Structure of Self-Assembled Monolayers of Partially Fluorinated Alkanethiols on GaAs(001) Substrates

A series of self-assembled monolayers (SAMs) of partially fluorinated alkanethiols (PFAT) with variable length of the fluorocarbon segment, viz. F(CF2)(n)(CH2)(11)SH (FnH11SH, n = 6, 8, and 10) were prepared on GaAs(001) and characterized by several complementary spectroscopic techniques. The SAMs were found to be well ordered and densely packed, and thus able to protect the substrate from oxidation, with the highest quality F10H11SH monolayer being most effective in this regard. The packing density of the SAMs was governed by the bulky fluorocarbon segments having a helical conformation. With decreasing length of these segments, a slight decrease in the packing density accompanied by progressive deterioration of the orientational order and a slight disturbance of the conformational order in the fluorocarbon part of the films occurred. In contrast, the hydrocarbon segments of the FnH11SH SAMs exhibited similar average orientation in all studied monolayers, accompanied by a partial conformational disorder. This was explained by the effect of a strong bending potential favoring a substrate-S-C angle of similar to 104 degrees and predefining, thus, the orientation of the hydrocarbon segments.