Article
pubs.acs.org/JPCC

Direct Synthesis of Vanadium Phthalocyanine and Its Electronic andMagnetic States in Monolayers and Multilayers on Ag(111)
Keitaro Eguchi,† Takeshi Nakagawa,†,‡ Yasumasa Takagi,†,‡ and Toshihiko Yokoyama*,†,‡†

Department of Structural Molecular Science, SOKENDAI, The Graduate University for Advanced Studies, Myodaiji, Okazaki444-8585, Japan‡
Department of Materials Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, JapanS Supporting Information*

ABSTRACT: Vanadium phthalocyanine (VPc) monolayers and multilayers weresynthesized on Ag(111), and the electronic and magnetic states of an unachieved VPc
with a divalent state of V were investigated. The VPc monolayer was fabricated by directly
depositing the V atoms on a metal-free phthalocyanine (H2Pc) monolayer under ultrahigh-vacuum conditions. The VPc multilayer was synthesized by repeated VPc monolayer
deposition and subsequent sample annealing at approximately 450 K. The N 1s X-ray
photoelectron spectra (XPS) of these samples showed a remarkable reduction in the peakassigned to H-bonded N atoms, concomitant with the appearance of a new peak attributed
to V-bonded N atoms close to the peak of iminic N. Additionally, the oxidation state of V
estimated from the V 2p XPS peak position corresponded to 1.6 and 2.4 in the monolayer
and multilayer samples, respectively. These results clearly imply that VPc monolayers and multilayers were successfully obtained.
The main ground-state electronic conﬁguration of the V center was found to be 2Eg by angle-dependent V L-edge X-ray
absorption spectroscopy. Furthermore, X-ray magnetic circular dichroism (XMCD) measurements suggest that this 2Eg state was
mixed with the 2A1g state by spin−orbit coupling in the ground state. Data revealed that VPc shows a paramagnetic state on the
Ag surface and in an H2Pc ﬁlm but an antiferromagnetic state in the multilayer. Partial electron charge transfer was also observed
from the Ag surface to the V center at the VPc/Ag(111) interface, leading to a signiﬁcant decrease in XMCD signals in themonolayer.

1. INTRODUCTION
Superior and unique characteristics, such as relatively highthermal and chemical stabilities as well as optical andsemiconducting properties, have contributed to the wide useof metal phthalocyanines (MPcs) in industrial products likedyes,1 gas sensors,2 organic light-emitting diodes,3 and organic
photovoltaics.4 While these properties have attracted interest inorganic and molecular electronics,5 the recent discovery of
exciting magnetic properties in MPcs has extended theirapplication to molecular spintronics.6,7 To functionalize andmanipulate intentionally their electronic and magnetic states inorganic ﬁlms and/or on substrate surfaces, it is important tosystematically understand these properties of MPcs.
Typically, MPcs consist of a central metal ion and a πconjugated macrocycle. Since the discoveries of CuPc and FePc
in 1927 and 1928, respectively,1 several types of MPcs havebeen synthesized by changing central metal ions andcoordination atoms and/or modifying the phthalocyanine
ligand. In the 3d transition metal series, all metal ions could
be coordinated to the phthalocyanine ligand. However, thesynthesis of MPcs in which the central metal ion presents a
simple divalent state has been limited to Ti, Cr, Mn, Fe, Co, Ni,
Cu, and Zn.8 Although Sc and V have displayed trivalent and/or quadrivalent states in scandium diphthalocyanine (ScPc2),9iodo(phthalocyaninato)vanadium(III) (VIPc),10 and oxyvanadium phthalocyanine (VOPc),11 ScPc and VPc comprising a© 2015 American Chemical Society

divalent metal center have not been synthesized to date becauseof their instability in air. The absence of these molecules limits
our understanding of MPc electronic and magnetic structures,making their synthesis necessary. This study focused on VPc as
a target compound because chemically stable VOPc is easily
obtained by a chemical method and its electronic and magnetic
properties have been investigated in bulk,12 thin ﬁlms,13 andmonolayers.14,15
One way to generate VPc is direct synthesis on a substrate in
the absence of air, as reported by Gottfried et al.16 In thisprevious report, Co-tetraphenylporphyrin (CoTPP) was
obtained by vapor deposition of tetraphenylporphyrin(H2TPP) molecules and Co atoms on Ag(111) under ultrahighvacuum (UHV) conditions. This approach has also provenuseful for MPc and metal porphyrin (MPor) synthesis,
particularly FeTPP, 1 7 − 2 1 CoTPP, 2 1 , 2 2 NiTPP, 2 3 , 2 4
ZnTPP,22,25,26 CeTPP,27,28 iron tetramethylporphyrin,29 and
FePc,30 which have been characterized by X-ray photoelectron
spectroscopy (XPS) and scanning tunneling microscopy(STM). The chemical reaction involved in this direct synthesis
has also been studied in detail by theoretical calculations andthermal desorption spectroscopy (TDS).22,26 Gas-phase
Received: December 28, 2014
Revised: April 15, 2015
Published: April 16, 2015
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on sapphire, Si, and Au substrates.52,53 In addition, large gvalues of 2.4−2.6 have been reported in α- and β-FePc,41,44
suggesting a large contribution of an unquenched orbital
magnetic moment. Magnetic susceptibility measurements of
MnPc single crystals41,54−56 and theoretical calculations31 haveindicated that the 4A2g (b2g2 eg2 a1g1 b1g0) state is a ground statefor MnPc. In addition, XAS57 and XMCD58 of MnPc thin ﬁlms,
UV−visible MCD spectra of MnPc in Ar gas,59 and theoretical
calculations60,61 have proposed the 4Eg (b2g1 eg3 a1g1 b1g0) state
as a ground state. In contrast to FePc, MnPc has presented aferromagnetic state in the β phase below 8.3−8.6 K.55,56,58 The
α-MnPc phase showed a Weiss temperature of −3 K inmagnetic susceptibility measurements.62
The eﬀective magnetic moment of CrPc was estimated as
3.49μB at room temperature, which lay between high (S = 2)
and intermediate (S = 1) spin states.41 However, the details of
its electronic and magnetic properties remain unclear. The
electronic and magnetic properties of TiPc are also unsolved,
although its synthesis has been reported.8 To the best of ourknowledge, no experimental or theoretical study has beenreported for VPc or ScPc. The occupancy of 3d orbitals in lessthan half the 3d electron systems, the order of the split 3dorbitals, and their spin multiplet and magnetic states also
remain ill-understood, making the electronic and magnetic
states of VPc worth investigating.
In this study, VPc monolayers and multilayers weresynthesized by depositing V atoms and metal-free phthalocyanine (H2Pc, Figure 1a) on Ag(111) under UHV conditions.

theoretical calculations of an unsubstituted porphyrin haveprovided a three-step mechanism for this reaction. Brieﬂy, themetal atom is ﬁrst captured by four central N atoms, and one H
atom bound to a pyrrolic N atom is subsequently transferred tothis central metal. Finally, the other H atom is also transferredto the metal and H2 molecule desorption produces MPor.22,26
As expected from the theoretical calculations, D2 desorption
from D2TPP was conﬁrmed by TDS.26 In addition, theseprevious reports showed that the reactivity depends on thenature of the central metal and the activation barrier decreases
in the order Zn, Cu, Ni, Co, and Fe.26 Therefore, this wellestablished method may be suitable for the synthesis of airunstable metal complexes.
The electronic and magnetic properties of MPcs, particularly
those containing 3d metal centers, have been investigatedexperimentally and theoretically. In typical planar MPcs, the 3dorbital of the metal ion is split into a1g (dz2), b1g (dx2−y2), b2g(dxy), and eg (dxz, dyz) levels because of the D4h symmetricalmolecular structure. Although these MPcs presented various
unique properties, the b1g orbital exhibited the highest energy
level irrespective of the central metal ion,31,32 while the otherthree orbitals were considered close to each other, making theelectronic state diﬃcult to determine. Theoretical calculations
suggested that the 3d electrons displayed the conﬁgurations a1g2
b2g2 eg4 b1g2 in ZnPc, a1g2 b2g2 eg4 b1g1 in CuPc, and a1g2 b2g2 eg4
b1g0 in NiPc.31,32 These electronic conﬁgurations have beenvalidated by X-ray absorption spectroscopy (XAS).33,34 Theysuggest the existence of spin in CuPc but no spin in ZnPc andNiPc, which is in good agreement with electron paramagnetic
resonance (EPR)35 and magnetic susceptibility36,37 measurements.
The electronic conﬁguration of the central metal ion
becomes more complicated when the number of 3d electrons
is less than seven. Theoretical calculations have indicated thatthe ground state of Co corresponded to 2A1g (b2g2 eg4 a1g1
b1g0)31,38,39 and 2Eg (b2g2 a1g2 eg3 b1g0)32 in CoPc. Angledependent XAS33 and magnetic susceptibility40 measurements
have suggested the 2A1g state as the ground state. A mixed stateof 2A1g and 2Eg has also been proposed to explain the large
eﬀective magnetic moments of CoPc obtained by magnetic
susceptibility measurements.41 CoPc has exhibited paramagnetic behavior in its α and β phases above 80 K. Further,
Serri et al.42 have observed high-temperature (∼100 K)
antiferromagnetic coupling in an α-CoPc thin ﬁlm.
Similarly, several electronic conﬁgurations have beensuggested for FePc and MnPc. Theoretical calculations havepredicted 3EgA (b2g2 eg3 a1g1 b1g0)31 and 3A2g (b2g2 a1g2 eg2
b1g0)32,43 as ground states for FePc. The 3EgA ground state in αand β-FePc has been widely supported by magnetic
susceptibility,44 Mössbauer,45 XAS,33,46 and X-ray magnetic
circular dichroism (XMCD)47 measurements, whereas few
experimental results have observed the 3A2g state.48 In addition
to these states, a potential 3B2g ground state (eg4 b2g1 a1g1 b1g0)49
and a spin-mixed state of the triplet and quintet states50 havebeen proposed by magnetic susceptibility and a combined
experimental XAS−theoretical approach, respectively. Magnetic
properties of FePc strongly depend on its crystal structure. In
the most stable β-FePc phase, magnetic interactions between
FePc molecules were moderate and Weiss temperatures of −9
K41 and 7 K51 were reported. On the other hand, Evangelisti etal.44 have observed ferromagnetic behavior in α-FePc at a Curie
temperature of approximately 10 K. This ferromagnetic
behavior has also been detected in FePc thin ﬁlms fabricatedFigure 1. Molecular structures of (a) H2Pc and (b) VPc. Each N atom
appears as Nimi (−N) and NHpyr (−NH−) for H2Pc and NV for VPc.
(c) Low-energy electron diﬀraction (LEED) pattern of the H2Pc
monolayer on Ag(111) obtained at an incident electron energy of 11
eV. Yellow lines represent the H2Pc lattice. The red arrow shows the
direction of Ag [112̅]. The green arrow marks a sham spot originatingfrom the ﬁlament’s artiﬁcial light.

The metalation reaction was evaluated by in situ XPS.
Subsequently, the electronic and magnetic states of VPc were
investigated by XAS and XMCD. This is the ﬁrst report on
synthesis of VPc and characterization of its electronic andmagnetic states.

2. EXPERIMENTAL SECTION
Sample preparation was performed under UHV conditions(base pressure of ∼2 × 10−8 Pa). The VPc monolayer was
synthesized in the XPS chamber as follows. Clean Ag(111) was
obtained by repeating a cycle of Ar ion sputtering and annealing
at 800 K. Its ordering structure and cleanliness were checked by
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of VPc with Ag(3 ML)/Cu(111) may be almost identical tothat with a Ag(111) single crystal.
VPc monolayers and multilayers for XAS and XMCD
measurements were prepared according to the same procedures
as XPS sample preparation. Additionally, a VPc monolayer was
prepared on an H2Pc ﬁlm (3 ML) formed on Ag/Cu(111) andcovered by 1 ML H2Pc to investigate the absence of interaction
between central V and substrate or between V ions in the upperand lower layers.
XAS and XMCD measurements were performed on an
XMCD system (P < 1 × 10−8 Pa) equipped with a 7 T
superconducting magnet and a liquid He cryostat (T = 3.8 K)
at the end station of the bending magnet beamline 4B of
UVSOR-III (Institute for Molecular Science, Okazaki, Japan).
This XMCD measurement system has been detailed
previously.70,71 Linearly polarized XAS measurements in the
N K-edge and V L2,3-edge regions were conducted at incident
angles θ of 0°, 30°, and 55° with respect to the surface normal
at a temperature of 5 K. The photon energy in each spectrum
was set by use of identical features in N and O K-edge regions,which originate from contaminations in the beamline optics. Xray energies were calibrated by use of the metallic V L3 and L2
peaks at 512.1 and 519.8 eV, respectively. The nominal energy
resolution (E/ΔE) of the linearly polarized X-rays in the N Kedge region was estimated as 2000 for 10 ML samples and 1300
for the other samples, and those in the V L2,3-edge and O Kedge regions were 2000 for 10 ML samples and 1000 for theother samples. Circularly polarized XAS measurements at the V
L2,3 and O K edges were performed in a magnetic ﬁeld of ±5 T
at incident angles θ of 0° and 55° at a temperature of 5 K. The
magnetic ﬁeld was applied to the sample along the parallel andantiparallel directions with respect to the X-ray beam. XMCD
spectra were obtained by subtracting μ− from μ+. Here, μ+ (μ−)
represents the applied magnetic ﬁeld antiparallel (parallel) tothe helicity of the X-rays. The estimated degree of circularpolarization of the X-rays was approximately 0.6 from thebeamline parameter. The E/ΔE value of the circularly polarized
X-rays in the V L2,3-edge and O K-edge regions was set to 1000
for 10 ML samples and 600 for the other samples. All XAS andXMCD spectra were recorded in the total electron yielddetection (TEY) mode, and several spectral scans wereaveraged to enhance the signal-to-noise ratio. TEY spectrafrom the sample (Isample) were divided by that of the clean
substrate (Isubst). Here, these signals were ﬁrst divided by the I0
signals recorded with an Au mesh in the upstream of thesample. Note that the resultant normalized spectrum μ is thus
given as μ = [Isample/I0]/[Isubst/I0]. No signiﬁcant beam damage
was observed although the synchrotron radiation typicallylasted more than 10 h.

low-energy electron diﬀraction (LEED) and Auger electron
spectroscopy (AES). No contamination of Ag(111) with asharp (1 × 1) LEED pattern was detected. Commercial H2Pc
(purity >98%) was puriﬁed by sublimation and introduced in ahomemade Knudsen cell equipped with an alumel−chromel(K-type) thermocouple. An H2Pc monolayer was formed on
Ag(111) at room temperature (RT) by using the Knudsen cell.
The deposition rate of H2Pc was set at approximately 0.1
monolayer (ML)/min by adjusting the cell temperature to 470
K. The molecular density of the H2Pc monolayer was deﬁned as
0.5 molecule/nm2, and the H2Pc monolayer self-assembly wasassessed by LEED (Figure 1c). The obtained LEED pattern wasalmost the same as in previous reports.63,64 Next, astoichiometric amount of V (0.5 atom/nm2) was depositedon the H2Pc ﬁlm by electron bombardment evaporation. The
deposited amount of V was estimated by use of the monitor ofa quartz crystal oscillator and by comparing the V 2p XPS
intensity with that of the VOPc monolayer on Ag(111).14 The
LEED pattern showed no signiﬁcant change after thedeposition of V. The VPc multilayer (10 ML) was obtainedby repeating the cycle of deposition of H2Pc and V 10 times
and was subsequently annealed at approximately 450 K for 15min.
XPS measurements were performed at 300 K in a UHVchamber (P < 4 × 10−8 Pa). The sample was irradiated with a
non-monochromatized Mg Kα beam (hν = 1253.6 eV) at anincidence angle of 45° with respect to the surface normal.
Photoelectron signals were detected by a hemisphericalelectron analyzer (SPECS, Phoibos 100) that faced the sample
surface (normal emission detection). Binding energies were
calibrated by use of the Ag 3d5/2 and 3d3/2 lines at 368.2 and374.2 eV, respectively.65 Total energy resolution of themeasurement condition was estimated to be 0.9 eV from thefull width of the Ag 3d5/2 line of 1.0 eV.14 Samples did not show
any radiation damage during XPS measurements.
An XPS peak-ﬁtting analysis was performed with the Voigt
functions and the Shirley background. Lorentzian widths of N
1s were set to 0.84 and 0.50 eV for the multilayer andmonolayer samples, respectively.19 The N 1s X-ray photoelectron spectra of H2Pc and VPc on Ag(111) were obtained bysubtracting the spectrum of the clean substrate from those of
H2Pc and VPc as well as from those of VOPc,14 because the
XPS N 1s region overlapped with the plasmon loss peaks ofAg(111).66
XAS and XMCD measurements were conducted on a 3 ML
Ag ﬁlm grown on Cu(111) as substrate to reduce the Ag
background signals. Cu(111) was cleaned by repeated cycles of
Ar ion sputtering and annealing at 800 K. Next Ag was
deposited in a layer-by-layer manner at RT on the Cu. The
thickness of Ag was estimated to be 3 ML from the AES
intensities of Cu MVV. As reported previously,67 the (9 × 9)
superstructure was observed by LEED. Note here that Ag(3ML)/Cu(111) shows an almost equivalent electronic structure
to a Ag(111) single crystal. It is well-known that face-centeredcubic (fcc) (111) surfaces exhibit speciﬁc surface states and thatbinding energy of the surface state in Ag/Cu(111) decreases
with Ag thickness. The diﬀerence of binding energies of thesurface state between Ag(3 ML)/Cu(111) and Ag(111) has
been reported as ∼50 meV, which is 6 times smaller than thatbetween Ag(111) and Cu(111) (∼300 meV).68 Moreover, theself-assembled structure of a porphyrin ﬁlm deposited on Ag(3ML)/Cu(111) is found to be the same as that on Ag(111).69
These previous remarks indicate that the interfacial interaction

3. RESULTS AND DISCUSSION
3.1. Direct Synthesis of VPc. Metalation of H2Pc on asubstrate is an excellent MPc synthetic technique. The
formation of MPc can be conﬁrmed by X-ray spectroscopy
because H2Pc and MPc exhibit a remarkable diﬀerence inelectronic states in the N 1s region.30 Therefore, N 1s X-ray
photoelectron spectra were determined for H2Pc and VPc
samples.
Figure 2 shows N 1s X-ray photoelectron spectra for H2Pc
and VPc monolayers. Two main peaks at 398.4 and 400.1 eVand one satellite peak at approximately 402.5 eV were observedfor H2Pc, which is in good agreement with previous
reports.30,72,73 According to the computational results,72 the9807

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Figure 2. N 1s X-ray photoelectron spectra of 1 ML H2Pc and VPc
taken at room temperature. The components of Nimi and NHpyr in H2Pc
and NV in VPc in the main peaks are shown by blue and red lines,
respectively. Shake-up satellite peaks at approximately 402.5 eV in
H2Pc and approximately 401 eV in VPc appear as gray lines. Greenlines represent peak ﬁtting residues.

Figure 3. N 1s X-ray photoelectron spectra of 10 ML H2Pc and VPc
multilayers (10 ML) taken at room temperature. Spectra of VPc before
and after annealing are shown at the center and bottom, respectively.
Components of Nimi and NHpyr in H2Pc and NV in VPc in the main
peaks are represented by dark blue and red lines, respectively. Light
blue lines correspond to the unidentiﬁed peak NX. Shake-up satellite
peaks at approximately 402.5 eV in H2Pc and approximately 401 eV in
VPc appear as gray lines. Green lines indicate peak ﬁtting residues.

main peaks at 398.4 and 400.1 eV were assigned to iminic
H(−N, N imi ) and pyrrolic N atoms (−NH−, N pyr),
respectively. Although two diﬀerent peaks corresponding toiminic N atoms were expected on each side of the Nimi peak in
H2Pc, the peak ﬁtting was performed as a single contribution
because the energy diﬀerence only equaled 0.4 eV.72 The Nimi/
NHpyr ratio amounted to 2.99, consistent with the stoichiometryof H2Pc. The peak at approximately 402.5 eV was attributed tothe shakeup satellite.73
After V deposition on H2Pc at RT, the NHpyr peak intensity
decreased drastically, while a main peak at 398.5 eV and asatellite peak at approximately 401 eV were observed in the N
1s spectrum. Here, the peak ﬁtting was performed by assumingthat the peak at 400.1 eV is the component of H2Pc and theintensity ratio Nimi/NHpyr is ﬁxed at 3.00, because it is hard to
separate the components of NV in VPc and Nimi in H2Pc due tothe low energy resolution of the experimental system. The
spectrum presented a very similar shape to that of MPcs, suchas TiOPc,74 VOPc,13,14 FePc,30 and CuPc,75 clearly implyingthat the deposited V was coordinated by the phthalocyanine
ligand and N−V bonds formed between V and the inner N
atoms. The contribution of all N atoms in VPc (NV, see Figure1b) to the total intensity of the N 1s spectrum equaled 0.88
after peak ﬁtting. Therefore, it is found that H2Pc easily reactedwith the deposited V without annealing, unlike the direct
synthesis of Zn porphyrin, which required annealing.25
Although the metalation of H2Pc occurs readily for VPc and
FePc30 monolayers, the metalation of multilayers usuallyproceeds with diﬃculty when metallic atoms are deposited on
H2Pc multilayers.30 Therefore, the VPc multilayer (10 ML) was
prepared by repeating the VPc monolayer synthesis 10 times tosolve this problem. Figure 3 shows the N 1s X-ray
photoelectron spectra of the 10 ML ﬁlm. Similar to the H2Pc
monolayer, two peaks corresponding to Nimi (398.6 eV) andNHpyr (400.2 eV) were detected in this N 1s spectrum, in
addition to a satellite peak (∼402.5 eV). The NHpyr peakintensity decreased concomitant with the appearance of the NV
peak for the as-deposited VPc multilayer. However, the ratio
between the NV peak intensity and the total N 1s peak intensity
amounted to 0.61. Furthermore, the appearance of a new NX
peak was conﬁrmed by peak ﬁtting, although the origin of thispeak was unclear (it might be a V−NH state but we could not
identify its origin). Therefore, VPc partially formed in themultilayer but the quality of the ﬁlm was insuﬃcient. Here, thepeak ﬁtting was performed in a similar manner to the case of

the VPc monolayer; namely, we assumed that the peak at 400.2
eV originates from H2Pc and the intensity ratio Nimi/NHpyr is
3.00. The NX peak was used to give a reasonable ﬁt because noconvincing ﬁt was obtained without the NX peak. To promote
the reaction of V with H2Pc and the quality of the VPc ﬁlm, thesample was annealed at ∼450 K. Annealing further reduced theintensity of the NHpyr peak and enhanced that of the NV peak for
the VPc multilayer. The NV peak intensity corresponded to
81% of the total N 1s XPS intensity, suggesting that annealingenhanced the VPc ﬁlm quality.
XPS measurements were also performed in the V 2p region
to investigate the oxidation state of the central V in VPc
monolayer and multilayer. Figure 4a shows the V 2p X-ray
photoelectron spectra for VPc monolayer, as-deposited andannealed VPc multilayers (10 ML), and VOPc multilayer (10ML) acting as a reference. The oxidation state of V in VPc canbe estimated from the V 2p3/2 peak position.76−84 The VOPc
multilayer displayed a quadrivalent formal oxidation state for V
and a V 2p3/2 binding energy of 516.4 eV, which is close to thatof V(IV) in VO2 (Figure 4b). The annealed VPc multilayer
exhibited a V 2p3/2 peak at 514.8 eV, representing a lower
energy than that for V(III) in V2O3 but higher energy than thatfor metallic V. The oxidation state of V in the VPc multilayer
was thus expected to be smaller than trivalent. Furthermore, if alinear relationship between the oxidation state of V and thebinding energy of V 2p is assumed, the nominal oxidation stateof V in the VPc multilayer was estimated as 2.4 at theintersection of the line and the V binding energy. Here astraight line was obtained by connecting the average binding
energies of V in the metal and in VOPc, and the standard
deviation of the binding energies of 0.14 eV for V(0) might givean estimated error of ±0.12 in the oxidation states of V. This
result supports the formation of VPc. In addition, the peak of
the as-deposited multilayer at approximately 513 eV reduced
drastically in the annealed sample and shifted toward higher
binding energy upon annealing. This means that the annealing
process accelerates VPc formation, consistent with the N 1s
XPS results.
In the VPc monolayer, the peak position was estimated to be
513.9 eV by peak ﬁtting (see Figure 4a). The peak showed ashift of 0.9 eV lower than the binding energy of VPc multilayer,
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indicates that the ﬁnal-state relaxation eﬀect is less important
for the shift of the XPS binding energy and that charge transfer
in the initial state should be a major contribution to the shift ofthe binding energy.
C 1s XPS measurements were also conducted for H2Pc and
VPc monolayers and multilayers (see Supporting Information).
When the spectra of 10 ML VPc and 10 ML H2Pc arecompared, the benzene C peak of VPc shifts 0.2 eV toward ahigher binding energy side, although no peak shift was observedin that of pyrrole C. On the other hand, the shifts of benzene C
and pyrrole C of the VPc monolayer are 0.1 and 0.3 eV,
respectively, in comparison with those of H2Pc. In addition,
both benzene and pyrrole C in the VPc monolayer appear at alower binding energy side (0.3 eV), compared with those in the
VPc multilayer. Thus, we observed site-speciﬁc chemical shifts.
In summary, VPc monolayer and multilayer were directly
synthesized by deposition of H2Pc molecules and V atoms on
Ag(111) under UHV conditions. The VPc monolayer was
obtained by V deposition on the self-assembled H2Pc
monolayer without sample annealing. On the other hand, themetalation reaction alone was insuﬃcient for the 10 ML sample
but was promoted by annealing, leading to the VPc multilayer.
3.2. Electronic Conﬁguration and Magnetic States of
VPc. Angle-dependent linearly polarized XAS measurements,
which typically provide molecular orientation and unoccupied
orbitals,33,96−98 were performed to elucidate the electronic
conﬁguration of the central V in VPc. Furthermore, XMCD
measurements were conducted with circularly polarized X-rays
to investigate the magnetic properties of VPc. In addition, themagnetic property of a VPc monolayer embedded in an H2Pc
ﬁlm (see Experimental Section) was also investigated to clarify
the interfacial interaction between VPc and the Ag surface andthe intermolecular interaction between VPc molecules inneighboring layers. All XAS and XMCD measurements were
performed at 5 K because ferromagnetic or antiferromagnetic
states of MPcs (M = Mn,56 Fe,44 and Co42) have interestingly
been observed at low temperature. Although the vibration ofmolecules is suppressed at low temperature, no structuralchange is expected in this temperature region.
The molecular orientation of VPc in monolayers andmultilayers was ﬁrst investigated to determine the electronic
conﬁguration of the central V. Angle-dependent N K-edge Xray absorption spectra is shown in Figure 5. The spectrum of
VPc exhibited at least eight peaks (A−H), similar to those ofother MPcs.14,98−100 Here, we notice that VPc provides two
peaks (peaks C and D) in the energy region between 401 and405 eV, although 10 ML H2Pc gives three peaks (seeSupporting Information) in this region. This diﬀerence between
nonmetallic and metal phthalocyanines has been also observedin other MPcs (M = H2, Fe, Co, and Cu),98 supporting theformation of VPc. On the other hand, we could not detect areliable diﬀerence between X-ray absorption spectra of VPc and
H2Pc monolayers (see Supporting Information). In addition, abump structure at ∼403 eV in the spectrum of the VPcmonolayer at 55° (green arrow in Figure 5) might be an artifact
originating from incomplete elimination of the contaminationeﬀect on the beamline optics in the spectral normalization
process (I/I0) due to small signals from the monolayer sample.
Peaks A−D were mainly attributed to N 1s → π* molecularorbital transitions, and peaks E−H corresponded to transitions
involving σ* resonances.98,99,101 In π-conjugated molecularsystems, the transition from 1s to a p-type molecular orbital ismost intense when an electric ﬁeld vector of linearly polarized

Figure 4. (a) V 2p and O 1s X-ray photoelectron spectra of VPc
monolayer, VPc multilayer before and after annealing, and VOPc
multilayer at room temperature. V 2p spectra were split into 2p3/2 and2p1/2 because of spin−orbit interactions. Experimental data are shownas dots, while ﬁtting curves are represented by black lines. The guidelines indicating peak positions are shown in orange. (b) Relationshipbetween oxidation state of V and binding energy of V 2p3/2 in XPS.
The triangle corresponds to the binding energy of VOPc obtained inthis study. Circles represent the reported binding energies of clean V
metal and vanadium oxides.76−84 The binding energy of V 2p3/2 in VPc
monolayer and multilayer are shown by red and blue dashed lines,
respectively.

and the nominal oxidation state of V in this monolayer was
estimated as 1.6 by using the relationship between oxidation
state and binding energy (Figure 4b). The XPS peak shift of themetal center was also observed in phthalocyanine or porphyrinsystems, such as MnPc,85 FePc,30,86 CoPc/Ag(111),85 CoTPP/
Ag(111),16,87,88 CoPc on polycrystalline Au,89 FePc,86 ZnPc/
Au(100),90 CuPc/Al(100),91 ZnPc/Pt(111),92 MgPc on the Au
foil,93 and PbPc/Ag(111).94 Spectroscopic methods havesuggested that the shift toward lower binding energies
originates from charge-transfer eﬀects. Although the contribution of the polarization screening eﬀect is also expected, theeﬀect has been clariﬁed to be considerably smaller than the
charge-transfer screening eﬀect in ZnPc and MgPc on Au
systems.90,93 The charge-transfer eﬀects generally depend on a
combination of adsorbed molecules with metal substrates. The
V 2p3/2 line of the VPc monolayer shows a 0.9 eV shift, which isrelatively smaller than those of FePc (1.5 eV)30,86 and CoPc
(1.8 eV)85 on Ag(111), CoPc (∼1.6 eV) on Au,95 and CuPc
(1.6 eV) on Al(100).91 Interaction of the VPc monolayer with
the Ag surface is thus expected to be as weak as that for MnPc
on Ag(111),85 which may result from the small electron aﬃnityof the metal center. The shift of binding energy also originatesfrom the ﬁnal-state charge-transfer (relaxation) eﬀect. As shownin the next section, however, a similar peak shift is observed in
X-ray absorption spectra of the VPc monolayer on Ag, wherethe ﬁnal-state relaxation eﬀect should be smaller due to theexistence of the excited electron within the V orbitals. This
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The Journal of Physical Chemistry C

Figure 5. Angle-dependent linearly polarized X-ray absorption spectraat the N K edge for VPc multilayer, H2Pc-embedded VPc monolayer,
and VPc monolayer on Ag(111). The green arrow indicates an artifact
bump structure. Measurements were performed at 5 K. The energy
resolution (E/ΔE) amounted to 2000 for the VPc multilayer andH2Pc-embedded VPc and 1300 for the VPc monolayer.
Figure 6. Angle-dependent linearly polarized X-ray absorption spectraat V L2,3 edge and O K edge for (a) metallic V, (b) VPc monolayer, (c)H2Pc-embedded VPc monolayer, (d) VPc multilayer, and (e) VOPc
multilayer. Peak positions are represented by green dashed lines.
Measurements were conducted at 5 K. The energy resolution (E/ΔE)
equaled 2000 for metallic V and VPc and VOPc multilayers and 1000
for VPc monolayers. Molecular orientation of VPc is schematicallyshown inside the graph for spectra b−d. Schematic energy diagrams of
V 3d orbitals in VPc and VOPc are shown in insets to the right. Peaks
A, B, C, and D (A′, B′, C′ and D′) correspond to eg, a1g, and b1g (b2g,eg, a1g, and b1g) orbitals, respectively, in VPc (VOPc).

X-rays points in the direction of the unoccupied orbital, and notransition occurs when the vector is perpendicular to theorbital.96,97 The π* peak intensity of the VPc monolayer was
extremely weak at an incident X-ray angle of 0° and increased
when the X-ray electric ﬁeld vector was gradually tilted awayfrom the surface, indicating that the phthalocyanine framework
lay ﬂat with respect to the surface. Similarly, the π* intensity of
the H2Pc-embedded VPc monolayer was small at 0° andincreased with the incident X-ray angle. Therefore, VPc wasadsorbed in a ﬂat orientation with respect to the surface eventhough the signals originating from the upper and lower layersof the H2Pc ﬁlm were also included in the spectra. On the other
hand, the π* intensity of the VPc multilayer was large at 0° anddisplayed smaller angular dependence, meaning that VPc wastilted with respect to the surface in the upper side of the ﬁlm.
This result might imply the unreacted V disturbs the adsorptionof H2Pc overlayer with ﬂat orientation. The estimatedmolecular orientation was 40° with respect to the substratesurface, calculated from the maximum intensity ratio between
I0° and I55° (0.63).97 This angle is close to that observed in the βphase.42
Angle-dependent V L-edge X-ray absorption apectra of VPc,
metallic V, and VOPc are shown in Figure 6. All spectra weresplit into L3 and L2 regions because of the spin−orbit
interactions in the V 2p orbitals. In agreement with the XPS
results, the V L-edge XAS peak position correlated linearly withthe oxidation state of V,102 enabling an estimate of theoxidation state of V in VPc. Here, this estimate used the main
peak position for the V L3 edge, and the V metal peak position
was assumed to be 512.1 eV. The VOPc multilayer (10 ML)
and VPc monolayer peaks appeared at 514.4 and 513.1 eV,
respectively, while the VPc multilayer and H2Pc-embedded VPcmonolayer exhibited a peak at 513.4 eV. With the assumptionthat V displayed formal oxidation states of 0 and +4 in themetal and VOPc, respectively, its oxidation state was estimatedto be +2.3 for the VPc multilayer and the H2Pc-embedded VPcmonolayer and +1.7 for the VPc monolayer, which were ingood agreement with the XPS estimates.
Next, angle-dependent V L-edge X-ray absorption spectra
and their characteristics were addressed. Metallic V exhibitedbroad XAS peaks and no angular dependence with respect tothe incident X-ray. On the other hand, XAS presented ﬁne
structures assigned to peaks A−D (A′−D′) and angulardependence for VPc (VOPc). These angular dependences
provide the unoccupied 3d orbitals in the ﬁlms. Peaks A′−D′
observed for VOPc have been previously attributed totransitions from V 2p cores to b2g, eg, a1g, and b1g orbitals,
respectively.14 The peak assignment for VPc was conductedwith the X-ray absorption spectrum of H2Pc-embedded VPcmonolayer because of its clear angular dependence. In this case,the b1g and b2g orbitals were spread parallel to the surface, while
the a1g orbital is perpendicular to the surface, because VPc
adsorbs in ﬂat orientation with respect to the surface. The main
peak D clearly showed angular dependence. Its intensity was
maximized at 0° and decreased with increasing incident X-ray
angle. Peak D was located at a higher energy than the otherpeaks, enabling its attribution to b1g. This is in accordance withthe previous observations for MnPc,57,85 FePc,86,103 and
CoPc.85,103 In addition, this result excludes the possibility of
the formation of a double-decker structure because the b1g andb2g orbitals are expected to be degenerate in the double-decker
structure. In contrast, the intensity of peak C increased withrising incident X-ray angle, suggesting its assignment to a1g.
This intensity was almost the same as that of peak D, indicatingthat the a1g orbital was unoccupied. Peaks A and B display little
angular dependence and appear at a lower photon energy sidethan peaks C and D, implying that peaks A and B can be
assigned to the eg orbital and its energy level was lower thanthose of b1g and a1g orbitals. Furthermore, the transition to b2g(peak A′) observed in the X-ray absorption spectrum of VOPc
was not detected in that of VPc. Therefore, the b2g orbital of V
in VPc was expected to be fully occupied and an unpaired
electron may occupy the eg orbital, suggesting that theelectronic conﬁguration of V in VPc corresponded to 2Eg(b2g2 eg1 a1g0 b1g0). Therefore, the lowest 3d level in VPc was
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The Journal of Physical Chemistry Cinteraction occurred between 2Eg and 2A1g (b2g2 eg0 a1g1 b1g0)
states because 2A1g exhibits no orbital magnetic moment. This
interpretation of the ground state is consistent with theprevious reports for FePc105 and CoPc.41,105 In these previous
systems, FePc and CoPc ground states were expressed as3
B2g−3Eg and 2A1g−2Eg mixed states, respectively. The groundstate of V in VPc was also expected to be the 2Eg−2A1g mixed
state and its small orbital magnetic moment might have resulted
from the covalent bond character and conﬁguration interaction
with the 2A1g state. The Jahn−Teller eﬀect may also reduce theorbital magnetic moment but was not very eﬀective for the
H 2 Pc-embedded VPc monolayer because of the rigid
phthalocyanine ligand.
The XMCD signals of V decreased drastically for the VPc
multilayer, indicative of the antiferromagnetic state of VPc in
this sample as a result of the antiferromagnetic interaction
between VPc in the upper and lower layers. The magnetic
interaction in the multilayer agrees with the previous
observation in antiferromagnetic α-CoPc42 and β-CoPc106 as
well as ferromagnetic α-FePc44,47 and β-MnPc.54,58 Here, thepossible antiferromagnetic coupling of VPc was addressed byassuming that the VPc multilayer adopts a β structure (metal−
metal distance of 4.8 Å)107 from the molecular orientation of
the multilayer ﬁlm. In this case, the eg orbital of the V center
pointed toward other V centers in the upper and lower layers.
This made the direct overlap of the V center half-ﬁlled eg
orbitals possible, leading to antiferromagnetism.108 Additionally, antiferromagnetic interactions between the ferromagnetic
VPc chains may also exist but their contributions may be
negligible because the distance between VPc chains exceeded
approximately 1 nm, which is estimated by assuming a lateralmolecular distance of 1.4 nm in the ﬂat orientation and a tilt
angle of 42.9° in the β phase (of CoPc),42 and the interchaininteraction was estimated to be less than 2 K, as suggested byprevious reports on the magnetic properties of CuPc109 and
FePc44 ﬁlms.
Finally, the interaction between VPc and Ag surface wasinvestigated. A comparison between X-ray absorption spectra of
VPc monolayer and H2Pc-embedded VPc revealed that peak D
appeared at a lower photon energy for the 1 ML sample (seeFigure 6). In addition, the VPc monolayer produced broader
XAS peaks than H2Pc-embedded VPc. These suggest thatcharge transfer occurred from the Ag surface to the V centerand the produced 2p core hole in V displayed a short lifetime at
the VPc−Ag interface. The charge-transfer eﬀect has also been
observed in other MPc systems deposited on metallic
substrates, and the transition intensities decreased byoccupation of the 3d orbitals.46,85,86,105 The intensity of peakC decreased for VPc, indicating the important role of the a1g
orbital in the electron charge transfer and the formation of a
hybridized state with the Ag surface. However, this reduction inpeak intensity was not signiﬁcant compared with CoPc/
Ag(111),85 FePc/Ag(111),86 and MnPc/Au(100).57 Moreover,the peak intensities related to the b1g orbital diminished in
CoPc/Ag(111),85 FePc/Ag(111),86 and MnPc/Au(100).57 In
contrast, no clear reduction in peak D (b1g) was observed in
VPc/Ag(111), as for CuPc/Au(110).46 Despite the existence of
charge transfer in VPc/Ag(111), this charge transfer was notvery eﬀective from the Ag surface to V.
XMCD results were examined to get an insight into theinﬂuence of interface interactions on VPc magnetic properties.
Figure 7c shows the XMCD signals of VPc monolayer, andFigure 7a shows those of H2Pc-embedded VPc. The VPc

b2g, in agreement with previous results obtained for other MPc
systems.46,57,103 VPc was expected to display a spin state of 1/2
as a result of the electronic state of V.
XMCD measurements were performed at 5 K for these threesamples to investigate the magnetic properties of VPc.
Circularly polarized XAS and XMCD spectra of VPc are
shown in Figure 7a−c. Those of VOPc are also shown in Figure

Figure 7. Circularly polarized XAS (red and blue lines) and XMCD
spectra (black lines) at the V L edge of (a) H2Pc-embedded VPcmonolayer, (b) VPc multilayer, (c) VPc monolayer, and (d) VOPcmonolayer in a magnetic ﬁeld of ±5 T at 5 K and incident X-ray angles
of 0° (normal incidence) and 55° (grazing incidence). All XMCD
signals were magniﬁed twice. Green dashed lines represent the
integrals of the XMCD signals from 507 to 527 eV. The energy
resolution (E/ΔE) of the circularly polarized X-rays in V L2,3-edge andO K-edge regions was set to 1000 for multilayers and 625 for the othersamples.

7d as references for paramagnetic V exhibiting an S value of 1/2.
The magnetic interaction between the central V was ignored in
the H2Pc-embedded VPc monolayer because VPc was
sandwiched between the H2Pc layers and the distance between
neighboring V ions in the layer might be close to 1.4 nm in theﬂat orientation. XMCD spectra of the H2Pc-embedded VPcmonolayer presented explicit signals at incident angles of 0°
and 55°. The XMCD signal of the H2Pc-embedded VPc at 0°
displays almost the same intensity as that of the VOPcmonolayer. Therefore, the spin state of the H2Pc-embedded
VPc was considered to be 1/2, consistent with the electronic
state suggested from the linearly polarized XAS measurements.
The orbital magnetic moment of VPc was approximately twicethat of VOPc, as obtained by the integration of XMCD signals
at L3 and L2 edges (green dashed lines, Figure 7). Because theorbital magnetic moment of VOPc amounted to ca.
0.02μB,12,104 that of VPc was expected to be approximately
0.04μB, although the Eg conﬁguration of a divalent V ion
predicts a much larger orbital magnetic moment of 1μB,
implying quenching of the orbital magnetic moment byformation of the VPc complex. This discrepancy may result
from the signiﬁcant contribution of the covalent bond(overlapping) between V 3dπ and N 2pπ in VPc. However,this explanation may be insuﬃcient. The proposed 2Eg state
contributed to the electronic structure of VPc to a great extent
but did not completely describe its ground-state electronic
conﬁguration. This suggests that a spin−orbit conﬁguration
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monolayer displayed slightly weaker signals than H2Pcembedded VPc, consistent with the XMCD data for CuPc/
Au(110)46 and MnPc/Cu(001).110 However, this result
disagrees with the XMCD signals obtained for CoPc/
Au(111),105 CoPc/Au(110),46 and FePc/Au(110),46 which
decreased dramatically and indicated the disappearance of thespin(s). The XMCD intensity of V decreased at the VPc/
Ag(111) interface, but this signal reduction was not signiﬁcantbecause of the insuﬃcient charge-transfer eﬀect and the small
electron aﬃnity of V as compared to Fe and Co.

ACKNOWLEDGMENTS■

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ASSOCIATED CONTENT

S Supporting Information*

Additional text and two ﬁgures showing C 1s XPS and N Kedge XAS spectra of VPc and H2Pc monolayers andmultilayers; complete refs 29, 46, 58, 72, and 110. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/jp512935v.

■■

We thank Professor Fujikawa, Dr. Matsunami, and Dr. Yamane
for their helpful discussions.

4. SUMMARY
It is not straightforward to study the electronic and magnetic
states of early transition metal phthalocyanines because theirinstability to air makes the development of synthetic
procedures challenging. To overcome this issue, VPc was
directly synthesized on an Ag surface under UHV conditions.
After V deposition on the H2Pc monolayer ﬁlm, the XPS peakascribed to the H-bonded N atoms clearly decreased, while a
new peak assigned to the V−N bond formation appeared.
Similar peak changes were observed in the VPc multilayer,
although it required subsequent annealing after the depositionof V. UHV conditions protected the V ions from oxidation andthe oxidation state of V was divalent, demonstrating the
successful synthesis of VPc on the Ag surface. This also madeinvestigations of the electronic and magnetic states of VPc
possible.
These electronic and magnetic states were evaluated by XAS
and XMCD measurements under UHV conditions. The
electronic conﬁguration of the V ion in VPc was mainly
attributed to 2Eg by angle-dependent XAS. Although this 2Eg
conﬁguration satisﬁed a spin state of 1/2, the small orbital
magnetic moment obtained by XMCD was diﬃcult tointerpret. Mixed conﬁgurations involving 2Eg and 2A1g resulting
from spin−orbit coupling were consequently suggested for the
ground state of VPc. The b2g orbital displayed the lowest energy
level and was occupied in VPc. VPc showed paramagnetic
behavior in the H2Pc-embedded VPc system while it formed an
antiferromagnetic state in the VPc multilayer as a result of
antiferromagnetic interaction between the layers. The VPc/Ag
interface exhibited charge transfer from the Ag surface to V,
similar to other divalent MPcs deposited on metal substrates.
The XMCD signals persisted at the interface, unlike for CoPc
on Au(111)105 and Au(110).46■

ArticleAUTHOR INFORMATION

Corresponding Author*E-mail yokoyama@ims.ac.jp; phone +81 (564)55 7341; fax+81 (564)55 7337.
Notes

The authors declare no competing ﬁnancial interest.
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