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ONIOM Modeling of Si(100) Surface Structure and Reactivity
AdvisorCasey, Sean M
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Reactions of ammonia and ethylene with Si<sub>71</sub>H<sub>46</sub> and Si<sub>270</sub>H<sub>122</sub> clusters that modelthe Si(100) surface were examined to understand the advantages and disadvantages ofusing larger clusters and the ONIOM method for these types of modeling studies. All thecomputational studies were performed by using the ONIOM method as implemented inthe Gaussian suite of program. In the ONIOM method, the Si<sub>71</sub>H<sub>46</sub> and Si<sub>270</sub>H<sub>122</sub> clusterswere divided into a high-level part calculated at the B3LYP level of theory with 6-31G*basis sets and a low-level part calculated using the semiempirical AM1 method.The Si<sub>71</sub>H<sub>46</sub> and Si<sub>270</sub>H<sub>122</sub> clusters allow for the study of dissociative adsorption ofammonia and [2+2] cycloaddition of ethylene on neighboring surface dimers, both acrossthe trench between dimers rows and along the dimer row, besides allowing for the studyof the intradimer reactions. Two different high and low level selections were used tostudy the reactions of ammonia and ethylene on these neighboring dimers.The relevant states in the intradimer and interdimer ammonia and ethylenereactions for both the Si<sub>71</sub>H<sub>46</sub> and Si<sub>270</sub>H<sub>122</sub> silicon clusters mostly completed using theONIOM method. Also, two buckled dimer arrangements were studied on the Si<sub>71</sub>H<sub>46</sub>cluster: the p(2x2) and c(4x2) reconstructions. The energetics for physisorption anddissociative adsorption of ammonia for both the p(2x2) and c(4x2) arrangements for theintradimer reaction and for the across the trench interdimer reaction were found to bevery similar, although in both cases the reaction on the c(4x2) reconstruction was slightlyfavored. For the ethylene adsorption studies, the along-the-row interdimer cycloadditionstate had an ONIOM energy that was in better agreement with the experimental data thanthe intradimer cycloaddition value.