Chlorine-induced restructuring processes on Cu/Si(111) ``5\ifmmode\times\else\texttimes\fi{}5'' surfaces were studied with Auger electron spectroscopy and scanning tunneling microscopy techniques. The copper Auger signal intensities from these surfaces (each with different chlorine exposures and subsequent 450 \ifmmode^\circ\else\textdegree\fi{}C annealing), decreased with increasing initial chlorine exposures. At a Cl-coverage equal to \ensuremath{\sim}1 ML (after annealing) the Cu signal decreased below the detection limit. The associated restructuring processes were also explored. Initially homogeneous Cu/Si(111) ``5\ifmmode\times\else\texttimes\fi{}5'' surfaces, with Cl-exposures and annealing, separate into coexisting areas of Cu-free Cl/Si(111) 1\ifmmode\times\else\texttimes\fi{}1 and nearly chlorine-free areas of Cu/Si(111) ``5\ifmmode\times\else\texttimes\fi{}5.'' The original copper density is conserved by formation of three-dimensional ${\mathrm{Cu}}_{3}\mathrm{Si}$ crystallites on the surface. The restructuring is completely reversible; upon desorption of all chlorine at \ensuremath{\sim}600 \ifmmode^\circ\else\textdegree\fi{}C the surface resumes its original Cu/Si(111) ``5\ifmmode\times\else\texttimes\fi{}5'' structure. Possible implications for the atomic scale mechanisms in the direct synthesis of methylchlorosilanes from Cu/Si surfaces are discussed.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The adsorption of chlorine and desorption of chlorosilanes from chlorine-covered Si(111) and Cu/Si surfaces have been studied. The latter include annealed "5 x 5" Cu2Si thin films as well as room-temperature deposited copper films, on Si(111). Techniques employed include low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and temperature-programmed desorption (TPD). A Langmuir adsorption mechanism was observed for Cl on the Si(111) 7 x 7 surface, but a mobile precursor mediated process was observed for L adsorption on the Cu/Si surfaces. Chlorine-exposed Si(111) 7 x 7 surfaces yield TPD peaks of SiCl2 at similar to650 degreesC with second-order desorption kinetics. For Cu-containing surfaces, similar TPD peaks were observed at slightly lower temperatures and with different desorption kinetics. The desorption rate includes a dependence on Cl-free sites. The presence of Cu on Si(111) also led to the appearance of two additional low-temperature TPD peaks at 200 degreesC and 300 degreesC, both consisting of SiCl4 and SiCl2 species. We propose that the lower temperature desorption occurs through the formation of an activated SiCl2 precursor on the copper-containing surface.

The chemistry of Cl2 on a Cu/Si(100) (at relatively high near surface concentrations of Cu) surface has been investigated using X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and ion scattering spectroscopy (ISS). XPS and ISS suggested that the deposition of Cu at 120 K on Si(100) resulted in Cu/Si intermixed layers. Heating this surface led to the rapid decrease of Cu in the outermost layer based on ISS, and by 600 K there was no Cu-derived scattering peak even though XPS showed there was a significant amount of Cu in the near surface region. Based on these experimental observations it is postulated that islanding of Cu/Si particles occurred, which is supported by prior studies. Adsorption of Cl2 on the 120 K-Cu deposited Si(100) surface led to some Cu agglomeration, based on XPS. TPD results showed that SiCl4 desorbed from this surface at 530 K and SiCl2 desorbed near 1000 K. The latter peak occurs at the same desorption temperature as SiCl2 for Cl2/Si(100) and is thus attributed to the thermal chemistry of Cl2 on bare Si(100). It is likely, however, that the 530 K desorption feature was a direct result of the weakened surface bonding of Si in Cu/Si islands, compared to Si on bare Si(100).

Photoelectron spectroscopy (PES), thermal programmed desorption (TPD) studies, and scanning tunneling microscopy (STM) investigated the interaction and chemistry of CH3 (generated by the thermal cracking of azomethane) on Si/Cu(100). Si was deposited on Cu(100) by the thermal decomposition of SiH4 at 420 K. STM of adsorbate-free Si/Cu(100) at a less than saturation coverage of Si revealed a surface that contained large domains of a Cu2Si structure. These Cu2Si domains coexisted with regions that were believed to be lower in fractional Si coverage. TPD results showed that (CH3)(3)SiH desorbed near 200 K from CH3/Si/Cu(100) prepared with a low Si concentration. With increasing Si concentration a (CH3)(3)SiH desorption state appeared near 420 K, in addition to the 200 K state. The two observed TPD states of (CH3)(3)SiH at 200 and 420 K were believed to be due to the thermal reaction of CH3 with the low Si density and high Si density (i.e., Cu2Si) regions, respectively. At a saturation coverage of Si, when the well ordered Cu2Si phase covered the surface, only the 420 K peak was present during CH3/Si/Cu(100) TPD. Results also suggested that (CH3)Si and possibly some (CH3)(2)Si intermediates predominated on the surface below room temperature, and (CH3)(3)-Si species were formed on the surface only at temperatures between 250 and 390 K. Surface hydrogen needed for the final evolution of (CH3)3SiH was generated from methyl groups at temperatures above 390 K on the Si-saturated Cu(100).

The Direct Synthesis of methylchlorosilanes from methyl chloride and silicon, catalyzed by copper and minor promoter elements was reviewed with respect to use of ultra-high vacuum (UHV) surface reaction techniques to uncover the mechanism of the reaction. In particular, recent results were presented for sequentially adsorbing methyl radicals and chlorine on polycrystalline Cu3Si alloy under ultra-high vacuum conditions. Methyl monolayers in the absence of chlorine produced primarily trimethylsilane, and chlorine monolayers in the absence of methyl produced SiCl4. However, mixed monolayers of methyl groups with chlorine atoms abandoned these separate pathways and instead reacted at similar temperatures on the surface to produce methylchlorosilanes with selectivities to 85% Me2SiCl2 with Zn, Sn, and Al as promoters.

The direct process for synthesizing methylchlorosilanes from methyl chloride + silicon in the presence of catalytic amounts of copper has been studied in vacuum using a sample of Cu 3 Si alloy, the bulk phase that is present in active regions of the catalytic direct process. From the melt containing 23% of excess silicon, a two-phase Cu 3 Si + Si sample was prepared. Free silicon phase served both to replenish silicon reacted from Cu 3 Si and to provide the grain boundaries found in an industrial process. Atomically clean surfaces of this material with varying Cu/Si atomic ratios were prepared by ion bombardment over a range of temperatures. While the dissociative adsorption of CH 3 Cl was observed to be immeasurably slow on these surfaces under ultrahigh vacuum conditions, methyl + chlorine monolayers generated by the coadsorption of methyl radicals and Cl 2 led to selective formation of dimethyldichlorosilane. Adsorption of methyl groups alone produced trimethylsilane from two different active sites with very different kinetics. Adsorption of chlorine alone produced SiCl 4 .

The synthesis of new fluorinated α,ω-diiodo telomers I(TFE)x(VDF)y(HFP)zI (A) where TFE, VDF and HFP represent tetrafluoro-ethylene, vinylidene fluoride and hexafluoropropene, respectively, was carried out by thermal telomerization of HFP with VDF oligomers, I(TFE)x(VDF)yI, as transfer agents. Compound A was obtained in 55% yield when x = y = 1, whereas almost no reaction occurred when x = 1 and y = 2. Interestingly, HFP reacted with IC2F4CH2CF2 · selectively and not with ICF2CH2CF2CF2 · whereas, from IC4F8CH2CF2I, HFP could be introduced on both end-groups but produced a higher amount of I(TFE)2(VDF)(HFP) I. The reactivity of HFP is compared with that of VDF in reactions with fluorinated α,ω-diiodinated telogens and it is shown that the environment of the terminal C-I bond in the telogen influences the orientation of the reaction.

The Rochow Process refers to the synthesis of liquid methylchlorosilanes from metallurgical silicon powder and gaseous methyl chloride. This chapter discloses a new approach for applying vacuum surface analysis techniques to understand the Rochow Process. Prior attempts to study the molecular details of this reaction in vacuum on polycrystalline Cu3Si have been hampered by the desorption of physisorbed CH3Cl before the temperature can be raised high enough to effect dissociative chemisorption. In the present study, the sluggish C–Cl bond scission step is circumvented by adsorbing methyl radicals and chlorine separately onto cold Cu3Si surfaces. It is found that these methyl + chlorine monolayers are active in forming methylchlorosilanes. In addition, the studies of samples with and without promoters show changes in activity and selectivity that parallel those found over real catalysts, and the results are beginning to show the ways in which these additives influence the catalytic process.

The synthesis of a new fluorinated αω-diiodo telomer I[(TFE)(VDF),]I (A), where TFE and VDF represent tetrafluoroethylene and vinylidene fluoride respectively, has been carried out by telomerization of VDF with αω-diiodoperfluoroalkanes. The thermal telomerization of VDF with 1,2-diiodoperfluoroethane IC2 F4I led to good yields of the monoadduct IC2F4CH2CF2I and of the diadduct which was composed of an almost equimolar ratio of ICF2CH2C2F4CH2CF2I and IC2F4(CH2CF2)2I, together with a small amount of IC2F4CH2CF2CF2CH2I and a triadduct containing I (VDF) C2F4(VDF)2I and IC2F4(VDF)3. This procedure has been applied successfully to αω-diiodo-n-perfluorobutane and α,ω-diiodo-n-perfluorohexane. The reactivity of VDF relative to the fluorinated α,ω-diiodo telogens has been studied from which it is shown that the environment of the terminal group of the telogen is a major factor affecting the reactivity.