Reactive etching of positive and negative silicon cluster ions by nitrogen dioxide

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Abstract
Positively and negatively charged silicon cluster ions, Si+1–8 and Si−1–6, react exothermically with NO2. The predominant reaction is loss of a single silicon atom in the form of SiO. This reaction repeats sequentially such that the clusters are ‘‘etched’’ down to monatomic silicon ions. Charge transfer to form NO−2 also occurs for Si−1–4. Previous results have shown that all Si−1–6 exhibit electron transfer to WF6. Together, these observations bracket the electron affinities of the neutral silicon clusters: EA (Si2–4)≤2.6 eV and 2.6 eV≤EA (Si5–6)≤3.7 eV. The reaction rates for the silicon cluster cations decrease slowly from 7.3±0.8×10−10 to 0.4±0.2×10−10 cm3 molecule−1 s−1 with increasing cluster size for Si+1–8. Silicon cluster anion reaction rates show a slight decrease from 15±1×10−10 to 5.0±0.1×10−10 cm3 molecule−1 s−1 with increasing cluster size for Si−1–6. The ionic silicon cluster reactivity is explained by a radical–radical coupling mechanism involving the unpaired electron on NO2 and an unpaired electron on a silicon atom at a charged or trivalent center in the clusters. Involvement of a radical electron on the silicon cluster contrasts with ionic silicon cluster reactivity with CH3SiH3 which requires divalent, silylene reactive centers on the clusters. Previously identified ‘‘magic number’’ clusters, Si+4 and Si+6, do not exhibit atypical reaction rates or products in spite of a theoretically derived 1–2 eV lower reaction exothermicity relative to the other cluster sizes.