Sequential clustering reactions of Si+ with SiD4: Identification of a bottleneck preventing rapid growth of hydrogenated silicon particles

Abstract

Sequential clustering of atomic silicon-29 cations with SiD4 is observed at room temperature in the ion cell of a Fourier transform mass spectrometer. The clustering reactions proceed in a highly specific fashion. Si+ grows initially by sequential addition of three –SiD2 units. The measured reaction rates for these three steps are, respectively, 8.1±0.4, 0.36±0.04, and 2.0±0.3×10−10 cm3 molecule−1 s−1. A back reaction which results in loss of the silicon-29 isotopic label also occurs for these three reactions and represents ∼12%–15% of the reaction products, depending on the reaction step. This cluster growth mechanism then encounters a critical bottleneck and ceases. Further aggregation occurs only by slow bimolecular attachment of SiD4 at a rate of 1.0±0.3×10−13 cm3 molecule−1 s−1 at p(SiD4)=2.0×10−7 Torr. The fundamental mechanisms and energetics for the individual reaction pathways have been calculated by Raghavachari using ab initio electronic structure theory and are presented in a companion paper. The clustering mechanism involves insertion of a cluster ion into a SiD bond of SiD4 followed by elimination of D2. Addition of –SiD2 serves to increasingly saturate the bonds of all of the silicon centers which leads to the observed growth limitations at Si4D+6. The accuracy of these calculated potential surfaces is tested using statistical phase space theory. Since both the forward and reverse reaction rates are measured using isotopic labeling, the phase space theory calculations are used to determine both the insertion and the elimination transition state energies for each of the first three clustering steps as well as a lower limit for the well depth of the [Si5D+10] intermediate complex. Overall, good agreement is found between the transition state energies obtained from phase space theory and those determined by Raghavachari. These results indicate that Si+ clustering with SiD4 encounters an early bottleneck which prevents rapid formation of the critical nucleus size required for spontaneous growth of large hydrogenated silicon particulates.