Some atomically thin 2D materials can emit customizable light in the color desired when the materials are combined together to create an “artificial semiconductor.” The discovery, made by researchers at the University of Geneva (UNIGE) and the University of Manchester, could spur use of 2D materials on an industrial scale. Finding new semiconductor materials that emit light is essential for developing a wide range of electronic devices. However, light emission in a semiconductor only occurs when certain conditions are met. Light emission occurs when an electron jumps inside the semiconductor from a higher to a lower energy level. The color of the emitted light is determined by the difference in energy. For light to be produced, the velocity of the electron before and after the jump must be exactly the same. Artistic view of a junction of different 2D light-emitting materials. Courtesy of Xavier Ravinet. Different 2D materials can be stacked on top of each other to form artificial structures that behave like semiconductors. The advantage of these “artificial semiconductors,” the researchers said, is that their energy levels can be controlled by selecting the chemical composition and thickness of the materials that make up the structure. “When the two-dimensional materials have exactly the same structure and their crystals are perfectly aligned, this type of artificial semiconductor can emit light. But it’s very rare,” researcher Nicolas Ubrig said. The researchers aimed to find a way to make artificial semiconductors easier to use for light emission. “Our objective was to manage to combine different two-dimensional materials to emit light while being free from all constraints,” professor Alberto Morpurgo said. A class of materials where the velocity of electrons was zero both before and after a change in energy level would always meet the conditions for light emission, regardless of how the atoms aligned, the researchers surmised. The scientists found that quite a large number of known 2D semiconductors had a zero-electron velocity in the relevant energy levels. They have used various transition metal dichalcogenides (TMDs) in their research, such as MoS2, MoSe2, and WS2, and indium selenide (InSe), and they have identified additional materials that could be useful for widening the range of colors of the light emitted by artificial semiconductors. Thanks to this diversity of compounds, many different materials can be combined, and each combination is a new artificial semiconductor, emitting light of a specific color. “Once we had the idea, it was easy to find the materials to use to implement it,” professor Vladimir Fal’ko at the University of Manchester said. “The great advantage of these 2D materials, thanks to the fact that there are no more preconditions for the emission of light, is that they provide new strategies for manipulating the light as we see fit, with the energy and color that we want to have,” Ubrig said. Using the new artificial semiconductors, it could be possible to devise future applications on an industrial level, since the emitted light is robust and there is no longer any restriction on the alignment of atoms when materials are combined. The collaboration between UNIGE and the University of Manchester took place within the framework of the EU Graphene Flagship Project. The research was published in Nature Materials (www.doi.org/10.1038/s41563-019-0601-3).