BLOG: Graphene cracks the case of brittle phone screens
How many of us have dropped our phones, accidentally cracking the screen? With the latest advances in nanotechnology, this may soon become a pain of the past. By adding graphene to silver nanowires, researchers have created a composite material that could make smartphone touchscreens less fragile.1
Dubbed the world’s “wonder material”, graphene is a one-atom thick layer of carbon atoms, arranged in a hexagonal honeycomb lattice: see Figure 1.2 Due to its unique structure, graphene boasts many superlative properties, most notably being the thinnest, most conductive material. The two-dimensional carbon-carbon bonds make it 200 times stronger than steel, yet extremely flexible.3
Currently, most standard smartphone screens consist of two layers coated with indium-tin oxide (ITO). When a user applies pressure, the two ITO layers are brought closer, creating an electrical circuit.4 Unfortunately, indium is a rare metal that is environmentally harmful to mine, resulting in a manufacturing process that is both expensive and unsustainable.1 The sputtering method used to apply ITO to screen surfaces is also flawed as it involves vaporising the ITO, causing more than half of the material to stick to the walls of the chamber. Because sputtering requires high temperatures, ITO cannot be applied directly to flexible plastic surfaces.4 Any replacement technology will, however, have to emulate or exceed the conductivity, transparency and robustness of ITO.5
Silver nanowire networks have been heavily considered as a replacement. Nanowires, as their name suggests, are thin wires with diameters of a few nanometres or less. They form junctions with one another in an interconnected mesh. In fact, silver exhibits a conductivity 50 times higher than that of ITO.6 However, silver metal corrodes when exposed to air, causing a decrease in transparency and conductivity. Furthermore, these meshes have yet to make a dent in the market due to their high prices.4
When paired with graphene, it is possible to make low-density silver nanowire (AgNW) films that match the optoelectronic performance at a lower cost. In the Langmuir journal, researchers recently proposed AgNW/graphene hybrid films.7 Since graphene is formed from graphite, a relatively abundant resource, supply shortage is less likely. Professor Dalton from the University of Sussex claims that the addition of graphene to the AgNW network improves conductivity ten-thousandfold. Thus, only a fraction of silver is required to achieve equal or better performance. Coupled with the flexibility of graphene, researchers claim that the new screens will be more responsive, less prone to breakage upon impact, and consume less energy.1
First, Langmuir films (i.e., monolayers at the air-water interface8) are formed at the surface of water by spreading graphene powder over acetone solvent.7 The film is then compressed and transferred to the substrate using a polydimethylsiloxane (PDMS) stamp. PDMS is a rubber-like polymer comprising chains of silicon linked to oxygen atoms.9 In experiments, the PDMS stamp was able to reproduce the pattern of graphene on sample surfaces, including AgNW films.7 Researchers are optimistic that spraying machines and patterned rollers will enable this technique to be replicated on an industrial scale.1
There are many advantages of using AgNW/graphene hybrid films over pure AgNW films, for example, their stability against atmospheric degradation. After one month, researchers noted that the relative increase in sheet resistance of the hybrid film was three times lower than that of the pure film. This is due to graphene acting as a barrier for atmospheric elements that could tarnish silver.7 Another issue with pure AgNW films was that straining caused fatigue in the nanowires, resulting in fluctuating electrical properties.1 To test this, researchers observed the response of both films to flexing at different applied stresses. With the hybrid film, those effects were mitigated.7
Nevertheless, AgNW/graphene hybrid films come with their own share of challenges. The main drawback is their surface roughness. ITO has the upper hand here as it produces smooth, continuous surfaces compatible with thin-film electronic devices.6 In contrast, nanowire surfaces have high roughness due to the random stacking of wires on top of each other. This may lead to short circuits.10 Efforts to overcome this include mechanical pressing and embedding in polymers.6 Another problem is that although graphite is abundantly available, pure graphene itself is difficult to isolate.
With the discovery of this hybrid film technology, researchers anticipate low-cost sensors that could pave the way towards developing completely flexible devices. Scientists are already looking to the future, and it is not just restricted to smartphones. Ideas such as electronic newspapers that you can put in your pocket, or even wearable devices embedded in clothing are becoming increasingly feasible.11 The prospects for AgNW/graphene hybrid films are exciting, but more improvements are required to overcome limitations such as surface roughness, manufacturing cost and the current difficulties surrounding the mass production of graphene. For now, though, the work covered here represents a step in the right direction.
- Ford, A. (2017). Sussex physicists have breakthrough on brittle smart phone screens. [online] EurekAlert Science News. Available at: https://www.eurekalert.org/pub_releases/2017-10/uos-sph102517.php [Accessed 1 Dec. 2017].
- Cervenka, J. (2012). Harder than diamond, stronger than steel, super conductor … graphene’s unreal. [online] The Conversation. Available at: https://theconversation.com/harder-than-diamond-stronger-than-steel-super-conductor-graphenes-unreal-5123 [Accessed 1 Dec. 2017].
- Lambert, R. (2017). What can graphene do?. [online] The Home of Graphene. Available at: http://www.graphene.manchester.ac.uk/explore/what-can-graphene-do/ [Accessed 1 Dec. 2017].
- Byrley, P. (2017). Making Flexible Electronics with Nanowire Networks. [online] Scientific American. Available at: https://www.scientificamerican.com/article/making-flexible-electronics-with-nanowire-networks/ [Accessed 1 Dec. 2017].
- Patel-Predd, P. (2009). The trouble with touch screens. IEEE Spectrum, [online] 46(1), pp.11-12. Available at: https://spectrum.ieee.org/consumer-electronics/gadgets/the-trouble-with-touch-screens [Accessed 1 Dec. 2017].
- Ye, S., Rathmell, A., Chen, Z., Stewart, I. and Wiley, B. (2014). Metal Nanowire Networks: The Next Generation of Transparent Conductors. Advanced Materials, [online] 26(39), pp.6670-6687. Available at: http://onlinelibrary.wiley.com/doi/10.1002/adma.201402710/epdf [Accessed 1 Dec. 2017].
- Large, M., Ogilvie, S., Alomairy, S., Vöckerodt, T., Myles, D., Cann, M., Chan, H., Jurewicz, I., King, A. and Dalton, A. (2017). Selective Mechanical Transfer Deposition of Langmuir Graphene Films for High-Performance Silver Nanowire Hybrid Electrodes. Langmuir, [online] 33(43), pp.12038-12045. Available at: http://pubs.acs.org/doi/abs/10.1021/acs.langmuir.7b02799 [Accessed 1 Dec. 2017].
- Oliveira, O. (1992). Langmuir-Blodgett Films – Properties and Possible Applications. Brazilian Journal of Physics, 22(2), p.60.
- Moretto, H., Schulze, M. and Wagner, G. (2012). Silicones. In: Ullmann’s Encyclopedia of Industrial Chemistry. Leverkusen, Federal Republic of Germany: Wiley, p.675.
- Nam, S., Song, M., Kim, D., Cho, B., Lee, H., Kwon, J., Park, S., Nam, K., Jeong, Y., Kwon, S., Park, Y., Jin, S., Kang, J., Jo, S. and Kim, C. (2014). Ultrasmooth, extremely deformable and shape recoverable Ag nanowire embedded transparent electrode. Scientific Reports, [online] 4(1), p.1. Available at: https://www.nature.com/articles/srep04788.pdf [Accessed 1 Dec. 2017].
- University of Sussex (2017). What next for touchscreen technology? Available at: https://www.youtube.com/watch?v=W1y4kObAxSw [Accessed 1 Dec. 2017].