2D Crystals Research. Graphene and More

Synthesis

We focus on synthetic 2D materials grown by chemical vapour deposition (CVD) and their application in electronics, transparent conducting electrodes and spintronics.

The 2D crystals explored are Graphene, hexagonal Boron Nitride, Transition Metal Dichalcogenides (MoS2, WS2, etc), 2D oxides (Silicon dioxide), and new semiconductors. We have been growing graphene by CVD since 2010 and have established four CVD systems for growing a 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 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.

CVD is also used to grow semiconducting 2D Crystals within the transition metal dichalcogenide family: MoS2, WS2, MoTe2, WTe etc. 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.

We have recently started expanding graphene from planar 2D structures to 3D networked scaffolds. This is achieved by using porous 3D catalytic foams that help template the growth in 3D. Removing the catalyst through chemical etching leaves a porous 3D structure consisting of continuously connected graphene sheets that retain high electrical conductivity. The thickness of the graphene layers can be controlled by reaction conditions. These graphene scaffolds can be used as electrodes in energy applications such as Fuel cells and Batteries, as catalyst supports for hydrogen reactions. We study how to integrate new inorganic materials with these 3D graphene structures. The figure below shows a SEM image of a freestanding porous 3D graphene network with visible wrinkling of the sheets.

Transfer of 2D Materials to any substrate.

A polymer support film is typically used to transfer the fragile 1 atomic layer material from the metal catalyst onto any other substrate of choice. Solution based etchants are used to remove the metal catalyst during transfer, as shown in the figure. The polymer is finally removed to leave a clean transferred 2D material on the target substrate.

Graphene is transferred to a wide range of substrates, such as glass, plastic, silicon wafers and also suspended in free space. Suspending graphene on TEM grids, shown below, enables us to study its properties without any substrate interaction. We can produce ultra-clean suspended graphene that is free from surface contamination on SiN TEM grids. Recent work has demonstrated our ability to form monolayer closed-packed arrays of PbTe nanocrystals on suspended graphene monolayers. We have developed large scale solution processing methods to increase the conductivity of graphene, whilst only marginally reducing the transparency.

Confirming monolayer graphene

We have performed tilt-dependent electron diffraction measurements and tracked the intensity of peaks in the patterns to confirm monolayer graphene. 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 and other 2D crystals

The structure of the synthetic graphene is studied primarily using atomic-resolution TEM imaging using state-of-the-art aberration-corrected machines. We primarily use the Oxford-JEOL 2200MCO FEG-TEM in the Department of Materials. this machine has monochromation of the electrons within the gun, combined with both probe and imaging CEOS aberration correctors. It can run from 80-200kV acceralating voltages. When running in full alignment, we have achieved imaging of monolayer graphene with 80 picometer spatial resolution. This ultra-high resolution enabled the first report of measuring bond length changes between two carbon atoms reported in Science, Vol. 337 no. 6091 pp. 209-212 (2012) and further refined in ACS Nano, Vol. 7 (11), pp 9860–9866 (2013).

We have conducted extensive studies into the variety of defects that exist in graphene and their stability, dynamics and transformations. These include the Stone-Wales bond rotation, vacancy defects (mono to tetra to larger), dislocation pairs and their resulting structural deformations, to excess C atoms forming self-interstitual defects. Our results how revealed that bond rotations are active in defects at room temperature to lower the total energy in the system through strain relief. This work is built upon previous work we did on understanding defects in carbon nanotubes. Understanding the behaviour of defects in 2D materials is now under investigation.

Aberration corrected TEM image of a 6-atom vacancy linear arm-chair defect in graphene

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. Using HRTEM image combined with image simulations we were able to prove that graphene edges could be hydrogen free and armchair edges can have triple-bond-like terminations. This was reported in Nature Communications 5, Article number: 3040 doi:10.1038/ncomms4040 (2013)

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).

Atomic Resolution Imaging of Dopants in Graphene

We study impurity atoms, such as Fe and Si, 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 of these heavier atoms. These movies how single Fe atoms are trapped in vacancies intentionally created in graphene by our focussed electron beam irradiation technique. We have demonstrated our ability to add a single metal dopant into the lattice of graphene with a 10nm spatial precision. This was published as Robertson et al. Nano Lett., 2013, 13 (4), pp 1468–1475 and as He at al. Nano Lett., 2014, 14 (7), pp 3766–3772.

Structural Studies of WS2.

Large WS2 single crystals are transferred onto SiN TEM grids to study their atomic structure. We map our the crystal continuity by taking selected area electron diffraction patterns from each hole in the TEM grid to obtain a spatial map of crystal orientation. Our measurements show long range crystallinity.

Utilizing structural knowledge.

Once the structure of the graphene and other 2D crystals are evaluated, we then measure the electronic properties. We utilized electron beam lithography to pattern Hall Bars and FET devices to measure both the field effect mobility and the Hall mobility. These values give insights into the electronic quality of the material and semiconductor properties. Our aim is to produce synthetic graphene and semiconducting 2D crystals 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 2D materials.