Article
pubs.acs.org/JPCC

Site-Speciﬁc Organic/Metal Interaction Revealed from Shockley-Type
Interface State
Hiroyuki Yamane* and Nobuhiro Kosugi
Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki 444-8585, JapanSOKENDAI, The Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan
ABSTRACT: In order to discuss the site speciﬁcity in the organic/metalinterfacial interaction, the electronic structure of the well-ordered interface of
cobalt phthalocyanine (CoPc) adsorbed on Au(111) is studied by angleresolved photoemission spectroscopy (ARPES) with synchrotron radiation. The
core-level and valence-band ARPES spectra show an evidence for the strong
CoPc/Au(111) chemical interaction localized at the Co site and the resultant
formation of the interface states. The CoPc/Au(111) interface states are formed
around the highest occupied molecular orbital (HOMO) and just below theFermi level (EF) with the nondispersive and dispersive characteristics,
respectively. The nondispersive interface state around the HOMO is attributed
to the intramolecular πPc−dCo mixed orbital induced by the site-speciﬁc
interfacial interaction. The dispersive interface state below EF originates fromthe adsorbate-induced modiﬁcation of the Au(111) Shockley state, which shows diﬀerence between CoPc and metal-free Pc(H2Pc) and dependence on the temperature. The Shockley-type interface state enables us to characterize organic/metal interface
energetics quantitatively in terms of the site-speciﬁc interfacial interaction.

1. INTRODUCTION
The Shockley state is a well-known surface state associated withthe breakdown of the periodic bulk crystal potential and thesymmetry at the surface and can be observed as a free-electronlike parabolic dispersion for normal metal and semiconductor
surfaces by using angle-resolved photoemission spectroscopy(ARPES). Recent technical advances of the photoelectron
analyzer and the photon source (e.g., synchrotron radiation andlaser) improve experimental data signiﬁcantly in energy
resolution, in angular (wavevector) resolution, and in photoelectron detection eﬃciency (i.e., surface sensitivity),1−3 giving
the opportunity to discuss new surface physics and chemistrysuch as the Rashba spin−orbit splitting and the kink structureby the electron−phonon (e−ph) interaction. The Shockleystate is in principle observable also in adsorbed-surface systems,
and indeed the Shockley state has been recently reported forsingle-crystalline surfaces adsorbed by rare gases,2,4 metals,4,5and organic molecules6−10 with modiﬁed dispersion parameters
relative to the parameters of clean surfaces. It is understood thatthe modiﬁed Shockley states are derived from the rearrangement of surface electron systems and the resultant surface
potential induced by complex interfacial interactions between
adsorbates and solid surfaces. In some cases, the Shockley stateshows a band gap opening at the Brillouin zone boundaries ofadsorbate’s superlattices.4
Organic monolayers adsorbed on metal surfaces are regarded
as a model system for the study of fundamental physical andchemical properties related to organic electronics. In particular,
the energetics in organic/metal interfaces is dominated by thecomplex interplay of various interfacial electronic phenomena© 2016 American Chemical Society

such as the charge transfer, the mirror force eﬀect, the Pauli
repulsion (exchange interaction), the chemical reaction, and theintramolecular dipole.11 Some of these factors have been
already demonstrated for the control of organic/metal interface
energetics. As a surface-sensitive method, the Shockley statemeasurement has recently been thus applied to organic/metalinterfaces for the quantitative characterization of the interfacial
bonding strength,6 ultrafast dynamics,7−9 and doping eﬀects10
related to organic/metal interface energetics.
In the present work, we have carried out the Shockley statemeasurement of well-ordered organic/metal interfaces of metal
phthalocyanine [MPc; M = Co, H2 (metal-free)] molecules
adsorbed on Au(111), in combination with high-resolution
core-level and valence-band ARPES measurements. MPc
molecules are a class of important organic semiconductors
especially for organic photovoltaics, and in particular the planar
CoPc molecule shows unique molecular electronic andmagnetic properties.12−14 Therefore, various CoPc/metalinterfaces have been investigated by using photoelectron/Xray spectroscopies and the scanning tunneling microscopy(STM).15−25 The present high-resolution core-level andvalence-band ARPES spectra show a clear evidence for thesite speciﬁcity in the CoPc/Au(111) interfacial interaction. The
relatively strong chemical interaction at the Co site in themolecule with the Au(111) surface introduces the core-level
shift and the interface state formation around the uppermost
Received: September 12, 2016
Revised: October 7, 2016
Published: October 11, 2016
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molecular electronic states. Moreover, we have succeeded inobservation of the diﬀerence in the shape of the modiﬁed
Shockley state at the CoPc/Au(111) interface as compared tothat at the H2Pc/Au(111) interface. On the basis of the present
observations for the core-level and valence-band electronic
structure, we reveal a possible origin of the modiﬁed Shockleystate at the CoPc/Au(111) interface in terms of the site-speciﬁcorganic/metal interfacial interaction and discuss its impact onorganic/metal interface energetics.

2. EXPERIMENTAL SECTION
The present experiment was performed on a highly brilliant invacuum undulator beamline of BL6U, which covers the photon
energy (hv) from 40 to 700 eV, of the UVSOR Synchrotron in
the Institute for Molecular Science (Okazaki, Japan). The
ARPES endstation consists of the measurement, preparation,and load−lock chambers. The ARPES spectra were acquired
from the energy-versus-angle image by using a microchannelplate (MCP) detector of a hemispherical electron energy
analyzer (MB Scientiﬁc A-1).
In the present work, the Au(111) single crystal (5N purity)and the highly oriented pyrolytic graphite (HOPG, ZYA grade)were used for the substrate. The clean Au(111) substrate wasobtained by repeated cycles of the Ar+ sputtering and thesubsequent annealing at 700 K. The HOPG substrate wascleaved in air just before loading into the load−lock chamberand cleaned by heating at 700 K for 5 h in the preparationchamber. The cleanliness of these substrates was conﬁrmed by
the core-level and valence-band ARPES spectra and the lowenergy electron diﬀraction (LEED) with the MCP detector(OCI BDL800IR-MCP).
The well-ordered organic layers of CoPc and H2Pc wereprepared by the vacuum deposition of highly puriﬁed powders
onto the clean substrate kept at the room temperature. Afterthe deposition, the sample was annealed at about 420 K to
improve the lateral ordering. The deposition rate (<1 Å/min)and the layer thickness were measured using a quartz crystal
microbalance, which was calibrated by the spectral shape in
ARPES. The lateral ordering of organic layers was conﬁrmed by
MCP-LEED. Note that, because of the use of MCP, alldisplayed LEED images in this paper were distorted.

Figure 1. High-resolution core-level photoemission spectra at thenormal emission measured for the clean Au(111) surface [labeled(0)], the CoPc thin ﬁlms on Au(111) with the ﬁlm thicknesses of 2.5,
15, and 300 Å [labeled (1), (1)′, and (2), respectively], and the 2.5 Åthick CoPc thin ﬁlm on HOPG [labeled (3)] at 20 K. hν = 500 eV for
the N 1s and C 1s measurement and hν = 170 eV for the Co 3p
measurement were used. The molecular structure of CoPc is shown in
the inset.

Au(111) interface are observed at the lower Eb side with respect
to those for the CoPc thick multilayer. The Eb diﬀerence for N
1s and C 1s between the CoPc/Au(111) interface and the
CoPc thick multilayer of 0.6 eV is explained by the core−holescreening in the photoemission ﬁnal state.16 The Co 3p signal
for the CoPc/Au(111) interface appears as a shoulder structurein the Au 5p3/2 peak, which is not observable in the Au 5p1/2
peak, excluding the possibility of the surface core-level shift byadsorbates. The Co 3p signal appears at Eb = 59.2 eV in thesecond derivative curve (−d2I/dE2) at the CoPc/Au(111)interface, and is getting strong and shifted to the higher Eb sidewith increasing the ﬁlm thickness; Eb = 60.1 and 60.6 eV for the
CoPc thin and thick multilayers, respectively. The Eb diﬀerence
for Co 3p between the CoPc/Au(111) interface and the CoPc
thick multilayer of 1.4 eV is larger than that for N 1s and C 1s
of 0.6 eV. This evidence indicates the presence of the sitespeciﬁc interaction at the CoPc/Au(111) interface, where thestrong charge-transfer interaction with Au(111) exists at the Cosite via an unpaired electron, as is pointed out using STM.24,25
A similar evidence has been demonstrated for the ZnPc/
Cu(111) interface using core-level photoemission and X-ray
standing wave (XSW) experiments.26
On the basis of the presence of the site-speciﬁc chemicalinteraction at the CoPc/Au(111) interface, we discuss thevalence electronic structure. Figure 2 shows the photoemission
angle (θ) dependence of ARPES and the LEED image
measured for the CoPc/HOPG, CoPc/Au(111), and H2Pc/
Au(111) interfaces at 20 K. The LEED image of CoPc/
Au(111) and H2Pc/Au(111) shows a typical diﬀraction patternof ﬂat-lying MPc molecules on the Au(111) surface,27
indicating the formation of the well-ordered MPc/Au(111)interface. The hexagonal spots just around the (0,0) spot are
ascribed to the 22 × √3 herringbone reconstruction of theAu(111) surface. The LEED spots arising from the molecularordering, indicated by the arrows in the LEED image, suggestthat the CoPc (H2Pc) molecules on Au(111) form a squarelattice with a distance of ca. 14.7 Å. At the CoPc/HOPG
interface, the LEED spots due to the molecular ordering areappeared as a ringlike pattern by the azimuthally averaging
eﬀect of the HOPG surface. These well-ordered organic layers
introduce the clear θ dependence in the ARPES peak intensity

3. RESULTS AND DISCUSSION
Figure 1 shows high-resolution N 1s, C 1s, and Co 3p
photoemission spectra measured for the clean Au(111) surface[labeled (0)], the CoPc/Au(111) interface [labeled (1)], the
CoPc thin (15 Å) and thick (300 Å) multilayers on Au(111)[labeled (1)′ and (2), respectively], and the CoPc/HOPG
interface [labeled (3)] at 20 K. The CoPc thick multilayershows well-known spectral features, where the N 1s peak is
attributed to both the azaporphyrin and the pyrrole nitrogensites with a small energy diﬀerence, the ﬁrst and second C 1s
peaks are attributed to the benzene ring and pyrrole carbon
sites, respectively, and the third C 1s peak is attributed to the
π−π* shakeup satellite at the pyrrole carbon site. Although theﬁrst C 1s peak for the CoPc/HOPG interface consists ofcarbon components of both the molecule and the substrate, thebinding energy (Eb) positions of the N 1s, second C 1s, and Co
3p peaks for the CoPc/HOPG interface agree well with thosefor the CoPc thick multilayer, suggesting a weak core−holescreening at the physisorbed CoPc/HOPG interface. On theother hand, the N 1s, C 1s, and Co 3p peaks for the CoPc/
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peak H′ is slightly diﬀerent from that of peak H, suggesting thediﬀerent orbital character between peaks H and H′. If peak H′
is mainly derived from the Co 3d orbital, peak H′ should bestronger than peak H at hv > 60 eV;30 however, peak H′ isweaker than peak H even at hv = 90 eV and θ = 21° (k∥ ∼ 1.7
Å−1). Judging from the previous STM24,25 and present ARPES
data, peak H′ is attributed not to the predominant Co 3d
orbital but to the interface-induced πPc−dCo mixed orbital
derived from the strong interfacial interaction at the Co site.
On the other hand, the e−ph coupling is visible for peak H of
CoPc/Au(111) as CoPc/HOPG and H2Pc/Au(111), suggesting the weak interfacial interaction at the Pc-ligand site.
Therefore, the observed HOMO line shape is explained by thesite-speciﬁc interfacial interaction with the weak interaction at
the Pc-ligand site and the relatively strong interaction at the Co
site.
In addition to the HOMO line shape, the other CoPc/
Au(111) interface state is observed. In Figure 2, for both the
CoPc/Au(111) and H2Pc/Au(111) interfaces, the Shockleytype electronic state (S′) appears at θ = 0° and disappears when
θ = 4° due to its free-electron-like dispersion to the lower Ebside with increasing θ. We found that the dispersion of band S′
is deduced to originate from the modiﬁed Au(111) Shockleystate as discussed below. Furthermore, only at the CoPc/
Au(111) interface, a quite weak adsorption-induced feature isobserved just below the Fermi level (EF) at θ = 0° as indicatedby the upper arrow in Figure 2.
In order to discuss the Shockley-type electronic state below
EF at the CoPc/Au(111) interface in more detail, the energyversus-wavevector (E−k∥) maps of the CoPc/Au(111), H2Pc/
Au(111), and Au(111) at 20 K are shown in Figure 3, as
obtained using the relation of

Figure 2. Photoemission angle (θ) dependence of ARPES (hv = 45eV) and the LEED image (E = 60 eV) measured for the interfaces of
CoPc/HOPG, CoPc/Au(111), and H2Pc/Au(111) at 20 K. ARPES
spectrum with hv = 90 eV and θ = 21° for CoPc/Au(111) is shown forcomparison.

since the angular distribution of the photoelectron intensity
reﬂects the spatial distribution of molecular orbitals, as has been
applied to the determination of the molecular orientation andthe orbital tomography.28,29 At the CoPc/HOPG interface, thedispersive and nondispersive ARPES peaks appear. The
dispersive feature at Eb = 0−1 eV around θ = 30° is attributed
to the well-known π band of the underlying HOPG substrate(πHOPG). The nondispersive peaks at Eb = 1.2 and 2.1 eV are
attributed to the highest occupied molecular orbital (HOMO)
of a C 2p character and the HOMO − 1 of a Co 3d character,
respectively. The HOMO intensity shows a sharp θ dependence with the maximum at θ = 32°, which corresponds to k∥ ∼1.7 Å−1. This is the reﬂection of the spatial electron distributionof the HOMO in the well-ordered CoPc monolayer. The
maximum intensity of the HOMO is stronger than that of the
HOMO − 1 when hv = 45 eV, which is ﬂipped when hv > 60
eV due to the diﬀerent photoionization cross section between
C 2p and Co 3d orbitals.30 On the other hand, the tail structure
at the higher Eb side in the HOMO peak is attributed to the e−ph coupling,31,32 of which the energy separation is about 170
meV (1370 cm−1) from the main peak and corresponds to the
CH and CNC in-plane bending vibrations of MPc molecules.
The conservation of vibrational progressions in the HOMO
peak rules out the presence of the strong hybridization between
the molecular orbital at the Pc ligand and the substrate wave
function.
At the CoPc/Au(111) interface, the nondispersive HOMO
peak exhibits the clear θ dependence with the intensity
maximum at θ = 32° (k∥ ∼ 1.7 Å−1) as CoPc/HOPG, while
no clear Co 3d-derived peaks appear below the HOMO-derived
peak, indicating the modiﬁcation of the Co 3d orbital. The
HOMO-derived peak at CoPc/Au(111) has two prominent
components with the main peak H at Eb = 0.6 eV and theshoulder peak H′ at Eb = 0.4 eV, which is not observable for
CoPc/HOPG, H2Pc/Au(111), ZnPc/Au(111),33 and CoPc
multilayers on Au(111).30 The intensity distribution with θ ofk =2m0(hυ − E b − Φ) sin θ /ℏ

where m0 and Φ are the free-electron mass and the work
function, respectively. From the LEED analysis as discussed
above, the Brillouin zone boundaries of the CoPc (H2Pc)
molecular unit cell exist at varied k∥ positions of ±0.15−0.21
Å−1 due to the multidomain ordering structure. The Shockleystate of the clean Au(111) surface (S) shows a parabolic
dispersion with the Rashba spin−orbit splitting, which can beﬁtted by two free-electron-like parabolas with the eﬀective mass(m*) of 0.27m0, the surface state energy at k∥ = 0 Å−1 (E0) of
465 meV, and the Rashba momentum oﬀset (ΔkR) of ±0.013
Å−1. Upon adsorption of H2Pc on Au(111), the Au(111)Shockley state S is quenched and the new Shockley state (S′)
appears at the lower Eb side. The observed Shockley state S′ at
the H2Pc/Au(111) interface can be ﬁtted by modiﬁed
dispersion parameters of the increased m* (0.27m0 →0.29m0) and the upshifted E0 of 345 meV, while ΔkR is
unchanged, with respect to the original Au(111) Shockley state.
According to the earlier ARPES studies on the Shockley state at
rare-gas/Au(111) and organic/Au(111) interfaces,6 the modiﬁed Shockley state with the upshift in E0 is in general ascribedto the Pauli repulsion of surface electron systems by adsorbates,
and has been interpreted as the Shockley-type interface state.
This modiﬁcation of the Shockley state enables us to determine
the interfacial adsorption energy (Ea) quantitatively, and theobserved shift in E0 (ΔE0 = 120 meV) at the H2Pc/Au(111)interface gives Ea = 12.7 meV/Å2 (= 0.106ΔE0), suggesting aphysisorption-type interaction as in rare-gas/Au(111) systems.6
The CoPc/Au(111) interface also forms the modiﬁed Shockley
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Table 1. Fitting Parameters for the Shockley State ofAu(111), H2Pc/Au(111), and CoPc/Au(111) at 20 K in
Figure 3 (Eﬀective Mass m*, Surface State Energy E0, andRashba Momentum Oﬀset ΔkR) and the Determined
Interfacial Adsorption Energy Ea
Au(111)
H2Pc/Au(111)
CoPc/Au(111)
CoPc/Au(111)
a

S
S′
S′
X

m*/m0

E0 (meV)ΔkR (Å−1)

Ea (meV/Å2)

0.27
0.29
0.29
3.46

465
345
337
32

0.013
0.013
0.013
0

N/Aa
12.7
13.6
45.9

N/A, not available.

peak of the CoPc/Au(111) interface should be diﬀerent fromthat of the CoPc multilayer of 1.4 eV due to the diﬀerent
chemical structure. The observed C 1s peak of the CoPc/
Au(111) interface in Figure 1 shows no evidence for thedehydrogenation. Furthermore, it was reported in the case of
the CuPc/Ag(111) interface that the Kondo resonance peak
was observed for all emission angle (i.e., all-k∥ region) with thenondispersive behavior.36 Such k∥-dependent intensity andenergy distributions related to the Kondo resonance cannot be
observed in the present case as judged from Figures 2 and 3.
Therefore, the possibility of the Kondo resonance is completely
ruled out for the origin of band X. From the analysis of the line
proﬁle in energy (i.e., energy distribution curve) for band X, wefound that band X consists of a single component within thepresent energy and angular resolution, which is modestly visible
in the temperature-dependent dispersion of band X (see, Figure4b). Since band X shows a weak parabolic dispersion aroundthe Γ̅ point (k∥ = 0 to ±0.2 Å−1) at the CoPc/Au(111)interface, the most appropriate origin of band X is the Shockleystate, which might be modiﬁed by the site-speciﬁc adsorptionenergy as has been observed for rare-gas/metal interfaces.4,5 In
this scenario, since band S′ is observable for both the CoPc/
Au(111) and H2Pc/Au(111) interfaces, bands X and S′ can be
ascribed to the Shockley state modiﬁed by the Co site and the
Pc-ligand site in the molecule, respectively. The curve ﬁtting ofband X using a single parabola gives m* = 3.46m0 and E0 = 32
meV (ΔE0 = 433 meV), which suggests the large Ea of 45.9
meV/Å2 localized at the Co site by the site-speciﬁc interfacialinteraction, as summarized in Table 1.
In order to examine the site-speciﬁc interfacial interaction at
the CoPc/Au(111) interface, we measured the temperaturedependence of the Shockley-type interface state at the Γ̅ point(k∥ = 0 Å−1), as shown in Figure 4a. The Shockley state of theclean Au(111) surface, blue dotted curve (labeled S), shifts tothe higher Eb side with decreasing the temperature due to theshrink of the bulk crystal lattice.38 A similar evidence isobserved for band S′ of CoPc/Au(111); however, the amountsof the energy shift are diﬀerent, wherein ΔE0S(T) = 60 meV for
Au(111) and ΔE0S′(T) = 49 meV for CoPc/Au(111) from 299
to 23 K. On the other hand, band X shifts to the lower Eb sideby ΔE0X(T) = −25 meV from 299 to 23 K, resulting in thechange in the dispersion curve as shown in Figure 4b. In thecase of organic/metal interfaces, the temperature-dependent
shift in the Shockley state originates from the sum of thechanges in the substrate lattice constant and in the interfacial
bonding distance, which modiﬁes surface electron systems andpotentials. Since ΔE0S(T) is regarded as the substrate−latticecontribution, the energy diﬀerences of ΔE0S′(T) − ΔE0S(T) =11 meV and ΔE0X(T) − ΔE0S(T) = 85 meV are attributed tothe magnitude of the site-speciﬁc change in the Pauli repulsion

Figure 3. E−k∥ map of the Shockley-type electronic state along Γ̅ −M̅
of CoPc/Au(111), H2Pc/Au(111), and Au(111) at 20 K, measured
with hv = 45 eV, which gives a maximum intensity for the Shockleystate (ref 34). The black dashed line in the E−k∥ map represents theparabolic ﬁtting to the observed dispersion. The energy distribution
curves at (i) k∥ = 0 Å−1 and (ii) k∥ = 0.26 Å−1 are shown in the right.

state S′ with the dispersion parameters of m* = 0.29m0 and E0= 337 meV (ΔE0 = 128 meV). The determined Ea = 13.6 meV/
Å2 at the CoPc/Au(111) interface is slightly stronger than thatat the H2Pc/Au(111) interface probably due to the strong sitespeciﬁc interfacial interaction at the Co site in the CoPc
molecule, which may collaterally aﬀect the overall adsorption
strength and the resultant interface energetics.
In addition to the presence of band S′ at the CoPc/Au(111)interface, an additional dispersive band is observed just below
EF as labeled X, which is not observable at the H2Pc/Au(111)interface. One of the possible origins for band X is a partially
ﬁlled lowest-unoccupied molecular orbital (LUMO) by thestrong charge transfer from the substrate to the molecule (i.e.,electron doping).23 If the LUMO is partially ﬁlled by the chargetransfer from electron donors, the C 1s and N 1s photoemission peaks have to be signiﬁcantly broadened asdemonstrated previously.35 In the present work, the C 1s andN 1s photoemission line shapes for the CoPc/Au(111)interface agree well with those for the CoPc thick multilayer
as judged from Figure 1; therefore, the possibility of the ﬁlled
LUMO is ruled out for the origin of band X. The other possible
origin for band X is the Kondo resonance, which has been
observed for some MPc/metal interfaces by using ARPES and
STM.12,36,37 In the case of CoPc adsorbed on Au(111), theKondo resonance was realized upon dehydrogenation of themolecule by using voltage pulses from the STM tip, which
recovers the localized spin state at the interface.12 If the CoPc
molecule is dehydrogenated just by the deposition on Au(111),
the relative energy between the benzene ring and pyrrole C 1s
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H′ with the temperature might be induced by the interplay ofthe molecule−substrate and intermolecular interactions.40
The expected model of the site-speciﬁc interfacial interaction
and the possible molecular distortion at the CoPc/Au(111)interface are also supported by the work function Φ, which canbe determined from the secondary electron cutoﬀ in ARPES,
shown in Figure 4c. The determined Φ for Au(111) of 5.46 eV
is decreased by ca. 0.6 eV upon formation of the dipole layer at
the CoPc/Au(111) interface due to the predominant
contribution of the Pauli repulsion of surface electrons. At
the CoPc/Au(111) interface, Φ at 299 K of 4.81 eV is increased
to 4.86 eV at 23 K. The increase in Φ at the lower temperature
is explained by the formation of the intramolecular dipole26 andthe local electron donation from the substrate at the Co site
accompanied by the molecular distortion due to the site-speciﬁc
interfacial interaction, which overwhelms the additional Pauli
repulsion.
In terms of the site-speciﬁc interfacial interaction, now we
discuss the energy position of the HOMO-derived peak forwell-ordered MPc monolayers on Au(111). It has been
demonstrated that the organic−metal bonding distance plays
a crucial role in interface energetics.41 Judging from thedetermined site-speciﬁc adsorption energies, the overall CoPc−
Au(111) bonding distance might be shorter than the H2Pc−
Au(111) bonding distance due to the relatively strong
interfacial interaction at the Co site in the CoPc/Au(111)interface. Here, the C 2p-derived HOMO peak of physisorbed
monolayers of CoPc, CuPc, ZnPc, and H2Pc on HOPG appears
at Eb = 1.20 ± 0.03 eV.31,32 On the other hand, the C 2pderived peak at the CoPc/Au(111) interface (peak H in Figure2) appears at Eb = 0.61 eV, which is 0.59 eV lower than that at
the CoPc/HOPG interface. Unlike the CoPc/Au(111) interface, the C 2p-derived peak of the CuPc, ZnPc, and H2Pc
monolayers on Au(111), all of which do not form the interfaceinduced πPc−dmetal mixed state, like peak H′ in Figure 2,
appears at Eb = 0.80 ± 0.03 eV,42 which is 0.2 eV higher thanthe case of the CoPc/Au(111) interface. It is considered thatthe low-Eb tailing in the interface state H′ due to the sitespeciﬁc organic/metal interaction plays a key role in the Fermilevel pinning at the organic/metal interface and the resultant
interface energetics, which requires further systematic examination for more detailed understanding.
Finally, we brieﬂy discuss the dispersion shape and intensityof the Shockley-type interface state S′ using Figure 3 and Figure
4b. In contrast to the H2Pc/Au(111) interface, the dispersion
curve of band S′ in the low-Eb region at the CoPc/Au(111)interface cannot be well-ﬁtted by the free-electron-like parabolabecause of the presence of a kinklike point at around Eb = 165± 10 meV and k∥ = 0.14 ± 0.02 Å−1, as indicated in Figure 4b
using an eye-catching curve and arrow. The observed Eb
position at the kinklike point corresponds to the e−ph couplingenergy by CoPc intramolecular vibrations as indicated in theinset of Figure 4a. This evidence suggests the interaction
between surface electron systems and intramolecular vibrations,which might be dominated by the site-speciﬁc molecule−
substrate bonding strength. Furthermore, as seen in Figure 3,
the intensity of band S′ at the CoPc/Au(111) interface isnoticeably decreased at k∥ = 0.14 ± 0.02 Å−1, whichcorresponds to the Brillouin zone boundaries of the CoPc
molecular unit cell as determined from LEED. The reduced
intensity of band S′ near EF can be explained by the band gap
opening at the adsorbate’s zone boundaries.4 Since such a
reduced intensity of band S′ is not observable at the H2Pc/

Figure 4. (a) Temperature dependence of the Shockley-type interface
state of CoPc/Au(111) at θ = 0° (k∥ = 0 Å−1), together with the
Shockley surface state of the clean Au(111) surface at 23 and 299 K(dotted line). The oﬀ-normal ARPES spectra (θ = 33°) of CoPc/
Au(111) at 23 and 299 K are shown in the inset. (b) E−k∥ map of the
Shockley-type interface state of CoPc/Au(111) at 23, 133, and 299 K.
(c) Secondary electron cutoﬀ region in ARPES at hv = 45 eV,
measured with a −5 V bias voltage to detect photoelectrons aroundzero kinetic energy.

due to the temperature-dependent interfacial bonding distance,yielding the change in the site-speciﬁc adsorption energy ΔEa =1.2 meV/Å2 at the Pc-ligand site and ΔEa = 9.0 meV/Å2 at theCo site from 299 to 23 K. The determined site-speciﬁc ΔEawith positive values can be explained by the shortening in themolecule−substrate bonding distance at the low temperature
with the site speciﬁcity. The stronger interfacial interaction at
the Co site than the Pc-ligand site is supported by thephotoemission and XSW experiments on the CoPc/Ag(111)interface,18,39 which suggests that the planar CoPc is distorted
and exhibits a nonplanar conformation with the Co atom
protruding toward the substrate. Such a site-speciﬁc strong
interfacial interaction introduces the increase in the desorption
temperature of MPc monolayers at metal surfaces.17
On the other hand, the molecular electronic states H and H′
are getting intense with decreasing the temperature, as shownin the inset of Figure 4a, due to the possible increase in themolecular ordering and the interfacial electronic coupling at 23
K, which aﬀects the matrix element. Furthermore, from 299 to
23 K, peaks H and H′ slightly shift to the higher Eb side (ca. 16
meV). If band X originates from the partially ﬁlled LUMO by
the charge transfer at the interface, band X should shift to thehigher Eb side together with peaks H and H′ with decreasing
the temperature, which is opposite to the observed evidence;
therefore, the possibility of the partially ﬁlled LUMO is again
ruled out for the origin of band X. The observed shift in H and
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J. Phys. Chem. C 2016, 120, 24307−24313

The Journal of Physical Chemistry C■

Au(111) interface, the Co atom in the molecule may play a
crucial role as a scatterer. The resultant band gap opening may
also aﬀect the intensity distribution of band X, which shows thehigher intensity at around the zone boundaries probably due tothe overlap with the low-Eb component in the gap-opened band
S′. In the present work, it is quite diﬃcult to discuss thedispersion feature of band S′ at the CoPc/Au(111) interface
precisely due to the broad line shape. Low-energy ARPES
experiments on high-quality samples may give a new insight
into the e−ph interaction at organic/metal interfaces, as has
been demonstrated for the Shockley state of the clean Cu(111)
surface.3

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4. CONCLUSION
We have characterized the electronic structure at the CoPc/
Au(111) interface in terms of the site-speciﬁc interfacialinteraction. The high-resolution C 1s, N 1s, and Co 3p
photoemission measurements on the CoPc/Au(111) interface,as compared with those on the CoPc/HOPG interface and the
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■

ArticleAUTHOR INFORMATION

Corresponding Author*E-mail: yamane@ims.ac.jp. Phone: +81-564-55-7394.
Author Contributions

The manuscript was written through contributions of all
authors. All authors have given approval to the ﬁnal version of
the manuscript.
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
The authors would like to thank the staﬀ of UVSOR for theirkind support in the present ARPES experiment. We also thankProfessor Masahiro Hiramoto (Department of Materials
Molecular Science, IMS) for the assistance of the puriﬁcationof CoPc and H2Pc. This work is supported by a Grant-in-Aid
for Young Scientists (A) 24685032 and Grant-in-Aid for
Scientiﬁc Research (A) 23245007 from the Japan Society for
the Promotion of Science and Grant for Basic Science Research
Projects from the Sumitomo Foundation 150301.
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Article

The Journal of Physical Chemistry C

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