Highly conductive ∼40-nm-long molecular wires assembled by stepwise incorporation of metal centres

Abstract

One of the main goals of molecular electronics is to achieve electronic functions from devices consisting of tailored organic molecules connecting two metal electrodes. The fabrication of nanometre-scale spaced electrodes still results in expensive, and often scarcely reproducible, devices. On the other hand, the 'conductance' of long organic molecules--generally dominated by the tunnelling mechanism--is very poor. Here, we show that by incorporating a large number of metal centres into rigid molecular backbones we can obtain very long (up to 40 nm) and highly 'conductive' molecular wires. The metal-centre molecular wires are assembled in situ on metal surfaces via a sequential stepwise coordination of metal ions by terpyridine-based ligands. They form highly ordered molecular films of elevated mechanical robustness. The electrical properties, characterized by a junction based on Hg electrodes, indicate that the 'conductance' of these metal-centre molecular wires does not decrease significantly even for very long molecular wires, and depends on the nature of the incorporated redox centre. The outstanding electrical and mechanical characteristics of these easy-to-assemble molecular systems open the door to a new generation of molecular wires, able to bridge large-gap electrodes, and to form robust films for organic electronics.

Activationless charge transport across 4.5 to 22 nm in molecular electronic junctions

“…Temperature-dependent transport with a small β-value has been associated with activated hopping between sites in both single molecules (35,36) and ensemble junctions (16,23), presumably governed by MarcusLevich kinetics. The low-V, high-T Arrhenius slopes of 150-300 meV listed in Table 1 are comparable to those reported for bulk polythiophene, e.g., 128-143 meV (37) and 280 meV (38), in which interchain charge transfer dominates transport (39,40).…”

“…For this distance range, there is general agreement that the conductance scales exponentially with length, with an attenuation coefficient (β), defined as the slope of ln J vs. thickness (d), equal to 8 to 9 nm −1 for aliphatic molecules (3-6) and 2-3 nm −1 for aromatic molecules (7)(8)(9)(10)(11)(12)(13)(14). A few molecular electronic systems have been investigated beyond 5 nm (15,16), some of which exhibit a decrease in β to less than 1 nm −1 . Such small values of β are significant both practically (17,18) and fundamentally (19)(20)(21)(22), because they imply transport across distances much greater than the dimensions of single molecules and may enable complex molecular circuits.…”

Metal complexes in molecular junctions

“…6 Rampi et al reported electrical measurements of such metal incorporated nanowires up to 40 nm in length with extremely low β values of 0.28 nm −1 (Fe II ) and 0.01 nm −1 (Co II ) implicating a multistep charge hopping process between the metal centres in the backbone. 7 Thus conductivity does not decrease significantly with length and depends on the metal atom. Wires up to 5 nm incorporating Zn porphyrins were also found to display very low attenuation factors (0.4 nm −1 ).…”

Luminescent acetylthiol derivative tripodal osmium(II) and iridium(III) complexes: Spectroscopy in solution and on surfaces

“…Monolayers of electroactive molecules on bulk metallic surfaces, in particular, Au and Pt have shown interesting behavior and some promising results [8][9][10]. The central key feature is to have electro/photoactive organic and organometallic fragments assembled onto these surfaces and study their charge conductivity [11][12][13][14][15] or eventually their electroluminescence in molecular junctions [16]. Different chemical functionality is required to assem-ble these molecules on the bulk surfaces through covalent and noncovalent interactions.…”