Chemical unzipping of WS2 nanotubes

Toward Edge Engineering of Two-Dimensional Layered Transition-Metal Dichalcogenides by Chemical Vapor Deposition

The manipulation of edge configurations and structures in atomically-thin transition metal dichalcogenides (TMDs) for versatile functionalization has attracted intensive interest in recent years. The chemical vapor deposition (CVD) approach has shown promise for TMD edge engineering of atomic edge configurations (1H, 1T or 1T'-zigzag or armchair edges) as well as diverse edge morphologies (1D nanoribbons, 2D dendrites, 3D spirals, etc.). These edge-rich TMD layers offer versatile candidates for probing the physical and chemical properties and exploring potential applications in electronics, optoelectronics, catalysis, sensing, and quantum technologies. In this Review, we present an overview of the current state-of-the-art in the manipulation of TMD atomic edges and edge-rich structures using CVD. We highlight the vast range of distinct properties associated with these edge configurations and structures and provide insights into the opportunities afforded by such edge-functionalized crystals. The objective of this Review is to motivate further research and development efforts to use CVD as a scalable approach to harness the benefits of such crystal-edge engineering.

Synthesis and Characterization of Transition Metal Dichalcogenide Nanoribbons Based on a Controllable O2 Etching

Although the synthesis of monolayer transition metal dichalcogenides has been established in the last decade, synthesizing nanoribbons remains challenging. In this study, we have developed a straightforward method to obtain nanoribbons with controllable widths (25-8000 nm) and lengths (1-50 μm) by O₂ etching of the metallic phase in metallic/semiconducting in-plane heterostructures of monolayer MoS₂. We also successfully applied this process for synthesizing WS₂, MoSe₂, and WSe₂ nanoribbons. Furthermore, field-effect transistors of the nanoribbons show an on/off ratio of larger than 1000, photoresponses of 1000%, and time responses of 5 s. The nanoribbons were compared with monolayer MoS₂, highlighting a substantial difference in the photoluminescence emission and photoresponses. Additionally, the nanoribbons were used as a template to build one-dimensional (1D)-1D or 1D-2D heterostructures with various transition metal dichalcogenides. The process developed in this study offers simple production of nanoribbons with applications in several fields of nanotechnology and chemistry.

WS2 Nanotube-Embedded SiOC Fibermat Electrodes for Sodium-Ion Batteries

Layered transition metal dichalcogenides (TMDs) such as tungsten disulfide (WS₂) are promising materials for a wide range of applications, including charge storage in batteries and supercapacitors. Nevertheless, TMD-based electrodes suffer from bottlenecks such as capacity fading at high current densities, voltage hysteresis during the conversion reaction, and polysulfide dissolution. To tame such adverse phenomena, we fabricate composites with WS₂ nanotubes. Herein, we report on the superior electrochemical performance of ceramic composite fibers comprising WS₂ nanotubes (WS₂NTs) embedded in a chemically robust molecular polymer-derived ceramic matrix of silicon-oxycarbide (SiOC). Such a heterogeneous fiber structure was obtained via electrospinning of WS₂NT/preceramic polymer solution followed by pyrolysis at elevated temperatures. The electrode capacity fading in WS₂NTs was curbed by the synergistic effect between WS₂NT and SiOC. As a result, the composite electrode exhibits high initial capacity of 454 mAh g^-1 and the capacity retention approximately 2-3 times higher than that of the neat WS₂NT electrode.

Longitudinal unzipping of 2D transition metal dichalcogenides

Unzipping of the basal plane offers a valuable pathway to uniquely control the material chemistry of 2D structures. Nonetheless, reliable unzipping has been reported only for graphene and phosphorene thus far. The single elemental nature of those materials allows a straightforward understanding of the chemical reaction and property modulation involved with such geometric transformations. Here we report spontaneous linear ordered unzipping of bi-elemental 2D MX₂ transition metal chalcogenides as a general route to synthesize 1D nanoribbon structures. The strained metallic phase (1T') of MX₂ undergoes highly specific longitudinal unzipping owing to the self-linearized oxygenation at chalcogenides. Stable dispersions of 1T' MoS₂ nanoribbons with widths of 10-120 nm and lengths up to ~4 µm are produced in water. Edge abundant 1T' MoS₂ nanoribbons reveal the hidden potential of idealized electrocatalysis for hydrogen evolution reactions at a competitive level with the precious Pt catalyst.

Confinement Growth of Layered WS2 in Hollow Beaded Carbon Nanofibers with Synergistic Anchoring Effect to Reinforce Li+ /Na+ Storage Performance

Novel nitrogen doped (N-doped) hollow beaded structural composite carbon nanofibers are successfully applied for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). Tungsten disulfide (WS₂ ) nanosheets are confined, through synergistic anchoring, on the surface and inside of hollow beaded carbon nanofibers (HB CNFs) via a hydrothermal reaction method to construct the hierarchical structure HB WS₂ @CNFs. Benefiting from this unique advantage, HB WS₂ @CNFs exhibits remarkable lithium-storage performance in terms of high rate capability (≈351 mAh g^-1 at 2 A g^-1 ) and stable long-term cycle (≈446 mAh g^-1 at 1 A g^-1 after 100 cycles). Moreover, as an anode material for SIBs, HB WS₂ @CNFs obtains excellent long cycle life and rate performance. During the charging/discharging process, the evolution of morphology and composition of the composite are analyzed by a set of ex situ methods. This synergistic anchoring effect between WS₂ nanosheets and HB CNFs is capable of effectively restraining volume expansion from the metal ions intercalation/deintercalation process and improving the cycling stability and rate performance in LIBs and SIBs.

TMDs beyond MoS2 for Electrochemical Energy Storage

Atomically thin sheets of two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted interest as high capacity electrode materials for electrochemical energy storage devices owing to their unique properties (high surface area, high strength and modulus, faster ion diffusion, and so on), which arise from their layered morphology and diversified chemistry. Nevertheless, low electronic conductivity, poor cycling stability, large structural changes during metal-ion insertion/extraction along with high cost of manufacture are challenges that require further research in order for TMDs to find use in commercial batteries and supercapacitors. Here, a systematic review of cutting-edge research focused on TMD materials beyond the widely studied molybdenum disulfide or MoS₂ electrode is reported. Accordingly, a critical overview of the recent progress concerning synthesis methods, physicochemical and electrochemical properties is given. Trends and opportunities that may contribute to state-of-the-art research are also discussed.

Deriving MoS2 nanoribbons from their flakes by chemical vapor deposition

Two-dimensional (2D) materials have attracted great interest due to their unique structures and exotic properties related to promising applications and fundamental research. Reducing the dimensionality of 2D materials into their 1D nanostructure is also highly desirable for the exploitation of novel properties and offers new research opportunities. In this work, we demonstrate a bottom-up synthesis of molybdenum disulfides (MoS₂) nanoribbons on graphene substrate via chemical vapor deposition (CVD) by precisely tuning the growth parameters into a sulfur-enriched condition. MoS₂ nanoribbons are mainly formed from the CVD grown MoS₂ flakes along the armchair (AC) direction. Atomic resolution ADF-STEM imaging characterizations show an alternating presence of molybdenum and sulfur zigzag edge terminations at the edges of MoS₂ nanoribbons. While at the apex of the nanoribbon, sulfur terminated zigzag edges become dominant. Taking these results together, we revealed the underlying growth mechanism of MoS₂ nanoribbons. Electronic transport properties of the MoS₂ nanoribbons were also measured by fabricating back-gate-effect transistors (FETs). The nanoribbon FETs present n-type behavior with a current on/off ratio higher than 10⁴ at V _DS = 1 and a carrier mobility of 1.39 cm² V^-1 s^-1. This work offers a new route to synthesize 1D MoS₂ nanoribbons, which has great potential in fabricating other 2D materials-derived 1D nanostructures.

WS2 Nanotubes, 2D Nanomeshes, and 2D In-Plane Films through One Single Chemical Vapor Deposition Route

We demonstrate a versatile, catalyst free chemical vapor deposition process on insulating substrates capable of producing in one single stream one-dimensional (1D) WO_{3- x} suboxides leading to a wide range of substrate-supported 2H-WS₂ polymorphs: a tunable class of out-of-plane (of the substrate) nanophases, with 1D nanotubes and a pure WS₂, two-dimensional (2D) nanomesh (defined as a network of webbed, micron-size, few-layer 2D sheets) at its extremes; and in-plane (parallel to the substrate) mono- and few-layer 2D domains. This entails a two-stage approach in which the 2WO₃ + 7S → 2WS₂ + 3SO₂ reaction is intentionally decoupled. First, various morphologies of nanowires or nanorods of high stoichiometry, WO_2.92/WO_2.9 suboxides (belonging to the class of Magnéli phases) were formed, followed by their sulfurization to undergo reduction to the aforementioned WS₂ polymorphs. The continuous transition of WS₂ from nanotubes to the out-of-plane 2D nanomesh, via intermediary, mixed 1D-2D phases, delivers tunable functional properties, for example, linear and nonlinear optical properties, such as reflectivity (linked to optical excitations in the material), and second harmonic generation (SHG) and onset of saturable absorption. The SHG effect is very strong across the entire tunable class of WS₂ nanomaterials, weakest in nanotubes, and strongest in the 2D nanomesh. Furthermore, a mechanism via suboxide (WO_{3- x}) intermediate as a possible path to 2D domain growth is demonstrated. 2D, in-plane WS₂ domains grow via "self-seeding and feeding" where short WO_2.92/WO_2.9 nanorods provide both the nucleation sites and the precursor feedstock. Understanding the reaction path (here, in the W-O-S space) is an emerging approach toward controlling the nucleation, growth, and morphology of 2D domains and films of transition-metal dichalcogenides.

Flexible Molybdenum Disulfide (MoS2) Atomic Layers for Wearable Electronics and Optoelectronics

Flexible, stretchable, and bendable materials, including inorganic semiconductors, organic polymers, graphene, and transition metal dichalcogenides (TMDs), are attracting great attention in such areas as wearable electronics, biomedical technologies, foldable displays, and wearable point-of-care biosensors for healthcare. Among a broad range of layered TMDs, atomically thin layered molybdenum disulfide (MoS₂) has been of particular interest, due to its exceptional electronic properties, including tunable bandgap and charge carrier mobility. MoS₂ atomic layers can be used as a channel or a gate dielectric for fabricating atomically thin field-effect transistors (FETs) for electronic and optoelectronic devices. This review briefly introduces the processing and spectroscopic characterization of large-area MoS₂ atomically thin layers. The review summarizes the different strategies in enhancing the charge carrier mobility and switching speed of MoS₂ FETs by integrating high-κ dielectrics, encapsulating layers, and other 2D van der Waals layered materials into flexible MoS₂ device structures. The photoluminescence (PL) of MoS₂ atomic layers has, after chemical treatment, been dramatically improved to near-unity quantum yield. Ultraflexible and wearable active-matrix organic light-emitting diode (AM-OLED) displays and wafer-scale flexible resistive random-access memory (RRAM) arrays have been assembled using flexible MoS₂ transistors. The review discusses the overall recent progress made in developing MoS₂ based flexible FETs, OLED displays, nonvolatile memory (NVM) devices, piezoelectric nanogenerators (PNGs), and sensors for wearable electronic and optoelectronic devices. Finally, it outlines the perspectives and tremendous opportunities offered by a large family of atomically thin-layered TMDs.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS₂, MoSe₂, MoTe₂, WS₂ and WSe₂, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1-2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

TMD-based highly efficient electrocatalysts developed by combined computational and experimental approaches

A thorough review on combined computational and experimental approaches to develop TMD-based highly efficient electrocatalysts by site doping, phase modulation, control of growth morphology and construction of heterostructures.

Synthesis of 2D-Mesoporous-Carbon/MoS2 Heterostructures with Well-Defined Interfaces for High-Performance Lithium-Ion Batteries

A sandwich-like 2D-mesoporous-carbon/MoS₂ -nanosheet heterostructure is fabricated for the first time. The hybrid structure is composed of three well-stacked monolayers: an ordered-mesoporous-carbon monolayer, a MoS₂ monolayer, and a further ordered-mesoporous-carbon monolayer. This unique heterostructure exhibits excellent electrochemical performance as an anode material for lithium-ion batteries.

Multifunctional heterostructures constructed using MoS2 and WS2 nanoribbons

Using first-principles calculations based on nonequilibrium Green's function together with density functional theory, we investigated the electronic transport properties of some devices consisting of armchair and zigzag MoS₂NRs/WS₂NRs in-plane heterostructures. The results indicate that these heterostructures exhibit rectifying performance, which are depressed as the number of WS₂NR unit cell decreases. In addition, the NDR effect is observed and can be modulated. Importantly, the zigzag MoS₂NRs/WS₂NRs heterostructures can be used as spintronic devices because of their tunable spin filtering behavior and NDR effect. The devices with WS₂NRs electrode display a better NDR effect and spin filtering behavior. The unique properties of these heterostructures suggest promising applications in the next generation nanoelectronic devices.

Electronic and optical properties of graphene nanoribbons in external fields

A review work is done for the electronic and optical properties of graphene nanoribbons in magnetic, electric, composite, and modulated fields. Effects due to the lateral confinement, curvature, stacking, non-uniform subsystems and hybrid structures are taken into account. The special electronic properties, induced by complex competitions between external fields and geometric structures, include many one-dimensional parabolic subbands, standing waves, peculiar edge-localized states, width- and field-dependent energy gaps, magnetic-quantized quasi-Landau levels, curvature-induced oscillating Landau subbands, crossings and anti-crossings of quasi-Landau levels, coexistence and combination of energy spectra in layered structures, and various peak structures in the density of states. There exist diverse absorption spectra and different selection rules, covering edge-dependent selection rules, magneto-optical selection rule, splitting of the Landau absorption peaks, intragroup and intergroup Landau transitions, as well as coexistence of monolayer-like and bilayer-like Landau absorption spectra. Detailed comparisons are made between the theoretical calculations and experimental measurements. The predicted results, the parabolic subbands, edge-localized states, gap opening and modulation, and spatial distribution of Landau subbands, have been identified by various experimental measurements.

Nanoscale characterization of the thermal interface resistance of a heat-sink composite material by in situ TEM

We developed an original method of in situ nanoscale characterization of thermal resistance utilizing a high-resolution transmission electron microscope (HRTEM). The focused electron beam of the HRTEM was used as a contact-free heat source and a piezo-movable nanothermocouple was developed as a thermal detector. This method has a high flexibility of supplying thermal-flux directions for nano/microscale thermal conductivity analysis, and is a powerful way to probe the thermal properties of complex or composite materials. Using this method we performed reproducible measurements of electron beam-induced temperature changes in pre-selected sections of a heat-sink α-Al(2)O(3)/epoxy-based resin composite. Observed linear behavior of the temperature change in a filler reveals that Fourier's law holds even at such a mesoscopic scale. In addition, we successfully determined the thermal resistance of the nanoscale interfaces between neighboring α-Al(2)O(3) fillers to be 1.16 × 10(-8) m(2)K W(-1), which is 35 times larger than that of the fillers themselves. This method that we have discovered enables evaluation of thermal resistivity of composites on the nanoscale, combined with the ultimate spatial localization and resolution sample analysis capabilities that TEM entails.

Clean WS2 and MoS2 Nanoribbons Generated by Laser-Induced Unzipping of the Nanotubes

The preparation of 1D WS(2) and MoS(2) flexible nanoribbons by laser-induced unzipping of the nanotubes is reported. The nanoribbons are of high quality, uniform width, and devoid of surface contamination. The zig-zag edges in WS(2) nanoribbons give rise to ferromagnetism at room temperature.

Electronic transport properties of in-plane heterostructures constructed by MoS2 and WS2 nanoribbons

A new and simple kind of in-plane heterostructure is constructed by MoS₂ nanoribbons (MoS₂NRs) and WS₂ nanoribbons (WS₂NRs) arranged both perpendicularly and in parallel.

Facile one-step mechanochemical synthesis of [Cu(tu)]Cl·1/2H2O nanobelts for high-performance supercapacitor

A facile one-step mechanochemical process from CuCl₂·2H₂O and thiourea to fabricate novel [Cu(tu)]Cl·1/2H₂O nanobelts has been observed for the first time, and the nanobelts were used as an electrode material for a supercapacitor.

Scalable large nanosheets of transition metal disulphides through exfoliation of amine intercalated MS2 [M = Mo, W] in organic solvents

Amine intercalated MS₂ exfoliate in organic solvents to give large 2D nanosheets.

A universal method for preparation of noble metal nanoparticle-decorated transition metal dichalcogenide nanobelts

MoS2, TaS2, TiS2, WSe2 and TaSe2 nanobelts decorated with a PtAg alloy or Pt NPs have been successfully synthesized by etching 2D nanosheets under a mild reaction condition followed by a subsequent nanosheet-to-nanobelt transformation mediated by the PVP template. The PtAg-MoS2 hybrid nanobelt coated with PVP is used as the active material in a memory device, which exhibits hysteresis behavior with the function of dynamic random access memory.

Component-controllable WS(2(1-x))Se(2x) nanotubes for efficient hydrogen evolution reaction

Owing to the excellent potential for fundamental research and technical applications in optoelectronic devices and catalytic activity for hydrogen evolution reaction (HER), transition metal dichalcogenides have recently attracted much attention. Transition metal sulfide nanostructures have been reported and demonstrated promising application in transistors and photodetectors. However, the growth of transition metal selenide nanostructures and their applications has still been a challenge. In this work, we successfully synthesized high-quality WSe2 nanotubes on carbon fibers via selenization. More importantly, through optimizing the growth conditions, ternary WS2(1–x)Se2x nanotubes were synthesized and the composition of S and Se can be systematically controlled. The as-grown WS2(1–x)Se2x nanotubes on carbon fibers, assembled as a working electrode, revealing low overpotential, high exchange current density, and small series resistance, exhibit excellent electrocatalytic properties for hydrogen evolution reaction. Our study provides the experimental groundwork for the synthesis of low-dimensional transition metal dichalcogenides and may open up exciting opportunities for their application in electronics, photoelectronics, and catalytic electrochemical reactions.

Theoretical aspects of WS₂ nanotube chemical unzipping

Theoretical analysis of experimental data on unzipping multilayered WS₂ nanotubes by consequent intercalation of lithium atoms and 1-octanethiol molecules [C. Nethravathi, et al., ACS Nano, 2013, 7, 7311] is presented. The radial expansion of the tube was described using continuum thin-walled cylinder approximation with parameters evaluated from ab initio calculations. Assuming that the attractive driving force of the 1-octanethiol molecule is its reaction with the intercalated Li ions ab initio calculations of a 1-octanethiol molecule bonding with Li(+) were carried out. In addition, the non-chemical interactions of the 1-octanethiol dipole with an array of positive point charges representing Li(+) were taken into account. Comparing between the energy gain from these interactions and the elastic strain energy of the nanotube allows us to evaluate a value for the tube wall deformation after the implantation of 1-octanethiol molecules. The ab initio molecular dynamics simulation confirmed our estimates and demonstrated that a strained WS₂ nanotube, with a decent concentration of 1-octanethiol molecules, should indeed be unzipped into the WS₂ nanoribbon.