Surface Mobility and Nucleation of a Molecular Switch: Tetraaniline on Hematite

Abstract

Understanding the dynamics of organic thin film formation is crucial to quality control in organic electronics and smart coatings. We have studied the nucleation and growth of the reduced and the oxidized states of phenyl-capped aniline tetramer (PCAT) deposited on hematite(1000) surfaces by physical vapor deposition. The fully reduced PCAT molecules form 2D islands on the surface, whereas the fully oxidized molecules form 3D islands. Through scaled island size distribution, it was found that the critical island sizes, i, for the reduced and oxidized molecules are i = 4 and 5, respectively. From the dependence of the island density on substrate temperature, the activation energies for the diffusion of the molecules away from the critical cluster were calculated to be 1.30 and 0.55 eV, respectively. At low temperatures, the reduced and the oxidized PCAT molecules form compact islands on the surface. At higher temperatures, the reduced islands become dendritic, whereas the oxidized islands become slightly dendritic. The attempt frequencies for surface diffusion of the reduced and the oxidized islands were estimated to be about 5 × 1025 and 8 × 1011 s–1, respectively. The former value is in line with the high degree of surface wetting by the reduced PCAT, whereas the latter value shows the higher degree of intermolecular interaction in the fully oxidized PCAT and the low degree of its interaction with the iron oxide surface. Interconversion between oxidized and reduced islands through exposure to a reducing environment, and its impact on island morphology was examined. We also found that the presence of Fe2+ defects on the hematite surface did not impact the nucleation and growth of the molecular islands, likely due to a discrepancy in time scale. This study elucidates the interactions between an oligoaniline-based molecular switch (PCAT) and hematite surfaces as a function of molecular oxidation state, with applications in molecular electronics, chemical sensors, and smart coatings.