Ultraviolet light is used to kill bacteria and viruses, but UV lamps contain toxic mercury. A newly developed nanomaterial is changing that.
The nano research team led by professors Helge Weman and Bjørn-Ove Fimland at the Norwegian University of Science and Technology (NTNU)’s Department of Electronic Systems has succeeded in creating light-emitting diodes, or LEDs, from a nanomaterial that emits ultraviolet light (Nano Letters, "GaN/AlGaN Nanocolumn Ultraviolet Light-Emitting Diode Using Double-Layer Graphene as Substrate and Transparent Electrode").It is the first time anyone has created ultraviolet light on a graphene surface.
“We’ve shown that it’s possible, which is really exciting,” says PhD candidate Ida Marie Høiaas, who has been working on the project with Andreas Liudi Mulyo, who is also a PhD candidate.
“We’ve created a new electronic component that has the potential to become a commercial product. It’s non-toxic and could turn out to be cheaper, and more stable and durable than today’s fluorescent lamps. If we succeed in making the diodes efficient and much cheaper, it’s easy to imagine this equipment becoming commonplace in people’s homes. That would increase the market potential considerably,” Høiaas says.
Dangerous – but useful
Although it’s important to protect ourselves from too much exposure to the sun’s UV radiation, ultraviolet light also has very useful properties. This applies especially to UV light with short wavelengths of 100-280 nanometres, called UVC light, which is especially useful for its ability to destroy bacteria and viruses. Fortunately, the dangerous UVC rays from the sun are trapped by the ozone layer and oxygen and don’t reach the Earth. But it is possible to create UVC light, which can be used to clean surfaces and hospital equipment, or to purify water and air.
The problem today is that many UVC lamps contain mercury. The UN’s Minamata Convention, which went into effect in 2017, sets out measures to phase out mercury mining and reduce mercury use. The convention was named for a Japanese fishing village where the population was poisoned by mercury emissions from a factory in the 1950s.
Building on graphene
A layer of graphene placed on glass forms the substrate for the researchers’ new diode that generates UV light.
Graphene is a super-strong and ultra-thin crystalline material consisting of a single layer of carbon atoms. Researchers have succeeded in growing nanowires of aluminium gallium nitride (AlGaN) on the graphene lattice.
The process takes place in a high temperature vacuum chamber where aluminium and gallium atoms are deposited or grown directly on the graphene substrate – with high precision and in the presence of nitrogen plasma. This process is known as molecular beam epitaxy (MBE) and is conducted in Japan, where the NTNU research team collaborates with Professor Katsumi Kishino at Sophia University in Tokyo.
Let there be light
After growing the sample, it is transported to the NTNU NanoLab where the researchers make metal contacts of gold and nickel on the graphene and nanowires. When power is sent from the graphene and through the nanowires, they emit UV light. Graphene is transparent to light of all wavelengths, and the light emitted from the nanowires shines through the graphene and glass.
“It’s exciting to be able to combine nanomaterials this way and create functioning LEDs, says Høiaas.
An analysis has calculated that the market for UVC products will increase by NOK 6 billion, or roughly US $700 million between now and 2023. The growing demand for such products and the phase- out of mercury are expected to yield an annual market increase of almost 40 per cent.
Concurrently with her PhD research at NTNU, Høiaas is working with the same technology on an industrial platform for CrayoNano. The company is a spinoff of NTNU’s nano research environment.
Use less electricity more cheaply
UVC LEDs that can replace fluorescent bulbs are already on the market, but CrayoNano’s goal is to create far more energy-efficient and cheaper diodes. According to the company, one reason that today’s UV LEDs are expensive is that the substrate is made of costly aluminium nitride. Graphene is cheaper to manufacture and requires less material for the LED diode.
Høiaas believes that the technology needs to be improved quite a bit before the process developed at NTNU can be scaled up to industrial production level.
Among the issues that need to be addressed are conductivity and energy efficiency, more advanced nanowire structures and shorter wavelengths to create UVC light. CrayoNano has made progress, but results documenting their progress have not yet been published. “CrayoNano’s goal is to commercialize the technology sometime in 2022,” says Høiaas.
The Mobile World Congress (MWC) held annually in Barcelona, Spain is one of the largest technology conferences in the world. For the last three years, the MWC has been hosting the Graphene Pavilion that showcases the research institutes and technologies that they have developed under the EU’s Graphene Flagship.
The Graphene Council visited the Graphene Pavilion last month in Barcelona and we came back with some videos. One of the anchor institutions at the Pavilion is The Institute of Photonics (ICFO) located just outside of Barcelona. The Graphene Council has been speaking to Frank Koppens at ICFO since 2015 about how graphene was impacting photonics and optoelectronics.
In our latest visit with them at MWC this year, we got an update on some of the ways they are applying their technologies to various technologies.
In the one shown in the video below, the researchers have developed ultraviolet (UV) sensors for protecting the wearers from overexposure to the sun.
What the ICFO discovered six years ago was that while graphene generates an electron-hole pair for every single photon the material absorbs generates, it doesn’t really absorb that much light. To overcome this limitation of graphene, they combined it with quantum dots with the hybrid material being capable of absorbing 25 percent of the light falling on it. When you combine this new absorption capability with graphene’s ability to make every photon into an electron-hole pair, the potential for generating current became significant.
The ICFO has been proposing applications like this for this underlying technology for years, and producing working prototypes. At the MWC in 2016, the ICFO was exhibiting a heart rate monitor. In that device, when a finger is placed on the photodetector, the digit acts as an optical modulator, changing the amount of light hitting the photodetector as your heart beats and sends blood through your fingertip. This change in signal is what generates a pulse rate on the screen of the mobile device.
This same basic technology is at the heart of another technology ICFO was exhibiting this year (see video below) in which the graphene-based photodector can determine what kind of milk you are about to drink. This could conceivably be used by someone who has a lactose intolerance that could threaten their lives and by using the detector could determine if it was cow’s milk or soy milk, for instance.
While ICFO goes so far as to discuss prices for the devices, it’s not clear that ICFO is really committed to any of these technologies for its wide-spectrum CMOS graphene image sensor, or not. In the case of the heart monitor, the researchers claimed at the time it was really just intended to demonstrate the capabilities of the technology.
The long-range aim of the technology is to improve the design of these graphene-based image sensors to operate at a higher resolution and in a broader wavelength range. Once the camera is improved, the ICFO expects that will be used inside a smartphone or smart watch. In the meantime, these wearable technologies offer intriguing possibilities and maybe even a real commercial avenue for the technology.