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J. Phys. Chem. C 2007, 111, 7772-7782
Tuning the Exchange Reaction between a Self-assembled Monolayer and Potential Substituents by Electron Irradiation Nirmalya Ballav,† Andrey Shaporenko,† Simone Krakert,‡ Andreas Terfort,‡ and Michael Zharnikov*,† Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, 69120 Heidelberg, Germany, and Anorganische und Angewandte Chemie, UniVersita¨t Hamburg, 20146 Hamburg, Germany ReceiVed: January 4, 2007; In Final Form: March 15, 2007
Self-assembled monolayers (SAMs) can undergo an exchange-reaction with the molecules capable of building a SAM on the same substrate upon the immersion into the respective solution. However, for most systems, the exchange reaction is very slow at normal conditions and occurs to a limited extent. Using the primary SAMs on nonsubstituted alkanethiols (ATs) and several ω-substituted ATs as the potential substituents, we show that the rate and extent of the exchange reaction can be significantly enhanced and precisely tuned by electron irradiation with a small dose. We assume that the irradiation results in the appearance of subtle structural and chemical defects in the target SAM, which promote the molecular exchange. The effect of irradiation and exchange reaction were monitored in detail by contact angle goniometry, infrared reflection absorption spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and X-ray photoelectron spectroscopy. The developed approach, irradiation-promoted exchange reaction, can be considered as a platform for the preparation of mixed SAMs and, in combination with e-beam lithography, for the fabrication of chemical patterns, including gradient ones.
1. Introduction Self-assembled monolayers (SAMs) are well-ordered and densely packed monomolecular organic films, which provide a convenient and flexible means to tailor the interfacial properties of metal, semiconductor, and oxide surfaces.1-3 The most extensively investigated class of SAMs are films of alkanethiols (ATs) on various noble and coinage metal substrates.1-15 In SAMs of ω-functionalized ATs, X-(CH2)n-SH, the molecules are chemically bound to the substrate via their thiolate “headgroups” while the molecular chain (“spacer”) directs away from the substrate. Within such a structure, functionality at the end of the spacer (“tail group“) builds the SAM-ambient interface and defines, thus, the surface properties of the entire system, such as, e.g., wetting, adhesion, lubrication, corrosion, and biocompatibility.1-3 In particular, depending on the chemical nature of X (e.g., polarity), the SAM-ambient interfaces can be either hydrophobic or hydrophilic.16,17 As typical examples, methyl (-CH3) terminated ATs generate hydrophobic surfaces with an advanced water contact angle of 112-115°, whereas hydroxyl (-OH) or carboxyl (-COOH) terminated ATs produce hydrophilic surfaces with the contact angles of 15-25° and
