Graphene is a wonder material poised to move out of the laboratory and into advanced products such as lightweight composites, anti-corrosion coatings, fast-charging batteries, and filters for water purification or food processing. Now the first collaborative centre of its kind in the Southern Hemisphere has been established at Monash University, combining research excellence, design and industry to realise the graphene revolution. By Michael Fuhrer and Dan Li.
The Monash Centre for Atomically Thin Materials (MCATM), a joint initiative between Monash’s Science and Engineering faculties, is an international hub for research in novel atomically thin materials such as graphene. The multidisciplinary centre brings together world-leading expertise from across Monash with national and international partners and industry. With state-of-the-art facilities and technology, it offers a platform for researchers to gain a deeper understanding of how atomically thin materials integrate with each other or with other materials, to achieve engineering solutions and realise new applications to meet the needs of the Australian manufacturing industry.
What is graphene?
Graphene is a plane of carbon just one atom thick arranged in a hexagonal lattice reminiscent of a honeycomb. Graphene is the basic constituent of graphite, which consists of graphene layers stacked one on another, with only very weak bonds between them. It is these weak bonds that make graphite useful as the writing material in pencils: the layers are easily separated, allowing thin flakes of graphite to be rubbed off onto the paper as the pencil mark. The weak interlayer bonds also allow graphite to be quite stable in its single-layer form – graphene – without chemical reactions with the elements around it.
Graphene was first isolated by physicists Andre Geim and Konstantin Novoselov at Manchester University in the UK in the early 2000s. Inspired by the pencil, Geim and Novoselov used a technique only slightly more sophisticated: they used common sticky tape to peel apart a graphite crystal until it was extremely thin. They thin pressed the thin graphite residue on the sticky tape against a flat surface (a silicon wafer coated with silicon dioxide), and to their surprise when they examined the material on the silicon wafer, they found that sometimes layers only a single atom thick could be found among the debris.
Geim and Novoselov were particularly interested in the electronic properties of graphene, and they were able to quickly demonstrate a graphene transistor. Michael Fuhrer, Director of MCATM, found in 2008 that graphene is an extraordinarily good conductor of electricity, with an intrinsic electrical conductivity higher than copper or silver at room temperature. He was also able to show that electrons in a graphene transistor can move more than 100 times faster than in silicon, making graphene very promising for ultra-high speed electronic devices.
The high electrical conductivity of graphene, coupled with the fact that it is almost perfectly transparent to light, makes graphene an ideal material for transparent conducting electrodes. Transparent conducting electrodes are an essential part of touch-screen displays and are also important in the construction of most types of photovoltaic cells.
The current widely adopted technology for transparent conducting electrodes uses indium-doped tin oxide (ITO) as the conducting layer. However ITO has several challenges: indium is expensive (though recycling of electronic devices has helped keep the cost down); ITO requires an expensive vacuum deposition step to manufacture; and, most importantly, ITO is relatively brittle. This last aspect makes thin flexible graphene attractive for new applications such as rollable or foldable touchscreens.
One technique to make graphene over large areas is to grow graphene on a copper foil by chemical vapour deposition (CVD) of carbon from methane, and then transfer the graphene from copper to the desired substrate either by dry or wet processing. But the CVD process still requires high temperatures, and reliably obtaining graphene over large areas with the uniformity required for electronics remains challenging. Processing graphene flakes in solution to form thin films could be an alternative that avoids the high-temperature step.
Graphene was first investigated for its electronic properties, but its superlative properties extend to much more than electrical conduction. The carbon-carbon bond in the graphene honeycomb is the strongest bond in nature, making graphene the strongest material known when stretched in the direction of the plane of atoms. Graphene is also exceedingly tough, and can be stretched over 25% before breaking. Graphene is an exceptionally good conductor of heat, with a thermal conductivity comparable to diamond at room temperature. As the ultimate limit in thin materials – just one atom thick – graphene has enormous surface area; just three grams of graphene have the same surface area as a soccer pitch. And as it is made of lightweight carbon atoms, it performs exceptionally well when compared to a given weight of another material.
Graphene as an additive
Graphene can be added to other materials to enhance their properties. Because of the atomic thinness of graphene, a little goes a long way; the threshold for loading of graphene into another material to achieve percolation (when the additive components begin to form a continuous network) is extremely low. Graphene added to plastics can dramatically increase their electrical and thermal conductivity as well as mechanical strength.
Challenges remain in dispersing graphene uniformly through the host matrix, and in ensuring the proper level of interaction between the matrix and the graphene. The weak bonding of graphene to other materials, which allows its stability, also means a weak mechanical coupling between graphene and matrix. This coupling can be strengthened through direct chemical bonding between matrix and graphene, but this comes at the expense of reducing graphene’s electrical and thermal conductivity as well as mechanical strength.
Graphene composites are in their infancy, but are already seeing some commercial use in niche areas such as the sporting goods industry, where the ultimate in performance is desired even at high cost. Graphene has appeared in a tennis racket designed by Austria’s Head, and bicycle wheels from Italy’s Vittoria, though the amount of graphene in the products, and the resulting increases in performance, are closely guarded secrets. But novel graphene-based materials, such as printable conducting inks or conducting elastomers, are entering the marketplace and are expected to become increasingly important – especially as the popularity of additive manufacturing increases.
Constructing new graphene materials
Graphene can be assembled into macroscopic three-dimensional materials in myriad ways. Dan Li, Co-Director of MCATM, has used this idea to create hierarchical structures of pure graphene that resemble flexible paper or extremely lightweight but elastic foam.
These materials can be designed to have exceptional surface area, which makes them superb as electrodes for supercapacitors, which can store similar amounts of electrical energy as conventional batteries but can be charged and discharged much faster. Supercapacitors can supply bursts of energy and are able to absorb energy rapidly in energy recovery systems, and are finding uses ranging from electric tools to electric cars and buses.
Research shows that graphene-based nanostructured paper or foam are also promising for next-generation separation membranes for water purification and chemical separation, as electrically conductive tissue scaffold for bone regeneration, and extremely sensitive yet fast responding flexible pressure sensors.
Large-scale use of these new graphene materials will require the availability of large quantities of graphene itself. Directly synthesising graphene from methane is far too expensive for most bulk applications, so graphene should be obtained from graphite, an inexpensive mineral rich in Australia. Li has pioneered techniques to disassemble graphite into graphene in solution in a scalable manner, making it useful as an additive for composites or for constructing graphene-based bulk materials at a reasonable cost.
Graphene not the only ‘wonder’ material
Taking a lesson from graphene, MCATM researchers are also studying other materials that can be disassembled into atomically thin layers. Molybdenum disulphide is a naturally occurring mineral (‘molybdenite’) commercially extracted as the primary source of molybdenum, but in single-layer form it is a direct band-gap semiconductor that can be used to make transistors, light-emitting diodes, and photovoltaic cells. Hexagonal boron nitride has a similar honeycomb structure to graphene, but is electrically insulating yet still highly thermally conductive. Graphene oxide is a modified form of graphene that is much more chemically active.
These materials can be used to add new properties to composites or hierarchically structured materials. MCATM researchers are working on ‘smart’ composites containing molybdenum disulphide that can report stress through a change in colour, and graphene oxide-based materials that can readily absorb toxic substances in water yet can be easily recycled.
Europe and the USA have already invested heavily in research and design in atomically thin materials – arguably the most disruptive and transformative new materials to have been discovered. Significant research expertise in Australia and more widely in the Southern Hemisphere, combined with rich reserves of the raw materials, need to be harnessed in such a way that Australia becomes recognised as a world leader in atomically thin materials research and commercialisation. By connecting research expertise and industry in one focused ‘hub’, the new Monash Centre for Atomically Thin Materials aims to do just that.
Michael Fuhrer and Dan Li are Directors at the Monash Centre for Atomically Thin Materials.