Thin layers of cobalt and ruthenium polypyridyloligomers with thicknesses between 2 and 8 nm were deposited on gold by electrochemical reduction of diazonium salts. A scanning tunneling microscope was used to create single-molecule junctions (SMJs). The charge transport properties of the Au-[Co(tpy) 2 ] n -Au (n = 1−4) SMJs do not depend markedly on the oligomer length, have an extremely low attenuation factor (β ∼ 0.19 nm −1 ), and do not show a thickness-dependent transition between two mechanisms. Resonant charge transport is proposed as the main transport mechanism. The SMJ conductance decreases by 1 order of magnitude upon changing the metal from Co to Ru. In Au-[Ru(tpy) 2 ] n -Au and Au-[Ru(bpy) 3 ] n -Au SMJs, a charge transport transition from direct tunneling to hopping is evidenced by a break in the length-dependent β-plot. The three different mechanisms observed are a clear molecular signature on transport in SMJs. Most importantly, these results are in good agreement with those obtained on large-area molecular junctions.

Various architectures have been generated and observed by STM at a solid/liquid interface resulting from an in situ chemical reaction between the bipyridine terminal groups of a ditopic ligand and Co(II) ions. Large monodomains of one-dimensional (1D) double wires are formed by Co(II)/ligand coordination, with polymer lengths as long as 150 nm. The polymers are organized as parallel wires 8 nm apart, and the voids between wires are occupied by solvent molecules. Two-dimensional (2D) grids, showing high surface mobility, coexist with the wires. The wires are formed from linear chain motifs where each cobalt center is bonded to two bipyridines. 2D grids are generated from a bifurcation node where one cobalt bonds to three bipyridines. Surface reconstruction of the grids and of the 1D wires was observed under the STM tip. As an exciting result, analysis of these movements strongly indicates surface reactions at the solid/liquid interface.Letter pubs.acs.org/JPCL

Single-molecule junctions (SMJs) have been fabricated using layers generated by diazonium electroreduction. This process immobilizes the molecule and creates a direct C-Au covalent bond between the molecule and the electrode. As a result, rigid oligomers of variable length, mainly perpendicular to the surface, are formed. The oligomers are based on porphyrin derivatives in their free base or cobalt complex forms. The conductance of the grafted oligomers has been studied by means of the scanning-tunneling-microscopy break-junction (STM-bj) technique and G(t) measurements, and the lifetime of the single-molecule junctions has been investigated. The conductance histograms indicate that charge transport in the porphyrins is relatively efficient and influenced by the presence of the cobalt center. With both systems, random telegraph G(t) signals are easily recorded showing SMJ on off states. The SMJs then stabilize and exhibit a surprisingly long lifetime around 10 s, and attenuation plots, obtained by both G(t) and STM-bj conductance measurements, give identical attenuation values. This work shows that highly stable SMJs can be prepared using diazonium grafting approach.

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On-surface 4-(bis-ethylenedioxythiophene)benzoic acid (2EBCOOH) dimerization and coordination with copper have been generated spontaneously and visualized by scanning tunneling microscopy at the solid/liquid interface. The home-synthesized monomers of 2EBCOOH self-assemble first into stable two-dimensional (2D) chain networks. By introducing Cu(II) ions, the preformed 2D chains turn into packets of short stripes through multistep chemical reactions: (i) oxidation of BiEDOT units triggered by Cu(II) reduction; (ii) formation of a COOH-phenyl-4EDOT-phenyl-COOH dimer by covalent coupling and formation of a C−C bond between two adjacent BiEDOT units; (iii) coordination between carboxylate groups and Cu(II); and (iv) formation of metal−dimer frameworks leading to a −[Cu-COO-phenyl-4EDOT-phenyl-COO] n − coordination polymer. This is a very first report of on-surface dimerization from BiEDOT oxidation generated using a novel input of Cu(II) ions at the solid/liquid interface.

Three-dimensional arrays of multilamellar silica nanoparticles are prepared through confined synthesis and enhance the catalytic activity of supported gold.