Download self-assembled monolayers lecture

• The Au-thiolate bond is strong - homolytic bond strength 44 kcal/mol
- and contributes to the stability of the SAMs together with the van
der Waals forces between adjacent methylene groups, which
amount to 1.4-1.8 kcal/mol. The latter forces add up to significant
strength for alkyl chains of 10-20 methylenes and play an important
role in aligning the alkyl chains parallel to each other in a nearly alltrans
configuration. At low temperatures, typically 100 K, the order is
nearly perfect, but even at room temperature there are only few
gauche defects, concentrated to the outermost alkyl units.
One convenient method of checking a SAM for well-ordered and
dense structure is infrared reflection-absorption spectroscopy
(IRAS). The CH stretching vibrations of the alkyl chain are very
sensitive to packing density and to the presence of gauche defects,
which makes them ideally suited as probes to determine SAM
quality. In particular, the antisymmetric CH2 stretching vibration (d-)
at ~2918 cm-1 is a useful indicator; its position varies from 2916 or
2917 cm-1 for SAMs of exceptional quality or cooled below room
temperature, via 2918 cm-1 which is the normal value for a highquality
SAM, to ~2926 cm-1 which is indicative of a heavily
disordered, "spaghetti-like" SAM. A typical IRAS spectrum of the CH
stretching region of a hexadecanethiolate (HS(CH2)15CH3 ) SAM is
shown in the following Figure...