Chemistry Reference
In-Depth Information
could contribute to the measured optical anisotropy. The RAS peak is clearly
asymmetric with a maximum at about 440 nm.
After exposing the 1-nm-thick film to HCl to dissolve the 3D structures [
36
],
then thermally treating the film at 60
C to recover tautomerism and optical anisot-
ropy [
36
,
81
,
82
], the RAS response and AFM surface morphology of the film
change. In panel b, after HCl exposure (i.e., after protonation switches off tautom-
erism), the RAS spectrum is almost zero, while AFM shows that 3D crystals have
been nearly dissolved. In panel c, after a proper thermal treatment, the optical
anisotropy is recovered (but the line shape is now more symmetric), and only few
3D structures are still visible. This demonstrates that by applying these two steps
(HCl exposure then annealing) to an extremely thin porphyrin layer, one should be
able to isolate the pure 2D layer and then the corresponding RAS anisotropy.
The results of the RAS and AFM characterization of a 0.05 ML sample are
reported in Fig.
36
. In panel a, the surface morphology only shows a 2D flat layer,
substantiating the conclusion that the corresponding RAS spectrum can be consid-
ered characteristic for such a phase. In the RAS spectrum, centered on the Soret
band of the molecule, a single symmetric peak at 432 nm, plus a small negative
contribution at higher wavelength (reminiscent of a similar line shape in Fig.
30
[
69
]) is dominant. An interpretation and modeling of the layer optical anisotropy, as
measured by RAS, is then possible within the three-layer model [
37
], in terms of the
porphyrin layer dielectric function and of properly chosen Lorentz oscillators,
respectively. In the limit of a small layer thickness (a condition that is certainly
satisfied in this case), the
R
/
R
RAS
signal can be expressed in terms of the dielectric
function of the anisotropic layer (the porphyrin 2D layer):
Δ
Δ
R
=
R
RAS
¼
ð
8
ˀ
d
=ʻ
Þ
ð
A
ʔε
2
B
ʔε
1
Þ:
ð
7
Þ
ε
2
]
ʱ
ε
2
]
ʲ
and
ε
1
]
ʱ
ε
1
]
ʲ
are the anisotropy of the imag-
Here,
Δε
2
¼
[
[
Δε
1
¼
[
[
inary and real parts of the layer along the
ʱ
and
ʲ
directions, respectively. In Eq. (
7
),
d
is the film thickness and
). A and
B express the optical properties of the isotropic graphite substrate and are known
from literature [
13
,
47
]. The single porphyrin molecule has been then modeled in
order to reproduce the experimental
ʻ
is the wavelength of the impinging light (
d
ʻ
Δε
2
, by using two Lorentz oscillators
to represent the two main optical transitions (at two different photon energies),
which are preferentially excited by perpendicularly oriented linearly polarized
light. The best fit result is reported in terms of
Δε
1
and
Δε
1
and
Δε
2
in Fig.
36
, showing a
very good agreement with the experimental data.
For a pure 2D porphyrin layer, this spectral analysis reaches two definitive
results: (1) the RAS signal line shape is due to optical transitions placed along
orthogonal directions and found at different wavelengths, and (2) on the basis of
ab
initio
calculation, the two distinct optical transitions describing the optical proper-
ties of a 2D porphyrin layer are present in a single H
2
TPP molecule, polarized along
orthogonal directions (the one at higher wavelength polarized along the fixed H-H
tautomer direction) [
83
].
