Chemistry Reference
In-Depth Information
Fig. 1 Differential reflectance (SDR) spectra of PTCDA on mica during the growth from 0.1 to
2.7 ML. 2.7 ML corresponds to the total final thickness. Note the clear change in the characteristic
spectral shape and the two intersection points at about 2.30 and 2.38 eV. For comparison, the
absorption spectrum of PTCDA dissolved in dimethylsulfoxide is shown in the
bottom
(
dashed
line
, shifted, arbitrary units). All the spectra are referred to the clean mica substrate (From Proehl
et al. [
16
]. Reprinted with permission. Copyright 2004 by the American Physical Society.)
in the range 1.8-3.2 eV, and consequently, the reported
R
/
R
SDR
signal can be
straightforwardly connected to the aggregation of the PTCDA molecules in the
film. At a nominal thickness of 1 molecular layer, the line shape of the
Δ
R
/
R
SDR
curve suddenly changes: a significant alteration occurs in the relative height of the
three peaks (at 2.35, 2.55, and 2.75 eV), the second becoming considerably more
evident and broader. The spectral change is interpreted in terms of a monomer-
dimer transition, occurring just above the completion of the first monolayer, that
completely wets the substrate. In the first layer, only a weak electronic coupling is
present among PTCDA molecules, but when the first layer is completed, the
molecules stack on each other, forming dimers (Fig.
2
).
Differently from mica, highly oriented pyrolitic graphite (HOPG) substrate is
not optically transparent in the range 2-4 eV. It is often chosen for the deposition of
organics: it is planar, easy to prepare (by simple scotch cleaving or exfoliation),
inert enough to avoid complicating reactions with the ambient and with the depos-
ited molecules, and compatible with organic and biological molecules because of
its carbon termination.
In Fig.
3
, the multilayer growth onto graphite of hexa-peri-hexabenzocoronene
(HBC, C
42
H
18
), a disk-like polycyclic aromatic hydrocarbon, has been monitored
by SDR [
17
]. At first glance, the thickness-dependent evolution of the spectra
should suggest that new peaks at certain photon energies are apparently developing.
However, given the non-negligible optical absorption of graphite, before discussing
Δ
