Metal-organic coordinated multilayer film formation: Quantitative analysis of composition and structure

Metal-organic coordinated multilayers are self-assembled thin films fabricated by alternating solution-phase deposition of bifunctional organic molecules and metal ions. The multilayer film composed of α,ω-mercaptoalkanoic acid and Cu (II) has been the focus of fundamental and applied research with its robust reproducibility and seemingly simple hierarchical architecture. However, internal structure and composition have not been unambiguously established. The composition of films up to thirty layers thick was investigated using Rutherford backscattering spectrometry and particle induced X-ray emission. Findings show these films are copper enriched, elucidating a 2:1 ratio for the ion to molecule complexation at the metal-organic interface. Results also reveal that these films have an average layer density similar to literature values established for a self-assembled monolayer, indicating a robust and stable structure. The surface structures of multilayer films have been characterized by contact angle goniometry, ellipsometry, and scanning probe microscopy. A morphological transition is observed as film thickness increases from the first few foundational layers to films containing five or more layers. Surface roughness analysis quantifies this evolution as the film initially increases in roughness before obtaining a lower roughness comparable to the underlying gold substrate. Quantitative analysis of topographical structure and internal composition for metal-organic coordinated multilayers as a function of number of deposited layers has implications for their incorporation in the fields of photonics and nanolithography.