The structure of self-assembled monolayers of long n-alkane molecules depos
ited on an Au(111) surface has been studied by in situ scanning tunnelling
microscopy at 300 K, These monolayers are formed on Au(111) either at the i
nterface between gold and a solution of hexatriacontane (C36H74) in tetrade
cane, or by ultrahigh vacuum (UHV) deposition of octatriacontane (C38H78).
At the liquid/solid interface, the monolayer consists of a close-packed arr
angement of lamellae separated by troughs. Perfect side-to-side packing of
C36H74 molecules, the long axis of which stays parallel to Au [110] directi
ons, forms each lamella, Lamella present two different widths (4.8 +/- 0.1
and 4.5 +/- 0.1 nm) determined by the molecular orientation with respect to
troughs. In contrast, the monolayers obtained after UHV deposition at 300
K show poor organization with meandering lamellae and troughs that present
strong variations in width. Nevertheless, all the C38H78 molecules stay par
allel to the [110] directions of Au(111), Annealing such monolayers at 350
+/- 5 K yields a structure that looks like the one observed at the liquid/s
olid interface.
These results show a directional epitaxy relationship perpendicular to the
[110] directions of Au(111) in a monolayer of C36H74 and C38H78 molecules.
We discuss the origin of the differences observed between the molecule pack
ing at the liquid/solid interface and the one in monolayers built in UHV at
300 K. Copyright (C) 2000 John Wiley & Sons, Ltd.