Graphene Research.

Synthesis

We have experience in both top-down and bottom-up methods for obtaining graphene. Our first work started in 2007 with the chemical exfoliation of graphene flakes in 1,2-dichloroethane and then its extension to BN in 2010. Other solvents have been explored.

We now focus on synthetic graphene grown by chemical vapour deposition and its application in electronics, such as transistors, transparent conducting electrodes and spintronics.

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 are focussing on 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. 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.

Graphene is transferred to a wide range of substrates, such as glass, plastic, silicon wafers and also suspended. Suspending graphene on TEM grids, shown below, enables us to study its properties without any substrate interaction. Our research aims to provide ultra-clean suspended graphene that is free from surface contamination. The atomic structure of inorganic nanomaterials, such as the highly fluorescent Mn doped ZnSe quantum dots, are studied in high detail using suspended graphene membranes. We have studied small molecules and atoms on the surface of few layer graphene and used focused electron beam irradiation to catalyse the formation of organic linear chain molecules with direct anchor attachment to the surface.

We undertake tilt-dependent electron diffraction measurements and then track the intensity of peaks in the patterns to confirm monolayer formation. The intensity of SAED peaks in few-layer graphene can oscillate as the sample is tilted. Monolayer graphene exhibits a gradual reduction in the SAED peak intensity as tilt is increased, and broadening in the peak FWHM from the rippling.

Structure of Graphene

The structure of the synthetic graphene is studied primarily using atomic-resolution HRTEM imaging using state-of-the-art aberration-corrected machines. Most recently we have been part of the microscopy activities being undertaken using the Oxford-JEOL 2200MCO FEG-TEM in the Department of Materials. Monochromation of the electrons from the gun, combined with both probe and imaging CEOS spherical aberration correctors enable us to image graphene with resolution ~ 80 picometers. Fully resolved HRTEM images of single atoms in graphene are obtained.

We have revealed how few layer graphene sheets merge together, and strain that arises in distorted regions of few layer graphene from pinches in the film.

We have studied the dynamics of edge atoms in graphene, shown below. The edges of graphene are unique in that they are not fully bonded and therfore highly reactive with motion confined to 1D.

The image below is a series of images 10 seconds apart showing an extra atom arriving at the edge in triangular bonding form, then ejecting.

The image below shows a single atom edge, as well as bond breaking and reforming after 10 seconds of electron beam irradiation.

Stone-Wales bond rotations were captured at the edge. (10 s between images).

We study impurity atoms, such as Fe, and their interaction with vacancies in graphene (See HRTEM movies below). Sequences of HRTEM images taken every 10 seconds can be compiled into a time-series that reveals dynamics. These movies show the dynamics of single Si atoms being trapped in mono and di vacancies in graphene.

Once the structure of the graphene is evaluated, we then measure the electronic properties. We utilized electron beam lithography to pattern Hall devices to measure both the field effect mobility and the Hall mobility. These values give insights into the electronic quality of the material. Our aim is to produce synthetic graphene of the highest possible quality. This requires large single crystal size, uniform layer number coverage over large distances, control of the number of layers, tailoring properties by electron beam irradiation and chemical doping, plus many more.....

The structural and electronic knowledge is then fed back into the synthesis process to lead to improved materials.