A scanning tunnelling microscope image of the photopolymerization process that produces the 2D polymer. Image: Markus Lackinger.
A scanning tunnelling microscope image of the photopolymerization process that produces the 2D polymer. Image: Markus Lackinger.

The quest for new two-dimensional (2D) materials has rapidly intensified after the discovery of graphene – a supermaterial whose excellent properties include high conductivity and strength, making it incredibly versatile.

Two main approaches are used to create ultrathin 2D materials. In the first, a continuous layer of molecules or atoms is 'peeled off' from the bulk of the material. Graphene can be derived from graphite using such a process.

The other approach, in contrast, involves constructing the material molecule-by-molecule by producing bonds between the molecules in various ways. The problem is that the materials are often small and fragile, and can contain many defects, which limits their potential areas of application.

An international research team with members from Linköping University in Sweden, and the Technical University of Munich and the Deutsches Museum in Germany, among others, has now developed a new method for manufacturing 2D polymers. Their discovery, which they report in a paper in Nature Chemistry, makes it possible to develop new ultrathin functional materials with highly defined and regular crystalline structures.

The manufacture, or polymerization, of this novel 2D material takes place in two steps. The researchers use a molecule known as 'fantrip' – a contraction of 'fluorinated anthracene triptycene' – which is a merger of two different hydrocarbons, anthracene and triptycene. The specific properties of fantrip cause the molecules to spontaneously arrange themselves into a pattern suitable for photopolymerization when they are placed onto a graphite surface covered with an alkane.

The next step is the photopolymerization itself, when the pattern is fixed with the aid of light. The molecules are illuminated by a violet laser that excites the electrons in the outermost electron shells of their component atoms, causing strong and durable covalent bonds to form between the molecules. The result is a porous 2D polymer, half a nanometre thick, consisting of several hundred thousand molecules identically linked. In other words, a material with nearly perfect order, right down to the atomic level.

"Creating covalent bonds between molecules requires a lot of energy," says Markus Lackinger, research group leader at the Deutsches Museum and the Technical University of Munich. "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. Using light to create covalent bonds preserves the pattern and fixes it precisely as we want it.

Since the photopolymerization is carried out on a surface of solid graphite, it is possible to follow the process at the molecular scale using scanning tunnelling microscopy. This shows the newly formed bonds creating a persistent network. In order to confirm this structure, the research group simulated the appearance of the molecular networks in the microscope at different stages of the reaction.

Jonas Björk, assistant professor in the Materials Design Division at the Department of Physics, Chemistry and Biology at Linköping University, used high-performance computing resources at the National Supercomputer Centre in Linköping to validate the experiments and understand the key factors that make the method successful.

"We see that the simulations agree well with reality down to the tiniest detail, and we can also understand why our specific system gives such useful results," says Björk. "The next step of the research will be to see whether the method can be used to link other molecules for new two-dimensional 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."

The polymerization takes place in a vacuum to ensure the 2D material is not contaminated. However, the final 2D polymer film is stable under atmospheric conditions, which is an advantage for future applications.

Lackinger believes that the material will find many conceivable applications. "The most obvious application is to use the material as 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," he says.

This story is adapted from material from Linköping University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.