Topological Band Engineering of Graphene Nanoribbons (1805.06470v1)

Abstract: Topological insulators (TIs) are an emerging class of materials that host highly robust in-gap surface/interface states while maintaining an insulating bulk. While most notable scientific advancements in this field have been focused on TIs and related topological crystalline insulators in 2D and 3D, more recent theoretical work has predicted the existence of 1D symmetry-protected topological phases in graphene nanoribbons (GNRs). The topological phase of these laterally-confined, semiconducting strips of graphene is determined by their width, edge shape, and the terminating unit cell, and is characterized by a Z2 invariant (similar to 1D solitonic systems). Interfaces between topologically distinct GNRs characterized by different Z2 are predicted to support half-filled in-gap localized electronic states which can, in principle, be utilized as a tool for material engineering. Here we present the rational design and experimental realization of a topologically-engineered GNR superlattice that hosts a 1D array of such states, thus generating otherwise inaccessible electronic structure. This strategy also enables new end states to be engineered directly into the termini of the 1D GNR superlattice. Atomically-precise topological GNR superlattices were synthesized from molecular precursors on a Au(111) surface under ultra-high vacuum (UHV) conditions and characterized by low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Our experimental results and first-principles calculations reveal that the frontier band structure of these GNR superlattices is defined purely by the coupling between adjacent topological interface states. This novel manifestation of 1D topological phases presents an entirely new route to band engineering in 1D materials based on precise control of their electronic topology, and is a promising platform for future studies of 1D quantum spin physics.

• The paper presents a novel method that engineers graphene nanoribbons via topological design to create controllable 1D electronic structures.
• The authors utilize ultra-high vacuum synthesis and STM/STS characterization to confirm the presence of localized, symmetry-protected edge states.
• The study reports an experimental bandgap of 0.74 ± 0.06 eV, highlighting potential applications in quantum devices and spintronics.

Topological Band Engineering of Graphene Nanoribbons

The paper "Topological Band Engineering of Graphene Nanoribbons" presents a detailed investigation into the topological phases in graphene nanoribbons (GNRs) and their potential for novel electronic applications. The paper focuses on developing a method for engineering the electronic structure of GNRs via topological considerations, which provides an alternative strategy to traditional bandgap engineering techniques.

The authors explore the theoretical predictions that laterally confined graphene nanoribbons exhibit 1D symmetry-protected topological phases, characterized by a Z2 invariant. These systems can host half-filled in-gap localized electronic states at interfaces between topologically distinct GNRs segments, paving the way for innovative material engineering approaches.

Experimental Approach

The research employs a strategy of synthesizing topologically-engineered GNR superlattices from molecular precursors under ultra-high vacuum (UHV) conditions. The GNRs were synthesized on a Au(111) surface, characterized by the precision with which the atomic structures are formed. The synthesis involves a multi-step process, integrating thermal cyclodehydrogenation and radical polymerization, thus forming periodic superlattices of alternating topologically trivial and nontrivial segments.

Characterization through low-temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) allowed the investigation of the electronic properties of these GNR superlattices. The experimental band structure identified in the paper was dominated by coupling between adjacent topological interface states, resulting in novel 1D electronic structures.

Results and Discussion

The article provides significant numerical results detailing the band structure of the engineered GNR systems. Notably, the engineered GNR superlattice exhibited an experimental band gap of 0.74 ± 0.06 eV, which is notably narrower than the band gaps for uniform 7-AGNRs and 9-AGNRs. The paper highlights the importance of the localized end states which are introduced at the GNR/vacuum termination region. The existence of these states confirms the presence of topologically protected properties dictated by the nontrivial Zak phase of the bands involved.

The results were corroborated with density functional theory (DFT) calculations, which demonstrated congruence with the experimental findings. The theoretical calculations also provided insights into the underlying mechanisms by which the precisely controlled atomic structure of the GNR superlattice contributes to its unique electronic properties.

Implications and Future Prospects

The implications of this research are significant for the field of quantum materials and nanotechnology. By harnessing the topological properties of GNRs, this work opens new avenues in band engineering that could be instrumental in designing electronic components with specific, customizable characteristics. Furthermore, the ability to induce characteristics such as metallicity and magnetism in previously semiconducting structures presents potential applications in spintronics and quantum computing.

Future developments may involve exploring the scalability of this synthesis method for larger-scale production of topologically-engineered GNRs and integrating these materials with other quantum materials, such as superconductors. The prospect of creating Majorana fermion states at the ends of these GNRs holds promise for topological quantum computing applications.

In conclusion, the research delineated in this paper showcases a formidable step towards mastering the electronic properties of nanoscale materials through topological design, setting the stage for future studies that may implement these principles in practical devices and further understand their fundamental quantum mechanical behaviors.