Quantum Mechanics Calculations of the Thermodynamically Controlled Coverage and Structure of Alkyl Monolayers on Si(111) Surfaces

E. Joseph Nemanick, Santiago D. Solares, William A. Goddard, III,* and Nathan S. Lewis*
Division of Chemistry and Chemical Engineering, Kavli Nanoscience Institute and Beckman Institute, 127-72, Noyes Laboratory, California Institute of Technology, Pasadena, California 91125
J. Phys. Chem. B, 2006, 110 (30), pp 14842–14848
DOI: 10.1021/jp060640+
Publication Date (Web): July 13, 2006
Copyright © 2006 American Chemical Society
*

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

Abstract

The heat of formation, ΔE, 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, ΔG, for the formation of both Si−C and Si−H bonds from Si−Cl model componds was also calculated using QM, with four separate Si−H formation mechanisms proposed, to give overall ΔGS 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, ΔGH, corresponding to a reaction mechanism including the elimination of two H atoms and the formation of a CC double bond in either unreacted alkyl Grignard groups or tetrahydrofuran solvent.

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History

  • Published In Issue August 03, 2006
  • Received January 30, 2006
    Revised April 5, 2006

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