Elaboration and characterization of transition metal-functionalized surfaces: influence of the cluster immobilization on electronic properties – CLUSTSURF

This proposal aims at elaborating and studying the electronic properties of redox-active molecular junctions (cluster/organic bridge/semiconductor) obtained by the attachment of octahedral metal cluster on semiconducting surfaces (Si, C) through organic bridges. The electronic, optical and magnetic properties of inorganic metal clusters are related to the nature of the metal and the nature of ligands to which they are bonded as well as to the geometry of the units (unit = cluster of 4d or 5d transition metal (M) + ligands (L) (halogen or chalcogen)). Among the cluster families, we will focus on M6 octahedral clusters chosen for their wide range of physico-chemical properties. The latter depend on the number of electrons available for metal-metal bonds (VEC). The edge bridged [(M6Li12)La6] and face capped [(M6Li8)La6] cluster units (La = apical ligand, Li = inner ligand) constitute the basic building blocks in octahedral cluster chemistry. These units are characterized by rigid M6Li12 and M6Li8 cluster cores bonded to six additional apical ligands La. In this proposal, different units covering a wide range of redox potentials will be syntesized by solid-sate chemistry. In solution, they show reversible electron oxidation processes that influence their luminescent (eg: M6L14, M = Mo or Re) or magnetic (eg: Nb6L18, VEC = 15) properties. The 6 La ligands can be replaced by coordinating organic entities by solution chemistry. The association of transition metal clusters, synthesized by solid state chemistry, with functional ligands is a field of research started in the 90's in Holm's group (Harvard, USA). This chemistry is actually in a stage of rapid development and should lead to nanostructured systems with optical, electric, magnetic and catalytic specific properties. Whithin this frame, the integration of functional cluster units directly with semiconductor surfaces is an important challenge for future applications (e.g. addressable optical or magnetic arrays, hybrid junctions for charge storage devices…). This proposal is a prerequisite to evaluate the qualities and drawbacks of metallic clusters as candidates for further integration in electronic devices based on redox-active molecular junctions and constitutes a first step before future investigation of luminescence and magnetic properties of cluster-functionalized surfaces. The strategy consists in the immobilization of clusters on surfaces. We focus here on the grafting of functional clusters on semiconducting surfaces (crystalline silicon or amorphous carbon) via covalent binding using an organic bridge. Two aspects will be considered : (i) the effects of the grafting on the electronic properties of the cluster that will be compared with those observed in solution, and (ii) consequently the control of the properties, in particular electronic, of the resulting functional surfaces by the choice of cluster units (nature of M and L) and/or the nature of surface. The clusters will be attached to the semi-conducting surface through an alkyl chain, functionalized by a group that will complex the cluster in apical position, using different grafting strategies (one step or multistep routes). The thickness of the layer of organic chains, between the metallic cluster and the underlying semiconductor surface, will be in the range 1-2 nm in order to obtain an electron tunneling sufficiently fast to observe a reversible electrochemical signal. The evaluation of the role of packing density and molecular characteristics (nature of the units, units-substrate bond….) on the electronic and redox properties of the hybrid junctions will be investigated in detail and compared using Mo6, Re6 and Nb6 clusters. The measurements of electronic properties of the metal cluster-modified surfaces will be performed at the macroscopic and nanometric scales using different techniques (XPS, UPS, conducting AFM, Hg drop transport, cyclic voltammetry). This proposal is built on the experience and skills of scientists with complementary knowledge that have already co-signed several publications: chemists for clusters synthesis and characterization (P1), for chemical or electrochemical molecular grafting processes (P2), for thin films deposition (P1), and physicists (P3) for semiconductor surface and interface characterizations combining surface physics methods (photoemission spectroscopies, AFM) and electron transport studies. It is coherent with the pluridisciplinary scientific policy of the University of Rennes involving chemists and physicists who collaborate through the new 2007-2011 funding program (CPER, PRIN2TAN Nanosoft proposal). Acceptance of this proposal would promote collaborations between these partners and would give them the required means and critical size to be more present at an international level in the field of nanomaterials. This proposal will constitute an excellent opportunity of consolidating national and international collaborations or to open new ones.