Room-temperature chemisorption and thermal evolution of perchloroethylene and trichloroethylene on Si(1 1 1)7 × 7: Formation of chlorinated vinylene and vinylidene and acetylide adspecies, and thermal etching reactions

Abstract

The prominent surface species and their corresponding bonding structures of perchloroethylene (PCE, C 2Cl 4) and trichloroethylene (TCE, C 2HCl 3) upon room-temperature adsorption and thermal excitation on Si(1 1 1)7 × 7 have been examined by vibrational electron energy loss spectroscopy (EELS) and thermal desorption spectrometry (TDS). The presence of the Si–Cl stretch at 510 cm −1 shows that both PCE and TCE undergo dechlorination on Si(1 1 1)7 × 7, while the presence of the C C stretch near 1500 cm −1 indicates that the C C bond remains intact upon adsorption. The mono-σ bonded trichlorovinyl ( Cl C ˙ CCl 2 ) and 1,2-dichlorovinyl ( Cl C ˙ CHCl ) adspecies have been proposed as plausible adsorption intermediates for PCE and TCE, respectively. Furthermore, the trichlorovinyl adspecies is found to further dechlorinate at 350 K, and becomes carbon clusters and/or SiC after complete dechlorination above 450 K. On the other hand, further dechlorination of the dichlorovinyl adspecies above 350 K is found to produce a di-σ bonded chlorovinylene adspecies that is stable up to 450 K. As supported by the presence of the C C stretch at 3300 cm −1, an additional adspecies in the form of acetylide ( C ˙ CH ) is also observed. Above 580 K, hydrocarbon fragments and/or SiC are evident, likely due to dehydrogenation of the chlorovinylene and acetylide adspecies. In addition to these novel adspecies, our TDS data reveal desorption of SiCl 2 over 800–950 K and HCl over 700–900 K for both PCE and TCE, which supports thermal etching and recombinative dehydrochlorination reactions at a higher temperature. The TDS data also provide evidence for weak molecular adsorption, likely related to dative bonding as a result of inductive effects.