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 (ΓSOL) 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 ΓSOL < 0.1, the acid chain fraction within the ML becomes quasi-independent of the solution composition and may become much larger than ΓSOL. 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.