A new positive approach could be the key to next-generation transparent electronics

The optical transparency of the new materials could allow for futuristic, flexible, transparent electronics. Photo: RMIT University

A new study published this week could pave the way for revolutionary, transparent electronics. Such end-to-end devices could potentially be integrated into glass, flexible displays, and smart contact lenses, bringing to life futuristic devices that seem like a product of science fiction.

For several decades, researchers have been looking for a new class of electronics based on semiconducting oxides, whose optical transparency could allow these fully transparent electronics.

Oxide-based devices can also find applications in electronics and communication technologies, reducing the carbon footprint of our utility networks.

A team led by RMIT has now introduced ultrathin beta-tellurite into a two-dimensional (2D) semiconducting material family, providing the answer to this multi-year search for high mobility p-type oxide.

"This new, highly mobile p-type oxide fills a critical gap in the material spectrum to provide fast, transparent circuits," says group leader Dr. Torben Daeneke, who led the collaboration across the three FLEET nodes.

Other key advantages of long-sought-after oxide-based semiconductors are their stability in the air, less stringent purity requirements, low costs, and easy deposition.

"The missing link in our promotion was to find the right, 'positive' approach," says Torben.

Positivity is not enough

There are two types of semiconducting materials. N-type materials have abundant negatively charged electrons, while "p" type semiconductors have a large number of positively charged holes.

It is the stacking together of additional N-type and P-type materials that enable electronic devices such as diodes, rectifiers, and logic circuits.

Modern life is critically dependent on these materials, as they are the building blocks of every computer and smartphone.

An obstacle to oxides is that, although many high-performance n-type oxides are known, there is a significant shortage of high-quality p-type oxides.

The molten mixture of tellurium and selenium flipped over the surface deposits with an atomically thin sheet of beta-tellurite. Photo: FLEET

Theory prompts action

However, a 2018 computational study found that beta-tellurite (β-TeO2) may be an attractive candidate for p-type oxide, with goturium's special place in the periodic table meaning it can behave as both a metal and a non-metal, providing its oxide with unique beneficial properties.

"This prediction prompted our team at RMIT University to study its properties and applications," says Dr. Torben Daeneke, who is an assistant investigator at FLEET.

The Liquid Metal Path for studying 2D materials

Dr. Daeneke's team demonstrated the isolation of beta-tellurite with a specially developed synthesis technique that relies on liquid metal chemistry.

"The molten mixture of goturium (Te) and selenium (Se) is prepared and allows you to flip the surface," explains co-author Patjaree Aukarasereenont.

"Thanks to the oxygen in the surrounding air, the molten drop naturally forms a thin surface oxide layer of beta-tellurium. As the liquid droplet is rolled over the surface, this oxide layer sticks to it, storing atomically thin sheets of oxide in its path.»

"The process is similar to drawing: you use a glass rod as a pen and liquid metal your ink," explains Ms. Aukarasereenont, who is a FLEET graduate student at RMIT.

While the beta phase of tellurite grows below 300 degrees Celsius, pure tellurium has a high melting point, above 500 degrees Celsius. Therefore, selenium was added to develop an alloy that has a lower melting point, making synthesis possible.

"The ultra-thin sheets we have obtained are only 1.5 nanometers thick, which corresponds to only a few atoms. The material was very transparent across the entire visible spectrum, having a range of 3.7 eV, which means they are essentially invisible to the human eye, " explains co-author Dr. Ali Zavabeti.

Beta-tellurite score: up to 100 times faster

To evaluate the electronic properties of the developed materials, field-effect transistors (FETs) were manufactured.

"These devices showed a characteristic p-type switching, as well as high hole mobility (approximately 140 cm2V-1sec-1), showing that beta-tellurite is ten to a hundred times faster than existing p-type oxide semiconductors. The excellent ratio between it and the switchable one (more than 106) also indicates that the material is suitable for energy-efficient, fast devices," said Ms. Patiari Aucaraserinont.

RMIT team from left, Ali Zavabeti, Patiari Aukarasereenont, and Torben Daeneke with transparent electronics. Photo: FLEET

"The findings close a critical gap in the electronic materials library," said Dr. Ali Zavabeti.

"Having a fast, transparent p-type semiconductor at our disposal has the potential to revolutionize transparent electronics, while also enabling better displays and improved energy-efficient devices."

The team plans to continue exploring the potential of this new semiconductor. "Our further research on this exciting material will explore integration into existing and next-generation consumer electronics," says Dr. Torben Daeneke.

The paper": "High mobility of p-type semiconducting two-dimensional β-TeO2", was published in Nature Electronics in April 2021.

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