Ⅲ-4
Surface andThin Films

Ⅲ -4. Surface and Thin Films

BL6U

Substituent-Induced
Induced Intermolecular Interaction in Organic Crystals
Revealed by Precise Band-Dispersion Measurements
H. Yamane and N. Kosugi

Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan

127

this systematic experiment, we revealed quite small
but different E(k⊥) relations. The transfer integral (t⊥)
of the C 2p band is found to be dependent on theintermolecular distance (a⊥) with the 75±5 meV/Å
relation (see, Fig. 2). Furthermore, we observed thedifferent dispersion phase and periodicity, dependingon the terminal group and central metal in MPc,
which originate from the site-specific intermolecularinteraction induced by substituents [1].
As described above, precise and systematic E(k)studies provide deeper insights into the nature of theintermolecular interaction, which further represents
the importance of the site specificity in the intermolecular interaction as a possible origin of uniquemolecular electronic and magnetic properties.
!!M
60

1 ML
ZnPc

34

! !M
Intensity / normalized

e–

0

SS

H

Ⅲ- 4

HOMO

L

!!M

Cryst.
ZnPc

60
34
0
HOMO

2

1

Binding Energy (vs. EF) / eV0

Fig. 1. The ! dependence of the ARPES spectra (hv =45 eV) and its intensity map for the monolayer andcrystalline films of ZnPc on Au(111) at 15 K.

t!   

Solid-state functionalities of organic molecules aregoverned not only by individual molecular properties
but also by their intermolecular interactions. This
concerted interplay dominates a key process of theelectric conduction in functional molecular systems.
In this work, we have investigated the intermolecularenergy-vs-momentum E(k) relation, originating fromthe molecular stacking periodicity, of sub-100-meV
scale in metal phthalocyanine (MPc) crystalline films.
The small E(k) relation of MPc with different
terminal groups and central metals are sensitive andessential to characterize the intermolecular interactionin terms of the intermolecular distance, the molecularconformation, and the orbital symmetry.
Figure 1 shows the emission angle (!) dependenceof the angle-resolved photoemission (ARPES) spectraand its intensity map for the flat-lying monolayer andcrystalline films of ZnPc on Au(111) at 15 K. For themonolayer, the dispersive and non-dispersive peaks
appear around the binding energy (Eb) of 0~0.32 eV
and 0.74 eV, respectively. The parabolic dispersion at
Eb = 0~0.32 eV is derived from the Shockley state(SS) of the Au(111) surface, which is modified by thecomplex interplay of molecule-substrate interactions.
The non-dispersive peak at Eb = 0.74 eV is derived
from the highest occupied molecular orbital (HOMO)
of C 2p (!) character in ZnPc. The observed HOMOpeak intensity shows a sharp ! dependence with the
maximum at ! = 34°. This is due to the reflection of
the spatial electron distribution of HOMO. For the
ZnPc crystalline film, the SS band of Au(111) issuppressed and the HOMO peak is stabilized as Eb ~1.3 eV. Since the ZnPc molecule deposited on
Au(111) shows the Stranski-Krastanov growth, thequite weak substrate signal of EF appears and isutilized for the energy calibration for the precise E(k)measurement. The ! dependence of the HOMO-peakintensity in the ZnPc crystalline film is almost thesame as that in the ZnPc monolayer; that is, themolecular orientation indicates the layer-by-layer
growth in the crystalline domain and induces orbital
delocalization. Indeed, the HOMO peak of the ZnPc
crystalline film shows a small dispersive behavior
with !. Such a dispersive behavior is not observed in
the monolayer film and is related to the delocalizedband formation.
In order to investigate the k component along the!-! stacking direction (k⊥), we measured the normal
emission ARPES as a function of the photon energy(hv) for crystalline films of various MPc (H2Pc, MnPc,
CoPc, ZnPc, and F16ZnPc) on Au(111) at 15 K. From

Crystalline MPc films
C 2p band @ T = 15 K

F16ZnPc
MnPc

CoPc



ZnPc




a!


H2Pc

a! / Å (from ARPES & XRD exp.)

Fig. 2. The t⊥-vs-a⊥ relation for the C 2p band in MPc
crystals at 15 K, with t⊥/a⊥ = 75 meV/Å line.
[1] H. Yamane and N. Kosugi, Phys. Rev. Lett. 111(2013) 086602.

Ⅲ -4. Surface and Thin Films

BL2A

Near-Edge X-Ray Absorption Fine Structure Spectroscopic Study of the
Annealed Metal Oxide Thin Film
1

E. Kobayashi1, K. K. Bando2 and T. Okajima1,3

Kyushu Synchrotron Light Research Center, Tosu 841-0005, Japan
National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565, Japan
3
Research Center for Synchrotron light applications, Kyushu University, Kasuga 816-8580, Japan2

Ⅲ- 4

Metal oxide is an important material from afundamental physical point of view and is widelyused in the field. It is recognized that the surface
structure and surface chemical states are important for
applications because it can significantly affect thematerial characteristics. In this study, the annealing
behavior of surface chemical states on Magnesiumoxide (MgO) thin film was studied by usingnear-edge X-ray absorption fine structure (NEXAFS).
MgO plays a prototype of metal oxides and is also awide-gap insulator [1]. In addition, MgO is widelyused in the field of catalytic, optical and electricalapplications [2, 3].
Mg K-edge NEXAFS spectra of MgO thin film on
Si(100) wafer (MgO/Si) were measured at thebeamline 2A of the UVSOR in the Institute of
Molecular Science. MgO thin films of about 10 nm inthickness were prepared by RF magnetron sputtering.
The NEXAFS spectra were obtained using the total
electron yield (TEY) mode and the partial
fluorescence yield (PFY) mode at room temperature.
The incident angle of the synchrotron radiation was
0° from the surface normal. The spectra werenormalized to the incident photon flux, recorded as aphotocurrent at a photon-flux monitor consisting of agold-evaporated mesh.
The MgO/Si thin film was annealed at several
temperatures with 373 K ~ 833 K of 10-5 ~ 10-6 Paorder in vacuum. After the annealing, the film wasslow cooled in vacuum.
Figure 1 shows the Mg K-edge NEXAFS spectra of
MgO/Si before and after annealing obtained from
PFY mode. Three strong peaks are observed ataround 1309.1 eV (A), 1314.7 eV (B) and 1316.9 eV(C), respectively. The spectral features are similar to
the experimentally obtained for polycrystalline MgO[4]. However, the intensity ratio of peak A, peak B
and peak C change gradually with a rise in annealing
temperature. Similar changes were also observed in
TEY spectrum.
According to first-principles density functional
calculations, the intensity of peak C is stronger thanpeak B. It is considered that the carbonate andhydroxide on the thin film surface was decomposedand desorbed by heating, the crystallinity of the thinfilm was improved.

A

BC

Intensity (arb. units)

833 K
773 K
673 K
573 K
473 K
373 K

as prepared
1300

1310

1320

1330

1340

Photon Energy (eV)Fig. 1. Mg K-edge NEXAFS spectra of MgO/Si
before and after annealing obtained from PFY mode.
[1] D. M. Roessler and W. C. Walker, Phys. Rev. 159(1967) 733.
[2] E. A. Colbourn, Surf. Sci. Rep. 15 (1992) 281.
[3] G. H. Rosenblatt et al., M. W. Rowe, G. P.
Williams, Jr., R. T. Williams, and Y. Chen, Phys. Rev.
B 39 (1989) 10309.
[4] P. Luches, S. D. Addato, S. Valeri, E. Groppo, C.
Prestipino, C. Lamberti and F. Boscherini, Phys. Rev.
B 69 (2004) 045412.

128

Ⅲ -4. Surface and Thin Films

BL3U

In Situ Observation of Nickel-Borate Catalyst for Oxygen Evolution
Reaction by Soft X-Ray Electrochemical XAFS
M. Yoshida1, M. Nagasaka2, T. Iida1, T. Mineo1, T. Yomogida1, H. Yuzawa2,
N. Kosugi2 and H. Kondoh1
1

Department of Chemistry, Keio University, Yokohama 223-8522, Japan
Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan

Electrochemical hydrogen production from water
has attracted considerable attention due to thepotential toward highly efficient energy conversion.
This reaction consists of two half reactions ofhydrogen and oxygen evolution. However, theefficiency of oxygen evolution reaction (OER) is
insufficient for many electrode materials because of
the high overpotentials. Recently, Bediako et al.
reported that a nickel-borate thin film can function as
an efficient electrocatalyst for OER and the activity
was likely to be dependent on the concentration ofpotassium borate (KBi) in electrolyte aqueous
solution [1]. Therefore, in this study, thenickel-borate thin film was investigated by in situ O
K-edge XAFS measurements under potential control
conditions with changing the electrolyte aqueous
solution.
The
soft
X-ray
electrochemical
XAFS
measurements were performed with the transmission
mode at BL3U of UVSOR, according to the previous
works [2]. Au/Cr/SiC thin film substrates were used
as working electrodes. A home-made electrochemicalcell was used with a Pt mesh counter electrode and a
Ag/AgCl (saturated KCl) reference electrode.
O K-edge XAFS spectra were taken for theelectrodeposition reaction of nickel-borate thin film at
1.0 V vs. Ag/AgCl in a 0.1 M KBi aqueous solution
containing 0.4 mM Ni(NO3)2, as shown in Fig. 1. A
peak associated with oxygen species was observed at
ca. 528.5 eV and kept to grow for 90 min. Next, theelectrolyte solution was changed to 0.5 M KBi
aqueous solution without nickel ions and the XAFS
measurements were tested with changing the applied
electrode potential (Fig. 2). The peak at ca. 528.5 eV
disappeared at the lower potential (0.5 V) andregenerated at the higher potential (1.0 V)
accompanying with the OER activity. In previous
works of Ni K-edge XAFS [1], it is indicated that thenickel borate electrocatalyst forms µ–oxo/hydroxo
nickel centers organized into higher-order domains ofedge sharing NiO6 octahedra at the higher potential.
Thus, our present study demonstrated the presence of
the NiO6 octahedra domain by the direct observationof oxygen species in the nickel borate thin film.
When the concentration of KBi in electrolyte solution
decreased, the XAFS peak was not observed even at
1.0 V accompanying with the decrease of the OER
activity, which indicates that the formation of theNiO6 octahedra domain was suppressed. Therefore,
we found that the high activity of nickel borate thinfilm for OER is derived from the formation of theNiO6 octahedra domain and related with the KBi
concentration in the electrolyte aqueous solution.
Intensity / a.u.

90 min

60 min15 min1.0 V
0.4 mM Ni(NO3)2
0.1 M KBi aq

blank

Ⅲ- 4

526
528
530
532
Photon energy / eV

Fig. 1. Time course of in situ O K-edge XAFS spectra
during electrodeposition reaction of nickel-borate thin
film.


1.0 V

Intensity / a.u.

1.0 V

0.5 V

1.0 V

0.5 V



2

1.0 V

0.5 M KBi aq

526 528 530 532
Photon energy / eV

Fig. 2. In situ O K-edge XAFS spectra underelectrochemical control in 0.5 M KBi aqueous
solution.
[1] D. K. Bediako et al., J. Am. Chem. Soc. 134(2012) 6801., 135 (2013) 3662.
[2] M. Nagasaka et al., J. Electron. Spectrosc. Relat.
Phenom. 177 (2010) 130., J. Phys. Chem. C 117(2013) 16343.

129

Ⅲ -4. Surface and Thin Films

BL3U
BL3U

Polarized
Polarized NEXAFSNEXAFS Study
Study onon Structure
Structure ofof Nitrogen
Nitrogen Doped
Doped Rutile
Rutile TiO
TiO22(110)(110)
Y. Monya1, M. Yoshida1, M. Nagasaka2, H. Yamane2, N. Kosugi2 and H. Kondoh1

Ⅲ- 4

Graduate School of Science and Technology, Keio University, Yokohama 223-8522, Japan
2
Institute for Molecular Science, Okazaki 444-8585, Japan

130

that the nitrogen dopants are not only in the form of
N but also in the form of NH. Thus, peaks X and Y
can be attributed to excitations to NH-derived
unoccupied states. Peak Z could be associated with anedge structure.

 !&!& #"#%
#"#% $
$

Nitrogen doped TiO2, which is one of mostpromising visible-light-response photocatalysts, hasbeen extensively studied to understand themechanism of its visible-light-response. Althoughmany structural studies on nitrogen dopants in TiO22
have been conducted with various techniques, neither
doping site nor chemical state of the nitrogen dopant
is known in detail. In this work, we measured
polarized NEXAFS spectra for nitrogen doped rutile
TiO(110) toto elucidate
elucidate its
its structure.
structure.
TiO22(110)The
The samples
samples were
were preparedprepared byby heating
heating rutile
rutileTiO(110) substrates
substrates under
under NH
NH33 atmosphere
atmosphere at
at 1.0
1.0
TiO22(110)
Torr.
Torr. Polarized
Polarized NEXAFSNEXAFS measurements
measurements were
were
performed
performed at
at BL3U
BL3U with
with using
using thethe partial
partial electron
electronyield
method.
The
photonenergies
were
yield method. The photon energies were calibrated
calibrated bybytheenergy ofof thethe first
first peak
peak (530.6(530.6 eV)eV) atat O-KO-K edge.
edge.
the energy
Fig. 1 shows
O-KO-K
edgeedge
NEXAFS
spectraof rutile
Figure1 shows
NEXAFS
spectra
of
TiO
(110)
with
different
polarization
directions.
For2rutile TiO2(110) with different polarization directions.
example,
NI NI
[001]
indicates
incidenceFor
example,[001]
indicatesthatthat x-ray
x-ray incidence
angle
angle isis 90°
90° fromfrom thethe surface
surface parallel
parallel andand its
its electric
electric
vectorvector isis lying
lying along
along thethe [001][001] direction
direction (see(see Fig.
Fig. 2).
2).
In
In thethe grazing
grazing incidence
incidence (GI)(GI) geometry,geometry, thethe incidence
incidenceangle
angle waswas 30°.
30°. For
For thethe O-KO-K edgeedge spectra,
spectra, we
we observe
observe
five peaks (a-e) and they are seen in the typicalfive peaks (a-e) and they are seen in the typical
spectra of rutile TiO2(110). Considering the previous
spectra of rutile TiO2(110). Considering the previous
assignments for these peaks [1,2], peaks a and b canassignments for these peaks [1, 2], peaks a and b can
be attributed to excitations to unoccupied states: (Tibe attributed to excitations to unoccupied states: (Ti
3d + O 2p!) and (Ti 3d + O 2p"), respectively. Peaks
3d + O 2p!) and (Ti 3d + O 2p"), respectively. Peaks
c-e are assigned to (Ti 4sp + O 2p). In NI[001]c-e are assigned to (Ti 4sp + O 2p). In NI[001]spectrum, peak b exhibits different polarization
spectrum, peak b exhibits different polarization
dependence from the other peaks. It might be because
dependence from the other peaks. It might be becausepart of peak b is associated with a surface oxygenpart of peakwith a surface
oxygenspeciesshownb isin associatedFig. 2. Considering
its bonding
species
shown
in
Fig.
2.
Considering
itsbonding
direction, the excitation to an unoccupied state of (Tidirection,
the character
excitation for
to anunoccupiedof (Ti3d + O 2p")
surfaceoxygenstateshould
be
3d
+
O
2p")
characterforsurfaceoxygenshouldbeobserved only in the [001] direction and particularly
observed
only
in
the
[001]
directionandparticularly
strong in NI[001]. Therefore, it can be said that the N
strong in
NI[001].
be saidthat the toN
dopedrutileTiO2Therefore,(110) has ita cansimilarstructure
(110)
has
a
similarstructureto
dopedrutileTiO
2pristine TiO2 and the surface oxygen species bridging
the surface
oxygenreactions.
species bridging
pristine
2 and
two Ti TiO
atoms
remainafter doping
Fig. 3twoTi
atoms
remainafter
dopingreactions.
3
shows N-K edge NEXAFS spectra of the Figure
nitrogen
shows
N-K
edge
NEXAFS
spectraofthenitrogendopants in the rutile TiO2(110), where the incidence(110),
thetheincidence
dopants
in 15°
the forrutileangle
was
GI TiOand 290°
for where
NI fromsurfaceangle wasAs15°
for GI sevenand 90°for NI
from
theX,surface
parallel.
a result,
peaks(a’-d’andY, Z)
parallel.
As a result,peaks(a’-d’X, Y, Z)
wereobserved.
Peaksevena’ andb’ canbe andattributed
towere observed.
Peak a’ andb’ (Tican 3dbe+attributed
toexcitations
to unoccupied
states:N 2p!) andexcitationstounoccupied
states:
(Ti
3d
+
N
2p!)
and
(Ti 3d + N 2p"), respectively. Peaks c’ and d’ are(Ti 3d + toN (Ti
2p"),
Peaks
c’ results,
and d’ theareassigned
4sprespectively.
+ N 2p). FromtheseassignedN tospecies(Ti 4spare+ Nlikely2p). Fromthese results,
the
doped
to occupy
the lattice
doped Nsites
speciesare likely Ittoshould
occupylatticeoxygenvia substitution.
be thenotedthatoxygensubstitution.
It should
be N-K
notededge
thatpeaks X,sites
Y, viaZ appearexclusively
in thepeaks X, Based
Y, Z appear
exclusively
N-K DFT
edge
spectra.
on theresults inof theXPS,
spectra.
Based
the resultsof it XPS,
DFT
calculations
and aonprevious
report [3],is proposed
calculations and a previous report [3], it is proposed








 
 

 #  !% Fig. 1. O-K NEXAFS spectra of N doped rutile
TiO22(110).
surface oxygen
110
surface oxygen
110
[110]
001
[110]
plane
001
plane
Fig. 2. Structure model for rutile TiO2(110) surface.
Fig. 2. Structure model for rutile TiO2(110) surface.

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$%"!
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 






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


$'$# #%) 


$'$##%)
 dopants in
Fig. 3. N-K NEXAFS
spectraof nitrogen
Fig. 3.TiO
N-K
NEXAFS
spectra of nitrogen dopants in.
rutile2(110)
rutile TiO2(110) .
[1] J. G. Chen, Surf. Sci. Rep. 30 (1997) 1.
[1] J.
Chen, etSurf.
Rep.
30 (1997)
1. 246 (2009)
[2]
E. G.
Filatova
al., Sci.
Phys.
StatusSolidi B[2]
E.
Filatova
et
al.,
Phys.
StatusSolidiB
246 (2009)
No.7, 1454.
No.7,
1454.
[3] Y. Kim et al., J. Phys. Chem. C 115 (2011)[3]
Y. Kim et al., J. Phys. Chem. C 115 (2011)
18618.
18618.

Ⅲ -4. Surface and Thin Films

BL4B

Perpendicular Magnetic Anisotropy in Ultrathin Fe Film on MgO Studied
by Angular-Dependent X-Ray Magnetic Circular Dichroism
J. Okabayashi1, J. W. Koo2, H. Sukegawa2, S. Mitani2, Y. Takagi3 and T. Yokoyama3
1

Research Center for Spectrochemistry, The University of Tokyo, Tokyo 113-0033, Japan
2
National Institute for Materials Science (NIMS), Tsukuba 305-0047, Japan
3
Institute of Molecular Science, Okazaki 444-8585, Japan

Perpendicular magnetization is one of the crucial
issues in spintronics research field because it has
benefits for thermal stability enhancement and the
low current magnetization switching, which are very
important for the development of spintronics devices.
MgO-based magnetic tunnel junctions have beendeveloped by exploiting the strong perpendicularmagnetic anisotropy (PMA) at the interfaces between
MgO and CoFeB transition metal alloys, measuring
2.1!105 J/m3 [1], which is comparable to PMA in
Co/Pt multilayers. As a fundamental understanding of
PMA induced at the interface between ferromagnetic
layer and MgO barrier layer, the electronic andmagnetic structures of interfaces between ultrathin Fe
layer and MgO have to be clarified explicitly [2]. It
brings the understanding for the origin of FeCo alloys
on MgO [3]. In order to investigate PMA energy (K),
it is necessary to evaluate the orbital magnetic
moments along parallel and perpendicular directions
to the surface. Here, we report the anisotropic orbital
moments of Fe/MgO by using angular dependent
x-ray magnetic circular dichroism (XMCD).
Samples were grown by electron-beam evaporation
methods on MgO substrates. The 0.7-nm-thick Fe
layer was deposited on Cr buffer layer and MgO layer
was also grown on thin Fe layer. Post annealing at
450 °C was performed to enhance PMA, which wasestimated to be 1.4 MJ/m3 by vibrating sample
magnetometer (VSM) at room temperature [2].
Saturation magnetic field of 0.7 nm Fe layer on MgO
was estimated to be 2 T. XMCD measurements weredone at UVSOR BL-4B under the conditions of 5 K[4]. Circular polarization was evaluated to be 71%. A
magnetic field of 5 T using superconducting magnet
was applied along the incident polarized soft x-ray,which is enough to saturate the magnetization along
magnetically hard axis direction. The total electron
yield mode was adopted. Normal incidence (NI) andgrazing incidence (GI) geometries were employed todeduce the anisotropic orbital magnetic moments.
Figure 1 shows Fe L-edge x-ray absorption spectra
and XMCD. Clear metallic peaks in absorption
spectra reveal no mixing with oxygen atoms. XMCD
spectra in NI and GI setups reveal the difference in L3
edges, suggesting that the large orbital moments are
induced in NI geometry. Using orbital sum rules, 0.22
and 0.17 µB are estimated for NI and GI geometries,
respectively. Considering the Bruno relationship;

K=(!/4)!ml, where ! is spin-orbit coupling, we obtain
K=0.9 MJ/m3, which is comparable to the results
obtained by VSM. We note that VSM measurements
at low temperature are difficult due to thediamagnetic contribution from MgO substrates.
Therefore, element specific XMCD measurements
become a unique technique for probing ultrathin
magnetic layer on MgO.
Considering above results, the origin of PMA in
Fe/MgO interface can be understood by theanisotronic orbital moments induced by spin-orbit
interaction at the interface. Fe 3dz2 states are pushed
up above the Fermi level through the hybridization
with O 2pz orbitals and large perpendicular orbital
moments are induced. Therefore, we conclude that
anisotropic orbital moments are the origin of large
PMA in Fe/MgO interface.

Ⅲ- 4Fig. 1. X-ray absorption spectra of NI and XMCD ofboth NI and GI setups in Fe/MgO at 5 K and 5 T.
[1] S. Ikeda et al., Nature Mater. 9 (2010) 721.
[2] J. W. Koo et al., Appl. Phys. Lett. 103 (2013)
192401.
[3] J. Okabayashi et al., Appl. Phys. Lett. 103(2013) 102402.
[4] T. Yokoyama et al., Int. Rev. Phys. Chem. 27(2008) 449.

131

Ⅲ -4. Surface and Thin Films

BL4B

XAS Study of Magnetic Atoms on Pb/Ge(111)
S. Hatta, N. Sakata, H. Okuyama and T. Aruga

Graduate School of Science, Kyoto University, Kyoto 606-8502, JapanⅢ- 4

The Pb monolayer on Ge(111) has surface states
exhibiting spin splitting due to the Rashba spin-orbit
interaction [1, 2]. One of the spin-split surface states
makes spin-polarized Fermi surfaces with the spin
splitting of 200 meV. Our four-point-probe
conductivity measurements of the monolayer
demonstrated the metallic temperature dependence ofthe conductivity and the high conductivity of ~10 mS.
Therefore the monolayer is actually suitable to study
the role of the spin-orbit interaction and spinpolarization of two-dimensional states in electrontransport. Recently, the spin-orbit interaction is
expected to have a significant effect in the couplingwith conduction electrons and a local magnetic
moment, resulting in the increase or decrease of the
Kondo temperature [3, 4]. In order to address such a
issue, we have investigated the magnetic properties of
Co and Fe atoms adsorbed on the Pb monolayer.
We performed X-ray absorption spectroscopy(XAS) experiments at BL4B using the x-ray magnetic
circular dichroism (XMCD) end station. The Ge(111)substrate was cleaned by cycles of annealing and Ar
ion sputtering until a sharp c(2!8) low-energy
electron diffraction (LEED) pattern was observed.
The Pb deposition of ~3 ML was done from analumina crucible heated by tungsten loops, followedby annealing at 550 K. Figure 1 shows the obtained("3!"3)R30˚ LEED pattern of the Pb monolayer. A
small amount of Co or Fe was evaporated by electron
bombardment with a rate of ~0.005 ML/sec.
Immediately after the deposition on the room
temperature Pb/Ge(111) surface, the sample wastransferred to the XMCD chamber and cooled down
to 6 K.
Figures 2 show L2,3-edge XAS spectra of Co (0.22
ML) on Pb/Ge(111) at ±1 T and the incidence anglesof 0˚ and 55˚ from the surface normal. The broadfeature of the adsorption peaks seemed to beunchanged from 0.07 to 0.22 ML. This indicates thesignificant interaction between the Co 3d and surface
states. For XMCD, unexpectedly little signal wasobserved both at normal and grazing incidence, as
shown in Fig. 2. It was same in the measurementswith ±3T. This result shows that the magnetic
moment of the adsorbed Co atoms is quenched. On
the other hand, a XMCD signal was observed for Fe(0.17 ML) on Pb/Ge(111). The magnetic field and theincidence angle dependence of the XMCD signal was
found. We are proceeding with further analysis of themagnetic moments of the adsorbed Fe atoms.

Fig. 1. A ("3!"3)R30˚ LEED pattern observed for
the Pb monolayer on Ge(111) (Ep=90 eV).

Fig. 2. L2,3-edge XAS spectra of Co (0.22 ML) on the
Pb/Ge(111) recorded at 6 K and ±1 T.
[1] K. Yaji et al., Nat. Comm. 1 (2010) 17.
[2] K. Yaji et al., Phys. Rev. B 86 (2012) 235317.
[3] M. Zarea, S. E. Ulloa and N. Sandler, Phys. Rev.
Lett. 108 (2012) 046601.
[4] T. Yanagisawa, J. Phys. Soc. Jpn. 81 (2012)
094713.

132

Ⅲ -4. Surface and Thin Films

BL4B

Study on Magnetic Properties of Metal-Coordinated Phthalocyanine
Molecules Self-Assembled on Semiconductor Surface Reconstructions
T. Uchihashi and S. Yoshizawa

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science,
Tsukuba 305-0044, Japan
Thin film complex molecules fabricated by thermal
evaporation onto a metal surface have been
extensively studies using X-ray absorption
spectroscopy (XAS) and X-ray magnetic circulardichroism (XMCD) for the purpose of device
applications [1]. In this work, we studied themagnetic properties of Co-Phthalocyanine (Pc) and
MnPc self-assembled on the Si(111)-(!7"!3)-In
surface reconstruction [referred to as (!7"!3)-In],
which is made of monatomic indium layers on a clean
silicon surface. Since the (!7"!3)-In surface exhibits
superconducting phase transition at 3 K [2],interesting
competition
effects
betweensuperconductivity and magnetism are expected tooccur in this system.
The experiment was performed in the ultrahighvacuum XMCD system with a superconducting
magnet, which was set at the UVSOR beam line
BL4B in the Institute for Molecular Science. Figure 1
shows the LEED patters of the (!7"!3)-In surface
before (left) and after (right) MnPc was evaporated toa monolayer level. High crystallinity was confirmed
for both cases. Similar results were obtained for
CoPc/(!7"!3)-In. XMCD spectra were determinedfrom the difference between the XAS spectra taken at
B = ±5 T.
For the CoPc/(!7"!3)-In sample, no XMCD signal
was obtained. Since CoPc has a spin 1/2 at the Co
atom in a free state, this spin must be quenched, forexample, by charge transfer between the moleculeand the substrate. In contrast, clear XMCD signals
were obtained for the MnPc/(!7"!3)-In sample (seeFig. 2). This clearly indicates that spins in the Mn
atom still survive when the molecule is adsorbed on
this surface. These behaviors are similar to the ones
observed for their adsorption on noble metal surfaces.
Accordingtoourrecentexperiments,
superconducting energy gap is retained on the first
and the second layers of CoPc on (!7"!3)-In. This isconsistent with our observation that the spin of CoPc
is quenched on absorption. On the other hand, thespins surviving in the MnPc molecule can suppress
the superconductivity of the (!7"!3)-In surface. This
will be further studied by combining the XMCD
experiment,
STM,
andelectrontransport
measurement.

Fig. 1. LEED patterns of the CoPc/(!7"!3)-In
sample.

Ⅲ- 4

Fig. 2. XAS and XMCD spectra of the CoPc/(!7"!3)-In sample taken at the incident angle # = 0 .
[1] Y. Takagi et al., UVSOR ACTIVITY REPORT
38 (2011) 67.
[2] T. Uchihashi et al, Phys. Rev. Lett., 107 (2011)
207001.

133

Ⅲ -4. Surface and Thin Films

BL4B

Magnetic Linear Dichroism Study of Non-Ferromagnetic Co on W(110)
H. Nakano1, T. Nakagawa2, K. Eguchi1, N. Takagi1 and T. Yokoyama1
1

2

Ⅲ- 4

Institute for Molecular Science, Okazaki 444-8585, Japan
Department of Molecular and Material Sciences, Kyushu University, Kasuga 816-8580, Japan

Metallic Co is known as a robust ferromagnetic
material, irrespective of its crystal structures andpressure. Thus antiferromagnetic phase of Co has notbeen found so far both for bulk and thin films.
However, the magnetism of Co single layer grown on
refractory metals, W and Mo, has been a puzzlingissue since no evidence of magnetic ordering has
been found [1]. On the other hand, Fe, a typicalferromagnetic material at ambient conditions, shows
variety of magnetic phases such as ferromagnetic,
antiferromagnetic, spin density waves, depending on
the crystal structure, lattice constant, and thickness.
Here we report non parallel alignment of Co magnetic
moment proved by x-ray magnetic linear dichroism(XMLD).
Experiments were done in a x-ray magnetic circulardichroims (XMCD) chamber with superconducting
magnet (H ~ 6 T). Co was deposited on W(110),
which was cleaned by repeated cycles of oxidationand high temperature flash upto 2200 K. The x-ray
linear dichroism (XLD) was measured with thepolarization of the light fixed and the azimuthal angleof the crystal rotated.
Figure 1(a) shows XMCD spectra taken on single
layer Co grown on W(110) at Ts = 5 K and amagnetic field of 5 T. The spectra for the parallel andantiparallel alignment between the light helicity andthe magnetic field are nearly identical, indicating thedisappearance of the macroscopic spin moments. The
zero spin magnetic moment obtained in XMCD has
several scenarios. One is the non-parallel alignmentof the spin moment such as antiferromagnetic
structure so as to give the macroscopic zero moment,
and another is the missing of the magnetic moment in
Co due to the strong hybridization with W substrate.
Linear dichroism could be observed for
non-parallel alignment of the magnetic moment
owing to the asymmetry of the magnetic structure.
Figure 1(b) shows XLD spectra taken between 5 K
and 60 K, where the dichroic signal is shown as thedifference between the light polarization (E) //[110]and E//[001] directions. In Fig. 1(c) the XMLD
intensity is largest at Ts = 5 K, and decreases withincreasing the temperature. Above 20 K, the XMLD
stays at the almost constant value, which comes fromC2 symmetry of Co/W(110) and the instrumental
asymmetry. The temperature dependence of the XLD
suggests an additional ordering below 20 K.
It is unlikely that the Co magnetic moment adjacentto W atoms vanishes due to the strong banding. Since
the thickness dependence of Co magnetic moment(not shown) demonstrates a rapid increase above 1

ML Co thickness and recover to ~ 80% of bulk Co
spin moment at 2 ML, which cannot be explained by
the zero magnetic moment in the first Co layer.
In summary, we show for the first time that themonolayer Co on W(110) is not non-magnetic, but it
exhibits the non-parallel alignment of the magnetic
moment, which cancels with each other such as an
antiferromagnetic structure.

Fig. 1. (a) XAS and XMCD spectra for 0.6 ML Co on
W(110) at Ts = 5 K. (b) Temperature dependent XLD
spectra for 0.6 ML Co on W(110). (c) XMLD
intensity as a function of temperature. The inset is aschematic drawing of the Co spin structure.
[1] G. Garreau, M. Farle, E. Beaurepaire and K.
Baberschke, Phys. Rev. B 55 (1997) 330.

134

Ⅲ -4. Surface and Thin Films

BL4B

Magnetic Property of Vanadyl Phthalocyanine on Ferromagnetic Iron
Surface Studied by XMCD
1

K. Eguchi1, Y. Takagi1,2, T. Nakagawa1,2 and T. Yokoyama1,2

The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan
2
Institute for Molecular Science, Okazaki 444-8585, Japan

It is important to understand the magnetic
interactions between phthalocyanine compounds
containing
transition
metals(TMPcs)andferromagnetic surfaces for realizing molecular
spintronics. Recently we have reported on the
ferromagnetic coupling of central metal ions likemanganese [1] and iron [2] with ferromagnetic Co
films. In this study, we investigated the molecularorientation and magnetic properties of vanadyl
phthalocyanine (VOPc) deposited on a ferromagnetic
iron surface by using x-ray absorption spectroscopy(XAS) and x-ray magnetic circular dichroism(XMCD).
Sample preparation was carried out in an ultrahighvacuum condition. First, a ferromagnetic iron film
was prepared by the electron bombardment
evaporation on a clean Cu(001) substrate at low
temperature (T~110 K). The thickness of the iron film
was confirmed by the reflection high-energy electron
diffraction oscillations during the evaporation.
Subsequently, purified VOPc was deposited on theiron film using a homemade Knudsen cell at 570 K.
During the sublimation deposition, the substrate was
kept at room temperature. The deposition rateestimated by a quartz crystal oscillator was 0.1
ML/min and the amount of deposition was 0.9 ML (1ML = 0.5 molecule/nm2). XAS and XMCD
measurements were performed at 5 K by means of thetotal electron yield detection mode. In the XMCD
measurement, the helicity of the circularly polarized
x-rays was fixed positively while the magnetic field
was reversed.
Figure 1(a) shows angle dependence of the linearlypolarized N K-edge XAS of 0.9 ML VOPc depositedon the iron surface. The peaks labeled A, B and C aremainly attributed to transitions from N 1s to the !*molecular orbitals and the intensity of the !* peaks
increases with increasing the x-ray incidence angle.
This indicates that the framework of phthalocyanine
is adsorbed with a flat orientation. In addition, thesatellite peak S is observed due to strong interactionof the N atoms with the iron surface.
Figure 1(b) shows V L-edge and O K-edge
circularly polarized XAS and XMCD spectra at theincident angle of 0° and 55°. Although the XMCD
spectra or the magnetization curves for the iron film
were not shown, we confirmed that the magnetizationof the iron film is saturated at 5 T in both the 0° and
55° configurations. The XMCD signal is detected at
the V L-edge, which means that the V spin in VOPc
is maintained. Then, the XMCD signal shows apositive sign at the V L3-edge, while it is negative at
the Fe L3-edge. This implies that the central
vanadium ions are magnetized to the opposite
direction to the magnetization of the iron film. In
contrast to the previous cases of MnPc/Co and
FePc/Co systems, the spins of the central vanadiumions are antiferromagnetically coupled with those of
the iron atoms in the ferromagnetic iron film.

Ⅲ- 4Fig. 1. (a) The linearly polarized N K-edge XAS
intensities of 0.9 ML VOPc on Fe (3 ML)/Cu(001).
(b) V L-edge and O K-edge XAS and XMCD spectraof 0.9 ML VOPc on Fe (3 ML)/Cu(001) taken at theincident angle of 0° and 55° from the surface normal
at T=5 K and µ0H=5 T.
[1] I. Yamamoto et al., UVSOR ACTIVITYREPORT 37 (2010) 74.
[2] Y. Takagi et al., UVSOR ACTIVITY REPORT
38 (2011) 74.

135

Ⅲ -4. Surface and Thin Films

BL5U

Photoemission Study of CaCu3Ru4O12
H. J. Im1, M. Iwataki1, M. Tsunekawa2, T. Watanabe1, M. Matsunami3,4 and S. Kimura3,4,5
1

Department of Advanced Physics, Hirosaki University, Hirosaki 036-8224, Japan
2
Faculty of Education, Shiga University, Hikone 522-8522, Japan
3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan.
4
School of Physical Sciences, The Graduate University for Advanced Studies, Okazaki 444-8585, Japan.
5
Graduate School of Frontier Biosciences, Osaka University, Suita, 565-0871, Japan.

Ⅲ- 4

A-site ordered perovskite compounds of the
stoichiometry of AA’3B4O12 are formed by thesubstitution of three-fourths of A-site ions by A’-siteions in the typical ABO3 structure. Particularly,
CaCu3B4O12 compounds (B: Ti4+, Ru4+, Mn4+, Fe3+)
have been attracted much attention due to a variety of
intriguing physical properties which come from thestrong correlation effects. For instance, CaCu3Ti4O12(CCTO), which shows extremely high dielectric
constant [1], is Mott-insulator which has turned out tobe caused by the band shift and the band narrowing inangle-resolved photoemission studies [2, 3]. And
CaCu3Ru4O12 (CCRO) have extensively studied dueto heavy-fermion like behaviors and non-Fermi liquidbehaviors [4, 5].
Here, in order to understand the underlying physics
of A-site ordered perovskites, we have performed
photoemission measurements on CCRO at BL5U andcompared it with the previous results of CCTO.
The polycrystalline sample has been synthesized byconventional solid state reaction technique using
CaCO3, CuO, and TiO2. In order to decompose thecalcium carbonate, stoichiometric mixtures were
initially calcined at 700 °C for 12 h. The pressed
pallets were annealed at 1050 °C for 24 h. Single
phase has confirmed by X-ray diffraction pattern.
The photon energy of 90 eV was used in PES
measurements. The clean sample surface wasprepared by in situ cleaving in the ultra-high vacuum.
The measurement temperature is 300 K.
Figure 1 shows the PES spectra of CCRO andCCTO in the valence band regime obtained byangle-integrated analyzer mode. We recognized thespectral shape of CCRO is similar with CCTO except
for the Fermi level. The most intensive peaks areobserved around 4 eV which come from the mainly
Cu 3d states. Interestingly, the peak positions arealmost same in CCRO and CCTO. It was alreadyreported in CCTO that the Cu 3d peaks are shifted tohigher binding energy due to strong correlation
effects, indicating the localized character [2]. In 5 – 8eV, the broad peak are mainly O 2p states. The humparound 2.5 eV are observed. Particularly, in CCRO
the spectral weights are clearly observed near the
Fermi-level where there are mainly Ru 4d-states inthe band calculations, indicating the itinerant
character.
We have reported that most Ti 3d states in CCTO
are located in the unoccupied regime [2]. In contrast,

Ru 4d states are expected to be exist in the occupied
regime due to strong hybridization of broader Ru 4d
states with Cu 3d and O 2p states in the bandcalculations. Therefore, we suggest that thedifferences of the spectral shape between CCTO and
CCRO are attributed to the participation of Ru 4d
states in the occupied regime.
For future work, it should be studied how theitinerant Ru 4d states and the localized Cu 3d statesplay a role in heavy-fermion behaviors and non-Fermi
liquid behaviors.

Fig. 1. Photoemission spectra of CaCu3Ru4O12 andCaCu3Ti4O12 in the valence band regime at room
temperature, obtained at hv = 90 eV.
[1] M. A. Subramanian et al., J. Solid State Chem.
151 (2000) 323.
[2] H. J. Im et al., Phy. Rev. B 88 (2013) 205133.
[3] H. J. Im et al., UVSOR Activity Report 40 (2013)
100.
[4] W. Kobayashi et al., J. Phys. Soc. Jpn. 73 (2004)
2373.
[5] A. Krimmel et al., Phys. Rev. B 78 (2008)
165126.

136

Ⅲ -4. Surface and Thin Films

BL5U

Soft X-Ray Photoemission Spectroscopy Study of Fe2P(0001) and Ni2P(0001)
Y. Sugizaki1, S. Ishida1, T. Nakamura1 and K. Edamoto1,2

1

Department of Chemistry, Rikkyo University, Tokyo 171-8501, Japan
Research Center for Smart Molecules, Rikkyo University, Tokyo 171-8501, Japan

137

energy side and the growth of a new peak at 3.4eV,
which is induced by bonding between Ni andsegregated P atoms. These result shows that thesurface Ni atoms are stabilized due to a ligand effect.
On the other hand, valence band spectra of Fe2P
shows little change by annealing the sputtered surface,indicating that the ligand effect is not effective on
Fe2P(0001). These results suggest that the control ofthe electronic structure of active metal sites isimportant to improve the catalytic performance of
TMPs.
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Fig. 1. P2p core-level spectra (hν=170eV) for (a)Ni2P(0001) and (b) Fe2P(0001).
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The transition metal phosphides (TMPs) haveattracted much attention because these materials showhigh catalytic activity for hydrodesulfurization (HDS)
and hydrodenitrization (HDN)[1]. Of all TMPs, Ni2P
shows the highest catalytic activity for HDS, whilethat of Fe2P is extremely low[1]. The difference isthought to originate from the difference in electric
structure of surface metal atoms. The surface
electronic structure of Ni2P has been investigated by
XPS, and it has been proposed that active Ni sites arestabilized through the bonding with segregated P
atoms[2]. As for Fe2P, very little information isavailable at present. To comprehend the origin ofdifference in catalytic activities, we investigatedsurface properties of Ni2P(0001) and Fe2P(0001) byusing soft X-ray photoemission spectroscopy(SXPES).
The experiments were performed at BL-5U of
UVSOR, Institute for Molecular Science. The
photoelectrons were collected by an electron energy
analyzer of hemispherical type (MBS A-1), using anangle-integrated mode (acceptance angle of ±20°).
The Ni2P and Fe2P single-crystals were grown by Dr.
S. Otani of National Institute for Materials Science.
The samples were cut at an orientation of (0001) byspark erosion, and the surfaces were polishedmechanically to a mirror finish. The clean surfaces
were prepared by several cycles of Ar+ ion sputteringand several temperature annealing (600 ∼ 900 K).
Figure 1 shows the P2p core-level spectra of Ni2P(a) and Fe2P (b) measured at hν=170eV. The
background drawn by a Shirley procedure has been
subtracted from each raw data. For Ni2P, major peaks(2p3/2 and 2p1/2 levels) are observed at 129.6 and
130.4eV, respectively, together with shoulder peaks
around 129.9 and 130.7eV. Both shoulder peaks
disappear by slight sputtering, and thus the shoulders
are associated with segregated P atoms. For Fe2P, in
addition to the major peaks at 129.3 and 130.1eV,several minor peaks are observed in the region
between 129 and 128eV. These peaks disappear byslight sputtering, and thus these are associated withsegregated P atoms. These results indicate that P
segregation occurs by annealing for both Ni2P(0001)
and Fe2P(0001).
Figure 2 shows valence band spectra of Ni2P (a)and Fe2P (b). The bands observed at 0-4.5eV for Ni2P
and at 0-4eV for Fe2P are Ni3d-P3p hybrid band andFe3d-P3p hybrid band, respectively. For Ni2P,
annealing the sputtered surface at 900K induces theshift of the whole band toward the higher binding  

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Fig. 2. Valence band spectra of Ni2P (a) and Fe2P (b).
Ni2P was measured at hν=60eV and Fe2P wasmeasured at hν=49eV.
[1] S. T. Oyama, J. Catal. 216 (2003) 343.
[2] K. Edamoto, et al., Solid. Statate. Communication.
148 (2008) 135.
[3] Y. Sugizaki, et al., Surf. Sci. 624 (2014) 21.

Ⅲ- 4

Ⅲ -4. Surface and Thin Films

BL5U

Electronic Structure of a Magnetic Insulator / Topological Insulator Thin
Film Heterostructure: MnSe / Bi2Se3

1

2

Department of Physics, University of Tokyo, Tokyo 113-0033, JapanUVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, JapanThe metallic surface states of three-dimensional
topological insulators are protected by time reversal
symmetry (TRS). Although breaking TRS isgenerally detrimental to these states, it may also leadto exotic topological quantum effects, such as
magnetic monopoles, quantized anomalous Hall
effect, and giant magneto-optical effects. In terms ofelectronic structure, it has been predicted that themassless Dirac-cone (DC) surface states will undergo
gap opening and become massive [1].
Up to now, most studies that have tried to break
TRS in topological insulators have been performed
with magnetic impurity doping in the bulk or at thesurface [2]. Although this is a straightforward andsimple strategy, various results have been reported
that are in favor/unfavor of the TRS breaking in termsof gap-opening of the DC [3]. One study has evenshown that a gap-opening is observed even whennonmagnetic impurities are incorporated [4].
Therefore, an alternative method to induce TRS
breaking in topological insulators with well-definedstructures is called for.
In the present study, we have fabricated aheterostructure of thin films of magnetic (MnSe) andtopological insulators (Bi2Se3) and tried to see
whether a gap opening can be observed in the DC.
These two materials have nearly the same lattice
constant and can be made into a heterostructure asconfirmed with diffraction measurements. Although
MnSe is an antiferromagnetic insulator, theoretical
calculation predicts that a magnetic interaction at theinterface is expected to induce a gap opening in theDC [5]. Figure 1 shows the band dispersion of a 10
quintuple layer (QL) Bi2Se3 film (a) and aheterostructure of 2.4 bilayer (BL) MnSe / 10 QL
Bi2Se3 film (b). One can notice that the DC remains
after MnSe deposition although its intensity has
decreased drastically. Furthermore, the Dirac point
has shifted towards the Fermi level, making the bulk
insulating. The origin of this shift is unclear at the
moment. Moreover, whether a gap has opened at the
Dirac point is also not clear and needs higher
resolution measurements.

264
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Ⅲ- 4

T. Hirahara1, T. Kubo1, T. Nakamura1, T. Hajiri2,
M. Matsunami2, S. Kimura2 and S. Hasegawa1

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Fig. 1. Band dispersion of a 10 quintuple layer (QL)
Bi2Se3 film, (a) and a heterostructure of 2.4 bilayer(BL) MnSe / 10 QL Bi2Se3 film, (b). S and B
correspond to surface and bulk states and the arrows
indicate the Dirac point.
[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82(2010) 3045.
[2] S.-X. Xu et al., Nat. Phys. 8 (2012) 616.
[3] A. Varykhalv et al., Phys. Rev. X 2 (2012)
041017.
[4] T. Sato et al., Nature Physics 7 (2011840).
[5] S. V. Eremeev et al., Phys. Rev. B 88 (2012)
144430.

138

Ⅲ -4. Surface and Thin Films

BL5U

Angle-Resolved Photoemission Study of Ba3Co2O6(CO3)0.7
T. Hosokawa1, T. Hajiri1,2, T. Ito1,3, M. Matsunami2,4, S. Kimura2,5,
Y. Shimizu6, Y. Kobayashi6 and M. Ito6
1

Graduate School of Engineering, Nagoya University, Nagoya 464-0818, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
3
Nagoya University Synchrotron Radiation Research Center, Nagoya University, Nagoya 464-8603, Japan
4
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585,
Japan
5
Graduate School of Frontier Biosciences and Physics Department, Osaka University, Osaka 565-0871, Japan
6
Graduate School of Science, Nagoya University, Nagoya 464-8603, Japan
Cobalt oxides have attracted attention due to theiranomalous electronic/magnetic properties such as alarge thermoelectric power. Recently it has beenfound that a quasi-one-dimensional cobaltate
Ba3Co2O6(CO3)0.7 shows a fairly large thermoelectric
power comparable to one of Na0.75CoO2. In order tounderstand its physical properties, we haveinvestigatedtheelectronic
structureof
Ba3Co2O6(CO3)0.7 by utilizing angle-resolved
photoemission spectroscopy (ARPES).
ARPES measurements were performed at thebeamline BL5U of UVSOR-III. Clean surfaces wereobtained by in situ cleaving the samples inperpendicular to the c axis.
Figure 1 shows the ARPES image near the Fermi
level (EF) along the !A high-symmetry line measured
at T = 20 K. We successfully obtain the clear
dispersive features from 0.5 to 3 eV binding energy
range. From the comparison with the Co 3p-3d
resonant photoemission result of Ba3Co2O6(CO3)0.7,
the observed bands mainly consist of the Co 3d
orbital character [3]. To show the symmetry of theelectronic structure clearly, we have mapped out theintensity of ARPES spectra around 0.5 eV (valenceband maximum) in the !A-!KM plane as indicated inFig. 2. From the constant energy intensity map, we
can find that the line-shape contrast along the AHL
high-symmetry line. Furthermore, the intensity
increases from the A to L point. It is expected that the
one-dimensional atomic (molecular) chain form theplate-like electronic structure perpendicular to the
one-dimensional conducting axis. Thus, the observed
symmetry of the ARPES intensity ensures theappropriate sample setup along the quasi-one
dimensional face-sharing CoO6 chains.
In contrast to the metallic/semimetallic properties
on electronic conductivity along c axis, the valenceband maximum seems to be located around 0.5 eV in
Fig. 1. It should be noted that the ARPES spectra
exhibit a low energy tail from 0.5 eV to EF aroundthe A point, though the tail feature does not cross EF.
To understand the origin of this discrepancy, moredetailed ARPES measurements are intended.

Fig. 1. ARPES image of Ba3Co2O6(CO3)0.7 along the!A high-symmetry line measured at T=20K. Whitelines are guide for eyes to trace a main dispersive
feature.

Fig. 2. The Valence Band Maximum (EB=0.5eV)surface in the !A-!KM plane of Ba3Co2O6(CO3)0.7
mapped by ARPES at T=20K.
[1]K. Iwasaki et al., JOURNAL OF APPLIEDPHYSICS 106 (2009) 034905.
[2]K. Igarashi et al., Journal of Physics: Conference
Series 400 (2012) 032024.
[3]K. Soda et al., UVSOR Activity Report 37 (2010)
99.

139

Ⅲ- 4

Ⅲ -4. Surface and Thin Films

BL5B

Photoinduced Changes of Amorphous Chalcogenide Thin Films Studied by
Vacuum Ultraviolet Transmission Spectroscopy
K. Hayashi

Department of Electrical, Electronic and Computer Engineering, Gifu University, Gifu 501-1193, JapanⅢ- 4

It is well-known that amorphous chalcogenide
semiconductor materials show a variety ofphotoinduced effects [1-3] and are very expected as apotential material for optoelectronic devices because
these materials are very sensitive to the light. For thedevice application, it is necessary to sufficiently
understand the physical property of these materials.
Although a large number of studies have been doneon the photoinduced phenomena of these materials,
little is known about the details of these mechanisms.
These phenomena were studied by exciting outer coreelectrons with the irradiation of light with the energy
corresponding to the optical bandgap or sub-bandgap.
The interest has been attracted for the change of the
optical properties in the energy region of the visible
light. We are interesting for the changes of the opticalproperties in the higher energy region. To obtain awide knowledge of the photoinduced phenomena, it isnecessary to investigate to the photoinduced effects
on wide energy region. Recently, we became able tomeasure directly the VUV transmission spectra ofamorphous thin films [4]. In this paper we report on
the study of the photoinduced changes of the
as-deposited amorphous chalcogenide thin films
before and after irradiation of the bandgap (BG) lightby the VUV transmission spectroscopy.
Samples used for the measurement of the VUVtransmission spectrum were amorphous chalcogenide(a-As2Se3 and a-As2S3) semiconductor thin films
prepared onto aluminum thin films by conventional
evaporation technique. Typical thickness of thesamples and the aluminum films were around 180nm
and 200nm respectively. The aluminum film was also
used in order to eliminate the higher order light fromthe monochromator in the VUV region. The
measurements were carried out at room temperature
at the BL5B beam line of the UVSOR facility of theInstitute for Molecular Science. And the spectrum
was measured by using the silicon photodiode as adetector. A pinhole of 1.5mm in a diameter wasinserted between the monochromator and sample toremove stray light. The intensity of the VUV light
was monitored by measuring the TPEY of a gold
mesh. The positions of the core levels for the samples
were calibrated by referencing to the 2p core levelabsorption peak of the aluminum film.
Figure 1 shows the VUV transmission spectra of
the 3d core levels of Se and As atoms in a-As2Se3
film before and after the irradiation of the BG light.
As shown in the figure, the photoinduced changes areobserved on the transmission spectra before and after

the irradiation of the BG light.
Those changes might relate to the photoinduced
defects created by the irradiation of the BG light.
More detailed experiments are necessary to clarify theorigin of the photoinduced changes of the VUVtransmission spectra.

Fig. 1. VUV transmission spectra of (a) Se 3d and (b)As 3d core levels of as-deposited a-As2Se3 thin film
before and after the irradiation of the BG light.
[1] K. Tanaka, Rev. Solid State Sci. 4 (1990) 641.
[2] K. Shimakawa, A.Kolobov and S. R. Elliott, Adv.
Phys., 44 (1995) 475.
[3] K. Tanaka, Encyclopedia of Nanoscience andNanotechnology, 7 (2004) 629.
[4] K. Hayashi, UVSOR Activity Report 34 (2007)
79.

140

Ⅲ -4. Surface and Thin Films

BL6U

Hydrogen Interaction on MoS2 Surface
1

S. W. Han1,2, H. Yamane3, N. Kosugi3 and H. W. Yeom1,2Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), 77 Cheongam-Ro,
Pohang 790-784, Korea
2
Department of Physics, Pohang University of Science and Technology, 77 Cheongam-Ro, Pohang 790-784,
Korea
3
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japanas shown in Fig. 1(c).
In contrast, after postannealing at 300 °C for 1 h[Fig. 1(d)], the VBM is located at the binding energyof 0.75 eV, which reversely shifted away from EF.
In order to elucidate the hydrogenation, another
MoS2 sample was cleaved and annealed at 300 °C
and then exposed to the H2 gas for 1.5 h (5400 L). In
Fig. 1(e), the VBM is obtained at a much higher
binding energy of 1.24 eV with slightly broadened
spectral features. Especially S pz derived bands areenhanced at the higher binding energy side.
Figure 1(f) represents the shift of valence-band
spectra, which were integrated from the ARPES dataand normalized by the intensity of the first peakindicated by the (red) lines. Without a significant
change of band structure, the H2 exposure leads toshift the VBM toward EF while the annealing shifts
the VBM away from EF.
These results suggest that the thermal annealing
promotes the dissociation of H2 molecules on the
MoS2 surface and then the atomic hydrogens areintercalated between van der Waals gaps.

Recently, it has been reported that thehydrogenated MoS2 induced a weak ferromagnetism
persisting up to room temperature (RT) with the
improved transport property [1]. It is important tounderstand the interaction between the H2 moleculeand the MoS2 surface.
Angle-resolved
photoemission(ARPES)
experiments were performed at the undulator
beamline BL6U of UVSOR-III in the Institute for
Molecular Science. Natural and single crystalline
MoS2 samples were cleaved in UHV. The orientationof samples was confirmed by a hexagonal pattern in
low-energy electron diffraction and the band structurefrom the ARPES data, which were collected at 140 K
in the main chamber with a base pressure of 1!10"10
torr. The total energy and angular resolutions of the
ARPES apparatus were better than 25 meV and 0.1°.
The MoS2 surface was exposed to the H2 gas at RT byback filling the chamber with a pressure of 1!10"6
torr and postannealed in the preparation chamber.
Figure 1(a) shows the ARPES data of a cleaved
MoS2 surface along the #-K high-symmetry line of
the hexagonal Brillouin zone. ARPES data wererecorded with a photon energy of 100 eV and scaledby the maximum intensity. A valence band maximum(VBM) is located at the #-point with a binding energyof 0.34 eV. Below the VBM at #, there exists a strong
band dispersing down (red rectangle) from thebinding energy of 0.60 eV. This state is known to bea surface state, with the main contribution from the S
3pz orbital on the top S layer above the Mo layer
within a topmost layer of bulk MoS2, while the VBM
originates mainly from the Mo 4dz2 orbital [2]. On theother hand, the top of the valence band dispersion at
the K point (1.325 Å"1, vertical dashed lines), stems
from the mixed states of Mo 4dxy/x2"y2 and S 3pxy
orbitals and is located at a higher binding energy of
0.80 eV than that of VBM. This confirms the indirect
bandgap of bulk MoS2.
Figure 1(b) exhibits the band structure of MoS2
surface after an exposure to hydrogen gas for 600 s,
corresponding to 600 L (1 L = 10"6 Torr·s). Despiteof the hydrogen exposure, band structure remains
unchanged. Instead, the VBM slightly shifted towardFermi energy (EF) and it is located at 0.26 eV with aquite obvious band dispersion. The VBM further
shifts to 0.20 eV in the case of a longer exposure of a
new MoS2 surface to the H2 gas for 3600 s (3600 L)

Fig. 1. ARPES intensity maps along the #-K directionof the Brillouin zone for three MoS2 samples.
[1] S. W. Han et al., Phys. Rev. Lett. 110 (2013)
247201.
[2] S. W. Han et al., Phys. Rev. B. 86 (2012) 115105.

141

Ⅲ- 4

Ⅲ -4. Surface and Thin Films

BL6U

Electronic Structure of Epitaxial, Metallic Germanium Nanofilms on
Zirconium Diboride Thin Film Substrates –Identification of a New Form of Crystalline Germanium
1

Ⅲ- 4

R. Friedlein1, H. Yamane2, N. Kosugi2 and Y. Yamada-Takamura1

School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi 923-1292, Japan
2
Dept. of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan

While Si and Ge are right below C in the periodic
table of elements, valence orbitals of Si and Ge atoms
do not sp2 hybridize as easily as for their smaller
counterpart. However, considering that theexperimental evidence for the existence of the largely
sp2-hybridized form of silicon called “epitaxial
silicene” mounting [1-3], the verification of layered,hexagonal germanium structures with Dirac-cone likeelectronic signatures – in analogy to graphene andsilicene conveniently coined “germanene” – would
represent another important step towards novel twodimensional nano-materials.
In this context, recently, in our home laboratory,
we have succeeded in the preparation of nanofilms ofgermanium on the surface of ZrB2(0001) thin films
grown on Ge(111) wafers. These epitaxial, singlecrystalline-like Ge nanofilms are formed by surface
segregation at elevated temperatures and oxidize
upon exposure to air.
However, the native oxide can be removed by anannealing procedure under ultra-high vacuum
conditions upon which again Ge nanofilms areformed. At BL6U, these films have then been studiedin situ by angle-resolved valence band (ARPES) andcore-level (XPS) photoelectron spectroscopy as well
as by low-energy electron diffraction (LEED).
The measured electronic and structural propertiesof the films are consistent with a layered structure ofhexagonal symmetry and a (1!1) in-plane lattice
constant that is about 20 % smaller than that of bulk
germanium in the diamond crystal structure.
1 shows the valence band structure along the_ Figure_"-K direction of the (1!1) diboride Brillouin zone,
as obtained at 15 K. A manifold of well-defined stateswith upwards curvature are observed. These states arelikely thin film slab states that are related to thelayered structure of the nanofilms. Some of thesestates cross the Fermi level indicating that the films
are metallic. This behavior is expected forhighly-strained germanium layers in which ! states
are partially pushed above EF [4].
At room temperature, the films are (#3!#3)reconstructed; on the other hand, below about 100 K,
the films undergo a phase transition into a 3#3!3#3
structure. This phase transition is connected to asignificant change in the low-energy band structure
which may be associated with a charge-density wave
ground state related to an electronic instability

characteristic of low-dimensional systems. This may
indicate that the interlayer coupling is weaker than
anticipated.
At present, the results are compared with those ofquantum-chemical calculations in order to understandboth the structural and electronic properties.
This work was supported by the Joint Studies
Program (No. 615, 2012~2013) of the Institute for
Molecular Science.

Fig. 1. ARPES spectrum of a Ge nanofilm _ on_
ZrB2(0001) thin films on Ge wafers, along the "-K
direction, with the sample held at 15 K. The photon
energy of 43 eV was used.
[1] P. Vogt, P. De Padova, C. Quaresima, J. Avila, E.
Frantzeskakis, M. C. Asensio, A. Resta, B. Ealet and
G. Le Lay, Phys. Rev. Lett. 108 (2012) 155501.
[2] A. Fleurence, R. Friedlein, T. Ozaki, H. Kawai, Y.
Wang and Y. Yamada-Takamura, Phys. Rev. Lett.
108 (2012) 245501.
[3] R. Friedlein, A. Fleurence, J. T. Sadowski andY. Yamada-Takamura, Appl. Phys. Lett. 102 (2013)
221603.
[4] Y. Wang and Y. Ding, Solid State Commun. 155(2013) 6.

142

Ⅲ -4. Surface and Thin Films

BL6U

Formation of Interface State and Intermolecular Band Dispersion
in Coronene/Au(111) Superstructure
H. Yamane and N. Kosugi

Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan143

The in-plane band dispersion of molecular electronic
states has been observed for strongly chemisorbed
interfaces with larger dispersion of 0.2~0.3 eV due tothe interfacial orbital hybridization [1, 2]. By
considering the quite weak interaction between
coronene and Au(111), the observed in-plane band
dispersion is ascribed to the genuine intermolecularinteraction in two-dimensional sheets of aromatichydrocarbons.
(a)

(b)

_
K

_
!

(0,0)

_
M
k!K
k!M

Fig. 1. (a) The MCP-LEED image of the coronene
monolayer on Au(111) at 15 K, taken with the 70-eV
incident electron beam. The red spot indicates thesubstrate's spot. (b) The surface Brillouin zone (SBZ)of the monolayer superstructure of coronene on
Au(111). The black and blue solid lines indicate thesubstrate and monolayer SBZs, respectively. The reddashed line indicates the scan region of ARPES.
(a) Au(111)(b) Coronene/Au(111)EF
Binding Energy Eb / eV

Study of organic/metal interfaces is essential toinvestigate electronic phenomena derived from thecomplex interplay between van der Waals interactionand exchange-correlation interaction. Previous studies
on organic/metal energetics have been performed
mainly for non-ordered or multi-domain systems due
to experimental difficulties. In the present work, weapplied the precise angle-resolved photoemission
spectroscopy (ARPES) to the single-domain monolayer of coronene physisorbed on Au(111).
The present experiments were performed at BL6U.
The cleanliness of the Au(111) surface was confirmedby the low-energy electron diffraction with a microchannel plate (MCP-LEED) and the Shockley surface
state in ARPES, as obtained from the repeated cyclesof the Ar+ sputtering (I ~ 2 µA) and the subsequentannealing (T = 700 K). The total energy resolution in
the present ARPES measurement was 16 meV.
Figure 1 shows (a) the LEED image and (b) the
surface Brillouin zone (SBZ) of the coronene monolayer deposited on Au(111). The LEED image showsthe (4!4) single-domain superstructure with respect
to the Au(111) hexagonal surface lattice.
__
The energy-vs-momentum E(k) map along the "-K
direction (k"K) of the clean Au(111) surface and thecoronene/Au(111) interface, obtained from ARPES,
is shown in Figs. 2(a) and 2(b), respectively. Uponthe formation of the coronene/Au(111) superstructure,some new electronic states are observed. The freeelectron-like dispersive bands are weakly appeared
below the Fermi level EF (Eb = 0~0.4 eV) and above
the Au 5d band (Eb = 1.7~2.4 eV). The inflection_
point of the parabolic dispersions appears at the "point of the monolayer's SBZ (k"K = 1.08 Å#1). The
parabolic dispersions at the low- and high-Eb side
may originate from the Shockley- and Tamm-type
interface states, respectively, both of which are
induced by the surface potential due to the coronene
superstructure. Here, the Eb position of the parabolic
dispersions are almost the same for the original
Shockley and Tamm surface states of the clean
Au(111) surface. Therefore, the interface interaction
between coronene and Au(111) is considered to bequite weak.
At Eb ~ 1.6 eV, a highest occupied molecularorbital (HOMO) peak is observed. The energy
distribution curve at k"K = 1.40 Å#1 shows a sharp
HOMO line shape with the high-Eb satellite due to thehole- vibration coupling. This observation alsosuggests the weak physisorption between coronene
and Au(111). Note that, the HOMO peak shows aquite weak but non-negligible dispersion of ~20 meV.

(i)

0

1

(ii)

(i) (ii)

1

HOMO

2

2

Au 5d
0.8

1

1.2

1.4

Momentum k!K / Å"1

0.8

1

1.2

1.4

Momentum k!K / Å"1 Low

High_

Fig. 2. The E(k) map at T = 15 K around the 2nd "point in the monolayer's SBZ [cf. Fig. 1(b)]; (a) The
clean Au(111) surface. (b) The coronene monolayer
on Au(111) with the energy distribution curve at (i)k"K = 1.08 Å#1 and (ii) k"K = 1.40 Å#1. The molecular
structure of coronene is also shown.
[1] H. Yamane et al., Phys. Rev. B 76 (2007) 165436.
[2] M. Wie$ner et al., Nat. Commun. 4 (2013) 1514.

Ⅲ- 4

Ⅲ -4. Surface and Thin Films

BL6U

Linearly-Dispersive Electronic States at the Interface ofOrganic Monolayers on Graphite
H. Yamane and N. Kosugi

Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan

144

H2Pc/Au(111) interface. These results suggest theimportance of the electronic coupling between
organic monolayers and Gr(0001). Furthermore, thelinearly-dispersive interface state is observed also forother monolayers of pentacene, coronene, and C60
grown on highly oriented pyrolytic graphite (HOPG)
with different dispersion parameters such as the
Fermi momentum (kF) and the Fermi velocity (vF).
On the other hand, both kF and vF at the CoPc/HOPG
interface are almost the same as those at the
H2Pc/HOPG interface, suggesting the importance ofthe molecular unit cell.
Judging from the present observations, the linearlydispersive interface state is governed by both the sizeof the molecular unit cell and the electronic couplingat molecule/graphite interfaces. The Dirac band of thegraphite surface could be scattered and modified by
the intermolecular phonons of adsorbate's lattices as aresult of the surface Umklapp process, which plays acrucial role in the charge/spin extraction in moleculegraphene hybrid systems.
(c)Binding Energy Eb / eV

(a)

(b)

b2
b1 !

EDC
k = 0 Å!1

0

1
HOMO
2

Intensity

Ⅲ- 4Organic monolayers on solid surfaces show various
electronic states and complex electronic phenomena,depending on electronic interactions at their interface.
Some interfacial electronic phenomena, such as the
charge transfer, have been applied to the control ofinterface energetics in functional molecular systems.
In this work, by using angle-resolved photoemission
spectroscopy (ARPES), we succeeded in observationof a linearly-dispersive electronic state at the interfaceof organic monolayers on graphite.
This work was performed at BL6U. The singlecrystalline graphite [Gr(0001)] was obtained by the
direct resistive heating of a 6H-SiC(0001) wafer at
1500°C [1], as confirmed by the low-energy electron
diffraction (LEED) and the valence band dispersion.
In order to obtain the well-ordered organic monolayer,
the Gr(0001) substrate was heated at ~120°C during
the deposition at 1~2 Å/min. The energy resolution in
the present ARPES was 16 meV at 15 K.
Figure 1(a) shows the LEED image of the metalfree phthalocyanine (H2Pc) monolayer on Gr(0001) at
15 K. The observed LEED image indicates the wellknown multi-domain structure for Pc molecules on
six-fold symmetric surfaces. The molecular unit cellof H2Pc/Gr(0001) determined from LEED [Fig. 1(b)]agrees with the previous STM study [2]. Considering
the symmetry of the molecular unit cell, we measured
the azimuthal-angle-dependent ARPES.
Figure 1(c) shows the energy-vs-momentum E(k)map and its energy- and momentum-distribution
curves (EDC and MDC) for H2Pc/Gr(0001) at 15 K
along the b1* direction (hv = 45 eV). The highest
occupied molecular orbital (HOMO) peak appears at
the binding energy (Eb) of 1.4 eV. In addition, as
indicated by the dashed line in the E(k) map and
MDC, the linearly-dispersive feature appears weakly
but undoubtedly at Eb = 0~2.2 eV, like Dirac cone.
This is the interface-specific electronic state, which isobservable for neither the clean Gr(0001) surface nor
the thick multilayer film. In general, the Dirac cone ina honeycomb structure, such as graphene, is appeared
not at the edge but at the corner of the Brillouin zone.
In the present case, we found that the linearlydispersive interface state appears at all azimuth
directions. Therefore, the observed linearly-dispersiveinterface state could be due to the scattering from the
Dirac band of the underlying Gr(0001) substrate.
In order to elucidate the origin of the linearlydispersive interface state, we measured ARPES asfunctions of temperature, substrate, and molecule. We
found that the linearly-dispersive interface state isgetting weak at 300 K, and is not observable for the

-0.2

0

0.2

MDC
0 (Eb/eV)
0.40
0.74
1.20
1.58

Momentum k / Å!1

Fig. 1. (a) The LEED image of the H2Pc monolayeron Gr(0001) at 15 K, taken with 20-eV incident
electrons. (b) The molecular unit cell of
H2Pc/Gr(0001), where b1 = 13.8 Å, b2 = 13.1 Å, and !
= 87.6° [2]. (c) The E(k) map along the b1* direction
with the energy- and momentum-distribution curves(EDC and MDC) of H2Pc/Gr(0001) at 15 K (hv = 45eV).
[1] I. Forbeaux et al., Phys. Rev. B 58 (1998) 16396,
and references therein.
[2] K. Nilson et al., J. Chem. Phys. 127 (2007) 114702.

Ⅲ -4. Surface and Thin Films

BL7U

Low-Energy Angle-Resolved Photoemission Study on Ultrathin Bi Films II
T. Hirahara1, T. Shirai1, M. Matsunami2, T. Hajiri2, S. Kimura2 and S. Hasegawa1
1

2

Department of Physics, University of Tokyo, Tokyo 113-0033, JapanUVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

Semimetal bismuth (Bi) is one of the mostextensively studied elements in solid state physics
because of its extreme physical properties, such as thehighest resistivity and Hall coefficient of all metals.
Bi has tiny hole and electron pockets at T and L
points, respectively, [Fig.1(a) and 1(b)], and therefore
the Fermi wavelength F is very large (about 30 nm).
Because of this large F, the quantum size effectshows up in the properties of Bi films, and theoscillation of the film resistance with the film
thickness d was reported [1]. Furthermore, it waspredicted that when the lowest quantized subband of
the electron pocket is raised to an energy higher thanthe highest hole subband, a band gap will develop[semimetal-to-semiconductor (SMSC) transition at
d~30 nm, Fig. 1(c)][2]. However, although many
angle-resolved
photoemission(ARPES)
measurements have been performed on ultrathin Bi
films, none has actually been able to verify whether
this SMSC transition occurs or not [6-9]. The main
reason is that the cross section for the bulk bands
depends dramatically on the photon energy andpolarization.
Therefore in the present study, we have performed
ARPES measurements on ultrathin Bi films bysystematically changing the photon energy (7-40 eV)
and polarization (P- and S- polarization). Figures 2(a)and (b) show the Fermi surface and band dispersion
near the Fermi level (h! = 9.5eV) of a 25Å thickBi(111) film measured with S-polarized photons.
There is a circular structure at the  point. However

when the polarization is switched to P-polarization((c) and (d)), it becomes hexagonal. These features
correspond to the bulk (B) and surface (S) states,
respectively. This suggests that the bulk bands remain
metallic and the SMSC transition does not occur.
Further sophisticated study is needed to reach adefinite conclusion.
9.5eV(a) S polarization(c) P polarization
Surface(S)Surface(S)Bulk(B)ΓΓ

Surface(S)Ⅲ- 4Fermi surface

Fermi surface
Surface(S)Surface(S)
(d)

(b)
S
S

B
B
Γ

S

S
S

S

S
S
Γ

S
S

Fig. 2. The Fermi surface and band dispersion of a 25
Å thick Bi(111) film measured at h!=9.5eV with S(a), (b) and P- (c), (d) polarized photons, respectively.
S (B) represents surface (bulk) states.
[1] Yu. F. Ogrin et al., JETP Lett. 3 (1966) 71.
[2] V. B. Sandomirskii, Sov. Phys. JETP 25 (1967)
101.
[3] T. Hirahara et al., Phys Rev. Lett. 97 (2006)
146803; Phys. Rev. B 75 (2007) 035422; Phys. Rev.
B 76 (2007) 153305; New Jour. Phys. 10 (2008)
083038.
[4] A. Takayama et al., Phys. Rev. Lett. 106 (2011)
166401; Nano Lett. 12 (2012) 1766.
[5] H. Miyahara et al., eJSSNT 10, 153 (2012); T.
Okuda et al., Rev. Sci. Inst. 82 (2011) 103302.

Fig. 1. (a) The Fermi surface of bulk Bi depicted inbulk (orange) and its projection to the surface (red)Brillouin zone. Electron (hole) pockets are filled withlight blue (purple). (b), (c) Schematic drawing of theBi bulk band projection near the Fermi level before(b) and after (c) the SMSC transition [2].

145Ⅲ -4. Surface and Thin Films

BL7B

Filterless Vacuum Ultraviolet Photoconductive Detector Based on YF 3 Thin
Film Prepared by Pulsed Laser Deposition
1

Ⅲ- 4

M. Yanagihara1, H. Ishikawa1, T. Tsuji1, H. Ohtake2 and S. Ono1

Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
2
Aisin Seiki Co.,Ltd., 2-1 Asahi-machi, Kariya 448-8650, JapanDue to potential applications such as monitoringfor industrial processes and gas sensing, considerable
attention has been focused on the development ofvacuum ultraviolet (VUV) detectors. Currently, thestudy of photoconductive detectors using diamond,
nitride, and oxide are demonstrated. However, thesedevices response deep-UV (DUV) range not only
VUV range because of limitation by the band gap.
Fluorides are relatively wide band gap in contrast tothese materials[1, 2]. Here, we report on thedevelopment of VUV photoconductive detector,
which has no sensitivity in DUV region without any
filters, by utilizing the YF3 thin films.
YF3 thin film was grown by pulsed laser deposition(PLD) on the quartz glass substrate. YF3 ceramic
target was irradiated with the femtosecond laser
pulses (wavelength: 790 nm, laser fluence: 13.5
J/cm2). Growth was carried out 3 h under highvacuum condition (2!10-4 Pa). The substrate heating
temperature was controlled at 670 K. Figure 1 isscanning electron microscope (SEM) images of YbF3
thin film. A sub-micron-sized particulates make up
the majority of the deposition layer. The thickness ofthe thin film was about 170 nm.
To evaluate these thin films as photoconductive
detectors, a pair of interdigitated aluminum electrodes
was fabricated onto the thin film by vacuum
deposition. The patterned area of interdigitatedelectrodes was 5!4 mm2. Both the gap between theelectrodes and the width of the electrodes were 0.2
mm. In addition, we covered them with YF3 thin film
to prevent photoelectron emission from electrodes.
Figure 2 shows I-V characteristic measured in thedark (unirradiation) and under illumination of VUVlight from deuterium lamp (irradiation). The darkcurrent was below 1 pA. The current value increased
3-digit before and after VUV illumination at an
applied bias of 300 V.
To evaluate the spectral responsivity of the detector
using YbF3 thin film, a bias voltage of 200 V wasapplied to a pair of interdigitated electrodes.
Photocurrent was measured at each wavelength ofirradiative light from 100 to 300 nm. Figure 3 shows
the results of the spectral responsivity evaluation
combined with the transmission of the sample. It
turned out that this detector responded only to VUVwavelengths shorter than 170-nm wavelength. The
170-nm wavelength corresponds to band gap of YbF3.
Consequently, these results indicated that wedeveloped a VUV photoconductive detector.

146

Fig. 1. SEM images of the surface and section of
YbF3 thin films.

Fig. 2. I-V characteristic of YF3 thin film.

Fig. 3. Transmission spectrum and spectral responseof the detector.
[1] M. Ieda, T. Ishimaru, S. Ono, K. Yamanoi, M.
Cadatal, T. Shimizu, N. Sarukura, K. Fukuda, T.
Suyama, Y. Yokota, T. Yanagida and A. Yoshikawa,
Jpn. J. Appl. Phys. 51 (2012) 022603.
[2] T. Ishimaru, M. Ieda, S. Ono, Y. Yokota, T.
Yanagida and A. Yoshikawa, Thin Solid Films, 534(2013) 12-14.

Ⅲ -4. Surface and Thin Films

BL8B

ARUPS Study of Molecular Orientation of Pentacene Thin Film on
SAM-Covered SiO2
K. K. Okudaira, N. Kanayama and N. Ueno

Association of Graduate Schools of Science and Technology, Chiba Univererity,
Chiba 263-8522, Japan
Field effect transistors (FETs) based on organic
semiconducting materials have made rapid progressin recent years. Pentacene (Pn) is one of the best
candidates for use in fabricating thin film transistors
because of their high field effect charge mobility. A
more efficient device is expected if all the molecules
are packed orderly with the conducting channels incorrect alignment relative to the electrodes. The
self-assembled monolayers (SAMs) provide asystematic way to modify surface properties such as
wetting and polarity of a substrate, which, in turn,
may affect the orientation of a film deposited on topof it.
In this work we used methyl and phenyl group
terminated SAMs for substrate and examined themolecular orientation of pentacene thin films crystals
thermally deposited on these SAM surfaces byangle-resolved ultraviolet photoelectron spectroscopy(ARUPS) measurements. To obtain the quantitative
analysis on the molecular orientation; we compare
observed take-off angle dependence of ! band andcalculated ones by the independent-atomic-center(IAC)/MO approximation.
ARUPS measurements were performed at the beamline BL8B2 of the UVSOR storage ring at theInstitute for Molecular Science. The take-off angle(") dependencies of photoelectron spectra were
measured at incident angle of photon (#) = 45O withthe photon energy (h$) of 28 eV. We use
octadecyltrichlorosilane
(OTS)
and
trichloro(phenetyl)silane (phenyl) as coupling agent
for SAM. The SAMs were prepared by dipping SiO2
substrate at room temperatures. Pentacene thin film
was deposited on these SAMs.
We observed take-off angle (") dependence of
HOMO peak in UPS of pentacene thin film (thicknessof about 2.0 nm on OTS-SAM and 1.5nm on
phenyl-SAM).
In Fig. 1 (a) the " dependences ofPn(2.0nm)/OTS-SAM have a sharp maximum at "=50°,
while " dependences of Pn(1.5nm)/phenyl-SAM show
two maxima at "=50° and 20°, which are different
from those of Pn(2.0nm)/OTS-SAM. From thecomparison with calculated results it is found that on
the OTS-SAM pentacene molecules have stand-up
orientation, while on the phenyl-SAM, pentacenemolecules have not only stand-up orientation but also
lie-flat orientation. This difference of film structure is
responsible for the interaction between substratesurface and molecules since the surface energy of
OTS-SAM is smaller than that of phenyl-SAM.[1]h!=28eV
E
0°

(a)

:
: observed:

h!=28eVE ::calculated ( #=75°)
0°
30°

"

60°

Intensity (normalized)

90°

:

(b)
observed
calculated ( #=30°)
calculated ( #=75°)
30°

"
60°

Intensity (normalized)

90°

Fig. 1. Take-off angle (a) dependences of photoelectron
intensities of HOMO peak of Pn(2.0nm)/OTS-SAM (a)and Pn(1.5nm/ phenyl-SAM (b). () and (
) :calculated a-dependence for inclination angle (b) = 75 
and 30
.
[1] D. Janssen, et al., Thin. Solid. Films 515 (2006)
1433.

147

Ⅲ- 4

Ⅲ -4. Surface and Thin Films

BL8B

Photoelectric Emission from Au on Zinc-Phthalocyanine Film
1

Ⅲ- 4

S. Tanaka1, T. Otani2 and I. Hiromitsu2

Department of Electric and Electronic Engineering, Faculty of Science and Engineering, Kinki University,
Higashiosaka 577-8502, Japan2
Department of Physics and Materials Science, Graduate School of Science and Engineering, Shimane
University, Matsue 690-8504, Japan

In general, the work function of thin films of metals
and the ionization potential of organic thin films are
no less than 4 eV. It is therefore expected that lightenergy of less than 4 eV cannot generate
photoelectric emission from the surfaces of metal ororganic films. However, we observed photoelectric
emission from Ag on a zinc-phthalocyanine (ZnPc)
layer at a photon energy of 3.4 eV. [1, 2] The
threshold energy for this photoelectric emission ismuch smaller than the work function of Ag. Although
the mechanism of this anomalous photoelectric
emission is not clear, the previous results suggest thatthe vacuum level shift at the Ag/ZnPc interface play a
key role. To investigate the mechanism of theanomalous photoelectric emission, we studied thephotoelectric emission from Au on ZnPc film underUV-visible light irradiation.
The ZnPc was purchased from Kanto Chemical andsublimed three times for purification. ZnPc thin films
were fabricated by vacuum deposition on apolycrystalline Ag substrate. The pressure usedduring the deposition was in the range of 10–5 Pa. Au
was used as the metal layer on the ZnPc film. The
thicknesses of the ZnPc and Au were measured using
a quartz microbalance. A white light source (a Xelamp with an air mass 1.5 filter, HAL-320, Asahi
Spectra Co. Ltd.) was used as UV-visible region lightsource. The UV-visible light was introduced to themeasurement chamber through a glass viewport thatcut off the light with wavelength shorter thanapproximately 320 nm. The power of the white lighton the sample surface was 100 ±10 mW/cm2.
First, we fabricated the ZnPc thin film with the
thickness of 10 nm. Under the white light irradiation,no photoelectric emission was observed from the
ZnPc film as expected. The ionization potential of
ZnPc film was estimated to be 5.0 eV byphotoelectron spectroscopy using synchrotron
radiation (h! = 39.6 eV) (Fig. 1). On the other hand,in the case of the Au (0.2 nm)/ZnPc sample, we
observed weak but obvious photoelectric emission
under the white light irradiation. This is surprisingsince both of the work function of Au and theionization potential of ZnPc are much larger than thehighest energy of white light. Interestingly, no
photoelectric emission under the white lightirradiation was observed from the Au (1.0 nm)/ZnPc
sample. In the case of Ag, the photoelectric emission
under the white light irradiation was observed even at
the Ag thickness of 20 nm.

Figure 1 shows the variation in the photoelectron
spectrum of the ZnPc thin film (10 nm) during theincremental deposition of Au. The secondary cutoff
was shifted toward higher kinetic energy bydeposition of 1.0 nm Au. The HOMO peak of ZnPc
(indicated by the bars in Fig. 1) showed a similarvariation to that of the secondary electron cutoff.
Vacuum level shift at metal/organic interfaces has
been reported in many systems, and is attributed to
the formation of an interfacial dipole layer. The
direction and magnitude of interfacial dipole isaffected by the work function of metal. Au has larger
work function than Ag. Basically, the large work
function of Au increases the threshold energy for thephotoelectric emission. Indeed, the magnitude of theanomalous photoelectric emission on the Au/ZnPc
surface under the white light irradiation was weaker
than that on the Ag/ZnPc surface. However, the verysmall amount of Au on ZnPc surface caused theanomalous photoelectric emission. Further studies areneeded to understand the role of Au and Ag in theanomalous photoelectric emission.

Fig. 1. Variation of the photoelectron spectra of the
ZnPc film under incremental Au deposition (0.2, 1.0nm). Left panel: Secondary electron cutoff region.
The sample was biased at –5.0 V during themeasurements. Right panel: Around the Fermi level.
No bias voltage was applied to the sample.
[1] S. Tanaka et al., UVSOR Activity Report 40(2013) 144.
[2] S. Tanaka et al., submitted.

148

Ⅲ -4. Surface and Thin Films

BL8B

Electronic Structures of Organic Donor-Acceptor Interfacesof a Squaraine Dye and C60
1

Y. Nakayama1, K. R. Koswattage2, Y. Ozawa1, N. Ohashi3 and H. Ishii1,2

Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan
2
Center for Frontier Science, Chiba University, Chiba 263-8522, Japan3
School of Engineering, Tohoku University, Sendai 980-8579, Japan

Squaraine dyes are recently attracting growinginterest as promising donor materials for organic solar
cell application owing to their efficient far red - near
IR absorption [1, 2]. Kido et al. reported that blendsof fullerenes with a member of the squaraine family,
2,4-bis[4-(N,N-di-isobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ; see Fig. 1 inset), exhibited
excellent solar cell efficiency [3]. In the present study,
we investigated these organic donor-acceptor
interfaces of SQ and C60 fullerene by means ofultraviolet photoelectron spectroscopy (UPS).
SQ and C60 were successively deposited onto
indium-tin oxide (ITO) substrate precleaned byultrasonication followed by UV/O3 treatment. UPS
measurement was conducted at BL8B of UVSOR.
Firstly, we determined the ionization energy (Is) of
SQ accurately. In this experiment, the excitation
photon energy was carefully calibrated to be 29.76 eV
by using the Fermi edge position of the second-order
light. This allows one to decide the absolute value ofthe work function (WF) of the sample from thesecondary electron cut-off (SECO) position. Figure 1
shows a UPS spectrum of a SQ film (5 nm-thick)plotted with respect to the vacuum level. Accordingly
Is value of SQ is settled to be 5.15 eV.
Figure 2 shows evolution of the UPS spectra of the
C60-on-SQ and its reverse interfaces. For the former,
stacking of C60 lifted the vacuum level up as shown in
Fig. 2(a), and the highest occupied molecular orbitals(HOMO) of both SQ and C60 also shifted to the lowbinding energy (BE) side (Fig. 2(b)). In contrast, forthe reverse interface (Fig. 2(c,d)), WF decreased on
coverage of SQ over C60. The SQ HOMO exhibitedthe similar behavior to WF, while the C60 HOMO
stuck to its original position.
According to the UPS results, the energy level
diagrams of the C60-on-SQ and SQ-on-C60 interfaces
can be drawn as shown in Fig. 3. Here the energy gap
width of C60 is assumed to be 2.3 eV [4]. The joint
upward shift observed in the former system should be
attributed to a band bending across the interface.
Meanwhile, the downward shift of the vacuum level
Evac and SQ-HOMO apart from C60 may be caused byorientation polarization of the SQ molecules [5].

149

Fig. 1. UPS spectra of SQ film (5 nm) on ITO. A
simulated density-of-states curve derived from thedensity functional theory calculation is also shown
for reference. (Inset) The chemical structure of SQ.

Ⅲ- 4

Fig. 2. Series of UPS spectra of (a,b) C60-on-SQ and(c,d) SQ-on-C60 interfaces.

Fig. 3. Energy level diagrams of the (left) C60-on-SQ
and (right) SQ-on-C60 interfaces (units: eV).
[1] K. Y. Law, Chem. Rev. 93 (1993) 449.
[2] G. Wei et al., ACS Nano 6 (2012) 972.
[3] G. Chen et al., Adv. Mater. 24 (2012) 2768; Appl.
Phys. Lett. 101 (2012) 083904.
[4] R. W. Lof et al., Phys. Rev. Lett. 68 (1992) 3924.
[5] Y. Nakayama et al., Org. Electron. 13 (2012) 2850.

Ⅲ -4. Surface and Thin Films

BL8B

Efficiency of HATCN as a Hole Injection Layer in a Practical System
A . Yoneyama1, K. R. Koswattage2, Y. Nakayama1,3 and H. Ishii1,2,3
1

Faculty of Engineering, Chiba University, Chiba 263-8522, Japan
Center for Frontier Science, Chiba University, Chiba 263-8522, Japan
3
Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan
2

4.0

(a)

(b)

hv=22eVC60 10nm
C60 4nm
C60 2nm
C60 1nm
C60 0.4nm
ITO

hv=22eV

Intensity / arb.units

Intensity / arb.units

in the in the C60 layer was observed toward the
C60/HATCN interface, supposedly there are electric
dipoles at the interface.
In summary, comparison of the UPS spectra
between C60/HATCN/ITO and C60/ITO has revealed
that insertion of the HATCN layer results in a 0.3eVlower HIB from ITO to C60. This supports that hole
injection becomes advantageous by a HATCN
interlayer in practical organic opto-electronic devices.

C60 10nm
C60 4nm
C60 2nm
C60 1nm
C60 0.4.nm
HATCN 1nm
ITO
4.0

3.0 2.0 1.0 0.0
Binding Energy / eV

3.0 2.0 1.0 0.0
Binding Energy / eV

4.0

(a)
C60 10nm
C60 4nm
C60 2nm
C60 1nm
C60 0.4nm
ITO

hv=22eVbias=-5V

5.0
6.0
Kinetic Energy / eV

Normalized Intensity

Fig. 1. UPS spectra in the SECO region of the C60
overlayers on (a) pristine and (b) passivated ITO.
Normalized Intensity

Ⅲ- 4

HATCN(dipyrazino[2,3-f:2’,3’-h]quinoxaline-2,6,
7, 10,11-hexacarbonitrile) is knwon as a strong
accepter and actually has been reported to play a roleof a hole injection layer in real devices [1].
Niederhausen et al clearly proved that this molecularlayer actually lifted the highest occupied molecularorbital (HOMO) energies of ad-layers of several
organic materials up toward the Fermi level of themetal substrate [2]. In this study, we examined how
efficient the HATCN interlayer play a role of a hole
injector in a more practical system ; that is HATCN
on indium-tin oxide (ITO) substrates.
Clean ITO substrates were prepared throughultrasonication in acetone followed by UV/O3
treatment. The work function of the ITO substrates
was 4.3 - 4.4 eV. One-nanometer thick HATCN wasthen deposited at the evaporation rate of 0.1 nm/sec
onto an ITO substrate to prepare a ‘passivated ITO’
substrate, which increased the work function of thesubstrate to 4.9 eV. C60 was successively deposited
onto the passivated and pristine (w/o HATCN) ITO
substrates in a step-by-step manner up to 10 nm thick.
All UPS spectra shown below were taken with thephoton energy of 22 eV at BL-8B in UVSOR.
Figure 1 shows the evolutions of the UPS spectra in
the secondary electron cut-off (SECO) region. On thepristine ITO the C60 overlayer induced almost no
vacuum level shift as seen in Fig. 1(a). Meanwhile,on the passivated ITO, one can find a downward
vacuum level shift by 0.1eV as shown in Fig. 1(b) .
Evolution of the valence band UPS spectra on
growth of the C60 overlayers is shown in Fig. 2. Aseach abscissa axis are taken with respect to the Fermi
level, the onset binding energy of the HOMO peakcorresponds to the hole injection barrier (HIB) fromthe electrode into C60. On the pristine ITO, the
HOMO peak stayed at a constant energy position
independent of the C60 thickness (Fig. 2(a)). The HIB
is estimated to be around 1.95eV for this case. On theother hand, as observed in Fig. 2(b), the HOMO peakof C60 appeared at a smaller binding energy positionon the HATCN interlayer; it emerged at the binding
energy of 2.20 eV in the peak position, while it
shifted further closer to the Fermi level by 0.15eV on
growth to 2 nm or thicker. Accordingly, the HIB of
C60 on HATCN is determined to be 1.58eV from the
HOMO position the 0.4 nm-thick overlayer, while the
HOMO onset lifts up by 0.07 eV in the bulk region.
Figure 3 shows the energy level diagrams based on
the values extracted from SECO/valence spectra anda literature [3]. C60 on passivated ITO exhibited a
0.3eV lower HIB compared with the pristine ITO
case. Also, in the passivated ITO case, band-bending

4.0

(b)
C60 10nm
C60 4nm
C60 2nm
C60 1nm
C60 0.4nm
HATCN 1nm
ITO

hv=22eVbias=-5V

5.0
6.0
Kinetic Energy / eV

Fig. 2. UPS valence spectra of C60 deposited onto the(a) pristine and (b)Fig. 3. Schematic energy level diagram for thedeposition of C60 on (a) pristine ITO and (b)passivated ITO. The value of the band gap of C60 istake from Ref.3.
[1] S. M. Park, Y. H. Kim, Y. Yi, H. Y. Oh and J. W.
Kim, Appl. Phys. Lett. 97 (2010) 063308.
[2] J. Niederhausen et al., Phys. Rev. B 84 (2011)165302.
[3] R. W. Lof, et al., Phys. Rev. Lett. 68 (1992) 3924.

150

Ⅲ -4. Surface and Thin Films

BL8B

HATCN on ITO: Work Function Variation and Influence of Air Exposure
K. R. Koswattage1, A. Yoneyama2, Y. Nakayama2 and H. Ishii1,2
1

2

Center for Frontier Science, Chiba University, Chiba, Japan
Graduate School of Advanced Integration Science, Chiba University, Chiba, Japan,

HATCN (Hexaazatriphenylene-Hexanitrile) is used
as an interlayer to increase the work function of metalelectrodes for the application of organic electronics.
In this work, we mainly examined impact of air
exposure on the valance electronic structure of
HATCN prepared under UHV condition in order toevaluate its ionization potential (IP) because
contradictory values are found in literatures and suchdiscrepancy is likely to arise from variations in theexperimental conditions. e.g. vacuum condition[1-3].
In this work, UPS measurements were done at
BL8B, UVSOR. The excitation photon energy was
carefully calibrated by using the Fermi edge energiesof the first and second order light. ITO was cleaned
by sequential ultra-sonication in acetone. The work
function of the ITO substrate was determined to be
4.6 eV. Then, HATCN was subsequently depositedon the ITO, and a set of UPS measurements were
performed at each step of the thickness.
Figure 1(a) shows the thickness-dependent UPS
spectra of HATCN on ITO at the photon energy of 30
eV, and the top spectrum (dotted) of the Fig. 1(a)shows a calculated spectrum of HATCN using
Gaussian. After about 1 nm, HOMO peak of HATCN
can be distinguished from the O-2p feature of ITO
and for the thickest film IP value of HATCN isestimated to be 9.1 eV. Kang et al. [3] performed
similar works to investigate interface and molecularlevel alignment of HATCN on ITO and their results
were almost consistent with our present findings. In
Fig. 1(b), the work function change is plotted as afunction of film thickness considering the energy
position of the secondary energy cutoff (SECO; at -5
V sample bias). It is clear from the Fig. 1(b) that thework function can be controlled by HATCN.
To clarify the reason for the contradictory IP valuesof HATCN have been reported previously, we
exposed this 10 nm-thick HATCN bulk film to the air
in the dark for 12 h. The UPS measurements were
subsequently performed and the results are shown in
the Fig. 2. Figure 2(a) shows the HOMO region
spectra before and after air exposure. The photon
energy was set at 45 eV. Two additional features canbe observed in the HOMO-LUMO gap region after
air exposure. Considering the features separately,
on-sets of them can be estimated as IP= 7.4 eV and
6.4 eV, while the original onset was IP = 9.1 eV. The
value of 7.4 eV is consistent with the reported IP(7.5) by PYS [1] where the value might correspond to
this gap states due to experimental condition of PYS.
Further, it was observed that the SECO value changed
to the low kinetic energy side after the air exposure asshown in Fig. 2(b). Such changes should be essential
to consider the application of HATCN for practicaldevices.
(a)h! = 30 eV(b)Fig. 1. (a) Thickness-dependent UPS of HATCN
on ITO & simulated spectrum (top-dotted line).
(b) Work function change of ITO as the functionof HATCN thickness.
(a)h! = 45 eV(b)Fig. 2. UPS spectra of (a) HOMO and (b) SECO
region before and after air exposure, respectively.
[1] T. Chiba et al., Org. Electron. 12 (2011) 710.
[2] Y. K. Kim et al., Appl. Phys. Lett. 94 (2009)06330.
[3] H. Kang et al., J. Korean Phys. Soc. 59 (2011)
3205.

151

Ⅲ- 4

Ⅲ -4. Surface and Thin Films

BL8B

Structural Requirements for the Charge-Transfer State Formation throughSurface-Induced Aromatic Stabilization
T. Hosokai1, K. Yonezawa2, K. Kato2, R. Makino2, K. R. Koswattage2, N. Ueno2 and S. Kera2
1

Ⅲ- 4

Department of Materials Science and Technology, Iwate University, Morioka 020-8551, Japan
2
Graduate School of Advanced Integration Science, Chiba 263-8522, Japan

Recently Heimel et al. and we found a newphenomenon occurred at π-conjugated organic
molecules/noble
metal
interfaces,i.e.,surface-induced aromatic stabilization (SIAS), andsucceeded to explain a mechanism leading to aformation of charge-transfer (CT) states at theinterfaces [1]. However, there still remain questions
on the condition inducing SIAS. Here, weinvestigated a role of chemical structure on the SIAS
by employing three kinds of polycyclic aromatic
molecules, diindenoperylene (DIP: C32H16), perylene(C20H12) and picene (C22H14), and angle-resolved
ultraviolet photoelectron spectroscopy (UPS).
Figures 1(a) ~ 1(c) show the UPS results of
DIP-monolayer (ML) on highly oriented pyrolytic
graphite(HOPG),
Ag(111)andCu(111),perylene-ML on those substrates, and picene-ML andmultlayer on Cu(111), respectively. All themeasurements were performed at 295 K with thegeometry illustrated in Fig. 1(d). The DIP (andperylene)/HOPG systems and the picene multilayer
were measured for the reference of electronic
structure at their physisorption, respectively. A
lying-flat orientation was confirmed for all themonolayer systems by angular-dependence ofphotoelectron intensities (not shown). It is clear in
Fig. 1(a) that DIP on Ag(111) and Cu(111) shows gap
states (bold arrows), while other molecules do not(see Figs. 1(b) and 1(c)). As we discussed for
DIP/Ag(poly) and Cu(poly) systems [2], the observed
gap states for the DIP/Ag(111) and Cu(111) systems
are attributed to CT states through SIAS due toelectron transfers to the lowest unoccupied molecularorbital (LUMO) of the DIP monolayer from thesubstrates.
According to Ref. [1], the formation of CT states
through SIAS requires a formation of resonance stateof the adsorbate molecules; the resonance statebrought by a site-specific substrate-moleculeinteraction reduces the LUMO binding energy, andresults in the CT. As shown in Figs. 2(a) and 2(b), the
DIP could have such a state, while its mothermolecule, perylene, does not because, otherwise, two
radicals remain at the center. However, as seen in Fig.
2(c) picene can also be drawn the entire resonancestructure, although no CT state is observed in UPS(Fig. 1(c)). To ensure this, further experiments
together with a theoretical calculation are strongly
required.

152

Fig. 1. Valence band structure of DIP-ML (a),perylene-ML (b), and picene-ML and -multilayer (c).
Their chemical structure is depicted in each graph.
The thin black arrows indicate the position of thehighest-occupied molecular orbitals (HOMO). (d)The schematic at bottom right-hand side is themeasurement geometry.

Fig. 2. A sketch of a resonance structure of DIP (a),perylene (b), and picene (c). All the molecules areconnected with benzene and/or naphthalene rings (left,
black lines).
[1] G. Heimel, et al., Nature Chem. 5 (2013) 187.
[2] T. Hosokai, et al., MRS proceedings, 1647 (2014)
mrsf13-1647-gg01-03.