Scanning tunneling microscopy investigation of spherosiloxane- and alkylsilane-based monolayers

Ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) investigations of octahydridosilsesquioxane (H₈Si₈O₁₂) monolayer formation on Si(100)-2×1, Si(100)-7×7, and Au(111)-23×√3 and octylsilane (C₈H₁₃SiH₃) monolayer formation on Au(111)-23×√3 are presented. Supporting data, including results from X-ray photoemission spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIRS), and density functional theory (DFT), are included.; Cluster reaction on Si(100)-2×1 occurs via Si-H bond activation and results in attachment via the “monovertex attachment” model leaving the cluster cage intact. Four key features in the STM data are: (1) the attached H₈Si₈O₁₂ cluster is comprised of four features arranged in a square, (2) the square spans the Si(100)-2×1 dimer vacancy trench and overlaps two dimer rows, (3) the square edges are oriented parallel and perpendicular to the dimer row, and (4) there exists a pronounced asymmetry in the pairs of cluster features oriented parallel to the dimer rows. Conversely, H₈Si₈O₁₂ clusters decompose upon reaction with Si(111)-7×7. A radical-based reaction mechanism is proposed for cluster reaction with the two silicon substrates.; Adsorbate layer formation occurs upon reaction of H₈Si ₈O₁₂ with Au(111) 23×√3. The 23×√3 surface reconstruction is preserved upon chemisorption of H₈Si ₈O₁₂ clusters. Approximately 10–15% H₈Si ₈O₁₂ clusters desorb from the Au/H₇Si₈O ₁₂ adsorbate layer following evacuation of excess H₈Si ₈O₁₂ cluster pressure from the UHV chamber. At saturation coverage the clusters are preferentially bound to, and predominantly desorb from, face-centered cubic (fcc) sites on Au(111) 23×√3 creating a pattern of holes in the adsorbate layer that provide open binding sites for impinging clusters.; Octylsilane monolayer formation relaxes the Au(111) 23×√3 surface reconstruction and forms mobile Au adatoms and islands. Island diffusion ceases upon impact with terrace edges. Remaining Au islands are trapped within the chemisorbed monolayer at saturation coverage. Octylsilane monolayer formation closely resembles monolayer self-assembly dynamics simulated on an elastic substrate with isotropic surface stress, suggesting the monolayer pattern may result from a self-assembly process involving surface phase separation, size selection, and spatial ordering. STM data indicate the monolayer pattern results from surface Au-Si interactions and suggest the alkyl chains impart a negligible contrast contribution to the monolayer features. Current-image tunneling spectroscopy (CITS) data indicate uniform monolayer coverage. Molecular sticking dynamics simulations successfully generate patterns similar to the octylsilane monolayer and suggest octylsilane chemisorption occurs atop Au atoms. Room temperature monolayer oxidation via ambient atmosphere exposure produces a physisorbed alkylsiloxane adlayer and spontaneously regenerates the Au(111) 23×√3 surface reconstruction. The physisorbed alkylsiloxane adlayer prohibits further substrate reactions and is “transparent” to typical STM imaging conditions. Scanning tunneling spectroscopy (STS) can detect both the alkylsilane and alkylsiloxane monolayers.