ABSTRACT: MoS2 is a two-dimensional material that is gaining prominence due to its unique electronic and chemical properties. Here, we demonstrate ligand conjugation of chemically exfoliated MoS2 using thiol chemistry. With this method, we modulate the ?-potential and colloidal stability of MoS2 sheets through ligand designs, thus enabling its usage as a selective artificial protein receptor for ?-galactosidase. The facile thiol functionalization route opens the door for surface modifications of solution processable MoS2 sheets.
Project description:Chemical exfoliation approaches such as Li-intercalation for the production of two-dimensional MoS2 are highly attractive due to their high yield of monolayer forms, cost-effectiveness, and mass-scalability. However, the loss of the semiconducting property and poor dispersion stability in solvent have limited the extent of their potential applications. Here, we report simultaneous phase recovery and surface functionalization for the preparation of a highly stable 2H-MoS2 dispersion in water. This study shows that high-yield restoration of the semiconducting 2H phase from a chemically exfoliated MoS2 (ce-MoS2) can be induced by a mild-temperature (180 °C) solvent thermal treatment in N-methyl-2-pyrrolidone (NMP). In addition to a phase transition, this solvent thermal treatment in NMP realizes concurrent surface functionalization of the 2H-MoS2 surface, which provides an outstanding dispersion stability to 2H-MoS2 in water for more than 10 months. Finally, we report the humidity sensor based on the functionalized 2H-MoS2, which shows a substantial response enhancement compared with a nonfunctionalized 2H-MoS2 or ce-MoS2.
Project description:We report the different oxidation behavior between polycrystalline chemical-vapor-deposited and mechanically exfoliated single crystal MoS2 monolayers by ultraviolet-ozone treatment. As ultraviolet-ozone treatment time increased from 0 to 5 min, photoluminescence emission and Raman modes of both MoS2 disappeared, suggesting structural degradation by oxidation. Analysis with optical absorbance and X-ray photoelectron spectroscopy suggested the formation of MoO3 in both MoS2 after ultraviolet-ozone treatment. In addition, ultraviolet-ozone treatment possibly led to the formation of oxygen vacancies, molybdenum oxysulfide, or molybdenum sulfates in chemical-vapor-deposited MoS2. The measurement of electrical resistance after ultraviolet-ozone treatment suggested the transformation of chemical-vapor-deposited MoS2 into doped MoO3 and of mechanically exfoliated MoS2 into negligibly doped MoO3. These results demonstrate that the crystallinity of monolayer MoS2 can strongly influence the effect of ultraviolet-ozone treatment, providing important implications on the device integration of MoS2 and other two-dimensional semiconductors.
Project description:Three dimensional (3D) MoS2 nanoflowers are successfully synthesized by hydrothermal method. Further, a composite of as prepared MoS2 nanoflowers and rGO is constructed by simple ultrasonic exfoliation technique. The crystallography and morphological studies have been carried out by XRD, FE-SEM, TEM, HR-TEM and EDS etc. Here, XRD study revealed, a composite of exfoliated MoS2 with expanded spacing of (002) crystal plane and rGO can be prepared by simple 40 minute of ultrasonic treatment. While, FE-SEM and TEM studies depict, individual MoS2 nanoflowers with an average diameter of 200 nm are uniformly distributed throughout the rGO surface. When tested as sodium-ion batteries anode material by applying two different potential windows, the composite demonstrates a high reversible specific capacity of 575 mAhg(-1) at 100 mAg(-1) in between 0.01 V-2.6 V and 218 mAhg(-1) at 50 mAg(-1) when discharged in a potential range of 0.4 V-2.6 V. As per our concern, the results are one of the best obtained as compared to the earlier published one on MoS2 based SIB anode material and more importantly this material shows such an excellent reversible Na-storage capacity and good cycling stability without addition of any expensive additive stabilizer, like fluoroethylene carbonate (FEC), in comparison to those in current literature.
Project description:The ultrathin two-dimensional nanosheets of layered transition-metal dichalcogenides (TMDs) have attracted great interest as an important class of materials for fundamental research and technological applications. Solution-phase processes are highly desirable to produce a large amount of TMD nanosheets for applications in energy conversion and energy storage such as catalysis, electronics, rechargeable batteries, and capacitors. Here, we report a rapid exfoliation by supercritical fluid processing for the production of MoS2 and MoSe2 nanosheets. Atomic-resolution high-angle annular dark-field imaging reveals high-quality exfoliated MoS2 and MoSe2 nanosheets with hexagonal structures, which retain their 2H stacking sequence. The obtained nanosheets were tested for their electrochemical performance in a hybrid Mg-Li-ion battery as a proof of functionality. The MoS2 and MoSe2 nanosheets exhibited the specific capacities of 81 and 55 mA h g-1, respectively, at a current rate of 20 mA g-1.
Project description:Material stability and dissolution in aqueous media are key issues to address in the development of a new nanomaterial intended for technological application. Dissolution phenomena affect biological and environmental persistence; fate, transport, and biokinetics; device and product stability; and toxicity pathways and mechanisms. This article shows that MoS2 nanosheets are thermodynamically and kinetically unstable to O2-oxidation under ambient conditions in a variety of aqueous media. The oxidation is accompanied by nanosheet degradation and release of soluble molybdenum and sulfur species, and generates protons that can colloidally destabilize the remaining sheets. The oxidation kinetics are pH-dependent, and a kinetic law is developed for use in biokinetic and environmental fate modeling. MoS2 nanosheets fabricated by chemical exfoliation with n-butyl-lithium are a mixture of 1T (primary) and 2H (secondary) phases and oxidize rapidly with a typical half-life of 1-30 days. Ultrasonically exfoliated sheets are in pure 2H phase, and oxidize much more slowly. Cytotoxicity experiments on MoS2 nanosheets and molybdate ion controls reveal the relative roles of the nanosheet and soluble fractions in the biological response. These results indicate that MoS2 nanosheets will not show long-term persistence in living systems and oxic natural waters, with important implications for biomedical applications and environmental risk.
Project description:Charge transport in MoS2 in the low carrier density regime is dominated by trap states and band edge disorder. The intrinsic transport properties of MoS2 emerge in the high density regime where conduction occurs via extended states. Here, we investigate the transport properties of mechanically exfoliated mono-, bi-, and trilayer MoS2 sheets over a wide range of carrier densities realized by a combination of ion gel top gate and SiO2 back gate, which allows us to achieve high charge carrier (>10(13) cm(-2)) densities. We discuss the gating properties of the devices as a function of layer thickness and demonstrate resistivities as low as 1 k? for monolayer and 420 ? for bilayer devices at 10 K. We show that from the capacitive coupling of the two gates, quantum capacitance can be roughly estimated to be on the order of 1 ?F/cm(2) for all devices studied. The temperature dependence of the carrier mobility in the high density regime indicates that short-range scatterers limit charge transport at low temperatures.
Project description:To progress from laboratory research to commercial applications, it is necessary to develop an effective method to prepare large quantities and high-quality of the large-size atomically thin molybdenum dichalcogenides (MoS2). Aqueous-phase processes provide a viable method for producing thin MoS2 sheets using organolithium-assisted exfoliation; unfortunately, this method is hindered by changing pristine semiconducting 2H phase to distorted metallic 1T phase. Recovery of the intrinsic 2H phase typically involves heating of the 1T MoS2 sheets on solid substrates at high temperature. This has restricted and hindered the utilization of 2H phase MoS2 sheets suspensions. Here, we demonstrate that the synergistic effect of the rigid planar structure and charged nature of organic salt such as imidazole (ImH) can be successfully used to produce atomically thin 2H-MoS2 sheets suspension in water. Moreover, lateral size and area of the exfoliated sheet can be up to 50??m and 1000??m(2), respectively. According to the XPS measurements, nearly 100% of the 2H-MoS2 sheets was successfully prepared. A composite paper supercapacitor using the exfoliated 2H-MoS2 and carbon nanotubes delivered a superior volumetric capacitance of ~410?F/cm(3). Therefore, the organic salts-assisted liquid-phase exfoliation has great potential for large-scale production of 2H-MoS2 suspensions for supercapacitor application.
Project description:Using a conventional Raman experimental apparatus, we demonstrate that the photoluminescent (PL) yield from ultrasonication-exfoliated transition metal dichalcogenides (TMDs) (MoS2 and WS2) can be increased by up to 8-fold by means of a laser etching procedure. This laser etching process allows us to controllably pattern and reduce the number of layers of the solution-exfoliated material, overcoming the key drawback to solvent-based exfoliation of two-dimensional (2D) semiconducting materials for applications in optoelectronics. The successful laser thinning of the exfoliated 2D crystals was investigated systematically by changes in both Raman and PL spectra. A simple proof-of-principle of the scalability of this laser etching technique for solution-exfoliated TMD crystals was also demonstrated. As well as being applicable for individual materials, it is also possible to use this simple laser etching technique to investigate the structure of solution-generated van der Waals heterostructures, consisting of layers of both MoS2 and WS2.
Project description:MoS2 has been studied intensively during recent years as a semiconducting material in several fields, including optoelectronics, for applications such as solar cells and phototransistors. The photoresponse mechanisms of MoS2 have been discussed but are not fully understood, especially the phenomenon in which the photocurrent slowly increases. Here, we report on a study of the photoresponse flash-light-processed MoS2 films of different thicknesses and areas. The photoresponse of such films under different light intensities and bias voltages was measured, showing significant current changes with a quick response followed by a slow one upon exposure to pulsed light. Our in-depth study suggested that the slow response was due to the photothermal effect that heats the MoS2; this hypothesis was supported by the resistivity change at different temperatures. The results obtained from MoS2 films with various thicknesses indicated that the minority-carrier diffusion length was 1.36?µm. This study explained the mechanism of the slow response of the MoS2 film and determined the effective thickness of MoS2 for a photoresponse to occur. The method used here for fabricating MoS2 films could be used for fabricating optoelectronic devices due to its simplicity.
Project description:Highly sensitive hydrogen detection at room temperature can be realized by employing solution-processed MoS2 nanosheet-Pd nanoparticle composite. A MoS2-Pd composite exhibits greater sensing performance than its graphene counterpart, indicating that solvent exfoliated MoS2 holds great promise for inexpensive and scalable fabrication of highly sensitive chemical sensors.