NUS scientists have devised a new method for the synthesis of nanographene molecules with a high product yield for the development of next generation quantum devices.

On-surface chemical reactions have shown potential in the synthesis of new organic functional materials such as atomically-precise nanographenes. The core concept of this strategy relies on the rational design of specific molecular precursors, which subsequently undergo chemical transformation along certain reaction pathways toward the desired product. The electronic, magnetic and optical properties of these nanographene molecules can be precisely tuned for the development of next-generation quantum devices. Unfortunately, conventional surface-assisted synthetic routes often involve a series of cascade reactions with competing reaction pathways. This inevitably leads to the formation of numerous undesired products and lowers the yield. The limited yield of the targeted products poses a challenge for practical applications of the nanographenes.

A NUS research team led by Prof Jiong Lu, in collaboration with Prof Jishan Wu’s research group, both from the Department of Chemistry, NUS has developed a route for synthesizing the hexagonal zigzag-edged nanographene, known as circumcoronene, on a copper (111) substrate. The reaction route relies on the robust dehydrogenative coupling of the methyl groups at the adjacent sites of the rationally-designed precursor molecules, followed by the ring closure reactions on the metallic substrate. This forms the elusive circumcoronene molecule consisting of 19 fused benzene rings. Importantly, such a synthetic route allows for an ultra-high yield of the reaction product (up to 98%), which has not been attained to date.

The electrostatic interactions between the large number of circumcoronene molecules and the copper substrate enabled the molecules to self-assemble into extended superlattices. This was observed by the team using bond-resolved scanning probe microscopy measurements. The researchers demonstrate that the unique hexagonal zigzag topology of circumcoronenes, along with their periodic electrostatic landscape, confines the two-dimensional (2-D) electron gas on the copper (111) surface. This creates a chiral electronic Kagome-honeycomb lattice with two emergent electronic flat bands. This arrangement of the circumcoronene molecules in a regular grid of hexagons and triangles can be particularly interesting in a wide range of condensed matter physics because of their favorable potential in the realization of a variety of exotic many-body phenomenon, including anomalous quantum Hall states, Wigner crystallization, and topological insulating transitions.

Prof Lu said, “Our findings open up a new route for the ultra-high yield synthesis of nanographene and atomically-precise fabrication of synthetic two-dimensional lattices with unique electronic properties for future technological applications.”

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An international multi-institution team of scientists has synthesized graphene nanoribbons—ultrathin strips of carbon atoms—on a titanium dioxide surface using an atomically precise method that removes a barrier for custom-designed carbon nanostructures required for quantum information sciences.

Graphene is composed of single-atom-thick layers of carbon taking on ultralight, conductive and extremely strong mechanical characteristics. The popularly studied material holds promise to transform electronics and information science because of its highly tunable electronic, optical and transport properties.

When fashioned into nanoribbons, graphene could be applied in nanoscale devices; however, the lack of atomic-scale precision in using current state-of-the-art “top-down” synthetic methods—cutting a graphene sheet into atom-narrow strips—stymie graphene’s practical use.

Researchers developed a “bottom-up” approach—building the graphene nanoribbon directly at the atomic level in a way that it can be used in specific applications, which was conceived and realized at the Center for Nanophase Materials Sciences, or CNMS, located at the Department of Energy’s Oak Ridge National Laboratory.

This absolute precision method helped to retain the prized properties of graphene monolayers as the segments of graphene get smaller and smaller. Just one or two atoms difference in width can change the properties of the system dramatically, turning a semiconducting ribbon into a metallic ribbon. The team’s results were described in Science.

ORNL’s Marek Kolmer, An-Ping Li and Wonhee Ko of the CNMS’ Scanning Tunneling Microscopy group collaborated on the project with researchers from Espeem, a private research company, and several European institutions: Friedrich Alexander University Erlangen-Nuremberg, Jagiellonian University and Martin Luther University Halle-Wittenberg.

ORNL’s one-of-a-kind expertise in scanning tunneling microscopy was critical to the team’s success, both in manipulating the precursor material and verifying the results.

“These microscopes allow you to directly image and manipulate matter at the atomic scale,” Kolmer, a postdoctoral fellow and the lead author of the paper, said. “The tip of the needle is so fine that it is essentially the size of a single atom. The microscope is moving line by line and constantly measuring the interaction between the needle and the surface and rendering an atomically precise map of surface structure.”

In past graphene nanoribbon experiments, the material was synthesized on a metallic substrate, which unavoidably suppresses the electronic properties of the nanoribbons.

“Having the electronic properties of these ribbons work as designed is the whole story. From an application point of view, using a metal substrate is not useful because it screens the properties,” Kolmer said. “It’s a big challenge in this field—how do we effectively decouple the network of molecules to transfer to a transistor?”

The current decoupling approach involves removing the system from the ultra-high vacuum conditions and putting it through a multistep wet chemistry process, which requires etching the metal substrate away. This process contradicts the careful, clean precision used in creating the system.

To find a process that would work on a nonmetallic substrate, Kolmer began experimenting with oxide surfaces, mimicking the strategies used on metal. Eventually, he turned to a group of European chemists who specialize in fluoroarene chemistry and began to home in on a design for a chemical precursor that would allow for synthesis directly on the surface of rutile titanium dioxide.

“On-surface synthesis allows us to make materials with very high precision and to achieve that, we started with molecular precursors,” Li, a senior author of the paper who led the team at CNMS, said. “The reactions we needed to obtain certain properties are essentially programmed into the precursor. We know the temperature at which a reaction will occur and by tuning the temperatures we can control the sequence of reactions.”

“Another advantage of on-surface synthesis is the wide pool of candidate materials that can be used as precursors, allowing for a high level of programmability,” Li added.

The precise application of chemicals to decouple the system also helped maintain an open-shell structure, allowing researchers atom-level access to build upon and study molecules with unique quantum properties. “It was particularly rewarding to find that these graphene ribbons have coupled magnetic states, also called quantum spin states, at their ends,” Li said. “These states provide us a platform to study magnetic interactions, with the hope of creating qubits for applications in quantum information science.” As there is little disturbance to magnetic interactions in carbon-based molecular materials, this method allows for programming long-lasting magnetic states from within the material.

Their approach creates a high-precision ribbon, decoupled from the substrate, which is desirable for spintronic and quantum information science applications. The resulting system is ideally suited to be explored and built upon further, possibly as a nanoscale transistor as it has a wide bandgap, across the space between electronic states that is needed to convey an on/off signal.

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