Scanning tunneling microscopy is used to determine the bonding geometry of the spherosiloxane cluster, H(8)Si(8)O(12) , on Si(100)-2 x 1. The images obtained are consistent with monovertex bonding to the Si(100)-2 x 1 surface via activation of a single Si-H bond. Filled and empty state images show good agreement with calculations of the electron density distribution of the cluster as well as the Psi(2) highest occupied molecular orbital and lowest unoccupied molecular orbital surface plots of the cluster.
Scanning tunneling microscopy (STM) data are presented in conjunction with reflection-absorption infrared spectroscopy and X-ray photoemission spectroscopy data investigating the adsorbate layer formation of H8Si8O12 clusters on a clean Au(111) 23× 3 surface. All three experimental techniques independently support desorption of approximately 10-15% of the H8Si8O12 clusters from the Au/H7Si8O12 adsorbate layer surface following evacuation of excess H8Si8O12 cluster pressure from the ultrahigh vacuum reaction chamber. Surprisingly, the STM data indicate that unlike all other reported molecular adsorbates having a strong chemical interaction with the Au(111) surface, the 23× 3 surface reconstruction is preserved following chemisorption of H 8Si8O12 clusters. Additionally, at saturation coverage the clusters are preferentially bound to, and predominantly desorb from, specific sites on the Au(111) 23× 3 reconstructed surface. The preferential cluster adsorption/desorption behavior creates a unique pattern of holes and channels in the Au/H7Si8O12 adsorbate layer surface.
Ultrahigh vacuum scanning tunneling microscopy data investigating octylsilane (C8H17SiH3) monolayer pattern formation on Au(111) are presented. The irregular monolayer pattern exhibits a 60 A length scale. Formation of the octylsilane monolayer relaxes the Au(111) 23 x square root3 surface reconstruction and ejects surface Au atoms. Au adatom diffusion epitaxially extends the Au(111) crystal lattice via step edge growth and island formation. The chemisorbed monolayer covers the entire Au surface at saturation exposure. Theoretical and experimental data suggest the presence of two octylsilane molecular adsorption phases: an atop site yielding a pentacoordinate Si atom and a surface vacancy site yielding a tetracoordinate Si atom. Theoretical simulations investigating two-phase monolayer self-assembly dynamics on a solid surface suggest pattern formation results from strain-induced spinodal decomposition of the two adsorption phases. Collectively, the theoretical and experimental data indicate octylsilane monolayer pattern formation is a result of interfacial Au-Si interactions and the alkyl chains play a negligible role in the monolayer pattern formation mechanism.
The oxidation of alkylsilane monolayers on Au has been studied by X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, contact-angle measurements, and scanning tunneling microscopy. Exposure of the monolayers at 298 K to pure O(2) or H(2)O (>5 x 10(-5) Torr and >150 000 L) does not cause oxidation. Ambient atmosphere only causes oxidation if direct sight lines are maintained to the sample. Ozone exposure results in rapid monolayer oxidation. Oxidation initially occurs only at the Si atom, resulting in formation of a cross-linked siloxane monolayer that retains alkyl surface termination. Prolonged ozone exposures result in the oxidation and subsequent loss of the alkyl chain.
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