Covalent Anchoring of Re6Se8i Cluster Cores Mono layers on Modified n- and p-Type Si(111) Surfaces: Effect of Coverage on Electronic Properties

The electronic properties of redox-active transition metal clusters (Re6Se8) covalently immobilized on modified Si(111) surfaces through linear alkyl spacers have been studied as a function of the cluster coverage (1 x 10(13)-6 x 10(13) cm(-2)). The latter is controlled by using Si(111)/H surfaces modified by dense mixed alkyl/acid-terminated monolayers with variable fraction of the acid grafting sites from 5 to 100% in solution. Quantitative X-ray photoemission analysis, spectroscopic ellipsometry, and scanning tunnelling microscopy indicate a covalent attachment of a submonolayer to densely packed monolayer of Re6Se8 clusters, while the vibrational Raman signature confirms the cluster integrity within the monolayer. Electrical band gaps as deduced from scanning tunnelling spectroscopy have been obtained for low Re6Se8 cluster coverage. Using ultraviolet photoemission spectroscopy, electronic properties such as ionization potential changes and energy level alignments at organic/inorganic interfaces are studied. We show that the lowest unoccupied molecular orbital of the Re6Se8 cluster is close to the bottom of the Si conduction band. At high cluster coverage, this affects the current voltage characteristics measured using a weakly interacting top mercury contact onto the organic monolayer/silicon junctions. Indeed, on n-type silicon, the high level current at low bias and the shape of the conductance G(V) curve indicate a Schottky barrier height lowering. On the other hand, the current voltage characteristics are the same for both acid-terminated and low coverage Re6Se8 cluster junctions at low bias; the high Schottky barrier height limits the current at low bias. When the forward bias increases, the current is tunnelling limited. As expected from the band alignment deduced from photoemission data, the opposite behavior is obtained on p-type silicon.