Chemistry Reference
In-Depth Information
360
L
340
S
320
IS
300
C
280
0.0
0.2
0.4
0.6
0.8
1.0
Coverage (ML)
Figure 4.16. Schematic
T
-
phase diagram of decanethiol on Au(111). The dif-
ferent regions and phases are denoted as S (stripes), IS (intermediate structures),
C[
c
(4
ϑ
2)] and L (liquid). The broken lines indicate phase boundaries of the IS,
which are not yet fully established. The solid curve between C and L (melting
transition) exhibits a sharp rise near full coverage. Adapted from Schreiber
et al.
,
1998.
×
When adsorbed on the atomically highly corrugated, anisotropic Cu(110) sur-
face N
2
orders commensurably in a complex seven-sublattice pinwheel structure
with a transformation matrix
41
13
(Zeppenfeld
et al.
, 1997). At this point we
should conclude that at the ML and sub-ML level the registry of even a sim-
ple model molecule such as N
2
on a simple substrate such as graphite exhibits
commensurism of different types and incommensurism depending on the nitrogen
coverage and temperature. This should warn us about the structural complexity that
organic/inorganic interfaces can achieve.
Let us move now to the case of alkanethiol SAMs on gold surfaces. The stability
of these SAMs originates from the high affinity of the thiol groups (-SH, also called
mercapto groups) to gold. Strictly speaking, SAMs are not pure 2D systems due to
the chains extending out-of-plane, and in fact the 3D character is essential for the
molecular packing. Although the phase diagram of thiols on Au(111) is obviously
a very fundamental issue, and several groups have studied it, important questions
remain unresolved. Figure 4.16 shows the schematic phase diagram of decanethiol
on Au(111) in temperature and coverage space.
