Graphene the wonder material – potentially
By Jason Major
TechNyou
It all started with sticky tape and pencil lead to make a single layer of carbon atoms called graphene. Soon after came the Nobel Prize.
Enter the real Flatland, the 2-dimensional molecular marvel that will revolutionize the electronics and communications industry; the basis of the next industrial revolution – apparently. There is lot of hype and hope for graphene. There was similar sentiment for graphene’s cousins the carbon nanotube and carbon buckyball. Extrapolating their unique properties into something useful, however, has been more difficult than thought. Diamond, another form of carbon, at least sparkles and impresses women. If only I could afford them.
So what is graphene and will it live up to expectations?
Simple beauty, strange behaviour
If you were to draw a model of graphene it would look like a 2-dimensional honeycomb. It was first obtained in 2004 by Andre Geim and Konstantin Novoselov by taking graphite – the form of carbon your pencil lead is made of – and using sticky tape to peel off carbon layers from it. The sticky part of the tape was then repeatedly folded onto this initial carbon peel until a single layer was obtained. They received their Nobel Prize for this in 2010.
Atomic structure of graphene
This deceptively simple structure has some extraordinary properties.
The various forms of carbon are a bit like Mexican food, the same stuff just wrapped in a different way. The difference is a tortilla tastes the same as an enchilada or a taco. Carbon wrapped different ways, or tweaked ever so slightly produces mind-boggling warps in physics. Don’t get me wrong, I like Mexican food.
By the way, apparently Andre Geim won an Ignoble prize in 2000 as part of a team that levitated live frogs in mid air using giant magnetic fields. (And see the video demonstation It is hard to tell with frog, but maybe they are having fun?) I believe Geim is the first to win both the Nobel and IgNoble prize. And his colleague Novoselov, in 2010, was only 36 years old, a baby relative to his fellow Nobel Prize winners. Take that to your next trivia night.
Carbon nanotube
Carbon Bucky Ball
The basics
Graphene is the thinnest and strongest material out there. It is 2-dimensional, so it has no internal structure and it means all of it is an exposed surface giving it the largest surface area of any material relative to its weight and volume – an important thing since chemical reactions occur on the surface of a material. It is really stretchy and, despite being only one carbon atom thick, it is impermeable to gases or liquids. It conducts heat and electricity better than copper.
The practical stuff
Potentially most exciting is graphene’s exotic electronic properties. Down at the nano scale carbon in this form takes on the weirdness of the quantum world. Graphene allows electrons to travel through it as though they were weightless (ie have no mass), and they travel immense distances without scattering, an ability thought to be because of graphene’s near-perfect atomic structure. Graphene has the highest known electron mobility (the speed at which electronic information is transmitted by a material). These traits promise faster electronics and can be made into transistors which are faster than those made from silicon. From here you get into improved solar cells, flexible electronics and speedier computers.
Companies such as chip manufacturers, and research groups are now racing to find ways to produce single layers of graphene reliably. Today a growing number of groups are successfully fabricating graphene transistors.
But…there is always a but
Zero band gap
Key among these ‘buts’ is that graphene lacks what is known as an energy band gap. That is, unaltered graphene has a bandgap of zero. In digital electronics this is a major problem because this lack of an energy band gap prevents transistors from switching off.
Energy band gap? An attempt to explain it
I was a biologist before I became a science communicator, so my knowledge of physics is disgraceful, hence I have sought some help from elsewhere including Casey Johnson at Ars Technica, though a basic understanding of chemistry and physics will help here.
A substance’s energy bandgap dictates the minimum energy an electron needs to escape an atom and become a free particle – a mobile charge carrier able to move freely within a solid material. Another description could be to say it is the energy difference between a material’s non-conductive and conductive state.
So the band gap is a major factor determining the electrical conductivity of a solid. Likewise, the bandgap will prevent electrons with too much energy from joining the atom. By manipulating the band gap, scientists can indirectly control the photons produced or absorbed when electrons undergo energy changes. Materials are frequently produced with a specific bandgaps for use in applications such as laser diodes and solar cells.
Changes in the bandgap also cause the electrons themselves to behave differently. Depending on the size of the bandgap, the electrons will act if they have different masses—a bandgap of zero makes them act as if they are without mass (eg, graphene), allowing them to move as quickly as photons. Larger bandgaps cause them to move slower than a regular electron, increasing the resistance.
Keeping it pure
One reason graphene hasn’t yet swamped our lives is that when you stack these single sheets of graphene into a usable macrostructure they immediately bond together, reforming graphite. When this happens most of the surface area is lost and it no longer behaves like graphene.
The economics
A way also needs to be found to economically mass produce graphene that is of high crystalline quality with no or virtually no defects. And it will need to be processed with atomic precision.
But as you will see below we are finding ways of overcoming these obstacles
Some latest advances
Graphene etching to usher in computing revolution
New Scientist 10 March 2011 by Jessica Griggs
Thankfully sticky tape is being replaced by more efficient ways of getting single layers of graphene: a spray of zinc atoms followed by a dash of acid is one of the latest versions. Such precise control is vital if the material is to be used in super-fast electronic devices.
Scientists from Rice University in Houston, Texas, found that if they spray zinc atoms onto a stack of graphene sheets they will only merge with the first layer. Adding acid dissolves the zinc and removes this weakened layer, leaving the rest of the stack intact (Science, DOI: 10.1126/science.1199183). The technique can be used to scrape off a specific number of layers from multi-layer stacks, leaving behind spots that are exactly one, two or three layers thick wherever you want.
The exact number defines graphene’s properties: a single layer behaves like a metal, and could form a wire, whereas a double layer is like a semiconductor and could be built into a transistor.
Getting control of the energy band gap
New Scientist February 2011 by Duncan Graham-Rowe
A new type of design suggests that simply creating “U” bends in the graphene could overcome the problem of having a zero energy band gap.
To make computers faster, circuits need to be turned on and off with an extremely high switching speed, something at which graphene excels. Indeed, just last year IBM scientists demonstrated a graphene transistor with a switching rate of 100 gigahertz – that is, capable of switching between a “1″ and “0″ state 100 billion times a second, more than twice that of even the fastest silicon transistors.
Ideally these binary states would correspond to a current flowing (“1″) and zero current (“0″). However graphene’s structure means that a current flows through the device in both states, even when the transistor is supposed to be switched off.
The difference between these two states is called the current on/off ratio, and graphene’s low on/off ratio has long been a major barrier to using it in transistors for logic gates and ultimately computer chips.
By making a normally flat transistor similar to a U-shape, but with corners instead of a curve at the bottom, he has found that he can switch it off entirely, increasing the current on/off ratio thousandfold.
Other research groups are also working on this problem
Researchers at the University of California LA (UCLA) have developed a fabrication process for graphene transistors using a nanowire, which is thinner than a human hair, as the gate that switches the transistor on and off.
Stretchy electronics
Researchers have fabricated a stretchable, transparent graphene-based transistor.
When it comes to fabricating stretchable, transparent electronics, finding a material to make transistors from has been a significant challenge for researchers. In fact, it is nearly impossible to fabricate transistors that offer both mechanical stretchability and high optical transparency on unusual substrates such as rubber slabs or balloons by using conventional materials. In particular, graphene devices have the advantage that they can be integrated using printing processes at room temperature without vacuum or high-temperature steps. The capabilities of these systems go far beyond conventional material-based systems.
Stretchable electronics could be useful for various current and future applications, such as wearable displays and communication devices, conformal and stretchable biosensors (brain sensors, balloon catheters, etc.), sensory skin for robotics, and structural health monitors and eye-ball cameras,. Stretchable interconnects and devices would create foldable, rollable and wearable displays. Stretchable sensors could be embedded into gloves and clothing without bulkiness. Surgeon gloves could constantly monitor blood pH and other chemical levels.
Energy storage
Nature, 475, 269 (21 July 2011) doi:10.1038/475269e Published online 20 July 2011
Porous textiles coated with graphene could underpin cheap and long-lasting energy-storage systems.
Researchers at Stanford University in California dipped polyester fibres into a graphene solution and then deposited manganese dioxide onto the resulting structure. This acted as one electrode, combined with another made from carbon-nanotube-coated textiles, in a sodium sulphate solution. The result was a *supercapacitor that maintained a high level of energy storage and power delivery over 5,000 charge and discharge cycles, which is unusually long-lasting for manganese-dioxide-based electrodes.
And in Oz
In Australia, researchers at Monash University have used graphene in combination with water in a way that could produce energy storage systems that perform on par with lithium ion batteries, but recharge in a matter of seconds and have an almost indefinite lifespan. In fact these guys have achieved two cool things. To achieve their energy storage system they had to find a way of keeping graphene sheets apart. This is where the water comes in. Keeping graphene moist – in gel form – provides repulsive forces between the sheets and prevents re-stacking.
In their paper published in Advanced Materials (May 2011), the researchers state that as with other polymeric or molecular materials, the performance of graphene-based materials is strongly affected by the way the individual sheets are arranged. Due to the forces that cause graphene sheets to attract each other, aggregation or restacking is inevitable. Consequently, many of the unique properties that individual sheets possess, such as high specific surface area and peculiar electron transport behaviors, are significantly compromised or even unavailable in an assembly
Their water-based graphene film exhibits unprecedented electrochemical performance, making it possible to make a new generation of *supercapacitors that can combine high energy density, high power density, and high operation rates.
*Capacitors and supercapacitors store electrical charge. Supercapicitors have an high energy density that can be thousands of times greater than common capacitors. CSIRO have a nice explanation
Keeping graphene layers apart with water (Monash Uni
Other references worth a look
New Scientist 28 Feb 2007
Atom-thick carbon transistor could succeed silicon
COSMOS magazine
New graphene chips much faster than silicon