Influence of Subsurface Oxidation on the Structure, Stability, and Reactivity of Grafted Si(111) Surfaces

Abstract

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 role of the organic layer functionalization in the formation of silicon/organic layer/metal junctions with coinage metals

“…In a previous work, we obtained a Si-C bond dissociation energy of 75.1 kcal mol À1 for an ethyl monolayer on Si(111). 29 However, metal atoms greatly catalyze the Si-C bond breakage during the metal insertion reaction (3). Considering a propyl layer with a 50% coverage as a prototype for long chain monolayers, we obtained an energy barrier of 21.6 kcal mol À1 for the reaction in which an Au atom in an adjacent H3 site breaks the Si-C bond to yield the Si-Au-CH 2 CH 2 CH 3 moiety.…”

“…These values are around three times smaller than the Si-C bond energy of 75.1 kcal mol À1 . 29 Therefore, if gold atoms reach the silicon/monolayer interface, they will insert into Si-H bonds (barrier of 6.2 kcal mol À1 , Table 3) as well as into the Si-C bonds if the surface temperature increases during the deposition process. We recall that when thermally vaporized atoms condense on a surface, they release a large amount of energy which is roughly the negative of the heat of sublimation of the metal.…”

“…All these values are much lower than the Si-C bond strength of 75.1 kcal mol À1 . 29 Therefore, we attribute the wide dispersion in the experimental data concerning the integrity of monolayers during the deposition process to the fact that the barriers for the breakage of the Si-C bond are much lower than the bond energy. In this respect, copper is the worst option.…”

“…We used a 0 = 5.48 A ˚for the lattice constant as determined in a previous work. 29 The positions of all the adsorbate atoms as well as those of the four topmost Si layers were fully optimized. The silicon atoms of the lower bilayer were kept fixed in a bulk configuration.…”

First-principles study of silicon bulk and nanowire (111) surfaces terminated with trihydrides: Symmetric, rotated, and tilted

“…The Si͑111͒ :CH 3 surface with a 30°rotation of the CH 3 radical is an energetically favorable structure, which is consistent with other DFT results. 22 Next, we studied the ͗110͘ SiNWs with their facets terminated with monohydride, dihydride, and trihydride. The structures studied here include ͑a͒ a ͗110͘ wire with four monohydride-terminated ͑111͒ facets ͓Fig.…”