Synthesis of single-layered fluorographdiyne nanosheets via selective on-surface 2D covalent polymerization

Abstract: Two-dimensional conjugated polymers (2DCPs) are significant macromolecular materials with intriguing and tunable physicochemical properties that depend on their geometries. Graphdiyne and its derivatives are exemplary 2DCPs featuring sp-sp2 hybridized skeletons. However, achieving single-layered, large-domain/regular graphdiyne and its derivatives on surfaces remains a formidable challenge due to the lack of selective 2D covalent polymerization methods. Here, we report a selective on-surface 2D covalent polymerization method via the combination of cobalt catalysis and coronene templating, achieving the synthesis of single-layered fluorographdiyne nanosheets up to 60*60 nm2 on Au(111) surface. Using scanning probe techniques, we visualize the sequential polymerization process and characterize cobalt-activated coupling intermediates at the atomic level. Experimental and theoretical analyses suggest that strong d-π coupling between cobalt and alkynyl transforms a robust Csp-Au bond into a weaker Csp2-Au bond, thereby facilitating the demetallization C-C coupling. Besides, the templating effect of coronene suppresses kinetically trapped defects and improves the selectivity of hexagonal-ring formation in the complex 2D covalent polymerization process.

• The paper presents a cobalt-catalyzed, coronene-templated method achieving single-layer fluorographdiyne nanosheets with >95% coupling efficiency and >90% hexagonal ring selectivity.
• It demonstrates that Co reduces the C–C coupling barrier from 2.04 eV to 1.27 eV and coronene significantly suppresses defects, enabling large domains up to 60×60 nm².
• Scanning probe experiments and DFT calculations validate the synthesis mechanism, revealing a semiconducting bandgap near 3.05 V and offering insights for scalable 2D material production.

Selective Synthesis of Single-Layered Fluorographdiyne Nanosheets via Cobalt-Catalyzed On-Surface 2D Covalent Polymerization

Abstract and Motivation

The synthesis of two-dimensional conjugated polymers (2DCPs) with large domain size, regularity, and controlled layer number remains a fundamental challenge in surface chemistry and materials science due to the complexity of on-surface 2D covalent polymerization protocols. The paper "Synthesis of single-layered fluorographdiyne nanosheets via selective on-surface 2D covalent polymerization" (2606.00495) introduces a synergistic strategy combining cobalt (Co) catalysis and coronene templating to enable selective polymerization and the fabrication of large-area, single-layer fluorographdiyne nanosheets on Au(111). The work rigorously elucidates the physicochemical and mechanistic underpinnings of this process, establishing practical advances in 2DCP synthesis and offering theoretical insight into polymerization regulation.

Mechanistic Insights into On-Surface 2D Covalent Polymerization

The study addresses the intrinsically high barriers and selectivity issues associated with alkynyl-based sp-metal–organic networks (sp-MONs) conversion into 2DCPs. The robust Csp-Au bond character in fluorinated alkynyl systems impedes domain growth and results in polycrystalline structures or defective regions unless precise regulation of demetallization and ring formation steps is implemented. Cobalt acts as a catalyst through strong d-orbital coupling with alkynyl groups, transiently transforming Csp-Au bonds into weaker Csp2-Au bonds, thereby lowering the energy barrier for demetallization C–C coupling (from 2.04 eV to 1.27 eV as per DFT calculations). Coronene, as a template, introduces noncovalent C–H···F interactions, reducing defect formation and markedly improving hexagonal ring selectivity during polymerization.

The process employs sequential deposition and annealing protocols: 1,3,5-tris(chloroethynyl)-2,4,6-trifluorobenzene (tFtCEB) is pre-assembled on Au(111), followed by Co and coronene vapor deposition. Annealing to 473–553 K induces Co-catalyzed coupling, coronene-guided ring formation, and yields nanosheets with domain sizes up to 60×60 nm². Scanning probe techniques (STM, nc-AFM) visualize polymerization intermediates at atomic resolution, confirming >95% coupling efficiency and >90% ring-formation selectivity.

Experimental and Theoretical Validation

The STM/nc-AFM experiments capture the stepwise progression from alkynyl-Au-alkynyl dimers to Co-activated intermediates, polycrystalline domains, and finally ordered fluorographdiyne nanosheets. Defect suppression by coronene is empirically validated by statistical domain analyses: with coronene templating, the fraction of hexagonal rings rises from 72.7% to 92.3%, with a corresponding increase in large ordered domains (up to 1.2% in the 652–802 nm² size range). DFT-calculated reaction pathways and atom-in-molecule (AIM) analyses identify critical binding energies (0.19–0.46 eV for noncovalent interactions), clarify the energetic favorability of Co-catalyzed bond conversion, and rationalize coronene’s spatial restriction effects in reducing defect probabilities.

Electronic structure characterization via differential conductance spectroscopy (dI/dV) and DFT-derived LDOS maps confirms the semiconducting nature of fluorographdiyne with a bandgap near 3.05 V. Coronene regions exhibit unique resonance signatures, further demonstrating the distinct electronic modulation afforded by templating.

Implications for 2D Materials Design

This protocol achieves single-layered fluorographdiyne nanosheets with domain sizes up to 60×60 nm², coupling efficiency exceeding 95%, and hexagonal ring selectivity over 90%, surpassing prior attempts limited by uncontrolled defect growth and polycrystallinity. The study provides atomic-level mechanistic understanding of synergistic catalysis-templating regulation, which is generally applicable to other alkyne-bridged 2D carbon-based materials and offers a pathway for precise electronic and structural property control.

The theoretical framework established for Co/alkynyl bond weakening and coronene-mediated spatial regulation is extensible to surface polymerization strategies employing alternative transition metals or molecular templates, potentially enabling tunable synthesis of designer 2DCPs, heterostructures, or functionalized nanomaterials. The demonstrated suppression of kinetic ring defects and improved domain size directly impact applications in nanoelectronics, energy storage (via enhanced charge mobility, as corroborated in references [1,5]), and quantum materials platforms.

Future Research Directions

• Transferability and Extension: Investigations can focus on the adaptation of this approach to other metal substrates or precursors and scaling to macroscopic chip-level domains.
• Functionality Integration: The role of coronene or other polycyclic aromatic hydrocarbons in modulating electronic band structures can be systematically explored for customized optoelectronic devices.
• Defect Dynamics: In situ studies on the mobility and evolution of radical intermediates under varying template or catalyst concentrations may further optimize selectivity and efficiency.
• Device Fabrication: Integration of these nanosheets into FETs, sensor platforms, and heterojunction architectures is a direct application leveraging their unique electronic properties.
• Theory and Computation: Advanced multiscale simulations combining DFT and molecular dynamics will elucidate kinetic vs. thermodynamic factors governing ring closure and defect migration.

Conclusion

The combination of Co catalysis and coronene templating enables the highly selective, efficient synthesis of single-layer fluorographdiyne nanosheets via surface-confined 2D covalent polymerization. This approach resolves key barriers in controlling ring formation and domain size, validated by strong numerical outcomes in coupling/selectivity, and sets a reference point for further functionalization and scale-up of 2D materials. The mechanistic and electronic insights provided guide future strategies for precision synthesis and application of 2DCPs with tailored properties (2606.00495).