Growth of 2D Electronics: Development of Van Der Waals Heterostructures for Flexible and Transparent Electronics.

Electrically conductive 2D materials are currently of great interest due to their potential for use in flexible electronics and miniaturisation to the sub-nanometre level. Currently, photolithography methods utilise light to etch a pattern into these materials to fabricate electronic devices. However, the diffraction limit of light limits the minimum resolution of these top-down devices to 7 nm. As such, new materials that can be synthesised in a bottom-up fashion are required to create smaller electronic devices.

Graphene, first isolated in 2004, has been of great interest recently due to its electrical properties. As a single atom thick layer of hexagonally bonded carbon atoms, it is the thinnest known material, making it very mechanically flexible. Pristine graphene is also one of the best-known electrical conductors. However, despite Graphene's outstanding material properties, it has one major drawback. Its lack of a bandgap means it cannot be used to fabricate transistors, since it cannot maintain an off state due to its metal-like conductivity. Despite this, Graphene remains attractive for use in the next generation of electronic devices due to its synergy with another class of semiconducting 2D materials, the Transition Metal Dichalcogenides (TMDs).

TMDs are a class of materials that follow the general formula MX2, where M is a transition metal (W and Mo in this research) and X is a chalcogen, an element from group 16 of the periodic table (S in this research). These materials are semiconducting; they support a bandgap but still have the potential to be electrically conductive when excited. This allows them to maintain an off state until a stimulus is applied, at which point they become electrically conductive. This makes them perfect for use in transistors, especially since these materials show potential for bottom-up synthesis. A monolayer of TMD material consists of a metal atom sandwiched between two chalcogen atoms and have thicknesses below the nanometre scale (MoS2 0.7 nm, WS2 0.8 nm). Although TMDs do not exhibit conductivities as high as that of Graphene, the two materials could be used in conjunction with one another to form a so-called Van der Waals heterostructure, a layered structure of two different materials held together by Van der Waals forces. Such heterostructures could allow for the production of electronics with Graphene like conductivities, but TMD like on/off control.

The initial aim of this work is to synthesise high quality monolayers of MoS2 and WS2 using Chemical Vapour Deposition methods, such as Metal-Organic CVD and Plasma Enhanced CVD, from known precursor molecules. Following a successful deposition, the synthesis of functional heterostructures will be explored, as well as the potential for development of new precursor molecules to control properties of the monolayers. The final goal of the project is to develop a series of new 2D heterostructure materials, ultimately to be supported on mechanically flexible substrates, that can be used in a multitude of electronic applications and drive forward miniaturisation while maintaining, or improving on, the electrical properties of current 2D electrical materials.