An STM study of the localized atomic reaction of 1,2-and 1,4-dibromobenzene at Si(111)-7 x 7

A comparative Study is reported of the thermal reaction of 1,2- and 1,4-dibromobenzene (1,2- and 1,4-diBrPh) on Si(111)-7 x 7, investigated by STM. Some results are given for the intermediate case of 1,3-diBrPh. The STM images eave evidence of a different pattern of reaction to yield pairs of Br-Si for 1,2-, 1,3- and 1,4-diBrPh. The ratio of pairs of Br-Si to single bromination events was 1:2 for 1,2-diBrPh and 1:3 for 1,4-diBrPh. In many cases organic residue from the bromination reaction, R-Si, was evident in the STM image. The products R-Si and Br Si were found to be bound to adjacent Si, for both 1,2- and 1,4-diBrPh. The mean Br. - -Br pair separation at the Surface depended on the parent molecule. beine 7.6Angstrom for 1.2-diBrh, 10.3Angstrom for 1,3-diBrPh, and 11.3Angstrom for 1,4-diBrPh. These separations are, in each case, about 4 A greater than the separation of the Br-atoms in the intact parent molecule, which increases systematically down the series. There was a marked decrease in the percentage of R-Si accompanying the Br-Si in going down the series. decreasing from 70% for 1,2- to 20% for 1,4-diBrPh; this was interpreted as being due to a decrease in the percentage of 'benzene-mediated' reaction dynamics, in which the benzene ring was bound to the Surface. At model-ately increased surface temperature (45degreesC) the reaction of 1,2- and also 1,4-diBrPh no longer resulted in R-Si formation. sugLyesting that the dynamics had altered from benzene-mediated to 'bromine-mediated'. (C) 2004 Elsevier B.V. All rights reserved.