(153f) The Role of Multifunctional Kinetics During Early-Stage Silicon Hydride Pyrolysis: Reactivity of Si2H2 Isomers with SiH4 and Si2H6

The
four lowest energy Si₂H₂ isomers on the singlet potential
energy surface, i.e., trans-HSiSiH, SiSiH₂, Si(H)SiH, and Si(H₂)Si,
have been suggested as precursor molecules to silicon wafer growth during
silicon hydride pyrolysis.  Pyrolysis
of the feed gas, typically SiH₄ or Si₂H₆, is a
standard protocol to create polycrystalline or amorphous silicon nanoparticles
in the gas phase or controlled growth of silicon wafers at a gas-solid
interface.  Si₂H₂
isomers are the simplest multifunctional silicon hydrides, i.e., these
molecules contain multiple functionality by way of divalent silicon centers
and/or Si-Si bonds with bond orders greater than one.  A detailed understanding of the microkinetics for the gas-phase
nucleation of silicon hydrides will allow for the improvement of applications
in which silicon nanoparticles are desired or side products such as biological
imaging and chemical vapor deposition (CVD), respectively. 

Kinetic
parameters for the dominant pathways during the addition of the four lowest
energy Si₂H₂ isomers to monosilane, SiH₄, and
disilane, Si₂H₆, were calculated using G3//B3LYP,
statistical thermodynamics, conventional and variational transition state
theory, and internal rotation corrections.  These simple multifunctional
silicon hydrides were used as model chemical species to probe the nature of the
reactive center with multifunctionality to determine if multifunctional
kinetics follow the reaction family concept.  Each of the four isomers was
found to be reactive with monosilane and disilane under pyrolytic conditions. 
These relatively unstable and unsaturated silicon hydride species can
potentially contribute to gas-phase silicon nanoparticle nucleation due
to high reactivity.  Si₂H₂ isomers may play a critical
role in the early stages of silicon nanoparticle nucleation, and filling the
gap in knowledge for the reactivity of multifunctional silicon hydrides under
pyrolytic conditions is needed.

Keywords: kinetics, nanoparticles, intermediates, G3//B3LYP,
chemical vapor deposition