Formation of vinylidene and vinylene by selective reactivity of Si(100)2 x 1 towards iso, cis and trans isomers of dichloroethylene

摘要

The room temperature (RT) chemisorption of three (wo, cis and trans) isomers of dichloroethylene (DCE) on Si(l 00)2 x 1 have been investigated by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Unlike ethylene, the lack of molecular desorption features in the TPD data effectively rules out the cycloaddition adsorption mechanism for all three isomers. XPS spectra show that cis- and trans-DCE adsorb dissociatively on the 2 x 1 surface in equal proportion as mono-σ bonded 2-chlorovinyl and di-σ bonded vinylene adspecies, which could be produced by dechlorination mechanisms involving the proposed tri-atom π-complex and diradical intermediates, respectively. Acetylene (m/z 26) evolution from 2-chlorovinyl adspecies at 590 K and vinylene at 750 K are also observed for both cis- and trans-DCE, further confirming the common adsorption mechanisms for these geometrical isomers and the relative stabilities of the adspecies. In contrast, only vinylidene adspecies is found for iso-DCE, which indicates that the high ionicity of the =CCl_2 group favours the diradical dechlorination mechanism. The single m/z 26 desorption peak for iso-DCE adspecies observed at a higher temperature (780 K) than cis and trans isomers is consistent with the higher adsorption energy of vinylidene than vinylene on Si(100) obtained in our ab initio calculations. The different relative locations of the Cl atoms in these isomers therefore play a crucial role in controlling the adsorption and thermal evolution on Si(100)2 x 1. The selective reactivity of the 2 x 1 surface towards these isomers can be used to generate vinylene or vinylidene templates from their corresponding adspecies.