Ⅲ-4
Surface,
Interface andThin Films

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

Anomalous Interfacial Interactionn at Polycyclic Aromatic Hydrocarbon
Monolayers on Graphene Layer
H. Yamane and N. Kosugi

Institute for Molecular Science, Okazaki 444-8585, Japan
Two-dimensional sp2 carbon layer, graphene, has ahoneycomb network, which induces unique lowdimensional electronic properties such as an in-planehigh carrier mobility (ca. 15,000 cm2/Vs at 300 K).
The ABAB-stacking graphene multilayer shows theelectronic structure of the single-crystalline graphite.
As the graphene sheet can be obtained simply bymultiple peeling of graphite with the sticky tape, theinterlayer (out-of-plane) interaction in graphite isdominated by the weak π-π interaction. Therefore,
organic/graphite interfaces have been regarded as anarchetypal model system for electronic and structural
studies of physisorbed organic molecules. On the other
hand, hexa-peri-hexabenzocoronene [HBC, see inset
of Fig. 1(b)] is a molecular unit graphene. In thepresent work, we have succeeded to observe theanomalous interfacial interaction in the HBC monolayers on graphene and graphite by angle-resolved
photoemission spectroscopy (ARPES).
The present experiment was performed at BL6U.
The photon energy (hv) for ARPES was 50 eV, which
can probe kz = naz* for graphite. The single layer
graphene (SLG) and the single crystalline graphite(SCG) were grown on a 6H-SiC(0001) substrate byannealing at 1150°C under the Si flux and 1600°C,
respectively [1]. The surface crystallinity of SLG and
SCG was characterized using low-energy electron
diffraction (LEED) and ARPES.
Figure 1 shows the LEED image and ARPES (E-k||)
map around the K point in the Γ-K direction for (a)HBC/SLG and (b) HBC/SCG at 20 K. The LEED
images indicate the p(√31×√31)R9.0 ordered structure
for both interfaces. The E-k|| map shows a well-known
π-band dispersion of graphene and graphite (πGr)
around the K point. The highest occupied molecularorbital (HOMO)-derived peak of HBC appears at
different binding energy (Eb) position depending on
the substrate; Eb = 2.1 eV for HBC/SLG and Eb = 1.8
eV for HBC/SCG. The charge transfer from SiC
through the graphene layer might exist at HBC/SLG.
It is of note that the HOMO-derived peak shows adispersion for both interfaces, which does not dependon the graphene layer thickness. As shown in Fig.2(a),the HOMO-derived peak at HBC/SCG disperses to thelower-Eb side with increasing k|| from 1.56 Å−1 to 1.86
Å−1. The total dispersion width is 105 meV.
Furthermore, the HOMO-derived peak involves thevibrational satellites at the higher-Eb side, which also
show the dispersive behavior with respect to the 0-0transition peak, as shown in Fig. 2(b). The observed
evidence suggests that the upper (bottom) HOMO
band with the anti-bonding (bonding) character aroundk|| = 1.82 Å−1 (1.58 Å−1) can effectively couple withhigh (low) frequency phonons.
Since such dispersive behaviors for HOMO and its
vibrational satellites are not observable for ironphthalocyanine/SLG [2] and HBC/Au(111) [3], themolecule-substrate interfacial π-π coupling can play a
crucial role in the MO delocalization.

Ⅲ- 4
Fig. 1. LEED image and ARPES (E-k||) map aroundthe K point in the Γ-K direction for (a) HBC/SLG and(b) HBC/SCG, measured at hv = 50 eV and T = 20 K.

Fig. 2. Energy distribution curves for the HOMO
region at HBC/SCG with the k|| step of 0.02 Å−1.
wherein the energy scale is measured from (a) theFermi level and (b) the 0-0 transition peak.
[1] I. Forbeaux, J.-M. Themlin and J.-M. Debever,Phys. Rev. B 58 (1998) 16396.
[2] M. Scardamaglia et al., J. Phys. Chem. C 117(2013) 3019.
[3] H. Yamane and N. Kosugi, to be published.

1211 Ⅲ04-

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

ARUPS of DNTT on a-TES Film Prepared on Copper Oxide
K. K. Okudaira and S. Nomoto

Graduate School of Science and Engineering, Chiba University, Chiba 263-8522, Japansurface energy of a-TES and CuO indicates that thesurface modification by a-TES affects the molecularorientation of organic semiconductor layer, which
would have an effect on the characteristics of OTFT
such as mobility.

Intensity (arb, units)

(a)
=60°

=38°
=23°

=0°
2.5

2
1.5
1
0.5
EB (relative to E f) (eV)(b)
Intensity (arb. units)

Ⅲ- 4

An organic thin-film transistor (OTFT) is likely tohave suitable applications requiring large-areacoverage, structural flexibility and low-cost. However,there are still many issues such as low mobility to beaddressed for practical organic electronics. To achieve
high mobility it is important to improve the charge
injection efficiency at the interface between the metalelectrode and the organic semiconductor layer andchargetransfer
probability.
Self-assembled
monolayers (SAMs) has been introduced on metalelectrode in order to modify the work function of themetal [1].
A triazine-based molecular adhesion agent (a-TES :6-triethoxysilyl propylamino-1,3,5- triazine- 2,4dithiol (TES)) was used for preparing thin film like
self- assembly monolayers on metal surface as well as
SiO2 insulator layer.
In this work we used a-TES modified copper oxide
substrate 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 surface
by angle- resolved ultraviolet photoelectron
spectroscopy (ARUPS) measurements. To obtain thequantitative analysis on the molecular orientation; wecompare observed take-off angle dependence of πband and calculated ones by the independent-atomiccenter (IAC)/ MO approximation [2].
ARUPS measurements were performed at the BL2B
of the UVSOR. The take-off angle (θ) dependencies ofphotoelectron spectra were measured with the photon
energy (hν) of 28 eV. We use a-TES modified copperoxide (CuO) and CuO as substrate. Copper substrate
was subjected to ultrasonic cleaning for 10 minutes in
acetone, isopropanol, and then ozone cleaning for 30
minutes. a-TES modified CuO substrate was obtainedby dipped in 0.1% solution at 20 °C for 30 minutes.
DNTT was deposited on 1.5 nm on both substrates.
We observed take-off angle (θ) dependence of
HOMO peak in UPS of DNTT deposited on CuO anda-TES/(CuO) and (thickness of 1.5 nm ).
The HOMO peaks of DNTT on CuO and on aTES/C appear at binding energy of about 1.2 eV and
1.5eV, respectively. The HOMO peaks of DNTT bothon SiO2 and on a-TES/SiO2 show intense peaks at
higher take- off angle (θ = 60°), and at lower θ these
intensities become small. For DNTT/CuO, HOMO
intensity at θ = 38° (about 40°) is almost same as thatat θ = 60°. On the other hand, for DNTT/a-TES/CuO,
HOMO intensity at θ = 40° is smaller than that at θ =60°. Due to the difference of θ dependence of HOMO
peak of DNTT on CuO and on a-TES/CuO, it is foundthat the molecular orientation of DNTT on CuO isdifferent from that on a-TES/CuO. The difference of

2.5

=60°
=40°
=20°
=0°
2
1.5
1
0.5
EB (relative to E f) (eV)Fig. 1. ARUPS of DNTT(1.5nm)/CuO (a) and
DNTT(1.5nm)/a-TES/CuO (b).
[1] F. Wang et al., Nanoscale Res. Lett. 6 (2011) 483.
J. –P. Hong et al., Appl. Phys. Lett. 92 (2008)
143311.
[2] N. Ueno et al., J. Chem. Phys. 99 (1993) 7169.

1222 Ⅲ04-

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

Phase Stability of Quasi-1 Dimensional Columnar Structure of
Tin-phthalocyanine Films
2

Y. Kashimoto1, H. Ichikawa1 and H. Yoshida1, 2

Graduate School of Engineering, Chiba University, Chiba 263-8522, Japan
Molecular Chirality Research Center, Chiba University, Chiba 263-8522, Japan
1

In organic semiconductors, the carrier transport
occurs through the overlap of molecular orbitals. Theintermolecular orbital interaction results in the split ofenergy levels. For the highest occupied molecularorbital (HOMO), the split of lead phthalocyanine(PbPc) dimer has been observed by ultraviolet
photoelectron spectroscopy (UPS) [1]. Similar
splitting can be observed for lowest unoccupied
molecular orbital (LUMO) using low-energy inverse
photoelectron spectroscopy (LEIPS) [2] .
Recently, we demonstrated that tin-phthalocyanine(SnPc) film grows layer by layer into a quasi-1Dcolumnar structure on graphite. From the low energy
electron diffraction and UV-visible absorption
measurements, we concluded that the structure issimilar to the monoclinic phase of PbPc crystal [3]. We
applied UPS and LEIPS to the SnPc films with the
thickness between 2 and 5 monolayers (MLs). The
observed characteristic broadening of peaks wereinterpreted as the energy level splits due to theintermolecular electronic coupling [4]. The initial aimof this study was to observe an energy-momentum
dispersion relation of splitted HOMO peaks of SnPc
bilayer using energy-dependent UPS. In the
momentum space, the intensities of the twocomponents are expected to vary [5] which should be
decisive evidence of our interpretation.
In the experiment, a highly oriented pyrolytic
graphite (HOPG) was cleaved in air and annealed at
873 K for 2 hours in vacuum. The 2 ML thick SnPc
film was prepared by the vacuum deposition onto the
HOPG at room temperature (300 K) for the UPS study.
Figure 1 shows energy-dependent UPS spectra. We
observed a peak at 1.5 eV and a shoulder at 1.3 eV atthe photon energy hv of 40 eV at 25 min after the film
preparation. The line shape is similar to our previous
UPS results [4] suggesting that the spectral features
originate from the split of SnPc dimer interaction.
Subsequently, we measured the spectra at different
photon energies, hv = 105, 85 and 70 eV. The line
shapes became a single broad peak without shoulder.
Finally, we measured again the spectrum at hv=40 eV
205 min after the sample preparation. The observed
spectrum is completely different from the previous one
as shown in Fig. 1.
The SnPc film shows polymorphs. When SnPc isdeposited at the room temperature, the 1D columnar
structure is observed. However, a different film having
a head-to-tail molecular arrangement is observed
when the film is prepared on a substrate at elevatedtemperature or the film is annealed after the deposition[4]. This indicate that the 1D columnar structure is ametastable and transformed to the more stable headto-tail arrangement. The relation of the two phases issimilar to the monoclinic [3] and triclinic [6] phases of
PbPc.
The observed difference in the spectral line shapes
in this work is most likely due to the phase transitionof Sn bilayer though the reason is not clear at the
moment. Our previous UPS measurement was made at
290 K which is 10 K lower than the temperature of thepresent work at BL2B. This subtle difference may
cause the phase transition. Also the contamination on
the HOPG surface may affect the stability of the 1Dcolumnar phase.

Ⅲ- 4Fig. 1. Energy-dependent UPS spectra of 2 ML SnPc
film on HOPG. Spectral features changed by elapsed
time after deposition.
[1] S. Kera et al., Phys. Rev. B 75 (2007) 121305.
[2] H. Yoshida, Chem. Phys. Lett. 539–540 (2012) 180.
[3] Ukei, Acta Cryst. B 29 (1973) 2292.
[4] Y. Kashimoto et al., (submitted).
[5] Moser, J. Electron Spectrosc. Relat. Phenom. 214(2017) 29.
[6] Iyechika et al., Acta Cryst. B 38 (1982) 766.

1233 Ⅲ04-

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

Internal Electric Potential Distributions in Metallo-phthalocyanine Thin
Films: Magnesium Phthalocyanine and Aluminum Phthalocyanine Chloride
Y. Nakayama

Department of Pure and Applied Chemistry, Tokyo University of Science, Noda 278-8510, JapanⅢ- 4

Phthalocyanines have been used as holetransporting materials in various prototypical organic
semiconductor devices such as organic solar cells [1].
A striking point of metallo-phthalocyanines is thetunablility of their electronic properties, e.g., energy
gap width (color), molecular shape, and permanent
dipole moment, by switching the metal species at thecenter of the ligand ring. In the present work, we
conducted photoelectron spectroscopy (PES)
measurements on thin films of two kinds of metallophthalocyanine molecules (Fig. 1), magnesium
phthalocyanine (MgPc) and aluminum phthalocyanine
chloride (AlClPc), to study electronic states of thesematerials.
MgPc and AlClPc thin film were deposited in a stepby-step manner under ultra-high vacuum (basepressure below 2 × 10-7 Pa) conditions onto Au-coated
Si substrates. It should be noted that the Au/Si
substrates had been prepared in a separate system andtransferred in the ambient atmosphere, so that thesurface of these was not clean but contained somecontaminants. PES measurements were conducted at
BL2B of UVSOR, IMS. The excitation photon energy
was fixed at 120 eV throughout this work. In order toavoid any unfavorable shift or broadening of thespectra due to sample charging or radiation damage,measurement duration of each spectrum was
minimized and also the photon flux was reduced torestrict the photoemission current not to exceed 100
pA. All the measurement were carried out in thenormal emission geometry and at room temperature.
As shown in Fig. 2, the vacuum level and Mg2p
states shifted to the lower energy side in increasing the
MgPc thickness suggesting a downward band bending
in the MgPc film toward the surface. The similar trend
was previously reported for CuPc [2, 3]. In contrast,
AlClPc exhibited substantially no (or slightly upward[4]) band bending in its thin film. As a result of thisexistence/absence of the band bending, the holetransporting state of MgPc shifted closer to the Fermi
level as approaching the substrate (Fig. 3(a)), which isadvantageous for extraction of holes to the Au
electrode, whereas that of AlClPc are almost constantregardless of the film thickness (Fig. 3(b)).

Fig. 1. Structures of MgPc (left) and AlClPc (right).

Fig. 2. Vacuum level shifts (upper panels) and corelevel peak positions (lower panels) of MgPc (leftpanels) and AlClPc (right panels) thin films on Au
substrates plotted as a function of the film thickness.

Fig. 3. Evolution of the PES spectra of the (a) MgPc
and (b) AlClPc thin films.
[1] C. W. Tang, Appl. Phys. Lett. 48 (1986) 183.
[2] H. Yamane et al., J. Appl. Phys. 99 (2006) 093705.
[3] Y. Nakayama et al., Adv. Energy Mater. 4 (2014)
1301354.
[4] M.-K. Lin et al., Phys. Rev. B 95 (2017) 085425.

1244 Ⅲ04-

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

Effects of Water Exposure on Materials for Organic Light Emitting Diodes
Y. Nakayama, F. Minagawa and M. Hikasa

Department of Pure and Applied Chemistry, Tokyo University of Science, Noda 278-8510, Japan
Deterioration of organic semiconductor materials is
a common drawback for organic electronic devices.
Humidity is widely believed as a major factor that
degrades functionalities of the materials, yet accuratemechanisms how water molecules impact theelectronic states of the organic semiconductormaterials are still not well understood. In the present
works, effects of exposure to H2O on representative
organic materials for light emitting diodes (Fig. 1),2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamino)9,9’-spirobifluorene (spiro-OMeTAD) as a holetransporter [1], tris(8-hydroxyquinolinato)aluminium(Alq3) as an electron transporter and emitter [2], anddipyrazino[2,3-f:2’,3’-h]quinoxaline-2,3,6,7,10,11hexacarbonitrile (HATCN) as a hole injector [3], arestudied by photoelectron spectroscopy (PES).
10 nm-thick spiro-OMeTAD, Alq3, and HATCN
were deposited onto Au/Si substrate in ultra-highvacuum conditions. Each sample was subsequently
exposed to H2O up to the dosage of 108 L (1 L = 1 ×
10-6 Torr∙s) in a step-by-step manner. PES
measurements were conducted at BL2B of UVSOR.
Figure 2 shows variation in the work function valuesof these three materials plotted as a function of dosageof H2O. It was revealed that spiro-OMeTAD exhibits
reduction of the work function by exposure to H2O,
whereas the work function of the Alq3 thin film waspreserved at the pristine value even after a 108 L
dosage of H2O. For the HATCN case, the work
function exhibited irregular jumps, which may beascribed to position by position inhomogeneity of the
HATCN film. However, the similar trend to the case of
spiro- OMeTAD can be recognized, which supports
reduction of the work function of HATCN due toexposure to the ambient atmosphere.

PES spectra in the highest occupied molecularorbital (HOMO) regions of the spiro-OMeTAD and
Alq3 films evolved upon exposure to H2O as shown in
Fig. 3, in which each spectrum is plotted with respect
to the vacuum level. The HOMO positions in these
spectra are substantially maintained independent of
the H2O dosage, namely the water vapor in this dosage
range does not rise any significant effects to theionization energies of spiro-OMeTAD and Alq3. The
Fermi level (EF) separated away from the HOMO ofspiro-OMeTAD in increasing the dosage of H2O,
which may result in a disadvantage for the efficiency
for hole injection into this material from electrodes.

Ⅲ- 4Fig. 2. Work function variation on exposure to H2O.

Fig. 3. Evolution of the PES spectra of 10 nm-thick(a) spiro-OMeTAD and (b) Alq3 thin films upon
exposure to H2O. The EF positions are indicated by
downward arrows on the respective spectra.

Fig. 1. Molecular structures of (a) spiro-OMeTAD,(b) Alq3, and (c) HATCN.

[1]O. A. Jaramillo-Quintero et al., J. Phys. Chem. Lett.
6 (2015) 1883.
[2] C. W. Tang et al., Appl. Phys. Lett. 51 (1987) 913.
[3] S. Park et al., Appl. Phys. Lett. 97 (2010) 063308.

1255 Ⅲ04-

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

Cobalt Oxide Catalyst Containing Glycine Molecule Studied by Operando
O K-edge XAFS Measurement
M. Yoshida1, 2, *, T. Hiue3, M. Nagasaka4, H. Yuzawa4, N. Kosugi4 and H. Kondoh3

Yamaguchi University, Yamaguchi, 755-8611, Japan
Cooperative Research Fellow, Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan
3
Keio University, Yokohama 223-8522, Japan
4
Institute for Molecular Science, Okazaki 444-8585, Japan
1

2

Ⅲ- 4

Electrochemical water splitting using renewableenergies such as solar, wind, and hydroelectric powers
is one of the promising methods for sustainablehydrogen production. In this system, oxygen evolution
reaction (OER) limits the efficiency of overall water
splitting due to the high overpotential. Thus, thedevelopment of highly active OER electrocatalyst is
strongly required. Recently, Nocera and co-workers
reported cobalt-phosphate-based catalyst (Co-Pi)
could function as the efficient OER catalyst [1]. On theother hand, our group found that nickel oxide clusters
were integrated by amino acids and the catalytic
activity was enhanced [2]. Herein, we examined
whether the OER activity was improved by addition oforganic molecules to Co-Pi, and the function of theorganic molecule was investigated using operando
UV-vis absorption, X-ray absorption fine structure(XAFS), and infrared absorption spectroscopy in an
attenuated total reflection mode (ATR-IR).
A Teflon electrochemical cell was equipped with a
Pt wire counter electrode and a Ag/AgCl referenceelectrode for all electrochemical experiments. Cobalt
oxide thin films were electrodeposited on an indium
tin oxide (ITO), a Pt/Pd thin film, or a Au thin film in
potassium phosphate (K-Pi) containing Co(NO3)2 andglycine (Co-Gly). The prepared sample wasinvestigated using operando UV-vis absorption, XAFS,
and ATR-IR.
Constant potential electrolysis was conducted in a
potassium phosphate (K-Pi) aqueous electrolyte. The
OER current of Co-Gly was higher than that of Co-Pi,
indicating that the glycine molecule enhanced the
OER activity of Co-Pi. To assess the film thickness ofthese catalysts, the operando UV-vis absorption
spectra were taken and we found that the glycine
molecule promoted the electrodeposition of cobalt
oxide catalyst. Next, to investigate the local structureof Co species in the Co-Gly, Co K-edge extended Xray absorption fine structure (EXAFS) spectra weretaken for Co-Gly and CoOOH reference powder. This
result suggested that the local structure of Co species
in the Co-Gly was composed of an edge-sharing
CoOOH octahedral clusters a few nanometers in size.
Moreover, to observe the glycine species in Co-Gly,
ATR-IR spectra were obtained and two upward bands
attributed to the adsorbed glycine species wasobserved during the electrodepositon of Co-Gly

catalysts.
Finally, operando O K-edge XAFS spectra for CoGly were taken under electrochemical control withtransmission mode at BL3U in the UVSOR
Synchrotron, according to the previous works [3]. An
absorption peak assigned to CoOOH was observed
around 530 eV. Meanwhile, when the electrodepotential was changed from 0.5 V to 1.0 V, a peak
attributed to the CoO2 speceis was observed around529 eV as the active species for OER process.
In conclusion, we found that the glycine molecule
combined between cobalt oxide clusters composed of
CoOOH octahedral structure and the OER activity wasenhanced because the number of active reaction sites(CoO2) increased.

Fig. 1. Operando O K-edge XAFS spectra for Co-Gly
catalyst.
[1] D. G. Nocera et al., Science 321 (2008) 1072.
[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.

126
Ⅲ04- 6 -Ⅲ -4. Surface, Interface and Thin Films
BL3B
BL3B

Effects of UV Irradiation and Annealing on the Oxygen Vacancies in
Solution-processed IGZO Thin Films
Y. Takamori1, 3, T. Morimoto1, 3, N. Fukuda3 and Y. Ohki1, 2

Waseda University, Tokyo 169-8555, Japan
Research Institute for Materials Science and Technology, Waseda University, Tokyo 169-8555, Japan
3
Flexible Electronics Research Center (FLEC), National Institute of Advanced Industrial Science andTechnology (AIST), Tsukuba 305-8565, Japan1

2

We fabricated solution-processed IGZO thin filmtransistors (TFTs) [1], and found that the drain current
in the TFTs decreases by UV irradiation and recovers
by thermal annealing. In this paper, the mechanism of
this phenomenon was analyzed by photoluminescence(PL) and XPS measurement.
The IGZO precursor solution was prepared bydissolving nitrate hydrates of In, Ga and Zn with amolar ratio of 6:1:3 in a solvent consisting of 2methoxyethanol and 2, 2, 2-trifluoroethanol with avolume ratio of 4:1. This solution was spin-coated on
a 300-nm thick thermally oxidized SiO2 fabricated on
a p-type Si substrate, and sintered at 250 °C on a hot
plate in air for 1 h. Then, Al electrodes were vacuumevaporated onto the IGZO film to fabricate a bottomgate top-contact TFT with a channel length of 50 μm
and a channel width of 500 μm.
Figure 1 shows transfer characteristics of the IGZO
TFTs at drain voltage VD = 40 V. Their drain currents
decrease by 7.21 eV UV irradiation from a Xe2*
excimer lamp (UER20-172, Ushio) and recover by thesubsequent thermal annealing at 250 °C on a hot platein air for 30 min.
Figure 2 shows broad PL spectra of the IGZO films,
excited by 4.6-eV photons at 10 K. The intensity ofthis PL peak centered at nearly 2.25 eV increases by
the UV irradiation and decreases by the subsequent
thermal annealing. This PL originates from oxygen
vacancies [2]. Next, XPS O 1s peaks observed in the
IGZO films were separated into three components by
Gaussian fitting. The lowest component OI at 530.2
eV is due to lattice oxygens. The middle component
OII at 531.7 eV is due to oxygens in oxygen-deficientregions. The highest component OIII at 532.7 eV is due
to loosely bound oxygens on the surface of the films,
such as –CO3, adsorbed H2O, or adsorbed O2 [3].
Figure 3 shows the intensity ratio of each component.
The OII increases by the UV irradiation and decreases
by the subsequent thermal annealing.
We have reported that oxygen vacancies can act aselectron scattering centers and that the increase ofoxygen vacancies occurs as the Ga content in solutionprocessed IGZO films increases. Namely, oxygen
vacancies decrease the drain current [2]. These results
indicate that the decrease in drain current induced by
the UV irradiation is also due to the increase in numberof oxygen vacancies.

Fig. 1. Transfer characteristics of the IGZO TFTs at
VD = 40 V before the UV irradiation (―), after the UV
irradiation for 1 (---), 5 (···), and 10 min (-·-), and after
the subsequent thermal annealing (·-·).

Ⅲ- 4

Fig. 2. PL spectra of the IGZO thin films, excited by
4.6-eV photons at 10 K before the UV irradiation (―),
after the UV irradiation for 1 (---), 5 (···), and 10 min(-·-), and after the subsequent thermal annealing (·-·).

Fig. 3. Intensity ratios of the three components of
XPS O 1s peaks: OI due to lattice oxygens (●), OII due
to oxygens in oxygen-deficient regions (▲), and OIII
due to loosely bound oxygens on the surface of the
IGZO films (■), before and after the UV irradiation for
10 min, and after the subsequent thermal annealing.
[1] S. Ogura et al., Flex. Print. Electron. 1 (2016)
045001.
[2] Y. Ochiai et al., UVSOR Activity Report 44 (2017)
128.
[3] G. H. Kim et al., Thin Solid Films, 517 (2009) 4007.

127
Ⅲ04- 7 -Ⅲ -4. Surface, Interface and Thin Films
BL4B
BL4B

Substrate-symmetry Driven Electronic and Magnetic Modifications
in a Monatomic Layer Iron Nitride
T. Miyamachi1, T. Hattori1, S. Nakashima1, Y. Takagi2, 3, T. Yokoyama2, 3 and F. Komori1

Institute for Solid State Physics, 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

Iron nitrides have a variety of compositions andcrystal structures depending on the nitrogenconcentration. Especially, the iron rich Fe4N phaseattracts great attention toward practical application for
its high coercivity and room temperature
ferromagnetism in the form of thin films [1]. Fe4N thinfilms can be grown on Cu(001) with alternating Feand Fe2N- planes in the fcc Fe sublattice with a N atom
centered in the cubic cell. At the thinnest limit, i.e., amonatomic layer, the Fe2N plane preferentially grows
on the surface and the surface reconstruction from thesquare c(2×2) lattice in bulk to the p4gm(2×2) one
takes place, which is so called clock reconstruction [1,2].
Using scanning tunneling microscopy (STM) andlow energy electron diffraction (LEED), we haverecently revealed the high stability of the p4gm(2×2)
lattice in the monatomic layer Fe4N (Fe2N) due to thestrong bonding between Fe and N atoms [3,4].
Furthermore, with the help of x-ray absorption
spectroscopy/x-ray magnetic circular dichroism(XAS/XMCD) and the first principles calculations, thestrong in-plane magnetic anisotropy observed for the
Fe2N is successfully associated with the change in the
distribution of Fe 3d electrons by the electronic
hybridization with Cu (001) [5]. In this work, taking
the advantage of the robust surface lattice of the Fe2N,
we intend to tune its magnetic properties by changing
the supporting substrate symmetry from fourfold
Cu(001) to threefold Cu(111). This can modify the
degrees of the binding and thus of the electronic
hybridization between the Fe2N and the substrate.
The growth and structural properties of the Fe2N on
Cu(111) were pre-checked by STM before
XAS/XMCD [6]. Likewise on Cu(001) [3-5], N+ ionswith an energy of 500 eV were firstly bombarded tothe clean Cu(111) surface and Fe was additionallydeposited at room temperature. By subsequentannealing at 620 K, well-ordered Fe2N was grown on
the surface. We find that the Fe2N keeps the clock
reconstruction even on Cu(111), while the moiré
pattern additionally appears on the surface due to theslight lattice distortion of the Fe2N. Note that the

sample quality for XAS/XMCD is confirmed from theconsistency of Fe2N LEED patterns in STM and
XAS/XMCD measurements.
The measurements were performed at BL4B in
UVSOR by total electron yield mode at B = ± 5 T and
T = 6.2 K. The XMCD spectra are obtained at thenormal (NI: θ = 0°) and the grazing (GI: θ = 55°)
geometries by detecting μ+ – μ–, where μ+ (μ–) denotes
the XAS recorded at Fe L-adsorption edges with thephoton helicity parallel (antiparallel) to the sample
magnetization. Note that θ is the angle between thesample normal and the incident x-ray.
We find that the XMCD intensity is slightly greaterin the GI geometry than in the NI geometry. Thisreveals that the magnetic easy axis of the Fe2N on
Cu(111) is towards in-plane direction as in the case on
Cu(001). However, the spin magnetic moment of the
Fe2N on Cu(111) is quantitatively evaluated to be ~ 0.5
μB/atom from XMCD sum rules, which is considerably
smaller than that on Cu(001) of ~ 1.1 μB/atom [5].
Furthermore, the magnetization curves of the Fe2N on
Cu(111) recorded in the NI and GI geometries behave
in a similar fashion, while the strong in-plane magnetic
anisotropy is confirmed for the geometry dependenceof the magnetization curve on Cu(001) [5]. The results
indicate that changing the symmetry of the supporting
substrate from Cu(001) to Cu(111) modifies electronic
properties of the Fe2N especially for Fe 3d states, andaccordingly magnetically weakens the Fe2N.
In future work, combining results of XMCD withthose of STM and the first principles calculations, wewill discuss the impacts of the atomic-scale distortionand surface inhomogeneity including moiré
corrugation on the electronic and magnetic propertiesof the Fe2N on Cu(111).
[1]J. M. Gallego et al., Phys. Rev. B 70 (2004) 115417.
[2] Y. Takagi et al., Phys. Rev. B 81 (2010) 035422.
[3] Y. Takahashi et al., Phys. Rev. Lett. 116 (2016)056802.
[4] K. Ienaga et al., Phys. Rev. B 96 (2017) 085439.
[5] Y. Takahashi et al., Phys. Rev. B 95 (2017) 224417.
[6] T. Hattori et al., Phys. Rev. M accepted.

128
Ⅲ04- 8 -Ⅲ -4. Surface, Interface and Thin Films
BL4B
BL4B

Interfacial Spin and Orbital Magnetic Moments and their Anisotropies in
Pd Studied by X-ray Magnetic Circular Dichroism in Co/Pd Multilayers
J. Okabayashi1,*, Y. Miura2 and H. Munekata3

Research Center for Spectrochemistry, The University of Tokyo, Tokyo 113-0033, Japan
Electrical Engineering and Electronics, Kyoto Institute of Technology, Kyoto 606-8585, Japan
3
Laboratory for Future Interdisciplinary Research Science and Technology, Tokyo Institute of Technology,
Yokohama 226-8503, Japan1

2

Ultrathin
Co/Pd
multilayers
areartificial
nanomaterials that exhibit perpendicular magnetic
anisotropy (PMA), due to the spin-orbit interactions,achieved through interfacial chemical bonding.
Extensive efforts have been made for studying interfacesof ultra-thin magnetic multilayers and nanostructures.
Studies on Co atoms performed using X-ray magnetic
circular dichroism (XMCD) have suggested theenhancement of orbital magnetic moments at theinterfacial Co that is adjacent to Pd. It has been reported
that the PMA emerges due to the cooperative effects
between spin moments in 3d TMs and large spin-orbit
interactions in the non-magnetic 4d or 5d TMs. However,
the interfacial PMA, including the anisotropic orbital
magnetic moments, has not been fully understood forboth Co and Pd sites. Bruno and van der Laantheoretically proposed an orbital moment anisotropy in
3d TMs within the second-order perturbation of the spinorbit interaction for more than half-occupied electrons[1]. However, in the case of strong spin-orbit coupling in
4d or 5d TMs, the validity of this perturbative formula
has been debated [2]. In order to study the mechanismsof PMA in Co/Pd multilayers, the orbital magnetic
moments contributions of each element have to be
explicitly considered. It is challenging to study theanisotropy of the orbital magnetic moments of both Co
and Pd elements using one specific experiment, due to thechallenges in detection of the induced magnetic
moments, of Pd in particular.
For the interfacial Pd, the magnetic dipole
contribution through the quadrupole interactions
between dipoles is assessed quantitatively. We focus
on the anisotropy of orbital moments at the Co/Pd
interfaces using XMCD and first-principles density
functional theory (DFT) calculations, which provide
the element- and layer-resolved contributions thatreveal the mechanism of PMA.
We prepared the sample of Co/Pd multilayered
structures: Co (0.69 nm)/Pd (1.62 nm) for PMA withstacking five periods on the Si substrates [3]. Sample
surfaces were sputtered by Ar ions before the XMCD
measurements in order to remove the oxygen
contamination. We performed XMCD experiments at
BL4B, UVSOR, Institute of Molecular Science. Totalelectron yield mode by directly detecting the sample
current was adopted. A magnetic field of ±5 T wasapplied along the direction of the incident polarized
soft x-ray.
We successfully observed clear XMCD signals in Pd

M-edges after the removal of surface contamination asshown in Fig. 1. Although the XAS line shapes overlap
with those of O K-edge absorption, clear XMCD signals
induced by the proximity with Co layers are observed.
The Pd M-edge XMCD line shapes are almost identical,
which suggests isotropic orbital moments in Pd, within
the detection limits. This indicates that the isotropic finite
orbital moments in Pd do not directly contribute to the
PMA. On the other hand, clear Co L-edge XAS andXMCD with angular dependence reveal the enhancementof orbital moments in the surface normal direction
because of PMA.
Next, we discuss the quadrupole-like contributionof the interfacial Pd layer. Our Pd XMCD results
indicate that the Pd orbital moments induced at theinterface are isotropic. Note that the magnetic dipole
term mT is an order of magnitude smaller than theorbital moments, i.e., less than 0.005 μB, comparablewith the detectable limits. Therefore, the relatively
large spin-orbit coupling constant and small Pd
exchange splitting contribute to the appearance of
PMA by means of quadrupole-like interactions
through the interfacial proximity effects Furthermore,these results are well explained by DFT calculations.
We acknowledge to Prof. T. Yokoyama, Dr. Y. Takagi,
and Dr. K. Uemura for the technical supports at BL4B.

Fig. 1. XAS and XMCD of Pd M-edge and Co L-edge
in perpendicularly magnetized Co/Pd multilayer.
[1] P. Bruno, Phys. Rev. B 39 (1989) 865.
[2] C. Andersson et al., Phys. Rev. Lett. 99 (2007) 177207.
[3] K. Yamamoto et al., IEEE TRANSACTIONS ON
MAGNETICS 49 (2013) 3155.
*e-mail: jun@chem.s.u-tokyo.ac.jp

129Ⅲ04- 9 -Ⅲ- 4

Ⅲ -4. Surface, Interface and Thin Films
BL4B
BL4B

Magnetic Properties of Magnetic Ionic Liquid Thin Films Prepared on
Cu(001) and Ferromagnetic Co/Cu(001)
K. Eguchi1, T. Yokoyama2, 3 and K. Awaga1

Department of Chemistry, Nagoya University, Nagoya 464-8602, Japan
Department of Materials Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan3
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585,
Japan
1

2

Ⅲ- 4

Ionic liquids (ILs), which consist of mobile cations andanions at near-room temperature, have attracted muchattention in a wide range of applications because of theirunique properties such as low melting points, nonvolatility, and high capacitance. ILs are generally nonmagnetic; however, incorporating magnetic ions into ILs
at room temperature yields paramagnetic ILs [1, 2].
Magnetic susceptibility measurements have been
performed to understand the magnetic properties ofmagnetic ILs [1, 2]. However, to the best of ourknowledge, their magnetic properties on metal substrates
have not been reported. This study investigates themagnetic properties of 1-ethyl-3-methylimidazolium
tetrachloroferrate ([Emim][FeCl4], Fig. 1(a)) thin films
prepared on Cu(001) using X-ray magnetic circulardichroism (XMCD) measurements. These measurements
were also performed for [Emim][FeCl4] thin films
prepared on ferromagnetic Co/Cu(001) to examine themagnetic interaction between trivalent Fe ions in FeCl4−
and Co thin films at the interfaces.
A commercially available [Emim][FeCl4] (Tokyo
Chemical Industry) was deposited on Cu(001) andCo(3 ML)/Cu(001) [3] at room temperature using apulse valve under ultrahigh vacuum (P ~ 10–7 Pa)
conditions. The thickness of [Emim][FeCl4] was set as
0.8 ML by controlling the opening time of the pulse
valve. X-ray absorption spectroscopy (XAS) and
XMCD measurements were performed using an
XMCD system (P < 1 × 10–8 Pa, B = 7 T, and T = 3.8
K). XMCD spectra were obtained by the total electron
yield detection mode in a magnetic field of ±5 mT or±5 T at a temperature of 6.5 K and an X-ray incidence
angle of 55° with respect to the surface normal.
The [Emim][FeCl4] thin film prepared on Cu(001)showed an XMCD signal at the Fe L-edge in a magnetic
field of 5 T (Fig. 1(b)). The effective spin (mspin) andorbital (morb) magnetic moments of the trivalent Fe ion
in the thin film were calculated using the sum rules[4,5], wherein mspin = 1.82 μB and morb = 0.15 μB; here,
the number of d-holes was assumed as 5. Considering
the Brillouin function at T = 6.5 K and B = 5 T, the valueof mspin is almost identical to the magnetic moment of
2.03 μB for S = 3/2. The magnetic property of the[Emim][FeCl4] thin film prepared on Co/Cu(001) was
similar compared with that prepared on Cu(001),
indicating weak magnetic interactions of Fe with Co at
T = 6.5 K (Figs. 1(c) and 1(d)).

Fig. 1. (a) Chemical structure of [Emim][FeCl4]. Fe
L-edge X-ray absorption and X-ray magnetic circulardichroism spectra of [Emim][FeCl4] on (b) Cu(001)and (c,d) Co (3 ML)/Cu(001).
[1] S. Hayashi and H. Hamaguchi, Chem. Lett. 33(2004) 1590.
[2] Y. Yoshida, et al., Bull. Chem. Soc. Jpn. 78 (2005)
1921.
[3] K. Eguchi, et al., J. Phys. Chem. C 118 (2014)
17633.
[4] B.T. Thole, et al., Phys. Rev. Lett. 68 (1992) 1943.
[5] P. Carra, et al., Phys. Rev. Lett. 70 (1993) 694.

13010 Ⅲ04-

Ⅲ -4. Surface, Interface and Thin Films
BL4B
BL4B

XMCD Measurements on a Magnetic Topological Heterostructure:
Mn and Te Deposited Bi2Te3
S. Kusaka1, K. Yokoyama1, Y. Uemura2, T. Yokoyama2 and T. Hirahara1

Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
Department of Materials Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan
1

2

Topological insulators (TI) are extensively studied
recently due to its peculiar properties [1]. The Diraccone surface states of TI are protected by time-reversal
symmetry (TRS) and backscattering among thesesurface states is prohibited. But when TRS is broken
by application of a magnetic field or incorporating
magnetic materials, a gap opening in the Dirac cone is
expected and an intriguing phase called the quantum
anomalous Hall state can be realized [2]. This phase is
expected to show even more exotic phenomena suchas the topological magnetoelectric effect. To realize
such state, two types of sample fabrication techniques
have been employed up to now: (1) magnetic doping
while growing the single crystal or thin film of TI [3],and (2) magnetic impurity deposition on the surface ofTI [4]. While method (1) was successful and showed
evidence of the TRS violation, no one has succeededusing method (2), which should be a more direct way
to examine the interaction between the topologicalsurface states and magnetism. Recently, we havesucceeded in inducing a gap in the TI Dirac surface
states by a new method called ‘magnetic extension’ [5].
An ordered Mn layer was incorporated into the
topmost quintuple layer of a prototypical TI, Bi2Se3 by
Mn and Se deposition and a MnBi2Se4/Bi2Se3
heterostructure was formed. Due to the large wave
function overlap between the Dirac cone and theferromagnetic Mn layer, a large gap of 85 meV wasobserved and its origin was ambiguously assigned to
the ferromagnetic nature of MnBi2Se4 as proven by
XMCD measurements [6].
We have recently found that we can also employ
magnetic extension to break the time reversal
symmetry of another prototypical TI, Bi2Te3. We
deposited Mn and Te on Bi2Te3 and found a clear gap
opening of ~ 70 meV at the Dirac point. In the present
work, we have performed XMCD measurements at
BL-4B to experimentally prove that this system isferromagnetic. Figure 1 shows the X-ray absorption(XAS) spectra and the corresponding XMCD spectra
at 5 T (~5 K). One can find clear XMCD peaks at theMn L3 and L2 edges. The measurements were also
performed at remanence, and the result is shown in Fig.
2. Although very weak, one can find a dip structure at
the L3 edge and this shows that this system is indeed
ferromagnetic.

L3

5Tµ+ XASµ- XAS
XMCD

L2

620

630

640
650
Photon energy(eV)660

Fig. 1. XAS and XMCD spectra at 5 T.

0 T (residual magnetization)µ+ XASµ- XAS
XMCD

620

630

640
650
Photon energy (eV)660

Fig. 2. XAS and XMCD spectra at 0 T (residual
magnetization).
[1] M. Hasan and C. Kane, Reviews of Modern
Physics 82 (2010) 3045.
[2] X.-L. Qi and S.-C. Zhang, Reviews of Modern
Physics 83 (2011) 1057.
[3] For example, C. Z. Chang et al., Science 340 (2013)
167.
[4] For example, M. Ye et al., Physical Review B 85
(2012) 205317.
[5] M. Otrokov et al., 2D Mater. 4 (2017) 025082.
[6] T. Hirahara et al., Nano Letters 17 (2017) 3493.

13111 Ⅲ04-

Ⅲ- 4

Ⅲ -4. Surface, Interface and Thin Films
BL4B
BL4B

Oxygen Induced Magnetic Transition in Single Layer Iron Film
1

Ⅲ- 4

T. Nakagawa1, T. Nomitsu1, S. Mizuno1 and T. Yokoyama2

Department of Molecular and Material Sciences, Kyushu University, Kasuga, Fukuoka 816-0955, Japan
2
Institute for Molecular Science, Okazaki 444-8585, Japan

Molecular adsorption on magnetic thin films
changes the magnetic anisotropy and themagnetization easy axis through the modification of
the magnetic orbital moment, and thus induces a spin
reorientation transition (SRT). It is not clear, however,whether molecular adsorption induces magnetictransitions, except the SRT, such as a ferromagnetic to
antiferromagnetic transition since the exchangeinteraction is larger compared with the magnetic
anisotropy energy.
Experiments were performed in the x-ray magnetic
circular dichroism (XMCD) endstation with the basepressures of 4×10-8 Pa for sample preparation andbetter than 1×10-8 Pa for XMCD measurement. Single
Fe layers were grown on W(110). Oxygen was dosedwith the sample kept at the room temperature, via a
variable leak valve at 2×10-7 Pa. The Fe films exposed
up to 150 L(1 L = 1.3×10-4 Pa s) oxygen were
examined by XMCD. The oxygen coverage wasestimated by the appearance of an ordered 3×2
structure with 0.33 ML oxygen on Fe/W(110) using
low energy electron diffraction [1]. Fe L edge XMCD
was taken with a circularly polarized light (circularly
polarization ~ 0.6) at H = ±5 T at a sample temperatureof 6 K. NEXAFS spectra were measured at H = 0 T.
All the measurements were performed for a grazing
incidence angle of 30° from the surface.
Figure 1 (a) shows magnetization curves measured
at Fe L edge. On the clean single layer Fe, themagnetization curve exhibits hysteresis and a large
coercivity of ~ 2 T. With increasing the oxygen
coverage, the magnetization and the coercivity
decrease up to 0.33 ML oxygen coverage. At 0.33 ML,
the Fe film is not ferromagnetic, and it is likely
paramagnetic. The oxygen adsorption of 0.6 ML
restores the Fe film to the ferromagnetic state with ~2
T coercivity. Further increase of the oxygen coverage,
leading to the formation of iron oxide, transforms theFe film to a non-ferromagnetic state.
In order to investigate the origin of the magnetictransition, we have measured NEXAFS spectra of O K
edge with increasing the oxygen coverage. Figure 1(b)shows NEXAFS for the oxygen coverage of 0.33, 0.6
and 1 ML. For all the coverages studied the oxygen
dissociates on the Fe film. At 0.33 ML, the peak at 530
eV is assigned to the transition to O(2p). The peakshifts to a higher energy with increasing the oxygen
coverage. This energy shift is due to the strong
hybridization between O 2p and Fe 3d, which would
drive the transition from the paramagnetic toferromagnetic state.

Fig. 1. (a) Magnetization curves measured at Fe-L
edges. (b) NEXAFS for oxygen K edge. Indicatedinside are the oxygen coverages.
[1] K. Feindl et al., Surf. Sci. 617 (2013) 183.

13212 Ⅲ04-

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

Change in the Electronic Structure of Fe2P(10-10) Induced by Phosphorus
Segregation: Soft X-ray Photoemission Spectroscopy Study
H. Motoyama1, Y. Sugizaki1, Y. Shimato1, T. Yoshida1, T. Takano1 and K. Edamoto1, 2
Department of Chemistry, Rikkyo University, Tokyo 171-8501, Japan
Research Center for Smart Molecules, Rikkyo University, Tokyo 171-8501, Japan1

2

The surface properties of transition metal
phosphides (TMPs) have attracted much attention
because of their high catalytic performance for HDS
and HDN for petroleum fuels. It has been found that
Ni2P has the highest catalytic activity among TMPs
while that of Fe2P is extremely low [1]. Therefore, inorder to elucidate the origin of the high catalytic
activity of TMPs, it is useful to explore the differencein the surface electronic structures of Ni2P and Fe2P. In
this work, we investigated the phosphorus segregation
process and its effect on the electronic structure of
Fe2P(10-10).
Our previous Auger electron spectroscopy (AES)
study revealed that the P atoms on Fe2P(10-10) areselectively removed by Ar+ ion sputtering. When the
sputtered surface is subsequently annealed, theintensity ratio of P LMM and Fe MVV peaks in AES
spectra decreases with elevating annealing
temperature up to 400 °C, and increases attemperatures higher than 400 °C, as shown in the inset
of Fig. 1. The P 2p spectrum of Fe2P(10-10) after Ar+
ion sputtering, which was measured under a highly
surface sensitive condition (hν= 170 eV), is shown in
Fig. 1. The spectrum can be fit by seven components
denoted as α – η. The δ peak is assigned as a bulk
component [2]. The analyses of annealing temperaturedependence of the spectra show that the ε’, ζ, η peaks
should be associated with weakly bonded surface
species which would be formed through the sputtering,and α, β, γ, ε peaks are associated with surface P atoms
segregated from the bulk. (The ε’ and ε peaks arethought to be observed in the same binding energy
region accidentally.) The former species are easily
removed by annealing at lower than 400 °C, and thelatter species are increased in intensities by annealing
at higher than 300 °C, which are consistent with the
AES results shown in the inset of Fig. 1.
Figure 2 shows the valence band spectra of Fe2P(1010) after Ar+ ion sputtering and subsequent annealing
at various temperatures at 300 – 700 °C. The band
observed at 0 – 4 eV is associated with the Fe 3d – P
3p hybrid band [2]. As shown in Fig. 2, the spectral
shape of the band in the vicinity of EF is not changedby the segregation of P atoms induced by heating
above 400 °C, suggesting that the Fe 3d states are notstabilized by the bonding with segregated P atoms.
This is contrary to the case of Ni2P surfaces where theNi 3d states are stabilized through the bonding withsegregated P atoms forming a pseudo gap around EF[3]. These results suggest that the stabilization of theactive metal sites through the bonding with P atom is
crucial to maintain the activity during HDS throughpreventing the accumulation of S atoms on active
metal sites.

Fig. 1. The P 2p spectrum of Fe2P(10-10) fit by Voigt
functions. The change in the P LMM and Fe MVV
Auger peak intensity ratio as a function of annealing
temperature is shown in the inset.

Fig. 2. Valence band spectra of Fe2P(10-10) after Ar+
ion sputtering and subsequent annealing at various
temperatures at 300 – 700 °C.
[1] S. T. Oyama et al., Catal Today 143 (2009) 94.
[2] Y. Sugizaki et al., Surf. Sci. 664 (2017) 50.
[3] K. Edamoto, Appl. Surf. Sci. 269 (2013) 7.

13313 Ⅲ04-

Ⅲ- 4

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

Valence-Band Electronic Structures of Ultra-Thin Fe Layers
on Rashba-Splitted Au (111) Surfaces
1

J. Okabayashi1*, K. Tanaka2 and S. Mitani3

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

2

Ⅲ- 4

When ferromagnetic transition metals (TMs) aredeposited on the Rashba-type spin-orbit coupledsurface, novel properties such as perpendicularmagnetic anisotropy (PMA) are emerged at theinterfaces, which are derived from the symmetry
broken spin-orbit effects. The gold Au (111) surfaces
have been investigated extensively by means ofscanning tunneling microscopy and angle-resolved
photoemission spectroscopy (ARPES) because thissurface exhibits the large Rashba-type spin-orbit
splitting. Large spin-orbit interaction in the heavy
element of gold provides the wide varieties for the
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
LaShell et al. [1]. Recently, the interfaces between
Au(111) and other heavy elements such as Bi or Ag
have been extensively investigated [2,3]. Here, we
focus on the interfaces 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, toclarify the origin of the PMA at these interfaces is a
crucial role. The relationship between Au (111)
Rashba-type spin-orbit interaction and PMA in 3d
TMs has not been clarified explicitly. In order toinvestigate the orbital- resolved states in the Fe films
showing the PMA, ARPES at the interfaces becomespowerful techniques through the photon-energy andpolarization 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 theresearches of novel PMA using spin-orbit coupledinterfaces.
We prepared the clean Au (111) surface for the Fe
deposition. The commercialized single-crystalline
100-nm-thick Au (111) films on Mica were used. At
the BL5U in UVSOR, we repeated the Ar-ion
sputtering at 1-2 kV accelerating voltage andsubsequent annealing at 400 °C under the high vacuum
conditions. After the sample preparation, the Fe 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 45 and 120 eV because of the large
cross-section of Au (111) surface states and detectionsof Fe 3p and Au 4f intensity ratios, respectively.
Figure 1 displays the Fe deposition dependence on
Au (111). As shown in Fig. 1a, the intensity ratio
between Fe 3p and Au 4f peaks systematically changes.
The Au (111) surface clearly exhibits the Rashba-typesurface states (Fig. 1c). The Fe 3d states appear near
the Fermi level (EF) and the surface states disappear as
shown in Fig. 1d. As shown in Fig. 1b, with increasing
the Fe layer thickness, spectral line shapes cannot be
explained by the overlap between Fe and Au,
suggesting that the chemical bonding at the interface
is modulated from the Au (111) surface and Rashbatype spin-orbit interaction affects to the Fe layer.
Therefore, the interfacial chemical reaction between
Fe and Au brings the novel properties on the ultrathin
Fe 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. Photoemission spectra for various Fe depositedlayers. a. Angle-integrated wide-range spectra taken at
120 eV. b. valence-band spectra taken at 45 eV.
c. ARPES near EF in Au surface taken at 45 eV.
d. ARPES near EF after 2 ML Fe deposition taken at 45 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

13414 Ⅲ04-

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

Photoelectron Angular Distribution Maps of Perfluoropentacene on Ag (111)
T. Yamaguchi1, 2, S. Ouchi3, M. Meissner1, T. Ueba1, 2, R. Shiraishi1, 2, S. Ideta1, 4, K. Tanaka1, 4
and S. Kera1, 2, 3
1School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585
2
Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan
3
Graduate School of Engineering, Chiba University, Chiba 263-8522, Japan
4
UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

The interface between a molecular semiconductor
and a metal greatly affects the electronic properties ofthe molecular film and it is thus of great importance tofundamentally study the large variety of effects taking
place at such an interface. Photoelectron angular
distribution (PAD) maps generated from angleresolved ultraviolet photoemission spectroscopy(ARUPS) provide insight into the orbital shapes ofadsorbed molecules. By comparing experiments withtheoretical data allows us to discuss the origin ofmodifications of orbitals and the implications for theelectronic properties of the film. Here, we aimed to test
the suitability of BL5U with the deflector mode forthis purpose, using monolayers of perfluoropentacene(PFP) adsorbed on Ag(111).
We prepared monolayers of PFP on Ag(111) byannealing slightly thicker films at 130 °C for 10 min.
These samples were then measured at room
temperature as well as at 5 K, using different photon
energies of 60 eV and 120 eV. An exemplary PAD map
is shown in Fig. 1a for a sample that is tilted away fromthe normal-emission geometry, such that the Γ point is
outside the upper left corner of the image. The arc-likefeature is part of the Ag sp-band, whereas the featurein the center belongs to the second-highest occupiedΓ𝐾𝐾 direction ofmolecular orbital (HOMO-1). The ����Ag(111) is aligned horizontally.
By probing the photoelectrons at different binding
energies, we generated three-dimensional energyversus-angle data, enabling us to discuss spectroscopic
details and dispersion behavior of the films. Fig. 1bshows a cut through that data in the direction indicatedby the red line in panel a. The HOMO and HOMO-1
are marked by horizontal lines.
Building on experiences from a previous beamtime
at BL5U, we aimed to reduce film damages due to theintense beam by measuring during single-bunch
operation. Obviously, however, there are low-energy
shoulders next to both the HOMO and HOMO-1,
which is a typical indication of molecules broken up
by the irradiation. Moreover, the field of view (FoV)of this setup is quite limited compared to the angulardimensions of typical molecular orbitals. It issufficient to discuss fine features of individual orbital
lobes. However, a stitching of images taken under
different rotation/tilting angles of the sample in orderto obtain a more complete PAD map is hardly possible
or meaningful due to the changing beam incidence
angle.

Fig. 1. Angle-resolved photoelectron map of HOMO-1
for monolayer PFP on Ag(111) at hv= 60 eV. The blueline marks the cut direction for the image in Fig. 2.

Fig. 2. Binding energy versus emission angle in the
direction highlighted in Fig. 1, overlayed by thehorizontally integrated energy distribution curve. The
HOMO and HOMO-1 of PFP are marked by horizontal
dashed lines.

13515 Ⅲ04-

Ⅲ- 4

Ⅲ -4. Surface, Interface and Thin Films
BL5U, BL7U
BL5U, BL7UElectronic Structure of Mn and Te Deposited Bi2Te3:
A New Magnetic Topological Heterostructure
K. Yokoyama1, Y. Okuyama1, S. Ideta2, K. Tanaka2 and T. Hirahara1

Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, JapanUVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
1

2

Ⅲ- 4

Topological insulators (TI) are extensively studied
recently due to its peculiar properties [1]. The Diraccone surface states of TI are protected by time-reversal
symmetry (TRS) and backscattering among thesesurface states is prohibited. But when TRS is broken
by application of a magnetic field or incorporating
magnetic materials, a gap opening in the Dirac cone is
expected and an intriguing phase called the quantum
anomalous Hall state can be realized [2]. This phase is
expected to show even more exotic phenomena suchas the topological magnetoelectric effect. To realize
such state, two types of sample fabrication techniques
have been employed up to now: (1) magnetic doping
while growing the single crystal or thin film of TI [3],and (2) magnetic impurity deposition on the surface ofTI [4]. While method (1) was successful and showed
evidence of the TRS violation, no one has succeededusing method (2), which should be a more direct way
to examine the interaction between the topologicalsurface states and magnetism. Recently, we havesucceeded in inducing a gap in the TI Dirac surface
states by a new method called ‘magnetic extension’ [5].
An ordered Mn layer was incorporated into the
topmost quintuple layer of a prototypical TI, Bi2Se3 by
Mn and Se deposition and a MnBi2Se4/Bi2Se3
heterostructure was formed. Due to the large wave
function overlap between the Dirac cone and theferromagnetic Mn layer, a large gap of 85 meV wasobserved [6].
In the present work, we have tried to employ
magnetic extension to break the TRS in anotherprototypical TI, Bi2Te3. We deposited Mn and Te on
Bi2Te3 and measured its electronic structure. Figures 1(a) and (b) show the band dispersion of the surface
states of the pristine Bi2Te3 and the sample with Mn
and Te deposited on top measured at 30 K (hν = 8 eV).
The Dirac-cone surface states are massless in (a) withno gap. On the other hand, one can find a clear gap of~ 70 meV at the Dirac point in (b) as indicated by theenergy distribution curve, meaning that it has turned
into in a massive Dirac cone. This result shows that wehave successfully induced TRS breaking in the surface
states of Bi2Te3. However, comparison with ab initio
calculations [5] showed that the formed structure is not
MnBi2Te4/Bi2Te3 and a structural study is under way todetermine the real crystal structure of the system.

Fig. 1. The band dispersion of Bi2Te3 (a), and thatwith Mn and Te deposition (b), respectively, measured
at 30 K with hν = 8 eV photons of p-polarization.
[1] M. Hasan and C. Kane, Rev. Mod. Phys. 82 (2010)3045.
[2] X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. 83(2011) 1057.
[3] For example, C. Z. Chang et al., Science 340 (2013)
167.
[4] For example, M. Ye et al., Phys. Rev. B 85 (2012)205317.
[5] M. Otrokov et al., 2D Mater. 4 (2017) 025082.
[6] T. Hirahara et al., Nano Letters 17 (2017) 3493.

13616 Ⅲ04-

Ⅲ -4. Surface, Interface and Thin Films
BL5B
BL5B

Optical Properties of Amorphous Arsenic Trisulfide
K. Hayashi

Department of Electrical, Electronic and Computer Engineering, Gifu University, Gifu 501-1193, Japan
Amorphous chalcogenide semiconductor materials,
such as a-As2S3, a-As2Se3 and a-Se etc., show a varietyof photo-induced phenomena. These materials are
expected as materials for optoelectronic devices. A lot
of work have been done on the photo-induced
phenomena of these amorphous semiconductormaterials and various mechanisms have been proposed
for these photo-induced phenomena [1-3]. However,the details of the mechanisms are still unknown. For
device applications, it is necessary to sufficiently
understand the fundamental properties of thesematerials. The interest has been attracted for thechange of the optical properties in the energy region of
the visible light. We are interesting for the changes ofthe optical properties in the higher energy region. To
our knowledge, little attention has been given to
photo-induced changes at the vacuum ultra-violet(VUV) absorption spectrum. In previous reports, wereported the photo-induced change at VUVtransmission spectra of as-deposited evaporated
amorphous As2Se3 thin films [4]. In this report, wemeasure the VUV transmission spectra on asdeposited evaporated amorphous As2S3 thin films.
Samples used for the measurement of the VUVtransmission spectra were amorphous As2S3 thin films
prepared onto aluminum thin films by conventional
evaporation technique. Typical thickness of theamorphous film and the aluminum film were around200 nm and 100 nm, respectively. The measurements
were carried out at room temperature at the BL5B of
the UVSOR Synchrotron Facility of the Institute for
Molecular Science. And the spectrum was measured
by using the silicon photodiode as a detector. Two
pinholes of 1.5 mm in a diameter were inserted
between the monochromator and sample to remove
stray light. The intensity of the VUV light wasmonitored by measuring the total photoelectron yieldof a gold mesh.
Figure 1 shows the transmission spectra of asdeposited a-As2S3 thin film at room temperature in the
wavelength range (a) 7.5-8.9nm and (b) 25-30nm.
Several peaks were observed at this wavelength range.
The absorption peaks around 7.7nm correspond to the
2p core level of S atom. Other peaks correspond to the
3p and 3d core level of As atom. The broad structures
observed in the absorption spectra. The origin of eachcomponent is not clear now. I would like to clarify theorigin of each component in the future. Further
analysis of these spectra is now in progress. We pay
attention to the photo-induced effects near thiswavelength range. We now are investigating photoinduced change on these spectra. The detailed
experiments and analysis will be done in the next step.

Ⅲ- 4

Fig. 1. The transmission spectra of as-deposited aAs2S3 thin film in the wavelength range (a) 7.5-8.9nm
and (b) 25-30nm.
[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 2014 42(2015) 134.

13717 Ⅲ04-

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

Band Engineering of Silicene-Ge Alloy
1

Ⅲ- 4

A. Fleurence1, T. Yonezawa1, H. Yamane2, N. Kosugi2 and Y. Yamada-Takamura1

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

Alloying plays an important role for applications as
it gives solution to tune the properties of materials. For
instance, the band gap can be tuned in bulk SiGe alloy
by means of the concentration of Ge atoms [1].
Silicene is a graphene-like buckled honeycomb
structure made of silicon atoms featuring π bands at
the K points of its Brillouin zone [2]. As a twodimensional material, silicene is of high interest for Sibased nanoelectronics.
Silicene only exists in epitaxial forms. Among them,epitaxial silicene on ZrB2 thin films grown on Si(111)
has the particularity to form in a spontaneous and selfterminating way by segregation of Si atoms from thesilicon substrate [3]. Epitaxial silicene adopts asilicene-(√3×√3) reconstructed unit cell in which 5 Si
atoms are in the same plane and a single atom is
protruding. This silicene is semiconducting as a 350
meV-wide gap is opened in the Dirac cones [3].
We demonstrated by means of scanning tunnelingmicroscopy at our home laboratory that the depositionof germanium on silicene can give rise to a siliceneGe alloy resulting from the incorporation of Ge atoms
into the silicene lattice [4].
The experiments done at the BL6U of UVSOR were
aimed at determining the evolution of the band
structure of this silicene-Ge alloy with theconcentration of Ge atoms.
Water-cooled mini K-cell, which we used for theexperiments in our home-based setup, was installed in
the preparation chamber of the end station forevaporating Ge. Epitaxial silicene samples wereprepared on-site by annealing ZrB2/Si(111) samples at
800 oC under ultrahigh vacuum [2]. Ge growths wererealized with a substrate temperature of 350 °C. All
photoemission spectra were recorded at 15 K.
The core-level of Fig. 1(a) shows a single Ge 3dcomponent which indicates that the Ge atoms areincorporated in a unique site in the silicene lattice. The
saturation of this peak for a Ge coverage of about 1/6
monolayer (ML) silicene points out that Ge atoms
occupy the protruding sites in the silicene lattice. More
importantly, a clear effect of the Ge atoms on the band
structure was found as the top of the valence band
shifts up monotonously with the concentration. Figs.
1(c) and (d) compare angle-resolved photoemission(ARPES) spectra of pristine silicene and of siliceneGe alloy at saturation. In the latter, the top of the bandis shifted by 160 meV above that of pristine silicene.
These experiments provide an experimental
demonstration of the possibility of band gap
engineering in SiGe two-dimensional materials as fortheir bulk counterparts.
This work was supported by the Joint Studies

Program (No. 206, 2017-2018) of the Institute for
Molecular Science.

Fig. 1. (a): Core-level spectra (hν=80 eV) in the Zr 4p
and Ge 3d region before and after deposition of 0.04
ML Ge on silicene. (b): Evolution of the Ge3d peakwith the Ge coverage. (c) and (d) ARPES spectra(hν=45 eV) of pristine silicene and after deposition of
0.18 ML Ge.
[1] J. C. Bean, Proc. IEEE 80 (1992) 571.
[2] K. Takeda and K. Shiraishi, Phys. Rev. B 50 (1994)
14916.
[3] A. Fleurence et al., Phys. Rev. Lett. 108 (2012)
245501.
[4] Y. Awatani, et al., Bulletin of the American PhysicalSociety, 2016 March Meeting, T1.307.

138
Ⅲ04- 18 -Ⅲ -4. Surface, Interface and Thin Films
BL6U
BL6U

Self-Encapsulation of Epitaxial Silicene on ZrB2 under a h-BN Monolayer
School of Materials Science, Japan Advanced Institute of Science and Technology, Nomi 923-1292, JapanMESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands
3
Institute for Molecular Science, Okazaki 444-8585, Japan
1

2

A. Fleurence1, T. Yonezawa1, M. P. de Jong2, M. W. Ochapski2, H. Yamane3,
N. Kosugi3 and Y. Yamada-Takamura1

Silicene, as a graphene-like silicon two-dimensional
material has great expectation to give solution to scale
down further the Si-based nanotechnologies. Even if afirst transistor based on silicene could be fabricated [1],its quick oxidation in air indicates that silicene
encapsulation is a crucial point to address forprocessing it routinely and fabricate reliable devices.
Silicene only exists in epitaxial forms. Among them,epitaxial silicene on ZrB2 thin films grown on Si(111)
has the particularity to form in a spontaneous and selfterminating way by the segregation of Si atoms fromthe silicon substrate [2].
It is also possible to fabricate a monocrystalline hBN single layer covering the entire ZrB2 thin film
surface [3]. The deposition of silicon on this hBN/ZrB2 surface gives rise to a surface structure
resembling that of epitaxial silicene. Si2p core-leveland band structure, indicate that a silicene layer similar
to that observed on bare ZrB2 surface intercalates
between the ZrB2 thin film and the h-BN layer.
The purpose of these experiments at BL6U was todetermine whether the h-BN layer is capable ofpreventing the oxidation of silicene.
Epitaxial silicene and oxide-free h-BN samples
were prepared on-site by annealing at 800 oC underultrahigh vacuum ZrB2/Si(111) [2] and hBN/ZrB2/Si(111) samples we elaborated at our homelaboratory. Silicon was evaporated by heating a pieceof silicon in front of the h-BN/ZrB2 surface held atroom temperature (RT). Core-level spectra in the Si2p
region were recorded with a surface-sensitive photon
energy of hν=130 eV at RT.
The comparison of the core-level spectra of pristinesilicene before and after a 5 min exposure to air (Figs.
1(a) and (b)) shows that the typical spectrum ofsilicene has turned into a Si oxide peak, which
confirms the destruction of silicene in air. The
spectrum recorded after deposition of silicon on hBN/ZrB2 is visible in Fig. 1(c). It differs from that ofFig. 1(a) due to an excess of silicon forming
amorphous islands. Most importantly, after exposure
to air with duration up to 60 min (Figs. 1(d)-(e)), thesilicene component remains while a small oxide peak
appears. It can be noticed that the line shape of thesilicene component now more closely resembles thatof pristine silicene, which indicates that the siliconoxide peak corresponds to the excess of Si.
The fact that the spontaneously encapsulated
silicene survived an exposure long enough to allow fortaking silicene in air opens the way to the possibilityof processing silicene ex-situ in a controlledenvironment.
This work was supported by the Joint Studies
Program (No. 206, 2017-2018) of the Institute for
Molecular Science.

Ⅲ- 4

Fig. 1. (a) and (b): Normalized core-level spectra(hν=130 eV) in the Si2p region before and after a 5
min exposure to air of unencapsulated silicene. (c):Core-level spectrum of intercalated silicene. (d) and(e): Core-level spectra of intercalated silicene
recorded after 5 and 60 min exposures to air.
[1] L. Tao et al., Nature Nanotech. 10 (2015) 227.
[2] A. Fleurence et al., Phys. Rev. Lett. 108 (2012)
245501.
[3] Z.-T. Wang et al., J. Appl. Phys. 100 (2006) 033506.

139
Ⅲ04- 19 -Ⅲ -4. Surface, Interface and Thin Films
BL6U
BL6U

Thin-film Structure Depending on the Sulfur Substitution of Phenanthrodithiophene Isomers
S. Ouchi1, C. Numata1, T. Yamaguchi2, 3, M. Meissner3, T. Ueba2, 3, H. Yamane2, 3,
N. Kosugi2, 3, H. Yoshida1, K. Hyodo4, Y. Nishihara4 and S. Kera1, 2, 3

Graduate School of Engineering, Chiba University, Chiba 263-8522, Japan
2
Institute for Molecular Science, Okazaki 444-8585, Japan
3
The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan
4
Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan1

Ⅲ- 4

Even a subtle difference of intermolecularinteraction can impose a large change in molecularordering and electronic properties at organic
semiconductor/metal interface. We examined two
picene analogues, phenanthro-dithiophene isomers(PDT1 and PDT2), where the sulfur atoms aresubstituted differently as shown in Fig.1. We
investigated the effect of the isomeric structure on thethin film structure as well as the electronic state.
The Au(110) substrate was cleaned by the repeatedcycles of the Ar-ion sputtering and the subsequentannealing at 873 K. The monolayer (ML) films wereprepared by the vacuum deposition onto the clean
Au(110) surface at 300 K. The deposition rate was
0.05 nm/min. After depositing the ML, the PDT1 andPDT2 samples were annealed at 373 K.
Figure 2 shows the LEED images of PDT1 andPDT2 MLs on Au(110). Before the annealing, no sharp
spots appear, while the sharp spots appear after theannealing for the PDT1 film. On the other hand,exhibits the stripe pattern for both as-grown andannealed films. The observed LEED images indicatethat the PDT1 molecules on Au(110) form a longrange ordered structure in two directions, while the
PDT2 molecules on Au(110) form it in only onedimensional ordered structure. This difference iscaused by a different intermolecular interaction
assisted by the largaer permanent dipole of PDT1.
Figure 3 shows the comparison of XPS spectra of
the MLs of PDT1 and PDT2 (hv = 335 eV). The upper
panel (a) show the spectra before the annealing and thebottom panel (b) show the one after the annealing.
While no differences are dicernible between PDT1 andPDT2 for the as-grown film, two changes are clearlyobserved in the spectra upon the annealing. First, thebinding energy of PDT1 is smaller than that of PDT2
by about 100 meV in S2p spectra. Second, comparing
C1s spectra, we also observed the binding energy shiftof 30 meV and the different line shapes between PDT1
and PDT2. The observed XPS spectra indicate that theinteraction between gold and sulfer of the PDT1 isstronger than that of the PDT2.
Understanding the relationship between thesubstitution position of the sulfur and the thin film
structure more precisely, we would discuss the impactsof the thin film structure on the electronic states.

Fig. 1 Molecular structures of PDT1 (left) and PDT2
(right).
(a)(b)

Fig. 2 Comparison of LEED images of PDT1 (left)
and PDT2 (right). (a) before the annealing. (b) after theannealing.
(a)

(b)

Fig. 3 Comparison of XPS spectra of PDT1(red) and
PDT2 (blue) MLs on Au(110). (a) before the annealing.
(b) after the annealing (S2p and C1s peaks are alignedby 100 meV and 30 meV, respectively).

140
Ⅲ04- 20 -Ⅲ -4. Surface, Interface and Thin Films
BL6U
BL6U

Effects of Surface Adsorbates on the Electronic Structure of
Excitonic Insulator Ta2NiSe5
K. Fukutani1, R. Stania1, J. Jung1, 2, H. Yamane3, N. Kosugi3 and H. W. Yeom1, 2Center for Artificial Low Dimensional Electronic Systems, Instutute for Basic Science (IBS), Pohang 37673,
Republic of Korea
2
Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of
Korea
3
Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan
1

Bound pairs of conduction band electrons andvalence band holes, so-called excitons, can be formed
within solids as a meta-stable state induced by external
excitations, such as light irradiation. On the other hand,
it has also been proposed theoretically for more thanhalf a century that, for narrow-gap semiconductors andsemimetals, a finite number of excitons can condense in
the ground state to form what is called an excitonic
insulator [1, 2]. However, due to number ofexperimental difficulties, the direct elucidation of such
excitonic condensate was revealed only recently [3].
Ta2NiSe5 is among the few materials that areproposed to be excitonic insulators, and a number ofstudies have recently been performed to explore thenature of excitonic phase of this material, including thepresence of many-body band gap near the Fermi level[4], band gap tuning by chalcogen substitutions [5],and layer-confined nature of excitonic phases [6].
Our previous studies on Ta2NiSe5, utilizing the lowenergy photon source, revealed that some surface
adsorbates on this material induce a renormalization of
the excitonic band structure. While the small excitonic
radii in the surface-normal direction and their, layerconfined nature likely makes it possible for the surface
adsorbates to modify the excitonic insulator phase at
the topmost surface, the detailed mechanisms bywhich the renormalization occurs required further
investigation.
In our experiments conducted at BL6U at UVSOR,
the excitonic insulator phase of Ta2NiSe5 wasinvestigated using the angle-resolved photoelectron
spectroscopy (ARPES). As shown in Fig. 1(a), theunusual flattening of the topmost valence band isobserved, which is believed to be a signature of theexcitonic condensate for T < Tc. Our ARPES data also
reveal the band structure perpendicular to the quasi-onedimensional atomic chain as shown in Fig. 1(b)indicating small, yet finite inter-atomic chaininteractions. Upon adsorption of potassium (K) as
surface dopants, the Ta 4f core levels are seen to shift
towards higher binding energy by ~40 meV (see Fig. 2),
indicating that K adsorption results in the electrondoping at the surface. While further investigations,
including model calculations, are required to makedefinite conclusions, the experimental results obtainedat BL6U likely indicate the change in the free carrier
concentration induced by the surface dopants screens
the attractive Coulomb interaction between theelectrons and holes, thereby modifying the binding
energy of excitons at condensate.

Fig. 1. (a) The ARPES band mapping taken at photon
energy of hv = 60 eV along the Γ-X direction (parallelto the quasi-one-dimensional atomic chain) and (b)along the Γ-Y direction (perpendicular to the atomic
chain). The sample temperature was maintained at T =40 K.

Fig. 2. The evolution of the Ta 4f core level spectrawith upon K adsorption, taken at hv = 60 eV, T = 40 K.
The both spin-orbit split pair of Ta 4f core levels areseen to move towards the higher binding energy with
K adsorption, indicating the n-type doping on thesample surface.
[1] N. F. Mott, Phil. Mag. 6 (1961) 287.
[2] W. Kohn, Phys. Rev. Lett. 19 (1967) 439.
[3] C. Monney et al., Phys. Rev. B 79 (2009) 045116.
[4] Y. Wakisaka et al., Phys. Rev. Lett. 103 (2009)026402.
[5] Y. F. Lu et al., Nat. Commun. 8 (2017) 1.
[6] S. Y. Kim et al., ACS Nano 10 (2016) 8888.

141
Ⅲ04- 21 -Ⅲ- 4

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

Photoelectron Angular Distribution of Anthradithiophene on Graphite
Investigated by Low-energy Angle Resolved Photoelectron Spectroscopy
1

Ⅲ- 4

T. Yamaguchi1, 2, M. Meissner2, T. Ueba1, 2, R. Shiraishi1, 2, S. Ideta1, 3,
K. Tanaka1, 3 and S. Kera1, 2

School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585
2
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

Ultraviolet photoelectron spectroscopy (UPS) is avery useful technique for studying the electronic
structure of surfaces and interfaces. However, UPS
with low energy photons is not well understoodtheoretically and experimentally because of thecomplicated photoemission process. For instance,
resonances excitation between initial states and final
states occur which cause unexpected scattering due tothe relatively long dwell time of low-energy electrons
at the surface/interface (potential trapping inintermediate states) [1]. Moreover, the asymmetricparameter of the photoelectron intensity greatly
depends on the photon energy (hv). In this work, weinvestigated the photoelectron angular distribution(PAD) of the highest occupied molecular orbital(HOMO) for anthradithiophene (ADT), using lowenergy angle-resolved UPS (LE-ARUPS), aiming to
improve the understanding of the photoemission
process in the low energy region at organic-metalinterfaces.
Highly oriented pyrolytic graphite (HOPG) as thesubstrate was cleaned by annealing at 600 K for 2 h.
An ADT molecule film of 5 Å was deposited on the
HOPG at room temperature in a custom UHV chamber
designed for organic layer deposition.
Figures 1 (a) and (c) show a LE-ARUPS intensity

map and energy distribution curves, respectively, taken
at hv = 21 eV. The HOMO peak was observed at 1.40
eV and around -1.5 Å-1 in Fig. 1 (a). Around the Γ point,
there is no momentum dependence of the HOMO
intensity. This is a typical PAD of the HOMO of πconjugated, planar molecules in a flat-lying orientation.
Figures 1 (b) and (d) show a LE-ARUPS intensity
map and energy distribution curves, respectively, taken
at 11 eV. It is impossible to observe the main HOMOintensity lobe by 11 eV due to the low kinetic energyof the photoelectrons. Even though the photons comefrom right side of the image, the HOMO intensity at
0.5 Å-1 is 3 times stronger than that at -0.5 Å-1.
Moreover, a sudden change of intensity occurs at the Γpoint. The direction of the p-polarized incident beam
normally causes the opposite intensity distribution as
found in Fig. 1 (a).
We clearly observed an interesting difference of
PAD between hv = 21 eV and 11 eV which will becaused by non-trivial multi-particle events in thephotoemission. The multiple scattering theory will be
helpful to understand the origin of the PAD, hence toreveal the modification of the molecular orbital upon
weak interactions.
[1] S. Tanaka et al., Sci. Rep. 3 (2013) 3031.

Fig. 1. LE-ARUPS intensity map for ADT/HOPG taken at room temperature using hv = 21 eV (a) and 11 eV (c).
Both images were prepared by combining 8 and 7 images taken every 12°. Energy distribution curves at -0.5 Å-1,
0.0 Å-1 and 0.5 Å-1 taken for hv = 21 eV (b) and 11 eV (d). Red arrows represent the incident light direction. The
structure of the ADT molecule is shown as an inset in (b).

14222 Ⅲ04-

UVSOR User 5

UVSOR User 6