Graphene holds tremendous potential as a material for creating integrated circuitry with atomic-level thickness. However, its zero bandgap has been a roadblock in its device applications. One solution to this challenge lies in graphene nanoribbons (GNRs). With a tunable bandgap that can be adjusted through modifications to width, chirality and edge structure, GNRs represent a significant step forward in device functionality.¹ Decorating GNR edges with metal atoms provides even greater enhancement of their functionality, a fact confirmed by numerous theoretical studies.² Despite this, experimental studies of GNR edge decoration are limited, leaving a significant opportunity for exploration and discovery.

Using advanced electron microscopy techniques, we have a unique opportunity to advance our understanding of GNRs and their potential. The synthesis and high-resolution transmission electron microscopy (TEM) characterization of GNRs with metal atom chains at their edges represents a major challenge. By investing in cutting-edge experimentation and characterization, we can unlock the full potential of GNRs. The time is now to explore the uncharted territory of GNRs with linear metal chain decoration.

We here present a pioneering approach to address the challenges faced in synthesizing linear chains of indium (In) atoms along the edges of graphene-supported GNRs. Elemental In anchored on graphene not only alters its electronic properties but also exhibits remarkable potential as a single-atom catalyst for CO₂ reduction.^{3, 4} As a background, our previous report has provided comprehensive in-situ synthesis and atomic-resolution characterization details of single In atoms and few-atom In nanoclusters anchored on monolayer graphene.⁵

In the present work, we demonstrate the in-situ formation of small GNR-like patches on suspended monolayer graphene membranes using laser-induced high-temperature crystallization of adsorbed hydrocarbons. Our cutting-edge setup, which combines a scanning transmission electron microscope (STEM) and an ultra-high-vacuum (UHV) preparation system equipped with deposition and laser-annealing chambers, allows us to synthesize and image atom chains in a controlled environment. This system enables transferring samples from the UHV preparation chamber to the microscope without breaking the vacuum. Before the synthesis of atom chains, we first crystallized adsorbed hydrocarbons into small ribbon-like patches on the graphene membrane by laser annealing. After the In deposition onto the GNR-covered graphene membrane by physical vapor deposition (PVD), we transfer the sample into a STEM without ambient air exposure and laser-anneal the sample once more to trigger the diffusion of the deposited In across the sample surface. Figure 1a shows the atomically resolved medium-angle annular dark-field (MAADF) STEM image of In-decorated GNR edges formed on a supporting graphene monolayer. A simplified model corresponding to the experimental structure is shown in Figure 1b.  The high-resolution STEM images reveal the formation of linear In-atom chains along both near-ZZ and near-AC GNR edges, and the electron beam irradiation in STEM promotes the formation of long In atom-chains. Our experimental observations have been verified through density functional theory (DFT) and image simulations. The electronic band structure calculations based on DFT indicate that the electronic properties of ZZ- and AC-GNRs can be modified via In-atom termination at their edges. Although there is no explicit difference in the electronic structures of bare and In-terminated ZZ-GNRs (Figure 1c), In atoms decorating the edges of AC-GNRs lead to changes in the lowest unoccupied bands and induce doping in the otherwise semiconducting AC-GNR (Figure 1d). This represents a significant advancement in our understanding of graphene nanoribbons and their potential for modification through the use of single-atom catalysts.

In this study, we make a significant contribution to the field by demonstrating a new method for synthesizing long linear metal-atom chains at graphene and graphene nanoribbon (GNR) edges. Our findings not only expand upon previous literature, which was limited to isolated metal atoms at graphene edges, but also highlight the potential for these long linear metal-atom chains to tune the electronic properties of GNRs. Our use of indium, a potent single-atom catalyst, further highlights the potential for these linear In-atom chains to play a role in future catalysis studies. These results offer a deeper understanding of how to create linear metal-atom chains at graphene and GNR edges, providing exciting opportunities for further exploration and advancement in the field.

References

1. Wang H, et al. Graphene nanoribbons for quantum electronics. Nature Reviews Physics 3, 791-802 (2021).

2. Chuvilin A, et al. Self-assembly of a sulphur-terminated graphene nanoribbon within a single-walled carbon nanotube. Nature Materials 10, 687-692 (2011).

3. Yeh C-H, et al. Ultrafast Monolayer In/Gr-WS₂-Gr Hybrid Photodetectors with High Gain. ACS Nano 13, 3269-3279 (2019).

4. Guo W, et al. Atomic Indium Catalysts for Switching CO₂ Electroreduction Products from Formate to CO. Journal of the American Chemical Society 143, 6877-6885 (2021).

5. Elibol K, et al. Single Indium Atoms and Few-Atom Indium Clusters Anchored onto Graphene via Silicon Heteroatoms. ACS Nano 15, 14373-14383 (2021).