Although
methods for a periodic perforation and heteroatom doping
of graphene sheets have been developed, patterning closely spaced
holes on the nanoscale in graphene nanoribbons is still a challenging
task. In this work, nitrogen-doped porous graphene nanoribbons (N-GNRs)
were synthesized on Ag(111) using a silver-assisted Ullmann polymerization
of brominated tetrabenzophenazine. Insights into the hierarchical
reaction pathways from single molecules toward the formation of one-dimensional
organometallic complexes and N-GNRs are gained by a combination of
scanning tunneling microscopy (STM), atomic force microscopy (AFM)
with CO-tip, scanning tunneling spectroscopy (STS), and density functional
theory (DFT).
The atomic buckling in 2D “Xenes” (such as silicene) fosters a plethora of exotic electronic properties such as a quantum spin Hall effect and could be engineered by external strain. Quantifying the buckling magnitude with subangstrom precision is, however, challenging, since epitaxially grown 2D layers exhibit complex restructurings coexisting on the surface. Here, we characterize using low-temperature (5 K) atomic force microscopy (AFM) with CO-terminated tips assisted by density functional theory (DFT) the structure and local symmetry of each prototypical silicene phase on Ag(111) as well as extended defects. Using force spectroscopy, we directly quantify the atomic buckling of these phases within 0.1-Å precision, obtaining corrugations in the 0.8- to 1.1-Å range. The derived band structures further confirm the absence of Dirac cones in any of the silicene phases due to the strong Ag-Si hybridization. Our method paves the way for future atomic-scale analysis of the interplay between structural and electronic properties in other emerging 2D Xenes.
Nitrogen‐doped Kagome graphene (N‐KG) has been theoretically predicted as a candidate for the emergence of a topological band gap as well as unconventional superconductivity. However, its physical realization still remains very elusive. Here, we report on a substrate‐assisted reaction on Ag(111) for the synthesis of two‐dimensional graphene sheets possessing a long‐range honeycomb Kagome lattice. Low‐temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM) with a CO‐terminated tip supported by density functional theory (DFT) are employed to scrutinize the structural and electronic properties of the N‐KG down to the atomic scale. We demonstrate its semiconducting character due to the nitrogen doping as well as the emergence of Kagome flat bands near the Fermi level which would open new routes towards the design of graphene‐based topological materials.
Bending and twisting around carbon–carbon single
bonds are
ubiquitous in natural and synthetic polymers. Force-induced changes
were so far not measured at the single-monomer level, owing to limited
ways to apply local forces. We quantified down to the submolecular
level the mechanical response within individual poly-pyrenylene chains
upon their detachment from a gold surface with an atomic force microscope
at 5 K. Computer simulations based on a dedicated force field reproduce
the experimental traces and reveal symmetry-broken bent and rotated
conformations of the sliding physisorbed segment besides steric hindrance
of the just lifted monomer. Our study also shows that the tip–molecule
bond remains intact but remarkably soft and links force variations
to complex but well-defined conformational changes.
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