Cr₂O₃ layer inhibits agglomeration of phosphine-protected Au₉ clusters on TiO₂ films

The properties of semiconductor surfaces can be modified by the deposition of metal clusters consisting of a few atoms. The properties of metal clusters and of cluster-modified surfaces depend on the number of atoms forming the clusters. Deposition of clusters with a monodisperse size distribution thus allows tailoring of the surface properties for technical applications. However, it is a challenge to retain the size of the clusters after their deposition due to the tendency of the clusters to agglomerate. The agglomeration can be inhibited by covering the metal cluster modified surface with a thin metal oxide overlayer. In the present work, phosphine-protected Au clusters, Au₉(PPh₃)₈(NO₃)₃, were deposited onto RF-sputter deposited TiO₂ films and subsequently covered with a Cr₂O₃ film only a few monolayers thick. The samples were then heated to 200 °C to remove the phosphine ligands, which is a lower temperature than that required to remove thiolate ligands from Au clusters. It was found that the Cr₂O₃ covering layer inhibited cluster agglomeration at an Au cluster coverage of 0.6% of a monolayer. When no protecting Cr₂O₃ layer was present, the clusters were found to agglomerate to a large degree on the TiO₂ surface.