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Over the past 15 years, two-dimensional (2D) materials have been studied and exploited for many applications. In many cases, 2D materials are formed by the exfoliation of layered crystals such as transition-metal disulfides. However, it has recently become clear that it is possible to exfoliate nonlayered materials so long as they have a nonisotropic bonding arrangement. Here, we report the synthesis of 2D-platelets from the earth-abundant, nonlayered metal sulfide, iron pyrite (FeS2), using liquid-phase exfoliation. The resultant 2D platelets exhibit the same crystal structure as bulk pyrite but are surface passivated with a density of 14 × 1018groups/m2. They form stable suspensions in common solvents and can be size-selected and liquid processed. Although the platelets have relatively low aspect ratios (∼5), this is in line with the anisotropic cleavage energy of bulk FeS2. We observe size-dependent changes to optical properties leading to spectroscopic metrics that can be used to estimate the dimensions of platelets. These platelets can be used to produce lithium ion battery anodes with capacities approaching 1000 mAh/g.

Liquid-phase exfoliation has progressed in recent years to become a common method for production of 2D materials. During exfoliation, surfactants can be used to stabilize the nanosheets against reaggregation. Here, using WS2as a model system, we explore the effect of varying surfactant type and concentration on the yield and dimensions of the exfoliated nanosheets. For ionic surfactants, the mass of nanosheets produced was widely constant for low surfactant concentrations but decreased sharply for surfactant concentrations above ∼10 mM, regardless of surfactant. Very similar surfactant–concentration dependence was observed for both nanosheet length and thickness. Contrary to previous reports, this data implies that the optimum surfactant concentration is not linked to the critical micelle concentration. In addition, we found that surfactant concentrations as low as 0.07 mM yielded stable nanosheet dispersions with zeta potentials above 40 mV. By decoupling the exfoliation and stabilization effects of the surfactant, we have shown that it is the (de)stabilization process, rather than the exfoliation process, which links nanosheet concentration, size, and thickness to surfactant concentration.

Nanocomposite strain sensors, particularly those consisting of polymer–graphene composites, are increasingly common and are of great interest in the area of wearable sensors. In such sensors, application of strain yields an increase in resistance due to the effect of deformation on interparticle junctions. Typically, widening of interparticle separation is thought to increase the junction resistance by reducing the probability of tunnelling between conducting particles. However, an alternative approach would be to use piezoresistive fillers, where an applied strain modifies the intrinsic filler resistance and so the overall composite resistance. Such an approach would broaden sensing capabilities, as using negative piezoresistive fillers could yield strain-induced resistance reductions rather than the usual resistance increases. Here, we introduce nanocomposites based on polyethylene oxide (PEO) filled with MoS2nanosheets. Doping of the MoS2by the PEO yields nanocomposites which are conductive enough to act as sensors, while efficient stress transfer leads to nanosheet deformation in response to an external strain. The intrinsic negative piezoresistance of the MoS2leads to a reduction of the composite resistance on the application of small tensile strains. However, at higher strain the resistance grows due to increases in junction resistance. MoS2–PEO composite gauge factors are approximately −25 but fall to −12 for WS2–PEO composites and roughly −2 for PEO filled with MoSe2or WSe2. We develop a simple model, which describes all these observations. Finally, we show that these composites can be used as dynamic strain sensors.

Liquid phase exfoliation is a commonly used method to produce 2D nanosheets from a range of layered crystals. However, such nanosheets display broad size and thickness distributions and correlations between area and thickness, issues that limit nanosheet application potential. To understand the factors controlling the exfoliation process, we have liquid-exfoliated 11 different layered materials, size-selecting each into fractions before using AFM to measure the nanosheet length, width, and thickness distributions for each fraction. The resultant data show a clear power-law scaling of nanosheet area with thickness for each material. We have developed a simple nonequilibrium thermodynamics-based model predicting that the power-law prefactor is proportional to both the ratios of in-plane-tearing/out-of-plane-peeling energies and in-plane/out-of-plane moduli. By comparing the experimental data with the modulus ratio calculated from first-principles, we find close agreement between experiment and theory. This supports our hypothesis that energy equipartition holds between nanosheet tearing and peeling during sonication-assisted exfoliation.