Water Exclusion at the Nanometer Scale Provides Long-Term Passivation of Silicon (111) Grafted with Alkyl Monolayers

• Published in: The journal of physical chemistry. B 2006-03, Vol.110 (11), p.5576-5585
• Main Authors: Gorostiza, P, Henry de Villeneuve, C, Sun, Q. Y, Sanz, F, Wallart, X, Boukherroub, R, Allongue, P
• Format: Article
• Language: English
• Subjects: Alkenes - chemistry; Carbon - chemistry; Electrochemistry; Hydrogen - chemistry; Membranes, Artificial; Microscopy, Atomic Force; Nanotechnology; Organosilicon Compounds - chemistry; Silicon - chemistry; Spectrum Analysis; Surface Properties; Water - chemistry; X-Rays
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp054825c

This work is a quantitative study of the conditions required for a long-term passivation of the interface silicon−alkyl monolayers prepared by thermal hydrosilyation of neat 1-alkenes on well-defined H−Si(111) surfaces. We present electrochemical capacitance measurements (C − U) in combination with ex situ atomic force microscopy (AFM) observations and X-ray photoelectron spectroscopy (XPS) measurements. Capacitance measurements as a function of the reaction time and XPS data reveal close correlations between the chemical composition at the interface and its electronic properties. A very low density of states is found if suboxide formation is carefully prevented. The monitoring of C − U plots and AFM imaging upon exposure of the sample in diverse conditions indicate that the initial electronic properties and structure of the interface are long-lasting only when the monolayer surface coverage is θ > 0.42. A model demonstrates that this threshold value corresponds to a monolayer with intermolecular channels narrower than ∼2.82 Å, which is equal to the diameter of a water molecule. Water exclusion from the monolayer promotes long-term passivation of the silicon surface against oxidation in air and water as well as perfect corrosion inhibition in 20% NH4F. We provide two criteria to assess when a sample is optimized:  The first one is an effective dielectric constant