BN on Pt foil through a low-pressure chemical vapor deposition method that allowed h-BN to be grown over a wide area (8 × 25 mm(2)). An electrochemical bubbling-based method was used to transfer the grown h-BN layer from the Pt foil onto an arbitrary substrate. This allowed the Pt foil, which was not consumed during the process, to be recycled repeatedly. The UV-visible absorption spectrum of the single-layer h-BN suggested an optical band gap of 6.06 eV, while a high-resolution transmission electron microscopy image of the same showed the presence of distinct hexagonal arrays of B and N atoms, which were indicative of the highly crystalline nature and single-atom thickness of the h-BN layer. This method of growing single-layer h-BN over large areas was also compatible with use of a sapphire substrate.

Conveyor CVD to high-quality and productivity of large-area graphene and its potentiality.

The mass production of high-quality graphene is required for industrial application as a future electronic material. However, the chemical vapor deposition (CVD) systems previously studied for graphene production face bottlenecks in terms of quality, speed, and reproducibility. Herein, we report a novel conveyor CVD system that enables rapid graphene synthesis using liquid precursors. Pristine and nitrogen-doped graphene samples of a size comparable to a smartphone (15 cm × 5 cm) are successfully synthesized at temperatures of 900, 950, and 1000 °C using butane and pyridine, respectively. Raman spectroscopy allows optimization of the rapid-synthesis conditions to achieve uniformity and high quality. By conducting compositional analysis via X-ray photoelectron spectroscopy as well as electrical characterization, it is confirmed that graphene synthesis and nitrogen doping degree can be adjusted by varying the synthesis conditions. Testing the corresponding graphene samples as gas-sensor channels for NH3 and NO2 and evaluating their response characteristics show that the gas sensors exhibit polar characteristics in terms of gas adsorption and desorption depending on the type of gas, with contrasting characteristics depending on the presence or absence of nitrogen doping; nitrogen-doped graphene exhibits superior gas-sensing sensitivity and response speed compared with pristine graphene.

Tailored Synthesis of Heterogenous 2D TMDs and Their Spectroscopic Characterization.

Two-dimensional (2D) vertical van der Waals heterostructures (vdWHs) show great potential across various applications. However, synthesizing large-scale structures poses challenges owing to the intricate growth parameters, forming unexpected hybrid film structures. Thus, precision in synthesis and thorough structural analysis are essential aspects. In this study, we successfully synthesized large-scale structured 2D transition metal dichalcogenides (TMDs) via chemical vapor deposition using metal oxide (WO3 and MoO3) thin films and a diluted H2S precursor, individual MoS2, WS2 films and various MoS2/WS2 hybrid films (Type I: MoxW1-xS2 alloy; Type II: MoS2/WS2 vdWH; Type III: MoS2 dots/WS2). Structural analyses, including optical microscopy, Raman spectroscopy, transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy, and cross-sectional imaging revealed that the A1g and E2g modes of WS2 and MoS2 were sensitive to structural variations, enabling hybrid structure differentiation. Type II showed minimal changes in the MoS2's A1g mode, while Types I and III exhibited a ~2.8 cm-1 blue shift. Furthermore, the A1g mode of WS2 in Type I displayed a 1.4 cm-1 red shift. These variations agreed with the TEM-observed microstructural features, demonstrating strain effects on the MoS2-WS2 interfaces. Our study provides insights into the structural features of diverse hybrid TMD materials, facilitating their differentiation through Raman spectroscopy.

Toward Ideal Low-Frequency Noise in Monolayer CVD MoS2 FETs: Influence of van der Waals Junctions and Sulfur Vacancy Management.

The pursuit of sub-1-nm field-effect transistor (FET) channels within 3D semiconducting crystals faces challenges due to diminished gate electrostatics and increased charge carrier scattering. 2D semiconductors, exemplified by transition metal dichalcogenides, provide a promising alternative. However, the non-idealities, such as excess low-frequency noise (LFN) in 2D FETs, present substantial hurdles to their realization and commercialization. In this study, ideal LFN characteristics in monolayer MoS2 FETs are attained by engineering the metal-2D semiconductor contact and the subgap density of states (DOS). By probing non-ideal contact resistance effects using CuS and Au electrodes, it is uncovered that excess contact noise in the high drain current (ID) region can be substantially reduced by forming a van der Waals junction with CuS electrodes. Furthermore, thermal annealing effectively mitigates sulfur vacancy-induced subgap density of states (DOS), diminishing excess noise in the low ID region. Through meticulous optimization of metal-2D semiconductor contacts and subgap DOS, alignment of 1/f noise with the pure carrier number fluctuation model is achieved, ultimately achieving the sought-after ideal LFN behavior in monolayer MoS2 FETs. This study underscores the necessity of refining excess noise, heralding improved performance and reliability of 2D electronic devices.

Hybrid energy harvester based on nanopillar solar cells and PVDF nanogenerator.

A tandem device which integrates a PVDF nanogenerator and silicon (Si) nanopillar solar cell is fabricated. The Si nanopillar solar cell was fabricated using a mask-free plasma etching technique and annealing process. The PVDF nanogenerator was stacked on top of the Si nanopillar solar cell using a spinning method. The optical properties and the device performance of nanowire solar cells have been characterized, and the dependence of device performance versus annealing time or method has been investigated. Furthermore, the PVDF nanogenerator was operated with a 100 dB sound wave and a 0.8 V peak to peak output voltage was generated. This tandem device can successfully harvest energy from both sound vibration and solar light, demonstrating its strong potential as a future ubiquitous energy harvester.

Large-scale graphene micropatterns via self-assembly-mediated process for flexible device application.

We report on a method for the large-scale production of graphene micropatterns by a self-assembly mediated process. The evaporation-induced self-assembly technique was engineered to produce highly ordered graphene patterns on flexible substrates in a simplified and scalable manner. The crossed stripe graphene patterns have been produced over a large area with regions consisting of single- and two-layer graphene. Based on these graphene patterns, flexible graphene-based field effect transistors have been fabricated with an ion-gel gate dielectric, which operates at low voltages of < 2 V with a hole and electron mobility of 214 and 106 cm(2)/V·s, respectively. The self-assembly approach described here may pave the way for the nonlithographic production of graphene patterns, which is scalable to large areas and compatible with roll-to-roll system.

Stress-induced domain dynamics and phase transitions in epitaxially grown VO2 nanowires.

We demonstrate that surface stresses in epitaxially grown VO2 nanowires (NWs) have a strong effect on the appearance and stability of intermediate insulating M2 phases, as well as the spatial distribution of insulating and metallic domains during structural phase transitions. During the transition from an insulating M1 phase to a metallic R phase, the coexistence of insulating M1 and M2 phases with the absence of a metallic R phase was observed at atmospheric pressure. In addition, we show that, for a VO2 NW without the presence of an epitaxial interface, surface stresses dominantly lead to spatially inhomogeneous phase transitions between insulating and metallic phases. In contrast, for a VO2 NW with the presence of an epitaxial interface, the strong epitaxial interface interaction leads to additional stresses resulting in uniformly alternating insulating and metallic domains along the NW length.

Tuning Schottky Barrier of Single-Layer MoS2 Field-Effect Transistors with Graphene Electrodes.

Two-dimensional materials have the potential to be applied in flexible and transparent electronics. In this study, single-layer MoS2 field-effect transistors (FETs) with Au/Ti-graphene heteroelectrodes were fabricated to examine the effect of the electrodes on the electrical properties of the MoS2 FETs. The contact barrier potential was tuned using an electric field. Asymmetrical gate behavior was observed owing to the difference between the MoS2 FETs, specifically between the MoS2 FETs with Au/Ti electrodes and those with graphene electrodes. The contact barrier of the MoS2 FETs with Au/Ti electrodes did not change with the electric field. However, the contact barrier at the MoS2-graphene interface could be modulated. The MoS2 FETs with Au/Ti-graphene electrodes exhibited enhanced on/off ratios (~102 times) and electron mobility (~2.5 times) compared to the MoS2 FETs with Au/Ti electrodes. These results could improve the understanding of desirable contact formation for high-performance MoS2 FETs and provide a facile route for viable electronic applications.

Observation of Strong Interlayer Couplings in WS2/MoS2 Heterostructures via Low-Frequency Raman Spectroscopy.

Van der Waals (vdW) heterostructures based on two-dimensional (2D) transition metal dichalcogenides (TMDCs), particularly WS2/MoS2 heterostructures with type-II band alignments, are considered as ideal candidates for future functional optoelectronic applications owing to their efficient exciton dissociation and fast charge transfers. These physical properties of vdW heterostructures are mainly influenced by the interlayer coupling occurring at the interface. However, a comprehensive understanding of the interlayer coupling in vdW heterostructures is still lacking. Here, we present a detailed analysis of the low-frequency (LF) Raman modes, which are sensitive to interlayer coupling, in bilayers of MoS2, WS2, and WS2/MoS2 heterostructures directly grown using chemical vapor deposition to avoid undesirable interfacial contamination and stacking mismatch effects between the monolayers. We clearly observe two distinguishable LF Raman modes, the interlayer in-plane shear and out-of-plane layer-breathing modes, which are dependent on the twisting angles and interface quality between the monolayers, in all the 2D bilayered structures, including the vdW heterostructure. In contrast, LF modes are not observed in the MoS2 and WS2 monolayers. These results indicate that our directly grown 2D bilayered TMDCs with a favorable stacking configuration and high-quality interface can induce strong interlayer couplings, leading to LF Raman modes.

Morphologically Controlled Synthesis of Reduced-Dimensional ZnO/Zn(OH)2 Nanosheets.

Conventional two-dimensional materials either have natural layered structures or are produced, with large surface areas, via physical or chemical synthesis. However, to form a two-dimensional material from a non-layered material, a method different from the existing ones is required. In this study, a surfactant-assisted method was utilized to synthesize Zn(OH)2 (a nonlayered transition metal oxide) nanosheets. This study described the synthesis of Zn(OH)2 nanosheets using an anionic sulfate layer and demonstrated a method of controlling the thickness and shape of the synthesized nanosheets by varying the surfactant concentration. Further, the characteristics of oxygen evolution reaction using ZnO/Zn(OH)2 nanosheets, obtained by annealing the synthesized sheets, as catalysts were studied.

Electrochemical Transparency of Graphene.

In the present study, we used the electrochemical transparency of graphene to show that the direct intercalation of alkali-metal cations is not a prerequisite for the redox reaction of Prussian blue (PB). PB thin films passivated with monolayer graphene still underwent electrochemical redox reactions in the presence of alkali-metal ions (K+ or Na+) despite the inability of the cations to penetrate the graphene and be incorporated into the PB. Graphene passivation not only preserved the electrochemical activity of the PB but also substantially enhanced the stability of the PB. As a proof of concept, we showed that a transparent graphene electrode covering PB can be used as an excellent hydrogen peroxide transducer, thereby demonstrating the possibility of realizing an electrochemical sensor capable of long-term measurements.