Chemical Vapor Deposition of High‐Quality and Atomically Layered ReS2

Recently, anisotropic 2D materials, such as black phosphorus and rhenium disulfides (ReS₂), have attracted a lot attention because of their unique applications on electronics and optoelectronics. In this work, the direct growth of high‐quality ReS₂ atomic layers and nanoribbons has been demonstrated by using chemical vapor deposition (CVD) method. A possible growth mechanism is proposed according to the controlled experiments. The CVD ReS₂‐based filed‐effect transistors (FETs) show n‐type semiconducting behavior with a current on/off ratio of ≈10⁶ and a charge carrier mobility of ≈9.3 cm² Vs⁻¹. These results suggested that the quality of CVD grown ReS₂ is comparable to mechanically exfoliated ReS₂, which is also further supported by atomic force microscopy imaging, high‐resolution transmission electron microscopy imaging and thickness‐dependent Raman spectra. The study here indicates that CVD grown ReS₂ may pave the way for the large‐scale fabrication of ReS₂‐based high‐performance optoelectronic devices, such as anisotropic FETs and polarization detection.     

Epitaxial Growth of Rectangle Shape MoS2 with Highly Aligned Orientation on Twofold Symmetry a‐Plane Sapphire

Research on transition metal dichalcogenides (TMDs) has been accelerated by the development of large‐scale synthesis based on chemical vapor deposition (CVD) growth. However, in most cases, CVD‐grown TMDs are composed of randomly oriented grains, and thus contain many distorted grain boundaries (GBs), which seriously degrade their electrical and photoelectrical properties. Here, the epitaxial growth of highly aligned MoS₂ grains is reported on a twofold symmetry a‐plane sapphire substrate. The obtained MoS₂ grains have an unusual rectangle shape with perfect orientation alignment along the [1‐100] crystallographic direction of a‐plane sapphire. It is found that the growth temperature plays a key role in its orientation alignment and morphology evolution, and high temperature is beneficial to the initial MoS₂ seeds rotate to the favorable orientation configurations. In addition, the photoluminescence quenching of the well‐aligned MoS₂ grains indicates a strong MoS₂?substrate interaction which induces the anisotropic growth of MoS₂, and thus brings the formation of rectangle shape grains. Moreover, the well‐aligned MoS₂ grains splice together without GB formation, and thus that has negligible effect on its electrical transport properties. The progress achieved in this work could promote the controlled synthesis of large‐area TMDs single crystal film and the scalable fabrication of high‐performance electronic devices.     

Colloidal Single‐Layer Photocatalysts for Methanol‐Storable Solar H2 Fuel

Molecular surfactants are widely used to control low‐dimensional morphologies, including 2D nanomaterials in colloidal chemical synthesis, but it is still highly challenging to accurately control single‐layer growth for 2D materials. A scalable stacking‐hinderable strategy to not only enable exclusive single‐layer growth mode for transition metal dichalcogenides (TMDs) selectively sandwiched by surfactant molecules but also retain sandwiched single‐layer TMDs' photoredox activities is developed. The single‐layer growth mechanism is well explained by theoretical calculation. Three types of single‐layer TMDs, including MoS₂, WS₂, and ReS₂, are successfully synthesized and demonstrated in solar H₂ fuel production from hydrogen‐stored liquid carrier—methanol. Such H₂ fuel production from single‐layer MoS₂ nanosheets is CO_x‐free and reliably workable under room temperature and normal pressure with the generation rate reaching ≈617 µmole g^?1 h^?1 and excellent photoredox endurability. This strategy opens up the feasible avenue to develop methanol‐storable solar H₂ fuel with facile chemical rebonding actualized by 2D single‐layer photocatalysts.     

A General Method for the Chemical Synthesis of Large‐Scale,Seamless Transition Metal Dichalcogenide Electronics

The capability to directly build atomically thin transition metal dichalcogenide (TMD) devices by chemical synthesis offers important opportunities to achieve large‐scale electronics and optoelectronics with seamless interfaces. Here, a general approach for the chemical synthesis of a variety of TMD (e.g., MoS₂, WS₂, and MoSe₂) device arrays over large areas is reported. During chemical vapor deposition, semiconducting TMD channels and metallic TMD/carbon nanotube (CNT) hybrid electrodes are simultaneously formed on CNT‐patterned substrate, and then coalesce into seamless devices. Chemically synthesized TMD devices exhibit attractive electrical and mechanical properties. It is demonstrated that chemically synthesized MoS₂–MoS₂/CNT devices have Ohmic contacts between MoS₂/CNT hybrid electrodes and MoS₂ channels. In addition, MoS₂–MoS₂/CNT devices show greatly enhanced mechanical stability and photoresponsivity compared with conventional gold‐contacted devices, which makes them suitable for flexible optoelectronics. Accordingly, a highly flexible pixel array based on chemically synthesized MoS₂–MoS₂/CNT photodetectors is applied for image sensing.     

Lowering the Schottky Barrier Height by Graphene/Ag Electrodes for High‐Mobility MoS2 Field‐Effect Transistors

2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post‐silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high‐performance devices while adapting for large‐area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS₂ devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD‐grown MoS₂ film and a Ag electrode as an interfacial layer. The MoS₂ field‐effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field‐effect mobility of 35 cm² V^?1 s^?1, an on/off current ratio of 4 × 10⁸, and a photoresponsivity of 2160 A W^?1, compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n‐doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS₂ and electrodes. This demonstration of contact interface engineering with CVD‐grown MoS₂ and graphene is a key step toward the practical application of atomically thin TMDC‐based devices with low‐resistance contacts for high‐performance large‐area electronics and optoelectronics.     

Stabilizing MoS2 Nanosheets through SnO2 Nanocrystal Decoration for High‐Performance Gas Sensing in Air

The unique properties of MoS₂ nanosheets make them a promising candidate for high‐performance room temperature sensing. However, the properties of pristine MoS₂ nanosheets are strongly influenced by the significant adsorption of oxygen in an air environment, which leads to instability of the MoS₂ sensing device, and all sensing results on MoS₂ reported to date were exclusively obtained in an inert atmosphere. This significantly limits the practical sensor application of MoS₂ in an air environment. Herein, a novel nanohybrid of SnO₂ nanocrystal (NC)‐decorated crumpled MoS₂ nanosheet (MoS₂/SnO₂) and its exciting air‐stable property for room temperature sensing of NO₂ are reported. Interestingly, the SnO₂ NCs serve as strong p‐type dopants for MoS₂, leading to p‐type channels in the MoS₂ nanosheets. The SnO₂ NCs also significantly enhance the stability of MoS₂ nanosheets in dry air. As a result, unlike other MoS₂ sensors operated in an inert gas (e.g. N₂), the nanohybrids exhibit high sensitivity, excellent selectivity, and repeatability to NO₂ under a practical dry air environment. This work suggests that NC decoration significantly tunes the properties of MoS₂ nanosheets for various applications.     

Synthesis of Few‐Layer MoS2 Nanosheet‐Coated TiO2 Nanobelt Heterostructures for Enhanced Photocatalytic Activities

MoS₂ nanosheet‐coated TiO₂ nanobelt heterostructures—referred to as TiO₂@MoS₂—with a 3D hierarchical configuration are prepared via a hydrothermal reaction. The TiO₂ nanobelts used as a synthetic template inhibit the growth of MoS₂ crystals along the c‐axis, resulting in a few‐layer MoS₂ nanosheet coating on the TiO₂ nanobelts. The as‐prepared TiO₂@MoS₂ heterostructure shows a high photocatalytic hydrogen production even without the Pt co‐catalyst. Importantly, the TiO₂@MoS₂ heterostructure with 50 wt% of MoS₂ exhibits the highest hydrogen production rate of 1.6 mmol h^?1g^?1. Moreover, such a heterostructure possesses a strong adsorption ability towards organic dyes and shows high performance in photocatalytic degradation of the dye molecules.     

Layer Thinning and Etching of Mechanically Exfoliated MoS2 Nanosheets by Thermal Annealing in Air

A simple thermal annealing method for layer thinning and etching of mechanically exfoliated MoS₂ nanosheets in air is reported. Using this method, single‐layer (1L) MoS₂ nanosheets are achieved after the thinning of MoS₂ nanosheets from double‐layer (2L) to quadri‐layer (4L) at 330 °C. The as‐prepared 1L MoS₂ nanosheet shows comparable optical and electrical properties with the mechanically exfoliated, pristine one. In addition, for the first time, the MoS₂ mesh with high‐density of triangular pits is also fabricated at 330 °C, which might arise from the anisotropic etching of the active MoS₂ edge sites. As a result of thermal annealing in air, the thinning of MoS₂ nanosheet is possible due to its oxidation to form MoO₃. Importantly, the MoO₃ fragments on the top of thinned MoS₂ layer induces the hole injection, resulting in the p‐type channel in fabricated field‐effect transistors.     

Gram‐Scale Aqueous Synthesis of Stable Few‐Layered 1T‐MoS2: Applications for Visible‐Light‐Driven Photocatalytic Hydrogen Evolution

Most recently, much attention has been devoted to 1T phase MoS₂ because of its distinctive phase‐engineering nature and promising applications in catalysts, electronics, and energy storage devices. While alkali metal intercalation and exfoliation methods have been well developed to realize unstable 1T‐MoS₂, but the aqueous synthesis for producing stable metallic phase remains big challenging. Herein, a new synthetic protocol is developed to mass‐produce colloidal metallic 1T‐MoS₂ layers highly stabilized by intercalated ammonium ions (abbreviated as N‐MoS₂). In combination with density functional calculations, the X‐ray diffraction pattern and Raman spectra elucidate the excellent stability of metallic phase. As clearly depicted by high‐angle annular dark‐field imaging in an aberration‐corrected scanning transmission electron microscope and extended X‐ray absorption fine structure, the N‐MoS₂ exhibits a distorted octahedral structure with a 2a ₀ × a ₀ basal plane superlattice and 2.72 Å Mo–Mo bond length. In a proof‐of‐concept demonstration for the obtained material's applications, highly efficient photocatalytic activity is achieved by simply hybridizing metallic N‐MoS₂ with semiconducting CdS nanorods due to the synergistic effect. As a direct outcome, this CdS:N‐MoS₂ hybrid shows giant enhancement of hydrogen evolution rate, which is almost 21‐fold higher than pure CdS and threefold higher than corresponding annealed CdS:2H‐MoS₂.     

Liquid Exfoliation of Colloidal Rhenium Disulfide Nanosheets as a Multifunctional Theranostic Agent for In Vivo Photoacoustic/CT Imaging and Photothermal Therapy

Near‐infrared light‐mediated theranostic agents with superior tissue penetration and minimal invasion have captivated researchers in cancer research in the past decade. Herein, a probe sonication‐assisted liquid exfoliation approach for scalable and continual synthesis of colloidal rhenium disulfide nanosheets, which is further explored as theranostic agents for cancer diagnosis and therapy, is reported. Due to high‐Z element of Re (Z = 75) and significant photoacoustic effect, the obtained PVP‐capped ReS₂ nanosheets are evaluated as bimodality contrast agents for computed tomography and photoacoustic imaging. In addition, utilizing the strong near‐infrared absorption and ultrahigh photothermal conversion efficiency (79.2%), ReS₂ nanosheets could also serve as therapeutic agents for photothermal ablation of tumors with a tumor elimination rate up to 100%. Importantly, ReS₂ nanosheets show no obvious toxicity based on the cytotoxicity assay, serum biochemistry, and histological analysis. This work highlights the potentials of ReS₂ nanosheets as a single‐component theranostic nanoplatform for bioimaging and antitumor therapy.     

Site‐Selective and van der Waals Epitaxial Growth of Rhenium Disulfide on Graphene

The surface property of growth substrate imposes significant influence in the growth behaviors of 2D materials. Rhenium disulfide (ReS₂) is a new family of 2D transition metal dichalcogenides with unique distorted 1T crystal structure and thickness‐independent direct bandgap. The role of growth substrate is more critical for ReS₂ owing to its weak interlayer coupling property, which leads to preferred growth along the out‐of‐plane direction while suppressing the uniform in‐plane growth. Herein, graphene is introduced as the growth substrate for ReS₂ and the synthesis of graphene/ReS₂ vertical heterostructure is demonstrated via chemical vapor deposition. Compared with the rough surface of SiO₂/Si substrate with dangling bonds which hinders the uniform growth of ReS₂, the inert and smooth surface nature of graphene sheet provides a lower energy barrier for migration of the adatoms, thereby promoting the growth of ReS₂ on the graphene surface along the in‐plane direction. Furthermore, patterning of the graphene/ReS₂ heterostructure is achieved by the selective growth of ReS₂, which is attributed to the strong binding energy between sulfur atoms and graphene surface. The fundamental studies in the role of graphene as the growth template in the formation of van der Waals heterostructures provide better insights into the synthesis of 2D heterostructures.     

Atomic‐Level Customization of 4 in. Transition Metal Dichalcogenide Multilayer Alloys for Industrial Applications

Despite many encouraging properties of transition metal dichalcogenides (TMDs), a central challenge in the realm of industrial applications based on TMD materials is to connect the large‐scale synthesis and reproducible production of highly crystalline TMD materials. Here, the primary aim is to resolve simultaneously the two inversely related issues through the synthesis of MoS_{2(1?x )}Se_2x ternary alloys with customizable bichalcogen atomic (S and Se) ratio via atomic‐level substitution combined with a solution‐based large‐area compatible approach. The relative concentration of bichalcogen atoms in the 2D alloy can be effectively modulated by altering the selenization temperature, resulting in 4 in. scale production of MoS_1.62Se_0.38, MoS_1.37Se_0.63, MoS_1.15Se_0.85, and MoS_0.46Se_1.54 alloys, as well as MoS₂ and MoSe₂. Comprehensive spectroscopic evaluations for vertical and lateral homogeneity in terms of heteroatom distribution in the large‐scale 2D TMD alloys are implemented. Se‐stimulated strain effects and a detailed mechanism for the Se substitution in the MoS₂ crystal are further explored. Finally, the capability of the 2D alloy for industrial application in nanophotonic devices and hydrogen evolution reaction (HER) catalysts is validated. Substantial enhancements in the optoelectronic and HER performances of the 2D ternary alloy compared with those of its binary counterparts, including pure‐phase MoS₂ and MoSe₂, are unambiguously achieved.     

Colloids of Holey Gd2O3 Nanosheets Converted from Exfoliated Gadolinium Hydroxide Layers

This paper proposes a confined solid‐state conversion approach using layered metal‐hydroxides for the production of a colloidal suspension of porous 2D crystalline metal oxide layers with superior electrochemical H₂O₂ sensing performance. This study investigates the conversion chemistry of delaminated layers of gadolinium hydroxide (LGdH), [Gd₂(OH)₅]⁺, encapsulated in a silica nanoshell that provides an antistacking and antisintering environment during the phase‐transition at high temperature. Thermal treatment of the LGdH layers within the protected environment results in a dimensionally confined phase‐transition into crystalline Gd₂O₃ nanosheets with an isomorphic 2D structure. Furthermore, annealing at higher temperatures leads to the evolution of in‐plane mesoporous structure on the Gd₂O₃ nanosheet. Based on insight acquired from in‐depth investigation, the evolution of in‐plane porosity proceeds through the in‐plane dominant silicate‐formation reaction at the interface with the surrounding silica shell. Their 2D‐anisotropic and mesoporous morphological features are preserved, producing a colloidal suspension of holey nanosheets that can be used to fabricate a thin and porous film through wet‐coating deposition. This study also demonstrates the superior electrochemical H₂O₂ sensing ability of the resultant porous Gd₂O₃ film, which represents a ≈1000‐ and 10‐fold enhancement of the detection limit and sensitivity, respectively, in comparison to previously reported Gd₂O₃ films.     

Two dimensional MoS<Subscript>2</Subscript>/CNT hybrid ink for paper-based capacitive energy storage

Developing two dimensional (2D) materials based ink is an advanced method for fabricating printable and flexible electronic devices. 2D few-layered molybdenum disulfide (MoS₂) reveals a great potential for capacitive energy storage because of its layered structure (for ion intercalation), high surface area (provide active sites) and multi-valence state of Mo (introduce pseudocapacitive reactions). These unique properties could intensively improve the potential of MoS₂ for supercapacitors. However, MoS₂ is a semiconductor with low conductivity, which limits its performance in electrochemistry. In the meantime, MoS₂ based ink for flexible energy storage application has been barely investigated. In this work, we design a MoS₂ and carbon nanotube (MoS₂/CNT) hybrid ink that uses exfoliated MoS₂ nanosheet and CNT to fabricate a paper-based supercapacitor. A strong synergistic effect between MoS₂ and CNT in capacitive performance was observed due to the good conductivity of CNT and high capacitance of MoS₂. Paper-based solid-state device is also fabricated which reveals good flexibility and high capacitive performance. This hybrid ink represents a new road for flexible paper-based devices.     

二维纳米片层结构MoS2和其改性材料的合成及在环境领域的应用

类石墨烯的二维纳米片层状MoS_2作为过渡金属硫化物材料家族的典型代表,由于其各方面独特的性能尤其是超薄的厚度和二维结构,已成为近些年材料的研究热点。独特的三明治结构使其具有良好的物理化学性质,MoS_2及其复合材料在催化、传感器、润滑等方面有广阔的前景。总结了国内外MoS_2的合成及其改性方法、性能和应用。针对社会发展过程中日益严重的环境污染问题,重点介绍利用其吸附和催化性能去除环境中污染物,并对其未来发展进行了展望。     

Hierarchical MoS2 Nanosheet@TiO2 Nanotube Array Composites with Enhanced Photocatalytic and Photocurrent Performances

A novel type of hierarchical nanocomposites consisted of MoS₂ nanosheet coating on the self‐ordered TiO₂ nanotube arrays is successfully prepared by a facile combination of anodization and hydrothermal methods. The MoS₂ nanosheets are uniformly decorated on the tube top surface and the intertubular voids with film appearance changing from brown to black color. Anatase TiO₂ nanotube arrays (NTAs) with clean top surfaces and the appropriate amount of MoS₂ precursors are key to the growth of perfect compositing TiO₂@MoS₂ hybrids with significantly enhanced photocatalytic activity and photocurrent response. These results reveal that the strategy provides a flexible and straightforward route for design and preparation nanocomposites based on functional semiconducting nanostructures with 1D self‐ordered TiO₂ NTAs, promising for new opportunities in energy/environment applications, including photocatalysts and other photovoltaic devices.     

Synthesis of Ultrahigh‐Quality Monolayer Molybdenum Disulfide through In Situ Defect Healing with Thiol Molecules

Monolayer transition metal dichalcogenides are 2D materials with many potential applications. Chemical vapor deposition (CVD) is a promising method to synthesize these materials. However, CVD‐grown materials generally have poorer quality than mechanically exfoliated ones and contain more defects due to the difficulties in controlling precursors' distribution and concentration during growth where solid precursors are used. Here, thiol is proposed to be used as a liquid precursor for CVD growth of high quality and uniform 2D MoS₂. Atomic‐resolved structure characterizations indicate that the concentration of sulfur vacancies in the MoS₂ grown from thiol is the lowest among all reported CVD samples. Low temperature spectroscopic characterization further reveals the ultrahigh optical quality of the grown MoS₂. Density functional theory simulations indicate that thiol molecules could interact with sulfur vacancies in MoS₂ and repair these defects during the growth of MoS₂, resulting in high‐quality MoS₂. This work provides a facile and controllable method for the growth of high‐quality 2D materials with ultralow sulfur vacancies and high optical quality, which will benefit their optoelectronic applications.     

Solution‐Phase Epitaxial Growth of Perovskite Films on 2D Material Flakes for High‐Performance Solar Cells

The quality of perovskite films is critical to the performance of perovskite solar cells. However, it is challenging to control the crystallinity and orientation of solution‐processed perovskite films. Here, solution‐phase van der Waals epitaxy growth of MAPbI₃ perovskite films on MoS₂ flakes is reported. Under transmission electron microscopy, in‐plane coupling between the perovskite and the MoS₂ crystal lattices is observed, leading to perovskite films with larger grain size, lower trap density, and preferential growth orientation along (110) normal to the MoS₂ surface. In perovskite solar cells, when perovskite active layers are grown on MoS₂ flakes coated on hole‐transport layers, the power conversion efficiency is substantially enhanced for 15%, relatively, due to the increased crystallinity of the perovskite layer and the improved hole extraction and transfer rate at the interface. This work paves a way for preparing high‐performance perovskite solar cells and other optoelectronic devices by introducing 2D materials as interfacial layers.     

Thermally Induced Bending of ReS2 Nanowalls

Among the variety of stimuli‐responsive materials, temperature‐responsive materials (TRMs) can adapt to the surrounding environment in the presence of a thermal stimulus, and they have attracted considerable attention in sensors, actuators, and surface engineering. However, polymers, as the most representative TRMs, are far from ideal with respect to long‐term reliability and durability. Here, for the first time, an inorganic material, ReS₂, is analyzed, which possesses an unexpected temperature‐responsive behavior that is triggered by stable and reversible thermally induced bending (TIB). Due to thermal fluctuations in the ReS₂ layers, intrinsic ripples tend to aggravate rapidly with rising temperature. Then, the weak interlayer interaction of ReS₂ is further weakened, thus resulting in interlayer sliding. Due to a decrease in bending rigidity with increasing temperature, out‐of‐plane bending spontaneously occurs in the ReS₂ layers. Interestingly, this TIB of ReS₂ can recover to its initial configuration when the temperature drops, which is further confirmed by the reversible wetting measurement. Above all, the TIB behavior of ReS₂ exhibits great potential in smart applications, such as smart windows and microfluidic devices, and fills the significant gaps of inorganic TRMs.     

Tunable Fabrication of Molybdenum Disulfide Quantum Dots for Intracellular MicroRNA Detection and Multiphoton Bioimaging

Molybdenum disulfide (MoS₂) quantum dots (QDs) (size <10 nm) possess attractive new properties due to the quantum confinement and edge effects as graphene QDs. However, the synthesis and application of MoS₂ QDs has not been investigated in great detail. Here, a facile and efficient approach for synthesis of controllable‐size MoS₂ QDs with excellent photoluminescence (PL) by using a sulfuric acid‐assisted ultrasonic route is developed for this investigation. Various MoS₂ structures including monolayer MoS₂ flake, nanoporous MoS₂, and MoS₂ QDs can be yielded by simply controlling the ultrasonic durations. Comprehensive microscopic and spectroscopic tools demonstrate that the MoS₂ QDs have uniform lateral size and possess excellent excitation‐independent blue PL. The as‐generated MoS₂ QDs show high quantum yield of 9.65%, long fluorescence lifetime of 4.66 ns, and good fluorescent stability over broad pH values from 4 to 10. Given the good intrinsic optical properties and large surface area combined with excellent physiological stability and biocompatibility, a MoS₂ QDs‐based intracellular microRNA imaging analysis system is successfully constructed. Importantly, the MoS₂ QDs show good performance as multiphoton bioimaging labeling. The proposed synthesis strategy paves a new way for facile and efficient preparing MoS₂ QDs with tunable‐size for biomedical imaging and optoelectronic devices application.