An ink sprayed onto a white canvas almost instantaneously creates a green, glowing face—an image of Marie Curie. But the reaction that created this picture has another purpose. The ink reacts with nanocrystals on the surface to make perovskites—materials with a number of electronic and electrochemical applications. This process offers a simple method of making patterned structures that could be used to make electronic devices (Adv. Mater. 2021, DOI: 10.1002/adma.202005291).
Perovskites are semiconductors with an organometallic halide or oxide structure that can absorb and emit light and are useful in optoelectronic applications like solar cells and LEDs. But a simple method of making them into patterned three-dimensional shapes has been elusive. Most methods use cumbersome techniques like photolithography that require a series of steps, some even requiring clean rooms.
Willem L. Noorduin of AMOLF and colleagues had previously developed a method of converting carbonates of various shapes, including nanocrystals, into perovskites using an ion-exchange reaction. Now they show the technique can also be used to pattern perovskite structures on surfaces, with pattern sizes ranging from a few micrometers up to a few meters. The team calls it ion-exchange lithography.
To demonstrate the technique, the researchers painted a thin layer of lead carbonate nanocrystals onto a surface. They made an ink with methyl ammonium halides and applied it to the painted surface. The ink causes a very fast ion-swapping reaction: the carbonate leaves as carbon dioxide, while the organic halides insert into the lead crystal structure, forming methylammonium lead halide perovskites. Under ultraviolet light, the perovskites glow in various colors of the visible spectrum depending on which halide the team used in the ink.
“The nice thing about the reactive inks combined with the lithographic approach is the tunability of the emission wavelengths,” says Helmut Cölfen of the University of Konstanz, who was not part of the research. The technique could also be used with other methods for controlled liquid delivery like printing or stamping, he says.
Noorduin and team are already working on other reactions that involve inks patterned on surfaces—like converting materials to metals and insulators. “What we are thinking of now is using these reactions to build entire devices,” Noorduin says, where conductors, semiconductors, and insulators of specific patterns and sizes can be made quickly and simply. They are also looking at making three-dimensional catalysts using this approach, which could help boost the catalysts’ performance.
“I will be eager to see how this method translates to other ion-exchange and solution-phase conversion reactions,” says Mary Elizabeth Anderson of Furman University, who was not involved in the study, “especially those at high temperatures or with less volatile solvents.”