Graphene nanoribbons (GNRs)—narrow stripes of graphene—have emerged as promising building blocks for nanoelectronic devices. Recent advances in bottom-up synthesis have allowed production of atomically well-defined armchair GNRs with different widths and doping. While all experimentally studied GNRs have exhibited wide bandgaps, theory predicts that every third armchair GNR (widths of N=3m+2, where m is an integer) should be nearly metallic with a very small bandgap. Here, we synthesize the narrowest possible GNR belonging to this family (five carbon atoms wide, N=5). We study the evolution of the electronic bandgap and orbital structure of GNR segments as a function of their length using low-temperature scanning tunnelling microscopy and density-functional theory calculations. Already GNRs with lengths of 5 nm reach almost metallic behaviour with ∼100 meV bandgap. Finally, we show that defects (kinks) in the GNRs do not strongly modify their electronic structure.
On-surface
synthesis with molecular precursors has emerged as the
de facto route to atomically well-defined graphene nanoribbons (GNRs)
with controlled zigzag and armchair edges. On Au(111) and Ag(111)
surfaces, the prototypical precursor 10,10′-dibromo-9,9′-bianthryl
(DBBA) polymerizes through an Ullmann reaction to form straight GNRs
with armchair edges. However, on Cu(111), irrespective of the bianthryl
precursor (dibromo-, dichloro-, or halogen-free bianthryl), the Ullmann
route is inactive, and instead, identical chiral GNRs are formed.
Using atomically resolved noncontact atomic force microscopy (nc-AFM),
we studied the growth mechanism in detail. In contrast to the nonplanar
BA-derived precursors, planar dibromoperylene (DBP) molecules do form
armchair GNRs by Ullmann coupling on Cu(111), as they do on Au(111).
These results highlight the role of the substrate, precursor shape,
and molecule–molecule interactions as decisive factors in determining
the reaction pathway. Our findings establish a new design paradigm
for molecular precursors and opens a route to the realization of previously
unattainable covalently bonded nanostructures.
Among
current technologies for hydrogen production as an environmentally
friendly fuel, water splitting has attracted increasing attention.
However, the efficiency of water electrolysis is severely limited
by the large anodic overpotential and sluggish reaction rate of the
oxygen evolution reaction (OER). To overcome this issue, the development
of efficient electrocatalyst materials for the OER has drawn much
attention. Here, we show that organometallic Ni(II) complexes immobilized
on the sidewalls of multiwalled carbon nanotubes (MWNTs) serve as
highly active and stable OER electrocatalysts. This class of electrocatalyst
materials is synthesized by covalent functionalization of the MWNTs
with organometallic Ni bipyridine (bipy) complexes. The Ni-bipy-MWNT
catalyst generates a current density of 10 mA cm–2 at overpotentials of 310 and 290 mV in 0.1 and 1 M NaOH, respectively,
with a low Tafel slope of ∼35 mV dec–1, placing
the material among the most active OER electrocatalysts reported so
far. Different simple analysis techniques have been developed in this
study to characterize such a class of electrocatalyst materials. Furthermore,
density functional theory calculations have been performed to predict
the stable coordination complexes of Ni before and after OER measurements.
[reaction: see text] Copper bisphosphine complexes catalyze the intramolecular reductive aldol reaction of alpha,beta-unsaturated esters with ketones, affording five- and six-membered beta-hydroxylactones in high stereoselectivities. Utilization of chiral nonracemic bisphosphines render the cyclizations enantioselective.
[reaction: see text] Cobalt catalysis enables a new method for the generation of zinc enolates using diethylzinc to reduce alpha,beta-unsaturated amides. This method has been applied to a high-yielding diastereoselective reductive aldol cyclization.
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