RESUMEN
Nanographenes with zigzag edges, for example, anthenes, exhibit a unique nonbonding π-electron state, which can be described as a spin-polarized edge state that yields specific magnetic ground state. However, prior researches on the magnetism of anthenes with varying lengths on a surface is lacking. This study systematically fabricated anthenes with inherent zigzag carbon atoms of different lengths ranging from bisanthene to hexanthene. Their magnetic evolution on the Au(111) surface was analyzed through bond-resolved scanning probe techniques and density functional theory calculations. The analyses revealed a transition in magnetic properties associated with the length of the anthenes, arising from the imbalance between hybridization energy and the Coulomb repulsion between valence electrons. With the increasing length of the anthenes, the ground state transforms gradually from a closed-shell to an antiferromagnetic open-shell singlet, exhibiting a weak exchange coupling of 4â meV and a charge transfer-induced doublet. Therefore, this study formulated a chemically tunable platform to explore size-dependent π magnetism at the atomic scale, providing a framework for research in organic spintronics.
RESUMEN
The explorations to extend present chemical synthetic methods are of great importance to simplify synthetic routes of chemical species. Additionally, understanding the chemical reaction mechanisms is critical to achieve controllable synthesis for applications. Here, we report the on-surface visualization and identification of a phenyl group migration reaction of 1,4-dimethyl-2,3,5,6-tetraphenyl benzene (DMTPB) precursor on Au(111), Cu(111) and Ag(110) substrates. With the combination of bond-resolved scanning tunneling microscopy (BR-STM), noncontact atomic force microscopy (nc-AFM) and density functional theory (DFT) calculations, the phenyl group migration reaction of DMTPB precursor is observed, forming various polycyclic aromatic hydrocarbons on the substrates. DFT calculations reveal that the multiple-step migrations are facilitated by the hydrogen radical attack, inducing cleavage of phenyl groups and subsequent rearomatization of the intermediates. This study provides insights into complex surface reaction mechanisms at the single molecule level, which may guide the design of chemical species.
RESUMEN
Atomically precise fabrication of covalent-organic frameworks with well-defined heteroatom-dopant sites and further understanding of their electronic properties at the atomic level remain a challenge. Herein, we demonstrate the bottom-up synthesis of well-organized covalent-organic frameworks doped by nitrogen atoms on an Ag(111) substrate. Using high-resolution scanning tunneling microscopy and non-contact atomic force microscopy, the atomic structures of the intermediate metal-organic frameworks and the final covalent-organic frameworks are clearly identified. Scanning tunneling spectroscopy characterization reveals that the electronic bandgap of the as-formed N-doped covalent-organic framework is 2.45 eV, in qualitative agreement with the theoretical calculations. The calculated band structure together with the projected density of states analysis clearly unveils that the incorporation of nitrogen atoms into the covalent-organic framework backbone will remarkably tune the bandgap owing to the fact that the foreign nitrogen atom has one more electron than the carbon atom. Such covalent-organic frameworks may offer an atomic-scale understanding of the local electronic structure of heteroatom-doped covalent-organic frameworks and hold great promise for all relevant wide bandgap semiconductor technologies, for example, electronics, photonics, high-power and high-frequency devices, and solar energy conversion.
RESUMEN
Doped graphene nanoribbons (GNRs) with heteroatoms are a principal strategy to fine-tune the electronic structures of GNRs for future device applications. Here, we successfully synthesized the N=9 nitrogen-doped armchair GNR on the Au(111) surface. Due to the flexibility of precursor molecules, three different covalent bonds (C-C, C-N, N-N) are formed in the GNR backbone. Scanning tunneling spectroscopy analysis together with band structure calculations reveals that the band gap of the N-9-AGNRs (C-C) will be enlarged compared to pristine 9-AGNRs, and the C-N bond and N-N bond at the isolated site of N-9-AGNR (C-C) will introduce new defect states near the Fermi level. DFT calculations reveal that the electronic structure of N-9-AGNR (C-C) shows semiconductor character, while N-9-AGNR (C-N) and N-9-AGNR (N-N) display metallic character. Our results provide a promising route for creating more complex molecular heterostructures with tunable band gaps, which may be useful for future molecular electronics and memory device applications.
