Light-Based Method Creates 2D Polymer, Expedites Quest for New 2D Materials

A method that uses light to manufacture 2D polymers that have the thickness of a single molecule could create a path for the development of ultrathin, functional 2D materials with highly defined and regular crystalline structures.

The new method was developed by an international team of researchers from Linköping University, the Technical University of Munich (TUM), the Deutsches Museum, and other institutions. Using an on-surface photopolymerization process, the team developed and tested a way to manufacture a 0.5-nm-thick, 2D polymer consisting of several hundred thousand molecules identically linked — in other words, a 2D material with nearly perfect order, right down to the atomic level.

The two-step method takes advantage of the self-organizing properties of fluorinated anthracene triptycene (fantrip) molecules; first, the researchers placed fantrip molecules on alkane-coated graphite surfaces under an ultrahigh vacuum. The specific properties of fantrip caused the molecules to spontaneously arrange into a pattern suitable for photopolymerization when they were placed onto the graphite surface. The molecules were then irradiated with a violet laser. The light excited the electrons in the outermost electron shell, inducing complete covalent crosslinking between the molecules to form a 2D polymer, while preserving the long-range order of the structure.

“Using light to create covalent bonds preserves the pattern and fixes it precisely as we want it,” Markus Lackinger, research group leader at the Deutsches Museum and TUM, said. “Creating covalent bonds between molecules requires a lot of energy. The most common way of supplying energy is to raise the temperature, but this also causes the molecules to start moving, so it won’t work with self-organized molecules, since the pattern would blur.”

Because the polymerization takes place in a vacuum, the material is protected from contamination. The final 2D polymer film is also stable under atmospheric conditions.

The actual photopolymerization process in a scanning tunneling microscope. Courtesy of Linköping University via Markus Lackinger.

Scanning tunneling microscopy (STM) was used to study the network of newly formed bonds at the molecular level. To confirm the structure assignment, the researchers simulated the appearance of the molecular networks at different stages of the polymerization process. The team’s next step will be to determine whether its method could be used to link other molecules for new 2D and functional materials.

“By improving the method, we will also be able to control and tailor the type of ultrathin materials we aim to manufacture,” said Jonas Bjork, assistant professor in the Materials Design Division at the Department of Physics, Chemistry, and Biology at Linköping University.

“The most obvious application is to use the material as a filter or membrane, but applications that we have no idea of at the moment in entirely different contexts may appear on the horizon, also by chance. This is why basic research is so exciting,” Lackinger said.

The research was published in Nature Chemistry (www.doi.org/10.1038/s41557-021-00709-y).