Site-specific chemistry of ethylene on Si(114)-(2 x 1)

Si(114)-(2 x 1) is a stable high-index surface with a single domain-like surface reconstruction composed of parallel rows of 3-fold coordinated rebonded atoms (resembling adatoms), dimers, and tetramers (dimers combined with non-dimerized atoms in diene-like structures). The distinct chemical properties expected for the oriented array of different structural motifs make this surface an interesting template for direct, comparative studies of structure-dependent reactions. We have studied the room-temperature adsorption of ethylene on rebonded atoms, dimers, and tetramers on Si(114)-(2 x 1) by atomic-resolution scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations of structure, energetics, and simulated STM images. Ethylene chemisorbs in four distinct geometries on Si(114), all of which are di-sigma-bonded structures, with all adsorption sites occupied in a well-ordered monolayer at saturation coverage. In order of decreasing adsorption probability, the ethylene-Si reaction products were ethylene-bridged rebonded atoms, dimer [2 + 2] adducts, tetramer [4 + 2] adducts on one side of the tetramer, and (at high coverages only) tetramer [2 + 2] adducts on the dimer component of the tetramer. The calculations indicate that adsorption is kinetically limited at room temperature, with one energetically favorable reaction on the tetramer inhibited by an adsorption barrier. A particularly interesting aspect of the Si(114)-(2 x 1) surface demonstrated by this work is that it can act as both a dienophile (dimer sites). and a diene (tetramer sites) in cycloaddition adsorption reactions.