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Quantum mechanics calculations of the thermodynamically controlled coverage and structure of alkyl monolayers on Si(111) surfaces.

Nemanick EJ, Solares SD, Goddard WA, Lewis NS

Division of Chemistry and Chemical Engineering, Kavli Nanoscience Institute and Beckman Institute, 127-72, Noyes Laboratory, California Institute of Technology, Pasadena, California 91125, USA.

The heat of formation, Delta E, for silicon (111) surfaces terminated with increasing densities of the alkyl groups CH3- (methyl), C2H5- (ethyl), (CH3)2CH- (isopropyl), (CH3)3C- (tert-butyl), CH3(CH2)5- (hexyl), CH3(CH2)7- (octyl), and C6H5- (phenyl) was calculated using quantum mechanics (QM) methods, with unalkylated sites being H-terminated. The free energy, Delta G, for the formation of both Si-C and Si-H bonds from Si-Cl model compounds was also calculated using QM, with four separate Si-H formation mechanisms proposed, to give overall Delta G(S) values for the formation of alkylated Si(111) surfaces through a two step chlorination/alkylation method. The data are in good agreement with measurements of the packing densities for alkylated surfaces formed through this technique, for Si-H free energies of formation, Delta G(H), corresponding to a reaction mechanism including the elimination of two H atoms and the formation of a C=C double bond in either unreacted alkyl Grignard groups or tetrahydrofuran solvent.

Published 27 July 2006 in J Phys Chem B Condens Matter Mater Surf Interfaces Biophys, 110(30): 14842-8.
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