Have you ever imagined your smartphone or tablet without a touchscreen? This could soon be the case if we ran out of indium, one of the rarest minerals on Earth.
Indium is used in many high tech devices such as touch screens, smart phones, solar panels and smart windows, in the form of indium tin oxide. This compound is optically transparent and electrically conductive – both crucial characteristics required for touch screens to function.
But there is a problem: we have no guarantee of long-term supply of indium. It is only found naturally in tiny traces, and it is therefore impractical to exploit it directly. Almost all of the indium in the world is a byproduct of zinc mining.
Fortunately, we have a potential solution: My colleagues and I have developed a new way to make optically transparent and electrically conductive coatings without indium.
A worsening problem
Since the global supply of indium is linked to zinc mining, its availability and price will depend on the demand for zinc.
Possible declines in demand for zinc – already evident in the auto industry – along with the ever-increasing use of smartphones and touch screens – are expected to exacerbate the potential shortage of indium in the future.
One option is to try to recycle indium. But recovering it from used devices is expensive because of the tiny amounts involved.
Read more: Touchscreens: Why a new transparent conductive material is sorely lacking
When a crucial material is scarce, we must look for alternatives. And that’s exactly what my colleagues and I found.
How it works?
Our new coating, details of which are published in the journal Solar Energy Materials and Solar Cells, uses plasma technology.
Plasma is like a soup of charged particles in which electrons have been stripped from their atoms, and is often described as the fourth state of matter, after solid, liquid, and gas. It may sound like an exotic substance, but in fact it comprises over 99% of the objects visible in the universe. Our Sun, like most stars, is essentially a giant ball of glowing plasma.
Closer to home, fluorescent bulbs and neon signs also contain plasma. Our new touchscreen films do not contain plasma, but their manufacture uses plasma as a means to create new materials that would otherwise be impossible to manufacture.
Our coating consists of an ultra-thin layer of silver, sandwiched between two layers of tungsten oxide. This structure is less than 100 nanometers thick, or about a thousandth the width of a human hair.
These ultra-thin sandwich layers are created and applied to glass using a process called “plasma spraying”. This involves subjecting a mixture of argon and gaseous oxygen to a strong electric field, until this mixture turns into plasma. Plasma is used to bombard a solid tungsten target, detaching atoms and depositing them as an ultra-thin layer on the surface of the glass.
We then repeat this process using silver, then a third time with tungsten oxide encrusted with silver nanoparticles. The whole process only takes a few minutes, produces minimal waste, is cheaper than using indium, and can be used for any glass surface such as a phone screen or window.
Another interesting feature of the finished plasma coating is that it is electrochromic, which means that it can become more or less opaque, or change color, if an electrical voltage is applied.
This means that it could be used to create ultra-thin “printable screens” that can get darker or brighter, or change colors as you like. They would be flexible and use little power, which means they could be used for a variety of purposes including smart labels or smart windows.
Smart windows covered with our new films could be used to block the flow of light and thus heat as needed. Our plasma film can be applied to any glass surface, which can then be adjusted to adjust its transparency according to the weather outside. Unlike existing ‘photochromic’ spectacle lenses, which react to ambient light levels, our material responds to electrical signals, which means it can be manipulated at will.
Our new indium-free technology offers great potential to manufacture next-generation touchscreen devices such as smart phones or electronic papers, as well as smart windows and solar cells for environmental sustainability. This technology is ready to be extended to create coatings on commercial glass, and we are currently continuing research and development to adapt them to future portable electronic devices.
Read more: From cobalt to tungsten: How electric cars and smartphones are triggering a new gold rush