Chemistry Reference
In-Depth Information
do not create any electron DOS in the molecular energy gap. The organic layer is
thus treated as a non-interacting gas and only the molecule-substrate interactions
are taken into account. The CNL is calculated by integrating the induced local
density of states (LDOS) and imposing charge neutrality conditions: the total
number of electrons up to the CNL equals that of the isolated molecule. For
PTCDA/Au the CNL is located 2
1 eV above the centre of the HOMO
level of the molecule. The important outcome of the analysis is that there is a
significant DOS at the PTCDA/Au interface, in spite of the weak interaction
between the two materials. The implication is that the interface
E
F
is close to
the CNL. Thus, the formation of the interface barrier is related to the transfer of
charge across the interface. The charge transfer is associated with the IDIS, and
creates an electrostatic interface dipole, which tends to align the metal
E
F
and the
PTCDA CNL.
These calculations are in line with experiments performed on PTCDA/Ag(111),
where it is shown that the central PTCDA carbon ring is the part of the molecule
most effectively coupled to the substrate via CT from the metal surface to the
LUMO, as evidenced by measurements using the vibrational spectroscopies
in situ
Raman (Wagner, 2001) and HREELS (Eremtchenko
et al.
, 2003). The vibrational
frequencies depend on the bond strength and thus contain direct chemical infor-
mation. EELS is most sensitive to infrared-active modes (dipole-induced) and with
Raman spectroscopy dipole-inactive vibrations as well as in-plane vibrations can be
analyzed. However, intrinsically Raman-active modes of the free molecule become
infrared-active at the surface because of CT. Raman spectra of sub-ML coverages
of PTCDA on Ag(111) show a doublet at 1297 and 1310 cm
−
1
. The 1297 cm
−
1
component is attributed to PTCDAmolecules strongly bonded to the silver surface.
This strong bonding induces a down-shift of the
A
g
-mode originally located at
1310 cm
−
1
for the free molecule, essentially involving the centre of the molecule.
Therefore, this mode shift indicates that the central carbon ring constitutes the
molecular reaction centre. That carbon atoms are mainly involved in the bonding to
the silver substrate for a flat molecule is expected, because of the
.
45
±
0
.
-electronic struc-
ture. At the same time, the local nature of the reaction centre allows the molecular
orientation to adjust to the intermolecular lateral interactions, inducing the stability
of the PTCDA layers.
The comparison of PTCDA with its parent perylene molecule is extremely inter-
esting. For perylene MLs on Ag(111), electron diffraction suggests an orientational
liquid, in which the molecules are positionally ordered in an incommensurate close-
packed superlattice but orientationally disordered and mobile. The same activated
Raman peaks as for PTCDA are observed but they are, however, orders of mag-
nitude weaker, indicating that, while a molecular reaction centre may still exist in
the perylene backbone, its residual activity would be too small for the molecule to
recognize a preferred site.
π
