In the article “Anomalous Kondo Resonance Mediated by Semiconducting Graphene Nanoribbons in a Molecular Heterostructure,” Hla and his co-authors write that “atomically precise semiconducting graphene nanoribbons are one-dimensional graphene strips with varying bandgaps depending on their width and length. Unlike graphene, which has a semimetallic character, graphene nanoribbons are more suited for applications ranging from electronic and optoelectronic devices to sensors due to their semiconducting gap.
Graphene is very popular but there is a drawback for applications in electronic industry because it does not have a bandgap. This means that graphene is a good conductor but it is less useful for the electronic devices. In contrast, the graphene nanoribbons, which are narrow strips of graphene, have bandgaps and they are semiconductors. Therefore, the graphene nanoribbons are very attractive for the electronic device applications.
“In the current work, we use graphene nanoribbons to separate magnetic molecules from a gold crystal surface. In a way, we cover the gold surface with graphene nanoribbons (like carpets or floor mats) and then put magnetic molecules on top of the graphene nanoribbons,” Hla says. “Our goal is to prevent the interaction between the magnetic molecules and the gold surface underneath.”
The magnetic molecules here are a type of porphyrinmolecules with a cobalt atom caged at the center (TBrPP-Co). The cobalt atom has a magnetic moment with a spin ½. On a gold crystal surface, the molecule interacts with gold both electronically and spintronically. This means that there is an electrical conduction between the molecule and gold as well as magnetic interactions.
“In order to separate the molecules from the gold surface, we get graphene nanoribbons on the gold by synthesizing them on surface from a precursor molecule called DBBA. We put the DBBA on the gold surface and then by heating them to 200 degrees Celsius lead to link these molecules and form long polymer chains. Further heating to 400 degrees Celsius produces graphene nanoribbons. These graphene nanoribbons have atomically precise edges. We then deposit the magnetic molecules on these graphene nanoribbons,” Hla says.
“When we separate the molecules from the gold surface by placing graphene nanoribbons in between, they are electronically de-coupled as expected. This means that there is no electrical connection between the molecules and gold. However, surprisingly, we discovered that the molecules are spintronically coupled to the gold surface, i.e. there is a magnetic interaction. This spintronic coupling is very robust (almost 100%). Here, the magnetic molecules on graphene nanoribbons interact almost the same way as the ones directly located on top of gold surface without any graphene nanoribbons. We detect the spintronic interaction between the molecules and the gold surface as a form of resonance known as ‘Kondo effect’. It turns out that the graphene nanoribbons mediate the magnetic interaction between the molecules and the gold surface underneath although they prevent electrical connection between them.
“This unexpected discovery can have a profound impact not only for the fundamental understanding of the materials properties but also for potential applications in spintronic devices and magnetic sensors. The experimental research was conducted by Ohio University Physics & Astronomy graduate students Yang Li and Kyaw Zin Latt and Hla with the support of Argonne scientist Brandon Fisher. The theory work was done by Dr. Sergio E. Ulloa, Professor of Physics & Astronomy at OHIO together with Argonne National Laboratory scientists Ohio University alum Anh T. Ngo ’11Ph.D. and Peter Zapol. This work as supported by the grants from the U.S. Department of Energy and National Science Foundation in addition to the partial supports from OHIO’s NQPI and CMSS.
Additional co-authors on the article Ohio University alum Heath Kersell ’08B.S., ’11 M.S., ’16Ph.D. and alum Andrew DiLullo ’07B.S., ’13Ph.D.
Abstract: Kondo resonances in heterostructures formed by magnetic molecules on a metal require free host electrons to interact with the molecular spin and create delicate many-body states. Unlike graphene, semiconducting graphene nanoribbons do not have free electrons due to their large bandgaps, and thus they should electronically decouple molecules from the metal substrate. Here, we observe unusually well-defined Kondo resonances in magnetic molecules separated from a gold surface by graphene nanoribbons in vertically stacked heterostructures. Surprisingly, the strengths of Kondo resonances for the molecules on graphene nanoribbons appear nearly identical to those directly adsorbed on the top, bridge and threefold hollow sites of Au(111). This unexpectedly strong spin-coupling effect is further confirmed by density functional calculations that reveal no spin–electron interactions at this molecule-gold substrate separation if the graphene nanoribbons are absent. Our findings suggest graphene nanoribbons mediate effective spin coupling, opening a way for potential applications in spintronics.
Source: Ohio University