The design and fabrication of robust metallic states in graphene nanoribbons (GNRs) are challenging because lateral quantum confinement and many-electron interactions induce electronic band gaps when graphene is patterned at nanometer length scales. Recent developments in bottom-up synthesis have enabled the design and characterization of atomically precise GNRs, but strategies for realizing GNR metallicity have been elusive. Here we demonstrate a general technique for inducing metallicity in GNRs by inserting a symmetric superlattice of zero-energy modes into otherwise semiconducting GNRs. We verify the resulting metallicity using scanning tunneling spectroscopy as well as first-principles density-functional theory and tight-binding calculations. Our results reveal that the metallic bandwidth in GNRs can be tuned over a wide range by controlling the overlap of zero-mode wave functions through intentional sublattice symmetry breaking.
We demonstrate a highly efficient thermal conversion of four differently substituted polydiacetylenes (PDAs 1 and 2a-c) into virtually indistinguishable N = 8 armchair graphene nanoribbons ([8]GNR). PDAs 1 and 2a-c are themselves easily accessed through photochemically initiated topochemical polymerization of diynes 3 and 4a-c in the crystal. The clean, quantitative transformation of PDAs 1 and 2a-c into [8]GNR occurs via a series of Hopf pericyclic reactions, followed by aromatization reactions of the annulated polycyclic aromatic intermediates, as well as homolytic bond fragmentation of the edge functional groups upon heating up to 600 °C under an inert atmosphere. We characterize the different steps of both processes using complementary spectroscopic techniques (CP/MAS C NMR, Raman, FT-IR, and XPS) and high-resolution transmission electron microscopy (HRTEM). This novel approach to GNRs exploits the power of crystal engineering and solid-state reactions by targeting very large organic structures through programmed chemical transformations. It also affords the first reported [8]GNR, which can now be synthesized on a large scale via two operationally simple and discrete solid-state processes.
Rubin and colleagues describe the development of a simple, bottom-up synthetic approach to graphene nanoribbons (GNRs). In contrast to current methods, this process requires only two solid-state transformations. The key to this approach is the in-crystal topochemical polymerization of butadiyne-containing monomers to produce the corresponding polydiacetylene polymers. These polymers are subsequently fully aromatized in the solid state to GNRs at relatively mild temperatures.
The design of a spin imbalance within the crystallographic unit cell of bottom-up engineered 1D graphene nanoribbons (GNRs) gives rise to nonzero magnetic moments within each cell. Here, we demonstrate the bottom-up assembly and spectroscopic characterization of a one-dimensional Kondo spin chain formed by a chevron-type GNR (cGNR) physisorbed on Au(111). Substitutional nitrogen core doping introduces a pair of low-lying occupied states per monomer within the semiconducting gap of cGNRs. Charging resulting from the interaction with the gold substrate quenches one electronic state for each monomer, leaving behind a 1D chain of radical cations commensurate with the unit cell of the ribbon. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal the signature of a Kondo resonance emerging from the interaction of S = 1/2 spin centers in each monomer core with itinerant electrons in the Au substrate. STM tip lift-off experiments locally reduce the effective screening of the unpaired radical cation being lifted, revealing a robust exchange coupling between neighboring spin centers. First-principles DFT-LSDA calculations support the presence of magnetic moments in the core of this GNR when it is placed on Au.
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