Chemistry Reference
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Fig. 34 Optical anisotropy
of H
2
TPP films on HOPG.
The RAS spectrum
measured immediately after
H
2
TPP deposition in
vacuum is represented by
the
black line
(
open circles
).
The RAS spectrum acquired
after the exposure of the
porphyrin films to HCl
vapors (di-acid H
4
TPP) is
drawn by the
dash-dotted
line
. The
red line
(
full
squares
) represents the
spectrum measured on the
sample after exposure to
HCl vapors and subsequent
heating at 60
C. The
reference RAS signal of
clean HOPG is the
dashed
black line
(From Bussetti
et al. [
36
])
as the surface morphology is well different in two conditions: in the former, 3D
nanocrystals are spread on the surface, and in the latter, the 2D layer is totally
predominant. We could conclude that the peaked RAS spectrum originated from the
characteristic electronic structure of the true wetting layer alone.
Despite the strong effort in controlling the layer thickness pursued by authors in
Bussetti et al. [
36
], an issue remains to be discussed: is the square lattice imaged by
STM (Fig.
33
) the true first porphyrin monolayer onto graphite? By STM, the
answer could come from a careful and precise measurement of the step height with
respect to the underlying graphite. But (at least on the basis of the published data in
[
36
]) the bare hexagonal graphite has not been measured together with the organic
layer (both lattices apparently are never present in the same published image). In
addition, STM and AFM characterizations have been performed after the samples
(prepared in UHV by OMBE) have been extracted from the growth vacuum
chamber. In principle, one could doubt whether the sample has rearranged its
structure with time during its permanence in atmosphere. To be conclusive, two
further steps are then necessary: (1) to investigate the stability of the sample during
the preparation in vacuum and after its extraction from the chamber, by using a
technique able to monitor a significant property of the sample in both environments
(e.g., RAS), and (2) to demonstrate if the thickness is definitely compatible with a
single organic monolayer.
The application of RAS to extremely thin layers of H
2
TPP in UHV has brought
to convincing conclusions. But it was still necessary to disentangle the optical
properties of the 2D layer from the 3D phase, the latter covering about 20% of the
