Customisable 2D polymers and magnetic bow-tie graphene open door to new areas of research

​Graphene Flagship researchers have reported the first synthesis of an emblematic class of graphene-based magnetic nano-goblet: the Clar's goblet, named after chemist Erich Clar, who attempted the synthesis for the first time in 1972. They also created promising, ultra-thin conducting polymers that open the door to exciting opportunities for the integration of these materials into next-generation devices.

Conducting polymers, created with various different building blocks, are valued for their interesting combinations of structural and electronic features, combined with suitable mechanical strength and pore size. Despite the tremendous developments in synthetic chemistry to date, producing polymer sheets with the desired quality and size remains a challenging task.

Pioneering new methodologies

Researchers from Graphene Flagship partners TU Dresden and Ulm University, Germany, devised a new technique to produce 2D polymers.  This method, called surfactant-monolayer-assisted interfacial synthesis (SMAIS), consists of spreading the surfactant and monomers on water and allowed them to react at room temperature. After a short time, a thin layer of surfactant forms on the water's surface, and monomers begin to gather just below the water-surfactant interface and self-assembly took place. The team noticed that the surfactant plays a crucial role in guiding the pre-organisation of monomers, thus accelerating the 2D polymerisation process. In this way, the team achieved large-size 2D polymers with much higher crystallinity compared to those from conventional synthetic methods.

The team used their new technique, published in Nature Chemistry,¹ to fabricate 2D polyimide and polyamide crystals. The method enabled them to control the synthesis of polyimide from amine and anhydride monomers, resulting in a two-dimensional net with a large crystal domain size, resulting in enhanced performance. The team say that once perfected, they could use their strategy to fabricate molecular sieving membranes as well.

Creating 2D polymers with customisable properties

In a Nature Communications publication,² researchers at Graphene Flagship partners TU Dresden and Ulm University, Germany, and the Technion, Israel Institute of Technology, Israel, reported the fabrication of quasi-two-dimensional polyaniline (q2D PANI) sheets with an adjustable thickness and a large crystal domain size. After doping with hydrogen chloride, the scientists reported a high conductivity: the highest ever recorded for this type of material. The q2D PANI sheets were also applied to chemical sensors, which were able to detect ammonia and volatile organic compounds. The team expects their q2D PANI films to find applications in transparent electrodes, flexible supercapacitors and functional membranes – and moreover, their new method could be applicable to the synthesis of other conducting polymers, such as polypyrrole, polythiophene and their analogues.

"These methodologies and theories can pave the way to engineering 2D polymers with different structures and properties, which are interesting for the fabrication of molecular sieving membranes, and in the production of new electronic and photonic devices," says Xinliang Feng, leader of the Graphene Flagship's Functional Foams and Coatings Work Package and Professor at Graphene Flagship partner TU Dresden.

Overcoming decades-old challenges

Feng also led the team that published the first successful synthesis of Clar's goblets. The electron configuration of these bowtie-shaped nanographenes gives them unconventional properties, including magnetism. Although carbon is usually not thought to be magnetic, the presence of two unpaired electrons explains Clar's globets' magnetic properties.

The researchers gathered and presented direct evidence for carbon magnetism in Clar's goblets using low-temperature scanning tunnelling microscopy (STM) and spectroscopy (STS). Furthermore, the energy gap between non-magnetic and magnetic states in the material is slightly higher than the minimum heat dissipation caused by deleting a single bit of digital information – the so-called Landauer limit – while being much lower than the power needed by current silicon devices to perform the same operation. These findings, published in Nature Nanotechnology,³ could therefore be highly valuable for studies on spintronics-based switches and logic gates, which utilize the spin of electrons, instead of their charge, to store and process information.

"The SMAIS method is a powerful strategy to produce free-standing 2D polymer single crystals comprising a few layers, thus enabling new opportunities for 2D materials and their integration into working devices. The all-carbon based magnetic properties reported in these newly designed nanographenes is also highly relevant and timely in the burgeoning realm of carbon spintronics," explains Paolo Samori, the Graphene Flagship's Work Package Deputy for Functional Foams and Coatings.

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "The family of layered materials is ever-expanding. While much focus has been directed to inorganic materials, these studies open up new opportunities by synthesising ultrathin layered polymers, which are quasi-2D. They also apply approaches typical of polymer science to devise new members of the graphene family. The Graphene Flagship is yet again at the forefront, creating new areas of research with huge potential."

References

1. T. Zhang, H. Qi, Z. Liao, Y.D. Horev, L.A. Panes-Ruiz, P.S. Petkov, Z. Zhang, R. Shivhare, P. Zhang, K. Liu, V. Bezugly, S. Liu, Z. Zheng, S. Mannsfeld, T. Heine, G. Cuniberti, H. Haick, E. Zschech, U. Kaiser, R. Dong and X. Feng. Engineering crystalline quasi-two-dimensional polyaniline thin film with enhanced electrical and chemiresistive sensing performances. Nature Communications, 10(1), 1-9 (2019)
2. K. Liu, H. Qi, R. Dong, R. Shivhare, M. Addicoat, T. Zhang, H. Sahabudeen, T. Heine, S. Mannsfeld, U. Kaiser, Z. Zheng and X. Feng. On-water surface synthesis of crystalline, few-layer two-dimensional polymers assisted by surfactant monolayers. Nature Chemistry, 11(11), 994-1000 (2019)
3. S. Mishra, D. Beyer, K. Eimre, S. Kezilebieke, R. Berger, O. Gröning, C.A. Pignedoli, K. Müllen, P. Liljeroth, P. Ruffieux, X. Feng and R. Fasel. Topological frustration induces unconventional magnetism in a nanographene. Nature Nanotechnology, 15(1), 22-28 (2020)