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ArticleTerahertz Scanning Tunneling Microscopy for Visualizing Ultrafast
Electron Motion in Nanoscale Potential Variations
Shoji Yoshida, Yusuke Arashida, Hideki Hirori, Takehiro Tachizaki, Atsushi Taninaka, Hiroki Ueno,
Osamu Takeuchi, and Hidemi Shigekawa*

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ABSTRACT: Studying the microscopic behavior of free carriers in materials
at an ultrashort time scale is critical to developing semiconductor,
optoelectronic, and other technologies satisfying the ever-increasing requirements for smaller sizes and higher speeds. Understanding the eﬀect of local
potential modulations and localized states due to nanoscale microstructures
on carrier dynamics is essential to realize these requirements. Here, we used
time-resolved scanning tunneling microscopy/spectroscopy (STM/STS)
combined with a carrier-envelope phase (CEP)-controlled subcycle THz
electric ﬁeld, THz-STM, to probe the ultrafast motion of electrons
photoinjected into C60 multilayer structures grown on Au substrate. We
have succeeded in demonstrating the time-resolved measurement of ultrafast
electron dynamics with sub-nanoscale spatial resolution and subcycle time
resolution for the ﬁrst time and successfully visualized the electron motion triggered by the spatial variation in the lowest unoccupied
molecular orbital (LUMO). The diﬀerence in the eﬀects of molecular defects, such as a molecular vacancy and orientational
disorder, was also clearly distinguished with single-molecular-level spatial resolution. This method is expected to play an importantrole in the precise evaluation of local electronic structures and dynamics for the future development of new functional materials anddevice elements.
KEYWORDS: ultrafast electron dynamics, scanning tunneling microscopy, terahertz, organic ﬁlm, C60, time-resolved imaging

Iand charge transfer has not yet been realized using thesemethods.
In this study, we used THz-STM to probe the ultrafast
dynamics of free electrons photoinjected into a C60 multilayerﬁlm formed on a Au(111) substrate. Research on the ultrafast
dynamics of photoexcited carriers in a C60 multilayer ﬁlm has
been actively pursued from the perspective of optoelectronic
device applications such as solar cells and ﬁeld eﬀect transistors(FETs).9−11 Since a C60 thin ﬁlm is formed by the weak van
der Waals interaction between molecules, as is common forgeneral organic thin ﬁlms, charge transport occurs owing to the
hopping of electrons between molecular orbitals in neighborsites. Consequently, a change in the molecular interaction
owing to structural disorder will strongly aﬀect the motion ofelectrons. Thus, the molecular-scale measurement of electron
dynamics will provide insight into the charge transport inorganic materials. By measuring the spatiotemporal variation ofn the development of functional materials and devices,atomic-scale structures such as defects, steps, and impurities
have large eﬀects on carrier (electron and/or holes) dynamics
because of their local potential modulations and localized
electronic states.1 Therefore, understanding how nanoscale
structural disorders in materials aﬀect the dynamics of free
carriers has been an essential issue, for example, in semiconductor physics, optoelectronics, and photochemistry. In
particular, visualization of phenomena such as ultrafast electron
motion in a nanoscale potential variation and the dynamics ofcharge transfer between molecules with a high spatiotemporal
resolution has been challenging.2 The methods of imparting
high time resolution to scanning tunneling microscopy (STM)
have high potential for such purposes. Among the microscopy
techniques, optical pump−probe (OPP) STM, which
combines optical technologies and STM, is eﬀective.3−5
Recently, terahertz (THz)-STM, a measuring method ofusing a subcycle pulse electric ﬁeld in the THz region as anapplied voltage, has been developed and has been attracting
attention.6−8 In OPP-STM, a high spatiotemporal resolution
was realized, which allows us to probe various physicalphenomena on their intrinsic time and space scales. However,the visualization of the ultrafast dynamics of electron motion© 2021 American Chemical Society

Received: October 8, 2020
Published: January 8, 2021

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ArticleFigure 1. Measurement system and sample. (a) Schematic illustration of the measurement setup. (b) Waveform of incident electric ﬁeld for ΔΦCEP= 0 and π. (c) Waveform of near-ﬁeld electric ﬁeld for ΔΦCEP = 0 and π. (d) STM image of the sample. (e) Band diagram of the STM tunnel
junction at equilibrium (ELUMO − EF,Au = 0.9 eV, EF,Au − EHOMO = 2.2 eV).

THz-induced tunnel current (ITHz), we have succeeded invisualizing electron motion in real space for the ﬁrst time at a
sub-nanoscale spatial resolution under the inﬂuence of thenanoscale potential modulation.

■

RESULTS AND DISCUSSION
Measurement Setup and Sample Preparation. Figure
1 shows a schematic of the measurement setup (see Materials
and Methods and Figure S1 for details). In THz-STM, an IR
pulse (1035 nm, 309 fs width) was used as the pump to excite
the sample, and a carrier envelope phase (CEP)-controlled
single-cycle THz pulse was used as the probe. Figure 1b shows
the incident THz electric ﬁeld waveform measured by electrooptical (EO) sampling, and Figure 1c shows the THz near-ﬁeld
waveform at the probe tip measured by the photoemission
technique (see Figure S2 for details). The CEP can becontrolled between 0 and π using a THz-CEP shifter (seeFigure S1).8,12,13 The voltage VTHz‑Peak produced at the
maximum point of the THz near ﬁeld used here was 1.5 V(see Figure S3). Figure 1d shows a typical STM image of thesample. Figure 1e shows the band diagram of the tunnel
junction at the equilibrium condition. The energy position of
the lowest unoccupied molecular orbital (LUMO) of C60(ELUMO) is 0.9 eV above the Fermi energy of Au(111)(EF,Au), and that of the highest occupied molecular orbital(HOMO) of C60 (EHOMO) is 2.2 eV below EF,Au.
Relationship between THz Current and Electron
Dynamics. When an infrared (IR) pump pulse is irradiatedto the C60 multilayer ﬁlm sample, hot electrons excited in theAu(111) surface are injected into the C60 LUMO, as shown in
Figure 2a (II). The accumulation of electrons in the sample’soutermost surface reduces and ﬂattens the tip-induced band
bending (TIBB), similarly to the surface photovoltage (SPV)mechanism.14,15 After that, the LUMO electrons return to their
original state by diﬀusion to the Au(111) substrate, during
which the electrons move around and their distribution in the
LUMO simultaneously changes, but some electron tunnelingto the tip side is detected by THz-STM, enabling us to observe
the dynamics induced by IR laser pulse irradiation.

Figure 2. IR-laser-pulse-induced change in the tunneling junction. (a)Change in the band diagram of a tunnel junction with IR pulse
irradiation. (b) I−V curves with (red) and without (blue) IR
irradiation. The THz pulse was blocked and the IR pulse irradiation
was turned on and oﬀ during the measurement (set point VDC = +2.0
V, It = 10 pA). VDC represents the DC bias voltage applied to thesample. IHOMO and ILUMO are the tunnel currents from the HOMO
and LUMO states, respectively. At a high negative bias voltage (V <VHOMO) where IHOMO appears, the tunnel current becomes the sum of
ILUMO and IHOMO. (c) VDC dependence of the photocurrent obtainedby subtracting the I−V curve in the IR-oﬀ state from that in the IR-on
state.

To see the above-described process, we ﬁrst compared thetunnel current-bias voltage (I−V) curves measured with andwithout IR pulse excitation (Figure 2b). Here, only DC bias
voltage VDC was applied. The THz pulse was blocked and theIR pulse irradiation was turned on and oﬀ during themeasurement. In the laser-oﬀ (IR-oﬀ) state, tunneling fromthe HOMO (= IHOMO) to the tip determines the shape of theI−V curve for a negative bias voltage, including TIBB, as
shown in Figure 2a (I). On the other hand, in the laser-on (IRon) state, in addition to IHOMO, which is shifted by SPV as
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ArticleFigure 3. Analysis of the THz-STM signal. (a) Delay time dependence of THz current after IR-pulse excitation (ΔΦCEP = 0, VDC = −3.0 V, It = 10
pA, IR: 20 mW). (b) An enlargement of the initial change in (a). The blue and black curves show a Gaussian pulse with a width of 309 fs showing
the IR pulse and the THz waveform, respectively. (c) Schematic model showing the relationship between the waveform of the bias voltage VTHz(t)applied to the STM tunnel junction by the THz pulse (ΔΦCEP = 0) and the shape of the THz-induced current ITHz(t) obtained for the I−V curvesof the IR-on (red) and IR-oﬀ (blue) states, which correspond to those in Figure 2b, at delay time t = td. (d) Band diagram of tunnel junction attunnel voltage of VS = −3.0 V. (e) Delay time dependence of ITHz−z curve (ΔΦCEP = 0, VDC = −3.0 V, It = 10 pA, IR: 20 mW). (f) ITHz−z curves
obtained at 0 and 3 ps and exponential ﬁtting. (g) IDC−z curve measured under the same conditions as in (f).

considered to indicate that the THz pulse waveform wasprobed by this fast signal, which is similar to the process
performed in the previous study using the fast relaxing
electrons in Bi2Se3.8 The time scale of the fast decay iscomparable to the cooling time of hot electrons photoexcitedin a metal tip (about 100 fs)16 and those excited into higher
LUMO levels in a molecular multilayer (about 50 fs).9 Sincetime-resolved measurements using a shorter pulse system arenecessary to obtain more details,17−19 we leave this issue for
future investigation and do not pursue this phenomenon indetail in this paper.
In THz-STM, instead of a rectangular pulse voltage, a timedependent voltage VTHz(t) whose waveform corresponds tothat of the THz electric ﬁeld is applied. Therefore, ITHz(td) is
deﬁned here asshown in Figure 2a (III), an increase due to the electron
tunneling from the LUMO (= ILUMO) is expected. In fact, thephotocurrent from the sample to the tip was observed from a
voltage much lower than the energy position of the HOMO
(VHOMO), as shown in Figure 2b, which is considered to reﬂect
the electron tunneling from the LUMO to the probe tip.
Figure 2c shows the bias voltage dependence of thephotocurrent obtained by subtracting the I−V curve of theIR-oﬀ state from that of the IR-on state. ILUMO is expected toincrease with the bias voltage due to the reduction of theeﬀective tunnel barrier height shown in Figure 2a (III).
However, experimentally, saturation of the photocurrent wasobserved for a negative bias voltage higher than Vsat, as shownin Figure 2c. This is because the number of electrons excited
into the LUMO by each IR pulse was ﬁnite, which isconsidered to limit the number of electrons tunneling from the
LUMO to the probe tip during the measurement time.
Next, we carried out IR-pump and THz-probe measurements to observe the photoinjected electron dynamics in the
C60 multilayer. A THz pulse was applied in addition to VDC,
modulating the bias voltage. We used the lock-in measurement
method by chopping the THz laser pulses at a frequency of
415 Hz. Figure 3a shows a typical time-dependent THz-STM
signal ITHz(td) after IR excitation. We observed an abrupt
increase in ITHz(td) at td = 0, followed by exponential decay
with time (td > 0), thus showing the time-resolved signal.
Regarding the fast component, Figure 3b shows a magniﬁedview of the region near 0 ps in Figure 3a, where the IR pulse(309 fs pulse width) and THz pulse are shown superimposedon the initial peak. The apparent signal shows the decay at thesimilar width as that of the THz pulse. The result shows that
there exists a signal faster than the change in the temporal
resolution of the system. Namely, this signal was widened
during the measurement owing to the convolution with IR and
THz pulses. Therefore, the original signal is considered to havea shorter lifetime. In fact, the dip present around td = −0.8 ps is

ITHz(VDC , td) = frep∫t

td + 1/ frep

[I{VDC + VTHz(t )}d− I(VDC)]dt(1)

where f rep is the repetition frequency of the IR and THz lasers.
Because ITHz(t) depends on the nonlinearity of the I−V curve,
ITHz(t) also depends on VDC. We used the schematic model
shown in Figure 3c where two I−V curves corresponding tothose in Figure 2b are drawn for the analysis. The relationshipbetween the waveform of the bias voltage VTHz(t) applied to
the STM tunnel junction by the THz pulse and the shape of
the THz-induced current ITHz(t) obtained for the I−V curvesof the IR-on (red) and IR-oﬀ (blue) states at the delay time t =td is shown. A negative VDC was applied to the sample to injectelectrons from the Au substrate into the LUMO of the sample
by IR irradiation. When we applied a THz pulse, the transient
voltage VTHz(t) produced by the THz electric ﬁeld modulated
the tunnel bias voltage VDC, producing the transient THz
current ITHz(t) at each delay time td, as shown in Figure 3a.
Since it has become possible to experimentally obtain the near317

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ﬁeld waveform immediately below the probe tip,8 such analysis
has become possible.
For Φ = 0 (VTHz > 0), as VTHz,peak increases, the tunnelingprobability decreases, reducing the tunnel current. However, if
the temporal change in electron density in LUMO is slower
than that of the THz pulse, that is, if the electron density is
assumed to be constant during the period when the THz ﬁeld
is changing, ITHz(td) obtained using eq 1 is considered to bealmost proportional to the electron density for t = td.
Therefore, THz-STM can be eﬀectively used when Φ = 0,
and the eﬀects of electron extraction and the tunnel current
from HOMO are not apparent. For Φ = π (VTHz < 0),however, if the number of electrons is limited, as in the sample
used in this experiment, the eﬀect of electron extraction may
appear as shown in Figure 2c, which complicates the analysis of
ITHz(td). Leaving detailed analysis for future work, all thefollowing experiments were performed with the THz pulse ofΦ = 0.
Next, we performed ITHz−z measurement to investigate thedominant tunnel channel (IHOMO or ILUMO) in ITHz. IHOMO orILUMO can be distinguished by ITHz−z measurement because
IHOMO and ILUMO have their own eﬀective vacuum barrierheights, as shown in Figure 3d. Their apparent barrier heights
Φ can be obtained from the decay constant κ of the tunnel
current expressed as
I(z) ∝ exp( −2κz)

κ=

2mϕ/ℏ

ArticleFigure 4. Eﬀect of electron extraction. (a) Delay time dependence of
the THz current at diﬀerent set point currents IDC (ΔΦCEP = 0, VDC =−3.0 V, IR: 20 mW). (b) Set point current dependence of the lifetime
shown in (a).

to the observation that no fast component appeared in Figure
3f. These results suggest that the probability of tunneling thefast component in the short time may not be suﬃcient at thisdistance. The discussion on this issue is beyond the scope of
this paper, a more detailed analysis is left for future work.
We carried out the experiments under the conditions
discussed here. Thus, the signal observed for td > 0 in Figure
3a represents the photoinjected electron population in the
LUMO state below the STM tip. An electron lifetime of 6.4 ps
was obtained by ﬁtting using an exponential function.
Compared with the interlayer hopping rate (3 × 1013 s−1)
obtained using time-resolved two-photon photoemission,21 ourmeasured electron lifetime is long. This diﬀerence isconsidered to be caused by the negative sample bias voltage
applied in the experiments, as shown by the band diagram in
Figure 2a.
Imaging Electron Motion with Sub-nanoscale Resolution. Next, we measured the spatial variation of ITHz(td) toanalyze the motion of electrons injected into the LUMO,
which is the ﬁrst ever visualization of the spatial motion ofelectrons with sub-nanoscale resolution. Figure 5a shows an
STM image of the C60 multilayer structure in a region
including steps and terraces, and Figure 5b shows the timeresolved spectra obtained at diﬀerent points in the STM image.
The spectra were simply ﬁtted with the sum of threecomponents,(2)

where m is the electron mass and ℏ is the Dirac constant.
Figure 3e shows the delay time dependence of the ITHz−z
curve, and Figure 3f shows the ITHz−z curves obtained for td =0 and 3 ps. The values of Φ obtained by exponential ﬁtting
were 2.06 eV at td = 0 ps and 1.99 eV at td = 3 ps. The barrierheight measured from the IDC−z curve shown in Figure 3g was
5.48 eV.
The work function of the W tip, Φtip, and that of theAu(111) substrate, ΦAu, are 4.5 and 4.74 eV, respectively.
Since the LUMO and HOMO levels are 0.9 and −2.2 eV withrespect to the Au Fermi surface,20 ΦLUMO and ΦHOMO in
Figure 3d become 3.64 and 6.94 eV, respectively. Since VDC =−3.0 V in the experiment, the barrier height for IHOMO
becomes 5.32 eV. This is in good agreement with the value
obtained from the IDC−z curve shown in Figure 3g. Similarly,
the barrier for the electrons in ILUMO was obtained to be 2.22
eV, which is also in good agreement with the value obtained
from the ITHz−z curve shown in Figure 3f. The results
demonstrate the dominant contribution of ILUMO to ITHz, as
expected. The fast component in Figure 3a is not observed
here. Since the modulation by VTHz is not included in theabove consideration, this is not a strict treatment, but it isconsidered suﬃciently rigorous for a qualitative evaluation. If
the signal was stronger and the measurement with a longer
delay time than those in Figure 3f was possible, TIBB would
have been reproduced.
In the actual measurement, the electron extraction may
aﬀect the decay process. Therefore, we investigated the set
point current dependence of the time-resolved spectrum(Figure 4a), and the result is summarized in Figure 4b.
When the set point current was increased by one order and thetip was moved closer, the lifetime was reduced by about 10%.
Although an eﬀect of the set point current dependence existed,
it can be reduced by selecting appropriate measurement
conditions. When the set current It was decreased to 2 pA, thefast component disappeared. This is considered to correspondI(td) = −A exp( −td /τr) + Bexp(−td /τd) + C(3)

where τr, τd, td, and C represent the lifetimes of the initial
increase and the subsequent decay components, the delay time,and a constant term due to a lifetime longer than the system
limitation, respectively. As shown by the solid black lines on
the spectra, the agreement was good, and the values of τr andτd are shown to the right of the ﬁgure. τr is considered torepresent a process in which electrons accumulate immediately
below the probe tip after electron injection, and τd represents aprocess in which electrons return to the Au substrate.
The features of the signals are as follows.
1. The top layer (A to C) has no initially rising component.
2. From the second layer, the lifetime of the initial increasein τr (0.3 to 1.5 ps) is nonzero except at the upper stepedge (D).
3. The lifetime of the decay τd has a relatively large value
near the vacancy defect indicated by the arrow andaround the down step edge.
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ArticleFigure 5. Time-resolved analysis of electron motion for steps and a vacancy defect. (a) STM image of the C60 multilayer structure in a region
including steps and a vacancy defect, indicated by the arrow. (b) Time-resolved spectra obtained at A to I in (a). ITHz represents the THz-induced
current. See Materials and Methods for details of the ﬁtting. (c) Snapshots obtained over the area shown in (a). (d) Snapshots obtained over a
wider area including steps. (VDC, It) = (−2.0 V, 10 pA).

diﬀusion was fast and the distribution of injected electrons was
already formed at td = 1 ps, the initial horizontal change in the
red area with high electron density is not very clear in thesnapshots. However, a gradual decrease in the high-density
area can be seen in the images in Figures 5a and 5b, showing
the existence of the horizontal diﬀusion that is faster than thevertical diﬀusion (see Figure S4 for more details).
To consider the mechanisms behind the dynamics observed
here, the potential variations around two structures were
examined. Figure 6 shows STM images obtained at locations
including steps (Figure 6a) and a vacancy (Figure 6b) and the
I−V curves22 measured at diﬀerent points in the images(Figures 6c and 6d, respectively). The spatial variations of the
HOMO level (EH) and LUMO level (EL) around the two
structures were estimated from the rising parts of the I−V
curves and are plotted in Figures 6e and 6f, respectively. First,
for the step structure shown in Figure 6a, the apparent energy
gap between the HOMO and the LUMO (EHL= |EH − EL|)
gradually decreases from an upper step edge (0) to the down
step edge of the neighboring upper layer (4), widens at theupper step edge (5), and then the same change occurs toward
the down step edge of the next upper layer. These results showthat the band-structure change at the step edge is not a simple
dipole-like one across the step,23 and other mechanisms are
considered to be involved. At the upper step edge, C60 has asmaller number of neighboring molecules than C60 at theterrace. Therefore, at the upper step edge, the broadening of
the molecular state owing to their overlaps with neighboring
molecules becomes less eﬀective. Also, the screening of mobile
carriers is weakened, that is, the value of the Hubbard on-siteenergy increases.24 Both phenomena increase the HOMO−

4. From the second layer, except at the upper step edge(D) and around the white cluster in the middle right (I),the constant term C in eq 3 (a long-lifetime component
considered to be in the sub-ns to ns range) is nonzero.
Although our system cannot measure this long lifetime
because of the limited length of the optical delay line(<∼300 ps), the condition returned to the initial state
when the next IR pulse was irradiated.
To visualize the electron dynamics in more detail, 2D
imaging of the THz current was performed, as shown in Figure
5c. The snapshots were obtained by repeatedly scanning overthe same area while changing the delay time from −4 to 49 ps.
At −4 ps, the THz current was almost zero, indicating that theTHz current was observed only under excitation by the IR
pulse. At 2 ps after IR pulse irradiation, the electron density
was widely distributed except around the white cluster on theright-side step. The electron density rapidly decreased in the
top layer, whereas electrons accumulated near the down step
area and around the defect indicated by the arrow andremained even after 49 ps. By assembling these ITHz images
taken at diﬀerent delay times, we made a movie that directlyshows the nanoscale motion of electrons in the ps time scale(see Supplementary Movie S1). The characteristics of each
spectrum were clearly observed. The electron dynamics arounda step are shown more clearly in Figure 5d, which was obtainedby a wider-area scan.
Regarding the component that rises at the initial stage, thelifetime was obtained by ﬁtting the spectra in Figure 5b (E−I).
Although the lifetime of 0.3 ps is comparable to the resolutionof the system, a signal exists because its shape is diﬀerent fromthose of the spectra A to D in Figure 5b. Since the horizontal
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Article7d. For the bright molecule, EHL similarly decreased owing to
the formation of a shallow defect state,25 but the change
occurred within the range of a single molecule.
Finally, we review and discuss the time-resolved THz-STM
data in detail. Figure 8 shows a schematic illustration of theultrafast electron motion in the C60 multilayer sample. Afterelectron injection, electrons at the upper step edge movetoward the down step edge owing to the higher EL at the upper
step edge, as shown in Figure 6e. At the same time, someelectrons are trapped into defect-induced gap states.
As mentioned above, the eﬀect of VDC slows down therelaxation of electrons to the Au substrate. Even under suchconditions, the relaxation of electrons at the upper step edge
was very fast (several ps), so it is considered that the electrons
at the upper step edge move to the terrace, the lower step edge,
and so forth, before relaxing to the Au substrate. An increase inelectron density was observed on the terrace in the ﬁrst 1 ps,suggesting carrier accumulation due to horizontal diﬀusion.
These results correspond well to the spatial changes in band
structure. Since the horizontal diﬀusion was fast and the
distribution of injected electrons was already formed at td = 1
ps, the initial horizontal change in the red area with highelectron density is not very clear in the snapshots. Fineradjustment of conditions, such as electronic excitation densityand electron extraction is necessary, and we would like to makethis a future work. However, a gradual decrease in the highdensity area can be seen in the images in Figures 5 and 7,
showing the existence of the horizontal diﬀusion that is fasterthan the vertical diﬀusion, as described above (see Figure S4
for more details).
As shown in Figure 5, although the spectrum changedslightly with the location, single exponential decay wasobserved in the spectra obtained from the top layer. The
decay was fastest at step edges (C) and (D), where EL washigh, as shown in Figure 6e. It is also noted that the lifetime
was long at point (A) owing to the inﬂuence of the vacancy
defect indicated by the arrow. Regarding the change in thesecond layer from the top, an increase in electron density wasobserved, except at (D), on a short time scale of sub-ps to ps(τr) and is considered to be caused by the redistribution ofelectrons to smooth the in-plane potential. Then decay (τd)
occurred on the same time scale, as was observed for the top
layer. It can be seen that the lifetime was slightly longer at (E),(F), and (G) below the step (down step edge) on the second
layer and that the accumulation of electrons occurred aroundthe down step edges in both the second and third layers. The
injected electrons were trapped at the down step edges anddefects, and they eventually diﬀused to the Au(111) surface
and returned to their original state.
From Figure 6e, the change in the potential at the step edge
is about 40−70 meV for a distance of 2 nm, which gives us arough estimate of about 20−35 mV/nm for the electric ﬁeld.
On the other hand, from spectra (C) and (D) in Figure 4b, thetime required for electrons to spread away from the step edge
is about 4−5 ps. By assuming these values, the mobility isroughly estimated to be 0.2−0.5 cm2/(V s), which is higher
than the macroscopically obtained value (below 0.1 cm2/(V s)at 100 K26,27). This discrepancy could be due to ourmeasurements being carried out in a local region without
any structural disorder. Taking into account the temperaturedependence and the electric ﬁeld dependence allows a moredetailed consideration, but this is not carried out here. For
both the vacancy and the bright defect, electrons were

Figure 6. Analysis of the band structures of steps, a vacancy defect,
and a bright defect. (a,b) STM images obtained over steps and avacancy defect, respectively (2.0 V, 20 pA for c and 2.0 V, 10 pA forothers). The inset of (b) is an STM image obtained for a negative bias
voltage (−2.5 V, 10 pA). A 2 × 2 unit cell is drawn in (b). (c,d) I−V
curves obtained above the points shown in (a) and (b), respectively.
Here, variable-pacing STS24 was used to measure the energy level
distribution with high accuracy. The black line in each graph shows
the change in the tip−sample distance during the I−V curvemeasurement. (e,f) EH and EL obtained from the rising parts of theI−V curves shown in (c) and (d), respectively. When the number oflayers increases, larger EH and EL are observed owing to TIBB; thus,
the actual change may be smaller.

LUMO gap at the upper step edge. In addition, EHL increases
with increasing number of layers because the voltage drop in
the C60 layer owing to TIBB increases with increasing numberof layers. The observed gradual change might have been causedby the slight relaxation of the lattice near the step edge. Thisvariation is considered to have a strong eﬀect on electron
motion in materials, as clearly shown in Figures 5c and 5d (seeFigure S3 for a larger-area image). For the vacancy (Figures 6b,
6d, and 6f), EHL apparently decreased owing to the formationof an in-gap state, the eﬀect of which was observed over a
distance of two molecules from the defect.
Figure 7 shows 2D snapshots measured around anotherdefect. In the bottom topographic STM image in Figure 7(a),the C60 molecule indicated by the white arrow appears slightly
brighter than the other molecules and is considered to have adiﬀerent molecular angle from those the other equivalent C60
molecules in the (2 × 2) lattice.25 As shown in the snapshot
images in Figure 7(a), electrons were trapped only in the single
bright C60 at 53 ps, clearly indicating that the molecule acts as
a trap site for electrons. The rapid decrease in electron density
in the bottom left area is considered to have been caused by
the step existing in the area. Similar to the case shown in
Figure 6, we carried out I−V measurement around the brightdefect (Figure 7b), and the results are shown in Figures 7c and320

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ArticleFigure 7. Time-resolved analysis of electron motion for a bright defect. (a) STM image and snapshots obtained over an area including the brightdefect indicated by the white arrow. (b) STM image of the same bright defect (2.0 V, 20 pA). The inset is an STM image obtained with a negative
bias voltage (−2.5 V, 10 pA). (c) I−V curves (variable-pacing STS) obtained above the points shown in (b). The black line in the graph shows thechange in the tip−sample distance during the I−V curve measurement. (d) EH and EL obtained from the rising points of the I−V curves shown in(c).

technique, we made a movie that shows the ultrafast
movement of free electrons with nanoscale spatial resolution.
Signiﬁcant inﬂuences of atomic-scale surface structures withmodulated local potentials on the nanoscale electron dynamics
in the organic thin ﬁlm were clearly demonstrated.
This method is expected to play an important role in the
precise evaluation of local electronic structures and dynamics
to develop new functional materials and device elements.

■

MATERIALS AND METHODS
THz-STM System. A coherent Monaco femtosecond laser(1035 nm, 40 W, 309 fs, 1−50 MHz) was used as a lightsource, and most of the output (39 W) was used for THz pulse
generation with a LiNbO3 crystal.28 Here, high-repetition-rate
and high-intensity THz pulses were generated to realize
measurement with an improved S/N ratio. A repetition
frequency of 16.7 (50/3) MHz (the maximum for our present
setup) and a peak electric ﬁeld of 250 V/cm were used, which
was a suﬃcient electric ﬁeld for driving the tunnel current. By
assuming the tip−sample distance to be 1 nm and calculating
the ratio of the peak voltages shown in Figures 1b and 1c, thetip enhancement factor was estimated to be 60 000, which issimilar to a previously reported value.6,29
Sample Preparation. A C60 multilayer ﬁlm was formed by
vacuum deposition at room temperature using a clean Au(111)
surface as a substrate. The average ﬁlm thickness was fourmonolayers (MLs). A close-packed C60 multilayer structure
was grown. Up to the third layer from the substrate, theelectronic structure exhibited coupling with the Au substrate,
but from the fourth layer, it had an electronic state peculiar to a

Figure 8. Model for the observed electron motion. Schematic
illustration showing the ultrafast electron motion in the C60 multilayer
sample inﬂuenced by the local modulation of the band structure andthe in-gap state around surface steps and defects.

observed to be trapped for a long time. The spread sizes ofelectrons in the vacancy and the bright defects were ∼2 and 1
nm, as indicated by the arrows in the snapshots shown in
Figures 5c and 7a, respectively. The results were in good
agreement with the spread of the electronic states owing to
defects shown in Figures 6f and 7b, respectively.
In conclusion, we used THz-STM to probe the ultrafast
dynamics of free electrons optically excited into the LUMO ofa C60 multilayer formed on a Au substrate. ITHz was utilized toprobe the change in the population of free electrons under the
STM tip with sub-ps time resolution. By assembling ITHz
images taken at diﬀerent delay times using the pump−probe
321

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and O.T. provided technical advice. H.S. organized andsupervised the project and edited the paper with S.Y.

C60 molecule disconnected from the substrate, as reported in aprevious paper.30
The lowest unoccupied molecular orbital (LUMO) of theﬁrst layer C60 was located about 0.9 eV higher than the Fermi
level of the Au(111), and the highest occupied molecularorbital (HOMO)−LUMO gap of C60 was about 3.1 eV.20 On
the other hand, the energy of the photons of the IR pulse used
was 1.2 eV. Therefore, no carrier (electron−hole pair)generation occurred inside C60. To detect the weak THz
current superimposed on the DC tunnel current, the THz
pulse train was chopped at 415 Hz and the diﬀerence signal
was detected by a lock-in ampliﬁer. The entire experiment was
performed in a vacuum (1 × 10−7 Pa) at ∼100 K using a liquid
nitrogen cryostat.
Two-Dimensional Imaging. Measurements were carriedout 100 times at a rate of 1.3 s/scan over 200 × 100 points,
resulting in a total measurement time of 2 min to obtain animage. Although some ﬂuctuation in each measurement wasinevitable, the same process was expected to be repeatable, andsnapshots of the dynamics were satisfactorily obtained.

■

Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
H.S. and S.Y. acknowledge the ﬁnancial support of a Grant-inAid for Scientiﬁc Research (17H06088) and (20H00341,
20H05662) from Japan Society for the Promotion of Science.

■

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ASSOCIATED CONTENT

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsphotonics.0c01572.
Schematic illustration of electric ﬁeld strength control
system; Method for measuring THz near-ﬁeld waveformusing photoemission; Derivation of THz voltage intunnel region; Time-resolved images of electron motion
obtained over a large area (PDF)
Movie S1: Movie showing the motion of electronsobtained over a large area (MP4)■

ArticleAUTHOR INFORMATION

Corresponding Author

Hidemi Shigekawa − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan;
orcid.org/0000-0001-9550-5148; Email: hidemi@
ims.tsukuba.ac.jp

Authors

Shoji Yoshida − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Yusuke Arashida − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Hideki Hirori − Institute for Chemical Research, Kyoto
University, Kyoto 611-0011, Japan
Takehiro Tachizaki − Department of Optical and Imaging
Science and Technology, Tokai University, Hiratsuka-shi,
Kanagawa 259-1292, Japan
Atsushi Taninaka − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Hiroki Ueno − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Osamu Takeuchi − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Complete contact information is available at:https://pubs.acs.org/10.1021/acsphotonics.0c01572
Author Contributions

S.Y., Y.A., H.H., and T.T. contributed to advance the THzSTM system. S.Y. performed experiments with H.U.; Y.A., A.T.
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