Light-emitting diodes are electronic chips made of semiconductor materials that convert electrical current into light. "The diameter of the smallest OLED pixels we have developed to date is in the region of 100 nm. This makes them around 50 times smaller than the current state of the art," explains Jiwoo Oh, a doctoral student in ETH Professor Chih-Jen Shih's Nanomaterial Engineering research group. Oh developed the process for manufacturing the new nano-OLED with Tommaso Marcato. "The maximum density of the pixels is around 2500 times greater than before in a single step," adds Marcato, who works as a postdoc in Shih's group.
Screens, microscopes and sensors
Pixels in the size range of 100 to 200 nm form the basis for ultra-high-resolution screens that could display razor-sharp images in glasses, for example. To illustrate this, the researchers have depicted the ETH Zurich logo. This logo consists of 2800 nano-OLEDs and is similar in size to a human cell. Each of its pixels measures around 200 nm.
However, the tiny lights could also help to focus into the sub-micrometer range with high-resolution microscopes. "A nano-pixel field as a light source could illuminate the smallest areas of a sample - the individual images could then be put together in the computer to form a detailed image," says the professor of technical chemistry. He also sees nano-pixels as potential tiny sensors that could detect signals from individual nerve cells, for example.
Nano pixels generate optical wave effects
The small dimensions also open up possibilities that were previously not even feasible, as Marcato explains: "If two light waves of the same color move closer together than half their wavelength, they no longer oscillate independently of each other, but begin to interact with each other." For visible light, this limit is between around 200 and 400 nm, depending on the color - and nano-OLEDs can also be placed this close together.
In initial experiments, Shih's team was able to manipulate the direction of the emitted light with the help of such interactions. Instead of emitting light in all directions above the chip, the OLEDs then only emit their light at very specific angles. Polarized light - i.e. light that only oscillates in one plane - can also be generated using interactions, as the researchers have already shown. It is now used in medicine, for example, to distinguish healthy tissue from cancerous tissue.
In the production of OLEDs, the light-emitting molecules have so far been subsequently vapor-deposited onto the silicon chips. This is done using relatively thick metal masks, which produce correspondingly larger pixels. A ceramic material is now providing the boost in terms of miniaturization, as Oh explains: "Silicon nitride can form very thin yet resilient membranes that do not sag on surfaces in the square millimetre range."
This enabled the researchers to produce templates that are around 3000 times thinner for the placement of the nano-OLED pixels. "Our method also has the advantage that it can be directly integrated into standard lithography processes for the production of computer chips," says Oh.