The nano-fractal structured tungsten oxides films with high thermal stability prepared by the deposition of size-selected W clusters
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Size-selected Wn− clusters (n = 1650) were deposited on the highly ordered pyrolytic graphite surface at room temperature under high vacuum conditions by utilizing a magnetron sputtering source and a magnet sector field. Moreover, geometrical structure and surface chemical states of deposited clusters were analyzed by in situ scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy, respectively. The formation of 2-D islands (lateral size ~150 nm) with multiple dendritic arms was observed by STM, and the structure of the individual W1650 clusters survived within the dendritic arms. To study the thermal stability of the nano-fractal structure under the atmospheric conditions, the sample was brought to the ambient air conditions and sequentially post-annealed at 200, 300, and 500 °C in the air. The nano-fractal structure was maintained after the 1st post-annealing process at 200 °C for 1 h in the air, and the subsequent 2nd post-annealing at 300 °C (for 1 h, in the air) also did not induce any noticeable change in the topological structure of the sample. The topological changes were observed only after the further post-annealing at a higher temperature (at 500 °C, 1 h) in the air. We show high potential use of these nano-structured films of tungsten oxides in ambient conditions.
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