Versatile Post-Doping toward TwoDimensional Semiconductors

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Yuya Murai, Shaochun Zhang, Takato Hotta, Zheng Liu, Takahiko Endo, Hiroshi Shimizu,
Yasumitsu Miyata, Toshifumi Irisawa, Yanlin Gao, Mina Maruyama, Susumu Okada, Hiroyuki Mogi,
Tomohiro Sato, Shoji Yoshida, Hidemi Shigekawa, Takashi Taniguchi, Kenji Watanabe,
Ruben Canton-Vitoria, and Ryo Kitaura*
Cite This: ACS Nano 2021, 15, 19225−19232

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sı Supporting Information*

ABSTRACT: We have developed a simple and straightforward way torealize controlled postdoping toward 2D transition metal dichalcogenides (TMDs). The key idea is to use low-kinetic-energy dopant beams
and a high-ﬂux chalcogen beam simultaneously, leading to substitutional doping with controlled dopant densities. Atomic-resolution
transmission electron microscopy has revealed that dopant atoms
injected toward TMDs are incorporated substitutionally into thehexagonal framework of TMDs. The electronic properties of doped
TMDs (Nb-doped WSe2) have shown drastic change and p-type action
with more than 2 orders of magnitude increase in current. Positionselective doping has also been demonstrated by the postdoping toward
TMDs with a patterned mask on the surface. The postdoping method
developed in this work can be a versatile tool for 2D-based nextgeneration electronics in the future.
KEYWORDS: two-dimensional materials, doping, transistors, STEM, transition metal dichalcogenides

Tultrathin uniform structure of 2D semiconductors is ideal torealize ultrashort-channel devices. Past research on 2D semiconductors has already demonstrated that 2D semiconductors
work as high on/oﬀ ratio FETs with carrier mobility up to theorder of 102 cm2/(V s) at room temperature.9−11 In conjunction
with the recent advancement of growth techniques and thesuccessful formation of ohmic contacts, 2D semiconductors
provide an excellent opportunity to develop next-generation
nanoelectronics.12,13
The accurate postdoping method for 2D materials is crucial
for the development of 2D-material-based electronic devices.
The success in Si devices is based on reliable postdoping
techniques; p/n-doped Si works as electrodes/channels in Sibased metal-oxide-semiconductor FETs (MOSFETs) in integrated circuits. In the fabrication of Si MOSFETs, p/n dopants
are incorporated through ion implantation or diﬀusion, forming

he current limitation in Si-based devices, arising fromthe adverse short-channel eﬀect, has led to searching formaterials for next-generation electronic devices. The
short-channel eﬀect becomes serious when the electric ﬁeld
originating from the source/drain electrodes is comparable tothat of the gate electrode. In this case, the gate electric ﬁeld
cannot control the channel current properly, and adverse eﬀects,
such as a decrease of threshold voltage and degradation ofsubthreshold characteristics, inevitably emerge. Various approaches, including double-gate ﬁeld-eﬀect transistors (FETs)and FinFET, are now being investigated to overcome the shortchannel eﬀect.1,2 Among the various approaches, one of thepromising ways is to use new material, particularly atomicallythin semiconductors (two-dimensional (2D) semiconductors),
as semiconductor channels.3,4The discovery of 2D semiconductors has triggered research
toward next-generation nanoelectronics.5,6 Research on 2D
materials started with gapless graphene, and graphene researchsoon led to the isolation of various 2D semiconductors, such astransition metal dichalcogenides (TMDs, for example, MoS2,
WSe2, etc.), whose band gap ranges in energy from 1 to 2 eV.7,8
2D semiconductors possess a uniform thickness of less than 1
nm without, ideally, any dangling bonds at the surface, and the© 2021 American Chemical Society

Received: May 30, 2021
Accepted: November 22, 2021
Published: November 29, 2021

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Articlechange after the doping process (Figure S1), HAADF-STEM
images show signiﬁcant change. Spotlike contrasts observed in
the images can be roughly categorized into three groups: thebrightest, the middle, and the darkest ones. The brightest andthe middle ones correspond to W and two Se atoms standing
vertically, respectively. The darkest contrasts observed at the W
positions correspond to Nb atoms substituted into theframework of WSe2. Also, image contrasts darker than those of
Nb are seen at Se sites (Figure 2d shows one dark spot at a Sesite), probably originating from single Se vacancies; single Se
vacancies arise not only from the growth process of WSe2 but
also from the formation of Se defects induced by electron beamirradiation.21 In HAADF-STEM images, image contrasts
strongly depend on the atomic number (Z-contrast), andsubstitutions of W by Nb are expected to yield darker contrasts
than W, as seen in the HAADF-STEM images. Figure 2c and dshows cutouts from the white squares shown in Figure 2a and b,
respectively. The hexagonal network structure of WSe2 is clearlyvisible, and dark contrasts at W sites are assigned to Nb atoms
substituted to W sites as shown in the structure models in Figure
2c and d. To conﬁrm the assignment, we performed STEM
image simulations using WinHREM software (HREM Research). As seen in Figure 2e, the simulated image well matches
the observed one shown in Figure 2c. Also, the line proﬁles along
the direction indicated by the blue arrows in Figure 2c and e
agree well (Figure 2f: left, observed; right, simulated), whichfurther conﬁrms the substitution of W by Nb.22 We carefully
checked the HAADF-STEM images and found that no W or Nb
atoms exist on top of the 2D layers; the additional contrast fromsurface adsorbed W/Nb atoms has not been observed in any
HAADF-STEM images. Energy-dispersive X-ray spectroscopy
has revealed that the Nb atoms are distributed uniformly over
the WSe2 samples (Figure S2). Substitution of Se by Nb isenergetically unfavorable, and therefore, Nb atoms should
predominantly occupy W sites in the current doping process.
Direct counting the number of Nb atoms substituted gives
doping rates of 3.4 and 7.4 at % for 3- and 6-min exposures,
respectively; the corresponding doping rate, in this case, is 1.2 at%/min. Because the determined doping rate is slow enough, it ispossible to control the degree of doping precisely, and we caneasily make the doping rate slower just by using smaller Nb ﬂux
in this method.
Whereas most Nb atoms are isolated to form one-atom-sized
dark contrasts in Figure 2a and b, some Nb atoms form clusters
in WSe2 (particularly in Figure 2a). To see if Nb substituted in
WSe2 favors the formation of clusters or not, we calculated thealloying degree J, deﬁned as follows.23,24

electrodes and channels at desired locations. However, the same
technique cannot be applied for doping into 2D semiconductors
because high-energy ions or high-temperature diﬀusion
processes can damage the ultrathin structure of 2D semiconductors. Doping toward 2D materials has, therefore, been
performed through mixing dopants during growth processes.14−19 Using this method, various dopants, such as Re
and Nb, have been successfully doped to show p/n-type FET
characteristics.16,20 Although this is a versatile way to dope p/n
dopants substitutionally to frameworks of 2D materials, thedevelopment of accurate postdoping is essential to realize
position-controlled doping for future 2D-semiconductor-based
devices.
In this work, we have developed a straightforward way torealize controlled postdoping toward 2D transition metal
dichalcogenides (TMDs), including MoSe2, WSe2, etc. Figure
1 shows a schematic representation of the doping process

Figure 1. Schematic representation of the postdoping process. A
predeposited crystal of TMD (in this case, WSe2) is irradiated with adopant (Nb) and a chalcogen (Se) beam, which leads tosubstitutional postdoping of Nb toward WSe2.

developed in this work. The key idea is to use dopant beams(Nb, Re, etc.) with low kinetic energy generated by thermal
evaporation of high-melting-point metals. The kinetic energy
distribution P(Ekin, β) of atoms in dopant beams is expected to
be proportional to exp(−βEkin), where Ekin and β represent
kinetic energy and inverse temperature (1/kBT; kB, Boltzmann
constant; T, temperature), respectively. In high-melting-point
metals, such as Nb and Re, we need a temperature around themelting point (2750 and 3458 K for Nb and Re) to have a beam
ﬂux high enough for doping processes. In this case, thedistribution of Ekin ranges typically from several hundreds ofmeV to 1−2 eV, leading to substitutional doping without
signiﬁcantly damaging the original framework of host 2D
semiconductors. Also, we supply a Se beam with a high ﬂux rate
(Se/Nb > 3000) during the whole doping process to heal Se
vacancies and enhance structural reconstruction to form wellregulated substitutionally doped TMDs. In this method,
accurate and position-selective doping becomes possible
through accurate control of the dopant beam ﬂux with a mask
layer patterned on 2D TMDs.

JW = Pobs/Prand × 100%

where Pobs and Prand represent the averaged number of Nb
adjacent to a W atom and the atomic ratio of Nb in the examinedregion, respectively; concrete representations of Pobs and Prandare given in the Supporting Information. Whereas a JW of 100%means no preference in neighboring atoms for Nb substituted in
WSe2, JW smaller/larger than 100% means Nb atoms preferisolation/clustering. Counting 300 Nb atoms yields JW of 97 and
92% for doping rates of 3.4 and 7.4 at %, respectively, suggestingthat the present Nb substitution occurs almost randomly. The
observed random doping indicates that Nb atoms may exist as
single atoms or tiny clusters composed of 2−3 atoms in Nb
beams, and after landing on a WSe2 layer, Nb atoms are
immediately incorporated into the WSe2 layer without
signiﬁcant surface diﬀusion to form large Nb clusters.

RESULTS AND DISCUSSION
Figure 2a and b shows atomic-resolution high-angle annulardark-ﬁeld (HAADF)-scanning transmission electron microscope (STEM) images of a monolayer of WSe2 after 3- and 6min exposures of niobium (Nb) beams with a ﬂux of 0.67
monolayer/hour. Although optical images show almost no19226

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ArticleFigure 2. (a and b) Typical HAADF-STEM images of monolayer WSe2 after 3- and 6-min doping of Nb. After applying “subtract background”
with a rolling ball radius of 2 nm, a bandpass ﬁlter was applied to ﬁlter out large and small structures, whose sizes are 2 and 0.06 nm, respectively.
All these image processings were performed with ImageJ.25 The scale bars correspond to 2 nm. (c and d) Cutouts from the white squares in partsa and b, respectively. Corresponding structure models are also shown. In the structure models, blue, green, and yellow balls correspond to W,
Nb, and Se atoms, respectively. (e) Image simulated with the structure model shown in part c. (f) Line proﬁles of the simulated (right) and theobserved (left) HAADF-STEM images shown in parts c and e. The line proﬁles were made along the direction indicated by the blue arrows
shown in parts c and e.

formation of second layers, metallic NbSe2, and attachment of
impurities are not observed. X-ray photoelectron spectroscopy(XPS) has shown the existence of Nb atoms (Nb 3d peaks) anddoping-induced red shifts in the W 4f and Se 3d peaks in the XPS
spectra of Nb-doped WSe2, which is consistent with previous
works (Figure S3).20,28 Considering all results, including
HAADF-STEM, EDX, PL, AFM, and XPS, it is reasonable toconclude that substitutional Nb doping, which leads to hole
doping, is the dominant process in our postdoping method.
To address the mechanism of the present doping process, wehave performed an ab initio molecular dynamics (MD)simulation, where an Nb atom with a kinetic energy of about
300 meV is injected toward a monolayer of WSe2 (similar results
were obtained with diﬀerent kinetic energies, Figure S4). Figure
4 shows snapshots of the structural change upon the injection (amovie is available as a Supporting Information movie ﬁle). As
seen in the snapshots, the Nb atom hits a W atom, followed by

Figure 3a shows room-temperature photoluminescence (PL)spectra of monolayer WSe2 before and after a 6-min Nb beam
exposure. As clearly demonstrated, the doping process causes a
signiﬁcant decrease in PL intensity; the PL peak arises fromradiative recombination of bright excitons at the K/-K valley.
The observed reduction in PL intensity is consistent with carrier
doping induced by the substitutional doping reported
previously.17,26−28 In addition to the decrease in PL intensity,
PL peaks shift to the red side, which is consistent with theprevious works on substitutional Nb doping; the increase in holedensity and possible strain induced by Nb doping can cause the
PL red shift.28 Corresponding Raman spectra (Figure 3b),where Raman peaks arising from A′1 and E′ at around 250 cm−1
are seen, showing intensity reduction, also compatible withearlier works.17,29 Typical AFM image and height proﬁles before
and after doping (Figure 3c) show that the thickness of theoriginal WSe2 does not alter after a doping process; the19227

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ArticleFigure 3. (a, b, and c) Typical PL, Raman spectra, and AFM images/height proﬁles of a pristine monolayer of WSe2 and the monolayer of WSe2
after 6-min doping of Nb. Red and blue curves correspond to spectra of the pristine sample and Nb-doped sample, respectively. We used theexcitation wavelength of 532 nm to measure the PL and Raman spectra. Intensities of the Raman spectra are normalized by the Si peak at 520
cm−1. The maximum and minimum heights shown by the color scale in part c are 4.5 and 0 nm, respectively. The AFM height proﬁlescorrespond to the height along the white lines shown in part c.

formation of not the hexagonal network but mirror twinboundaries.30 As seen in the ab initio MD calculation, Nb atoms
injected are rapidly incorporated into WSe2 without signiﬁcantsurface diﬀusion. In this case, the formation of Nb clusters is not
likely, which is consistent with the analyses of STEM images,
random Nb doping.
One of the advantages of the present doping method is
position-controlled doping. For this purpose, we deposit apatterned mask on a monolayer of WSe2 by electron beamlithography with an inorganic resist (Hydrogen Silsesquioxane,
HSQ); HSQ is stable up to a process temperature of 823 K.
Figure 5 shows optical and Raman images of WSe2 with an HSQ
pattern after 6-min doping. The green and orange frames in
Figure 5a show a monolayer of WSe2 on a hBN ﬂake and HSQ
patterns (rectangular bars) fabricated on WSe2/hBN. To
conﬁrm the spatial distribution of the Nb-doped area, we have
performed Raman intensity mapping (Figure 5b), where image
contrasts correspond to Raman intensity. As clearly seen, theregion exposed to the dopant beam shows quenching of Raman
intensity, whereas the region covered by the HSQ patterns gives
strong Raman intensity, giving the dark/bright contrasts patternin the Raman image shown in Figure 5b. The observed pattern in
the Raman image matches the HSQ pattern well, consistent withspatially selective Nb doping via HSQ patterning. The spatiallyselective doping is expected because the doping processes are
performed at 823 K, which is low enough to inhibit the thermal
diﬀusion of doped Nb atoms in the framework of WSe2. Given

Figure 4. Snapshots of the ab initio molecular dynamics simulationof Nb atom injected toward monolayer WSe2. Parts a, b, and c show asnapshot after 0, 40, and 100 fs, respectively.

the release of the W atom from the original position. The W
atom released moves around the original position during thesimulation time (3 ps). As mentioned above, we observed no W
or Nb atom on the WSe2 layer after doping processes, whichmeans that released W atoms are incorporated into the WSe2
again. We perform the doping at 823 K with the supply of Se
with a high ﬂux rate of 2 Å/s (Se/Nb > 3000), and structural
reconstruction to recover the original hexagonal network occurs
under this condition. The high-ﬂux Se supply during the doping
processes is probably essential to form the hexagonal network
structure because injecting Nb atoms alone results in the19228

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ArticleFigure 5. (a) Optical microscope image and (b) corresponding Raman image of patterned Nb-doped WSe2. Raman intensities from A′1 and 2LA
are used to make the Raman image. We used a Nb doping ratio larger than 10 at % to enhance the diﬀerence in Raman intensity for thismeasurement. The scale bar corresponds to 10 μm. The excitation wavelength 532 nm, the excitation power 30.8 μW, and the exposure time 30 s
were used to obtain the Raman image. The Raman intensity is displayed by grayscale from 0−18000 counts.

Figure 6. (a) Transfer curves of monolayer WSe2 measured at room temperature. All devices are on the same substrate, and 6-min Nb doping
was performed before the measurements. The black curve represents a typical transfer curve of monolayer WSe2 before the doping. Red, blue,
pink, yellow, and green curves correspond to transfer curves of FET devices (after 6-min doping) located at diﬀerent positions in a substrate. We
use channel length = 20 μm and channel width = 20 μm in this measurement. (b) Transfer curves of pristine multilayer WSe2 (black) and WSe2
after 1-min (red), 2-min (green), and 3-min doping (blue). All transfer curves were measured at room temperature with a bias voltage of 10 V(channel length = 20 μm, channel width = 67 μm).

from lowering the Schottky barrier induced by the increase inhole density. The p-doping was also conﬁrmed by an upward
shift of Fermi energy in angle-resolved photoelectron spectraand scanning tunneling microscopy/spectroscopy (Figures S6and S7). A critical point here is that p-type doping occurs
uniformly all over the substrate because of the uniform intensityof Nb dopant beams; the current size of the Nb beam is at least 2
cm, which is larger than those of the substrate used.
Because the present doping method is a postdoping method,
it should be possible to increase the number of dopants
afterward. Figure 6b shows a transfer curve of a pristine bilayer ofWSe2 and transfer curves of the same bilayer of WSe2 after
several consecutive Nb postdoping processes; electrodes canwork even after multiple times Nb postdoping processes. As seenin Figure 6a, all transfer curves show explicit p-type action, andIon increases in a stepwise manner as the doping process was
repeated, ﬁnally reaching 10−6 A after three times doping;
additional doping led to a decrease in Ion, probably due toincreased Coulomb scatterings from increased impurities. The
observed increase in Ion demonstrates that we can tune the Nb
doping ratio afterward, making the present postdoping method
useful to fabricate various functional devices. One more criticalpoint of the present postdoping method is that the same processthat lateral TMD heterostructures grown at high temperatures(∼1000 K) give sharp junction interfaces, the temperatureneeded for thermal diﬀusion within the framework is supposed
to be much higher than the temperature in the current doping
processes.31,32 The high-temperature required for the thermal
diﬀusion means that substitutional doping with nanometer-scaleprecision is possible in the current method, leading to thepossibility of realizing arrays of FETs with ultrashort 2D
channels in the future.
Figure 6a shows two-terminal transfer curves of WSe2 before
and after a 6-min exposure to a Nb beam; we used ﬁvemonolayer WSe2 grown on a SiO2/Si substrate for this Nb
doping. The ab initio band structure calculation of Nb-doped
WSe2 (Figure S5) shows that the Nb atom doped yields a
localized state around the VBM of WSe2, showing that Nbshould work as a p-dopant. As seen in the transfer curves shownin Figure 6a, the on current (Ion) before doping is very small,typically ∼100 pA, probably due to considerable contact
resistance from the Schottky barrier at the interface between
Ti and WSe2. In contrast, after doping, Ion is greatly enhanced in
all cases, typically four orders in magnitude, and all devices
clearly show p-type behavior consistent with previous works(Figure 6a).16,33,34 The observed Ion increase should originate
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ArticleAb Initio Calculations. The geometric and electronic structures of
Nb-doped WSe2 were investigated using the STATE package based on
density functional theory.37 The generalized gradient approximation
with the functional forms of Perdew−Burke−Ernzerhof is adopted todescribe the exchange-correlation potential energy.38,39 The electron−
ion interaction is treated by ultrasoft pseudopotentials.40 The valence
wave function and deﬁcit charge density were expanded by plane-wave
basis sets with cutoﬀ energies of 25 and 225 Ry, respectively. Ab initio
molecular dynamics (MD) simulations were conducted with the use of
the velocity scaling method to maintain the temperature at 300 K to
implant Nb into WSe2. Nb atom has initial velocities of 0.01, 0.3, and 1
eV toward the WSe2 layer. Ab initio calculations were conducted with 2
× 2 and single γ points for static and MD calculations, respectively,
under the 6 × 6 lateral periodicity of WSe2 with Nb atom. Atomic
coordinates were optimized until the force acting on each atom becameless than 5 mRy/A.

should be transferred to dope various dopants to 2D
semiconductors; dopant atoms are expected to be doped
regardless of kinds of atoms unless atoms possess suﬃcientkinetic energy/momentum. We also performed Re doping witha Re beam to demonstrate this transferability and found that the
postdoping method developed also works for Re doping (FigureS8). PL spectra before and after Re doping show results similar
to those obtained in the Nb doping, where the decrease in PL
intensity is seen after Re doping. A HAADF-STEM image of a
Re doped sample shows a bright spot at a metal site, which is also
consistent with Re doping; we used MoSe2 to visualize doped Re
atoms. The FET characteristic shows n-type action after thedoping, which is expected in the case of Re doping.35

CONCLUSION
In conclusion, we have successfully developed a postdoping
method for 2D semiconductors, including MoSe2 and WSe2.
The key idea is to simultaneously use a low-energy dopant beam
and a high-ﬂux chalcogen beam, leading to accurate post dopingof p/n dopants substitutionally into 2D structures. Atomicresolution HAADF-STEM observations and ab initio molecular
dynamics simulations have revealed that low-energy dopant
atoms can be directly incorporated without damaging theoriginal lattice to achieve uniform random doping over thewhole sample. Also, position-selective doping has been
demonstrated by combining HSQ patterning and the postdoping method. In principle, the same approach can be applied toother high-melting-point elements, such as transition metals andrare earth metals, providing opportunities to tune the Fermi level
and the band structure of 2D materials. The postdoping methodwith controllability and broad applicability makes the present
method useful to develop the next-generation 2D-semiconductor-based nanoelectronics.

ASSOCIATED CONTENT
sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsnano.1c04584.
Optical images of a monolayer of WSe2 before and after
Nb doping, a typical EDS spectrum and corresponding
elemental mapping, the deﬁnitions of J, Pobs, and Prand,
XPS spectra of a typical WSe2 and Nb-doped WSe2,
snapshots of a Nb atom injected toward monolayer WSe2,
band structure of Nb-doped monolayer WSe2 andcorresponding wave functions, band dispersion of pristine
trilayer MoSe2 and Nb-doped trilayer MoSe2, STM andSTS observations of Nb in a Nb-doped WSe2, andcharacterization of Re-doped samples (PDF)
Movie of the structural change upon injection of an Nb
atom with a kinetic energy of about 300 meV toward amonolayer of WSe2 (AVI)

EXPERIMENTAL METHODSAUTHOR INFORMATION

Sample Preparations. We prepared WSe2 and MoSe2 by themechanical exfoliation or chemical vapor deposition (CVD) method on
a SiO2/Si substrate according to a previously reported procedure.36 The
prepared TMDs were put in a vacuum chamber (base pressure is 10−6
Pa) and were heated up to 823 K. We started supplying Se (99.999%
purity, SigmaAldrich) with a Knudsen cell when the sample
temperature becomes higher than 623 K to avoid unwanted formationof Se vacancies; the supply rate of Se was kept constant (2 Å/s)throughout the doping process. This preheating process makes the
sample surface clean, which is required for the following metal doping
process. After the preheating process, we decreased the sample
temperature to 523 K and start supplying dopant (Nb or Re) with anelectron beam evaporation (EFM 3, FOCUS GmbH) for 1−15 minwith the supply rate 0.67−1.11 monolayer/hour.
Characterizations. Raman and PL spectra were measured withmicrospectroscopy systems (Jovin-Yvon HR-800 and Renishaw InVia
Raman) with 488 or 532 nm CW laser excitation. The laser was focused
onto samples with objective lenses (×50−100 and 0.7−0.85 NA), andRaman and PL signals were detected with a charge-coupled device(CCD). All measurements were carried out at room temperature underatmospheric conditions. Electronic properties, including transfer
curves, were measured with a semiconductor parameter analyzer(Kethley 4200-SCS/F) at room temperature.
Optical responses were obtained with a homemade microspectroscopy system. HAADF- and ABF-STEM images were taken at room
temperature by using a JEM-ARM200F ACCELARM (cold ﬁeld
emission gun) equipped with a CEOS ASCOR corrector, operated at
120 kV. For each frame of the STEM image, the fast scan rate 3−5 μs
per pixel is used. Several frames up to ten are overlapped after driftcompensation to form a HAADF or ABF-STEM image. EDS elemental
mapping is performed by using JED-2300.

Corresponding Author

Ryo Kitaura − Department of Chemistry, Nagoya University,
Nagoya, Aichi 464-8602, Japan; orcid.org/0000-00018108-109X; Email: r.kitaura@nagoya-u.jp

Authors

Yuya Murai − Department of Chemistry, Nagoya University,
Nagoya, Aichi 464-8602, Japan
Shaochun Zhang − Department of Chemistry, Nagoya
University, Nagoya, Aichi 464-8602, Japan
Takato Hotta − Department of Chemistry, Nagoya University,
Nagoya, Aichi 464-8602, Japan
Zheng Liu − Innovative Functional Materials Research Institute,
National Institute of Advanced Industrial Science andTechnology (AIST), Nagoya, Aichi 463-8560, Japan;
orcid.org/0000-0001-9095-7647
Takahiko Endo − Department of Physics, Tokyo Metropolitan
University, Hachioji, Tokyo 192-0397, Japan
Hiroshi Shimizu − Department of Physics, Tokyo Metropolitan
University, Hachioji, Tokyo 192-0397, Japan
Yasumitsu Miyata − Department of Physics, Tokyo
Metropolitan University, Hachioji, Tokyo 192-0397, Japan
Toshifumi Irisawa − Device Technology Research Institute,
National Institute of Advanced Industrial Science andTechnology (AIST), Tsukuba, Ibaraki 305-8568, Japan
Yanlin Gao − Department of Physics, Graduate School of Pureand Applied Sciences, University of Tsukuba, Tsukuba 3058571, Japan; orcid.org/0000-0002-4587-5391

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Mina Maruyama − Department of Physics, Graduate School ofPure and Applied Sciences, University of Tsukuba, Tsukuba
305-8571, Japan; orcid.org/0000-0002-2872-5543
Susumu Okada − Department of Physics, Graduate School ofPure and Applied Sciences, University of Tsukuba, Tsukuba
305-8571, Japan; orcid.org/0000-0002-0783-3596
Hiroyuki Mogi − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8571, Japan
Tomohiro Sato − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8571, Japan
Shoji Yoshida − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8571, Japan
Hidemi Shigekawa − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8571, Japan;
orcid.org/0000-0001-9550-5148
Takashi Taniguchi − International Center for Materials
Nanoarchitectonics, National Institute for Materials Science,
Tsukuba 305-0044, Japan; orcid.org/0000-0002-14673105
Kenji Watanabe − Research Center for Functional Materials,
National Institute for Materials Science, Tsukuba 305-0044,
Japan; orcid.org/0000-0003-3701-8119
Ruben Canton-Vitoria − Department of Chemistry, Nagoya
University, Nagoya, Aichi 464-8602, Japan
Complete contact information is available at:https://pubs.acs.org/10.1021/acsnano.1c04584

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Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS
R.K. was supported by JSPS KAKENHI Grant Numbers
JP16H03825, JP16H00963, JP15K13283, JP25107002,
20H05664, and JST CREST, Grant Numbers JPMJCR16F3
and JPMJCR19H4, JST SICORP Grant Number JPMJSC2110,
and JST PRESTO Grant Number JPMJPR20A2. The ARPES
measurements were performed at BL25SU of SPring-8 with theapproval of the Japan Synchrotron Radiation Research Institute(Proposal No. 2020A1352 and 2019B1392). R.C. was
supported by JSPS Grant-in-Aid for JSPS Fellows 19F19368.
S.O. was supported by HSPS KAKENHI Grant Numbers
JP21H05233, JP21H05232, JP21K14484, JP20K22323,
JP20H00316, JP20H02080, JP20K05253, andJP20H05664.We thank T. Muro for his technical assistanceduring the ARPES measurement at SPring-8.
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