The nano-fractal structured tungsten oxides films with high thermal stability prepared by the deposition of size-selected W clusters

Abstract

Size-selected W ⁻ _n 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.