Elaboration, Characterizations, and Energetics of Robust Mo6 Cluster-Terminated Silicon-Bound Molecular Junctions

Reaction of a [Mo6Br8F6]2– octahedral cluster unit with functional carboxylic acid leads to the formation of the corresponding carboxylate along with the elimination of HF and the formation of a Mo–O bond between the cluster and the carboxylate. This offers an easy way to prepare Mo6 cluster functionalized surfaces by simple reaction of carboxylic acid-modified n-type Si(111) surfaces in a solution of [Mo6Br8F6]2– cluster units. The surface coverage of metallic clusters has been controlled in the range 0.3–5.8 × 1013 cm–2 by dilution of the carboxylic acid-terminated organic chains with inert n-dodecyl chains. The homogeneity of the grafted films on the oxide-free Si surface was followed by combined XPS and STM analyses whereas the electronic gap corresponding to the difference between the conduction band minimum (CBM) and the valence band maximum (VBM) of a single immobilized cluster was determined from tunneling spectroscopy data obtained from a surface with a low cluster coverage. The energy position of the VBM of the functional surface was determined by UPS for a surface with a high cluster coverage and charge transport characteristics through Hg//[Mo6Br8F6]-terminated organic monolayer (OML)//n-Si junctions were measured using the mercury drop technique. These studies allow the determination of the electronic band diagram at the cluster–organic layer–Si(111) interfaces. Results are discussed and compared with other covalent assemblies of metallic clusters onto Si surfaces differing by the nature of cluster cores, [Re6Sei8]2+ or [Mo6Ii8]4+, and linker functionality, carboxylic acid or pyridine.