2D Crystals Synthesis by Chemical Vapour Deposition
2D materials encompass a wide range of elemental compositions that give rise to properties spanning metallic, semiconducting and insulation behaviour. Graphene is well known as a semi-metal, and hexagonal boron nitride as an insulator. Recently, monolayer transition metal dichalcogenides have emerged as unique semiconductors with light emission in the visible spectrum. Combining these 2D materials as building blocks leads to the formation of heterojunctions and heterostructured devices in both lateral and vertical geometries. These mixed 2D layer heterostructures are particularly promising for ultrathin opto-electronics. Research is focused on how to grow large area 2D materials bottom up using chemical vapour deposition (CVD).
CVD Synthesis
Research within the Nanostructured Materials group involves the synthetic large scale growth of 2D materials using chemical vapour deposition (CVD) methods and their application in electronics, opto-electronics, and energy applications. The typical layout for a CVD reaction of TMD materials is shown below:
The 2D crystals explored are Graphene, hexagonal Boron Nitride, Transition Metal Dichalcogenides (MoS2, WS2, etc), 2D oxides (Silicon dioxide), 2D inorganic salts, 2D metals, and new semiconductors. We have been growing graphene by CVD since 2010 and have extended this into a larger range of 2D materials.
A systematic cycle of research involves:
Synthesis - Structure - Electronic properties - Application
Synthesis of Graphene by Chemical Vapour Deposition
Chemical vapour deposition is used to grow large area graphene with an aim to produce massive single crystals. We have researched using copper foils as a catalyst and developing a deep understanding of the factors that influence growth.
We showed that graphene grown on copper is not self-limiting to monolayer coverage and has potential for bilayer and other few-layer graphene structures through parameter tuning. It is possible to grow few layer graphene domains and we have shown using electron diffraction that these are single crystals. By appropriate control of hydrogen, we demonstrated that well defined hexagonal shaped few layer graphene domains on copper can be produced, as well as large monolayer hexagons. This has been extended to 10 inch scale production of electronic grade high quality graphene.
CVD of 2D semiconducting crystals
CVD is also used to grow semiconducting 2D Crystals within the transition metal dichalcogenide family: MoS2, WS2, MoSe2. By developing new growth reaction systems we have been able to grow large single crystal domains exceeding 300 microns in diameter. These WS2 domains exhibit a narrow photoluminescence spectrum in the red part of the visible spectrum (620-700nm). 2D scanning confocal Raman Spectroscopy can map the vibrational properties of these materials on the micron to millimeter length scales and determine layer number uniformity.
Transfer of 2D Materials to any substrate.
A polymer support film is typically used to transfer the fragile 1 atomic layer material from the growth substrate onto any other substrate of choice, including flexible plastics as shown to the right. Solution based etchants are used to remove the metal catalyst or substrate during transfer, as shown in the figure.
The polymer is finally removed to leave a clean transferred 2D material on the target substrate. We have utilized these transfer methods to create vertical stacked heterostructures with TMDs, graphene and hBN. These were used in tunneling photodetecting transistors.
The SEM image below shows graphene on a holey SiN grid, with one hole at the left being empty and shows the contrast from the suspended graphene. We have also transferred TMDs and graphene onto slits in SiN heating chips for TEM.
