Formation of alkanethiol and alkanedithiol monolayers on GaAs(001)

The formation of alkanethiol (H-(CH(2))(n)-SH, n = 8-18) and 1,8-octanedithiol (HS-(CH(2))(8)-SH) monolayer films on n-type GaAs(001) has been systematically Studied. We observed a nonlinear dependence of the film thickness on molecular length, which is drastically different from monolayer films of the same molecules oil metals. For 8 <= n <= 14. the films are only 3-4.5 angstrom thick, significantly smaller than the corresponding molecular length. For n = 16 and 18, the measured film thicknesses were 9 and 11 angstrom, respectively, consistent with molecules orienting with a tilt angle of similar to 60 degrees from the surface normal. Unlike the alkanethiols, the thickness of the 1,8-octanedithiol monolayer is almost the same as its molecular length, indicating that dithiol molecules orient vertically with only one thiol end croup bound to the GaAs surface. Additional support for this conclusion comes front the fact that X-ray photoelectron spectroscopy of the 1,8-octanedithiol monolayer clearly resolves two types of S atoms in the monolayer: those bound to the GaAs surface and those existing its free thiols. A suggestion was made oil the mechanisms for alkanethiol and alkanedithiol monolayer formation.