AVS 51st International Symposium
    Surface Science Wednesday Sessions
       Session SS1-WeM

Paper SS1-WeM7
STM/AFM Study of WO@sub 3@ Cluster Growth on Highly-Oriented Pyrolitic Graphite

Wednesday, November 17, 2004, 10:20 am, Room 210B

Session: Metal Oxides and Clusters I: Formation and Structure
Presenter: O. Bondarchuk, University of Texas at Austin
Authors: O. Bondarchuk, University of Texas at Austin
Z. Dohnalek, Pacific Northwest National Laboratory
B.D. Kay, Pacific Northwest National Laboratory
J. Kim, Pacific Northwest National Laboratory
J.M. White, University of Texas at Austin
Correspondent: Click to Email
With the ultimate goal of understanding the catalytically activity of WO@sub 3@ clusters supported on oxide surfaces, we have investigated WO@sub 3@ deposition on highly-oriented pyrolitic graphite (HOPG) using in-situ scanning probes. Our efforts were aimed at understanding how the WO@sub 3@ cluster size and morphology depend on deposited amount and substrate temperature. In this work we have studied tungsten oxide films grown on HOPG substrate at temperatures ranging from 300 to 650 K and having thickness from submonolayer up to ~1 µm. STM images of WO@sub 3@clusters formed from submonolayer WO@sub 3@ deposition at room temperature show fractal-like shape. This shape suggests that a diffusion-limited aggregation mechanism governs the growth. Scanning tunneling spectroscopy indicates that the WO@sub 3@ clusters are semiconducting in nature. Tip induced effect leading to complete removal of WO@sub 3@ clusters from HOPG are observed upon extended scanning. In contrast, non-contact AFM imaging does not does not perturb the WO@sub 3@clusters. Possible mechanisms of the tip induced removal of WO@sub 3@ on HOPG will be discussed. Deposition of a submonolayer amount of WO@sub 3@ at ~ 600 K leads to the formation of WO@sub 3@ needle like crystallites of relatively uniform width (~15-20 nm) and length (~100-300 nm). The crystallites are mostly attached to the steps indicating that steps are the preferred nucleation centers. The needle-like growth of WO@sub 3@ crystallites persists even for thicker deposits (~ 1 µm). Ex-situ X-ray diffraction (XRD) analysis performed on thick films suggests that the WO@sub 3@ crystals have cubic symmetry. @FootnoteText@ Pacific Northwest National Laboratory is operated for the Department of Energy by Battelle under Contract DE-AC06-76RLO 1830.