Facile charge-displacement at silicon gives spaced-out reaction.

Journal of the American Chemical Society (2011-09-02)
Maryam Ebrahimi, Kai Huang, Xuekun Lu, Iain R McNab, John C Polanyi, Zafar Waqar, Jody Yang, Haiping Lin, Werner A Hofer

Adsorbates on metals, but not previously on semiconductors, have been observed to display long-range repulsive interactions. On metals, due to efficient dissipation, the repulsions are weak, typically on the order of 5 meV at 10 Å. On the 7×7 reconstruction of the Si(111) surface, charge transport through the surface has been demonstrated by others using charge injection by STM tips. Here we show that for both physisorbed brominated molecules, and for chemisorbed Br-atoms, induced charge-transfer in the Si(111)-7×7 surface can lead to a strong repulsive interaction between adsorbates, calculated as 200 meV at 13.4 Å. This large repulsive interaction must be channeled through the surface since it causes widely spaced "one-per-corner-hole" patterns of physisorption (three cases--directly observed here) and subsequent chemisorption (four cases observed). The patterns were observed by ultrahigh vacuum scanning tunneling microscopy for four different brominated hydrocarbon adsorbates; 1,2-dibromoethane, 1-bromopropane, 1-bromopentane, and bromobenzene, deposited individually on the surface. In every case, adsorbates were overwhelmingly more likely to be found singly than multiply adjacent to a corner-hole, constituting a distinctive pattern having a probability p = 7 × 10(-5) compared to a random distribution.

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1,2-Dibromoethane solution, certified reference material, 200 μg/mL in methanol
Bromobenzene, ≥99.5% (GC)
Bromobenzene, ReagentPlus®, 99%
1-Bromopropane, 99%