Structure of Mixed Acid/Decyl Monolayers Grafted on Oxide-Free Si(111) Surfaces

The structure of mixed acid/decyl monolayers (MLs) grafted on oxide-free Si(111) surfaces by photochemical hydrosilylation in a mixture of neat undecylenic acid and 1-decene is studied in detail. After appropriate surface cleaning of the as-grafted surfaces, atomic force microscopy (AFM) (topography and phase imaging) and calibrated FTIR analysis demonstrate that a mixed monolayer is formed, free of residue. When the acid-molecule fraction (Γ_SOL) is >0.1, mixed MLs are homogeneous on the scale of observations and they are only slightly enriched in acid chains with respect to the solution. Conversely, when Γ_SOL < 0.1, the acid chain fraction within the ML becomes quasi-independent of the solution composition and may become much larger than Γ_SOL. In addition, dark domains are observed in AFM phase images. Correlations between the characteristic parameters of νCO IR bands and AFM phase images suggest a strong phase separation of acid and alkyl chains at various length scales. A model involving a structuration of the grafting solution is proposed to explain observations.