Work performed beyond state-of-art: A comprehensive library (>20) of semiconducting 2D materials was electrochemically exfoliated (Figure 1a) with large-aspect-ratio (>100), and the morphological and electrical properties were characterised. A route to increase 2D solution-processed material throughput from about 40 μg/day to about 2 mg/day was demonstrated, a 5000% increase in mass yield from when the proposal was written. Protocols were established to clean the 2D material flakes minimising residuals (Figure 1b), improving the flake-to-flake junctions and maximising our transistor performances. Langmuir-Schaefer (LS) deposition process was developed to minimise the amount of semiconducting material required to make arrays of devices (∼20 μg/cm2), achieve highly aligned and conformal flake networks, with minimal mesoporosity (∼2–5%), at low processing temperatures (120 °C) and without acid treatments. A model was developed that uses impedance spectroscopy data of LS networks to separate the flake-to-flake junction resistance from the nanosheet resistance, which helps the field understand the individual contribution from the nanosheets and flake-to-flake junctions for the first time. Electronic inks of semiconducting two-dimensional (2D) flakes such as molybdenum disulfide (MoS2) and tungsten dichalcogenides (WS2, WSe2) were made with network mobility μ > 10 cm2 V-1 s−1 (Figure 1c) and single flake mobility ~50 cm2 V-1 s−1 with current on/off ratio, Ion/Ioff ~ 103-104 at room temperature (20°C) in ambient atmosphere which is beyond the initial targets of the proposal. It was also the first time high μ solution processed WS2 and WSe2 were made on a flexible substrate (Figure 1d). Trade-offs between flake lateral size and performance were identified, and it was found that performance is maximised at flake lateral size >1 μm (Figure 1e). Our transistors displayed 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 (Figure 1f). Furthermore, the performance of our WSe2 transistors is maintained on polyethylene terephthalate (PET) even after 1000 bending cycles at 1% strain. Our transistors with TMD inks are now comparable to the wider literature of solution-processed materials (Figure 1g). The transistors were also fabricated onto textile fibres, which traditionally used rigid silicon components incompatible with textile surfaces (Figure 2a). Some of the first semiconducting 2D material fibre transistors were made with WSe2 and MoS2 achieving mobilities as high as μ ~ 15 cm2 V-1 s-1 (Figure 2b). This was achieved using a novel “knot” based architecture leveraging the fibre diameter to establish the length of the transistor channel, facilitating a route to scale down transistor channel dimensions (~ 100 μm) (Figure 2c). It was possible to use this architecture to make ionic transistors by hand using a human hair. The biocompatibility of solution processed 2D materials was extensively investigated so that it might be possible to use the technology in biomedical or wearable applications. For example, electrochemically exfoliated MoS2 and WSe2 flakes were biocompatible with human keratinocyte cells for at least 72 hours at 80 μg ml-1 and liquid phase exfoliated (LPE) graphene was biocompatible with human colon cells and human umbilical vein endothelial cells at high concentration ∼ 1 mg ml−1 facilitating a route for the use of the inks in wearable applications (Figure 2d). In addition, using the materials and chariterisation techniques that were developed in work package 1 and 2 other devices were made in collaboration with the Coleman group in Trinity College Dublin which resulted in both first author and co-authorship journal articles. We made li-ion battery anodes with graphene (Figure 2e) and FeF3 cathodes, sodium-ion red phosphourus anodes, inkjet printed graphene interconnects (Figure 2f), TMD gas sensors, electromagnetic interference shields, solution-processed diodes with TMDs, TMD and graphene strain sensors and developed equations to link gauge factor to volume fraction, conductivity, and network thickness, and photodiodes.
