Characterization of alkanethiol self-assembled monolayers on gold by thermal desorption spectroscopy.

SAM formation of undecanethiol (UDT) and mercaptoundecanoic acid (11-MUA) on Au(111) and on gold foils, using wet chemical preparation methods as well as physical vapor deposition (PVD) in UHV, has been studied by means of thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). The main aim of this study was to explore the possible application of TDS to characterize the quality of a SAM and to determine its thermal stability. The influence of various parameters, like substrate pretreatment, film formation method, and type of the functional end group, has been studied in detail. Three different temperature regimes can be identified in TDS, which yields specific information about the organic layer: Desorption of disulfides around 400 K can be shown to result from standing molecules in a well-defined SAM. Desorption of intact molecules and of molecules with split-off sulfur is observed around 500 K, resulting from lying molecules. Finally, desorption of an appreciable amount of gold-containing molecules is observed around 700 K. This is more pronounced for 11-MUA than for UDT, and in addition more pronounced for solution-based SAMs than for PVD prepared SAMs. These results emphasize the important role of gold adatoms in SAM formation, as recently discussed in the literature.

A study on the formation and thermal stability of 11-MUA SAMs on Au(111)/mica and on polycrystalline gold foils.

In this article we present a comprehensive study of 11-mercaptoundecanoic acid self-assembled monolayer (SAM) formation on gold surfaces. The SAMs were prepared in ethanolic solution, utilizing two different substrates: Au(111)/mica and polycrystalline gold foils. Several experimental methods (X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and atomic force microscopy) reveal a well-defined SAM. The main focus of this work, however, was to test the stability of these SAMs by thermal desorption spectroscopy. The spectra show different desorption peaks indicating different adsorption states and/or decomposition products on the surface. The assumed monolayer peak, which can be attributed to desorption of the intact molecule, is detected at 550 K. Further desorption peaks can be found, which result, e.g., from cracking of the S-C bond on the surface, depending on the substrate quality and on the residence time under ambient conditions.

A two-step method to covalently bind biomolecules to group-IV semiconductors: Si(111)/1,2-epoxy-9-decene/esterase.

A versatile two-step method has been developed that allows linking of biomolecules covalently to hydrogen-terminated group-IV semiconductors by means of epoxy-alkenes. First, the terminal C==C double bond of the alkene forms a covalent bond with the silicon, germanium, or diamond surface by UV-mediated hydrosilylation. The terminal oxirane moiety then reacts with the biomolecule. As a model system, we investigated the attachment of an esterase B to a Si(111) surface by means of the linker molecule 1,2-epoxy-9-decene. Samples were characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. The immobilized enzyme retained its activity and exhibited good long-term stability.