BL5U

Topological Phase Transition in Ultrathin Bi Films on Bi2Te3 Substrates
T. Hirahara1, M. Aitani1, M. Yamada1, H. Miyazaki2, M. Matsunami2, S. Kimura2
and S. Hasegawa1
1

Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

Topological insulators have become one of themodel systems to study Dirac physics in solids. They
are mathematically characterized by the Z2
topological number with edge modes that cannot bebackscattered [1]. An essential ingredient in realizing
a topological insulator is the parity inversion inducedby the strong spin-orbit coupling. From thisviewpoint, bismuth (Bi), which is virtually theheaviest nonradioactive element, has been the mainbuilding block. Nowadays many Bi alloys are known
as topological insulators. Although it is necessary to
make alloys, inhomogeneity may be introduced in thegrown samples. Therefore, searching for other ways
to produce novel topological materials with simple,
well-defined structures is important.
In this respect, driving a quantum topologicalphase transition [making a normal (trivial) material
into a topological (nontrivial) one] in a simple systemseems to be a rather smart approach. One example ofsuch a transition is the case of Bi, where thethree-dimensional (3D) trivial bulk becomes
topological in two dimensions by making thin filmsof 1–8 bilayers (BL) [2, 3]. Another way to induce a
topological phase transition is to apply pressure andchange the lattice parameters slightly, which isexperimentally not easy to do.
In this study, we have attempted to change thelattice constant by growing an ultrathin film. We havefabricated Bi films on Bi2Te3 substrates and measured
theband
dispersion
withangle-resolved
photoemission spectroscopy. Due to the small
in-plane lattice mismatch between Bi and Bi2Te3(~3.6 %), Bi can be grown epitaxially on Bi2Te3 from
1 BL [4]. Figure 1(a) shows the band dispersion of a 7
BL Bi film on Bi2Te3. Compared to the 7 BL Bi filmgrown on Si(111)-7x7 [Fig. 1(b)], the band dispersion
is significantly different near the * point.
The change in the band dispersion of the Bi films
shown in Fig. 1 can be explained by the change in thelattice constant. Our detailed LEED-IV measurements
showed that due to the contraction of the in-planelattice contraction by ~3.6 %, the out-of-plane lattice
constant is expanded ~4 % [5]. The Poisson ratio,
which is the ratio of the above two changes is as large
as 1, meaning a very large distortion. We have further
found that the lattice constant returns back to its
original bulk value at ~15 BL.
The band dispersion in Fig. 1(a) is reproduced by
ab initio calculations for a free standing Bi slab whenthe experimentally obtained lattice parameters are

Fig. 1. The band dispersion of a 7 BL Bi(111) filmgrown on Bi2Te3 (a), and that on Si(111) (b).
used. Furthermore, by calculating the 3D Z2
topological for this strained Bi, we have found thatthe originally trivial Bi becomes nontrivial. Thus it isshown that the topological properties of Bi can bechanged by inducing a small change in the lattice
parameters [5].
[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82 3045 (2010).
[2] S. Murakami, Phys. Rev. Lett. 97 (2006) 236805[3] Z. Liu et al., Phys. Rev. Lett. 107 (2011) 136805.
[4] T. Hirahara et al., Phys. Rev. Lett. 107 (2011)
166801.
[5] T. Hirahara et al., Phys. Rev. Lett. 109 (2012)
227401.

69

BL3U

Hydrogen Reduction of Pd Catalysts Studied by Transmission Soft-X-Ray
Absorption Spectroscopy
H. Kondoh1,2, R. Toyoshima1, K. Suzuki1, M. Yoshida1, H. Yuzawa2, M. Nagasaka2 andN. Kosugi2
1

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

Hydrogenation of unsaturated hydrocarbon is oneof key reactions both in the fundamentalorganosynthesis and in the chemical industry. Pd
nanoparticles supported on amorphous carbon are
well known as good catalysts for hydrogenation using
atmospheric H2 gas. Also Pd has a capability toabsorb hydrogen in the bulk. Recently it wasdemonstrated that the absorbed hydrogen has a close
relation to the catalytic activity for the hydrogenationof unsaturated hydrocarbon at the surfaces [1].
Furthermore, carbonaceous species included in the Pd
catalysts give significant effects on the hydrogen
absorption and hence on the hydrogenation activity.
The oxidation state of Pd should be also important tothe activity. However, chemical behavior of the Pd
catalysts has been hardly observed under gas flow
conditions. In this work, we studied Pd nanoparticles
and Pd porous films from the viewpoint of chemical
state of oxygen adsorbed on and/or included in the Pd
catalysts under atmospheric H2 gas flow conditions
by means of transmission soft-x-ray absorption
spectroscopy.
Transmission NEXAFS spectra were measured at
BL3U using a photodiode detector mounted in He
path [2]. The Pd nanoparticles were supported on
amorphous carbon and the hydrogenation activity waschecked for butene at 295 K with a flow cell andmass spectrometry. The Pd porous film was preparedby temporal immersion of a Pd foil into aqua regia
and subsequent rinses with pure water and acetone.
These Pd catalysts were loaded in the flow cell with asandwiched configuration between two Si3N4
windows of 100 nm thickness.
O K-edge transmission NEXAFS spectra for the Pd
nanoparticles supported on amorphous carbon weretaken at 295 K before and under H2 flow as shown in
Fig. 1. Before H2 flow, several peaks (A-D) are
observed at a lower energy range from 532 to 538 eV,
which are tentatively associated with adsorbed
O-containing species on the Pd surfaces and surface
oxides of Pd nanoparticles, while under H2 flow thesepeaks diminish significantly. This decrease inintensity indicates reduction and removal of the
O-containing surface species under H2 flow even atroom temperature. A prominent and broad peak isobserved at around 540 eV. This peak is attributed tooxygen atoms of a Pd oxide included in the particles.
This oxide peak does not disappear even under H2flow, which suggests that the Pd oxide included in theparticles is not reduced even under hydrogen-richconditions. A similar trend was observed for a Pd
porous film as shown in Fig. 2: The low-energy peaks(A-D) almost disappear, while main structures at 540
eV and 550-560 eV remain under H2 flow.
The next step is in-situ monitoring of behavior ofincluded carbon as well as included oxygen underhydrogenation reaction conditions, which will reveal
the relation between the presence of carbon andoxygen in the Pd catalysts and the hydrogenation
activity.

O K-edge transmission NEXAFS
Pd particle/carbon
D
295 K

A

C
B

before H2 flow㩷Absorbance [arb. units]㩷under H2 flow
530

540

550

560

570

Photon Energy [eV]Fig. 1. O-K transmission NEXAFS spectra taken for
Pd nanoparticles supported on amorphous carbon
before (upper) and under (lower) H2 gas flow (200
SCCM).

Absorbance [arb. units]

㩷O K-edge transmission NEXAFS
Pd porous film
295 K

D
C
A

before H2 flow

B

㩷

2

under H2 flow

530

540

550

560

570

Photon Energy [eV]Fig. 2. O-K transmission NEXAFS spectra taken for a
Pd porous film before (upper) and under (lower) H2
gas flow (200 SCCM).
[1] M. Wilde et al., Angew. Chem. Int. Ed. 47 (2008)9289.
[2] M. Nagasaka et al., J. Electron. Spectrosc. Relat.
Phenom. 177 (2010) 130.

70

BL3U

The Study on Nickel-Borate Oxygen Evolution Catalyst by Soft X-ray
Electrochemical XAFS
M. Yoshida1, T. Mineo1, T. Yomogida1, H. Yuzawa2, M. Nagasaka2, N. Kosugi2 andH. Kondoh1,212Department of Chemistry, Keio University, Yokohama 223-8522, Japan
Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan

Electrochemical water splitting to hydrogen andoxygen gases is an attractive and promising approach
toward highly efficient energy conversion. This
reaction is derived from two half reactions ofhydrogen evolution reaction (HER) and oxygen
evolution reaction (OER). However, many electrodematerials are known to be not active for OER due tothe high overpotentials. Recently, a nickel-borate thinfilm was reported as an efficient electrocatalyst for
OER [1]. The local structure around the nickel ions of
this thin film was revealed by Ni K-edge XAFS
measurements, while that around B species is still
unclear. The structure and chemical state of the B
species may change depending on the film growth.
Thus, in this study, the nickel-borate thin film were
investigated by B K-edge XAFS measurements underpotential control conditions.
The
soft
X-ray
electrochemical
XAFS
measurements were performed with the transmission
mode at BL3U of UVSOR, according to the previous
work [2]. Au/Cr/Si3N4 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 in a
potassium borate aqueous solution.
Figure 1 shows B K-edge XAFS spectra taken for
nickel-borate thin film electrodeposited at 2.0 V vs
Ag/AgCl. Prominent peaks associated with B species
were observed at ca. 192 eV. The peak positions wereslightly different between the samples before andafter cyclic voltammetry measurements, indicatingthat the chemical state of B in the nickel-borate thinfilm is likely to change by potential control.
In-situ B K-edge XAFS spectra for thenickel-borate deposition on the Au/Cr/Si3N4 electrodewere measured as a function of time in a potassium
borate aqueous solution containing nickel nitrate at
0.9 V, as shown in Fig. 2. A broad peak grows
gradually at ca. 192 eV, suggesting that thenickel-borate thin film was deposited on the
Au/Cr/Si3N4 electrode. The B species at thesolid-liquid interface was successfully detected underpotential control for the first time by the in-situ B
K-edge XAFS measurement. This in-situ techniquewill be applied to investigation of structures andchemical states of nickel-borate thin films underworking conditions as the OER electrocatalyst.

Absorption / a.u.

%1 SRZGHU

1L%L E

1L%L D
%ODQN

186 188 190 192 194 196 198
Photon energy / eV

Absorption / a.u.

Fig. 1. B K-edge XAFS spectra taken for thenickel-borate thin film and BN powder reference. The
samples were measured before (a) and after (b) cyclic
voltammetry.

PLQ
PLQ
PLQ
PLQ
PLQ
2SHQ FLUFXLW

185
190
195
Photon energy / eV
Fig. 2. Time course of in-situ B K-edge XAFS spectra
during electrodeposition of nickel-borate thin film.

[1] D. K. Bediako et al., J. Am. Chem. Soc. 134(2012) 6801.
[2] M. Nagasaka et al., J. Electron. Spectrosc. Relat.
Phenom. 177 (2010) 130.

71

BL3B

Photoionization Threshold Measurement of Porphyrin Derivatives at theSurface of Water
T. Ishioka, K. Tomita, T. Morimoto and A. Harata
Molecular and Material Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
respectively. P+ is always negative since thepolarization is exothermic. In the porphyrin case, theobserved value P+ is smaller than that for aromatichydrocarbon (perylene, Table 1) because polarizationenergy reciprocally depends on the size of moleculeand the size of porphyrin is approximately 2.1 times
larger than perylene. However, this value is notenough for explaining the small polarization energy(1/2.5-3.0) compared to perylene. Further researches
are now in progress for analyzing the result.

Porphyrin derivatives play import roles in naturalscience such as photosynthesis and energy transfer
systems in a cell. A number of research works havebeen carried out utilizing the ring molecule forconverting photon energy to chemical structure [1]. Insuch cases, porphyrin molecules are usually dissolved
into some kinds of solvents. Wavelength regions
effective for reactions are limited by light absorptionof the solvents. Water surfaces are ideal environments
for utilizing photon energies since loss of photon
energy can be minimized. Moreover, source
molecules for chemical synthesis are easily supplied
from both gas phase and solution. However, physicaland chemical properties of such photoactive
molecules are usually known for gas phase or solution
phase. The properties of surface phase are not easily
accessible, which are essential for designingphotoinduced reactions at the surface. In this work,
photoionization behavior of porphyrin derivatives has
been analyzed by utilizing synchrotron light source.
Solvation state of such molecules at the surface ofwater are also discussed by comparing the determinedthreshold values to those in vacuum.
In a typical photoionization measurement, monochromated light was emitted from the chamber to a
He-purged cell through an MgF2 window. The rangeof light energy was in 4-8 eV. The emitted light wasreflected with an Al mirror and vertically irradiatedon the sample surface. High voltage (400V) wasapplied between the mesh electrode that was 5 mmabove the liquid surface and a Pt cell with 25 mm indiameter. Benzene solutions of porphyrin derivatives(tetraphynylporphyrin (TPP), Zn complex of TPP(Zn-TPP)) were prepared at the concentration of
1*10-4 M. The solution was added by 100 PL
dropwisely on the surface of 5 mL water. Afterevaporating benzene for 5 min, the photoinduced
current (~ 0.1 pA) was measured by a picoammeter(Keithley model 428).
Measured photoionization spectra were analyzed
and threshold values were determined by fitting thespectra to the empirical formula of I = (E - Eth)2.5.
Figure 1 shows the obtained photoionization currentof Zn-TPP. The curve fits well and the thresholdvalue was determined to 5.50 ±0.01 eV from repeatedexperiments (n = 5). The threshold values are listed inTable 1 as well as the reported values in vacuum [2].
All measured threshold lies higher in a vacuum
because each molecule (Eth) satisfy the formula in thecase of emitting electron into the air, Eth = Ip + P+
where Ip and P+ are the ionization potential andpolarization energy of the photoionized molecule,1 x 10 -13

Normalized Current /A

8 x 10 -14

6 x 10 -14

4 x 10 -14

2 x 10 -14

0
4

4.5

5

5.5

6

6.5

Photon Energy /eV

Fig. 1. Measured photoinduced current of Zinc
Tetraphenylporphyrin at the surface of water. Inserted
curve is the nonlinear fit using the formula I = I0 (E –Eth)2.5.
Table 1. Photoionization threshold (Eth) ofporphyrin derivatives at the water surface and their
ionization potential (IP) in vacuum.
Eth / eV
IP / eVTPP
5.65 ± 0.04
6.3 ± 0.2*
Zn-TPP

5.50 ± 0.01

6.0 ± 0.2*

Pyrene5.30 ± 0.04**

6.960*) Ref.[2] **)Ref.[3][1] N. Inoue, T. Ishioka and A. Harata, Chem. Lett. 38(2009) 358.
[2] Y. Nakano et al., Chem. Phys. Lett. 39 (1976)
358.
[3] T. Ishioka, K. Tomita, Y. Imanishi and A. Harata,
UVSOR Activity Report 39 (2012) 63.

72

BL4B

Multi-Electron Spectroscopy for Condensed H2O Molecules
Y. Konosu, M. Sawa, K. Soejima and Y. Hikosaka
Niigata University, Niigata 950-2181, Japan
Slow electron formation has received particularattention, in connection with DNA strand breaks inliving cells exposed to ionizing radiations. The DNA
strand breaks are caused, not only by direct ionizationof the DNA itself, but also by the impact of secondary
particles produced by the radiation. Here, the mostabundant secondary particles are electrons emitted
from the ionization of water molecules andbiomolecules surrounding the DNA, where themajority of the electrons have kinetic energies of lessthan 20 eV. The efficiency of the DNA strand breaks
is strongly dependent on the incident electron energyof the impacting electrons, due to its resonant
character, and therefore the knowledge of the energy
distribution of the slow electrons is important inmodeling the radiation damages of living cells. In thisstudy, we have investigated the slow electron
emission process of condensed H2O irradiated with
soft x-ray.
The experiments were carried out by using amagnetic bottle-type electron energy analyzer. A
copper wire of a 0.3-mm diameter was placed at thefocus point of the monochromatized synchrotron
radiation, and was cooled with liquid nitrogen, inorder to condense H2O vapor admitted into thechamber. Figure 1 shows total electron yield spectra
around the O1s range, measured before and after thecooling of the copper wire. The spectrum measured
before the cooling shows structures attributable toelectrons from CuO formed on the copper wire
surface. After cooling, the spectral feature changes
drastically, implying that H2O molecules are adsorbedon the surface.
The energy correlation between two electrons
emitted from condensed H2O is shown in Fig. 2. On
the map clear structures due to coincidences between
photoelectrons and Auger electrons are seen, which
proves that the magnetic bottle-type electron energy
analyzer enables us to investigate the energy
correlations among electrons emitted from condensed
H2O. Figure 3 shows the energy correlation between
photoelectrons and slow-electron observed in triple
coincidence events including Auger electrons. The
right panel represents the slow electron spectrum
extracted by integrating the coincidence yields in thehorizontal range for the O1s photoelectrons. It isestimated, from the total coincidence counts in thiscurve and the photoelectron yields, that the decay
path emitting the slow electrons is about 0.18% of theO1s-1 creation. This ratio for gas-phase H2O isreported to about 13% [1], and the emission of slow
electron is reduced in the condensation. The reasonsof the less efficient sole electron formation from thecondensed H2O are as follows. In the case of73

gas-phase H2O, the main source of slow electron is
autoionizing O* fragment formed by double OH-bondbreaking at highly-excited H2O2+ states populated
after Auger decay of the O1s core-hole state. Thisdissociation at the H2O2+ states should be largely
inhibited in the condensed phase by the neighboring
H2O molecules. Moreover, while such dissociations
could occur and emit slow electrons, a considerable
part of the slow electrons would fail to escape fromthe condensed H2O molecules.

Fig. 1. Total electron yield spectra measured before
and after the cooling of the copper wire.

Fig. 2. Energy correlation between photoelectronsand Auger electron emitted from condensed H2O. The
photon energy was set to 900.1 eV.

Fig. 3. Energy correlation between photoelectronsand slow-electron.
[1] Y. Hikosak et al., J. Chem. Phys. 137 (2012)
191101.

BL4B

Molecular Orientation and Magnetic Property of Vanadium Phthalocyanine
Multilayer on a Silver Surface Studied by XAS/XMCD
K. Eguchi1, Y. Takagi1, 2, T. Nakagawa1, 2 and T. Yokoyama1, 2
1School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585,
Japan
2
Division of electronic structure, Institute for Molecular Science, Okazaki 444-8585, Japan

3d transition metal phthalocyanines (TMPcs) haveattracted much interest in recent years because theyshow characteristic electronic, optical and magnetic
properties. In our group, we have especially focusedon the magnetic properties of TMPcs to understandtheir magnetic coupling in their films and between
TMPcs and ferromagnetic surfaces [1-3]. For instance,iron phthalocyanine (FePc) in multilayer shows aferromagnetic behavior [2]. Recently, we havesucceeded in preparing a vanadium phthalocyanine(VPc) multilayer. In this study, we have investigatedthe molecular orientation and magnetic property of
the VPc multilayer by means of x-ray absorption
spectroscopy (XAS) and x-ray magnetic circulardichroism (XMCD).
The experiments were carried out in an ultra highvacuum condition at beam line 4B. A silver film was
prepared on a clean Cu(111) substrate by the electron
bombardment evaporation at room temperature (RT).
Purified H2Pc and Vanadium atoms were deposited on
the substrate at RT using Knudsen cell and electron
beam evaporation source, respectively. The N K-edge
and V L-edge XAS/XMCD measurements were doneusing a system (JANIS: 7THM-ST-UHV) with asuperconducting magnet and a liq. He cryostat. The
XAS and XMCD spectra were taken at 5 K and the
XMCD spectra were recorded with reversal magnetic
field.
Figure 1 shows angle dependence of N K-edge
XAS spectra of 10ML VPc. The ʌ* resonanceintensity is observed at normal incidence and theintensities increase with increasing the incident angle.
This result means that the framework of thephthalocyanines is slantingly tilted from the substratesurface plane.
Figure 2 shows V L2,3-edge XMCD spectra of
10ML-VPc and VOPc [4] taken at an external
magnetic field of 5 T and an incident angle of 55
degree. The spectra of VPc are obviously different
from ones of VOPc at both V L-edge and O K-edge.
The XMCD signal of VOPc is observed because it
has one electron in 3d-orbital. On the other hand, thatof VPc is very small in comparison to that of VOPc
even though a vanadium atom in VPc has 3 electronsin 3d-orbital. This result might indicate that vanadium
atoms in VPc are antiferromagnetically coupled. To
reveal it, we need to investigate for the case of
1ML-VPc which has no magnetic interaction between
them.

Fig. 1. The angle dependent N K-edge XAS spectra of
VPc multilayer.

Fig. 2. V L2,3- (and O K) edges XAS and XMCD
spectra of VPc and VOPc multilayers taken at theincident angle of 55 deg, ȝ0H=5 T and T=5 K.
[1] I. Yamamoto, K. Eguchi, Y. Takagi, T. Nakagawa
and T. Yokoyama, UVSOR Activity Report 37 (2010)74.
[2] Y. Takagi, K. Eguchi, T. Nakagawa and T.
Yokoyama, UVSOR Activity Report 38 (2011) 67.
[3] Y. Takagi, K. Eguchi, T. Nakagawa and T.
Yokoyama, UVSOR Activity Report 39 (2012) 66.
[4] K. Eguchi, Y. Takagi, T. Nakagawa and T.
Yokoyama, UVSOR Activity Report 39 (2012) 65.

74

BL4B

XMCD Studies of Magnetization of Co Ultrathin Films on W(110)
T. Nakagawa1, 2, 3, H. Nakano2, 4, K. Eguchi2, 3, Y. Takagi2, 3 and T. Yokoyama2, 3
1

Department of Molecule and Material Sciences, Kyushu University, Kasuga 816-8580, Japan
2
Institute for Molecular Science, Okazaki 444-8585, Japan
3
The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan
4
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
Figure 1(b) shows a magnetization curve taken on
1.7 ML Co film. The 1.7 ML Co film exhibits
perpendicular magnetic anisotropy with a coercive
field of 1.7 T, which is large compared to that of thebulk Co (~0.1 T). Thus the magnetization easy axis
for 1.7 ML Co film is perpendicular to the surface,which is revealed for the first time. In the previous
measurements the Co film did not show
ferromagnetic states between 1 and 2 ML. The failure
to detect the perpendicular magnetization may comefrom its large coercivity.
Figure 1(c) shows a magnetization curve for 4 ML
Co film. This shows in-plane magnetization with 1.5
ȝB, which is in agreement with the previous results [1,2].
These result demonstrates that the Co film grownon W(110) surface undergoes a spin reorientationtransition from the perpendicular to the in-plane axis
with increasing the Co coverage. Below 1 ML thetotal magnetization is almost zero. The XMCD
measurement rules out the existence of hugeanisotropy. The vanishing magnetization would becaused by quenching of Co magnetic moment due toa strong hybridization between Co and W or
anti-ferromagnetism (or spin density waves).

Magnetic properties in ultra thin films haveattracted interests because of their characteristics due
to low dimensionality [1-3]. Co ultrathin films,
including nano wire and nano dots, have been
extensively studied since they exhibit large magnetic
anisotropy [1, 2]. Co films grown on W surfaces havebeen investigated since it shows sharp interface
without no alloying. One of the long standing issues
on Co/W(110) is vanishing magnetization below 2monolayer (ML).
We performed experiments at BL4B using x-ray
magnetic circular dichroism (XMCD) end stationequipped with a superconducting magnet under H = 5
T and Ts = 5 K. All the measurements were done
below 1×10-10 Torr. The Co films were grown on
W(110) at room temperature, which was cleaned byrepeated cycles of oxidation at 1500 K andsubsequent annealing at 2200 K. The Co film shows
psedomorphic structure bwlow 0.6 ML, and above
this coverage it shows close packed structures. All the
XMCD results are taken on the close packed
structures.
Figure 1(a) shows x-ray absorption spectra (XAS)
using circularly polarized light with opposite
magnetization directions and XMCD spectrum for 0.9
ML. The 0.9 ML Co film shows no detective XMCD
signal even in the low temperature (5 K) and highmagnetic field (5 T). This result ensures that the Co
film is not in a ferromagnetic order or has extremely
high magnetic anisotropy, but the latter case isunlikely since the magnetic anisotropy greater than 50
T is necessary to explain the negligible XMCD signal.
This vanishing XMCD result is in good agreementwith previous studies [1, 2].

[1] G. Garreau, M. Farle, E. Beaurepaire and K.
Baberschke, Phys. Rev. B 55 (1997) 330.
[2] A. Kleibert, V. Senz, J. Bansmann and P. M.
Oppeneer, Phys. Rev. B 72 (2005) 144404.
[3] T. Nakagawa, Y. Takagi, T. Yokoyama, T.
Methfessel, S. Diehl and H.J. Elmers, Phys. Rev. B
101 (2012) 144418.

Fig. 1. XMCD results on Co/W(110) surfaces. (a) x-ray adsorption and XMCD result on Co(0.9 ML)/W(110)
surface, showing no dichroic signal. (b) A magnetization curve obtained from XMCD measurement on Co(1.7ML)/W(110) surface. Magnetic field is applied normal to the surface. (c) A magnetization curve obtained fromXMCD measurement on Co(4.0 ML)/W(110) surface. Magnetic field is applied along the direction which istilted 30° to the [1-10] direction.

75

BL4B

X-ray Magnetic Circular Dichroism Studies of Monolayer Iron
Phthalocyanine Molecules Grown on Single and Bilayer Graphene
M. Ohtomo1, Y. Matsumoto1, S. Entani1, P. V. Avramov1, H. Naramoto1,
T. Yokoyama2 and S. Sakai1
12Advanced Science Research Center, Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1195, Japan
Department of Materials Molecular Science, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japangraphene interact with magnetic defects (carbon
atomic vacancies) thorough Kondo effect [7]. This
report together with our XMCD results suggest thatthe transport property of FePc/graphene nanocomposite should be investigated as well in order tounderstand the nature of electron-electron interactionin the system.

Graphene is a promising spintronics material for its
long spin diffusion length as a consequence of small
spin-orbit coupling [1]. There are also numbers ofreports on the functionalization of graphene using
finite geometries, adatoms or defects [2]. The
adsorption of magnetic adatoms or molecules, forexample, open the possibility for controlling the
intrinsic spintronic properties of graphene through so-called Ruderman-Kittel-Kasuya-Yoshida  (RKKY)
coupling between magnetic molecules or defects.
Previous theoretical works on the RKKY couplingin graphene [3-5] have shown that for all bipartite
lattices at halffilling, the RKKY coupling isferromagnetic (FM) for impurities on the same
sub-lattice but antiferromagnetic (AFM) for
impurities on different sub-lattices. However, there
are no reports of the experimental observation of
RKKY interaction in graphene. In our beam time,
we performed series of X-ray magnetic circulardichroism (XMCD) measurements on ultra-thinmagnetic molecules/graphene hetero structure.
The monolayer film of Iron phthalocyanine (FePc)
was grown on three graphene samples: bilayer
graphene (BLG) on Cu(111), single layer graphene(SLG) on SiC(0001) and highly ordered pyrolytic
graphite (HOPG) as a reference. The samples were
thoroughly degassed in ultra-high vacuum chamber at
500C before the deposition of FePc.
Fig. 1 shows Fe L2,3-edge XMCD of monolayer
FePc/SLG hetero structure at magic angle (55)
incidence (MI). The spectrum shape of L2,3 edge
adsorption was identical with that of FePc textured
thin film [6], which suggest 3Eg ground state meaningsmall interaction with the substrate. Similar X-ray
adsorption spectra were observed for FePc/BLG and
FePc/HOPG as well. Due to large orbital momentof FePc molecules, smaller XMCD signal wasobserved in case of normal incidence (NI) condition.
The magnetic field dependence of the XMCD signals
is shown in Fig. 2 for FePc/BLG sample, which did
not show saturation behavior up to 5 T.
In order to confirm the possibility of superparamagnetic component, we assumed the blocking
temperature around 50 K and measured XMCD at“field cool” and “zero-field cool” conditions, whichshowed no differences within experimental error (notshown). This result indicates two possibilities, thatis, the blocking temperature was larger than 50 K, ormagnetic coupling was negligibly small. It wasrecently reported that the conduction electrons in

Fig. 1. Fe L2,3-edge XMCD spectra of monolayer
FePc/SLG hetero structure. The temperature was 5.4
K with applied magnetic field of ±50 kOe.

Fig. 2. The magnetic field dependence of XMCD
signals in case of FePc/BLG nano-composite. The
temperature was 5.5 K during the measurement.
[1] B. Dlubak et al., Nature Phys. 8 (2012) 557.
[2] O. V. Yazyev, Reports on Progress in Physics 73(2010) 056501.
[3] A. M. Black-Schaffer, Phys. Rev. B 81 (2010)
205416.
[4] E. Kogan, Phys. Rev. B 84 (2011) 115119.
[5] M. Sherafati et al., Phys. Rev. B 84 (2011)125416.
[6] J. Bartolomé et al., Phys. Rev. B 81 (2010)
195405.
[7] J. H. Chen et al., Nature Phys. 7 (2011) 535.

76

BL5U

Temperature Dependence of the Electronic Structureof K-doped EuO Ultrathin Film
H. Momiyama1, H. Miyazaki2, T. Hajiri1,3, M. Matsunami3,4,T. Ito1 and S. Kimura3,4
1

Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
Center for Fostering Young and Innovative Researchers, Nagoya Institute of Technology, Nagoya 466-8555,
Japan
3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
4
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585,
Japan
2

Europium monoxide (EuO), a ferromagnetic
semiconductor, with Curie temperature (TC) of 69 K,
is attracting attention due to its anomalous
magneto-optical and transport properties [1, 2].
Especially, TC achieves to as high as 200 K in theelectron doping case, recently. Therefore EuO is oneof the candidate compounds for next-generation
spintronics applications such as spinfilter. So far, we
successfully fabricated single-crystalline EuO
ultrathin films with a few atomic layers for device
development by using molecular beam epitaxy(MBE) method. Furthermore, we have performed
angle-resolved photoemission spectroscopy (ARPES)
to clarify the electronic structure and its relation to
the anomalous properties. Consequently, we revealed
that the TC decreases with reducing the thickness
because of the weakening of hybridization strength of
Eu 4f – O 2p and Eu 4f – Eu 5d [3].
In this study, we had fabricated K-doped EuO
ultrathin film to elevate TC. And we performed
ARPES at the beamline BL5U of UVSOR- Υ
combined with the MBE system.
Figure 1 shows the temperature-dependent ARPES
spectra of single-crystalline K-doped EuO ultrathin
film (5nm) measured at hQ = 38 eV, whichcorresponds to the trace around the X point. Eu 4f
state has two structures at 1.4 eV (A) and 2.2 eV (B)binding energies. According to the previous researchon EuO single-crystalline ultrathin film (5nm) [4], theformer originates from the Eu 4f – O 2p and Eu 5dhybridized states being attributed to the magnetic
property, while the latter from the bare Eu 4f states. In
Fig. 1, we found that the feature A gradually shifts
to the lower binding-energies below 100 K. The
observed shift could be originated in the exchange
energy splitting below TC of around 100 K. In turn, it
is suggestive of the elevated TC on the K-doped EuOthan EuO ultrathin film. To clarify the detailed effectof K-doping, further studies are intended.

Fig. 1. Temperature dependent ARPES spectra of
K-doped EuO ultrathin film (5nm) measured at hȞ =38 eV. The thin lines are ARPES spectra in 160 K forcomparison.
[1] N. Tsuda et al., electronid Conduction in Oxides(Springers College) (1976).
[2] A. Mauger et. al., J. Phys. (paris) 39 (1978) 1125.
[3] H. Miyazaki et al., UVSOR Activity Report 39(2012).
[4] H. Miyazaki et al., Phys. Rev. Lett. 101 (2009)
227203.

77

BL5U

Soft X-ray Photoelectron Spectroscopy Study of Fe2P(0001)
K. Edamoto1, 2 and Y. Sugizaki1
1

2

Department of Chemistry, Rikkyo University, Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan
Research Center for Smart Molecules, Rikkyo University, Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501,JapanThe surface properties of transition metal
phosphides (TMPs) have attracted much attention
because these materials have been shown to havehigh catalytic activities for HDS and HDN [1]. Of all
TMPs studied thus far, Ni2P has the highest catalytic
activity, while that of Fe2P has been found to be very
low [1]. The high catalytic performance of Ni2P has
been proposed to arise from the stabilization of active
Ni sites induced by segregation of substrate P atoms
toward the surface [2]. In order to elucidate the originof the high activity of Ni2P, we think that it is usefulto elucidate the difference in surface properties ofNi2P and Fe2P. However, in contrast to theaccumulation of data on Ni2P, very limited
information is available as to the Fe2P surfaces. In
this work, we performed a soft X-ray photoelectron
spectroscopy (SXPES) study on Fe2P(0001).
The experiments were performed at BL-5U of
UVSOR, Institute for Molecular Science. The
photoelectrons were collected by an electron energy
analyzed of hemispherical type (MBS A-1), using anangle-integrated mode (acceptance angle of ±20°).
The Fe2P single-crystal was grown by Dr. S. Otani of
National Institute for Materials Science. The sample
was cut at an orientation of (0001) by spark erosion,and the surface was polished mechanically to a mirror
finish. The surface was cleaned by several cycles of
Ar+ ion sputtering and annealing (600͠) in the UHV
system.
Figure 1 shows SXPES spectra of Fe2P(0001)
measured at various photon energies of 48 – 60 eV. A
clear cut-off at the Fermi edge is observed
independent of the photon energy, indicating a
metallic nature of Fe2P. A band is observed at 0 – 4
eV independent of hȞ. The band is ascribed to a Fe 3d– P 3p hybrid band (main band). Satellite peaks areobserved at 5 and 7 eV, and these peaks are increased
in intensity around the photon energy region of Fe 3p
photoexcitation threshold (hȞ = 54 eV). Therefore, thesatellites are associated with the photoemission
process including the Fe 3p – 3d photoexcitation andthe following Fe 3d electron emission through asuper-Coster-Kronig decay. At hȞ > 56 eV, an
additional band composed of photoelectrons withconstant kinetic energy (44 – 45 eV) is observed, andthe band is ascribed to a Fe MVV Auger peak.
The overall spectral features of Fe2P(0001) aresimilar to those of Ni2P(0001) [3]. In Fig. 2, thenormal-emission spectrum of Ni2P(0001) is shown
together with the spectrum of Fe2P(0001). Bothspectra are measured at hȞ = 48 eV. For Ni2P(0001),

the main band is also observed at 0 – 4 eV. However,
the main band is more stabilized and DOS around EF
is more suppressed for Ni2P(0001). It is consideredthat the stabilization of Ni 3d levels is essential toprevent the accumulation of S atoms on active metalsites in HDS for Ni2P, which is less effective for Fe2P.
Fe2P(0001)

60
59

Intensity (Arb. Units)

58
57
56
55
54
53
52
51
50
49
15

10

5

0

48

Binding Energy (eV)Fig. 1. Valence band spectra of Fe2P(0001).
Valence Band Spectra

Intensity (Arb. Units)

hQ = 48 eV

Fe2P(0001)

Ni2P(0001)

10

5

0

Binding Energy (eV)Fig. 2. Comparison of valence band spectra ofFe2P(0001) and Ni2P(0001).
[1] S. T. Oyama, J. Catal. 216 (2003) 343.
[2] K. Edamoto, Appl. Surf. Sci. 269 (2013) 7.
[3] K. Edamoto et al., Solid State Commun. 148(2008) 135.

78

BL5U

Characterization of Electronic Structures of Ge and Ge1-xSnx Surfaces
by Using Angle-Resolved Photoelectron Spectroscopy
O. Nakatsuka1, S. Ike1, T. Asano1, M. Kurosawa1,2, N. Taoka1,
M. Matsunami3, S. Kimura3 and S. Zaima1
1

Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
2
JSPS Research Fellow, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan
3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

Germanium (Ge), strained-Ge, and germanium-tin(Ge1-xSnx) alloy are attractive candidates for highmobility channel material alternative to silicon (Si) in
metal-oxide- semiconductor filed effect transistors(MOSFETs) in order to improve on ultra-large scale
integrated circuits (ULSIs). Ge1-xSnx alloy with a Sn
content high than 10% is also expected to be directtransition semiconductor. Theoretical calculation
promises the reduction in the effective mass of holes
in tensile strained Ge and compressive strained
Ge1-xSnx compared to bulk-Ge and strained Si.
Recently, we achieved the epitaxial growth ofGe1-xSnx thin films with wide Sn content (0~27%) on
various substrates such as Ge, Si, and InP [1-3]. It isessentially important to understand the electronic
properties of energy band dispersion, effective massof electrons and holes, and carrier mobility of strained
Ge and Ge1-xSnx alloy for ULSI applications.
However, there are few reports about the electronic
structure of strained Ge and Ge1-xSnx.
In this study, we prepared the compressivestrained Ge1-xSnx epitaxial layers on Ge substrates,
and investigated the electrical structures of Ge andGe1-xSnx by using angle-resolved photoelectron
spectroscopy (ARPES) measurement.
Ge(001) and Ge(110) wafers were used for
substrates. After chemically cleaning, a Ge substrate
was thermally cleaned in an ultra high vacuum
chamber. Then, a 200 nm-thick Ge1-xSnx layer was
deposited at 150~170qC on a Ge substrate. Sn content
was ranging from 4.6% to 6.0%, which was estimatedwith x-ray diffraction two dimensional reciprocal
space mapping. Successively, A 1~2 nm-thick Ge cap
layer was deposited. Samples were brought to
UVSOR/BL5U, and then introduced into a
preparation UHV chamber connected to the ARPES
measurement chamber. A sample was annealed at as
high as 460qC in the preparation chamber in order toremove surface native oxide. Clean surface wasconfirmed from the surface reconstruction structureby using reflective high-energy electron diffraction(RHEED) observation. Samples were cooled at 15K
duringthe
ARPES
measurement.
ARPES
measurement was performed with x-ray energies of
66 and 33 eV for Ge1-xSnx/Ge(001) andGe1-xSnx/Ge(110) samples, respectively.
Figure 1 shows a typical ARPES result for the
Ge/Ge0.968Sn0.032/n-Ge(001) sample. The spectra were

measured for various take-off-angles (TOA). We canclearly see the energy dispersion related to thevalence bands of light and heavy holes at the *-point.
We have also obtained photoemission spectra fromthe strained-Ge1-xSnx/Ge(110) and tensile-strained
Ge/ Ge1-xSnx/Ge(001) samples. The analysis ofelectronic structures and effective mass for holes are
ongoing and will be reported in future.
We acknowledge members of Prof. Ujihara’s group,
Nagoya University for their kind support in the
ARPES measurement.

Photoemission
intensity
Photoelectron
intensity(arb.(a.u.)units)

hȞ=66 eV

lh

-5

5

-4

-3

hh

-2

-1

0

4
3
2
1
0
Binding energy (eV)1Fig. 1. Photoemission spectra with various TOAs
(126 slices) for the Ge/Ge0.968Sn0.032/n-Ge(001)sample.
[1] S. Zaima, O. Nakatsuka, Y. Shimura, S. Takeuchi,
B. Vincent, F. Gencarelli, T. Clarysse, J.
Demeulemeester, K. Temst, A. Vantomme, M.
Caymax and R. Loo, ECS Trans. 41 (2011) 231.
[2] S. Zaima, O. Nakatsuka, M. Nakamura, W.
Takeuchi, Y. Shimura and N. Taoka, ECS Trans. 50(2012) 897.
[3] O. Nakatsuka, Y. Shimura, W. Takeuchi, N. Taoka
and S. Zaima, Solid-State Electron. 83 (2013) 82.

79

BL5B

Characterization of TiO2/Sapphire Beam Splitter for High-Order
Harmonics
H. Kumagai1, M. Sanjo1 and T. Shinagawa2
1

Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osakashi, Osaka
558-8585, Japan
2
Inorganic Materials Lab., Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya,Joto-ku,Osaka
536-8553, Japan
reflectivities of TiO2/sapphire BS were highreflectivity of 44.8% (incident angle șB =70.5° for
TiO2) and 31.9% (incident angle șB =65.56° forsapphire) at 21.24 nm, which corresponded to the37th order harmonics of the 800-nm pump pulse,
respectively. Thus, the incident angle șB was set at
70.5° for TiO2, which corresponded to the Brewster
angle șB of the 800-nm pump pulse.

High-intensity high-order harmonic generation has
been investigated intensively in recent years becauseof application to nonlinear optics with magnificent
spatial coherence and with an ultrashort pulse
duration for high peak power. In the development ofbeam line for high-intensity high-order harmonics,
however, utilizing a conventional beam splitter (BS)(Si or SiC) that absorbs fundamental waves causes
serious problems such as its thermal distortion [1]. To
resolve these problems, we proposed and investigatednovel BS with transparent materials that transmitted
the fundamental waves and then reflected thehigh-order harmonics. In BS for the high-orderharmonics, reflection of the fundamental waves
should be prevented by entering the p-polarized
fundamental waves at the Brewster̉s angle ǰ B,
which could improve the separation between thefundamental waves and the high-order harmonics.
Atomic layer deposition (ALD) and atomic layer
epitaxy (ALE) are gas phase deposition methods thatare based on sequential surface chemical reaction
where the growth rate is dependent solely on thenumber of growth cycles. These deposition methods
have emerged as the excellent techniques to fabricate
a wide variety of thin films [2-6]. The main difference
between ALE and chemical vapor deposition (CVD)is the way to introduce precursor into the reaction
chamber. Unlike CVD, each precursor is alternately
pulsed to the chamber in ALE process. ALE has
self-limiting nature of the surface saturation, allowing
the formation of a full or a sub-monolayer of atoms
because adsorption of precursor gases stops due tocovered surface reactive sites or steric hindrances ofligands of adsorbed molecules. It can coat complex
shapes with excellent uniformity and control
thickness with precision over large areas.
We have already studied and fabricated TiO2 thinfilms on sapphire (0001) substrates by controlled
growth with ALD or ALE. One of authors has alreadydemonstrated that rutile TiO2 (200) thin films weregrown epitaxially on sapphire (0001) substrates by
ALE using sequentially fast pressurized titanium
tetrachloride (TiCl4) and water (H2O) vapor pulses[7].
Figure 1 shows reflectivity (p polarization) of
TiO2/sapphire BS. Incident angle șB was set at 70.5°
for TiO2 and at 65.56° for sapphire, whichcorresponded to the Brewster angles șB of the 800-nm
pump pulse. Figure 1 indicated that the peakFig. 1. Reflectivities of TiO2/sapphire BS at the
Brewster angles for TiO2 (solid line) and sapphire(dashed line) at the pump pulse.
[1] E. J. Takahashi et al., Opt. Lett. 29 (2004) 507.
[2] S. M. George et al., J. Phys. Chem. 100 (1996)
13121.
[3] D. M. Hausmann et al., Thin solid Films 443(2003) 1.
[4] S. J. Kwon, Jpn. J. of Appl. Phys. 44 (2005) 1062.
[5] M. Ishii et al., J. crystal Growth 180 (1997) 15.
[6] H. Kumagai et al., J. Mater. Sci. Lett. 15 (1996)
1081.
[7] H. Kumagai et al., J. Crystal Growth 314 (2011) 1.

80

BL6U

Spin-Orbit Coupled Gold Atomic Chains on Vicinal Si Surfaces
S. Won Jung1, W. Jong Shin1, H. Yamane2, N. Kosugi2 and H. W. Yeom1
1

Department of Physics and Center for Low Dimensional Electronic Symmetry,
Pohang University of Science and Technology, Pohang 790-784, Korea
2
Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, JapanOne dimensional (1D) electronic system has beenstudied for its exotic phenomena such as Peierls
instability [1], and non-Fermi liquid behavior [2]. For
the non-Fermi liquid, most experimental researches
focused on the spin-charge separation [3] and thepower-law dependent density of states near the Fermi
level [4].
However, there has been almost no such study forthe non-Fermi liquid state with the spin-orbit
coupling [2-4]. Moreover, 1D systems with thespin-orbit coupling becomes important for spintronic
applications and for finding Majorana Fermions [5,6]. Therefore, 1D electronic system with a substantial
spin-orbit coupling would be an important topic witha chance to observe exotic states such as Spiral
Luttinger liquids and Helical Luttinger liqids [7].
In the present study, we performed angle-resolved
photoelectron spectroscopy (ARPES) measurements,using BL6U of UVSOR, in order to measure the band
structure of gold atomic wires formed on a vicinalsilicon surface, Si(553). Gold atomic wires on vicinal
Si surfaces has been known to possess multiple 1D
metallic bands [8], Peierls instability [9], andRashba-type spin-orbit coupling [10].
Fig. 1 shows low energy electron diffraction(LEED) images of two phases of Au atomic wires onSi(553) surface at different gold coverages. The
periods along [1 1 -2] direction (perpendicular withwires) is (4+1/3)a0 [Fig. 1 (a), A phase] , (9+2/3)a0[Fig. 1 (b), B phase]. The B phase is found for the
first time in this work. However, in Fig. 2, theelectronic structures have strong similarity withsurface states, S1 and S2 (known as the Rashba spin
splitting [10]) and S3. Therefore, we can assume thatthe two phases features the same gold chain structurewith different density.
In this study, we measured high-resolution band
spectra of the B phase, which is thought to have alower wire density and thus a large interwire distance.
We thus expect that this low density wire phasewould exhibit a more ideal 1D electronic property
and thus is a good candidate to investigate thepossibility of non-Fermi liquid state with thespin-orbit interaction. The analysis of the spectra
functions is underway along with a scanningtunneling microscopy study.

Fig. 1. Measured LEED images of Au atomic wires
on Si(553). The coverage of Gold is about (a)0.48ML, (b) 0.22ML. [1 -1 0] is the direction of
atomic wires.

Fig. 2. Measured ARPES Spectra of Au atomic wires
along [1 -1 0] direction with 46eV photon. The
coverage of Gold is about (a) 0.48ML , (b) 0.22ML.

[1] H. W. Yeom et al., Phys. Rev. Lett. 82 (2009)4898.
[2] T. Giamarchi, Quantum Physics in One
Dimension (Clarendon Press, OXFORD, 2003).
J. N. Crain and F. J. Himpsel, Appl. Phys. A 82
(2006) 431.
[3] C. Kim et al., Phys. Rev. Lett. 77 (1996) 4054.
[4] Marc Bockrath et al., Nature 397 (1999) 598.
C. Blumenstein et al. ,Nature Physics 7 (2011) 776.
[5] R. M. Lutchyn, J. D. Sau, and S. Das Sarma, Phys.
Rev. Lett. 105 (2010) 077011.
[6] J. Park et al., Phys. Rev. Lett. 110 (2013) 036801.
[7] B. Brauecker, C. Bena and P. Simon, Phys. Rev. B
85 (2012) 035136.
[8] J. N. Crain et al., Phys. Rev. Lett. 90 (2003)
176805.
J. N. Crain et al., Phys. Rev. B 69 (2004) 125401.
[9] J. R. Ahn, P. G. Kang, K. D. Ryang and H. W.
Yeom, Phys. Rev. Lett. 95 (2005) 196402.

[10] I. Barke et al., Phys. Rev. Lett. 97 (2006)
226405.

81

BL6U

Systematic Study on Intermolecular Interactionin Crystalline Films of Metal Phthalocyanine
H. Yamane and N. Kosugi
Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan(a)Intensity / normalized

The intermolecular electronic band dispersion,
originating from the periodicity of the molecular
stacking structure, is essential to investigate the chargetransport mechanism related to organic electronics.
Recently, we have succeeded in observation of quite
small intermolecular band dispersions for crystalline
films of metal phthalocyanines (MPc, M = metal),such as ZnPc, MnPc, and F16ZnPc, by the precise
angle-resolved photoemission spectroscopy (ARPES)
experiments [1]. These observations enable us to dosystematic study of the intermolecular interaction interms of the intermolecular distance along the ʌ-ʌstacking direction (aA) by changing terminal groups
or central metals in the MPc molecule.
Figure 1(a) shows the normal-emission ARPES
spectra as a function of the incident photon energy(hȞ) for a crystalline film of CoPc on Au(111) at 15K.
The relative peak intensity between the highest
occupied molecular orbital (HOMO) and HOMO-1
levels changes with hȞ. At hȞ = 50 eV, the HOMO
intensity is stronger than the HOMO-1 intensity. AthȞ  58 eV, the HOMO-1 intensity is getting stronger
than the HOMO intensity due to different ionization
cross sections between the HOMO and HOMO-1
characters. This evidence indicates that the HOMO of
CoPc is derived from the C 2p orbital; on the other
hand, the HOMO-1 of CoPc is derived from the Co 3d
orbital. These assignments agree well with theoreticalresults [2]. The HOMO (C 2p) and HOMO-1 (Co 3d)
derived peaks show the clear periodic shift with hȞ.
The periodicity of these dispersive peaks in the wavevector space is explained by aA of 3.318 Å, which isevaluated from the X-ray diffraction (XRD) shown in
Fig. 1(b). Therefore, it is concluded that the observed
dispersive behavior for the HOMO and HOMO-1
peaks is ascribed to the delocalized band formation
due to the intermolecular van der Waals interaction in
the crystalline CoPc film.
From the observed band dispersions of the C-2pderived band for crystalline films of various MPc
molecules, the intermolecular transfer integral (tA)can be evaluated with respect to aA as shown in Fig. 2.
From the tA-vs.-aA relation, we found that (i) the tA
value is getting large with decreasing the aA value dueto the stronger intermolecular electronic coupling andthat (ii) the tA-vs.-aA relation seems to be fitted by alinear function in the present narrow aA range. Fromthe least-squares fitting, the slope parameter of tA/aA
is determined to be 70 meV/Å for the C-2p-derived
band of Į-crystalline MPc films.

HOMO-1(Co 3d)CoPc(250Å)/Au(111)
T = 15 K hv

HOMO
(C 2p)

/eV
50
70
90
110
130
2

1

0

Binding Energy (vs. EF) / eV
0

10

20

(b)

30

Intensity / cps

CuKD XRD: CoPc(250Å)/Au(111)aA / Å
3.318

02T / °

40

1

2

Momentum Transfer / ÅAu(111)-1

3

Transfer Integral / meV

Fig. 1. (a) The photon energy (hȞ) dependence of thenormal-emission ARPES spectra shown with the hȞ
step of 4 eV and (b) the CuKĮ X-ray diffraction(XRD) spectrum, measured for the crystalline CoPc
film on Au(111). In panel (b), the low-energy electron
diffraction image, taken at the incident electronenergy of 34 eV, is also shown.

30: F 16 ZnPc
: MnPc: CoPc: ZnPc: H 2Pc

25

20
3.2

3.3

3.4

Intermolecular Distance / ÅFig. 2. The intermolecular transfer integral (tA) withrespect to the intermolecular distance along the ʌ-ʌstacking direction (aA) in flat-lying crystalline filmsof MPc on Au(111) at 15 K. The LCAO (linear
combination of atomic orbitals) pattern of the C 2p
derived level of MPc is shown in the inset.
[1] H. Yamane and N. Kosugi, submitted.
[2] For example, M.-S. Liao and S. Scheiner, J. Chem.
Phys. 114 (2001) 9780.

82

BL6U

Crystallinity-Dependent Excited-Electron Dynamics in F16ZnPc Films
H. Yamane and N. Kosugi
Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan

Resonant photoemission spectroscopy (RPES) has
been applied to the study of the charge transfer
dynamics at molecule/electrode interfaces [1, 2],which is called as a core-hole clock (CHC) experiment.
The CHC method is based on the normal decay time ofa core excitation (6 fs for N 1s) as a reference clock forother processes in the target system. That is, RPES
signals depend on the timescale of excited electron
motions with respect to that of the core-hole lifetime.
Here we have applied the CHC method to the thickmolecular films for the study of the excited-electron
dynamics by the weak intermolecular interaction.
In the present work, we have used an archetypaln-type organic semiconductor of F16ZnPc. In order tostudy the effect of the weak intermolecular interaction
on the excited state, we have prepared an amorphousand a crystalline film of F16ZnPc. The amorphous
F16ZnPc film is obtained by the molecular deposition
onto a polycrystalline Au surface kept at the lowtemperature (T ~ 180 K). On the other hand, thecrystalline F16ZnPc film is obtained by the moleculardeposition onto a single-crystalline Au(111) surface
kept at the elevated temperature (T ~ 360 K). The
sample crystallinity was confirmed by the N K-edge
X-ray absorption spectroscopy (XAS), the low-energy
electron diffraction, and the X-ray diffraction. For the
crystalline F16ZnPc film, furthermore, we confirmedthat the valence band dispersion by the intermolecularinteraction is observable. In order to avoid the effect ofthe molecular orientation on the core-level excitation,
the N K-edge RPES and XAS spectra were measured
at the magic-angle incidence geometry.
Figure 1 shows the color contour map of the N-K
RPES spectra for (a) the amorphous and (b) crystalline
F16ZnPc film, measured at T = 15 K. We found thatthe ionization energy and the RPES intensity dependon the sample crystallinity. Among these, the RPES
intensity, plotted in the upper panel of Fig. 1 together
with the N-K XAS spectrum, is related to the excitedelectron dynamics. The XAS spectra of both theamorphous and crystalline F16ZnPc films show the
first, second, and third N 1s ĺ ʌ* transition peaks at
the photon energy (hȞ) of 397.0, 399.4, and 401.4 eV,
respectively. At these hv, the RPES spectra for theamorphous F16ZnPc film also show the intensity
maxima. On the other hand, for the crystalline F16ZnPc
film, the RPES intensity at hȞ = 399.4 and 401.4 eV isdecreased significantly. This evidence in RPES and
XAS indicates that the timescale of the intermolecularcharge transfer in the crystalline F16ZnPc film is fasterthan that of the N 1s core-hole lifetime (6 fs) due to thedelocalization of the excited electron. The inter-molecular charge transfer time (ĲCT) can be determinedfrom the modified equation given in Ref. [1] of
ĲCT = ĲCH (Icry-PE/Icry-XA)(Iamo-PE/Iamo-XAí Icry-PE/Icry-XA)í1,
where ĲCH is the core-hole lifetime, Icry-PE/Icry-XA is the
RPES/XAS intensity ratio for the crystalline film, andIamo-PE/Iamo-XA is the RPES/XAS intensity ratio for theamorphous film. Using this equation, the ĲCT in the
crystalline F16ZnPc film is determined to be 2.1 fs.
(a): RPES (Eb ~ 3.0 eV): XAS ( Is / I0)

Imax/2@Magic Angle

Binding Energy Eb (vs.EF) / eV0hv =394 eV (397 eV (Amorphous F16ZnPc
0

)
)

S

5
10
15
20
25
394

396

398

400

402

404

Photon Energy hv / eV(b)

: RPES (Eb ~ 3.0 eV): XAS ( Is / I0)

Imax/2@Magic Angle

Binding Energy Eb (vs.EF) / eV0hv =394 eV (397 eV (Crystalline F 16ZnPc
0

)
)

S

5
10
15
20
25
394

396

398

400

402

404

Photon Energy hv / eV

Fig. 1. Color contour map of the N-K RPES spectra for(a) amorphous and (b) crystalline films of F16ZnPc at
15 K. Upper panel shows the intensity-vs-hȞ profile at
Eb = 3.0 eV (ż), together with the XAS spectrum (–).
Side panel shows the intensity-vs-Eb profile (i.e.,
RPES spectrum) at hȞ = 394 and 397 eV.
[1] P.A. Brühwiler, O. Karis and N. Mårtensson, Rev.
Mod. Phys. 74 (2002) 703.
[2] L. Wang, W. Chen and A. T. S. Wee, Surf. Sci Rep.
63 (2008) 465, and references therein.

83

BL7U

Anisotropy of Electron-phonon Coupling for Pentacene Monolayer
S. Kera1, F. Bussolotti1, H. Shinotsuka1, N. Ueno1, M. Matsunami2 and S. Kimura2
1

Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan
2
UVSOR, Institute for Molecular Science, Okazaki 444-8585, Japan

Over the past decade, organic electronic devices
have increased considerable attentions due to theirpotential applications. Fine features of the highest
occupied molecular orbital (HOMO) state inhighly-resolved ultraviolet photoelectron spectrum(UPS) of organic semiconductor can offer a variety of
key information that is necessary to unravel
fundamental mechanism in carrier-transport(transfer)properties in organic devices. Here, we present
high-resolution angle-resolved UPS (ARUPS) studies
on the HOMO hole in pentacene monolayer (ML) by
using low-energy photon to access dynamic events ofphotohole.
ARUPS spectra were measured at photon incidence
angle D=45o, hv = 7 to 10 eV and T=15 K at BL7U.
The molecules were evaporated onto a clean graphite(HOPG) substrate. The coverage/thickness of thepentacene ML was confirmed by the binding energy(EB) of HOMO[1,2]. No significant band dispersion isdetected for the ML.
Figure 1 shows the angle-integrated UPS (for 36o) of
pentacene ML. HOMO band is appeared at 1.3 eV.
Vibronic satellites bands denoted by v = 1, 2, 3 andLorentzian-like-tail feature are observed at higher-EB
and lower-EB side, respectively [2]. We found theintensity ratio of hole-vibration coupling and the tailfeature largely depends on the photoelectron kinetic
energy.
To reveal the differences more precisely, photon
energy dependences of ARUPS were recorded asshown in Fig.2. We found clear differences in the
ARUPS intensity maps by using different photon
energy of 0.8eV. The vibrational progression should
follow a Poisson distribution under Frank-Condon
approximation. The intensity of hole-vibration
coupling, however depends largely on the momentum
number, hence the momentum distribution curve(MDC) of vibronic peak is different from that of theFig.1. Photon energy dependence of angle-integrated
UPS of pentacene ML film prepared on a clean graphite(HOPG) (background subtracted).

84

main peak. It demonstrates that we have to consider aviolation of Born-Oppenheimer approximation anddynamic photoemission processes for large-S
conjugated materials.
Moreover, the low-EB tail feature appears at around* point, which was accessible by observing
low-kinetic-energy photoelectrons. The results may
give a chance to discuss the origin of the HOMO band
shape of molecular solids, which would be closely
related to following topics: (i) accurate contribution ofelectron-phonon (hole-vibration) coupling to the
hopping conduction. (ii) dynamics of polarization(molecular polaron and Holstein polaron) which isaffected by lifetime of the photohole to give a thehigh-kinetic-energy tail of HOMO shape. To elucidate
the anomalous features and their anisotropy, further
studies are intended.

Fig. 2. ARUPS intensity map for pentacene ML/HOPG
measured at hv=7.0eV a) and hv=7.8eV b). EDCs at k//= 0 and 0.18 ('k= 0.04ǖ-1) are shown in right panels.
MDCs at EB= 1.25 and 1.50 eV for 'E= 0.05eV areshown in top panels.
[1] H. Yamane et al., Phys. Rev. B 72 (2005) 153412.
[2] S. Kera et al., Prog. Surf. Sci. 84 (2009) 135.

BL7U

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

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

2

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 ǳF is very large (about 30 nm).
Because of this largeǳF, nanosized objects of Bi in
the range of several tens of nanometers have been
examined extensively toward the development of
quantum-size-effect-based devices [1–3]. For
example, the oscillation of the film resistance with thefilm thickness d was reported [4] and, furthermore, it
was predicted that when the lowest quantizedsubband of the electron pocket is raised to an energy
higher than the highest hole subband, a band gap willdevelop[semimetal-to-semiconductor(SMSC)
transition at d~30 nm, Fig. 1(c)] [1, 5]. However,
although many angle-resolved photoemission(ARPES) measurements have been performed on
ultrathin Bi films, none has actually been able toverify whether this SMSC transition occurs or not[6-14]. The main reason is that the cross section forthe bulk bands depends dramatically on the photon
energy and polarization.
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 band dispersion near the Fermi levelat the ǌ point (hQ =9 eV). Whereas the well-studied
surface states are observed clearly for the
P-polarization data (a), they completely disappear in
the S-polarization data (b). The hole-like dispersion in(b) suggests the detection of the bulk hole band andits maximum seems to be located below the Fermi
level. This may be suggesting the occurrence of a
SMSC transition. Further sophisticated study isneeded to reach a definite conclusion.

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 as predicted inRef. [5].

Fig. 2. The ARPES image of the band dispersion ofBi(111) films measured at hQ=9eV for P- (a) and S(b) polarized photons.

[7] T. Hirahara et al., Phys. Rev. B 75 (2007) 035422.
[8] T. Hirahara et al., Phys. Rev. B 76 (2007) 153305.
[9] T. Hirahara et al., New Jour. Phys. 10 (2008)
083038.
[10] A. Takayama et al., Phys. Rev. Lett. 106 (2011)
166401.
[11] A. Takayama et al., Nano Lett. 12 (2012) 1766.
[12] H. Miyahara et al., eJSSNT 10 (2012)153.
[13] T. Okuda et al., Rev. Sci. Inst. 82 (2011) 103302.
[14] G. Bian et al., Phys Rev. B 80 (2009) 245407.

[1] C. A. Hoffman et al., Phys. Rev. B 48 (1993)
11431.
[2] Z. Zhang et al., Phys. Rev. B 61, 4850 (2000).
[3] T. E. Huber et al., Appl. Phys. Lett. 84 (2004)
1326.
[4] Yu. F. Ogrin et al., JETP Lett. 3 (1966) 71.
[5] V. B. Sandomirskii, Sov. Phys. JETP 25 (1967)
101.
[6] T. Hirahara et al., Phys Rev. Lett. 97 (2006)
146803.

85

BL7U

Secondary Electron Emission from the Graphite Surface
S. Tanaka1, M. Matsunami2,3 and S. Kimura2,3
1

The institute of Scientific and Industrial Research, Osaka University, Ibaraki 567-0047, Osaka, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan3
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585,
Japan

When the condensed matter is electronically
excited, the following relaxation process leads to theemission of many particles/waves such as phonons,
photons, and electrons. For example, Graphite, one ofthe ‘textbook’ materials, emits 3.3-eV electrons when
excited no matter what photon energy is used asshown in the right panel of Fig. 1, where other peaks
change their kinetic energies as a function of thephoton energy. This phenomenon is important not
only from the scientific point of view, because it
reveals the relaxation dynamics in solids, but also
from the technical one, because it may leads to thedevelopment of the monochromatic electron emitter.
Previously, this 3.3eV peak has been ascribed tothe electron emission from the unoccupied band
located at 3.3eV above the vacuum level of theGraphite [1]. The photoexcitation process provides
the electrons located at the unoccupied band, and they
may be emitted with conserving their energies andmomentum if its wavefunction is connected to thefree-electron-like wavefunction in the vacuum. In thisstudy, we chalenge this interpretation with using thetwo-dimensional photoelectron spectroscopy where
both of the photon and kinetic energies are scanned.
The experiments were carried out at BL7U of
UVSOR-III. The HOPG (highly oriented pyrolytic
graphite) was cleaved in-situ and the surface-normal
photoelectron spectra were recorded with the photon
energy is scanned by 0.3-eV step for hQ=6-16eV and
1-eV step for hQ=16-26eV. The sample temperature
was 13K for hQ=6-16eV and 50K for hQ=16-26eV.
Significant differences were not observed whentemperature was changed except anomalous peaks
just below the Fermi level at lower temperature [2].
The photon intensity was calibrated by using thephoto-diode.
The results for the surface-normal photoelectron
emission, which correspond to the electron at the*-point in the Brillouin zone, are shown in center
panel of Figure 1 as a colored map (red is higher thanblue) where the x-axis indicates the photon energy
and the y-axis indicates the kinetic energy of thephotoelectron. The photoelectron intensities as afunction of the electron-kinetic energy are plotted at
the right panel at some photon energies and those as afunction of the photon energy at some photon
energies are shown in the upper panel. Dotteddiagonal lines in the center panel shows the positionof the peaks corresponding the photoexcitation from

Fig. 1. Two-dimensional photoelectron spectra for
HOPG Graphite.
the S-band (backfolded from the K-point [2]), theV-band and the S-band, respectively, from the top to
the bottom. The V-band is hardly seen because it isprohibited by the dipole selection rule. Meanwhile,the horizontal lines correspond to the photoexcitation
into a specific unoccupied state. Dotted lines are
kinetic energies where high intensities are observed
for several photon energies. The one who has thehighest intensity is the 3.3-eV peak. According to the
Fermi’s golden rule, the photoexcitation probability
resonantly increased when the photon energy agrees
with the difference in energy between the occupied
and unoccupied bands. These should be observed at
the intersection points among the diagonal andhorizontal lines. In the case of 3.3-eV peak, however,there is no particular enhancement at those photon
energies. It clearly excludes the interpretation that the
3.3-eV peak is due to the unoccupied state of theGraphite. Thus, there should be another truemechanism to emit the 3.3-eV electrons. Our next
goal will be finding it and is now under progress by
using
the
photoelectron-secondary
electroncoincidence spectroscopy.
[1] R. F. Willis, B. Fitton, and G. S. Painter, Phys. Rev.
B 9 (1974) 1926.
[2] S. Tanaka M. Matsunami and S. Kimura, Phys.
Rev. B 84 (2011) 121411(R) .

86

BL8B

Interface Electronic Structure of Diindenoperylene on Cu(111)1

S. Kera1, K. Kato1, K. Yonezawa1, K. Sato1, Y. Liu1, N. Ueno1 and T. Hosokai2

Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan
2
Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
analyzed by considering the LEED, STM andsimulated PAD. To reveal each orbital character ofISs, a full-spherical map of the PAD is requested tobe measured.

Electric property of organic semiconductormaterials is dominated by the density-of-states at theband gap and wavefunction spread of the electronic
states. The molecule-substrate interaction cruciallyinfluences electronic and geometric properties at theinterface [1]. The adsorption geometry of large
S-conjugated molecules on metal surfaces, therefore
is one of the key targets to understand the variety of
peculiar interface properties. Diindenoperylene (DIP:
C32H16) (inset in Fig 2b) is a typical organic
semiconductor. The DIP monolayer (ML) films areprepared on Cu(111) substrate to reveal the interface
electronic structure by using angle-resolved
ultraviolet photoelectron spectroscopy (ARUPS).
ARUPS spectra were measured at photon incidence
angle D=45o, hv=28 eV and T=295 K. The molecules
were evaporated onto the Cu(111). The
coverage/thickness of the monolayer was confirmed
by work function of the densely-packed ML film
which shows a clear LEED pattern.
Figure 1 shows the ARUPS of DIP 1ML on theCu(111). The azimuthal angle (I) dependence isrecorded at photoemission angle of T= 37o by rotating
the Is. The interface states (IS) denoted by B1, B2 andB3 are observed. The features B1 and B2 (B3) would
be related to the former LUMO and former HOMO of
DIP as reported similarly in PTCDA/Ag(111) and
PTCDA/Cu(111), respectively [2]. There is no clear
energy-band dispersion for the ISs. The feature B1 isappeared as a broadened feature and hardly to discuss
the origin of electronic state at the present results. On
the other hand, the features B2 and B3 show a clear
angular distribution in the I rotation.
Figure 2(a) shows the photoelectron angular
distribution (PAD) of features B2 and B3. The PAD ofB2 shows the maximum at around Is=0o, while B3
gives the minimum. Fig. 2(b) represents the simulated
PAD for the LUMO and HOMO distribution ofisolated DIP molecule by using multiple-scattering
theory combined with molecular orbital calculation(MS/MO) [3]. The molecule is placed that themolecular longer axis sets in the plane of electric
vector (Im=0o) as seen in Fig. 2(b). The observed
PAD of features B2 and B3 would be represented by
the PAD pattern of HOMO for different molecular
azimuthal orientation in the crystal domains. The
results of LEED and STM indicate that the ML film
consists of at least two different molecular unit cell.
Depending on the lattice constant and moleculararrangement the absorption energy would be
modified, hence they give the different binding
energies. A molecular azimuthal orientation will be

Fig. 1. I dependence of ARUPS for DIP ML film
prepared on Cu(111).

Fig. 2. Observed PAD of features B2(circles) andB3(squares) (a) and calculated PAD of LUMO and
HOMO distribution by MS/MO (b). Full-spherical
PAD images are also shown. The simulated patterns
are displayed by slicing the image at T=370. Scheme
view of DIP molecule at Im=0 is also shown.
[1] G. Heimel et al., Nature Chem. 5 (2013) 187.
[2] J. Ziroff et al., Phys. Rev. Lett. 104 (2010)
233004.
[3] S. Kera et al., Chem. Phys. 325 (2006) 113.

87

BL8B

Role of Oxygen Side Group on Interface State Formation in S-conjugatedOrganic Molecule/metal System
T. Hosokai1, K. Yonezawa2, K. Kato2, N. Ueno2 and S. Kera2
1

Department of Materials Science and Technology, Iwate University, Morioka 020-8551, Japan
2
Graduate School of Advanced Integration Science, Chiba 263-8522, Japan
carbons and substrate atoms, which may be realized
by a short bonding distance, can be also a key for the
IS.

Electronic structure of organic/metal (O/M)
interfaces is crucial for Organic Electronics, in whichcharge carrier injection at the interface dominates
device efficiency. Therefore, enormous effort has
been devoted for understanding the interface
electronic structure by using (angle-resolved (AR))
ultraviolet photoelectron spectroscopy (UPS). One
remaining issue in this research field is an interface
state (IS). In some O/M systems the IS appears
unexpectedly near the Fermi level. It has been
elucidated that this state is caused by acharge-transfer from metal substrate to the former
lowest unoccupied molecular orbital (LUMO) state of
the adsorbates. From x-ray standing waves (XSW)
experiments on monolayers of perylene compoundwith acyl groups (PTCDA: C24H8O6)(Fig. 2)) on
noble metal substrates (Au(111), Ag(111) andCu(111)), it was implied that a shorter bonding
distance of the adsorbates onto the substrates may
trigger to form the IS [1]. In such case, however, arole of chemical bonding of the side acryl groups tothe substrates for the IS is not yet clarified.
To address this issue, we have focused
diindenoperylene (DIP: C32H16) (Fig. 2), a perylene
compound consisting of only carbon and hydrogenatoms. From XSW we confirmed that the bonding
distance of DIP to noble metal substrates was similar
to that of PTCDA [2]. Here we performed ARUPS
study of DIP monolayer (ML) films deposited on
Ag(111) and Cu(111), and also on polycrystalline-Ag
and -Cu surfaces.
ARUPS spectra were measured at photon incidence
angle D=45°, hv=28 eV and 40 eV and T=295 K. The
molecules were evaporated onto each substrate. The
ML formation was confirmed by saturation of thework function shift.
As shown in Fig. 1, the ML spectrum of DIP on
Ag(111) and Cu(111) shows some spectral features(as denoted to A1 – A4 for Ag(111) and B1 – B3 forCu(111)) distinct from theoretical one or multilayer(island) film spectrum (b2). This is a direct evidenceof IS formation on Ag(111) and Cu(111) surfaces.
Notably, such ISs are also seen on polycrystalline
surfaces, suggesting the importance of IS in organic
devices employing those metal electrodes and DIP.
Figure 2 shows comparison of the energy levelalignments of DIP and PTCDA on Ag(111) andCu(111). ISs are located at nearly the same positions
for both molecular films. This clearly indicates that
the O-atoms are not necessarily required to form the
IS. A strong interaction between the S-conjugatedFig. 1. Valence band structure of DIP ML on
Ag(poly) (T=0o) (a), Ag(111) (T=40o) (b1), Cu(poly)(T=0o) (c) and Cu(111) (T=37o) (d). Multilayer(island) film (1 nm) of DIP/Ag(111) (T=30o,
hv=40eV) (b2). A calculated density-of-state (DOS)of DIP molecule based on B3LYP/6-31G(d) is also
shown at the top.

Fig. 2. Energy-level alignment of DIP/Ag(111) andCu(111) systems compared with PTCDA from ref [1].
The energy levels of multilayer films and orbital
pattern of HOMO and LUMO are depicted at theright-hand side, respectively.
[1] S. Duhm et al., Org. Electronics, 9 (2008) 111.
[2] C. Bürker et al., submitted.

88

BL8B

Quantitative Analysis of Anisotropic Molecular Orientation of Pentacene
Thin Film
K. K.Okudaira, T. Ishii and N. Ueno
Association of Graduate Schools of Science and Technology, Chiba Univererity,
1-33 Yayoi-cho Inage-ku, Chiba 263-8522, Japan

In recent years, organic semiconductor materials
have proven to be one of the most popular materials
used in the fabrication of thin-film transistor The
performance of organic-TFT such as the field-effectmobility depends not only on the molecular structureof the semiconducting films but also the degree ofmolecular orientation, since charge transport inorganic materials is realized by S-S interactions
between molecules through the hopping mechanism.
The anisotropic mobility could be useful in isolatingneighboring components so as to reduce cross-talk.
We apply a method to control both the molecular andcrystal orientation of organic semiconductors,
producing films with strong anisotropy. The
anisotropic orientation of pentacene (Pn) film can beobtained by evaporating pentacene on the uniaxiallyoriented PTFE thin films onto the uniaxially polishedCu plate To obtain the quantitative analysis on themolecular orientation; we compare observed
take-off angle dependence of S band and calculated
one by the independent-atomic-center (IAC)/MO
approximation [1].
ARUPS measurements were performed at the beamline BL8B of the UVSOR storage ring at the Institutefor Molecular Science. The take-off angle (T)dependencies of photoelectron spectra were measured
at incident angle of photon (D) = 45° with thephoton energy (hQ) of 40 eV. Pentacene was
evaporated to a final thickness of 10 nm on theuniaxially PTFE(5nm)/Cu (Pn(10nm)/ PTFE (5nm)/
Cu).
We observed take-off angle (T) dependence of
HOMO peak in UPS of (Pn(10nm)/ PTFE (5nm)/ Cu).
Figure 1 shows the T dependence of photoelectronfrom HOMO band with the parallel and perpendicularcondition Under the parallel and perpendicularcondition, the polarization plane of incidence photon
is parallel and perpendicular to the direction of thegroove of Cu plate, respectively. the T dependenceswith parallel condition have a sharp maxima at T
65°. On the other hand, with the perpendicularcondition, the T dependence of HOMO band does notshow a strong T dependence. (T for the maximum ofis lower than that with parallel condition).
Figure 2 (a) and (b) show the calculated
T-dependence of HOMO peak for two geometries.
With the geometry (1) and (2), the polarization plane
is parallel and perpendicular to the a-axis of bulk
phase pentacene crystal, respectively. For thegeometry (1). the calculated T-dependence of HOMO

e-

hQ

45° T

:
:
:

E

T

0°

Parallel Condition
Perpendicular Condition
30°

60°

Intensity (normalized)

90°

Fig. 1. Take-off angle (T) dependences ofphotoelectron intensities of HOMO peak of n(10nm)/
PTFE (5nm)/ Cu) with parallel (٨) and perpendicularcondition (٨).
peak shows a maximum of T =70°. It is similar withobserved one with parallel condition. On the other
hand, for the geometry (2) the calculated
T-dependence of HOMO peak shows a maximum of
T =40°, which is lower than that for the geometry (1).
From the comparison between observed and calculate
ones,. it can be expected that pentacene molecules
have the anisotropic molecular orientation on uniaxial
PTFE film, where the a-axis of pentacene crystal isparallel to the direction of groove of Cu plate.
hQ
45° T

E

0°
0.15

e-

hQ

(a)

T

E

0°

30°

0.03

45° T

e-

(b)

T
30°

0.1
60°
0.05
0
0

0.02

60°

0.01

0.05

0.1

Intensity(arb.units)

90°
0.15

0
0

0.01 0.02 0.03

90°

Intensity (arb.units)

Fig. 2. Calculated T-dependence of HOMO peak for
two geometries. where the polariaion plane ofincidence photon is parallel (a) and perpendicular (b)to the a-axis of bulk phase pentacene crystal.
[1] S. Hasegawa et al., Phys. Rev. B 48 (1993) 2596.

89

BL8B

Ultraviolet Photoelectron Spectroscopic Study on the Electronic Structureof the L-cysteine Ad-layers on the Noble Metal Surfaces
K. K. Rasika, Y. Nakayama and H. Ishii
Center for Frontier Science, Chiba University, Chiba 263-8522, Japan
originated by sulfur.
(a)

In recent years the possibility to use proteins inbioelectronic devices has attracted much attention. In
order to use proteins for bioelectronic applications, it
is essential to investigate on the electronic structuresof amino acids as the building blocks of proteins to
elucidate functionality such as transport properties,
adsorption properties, interface, and so on. As theinitial step, we selected L-cysteine from the aminoacids for the investigation, because it has beenreported that the cysteine with its sulfanyl as apromising candidate for bioelectronics [1].
In the present study, we systematically elucidated
the electronic structure at the interfaces of L-cysteineover layers and the noble metal surfaces Au(111),
Ag(111), and Cu(111) by means of ultraviolet
photoelectron spectroscopy (UPS) at BL8B.
Figure 1 shows the thickness-dependent UPS
spectra of L-cysteine on (a) Ag(111), (b) Au(111), and(c) Cu(111) at photon energy of 28 eV. In each figure,on the left, the secondary energy cutoff (SECO) is
shown on a magnified scale. On the right, the region
between Fermi level of the metal substrate and the
HOMO of the L-cysteine measured with relatively
small energy step is also shown.
It was observed that the electronic structures of the
L-cysteine thin films are different from each otherand also from that of the L-cysteine thick film, whichindicates strong dependence on the kind of substrate.
On the thin films, a new structure appears in between
Fermi level and the HOMO of the L-cysteine and thestructure may be due to bonding of S 3sp orbital with
Ag 4d, Au 5d, and Cu 3d states for each substrate,
respectively. Further, the peak at ~5 eV for thin films
shifts to the higher binding energy (e.g., Au by ~ 0.3eV) (Fig.1., dotted line), may be due to the interactionof O 2sp orbital with metal substrates. The maximum
SECO shifts were estimated to be 0.46, 0.83, and 1.01
eV to a higher binding energy for Ag, Au, and Cu,
respectively. The direction of the shifts corresponding
to a net transfer of negative charge from the cysteine
to the substrates and the low value for Ag may be due
to relatively large energy difference of S 3sp and Ag
4d states (a weak Ag-S bonding) [2].
Despite the difference in electronic structure forthin films, the electronic structure of thick film is
identical for each substrate. Thus, for the thick films,
photon energy-dependent UPS spectra were measured
in the range of 20-120 eV. The results are shown thatthe top of valance band (VB) has a different origin
from the other structure. By considering the results of
Ogawa et al. [2], and DV-XĮ molecular orbital
calculation, it is attributed that the top of VB is

L-cys(x)/Ag(111)hQ eV64 Å

Intensity (arb. uni.)

32 Å
16 Å
8Å
4Å
2Å

1Å

Ag(111)

24

(b)

23 12

10

8

6

4

2

Binding Energy (eV)

0 3 2 1 0

L-cys(x)/Au(111)hQ eV

Intensity (arb. uni.)

64 Å
32 Å
16 Å
8Å
4Å
2Å
1Å
Au(111)

24

(c)

23 12

10

8

6

4

2

0

2

1

0

Binding Energy (eV)L-cys(x)/Cu(111)hQ eV64 Å

Intensity (arb. uni.)

32 Å
16 Å
8Å
4Å
2Å
1Å

Cu(111)

24

23 12

10 8
6
4
2
Binding Energy (eV)

0 2

1

0

Fig. 1. Thickness-dependent UPS of L-cysteine on (a)Ag(111), (b) Au(111), and (c) Cu(111) at photon
energy of 28 eV. In each figure, on the left, the SECO
is shown on a magnified scale. On the right, it isshown that the region between Fermi level of thesubstrate and the HOMO of the cysteine, which wasmeasured with relatively small energy step and thedashed curves are plotted as a guide for eye.
[1] L. Willner and E. Katz, Bioelectronics: From
Theory to Applications (Wiley-VCH, Weinheim,2005).
[2] K. Ogawa et al., J. Appl. Phys. 112 (2010)
023715.

90