Self-Assembly of Organic Monolayers onto Hydrogen-Terminated Silicon: 1-Alkynes Are Better Than 1-Alkenes

• Published in: Langmuir Vol. 26; no. 13; pp. 10924 - 10929
• Main Authors: Scheres, Luc, Giesbers, Marcel, Zuilhof, Han
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 06.07.2010
• Subjects: alkenes; alkyl monolayers; Chemistry; covalently attached monolayers; Exact sciences and technology; extremely mild attachment; General and physical chemistry; Interfaces: Adsorption, Reactions, Films, Forces; metal-semiconductor diodes; molecular electronics; si surface; Solid-liquid interface; Surface physical chemistry; thermal-reactions; visible-light; Water; Self assembly; Monolayer; Acetylenic compound; Hydrogen; Stabilization; Activation; X ray; Hydrophobicity; Alkene; Energy; Photoelectron spectrometry; Silicon; Total reflection; Chemical reactivity; Structure; Hydrocarbon; Contact angle; Double bond; Infrared spectrometry; Preparation; Ethylenic compound; Room temperature; Alkyne; Electrons
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la100858q

Recently, a new method for the preparation of high-quality organic monolayers with 1-alkynes at room temperature in the dark (i.e., without any external activation) was reported. To pinpoint the precise origin of this self-assembly process and to compare the reactivity of 1-alkenes and 1-alkynes toward hydrogen-terminated Si(111) [H−Si(111)], we followed the gradual formation of both monolayers at room temperature by static water contact angle measurements. Subsequently, attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS) were used to obtain detailed information about the structure and quality of the resulting monolayers. Our data clearly demonstrate that 1-alkynes are considerably more reactive toward H−Si(111) than 1-alkenes. 1-Alkynes are able to self-assemble into densely packed hydrophobic monolayers without any external activation (i.e., at room temperature under ambient light and even in the dark) whereas for 1-alkenes under the same conditions hardly any reactivity toward H−Si(111) was observed. The self-assembly of 1-alkynes on H−Si(111) at room temperature is explained by three factors: the higher nucleophilicity of 1-alkynes, which results in a facile attack at the electron−hole pairs at the H−Si surface and easy Si−C bond formation, the stabilization of the β radical by delocalization over the double bond, and the lower-energy barrier encountered for H abstractions.