Low-temperature scanning tunneling microscopy (STM) has been used to image CH3-terminated Si(111) surfaces that were prepared through a chlorination/alkylation procedure. The STM data revealed a well-ordered structure commensurate with the atop sites of an unreconstructed 1 x 1 overlayer on the. silicon (111) surface. Images collected at 4.7 K revealed bright spots, separated by 0.18 +/- 0.01 nm. which are assigned to adjacent H atoms on the same methyl group. The C-H bonds in each methyl group were observed to be rotated by 7 +/- 3degrees away froth the center of an adjacent methyl group and toward. an underlying Si atom. Hence, the predominant interaction that determines the surface structure arises from repulsions between hydrogen atoms oil neighboring methyl groups, and secondary interactions unique to the surface are also evident.
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