BL1B

Salt Effect on the Photoelectron Emission Thresholdfrom the Wet Surface of TiO2
S. Hitaka, K. Tomita, T. Ishioka and A. Harata
Molecular and Material Sciences, Kyushu University, Kasugakoen 6-1, Kasuga,
Fukuoka 816-8580, Japanderived threshold energy is listed in Table 1. The
threshold energy for wet TiO2 with no salt is 5.20 eV.
As for single crystalline TiO2(111), the thresholdenergy is reported to be 5.1 eV. Since single crystal(111) surface is dominant for the used TiO2 powder, it
is considered the presence of moisture at the surface
has little effect on the band structure of TiO2.
The threshold energy is clearly dependent on thesalt species. The threshold value order of cation
Mg2+~Li+<Na+<no salt agrees well with the reversedorder of photocatalytic activity. Smaller thresholdsuggests narrower band gap for the surface. The
narrower band gap would improve the efficiency ofphotoexcitation because photons with weak energy
become available for excitation.
From above discussion, it is considered that theenhancement by the salt is originated from thedistortion of the valence band structure of TiO2. The
result can be applied to evaluate photocatalytic
activity before actual long-term photochemical
experiments.

Introduction
Titanium oxide (TiO2) has attracted much interestfor its unique characteristics of photocatalytic ability
such as oxidation of organic compounds without any
harmful oxidizing agent. It is widely applied todecompose various organic wastes with solar energy
by illuminating the pollutant-adsorbed surface of
TiO2. Though some wastewater treatment systems
with TiO2 have already been developed, theirapplication to seawater is limited because salt in theseawater usually affect the photocatalytic reaction.
Makita et al. [1] reported enhanced photocatalytic
reactions in aqueous solutions containing diluteinorganic salt. It is essential to understandsalt-enhancement or suppression mechanism to apply
photocatalytic reaction for seawater treatment.
However, it is still obscure and precise analysis on themechanism is needed.
In this report, threshold photon energy wasmeasured on photoelectron emission process on thesurface of wet TiO2 containing salts. From the results,
electronic structures on the TiO2 surfaces wereanalyzed.

㪈㪅㪈㪈Experimental
Monochromated synchrotron light (4-8) was
emitted from the beamline chamber to a He-purged
cell through an MgF2 window. The emitted light wasreflected on an Al mirror and vertical irradiated on thesample surface through a Cu-mesh electrode. The
electrode was set above the wet TiO2 surface and highvoltage (400 V) was applied to the electrode. The
emitted electron was trapped to the electrode and thetotal current was monitored by a picoammeter.
The sample was prepared by spreading TiO2
powder (Degussa P-25, anatase:rutile = 8:2) on filter
paper covering the bottom of a Pt cell and aqueous
salt solution was added to the cell with the volume
enough to keep all the powder wet. The salt
concentration was 5 mM.
Threshold energy value was derived from themathematical fitting of empirical formula:(1)
S(Q)=6Si(hQ-Eth,i)nwhere S(Q) is total signal intensity and Eth,i is thethreshold energy of the i th component. As for index
number, n = 2 was used in analogy with the case of
metal substrates.

㪇㪅㪐㫎㪼㫋㩷㪫㫀㪦㪉㪶㪼㫏㫇㪇㪅㪏㫎㪼㫋㩷㪫㫀㪦㪉㪶㪺㪸㫃

㪇㪅㪎
㪇㪅㪍
㪇㪅㪌
㪇㪅㪋
㪇㪅㪊
㪇㪅㪉
㪇㪅㪈
㪇
㪄㪇㪅㪈
㪋

㪌
㪍
㪧㪿㫆㫋㫆㫅㩷㪼㫅㪼㫉㪾㫐㩷㪆㩷㪼㪭㪎

Fig. 1. Photon energy dependence on photoelectron
intensity from wet TiO2 surface.
Table 1. Salt dependence on the threshold energy ofphotoelectron emission from wet TiO2.
Substrate

Results and Discussion
Photon energy dependence of the photoelectron
intensity is shown in Fig. 1. The intensity is wellfitted by formula (1) with single component andThreshold / eV

TiO2

5.20

TiO2+ 5 mM LiNO3

4.72

TiO2+ 5 mM NaNO3

5.02

TiO2+ 5 mM Mg(NO3)2

4.71[1] M. Makita and A. Harata, Chem. Eng. Proc. 47(2008) 859.

63

BL1B

Development of Lower Environmental Load and UVSOR Exciting
Luminescence Materials Using with Biomaterials Preparedby Soft Chemistry

M. Ohta
Department of Material Science and Technology, Faculty of Engineering, Niigata University,
Niigata 950-2181, Japan
It was known that rare earth ions dosed for oral
administration to mouse and rat are transferred toblood vessel through the ileum and deposited its teeth
and bone, which mainly consists of hydroxyapatite(Ca10(PO4)6(OH) 2) [1, 2]. Recently, rare earth is also
useful as a contrast medium for magnetic resonanceimaging, restriction enzyme, biocatalyst, and so on infields of biochemistry, physiology, medicine, etc.
However, the behavior of rare earth in the living bodysystem remains an open question until now. We havefound that Eu ion substituted Ba ion in Eu doped
Ba10(PO4)6Cl2 phosphor, which matrix is apatite
structure [3]. The rare earth ion is also found to
substitute easily for calcium ion in hydroxyapatitewhich is soaked in rare earth chloride aqueous
solution, and to play on emission center.
In this study, hydroxyapatite samples doped withrare earth phosphate were prepared in order to apply
to phosphor. Their characteristics were investigatedby photoluminescent property of rare earth ion-doped
hydoxyapatite samples excited by ultraviolet
synchrotron orbital radiation light.
Eu-dopedhydroxyapatite
andGd-dopedhydroxyapatite samples were prepared as follows:hydroxyapatite was added with EuPO4 or GdPO4 andmixed homogeneously, and then heated at 1373 K for
1 hr. EuPO4 or GdPO4 was prepared by reaction of
GdCl3 or YbCl3 and Na3PO4䍃12H2O and then fired at1373 K for 1 hr. GdCl3 or YbCl3 was prepared byreaction of Eu2O3 or Gd2O3 and HCl.
The photoluminescent property of each sample
exited by ultraviolet synchrotron orbital radiation
light (BL1B) was detected by using with amulti-channel analyzer.
Figure 1 shows photoluminescence spectra of rareearth ion-doped hydroxyapatite samples excited by
BL1B.
The data of X-ray powder diffraction indicated thatthe rare earth ion substituted for calcium ion inhydroxyapatite.
Figure 1 shows the photoluminescent spectra of Eu
ion or Gd ion-doped hydroxyapatite samples excitedby ultraviolet synchrotron orbital radiation light. Eu
ion-doped sample emitted strong peaks due to f-ftransition of Eu3+ and Gd ion-doped sample emitted avery strong peak due to f-f transition of Gd3+ by
excitation from 10 nm to 100 nm. These strong
emission phenomena due to rare earth ion suggest to
the origin of multiphoton excitation.

[1] S. Hirano and K. T. Suzuki, Environ. Health
Perspect. 104 (Supplement 1) (1996) 85.
[2] K. Kostial, B. Kargacin and M. Lendeka, Int. J.
Radiat. Biol. Relat. Stud. Phys. Chem. Med. 51(1987) 139.
[3] M. Sato, T. Tanaka and M. Ohta, J. Electrochem.
Soc. 141 (1994) 1851.

Intensity

Eu-doped

Ex

cit
at
ion

wa
ve
len
gt
h

/n

m

iss
Em

ion

ve
wa

h
gt
len

/n

m

Intensity

Gd-doped

Ex

cit
a

ti o
nw

av

el
en
gt

h

/n

m

Em

w
io n
s
s
i

av

e

h/
gt
n
le

nm

㩷
Fig. 1. These figures show the photoluminescent
spectra of Eu ion or Gd ion-doped hydroxyapatite
samples excited by ultraviolet synchrotron orbitalradiation light.

64

BL3U

Field Effect on the Electronic States of Organic Thin Films:
Incident Angle Dependence of Fluorescence-Yield XAS Spectra

H. S. Kato1, H. Yamane2, N. Kosugi2 and M. Kawai1,3
RIKEN (The Institute of Physical and Chemical Research), Wako 351-0198, Japan
2
Institute for Molecular Science, Okazaki 444-8585, Japan3
Department of Advanced Materials Science, University of Tokyo, Kashiwa 277-8501, Japan1

counters for each of the top and bottom bias levels,
which enabled us to get two different bias spectra in
one photon energy sweep and to compare details of
their spectra with sufficient reliability. In Fig. 1, thespectra at the bias of 0 V (blue lines) and -45 V (redlines) are plotted with their difference spectra (boldblack lines). It is clear that the spectral changes from
0 V to -45 V increased with increasing incident angle.
In our previous studies, the bias dependence wasobserved at the normal incidence, so that the electric
vector of the x-ray was perpendicular to the directionof the applied electric field. Since the spectral change
is claimed to be a result of distorted molecularorbitals under the applied electric field, the previous
experimental condition might not be sensitive to theorbital distortion. As the incident angle of thep-polarized x-ray increases, in contrast, the parallel
component of the electric vector to the direction of
applied electric field increases. Therefore, theincrease of the difference signals with increasing
incident angle, as shown in Fig. 1, rightly supports
our previous deduction of the origin of the spectralchange under applied bias.

Introduction
In order to extend new functionality of electronic
devices, the molecular devices have recently been
investigated with great efforts. The organic field effecttransistor (OFET) is a typical molecular device that
controls electric conductivity by injection of carriers
into the organic thin film under the applied electricfield. Since the organic materials consist of molecularunits having their own molecular orbitals, it is notclear that the energy diagram of OFET is exactly thesame as that of the inorganic semiconductors, i.e.,band bending at the interface in the semiconductorside. Therefore, the direct observation of electronic
states in the organic thin films under operative
conditions has been required.
In our study, recently, in situ element-specificobservation of electronic states of organic films
beneath metal electrodes is successfully achieved by
x-ray absorption spectroscopy (XAS) in thebulk-sensitive fluorescence yield (FY) mode, evenunder operational conditions. As a result, the bias
dependence of the FY-XAS spectra of oligothiophene
and pentacene films was detected. Since the obtained
spectral changes have included an incomprehensive
response, additional experiments were performed.
Experimental
To investigate the electronic states of OFET,
pentacene thin films on the SiO2-covered Si substrates
were fabricated at RIKEN. We confirmed that the
fabricated pentacene thin films show a p-type-like I-V
property in the FET configuration. The pentacene
films (35 nm thick) were covered homogeneouslywith a thin Au electrode (25 nm thick) to makeuniform electric field in the films under the appliedbias. The FY-XAS measurements were performed at
the BL3U beamline of the UVSOR facility in IMS.
The samples were set in a BL3U end-station through asample-entry system. The fluorescence intensities
were measured using a retarding field detector
consisting of MCP plates.
Results and Discussion
Figure 1 shows the incident angle dependence of
the C K-edge FY-XAS spectra of the pentacene films,
for which the spectra at the incidence angle of 0°, 30°
and 60°from the surface normal were measured. At
the measurements, the gate bias VG was applied as asquare wave (7 Hz) synchronized to two signal

Fig. 1. Incident angle dependence of the C K-edge
fluorescence-yield XAS spectra of the Au-covered
pentacene films. The spectra were measured at theincident angle of 0°, 30° and 60°from the surface
normal. Each set of the spectra at 0 V (blue line) and-45 V (red line) is plotted with their difference
spectrum (bold black line).

65

BL4B

X-Ray Magnetic Circular Dichroism Study of Suppression of Iron Silicide
Formation in Fe/Si3N4/Si(111)K. Eguchi1, Y. Takagi1,2, T. Nakagawa1,2 and T. Yokoyama1,2
1
The graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan
2
Institute for Molecular Science, Okazaki 444-8585, JapanThe spin and orbital magnetic moments were
evaluated using the XMCD sum rule and theangle-dependent
XMCD
measurements.
The
magnetization of Fe on Si3N4 was fully saturated
under the condition of 5 T (See Fig.1). The
angle-dependent XMCD spectra taken with theincident angles ș of 0° and 55° are shown in Fig. 2.
The spin magnetic moments of each thickness on
the Si3N4 surface are found to be much larger thanthat on the clean surface. Moreover, the spin
magnetic moments increases monotonically withincreasing the film thickness for the Fe/Si(111)system, because the Fe atom near surface does notform silicides in the thick film. On the other hand,that of the iron atom on Si3N4 surface is 2.62 PB at 1.6
ML and the value decreases toward the bulk value of
2.2 PB with increasing the film thickness. It isrevealed that the Fe atom deposited on the Si3N4
surface does not react chemically with the Si atom.
In summary, the XMCD and M-H curves of iron
were measured to evaluate the suppression effect of
the iron silicide formation on the Si3N4 surface. The
silicidation of iron was found to be suppressed wellusing the Si3N4 surface.

It is known that when transition metals aredeposited on silicon substrates, metal silicides areeasily formed, which are typically nonmagnetic
materials. It is important to suppress the chemicalinteraction of transition metal atoms with Si
substrates by modifying the Si substrate surface. An
efficient method to provide well defined surfaces has
not been report so far. Previously, we tried to suppress
iron silicidation using a Si(111)-(¥3×¥3)Ag substrate,
but its effect is not sufficient to exhibit large
magnetization [1]. In this work, we have measured Fe
L-edge X-ray magnetic circular dichroism (XMCD)of Fe films on a Si3N4/Si(111)-(8×8) substrate.
The experiments were carried out at BL4B. Fe wasdeposited onto Si3N4/Si(111)-(8×8) and Si(111)-(7×7).
The XMCD spectra were taken at H=±5 T and T=5 K
with the incident angles ș of 0° and 55°. The helicityof the X-rays was fixed positively (Pc:0.41-0.60),
while the magnetic field was reversed.
Figure 1 shows magnetization curves of 1.6 ML
Fe on Si3N4/Si(111)-(8×8) and Si(111)-(7×7), whichwere taken at the Fe L3 peaks with the magnetic fieldnormal to the surface. No magnetization can be seenin the Fe film on Si(111)-(7×7), because of thesilicidation of iron atoms, while the Fe film on
Si3N4/Si(111)-(8×8) was magnetized clearly. The factindicates that the chemical interaction between Fe and
Si atoms is suppressed effectively on the
Si3N4/Si(111)-(8×8) substrate.

Fig. 1. Magnetization curves for Fe on
clean Si(111) and Si3N4/Si(111). Bothmagnetic field and photon incident
direction are perpendicular to thesurface.

[1] Y. Takagi, K. Isami, I. Yamamoto, T. Nakagawa
and T. Yokoyama, UVSOR ACTIVITY REPORT 37(2010) 73.

Fig. 2. XMCD spectra at L3,2 edge of iron in Fe/Si(111) andFe/Si3N4/Si(111) systems. Thickness is 1.6, 3.7, 9.1 ML and 1.6,
3.9, 10.1 ML, respectively. Applied magnetic field is 5 T andtemperature of samples is 5 K. Incident angles of the light are
0° and 55° to normal direction of the surface.

66

BL4B

Magnetic Property of Iron Phthalocyanine on Co Films Studied by XMCD
Y. Takagi1,2, K. Eguchi2, T. Nakagawa1,2 and T. Yokoyama1,21Institute for Molecular Science, Myodaiji-chio, Okazaki 444-8585, Japan
2
The Graduate University for Advanced Studies, Okazaki 444-8585, Japan
with magnetic field. This result suggests the presenceof a ferromagnetic exchange coupling between the Fe
atom in the molecular and the Co atom in theunderlying layer.

In order to realize the molecular spintronic devices,
it is crucial to understand the coupling between
molecular spins and a magnetic metal surface. Metal
phthalocyanines (Pc's) have attracted much interest
due to their characteristic electronic and magnetic
properties. Especially, iron(II) phthalocyanine (FePc)
is one of these interesting and fundamental
compounds because it takes the unusual intermediate
triplet spin state and has large orbital moment. In thisstudy, we report on the magnetic property of FePc on
clean Cu(001) and magnetic Co layer grown on
Cu(001) by means of X-ray magnetic circulardichroism (XMCD) at BL4B, which equipped with asuper conducting magnet system.
Sample preparation and XMCD measurement werecarried out in UHV chambers. A Cu(001) single
crystal was cleaned by repeated cycle of Ar+
sputtering and annealing. A Co layer was depositedon the clean Cu(001) at room temperature (RT).
Purified FePc was deposited on these substrates at RT
by sublimation and the thicknesses of the films weremonitored by a quartz crystal oscillator. The intensity
dependence of N K-edge X-ray absorption spectra(XAS) for the FePc film with X-ray incident angle
indicated that the molecular was parallel to thesubstrate plane. The XAS and XMCD spectra weretaken at 5 K and the XMCD spectra were recordedwith reversal of magnetic field.
Figure 1 shows Fe L-edge XAS spectra of 25 ML
FePc on clean Cu(001) and 1 ML FePc on
Co(3ML)/Cu(001) at incident angle T= 55q. The
spectra of 25 ML film represent the electronic state of
only FePc film because XAS is very surface sensitive.
On the other hand, the shape of 1 ML FePc spectra isquite different from that of 25 ML FePc due tointeraction between the FePc molecule and the Co
layer.
The XMCD spectra of these films are showed inFig. 2, which were taken at H = r0 T (remanence)and r5 T (saturation) and at temperature of 5 K. Both
L2 and L3 XMCD signs of the 25 ML FePc film are
negative, which indicates that the FePc film has alarge orbital magnetic moment. In addition, themagnetic moments were detected in both films inremanence, which means that both films areferromagnetic at 5 K. The remanent magnetization in
the 1 ML FePc on Co layer is larger than that in the
25 ML FePc film on clean Cu(001). Additionally,magnetization curves measured for 1 ML FePc filmon Co layer, which were recorded with Fe L3-edge
intensity for FePc films and with Co L3-edge intensity
for underlying Co layer, behaved similar dependence

12

H=5T
H=5T

Normalized XAS

FePc(25ML)/Cu(001)108FePc(1ML)/Co(3ML)/Cu(001)40705 710 715 720 725 730
Energy (eV)Fig. 1. Fe L-edge XAS spectra of 25 ML FePc on
clean Cu(001) and 1 ML FePc on Co(3 ML)/Cu(001)
at incident angle T = 55º from the surface normal at
H = 5 T.

Normalized MCD

4

2

FePc(25ML)/Cu(001)0
FePc(1ML)/Co(3ML)/Cu(001)

-2

7
7

705 710 715 720 725 730
Energy (eV)Fig. 2. Fe L-edge XMCD spectra of 25 ML FePc on
clean Cu(001) and 1 ML FePc on Co(3 ML)/Cu(001)
at incident angle T = 55º from the surface normal.

67

BL5U

Electronic Structure of the Ferromagnetic Semiconductor
EuO Ultrathin Films
H. Miyazaki1, T. Hajiri2, M. Matsunami1,3, T. Ito2 and S. Kimura1,3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
2
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan3
School of Physical Sciences, The Graduate University for Advanced Studies,
Okazaki 444-8585, Japan1

Europium monoxide (EuO) is a ferromagnetic
semiconductor with the Curie temperature (TC) ataround 70 K [1, 2]. The magnetic moment originatesfrom the half-filled 4f shell of the Eu2+ ion with thespin magnetic moment of S = 7/2. Recently, we haverevealed that the magnetism of EuO is caused by thehybridizations of the Eu 4f – O 2p and Eu 4f – 5d[3-5]. Appling EuO for spintronics devices such as
spin filter tunnel barriers [3], it is important toinvestigate the physical properties as well as theelectronic and magnetic structure of EuO thin films
with a few nanometer-thicknesses. Three-dimensional
angle-resolved
photoemission
spectroscopy(3D-ARPES) using synchrotron radiation is the mostpowerful technique to directly determine theelectronic band structure. Using this technique, we
observed the change of the electronic structure acrossTC.
Single-crystalline EuO ultrathin films withthickness of about 2 nm were fabricated by amolecular beam epitaxy (MBE) method. Epitaxial
growth of the single-crystalline EuO ultrathin films
with the 1 x 1 EuO (100) patterns was confirmed by alow energy electron diffraction (LEED) method. TC
was evaluated to be about 40 K by a magneto-optical
Kerr effect (MOKE) in-situ. The 3D-ARPES
measurements were performed at the beam line BL5U
of UVSOR-II combined with the MBE system. The
EuO ultrathin films grown in the MBE chamber andwere transferred to a 3D-ARPES chamber under
UHV condition. The total energy and momentum
resolutions for the ARPES measurement were set to
45 meV and 0.014 Å-1 at the X point (hQ = 38 eV),
respectively.
Figure 1 (a) shows the energy distribution curves(EDCs) of EuO (100) ultrathin film with a thicknessof 2 nm near the X point at 5 K (ferromagnetic phase).
From a comparison with the band calculation, theobserved states at a binding energy EB of 1.0 – 3.5 eV
and 4.0 – 7.0 eV are attributed to the Eu 4f and O 2p
states, respectively. From our previous results [5], theobserved dispersive features of the Eu 4f statesaround the X point indicates the hybridization of theEu 4f and O 2p states, leading to the origin of theferromagnetic phase transition of EuO. Figure 1 (b)shows the temperature dependence of the peakenergies of the Eu 4f states at the X point derived
from the second-derivative EDCs. There are two Eu

4f bands at the X point, which shift to the lower
binding energy side on cooling across TC of 40 K.
The energy shifts of the higher binding energy peak
are small compared to the bulk ones, corresponding
to the decrease of TC. This result indicates that the
hybridization intensity between the Eu 4f and otherstates becomes weaker with decreasing thickness.
This result is consistent with our recent
thickness-dependent band calculation.

Fig. 1. (a) Energy distribution curves (EDCs) of an
EuO (100) ultrathin film (2 nm thickness) near the X
point. (b) Temperature dependence of the peakenergies at the X point derived from thesecond-derivative EDCs of the Eu 4f states.
[1] N. Tsuda et al., Electronic Conduction in Oxides(Springers College) (1976).
[2] A. Mauger et al., J. Phys. (paris) 39 (1978) 1125.
[3] H. Miyazaki et al., Physica B 403 (2008) 917.
[4] H. Miyazaki et al., Jpn. J. Appl. Phys. 48 (2009)055504.
[5] H. Miyazaki et al., Phys. Rev. Lett. 102 (2009)
227203.
[6] M. Müller et al., J. Appl. Phys. 105 (2009)
07C917.

68

BL5U

Quantum Topological Phase Transition in Ultrathin Bi2Se3 Films
Y. Sakamoto1, T. Hirahara1, H. Miyazaki2, S. Kimura2 and S. Hasegawa1
1
Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

Topological insulators (TI) are a novel state ofquantum matter in three dimensions (3D) that havebeen gaining increased attention. Along with its
two-dimensional (2D) counterpart, the quantum spin
Hall (QSH) phase, they are mathematicallycharacterized by the so-called Z2 topological number[1]. While the bulk is insulating with an inverted gap,
there is a metallic edge or surface state which is
topologically protected and hence robust against
weak perturbation or disorder.
One of the important aspect in the study of TI is thephase transition between the trivial (normal) and the
topological state. The principal issue here is theexchange of the parity of conduction and valencebands by changing some external parameter. One
example is the case of HgTe/CdTe quantum well. In
this case, it was reported that by reducing the
thickness of the well below a critical thickness of dc =60 Å, the inverted band gap (band gap '<0) once
closes ('=0) and becomes a normal band gap ('>0)
as schematically shown in Fig. 1 (a). Thus reducing
the system size and changing the dimensionality does
not only have the advantage that the surface-sensitivity is enhanced which will make thesurface-state property more apparent in terms of 3D
TI, but is also important in determining if the system
is trivial or topological in 2D.
Bulk Bi2Se3 is a 3D topological insulator with aninverted band gap and its surface states possess alinear band dispersion expressed by the Diracequation [Fig. 1 (b)]. In a recent theoretical work, it
was shown that this Hamiltonian possess similarity to
the 2D QSH effective Hamiltonian for the HgTe
quantum wells. The authors predicted that an energy
gap will open in the surface-state dispersion due tothe interaction between the top and bottom surfaces
for thin film thicknesses. Furthermore, it was shownthat the inverted gap will close at dc = 25 Å for Bi2Se3,
and below dc a normal band gap will develop so thesystem will become trivial [Fig. 1 (a)] [2]. Therefore
a topological quantum phase transition is predicted tooccur in the thin film limit. However this has not been
experimentally demonstrated yet.
In the present study, we have performed
angle-resolved photoemission spectroscopy (ARPES)
measurements on ultrathin Bi2Se3 films on silicon to
verify if the phase transition actually takes place.
Figure 2 shows the band dispersion for 2 (a), 3 (b)and 8 (c) quintuple layer (1 QL = 9.5 Å) ultrathin
Bi2Se3 films measured at the photon energy of 20 eV
using circularly-polarized light. The band structure
for the 8 QL film is similar to that found in bulk

Fig. 1. Schematic drawing of the topological quantum
phase transition in 2D. (b) Schematic drawing of thesurface-state linear band dispersion of the 3D TI.

Fig. 2. E-k band dispersion images for the 2 QL (a),3 QL, and 8 QL ultrathin Bi2Se3 films along the *-K
direction. The overlapped lines show the fitted curves
using Eq. (1).
single crystals, showing the bulk bands just near the
Fermi level as well as the Dirac surface states. On theother hand, the 2 and 3 QL films show a clear gap
opening in the surface-state dispersion. However, thegap size is larger for the 3 QL film than the 2 QL one,which is counterintuitive to our understanding forparticles confined in a potential, suggesting theoccurrence of a parity reversal. Moreover, by fitting
the measured images to (red solid lines in Fig. 2)

E r E 0  Dk 2 r (v F !k ) 2  ( ' / 2  Bk 2 ) 2 , (1)we are able to deduce the sign of 'and B which are
related to the edge-state Hall conductance. Thisquantitative analysis also supported the occurrence ofa transition from a QSH state (3 QL) to a trivial phase(2 QL). Thus we have found experimental evidenceof a quantum topological phase transition in thissystem [3].
[1] L. Fu and C. L. Kane, Phys. Rev. B 76(2007)
045302.
[2] H.-Z. Lu et al., Phys. Rev. B 81 (2010) 115407.
[3] Y. Sakamoto et al., Phys. Rev. B 81 (2010)
165432.

69

BL6U

Very Narrow Intermolecular Electronic Band Dispersion
in a Crystalline Film of Zn-Phthalocyanine

H. Yamane and N. Kosugi
Dept of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, JapanThe electronic band dispersion, energy versus wave
vector E(k), is a fundamental parameter to understandelectric properties of solids such as hole mobility (ȝh).
In the field of organic semiconductors, the study ofthe intermolecular E(k) shows a rapid progress due tothe needs of the interpretation of the charge transportmechanism in molecular electronic devices. However,
due to the very weak intermolecular interaction andthe difficulty in preparing crystallized films sufficientfor the E(k) measurement, the observation of theintermolecular E(k) has been limited to the case ofhigh-ȝh materials. In order to elucidate and control thefunctionality of organic semiconductors, a systematicand quantitative experiment on the intermolecularinteraction can play a crucial role. In this work, wehave succeeded to observe a narrow intermolecularE(k) in crystalline films of Zn-phthalocyanine (ZnPc),
which is one of the promising materials in the field oforganic electronics. The present observation may
pave the way for the systematic band structure
analysis in the field of organic semiconductors.
The present experiment was performed using theangle-resolved photoemission spectroscopy (ARPES)
system at BL6U. The crystallinity of the sample wasconfirmed by low-energy electron diffraction (LEED)and by the N K-edge X-ray absorption spectroscopy(XAS) in the sample-current mode. The E(k) relation
was measured by ARPES at normal emission as afunction of the incident photon energy (hȞ), whichscans the k component along the surface normal. The
sample temperature at all measurements was 15 K.
Figure 1 shows the incident angle (Į) dependenceof N K-edge XAS spectra for the 30-nm-thick filmsof ZnPc on Au(111). The sharp 1s ĺ ʌ* transition
peaks appear at hȞ = 398 ~ 405 eV. These peaks arestrongest at grazing incidence (Į = 60°) and are getting
weaker with decreasing Į. Other broad 1s ĺ ı*transition features (hȞ > 405 eV) show the opposite
trend. From the data analysis, we found that the ZnPc
film on Au(111) is well oriented with their molecularplane parallel to the substrate surface.
Figure 2 shows the hȞ dependent ARPES spectra at
the normal emission for the ZnPc crystalline film on
Au(111). Since the ZnPc crystalline film on Au(111)shows a Stranski-Krastanov growth mode, there are
the remanent substrate signal such as a Fermi edge,
which we used for the fine hȞ calibration. For the
ZnPc-derived peaks A, B, and C, we have observed aclear dispersive behavior with hȞ, wherein (i) the
periodicities of A–C are same in the k space and (ii)the bandwidth of peak A, originating from the highest
occupied molecular orbital (HOMO), is 120 meV.

Intensity / normalized

The present observation clearly indicates that the
band-like transport can be realized in phthalocyanine
films by the control of the geometric film structure.
Moreover, the present result can be a benchmark forthe systematic study on the intermolecular interaction,e.g., intermolecular E(k) as a function of the central
metal atom in the phthalocyanine molecule, whichenables to discuss experimentally the intermolecularinteraction in terms of the intermolecular distance andthe molecular orbital symmetry.
The authors like to thank the staff of UVSOR, in
particular, Mr. T. Horigome, Mr. E. Nakamura, andProf. E. Shigemasa, for their fruitful assistance.
ZnPc(30nm)/Au(111) @T ~ 15 K

hv = 398.2 eV: D = 0°
: D = 20°
: D = 35°
: D = 45°
: D = 50°
: D = 60°

E = 0°
E = 7°
E = 10°

hv

Į

E

400

410

420

0

30

60

90 – D / °Photon Energy hv / eV

90Fig. 1. The Į dependence of N K-edge XAS spectra
for the crystalline ZnPc films on Au(111) at 15 K.
ZnPc(30nm)/Au(111) @ T ~ 15 K
B

N

C

N

N

N

Zn

N
N

N

N

Intensity / normalizedHOMO
A (HOMO) hv

/eV

130

110

90

hv

60°

e–

70

E
6

5

50
4

3

2

1

0

Binding Energy (vs. EF) / eV

Fig. 2. The hȞ dependence of ARPES spectra (4 eVstep) at the normal emission for the crystalline ZnPc
films on Au(111) at 15 K.

70

BL8B

Interface Electronic Structure of Crownether-Derivatives on Cu(111)

H. Machida1, K. Yonezawa1, M. Yamamoto1, S. Hosoumi1, T. Hosokai2, T. Nishi3, N. Ueno1,
and S. Kera1,3
1
Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan
2
Institut für Angewandte Physik, Universität Tübingen, Tübingen 72076, Germany
3Institute for Molecular Science, Okazaki 444-8585, Japan
Studies on the adsorption of large S-conjugatedorganic molecules on metal surfaces have increased
considerable attentions, since the molecule/substrateinteraction crucially influences electronic functions oforganic devices. A missing study of the interface issue
is on a molecular system with structural flexibility by
sp3 bond (V bond) in its main backbone. To reveal
physical properties of such flexible molecules, westudied the valence electronic structure oftetrabromide-dibenzo-18-crown-6 (BDBC) thin films
prepared on Cu(111) by using angle-resolved
ultraviolet photoelectron spectroscopy (ARUPS).
Experimental
ARUPS spectra were measured at photon incidence
angle D=60o, hv=28 and 100 eV and T=295 K. The
molecules were evaporated onto the Cu(111). The
monolayer (ML) formation was confirmed by thework function of the densely packed ML that shows aclear LEED pattern. High-resolution UPS spectra
were also recorded using HeI light source.
Results and Discussion
Figure 1 (a) shows the HeI UPS of BDBC(ML) on
Cu(111) together with BDBC(ML)/HOPG and DOS
by DFT calculation. The spectrum (1: thick curve) isshown after the background subtraction. All the bandsof BDBC/Cu(111) are shifted to low-binding energy(Eb) side by 0.3-0.4 eV compared with BDBC/HOPG.
Bands A (HOMO) and B seem to split on Cu(111)

due to strong coupling. An interface state (G) appears
at the band-gap region.
Figure1 (b) shows the UPS at Br(3d) region. Bands
L and M are due to spin-orbit splitting and shifted tolow-Eb side by 2.1 eV (bands L’ and M’) for
BDBC(ML)/Cu(111), indicating Br atoms interact
more strongly with Cu(111).
Figures1 (c) and 1 (d) show ARUPS results forannealed-BDBC(ML)/Cu(111) along *K direction.
The photoelectron angular distributions (PAD) of split
bands A1 and A2 are evaluated by curve fitting. These
PAD patterns are different but show a nearly similar
distribution, indicating both states could originate
from the HOMO (Sstatedistributed at phenyl rings)
of a free BDBC and may split due to hybridization
with the substrate, which may give differences in the
S-MOs distribution. Moreover, the PADs have broad
distribution around T=0-30o, leading the phenyl rings(S planes) are inclined to the substrate surface. All
these show a clear evidence of chemisorptive
interaction at BDBC/Cu(111) interface. An expected
BDBC structure on Cu(111) is shown at the bottom of
Fig.1 (d). The molecule-substrate distance andmolecular deformation evaluated by x-ray standing
wave measurements remarks a closer distance to theCu(111) for Br atoms than other elements, which isconsistent with the present UPS results.

Fig. 1. (a) He I UPS of a clean Cu(111), BDBC(1ML)/Cu(111), and background subtracted BDBC(1ML)
spectrum(1) together with BDBC(0.9 ML)/HOPG(2) and DFT-simulated spectrum(3). (b) Br(3d)-region UPS
for 1ML/Cu(111)(4), thick-island film(7 nm)/Cu(111)(5) and 1ML/HOPG(6). Deconvoluted features in each
spectrum are shown at the bottom. (c) ARUPS along *K direction for BDBC(annealed ML)/Cu(111). (d)PAD for bands A1(orange triangles) and A2(blue circles). HOMO distribution on gas-phase and expected
molecular conformation on Cu(111) are also shown.

71

BL8B

ARUPS Study of Pentacene Thin Film on Uniaxially Oriented PTFE Film
K. K.Okudaira, K. Ito, K. Hotta and N. Ueno
Association of Graduate Schools of Science and Technology, Chiba Univererity,
Chiba 263-8522, Japan
perpendicular to polishing direction. It was foundthat pentacene molecules show anisotropic molecularorientation on uniaxially oriented PTFE film.

Introduction
Electroactive organic molecules have attracted
much attention owing to their versatility and uniqueelectronic properties of interest for the design ofoptoelectronic and electronic devices. In-planeorientation of pentacene films has been performed inorder to improve the transport properties in OFETs.
Fabrication of oriented structure such as uniaxiallyoriented crystallites is important to obtain the oriented
growth of materials as a substrate. The uniaxiallypoly(tetrafluoroethylene) (PTFE) films can be used asorienting media onto various substrate [1]. In thisstudy, we observe angle-resolved ultraviolet
photoelectron spectroscopy (ARUPS) and show theanisotropic orientation of pentacene (Pn) film on theuniaxially oriented PTFE thin films by evaporation
onto the uniaxially polished Cu plate.

[1] J. C. Wittmann et al., Nature 352 (1991) 414.

Experimental
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. At first (PTFE) was
deposited on uniaxially polished polycrystal Cu plateby polishing paste (alumina particles with mean
radius of about several ȝm) (PTFE(5nm)/Cu). An
additional organic layer of pentacene wassubsequently evaporated to a final thickness of 10 nm
on the uniaxially PTFE(5nm)/Cu (Pn(10nm)/ PTFE(5nm)/ Cu).

Fig. 1. Take-off angle (T) dependences of ARUPS ofPn(10nm)/PTFE(5nm)/Cu with (a) parallel and(b) perpendicular condition , respectively.
hQ

e
45° T

E

Results and Discussion
Figures 1 (a) and (b) show the take-off angle (T)dependences of ARUPS spectra of Pn(10nm)/ PTFE(5nm)/Cu with the parallel condition (the polarization
plane of incidence photon is parallel to the polishingdirection of Cu plate) and the perpendicular condition(the polarization plane of incidence photon isperpendicular to the polishing direction of Cu plate),
respectively. The peak located at binding energy ofabout 1 eV corresponds to the localized S state(HOMO). Figure 2 shows the T dependence ofphotoelectron from HOMO band with the parallel andperpendicular condition The T dependences withparallel condition have a sharp maxima at Tҏ= 65°.
On the other hand, with the perpendicular condition,
the intensity of HOMO band does not show a strong T
dependence. They indicate that the molecularorientation of pentacene film in parallel to polishingdirection for Cu plate is different from that in

0°

T

-

:
:
:

Parallel Condition
Perpendicular Condition
30°

60°

Intensity (normalized)

90°

Fig. 2. Take-off angle (T) dependences ofphotoelectron intensities of HOMO peak ofPn(10nm)/PTFE(5nm)/Cu with parallel (Ɣ) andperpendicular condition (Ɣ).

72