by Brian Clegg
Entertaining though the idea of floating strips of graphene may be, there are likely to be much more significant applications that derive not so much from its electrical properties, or even its strength, but as a result of graphene and the other ultrathin material providing such a unique atomic lattice.
Thin drinks
Specifically, the special nature of two-dimensional materials can provide benefits as a result of the way the two-dimensional lattices interact with other atoms and molecules.
This becomes apparent in developments being made to use these materials in the desalination of water. If it can be done simply and cheaply, desalination is a hugely important technology. The Earth is not exactly short of water. There’s about 1.4 billion cubic kilometres of the stuff. A single cubic kilometre contains a trillion litres of water. Yet areas of the world can be chronically short of drinking water, even if they have ready access to the sea – because the vast majority of those cubic kilometres are undrinkable salt water.
Desalination has been possible for a long time, either by variants on distillation or by using special membranes where pressure is used to force water through a material that holds the salts back in a process known as reverse osmosis, as mentioned above (page 108 ). However, the reverse osmosis method takes a considerable amount of power to run and features expensive membranes that have to be regularly cleaned or replaced. Molecular sieves based on ultrathin materials could be a far cheaper option. A first approach was to use graphene oxide membranes parallel to each other, so the gap between was small enough to let water through, but not big enough to allow the salts in the water to pass through.
This approach worked for larger salts and contaminants, but small salts – though they shouldn’t have been able to get through the gaps – stayed in the water. It was discovered that when the membranes were soaked in water they swelled sufficiently to let the smaller salts through some of the channels. With careful adjustments of the size, it has proved possible to make a largely effective molecular sieve for desalination. However, this wasn’t the end of the development.
A more controlled approach would be to make just the right sized holes or slits in a material to let only the water through – but this doesn’t usually work with conventional membrane materials, because the surfaces of the membrane are too uneven to be able to keep the incisions correctly sized. However, high-quality two-dimensional materials don’t have such irregularities – there is no variation in the surface.
Researchers at the Graphene Institute in Manchester have managed to produce slits less than a nanometre across using graphene, boron nitride and molybdenum disulfide sheets. These gaps are of the same scale as the molecules such as water that are being worked with for filtration. Cutting slits so accurately would prove technically challenging to say the least, but Andre Geim and his team came up with a clever alternative.
They produced two thin pieces of graphite, each with atomically smooth surfaces. These crystals, around 100 nanometres thick, would form either side of the slit. They then placed strips of two-dimensional material along two parallel edges of one of the crystals and rested the other crystal on top, producing a sandwich. The result was a pair of crystals, wedged apart by a gap no thicker than the two-dimensional material used. As Geim explained:
‘It’s like taking a book, placing two matchsticks on each of its edges and then putting another book on top. This creates a gap between the books’ surfaces with the height of the gap being equal to the matches’ thickness. In our case, the books are the atomically flat graphite crystals and the matchsticks the graphene or molybdenum disulfide monolayers.’
The whole structure is held together by van der Waals forces and the size of the slits is similar to the tiny gaps provided in living cells by proteins called aquaporins, which allow water and ions † to move through cell walls, a process that is essential for biological functions. When an electrical potential difference is applied from one side of the molecular sieve to the other, different ions move through the slits. Surprisingly, ions bigger than the slits can get through them, as atoms aren’t solid balls but have a degree of flexibility. The hope is that with a better understanding of how such slits can be used to control ion movement it will be possible to produce desalination plants that can process seawater much more quickly and with far less power consumption than would be possible using a simple membrane sieve approach.
In this case, the sieve action came from the slits between the blocks of graphite, held apart by ultrathin strips. However, a variant of graphene has provided an alternative approach that produced the remarkable property of automatically distilling liquor without any added energy required.
The magic still
Graphene continues to surprise those working on it with this kind of unexpected new trick. In 2012, Andre Geim’s team reported one of its strangest behaviours yet. Because of its continuous lattice structure, graphene is very good at keeping liquids and gases at bay. The team produced sheets of multi-layered graphene oxide – graphene with ‘hydroxyl’ structures (OH) randomly attached to the surface, forming a self-supporting membrane, still hundreds of times thinner than a human hair.
When the membrane was used to line a metal container it proved superbly effective at preventing liquids and gases escaping – even helium, which is notoriously hard to keep in place. This is impressive when you consider that a millimetre-thick coating of glass won’t stop helium slowly working its way through – but this wafer-thin membrane kept it in place.
Of itself, this wasn’t too much of a surprise, as graphene’s structure doesn’t allow much in the way of escape routes. But what was remarkable was that one thing could get through the graphene oxide membrane. Water. Liquid water couldn’t be poured through the membrane, but the team discovered that water would evaporate through it at the same rate as it evaporates to open air. This was a stunning discovery.
What appears to be happening is that graphene oxide sheets have just the right spacing between them for a single molecule-thick layer of water to slide through the gap. Bigger atoms or molecules don’t fit and smaller ones, such as helium, cause the structure to shrink and close up the space – only water seems to have the magic touch.
This being Andre Geim’s team, there was an irresistible application that suggested itself. If you place a water-based mixture into a container sealed with this membrane, over time the water will evaporate off, leaving the rest of the contents behind. One of Geim’s team, Rahul Nair, commented: ‘Just for a laugh, we sealed a bottle of vodka with our membranes and found that the distilled solution became stronger and stronger with time.’ Nair himself doesn’t drink, but it’s hard to imagine that the super-strong vodka went to waste.
While self-distilling alcoholic drinks have a certain appeal, there are plenty of other applications where it may be useful to reduce the water content of a mixture without allowing other volatile substances to escape, for example to remove contaminants from fuels, or to reduce water content of fruit juices for transport without losing any of the volatile compounds that give them the ‘freshly squeezed’ taste.
Moving away from liquids, though, one of the original graphene team has found another use for graphene that may be niche, but plays to its strengths.
The microscope’s friend
One of the smaller potential applications of graphene, but one that Konstantin Novoselov is fond of, is as a support structure for materials to be examined when using transmission electron microscopes – the kind of electron microscope wherein a stream of electrons – fulfilling the role performed by light in a traditional optical microscope – is passed through a material rather than reflecting off it as happens with a scanning electron microscope. What’s needed to support the sample is a strong material that isn’t damaged by radiation and that is a good conductor of electrons – and graphene ticks all the boxes.
The process would involve transferring a layer of graphene onto one of the metal support grids used in transmission electron microsco
py. It could be exposed to the substance to be studied in solution and would support and hold in place the biological material or other substance far more consistently than a traditional electron microscope slide.
This kind of application is likely to have some indirect medical use, but there is a way that graphene could have a much more direct benefit for medical teams, by providing diagnostic tattoos.
What use is an invisible tattoo?
There is considerable excitement in medical circles over the possibility of creating graphene tattoos. These are not the latest fashion in body modification – after all, graphene is pretty much transparent, so graphene tattoos are hardly showy – but rather a remarkable potential mechanism for producing health-monitoring devices that will hardly be noticed by the user, even when engaging in movements that would tend to rip away an ordinary sensor.
Although the term ‘tattoo’ sounds worryingly permanent, the graphene tattoos are not inserted under the skin, but use the same adhesive technology as temporary tattoos. Lasting around two days before they fall away, they can easily be removed earlier if required, by the familiar graphene transport trick of applying a piece of sticky tape.
In comparison to the kind of sensor found in fitness bands or smart watches, a graphene tattoo has the huge advantage of contorting to match the shape of the skin as the wearer moves about. This means that the electrical contact stays constantly in place, a requirement for medical-grade data, whether the tattoo is being used to monitor heart rate or bioimpedance (the skin’s electrical resistance), the latter being used both to give an idea of body composition levels – a more effective measure of body fat levels than BMI – and diagnostically for a range of cardiac, pulmonary, renal, neural and infection-based disorders.
The constant contact, thinness and flexibility mean that a graphene tattoo would be significantly less obtrusive than a fitness band but would have at least as good a connection as existing medical sensors, which have to be glued on with an application of conducting gel. The traditional sensors can cause skin damage when removed, particularly to elderly patients, and are relatively expensive to produce.
As we’ve seen many times already, graphene needs some kind of substrate to prevent it from crinkling up. In the graphene tattoo, that substrate sits above the graphene, rather than below it, in the form of a transparent polymer called polymethyl methacrylate (PMMA for short). The structure of the sensor is then laser-cut onto temporary tattoo paper, which is used to apply it to the skin.
Not only is the graphene tattoo nearly transparent, it is as flexible as human skin, so is hardly felt by the wearer, even when covering relatively extensive areas. In the first practical tests of the technology in 2017, the prototype tattoos were used to check skin temperature and hydration (measured from skin conductivity) and as electrodes for electrocardiograms, electromyograms and electroencephalograms, measuring activity in the heart, muscles and brain.
This flexibility makes graphene technology a natural for something attached to the skin, but there are plenty of other circumstances where having non-rigid electronics could be useful – and potentially doing far more than just acting as an invisible electrode.
Flexible fun fashion
The flexibility of graphene and the other thin film materials gives them a natural potential for producing wearable electronics. This is the same kind of technology as the graphene tattoos discussed in the previous section, but in this case could be applied either to the skin or to clothing, as a membrane like the tattoo or as graphene ink for less serious applications. It’s possible that such wearable electronics could enable anything from direct interaction between human and machine – controlling a machine by gestures, for example – to using the light-emitting capabilities of molybdenum disulfide to produce clothing that lights up with a still or moving image.
The researchers who were responsible for the graphene tattoo suggest that wearable graphene electronics could be used for interaction with smart houses and controlling wheelchairs and robots. They have already demonstrated the ability to control a drone wirelessly using signals from a graphene tattoo.
Another 2017 development was at the Tsinghua University in Beijing, where a graphene-based strain sensor was used to change the colouring of a layer above it. A layer of graphene acted both as a very sensitive measure of strain – since distorting the graphene lattice changes its electronic properties – and as an electrode to control an organic electrochromic device. This is a material that changes colour depending on the electrical voltage applied to it.
The result was a thin, flexible sheet of material giving a constant visual readout on the amount of strain applied to the sheet. The visual strain sensor has both potentially entertaining applications – for example, clothing that changes colour with your movements – and practical use as a sensitive strain gauge that could be used to monitor conditions in anything from high-risk construction to medical swelling.
As far as electronically enhanced clothing goes, in 2017, researchers at Cambridge, working with colleagues from Italy and China, produced fabric which had graphene circuitry directly printed onto the fibres of the material. The result was electronic circuitry which is as flexible as the garment and can survive up to twenty washes.
This development reflects one of the most significant steps in these kinds of graphene technology – the development of inks based on graphene and some of the other two-dimensional substances, which can be printed onto a fabric (or a piece of paper) from an inkjet printer. Even at this very early stage, the researchers have been able to produce fully functional, all-printed electronic circuits. Although inks for printed electronics had already been used to a limited extent, they use solvents that are dangerous for human contact, and are not effective on flexible materials. However, the materials based on layers of graphene and boron nitride have the usual flexibility of two-dimensional materials and are non-toxic and environmentally friendly.
Most importantly, the visual side is not all that can be provided using printable two-dimensional inks. Anyone who has ever worn a shirt with built-in technology for lights or other devices knows that the real problem is not so much the display as the power source. ‡ They come with a separate, clunky battery pack which has to be housed somewhere when the garment is worn, and removed for washing. However, in 2017, the Graphene Institute demonstrated supercapacitors (the ultrathin alternative to batteries – see the next section) produced using graphene oxide ink printed on to cotton fabric. The cotton fibres act as a substrate for the graphene oxide supercapacitors, which remain as flexible as the fabric. As well as making clothing-based electronics more practical, this would be equally valuable used in conjunction with the graphene tattoos to power medical diagnostic and fitness monitoring devices.
Although making a T-shirt light up is an entertaining use of ultrathin materials, the supercapacitor itself has the potential to do far more. It could even be the answer to the electric car problem.
Supercharged stores
One of the biggest drawbacks of our increasing dependence on batteries – whether it’s to power our smartphones or, at the other extreme, an electric car – is the time that it takes a battery to charge. Thanks to graphene’s electronic abilities, a team at the University of Waterloo have managed to make significant steps forward in the production of supercapacitors, which are devices that have the potential to replace batteries, but can be charged up in seconds.
A capacitor (the electronic component formerly known as a condenser) is a device that holds electrical energy in the form of an electrical field, as opposed to the chemical energy used for storage in a battery. This means that a capacitor can charge up and discharge extremely quickly – as quickly as you can get the power into it, without having to wait for a gradual chemical process to take place. At its simplest, a capacitor is just a pair of metal plates with a sheet of dielectric medium between them, usually a type of plastic. ‘Dielectric’ means that it is an insulator, but is able to hold an electrical cha
rge – so although it doesn’t allow a current to flow between the plates, the dielectric becomes positively charged on one side and negatively charged on the other.
Once a capacitor is charged up, it can hold that charge for some time before it is connected to a circuit and the charge is released. This ability makes capacitors a common component in electrical circuits, particularly as they will block a direct current from flowing, but will allow an alternating current through. Direct current (DC) is the kind of continuous electrical current in one direction produced by a battery, while alternating current (AC) is the type of electricity used in mains supplies, where the direction of flow alternates, its voltage usually varying like a sine wave. Signals in electrical form usually come in the form of waves and one of the benefits of the capacitor’s ability to lose DC is to filter out unwanted noise to leave the pure wave behind. But the capacitors face two problems if they are to replace batteries – the amount of charge they can hold and the speed with which they discharge it.
Just as a capacitor can be charged up far quicker than a battery, it can also let all its charge go as quickly as the resistance in the circuit allows it. When I constructed a laser flashtube in a sixth form project at school, we used capacitors borrowed from Manchester University which were the size of five-litre oil cans, could hold enough charge to kill, and discharged in a fraction of a second to make a flash of miniature lightning several centimetres long.