INVESTIGADORES
PATRITO Eduardo Martin
artículos
Título:
Influence of Subsurface Oxidation on the Structure, Stability, and Reactivity of Grafted Si(111) Surfaces
Autor/es:
M. F. JUÁREZ; F. SORIA; E. M. PATRITO; P. PAREDES OLIVERA
Revista:
JOURNAL OF PHYSICAL CHEMISTRY C
Editorial:
AMER CHEMICAL SOC
Referencias:
Lugar: Washington; Año: 2008 vol. 112 p. 14867 - 14877
ISSN:
1932-7447
Resumen:
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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-CH3, Si-CCH, Si-CN, Si-CH2CH3,
Si-OCH3, Si-OH, Si-NH2, Si-NHOH and Si-ONH2.
The highest hydrogen bond strength (7.5 kcal/mol) was observed for the (1x1)
Si-NHOH monolayer. The (1x1) Si-CH2CH3 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, -CH3
and -CH2CH3 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 -CH3 (40.3
kcal/mol) and -CH2CH3 (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, -OCH3 and -NH2 terminated surfaces due to the
increased positive charge of the silicon atom. The -NH2 and -Cl grafted
surfaces are the most reactive with activation energies of 7.9 and 13.4
kcal/mol.