Partial Hessian Vibrational Analysis of Organic Molecules Adsorbed on Si(100)

Abstract

A partial Hessian approach is used to study the vibrational spectroscopy of acetylene, ethylene, benzene, 1,3-butadiene, and naphthalene adsorbed on the Si(100) surface. The partial Hessian approximation introduces a very small error in the computed frequencies and intensities of the adsorbed molecules and results in a significant reduction in computational cost. Exploiting this reduced computational cost, molecular adsorbates on large cluster models of the Si(100) surface can be studied with density functional theory. Using clusters with two and four silicon−silicon dimers, the effects of surface coverage and the spectroscopy of large adsorbates that bind to more than one dimer are studied. For benzene and butadiene, molecules adsorbed on neighboring silicon dimers in the same dimer row can have a significant effect on the infrared spectrum in the C−H stretching region, changing both the frequency and intensity of the computed vibrational modes. A weaker effect is observed for molecules adsorbed in adjacent dimer rows. Comparison of the computed spectra with experiment shows that several binding configurations are present for benzene. For 1,3-butadiene, the adsorption product is dominated by the product of a Diels−Alder [4+2] addition. However, the product may have the hydrogens of the sp2 carbons cis or trans with respect to each other. Larger cluster models of the surface also allow large adsorbates that can bind to multiple silicon dimers to be studied. The infrared spectroscopy of naphthalene bound to one, two, and four silicon dimers is computed. Comparison with experiment shows that naphthalene preferentially binds within a dimer row.