A keener eye for the invisible

What do motion detectors, self-driving cars, chemical analyzers and satellites have in common? They all contain detectors for infrared (IR) light. At their core and besides readout electronics, such detectors usually consist of a crystalline semiconductor material. Such materials are challenging to manufacture: They often require extreme conditions, such as a very high temperature, and a lot of energy. Empa researchers are convinced that there is an easier way. A team led by Ivan Shorubalko from the Transport at the Nanoscale Interfaces laboratory is working on miniaturized IR detectors made of colloidal quantum dots.

The words "quantum dots" do not sound like an easy concept to most people. Shorubalko explains: "The properties of a material depend not only on its chemical composition, but also on its dimensions." If you produce tiny particles of a certain material, they may have different properties than larger pieces of the very same material. This is due to quantum effects, hence the name "quantum dots".

For the discovery and synthesis of these fascinating tiny particles, Moungi Bawendi, Louis E. Brus and Alexey Ekimov were awarded the Nobel Prize in Chemistry in 2023. But while the science behind quantum dots is complex, the simplicity instead lies in their processing. Colloidal quantum dots come in a solution and can be applied to different materials by means of spin coating or printing – cheaper, more energy-efficient and more flexible than conventional semiconductors.

Next, Shorubalko aims to improve the speed of the detector. Fast IR detectors are needed, for example, for lidar, the light-based distance detection technology that helps self-driving cars to find their way, among other things. "Today, lidars use silicon-based infrared detectors, which measure IR light with a wavelength of around 905 nanometers," says the researcher. The problem: Although this wavelength is invisible to the human eye, it is still harmful at high power. For this reason, the lidar's laser can only emit weak light, which limits the range of the entire system, however. Detectors for longer, safer wavelengths do exist, but they are too expensive to be used on a large scale. A fast detector based on quantum dots could provide an alternative and enable powerful, safe and cost-effective lidar systems.

So when will quantum dot-based IR detectors come onto the market? Unlike most novel technologies and materials, patience is not required in this case. "Qquantum dot IR detectors are already available, for example from the US manufacturer SWIR Vision Systems" says Shorubalko. "I've never seen a technology make the leap from the lab to the market so quickly." Nevertheless, the researchers' work is far from over. Their task now is to make this promising technology even faster, cost-effective, more flexible and more sustainable.