ARTICLE
pubs.acs.org/JPCC
Structure and Electron States of Co-phthalocyanine Interacting
With the Cu(111) Surface
E. Annese,*,† J. Fujii,† I. Vobornik,† and G. Rossi†,‡
†
‡
TASC Laboratory, IOM-CNR, SS 14, km 163.5, I-34149 Trieste, Italy
Dipartimento di Fisica, Universita di Modena e Reggio Emilia, via Campi 213/A, I-41100 Modena, Italy
ABSTRACT: We studied the growth and the electronic properties of Cophthalocyanine (CoPc) molecular layers on Cu(111). Scanning tunnelling
microscopy shows that the CoPc molecules adsorb first at the Cu(111) step
edges, lying “flat”, i.e., parallel to the substrate. At monolayer coverage, the
molecules distribute uniformly on the Cu(111) surface with short-range
ordering. Angle-dependent N 1s X-ray absorption spectroscopy (XAS) confirms the overall flat orientation. C 1s, N 1s, core-level photoemission, and Co
L3 edge XAS indicate selective participation of these atoms to the interface
bonds. Angle-resolved photoemission, for submonolayer coverage, shows that
the Cu(111) surface states undergo a shift to lower binding energies. This
binding energy shift and the increase in the effective mass indicate charge
redistribution at the CoPc/Cu(111) interface. At monolayer saturation, the Cu(111) surface state is quenched, and new molecular
states appear at binding energies of 0.9 eV.
’ INTRODUCTION
The control of organic nanostructures may lead to tuning of
the electronic, optical, catalytic, and magnetic properties of
interfaces suitable for nanoelectronics and sensor applications.
The choice of the molecules and the substrates are primary
parameters. Metal phthalocyanines (MPc) have been widely
used as pigments and dyes, as an active element in chemical
sensors, in optoelectronic devices, in solar cells, and in field-effect
transistors.1 MPcs with different metal atoms do have specific
behavior when interfaced with metal substrates displaying peculiar characteristics that have prompted several studies aiming at
understanding the electronic properties of the MPc single layer
or multilayer.2,3 Particular interest is raised by the control of the
magnetic properties of MPc. The magnetic anisotropy has been
studied for molecules (FePc and CuPc) on metallic substrates.4,5
Cobalt phthalocyanine (CoPc) is a typical example of a paramagnetic molecule where the metal ion interacts with its
surrounding giving rise to different electronic and magnetic configurations depending on temperature.6 It is therefore a prototype system in view of a possible implementation of molecular
magnetic junctions.
The interplay of moleculemolecule and moleculesubstrate
interactions determines the structure and growth kinetics of MPc
adsorbed layers. The metallic substrates favor flat lying adsorption geometry79 of MPcs at least in the contact layer. In the
present work, we focus the attention on the growth mode of
CoPc on the Cu(111) surface. CoPc magnetic and electronic
properties are strictly related to the molecular adsorption site10,11
and moleculesubstrate distance.3,12 We monitored the growth
of CoPc on Cu(111) by scanning tunnelling microscopy (STM),
UV angle-resolved and X-ray photoemission spectroscopies
r 2011 American Chemical Society
(ARPES and XPS, respectively), and X-ray absorption spectroscopy (XAS). Our STM data demonstrate that at the early stage of
growth the molecules decorate the step edges of the substrate
surface, while at one monolayer the complete 2D film shows
short-range order. The adsorption of the molecule is planar (flat
lying) as observed both by STM and by polarization-dependent
N K edge XAS spectra (“search light” effect). Upon adsorption
on Cu(111), CoPc modifies its electronic structure as indicated
by a core level binding energy shift of C 1s, N 1s, and Co L3 and N
K edges XAS. The charge redistribution at the interface induces a
continuous shift of the L-gap Cu(111) surface state to lower
binding energy.
Our results confirm the important role of the central host
atom in the interaction with the substrate at the contact layer.
Moreover, CoPc/Cu(111) coupling is stronger than molecule
molecule interaction up to 2.4 ML.
’ EXPERIMENTAL DETAILS
The Cu(111) substrate was prepared in situ by several cycles
of Ar ion sputtering and annealing. The thickness was estimated
by a quartz microbalance. CoPc was deposited on atomically
clean Cu(111) maintained at room temperature (RT) by molecular beam effusion from a resistively heated quartz crucible.
In situ synthesis and measurements were performed at the
APE beamline of IOM-CNR at the ELETTRA Synchrotron
Radiation Facility (Trieste, Italy), delivering both linearly and
circularly polarized light13 at UV and soft X-ray energies.
Received: April 6, 2011
Revised:
June 30, 2011
Published: August 16, 2011
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Figure 1. STM images of CoPc on the Cu(111) surface at different CoPc coverages: (a, b) submonolayer; (c) monolayer. STM image reveals molecules
pinned at the step edges at low coverages (panels a, b). Size: (a) 15 20 nm2 and (b) 3.7 8.6 nm2. To visualize the image (a), we have used a
perspective mode, which stretches the z axis. After the decoration of step edges, the formation of a uniform layer occurs at deposition corresponding to 1
ML, as shown in panel (c). Some of the CoPc molecules arrange along the principal crystallographic direction of the Cu(111) substrate (e) and others in
the rectangular lattice (d), but no long-range order is observed at room temperature. This is also demonstrated by the Fourier transform (f) of the STM
image reported in panel (c), where the ring is indicative of the molecule neighborhood and the different intensity throughout the ring is the signature of
some short-range ordering.
X-ray absorption spectroscopy (XAS) measurements were
obtained by measuring the sample drain current with an energy
resolution of 150 meV. We used linearly polarized radiation
oriented in the storage ring plane. XAS experiments were
performed in the following geometries (see Figure 2a): photon
beam impinges on the sample surface at different angles (θ = 0°,
15°, 30°, 45°, 60°) with respect to its normal and with an electric
vector in the plane containing the light direction and the surface
normal (horizontal polarization, HP). In this geometry, the
electric vector is oriented from parallel to almost perpendicular
to the sample surface, exploiting the “search light” effect.
The ARPES spectra were measured at RT using an SES2002
electron energy analyzer. We used linearly polarized light with
the polarization vector in the scattering plane containing the
sample surface normal and the analyzer entrance slit.
Scanning tunnelling microscopy (STM) images were acquired
with a homemade STM apparatus, operating in UHV with
atomic resolution in the connected suite of UHV chambers at
the APE Beamline.13 STM measurements were acquired in
constant current mode. The sample bias voltage is referred with
respect to the tip. STM data analysis and representation is made
using WSxM software.14
’ RESULTS AND DISCUSSION
CoPc Growth on Cu(111): STM and N 1s XAS Results. Our
first aim is to address the geometrical arrangement of CoPc
molecules on Cu(111). To this end, we have investigated
different stages of the CoPc growth by STM and by angledependent XAS.
Figure 1 shows high-resolution STM images of CoPc molecules deposited at RT on Cu(111) for submonolayer (a,b) and
monolayer (1 ML, panel (ce)). For very low coverages, the
molecules are exclusively observed at step edges. The profile of
STM images gave a Cu(111) step height of ∼2.50 Å that
corresponds to a step with one atom. The molecule at the step
is almost flat (see Figure 1a, where a perspective mode is used to
visualize the image). From the analysis of STM images, we
estimated a tilt angle of less than 9° between the main molecular
and the substrate surface.
The increase of the coverage results in increased step-edge
occupancy, until every step is fully decorated. The preferential
occupancy of step sites was also observed in the case of CuPc
adsorbed on Au(111),15 but this is not a general behavior for
CoPc (a counterexample is reported for co-evaporated CoPc
with Co-porphyrine on Au(111)7). A two-dimensional network
is observed at 1 ML coverage, only after the completion of the
step-edge decoration.
The arrangement of the molecules shows a clear influence of
the underlying substrate structure. Some of the molecules appear
to be oriented along the principal crystallographic directions of
the substrate (see Figure 1e). The local arrangements of the
molecules are highlighted in the panels (d,e) where it is possible
to distinguish a rectangular lattice (d) of size 1.34 1.4 nm2 as
well as a hexagonal lattice (e) of dimension 1.48 nm. These two
lattices are found in all images on different sample areas. The
enlarged images (d,e) of panel (c) show submolecular resolution.
CoPc molecules are recognized as a four-lobed pattern (related
to the four aromatic rings) and a brighter spot in the center
(attributed to Co states within the HOMOLUMO gap),
consistent with previous results on CoPc adsorbed on different
metallic substrates.7,8
A Fourier transform of the STM image for 1 ML (Figure 1f)
shows a clear ring consistent with the presence of a well-defined
correlation length (related to moleculemolecule distance).
Within the ring, spots are observed which identify regions of
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Figure 2. (a) Experimental geometry of XAS experiment. (b) Sketch of CoPc molecule. (c) Representative N 1s XAS spectra measured for CoPc films
of two thicknesses: 0.7 and 2.4 ML taken at θ = 45°(θ angle between the incident photon and the sample surface normal). (d) Representative N 1s XAS
spectra measured for CoPc film of thickness 0.7 ML deposited on Cu(111) taken at different incident angles θ with respect to the sample surface normal.
(e) Angular dependence of the intensity of the main π* resonances of CoPc at the N K edge. The expected intensity profile for flat lying molecules is also
indicated by a dashed line.
short-range order. The absence of sharp spots in Figure 1f and in
the low electron diffraction pattern shows that the local order
does not extend to long range.
XAS probes the unoccupied molecular orbital through dipoleselected core excitations. Figure 2c shows representative N 1s
XAS spectra (in the energy region from 394 to 399 eV) recorded
for CoPc 0.7 and 2.4 ML thick film grown on Cu(111). The
spectra are normalized to the intensity at energy greater than 406
eV. Despite the similar line shape of the N K edge spectra, we
noted the fine difference in the features A and B (i.e., the LUMO
component for the two different sites of N atoms). The energy
position and the intensity of the threshold are specific for N sites:
N1 (aza-bridge) is predicted to have a lower energy and higher
intensity than N2 (Fe-bound atom) according to a DFT calculation of N 1s XAS spectra for FePc.17 CoPc differs from FePc for
the host central atom. In CoPc, N2 features energy positions that
are different from those in FePc because of different binding with
the central atom. On the other hand, N1 sites are similar in the
two molecules, and their intensities and their energy positions
remain similar. N1 LUMO peak intensity is maximum in CoPc
2.4 ML thick film, and it reduces in CoPc 0.7 ML thick film. The
change in intensity ratio of the features A and B indicates a siteselective strength of the interaction between the molecule and
the substrate for submonolayer coverage. Moreover, we observed
an overall decrease (∼15%) of LUMO intensity (sites: N1, N2) in
the submonolayer CoPc film.
Figure 2d shows representative N 1s XAS spectra of CoPc 0.7
ML thick film as a function of θ (i.e., the angle between the
incident photons and the sample surface normal). In planar πconjugated systems, π* orbitals are expected to be out of the
molecular plane, while σ* are oriented in the molecular plane.
The discrimination between the states can therefore be obtained
by the “search light” effect in XAS with linearly polarized light.
The spectrum measured at θ = 60° (dashed line) shows sharp
features at energies about 395.5, 395.9, 396.6, 397.3, 399.5, and
400.8 eV below the N 1s threshold at 403.6 eV. These are core
excitation lines due to the transition 1sπ*, typical of planar
π-conjugated systems. The broader features at energies higher
than 403.6 eV are related to 1sσ* transitions. In CoPc, XAS at
the N threshold are given by the superposition of contributions
due to the different N atom sites (see Figure 2b).1619
Figure 2d reveals a very strong angular dependence of N
1sπ* intensities (395.5404 eV). At grazing incidence, the
intensities of π* features show a maximum and decrease with
decreasing θ reaching a minimum at normal incidence (θ = 0°).
The opposite verifies for 1sσ* transition. Similar findings are
observed for CoPc and FePc grown on Au(110).20 The spectrum
measured at θ = 0° manifests a residual signal at 396.6 eV. In
previous works on MPc, this residual intensity measured was
attributed to a hybridization of the LUMO states localized on
some N atoms or to a distortion of some N bound in the MPc
molecule.21
Figure 2e shows the angular dependence of the intensity for
the N π* resonance and the expected profile (dotted line) for flat
lying molecules. This demonstrates that CoPc molecules are
lying at the same orientation (flat) with respect to the surface.
Equivalent results were obtained for other molecules adsorbed
on metallic substrates.16,20
CoPc adsorbs in a flat lying configuration, with its macrocycle
parallel to the substrate plane in submonolayer up to 1 ML as
demonstrated by STM images. The molecules remain flat also for
higher thicknesses up to at least 2.4 ML as indicated by angledependent XAS N 1s (Figure 2e).
X-ray and Valence Band Photoemission Results. At the
onset of the organic film/metal interface, there are various
scenarios: formation of the dipole layer due to charge transfer,
redistribution of the electron cloud, and interfacial chemical
reaction.22 Moreover, the band bending (i.e., the energy level
deformation induced by the charge redistribution in the organic
layer to achieve electrical equilibrium) is possible when a
sufficient number of mobile charge carriers are available, either
in a rather thick organic layer or in an organic layer with good
semiconductor character.
By means of XPS, the charge transfer can be revealed as a shift
in the core-level binding energy. A peak shift to higher binding
energy indicates donation of electrons, whereas a peak shift to
lower BE indicates acceptance of electrons. However, to correctly
assign the origin of the peak binding energy change, it is
necessary to use a proper reference and account for: (i) the final
state relaxation of the core hole between the metal and the
molecule; (ii) the changes in the system work function; and (iii)
the total charge flow in or out of the atomic sites.
Here, we address the relative role of ligand atoms (C, N) and
the host central atom (Co) of CoPc in the interaction with the
substrate by exploiting the core level shifts and valence band of
CoPc adsorbed on Cu(111).
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Figure 3. Photoemission C 1s (a) and N 1s (b) spectra of CoPc/Cu(111) as a function of CoPc thickness, measured at 480 eV photon energy. The
fitting of the C 1s core levels (a) is done using five Gaussians, which accounts for the chemically different C atoms and their shakeup satellites. (c) Binding
energy of C1s, N1s core level as a function of CoPc deposition.
Table 1. Fitting Parameters of C 1s XPS Spectra Reported in
Panel (a) of Figure 3a
CoPc
Cbenz BE
Δ(BEbenzBEpyr)
fwhm
I(Cbenz)/
(thickness) ML
(eV)
(eV)
(eV)
I(Cpyr)
0.4
284.4(0.4)
1.16(4)
0.43
2
0.7
1
284.6(0.4)
284.76(4)
1.24(4)
1.23(4)
0.46
0.43
1.9
1.9
2.4
284.8(0.4)
1.34(4)
0.53
2.4
In the fit, benzene and pyrrole features are fitted with the same
Gaussian fwhm.
a
In Figure 3, the evolution of C 1s, N 1s core level spectra for
the CoPc/Cu(111) system is reported as a function of CoPc
layer thickness. The intensity of the spectra is normalized to the
peak height of the most intense component to highlight the
changes in the line shape. Further insight into the molecular
adsorption can be gained by following the evolution of the C 1s
line shape vs CoPc thickness. The C 1s XPS spectrum of the
thin film (2.4 ML) is composed by three components. The most
intense features are related to C atoms within the benzene ring
(284.4 eV) and pyrrole (285.6 eV); the weaker one is attributed
to the relative shakeup satellite. The two N atom sites of CoPc
have binding energy differing by ∼0.34 eV (as predicted for a
similar MPc molecule17,23,24), and they are not resolved in
photoemission spectra. The N 1s line shape consists of a main
feature resulting from the superposition of N atom sites and a
faint high binding energy contribution (∼400 eV) relative to the
shake up satellites. This line shape does not change by increasing
CoPc coverage. In both C 1s and N 1s core level spectra, shakeup
features are attenuated in the first layer in contact with the
substrate, giving a further indication of moleculesubstrate
interaction.
To describe the C 1s XPS spectrum, we used a fitting function
consisting of five Gaussians. No additional components at
monolayer coverage are present that may point to a chemical
interaction between C atoms and the substrate. The relevant
parameters of the fit are the CbenzCpyr peak relative energy shift
and the relative intensities of these spectral features. An increase
in CbenzCpyr peak separation from 1.16 to 1.3 eV is observed
with increasing CoPc thickness. Similar behavior was observed in
the CoPc/Au(100) system and was ascribed to the site-specific
screening effect.25 Concerning the photoemission intensity, we
compare the peak areas to the 3:1 expected ratio for benzene/
pyrrole features (see Table 1 for more details). A smaller ratio is
found in the spectra and is indicated by the fitting, similarly to
previous results on the MPc/metal interface.17,23,24 The presence
of benzene carbon related shakeup intensity close to the energy
position of pyrrole carbon causes the reduction in intensity ratio.
A change in binding energy position occurs as a function of
thickness for all the investigated core levels: C 1s and N 1s
spectra manifest a shift of 0.4 eV to higher binding energies. The
extent of C, N core-level binding energy shift differs from the one
of CoPc/Au(100) and CuPc/Au (polycrystalline gold) interfaces where the amount of total C 1s core level shift is ∼0.15 eV25
and ∼0.8 eV.26 In ref 26, authors showed that for a thick film of
CuPc adsorbed on polycrystalline Au the C 1s and N 1s core level
shifts can be interpreted within a framework of band bending
formation in the molecular film with a charge transfer from the
molecule to the substrate. In ref 25, combined XPS, UPS
measurements on the CoPc/Au (100) interface gave a sitedependent sense of charge transfer between the molecule and
substrate: the C 1s and N 1s core level shifts toward higher
binding energy are driven by polarization due to image charge at
the interface, whereas the Co core level and Auger results indicate
a charge redistribution between the molecule and substrate. In ref
27, polarization screening was observed mainly for the central
metal atom of the ZnPcF16, indicating a local charge-transfer
screening at the central metal atom due to the overlap of the host
central atom and substrate states, whereas a charge transfer to the
LUMO is not localized at the F atom.
To interpret the C 1s and N 1s core level shift, we need to
combine these results with valence band data, N K XAS and Co
L3 XAS, reported in the following.
In Figure 4, we report the dispersion plot of the Γ surface state
of Cu(111) for the clean surface and as a function of subsequent
deposition of CoPc molecules. The modification of the surface
states depends on the properties of both organic molecules and
metallic substrates and strongly relies on the nature and the
strength of the bonding and electronic coupling at the interface.
The character of the interaction between the π-conjugate
organic molecule and the substrate is reflected in the surface state
of the substrate. The electronic coupling at similar interfaces
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Figure 4. Dispersion of the surface state of the Cu(111) surface measured along the Γ-K high symmetry direction and visualized as a function of
subsequent CoPc depositions: clean Cu(111) (a) up to 1 ML (g). Bright corresponds to high intensity in the grayscale. The red dashed parabola
indicates the position of surface state dispersion of clean Cu(111). The change in surface state parabola as a function of CoPc content is highlighted by
the yellow parabolic line. (h) Binding energy of the bottom of the surface state as a function of CoPc deposition. The clean Cu(111) spectrum shows the
bottom of the surface state at a binding energy of 0.4 eV.
Figure 5. (a) Normal emission ultraviolet photoemission spectra in the binding energy region close to the Fermi level for CoPc deposited on Cu(111)
as a function of coverage. Spectra are measured at 25.5 eV. Binding energies are referred to the Fermi level. (b) Normal emission photoemission spectra
measured at photon energies: 25.5 and 50 eV for CoPc 1 ML film. Intensities are corrected with cross sections. (c) Normal-emission photoemission
spectra measured at photon energies: 25, 58.9, and 70 eV for CoPc 2.4 ML film. (d) Co L3 XAS spectra measured for CoPc films: 0.4 ML (line and
marker) and (thick line) 2.4 ML. The spectra are taken at incident angle θ = 45° between the polarization vector E and the normal to sample surface.
was observed to induce: (i) a shift of the surface states toward
lower binding energy (NTCDA physisorbed on Au(111)28); (ii)
a quenching of surface states (PTCDA chemisorbed on Ag(111)29); (iii) a shift of the surface states toward higher binding
energy (pentacene/Cu(110)30); and (iv) the hybridization of
the metal bulk state with the molecular state (pentacene/
Cu(119)30).
A parabolic surface-state band dispersion is observed near Γ
with a binding energy minimum of E0 = 0.4 eV for the clean
Cu(111) surface. The area within the surface state Fermi contour
corresponds to the number of electrons occupying the surface
state.32 In the case of Cu(111), the Fermi contour is centered at Γ
and has a circular shape that depends on both the band bottom
and the electron effective mass. The area of the surface state
Fermi contours amounts to 2.1% of the SBZ total area and
corresponds to 0.042 electrons.
Upon CoPc adsorption, the surface state band undergoes a
binding energy shift toward lower binding energy and reaches the
Fermi level at the completion of the CoPc monolayer as shown in
Figure 4.
We note that surface state dispersion crosses the Fermi level at
k// (∼(0.2 Å1), and this value does not change as a function of
CoPc content. In terms of electron population of the surface state
at the Fermi level, this implies that it remains constant with
increasing CoPc coverage. Besides the energetic shift, an increase
in band mass and a decrease of the signal-to-noise ratio are
observed in Figure 4. The effective mass doubles from 0.4me (the
effective mass associated to the bare Cu surface state band,
hereafter m0) to 0.8me (the effective mass at CoPc 1 ML). The
decrease of the signal-to-noise ratio is caused by the damping of
the photoemission signal by the adsorbate and by the additional
scattering of the photoelectrons in the overlayer.
Figure 5a exhibits a detailed view of the CoPc HOMO region
using 25.5 eV photon energy at normal emission vs CoPc
thickness. The photoemission spectrum of 1 ML CoPc exhibits
a new feature at 0.9 eV and still a residual intensity at the Fermi
level. The feature at 0.9 eV is present also in the photoemission
spectrum of CoPc film 2.4 ML thick in addition to features at 1.37
and 0.36 eV. These peaks are related to the molecular absorption on Cu(111). Figure 5b shows photon energy-dependent
spectra for 1 ML CoPc. The intensity of the feature at 0.9 eV
varies similarly to the one at the Fermi level, i.e., the residual of
the surface state. We note that photoionization cross sections for
Cu 4p electrons for hν = 50 eV are 101 the one at hν = 25.5 eV,
whereas photoionization cross sections for Cu 4s electrons do
not change much when going from 50 to 25.5 eV. On the other
hand, the Co 3d photoionization cross section doubles for energy
changes from 25.5 to 50 eV. This implies that the peak at 0.9 eV
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does not show 3d character but rather p-like; i.e., it is attributed to
a feature originating from the interaction of the substrate and the
pyrrole/benzene part of the adsorbed molecules. A similar
feature was also observed in the CoPc/Au(110) system where
a peak at 0.7 eV binding energy was attributed to interface states
originating from the mixing of the π states localized at the N
atom and the Au metallic states.2 Such states disappear when the
CoPc multilayer is formed. In another study,33 CoPc grown
on Au(100) manifested features at binding energies of 0.48 and
1.13 eV, which were attributed to the shift of the unoccupied
down-spin states of Co and of the former highest occupied
molecular orbital (fHOMO), respectively. The first peak is not
observed here, probably due to overlapping intensity from the
surface state, and the second peak appears at different binding
energy in the present system.
In the CoPc/Cu(111) system, the fHOMO localized on the
pyrrole ring appears clearly at 1.37 eV in the photoemission
spectrum of the 2.4 ML film. The feature at 0.36 eV is observed at
all the photon energies explored, from 25.5 to 70 eV. This cannot
be considered as reminiscent of surface states since it is measured
also at k// 6¼ 0; rather, it is associated to a molecular state that
crossed the Fermi level as a consequence of the interface charge
redistribution. In particular, the interaction between the Co
central atom and the metal substrate may induce an extra feature
below the Fermi level. To establish the nature of this feature, we
measured on the same samples the XAS spectra at the Co edge.
Co L3 XAS spectra probe the unoccupied s and d states of the
molecule projected to the Co site. The 3d band of Co is split
because of the molecular field. In the case of the free molecule,
the Co environment has D4h symmetry, and Co is divalent
(Co2+). DFT calculations indicate that 3dx2y2 states are fully
unoccupied, whereas 3dxy, 3dxz, 3dyz, and 3d3z2r2 orbitals are
close in energy to each other. Different calculations predict
different orbital occupancy.6,19 Figure 5d reports Co L3 XAS
spectra measured at photon incident angle θ = 45° for two
different CoPc film thicknesses: 0.4 and 2.4 ML. The spectrum
relative to 2.4 ML shows several spectral features consistent with
the Co2+ valence state and a complex electronic structure, which
we do not discuss here. We focus on the evolution of the spectra
versus CoPc thickness. The feature at 773.5 eV is quenched at the
lower coverages.
’ DISCUSSION
Important parameters that qualify the molecular/metal junction are the electron transfer between the molecule and metallic
electrodes and the magnetic state of the molecule (including the
central host atom and the ligands in the case of metallorganic
molecules). The molecule/metal interface properties strongly
depend on the adsorption site of the molecule and the corresponding charge redistribution at the interface. We discuss the
adsorption arrangement of CoPc on Cu(111) and its electronic
structure.
In the STM images, we observed that at the first stage of CoPc
adsorption on Cu(111) the molecules settle on the step edges
and subsequently fill the Cu(111) terraces. CoPc 1 ML on
Cu(111) does not show long-range but different local ordering.
Our findings confirm the recent observations of coverage-dependent arrangement of metal phthalocyanine on the Cu(111)
surface, where very low dose molecules organize along one line
and at higher coverage (from 0.8 ML) two-dimensional molecular arrays are preferred.34
ARTICLE
The molecules adsorb flat with their macrocycle parallel to the
Cu(111) surface as also probed by angle-dependent N K edge
XAS. The molecules maintain themselves flat up to the highest
investigated CoPc thickness (i.e., 2.4 ML). This implies that
CoPcsubstrate coupling is stronger than the CoPcCoPc
interaction; the latter, in fact, favors a high density stand-up
configuration as found for FePc on Si(100).17
The electron states at the first stage of CoPc growth on
Cu(111) are characterized by a continuous shift of the surface
states to lower binding energy. Since the surface states can be
viewed as electrons confined within the surface plane, the charge
redistribution and/or the interface dipoles are both expected to
change at the CoPc/Cu(111) interface, and the lateral confinement within the reducing uncovered portions of the Cu(111)
substrate.
The two-dimensional free electron gas of the Cu(111) surface
state was observed to be trapped within the pores of an organic
nanoporous network that can be regarded as a regular array of
quantum dots. An additional shallow two-dimensional dispersive
electronic band structure originating from electronic coupling
between neighboring pore states can be observed in the case of
periodic quantum confinement of the Cu(111) surface state.35
Although the STM result suggests a subsequent filling of the
Cu(111) terrace vs CoPc coverage, we observed a broadening of
the surface state peak-width, the quenching of the intensity at
monolayer completion, and an increase in the effective mass, but
no additional dispersive bands. On the other hand, the increase of
the effective mass with respect to m0 of the unperturbed band
indicates a charge confinement at the surface, not driven by
periodic quantum confinement on surfaces but induced by
charge redistribution at the interface due to the electrostatic
dipole formation.36 Ab initio calculations for CoPc/Cu(111) also
predict the formation of a dipole at the molecule/metal interface
and an additional Pauli repulsion between metal and molecular
electrons12 and a charge transfer from the substrate to the
molecule.
A charge transfer from the substrate to the molecule appears in
ARPES spectra as an increase of the electron density of states in
the valence band region close to the Fermi energy. In the case of
acenes adsorbed on the metal surface, features at the Fermi level
are understood in terms of hybridization involving metal states
and a non-negligible charge transfer to the lowest occupied
molecular orbital(LUMO).31,37 In the case of MPc, both the
LUMO of the macrocycle and Co 3d unoccupied orbitals can be
hybridized with substrate states or be filled by charge transfer
from the substrate since their energies are very close. In ref 34, a
partial filling of the former LUMO is observed for CuPc adsorbed
on Cu(111) at a binding energy of 0.4 eV. ARPES spectra for 1
ML of CoPc on Cu(111) show a feature at 0.9 eV that originates
from the interaction of the substrate and pyrrole/benzene part of
the molecule as deduced from the photon energy-depedent
photoemission intensity. This attribution is also supported by
optical absorption results on CoPc, where the lowest unoccupied
branch is the one relative to the macrocycle.38,39 The reduction of
LUMO intensity in N 1s XAS in a submonolayer film suggests
the role of these molecular states located in the formation of the
interface states at 0.9 eV.
Charge transfer in XPS spectra manifests as a binding energy
shift. However, the interpretation of core level binding energy
position at the interface is challenging; in fact, there are examples
in the literature where the same sign in core level binding energy
shift originates from different mechanisms. The C 1s core level
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The Journal of Physical Chemistry C
shift to higher binding energies vs film thickness was already
observed in the CuPc/Au interface and attributed to a charge
transfer from the molecule to the substrate. 26 Calculations3
indicate that electronic charge is donated by CoPc to the
Ag(111) surface and partially backdonated from the substrate
to the Co central atom.3 A dominant substrate-mediated
repulsive intermolecular interaction between MPc molecules
was found at the origin of the adsorption of two-dimensional
arrangement of SnPc and CuPc on Ag(111) (and CuPc on
Cu(111)) in terms of coverage and temperature.40 In these
cases, the overlap between molecular and substrate states
produced electron donation from the molecule to the substrate and a back-donation into the unoccupied molecular
orbital.40
We interpreted the C 1s and N 1s core level shift toward
higher binding energy in the XPS spectra, the interface state at 0.9
ML in the ARPES spectra, and the reduction of LUMO in N 1s
XAS spectra of CoPc 1 ML film as a signature of charge transfer
from substrate to molecule. However, we cannot exclude the
polarization screening as the origin of the core level binding
energy shift, as discussed in ref 26. The latter is site dependent,
and in metal phthalocyanine the screening is more effective at the
central atom.
In addition, the independence of surface state charge population from CoPc thickness excludes their involvement in charge
transfer processes that rather occur between molecular states and
bulk bands of the metal substrate. This is not the first observation
of this type of interaction between the molecule and the bulk
metallic state, already reported for pentacene deposited on
Cu(119).31
The chemical selectivity of XAS permits us to investigate Co
metal and ligand atom (N, C) sites of CoPc adsorbed on
Cu(111). At submonolayer, the N 1s XAS spectrum shows a
change in intensity ratio between N1/N2 spectral features and
indicates the role of N1 in the interaction with the Cu(111)
surface.
The electronic properties of CoPc and in particular the charge
density at the metal center are controlled by the energy alignment of Co 3d orbitals and LUMO relative to pyrrole/benzene
ring orbitals. CoPc molecules can be magnetic due to unpaired
spin residing in the 3d orbitals of Co. The free CoPc molecule has
an uncompensated magnetic moment of 1.09 μB, and it is
paramagnetic. The interaction with the metallic substrate, by
changing the electronic configuration of Co, has implications to
the molecular magnetism of CoPc.
The low-energy side of the Co L3 XAS spectrum is quenched
upon CoPc adsorption on Cu(111) at CoPc 1 ML. The disappearance of intensity in Co XAS spectra corresponds to the
presence of a Co 3d feature in the energy region close to Fermi
energy in ARPES spectra. A small feature is observed at a
binding energy of 0.36 eV in the ARPES spectrum for CoPc 2.4
ML. This feature is attributed to the Co partial filled state, even
though the photon energy dependent photoemission intensity
does not follow the expected cross-section change. This behavior can be explained with an overlap of the Co and the
substrate states which makes difficult to discern the two
components. A similar effect was already observed in FePc
and CoPc grown on Au(110).2 At CoPc 1 ML in this binding
energy region there is still a residual surface state component,
and the Co 3d feature is not clearly distinguishable. In the
spectrum of CoPc 2.4 ML, the HOMO peak is located at a
binding energy of 1.37 eV.
ARTICLE
Therefore, we observed a partial filling of the first Co unoccupied 3d states in the Co L3 XAS spectrum for submonolayer
CoPc coverage. The charge redistribution at the Co site can be
explained either by direct charge injection12 or by backdonation
between the molecule and the substrate.3
It can be noted that CoPc is also the MPc with the highest
affinity for electron doping, being capable of taking up to 56
electrons/molecule from K doping impurities at saturation, i.e.,
more charge than the other MPcs.41 The charge redistribution at
the Co site has certainly relevant effects on the magnetic state
of CoPc.
At 2.4 ML, the flat lying arrangement of CoPc molecules is
demonstrated by “search light” effect at N 1s XAS: it is the
signature of residual interaction at the CoPc/Cu(111) interface,
mediated by the interfacial monolayer. However, both N 1s and
Co 2p XAS spectra relative to 2.4 ML CoPc show an electronic
structure of CoPc which resembles the one of the weak interacting molecule. At 1 ML, the moleculesubstrate interaction
strongly modifies the CoPc electronic and magnetic structures,
with consequences extending to subsequent molecular layers.
’ CONCLUSIONS
We have characterized the CoPc/Cu(111) interface and
correlated the CoPc adsorption geometry with its electronic
properties. The molecules are lying flat with respect to the
Cu(111) surface up to 2.4 ML. The adsorption of the CoPc
molecule on the Cu(111) surface results in a charge imbalance at
the interface. The combined XPS, ARPES, and Co XAS results
indicate charge redistribution at the interface. A charge transfer
from the substrate to the molecule is suggested. The Cu(111)
surface state shifts toward the Fermi level with unaltered
population, and the increase of surface state effective mass
indicates charge confinement at the interface. Partial filling of
the Co projected states was observed, which involves a modification of magnetic properties of the molecule to be explored.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
’ ACKNOWLEDGMENT
This work was carried out in the framework of the PRIN 2008
525SC7 contract coordinated by M.G. Betti.
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