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Printed Optoelectronic Devices from Nanosheet Network Electrochemically-gated Field Effect Transistors

Periodic Reporting for period 1 - 2D-EG-FET (Printed Optoelectronic Devices from Nanosheet Network Electrochemically-gated Field Effect Transistors)

Reporting period: 2016-05-16 to 2018-05-15

Problem being addressed:

This project was focused on fabricating printed ensembles of semiconducting nanosheets (i.e. semiconducting nanosheet networks, SNNs) with controlled composition, and characterising their properties as the active material within solution-processable electrochemically-gated field-effect transistors (E-gated FETs).

Numerous layered semiconductors, such as members of transition metal dichalcogenides (e.g. WS2 and MoS2) are naturally abundant and can be made accessible in large quantities in a printable form by liquid phase exfoliation (LPE), which converts the parent crystal into liquid dispersed nanosheets. However, presently little is known about the influence of particular dispersion media, geometry of the flake constituents (i.e. mean flake length and number of layers), the chosen deposition approach, and post processing on the resulting SNN properties.

In particular, it is unknown how to produce sufficient quantities of monolayer enriched inks and a means of depositing them in a manner that prevents flake restacking, such that printed networks with direct bandgap characteristics can be retained. Such networks could then be investigated for optoelectronic applications, such as light-emitting FETs.

Importance for society:

By combining versatile and scalable solution-processing methods with the excellent solid state properties of layered crystalline nanomaterials, it is hoped that this work will lead to a wide range of applications in printed electronics. It is envisaged that this approach will give rise to applications that are impossible using traditional manufacturing approaches (i.e. based on silicon), such as devices that are mechanically deformable (i.e. flexible and stretchable) when deposited on plastic foil substrates, and easily customised for diverse purposes. Such high performance printed electronics, when realised, will enable the integration of electronic functionality in new locations and situations where numerous applications will be found, such as inexpensive hardware for the Internet of Things.

Overall Objectives:

The overall research objectives were: (1) to prepare size selected inks by liquid phase exfoliation, (2) process these into nanosheet networks with controlled morphology and composition using industrially relevant solution deposition methods, (3) fabricate electrolyte-gated field effect transistors based on these nanosheet networks and (4) characterise their electronic transport properties under electrochemical control, especially those produced to retain monolayer properties with the intent to demonstrate electroluminescence from these printed SNNs for the first time. Following this, we wish to (5) realise all-printed E-gated FETs by printing all of the device components, i.e. the SNN channel material, as well as nanomaterial based metal contact and gate electrodes.
Objectives 1-3 were completed and were used for a major research publication. While aerosol-jet printing was explored with some success, it was found that an alternative deposition approach, airbrush spraying, was more effective. Aspects of Objective 4 were completed, however the production of nanosheet networks based on monolayer enriched networks was unsuccessful. Aspects of this problem were explored through a monolayer production study, featuring a sediment recycling method. Objective 5 was not completed specifically as this work was published by other researchers during the project. Alternatively, studies featuring alternative all-printed nanomaterial based electrochemical devices were carried conducted using single walled carbon nanotube networks, the results of which were featured in two peer-reviewed publications.

The obtained results were primarily exploited through presentations (talks and posters) at a number international conferences, including a Hengstberger Symposium at the Internationales Wissenschaftsforum Heidelberg.
Through this project, the first n-type E-gated FETs based on printed nanosheet networks of WS2 were fabricated and a detailed study of their electronic transport properties were conducted. The devices were found to possess relatively low mobilities, however, the completed work sets a new state of the art for solution processed layered dichalcogenide based E-gated FETs. The devices were produced using inks stabilised in aqueous surfactant media, highly desirable due to the avoidance of organic solvents commonly used for exfoliation and stabilisation of layered nanomaterials (for example, N-Methyl-2-pyrrolidone and N-Cyclohexyl-2-pyrrolidone), which present environmental and health concerns Furthermore, the SNNs were prepared at low temperatures using an inexpensive and industrially relevant deposition approach, airbrush spraying. Some p-type behavior was also observed, which suggests that these devices could be optimised for balanced ambipolar transport behavior, and thus widens the potential applications of WS2 for printed electronics.

The demonstration of ambipolar transport behaviour (simultaneous injection of electrons and holes) is a requirement for subsequent demonstration of light-emitting FETs based on printed semiconducting nanosheet networks. Hence, the completed work marks important progress towards the realisation of high performance printed electronics based on layered nanomaterials.
Electrolyte-gated field effect transistor based upon printed semiconducting nanosheet networks