Roll up for a post-silicon future

In 2008, sales of televisions with liquid crystal displays (LCDs) overtook those that relied on cathode ray tubes for the first time. Thinner and lighter, LCD displays have continued to improve since. But behind the scenes, and behind the screens, the technological ground is shifting again.

Televisions and other electronic devices have relied on silicon transistors to shuffle electric current around for decades. But many experts now argue that silicon has run its course. It’s rigid, expensive, hard to recycle, and its manufacture is difficult — well controlled atmospheres, such as those in clean rooms are essential, and polluting — highly toxic gases, such as diborane, phosphine, and silane are involved in its production. Scientists are racing to find alternative materials and low- cost process technologies to replace it.

“We think we can exploit materials like metal oxides to replace silicon, which would come with many advantages,” says Rodrigo Martins, a materials scientist at NOVA University of Lisbon and a member of the Institute of Nanostructures, Nanomodelling and Nanofabrication (i3N). “We can process metal oxides at much lower temperatures than silicon. And another big advantage is their conduction mechanism, which is completely different from the classical covalent materials, in which the role of disorder does not interfere with the conduction mechanism.”

Silicon has sp3 covalent bonds, which weakens its ability to conduct electricity when its crystal structure becomes disordered. Because ionic metal oxides have isotropic, spherical bonds, they allow electrons to flow more easily, and can continue to conduct electricity well even within a disordered structure.

Martins and his colleagues at the i3N are using this property of metal oxides to build and explore ultra-thin semi-conductors. Just a nanometer thick, these materials are flexible and transparent, which, Martins says, raises some extraordinary possibilities, allowing for the fabrication of transistors, the key LEGO block for electronics, and from this, CMOS and memristor devices, from materials other than silicon, serving clearly the commitments towards sustainable circularity.

“You could have a roll-up computer,” he says. “Or we can weave metal oxide fibres into textiles and mount them on any surface.”

These applications are not ready for consumers yet, but commercial companies including Sharp and Samsung have already started to use metal-oxide based displays in a handful of consumer products.

Energy reduction

Researchers are still working on the best ways to manufacture and maximize the performance of these silicon replacements. But scientists at i3N have already demonstrated one important feature. While silicon semiconductors must be processed at 1200°C, which consumes great large energy and limits the type of substrate materials that they can be mounted upon, the Portuguese group has shown that metal oxide versions can be made at room temperature¹.

“Because we do this process at very low temperatures, we can use smaller amounts of low-cost materials as substrates,” Martins says. “We can even put transparent electronics on to paper. You could have bright wallpaper that changes colour or shows pictures.”

Paper is the original display material and has been used to save and transmit information for more than a thousand years. It’s cheap enough to make disposable devices and can be recycled. And because it’s made from cellulose, the most abundant organic polymer on Earth, there is no shortage of raw materials. But when it comes to making electronic gadgets there is a problem: paper doesn’t conduct electricity.

Research at the i3N, led by its director Elvira Fortunato, head of the Materials Research Centre at NOVA University of Lisbon, got around this conduction problem. Using a new low-temperature processing technique they developed, the team showed how semi-conducting metal oxides could be successfully mounted on paper².

Then they went further. “Since we knew that paper has an insulating effect, we also wondered if we could use paper as an active element inside a transistor,” Fortunato says. Transistors act as tiny electronic switches and need an insulating layer between their electrical connections. Usually pure silicon oxide, the i3N team swapped in a very thin sheet of paper³.

“I thought there was a very low probability it would work,” she says. “We were really happy when it did.”

Paper renaissance

The breakthrough expanded a nascent field of research: papertronics. Targeted at applications that cost little and need to be produced in large quantities, researchers at i3N, and colleagues around the world, are investigating how paper transistors, paper sensors, could be combined with solar cells and developed into everyday devices as part of the Internet-of-Things. Examples include price tags on supermarket goods that update to reflect sales and reductions, or business cards that switch language depending on who you hand them to.

Research aiming to push to the energy conversion efficiency to the limit is now being realized, so have available low-cost and flexible solar cells are now available to power all these commodities⁵.

“The greatest benefit of using paper in the electronics sector is the fact that it’s renewable,” Fortunato points out. “And the material is already there. Paper doesn’t need to be invented.”

There could be healthcare applications as well. Paper electronics could act as cheap and widely available biosensors to detect the presence of bacteria or viruses or simple diagnostic tests for glucose or other relevant biomarkers.

Physicist, João Veloso’s team at i3N, University of Aveiro, is developing functionalized nanoparticles that can simultaneously act as a label and deliver a drug. “This nanoparticle will fix in the place that you want to treat. And then you are treating and observing the impact of the treatment at the same time,” he says. They developed a super-resolution PET (easyPET technology) capable of efficiently assessing the effect of such nanoparticles in the subject.

Cancer treatment, television screens and supermarket labels are very different applications. But according to Fortunato, these and other projects at i3N to address technological, medical and societal challenges are united by a common goal.

“We are trying to improve the world,” she says. “To use new materials to address the challenge of how to enjoy more comfortable lives while respecting the environment. That means making sustainable products to shape the future.”