Ⅲ-4
Surface,
Interface andThin Films

Ⅲ -4. Surface, Interface and Thin Films
BL6U
BL6U

Bulk and Surface Band Dispersion Mapping of the Au(111) Surface
by Newly-developed Acceptance-cone Tunable PES System
F. Matsui1, H. Yamane2, T. Ueba1, T. Horigome1, S. Makita1, K. Tanaka1,3, S. Kera1,3
and N. Kosugi3.4
UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
2
RIKEN SPring-8 Center, Sayo 679-5148, Japan
3
The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan
4
Institute of Materials Structure Science, KEK, Tsukuba 305-0801, Japan1

Two-dimensional angle-resolved photoelectron(PE) spectroscopy is a powerful method to study theelectronic structure of crystal surfaces. The initial statebinding energy (E) and the wave vector (k) can be
determined from the kinetic energy and the directionof the detected PE. The latest version of ARPES
analyzer was installed (MB Scientific AB, A-1
analyzer Lens#5) at BL6U (in-house beamline). ThisPE spectrometer consists of a hemispherical electron
analyzer with a mechanical deflector and a meshelectrostatic lens near the sample to make theacceptance cone tunable. A constant energy PE angular
distribution of the valence band dispersion crosssection in the large k range can be efficiently obtainedby applying a negative bias voltage to the sample andusing the mechanical deflector. Details of thespecifications will be reported elsewhere [1].
Here, we introduce three-dimensional band dispersion
mapping of the Au single crystal to show the performanceof the current PE spectroscopy end station. The Au(111)
surface was cleaned by repeating the cycle of Ar+
sputtering and annealing. The cleanliness of thereconstructed Au surface was confirmed by LEED and
XPS. ARPES measurement was made at approximately
20 K. The incident photon axis, the electric vector, the￣ direction of the sample
normal direction and ￣surface and the analyzer entrance slit were all in thehorizontal plane. The incident photon axis was offset
60 ° from the surface normal direction.
Figure 1 shows the bias-voltage-dependent band￣ plane. The photon energy
dispersion map in the ￣was 90 eV. A well-known Au surface state across the
Fermi level is observed at ￣ point. The acceptanceangle was ±16 °, which corresponding to ±1.2 Å-1. The
detection range was expanded to ±3.0 Å-1 by applying
a negative bias to the sample.
Figure 2 shows the photon-energy-dependent Fermi
surface maps with simulated cross sections of bulk
Fermi surface. Photon energies of 60, 90, and 120 eV
were used for excitation. The bias voltage applied to thesample was fixed to -400 V. The Au(111) surface stateat the ￣ point was observed at the center. The
hexagonal cross section of the bulk Fermi surface wasobserved in the angular distribution excited with photon
energies of 90 and 120 eV. Note that the observed
patterns are more six-fold symmetric, although a threefold symmetric feature is expected for the second

adjacent Brillouin zone. This is probably due to the PE
scattering at the surface reconstructed structure. One
can not notice such a phenomenon just by measuring theband dispersion at a high symmetry plane.
We greatly acknowledge Dr. P. Baltzer and Ms. M.
Matsuki (MB Scientific AB) for their contribution in
the construction of the BL6U PE end station.

Ⅲ- 4

Fig. 1. Band dispersion of the Au(111) surface along￣￣ direction. Photon energy was 90 eV. Various

negative bias was applied to the sample.

Fig. 2. Fermi surface of the Au(111) surface. Photon
energy was varied from 60 eV to 120 eV. Bias voltageof -400 V was applied to the sample.
[1] H. Yamane, F. Matsui, T. Ueba, T. Horigome, S.
Makita, K. Tanaka, S. Kera and N. Kosugi, Rev. Sci.
Instrum. in Press (2019).

131
Ⅲ04- 1 -Ⅲ -4. Surface, Interface and Thin Films
BL2A
BL2A

Kinetics Investigation to Form Magnesium Hydroxide from Low
Crystalline Magnesium Oxide in the Solution
1

T. Kato1, K. Takahashi1, Y. Sawamura1, M. Kadokura2 and C. Tokoro3

Graduate School of Creative Science and Engineering, Waseda University, Tokyo 169-8555, Japan
2
School of Creative Science and Engineering, Waseda University, Tokyo 169-8555, Japan
3
Faculty of Science and Engineering, Waseda University, Tokyo 169-8555, JapanThe low crystalline magnesium oxide, which is
produced by heating at 600 degree in 1 hour, has thecapacity to remove toxic elements, such as the boron(B), fluorine (F) and so on [1]. In the solution, a partof low crystalline magnesium oxide forms tomagnesium hydroxide by the reaction described inequations (1) and (2).
MgO (s) + H2O ⇄ Mg2+ + 2OHMg2+ + 2OH- ⇄ Mg(OH)2

Ⅲ- 4

(1)
(2)

When a part of low crystalline magnesium oxide
precipitates as magnesium hydroxide in the solution, it
is suggested that the toxic elements, such as B, F andso on, remove by co-precipitation with magnesiumhydroxide. However, the ratio of magnesiumhydroxide in the low crystalline magnesium oxide
after experiments is not clarified because the structureof precipitation is amorphous. To achieve thequantitative modeling for toxic elements removal by
low crystalline magnesium oxide, it is desired that theratio of magnesium hydroxide in the low crystalline
magnesium oxide after experiments evaluates. Thus,
the objective of this study is to evaluate the ratio of
magnesium hydroxide in the low crystalline
magnesium oxide after experiments using x-ray
absorption fine structure (XAFS) analysis inmagnesium K-edge.
The precipitation after experiments, which the low
crystalline magnesium oxide reacted in the solution
after 0, 1, 5, 10, 20 and 120 min, was analyzed by
XAFS analysis in magnesium K-edge by fluorescencemethod (Fig. 1). The XAFS spectra of magnesiumoxide in magnesium K-edge has two peaks while thatof magnesium hydroxide has three peaks in the rangeof 1300 – 1320 eV [2]. Thus, the x-ray absorption nearedge structure (XANES) analysis was performed in
the range of 1300 – 1320 eV to evaluate the ratio of
magnesium hydroxide in the low crystalline
magnesium oxide (Table 1). From XANES results, it
was confirmed that almost low crystalline magnesiumoxide was precipitated as magnesium hydroxide after
20 min. Using above results, we can construct thequantitative model for toxic elements by low
crystalline magnesium oxide.

Fig. 1. Magnesium K-edge x-ray absorption finestructure spectra corresponding to x-ray absorption
near edge structure region of 0, 1, 5, 10, 20 and 120
min samples, showing resolved components ofmagnesium oxide and hydroxide based on magnesiumoxide and hydroxide reference spectra, respectively.
Table 1. Concentration of magnesium oxide andhydroxide in 0, 1, 5, 10, 20 and 120 min samples,
based on x-ray absorption near edge structure analysis
in magnesium K-edge [%]Reaction time
[min]
0
1
5
10
20
120

Concentrationof MgO
100
29.1
16.4
10.1
6.2
0

Concentrationof Mg(OH)2
0
70.9
83.6
89.9
93.8
100

[1] H. Fukuda, S. Hobo, G. Granata, C. Tokoro, Y.
Toba and M. Eguchi, Proceedings of the Conferenceof Metallurgists (2017) 9408.
[2] T. Yoshida, T. Tanaka, H. Yoshida, T. Funabiki andS, Yoshida, J. Phys. Chem. 100 (1996) 2302.

132
Ⅲ04- 2 -Ⅲ -4. Surface, Interface and Thin Films
BL2B
BL2B

Molecular Orientation of DNTT Thin Film on TES-derivatives Deposited
Au Substrate
K. K. Okudaira and A. Murafuji

Association of Graduate Schools of Science and Technology, Chiba Univererity, Chiba 263-8522, Japan
thin film was deposited on TES.
We observed take-off angle (β) dependence of
HOMO peak in UPS of DNTT thin film (thickness ofabout 6.0 nm on TES-derivates prepared on Au
substrate (Fig.1). The peaks originated from π-orbitals
including HOMO do not show take-off angle
dependence. On the contrary, the intensities ofphotoelectrons from the σ-states become large asincrease take-off angle. For the case of flat-lying
molecular orientation of π-conjugated aromatic
molecules such as phthalocyanine, the π-peaks show
strong take-off angle dependence [3]. It indicates that
DNTT molecules have the orientation (not parallelorientation) on TES-derivates Au substrate.

Intensity / a.u.

Organic devices such as organic light-emitting
diodes, organic thin-ﬁlm transistors (OTFTs), andorganic photovoltaic cells, have been attracting interest
concerning both fundamental research and practicalapplication for low-cost, large-area, lightweight andﬂexible devices Recently, OTFT’s with ﬁeld-effectmobility and on/off current ratio comparable tohydrogenated amorphous silicon thin-ﬁlm transistors.
Since source and drain contact resistance has been
identiﬁed as a major limitation in OTFTs, control of thisinterface is important. Several surface treatments havebeen reported to improve charge injection from metalelectrodes into organic semiconductors. A single
molecular layer, for example self-assembled monolayer(SAM), has been purposely introduced on top of theelectrode as well as insulator surfaces [1]. SAM
spontaneously forms by dipping the substrate in asolution of an appropriate reactive surfactant. For thecase of application of SAM to the bottom -contact OTFT,
where the substrates consist of not only electrodes suchas Au but also insulator, it is necessary to use of two
types of SAMs for the suitable surfaces of substrates. 6(3-(triethoxysilyl)
propylamino)-1,3,5-triazine-2,4dithiol monosodium (TES) derivatives have beenreported as molecular adhesion, which are able to form
the chemical bonds with both insulator such as plastic
and metal. TES can be considered as a kind of SAM,
which is available to form the chemical bonding(anchoring) to two different substrates.
The characteristics of OTFT such as mobility are
considered to be dependence on not only the electronic
structure but also the molecular orientation of organic
molecule.
In this work we deposited a TES derivative on Au
and examined the molecular orientation of
dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene(DNTT) thin films thermally deposited on the TESderivate surface by angle-resolved ultraviolet
photoelectron spectroscopy (ARUPS) measurements.
In the ARUPS take-off angle dependence of HOMO
peak for π-conjugated organic molecule provides animportant information on the molecular orientation [2].
ARUPS measurements were performed at the beamline BL2B of UVSOR III at the Institute for Molecular
Science. The take-off angle (β) dependencies ofphotoelectron spectra were measured with the photon
energy (hν) of 28 eV. The Au coated substrate wasprepared by depositing Au thin ﬁlms on natural-oxide
Si(001) wafer. We used TES derivatives as coupling
agent. The TES layer on Au substrates was preparedusing a dipping method at room temperatures. DNTT

=60°
=50°
=40°
=30°
=20°
=10°
=0°

e-

Substrate states

-20

-15

-10

 states-5

Binding Energy relative to vacume level /eV

Fig.1. Take-off angle (β) dependences of ultraviolet
photoelectron spectra DNTT (6.0nm)/TES/Au.
[1] P. Marmont et al., Org Electron. 9 (2008) 419,
Y. Horii et al., Thin Solid Films 518 (2009) 642.
[2] K. Mori and K. Abe, Hyomen Gijutsu 59 (2008)
299, K. Mori et al., J. Soc. Rubber Sci. Technol. 83(2010) 71.
[3] N. Ueno et al., J. Chem. Phys. 99 (1993) 7169.

133
Ⅲ04- 3 -Ⅲ- 4

Ⅲ -4. Surface, Interface and Thin Films
BL2B
BL2B

Electronic Structure of Disordered Molecular Heterointerfaces
1

Ⅲ- 4

K. Akaike 1, A. Onishi1, Y. Wakayama2 and K. Kanai1

Department of Physics, Faculty of Science and Technology, Tokyo University of Science, Noda 278-8510, Japan
2
International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science(NIMS), Tsukuba 305-0044, Japan

Intended design of organic heterointerfaces is crucial
to control functionality in organic devices based on
multi-stacked organic layers. Formation of theheterointerfaces via either vacuum deposition or
solution process leads to a variety of interfacial
structure; molecular exchange, molecular reorientation,and mixed heteromolecular arrangement. The formationof a well-defined heterointerface is a typically expectedconsequence, but it is not a sole case. The complicatedmolecular rearrangement that have been found should
introduce structure disorder into the system, but its
molecular origin is elusive.
Previously, using scanning tunneling microscopy(STM), we directly observed the disorder formed upon
the formation of sexithiophene (6T)/perfluorinatedcopper phthalocyanine (F16CuPc) interface on a cleaned
Ag(111) surface [1]. Deposition of 6T onto an
epitaxially grown F16CuPc monolayer perturbed thearrangement of F16CuPc along [1–2] direction. The
adsorbed 6T molecules adapted various bent shapes andwere randomly mixed with F16CuPc molecules. The
bent molecules are originated from cis-transisomerization, in which neighboring thiophene rings
rotate. The various molecular shapes of 6T significantly
reduced short- and long-range orders.
The STM results prompted us to characterize theelectronic structure of such a disordered heterointerfaces,
because the Gaussian broadening and tailing of thefrontier orbitals are often correlated with structural
disorder [2]. In this research, we performed ultraviolet
photoelectron spectroscopy (UPS) in BL2B to investigate
the energy distribution of the occupied states of
6T/F16CuPc interface.
Figure 1 shows the evolution of the UPS spectra for
the interface as a function of 6T coverage in secondary
electron cutoff (a) and valence band regions (b). Photon
energy of 28 eV was used to acquire the spectra at room
temperature. Photoelectron emission angle was fixed tobe 45º. The formation of F16CuPc monolayer on
Ag(111) reduced work function (WF) by 0.5 eV. Thisoriginates from the push-back effect. The highest
occupied molecular orbital (HOMO) peak appeared at
the binding energy of ~2.1 eV. At the same time, a broadfeature was observed just below the Fermi level. The
electronic states should be originated from partial
occupation of the lowest unoccupied molecular orbital(LUMO) of F16CuPc upon the adsorption on the silver
surface.
Increasing coverage of 6T lowered WF by less than
0.1 eV. The WF reduction accompanies with the shift of

the F16CuPc HOMO toward lower binding energy. The
analysis of the STM images suggested that the nominal
coverage of the organic layer reduced upon thedeposition of 6T. Therefore, because the bare silver
surface has a larger WF, the WF increased upon theinterface formation. Unexpectedly, the shape of the
HOMO was not broadened by the structural disordering.
Comparable UPS measurements were also carried out
for 6T/CuPc/Ag(111) interface. The interface states,
formed upon the adsorption of CuPc on the silver
surface, shifted toward higher binding energy withincreasing 6T coverage. STM images of 6T/CuPc
interface suggested the formation of phase-separated
binary structure. However, further investigation isnecessary to elucidate what molecular arrangement led
to the shift of the interface states.

Fig. 1. Evolution of UPS spectra for 6T/F16CuPc
interface as a function of 6T coverage.
[1] K. Akaike, A. Onishi, Y. Wakayama and K. Kanai,
J. Phys. Chem. C, in press.
[2] J.P. Yang, F. Bussolotti, S. Kera and N. Ueno, J.
Phys. D: Appl. Phys. 50 (2017) 423002.

134
Ⅲ04- 4 -Ⅲ -4. Surface, Interface and Thin Films
BL3U
BL3U

Cobalt Oxide Catalyst in Carbonate Aqueous Solution
Studied by Operando C K-edge XAFS Measurement
K. Yamada1, T. Hiue2, M. Nagasaka3, H. Yuzawa3, H. Kondoh2, Y. Sakata1 and M. Yoshida1,4
Faculty of Engineering, Yamaguchi University, Ube 755-8611, Japan
Graduate School of Science and Technology, Keio University, Yokohama 223-8522, Japan
3
Institute for Molecular Science, Okazaki 444-8585, Japan
4
Blue energy center for SGE technology (BEST), Yamaguchi University, Ube 755-8611, Japan1

2

In these days, the system to produce hydrogen bywater electrolysis utilizing renewable energy such as
solar, wind, and salinity concentration gains significant
attention due to increasing global awareness ofenvironmental issues. However, the efficiency of waterelectrolysis is not enough for commercial use, becauseof the high overvoltage on the oxygen evolution
electrocatalyst. In this situation, Takanabe and coworkers reported that cobalt oxide catalyst
electrodeposited in carbonate aqueous solution (Co-Ci)
could function as the efficient oxygen evolution
reaction (OER) catalyst in 2014 [1]. On the other hand,
our group find out the function of similar OER catalysts
by operando observation using X-ray absorption finestructure (XAFS) technique [2]. Herein, we examined
the operando XAFS measurements for Co-Ci catalysts
to reveal the function of carbonate species in the Co-Ci.
A Teflon electrochemical cell was equipped with a
Pt counter electrode and an Ag/AgCl referenceelectrode for all electrochemical experiments. The CoCi thin films were electrodeposited on Au thin film in
potassium carbonate aqueous solution (K-Ci)
containing Co(NO3)2 at 1.7 V vs. RHE. The operando
C K-edge XAFS spectra for Co-Ci were taken underelectrochemical control with transmission mode at
BL3U in the UVSOR Synchrotron, according to theprevious works [3].
First, we measured C K-edge XAFS for saturated
K2CO3 aqueous solution as reference sample (Fig. 1),
in order to find out the absorption peak of carbonate
species. The absorption peak was estimated around290.3 eV, corresponding with a previous research
about carbonate material [4].
Next, we measured operando C K-edge XAFS
spectrum for Co-Ci electrocatalyst. An absorption
peak assigned to carbonate adsorbed on the Co-Ci
catalyst was observed around 290.4 eV. Moreover,when the electrode potential was changed from 0.5 V
to 1.0 V, a new peak was observed around 290.6 eV. In
our previous work about operando O K-edge XAFS
spectra, we revealed that the CoO2 structure was
generated by the oxidation of CoOOH at higher
electrode potential. Therefore, we think the new peakin the C K-edge XAFS is assigned to the carbonate
species adsorbed on the CoO2 structure and functions
as the active site for OER process.

In conclusion, we found that the carbonate species
were adsorbed on the CoO2/CoOOH structure in the
Co-Ci electrocatalyst by operando C K-edge XAFS
and the interface of carbonate/CoO2 was likely to workas efficient OER site.

Ⅲ- 4Fig. 1. Operando C K-edge XAFS spectra for thesaturated K2CO3 aqueous solution (green line) and Co-Ci
at 1.7 V vs. RHE (red line).
[1] K. Takanabe et al., Adv. Energy Mater. (2014)
1400252.
[2] M. Yoshida et al., J. Phys. Chem. C 121 (2017) 255.
[3] (a) M. Nagasaka et al., J. Phys. Chem. C 117 (2013)
16343.
(b) M. Yoshida et al., J. Phys. Chem. C 119 (2015) 19279.
[4] Chris Jacobsen et al., J. Synchrotron Rad. 17 (2010)
676.

135
Ⅲ04- 5 -Ⅲ -4. Surface, Interface and Thin Films
BL3B
BL3B

Evaluation of Optical Basic Properties of Ultra-Violet Emitting Zinc
Aluminate Phosphor
H. Kominami1, M. Ohkawa1, Y. Kato1, M. Arimura1, K. Imagawa1, K. Warita2,
S. Takahashi2, K. Higashi2 and S. Nishibori2

Graduate School of Integrated Science and Technology, Shizuoka University, Hamamatsu, 432-8651 Japan
2
Faculty of Engineering, Shizuoka University, Hamamatsu, 432-8651 Japan
Figure 1 shows transmittance spectra of ZnAl2O4 thinfilm prepared on sapphire substrates annealed at 1000 oC
for 100 hours. The absorption edge of ZnAl2O4 on asubstrate was sifted to shorter wavelength. For
comparison, Fig. 2 shows transmittance spectra ofsubstrates. The change of absorption edge indicates that
ZnAl2O4 film on a-plane substrate was formed Al-rich
ZnAl2O4 spinel structure. From the figure, theinterference fringes were also obtained. Now we try to
calculate the refractive index from the spectra. We are
expecting that the optical parameters of ZnAl2O4 areobtain from the UVSOR measurement.
[10 ] 2.5
6Transmittance Intensity (a.u.)

Ⅲ- 4

The UV light is used for various applications
depending on the wavelength as well as thesterilization described above. The lights of 200-280
nm (UV-C) region as for the sterilization, 280-320 nm(UV-B) region as the treatment of the skin disease,320-400 nm (UV-A) region as application of
purification of water and air, and photocatalysts.
Recently, from the viewpoint of consideration to theenvironment, the mercury free UV emission devices
have been demanded for the application of catalyst andmedical situations. In our previous work, it wasclarified that ZnAl2O4 phosphor was suitable for theUV field emission lamp because of its stability andluminescent property. It shows strong UV emission
peaked around 250 nm which suitable for sterilization.
We are exploring the physical properties of ZnAl2O4,
because the optical properties of ZnAl2O4 have notbeen cleared. We tried the evaluation of ZnAl2O4 using
the powder phosphor. However, it is difficult toevaluate because of the influence of surface scattering,such as the absorption coefficient and refractive index
reflectance. Therefore, we thought that the thin film
analysis was suitable for correct measurements of thefundamental properties. In previous work, ZnAl2O4
thin film prepared by thermal diffusion process using
ZnO film on sapphire substrate. However, the opticalmeasurement was difficult, because the compositionof the film and each interface was not uniform.
In this study, to avoid the inarticulate interface ofeach layer, the multilayer film containing constituent
elements was formed by sputtering on sapphire
substrate, and tried to prepare the ZnAl2O4 thin film by
thermal diffusion.
In this experiment, we used an RF magnetron
sputtering system. Fig.1 shows the experimental
procedure of the ZnAl2O4 film formation. ZnO layer
was deposited on a, c, and m-plane sapphire substrates
at room temperature. Next, α-Al2O3 thin layer wascovered with ZnO to avoid re-evaporation by thermal
diffusion process because of high evaporation pressureof Zn. All ZnO layer was sandwiched by α-Al2O3
layers. The films were annealed at 1000 oC for several
hours in air to enhance thermal diffusion of Zn and Al
atoms in the film. The films were evaluated by X-ray
diffraction (XRD), scanning electron microscope(SEM), cross-sectional EPMA, and photo- andcathodoluminescence (PL, CL) systems were used.

2

1.5
1

1

a-sub.

0.5

0.5
0
150

200

250

0
170
300

Wavelength [nm]m-sub.

180
350

c-sub.
190
400

Fig. 1. Transmittance spectra of ZnAl2O4 thin film
prepared on several -plane sapphire substrates
annealed at 1000 oC for 100 hours.

6[10 ]

Transmittance Intensity (a.u.)

1

2

0.5

1

0

150

200

c-sub.
a-sub.

0
130
250
300

m-sub.
140
350

150
400

Wavelength [nm]Fig. 2. Transmittance spectra of sapphire substrate.

136
Ⅲ04- 6 -Ⅲ -4. Surface, Interface and Thin Films
BL4B, BL5B
BL4B, BL5B

Observation of Photoinduced Phenomenon in Amorphous Chalcogenide
Thin Films by Ultraviolet Synchrotron Orbital Radiation
K. Hayashi

Department of Electrical, Electronic and Computer Engineering, Gifu University, Gifu 501-1193, Japan
It is well known that amorphous semiconductormaterials are very sensitive to the light and show avariety of photoinduced phenomena [1-3]. Therefore,
amorphous semiconductor are very expected asmaterials for optoelectronic devices, such as solar cells,
thin film transistors, light sensors, and optical memory
devices etc. These applications require anunderstanding of the physical properties of amorphousmaterials. In our recent study, we observed interesting
photoinduced change in the photoconductivity ofamorphous chalcogenide films by the irradiation of thevisible light with the energy corresponding to theoptical bandgap. To obtain a wide knowledge of thephotoinduced phenomena, we are trying to investigate
photoinduced effects in the vacuum ultraviolet region
by the transmission spectra. In the previous report, wehave reported the transmission spectrum of amorphousarsenic selenium thin film measured at beamline
BL5B [4, 5]. The inner core level absorption of thismaterial is observed in the wavelength region between
16nm and 30nm. Amorphous thin films fortransmission spectrum measurement can be easily
prepared by conventional evaporation technique. The
aluminum thin film used as a filter for removing higher
order light from the monochromator in thiswavelength region was used as a substrate fortransmission spectrum measurement. However,beamline BL5B does not have very good wavelength
accuracy and spectral resolution. Beamline BL4B has
better wavelength accuracy and spectral resolution
than BL5B, but it is not suitable for measurement in
the lower energy region. In this report, we try tomeasure the transmission spectra of other materials
that have absorption levels in the higher energy region
and compare the results of BL4B and BL5B.
The metal thin films were prepared onto Corning
7059 glass plates with a pinhole of the diameter of
2.0mm. The transmission spectrum was measured
through this pinhole. The metal thin film was used asa filter to eliminate the higher order light from themonochromator in the VUV region and also as a
substrate of amorphous film. A gold thin film was usedin the wavelength region between 5nm and 12nm, andan aluminum thin film was used in the wavelength
region between 16nm and 36nm. A silicon photodiodewas used as a detector in the wavelength region. The
measurement of the transmission spectra in the VUVregion was performed at room temperature at BL4B
and BL5B of the UVSOR facility of the Institute for
Molecular Science.
Figure 1 shows the transmitted light intensity ofmetal thin films (Au and Al) measured at BL4B and
BL5B by a silicon photodiode. As can be seen in the
figure, the transmitted light intensity in the highenergy region at BL4B is stronger than at BL5B,
which is advantageous for the measurement of thetransmission spectrum. In fact, the transmission
spectra of amorphous films prepared on gold thin film
substrates were able to be measured with higher
resolution when measured with BL4B than with BL5B.
On the other hand, the transmitted light intensity of thealuminum thin film in the low energy region near
30nm was not much different between the two
beamlines. In the measurement of the transmission
spectrum in the low energy region, the SN was lower
in BL4B than in BL5B.

Ⅲ- 4

Fig. 1. Transmitted light intensity of thin metal films(Au and Al) measured at BL4B and BL5B by a silicon
photodiode.
[1] K. Tanaka, Rev. Solid State Sci. 4 (1990) 641.
[2] K. Shimakawa, A. Kolobov and S. R. Elliott, Adv.
Phys. 44 (1995) 475.
[3] K. Tanaka, Encyclopedia of Nanoscience andNanotechnology 7 (2004) 629.
[4] K. Hayashi, UVSOR Activity Report 2013 41(2014) 140.
[5] K. Hayashi, UVSOR Activity Report 2014 42(2015) 134.

137
Ⅲ04- 7 -Ⅲ -4. Surface, Interface and Thin Films
BL4B
BL4B

Electronic and Magnetic Properties of FeNi Alloy Thin Films
Grown on a Monatomic Layer Magnetic Nitride
T. Miyamachi1, K. Kawaguchi1, T. Hattori1, T. Koitaya2,3, T. Yokoyama2,3 and F. Komori1

Institute for Solid State Physics (ISSP), The University of Tokyo, Kashiwa 277-8581, Japan
Department of Materials Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan3
Department of Structural Molecular Science, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan
1

2

Ⅲ- 4

L10-ordered alloy thin films fabricated by thealternating deposition of magnetic or noble elements
attract great attention for their strong uniaxial
magnetic anisotropy. Especially, L10-ordered FeNi
alloy thin films, composed of cheap and abundantmaterials, raise the hope for rare-earth free permanent
magnets compatible with NdFeB-based rare earthmagnets. Previous studies revealed the close
relationship between the strength of the uniaxial
magnetic anisotropy and structural ordering. However,
the order parameter S reported so far still remains low(up to ~ 40 %) for L10-ordered FeNi alloy thin films
and the magnetic easy axis is still along in-plane
direction. The low S values could be caused by theatomic-scale disorder at the Fe/Ni interface during the
fabrication processes, but the lack of the appropriate
experimental method with high spatial resolution
hampered the intrinsic improvement of structural andresulting electronic and magnetic properties of FeNi
alloy thin films.
Toward realizing high quality FeNi alloy thin films,
we here focus on the fabrication method utilizing
nitrogen surfactant effects in a monatomic layer
magnetic nitride. In this method, high lateral latticestability, which is characteristic of a monatomic layer
magnetic nitride [1, 2], and surfactant nitrogen during
the deposition of Fe and Ni layers can effectively
suppress the interdiffusion at the Fe/Ni interface,leading to atomically flat and homogeneous surface
and interface of FeNi alloy thin films.
In this work, we investigate structural, electronic
and magnetic properties of 1 monolayer (ML) Fe on amonatomic layer nickel nitride (Ni2N) grown on
Cu(001), the initial stage for the fabrication of FeNi
alloy thin films by means of nitrogen surfactant effects,
using scanning tunneling microscopy (STM)
combined with x-ray absorption spectroscopy/x-ray
magnetic circular dichroism (XAS/XMCD). This
complementary experimental approach allows to link
macroscopic observations of element specific andquantitative electronic and magnetic properties by
XAS/XMCD with microscopic origins of the Fe/Niinterface characteristics revealed by STM [3].
The growth of the Ni2N on Cu(001) was checked by
STM before XAS/XMCD measurements. Likewise thefabrication process of a monatomic layer iron nitride(Fe2N) [1,2], N+ ions with an energy of 500 eV werefirstly bombarded to the Cu(001) substrate and 1 ML

Ni was deposited at room temperature. By subsequentannealing at ~ 670 K, well-ordered and flat 1 ML Ni2N
was grown on the surface [4]. 1 ML Fe was thereafter
deposited at a low temperature (~ 150 K).
XAS/XMCD measurements were performed at
BL4B in UVSOR by total electron yield mode at B =± 5 T and T = 7.1 K. The XMCD spectra are obtainedat the normal (NI: θ = 0°) and the grazing (GI: θ = 55°)
geometries by detecting μ+ – μ–, where μ+ (μ–) denotes
the XAS recorded at Ni and Fe L adsorption edges withthe photon helicity parallel (antiparallel) to the sample
magnetization. Note that θ is the angle between thesample normal and the incident x-ray.
We find no clear Ni L3 XMCD signal for bare Ni2N
both in the NI and GI geometries, reflecting its
paramagnetic character as previously reported [5].
However, the Ni magnetization is induced by adding 1
ML Fe as a consequence of the formation of
Fe2N/Ni/Cu(001) via nitrogen surfactant effects. The
electronic hybridization at the Fe/Ni interface modifies
the strong in-plane magnetic anisotropy of Fe2N [2]toward an out-of-plane direction, but the uniaxial
magnetic anisotropy has not yet been achieved.
In future work, combining STM and XAS/XMCD,
we will further improve structural ordering of FeNi
alloy thin films by annealing and discuss the impactsof surface/interface quality on their electronic andmagnetic properties at the atomic scale.
[1] Y. Takahashi et al., Phys. Rev. Lett. 116 (2016)056802.
[2] Y. Takahashi et al., Phys. Rev. B 95 (2017) 224417.
[3] S. Nakashima et al., Adv. Funct. Mater. 29 (2019)1804594.
[4] Y. Hashimoto et al., Surf. Sci. 604 (2010) 451.
[5] F. Tanaka et al., J. Appl. Phys. 120 (2016) 083907.

138
Ⅲ04- 8 -Ⅲ -4. Surface, Interface and Thin Films
BL4B
BL4B

Magnetic Coupling at the Interface ofFerromagnetic Iron and Nickel Nitride Monatomic Layers
T. Miyamachi1, K. Kawaguchi1, T. Gozlinski1,2, T. Koitaya3,4,
W. Wulfhekel2, T. Yokoyama3,4 and F. Komori1

1
Institute for Solid State Physics (ISSP), The University of Tokyo, Kashiwa 277-8581, Japan
Physikalisches Institut, Karlsruhe Institute of Technology (KIT), Karlsruhe D-76161, Germany
3
Department of Materials Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan
4
Department of Structural Molecular Science, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan2

L10-ordered FeNi alloy thin films attract great
attention as rare-earth free permanent magnets
compatible with NdFeB-based magnets for their
expected uniaxial magnetic anisotropy. Previous
studies revealed the importance of structural orderingof FeNi alloy thin films on the emergence of the strong
uniaxial magnetic anisotropy. However, the orderparameter S of up to ~ 0.4 reported so far still remains
low. The atomic scale disorder could result in the low
S values, but the details have been overlooked due tothe lack of the appropriate experimental method withhigh spatial resolution. Toward achieving FeNi alloy
thin films with high S values, we intend to utilize
nitrogen surfactant effects in a monatomic layer
magnetic nitride, which can effectively suppress theinterdiffusion at the Fe/Ni interface and keep
atomically flat surface/interface of FeNi alloy thinfilms.
For this purpose, we have recently performed scanningtunneling microscopy (STM) and x-ray absorption
spectroscopy/x-ray magnetic circular dichroism(XAS/XMCD) measurements on a monatomic layer
nickel nitride (Ni2N) on Cu(001) with 1 Fe overlayer,
which is the initial stage for the fabrication of FeNi alloy
thin films by means of nitrogen surfactant effects. The
combined STM and XAS/XMCD study gives thecomprehensive information on structural, electronic andmagnetic properties of the system both from microscopic
and macroscopic points of view. The results
demonstrated that nitrogen surfactant effects lead to theformation of a monatomic layer iron nitride (Fe2N: [1])on the surface [Ni2N/Cu(001)→Fe2N/Ni/Cu(001)], andthe strong in-plane magnetic anisotropy of the Fe2N [2] ispartially modified toward the out-of-plane direction. In
this work, we further investigate the impacts of annealingon the Fe/Ni interface structures and magnetic propertiesof the system by STM and XAS/XMCD.
The Ni2N was grown on Cu(001) by following
processes. First, N+ ions with an energy of 500 eV werebombarded to the Cu(001) substrate and 1 ML Ni was
deposited at room temperature. By subsequentannealing at 670 K, well-ordered and flat 1 ML Ni2N
was grown on the surface [3]. The quality of the Ni2N
surface was prechecked by STM and LEED before
XAS/XMCD measurements. 1 ML Fe was thendeposited on the Ni2N at 150 K. This “as-deposited
sample” was finally annealed up to 420 and 570 K.
XAS/XMCD measurements were performed at
BL4B in UVSOR by total electron yield mode at T =6.1 K and B = 0-5 T. The XMCD spectra are obtainedat the normal (NI: θ = 0°) and the grazing (GI: θ = 55°)
geometries by detecting μ+ – μ–, where μ+ (μ–) denotes
the XAS recorded at Ni and Fe L adsorption edges withthe photon helicity parallel (antiparallel) to the sample
magnetization. Note that θ is the angle between thesample normal and the incident x-ray.
We observed that the XMCD intensity at Fe L3 edgeof the as-deposited sample, Fe2N/Ni/Cu(001), increases
by annealing at 420 K. Furthermore, the magnetization
curves of Fe2N recorded in the NI and GI geometries
reveals the enhancement of the magnetic anisotropy.
Taking results of atomically resolved STM observations
into account, these changes could be attributed to thepromotion of the structural ordering at the Fe/Niinterface by annealing at an optimal temperature. In
contrast, we found that annealing the as-deposited
sample at 570 K degrades magnetic properties of thesystem. At this annealing temperature, the surface
segregation of substrate Cu atoms is activated, locallyweakening the magnetic coupling at the Fe/Ni interface.
In the future, we will grow high-quality thicker FeNi
alloy thin films by additional alternating Fe and Ni
depositions with controlled post-annealing processes,
and investigate their structural, electronic andmagnetic properties by STM and XAS/XMCD.
[1] Y. Takahashi et al., Phys. Rev. Lett. 116 (2016)056802.
[2] Y. Takahashi et al., Phys. Rev. B 95 (2017) 224417.
[3] Y. Hashimoto et al., Surf. Sci. 604 (2010) 451.

139
Ⅲ04- 9 -Ⅲ- 4

Ⅲ -4. Surface, Interface and Thin Films
BL5U
BL5U

Valence-Band Electronic Structures of Ultra-Thin Co Layers
on Rashba-Splitted Au (111) Surfaces
J. Okabayashi1,* K. Tanaka2 and S. Mitani3

Research Center for Spectrochemistry, The University of Tokyo, Tokyo 113-0033, JapanUVSOR synchrotron Facility, Institute for Molecular Science, Okazaki, 444-8585, Japan
3
National Institute for Materials Science, Tsukuba 305-0047, Japan

1
2

Ⅲ- 4

When ferromagnetic transition metals (TMs) aredeposited on the Rashba-type spin-orbit coupled surface,novel properties such as perpendicular magnetic
anisotropy (PMA) are emerged at the interfaces, which
are derived from the symmetry broken spin-orbit effects.
The gold Au (111) surfaces have been investigatedextensively by means of scanning tunneling microscopy
and angle-resolved photoemission spectroscopy(ARPES) because this surface exhibits the large Rashbatype spin-orbit splitting. Large spin-orbit interaction in
the heavy element of gold provides the wide varieties forthe topological physics and spin-orbit coupled sciences atthe surfaces and interfaces. The Rashba splitting of 110
meV in Au (111) surface was reported firstly by LaShellet al. [1]. Recently, the interfaces between Au(111) andother heavy elements such as Bi or Ag have been
extensively investigated [2, 3]. Here, we focus on theinterfaces between ferromagnetic materials and Au(111)interfaces because the thin Fe layers on the heavy
elements are expected to exhibit the PMA induced by the
Rashba-type spin-orbit interaction.
The spin-orbit coupling between the ferromagnetic
3d TMs Fe or Co and 5d heavy TM elements of nonferromagnetic materials such as Pt and Au has beenutilized for the PMA through the proximity at theinterfaces. It is believed that the future researches
concerning not only spins but also orbitals arerecognized as the spin-orbitornics. Therefore, to clarify
the origin of the PMA at these interfaces is a crucial role.
The relationship between Au (111) Rashba-type spinorbit interaction and PMA in 3d TMs has not beenclarified explicitly. In order to investigate the orbitalresolved states in the Fe films showing the PMA,
ARPES at the interfaces becomes powerful techniquesthrough the photon-energy and polarization
dependences in each 3d orbital. By using ARPES, we
aim to understand the orbital-resolved electronic
structures at the magnetic interfaces on the Rashba-type
Au (111) surface in order to develop the researches ofnovel PMA using spin-orbit coupled interfaces.
We prepared the clean Au (111) surface for the Co
deposition. The commercialized single-crystalline 100nm-thick Au (111) films on Mica were used. At thebeamline BL5U in UVSOR, we repeated the Ar-ion
sputtering at 1 kV accelerating voltage and subsequentannealing at 400 °C under the high vacuum conditions.
After the sample preparation, the Co layer was
deposited at room temperature and then annealed. By
in-situ transferring the samples into the ARPES
chamber, we performed ARPES at 10 K using thephoton energies of 60 and 120 eV because of the large
cross-section of Au (111) surface states and detectionsof Co 3s and Au 4f intensity ratios, respectively.
Figure 1 displays the Co deposition dependence on
Au (111). As shown in Fig. 1a, the intensity ratio
between Co 3s and Au 4f peaks systematically changes.
The Au (111) surface clearly exhibits the Rashba-typesurface states. The Co 3d states appear near the Fermi
level (EF) and the surface states disappear. As shown in
Fig. 1b, kx-ky plots taken by horizontal and verticalpolarized beams exhibit asymmetric feature, which canbe explained by the Co 3d orbital resolved states
through the overlap between Co and Au. Furthermore,
we confirmed the PMA at Co/Au interface by x-ray
magnetic circular dichroism. Therefore, the interfacialchemical reaction between Co and Au brings the novel
properties on the ultrathin Co electronic structures.
We acknowledge Dr. S. Ideta in IMS for technicalsupports of data analyses. This work was in part
supported by KAKENHI Kiban(S) project.

Fig. 1. (a) Photoemission spectra for various Co
deposited layers taken at 120 eV. (b) Polarizationdependent kx-ky Fermi surface mapping of Co 4 ML
taken at 60 eV.
[1] S. LaShell et al., Phys. Rev. Lett. 77 (1996) 3419.
[2] C. Tusche et al., Ultramicroscopy 159 (2015) 620.
[3] B. Yan et al., Nature Commun. 6 (2015) 10167.
*e-mail: jun@chem.s.u-tokyo.ac.jp

140Ⅲ04- 10 -Ⅲ -4. Surface, Interface and Thin Films
BL5U
BL5U

Effects of P Segregation on the Surface Electronic Structure of Fe2P(0001)
N. Maejima1,2, Y. Sugizaki1, Y. Shimato1, T. Yoshida1 and K. Edamoto1,2
Department of Chemistry, Rikkyo University, Tokyo 171-8501, Japan
Research Center for Smart Molecules, Rikkyo University, Tokyo 171-8501, Japan
1

2

Transition metal phosphides (TMPs) are goodcandidates for the catalyst in the next generation forhydrodesulfidation. Although Ni2P shows the highest
catalytic activity among TMPs, Fe2P, which have thesame crystal structure as that of Ni2P, shows the lowest
catalytic activity.[1] One of possible origins of theactivity difference is the difference in the surface
electronic structures of these catalysts. We havereported that surface segregation of P atoms in Ni2P
induces the change of surface electronic structure in thevicinity EF.[2, 3] On the other hand, P segregation in
Fe2P(10-10) had little effect on the surface electronic
structure around EF.[4] In this study, we investigated theprocess of P segregation induced by annealing and its
effect on the electronic structure of Fe2P(0001) using
photoelectron spectroscopy (PES) at BL5U, and theresults will be compared with those of Ni2P.
Figure 1 shows the peak intensity ratio of PLMM to
FeMVV obtained by Auger electron spectroscopy (AES)in various conditions of annealing temperature. The
ratio decreased by annealing at temperatures lower
than 400 °C and increased with annealing at higher
than 400 °C. It indicates that the surface segregationinduced by the annealing at higher than 400 °C. The P
segregation process was studied in detail by P 2p PES.
Figure 2 show the results of the fitting of the P 2p
spectrum of Fe2P(0001) clean surface. The spectrum
can be fit with seven P components. The inspection of
annealing temperature dependence of each peak showsthat a,b,d,f and g components are increased in intensity
by annealing at higher than 350 °C, while those of cand e components are decreased by annealing at higher
than 350 °C. PES and AES results indicate that
segregation and desorption of P atoms compete at
350 °C and segregation exclusively proceeds at higher
than 400 °C. Finally, we studied the change of surface
electronic structure by the heating treatment. Figure 3
shows the valence band spectra of Fe2P(0001)
annealed at various temperatures at 300 – 700 °C. All
spectra are normalized by the peak top intensity at 0.2
eV. Although the spectral line shape in the range of 1 4 eV is changed by annealing, the peak in the vicinityof the EF did not show any shifts. These results indicatethat segregated P atoms have little effect on the surface
electronic structure around EF of Fe2P(0001).
Although the Ni 3d states are stabilized through thesegregating P atoms for the Ni2P, the Fe 3d states are
not changed by P segregation for Fe2P, which would
be one of the possible origins of the stark contrast in
the catalytic activities of these materials.

Fig. 1. The results of Auger electron intensity ratio at
each annealing temperature.

Ⅲ- 4

Fig. 2. Fitting result of P 2p spectra obtained fromclean surface of Fe2P(0001).

Fig. 3. The change of valence band spectrum obtained
from the sample annealed at each temperature.
[1] S. T. Oyama et al., Catal Today 143 (2009) 94.
[2] K. Edamoto et al., Appl. Surf. Sci. 269 (2013) 7.
[3] S. Ishida et al., e-J. Surf. Sci. Nanotech. 13 (2015) 93.
[4] Y. Sugizaki et al., Jpn. J. Appl. Phys. 57 (2018)
115701.

141
Ⅲ04- 11 -Ⅲ -4. Surface, Interface and Thin Films
BL5U
BL5U

Measuring the Berry Curvature of Dresselhaus Semiconductor by Circular
Dichroism ARPES
S. Cho1, D. Oh2,3, K. Tanaka4, S. R. Park5 and C. Kim2,3

Institute of Physics and Applied Physics, Yonsei University, Seoul 03722, Korea
Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea
3
Department of Physics and Astronomy, Seoul National University (SNU), Seoul 08826, Republic of Korea
4
UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
5
Department of physics, Incheon National University, Incheon 406-772, Korea
1

2

Ⅲ- 4

Recently, Datta and Das [1] proposed that spin-fieldeffect transistor (spin-FET) can be shown by in light ofcontrollability of spin precession motion of electrons ofzinc blende semiconductors (ZBS) [2, 3, 4]. The ZBS
have unique features; broken inversion symmetry andstrong spin orbit coupling (SOC). The features lead toan effective momentum dependent magnetic field whichlifts the spin degeneracy of the electrons in the materials.
The helical spin structure can be described successfully
by Rashba and Dresselhaus model at a constant energy
contour of the valence band [5]. The unique spin
structure lead to the noble spin transport, spin Hall effect,
and this was shown by Kerr rotation measurement in
GaAs with zinc blende structure enerally imply that
there is a transvers velocity which proportional to the
Berry curvature [6, 7]. Understanding the Berrycurvature of the ZBS is considered important to develop
the spin-FET.
In this study, we try to inverstigate the Berrycurvature of HH and LH band of ZBS. We note that thesemiconductor with zinc blende structure generally
have the three hole bands which consist of 2-folddegenerate (j = 1/2) and 4-fold degenerate (j = 3/2) state,
resulting from the SOC and a negligible crystal field.
The j = 3/2 state split into heavy- (mJ = ±3/2) (HH) andlight hole (mJ = ±1/2) (LH) bands near the Fermi level.
The j = 1/2 state, called split-off band, is located at ahigher binding energy about the energy splitting (Δ)
than j = 3/2 state. This splitting Δ is determined by themagenitude of the spin-orbit interaction. Since the HH
and LH state have different value of mJ, it is expectedthat the Berry curvature have different behavior [8].
To show the Berry curvature of Bloch state in the ZBS,
we note a recent proposal, based on the study on therelation bewteen Berry curvature and circular dichroism(CD)-ARPES, that the CD-signal is approximatly

proportional to the Berry curvature in 2H-WSe2 [9]. It
implied that CD-ARPES can show the Berry curvatureof Bloch state in momentum space. From the previous
study, it is expected that the diffetent behavior of the
Berry curvature between HH and LH state can be shown
by CD-ARPES measurement.
Figures 1(a) and (b) show the ARPES intensity mapof InSb at a binding energy 0.6 eV obtained from righ(RCP) and left (LCP) circularly polarized light. Then,
we can have CD-signal, as shown in Fig. 1(c), from thedifference between the intensity map taken from RCP
and LCP light. In our result as depicted in Fig. 1(c),outer (inner) band indicate the CD-intensity map of HH(LH) band. But, they seem to illustrate a similar CDpattern. We think that our CD-intensity map refleced thefinal state effect because CD-ARPES data of HH and
LH band were taken with 2nd Brillouin zone (BZ). In
that experiments, since the ARPES intensities with RCP
and LCP light in first BZ were too low, we chose the 2nd
BZ.
[1] S. Datta et al., Appl. Phys. Lett. 56 (1990) 665[2] Y. A. Bychkov et al., JETP Lett. 39 (1984) 78.
[3] J. Schliemann et al., Phys. Rev. Lett. 90 (2003)
146801.
[4]X. Cartoixà et al., Appl. Phys. Lett. 83 (2003) 1462.
[5] G. Dresselhaus, Phys. Rev. 100 (1955) 2.
[6] S. Murakami et al., Science 301 (2003) 1348.
[7] Y. K. Kato et al., Science 306 (2004) 1910.
[8] M.-C. Chang and Q. Niu, J. Phys.: Condens. Matter
20 (2008) 193202.
[9] S. Cho et al., PRL 121 (2018) 186401.

Fig. 1. ARPES intensity map of InSb at a binding energy 0.6 eV taken with (a) right and (b) left circularly
polarized 70 eV light. (c) Circular dichroism (CD) intensity obtained from the difference between (a) and (b).

142
Ⅲ04- 12 -Ⅲ -4. Surface, Interface and Thin Films
BL6U
BL6U

Arpes Study of (BixSb1-x)2Se3 Topological Insulators
2

L. Yashina1 and F. Matsui2

Chemistry Department, Moscow State University, Moscow 119991, Russian Federation
UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

1

Band structure of crystals of a series of solid
solutions (BixSb1-x)2Se3 (x=1 - 0.7) between
prototypical topological insulator (TI) Bi2Se3 [1] andtrivial insulator (BI) Sb2Se3 was studied by means of
ARPES. In the Bi2Se3-Sb2Se3 system, topologicalphase transition was predicted theoretically. In thisproject we have aimed on revealing its relevant
aspects: spontaneous destroying or gap formation in
Dirac cone and x composition corresponding totopological phase transition. For better surface
sensitivity photon energy hν=60 eV was chosen.
Increasing an antimony content leads to Dirac point
shifting towards bulk valence band at range x=1-0.8.
This trend breaks at (Bi0.7Sb0.3)2Se3 composition(Fig.1a). In case of its electronic structure, Dirac point

is located approximately at the same energy that in
Bi2Se3 (Fig.1a). However, exact energy position of
Dirac point in at (Bi0.7Sb0.3)2Se3 is hard to estimate.
Bisthmus-like band structure near Dirac point allowed
us to suggest, that backscattering channels starts toopen near the (Bi0.7Sb0.3)2Se3 composition, but noenergy gap in topological surface state was notobserved (Fig.1b).
[1] Y. Xia et al., Nat. Phys. 5 (2009) 398.

Ⅲ- 4

Fig. 1. a) Energy dispersion curves along K-Г-K direction as a function of crystal composition. b) Constantmomentum cross-section at kГ-К=0 for (Bi0.7Sb0.3)2Se3 compound.

143
Ⅲ04- 13 -Ⅲ -4. Surface, Interface and Thin Films
BL6U
BL6U

Reversible and Irreversible Metal-insulator Transition of VO2
1

Ⅲ- 4

T. Zhai1, J. Yang1, F. Matsui2 and S. Duhm1

Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, P. R. China
2
UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

Vanadium Dioxide (VO2) is widely known for its firstorder metal-insulator transition (MIT) property. The MIT
goes along with a structural change from a monoclinic
phase to a rutile phase at 340 K, while the conductivity
increases 5 orders of magnitude comparing to theinsulating phase. Moreover, also oxygen vacancies [1] orsoft X-ray irradiation [2] can lead to a metallic state.
Figure 1 shows the electronic structure of insulating
and metallic VO2 based on a previous photoelectron
spectroscopy (PES) study [3]. For insulating VO2 theoccupied V 3d derived orbital (termed as 𝑑𝑑‖ ) is locatedbelow the Fermi level, while the other two V 3d orbitals,
𝑑𝑑‖∗ and π*, are unoccupied and provide a 0.6 eV band gap.
In the metallic phase the 𝑑𝑑‖ and π* orbitals are partiallyfilled. Interestingly, non-stoichiometric VO2-δ shows asimilar behavior. However, the details of the electronic
structure are not clarified yet. Especially the unoccupied
DOS of VO2-δ is largely unexplored. Thus, we performed
X-ray absorption spectroscopy (XAS), resonant
photoemission (RPES) and ultraviolet photoelectron
spectroscopy (UPS) experiments at BL6U.
VO2 thin films were fabricated by pulsed laser
deposition (PLD) [1,3] and cleaned by sputtering andannealing cycles in the ultra-high vacuum (UHV)
preparation chamber of the BL6U endstation. The
stoichiometry can be controlled by annealing at 423 K
in UHV (base pressure: ~8 x 10-10 mbar) or in partial
oxygen atmosphere (~9 x 10-5 mbar).
The XAS spectra in Fig. 2 show the V L-edge and theO K-edge regions of pristine, stoichiometric VO2 atroom temperature (RT), the sample after an exposure to
X-rays of 180 min and a pristine VO2 sample at 388 K.
The peak shift at the V L3 edge at ~511.6 eV is ascribedto the weight increase of t2g due to the irradiation, which
also narrows the band gap; the t2g of O K-edge region
from irradiation induced VO2 exhibits weight loss. In
the meantime the energy interval between the t2g and eg
becomes smaller. UPS (not shown) confirms a metallic
state of irradiated VO2. The irradiation induced phase isdifferent to the temperature induced metallic phase. In
particular, the main absorption peak at the V L-edge
shift does not show an apparent shift. The O K-edge
absorption features reveal that there is almost no oxygen
loss during the temperature induced reversible MIT.
In conclusion, our preliminary data analysis confirmsthat X-ray exposure of VO2 leads to a non-reversible
phase transition and to a metallic state at RT. Analyzing
the additional XAS data and the RPES and UPS datameasured at BL6U, will allow us to understand themechanism leading to this phase transition. Moreover,
our data will also shed light on the electronic structureof VO2 upon the temperature and the oxygen vacancy
induced MIT.

Fig. 1. Electronic structure of stoichiometric VO2(insulating and metallic phase) and VO2-δ. In theinsulating state a gap opens between the V 3d
derived 𝑑𝑑‖∗ , π* and 𝑑𝑑‖ orbitals. Taken from Ref. [3].

Fig. 2. XAS spectra for pristine VO2 at RT, the same
sample after 180 minutes of X-ray exposure andpristine VO2 at 388 K. In all cases, the V L-edge region
and the O K-edge region are shown. The peaks arelabeled according to Ref. [4].
[1] R.-B. Wang, T. Katase, K.-K. Fu, T.-S. Zhai, J.-C.
Yang, Q.-K. Wang, H. Ohta, N. Koch and S. Duhm,
Adv. Mater. Interfaces 5 (2018) 1801033.
[2] V. R. Singh, V. Jovic, I. Valmianski, J. G. Ramirez,
B. Lamoureux, I. K. Schuller and K. E. Smith, Appl.
Phys. Lett. 111 (2017) 241605.
[3] K.-K. Fu, R.-B. Wang, T. Katase, H. Ohta, N. Kochand S. Duhm, ACS Appl. Mater. Interfaces 10 (2018)
10552.
[4] R. Zimmermann, R. Claessen, F. Reinert, P. Steinerand S. Hüfner, J. Phys.: Condens. Matter 10 (1998) 5697.
.

144
Ⅲ04- 14 -Ⅲ -4. Surface, Interface and Thin Films
BL6U
BL6U

Resonant Photoemission Spectroscopy of Aligned Graphene Nanoribbons
1

D. Usachov1, B. Senkovskiy2 and F. Matsui 3

Saint Petersburg State University, 7/9 Universitetskaya Nab., St. Petersburg 199034, Russia
II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
3
UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan2

Absence of a band gap in graphene prevents its wideuse in electronic devices. This problem can be overcome
by reducing dimensionality from 2D to 1D. Due toquantum confinement graphene nanoribbons (GNRs)
have a notable band gap and a tunable electronic
structure, and therefore they are a promising material for
optoelectronic applications. Furthermore, thanks to thebottom-up nanofabrication technique it is possible tosynthesize atomically precise GNRs with definedelectronic properties. GNRs also represent significant
fundamental interest because of being an intermediate
system between a 2D crystal and molecules. Theirvalence band is formed by a set of subbands possessing
dispersion along the ribbon axis, while in perpendicular
direction the wave function is trapped in a narrow
quantum well [1]. Bottom-up synthesis allows
atomically precise fabrication of GNRs of fixed width.
The present work is devoted to resonant photoemission
study of 7-atom-wide armchair-edge GNRs (7-AGNRs)aligned on a stepped Au(788) surface.
The C K-edge NEXAFS spectrum of 7-AGNR
contains fine features, which can be associated withdifferent unoccupied subbands [2]. Therefore, it was
expected that photoemission from different subbands
could be selectively enhanced by tuning the photon
energy near the absorption resonance.
Figure 1 shows preliminary results of our study. The
upper row corresponds to such tilting angle of sample,
at which the topmost valence subband (indicated by thearrow) has maximal intensity. At the photon energy of
284.6 eV, which is just below the absorption edge, thissubband is not visible because of low photoemission
cross section. When the photon energy is increased to
285 eV the subband is resonantly enhanced (shown withwhite arrow). However, already at 285.8 eV it
disappears.
The second row corresponds to the second subband.
We have found that its behavior is different from the firstone. While it is resonantly enhanced at 285 eV, it does
not disappear at 285.8 eV. For the third subband (shownin the third row) the resonance seems to be shifted tohigher photon energies. This is a clear indication thatdifferent valence subbands are selectively excited withphotons of different energy at the C absorption K-edge.
Further theoretical calculations will be performed tounderstand this effect in more detail.
Another result of our study was the discovery of thefact that 7-AGNRs undergo radiation damage when thephoton energy is set to the C K-edges absorption
resonance. The valence bands rapidly vanish under thephoton beam. We suppose that photoexcited

nanoribbons undergo dehydrogenation and become
chemically active due to formation of carbon dangling
bonds. Such active edge carbon atoms may react withadsorbates. This opens a possibility for chemicalfunctionalization of GNRs by means of photostimulated
reactions. This possibility will be explored in moredetail in the future experiments.

Ⅲ- 4

Fig. 1. Resonance photoemission of 7-AGNR at the
carbon K-edge. Different rows correspond to different
sample orientations at which different subbands
exhibit the highest intensity.
[1] B. V. Senkovskiy et al., 2D Mater. 5 (2018) 035007.
[2] B. V. Senkovskiy et al., ACS Nano 12 (2018) 7571.

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Ⅲ04- 15 -Ⅲ -4. Surface, Interface and Thin Films
BL6U
BL6U

Photoemission Tomography of Organic Monolayer:
Substrate-assisted Intermolecular Dispersion of PTCDA on Cu(100)
A. Haags1,2, M. Meissner3, X. Yang1,2, F. Matsui3, S. Soubatch1,2, F. S. Tautz1,2 and S. Kera3
Peter Grünberg Institut (PGI-3), Forschungszentrum Jülich, 52425 Jülich, Germany
Jülich Aachen Research Alliance, Fundamentals of Future Information Technology, 52425 Jülich, Germany
3
UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
1

2

Ⅲ- 4

The transport properties of electronic materials are
crucial for device performances. This is particularly
important for organic-based electronics [1, 2], because
charge mobility in molecular films is usually restricted
due to a hopping transport, while exclusively band-like
transport along the π stacking direction is expected inwell-structured films. However, recently several reports
have been published about intermolecular band
dispersion of occupied states in the organic monolayers
on metal in the lateral direction. Since there is no
significant overlap of π states, the rise of dispersion inthese cases has been attributed to molecule-substrate
hybridization. An example is the strong intermoleculardispersion of the former lowest unoccupied molecularorbital(f-LUMO)ofperylene-tetracarboxylic
dianhydride (PTCDA) on Cu(100) [3]. Upon PTCDA
adsorption on Cu(100), its LUMO becomes occupied
due to the charge transfer from copper and forms a
dispersive band whose momentum-space (k-space)pattern clearly reflects the symmetry of the
PTCDA/Cu(100) interface [3].
An intriguing aspect in this context is the k-spacecharacter of the hybridized molecular electronic states.
Recently it has been shown by means of photoemission
tomography that the k-space patterns of particularmolecular states usually retain their shapes despite
hybridization and charge transfer [4]. Only in case of thedispersive mixed state, the modified symmetry of wavefunctions becomes apparent in the k-space patterns as anadditional fine-structure [3, 5]. So far it remains unclear,
why the symmetry of the hybridized molecular stateschanges in some cases and remains intact in others. To
shed light on this issue, a combined experimental andtheoretical approach is required. The experimental
setups used so far for photoemission tomography, suchas k-space imaging photoemission microscopes and atoroidal electron spectrometer, suffer from poor energy
and k-space resolution, which is particularly needed fora characterization of dispersive bands. On the other
hand, the analytical tools providing state-of-the-art
resolution, for example hemispherical analyzers, do nothave the k-space imaging option.
To address this issue, we initiated a series ofexperiments using the two-dimensional (2D) angleresolved photoemission end-station based on an MBS
A-1 hemispherical electron analyzer with modified lenssystem at the BL6U beamline of UVSOR. The 2D kspace resolution in this case is achieved by a mechanicaldeflection of the lens system perpendicular to theentrance slit of the analyzer. Correspondingly, thephotoemission distribution is recorded as a set of energy
distribution maps depending on the deflection angle
resulting in a three-dimensional dataset I(Ekin, φ, θ).
To demonstrate the capability of the setup forphotoemission tomography applications, we grew acommensurate monolayer of PTCDA on Cu(100) andcarried out angle-resolved photoemission spectroscopy
at room and low temperature. To visualize theexperimental results, we developed a python-based
software. The 2D angle-space resolved photoemission
patterns measured at the binding energy corresponding
to the f-LUMO, clearly resemble the contrast of the
major LUMO emission lobes as well as substraterelated features - copper sp-bands.

Fig. 1. Screenshot of the python-based software fordata evaluation. Top: slicing in different axes of the 3D
data cube I(Ekin, φ, θ) measured on PTCDA/Cu(100);
Bottom Left: corrected I(φ, θ) image for Ekin = 55 eV;
Bottom Right: corresponding image converted toreciprocal space.
[1] M. Schwarze et al., Nat. Mat. 18 (2019) 242.
[2] N. Ueno and S. Kera, Prog. Surf. Sci. 83 (2008) 490.
[3] D. Lüftner et al., Phys. Rev. B 96 (2017) 125402.
[4] S. Weiss et al., Nat. Commun. 6 (2015) 8287.
[5] T. Ules et al., Phys. Rev. B 90 (2014) 155430.

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Ⅲ04- 16 -Ⅲ -4. Surface, Interface and Thin Films
BL7U
BL7U

Fabrication of Bi1Te1 Ultrathin Films and the Surface Electronic Structure
S. Kusaka1, K. Yokoyama1, S. Ideta2, K. Tanaka2, S. Ichinokura1 and T. Hirahara1
Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, JapanUVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan1

2

A typical example of a compound composed of Bi
and Te is Bi2Te3 (Te-Bi-Te-Bi-Te), which is known as astrong topological insulator (TI), but compounds having
different composition ratios also exist. One of them,
Bi1Te1, is a superlattice (Te-Bi-Te-Bi-Te - Bi-Bi-Te-Bi)
in which two Bi2Te3 quintuple layer (QL) and one
bilayer (BL) of Bi stack alternately [1]. There can bethree types of terminations for this material, namely
1QL and 2QL Bi2Te3 terminations or a BL-Bi
termination. It is called a dual TI and has a surface stateof a topological crystal insulator (TCI) which isprotected by the mirror symmetry of a crystal and asurface state of a weak topological insulator protectedby time reversal symmetry. They are located at different
surface orientations. Thus it has been predicted that
Bi1Te1 can realize novel exotic properties by breaking
individual symmetries protecting different topologies
by applying a magnetic field or a lattice distortion andinducing a gap in the respective surface Dirac cone [2].
In the previous study, Bi1Te1 was prepared bydepositing Bi and Te at a ratio of 1:1 with molecularbeam epitaxy (MBE), and its electronic structure wasreported [2]. However, the surface termination seemed
to be mixed between 1QL and 2QL Bi2Te3. Therefore,
for a better understanding of Bi1Te1, establishment ofa stable sample preparation method with a welldefined surface termination is required. Therefore in
the present study, we tried to fabricate a Bi1Te1 thinfilm with controlled termination by a different method.
Namely, first we fabricated Bi2Te3(111) films bydepositing Bi and Te at a ratio of 1:5 with MBE (Fig.
1 (a)) and then annealed it to get rid of Te layers. The
samples thus prepared showed three different band

dispersions as shown in Fig. 1 (b-d) before breaking.
We found through comparison with ab initio
calculations for Bi1Te1 with different surface
terminations that (b) showed similarities with the
2QL-Bi2Te3 terminated Bi1Te1, whereas (d) was
similar to the 1QL-Bi2Te3 terminated Bi1Te1, although
there was no quantitative match [2]. (c) was identified
as an intermediate state of (a) and (b). Furthermore, we
also observed a Dirac point in the direction
perpendicular to Γ-M at a wavenumber not equal tozero (Γ point). This was also seen in the theoretical
calculation and thus this (111) surface was identified
as the surface possessing the TCI Dirac cone.
We are now trying to investigate the real atomic
structure of this system with low-energy electron
diffraction (LEED) as well as cross sectionaltransmission emission microscopy (X-TEM). There isa possibility that they are not actually Bi1Te1, butrather more complex structures composed of BL-Bi
and Bi2Te3, such as Bi4Te5, Bi6Te7, Bi4Te3, Bi2Te,
Bi7Te3 [3]. In any case, the combination of ARPES, abinitio calculations, and structure analysis should bevery useful in studying the topological properties of
Bi-Te compounds.
[1] T. Hirahara et al., Phys. Rev. Lett. 107 (2011) 166801.
[2] M. Eschbach et al., Nat. Comm. 8 (2017) 14976.
[3] O. Concepcion et al., Inorg. Chem. 57 (2018)
10090.

Fig. 1. Band dispersion of Bi2Te3 (a) and that obtained by annealing it (b-d), which is likely Bi1Te1 with different
surface terminations. The measurements were performed along the Γ-M direction.

147
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BL7U
BL7U

Photoelectron Angular Distribution of Pentacene/Graphite Interface
Investigated by Low-energy Angle-resolved Photoelectron Spectroscopy
T. Yamaguchi1,2, M. Meissner2, S. Ideta1,3, K. Tanaka1,3 and S. Kera1,2,3

School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan2
Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan
3
UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

1

Ⅲ- 4

Ultraviolet photoelectron spectroscopy (UPS) is avery useful technique for studying the electronic
structure of surfaces and interfaces. However, theorigin of UPS features obtained by low-energy photon
excitation is not well established theoretically andexperimentally because of the complicatedphotoemission process. Tanaka et al. demonstrated
resonance excitation between initial states and final
states which causes unexpected scattering due to therelatively long lifetime of low-energy electrons
potentially trapping them in intermediate states at theinterface [1]. A similar behavior was found for organicmonolayers on graphite, giving rise to inelastic
energy-loss features [2-4]. In addition, the lifetime ofphotogenerated holes may also lead to interesting
phenomena. Franck-Condon anomalies, where thevibronic progression does not follow that of thegaseous phase and shows a strong angular anisotropy,
and photoemission matrix anomalies, where highintensity appears at the surface normal, were observed
for monolayers of flat-lying organic molecules ongraphite [2-4]. Moreover, anomalous bands have beenreported, caused by conduction bands in the secondary
electron emission (SEE) background [5]. Therefore,
despite the weak interaction of physisorbed moleculeswith the substrate, the corresponding wavefunctions
may connect strongly at the interface; and the
combination of these effects could be used to study the
impact on the electronic states. In this work, weinvestigated the photoelectron angular distribution(PAD) of the highest occupied molecular orbital(HOMO) for pentacene (PEN), using low-energy
angle-resolved UPS (LE-ARUPS) to clarify thosephenomena.
Highly oriented pyrolytic graphite (HOPG) as thesubstrate was cleaned by annealing at 600 K for 2 h. A
PEN film of 4 Å was deposited on HOPG at room
temperature in a custom UHV chamber designed fororganic layer deposition.
Figure 1(a) shows a LE-ARUPS map taken at hv =7.7 eV and T = 15 K. In the HOMO region (1.6 ~ 2.0eV) some features stand out: (i) Strong photoemission
intensity around the Γ point which is unexpected for ppolarized excitation of weakly bounded, flat-lying, piconjugated molecules, (ii) the first vibronic satellite
has a higher intensity at Γ than the HOMO (dashedoval in Fig. 1(a)), which deviates from the spectral
shape found for higher photon energies such as HeI
light, and (iii) a faint background in form of a strongly

dispersed dark band and/or bright bands with constantkinetic energy is detected (dashed straight lines). The
latter can be seen more clearly when averaging LEARUPS maps for different photon energies (here 7.4 ~7.8 eV) and normalizing the averaged map by its
horizontally and vertically integrated profiles
separately (Fig. 1(b)). This way, a constant-kineticenergy feature becomes more prominent whilemolecular occupied states move in energy and “scan”
the unoccupied features.
In conclusion, we clearly observed an unusual
evolution of PAD in a photon energy range of hv = 7
eV ~ 8 eV which is caused by non-trivial multi-particle
photoemission events at weakly bounded interfaces.
Multiple scattering theory will be helpful tounderstand the origin of the PAD and hence to reveal
the modification of the molecular orbital upon weakinteractions.

Fig. 1. (a) LE-ARUPS intensity map for PEN(0.3nm)/HOPG taken at 15 K and normal emission using
hv = 7.4 eV; and (b) the average of 5 maps using hv =7.4 eV ~ 7.8 eV, normalized by its horizontally andvertically integrated profiles.
[1] S. Tanaka et al., Sci. Rep. 3 (2013) 3031.
[2] T. Yamaguchi et al., UVSOR Activity Report 2015
43 (2016) 135.
[3] T. Yamaguchi et al., UVSOR Activity Report 2016
44 (2017) 151.
[4] T. Yamaguchi et al., UVSOR Activity Report 201745 (2018) 142.
[5] S. K. Mahatha et al., Phys. Rev. B 84 (2011)
113106.

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BL7U
BL7U

Band Structure Modification of 2D TMDCs Using a Molecular Dopant
S. Park, T. Schultz, P. Amsalem, X. Xu and N. Koch

Humboldt-Universität zu Berlin, Institut für Physik, Brook-Taylor-Str. 6, 12489 Berlin, Germany
The aim of the proposed experiments was to achievea deep understanding of the microscopic phenomena
occurring upon interface formation between a MoS2
monolayer and a strong electron acceptor molecule,
F6TCNNQ, which can be employed for p-doping ofsemiconductors [1]. In the initially planned experiments,
several substrates including sapphire, HOPG and silver,were to be employed as support for the MoS2 film.
During the experiments at BL7U, we firstencountered charging issues when using sapphire assupport, though the very same sample could be
successfully measured in-house by photoemission using
a standard helium discharge lamp. Most of theremaining time was then dedicated to high resolution
measurements of MoS2/HOPG with and without
F6TCCNQ and at different temperatures ranging from 7
K to 300 K. For this system, we first observed by angleresolved photoemission (ARPES) that the electronic
band structure could be nicely resolved, as exemplified
in Fig. 1, though the MoS2 monolayer is azimuthally
disordered. Such an observation is enabled by selective
high photoemission intensity along Γ-K and Γ-M of thesingle crystal which singles out these features also after
azimuthal integration. These first results are included ina manuscript that has been submitted for publication [2].
Regarding the initial aim of this project, we could
successfully demonstrate that p-doping of the MoS2
film on HOPG occurs upon F6TCCNQ deposition. Thisis shown in Fig. 1 in which the MoS2 valence band (VB)
features are seen to shift to lower binding energies by
up to 0.26 eV upon molecular adsorption. Interestingly,
we also observed that the MoS2 VB shifts aretemperature-dependent and p-doping is found lesseffective at low temperatures. The measurements
indicate that p-doping results from charge transfer to the
F6TCNNQ, giving rise to partial filling of the lowest
unoccupied molecular orbital (LUMO). Further highresolution measurements of the low binding energy
region corresponding to the partially-filled LUMO
states, revealed temperature-dependent magnitude of
the charge transfer with a critical temperature of ca. 100
K as shown in Fig. 2. Deep understanding of the chargetransfer mechanisms as unraveled in this experimentwill lead to strong insight into the physics involved in
TMDC doping by adsorption of small organic
molecules with strong electron acceptor properties.

Ⅲ- 4
Fig. 1. a) and b) ARPES spectra of clean MoS2/HOPG
around the Κ point of the Brillouin zone at 300 K and 7
K, respectively. c) and d) corresponding ARPES spectra
after deposition of F6TCNNQ. (hν = 21 eV )Fig. 2. a) Temperature-dependent energy distribution
curve of MoS2/HOPG near the Fermi-level, before(black) and after (colored) F6TCNNQ deposition. The
additional density of states after molecular deposition isdue to charged molecules only. b) Area of the signal due
to charged molecules and energy shift of the MoS2 VB
as a function of temperature. The critical temperature
occurs at ca. 100 K.
[1] T. Schultz et al., Physical Review B 93 (2016) 125309.
[2] S. Park et al., Comm. Phys. (submitted).

149
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BL7U
BL7U

UPS Measurement of Valence Electronic Statesin Dinaphtho[2,3-b:2’, 3’-f]thieno[3,2-b]thiophene Single Crystals
R. Takeuchi1, S. Izawa2, R. Tsuruta1, S. Yamanaka1, M. Iwashita1, K. Tonami1, S. Ideta2,
K. Tanaka2, M. Hiramoto2 and Y. Nakayama1
1

Ⅲ- 4

Department of Pure and Applied Chemistry, Tokyo University of Scienece, Noda 278-8510, Japan
2
UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8787, Japan

Dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene(C22H12S2, DNTT, Fig. 1) attracts attention as a highmobility p-type organic semiconductor material [1].
One of the problems of organic semiconductor materials
such as pentacene is their low stability in theatmosphere. DNTT is proposed to be more robust
against oxidation than pentacene [2], and has been
regarded as a promising material for applications.
However, the valence electronic structure that
contributes to its charge transport is not fully understood,
and researches using the single crystals are necessary toobtain knowledge about the fundamental electronic
structure [3]. In this study, the valence electron structureof the single crystal of DNTT was measured byultraviolet photoelectron spectroscopy (UPS).
DNTT single crystals were prepared by a physicalvapor transport (PVT) technique. The crystals wereattached to gold-coated silicon substrates using silver
paste in order to avoid sample charging. UPS
measurements were conducted at BL7U, UVSOR,
using an electron spectrometer A-1 (MBS) or at an
offline apparatus equipped with DA-30 (ScientaOmicron). For the offline UPS measurements, He-IIα
light source (hν = 40.81 eV) was used, and the work
function value of the electron spectrometer wasdetermined to be 4.37 eV from the Fermi edge position.
In this study, a 375 nm laser was irradiated at the sametime in order to avoid sample charging. All UPS
measurements were conducted at room temperature.
Figure 2 shows a UPS spectrum of a DNTT single
crystal sample taken at hν = 10 eV. Whereas threecomponents were barely recognized in the binding
energy (BE) range of -3.5 ~ -0.5 eV, the highest
occupied molecular orbital (HOMO)-derived peak was
not resolved clearly.
In contrast, UPS results taken at hν = 40.81 eV clearly
exhibited the HOMO-derived peak as shown in Fig. 3(left), which allowed the determination of the valenceband maximum (VBM) of the DNTT single crystal tobe -0.69 eV. Figure 3 (right) shows a photoemission
spectrum in the secondary electron cutoff region fromthe sample being biased at −5 V, where the abscissa is
rescaled to indicate the work function. The work
function of the DNTT single crystal was determined to
be 4.30 eV from the onset position. From these results,
the ionization energy of the DNTT single crystals isdetermined to be 4.99 eV, which is significantly smaller
than that reported previously for the DNTT thin film
(5.44 eV [2]).

Fig. 1. Molecular structure of DNTT.

Fig. 2. UPS spectrum of the DNTT single crystal taken
at BL7U (hν = 10 eV).

Fig. 3. UPS spectra of the DNTT single crystal in the
HOMO region (left) and secondary electron cutoff
region (right) taken with He-IIα excitation.
[1] T. Yamamoto and K. Takimiya, J. Am. Chem. Soc.
129 (2007) 2224.
[2] H. Yagi et al., Chem. Phys. Lett. 563 (2013) 56.
[3] X. Wei et al., Adv. Mater, 25 (2013) 3478.

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BL7U
BL7U

Two-Dimensional Nature of Topological Surface States on SmB6(111) Clarified
by Synchrotron-Radiation Angle-Resolved Photoelectron Spectroscopy
Y. Ohtsubo1,2, Y. Yamashita2, T. Nakamura2, S. Ideta3, K. Tanaka3, W. Hirano4, F. Iga4
and S. Kimura1,2
1
Graduate School of Frontier Biosciences, Osaka University, Suita 565-0871, Japan
Department of Physics, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
3
UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
4
College of Science, Ibaraki University, Mito 310-8512, Japan

2

Topological Kondo Insulators (TKI) are gatheringmuch attention in these days as a promising playgroundto study the concert effect of the spin-polarized
topological surface states and strong electron
correlation resulting in the metal-insulator transition of
Kondo insulators [1]. SmB6 is the first and the mostextensively studied candidate of TKI. It has been
revealed that the (001) surface of the cleaved SmB6
single crystal hosts the metallic and spin-polarized
surface states as expected from topologicalclassification [1, 2]. However, the topological order of
SmB6 itself is still under debate because of the multiple
possible interpretations of the metallic surface bands [3].
We have studied the surface electronic structure of
SmB6 in these years to reveal its topological order aswell as the non-conventional electronic phenomena
driven by strong electron correlation [4, 5]. Before thisproject, we have already discovered the metallic
electronic states with spin polarization on the (111)
surface of SmB6 [4, 5]. However, its dimensionality,
two-dimensional (2D) surface-derived ones or threedimensional (3D) bulk bands, had not been clear yet.
In this project, we performed angle-resolved
photoelectron spectroscopy (ARPES) measurements on
the SmB6(111) surface and carefully traced the incident
photon energy dependence of the metallic band S1 andthe other band lying just below S1, namely S2, as shownin Fig. 1 (a), to elucidate kz-dependent band dispersions,
if it exists.
Figures 1(b) and 1(c) show the ARPES momentum
distribution curves (MDCs) corresponding to S1 and S2,
respectively. As evidently shown there, the S1 peak
positions shown in Fig. 1(b) are independent from theincident photon energy, indicating its 2D nature,
implying to originate from the surface electronic
structure. As for S2 in Fig. 1(c), the behavior is similar
to S1, suggesting the common 2D origin. However, one
can find broad humps changing its center positions
depending on the photon energies, such as that at +0.3Å-1 with hν = 20 eV. Because of these photon-energydependent features, we cannot exclude possible 3D
character for S2 from these data.
Based on the current data, we could conclude that themetallic electronic state at the Fermi level, S1, is fromthe surface electronic structure without any ambiguity.
Together with the former results, we propose thetopologically non-trivial picture of SmB6 [4, 5].

Ⅲ- 4

Fig. 1. (a) ARPES intensity plot of SmB6(111) taken athν = 26 eV and 15 K. (b, c) ARPES MDCs at 15 K withelevating photon energies. The binding energies are (b)0 meV (Fermi level) and (c) 60 meV, corresponding to
the bands S1 and S2 in (a), respectively.
[1] M. Dzero et al., Phys. Rev. Lett. 104 (2010) 106408.
[2] N. Xu et al., J. Phys.: Condens. Matt. 28 (2016)
363001 and references therein.
[3] P. Hlawenka et al., Nat. Commun. 9 (2018) 517.
[4] Y. Ohtsubo et al., arXiv: 1803.09433 (2018).
[5] Y. Ohtsubo et al., UVSOR Activity Report 2016 44(2017) 138.

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