Interfacial architecture on the fractal support: polycrystalline gold films as support for self-assembling monolayers

Abstract

Multifractal analysis is performed for description of the surface topography of thin polycrystalline gold film. Its structure was modified by annealing at different temperatures in the range 20÷200 ⁰C and films were imaged by Atomic Force Microscopy. Image was analyzed as a collection of layers taken parallel to the mean surface. Fractal subsets with different scaling properties were described by multifractal divergence (e.g. the difference between maximal and minimal values of the f (a) spectrum). This allowed to highlight the effect of the temperature of film annealing on the surface structure. We found that fractal diversity jumps down in the temperature range 130÷140 ⁰C. Therefore, phase transition occurs in the system. Below the temperature of the phase transition the surface topography is characterized by high roughness and existence of small-scale irregularities. At critical temperature the surface structure undergoes morphological transition caused by melting of small-scale irregularities. The melting also results in a decrease of the surface roughness due to the flowing down of gold crystallites. A notable feature of the approach is its ability to highlight a possible influence of substrate structure on the adsorption/self-assembling processes at the interface, which may be disturbed by the surface irregularities. The typical and expressive example taken from the self-assembling on the polycrystalline substrate. Particularly, substrate topography determines an order of thiols layers resulting in peculiarities of chemical functionality of obtained material. It was shown that formation of well-ordered monolayers of ω-substituted alkanethiols on gold films occurs only if the freshly evaporated gold films were annealed at temperature more than ca.120 ⁰C. The analysis of surface peculiarities allows suggesting that this behavior is caused by disappearance of short-scale multifractal structures. Therefore, the multifractal analysis opens a new avenue for both characterization and direct prediction of surface properties. Particularly, it gives a hint regarding a formation of the Euclidean two-imensional structures at the multifractal substrates.