Mechanism of Stoichiometrically Governed Titanium Oxide Brownian Tree Formation on Stepped Au(111)

Previously observed formation of sub-stoichiometric titanium oxide dendritic structures across terraces of Au(111) are computationally studied and shown to follow the classical fractal formation mechanism of diffusion limited aggregation (DLA). Globally optimized gas-phase oxide cluster structures were sampled in a variety of landing formations on gold surfaces and shown to favor isomers driving polymerization to Brownian tree fractal structures. Mobility of Ti3O5 monomers is shown to be extremely high, with diffusion barriers of 0.21 eV or less. Through bonding stabilization, polymerization of these monomers is energetically favorable and irreversible on the 111 terrace, but geometrically impossible to propagate along the step edge. Simulated STM images show strong similarity to experiment. By contrast, observation of Ti3O6 aggregating as wires along step edges is explained by affinity of oxygen to step edges and statistical arguments for aggregation entropy at the step, in addition to low barriers for monomer diffusion and polymerization.