Diaryl and dialkyl chalcogenide molecules
serve as convenient
precursors to silicon–chalcogenide bonds, Si–E–R
groups, on silicon surfaces, where E = S, Se, and Te. The 254 nm light,
coupled with gentle heating to melt and liquefy the chalcogenide precursors
for 15 min, enables formation of the resulting silicon–chalcogenide
bonds. R groups analyzed comprise a long alkyl chain, octadecyl, and
a phenyl group. Quantification of substitution levels of the silicon-hydride
on the starting Si(111)–H surface by an organochalcogen
was determined by XPS, using the chalcogenide linker atom as the atomic
label, where average substitution levels of ∼15% were found
for all Si–E–Ph groups. These measured substitution
levels were found to agree with 2-dimensional stochastic simulations
assuming kinetically irreversible silicon–chalcogen bond formation.
Due to the small bond angle about the chalcogen atom, the phenyl rings
in the case of Si–E–Ph effectively block otherwise
reactive Si–H bonds, leading to the observed lower substitution
levels. The linear aliphatic dialkyl disulfide version, Si–S–<i>n</i>-octadecyl, is less limited by steric blocking of surface
Si–H groups as is the case with a phenyl group and has a much
higher substitution level of ∼29%. The series, Si–S–Ph,
Si–Se–Ph, and Si–Te–Ph,
was prepared to determine the effect of chalcogenide substitution
on the electronics of the silicon, including surface dipoles and work
function. The electronics did not change significantly from the starting
Si–H surface, which may be due to the low level of
substitution that is believed to be caused by steric blocking by the
phenyl groups, as well as the relatively similar electronegativities
of these elements relative to silicon
