feature articles

AUGUST 2018 vol. 28 no. 4

DOI: 10.22661/AAPPSBL.2018.28.4.09

Laser-combined Scanning TunnelingMicroscopy and Its Applications
Hidemi Shigekawa, and Shoji Yoshida
Faculty of Pure and Applied Sciences, University of Tsukuba

ABSTRACTWith size reduction, the differences in the electronic
properties of materials and current devices, for example,
caused by the structural nonuniformity in each element,
have an ever more crucial effect on macroscopic functions. For further advances of nanoscale science and technology, development of a method for exploring transient
dynamics of local quantum functions in organized small
structures is essential. Here, we review laser-combined
scanning tunneling microscopy (STM) which enables real
space imaging of nanoscale quantum dynamics even in
the femtosecond range.
INTRODUNTION
The understanding and control of quantum dynamics, such as transition and transport in nanoscale structures, are the key factors for continuing the advance ofnanoscale science and technology. However, with size
reduction, the differences in the electronic properties ofmaterials and current devices, for example, caused by
the structural nonuniformity in each element, have anever more crucial effect on the macroscopic functions.
That is, atomic-scale defects, for example, have markedly changed the entire situation: defects, which wereonce considered as a problem to be avoided, are actively
designed and controlled to realize desired functions.
The fluctuation in the distribution of dopant materials governs the characteristic properties of macroscopic
functions. Therefore, for further advances in nanoscalescience and technology, development of a method for
exploring transient dynamics of local quantum functions
in organized small structures is essential.

Since the development of scanning tunneling microscopy(STM), the direct imaging of atomic-scale structures has
been lifting the veil from various longstanding problems
and extending the frontiers of science and technology[1-3]. In STM, a sharp tip is placed above the target material, and we obtain information just below the probe tip
through the measurement of tunnel current, spin, force
and so on (Fig. 1). In a basic STM, the bias voltage applied between the tip and the sample is adjusted and thecorresponding change in tunneling current is measured.
To obtain further information, additional parameters,such as temperature, magnetic field, and tip-sample
distance, are modulated depending on the purpose. An
attractive option is to combine optical technologies withscanning probe microscopy (SPM), and many efforts
have been devoted to developing various techniquesFig. 1: Schematic illustration imaging SPM combined with optical technologies.

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feature articles[4-6]. For example, near-field scanning optical microscopy (NSOM) is used not only for spectroscopy but also
for manufacturing nanoscale structures [7-9], and STM
combined with synchrotron radiation has been opening
the door to characterizing atomic species by exciting corelevel electrons [10-13]. In this paper, we review lasercombined STM based on what we have been developing.
STM combined with a continuous-wave laser
In this section, several techniques realized by combining
a continuous-wave (CW) laser with STM are overviewed.
Direct observation of single molecule
photoisomerization
Photoisomerization plays important roles in realizing
various attractive functions and has been extensively
studied [14-16]. Here, laser-combined STM was applied
to directly observe the structural change of azobenzene,a typical molecule with the characteristic of photoisomerization, which shows trans- or cis-conformation undervisible or ultraviolet photoillumination, respectively(Fig. 2(a)) [17]. Isolated azomolecules were prepared in aself-assembled monolayer (SAM) on a Au substrate withdodecanethiol molecules as a spacer, and a photoinducedchange was observed by STM. By alternately changing the
wavelength of a He-Cd laser between a visible (440 nm)
and an ultraviolet (360 nm), the photoinduced structural change of a single molecule between trans- and cisconformations was directly imaged, as shown in Fig. 2(b).
Using STM, the electronic structures associated with thestructural change can be investigated. Rectification behavior in current-bias voltage (IV) characteristics, which
was predicted by Ratner et al. [18], was produced by theswitching of the two conformations.

Bulletintures obtained by several groups showed large variations[21]. By analyzing the STM image of the phase transition
depending on the laser intensity and bias voltage, it was
shown that inhomogeneous charge transfer exists at themetal/semiconductor interface, depending on the distribution of the atomic-scale defects in each area (Fig. 3)[22]. The spatial variation in the band filling at the interface depending on the sample condition results in theobserved variation in critical temperature. On the basisof this mechanism, the phase transition was actively controlled by changing the amount of charge transfer using
the combination of the bias voltage applied between the
tip and the sample and the intensity of photoillumination. Namely, the balance between the tip-induced band
bending (TIBB) and the laser intensity determines surface photovoltate (SPV) that induces a rapid change in

Fig. 2: (a) Photoisomerization of azobenzene,(b) StM images of photoisomerization of single azobenzene molecule.

Optical doping for controlling phase transition
Charge transfer is one of the key factors for governing
the mechanisms in various physical systems. In-wire/Si
is a typical structure with a characteristic metal-insulator
phase transition [19, 20]. However, the critical tempera-

10

Fig. 3: StM images of in/Si surface obtained at 61 K (a) without and (b) with
illumination. the metal phase ((c) and (f )) changes into the insulator phase((e) and (f )) with photoillumination due to the change in band filling dependingon the balance between tiBB (tip-induced band bending) and SPv (surface
photovoltage) in each area ((g) to (i)). (d) Crossover phase. the balance wascontrolled by the combination of the bias voltage applied between the tip andthe sample and the laser intensity.

AUGUST 2018 vol. 28 no. 4

the band filling of the local area. Rapid and local switching of conductance is considered to make the phase transition more applicable than the control by temperature.
Light-modulated STS (LT-STS) analysis on local
potential
In STM on a semiconductor, a nanoscale metal-insulatorsemiconductor (MIS) junction is formed by the STM tip,tunneling gap, and sample (Fig. 4). For a reverse bias
voltage condition between the STM tip and sample, bias
voltage leakage causes TIBB. With optical illumination,
the redistribution of photocarriers reduces the electricfield, thereby flattening the band bending, i.e., SPV. By
comparing the I-V curves measured in the dark and inilluminated conditions, we can obtain information about
the band bending under the dark condition. When carriers are injected, owing to a mechanism similar to SPV,
the band bending is reduced. Therefore, by measuring
the value of SPV over the surface, we can map the carrier
distribution as well as the potential landscape in the corresponding area.

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curves under dark and photoilluminated conditions are
obtained simultaneously, and from the amount of shift
between the two IV curves, SPV at a required bias voltage
is determined.

Fig. 5: (a) Schematic illustrations of light-modulated StS (lt-StS). (b) the i-v
curve is measured under illumination with a chopped light (typically 1 kHz).
Bias-voltage-dependent SPv is determined to be related to the bias voltage forthe dark condition here from the shift of the two i-v curves ((c) and (d)).

Figure 6 shows an example of the imaging of the hole
injection process in a GaAs p-n junction in real space [27].
Holes are injected by increasing the forward bias voltage

Fig. 4: nanoscale metal-insulator-semiconductor (MiS) structure formed by the
StM junction under reverse bias voltage conditions. tiBB under the dark
condition is reduced by charge injection as well as SPv.

There are several ways to obtain the information on SPV[23-26]. One method we have developed is light-modulated STS (LM-STS), where the I-V curve is measured under chopped photoillumination (Fig. 5) [26, 27]. The IV

Fig. 6: nanoscale mapping of hole injection in GaAs pn junction with increasing
forward bias voltage (blue to red). Band structures corresponding to theconditions are shown on the right side. SCR: space charge region.

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Bulletintime-resolved (TR) analysis of such carrier dynamics forfurther advances.

Fig. 7: StM image and potential mapping around atomic-scale defects on GaAs
surface obtained by lM-StS.

applied to the p-n junction. Since a reverse bias voltage
condition is necessary to observe SPV, only the left halfof each image is considered here. To observe the carrier
dynamics on the right side, we need to switch the sign ofbias voltage. Instead of a uniform current flow, we cansee a large fluctuation in current on the nanoscale. Thismay be attributed to an interface problem, but as shownin the images in Fig. 7 obtained by the same technique,local potential is modulated by atomic-scale defects [28],which affects the local carrier dynamics, and therefore,
macroscopic functions. Furthermore, the size of the image in Fig. 7 is similar to the target size of current semiconductor devices, suggesting the importance of theFig. 8: Bias voltage dependence of SPv mapping obtained by lM-StS on
Ag/Si(001) surface (30 nm × 30 nm).

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In addition to the 2D information, the question of how
to analyze the depth profile of the electronic structures
should be answered. Since SPV originates from TIBB
related to the depletion layer of the sample surface induced by bias voltage leakage, one option is to control
the area of the depletion layer by changing the bias voltage. Figure 8 shows the SPV mapping of Ag/Si depending on the bias voltage applied between the STM tip andsample. The spatial distribution of SPV clearly changes
with the bias voltage, showing the possibility of threedimensional (3D) analysis on semiconductors with nanostructures. This bias-voltage-dependent analysis is considered to be applicable to the TR-STM measurement
discussed in 3.3.
STM combined with an ultrashort-pulse laser
In this section, ultrashort-pulse laser technology is combined with STM to obtain additional information on thelocal carrier dynamics in materials and devices.
Optical pump-probe (OPP) method
One of the most promising methods in laser-combined STM is the optical pump-probe method (Fig. 9)[29, 30]. When ultrashort laser pulses are produced, forexample, at a typical repetition frequency of 80 MHz,the original pulse train has a time difference of about 13
ns between two subsequent pulses, as shown in Fig. 9(b)(upper train). Each pulse is divided into two pulses witha certain delay time to form a train of pulse pairs (lower
train), which are called pump and probe pulses. The delay time can be controlled by changing the optical lengthof the delay line system (Fig. 9(c)). In general, the opticallength is changed by the mechanical drive of reflecting
mirrors along the delay line. How to use the pump andprobe pulses depends on the purpose.
Atomic-scale manipulation
A simple way to observe the change in an STM image
after photoexcitation is as follows. For example, we can
excite selected phonon modes coherently, as well as theelectronic transition [30, 31]. Here, the periodicity ofultrashort laser pulses was tuned to excite the phononmodes of Si-7×7 adatoms, which are site-dependentamong 4 different states, i.e., faulted, unfaulted, corner and center adatoms [32, 33]. After each irradiation
with a different repetition rate, the number of adatoms
desorbed was counted. Similar to the manipulation by

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ment, which has enabled us to observe ultrafast dynamics
in the femtosecond range. However, the spatial resolution of such optical methods is limited, in general, by
the wavelength. Therefore, it has been one of the mostchallenging goals to combine STM with ultrashort-pulse
laser technology since the development of STM in 1982
[1]. There is a long history on this issue, and many efforts were made in various ways [5, 40-45]. In this review,
we focus on the laser-combined STM developed basedon the OPP method.
OPP method for time-resolved measurement
To use the OPP method for time-resolved analysis, thefirst pulse is used as a pump to excite the sample surface
and the second pulse is used as a probe to observe the relaxation of the excited states induced by the pump pulse.
When carriers excited by the first pulse, for example,
remain in the excited states, absorption of the second
pulse is suppressed depending on the delay time, whichis called absorption bleaching.

Fig. 9: Schematic illustrations of optical pump-probe (oPP) method:(a) ultrashort optical pulse, (b) original and modified pulse trains, and (c) delayline system.

the inelastic tunneling technique [34, 35], adatoms wereselectively manipulated. With this technique, a large
area can be simultaneously modulated. Control of phasetransition by coherent phonon excitation is an attractive
target of current research [36].
Time-resolved STM (TR-STM)
The spatial resolution of STM is excellent. We can see
individual atoms and electronic structural properties
such as the local density of states with atomic resolution. Since the development of STM, the direct imaging of atomic-scale structures has been lifting the veil
from various longstanding problems and extending thefrontiers of science and technology [1-3]. However, since
the temporal resolution of STM is limited to less than
100 kHz owing to the circuit bandwidth [37-39], the ultrafast dynamics in materials has been beyond its field ofvision. In contrast, the advances in ultrashort-pulse laser
technology have opened the door to the world of ultrafast phenomena. A prominent method is OPP measure-

Therefore, if the reflectivity of the second pulse (probe
pulse), for example, is measured as a function of delaytime, we can obtain information on the relaxation of
the excited state induced by the first pulse (pump pulse)
through the change in the reflectivity of the probe pulse.
In this case, the time resolution is limited only by thepulse width, i.e., in the femtosecond range.
In the new microscopy technique, the sample surface
below the STM tip is excited by a paired-pulse train
with a certain delay time t d, that is similar to the OPP
method, but the signal is the tunnel current I, insteadof the change in the reflectivity of the probe pulse, asa function of delay time. The optical pulses give rise tocurrent pulses in the raw tunneling current I *, which reflects the excitation and relaxation of the sample. If thesecurrent pulses decay fast compared with the time scaleof the STM preamplifier bandwidth, they are temporally
averaged in the preamplifier and cannot be detected directly in the signal I. Even in such a case, the relaxation
dynamics can be probed through the delay time dependence of I. By combining STM with ultrashort pulse laser
technologies, the spatial resolution of STM and the temporal resolution of an ultrashort pulse laser in the femtosecond range are simultaneously achieved. Since signals
are weak, we need to use the lock-in detection method.
There, the excitation is oscillated at a certain frequency
and the corresponding signal change is measured. In
general, laser intensity is modulated in the OPP method,13

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Bulletinwhich, however, causes thermal expansion of the STM
tip and sample. Since a 0.1 nm change in tip-sample distance produces one order change in tunneling current,
it is difficult to detect the weak signal under this condition. A promising option is to modulate the delay time
between the pump and probe pulses instead of intensity.
With this modulation technique, laser intensity is notchanged and we can choose the modulation frequency
independent of the noise frequency originating fromthermal expansions [46].

due to absorption bleaching. In such a case, the amountof surface photovoltage, thus, the total tunneling current,changes depending on the delay time. On the other hand,when the second pulse arrives during the surface-side
decay, the photocarriers are less trapped at the surface
owing to the existence of SPV induced by the first pulse,
resulting in change in the total tunneling current depending on the delay time. Therefore, by measuring the tunneling current as a function of delay time, we can obtain
information about the carrier dynamics in both processes.

TR-signal from a semiconductor
What type of signal can be observed? TIBB appears
under a reverse bias voltage condition. With photoillumination, the redistribution of the photocarriers reduces
the band bending, i.e., SPV. Then, the excited state
subsequently relaxes to the original state through two
processes. One is the decay of the photocarriers on thebulk side via recombination, drift and diffusion. We callthis bulk-side decay. The other is the decay of the minority carriers trapped at the surface via recombination andthermionic emission, which we call surface-side decay.

Figure 10 shows time-resolved spectra of the bulk-side
decay obtained for various samples [46]. With the new
method, carrier dynamics can be measured over a wide
range of time scales using one microscope. Therefore,
we can evaluate the carrier dynamics in organizedsmall structures consisting of composite materials with
pinpoint accuracy. Furthermore, as has been shown, although the new microscopy technique is based on the
optical pump-probe method, since the probe is a tunneling current, we can obtain local information at the nanoscale level, which cannot be measured by a conventional
OPP method, such as diffusion, drift and the effects ofatomic-scale defects on the carrier dynamics.

When the second pulse arrives during the bulk-side decay,
the carrier density induced by the second pulse decreases

Applications of time-resolved STM
In this section, two examples of the OPP-STM measurement are shown.
Nanoparticle/semiconductor interface
The understanding and control of quantum dynamics
in nanoparticles and their interfaces with surrounding
materials are important and play essential roles in various
fields, such as semiconductor devices, catalyst actions, andenergy transfer. Here, as an example, OPP-STM is carriedout on a metal-nanoparticle/semiconductor structure [46].
When Co is deposited on a GaAs(110) surface, nanoparticles are formed with gap states, enhancing recombination.
Figure 11 shows an STM image and the spectra obtained
for a bare GaAs and above a Co nanoparticle. The decay
constants obtained for both cases are 223 ns and 42 ns,
respectively. As expected, the decay is faster at the Co site.

Fig. 10: oPP-StM spectra obtained for various samples. oPPR spectra are aalso
shown. (a) lt-GaAs, (b) GanAs, (c) n-GaAs, and (d) H-terminated Si.

14

Furthermore, Fig. 11 shows a topographic image (a) andthe map of the decay constant obtained over the surface(b). As shown in the overlap of these two images (c), positional agreement is good. As the cross section shows (d),the decay is fast in the Co regions, which is due to thefact that the gap states enhance recombination.

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Direct detection of photocurrent
To increase the generality of this microscopy techniqueand its applicability to various types of material andstructure, additional techniques are desired. For example, as an application of TR-STM based on a method
similar to the analysis of a molecular electronic structure,a two-photon absorption measurement was used together
with TR-STM [47]. In the future, the development of direct techniques for detecting photocurrents in transient
dynamics at the nanoscale level is expected to play animportant role in determining the optical characteristics
of materials and devices.

Fig. 11: (a) StM image and (b) decay constant mapping obtained for
Co-nanoparticle/GaAs structures. (c) overlay of (a) and (b). (d) Cross section
along line in (b). As shown in (d), a spatial resolution of sub-nm scale is obtained.

Here, OPP-STM measurements were carried out ontransient photocurrent dynamics in a layered n-typesemiconductor, p-WSe2 [48]. Since WSe2 has an indirectbulk band gap, the recombination lifetime of photoexcited carriers is significantly long (~10 μs) comparedwith the process of diffusion. Therefore, under a forward
bias voltage condition, i.e., negative sample bias voltage,excited electrons are considered to directly tunnel to the
STM tip as a photocurrent.

Fig. 12: (a) Decay constant as a function of setpoint current.
(b) Hole capture constant (saturated value in (a)) τ cap as a function of Co size.

Figure 13 shows the I-V curves measured by LM-STS (see2.3), where dashed and solid lines indicate the I-V curves
obtained without and with laser excitation, respectively.
Under an illuminated condition using a CW laser, thenegative sample bias-voltage region is divided into twoparts: (1) a photovoltaic region and (2) a photoamperic
region. Until the conduction band edge reaches the Fermi level (photovoltaic region), the bias voltage is mainly
applied to the tunnel gap; therefore, under photoillumination, the SPV, and thereby the tunneling current,
strongly depends on the bias voltage. When the negative
bias voltage is further increased, the tunneling gap con-There are two limitations in this process, i.e., the injection of tunneling current from the STM tip and thecapture rate of holes at the gap states. For a sufficientamount of tunneling current, the capture rate becomesthe limiting process. Figure 12(a) shows the relationshipbetween the decay constant and the tunneling current.
As expected, the decay constant decreased with increasing tunneling current, and had a saturated value of 6.9
ns, which is the hole capture rate of the system, τcap. The
decay process should depend on the gap-state density.
Figure 12(b) shows τcap as a function of nanoparticle size,i.e., density of gap states. As expected, the decay constantincreases with decreasing nanoparticle size.

Fig. 13: i-v curves measured under dark (solid line) and illuminated (dashed line)
conditions.

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ductance becomes higher than that in the space charge
region of WSe2 owing to the reduction in tunneling barrier height for electrons (photoamperic region). Under
such a condition, the tunneling current mainly consists ofphotogenerated carriers from the bulk side. Therefore,
the photocurrent depends on the photointensity and thevolume of TIBB, where photogenerated carriers drift
toward the STM tip. Since the region of TIBB gradually
increases as the bias voltage increases, the tunneling current slowly increases with the bias voltage. No current
is observed for a negative bias voltage under the dark
condition, indicating that the photocurrent is directly
detected as a signal via tunneling.
Figure 14 shows a series of time-resolved spectra obtained
under various laser intensities. A fast component (decay
constant ~5 ns) and a slow component (decay constant~200 ns) were observed. For low laser intensities, only
the fast component appears and the intensity of the slow
component increases with increasing laser intensity.
The photocurrent dynamics reflects the flow of excited
photocarriers at the surface, which is determined by thebalance between the diffusion and tunneling rates. For
low laser intensity, the diffusion rate becomes the limiting
factor in the balance, where the tunneling of the excess
electrons trapped at the surface during photoexcitation

Bulletinis detected dominantly as the signal (fast component,(e), (f), and (g) in Fig. 14). In contrast, for a high laser
intensity, the tunneling rate becomes the limiting factor and the diffusion process produces tunneling with alarge decay constant (slow component, (a) in Fig. 14). In
the intermediate range, the excess electrons trapped at
the surface first undergo tunneling. Then, the electrons
from the bulk side undergo tunneling at a certain rate ofdiffusion. As a result, the slow component appears in thetime-resolved spectra following the fast component ((b),(c) and (d) in Fig. 14).
To analyze the process for the fast component in moredetail, the tunneling current dependence of the fast component was measured. Figure 15(a) shows the series ofspectra obtained, and the decay constant τ as a functionof the tunneling current is shown in Fig. 15(b). The tunneling current was adjusted by changing the tip-sample
distance before each measurement under illumination
with the pulse laser. The decay constant τ decreases withincreasing tunneling current as expected. The excess
minority carriers transiently trapped at the surface for a
few nanoseconds, which produce a transient surface photovoltage and cannot be detected by conventional methods, are directly observed by TR-STM
The direct detection of a photoexcited current shown inthis section makes this microscopy technique more general and practical.

Fig. 15: (a) Series of oPP-StM spectra obtained for different setup tunnelingcurrents. (b) Decay constant as a function of setup tunneling currents.

Fig. 14: Series of oPP-StM spectra obtained for different laser intensities.
the maximum laser intensity is 0.9 mW and the other values are relative ratios
reduced by filters.

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Future prospects
We have overviewed laser-combined STM and its applications. For further advances, the development ofadditional techniques is expected. For example, as anapplication of TR-STM based on a method similar to
the analysis of a molecular electronic structure, a twophoton absorption measurement was used together with
TR-STM [47]. Direct detection of photoexcited current

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AUGUST 2018 vol. 28 no. 4

and chemistry of nanoscience, such as the molecular dynamics, chemical reactions, phase transitions, and carrier
dynamics in functional materials can be explored with
sub-cycle time resolution.
Summary

Fig. 16: Schematic illustration for explaining mechanism for measuring spin
dynamics, absorption bleaching.

may play an important role. Using circularly polarized
light for pump and probe optical pulses, spin dynamics,
which has been studied, for example, by spin polarized
STM [49, 50], can be observed as has been carried out
by the OPP method with optical orientation techniques[51, 52]. The mechanism is similar to that of absorption bleaching. When the down spins excited by the first
pulse remain in the excited state, the excitation of thedown spins by the second pulse is suppressed dependingon the delay time, which is schematically shown in Fig.
16. For example, we have succeeded in observing therelaxation of spins oriented in a single quantum well. By
applying an external magnetic field, spin precession was
also observed by STM [53].
When the energies of pump and probe pulses are chosen
appropriately, excited states may be included in the analysis as in the OPP method. The development of thesetechniques is considered to increase the generality of thismicroscopy technique, i.e., ehen it is more practical andapplicable to various phenomena. The essential mechanism of this microscopy technique is the nonlinear interference between the excitations in the transient tunneling current generated by the two laser pulses, therefore,
the introduction of new ideas is desired to make further
advances. In addition to the use of absorption bleachingmechanism, a new technique is to use terahertz (THz)pulses. Although it is difficult to apply a high bias voltage between the STM tip and sample in general, the tipenhanced THz monocycle pulses enable it and taking
a snapshot of ultrafast dynamics becomes possible [54,55]. Furthermore, control of the carrier envelope phases(CEP) in pump and probe pulses paves the way for thedevelopment of new time-resolved analyses. The physics

Spectroscopy techniques at the nanoscale level developed by the combination of laser technologies with STM
have been overviewed. In the combination of STM witha continuous wave laser, nanoscale analysis of the local
potential as well as molecular and atomic structures havebeen realized. On the other hand, for the combination of
STM with ultrashort pulse laser technologies, the spatial
resolution of STM and the temporal resolution of an ultrashort pulse laser in the femtosecond range have been
simultaneously achieved. With phase-controlled laser
pulses, sub-cycle analysis of dynamics can be realized.
Following the examples shown in this paper, the development of other new techniques is expected for further
advances in this field [54-57].
Acknowledgements: H. S. acknowledges the financial support of a Grant-in-Aid for Scientific Research (17H06088)
from the Japan Society for the Promotion of Science.
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Hidemi Shigekawa is the group leader of the tsukuba Research unit for extreme Quantum Metrology & Quantum Bioscience.
He received a doctorate from the university of tokyo. He was a visiting researcher at Bell laboratories and worked at the synchrotron
radiation facility at Brookhaven national laboratory. His current research interests are nanoscale science and technologies and he is now
leading specially promoted research, supported by Grants-in-Aid for Scientific Research from JSPS (Japan Society for the Promotion of
Science) for “Development of sub-cycle time-resolved StM (scanning tunneling microsopy) and its applications”.

Shoji Yoshida is an associate professor of university of tsukuba. He received a doctorate from university of tsukuba in 2006.
His current research target is to develop new microscopy techniques based on scanning tunneling microscopy and ultrashort pulse
technologies. He received the Article Award of the Applied Physics Society, Japan in 2015, and is now promoting a Young Scientific
Research Fund from JSPS (Japan Society for the Promotion of Science) for “Development of tHz-StM”.

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