No abstract

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 MoS2 nanosheets. Doping of the MoS2 by 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 MoS2 leads 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 MoSe2 or WSe2. We develop a simple model, which describes all these observations. Finally, we show that these composites can be used as dynamic strain sensors.

Extinction spectra of nanomaterial suspensions can be dominated by light scattering, hampering quantitative spectral analysis. No simple models exist for the wavelength-dependence of the scattering coefficients in suspensions of arbitrary-sized, high-aspect-ratio nanoparticles. Here, suspensions of BN, talc, GaS, Ni(OH)2, Mg(OH)2 and Cu(OH)2 nanosheets are used to explore non-resonant scattering in wide-bandgap 2D nanomaterials. Using an integrating sphere, scattering coefficient (σ) spectra were measured for a number of size-selected fractions for each nanosheet type. Generally, σ scales as a power-law with wavelength in the non-resonant regime: σ(λ)∝[λ/〈L〉]−m, where 〈L〉 is the mean nanosheet length. For all materials, the scattering exponent, m, forms a master-curve, transitioning from m = 4 to m = 2, as the characteristic nanosheet area increases, indicating a transition from Rayleigh to van der Hulst scattering. In addition, once material density and refractive index are factored out, the proportionality constant relating σ to [λ/〈L〉]−m, also forms a master-curve when plotted versus 〈L〉.

very important goal within this field is to continuously improve the performance of lithium-ion batteries. For example, increasing battery energy densities will accelerate the rollout of electric vehicles [1] and improve the potential for bulk energy storage. [2] Thus, battery electrode development is an important part of the technological component of climate stabilization. A significant component of battery research is the development of new electrode materials. [3] Among other things, such materials should have high specific lithium storage capacity combined with the potential to display high rate performance. [4] Ideally, they would also have the capability to store other ions beyond lithium, such as sodium or potassium. [5] Recently, much attention has focused on 2D layered materials for use in both anodes and cathodes. [6,7] Such 2D materials consist of van der Waals bonded few-layer stacks of atomically thin sheets with the most wellknown examples being graphene, boron nitride, and MoS 2. [8] Increasing the energy density of lithium-ion batteries requires the discovery of new electrode materials capable of achieving very high areal capacity. Here, liquid phase exfoliation is used to produce nanosheets of SnP 3 , a 2D material with extremely high theoretical capacity of 1670 mAh g −1. These nanosheets can be fabricated into solution-processed thin films for use as lithium storing anodes. To maximize their performance, carbon nanotubes are incorporated into the electrodes to simultaneously enhance conductivity and toughness. As a result, electrodes of thickness >300 µm can be produced, which display activemass-normalized capacities (≈1657 mAh g −1 Active) very close to the theoretical value. These materials show maximum specific (≈1250 mAh g −1 Electrode) and areal (>20 mAh cm −2) capacities, which are at the state-of-the-art for 2D-based electrodes, coupled with good rate performance and stability. In combination with commercial cathode materials, full-cells are fabricated with areal capacities of ≈29 mAh cm −2 and near-record energy densities approaching 1000 Wh L −1 .

The propensity of many 2D materials to oxidize in ambient conditions can complicate production and limit applications potential. Here we describe ambient liquid phase exfoliation of GeS, a layered material known for its chemical instability. Ambient exfoliation in organic solvents such as N-methyl-pyrrolidone yields good quality multi-layer GeS nanosheets. Although oxidation appears to occur with a time constant of ∼10 d, the data suggests it to be limited to nanosheet edges leaving the basal plane intact. The rate of oxidation is slow enough to allow processing of the dispersions. For example, it was possible to size-select GeS nanosheets and characterize the size-dependence of nanosheet optical properties, leading to the observation of significant changes in bandgap with nanosheet thickness. Additionally, we were able to fabricate the nanosheets into lithium ion battery anodes using carbon nanotubes as both binder and conductive additive. These electrodes were relatively stable, showing ∼0.2% capacity decay per cycle, and displayed low-rate capacity of 1523 mAh g −1 which is within 93% of the theoretical value. However, detailed analysis showed relatively poor rate performance, possibly due to nanosheet alignment.

While polymers are typically processed using methods such as compression molding, injection molding, extrusion, and thermoforming, [10] polymer nanocomposites are typically prepared by solution blending, melt mixing/compounding, in situ polymerization, and composite self-assembly. [11] Nanocomposite formation by printing is somewhat less common. [12] Depending on the matrix, nanocomposite materials can be very soft and so skin mountable. [1] They also have high working strain ranges making them ideal candidates for emerging areas such as wearable sensing. [13,14] Although their elec-Research data are not shared.

Although printed networks of semiconducting nanosheets have found success in a range of applications, conductive nanosheet networks are limited by low conductivities (<106 S m−1). Here, dispersions of silver nanosheets (AgNS) that can be printed into highly conductive networks are described. Using a commercial thermal inkjet printer, AgNS patterns with unannealed conductivities of up to (6.0 ± 1.1) × 106 S m−1 are printed. These networks can form electromagnetic interference shields with record shielding effectiveness of >60 dB in the microwave region at thicknesses <200 nm. High resolution patterns with line widths down to 10 µm are also printed using an aerosol‐jet printer which, when annealed at 200 °C, display conductivity >107 S m−1. Unlike conventional Ag‐nanoparticle inks, the 2D geometry of AgNS yields smooth, short‐free interfaces between electrode and active layer when used as the top electrode in vertical nanosheet heterostructures. This shows that all‐printed vertical heterostructures of AgNS/WS2/AgNS, where the top electrode is a mesh grid, function as photodetectors demonstrating that such structures can be used in optoelectronic applications that usually require transparent conductors.

The investigation of high-mobility two-dimensional (2D) flakes beyond molybdenum disulfide (MoS 2 ) will be necessary to create a library of high-mobility solution-processed networks that conform to substrates and remain functional over thousands of bending cycles. Here we report electrochemical exfoliation of large-aspect-ratio (>100) semiconducting flakes of tungsten diselenide (WSe 2 ) and tungsten disulfide (WS 2 ) as well as MoS 2 as a comparison. We use Langmuir− Schaefer coating to achieve highly aligned and conformal flake networks, with minimal mesoporosity (∼2−5%), at low processing temperatures (120 °C) and without acid treatments. This allows us to fabricate electrochemical transistors in ambient air, achieving average mobilities of μ MoSd 2 ≈ 11 cm 2 V −1 s −1 , μ WSd 2 ≈ 9 cm 2 V −1 s −1 , and μ WSed 2 ≈ 2 cm 2 V −1 s −1 with a current on/off ratios of I on /I off ≈ 2.6 × 10 3 , 3.4 × 10 3 , and 4.2 × 10 4 for MoS 2 , WS 2 , and WSe 2 , respectively. Moreover, our transistors display threshold voltages near ∼0.4 V with subthreshold slopes as low as 182 mV/dec, which are essential factors in maintaining power efficiency and represent a 1 order of magnitude improvement in the state of the art. Furthermore, the performance of our WSe 2 transistors is maintained on polyethylene terephthalate (PET) even after 1000 bending cycles at 1% strain.