The 'pixels' are 13µm wide circular indents in silicon covered by a double layer of graphene, which encloses air inside the cavities.
When the researchers observed the pixels, they saw their colours were not constant. Dr Samer Houri, a researcher at TU Delft, led the work. “We observed Newton rings and noticed their colour changing over time,” he said. "It became clear the pixels were behaving like tiny balloons – in some, pressure differences caused the graphene membrane to be pressed downwards; in others, the membrane was pushed upwards."
The colour change is caused by interference between light waves reflected from the bottom of the cavity and the membrane on top. When the membranes are closer to the silicon, they appear blue. When the membranes are pushed away from the silicon, they appear red.
By applying a pressure difference across the graphene membranes, the perceived colour can be shifted continuously and researchers are working on a way to control the colour electrically.
PhD student Santiago Cartamil-Bueno said: “These devices provide a means to implement display technology based on interferometric modulation – or IMOD. By using graphene, an IMOD could improve device performance – including power consumption, pixel response time and failure rates – while enabling electrical integration and even flexible devices.”
The researchers hope to have developed a prototype ready for demonstration at Mobile World Conference in Barcelona.
