Watching how a covalent polymer develops using a scanning tunneling microscope

A team of researchers from KU Leuven, the University of Aveiro, OLYMAT, the University of the Basque Country UPV/EHU and the Ikerbasque, Basque Foundation for Science has used a scanning tunneling microscope (STM) to observe the formation of a 2D covalent polymer. In their paper published in the journal Nature, the group describes how they captured images of the stages involved in polymer development and then used simulations to complement the progression.

The crystallization and polymerization process involved in the creation of polymers has been widely used to produce a variety of useful products, and the steps that are involved in such processes are well known. But until now, the process has not been directly observed. In this new effort, the researchers have found a way to observe the entire development process as a string of events.

The team used an STM to study a sample of boroxine 2D, a dynamic covalent polymer. They found that their choice proved to be a surprisingly good one—its reaction rate occurs over approximately one-minute intervals. And that was fortuitous because STMs create images once every minute. Thus, the researchers were able to create images for each step in the process.

Looking at the images, the researchers saw first the formation of crystallization nuclei. Next, they were able to observe the appearance of oligomers and dimers and then their disappearance as they grew to a size large enough to jumpstart the polymerization process. The researchers note that, taken together, the images show how dynamic the process is and how elongation occurs. The researchers then used the images they made to create simulations that more clearly showed the process from beginning to end.

A better understanding of the process, the researchers note, should lead to methods of reducing defects and perhaps to ways to create tailored orientations or even the development of heterostructures with sandwiching of different materials. Their work has also confirmed theories that have been developed to describe the parts of the process that were not yet fully understood.

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