Gold Clusters and Colloids in Alumina Nanotubes

The fabrication of a supported and insulated quantum wire would be of great interest, especially if electronic information could be accessed to determine charging and conductivity profiles. The feasibility of forming one‐dimensional configurations of ≈ 15 nm gold colloids and 1.4 nm gold clusters via template methods of synthesis has now been demonstrated. The template host material consisted of porous alumina membranes formed by an electrochemical anodic process. The pores of the membrane, and hence the parallel pore channels, were packed in a hexagonal array. Alumina membranes are excellent template materials because of their high degree of order, thermal and chemical stability, and optical clarity. Pore diameter was controlled by regulation of the applied anodic potential (ca. 1.4 nm V ‐1 ). The pore channels were filled by one of three methods: vacuum induction (colloids only), electrophoresis (clusters only), or immersion (clusters, which were then converted into colloids by heating). Rudimentary wires consisting of colloids and clusters were successfully formed. In both cases, the diameter of the pore channel exceeded that of the clusters or colloids. The wires thus formed conformed to the pore channel by forming helical secondary structures. It was not possible to form contiguous wires of clusters by immersion, or of colloids formed from clusters after heating. Composites (consisting of the gold‐alumina system) were a bright scarlet color with an absorption maximum (λ max ) at 519.5 nm. This is an unexpected result for spherical and small‐diameter (10 nm) gold colloids, which normally absorb at λ max 525–530 nm, a ruby‐red color. Possible causes of this small but remarkable blue shift are discussed below. A new Au 55 cluster ligand system consisting of a silsesquioxane‐derivatized thiol is also described.