Chemistry of semiconductor clusters: A study of the reactions of size selected Si+<i>n</i> (<i>n</i>=3–24) with C2H4 using selected ion drift tube techniques

Abstract

The chemical reactions of size selected Si+n (n=3–24) clusters with C2H4 have been studied at thermal energies using selected ion drift tube techniques. Except for Si+3 (which dehydrogenates C2H4 to yield Si3C2H+2) the dominant products arise from adsorption of C2H4 onto the silicon clusters. The reaction rates show large variations with cluster size. Si+13 and Si+14 were found to be particularly unreactive. The reactions were studied over a temperature range of 143–298 K. The reactivity increases as the temperature is lowered indicating that variation in the rates observed for the different cluster sizes is not due to an activation barrier, but reflects the stability of the SinC2H+4 adducts. Statistical phase space theory was used to model the reactions of Si+4–Si+10 and provide an estimate of the binding energy of the first C2H4 to these clusters. The binding energies vary between 0.8 and 2.0 eV. Binding energies of this magnitude are too small to be accounted for by strong di-σ bonding, suggesting that the C2H4 molecule is bound to the silicon clusters by a weaker π-bonding interaction. Kinetic evidence for the presence of structural isomers was found for several clusters. With Si+9 the relative abundance of an unreactive isomer could be changed from ∼1% up to 17% by changing the source conditions. More than one C2H4 molecule will adsorb onto the silicon clusters (up to eight adsorb on Si+4). Rate constants for the adsorption of the first few (up to seven) C2H4 molecules on Si+n (n=4–10) have been determined. They show large variations with the number of adsorbed C2H4 molecules.