Touchscreen technologies convert tactile input from fingers or a stylus into control signals for a computer. this is achieved using a matrix of conducting material over the display whose electromagnetic field alters with stimulii from a capacitive object. This field change is fed to a microprocessor that interprets these stimulii as a two-dimensional (x,y) location.
There is a rapid growth in the science behind touch screens, and presently there are more than 20 different touchscreen technologies in production. Almost all of these rely on transparent conductive materials such as indium tin oxide (ITO). This film is used to create the capacitive and resistive grid networks that capture the touch signals, without obstructing the display screen.
Rise of Touch Screen Devices
Today, the market is booming with advancements in touch technology. With Windows 8 hitting the market, ultrabooks, laptops and all-in-one-computers that have touch capabilities are now available. Touch screens have primarily been used in smartphones up until recently, but this new array of touch devices appearing on the market have considerably larger screen areas, putting additional strains on the material requirements for the touch sensors.
Presently, the most common material used in touch panels is indium tin oxide (ITO), which is quite challenging to work with. Chemical vapour deposition is used to deposit ITO on substrates, which can be quite costly for large area substrates. It is a brittle ceramic material, so it can easily crack if placed on a flexible substrate.
Whilst it is possible to apply ITO layers to thin films by CVD, the substrate materials are limited to those that can withstand the high temperatures required - constraints on annealing temperature also place an upper limit on the conductivity of thin-film ITO, limiting performance.
ITO films also block around 10-30% of the light from the display screen, reducing clarity and brightness. In addition, indium is an expensive and rare material - natural supplies of indium are likely to run out in just a few years, so aside from any technical problems, the raw material supply chain will simply not be able to cope with the predicted rapid growth in the touchscreen market.
Because of these limitations, researchers and manufacturers are actively exploring alternatives to ITO for the touch sensor layer in displays.
Touch screens are becoming more and more ubiquitous, but significant advances in the materials used will be required for the technology to fulfil its potential.
Graphene Touch Screens
In 2010, an international team of physicists, including co-inventor of graphene Dr Kostya Novoselov, published a paper in which they have described the use of the "wonder material" graphene in rollable e-paper, amongst other applications.
The article, titled “A Roadmap for Graphene”, published in Nature, predicts that the super-conductive material, which is made of a single sheet of carbon atoms, will be used in the development of many future electronic devices.
The researchers anticipate that graphene will replace indium tin oxide in touch screen devices in three to five years, with rollable e-paper emerging by the year 2015.
A Rice University team has already conducted trials of graphene touch screens by growing a fine graphene sheet on a metal nanowire grid.
Companies such as Bluestone Global Tech are already marketing sheets of graphene large enough to use in manufacturing of electronic devices, and many electronics manufacturers are investigating using this material in their future products.
Silver Nanowires
In an interview with AZoNano, John LeMoncheck, the CEO of Cambrios Technologies, talked about the company's work on a novel transparent conductive material using silver nanowires that can be deposited very easily, even on flexible substrates, and can reach high conductivities.
The nanowires are arranged randomly across each other, creating a wire network that forms a conductive surface on the substrate. Due to the thinness of the nanowires, the network has a lot of empty spaces, making the surface transparent. The process is solution-based hence the substrates can be coated rapidly using standard roll-to-roll printing techniques, and the use of silver makes the conductivity of the material very high.
A number of innovative display modules created with this novel material were displayed at CES Fall 2012 and CES 2013 in January, including models from LG, eTurboTouch and CNi. These devices are designed to take advantage of the touch capability in Windows 8.
The flexibility and performance of silver nanowires will enable leaps forward in industrial design, with touch-capable surfaces applied to any number of devices, even ordinary objects or structural features.
Copper Nanowires
Duke University researchers have developed a technique for producing copper nanowires at a scale that could make them a replacement for ITO in solar panels and touch screens.
The water-based production process, however, caused clumping of the nanowires, decreasing their conductivity. The researchers have conducted further studies and have solved the clumping problem and according to them copper nanowires will appear in cheaper solar cells, touch screens and flexible electronics in the coming years.
Copper nanowires can be coated in a roll-to-roll process unlike ITO films that use a costly and slow process. Also, copper nanowires are flexible and can be used to build flexible electronics. After just a few bends, ITO films break, but copper nanowires retain their conductivity and form even if bent 1000 times.
Copper nanowires are more economical than ITO and silver. Researchers believe that with constant development, they can be used in solar cells and screens leading to more reliable and lighter displays and make solar energy more competitive when compared with fossil fuels.
Conclusion
Touch screen displays are currently going through a phase of rapid transformation. The touch-enabled products that come onto the market in the next few years will represent a paradigm shift in the technology.
Electronics manufacturers are investing heavily in materials like nanowires and graphene, and are actively developing manufacturing processes and products that take advantage of their properties. It is likely that the expensive and limiting ITO will begin to be replaced in the near future - 2013 is expected to see the launch of a number of touch devices using these new materials.
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