Chemistry Reference
In-Depth Information
4
Interfaces
I call our world Flatland, not because we call it so, but to make its nature
clearer to you, my happy readers, who are privileged to live in Space.
Edwin A. Abbott,
Flatland
This chapter is devoted to interfaces involving small organic molecules. We start
with the ideal 0D case of a single molecule on an inorganic surface. The simple
theoretical arguments from R. Hoffmann based on MOs will be discussed and
complementedwith experimentsmaking use of STMs, demonstrating the incredible
capabilities of this technique, which permits not only the chemical identification
of such isolated molecules through the determination of their vibrational spectrum
but the possibility of directly imaging the MOs in real-space. The 0D case will be
followed by examples of the formation of small 2D aggregates, allowing molecule-
molecule lateral interactions. Finally, the 2D case where molecules form ordered
compact layers on top of the substrate surfaces within the ML regime will be
analysed.
A ML can be simply defined as a one-molecule thick 2D film, but the molecular
surface density has to be defined for each molecule-substrate system because it
depends on the shape, size and relative orientation of the molecules. To clarify this
point let us consider the examples of PTCDA and C
60
on the Ag(111) surface. The
surface density of the substrate is 1
10
15
atoms cm
−
2
, which is usually defined
as 1 ML as a reference limit. The surface density of the (102) plane of PTCDA,
the cleavage plane, is 8
.
×
4
10
13
and 8
10
13
cm
−
2
(molecules cm
−
2
) for the
.
4
×
.
3
×
monoclinic
polymorphs, respectively. Therefore, full coverage corresponds
to 0.02 ML according to this definition. On the other hand, the surface density of
a full hexagonal layer of closed-packed C
60
molecules corresponding to the (111)
plane in the fcc-C
60
crystal is 1
α
and
β
10
14
cm
−
2
. Thus, C
60
would fully cover the
Ag(111) surface at a coverage of 0.09 ML. However, other authors define 1 ML as
.
2
×
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