On-surface synthesis of graphene nanoribbons with zigzag edge topology (1511.05037v1)

Abstract: Graphene-based nanostructures exhibit a vast range of exciting electronic properties that are absent in extended graphene. For example, quantum confinement in carbon nanotubes and armchair graphene nanoribbons (AGNRs) leads to the opening of substantial electronic band gaps that are directly linked to their structural boundary conditions. Even more intriguing are nanostructures with zigzag edges, which are expected to host spin-polarized electronic edge states and can thus serve as key elements for graphene-based spintronics. The most prominent example is zigzag graphene nanoribbons (ZGNRs) for which the edge states are predicted to couple ferromagnetically along the edge and antiferromagnetically between them. So far, a direct observation of the spin-polarized edge states for specifically designed and controlled zigzag edge topologies has not been achieved. This is mainly due to the limited precision of current top-down approaches, which results in poorly defined edge structures. Bottom-up fabrication approaches, on the other hand, were so far only successfully applied to the growth of AGNRs and related structures. Here, we describe the successful bottom-up synthesis of ZGNRs, which are fabricated by the surface-assisted colligation and cyclodehydrogenation of specifically designed precursor monomers including carbon groups that yield atomically precise zigzag edges. Using scanning tunnelling spectroscopy we prove the existence of edge-localized states with large energy splittings. We expect that the availability of ZGNRs will finally allow the characterization of their predicted spin-related properties such as spin confinement and filtering, and ultimately add the spin degree of freedom to graphene-based circuitry.

• The paper demonstrates an innovative bottom-up synthesis achieving atomic precision in zigzag graphene nanoribbons.
• It employs tailored monomers and surface-assisted reactions to fabricate ZGNRs with clearly defined edge-localized states.
• Differential conductance spectroscopy confirms key theoretical predictions by revealing energy splittings of 1.5 eV and 1.9 eV.

Insights into On-surface Synthesis of Graphene Nanoribbons with Zigzag Edge Topology

The paper investigates an advanced methodology for synthesizing zigzag-edge graphene nanoribbons (ZGNRs) through an innovative bottom-up approach. This paper demonstrates a significant advancement in graphene nanostructures, especially in terms of achieving atomic precision for zigzag edges, which have long been regarded as crucial for exploring spintronic applications. The synthesis involves a novel utilization of surface-assisted colligation and cyclodehydrogenation of tailor-made precursor monomers, thereby opening avenues for detailed examination of predicted spin-polarized edge states in ZGNRs.

Key Contributions and Methodology

The research primarily revolves around two pivotal components: the chemical design of monomers allowing for bottom-up synthesis, and the analysis of the electronic and structural properties of the synthesized ZGNRs. The use of a unique U-shaped monomer with specific halogen functionalities at the R positions enables polymerization in the zigzag direction, a task that conventional top-down approaches have failed to achieve with precision. Specifically, the use of monomer 1a, with additional methyl groups at strategic positions, facilitates a cyclization process that yields atomically precise zigzag edges. The synthesis is closely monitored with tools such as scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM), ensuring the accuracy of structural attributes and the absence of undesired terminations such as radical-edge or H-dimer terminations.

Numerical Results and Analysis

One of the most pronounced achievements of this paper is the identification of edge-localized states with large energy splittings (∆ = 1.5 eV and ∆ = 1.9 eV) observed using differential conductance spectroscopy. Such results align with quasiparticle band structures and density of states computations conducted using the G$_0$W$_0$ approximation. This synthesis enables unprecedented insights into the localized edge states of ZGNRs, as evidenced when the structures are electronically decoupled from the metal substrate by transferring them to insulating NaCl islands.

Practical and Theoretical Implications

The mastery in synthesizing ZGNRs with perfect zigzag edges paves the way for empirical validation of theoretical predictions regarding ZGNR electronic, optical, and magnetic properties. The paper suggests potential applications in graphene-based spintronic devices, such as spin valves and quantum computing elements, which capitalize on the spin degree of freedom. Furthermore, the research hints at prospects of engineered ZGNR variants through monomer design modifications, exemplified by the introduction of monomer 1b that potentially reduces substrate interaction due to its steric structure.

Future Prospects

Looking ahead, the paper opens several pathways in the domain of nanomaterials and spintronics. The authors indicate that reducing metal substrate interactions and improving ambient condition stability remain key challenges. There's a foreseeable trajectory towards engineering defect bands in ZGNRs for customized electronic properties and integrating these nanoribbons into practical devices. The continued refinement of monomer synthesis and polymerization strategies promises richer insights into electronic states and a greater ability to manipulate these properties for technological advancements.

Overall, the paper represents a substantial progression in graphene nanotechnology, providing a foundation for further research aimed at exploiting the unique characteristics of ZGNRs in frontier applications like spintronic circuits and quantum information processing.