Researchers from Empa, Chinese Academy of Sciences, the Chinese University of Hong Kong and Max Planck Institute for Polymer Research have developed a hybrid system in which porphyrins are attached to graphene nanoribbons (GNRs) in a precise and well-defined manner.
Image credit: Credit: Swiss Federal Laboratories for Materials Science and Technology
Graphene nanoribbons with zigzag edges are promising materials for spintronic devices, owing to their tunable bandgaps and spin-polarized edge states. Porphyrins offer complementary optoelectronic benefits. In the new system, a graphene ribbon just one nanometer wide with zigzag edges is used as a molecular wire, along which porphyrin molecules are docked at perfectly regular intervals, alternating between the ribbon’s left and right sides.
“Our graphene ribbon exhibits a special type of magnetism thanks to its zigzag edge,” explains Feifei Xiang, lead author of the study. The metal atoms in the porphyrin molecules, on the other hand, are magnetic in a more “conventional” way. The key difference lies in the electrons that provide the spin responsible for magnetism.
While the spin-carrying electrons in the metal center stay localized on the metal atom, the corresponding electrons in the graphene ribbon “spread out” along both edges.
“Thanks to the coupling of the porphyrins to the graphene backbone, we have succeeded in combining and connecting both types of magnetism in a single system,” explains co-author Oliver Gröning, deputy head of the nanotech@surfaces laboratory at Empa.
This coupling opens many doors in the field of molecular electronics. The graphene ribbon serves as both an electrical and magnetic conductor—a kind of nanoscale “cable” between the porphyrin molecules. The correlated magnetism of such graphene nanoribbons is considered particularly promising for quantum technology applications, where the spin underlying magnetism acts as an information carrier.
“Our graphene ribbon with the porphyrins could function as a series of interconnected qubits,” says Roman Fasel, head of the “nanotech@surfaces” laboratory.
In addition, porphyrins are also natural pigments, as seen in molecules like chlorophyll and hemoglobin. For materials scientists, this means that “the porphyrin centers are optically active,” says Gröning. And optics is an important way of interacting with the electronic and magnetic properties of such molecular chains. Porphyrins can emit light whose wavelength changes with the magnetic state of the entire molecular system—a kind of molecular string of lights, where information could be read out by subtle shifts in color.
The reverse process is also possible: The porphyrins could be excited by light, thereby influencing the conductivity and magnetism of the graphene backbone. These molecular all-rounders could even serve as chemical sensors.
Porphyrin molecules can be easily functionalized—that is, chemically modified by attaching specific chemical groups. If one of these added groups binds to a target chemical substance, this interaction also affects the conductivity of the graphene ribbon.
“Our system is a toolbox that can be used to tune different properties,” says Fasel. Next, the researchers plan to explore different metal centers inside the porphyrins and investigate their effects. They also aim to broaden the graphene ribbon backbone, giving their molecular system an even more versatile electronic base.
The synthesis of these “string lights” is anything but trivial. “Our partners at the Max Planck Institute were able to produce precursor molecules consisting of a porphyrin core complemented by a few carbon rings placed at exactly the right positions,” says Gröning.
These complex molecules are then “baked” at several hundred degrees Celsius under ultra-high vacuum to form the long chains. A gold surface serves as the “baking sheet.” This is the only way to achieve these nanometer-fine structures with atomic precision. The team is now working to make these novel designer materials usable for future quantum technologies.