Chemistry Reference
In-Depth Information
Fig. 33 (a)36nm
36 nm STM image of a H
2
TPP 2D domain on HOPG, showing the
arrangement of molecules in a square unit cell (V
bias
= 0.4 V, I
tunnel
= 50 pA). The center of each
porphyrin molecule in highlighted in
red
. The alignment of the inner H atoms of the molecules
(frozen tautomers along the
direction) composing the domain is evident. The HOPG exfoliation
direction is also indicated. The reported scale bar (
bottom left
) is 7 nm long. (b) STM signal
profiles acquired along the
ʲ
directions, respectively. The vertical and the horizontal
scale
bars
are 50 pm and 1 nm, respectively. Data have been acquired at RT. In the inset:
ʱ
and
ʲ
ʲ
directions of the RAS polarized electric fields, used to maximize the optical anisotropy signal of
the sample (From Bussetti et al. [
36
])
ʱ
and
¼
(
R
ʱ
-
R
ʲ
)/
R
(R being the average value
of the two polarized reflected intensities
R
ʱ
and
R
ʲ
) for linearly polarized electric
fields aligned along the
The RAS spectrum is measured as
Δ
R
/
R
directions reported in Fig.
33
. The curve of the “as-
deposited” H
2
TPP layer, just after sublimation in the OMBE chamber, exhibits a
sharp maximum at about 434 nm, consistent with the Soret band of the porphyrin.
After exposure of the porphyrin layer to HCl vapors (protonation), the formation of
di-acid H
4
TPP molecules is induced as well as the extended and strong modification
of morphology (Fig.
31
, panel c). Also the RAS spectrum is radically changed
(dash-dotted line) after sample protonation, with a well lower intensity and a broad
appearance without any marked peak. This result agrees with the overall optical
isotropy [
80
] of the H
4
TPP film. After a successive heating step (flashing the sample
at about 60
C), the pristine RAS line shape is practically recovered (full squares in
Fig.
34
), apart from a little but measurable red shift of the whole peak. The
similarity of spectra for the “as-deposited” and the “annealed” layer is significant,
ʱ
and
ʲ
