3D printing of transition metal dichalcogenides

Abstract

The discovery of graphene initiated a surge of interest in other two-dimensional atomically thin materials. A class of materials that is attracting tremendous attention since 2011 is the family of layered transition metal dichalcogenides (TMDs), specifically group IV TMDs, namely MoS2, WS2, MoSe2 and WSe2. Their fascinating main characteristic is a varied electronic structure with layer number. This characteristic manifests as varied properties in their atomically thin form where they can be tuned to be direct band gap semi-conductors or electrically conductive, chemically stable, mechanically strong, flexible, catalytically and photo-catalytically active, and have very high specific surface area. All these properties make them very promising materials for applications in optoelectronics on flexible substrates, photonics, water splitting, and energy storage device electrodes. The objective of this thesis is the development of a scalable templating method to fabricate 3D architectures based on atomically thin layers of TMDs. We have chosen to utilize three-dimensional (3D) printing via robocasting (or direct ink writing) to fabricate 3D architectures of assembled 2D nanosheets. To this end, firstly we develop a liquid-phase exfoliation method of bulk TMDs which allows the achievement of nanosheets with lateral sizes greater than 1 micron in water with high yield. This lateral size is essential to achieve cohesive and mechanically stable 3D networks of 2D nanosheets and the use of water as dispersant agent is necessary to enable the scalability of the process. Secondly, we have formulated inks of exfoliated TMDs with the desirable rheological properties for printing, which are the shear thinning behaviour and high viscosity. 3D printed devices with footprint of mm2 and features in the hundreds of micrometre range have been successfully obtained based on exfoliated MoS2 and TiS2 nanosheets. Different architectures have been demonstrated, including woodpile and stacked and some of them have been tested as electrodes for supercapacitors. In proof of concept devices for electrochemical energy storage, they exhibit superior areal capacitance and energy density as compared to the existing planar microsupercapacitors, and enhanced performance to hybrid supercapacitor-battery devices. This work opens new directions towards additive manufacturability of 3D miniaturized devices with TMDs.