Preparation and Applications of Mechanically Exfoliated Single-Layer and Multilayer MoS2and WSe2Nanosheets

Adsorption studies of alcohol molecules on monolayer MoS2 nanosheet—A first-principles insights

Molybdenum disulfide (MoS2) is an extremely intriguing material because of its unique electrical and optical properties. The preparation of large-area and high-quality MoS2 nanosheets is an important step in a wide range of applications. This study demonstrates that monolayer and few-layer MoS2 nanosheets can be obtained from electrochemical exfoliation of bulk MoS2 crystals. The lateral size of the exfoliated MoS2 nanosheets is in the 5-50 μm range, which is much larger than that of chemically or liquid-phase exfoliated MoS2 nanosheets. The MoS2 nanosheets undergo low levels of oxidation during electrochemical exfoliation. In addition, microscopic and spectroscopic characterizations indicate that the exfoliated MoS2 nanosheets are of high quality and have an intrinsic structure. A back-gate field-effect transistor was fabricated using an exfoliated monolayer MoS2 nanosheet. The on/off current ratio is over 10(6), and the field-effect mobility is approximately 1.2 cm(2) V(-1) s(-1); these values are comparable to the results for micromechanically exfoliated MoS2 nanosheets. The electrochemical exfoliation method is simple and scalable, and it can be applied to exfoliate other transition metal dichalcogenides.

High-yield exfoliation of MoS2 (WS2) monolayers towards efficient photocatalytic hydrogen evolution

The low light absorption of transition-metal dichalcogenide (TMDC) nanosheets hinders their application as high-performance optoelectronic devices. Rolling them up into one-dimensional (1D) nanoscrolls and decorating them with plasmonic nanoparticles (NPs) are both effective strategies for enhancing their performance. When these two approaches are combined, in this work, the light-matter interaction in TMDC nanosheets is greatly improved by encapsulating silver nanoparticles (Ag NPs) in TMDC nanoscrolls. After the silver nitrate (AgNO3) solution was spin-coated on monolayer (1L) MoS2 and WS2 nanosheets grown by chemical vapor deposition, Ag NPs were homogeneously formed to obtain MoS2-Ag and WS2-Ag nanosheets due to the TMDC-assisted spontaneous reduction, and their size and density can be well controlled by tuning the concentration of the AgNO3 solution. By the simple placement of alkaline droplets on MoS2-Ag or WS2-Ag hybrid nanosheets, MoS2-Ag or WS2-Ag nanoscrolls with large sizes were obtained in large area. The obtained hybrid nanoscrolls exhibited up to 500 times increased photosensitivities compared with 1L MoS2 nanosheets, arising from the localized surface plasmon resonance effect of Ag NPs and the scrolled-nanosheet structure. Our work provides a reliable method for the facile and large-area preparation of NP/nanosheet hybrid nanoscrolls and demonstrates their great potential for high-performance optoelectronic devices.

In this work, MoS2 nanosheets were obtained successfully using the liquid phase exfoliation method assisted by formamide solvothermal treatment. The exfoliation efficiency in N-methyl-2-pyrrolidone (NMP) was enhanced by the synergetic effect of easier intercalation of polar solvent and higher repulsive force of the treated bulk MoS2. The exfoliated MoS2 nanosheets were assembled alternately with H3Mo12O40P (PMo12) into a multilayer heterostructure by the layer-by-layer (LBL) method, in which PMo12 with high electron mobility bridges the adjacent catalytically active MoS2 layers. Based on the heterostructure, the electrocatalytic performance for hydrogen evolution was substantially enhanced over multilayer MoS2 nanosheets alone. Moreover, it was found that the electrocatalytic performance was influenced by the layer number, indicating that an optimum balance between the mass transfer (MoS2 layer) and electron conductivity (PMo12 layer) was needed for the construction of efficient electrocatalysts. In addit...

Van der Waals MoS2/graphene heterostructures are promising candidates for advanced electronics and optoelectronics beyond graphene. Herein, scanning probe methods and Raman spectroscopy were applied for analysis of the electronic and structural properties of monolayer (ML) and bilayer 2H-MoS2 deposited on single-layer graphene (SLG)-coated sapphire (S) substrates by means of an industrially scalable metal organic chemical vapor deposition process. The SLG/S substrate shows two regions with distinctly different morphology and varied interfacial coupling between SLG and S. ML MoS2 nanosheets grown on the almost free-standing graphene show no detectable interface coupling to the substrate, and a value of 2.23 eV for the MoS2 quasiparticle bandgap is determined. However, if the graphene is involved in hydrogen bonds to the hydroxylated sapphire surface, an increased MoS2/graphene interlayer coupling results, marked by a shift of the conduction band edge toward Fermi energy and a reduction of the ML MoS2 quasiparticle bandgap to 1.98 eV. The surface topography reveals a buckle structure of ML MoS2 in conformity with SLG that is used to determine the dependence of the ML MoS2 bandgap on the interfacial spacing of this heterostructure. In addition, an in-gap acceptor state about 0.9 eV above the valence band minimum of MoS2 has been observed on locally elevated positions on both SLG/S regions, which is attributed to local bending strain in the grown MoS2 nanosheets. These fundamental insights reveal the impact of the underlying substrate on the topography and the band alignment of the ML MoS2/SLG heterostructure and provide the possibility for engineering the quasiparticle bandgap of ML MoS2/SLG grown on controlled substrates that may impact the performance of electronic and optoelectronic devices therewith.

Layered transition metal dichalcogenides (TMDs) are thrust into the limelight within the scientific community thanks to their properties that can be exploited for a myriad of applications in electrochemistry and electrocatalysis of hydrogen evolution reaction (HER). Of interest, we investigate the correlation between electrochemical treatment of exfoliated MoS2, WS2, MoSe2 and WSe2 nanosheets in the hopes of electrochemically activating their electrochemical and electrocatalytic properties. In the electrochemical aspect, we show that electrochemical activation is achieved, for all TMDs except WS2, via electro-reduction at selected reductive potentials based on their innate electrochemistry while all TMDs become electrochemically deactivated upon electro-oxidation. Across all TMDs, MoSe2 exhibits most prominent charge transfer activation. Scrutinizing further, we conclude that molybdenum metal and selenium chalcogen type are more prone to electrochemical activation than tungsten metal and sulfur chalcogen type. Contemporary research into TMDs as prospective electrocatalysts for hydrogen evolution has been overwhelming; driven by the ardent pursuit of cheaper alternatives to the rare precious metal platinum, which is the best electrocatalyst to date. TMDs are abundant in nature and also exhibit promising electrocatalytic performance for HER. It provides an interesting perspective to explore the influence of electrochemical potential on the efficiency of the TMDs as a HER electrocatalyst. We found that MoS2 demonstrated better HER performance upon electro-reduction whereas the electro-oxidation of WS2 deteriorated the HER performance significantly. Conversely, the HER efficiency of MoSe2 and WSe2 remained largely unperturbed by electrochemical pretreatment. Therefore, in terms of HER electrocatalysis, we conclude that sulfur-containing TMDs are more receptive to electrochemical redox treatment compared to selenium-containing TMDs. These findings are advantageous towards the electrochemistry of TMDs and provide insights into the effectiveness and feasibility of electrochemical activation for charge transfer and HER electrocatalysis applications. The administration of such knowledge to electrochemical and energy applications in future will be indefinitely beneficial.

Synthesizing transition metal dichalcogenide (TMDC) monolayers through the liquid exfoliation of bulk crystals in low boiling point polar micromolecular solvents, such as water, is paramount for their practical application. However, the resulting hydrodynamic forces only appear on the crystal edges due to the mismatch in surface tension between the polar micromolecular solvents and the bulk crystals and are insufficient to overcome the strong van der Waals attraction between adjacent microscale layers. Herein, we present the novel strategy of engineering the lateral size of TMDC (MoS2 and WS2) crystals in the nanoscale to increase the fraction of edges, leading to their direct and ready exfoliation in polar micromolecular solvents, even in pure water, to produce monolayer MoS2 and WS2 nanosheets in high yield. To examine one of their important applications, their catalytic hydrogen evolution activities were evaluated when used as cocatalysts with a photoharvester semiconductor (cadmium sulfide, CdS) in a reaction driven by solar energy. These exfoliated MoS2 (WS2) monolayers exhibited superior cocatalytic performance in the photocatalytic hydrogen evolution reaction (HER). Notably, the cocatalytic performance of monolayer WS2 nanosheets is even higher than that of platinum (Pt), which is a state-of-the-art catalyst for catalytic hydrogen evolution. This work elucidates the importance of decreasing the lateral size of layered crystals to significantly enhance their exfoliability, providing a new strategy for the large-scale preparation of nanoscale TMDC monolayers by liquid exfoliation.

Monolayer MoS2 nanosheets (NSs) are promising anode materials for lithium-ion batteries because all redox reactions take place at the surface without lithium-ion diffusion limit. However, the expanded band gap of monolayer MoS2 NSs (∼1.8 eV) compared to their bulk counterparts (∼1.2 eV) and restacking tendency due to the van der Waals forces result in poor electron transfer and loss of the structure advantage. Here, a facile approach is developed to fabricate the MoS2-graphene aerogels comprising controlled three-dimensional (3D) porous architectures constructed by interconnected monolayer MoS2-graphene hybrid NSs. The robust 3D architectures combining with the monolayer feature of the hybrid NSs not only prevent the MoS2 and graphene NSs from restacking, but also enable fast electrode kinetics due to the surface reaction mechanism and highly conductive graphene matrix. As a consequence, the 3D porous monolayer MoS2-graphene composite aerogels exhibit a large reversible capacity up to 1200 mAh g(-1) as well as outstanding cycling stability and rate performance, making them promising as advanced anode materials for lithium-ion batteries.

Patterning of transition metal dichalcogenides catalyzed by surface plasmons with atomic precision

In this communication, we demonstrate uniaxial strain relaxation in monolayer (1L) MoS2 transpires through cracks in both single and double-grain flakes. Chemical vapour deposition (CVD) grown 1L MoS2 has been transferred onto polyethylene terephthalate (PET) and poly(dimethylsiloxane) (PDMS) substrates for low (∼1%) and high (1-6%) strain measurements. Both Raman and photoluminescence (PL) spectroscopy revealed strain relaxation via cracks in the strain regime of 4-6%. In situ optical micrographs show the formation of large micron-scale cracks along the strain axis and ex situ atomic force microscopy (AFM) images reveal the formation of smaller lateral cracks due to the strain relaxation. Finite element simulation has been employed to estimate the applied strain efficiency as well as to simulate the strain distribution for MoS2 flakes. The present study reveals the uniaxial strain relaxation mechanism in 1L MoS2 and paves the way for exploring strain relaxation in other transition metal dichalcogenides (TMDCs) as well as their heterostructures.

Preparation of layered transition metal dichalcogenides (TMDs) with unconventional metastable phases has increased in importance thanks to their fascinating physicochemical properties that are promising in various practical applications. Many considerable and rapid advances in the development of synthesis and preparation methods for this new family of layered TMDs have been made in recent decades, yet a universal, reliable, and comprehensive strategy to obtain metastable layered TMDs with high yield, high phase purity, and absence of thermodynamically stable phase impurity is still lacking. Therefore, it is desirable to design a preparation method that can successfully address these issues using the existing understanding of the metastable layered TMDs, which will subsequently open up many opportunities for in-depth studies on their intrinsic properties and the exploration of their potential practical applications. In the effort to achieve this goal, this thesis focuses specifically on the development of a bottom-up synthesis approach for group VIB layered TMDs with metastable 1T′ phase, the evaluation of the phase purity of the products, the investigation of their crystal structures and intrinsic properties, and their applications in other fields of materials science research. Firstly, a general and facile synthetic method was developed to prepare metastable group VIB layered TMD crystals including 1T′-phase WS2, WSe2, MoS2, MoSe2, WS2xSe2(1-x), and MoS2xSe2(1-x) with the use of alkali metal-based precursors. The distorted structure, the high phase purity, and the metastable nature of the as-prepared 1T′-TMD crystals were confirmed with various characterization techniques such as X-ray diffraction, transmission electron microscopy, Raman spectroscopy, etc. Importantly, the complete structure solutions of the metastable 1T′-phase WS2, WSe2, MoS2, and MoSe2 were obtained with single-crystal X-ray diffraction technique, which laid a foundation for future in-depth studies on structure-related properties of these materials. A proposed formation mechanism of the metastable 1T′ phase among group VIB TMDs was also discussed, which could be beneficial for the development of synthetic methodologies to prepare layered TMDs with unconventional phases. Overall, this work broadens the understanding of the formation of metastable 1T′ phase among group VIB layered TMDs and their alloys, provides an effective strategy to explore their intrinsic properties and applications, and paves a way for the development of preparation methods for various layered TMDs with novel unconventional crystal structures. Secondly, a facile synthetic process was developed to study the anisotropic growth of Au nanostructures on metastable 1T′-MoS2 and 1T′-WS2 nanosheets, which have an anisotropic quasi-1D lattice arrangement on their basal planes. The 2D ultrathin 1T′-MoS2 and 1T′-WS2 nanosheets were prepared by applying the lithium intercalation and exfoliation method to their corresponding 1T′-TMD bulk crystals. The formation of the anisotropic pattern of the metal nanostructures was successfully realized on the 1T′-MoS2 nanosheets and further investigated with varying reaction time, reaction temperature, and growth substrate, which provides an important insight into the nucleation and formation of the anisotropic Au nanostructures. Additionally, the obtained 1T′-MoS2/Au heterostructure exhibited significantly enhanced electrocatalytic performance toward hydrogen evolution reaction in aqueous 0.5 M H2SO4 electrolyte. The incorporation of Au showed a substantial improvement in the intrinsic activity and the charge-transport properties of the heterostructure compared with the as-prepared 1T′-MoS2 nanosheets. This work demonstrates the ability of metastable 1T′-phase layered TMDs in controlling the growth behavior of other materials and the promising potential of the resulting heterostructure in electrocatalysis application.

One-dimensional (1D) nanoscrolls derived from two-dimensional (2D) nanosheets own unusual physical and chemical properties that arise from the spiraled 1D morphology and the atomic thin 2D building blocks. Unfortunately, preparation of large-sized nanoscrolls of transition-metal dichalcogenides (TMDCs) remains a big challenge, which greatly restricts the fabrication of single-scroll devices for their fundamental studies and further applications. In this work, we report a universal and facile method, by making use of the evaporation process of volatile organic solvent, to prepare TMDC (e.g., MoS2 and WS2) nanoscrolls with lengths of several tens to one hundred micrometers from their 2D precursors presynthesized by chemical vapor deposition on Si/SiO2. Both atomic force microscopy and electron microscopy characterizations confirmed the spirally rolledup structure in the resulting nanoscrolls. An interlayer spacing of as small as ∼0.65 nm was observed, suggesting the strong coupling between adjacent layers, which was further evidenced by the emergence of new breathing mode peaks in the ultralow frequency Raman spectrum. Importantly, compared with the photodetector fabricated from a monolayer MoS2 or WS2 nanosheet, the device based on an MoS2 or WS2 nanoscroll showed the much enhanced performance, respectively, with the photosensitivity greatly increased up to 2 orders of magnitude. Our work suggests that turning 2D TMDCs into 1D scrolls is promising in achieving high performances in various electronic/optoelectronic applications, and our general method can be extended to the preparation of large-sized nanoscrolls of other kinds of 2D materials that may bring about new properties and phenomena.

Layered transition metal dichalcogenides (TMDs) have been the center of attention in the scientific community due to their properties that can be tapped on for applications in electrochemistry and hydrogen evolution reaction (HER) catalysis. We report on the effect of electrochemical treatment of exfoliated MoS2, WS2, MoSe2 and WSe2 nanosheets toward the goal of activating the electrochemical and HER catalytic properties of the TMDs. In particular, electrochemical activation of the heterogeneous electron transfer (HET) abilities of MoS2, MoSe2 and WSe2 is achieved via reductive treatments at identified reductive potentials based on their respective inherent electrochemistry. Comparing all TMDs, the charge transfer activation is most accentuated in MoSe2 and can be concluded that Mo metal and Se chalcogen type are more susceptible to electrochemical activation than W metal and S chalcogen type. With regards to the HER, we show that while MoS2 displayed enhanced performance when subjected to electrochemical reduction, WS2 fared worse upon oxidation. On the other hand, the HER performance of MoSe2 and WSe2 is independent of electrochemical redox treatment. We can conclude therefore that for the HER, S-containing TMDs are more responsive to redox treatment than compounds with the Se chalcogen. Our findings are beneficial toward understanding the electrochemistry of TMDs and the extent to which activation by electrochemical means is effective. In turn, when such knowledge is administered aptly, it will be promising for electrochemical uses.

Surfactant-free exfoliation of multilayer molybdenum disulfide nanosheets in water