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While liquid phase exfoliation can be used to produce nanosheets stabilized in polymer solutions, very little is known about the resultant nanosheet size, thickness, or monolayer content. The present study uses semiquantitative spectroscopic metrics based on extinction, Raman, and photoluminescence (PL) spectroscopy to investigate these parameters for WS2 nanosheets exfoliated in aqueous poly(vinyl alcohol) (PVA) solutions. By measuring Raman and PL simultaneously, the monolayer content can be tracked via the PL/Raman intensity ratio while varying processing conditions. The PL is found to be maximized for a stabilizing polymer concentration of 2 g L−1. In addition, the monolayer content can be controlled via the centrifugation conditions, exceeding 5% by mass in some cases. These techniques have allowed tracking the ratio of PL/Raman in a droplet of polymer-stabilized WS2 nanosheets as the water evaporates during composite formation. No evidence of nanosheet aggregation is found under these conditions although the PL becomes dominated by trion emission as drying proceeds and the balance of doping from PVA/water changes. Finally, bulk PVA/WS2 composites are produced by freeze drying where >50% of the monolayers remain unaggregated, even at WS2 volume fractions as high as 10%. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs) have recently attracted attention due to their interesting electronic and optical properties. Fabrication of these materials in a reliable and facile method is important for future applications, as are methods to characterize material quality. Here we present the chemical vapor deposition of MoSe2 monolayer and few layer crystals. These results show the practicality of using chemical vapor deposition to reliably fabricate these materials. Low frequency Raman spectra and mapping of shear and layer breathing modes of MoSe2 are presented for the first time. We correlate the behavior of these modes with layer number in the materials. The usefulness of low frequency Raman mapping to probe the symmetry, quality, and monolayer presence in CVD grown 2D materials is emphasized. Raman map of maximum intensity position for low wavenumber shear and layer breathing modes of MoSe2. [ABSTRACT FROM AUTHOR]

Here,we demonstrate a simple method to exfoliate layered molybdenumtrioxide (MoO3) crystallites to give multilayer MoO3nanosheets dispersed in solvents. Exfoliation is achievedby sonicating MoO3powder in the presence of suitable solventsfollowed by centrifugation to remove undispersed material. This procedureworks well in a range of solvents with Hildebrand solubility parametersclose to 21 MPa1/2and is consistent with the predictionsof classical solubility theory. We have fully optimized this processand demonstrated methods to separate the resultant nanosheets by size.Raman spectroscopy suggests the exfoliation process does not damagethe MoO3. This is supported by measurements showing thatthe reaggregated nanosheets display very similar photoluminescenceto bulk MoO3. However, the dispersed nanosheets had distinctlydifferent photoluminescence, indicating a decoupling of the monolayerson exfoliation. We have used liquid-exfoliated MoO3toprepare supercapacitor electrodes that have relatively low capacitance(∼2 F/g at 10 mV/s) because of the low electrical conductivityof the MoO3. However, addition of carbon nanotubes beyondthe percolation threshold yielded a 100-fold increase in capacitance.Some MoO3/nanotube composites displayed a capacitance ashigh as 540 F/g at 0.1 mV/s. This is the first example of solventexfoliation of a layered metal oxide. We believe this work opens theway to liquid exfoliation of a wide range of layered compounds, leadingto an array of new solution-processed 2D materials. [ABSTRACT FROM AUTHOR]

Solution-exfoliated MoS2 nano-platelets were formed into thin films by deposition onto a water surface followed by transfer to indium tin oxide coated glass. After drying, a gold electrode was evaporated on top to give a sandwich structure with quasi-Ohmic contacts. Illumination of this device with broadband light of ∼1 kW m−2 intensity gave a fourfold increase in conductivity. The photocurrent increased sub-linearly with intensity and exponentially with time indicating the presence of traps. The photo-responsively at low intensity was ∼10−4 A per W at 15 V. This work demonstrates the potential for liquid-exfoliated, inorganic nanosheets to be fabricated into low-cost optoelectronic devices. [ABSTRACT FROM AUTHOR]

Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement. [ABSTRACT FROM AUTHOR]

Molybdenum dioxide (MoO 2 ) is a layered material which shows promise for a number of applications in the electrochemical energy storage arena. Mostly studied as a bulk layered material, MoO 2 has not previously been exfoliated in large quantities. Here we demonstrate the liquid phase exfoliation of MoO 2 in the solvent isopropanol, yielding reasonable amounts of good quality nanosheets. However, we found that, when dispersed in isopropanol under ambient conditions, MoO 2 nanosheets are gradually oxidized to higher oxides such as MoO 3 over a period of days. Conversely, if the nanosheets are processed into dried films immediately after exfoliation, and before oxidation has had a chance to progress, the nanosheets are relatively stable under ambient conditions, remaining unoxidised unless the films are heated. We also found that MoO 2 nanosheets can be size selected by controlled centrifugation and show size-dependent optical properties. This allows us to propose spectroscopic metrics which allow concentration- and size-estimation from extinction spectra. Finally, we found that liquid-exfoliated MoO 2 nanosheets could be used to produce lithium ion battery anodes with capacities of up to 1140 mA h g -1 ., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

While 2D transition metal dichalcogenides are known to be promising materials for electrocatalysis of hydrogen production, it is not clear which member of this family of materials is the most effective catalyst. Here we perform a comprehensive study, comparing the catalytic performance of electrodes consisting of porous arrays of liquid exfoliated MX2 nanosheets (M = Mo, W; X = S, Se, Te). We find a clear hierarchy with selenides > sulphides > tellurides with MoSe2 clearly out-performing the other materials. In all cases the performance, as characterised by current density at a given potential, can be improved by increasing the number of active sites (via control of the electrode thickness) and/or by adding carbon nanotubes to the electrode (i.e. increasing the electrode conductivity). While all materials formed reasonably stable electrodes, addition of nanotubes tended to improve mechanical cohesion. In an attempt to maximise performance, we prepared thick (∼15 μm), free standing MoSe2/SWNT composite electrodes which displayed Tafel slopes of ∼77 mV per decade and exchange current densities of ∼0.1 mA cm(-2). These electrodes had low onset potentials, reaching -2 mA cm(-2) at -41 mV (vs. RHE) and generated high current densities of -35 mA cm(-2) at -200 mV (vs. RHE).

We have used solution processing techniques to fabricate thin-film networks of nanosheets of six different transition metal dichalcogenides; MoS2, MoSe2, MoTe2, WS2, WSe2 and WTe2. We have measured both the dark conductivity and the photoconductivity under broad band illumination in the intensity range from 0-1500 W m(-2). The dark conductivity varied from ∼ 10(-6) S m(-1) for MoS2 to ∼ 1 S m(-1) for WTe2, with an apparent exponential dependence on bandgap. All materials studied show photocurrents which rise slowly with time and depend sub-linearly on light intensity, both hallmarks of trap limited processes. Because the photoresponse depends relatively weakly on bandgap, the ratio of photo- to dark conductivity is largest for the sulphides because of their lower dark conductivities. As such, MoS2 and WS2 may be best suited to photo-detection applications. However, due to their lower bandgap and superior conductivity, WSe2 and WTe2 might prove more effective in other applications, for example in photovoltaic cells.