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The nano-sandwiches that could transform electronics

Two-dimensional sheets are stacked on top of each other to create materials with novel properties. Shutterstock/nobeastsofierce
Two-dimensional sheets are stacked on top of each other to create materials with novel properties. Shutterstock/nobeastsofierce

Researchers have developed a transparent light sensor that could be integrated into windows or mobile phones, in a demonstration of how two-dimensional materials can be sandwiched together to generate on-demand properties.

Two-dimensional materials hit the headlines in 2004 following groundbreaking experiments into graphene, a one-atom-thick sheet of carbon that is renowned for its ability to conduct electricity.

However, scientists now know that there are at least 500 materials that can be reduced to the thickness of a single layer and they are beginning to combine these two-dimensional materials into three-dimensional stacks to see what emerges.

‘It is like a sandwich,’ said Professor Andrea Ferrari at the University of Cambridge, UK. ‘You know very well that if you eat a piece of bread and then a piece of lettuce and then a piece of cheese it is not the same thing as eating them in a sandwich at the same time.

‘It is exactly the same for two-dimensional materials. Each of the two-dimensional materials is an ingredient of the sandwich. Once you put them together you create a new material that has properties that are different and are not just a combination of the properties of each individual material.’

Prof. Ferrari is a partner on the EU-funded HETERO2D project, which aims to determine the properties of some of these nano-sandwiches and show that they can be used in a variety of devices. The project has already resulted in several novel electronic devices, including a transparent light sensor.

‘A photodetector is usually black because it absorbs the light, but in this case more than 80 % of the light goes through,’ said Prof. Ferrari. ‘At the same time it has comparable efficiency as some traditional black detectors. That can be very useful because you could incorporate these detectors into windows or even on the surface of your mobile phone.’

Superior properties

Over the next few years, the researchers hope that they can demonstrate that stacked 2D materials can be used in photovoltaic devices, such as solar panels, as well as new kinds of displays, lasers and sensors. However Prof. Ferrari cautions that it will be around 10 years before current research is developed enough to make its way into consumer electronics.

In the meantime, the HETERO2D partners are focusing on basic research. A team at the University of Manchester, led by Nobel prize winner Professor Konstantin Novoselov, is exploring which combination of materials produces which properties. They are helped by theoretical input from their project partners at the University of Lancaster, UK, which Prof. Ferrari says is vital.

‘Theory is essential for us to get an idea of what to do because otherwise the phase space is too large. The number of possible combinations is essentially infinite.’

Stacking the 2D layers is done by hand in the lab’s clean room. While this enables many layers to be built with great precision, it is a time-consuming and highly skilled process. As a result, the researchers are also working on other methods of creating stacks that may be more useful in a commercial setting.

Overcoming limitations

One of the hopes for new nanomaterials is that they will be able to overcome some of the limitations of graphene, notably that it is too good at conducting electricity. Graphene has no band gap – the minimum amount of energy required to make a material conductive – which means that its conductivity cannot be switched off and it needs to be modified before it can be used in semi-conductors.

Inspired by the existence of graphene, researchers on the 2DNANOLATTICES project, funded by the EU, set out to find the silicon equivalent, silicene. Silicon is already widely used in the electronics industry and they speculated that a lattice made from a single layer of silicon atoms might possess some of the advantages of graphene but without the band gap problem.

However, unlike graphene, where a layer of carbon atoms can be easily lifted from a lump of graphite, silicene cannot be exfoliated from silicon and in fact does not exist in nature in free-standing form. The researchers tried instead to grow silicene on another material, known as a substrate.

Dr Athanasios Dimoulas from the National Center for Scientific Research ‘Demokritos’ in Greece explained that they did this by melting silicon crystals. ‘When you melt it some of the silicon atoms evaporate. These atoms just arrive at your substrate and then they stick. The tricky thing is that you have to control the temperature of the substrate in order to be able to order the silicon atoms in the silicene lattice.’

Using a single crystal of silver as a substrate, the researchers succeeded in producing a single layer of silicon atoms – silicene – for the first time. But growing silicene on silver is of limited practical use as silver conducts electricity, and to be used in electronics, silicene needs to be surrounded by an insulator.

Recently, however, other researchers on the project have succeeded in moving silicene from the silver substrate onto a layer of silicon dioxide, and used this to make the first silicene-based transistors.

While these results are promising, research is still at the very early stages and Dr Dimoulas says that more modification is needed to achieve the desired properties of silicene. However, he says there is good potential. ‘The trend (in electronics) is to find new ways to reduce the power consumption; you want to make devices that are not energy-hungry devices. I think silicene can serve that purpose.’

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