© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1688 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Nanoscale Electrical Investigation of Layer-by-Layer Grown Molecular Wires Chiara Musumeci, Gabriella Zappalà, Natalia Martsinovich, Emanuele Orgiu, Swen Schuster, Silvio Quici, Michael Zharnikov, Alessandro Troisi,* Antonino Licciardello,* and Paolo Samorì* The promise of new technological breakthroughs, motivated by the exponential miniaturization trend of conventional elec- tronics, has been the major driving force in the field of molec- ular electronics. Nevertheless important fundamental issues aiming to establish the conceptual framework of the electron transfer processes have been solicited. [1] Indeed the under- standing of the charge transfer and charge transport mecha- nisms is of central interest in the field of chemistry, physics and biology, because of their crucial role in many processes involving molecular, supramolecular and biological systems, such as reactions across bridged species, reactions at interfaces, quantum transport phenomena, photoinduced electron transfer and photosynthetic reactions. [2] Because of their tunable composition, metallo-ligand com- plexes are multifunctional nanoscale architectures. Among them, chelate complexes based on N-heteroatomic ligands, like 2,2′:6′2″-terpyridine derivatives, are considered an ever- expanding synthetic and structural frontier because of their unique electrochemical, photophysical, catalytic and magnetic properties. [3] Hitherto, a great effort has been devoted to the under- standing of the photophysical properties of these compounds by looking at the photoinduced electron transfer reactivity, [4] which in turn has led to their use in optoelectronic devices, for example, as dye sensitizers. [5] Conversely, their application in electronics has been hampered by a relative lack of knowledge on their charge transport behavior in the solid state. Some elec- trochemical studies aimed at exploring the redox conduction properties of polymers bearing terpyridine complexes in the side chains; for these systems the measured fast charge trans- port dynamics suggested sequential electron hopping between discrete localized valence states as the most probable charge transport mechanism. [6] Since the structural versatility of the coordination compounds allows for the fabrication of well- defined architectures, such as quasi-1D structures, they are also being considered possible building blocks for the fabrication of conductive molecular wires. [7] Stepwise coordination reactions involving terpyridine functions have been successfully exploited to connect the molecular wires to solid surfaces such as gold, [8] silicon, [9] carbon [10] and oxides, [11] and to lengthen the wires up to tens of nanometers. [12] By taking advantage of the redox- active properties of these compounds, Nishihara et al. analyzed how the redox currents for the metal-bis(terpyridine) moieties flow through the complex wires, postulating an intrawire redox conduction in which the electrons hop sequentially between the neighboring metal-bis(terpyridine) sites. [13] Herein we focus on a Fe(II)-bis(terpyridine) molecular system based on 4′,4′′′′-(1,4-phenylene)bis(2,2′:6′,2′′-terpyridine) (TPT) ligand attached to planar gold substrates through a 4-[2,2′:6′,2′′-terpyridin]-4′-yl-benzenethiol (MPTP) self-assembled monolayer ( Figure 1). This system has been proved to self- assemble via a layer-by-layer growth along the direction perpen- dicular to the basal plane of Au planar electrodes, up to about 40 nm in length; [12a] in these assemblies charge hopping through the metal centers was suggested as the most likely charge transport mechanism, in light of the low attenuation factor calculated from electrical measurements by using macroscopic Hg drop junc- tions. In this paper, we characterize electrically the system on the nanoscale by conductive atomic force microscopy (C-AFM), [14] corroborated by theoretical calculations, synchrotron-based X-ray photoelectron spectroscopy (XPS) and angle-resolved near edge X-ray absorption fine structure (NEXAFS) spectroscopy to gain deeper insight into its charge transport mechanism, composition and structural order within the assembly. The molecular self-assembled platform was prepared by incubating for 24 hours the Au(111) substrates in a mixed solution of benzenethiol (MB) and MPTP in ethanol in a 1:1 ratio, the presence of the MB allowing for a better assembly of the MPTP on the surface. [15] The stepwise construction of the molecular layers was carried out by soaking previously washed Au-MB/MPTP samples (hereafter MB/MPTP): (i) in a solution of iron(II) sulfate and then, after accurate rinsing, (ii) in a DOI: 10.1002/adma.201304848 C. Musumeci, Dr. E. Orgiu, Prof. P. Samorì Nanochemistry Laboratory, ISIS & icFRC Université de Strasbourg & CNRS 8 allée Gaspard Monge, 67000, Strasbourg, France E-mail: samori@unistra.fr G. Zappalà, Prof. A. Licciardello Dipartimento di Chimica Università di Catania v.le A. Doria 6, 95125, Catania, Italy E-mail: alicciardello@unict.it Dr. N. Martsinovich, Prof. A. Troisi Department of Chemistry University of Warwick Coventry, CV4 7AL, UK E-mail: A.Troisi@warwick.ac.uk Dr. S. Quici CNR – ISTM via Golgi 19, 20133, Milano, Italy S. Schuster, Prof. M. Zharnikov Angewandte Physikalische Chemie Universität Heidelberg Im Neuenheimer Feld 253, 69120, Heidelberg, Germany Adv. Mater. 2014, 26, 1688–1693