Can we cope without rare-earth elements?

Researchers look for materials for the future

A mobile phone is full of raw materials: its electronic components contain valuable precious metals, such as gold, silver and platinum, rare metals, like cobalt, gallium or indium, and rare-earth elements, such as neodymium. Often, the old devices end up in the bin - and, with them, their precious cargo. "These days, we already sometimes have higher concentrations of elements like gold or germanium in landfills for electronic scrap than in some of the mines they are extracted from," says materials chemist Michael J. Bojdys. And some of these materials could become scarce in the future.

Since 2011, the EU has been keeping a "list of critical raw materials", in which it enumerates all the substances there may no longer be enough of in the coming decades - either because there is only very little of them in the Earth’s crust or because they are very difficult to recycle. There are currently 30 materials on this list, including quite a few that ensure that the displays of mobile phones and laptops or flat screens light up. As Visiting Professor of Organic Materials Chemistry and head of the Functional Nanomaterials Group at IRIS Adlershof and the Department of Chemistry, Michael Bojdys and his team conduct research into which alternative materials could fill these voids in future.

Carbon is the basis for new materials

In their quest, the researchers also look at organic light-emitting diodes - OLEDs for short - which are considered a pioneering technology of the future and can be used as an energy-saving light source, for example, in flexible displays. They do not require any rare metals or rare-earth elements and are already being used today in the displays of smartphones and tablets and in screens. The principle is simple: an OLED is made up of several thin layers. At its core is a wafer-thin active layer of carbon-based molecules, an organic semiconductor that is excited by electrodes and made to light up. For researchers like the chemist David Burmeister, it is this active layer that is particularly interesting - because it can be made of very different materials.

In his doctoral thesis at IRIS Adlershof, David Burmeister concentrates on one such material that consists of layers of bonded carbon and nitrogen atoms stacked on top of one another. It is a so-called polymer, composed solely of identical building blocks so as to form a large network, and is synthesised in the laboratory. Its name: poly(triazine imide) - PTI for short. In the field of organic semiconductors, this layered polymer belongs to a completely new class of materials - graphitic carbon nitrides. The researchers are now investigating its properties, structure and function from the ground up in order to test whether it is suitable as a construction material in OLEDs or other future technologies.

From sketch to application 

"Today, OLEDs are not quite where the market would like them to be," says Michael Bojdys of the current state of affairs. This is mainly due to one major weakness: they are sensitive to air and water and oxidise quickly. They are also prone to defects, as certain chemical compounds can migrate through the material and impair its function. The lifespan of organic light-emitting diodes is therefore very short, and their luminous efficacy is still low. There is thus plenty of room for improvement and optimisation. The special structure of PTI gives the materials researchers hope that some of these problems can be circumvented with the new polymer. "There are strong chemical bonds - we call them covalent bonds - between the individual building blocks that make the system more stable," explains Michael Bojdys. PTI is less sensitive to heat and oxygen - light-emitting diodes that use this material may last longer than conventional systems. 

The team is still at the very beginning of its investigations. It will likely be a number of years before an optimised light-emitting diode with PTI actually comes onto the market. "It always starts with an idea on paper," says Michael Bojdys, outlining the beginnings of such a work process. He himself sketched out the substance PTI for the first time in 2008 in his own doctoral thesis at the Max Planck Institute of Colloids and Interfaces and noted down the chemical structural formula with a pencil. Much work was done in the laboratory before the polymer was successfully synthesised for the first time. The chemical and physical analysis and characterisation were ultimately provided by David Burmeister. In this process, all the properties of a new material like this are scrutinised: How are its atomic structures arranged? Is it soluble, acid-resistant, heat-resistant, elastic or brittle? How does it react with other elements? Does it absorb light, and at what wavelength? "It took me a year for that alone," says Burmeister. "But the material basically lives or dies on the quality of its characterisation. Only if that goes well can we ultimately build anything out of it."

Once the material has been analysed and characterised, engineering knowledge is needed in order to combine the individual components in a functional light-emitting diode. "The journey from the sketch to the prototype only works in an interdisciplinary setting," emphasises Michael Bojdys. "We have that here at IRIS Adlershof - with experts from chemistry, mathematics, physics and engineering."

Only one atomic diameter thick

The first prototype of the PTI light-emitting diode is now ready - it is the starting point for further investigations and developments to further optimise the luminous efficacy and energy efficiency. In addition, the researchers are examining what else the new material could be used for. What is particularly interesting here is that PTI has a graphitic structure: it is built up of several layers on top of one another. These can be split up so as to create an extremely thin nanolayer of exactly one atomic diameter. So far, only five compounds - based on light elements (carbon, boron, nitrogen, fluorine, oxygen) - are known for which this also applies. One of them is graphene, which consists of a single layer of linked carbon rings and for whose discovery the physicists Konstantin Novoselov and Andre Geim received the Nobel Prize in Physics in 2010. "What else can you do with this material?" asks Michael Bojdys. The researchers are determined to find out.