New nanomaterial to replace mercury

The nanotechnology research team led by Professors Helge Weman and Bjørn-Ove Fimland of the Electronic Systems Department of the Norwegian University of Science and Technology (NTNU) has succeeded in creating light-emitting diodes (LEDs) from a nanomaterial emitting ultraviolet light.

This is the first time that someone creates ultraviolet light on a graphene surface.

"We showed that it was possible, which is really exciting," said Ph.D. candidate Ida Marie Høiaas, who worked on the project with Ph.D. candidate Andreas Liudi Mulyo.

"We've created a new electronic component that can become a commercial product, it's nontoxic and could be cheaper, more stable and more durable than current fluorescent lamps, so if we can make diodes efficient and much cheaper, it is easy to imagine that this equipment is becoming commonplace among individuals, which would greatly increase the potential of the market, "said Høiaas.

Dangerous – but useful

While it's important to protect yourself from excessive exposure to the sun's UV rays, ultraviolet rays also possess very useful properties.

This particularly concerns ultraviolet rays with short wavelengths of 100 to 280 nanometers, called UVC rays, which are particularly useful for their ability to destroy bacteria and viruses.

Fortunately, the dangerous UVC rays of the sun are trapped by the ozone layer and oxygen and do not reach the Earth. But it is possible to create a UV-C light, which can be used to clean hospital surfaces and equipment, or to purify water and air.

The problem today is that many UVC lamps contain mercury. The UN Minamata Convention, which came into effect in 2017, provides for measures to phase out mercury extraction and reduce its use.

The convention owes its name to a Japanese fishing village where the population was intoxicated by mercury emissions from a plant in the 1950s.

Build on graphene

A layer of graphene placed on glass is the substrate of the new researcher diode that generates UV light.

Graphene is a super-strong, ultra-thin crystalline material consisting of a single layer of carbon atoms. The researchers succeeded in growing nanowires of aluminum and gallium nitride (AlGaN) on the graphene network.

The process takes place in a high temperature vacuum chamber where aluminum and gallium atoms are deposited or grown directly on the graphene substrate – with high precision and in the presence of nitrogen plasma.

This process, called molecular beam epitaxy (MBE), is conducted in Japan, where the NTNU research team is working with Professor Katsumi Kishino of Sophia University in Tokyo.

Let there be light

After sample growth, it is transported to the NTNU NanoLab where researchers establish metal contacts in gold and nickel on graphene and nanowires. When graphene and nanowires are energized, they emit UV light.

Graphene is transparent to light of all wavelengths, and light emitted by nanowires passes through graphene and glass.

"It's exciting to be able to combine nanomaterials in this way and create LEDs that work," said Høiaas.

Multi-million dollar market

An analysis calculated that the market for UVC products would increase by NOK 6 billion, or about US $ 700 million by 2023. The growing demand for these products and the elimination of mercury are expected to generate an annual increase in the market for nearly 40%.

At the same time as his doctorate Høiaas is working with the same technology on an industrial platform for CrayoNano. The company is from NTNU's nano research group.

Use less electricity at a lower cost

The UVC LEDs that can replace fluorescent bulbs are already on the market, but the goal of CrayoNano is to create much cheaper and cheaper LEDs.

According to the company, the high cost of UV LEDs today lies in the fact that the substrate is composed of expensive aluminum nitride. Graphene is cheaper to manufacture and requires less material for the LED diode.

Additional development needed

Høiaas believes that many improvements are needed before the process developed at NTNU can be extended to the level of industrial production. Necessary enhancements include conductivity and energy efficiency, more advanced nanowire structures, and shorter wavelengths to create UVC light.

CrayoNano has made further progress, but their results have not been published yet.

"The goal of CrayoNano is to market the technology during the year 2022," said Høiaas.