Surface Science 470 (2001) 265±274
www.elsevier.nl/locate/susc
High resolution XPS studies on hexadeca¯uoro-copperphthalocyanine deposited onto Si(1 1 1)7 7 surface
L. Lozzi *, L. Ottaviano, S. Santucci
INFM and Department of Physics, University of L'Aquila, Via Vetoio, I-67010 Coppito, L'Aquila, Italy
Received 10 March 2000; accepted for publication 19 September 2000
Abstract
High resolution X-ray photoelectron spectroscopy measurements have been performed onto ultrathin ®lms of
hexadeca¯uoro copper phthalocyanine deposited, at room temperature and in ultrahigh vacuum conditions, onto clean
Si(1 1 1)7 7 substrate (silicon, Si). The measurements, performed at various ®lm thicknesses, show a strong interaction
between the molecule and the Si substrate. All the core level peaks present strong modi®cations induced by the substrate
interaction. In particular the ¯uorine (F) spectrum clearly presents the e€ect of the interaction of some F atoms of the
molecule with the substrate, which determines the formation of F±Si bonds while the copper spectrum indicates a
charge transfer from the Si substrate. The changes observed in the other core level spectra have been attributed to a
di€erent charge distribution in the molecule, after the formation of F±Si bonds. We suggest a planar growth of these
molecules on the Si substrate starting from the ®rst layer. Ó 2001 Elsevier Science B.V. All rights reserved.
Keywords: X-ray photoelectron spectroscopy; Silicon; Chemisorption
1. Introduction
The metal phthalocyanine (MePc, C32 H16 N8 M) is a planar molecule constituted by four aromatic rings around a porphyrin-like central ring,
having in its centre a metal atom (usually one of
the ®rst transition metals row). They form twodimensional crystals with a columnar stacking
structure, in two di€erent crystalline forms, a and
b. In both cases the crystalline planes are parallel
to each other and the only di€erence is the angle
between the stacking direction and the normal to
*
Corresponding author. Tel.: +39-0862-433097; fax: +390862-433033.
E-mail address: [email protected] (L. Lozzi).
the molecular plane (for example, for copper phthalocyanine, CuPc, c ˆ 26° in the a form, c ˆ 45°
in the b one).
These molecular crystals are mainly used as thin
®lms, with a thickness ranging from 100 to 2000 A.
In this form they show some interesting electrical
properties, like rectifying behaviour [1,2], because
they are semiconductors whose conductivity can
be tuned by doping, for example with oxygen, by
varying the central metallic atom or by changing
the crystalline structure, for example causing a
transition between the di€erent phases or varying
the substrate [3]. An important application of
these molecular ®lms is as bu€er layers in organic
devices. For example it has been shown that introducing a monolayer (ML) of NiPc on other
organic compounds between aluminium and Si
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 0 8 6 6 - 9
266
L. Lozzi et al. / Surface Science 470 (2001) 265±274
electrodes, it is possible to tune the electrical
properties of the device [4]. Or they can be used in
organic light emitting devices (OLED) between the
light emitting ®lm and the transparent anode
(normally indium±tin-oxide, ITO, layer) [5]. In fact
it has been shown that thin Pc ®lms (CuPc or
doped vanadyl-phthalocyanine, VOPc) can improve the electrical properties of the OLED, for
example as bu€er layer in preventing sputter
damage during ITO deposition [5] or improving
the carrier injection eciency [6,7]. Another
promising application is as gas sensors, in particular for NOx and NH3 [3,8].
The electronic structure and the growth mode
of these molecules, and in particular of CuPc
one, and their interaction with substrates have
been widely studied by means of scanning tunnelling microscopy (STM) [9±11], X-ray photoelectron spectroscopy (XPS) [12,13], X-ray absorption
spectroscopy (XAS) [12], ultraviolet photoelectron spectroscopy [6,14], atomic force microscopy
[15], total re¯ection X-ray photoemission spectroscopy [16], transmission electron microscopy
[17], electron energy loss spectroscopy [18], and Xray di€raction [19].
Many studies on the growth mode of these
molecules, and in particular for CuPc, onto Si
surfaces have shown that these molecules grow
with the molecular plane parallel to the surface
[9,11], although XAS data suggest the possibility of a standing up geometry for CuPc molecules onto Si at intermediate ®lm thickness [12],
or for naphthalocyanine, H2 NPc, deposited onto
Si(1 0 0)2 1 [20].
A particular type of MePc is the hexadeca¯uoro-CuPc (CuFPc, C32 F16 N8 -Cu) molecule, in
which F atoms have substituted all the hydrogen
atoms around the benzene rings (Fig. 1). The
presence of these F atoms strongly in¯uences the
electronic properties of the molecule in the bulk
phase [21]. In particular the C 1s spectrum is
completely di€erent with respect to that observed
in bulk CuPc [12], suggesting a strong charge
transfer from the benzene rings towards the F atoms.
The aim of this work is to study the interaction
between the CuFPc molecules and the Si(1 1 1)7 7 substrate. We have studied the core levels of
Fig. 1. CuFPc molecule.
Si, Cu, N and C both for ultrathin ®lms compared
with those observed in bulk sample by using
monochromatic XPS spectroscopy and performing
high quality ®t analysis, in order to determine the
growth mode and the interfacial electronic structure. We will show that a parallel growth of the
®rst CuFPc layer onto Si(1 1 1)7 7 is likely to
occur, followed by other layers with a bulk-like
character.
2. Experimental
The measurements have been carried out in a
system composed by three di€erent ultrahigh
vacuum (UHV) chambers connected by transfer
systems, which allow us to move the samples from
one chamber to the others without breaking the
UHV condition, and with a load-lock system. In
all the chambers the base pressure is about
1±3 10ÿ10 Torr. One chamber is used for studying the electronic properties of the samples and is
equipped with a monochromatic X-ray source (Al,
hm ˆ 1486:6 eV), a double anode X-ray source (Al/
Mg), a ultraviolet source, a hemispherical analyser, a cylindrical mirror analyser (CMA) and a ion
L. Lozzi et al. / Surface Science 470 (2001) 265±274
gun. A second chamber is used for the structural
analysis of the surface samples. It is equipped with
a low energy electron di€raction system, a variable
temperature STM, which is able to perform measurements from 100 to 1300 K, and an electron
beam evaporator, which can evaporate materials
onto the sample while the STM tip is scanning.
The third chamber is used for preparing the samples by thermal or by electron beam evaporation.
In all the chambers the samples can be annealed
either by indirect heating (up to about 1100 K) or,
for semiconductors samples, by direct heating.
The Si substrate was cut from a Si(1 1 1) wafer
n-type P doped (q 1 X cm). The reconstructed
7 7 surface was prepared by repeatedly ¯ash
annealing the sample at 1200°C. After the reconstruction no traces of oxygen and C were detected.
The CuFPc was evaporated, in UHV conditions,
from a directly heated molybdenum boat. During the evaporation the Si substrate was kept at
room temperature in order to prevent the dissociation of the molecules after the deposition. The
267
XPS measurements have been performed using the
monochromatic X-ray source. The experimental
resolution was high enough to resolve the Si 2p
doublet (DE < 0:5 eV).
All the reported spectra have been carefully
analysed using a nonlinear least square ®t procedure. For this ®t analysis a linear combination of
Gaussian functions superimposed to a Shirley
background has been used. We did not try to obtain always the best ®tting results, but our aim was
to extract reasonable data from complex XPS
spectra, in order to have a self-consistent picture of
the interaction between CuFPc molecule and the
Si(1 1 1)7 7 surface. The results of these analyses
are reported in Table 1. The reported binding energies have been calibrated using the Si 2p3=2 peak,
located at 99.15 eV [22], for the thin ®lms, where
the substrate signal was still present. The Si 2p
spectra do not show any variation either in the
energy position or in the lineshape on going from
the thinnest to the thickest ®lms. This is reasonable
if we take into account the high escape depth of
Table 1
Results of the ®tting analysis on the experimental spectra
Bulk
F=Si ˆ 5:2
F=Si ˆ 0:2
E (eV)
FWHM
Int%
E (eV)
FWHM
Int%
Energy
FWHM
Int%
288.89
288
287.31
286.42
285.21
0.88
0.66
0.56
0.72
0.62
9
7
41
20
23
288.91
288.20
287.41
286.51
285.36
284.44
283.93
1.00
0.42
0.71
0.74
0.72
0.97
1.24
8
2
32
19
20
11
8
288.88
288
287.08
286.21
285.25
284.60
283.80
1.30
1.20
1.15
0.88
0.98
1.05
1.12
1
3
30
12
15
23
16
F 1s
SF1
F1
F2
689.12
687.49
1.14
1.02
10
90
689.2
687.55
686.36
1.14
1.09
1.93
6
80
14
688.77
687.27
685.62
0.75
1.48
1.17
1
73
26
Cu 2p
Cu (II)
Cu M
935.79
0.96
100
935.79
933.13
1.2
1.04
95
5
935.34
932.88
1.24
1.82
43
57
N 1s
SN1
N1
N2
N3
400.77
399.34
398.93
0.72
0.68
0.56
6
46
48
400.23
399.4
399.01
398.19
0.95
0.66
0.78
1.23
9
32
44
15
400.05
399.18
398.53
398
1.76
0.85
0.77
1.14
18
23
19
40
C 1s
SC2
SC3
C2
C3
C4
C5
C6
268
L. Lozzi et al. / Surface Science 470 (2001) 265±274
the Si 2p photoelectrons using the Al X-ray pho For the bulk sample, where the Si
tons (k 20 A).
signal was not detectable, the spectra have been
aligned using the Cu 2p3=2 sharp peak of a thinner
®lm. The error of the calculated energies is about
0.05 eV.
3. Results and discussion
In all the following ®gures the amount of the
deposited molecules will be indicated reporting the
ratio between the F 1s and Si 2p XPS intensity
signals, that is I…F†=I…Si† ˆ 5:2 (thin ®lm) and
I…F†=I…Si† ˆ 0:2 (ultrathin ®lm). This is the thinnest ®lm that was possible to study. If we take into
account the di€erent cross-section of the F 1s and
Si 2p core levels for the used photon energy,
hm ˆ 1486:6 eV, ((r…F†=r…Si† 5 [23]), and supposing a planar growth of the molecules, we can
estimate a thickness of about 1.5±2 ML for the
smallest deposition.
The assignment of the features observed in the
bulk spectra for C 1s and for the other elements
has been already discussed in a previous paper [21
and reference therein], after a careful ®t analysis
and a Hartree±Fock calculation, and here we will
report the conclusions only. 1
In Fig. 2 the C 1s spectra for a thick ®lm (bulk,
curve a) and for two thinner ®lms are shown. For
the C 1s bulk spectrum ®ve peaks have been
identi®ed: the most intense peak, named C2, is due
to the C atoms bonded to the F atoms, that is the
outer C atoms in the benzene rings (black circles,
Fig. 1). The C3 peak has been assigned to the C
atoms bonded to the nitrogen atoms (empty circles, Fig. 1) and the C4 feature to the C±C bonded
atoms (cross circles, Fig. 1). The broad feature in
the high binding energy side can be attributed to
the presence of two shake up excitations of the C3
(SC3 ) and C2 (SC2 ) peaks.
1
There is a discrepancy between the binding energy of the
bulk sample reported in Ref. [21] and those reported in this
paper. These one are the right one. In the Ref. [21] there was a
mistake in the reference energy. In fact there is an almost
constant shift of about 0.25±0.35 eV towards high binding
energy values in all the spectra.
For the thin ®lm (curve b, F=Si ˆ 5:2) the bulk
contribution is still clearly present but two other
features are detectable in the low binding energy
side (we will call them as interface states). In fact
the ®t analysis shows the presence of two new
peaks, C5 at 284.44 eV and C6 at 283.93 eV. A
strong decrease of the C2 peak (C±F bonds) with
respect to the C3 and C4 features is evident. This
decrease is followed by the increase of its width
and the enhancement of the SC2 shake up intensity,
determining the bulk ratio between C±F and C±N
or C±C peaks (about 16:8:8), within the statistical
error (few percents).
For the thinnest ®lm (curve c, F=Si ˆ 0:2) the
spectrum is completely changed. Two broad peaks
at about 284.5 and 287 eV are present. Under these
features it is still possible to obtain, by means of
the ®t analysis, the bulk components. In fact, we
can observe the three main bulk peaks and, under the high binding energy tail, the shake up features. However, there are some di€erences between
these bulk contributions and the same components
shown in the bulk spectrum (a). All the di€erent
peaks, C2, C3 and C4, are quite broader than in
the bulk phase. For example the full width at half
maximum (FWHM) of the C2 peak increases from
0.56 eV (bulk) to 1.15 eV, while C4 FWHM grows
from 0.62 eV (bulk) to 0.98 eV. These width
variations compensate the changed height ratios,
so the relative intensities of these bulk-like peaks,
including the related shake up peaks (SC3 and SC2 ),
are as for the bulk sample. The broadening of the
C 1s features was also observed in previously
published works on going from bulk to ultra thin
®lms. It was attributed to a variation of the charge
density on the C atoms after the deposition onto
the Si substrate [12]. This suggests that in the interaction with the substrate there is a sizeable
variation of the electronic charge on the C atoms
with respect to that in the bulk one in the bulk-like
peaks too. We can rule out any silicon carbide
formation, because this interaction should give a
peak at about 282.5±283 eV [24], while in our
spectrum of the thinnest ®lm the lowest binding
energy peak is at about 283.8 eV. The other big
di€erence is a shift towards lower binding energy
values of all the bulk-like peaks. This shift, with
respect to both the bulk spectrum (upper curve)
L. Lozzi et al. / Surface Science 470 (2001) 265±274
Fig. 2. C 1s spectra of (a) the bulk sample and (b,c) two different ®lm thicknesses.
and the thin spectrum (middle curve) is about
0.25±0.35 eV. This shift could be due to the e€ect
of the Si band bending that a€ects the spectrum of
the thinnest ®lm, while it is absent in the case of a
thicker one. Since we observe a movement of all
the peaks towards lower binding energies it should
be ascribed to a band bending in the upper direction, towards the Fermi level.
The two intense new structures determined by
the ®t analysis under the low binding energy broad
peak (the interface states) which are strongly related to the substrate interaction, are located at
about 283.8 and 284.6 eV and their intensities
decrease as the ®lm thickness increases. The shape
of the C 1s spectrum for the thinnest ®lm resembles that observed in CuPc [12]. The spectra weight
moves from high energy (about 287 eV) towards
low binding energy (284.5 eV). This resemblance
can be attributed to a di€erent e€ect of the F atoms on the C ones in the outer part of the aromatic rings. In fact the main di€erence in the bulk
phase between CuPc and CuFPc is in the C 1s
spectrum. Due to the high electronegativity of the
269
F atoms, a strong charge transfer from the 16
outer C atoms and the F ones is observed, determining the presence of the intense C2 peak (Fig. 2)
at high binding energy. In the thinnest ®lm, the
sizeable shift of the spectral weight towards low
binding energies can be attributed to a decreased
charge transfer from C to F atoms, that is the effect of the F atoms on the C ones is lowered, because of the interaction with the Si substrate. The
intensity ratio between the C5 and C6 peaks is
almost constant, on going from thin ®lm (C5=
C6 1:4) to the thinnest one (C5=C6 1:5). It is
interesting to note that also the shake up structures
related to the bulk-like peaks change their intensity as a function of the ®lm thickness. In our ®t
analysis we have not tried to include these shake
up features for the interface states, because of the
diculty to ®nd them clearly in the spectrum. So
we have included the shake up features for the
bulk part only, because they are always clearly
evident. This means that the presence of shake up
excitation related to the interface states (the two
low binding energy peaks) cannot be excluded.
The modi®cation of the shake up intensity, on
going from the bulk to the thinnest sample, is a
clear evidence of the e€ect of the molecule±Si
interaction on the electronic molecular structure. In fact this shake up feature is due to the
highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) excitation following the electron photoemission. The
intensity modi®cation is a ®ngerprint of the different charge density distribution in the molecule
[25±27]. For example, for thick ®lms, the shake up
of the C3 structure in the C 1s peak is about 1.6 eV
for CuFPc, while it is about 1.9 eV for CuPc [12].
Actually, in the molecule±substrate bonding, if an
overlap between adsorbate and substrate states
occurs, a sizeable charge transfer is possible,
modifying the screening properties of the core hole
in the molecule. This modi®cation changes the
relaxation after the core hole formation, determining an adjustment of the shake up spectra
weight and position with respect to the main line
(fully relaxed).
In Fig. 3 the F 1s spectra obtained for the bulk
sample (a) and for two di€erent ®lm thicknesses
(b, F=Si ˆ 5:2) and (c, F=Si ˆ 0:2) are reported.
270
L. Lozzi et al. / Surface Science 470 (2001) 265±274
(FWHM ˆ 2 eV), while it decreases for the (c)
spectrum (FWHM ˆ 1:2 eV). In principle the (b)
spectrum could be ®tted introducing two peaks
(instead of one) in the low binding energy side,
obtaining a good result, too. But the introduction
of two peaks instead of one only cannot be justi®ed, so we have preferred to use one peak only.
The missing of any variation in the Si 2p spectra, although we observe a quite strong change in
the F 1s spectrum attributed to an interaction
between the Si surface and the F atoms, can be
understood if we take into account the escape
the low
depth of the Si 2p electrons (k 20 A),
number of the Si atoms which can interact with the
F atoms in the molecule with respect to the number of the Si atoms on the substrate surface and
the low Si 2p cross-section (r…F†=r…Si† 5 [23]).
In Fig. 4 the Cu 2p3=2 spectra are reported. In
the upper part the bulk spectrum (curve a) shows a
single peak located at 935.8 eV. The Cu 2p spectrum is characteristic of Cu(II), like for copper
dihalides [29]. In this case the peak is attributed to
Fig. 3. F 1s spectra of (a) the sample bulk and (b,c) two different ®lm thicknesses.
The bulk spectrum shows two peaks, the main one
at 687.49 eV and the other at 689.17 eV. The ®rst
one is due to the F±C bonds in the benzene rings,
the other one to a shake up excitation [21]. Decreasing the ®lm thickness a new peak appears in
the low binding energy side. It is clearly evident in
the (c) spectrum. This peak is located at about
686.36 eV for the (b) curve and at about 685.62 eV
for the thinnest ®lm. In this spectrum the main
peak FWHM increases from about 1 eV (bulk) to
about 1.5 eV. As for the C 1s spectra, an overall
shift towards lower binding energy is observed. It
is about 0.3 eV for the main peak, in good agreement with that observed for the C 1s bulk-like
components, while the interface component shift is
about 0.7 eV. The bulk component always shows a
shake up structure, although its intensity changes
on going on from (a) to (c) spectra, as observed in
the C 1s spectra and, in the last spectrum, it appears as a shoulder of its parent line.
The low binding energy peak can be assigned to
a Si±F bond [28]. This peak, obtained by the ®tting
procedure, is quite large for the (b) spectrum
Fig. 4. Cu 2p spectra of (a) the sample bulk and (b,c) two
di€erent ®lm thicknesses.
L. Lozzi et al. / Surface Science 470 (2001) 265±274
the 2p5 3d10 L-1 ®nal state (L-1 indicates that there
is an hole in the ligand valence band). The Cu 2p3=2
spectrum for the thin ®lm …F=Si ˆ 5:2† is reported
in curve (b). In this spectrum a new peak appears
in the low binding energy side, at about 933.1 eV,
that is 2.6 eV far from the bulk-like peak. The
binding energies correspond to the presence of a
2p5 3d10 ®nal state, as for Cu metal [22]. This indicates that the interaction between the ®rst deposited ®lm and the substrate determines the
presence of Cu metal with a 3d10 initial electronic
con®guration. This behaviour has been already
observed for CuPc on Si(1 1 1)7 7 surface [12]
and for NiPc always on Si(1 1 1)7 7 [13]. Decreasing the CuFPc ®lm thickness, the metallic
peak intensity grows (spectrum c, F=Si ˆ 0:2) and
it is higher than the bulk-like peak, because of the
quite high FWHM, which is about 1.8 eV, compared to 1.2 eV of the bulk-like peak. As for the F
1s interface feature (Fig. 3b) we could try to introduce two peaks instead of one, in order to explain the broad interface feature, but there are no
indications to do this, so we have decided to use a
broad, single peak. As observed for the C 1s and F
1s peaks, in the thinnest case a shift of the spectrum is detected. This shift, attributed to a band
bending, is of about 0.3 eV towards lower binding
energy values on going from the (b) spectrum to
the (c) one for the metallic peak and of about 0.5
eV for the bulk-like peak. We have to underline
that the energetic distance between the new peaks
appeared at low binding energy in the thinnest ®lm
spectra and the bulk-like ones for both the Cu 2p
and F 1s data is di€erent. In fact this di€erence is
about 2.6 eV for the Cu 2p3=2 peak and 1.6 eV for
the F 1s ones. This di€erence rules out the possibility that the new peaks are due to a charging
e€ect of the molecule thin ®lm. The broad satellite
features corresponding to the 2p5 3d9 ®nal state,
not reported here and located between 942 and 947
eV, do not show any signi®cant di€erence, on
going from (a) to (c) spectra. The only sizeable
di€erence that can be observed, strongly a€ected
by the noise, is attributable to the presence of two
contributions, as for the main lines.
In Fig. 5 the N 1s spectra for the bulk sample (a) and for the ®lms (b, F=Si ˆ 5:2 and c,
F=Si ˆ 0:2, respectively) are reported. The bulk
271
Fig. 5. N 1s spectra of (a) the sample bulk and (b,c) two different ®lm thicknesses.
spectrum shows two peaks, one at 399.1 eV and
the other one at about 400.8 eV. The ®rst one is the
parent line, due to the eight nitrogen atoms in the
porphyrin-like central ring, while the small peak is
due to a shake up transition. As already reported
the main peak can be explained in terms of two
di€erent peaks (shown in the ®t result), one at
398.93 eV and the other one at 399.34 eV (to which
the shake up excitation has been assigned) [21].
These peaks are due to the two di€erent chemical
states. Four N atoms are bonded to C atoms only,
while the other four N atoms are bonded to two C
atoms and to the central Cu atom (see Fig. 1). The
N1 peak (the high binding energy peak) and its
shake up have been assigned to external N atoms,
while the low binding energy peak (N2) has been
assigned to the internal N atoms, that is the N
atoms bonded to the Cu atom.
Decreasing the ®lm thickness (curve b) the experimental peak broads and a shoulder appears in
the low binding energy side. This peak can be ®tted
introducing the three bulk components (N2, N1
and SN1 ) and an interface peak, at about 398.2 eV.
272
L. Lozzi et al. / Surface Science 470 (2001) 265±274
For the thinnest ®lm the N 1s interface peak is
e€ectively grown. The intensity ratio of the two
bulk components is changed and the shake up
satellite is quite broad and intense. Finally, as
observed for the other elements, a rigid shift of
about 0.2 eV towards lower binding energy values
of all the components and in particular for the
bulk-like is detectable. The observed broadening
of all the N 1s peaks, with respect to the bulk
spectrum, is in agreement with the previous published results on other MePc molecules [12,20], and
should be ascribed to a possible distortion of the
bonds because of the interaction with the substrate.
It is interesting to note that, on going from the
outer part of the molecule (benzene rings) to the
central part (metallic atom) the resemblance between the CuFPc results and those published for
CuPc data both for thick and for thin ®lms increases. Indeed, the qualitative evolution of both
N 1s and Cu 2p is close to that observed in CuPc,
while the variation observed for C 1s shows a
strong di€erence with respect to that obtained in
CuPc. This can be due to the fact that the strong
e€ect of the outer F atoms on the molecule electronic structure and on the interaction between the
molecule and the substrate decrease on going towards the molecule centre.
From XPS data only it is not possible to give a
detailed explanation of the results in terms of
molecule growth geometry. For this analysis XAS
spectroscopy should be used. However some hypothesis can be done.
Both F 1s and Cu 2p spectra allow an accurate
determination of the bulk-to-interface intensity
ratios, because of the presence of well evident interface structures, although the presence of a shake
up component of the interface states in the F
spectrum under the bulk component cannot be
excluded. For F 1s spectra (Fig. 3) of the thinnest
®lm this ratio is about 2.8, while for Cu 2p3=2
spectrum (Fig. 4) it is about 0.8. For the C 1s and
N 1s spectra this determination is much more
dicult, because the interface components are not
well separated from the bulk-like part of the
spectra. The reported ®t results (Figs. 2 and 5) give
a bulk-to interface intensity ratio of about 1.6 for
C and 1.5 for N. In our ®t analysis we could un-
doubtedly include the interface features only. But
we can not exclude the presence of any ``tail'' of
the interface signal under the bulk-like components or of any shake up structure for the interface
part of the spectrum, which has not been included
in our analysis, which would change the bulk-tointerface intensity ratio. Indeed, taking into account these components, we can be con®dent that
even for C 1s and N 1s the real bulk-to-interface
ratio could be close to 1, that is not far from Cu
ratio.
A possible explanation is that the ®rst molecular layer is parallel to the surface. So, for the
thinnest ®lm, we have almost two layers parallel to
Si. In this case we can explain the bulk-to-interface
intensity of the all core level peaks, which are close
to 1 considering the probable interface tails, except
for F, where this ratio is about 3. In particular the
Cu 2p metallic peak is attributed to a charge
transfer from the Si surface or by the formation of
Cu±Si bond [30]. The charge transfer should be
higher than for other MePc on Si, probably because the Si±F bonds are stronger than the weak
Si±C bonds, so the distance between the molecular
plane and the Si surface is lower than for other
MePc/Si interfaces. The presence of two Cu peaks
can be easily explained with this model. The metallic peak is due to the ®rst layer, interacting directly with the Si substrate, while the high energy
peak comes from the second layer, which has a
bulk-like character. The problem of the wrong
interface-to-bulk intensity ratio in the F spectrum
could be overcome if we suppose that not all the F
atoms are bonded to the Si atoms, but only eight
of them interact directly with the substrate. This
partial interaction could be explained taking into
account the dimension of the molecule, about 16 A
wide, so in the Si(1 1 1)7 7 unit cell no more than
two molecules can be hosted. Considering that in
the Si unit cell there are 19 dangling bonds, it is
likely that not all the F atoms are bonded with the
Si atoms. This is also true if there is only one
molecule for each Si cell, because of the quite great
distance between the silicon dangling bonds. In
this model the CuFPc molecule is ¯at on the surface and it is strongly bonded to the Si surface by
the F±Si bonds. There is also some Cu±Si interaction, which determines the presence of the me-
L. Lozzi et al. / Surface Science 470 (2001) 265±274
tallic Cu 2p state. We believe that this is the most
likely growth mode because it can explain all our
experimental data, although our data cannot be
conclusive.
Another possible explanation of the data is a
nonplanar growth of the ®rst molecular layer. For
the thinnest ®lm composed by 1.5±2 molecular
layers, for the F spectrum, if eight F atoms of one
molecule (around two benzene rings) are bonded
to Si and the other eight are bonded to the second
molecular layer we should obtain 24 bulk-like
(eight of the interface layer ‡ 16 of the second
layer) and eight interfaces atoms, that is about 3,
in agreement with the experimental ratio (2.8). But
in this case it is dicult to explain the presence of
two di€erent Cu components.
There are also some other possibilities. One
possibility is that there is not a unique growth
geometry, that is there are at the same time some
molecules of the ®rst layer parallel to the surface
and some others standing up. Another one is that
a lot of molecules lose the central copper atom
only. In this case the Cu metallic peak can be explained as due to the Cu atoms free on the Si
surface and the N 1s spectrum for the thinnest ®lm
could be compared with that observed for H2 NPc
[20]. And, in fact the N3 peak position (the interface peak) is quite in agreement with that of
H2 NPc and also the metallic Cu 2p3=2 peak binding
energy is comparable to that measured for Cu/
Si(1 1 1) interface [30]. So also the standing geometry of the CuFPc molecule could be considered. Up to now we cannot exclude this last
possibility, but we have no way to prove it.
4. Conclusions
In conclusion we have reported high resolution
XPS spectra obtained on ultra thin ®lms of CuFPc
molecules deposited at room temperature onto
Si(1 1 1)7 7 surface. The presence of the F atoms
instead of the hydrogen ones around the benzene
rings determines a stronger interaction with the Si
substrate than that observed for other MePc
molecules, like CuPc.
Our data indicates that there is a sizeable interaction between F and Si atoms and there is a
273
high charge transfer from the substrate and the
inner porphyrin-like ring, in particular toward Cu
atoms. The other atoms of the CuFPc molecule do
not clearly present any bond with the Si substrate.
In fact their spectral changes are attributed to a
variation of the electronic structure in the molecule
because of the charge transfer from F to the benzene rings, following the Si±F bond formation.
We suggest that the CuFPc molecules grow
parallel to the surface, starting from the ®rst layer.
Further measurements are in progress on this interface, in particular by using soft X-ray photoemission, in order to identify the Si adsorption
sites and by XAS spectroscopy to better identify
the adsorption geometry of the ®rst layer.
References
[1] M. Pomerantz, A. Aviram, R.A. McCorkle, L. Li, A.G.
Schrott, Science 255 (1992) 1115.
[2] L. Ottaviano, S. Santucci, S. Di Nardo, L. Lozzi, M.
Passacantando, P. Picozzi, J. Vac. Sci. Technol. A 15
(1997) 1014.
[3] R.A. Collins, K.A. Mohammed, J. Phys. D: Appl. Phys. 21
(1988) 154.
[4] D.-Q. Li, A. Bishop, Y. Gim, X.B. Shi, M.R. Fitzsimmons,
Q.X. Jia, Appl. Phys. Lett. 73 (1998) 2645.
[5] L.S. Hung, C.W. Tang, Appl. Phys. Lett. 74 (1999) 3209.
[6] S.T. Lee, Y.M. Wang, X.Y. Hou, C.W. Tang, Appl. Phys.
Lett. 74 (1999) 670.
[7] J. Blochwitz, M. Pfei€er, T. Fritz, K. Leo, Appl. Phys.
Lett. 73 (1998) 729.
[8] R.J. Ewen, C.L. Honeybourne, J. Phys.: Condens. Matter
3 (1991) S303.
[9] M. Kanai, T. Kawai, K. Motai, X.D. Wang, T. Hashizume, T. Sakura, Surf. Sci. 329 (1995) L619.
[10] P.H. Lipper, R.J. Wilson, M.D. Miller, Ch. W
oll, S.
Chiang, Phys. Rev. Lett. 62 (1989) 171.
[11] F. Rochet, G. Dufour, H. Roulet, N. Motta, A. Sgarlata,
M.N. Piancastelli, M. De Crescenzi, Surf. Sci. 319 (1994)
10.
[12] G. Dufour, C. Poncey, F. Rochet, H. Roulet, M. Sacchi,
M. De Santis, M. De Crescenzi, Surf. Sci. 319 (1994)
251.
[13] L. Ottaviano, S. Di Nardo, L. Lozzi, M. Passacantando, P.
Picozzi, S. Santucci, Surf. Rev. Lett. 4 (1997) 59.
[14] I.G. Hill, A. Kahn, J. Appl. Phys. 86 (1999) 2116.
[15] M. Nakamura, H. Tokumoto, Surf. Sci. 398 (1998) 143.
[16] J. Kawai, S. Kawato, K. Hayashi, T. Horiuchi, K.
Matsushige, Y. Kitajima, Appl. Phys. Lett. 67 (1995) 3889.
[17] A.J. Dann, H. Hoshi, Y. Maruyama, J. Appl. Phys. 67
(1990) 1371.
274
L. Lozzi et al. / Surface Science 470 (2001) 265±274
[18] M. Scrocco, C. Ercolani, A.M. Paletti, J. Electron Spectrosc. Relat. Phenom. 63 (1993) 155.
[19] L. Ottaviano, L. Lozzi, A.R. Phani, A. Ciattoni, S.
Santucci, S. Di Nardo, Appl. Surf. Sci. 136 (1998) 81.
[20] L. Ottaviano, L. Lozzi, A. Montefusco, S. Santucci, Surf.
Sci. 443 (1999) 227.
[21] L. Ottaviano, L. Lozzi, F. Ramondo, P. Picozzi, S.
Santucci, J. Electron Spectrosc. Relat. Phenom. 105
(1999) 145.
[22] J.F. Moulder, W.F.Stickle, P.E. Sbol, K.D. Bomben, in: J.
Chastain (Ed.), Handbook of X-ray Photoelectron
Spectroscopy, Perkin Elmer, Eden Prairie, MN, USA,
1992.
[23] J.J. Yeh, I. Lindau, Atomic Data Nucl. Tab. 32 (1985) 1.
[24] F. Rochet, G. Dufour, P. Prieto, F. Sirotti, F.C. Stedile,
Phys. Rev. B 57 (1998) 6738.
[25] S. Carniato, G. Dufour, F. Rochet, H. Roulet, P. Chaquin,
G. Giessner-Prettre, J. Electron Spectrosc. Relat. Phenom.
67 (1994) 189.
[26] T.T.P. Cheung, J. Appl. Phys. 55 (1984) 1388.
[27] H. Tillborg, A. Nillson, N. M
artensson, J. Electron
Spectrosc. Relat. Phenom. 62 (1993) 73.
[28] D. Rieger, F.J. Himpsel, U.O. Karlsson, F.R. McFeely,
J.F. Morar, J.A. Yarmo€, Phys. Rev. B 34 (1986) 7295.
[29] G. van der Laan, C. Westra, C. Haas, G.A. Sawatzky,
Phys. Rev. B 23 (1981) 4369.
[30] F. Ringeisen, J. Derrien, E. Daugy, J.M. Layet, P.
Mathiez, F. Salvan, J. Vac. Sci. Technol. B 1 (1983) 546.