Probing the Structure and Orientation of Carboxylic Acid-Terminated Self-Assembled Monolayers

Area of Science:

• Surface Science and Nanotechnology
• Materials Chemistry
• Physical Chemistry

Background:

• Self-assembled monolayers (SAMs) are crucial functional coatings and interfaces.
• Alkanethiols (AT) are common SAM-forming molecules.
• The structure of carboxyl-terminated alkanethiol SAMs is often assumed to mirror non-terminated alkanethiols.

Purpose of the Study:

• To analyze the structure of the outermost layer and near-surface region of carboxyl-terminated alkanethiol SAMs.
• To investigate the arrangement and alignment of hydrocarbon chains in these SAMs.
• To determine if the structure differs from non-terminated alkanethiol SAMs.

Main Methods:

• X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS (AR-XPS) for chemical composition and film thickness.
• Metastable induced photoelectron spectroscopy (MIES) for outermost layer analysis.
• Density functional theory (DFT) calculations for detailed structural insights.

Main Results:

• MIES and DFT revealed the topmost layer primarily consists of the SAM backbone.
• AR-XPS depth profiles indicated a tendency towards gauche orientation in the hydrocarbon chains.
• This gauche orientation facilitates interactions between the functional groups.

Conclusions:

• Carboxyl-terminated alkanethiol SAMs do not necessarily form structures identical to non-terminated alkanethiols.
• The SAMs favor a gauche conformation, exposing functional groups for interaction.
• This finding has implications for designing functional interfaces and coatings.