Self-assembly of (WO) clusters on a highly oriented pyrolytic graphite surface and nanowire formation: a combined experimental and theoretical study.

Tungsten oxide cluster anions (WO) produced from a magnetron source were soft-landed on highly oriented pyrolytic graphite (HOPG) surfaces at different temperatures, and the structures formed upon deposition were analyzed using ex situ atomic force microscopy. Under liquid nitrogen cooling conditions of 100 K, the clusters form small branched nanostructures extending over a few nanometers with a height not exceeding 1 nm. At 673 K, nanowires approximately 100 nm long and less than 10 nm in diameter are observed. Dedicated atomistic modeling of the early steps of the assembly process was carried out using a polarizable potential parametrized to reproduce electronic structure calculations on small clusters, and accounting for the corrugated interaction with the ideal HOPG surface. Molecular dynamics simulations reproduce the branched nanostructures formed at low temperature fairly well, and confirm the loss of ramification at higher temperatures. Additional simulations of coalescence processes between nanoislands containing a hundred (WO) units produce elongated shapes as observed experimentally, providing a typical size of the individual building blocks. The preferred orientation of nanowires and additional in situ X-ray photoelectron spectra both further suggest that the nanostructures grow along terraces and could bind through chemical processes and possibly tungsten carbide formation.