When it comes to smartphones, tablet computers and television displays, one construction material dominates them all: Indium tin oxide (ITO). It has been used for the last 60 years, and is a key component for more than 90 percent of all such displays. But a potential competitor to ITO has just been discovered, one that is both highly transparent and extremely conductive. Its workings are detailed in the journal Nature Materials.
Unlike processing chips and memory storage devices, the cost of manufacturing more touch-sensitive and fracture-proof displays has only increased over time, and ITO has proven to be the most adaptable material in this respect. It is easy to make, easy to shape, conducts electricity very efficiently, and has excellent optical properties. Unfortunately, its price has skyrocketed over the last 10 years, meaning that there’s room for competitors – none, however, have challenged ITO’s market dominance.
For this study, the team of researchers led by Roman Engel-Herbert, an assistant professor of materials science and engineering at Pennsylvania State University, looked at the problem from a different angle. The research team hoped to create a new material that provided the same properties as ITO, but using more readily accessible materials in order to make a cheaper variant.
The researchers first produced incredibly thin (10 nanometer) films made of two types of unusual metal compounds – strontium vanadate and calcium vanadate. Most metals, such as gold or silver, conduct electricity by passing charge along using the outer electrons of its metal atoms; these are able to move freely through the structure of the material.
But the two metals used for this study – known as correlated metals – behave differently, conducting electricity by taking advantage of a “hole” within the structure. This hole is a zone of positive charge, not a physical hole; this encourages the electrons to “hop” through, allowing for the conduction of electricity, somewhat flowing like a liquid. This efficient electrical conduction is a necessary requirement of all contemporary digital displays.
Spot the difference if you can. The two squares on the left are strontium vanadate, the right two are calcium vanadate, and the middle two are uncoated. Lei Zhang, Penn State
When combined with oxygen, these metals also show remarkable optical properties, allowing an impressive range of wavelengths of light to penetrate them, from the low-energy infrared to the high-energy ultraviolet. Essentially, it is transparent to a wide variety of colors of light, making it ideal for use in smartphone displays.
Currently, indium costs £500 ($750) per kilogram. It’s already a rare element in the Earth’s crust, one we have severely depleted over the last six decades. Strontium, calcium and vanadium – the elements of the correlated metal compounds used in this study – are far more abundant. Vanadium, for example, sells for only £17 ($25); strontium is even cheaper.
“Our correlated metals work really well compared to ITO,” said Engel-Herbert in a statement. “Now, the question is how to implement these new materials into a large-scale manufacturing process. From what we understand right now, there is no reason that strontium vanadate could not replace ITO in the same equipment currently used in industry.”