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The preparation and thermal stability of benzenethiol and benzeneselenol self-assembled monolayers (SAMs) grown on Au(111) have been investigated by electrochemical experiments and high-resolution photoemission spectroscopy. Both techniques confirm the formation of monolayers with high packing densities (θ = 0.27-0.29 ML) and good degrees of order in both cases. Despite many similarities between the two SAMs, the thermal desorption is distinctly different: whereas the benzenethiol SAM desorbs in a single steplike process, the desorption of the benzeneselenol SAM occurs with a much lower activation energy and involves the cleavage of some Se-C bonds and a change in molecular configuration from standing up to lying down. This behavior is explained by considering the different nature of the bonding of the headgroup with the metal surface and with the phenyl ring. Density functional theory calculations show that the breakage of the Se-C bond has a lower activation energy barrier than the breakage of the S-C bond.

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We investigated the formation and stability of layers of methylthiolate prepared on the Au(111) surface by the method of immersion in an ethanolic solution of dimethyl disulfide (DMDS). The surface species were characterized by electrochemical reductive desorption and high-resolution photoelectron spectroscopy. Both techniques confirmed the formation of a methylthiolate monolayer at short immersion times (around 1 min). As the immersion time increased, the electrochemical experiments showed the disappearance of the methylthiolate reductive desorption current peak and the appearance of a current peak at ca. -0.9 V which was attributed to sulfur species. At long immersion times, the XPS measurements showed two main components for the S 2p signal: a component at ca. 161 eV which corresponds to atomic sulfur and a component at ca. 162 eV which we attributed to polysulfide species. We propose that the breakage of the S-C bond of methylthiolate is responsible for the appearance of sulfur species on the surface. Density functional theory (DFT) calculations were performed to identify the elementary steps that may lead to the decomposition of methylthiolate. We found that the cleavage of the S-C bond is only activated by the oxidative dehydrogenation of the methyl group of methylthiolate. Thio-oxymethylene, SCH 2 O, is the key intermediate leading to the breakage of the S-C bond because it decomposes into atomic sulfur and formaldehyde with an activation energy barrier of only 1.1 kcal/mol.

We investigated the influence of intermolecular interactions and subsurface oxidation on the structure, surface bonding, and reactivity of compact monolayers of small organic and inorganic molecules bound to the Si(111) surface via Si-C, Si-N, and Si-O bonds. We considered the following modified surfaces: Si-CH 3 , Si-CCH, Si-CN, Si-CH 2 CH 3 , Si-OCH 3 , Si-OH, Si-NH 2 , Si-NHOH, and Si-ONH 2 . The highest hydrogen bond strength (7.5 kcal/mol) was observed for the (1 × 1) Si-NHOH monolayer. The (1 × 1) Si-CH 2 CH 3 monolayer had the highest repulsion at the DFT level, 9.1 kcal/mol. However, inclusion of dispersion interactions yielded a repulsion of only 1.8 kcal/mol. Subsurface oxidation was investigated for -H, -CH 3 , and -CH 2 CH 3 terminated surfaces with surface coverages of 100 and 50%. The oxidation of the third Si-Si backbond is considerably more exothermic than the oxidation of the first and second backbonds. For monolayers with a surface coverage of 50%, the oxidation of alkylated silicon atoms is more stable than the oxidation of hydrogenated silicon atoms. The oxidation of alkylated silicon atoms stabilizes the organic monolayer for two reasons: a decrease of repulsive interactions between adjacent alkyl chains (due to the increase in intermolecular separations) and a strengthening of the Si-C surface bond. The reactivity of the grafted surfaces was investigated in the low coverage limit for the surface hydroxylation reaction with water. The highest activation barriers are obtained for the -CH 3 (40.3 kcal/mol) and -CH 2 CH 3 (40.4 kcal/mol) terminated surfaces. The presence of conjugation in the organic molecule lowers the activation barrier. On the -CCH terminated surface, the activation energy decreases to 29.2 kcal/mol. The nucleophilic attack of silicon by water is facilitated on the -Cl, -OCH 3 , and -NH 2 terminated surfaces due to the increased positive charge of the silicon atom. The -NH 2 and -Cl grafted surfaces are the most reactive with activation energies of 7.9 and 13.4 kcal/mol.

The reactivity of the hydrogenated Si(111) surface toward H2O and O2 was investigated in order to elucidate the mechanism of oxidation of the first silicon bilayer in air. Density functional theory calculations were performed to identify elementary reaction steps and their corresponding activation energy barriers. The perfect surface is unreactive toward H2O and O2 at room temperature as deduced from the high energy barriers found. However, isolated Si dangling bonds, (Si3)Si·, surrounded by SiH groups, readily react with O2 (but not with H2O), producing a silanone intermediate of the form (Si2O)SiO· where one of the silicon back bonds is oxidized. This intermediate is reached after a series of elementary steps with very small activation energy barriers. In the next step, the oxygen atom of the silanone group inserts into a Si–Si back bond, and the surface silicon dangling bond is regenerated as a (SiO2)Si· moiety in which the silyl group has two oxidized back bonds. This initiates a surface chain reaction in which the oxidized silyl group abstracts a hydrogen atom from a neighboring SiH thus producing a new Si dangling bond that is oxidized by O2 in the next step of the chain reaction. This radical propagation mechanism explains the two-dimensional oxide growth in air and the lack of surface SiOH groups. Therefore, the oxidation of the H–Si(111) surface requires the presence of radicals in air that, upon reaction with the hydrogenated surface, produce silicon dangling bonds where the oxidation begins and propagates by hydrogen abstraction from nonoxidized neighboring SiH groups.