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Tungsten disulfide nanotubes (WS 2 -NTs), with their cylindrical structure composed of rolled WS 2 sheets, have attracted much interest because of their unique physical properties reflecting quasi-one-dimensional chiral structures. They exhibit a semiconducting electronic structure regardless of their chirality, and various semiconducting and optoelectronic device applications have been demonstrated. The development of techniques to fabricate arrayed WS 2 -NTs is crucial to realizing the highest device performance. Since the discovery of WS 2 -NTs, various synthesis techniques have been reported; however, horizontally arrayed WS 2 -NTs have never been successfully synthesized. Here, we demonstrate a simple technique to synthesize arrayed WS 2 -NTs. Through precise temperature and gas control, W 18 O 49 nanowires are grown along the [1̅101] direction on an r-plane sapphire substrate, and the nanowires are converted into nanotubes via sulfurization under optimized conditions. The demonstrated synthesis technique for arrayed WS 2 -NTs will play a central role in the fabrication of devices using transition-metal dichalcogenide nanotubes.

The surface sensitivity of high-resolution secondary electron (SE) imaging is examined using twisted bilayers of MoS2 stacked at an angle of 30-degree. High-resolution SE images of the twisted bilayer MoS2 show a honeycomb structure composed of Mo and S atoms, elucidating the monolayer structure of MoS2. Simultaneously captured annular dark-field scanning transmission electron microscope images from the same region show the projected structure of the two layers. That is, the SE images from the bilayer MoS2 selectively visualize the surface monolayer. It is noted that SE yields from the surface monolayer are approximately 3 times higher than those from the second monolayer, likely attributable to attenuation when SEs emitted from the second layer traverse the surface layer. Mini abstract: The surface sensitivity of atomic resolution secondary electron imaging is examined using MoS2 bilayers, the thinnest system composed of a surface layer and substrate. This study reveals that the secondary electrons visualize the atomic arrangement of the surface monolayer three times more intensely than that of the second layer., (© The Author(s) 2024. Published by Oxford University Press on behalf of The Japanese Society of Microscopy.)

Superatomic clusters - assemblies of atoms with various sizes, shapes, and compositions - can form hierarchical architectures that exhibit emergent electronic properties not found in their individual units. In particular, cubic M 4 X 4 clusters of chalcogenides (M = transition metal; X = chalcogen) are recognized as versatile building blocks for 3D structures with tunable morphologies and electronic properties. However, tetrahedral M 4 X 4 clusters rarely assemble into 2D architectures, which could offer a distinct class of functional materials from their 3D analogues. Here, this work reports the preparation of 2D Mo 8 S 8 Cl 11 , a superatomic layer with a sandwich structure consisting of Mo 4 S 4 clusters interconnected through Cl cross-linking. The vapor-phase reaction inside nanotubes promotes the selective growth of Mo 8 S 8 Cl 11 nanoribbons, allowing detailed characterization via transmission electron microscopy. This methodology can be applied to the growth of layered structures containing Mo 8 S 8 Cl 11 at the micrometer scale. This work has demonstrated that mono- and few-layer Mo 8 S 8 Cl 11 can be prepared by exfoliation of parent solids. Electronic structure calculations indicate that the 2D monolayer has quasi-flat bands, giving rise to an indirect-to-direct bandgap transition under mechanical strain. Furthermore, scanning electrochemical microscopy reveals the potential of the layered structures as highly efficient catalysts for the hydrogen-evolution reaction., (© 2024 The Author(s). Advanced Materials published by Wiley‐VCH GmbH.)

Two-dimensional (2D) materials stand as a promising platform for tunnel field-effect transistors (TFETs) in the pursuit of low-power electronics for the Internet of Things era. This promise arises from their dangling bond-free van der Waals heterointerface. Nevertheless, the attainment of high device performance is markedly impeded by the requirement of precise control over the 2D assembly with multiple stacks of different layers. In this study, we addressed a thickness-modulated n/p + -homojunction prepared from Nb-doped p + -MoS 2 crystal, where the issue on interface traps can be neglected without any external interface control due to the homojunction. Notably, our observations reveal the existence of a negative differential resistance, even at room temperature (RT). This signifies the successful realization of TFET operation under type III band alignment conditions by a single gate at RT, suggesting that the dominant current mechanism is band-to-band tunneling due to the ideal interface.

Tubular structures of transition metal dichalcogenides (TMDCs) have attracted attention in recent years due to their emergent physical properties, such as the giant bulk photovoltaic effect and chirality-dependent superconductivity. To understand and control these properties, it is highly desirable to develop a sophisticated method to fabricate TMDC tubular structures with smaller diameters and a more uniform crystalline orientation. For this purpose, the rolling up of TMDC monolayers into nanoscrolls is an attractive approach to fabricating such a tubular structure. However, the symmetric atomic arrangement of a monolayer TMDC generally makes its tubular structure energetically unstable due to considerable lattice strain in curved monolayers. Here, we report the fabrication of narrow nanoscrolls by using Janus TMDC monolayers, which have an out-of-plane asymmetric structure. Janus WSSe and MoSSe monolayers were prepared by the plasma-assisted surface atom substitution of WSe 2 and MoSe 2 monolayers, respectively, and then were rolled by solution treatment. The multilayer tubular structures of Janus nanoscrolls were revealed by scanning transmission electron microscopy observations. Atomic resolution elemental analysis confirmed that the Janus monolayers were rolled up with the Se-side surface on the outside. We found that the present nanoscrolls have the smallest diameter of about 5 nm, which is almost the same as the value predicted by the DFT calculation. The difference in work functions between the S- and Se-side surfaces was measured by Kelvin probe force microscopy, which is in good agreement with the theoretical prediction. Strong interlayer interactions and anisotropic optical responses of the Janus nanoscrolls were also revealed by Raman and photoluminescence spectroscopy.

Monolayers of transition metal dichalcogenides (TMDs) are an ideal 2D platform for studying a wide variety of electronic properties and potential applications due to their chemical diversity. Similarly, single-walled TMD nanotubes (SW-TMDNTs)-seamless cylinders of rolled-up TMD monolayers-are 1D materials that can exhibit tunable electronic properties depending on both their chirality and composition. However, much less has been explored about their geometrical structures and chemical variations due to their instability under ambient conditions. Here, the structural diversity of SW-TMDNTs templated by boron nitride nanotubes (BNNTs) is reported. The outer surfaces and inner cavities of the BNNTs promote and stabilize the coaxial growth of SW-TMDNTs with various diameters, including few-nanometers-wide species. The chiral indices (n,m) of individual SW-MoS 2 NTs are assigned by high-resolution transmission electron microscopy, and statistical analyses reveals a broad chirality distribution ranging from zigzag to armchair configurations. Furthermore, this methodology can be applied to the synthesis of various TMDNTs, such as selenides and alloyed Mo 1- x W x S 2 . Comprehensive microscopic and spectroscopic analyses also suggest the partial formation of Janus MoS 2(1- x ) Se 2 x nanotubes. The BNNT-templated reaction provides a universal platform to characterize the chirality-dependent properties of 1D nanotubes with various electronic structures., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Tungsten disulfide (WS 2 ) nanotubes exhibit various unique properties depending on their structures, such as their diameter and wall number. The development of techniques to prepare WS 2 nanotubes with the desired structure is crucial for understanding their basic properties. Notably, the synthesis and characterization of multi-walled WS 2 nanotubes with small diameters are challenging. This study reports the synthesis and characterization of small-diameter WS 2 nanotubes with an average inner diameter of 6 nm. The optical absorption and photoluminescence (PL) spectra of the as-prepared nanotubes indicate that a decrease in the nanotube diameter induces a red-shift in the PL, suggesting that the band gap narrowed due to a curvature effect, as suggested by theoretical calculations., (© 2023. Springer Nature Limited.)

Tip-enhanced photoluminescence (TEPL) microscopy allows for the correlation of scanning probe microscopic images and photoluminescent spectra at the nanoscale level in a similar way to tip-enhanced Raman scattering (TERS) microscopy. However, due to the higher cross-section of fluorescence compared to Raman scattering, the diffraction-limited background signal generated by far-field excitation is a limiting factor in the achievable spatial resolution of TEPL. Here, we demonstrate a way to overcome this drawback by using remote excitation TEPL (RE-TEPL). With this approach, the excitation and detection positions are spatially separated, minimizing the far-field contribution. Two probe designs are evaluated, both experimentally and via simulations. The first system consists of gold nanoparticles (AuNPs) through photoinduced deposition on a silver nanowire (AgNW), and the second system consists of two offset parallel AgNWs. This latter coupler system shows a higher coupling efficiency and is used to successfully demonstrate RE-TEPL spectral mapping on a MoSe 2 /WSe 2 lateral heterostructure to reveal spatial heterogeneity at the heterojunction., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

van der Waals (vdW) layered materials have attracted much attention because their physical properties can be controlled by varying the twist angle and layer composition. However, such twisted vdW assemblies are often prepared using mechanically exfoliated monolayer flakes with unintended shapes through a time-consuming search for such materials. Here, we report the rapid and dry fabrication of twisted multilayers using chemical vapor deposition (CVD) grown transition metal chalcogenide (TMDC) monolayers. By improving the adhesion of an acrylic resin stamp to the monolayers, the single crystals of various TMDC monolayers with desired grain size and density on a SiO 2 /Si substrate can be efficiently picked up. The present dry transfer process demonstrates the one-step fabrication of more than 100 twisted bilayers and the sequential stacking of a twisted 10-layer MoS 2 single crystal. Furthermore, we also fabricated hBN-encapsulated TMDC monolayers and various twisted bilayers including MoSe 2 /MoS 2 , MoSe 2 /WSe 2 , and MoSe 2 /WS 2 . The interlayer interaction and quality of dry-transferred, CVD-grown TMDCs were characterized by using photoluminescence (PL), cathodoluminescence (CL) spectroscopy, and cross-sectional electron microscopy. The prominent PL peaks of interlayer excitons can be observed for MoSe 2 /MoS 2 and MoSe 2 /WSe 2 with small twist angles at room temperature. We also found that the optical spectra were locally modulated due to nanosized bubbles, which are formed by the presence of interface carbon impurities. The present findings indicate the widely applicable potential of the present method and enable an efficient search of the emergent optical and electrical properties of TMDC-based vdW heterostructures., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) materials for next-generation optoelectronic devices; they can also provide opportunities for further advances in physics. Structuring 2D TMDC sheets as nanoribbons has tremendous potential for electronic state modification. However, a bottom-up synthesis of long TMDC nanoribbons with high monolayer selectivity on a large scale has not yet been reported yet. In this study, we successfully synthesized long W x O y nanowires and grew monolayer WS 2 nanoribbons on their surface. The supply of source atoms from a vapor-solid bilayer and chemical reaction at the atomic-scale interface promoted a self-limiting growth process. The developed method exhibited a high monolayer selection yield on a large scale and enabled the growth of long (∼100 μm) WS 2 nanoribbons with electronic properties characterized by optical spectroscopy and electrical transport measurements. The produced nanoribbons were isolated from W x O y nanowires by mechanical exfoliation and used as channels for field-effect transistors. The findings of this study can be used in future optoelectronic device applications and advanced physics research.

One-dimensional (1D) conducting materials are of great interest as potential building blocks for integrated nanocircuits. Ternary 1D transition-metal chalcogenides, consisting of M 6 X 6 wires with intercalated A atoms (M = Mo or W; X = S, Se, or Te; A = alkali or rare metals, etc .), have attracted much attention due to their 1D metallic behavior, superconductivity, and mechanical flexibility. However, the conventional solid-state reaction usually produces micrometer-scale bulk crystals, limiting their potential use as nanoscale conductors. Here we demonstrate a versatile method to fabricate indium (In)-intercalated W 6 Te 6 (In-W 6 Te 6 ) bundles with a nanoscale thickness. We first prepared micrometer-long, crystalline bundles of van der Waals W 6 Te 6 wires using chemical vapor deposition and intercalated In into the crystal via a vapor-phase reaction. Atomic-resolution electron microscopy revealed that In atoms were surrounded by three adjacent W 6 Te 6 wires. First-principles calculations suggested that their wire-by-wire stacking can transform through postgrowth intercalation. Individual In-W 6 Te 6 bundles exhibited metallic behavior, as theoretically predicted. We further identified the vibrational modes by combining polarized Raman spectroscopy and nonresonant Raman calculations.

van der Waals (vdW) heterostructures, which can be assembled with various two-dimensional materials, provide a versatile platform for exploring emergent phenomena. Here, we report an observation of the photovoltaic effect in a WS 2 /MoS 2 vdW heterostructure. Light excitation of WS 2 /MoS 2 at a wavelength of 633 nm yields a photocurrent without applying bias voltages, and the excitation power dependence of the photocurrent shows characteristic crossover from a linear to square root dependence. Photocurrent mapping has clearly shown that the observed photovoltaic effect arises from the WS 2 /MoS 2 region, not from Schottky junctions at electrode contacts. Kelvin probe microscopy observations show no slope in the electrostatic potential, excluding the possibility that the photocurrent originates from an unintentionally formed built-in potential.

The diverse series of transition metal dichalcogenide (TMDC) materials has been employed in various optoelectronic applications, such as photodetectors, light-emitting diodes, and lasers. Typically, the detection or emission range of optoelectronic devices is unique to the bandgap of the active material. Therefore, to improve the capability of these devices, extensive efforts have been devoted to tune the bandgap, such as gating, strain, and dielectric engineering. However, the controllability of these methods is severely limited (typically ≈0.1 eV). In contrast, alloying TMDCs is an effective approach that yields a composition-dependent bandgap and enables light emissions over a wide range. In this study, a color-tunable light-emitting device using compositionally graded TMDC alloys is fabricated. The monolayer WS 2 /WSe 2 alloy grown by chemical vapor deposition shows a spatial gradient in the light-emission energy, which varies from 2.1 to 1.7 eV. This alloy is incorporated in an electrolyte-based light-emitting device structure that can tune the recombination zone laterally. Thus, a continuous and reversible color-tunable light-emitting device is successfully fabricated by controlling the light-emitting positions. The results provide a new approach for exploring monolayer semiconductor-based broadband optical applications., (© 2022 Wiley-VCH GmbH.)

Rolling two-dimensional (2D) materials into 1D nanotubes allows for greater functionality. Boron-nitride nanotubes (BNNTs) can serve as insulating 1D templates for the coaxial growth of guest nanotubes, without interfering with property characterization. However, their application as 1D templates has been greatly hindered by their poor dispersibility, inevitably resulting in the formation of thick bundles. Here we present the facile preparation of well-dispersed BNNT templates via surfactant dispersions and synthesis of 1D van der Waals heterostructures based on the BNNTs. Comprehensive microscopic analyses show the isolation of clean, high-quality BNNTs. Statistical analyses revealed that small-diameter double-walled BNNTs are highly enriched by chemical peeling of BN sidewalls through the sonication process. We further demonstrate that the isolated BNNTs can template the coaxial growth of carbon and MoS 2 nanotubes by using chemical vapor deposition. The present strategy can be applied to the synthesis of a variety of nanotubes, thereby allowing for their characterization.

Controlling the direction of exciton-energy flow in two-dimensional (2D) semiconductors is crucial for developing future high-speed optoelectronic devices using excitons as the information carriers. However, intrinsic exciton diffusion in conventional 2D semiconductors is omnidirectional, and efficient exciton-energy transport in a specific direction is difficult to achieve. Here we demonstrate directional exciton-energy transport across the interface in tungsten diselenide (WSe 2 )-molybdenum diselenide (MoSe 2 ) lateral heterostructures. Unidirectional transport is spontaneously driven by the built-in asymmetry of the exciton-energy landscape with respect to the heterojunction interface. At excitation positions close to the interface, the exciton photoluminescence (PL) intensity was substantially decreased in the WSe 2 region and enhanced in the MoSe 2 region. In PL excitation spectroscopy, it was confirmed that the observed phenomenon arises from lateral exciton-energy transport from WSe 2 to MoSe 2 . This directional exciton-energy flow in lateral 2D heterostructures can be exploited in future optoelectronic devices.

The past decades of materials science discoveries are the basis of our present society - from the foundation of semiconductor devices to the recent development of internet of things (IoT) technologies. These materials science developments have depended mainly on control of rigid chemical bonds, such as covalent and ionic bonds, in organic molecules and polymers, inorganic crystals and thin films. The recent discovery of graphene and other two-dimensional (2D) materials offers a novel approach to synthesizing materials by controlling their weak out-of-plane van der Waals (vdW) interactions. Artificial stacks of different types of 2D materials are a novel concept in materials synthesis, with the stacks not limited by rigid chemical bonds nor by lattice constants. This offers plenty of opportunities to explore new physics, chemistry, and engineering. An often-overlooked characteristic of vdW stacks is the well-defined 2D nanospace between the layers, which provides unique physical phenomena and a rich field for synthesis of novel materials. Applying the science of intercalation compounds to 2D materials provides new insights and expectations about the use of the vdW nanospace. We call this nascent field of science '2.5 dimensional (2.5D) materials,' to acknowledge the important extra degree of freedom beyond 2D materials. 2.5D materials not only offer a new field of scientific research, but also contribute to the development of practical applications, and will lead to future social innovation. In this paper, we introduce the new scientific concept of this science of '2.5D materials' and review recent research developments based on this new scientific concept., Competing Interests: No potential conflict of interest was reported by the author(s)., (© 2022 The Author(s). Published by National Institute for Materials Science in partnership with Taylor & Francis Group.)

We have developed a simple and straightforward way to realize controlled postdoping toward 2D transition metal dichalcogenides (TMDs). The key idea is to use low-kinetic-energy dopant beams and a high-flux chalcogen beam simultaneously, leading to substitutional doping with controlled dopant densities. Atomic-resolution transmission electron microscopy has revealed that dopant atoms injected toward TMDs are incorporated substitutionally into the hexagonal framework of TMDs. The electronic properties of doped TMDs (Nb-doped WSe 2 ) have shown drastic change and p- type action with more than 2 orders of magnitude increase in current. Position-selective doping has also been demonstrated by the postdoping toward TMDs with a patterned mask on the surface. The postdoping method developed in this work can be a versatile tool for 2D-based next-generation electronics in the future.

A comprehensive understanding of the roles of various nanointerfaces in thermal transport is of critical significance but remains challenging. A two-dimensional van der Waals (vdW) heterostructure with tunable interface lattice mismatch provides an ideal platform to explore the correlation between thermal properties and nanointerfaces and achieve controllable tuning of heat flow. Here, we demonstrate that interfacial engineering is an efficient strategy to tune thermal transport via systematic investigation of the thermal conductance ( G ) across a series of large-area four-layer stacked vdW materials using an improved polyethylene glycol-assisted time-domain thermoreflectance method. Owing to its rich interfacial mismatch and weak interfacial coupling, the vertically stacked MoSe 2 -MoS 2 -MoSe 2 -MoS 2 heterostructure demonstrates the lowest G of 1.5 MW m -2 K -1 among all vdW structures. A roadmap to tune G via homointerfacial mismatch, interfacial coupling, and heterointerfacial mismatch is further demonstrated for thermal tuning. Our work reveals the roles of various interfacial effects on heat flow and highlights the importance of the interfacial mismatch and coupling effects in thermal transport. The design principle is also promising for application in other areas, such as the electrical tuning of energy storage and conversion and the thermoelectricity tuning of thermoelectronics.

Emerging transition metal dichalcogenides (TMDCs) offer an attractive platform for investigating functional light-emitting devices, such as flexible devices, quantum and chiral devices, high-performance optical modulators, and ultralow threshold lasers. In these devices, the key operation is to control the light-emitting position, that is, the spatial position of the recombination zone to generate electroluminescence, which permits precise light guides/passes/confinement to ensure favorable device performance. Although various structures of TMDC light-emitting devices have been demonstrated, including the transistor configuration and heterostructured diodes, it is still difficult to tune the light-emitting position precisely owing to the structural device complexity. In this study, we fabricated two-terminal light-emitting devices with chemically synthesized WSe 2 , MoSe 2 , and WS 2 monolayers, and performed direct observations of their electroluminescence, from which we discovered a divergence in their light-emitting positions. Subsequently, we propose a method to associate spatial electroluminescence imaging with transport properties among different samples; consequently, a common rule for determining the locations of recombination zones is revealed. Owing to dynamic carrier accumulations and p-i-n junction formations, the light-emitting positions in electrolyte-based devices can be tuned continuously. The proposed method will expand the device applicability for designing functional optoelectronic applications based on TMDCs.

To maximize the potential of transition-metal dichalcogenides (TMDCs) in device applications, the development of a sophisticated technique for stable and highly efficient carrier doping is critical. Here, we report the efficient n-type doping of monolayer MoS2 using KOH/benzo-18-crown-6, resulting in a doped TMDC that is air-stable. MoS2 field-effect transistors show an increase in on-current of three orders of magnitude and degenerate the n-type behaviour with high air-stability for ∼1 month as the dopant concentration increases. Transport measurements indicate a high electron density of 3.4 × 1013 cm-2 and metallic-type temperature dependence for highly doped MoS2. First-principles calculations support electron doping via surface charge transfer from the K/benzo-18-crown-6 complex to monolayer MoS2. Patterned doping is demonstrated to improve the contact resistance in MoS2-based devices.

The development of bulk synthetic processes to prepare functional nanomaterials is crucial to achieve progress in fundamental and applied science. Transition-metal chalcogenide (TMC) nanowires, which are one-dimensional (1D) structures having three-atom diameters and van der Waals surfaces, have been reported to possess a 1D metallic nature with great potential in electronics and energy devices. However, their mass production remains challenging. Here, a wafer-scale synthesis of highly crystalline transition-metal telluride nanowires is demonstrated by chemical vapor deposition. The present technique enables formation of either aligned, atomically thin two-dimensional (2D) sheets or random networks of three-dimensional (3D) bundles, both composed of individual nanowires. These nanowires exhibit an anisotropic 1D optical response and superior conducting properties. The findings not only shed light on the controlled and large-scale synthesis of conductive thin films but also provide a platform for the study on physics and device applications of nanowire-based 2D and 3D crystals.

Atomically thin one-dimensional (1D) van der Waals wires of transition metal monochalocogenides (TMMs) have been anticipated as promising building blocks for integrated nanoelectronics. While reliable production of TMM nanowires has eluded scientists over the past few decades, we finally demonstrated a bottom-up fabrication of MoTe nanowires inside carbon nanotubes (CNTs). Still, the current synthesis method is based on vacuum annealing of reactive MoTe 2 , and limits access to a variety of TMMs. Here we report an expanded framework for high-yield synthesis of the 1D tellurides including WTe, an previously unknown family of TMMs. Experimental and theoretical analyses revealed that the choice of suitable metal oxides as a precursor provides a useful yield for their characterization. These TMM nanowires exhibit a significant optical absorption in the visible-light region. More important, electronic properties of CNTs can be tuned by encapsulating different TMM nanowires.

High-harmonic generation (HHG), which is the generation of light with multiple optical harmonics, is an unconventional nonlinear optical phenomenon beyond the perturbation regime. HHG, which was initially observed in gaseous media, has recently been demonstrated in solid-state materials. Determining how to control such extreme nonlinear optical phenomena is a challenging subject. Here, we demonstrate the control of HHG through tuning the electronic structure and carrier injection using single-walled carbon nanotubes (SWCNTs). We reveal systematic changes in the high-harmonic spectra of SWCNTs with a series of electronic structures ranging from a metal structure to a semiconductor structure. We demonstrate enhancement or reduction of harmonic generation by more than 1 order of magnitude by tuning the electron and hole injection into the semiconductor SWCNTs through electrolyte gating. These results open a path toward the control of HHG in the context of field-effect transistor devices.

Molecularly thin two-dimensional (2D) semiconductors are emerging as photocatalysts owing to their layer-number-dependent quantum effects and high charge separation efficiency. However, the correlation among the dimensionality, crystallinity, and photocatalytic activity of such 2D nanomaterials remains unclear. Herein, a Ag photoreduction technique coupled with microscopic analyses is employed to spatially resolve the photocatalytic activity of MoS 2 as a model catalyst. Interestingly, we find that only monolayer (1L)-MoS 2 is active for a Ag photoreduction reaction. The photocatalytic activity of 1L-MoS 2 is enhanced by a built-in electrical field originated from the MoS 2 /SiO 2 interface, instead of by the specific surface structure and quantum electronic state of 1L-MoS 2 . Furthermore, we observe photocatalytic active sites to be geometrically distributed on triangular 1L-MoS 2 crystals, wherein the Ag particles are preferentially deposited on the outermost zigzag edges and defective inner parts of the triangular grains. The degradation of photocatalytic activity and electron mobility with the formation of Mo(VI) species indicates that the species inhibit the in-plane diffusion of the photogenerated electrons to the reductive sites. The monolayer-selectivity, activation, and inactivation mechanisms, unveiled in this work, will offer future directions in designing 2D nanophotocatalysts.

Two-dimensional (2D) transition-metal dichalcogenides have attracted a considerable amount of attention because of their potential for post-silicon device applications, as well as for exploring fundamental physics in an ideal 2D system. We tested the chemical vapour deposition (CVD) of WS 2 using the gaseous precursors WF 6 and H 2 S, augmented by the Na-assistance method. When Na was present during growth, the process created triangle-shaped WS 2 crystals that were 10 μm in size and exhibited semiconducting characteristics. By contrast, the Na-free growth of WS 2 resulted in a continuous film with metallic behaviour. These results clearly demonstrate that alkali-metal assistance is valid even in applications of gas-source CVD without oxygen-containing species, where intermediates comprising Na, W, and S can play an important role. We observed that the WS 2 crystals grown by gas-source CVD exhibited a narrow size distribution when compared with crystals grown by conventional solid-source CVD, indicating that the crystal nucleation occurred almost simultaneously across the substrate, and that uniform lateral growth was dominant afterwards. This phenomenon was attributed to the suppression of inhomogeneous nucleation through the fast and uniform diffusion of the gas-phase precursors, supported by the Na-assisted suppression of the fast reactions between WF 6 and H 2 S.

Atomically thin transition-metal dichalcogenides (TMDs) are attracting great interest for future electronic applications. Even though much effort has been devoted to preparing large-area, high-quality TMDs over the past few years, the samples are usually grown on substrate surfaces. Here, we demonstrate the direct growth of a MoS 2 monolayer at the interface between a Au film and a SiO 2 substrate. MoS 2 grains were nucleated below Au films deposited on SiO 2 via interface diffusion and then grown into a continuous MoS 2 film. By programming the Au pattern deposited, controlled growth of MoS 2 with the desired size and geometry was achieved over preferred locations, facilitating its integration into functional field-effect transistors. Our findings elucidate the fabrication of a two-dimensional semiconductor at the interface of bulk three-dimensional solids, providing a novel means for establishing a clean interface junction. It also offers a promising alternative to the site-selective synthesis of TMDs, which is expected to aid the fabrication of TMD-based nanodevices.

Heterostructures of two-dimensional (2D) atomic crystals provide fascinating molecular-scale design elements for emergent physical phenomena and functional materials, as integrating distinct monolayers into vertical heterostructures can afford coupling between disparate properties. However, the available examples have been limited to either van der Waals (vdW) or electrostatic (ES) heterostructures that are solely composed of noncharged and charged monolayers, respectively. Here, we propose a "vdW-ES heterostructure" chemical design in which charge-neutral and charged monolayer-building blocks with highly disparate chemical and physical properties are conjugated vertically through asymmetrically charged interfaces. We demonstrate vdW-ES heteroassembly of semiconducting MoS 2 and dielectric Ca 2 Nb 3 O 10 - (CNO) monolayers using an amphipathic molecular starch, resulting in the emergence of trion luminescence observed at the lowest energy among MoS 2 -related materials, probably due to interfacial confinement effects given by vdW-ES dual interactions. In addition, interface engineering leads to tailored exciton of the vdW/ES heterostructures owing to the pronounced dielectric proximity effects, bringing an intriguing interlayer chemistry to modify 2D materials. Furthermore, the current approach was successfully extended to create a graphene/CNO heterostructure, which verifies the versatility of the preparative method.

Monolayers of transition metal dichalcogenides (TMDCs) have attracted a great interest for post-silicon electronics and photonics due to their high carrier mobility, tunable bandgap, and atom-thick 2D structure. With the analogy to conventional silicon electronics, establishing a method to convert TMDC to p- and n-type semiconductors is essential for various device applications, such as complementary metal-oxide-semiconductor (CMOS) circuits and photovoltaics. Here, a successful control of the electrical polarity of monolayer WSe 2 is demonstrated by chemical doping. Two different molecules, 4-nitrobenzenediazonium tetrafluoroborate and diethylenetriamine, are utilized to convert ambipolar WSe 2 field-effect transistors (FETs) to p- and n-type, respectively. Moreover, the chemically doped WSe 2 show increased effective carrier mobilities of 82 and 25 cm 2 V -1 s -1 for holes and electrons, respectively, which are much higher than those of the pristine WSe 2 . The doping effects are studied by photoluminescence, Raman, X-ray photoelectron spectroscopy, and density functional theory. Chemically tuned WSe 2 FETs are integrated into CMOS inverters, exhibiting extremely low power consumption (≈0.17 nW). Furthermore, a p-n junction within single WSe 2 grain is realized via spatially controlled chemical doping. The chemical doping method for controlling the transport properties of WSe 2 will contribute to the development of TMDC-based advanced electronics., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The ideal quantum confinement structure of monolayer semiconductors offers prominent optical modulation capabilities that are mediated by enhanced many-body interactions. Herein, we establish an electrolyte-gating method for tuning the luminescence properties that are in transition metal dichalcogenide (TMDC) monolayers. We fabricate electric double-layer capacitors on TMDC/graphite heterostructures to investigate electric-field- and carrier-density-dependent photoluminescence. The exciton peak energy initially shows a slight quadratic red shift of ∼1 meV without carrier accumulations, which is caused by the quantum-confined Stark effect. In contrast, the exciton resonance exhibits a larger red shift up to 10 meV with the accumulated carrier density above 10 13 cm -2 . These results indicate that the optical transitions can be largely modulated by the carrier density control in S- and Se-based TMDCs, as triggered by the doping-induced band gap renormalization effect. To further inspire this modulation capability, we also apply our method to electrolyte-based TMDC light-emitting devices. Biasing solely in electrolyte-induced p-i-n junctions yields pronounced red shifts up to 40 meV for exciton and trion electroluminescence. Consequently, our approach reveals that the doping effects in the high-carrier-density regimes are potentially significant for efficient optical modulation in monolayer semiconductors.

The in-plane connection and layer-by-layer stacking of atomically thin layered materials are expected to allow the fabrication of two-dimensional (2D) heterostructures with exotic physical properties and future engineering applications. However, it is currently necessary to develop a continuous growth process that allows the assembly of a wide variety of atomic layers without interface degradation, contamination, and/or alloying. Herein, we report the continuous heteroepitaxial growth of 2D multiheterostructures and nanoribbons based on layered transition metal dichalcogenide (TMDC) monolayers, employing metal organic liquid precursors with high supply controllability. This versatile process can avoid air exposure during growth process and enables the formation of in-plane heterostructures with ultraclean atomically sharp and zigzag-edge straight junctions without defects or alloy formation around the interface. For the samples grown directly on graphite, we have investigated the local electronic density of states of atomically sharp heterointerface by scanning tunneling microscopy and spectroscopy, together with first-principles calculations. These results demonstrate an approach to realizing diverse nanostructures such as atomic layer-based quantum wires and superlattices and suggest advanced applications in the fields of electronics and optoelectronics.

This study investigated the intrinsic optical properties of MoS2 monolayers and MoS2/WS2 van der Waals (vdW) heterostructures, grown using chemical vapor deposition. To understand the effect of the growth substrate, samples grown on a SiO2/Si surface were transferred and suspended onto a porous substrate. This transfer resulted in a blue shift of the excitonic photoluminescence (PL) peak generated by MoS2 monolayers, together with an intensity increase. The blue shift and the intensity increase are attributed to the release of lattice strain and the elimination of substrate-induced non-radiative relaxation, respectively. This suspension technique also allowed the observation of PL resulting from interlayer excitons in the MoS2/WS2 vdW heterostructures. These results indicate that the suppression of lattice strain and non-radiative relaxation is essential for the formation of interlayer excitons, which in turn is crucial for understanding the intrinsic physical properties of vdW heterostructures.

Extending π-electron systems are among the most important topics in physics, chemistry and materials science because they can result in functional materials with applications in electronics and optics. Conventional processes for π-electron extension, however, can generate products exhibiting chemical instability, poor solubility or disordered structures. Herein, we report a novel strategy for the synthesis of π-conjugated polymers within the interiors of carbon nanotubes (CNTs). In this process, thiophene-based oligomers are encapsulated within CNTs as precursors and are subsequently polymerized by thermal annealing. This polymerization increases the effective conjugation length of the thiophenes, as confirmed by transmission electron microscopy and absorption peak red shifts. This work also demonstrates that these polythiophenes can serve as effective markers for individual CNTs during Raman imaging with single-wavelength laser excitation due to their strong absorbance. In addition, stable carrier injection into the encapsulated polythiophenes is found to be possible via electrochemical doping. Such doping has the potential to produce π-electron-based one-dimensional conductive wires and highly stable electrochromic devices.

A van der Waals (vdW) heterostructure composed of multivalley systems can show excitonic optical responses from interlayer excitons that originate from several valleys in the electronic structure. In this work, we studied photoluminescence (PL) from a vdW heterostructure, WS 2 /MoS 2 , deposited on hexagonal boron nitride (hBN) flakes. PL spectra from the fabricated heterostructures observed at room temperature show PL peaks at 1.3-1.7 eV, which are absent in the PL spectra of WS 2 or MoS 2 monolayers alone. The low-energy PL peaks we observed can be decomposed into three distinct peaks. Through detailed PL measurements and theoretical analysis, including PL imaging, time-resolved PL measurements, and calculation of dielectric function ε(ω) by solving the Bethe-Salpeter equation with G 0 W 0 , we concluded that the three PL peaks originate from direct K-K interlayer excitons, indirect Q-Γ interlayer excitons, and indirect K-Γ interlayer excitons.

Water in a nanoconfined geometry has attracted great interest from the viewpoint of not only basic science but also nanofluidic applications. Here, the rotational dynamics of water inside single-walled carbon nanotubes (SWCNTs) with mean diameters larger than ca. 1.4 nm were investigated systematically using 2 H nuclear magnetic resonance spectroscopy with high-purity SWCNTs and molecular dynamics calculations. The results were compared with those for hydrophilic pores. It was found that faster water dynamics could be achieved by increasing the hydrophobicity of the pore walls and decreasing the pore diameters. These results suggest a strategy that paves the way for emerging high-performance filtration/separation devices. Upon cooling below 220 K, it was found that water undergoes a transition from fast to slow dynamics states. These results strongly suggest that the observed transition is linked to a liquid-liquid crossover or transition proposed in a two-liquid states scenario for bulk water.

Making contact of transition metal dichalcogenides (TMDCs) with a metal surface is essential for fabricating and designing electronic devices and catalytic systems. It also generates strain in the TMDCs that plays significant role in both electronic and phonon structures. Therefore, detailed understanding of mechanism of the strain generation is important to fully comprehend the modulation effect for the electronic and phonon properties. Here, MoS 2 and MoSe 2 monolayers are grown on Au surface by chemical vapor deposition and it is demonstrated that the contact with a crystalline Au(111) surface gives rise to only out-of-plane strain in both MoS 2 and MoSe 2 layers, whereas no strain generation is observed on polycrystalline Au or SiO 2 /Si surfaces. Scanning tunneling microscopy analysis provides information regarding consequent specific adsorption sites between lower S (Se) atoms in the SMoS (SeMoSe) structure and Au atoms via unique moiré superstructure formation for MoS 2 and MoSe 2 layers on Au(111). This observation indicates that the specific adsorption sites give rise to out-of-plane strain in the TMDC layers. Furthermore, it also leads to effective modulation of the electronic structure of the MoS 2 or MoSe 2 layer., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Transition-metal dichalcogenide layered materials, consisting of a transition-metal atomic layer sandwiched by two chalcogen atomic layers, have been attracting considerable attention because of their desirable physical properties for semiconductor devices, and a wide variety of pn junctions, which are essential building blocks for electronic and optoelectronic devices, have been realized using these atomically thin structures. Engineering the electronic/optical properties of semiconductors by using such heterojunctions has been a central concept in semiconductor science and technology. Here, we report the first scanning tunneling microscopy/spectroscopy (STM/STS) study on the electronic structures of a monolayer WS2/Mo1-xWxS2 heterojunction that provides a tunable band alignment. The atomically modulated spatial variation in such electronic structures, i.e., a microscopic basis for the band structure of a WS2/Mo1-xWxS2 heterojunction, was directly observed. The macroscopic band structure of Mo1-xWxS2 alloy was well reproduced by the STS spectra averaged over the surface. An electric field of as high as 80 × 10(6) Vm(-1) was observed at the interface for the alloy with x = 0.3, verifying the efficient separation of photoexcited carriers at the interface.

Multiwalled carbon nanotubes (MWCNTs) have the potential for widespread applications in engineering and materials science. However, because of their needle-like shape and high durability, concerns have been raised that MWCNTs may induce asbestos-like pathogenicity. Although recent studies have demonstrated that MWCNTs induce various types of reactivities, the physicochemical features of MWCNTs that determine their cytotoxicity and carcinogenicity in mesothelial cells remain unclear. Here, we showed that the deleterious effects of nonfunctionalized MWCNTs on human mesothelial cells were associated with their diameter-dependent piercing of the cell membrane. Thin MWCNTs (diameter ∼ 50 nm) with high crystallinity showed mesothelial cell membrane piercing and cytotoxicity in vitro and subsequent inflammogenicity and mesotheliomagenicity in vivo. In contrast, thick (diameter ∼ 150 nm) or tangled (diameter ∼ 2-20 nm) MWCNTs were less toxic, inflammogenic, and carcinogenic. Thin and thick MWCNTs similarly affected macrophages. Mesotheliomas induced by MWCNTs shared homozygous deletion of Cdkn2a/2b tumor suppressor genes, similar to mesotheliomas induced by asbestos. Thus, we propose that different degrees of direct mesothelial injury by thin and thick MWCNTs are responsible for the extent of inflammogenicity and carcinogenicity. This work suggests that control of the diameter of MWCNTs could reduce the potential hazard to human health.

A simple and efficient technique is described for measuring photoluminescence (PL) maps of carbon nanotubes (NTs) in the extended IR range (1-2.3 mum). It consists of preparing an NT/surfactant/gelatin film and measuring PL spectra using a combination of a tunable Ti-sapphire laser excitation and FTIR detection. This procedure has been applied to a wide range of single- and double-wall NTs unveiling chirality and diameter distributions that have so far been very difficult to measure. The problems associated with deducing these distributions are discussed by comparing absorption and PL mapping data for NT samples prepared under different conditions.

Selective oxidation of single-wall carbon nanotubes (SWCNTs) by H2O2 was conducted at varying heating times and monitored by UV-vis-NIR spectroscopy. A major increase in the relative absorption intensity indicated a higher than 80% concentration of metallic SWCNTs in the final product. Here, it is suggested that semiconducting SWCNTs are more reactive than metallic SWCNTs because of hole-doping by H2O2, resulting in faster oxidation.