Xinliang Feng, Technische Universität, Dresden
Exploring New Matters with Soft and Hard Features
Written by Sean Langan
The EU-40 Materials Prize is given by the European Materials Research Society for researchers showing exceptional promise as leaders in materials science for their research done while in Europe, and this year was no exception. This Prize presentation given by Xinliang Feng of the Technische Universität on Thursday evening at the MRS Fall Meeting in Boston is a combined effort by MRS and E-MRS, and celebrates mid-career research.
Feng started with a discussion of various types of matter, hard matter and soft matter, and their various properties. Hard matter is characterized as rigid and crystalline, and often electrically conductive. Soft matter often consists of macromolecular assemblies, can be more amorphous, and is less rigid. He also described materials that were “in-between,” like carbon nanotubes, graphene, and organic and polymer crystals. Feng then transitioned into talking about his research into one of these materials that combines the properties of hard and soft matter, organic two-dimensional (2D) materials.
He described, first, 2D supramolecular molecular crystals, and then 2D polymers. Two-dimensional polymers are highly ordered, and single-monomer-thick. A porous graphene was shown as an example. Its monomers were made of rings of benzene, that when combined make a structure that is like graphene, but with large “holes” in it.
After this, a number of graphene nanoribbon structures were discussed. By varying the thicknesses of the nanoribbon, the bandgap can be controlled and thus its electrical properties. These nanoribbons shapes can be altered, from straight to curvy. Furthermore, other atoms can be added to rings within the structure to dope the material.
Feng then examined how to use interface chemistry to make these materials and move them onto other substrates. His group uses a number of techniques within interface chemistry, such as liquid/liquid interfaces, gas/solid interfaces, gas/liquid interfaces, and the Langmuir-Blodgett method. The nature of these interfaces allow for lower temperature reactions, changing the surface roughness of the material, and controlling the materials molecular orientation.
This led to a discussion on 2D supramolecular polymers with metal-dithiolene links. These were created with the Langmuir-Blodgett method, allowing for good control of thickness and the ability to create large sheets of the material that could then be transferred to another desired substrate. The material had desirable mechanical strength, electrical conductivity, and crystallinity. It was also found that in water they can lead to a hydrogen evolution reaction.
Two-dimensional polymers can also be used to make supercapacitors on the microscale, as is the case with PiCBA, a semiconducting 2D polymer. Similar materials can also be magnetic semiconducting through metal-organic frameworks, as happens when PSC monomers are turned into PTC monomers by hydrolyzing the sulfur, and then after coordinating with iron to make the sheets of the material. These networks have good electrical conductivity, are semiconducting, and at low temperatures can go from paramagnetic to ferromagnetic.
The next topic was polyimine-based 2D polymers. Depending on the interface they are created with, the thickness can be varied. These polymers were shown to absorb more light than graphene. When mechanical properties were tested, it was shown that it had a higher Young’s modulus than steel, and could be used for membrane applications.
Feng’s group is also making 2D organic crystals, which has proven quite difficult. Both polyamides and polyimides were made in this fashion, and single crystals were able to be formed.
Concluding his lecture, Feng emphasized the diverse nature of his work pursuing new materials. These materials can be made through many kinds of processes, and have unique properties and shapes.