Science and Technology of Advanced Materials 6 (2005) 582–588
www.elsevier.com/locate/stam

ReviewDevelopment of femtosecond time-resolved scanning tunnelingmicroscopy for nanoscale science and technology
Hidemi Shigekawa*, Osamu Takeuchi, Masahiro Aoyama
Institute of Applied Physics, CREST, 21st COE, University of Tsukuba, Tsukuba 305-8573, Japan
Received 29 March 2005; revised 15 June 2005; accepted 24 June 2005
Available online 26 September 2005

Abstract‘Smaller’ and ‘faster’ are the key words in the progress of current nanoscience and technology. Thus, a method of exploring the ultrafast
transient dynamics of the local quantum functions in organized small structures is eagerly desired. Ultrashort optical pulse technology has
allowed us to observe transient phenomena in the femtosecond range, which, however, has the drawback of a relatively low spatial resolution
due to the electromagnetic wavelength used. In contrast, scanning tunneling microscopy and its related techniques, although having a time
resolution limited by the circuit bandwidth (w100 kHz), enable us to observe spatial dynamics at the atomic level in real space. Our purpose
is to combine these two techniques to achieve a new technology which will advance the pursuit of future nanoscale scientific research interms of the ultimate temporal and spatial resolutions.
We proposed a promising new design for achieving ultimate spatial and temporal resolution, by combining a short-pulse laser and STM.
Using this method, time-resolved tunneling current measurement in the subpicosecond range was successfully demonstrated, this is shakenpulse-pair-excited STM (SPPX-STM) satisfies the requirements for exploring the ultrafast dynamics of the local quantum functions
occurring in organized small structures. We hope this new technology will promote the development of future research on the nanoscale.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: STM; Ultrashort-pulse laser; Pump-probe technique; Nanotechnology; Femtosecond laser

Contents
1.
2.
3.
4.
5.
6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582
Previous studies in this field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583
Pulse-pair-excited STM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585
Shaken-pulse-pair-excited STM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585
Experimental and results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588

1. Introduction* Corresponding author.
E-mail address: hidemi@ims.tsukuba.ac.jp (H. Shigekawa).
URL: http://dora.ims.tsukuba.ac.jp/

1468-6996/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.stam.2005.06.021

With the progress of nanoscale science and technology,
barriers between different research fields have been
eliminated and interdisciplinary fields have been emerging.
The ability to work at the molecular level, atom by atom, tocreate large structures with fundamentally new properties
and functions is leading to unprecedented understanding
and control over the basic building blocks and properties of
all natural and man-made things. However, as the size of thecomponents in the integrated structures is reduced toH. Shigekawa et al. / Science and Technology of Advanced Materials 6 (2005) 582–588

nanoscale order, there appears a growing and urgent need todevelop a new technique for analyzing the local electronic
dynamics, which determines the major device characteristics as well as fundamental functions.
At present, for example, the single-molecule observationis performed by several techniques. However, in order tounderstand and realize functional materials at the singlemolecule level, detailed study of the intramolecular
structure is important even for the single-molecule case.
Fig. 1 shows a scanning tunneling microscopy (STM) imageof a Si nanoparticle, and its cross sections obtained at
different bias voltages. Since, the Si nanoparticle isemissive, it is expected to be a future nanoscale opticalelement. The figure indicates that a Si nanoparticle is nothomogeneous and has various internal structures dependingon the bias voltage. Since STM images at different bias
voltages reflect the local density of electronic states ofdifferent energy levels, the observed results indicate that the
distributions of the local electronic structures are very
complicated even in a simple single molecule.
In the case of more complex materials such as functional
polymers, understanding the functions on the basis of themolecular structures becomes more difficult despite theirimportance. Fig. 2 shows the STM image and schematic
structure of an optically active p-conjugated polymer,
phlyphenylacetylene bearing menthoxycabonylaminogroups [(K)-poly(MtOCAPA)] [1]. Its high chiral hierarchical structure was clearly observed. If the p-conjugatedpolymer chain can be controlled in the higher order
structure, novel functions at the molecular level will become
possible due to the characteristic p-electron systems. In
order to realize this, however, we must characterize the local
molecular and electronic structures with respect to themacroscopic functions.
Another observation technique involves surface photovoltage (SPV), which reflects the characteristics of local
carrier dynamics. Fig. 3 shows an STM topographic imageof a Ag/Si(100)-2!3 surface (left) and its SPV map (right)
obtained by light-modulated STM [2]. Line profiles of the
SPV image obtained at different bias voltages are also
shown. The characteristic length of the spatial variation is583

Fig. 2. STM image of optically active p-conjugated polymer, phlyphenylacetylene bearing menthoxycabonylamino groups [(K)-poly(MtOCAPA)].

5–10 nm, indicating the importance of the dynamical
analysis of the electronic structure on the nanoscale.
These important local phenomena, observed in electronic
devices, signal transfer in biosystems and chemical
reactions, occur in the subpicosecond time range. However,resolutions in space and time have not yet been realized
simultaneously. Therefore, in order to explore thisintriguing world, the development of a new technique isurgently needed.
In this paper, we introduce our recent efforts indeveloping a new microscopy method, laser-combined
STM, which enables us to explore the ultrafast dynamics
of quantum functions with atomic resolution.
Near-field scanning optical microscopy (NSOM) may be
a complementary approach to the analysis of the dynamics
on the nanoscale in the future, the details of which will not
be discussed in this paper. We here concentrate on the workof laser-combined STMs which can realize the ultimate
spacial resolution through the measurement of the tunnelingcurrent.

2. Previous studies in this field

Fig. 1. STM image of Si nanoparticle and its cross sections at different bias
voltages.

Ultrashort optical pulse technology has allowed us toobserve transient phenomena in the femtosecond range,
which, however, has the drawback of a relatively low spatial
resolution due to the electromagnetic wavelength used. In
contrast, scanning tunneling microscopy and its related
techniques, although having a time resolution limited by thecircuit bandwidth (w100 kHz), enable us to observe spatial
dynamics at the atomic level in real space. Therefore, one of
the most challenging and exciting tasks is to combine these
two techniques to achieve unprecedented simultaneous

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H. Shigekawa et al. / Science and Technology of Advanced Materials 6 (2005) 582–588

Fig. 3. STM topograph of Ag/Si(100)-2!3 surface observed with the sample bias voltage of K2.0 V (left). SPV map for the sample voltage of 2.4 V and crosssections obtained at different bias voltages (right).

spatial and temporal resolution, i.e. femtosecond-angstrom
technology. In fact, since the development of STM, many
researchers have been making efforts to accomplish this task
[1–6]. The ultimate goal is to analyze the electronic andstructural dynamics of materials on a femtosecond timescale with atomic resolution.
There have been two major concepts proposed forachieving this goal. One is to introduce an ultrafast
photoconductive gate into the current detection line of
STM, as shown in Fig. 4. This type of microscopy is called‘photoconductively gated STM’ [3,4]. Two laser beams
consisting of a train of laser pulses are used to excite asample and switch the photoconductive gate, respectively,
as shown in Fig. 4. The photoconductive gate enables the
sampling of instantaneous tunneling current induced on thesample by the excitation laser pulses. When the current
value is recorded as a function of the delay time between
excitation and gating, the transient tunnel current ispresumed to be reproduced in a real-time scale. However,the detected signal was primarily due not to the tunnelingcurrent, but to the displacement current generated by the
coupling of two stray capacitances, one at the tunneling
junction and the other at the photoconductive gate [5].
When a signal arises from the displacement current, thelateral resolution of measurement is given by the geometricdimension of the capacitance at the tunneling junction,
typically about 1 mm, and is much worse than that of STM.
Up to now, several attempts have been made to improve thistype of STM [6]. However, since the time resolution of thistype of STM, in principle, cannot overcome the responsetime of the photoconductive gate, typically about 1 ps, thismethod is of rather limited application.
In another approach, the tunneling junction is excited bya sequence of laser pulses and the induced tunneling current
is measured as a function of the interpulse spacing, td(Fig. 5).
As a pioneering work, the carrier relaxation time at theSi(111)-7!7 surface was determined with about a 10 ns
time resolution and 1 mm spatial resolution [7]. The sample
surface just below the STM tip was irradiated by a train oflaser pulses. Since the surface potential is modulated by theirradiation due to the surface photovoltage effect, thedisplacement current can be probed by the STM tip, when
the irradiation is switched from on to off and vice versa. As
the amount of displacement current depends on the change
in band bending during the interpulse period of the laser
pulses, the signal as a function of the repetition time of the
STM
Photoconductive
gate switch

I

E0

Fig. 4. Schematic of a photoconductively gated STM.

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585

amplifier corresponds to the photoinduced tunneling current
Idiff(td) as
Idiff ðtd Þ Z Iillum ðtd ÞKIdark

Fig. 5. Schematic of a pulse-pair-excited STM.

laser pulses provides information about the band relaxation
mechanism. This technique is applicable, but the spatial
resolution is limited to w1 mm, since the signal is thedisplacement current. Moreover, since the repetition rate oflaser pulses is changed by thinning out laser pulses from theoriginal pulse train, the time resolution is limited by therepetition rate of the original pulse train, w100 MHz(10 ns).
In order to obtain a time resolution higher than therepetition rate, a new method, called ‘pulse-pair-excited
STM’ was proposed. Tunneling current measurement by
STM yields us the atomic resolution simultaneously.

3. Pulse-pair-excited STM
In this method, the illumination is composed of asequence of pulse pairs and the tunneling current ismeasured as a function of the delay time of the pulse pair.
The signal obtained by this process is complicatedcompared with signals obtained by the straightforward
approaches such as photoconductively gated STM, but it isfree from the drawback of the slow response of theelectronic circuit. Furthermore, since the time resolution isdetermined by the delay time between the two pulses inpulse pairs, instead of the repetition rate of the original laser,
it can be improved by shortening the laser pulse widthwithout any other limitation. Thus, in principle, pulse-pairexcited STM has the potential to achieve both the
femtosecond and angstrom resolutions.
However, in the measurement by pulse-pair-excited
STM, we must devise a way to detect the small componentof tunneling current that is dependent on the delay time. For
such a purpose, in general, the lock-in detection technique isused to realize the sensitive measurement. As a reference
signal for the lock-in detection, for example, the modulation
by a mechanical chopper or the repetition rate of the laser
pulses is used [6]. In both cases, the output from the lock-in

Here, Iillum(td) and Idark are the tunneling current with andwithout illumination, respectively. Td is the delay time of
the two pulses in pulse pairs. The lock-in detection method
reduces the broad-band noise from the current–voltage
converter of STM. Thus, the delay-time dependence of thephotoinduced current is expected to be clarified. However,the delay-time-dependent component is quite small
compared to the large background of the delay-timeindependent component. For example, when the laser
intensity is modulated at low frequency, thermal expansion
and shrinking of the STM tip at the modulation frequency
increases the background. Displacement current due to thephotovoltage effect, in the case of a semiconductor sample,
also increases the background. Thus, an unrealistic dynamic
range is required for the lock-in amplifier. What is worse isthat the large background fluctuates with the unavoidable
small fluctuation in the laser intensity at low frequency. The
lock-in amplifier does not eliminate this fluctuation.
Here, we summarize the requirements that must be
satisfied for the development of the time-resolved STM.
1. High signal-to-noise ratio.
2. Reduction of noise from the fluctuation of laser
intensity.
3. Removal of the thermal expansion effect of the tip andsample.
These requirements are the difficulties that remainunresolved.

4. Shaken-pulse-pair-excited STM [8–11]
To satisfy the requirements listed above, we proposed a
new technique, ‘shaken-pulse-pair-excited STM (SPPXSTM)’. Fig. 6 shows the schematic of the setup. The
tunneling junction is directly illuminated by a sequence ofFig. 6. Schematic of the shaken-pulse-pair-excited STM system.

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laser pulse pairs and average tunneling current, It (td), ismeasured as a function of the delay time between the twopulses, td. To decrease the broadband noise, the delay time td
is modulated with a small amplitude Dtd at a frequency u,
and the tunneling current is detected by a lock-in amplifier.
Since, the tunneling current It responds to the modulation as
It ðtd C Dtd sin utÞ Z It ðtd Þ C Dtd sin ut

dIt
C O2 ;
dtdthe coefficient Dtd (dIt/dtd) of the term sin u t is obtained bylock-in detection. Since, the laser intensity is not modulated
in this system, neither thermal expansion nor shrinking of
the STM tip occur. Then, with the numerical integration of
the dIt/dtd signal, It(td) can be obtained, conserving the
spatial resolution of STM.
In the pulse-pair-excited STM measurement, the firstlaser pulse in each pulse pair acts as the pump pulse to excite
and modulate the electronic structure of the sample surface,which causes deviation in the tunneling current. Then, thetunnel current is again deviated by the second laser pulse, as
shown in Fig. 7. In the case of a short delay time, the excitedstate induced by the first pulse partially remains when thesecond pulse impinges on the sample. Thus, the amount oftunneling current deviation due to the second pulse can bedifferent from that caused by the first pulse and dependent
on the delay time. Although such ultrashort spikes in thetunneling current cannot be resolved by any amplifier ofthe present STM systems, this change can be detected as theaverage value of the tunneling current, if the deviation alsochanges the total amount of tunnel current for each pulse
pair (Fig. 8). Thus, we can observe the ultrafast transientFig. 7. Relationship between the change in tunneling current and the delay
time of two laser pulses.

Fig. 8. Relationship between the average tunneling current (dotted line) andthe current photoinduced by the pulse pairs.

phenomenon by measuring the time-averaged tunnel current
as a function of the delay time.
In such measurement, the relationship between the
excitation/relaxation phenomena of interest and the timeresolved signal in the tunnel current is rather complex.
However, this method combines and conserves the ultimate
temporal resolution of the optical pump probe approach andthe ultimate spatial resolution of STM. Namely, SPPX-STM
satisfies the requirements for exploring the ultrafast
dynamics of the local quantum functions occurring inorganized small structures.

5. Experimental and results [8–10]
Here we show some experimental results. Experiments
were performed on GaNxAs1Kx (xZ0.36%), which is an
excellent candidate for future fast information transfer
systems and highly efficient solar cells, similar to other IIIV-N compound semiconductors [11,12].
For comparison, we first examined the dynamics of the
GaNxAs1Kx sample by the conventional optical pump-probe
method. The system setup was similar to that shown in
Fig. 6; Ti:sapphire excitation laser of 300 mW, central
wavelength of 800 nm, bandwidth of 30 nm, pulse width of
25 fs and oscillation frequency of 80 MHz.
The sample was first excited by an intense pump pulse
and the reflectivity just after pumping was measured by theprobe pulse with six times lower intensity of the pump pulse(Fig. 9). The intensity of the reflected probe pulse wasmeasured as a function of the delay time between pumping
and probing.
Fig. 10 shows the result. From the analysis of fitting byexponential functions, two exponential components withdecay times of 0.8 and 10 ps were obtained. These wererespectively attributed to the lifetimes of the intraband(faster one) and interband (slower one) carrier relaxation
processes, similar to the case of GaAs [12,13].

H. Shigekawa et al. / Science and Technology of Advanced Materials 6 (2005) 582–588

587

Fig. 11. Result obtained by SPPX for a GaNxAs1Kx (xZ0.36%) sample.

Fig. 9. Schematic of the conventional optical-pump-probe method.

Fig. 11 shows a result obtained by SPPX-STM for the
GaNxAs1Kx sample. The raw signal and the fitting result areplotted as a function of the delay time. The vertical axis
represents the amount of change in the tunneling current.
Here, the left-hand side of the graph is adjusted to the zero
point for simplicity, since the absolute value cannot be
determined by this experiment. The asymmetry of the graphfor the negative and positive delay times is due to thedifference between the light intensities of the two pulses (1:2 in this case; intense pulse impinging first for td!0 region).
The curves for td!0 and tdO0 were confirmed to have
similar shapes at various intensity ratios. The experimental
curve for each side of the x-axis was well reproduced by thesame function of two exponential components with fast
(w1.0 ps) and slow (w50 ps) decay times. The difference
between the baselines for the positive and negative sides
indicates the existence of a long-time-scale physical process
in the system, the origin of which is not yet clear.
The two decay times obtained by the SPPX-STM

Fig. 10. Result obtained by the optical-pump-probe technique for a
GaNxAs1Kx (xZ0.36%) sample.

Fig. 12. Relationship between time-resolved signal intensity and averagetunnel current, measured while varying of the tip-sample distance.

measurement are in similar ranges as those obtained by
the conventional pump-probe reflectivity measurement,
which suggests that this system is probing the same ultrafast
transient phenomena [14,15].
In the development of the time-resolved STM, what isalways questioned is whether the time-resolved signal isrelated to the tunnel current, because this is the key issue
that determines whether the method can have the same
superior spatial resolution as STM or not [4–8]. Fig. 12shows the time-resolved signal intensity of SPPX-STM as a

Fig. 13. Schematic of the pulse-pair-excited-multiprobe STM.

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H. Shigekawa et al. / Science and Technology of Advanced Materials 6 (2005) 582–588

function of the average tunnel current (reference current),
which was measured while varying the tip-sample distance.
There is a strong linear relationship, indicating that the timeresolved signal is indeed produced by tunneling current. In
other words, we are observing the transient phenomena
induced by photoexcitation just below the STM tip. Thus,
the result shown in Fig. 11, actually is the first successful
demonstration of probing subpicosecond dynamics using
pulsed-laser-combined STM.
In order to make the system more practical and efficient,
we are now constructing a shaken-pulse-pair-excitedmulitprobe STM, with which ultrafast electronic dynamics
at different points can be detected simultaneously (Fig. 13).
Studies on nanoscale materials for the application of our
new technique are also in progress [16–22].
6. Conclusion
A promising new design for achieving ultimate spatial
and temporal resolution, by combining a short-pulse laser
and STM, was proposed. Using this method, time-resolved
tunneling current measurement in the subpicosecond range
was successfully demonstrated. Namely, shaken-pulse-pairexcited STM (SPPX-STM) satisfies the requirements for
exploring the ultrafast dynamics of the local quantum
functions occurring in organized small structures. We hope
this new technology will promote the development of future
research on the nanoscale.

Acknowledgements
This work was supported in part by a Grant-in-Aid for
Scientific research from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan. We thank Ms Rie
Yamashita, in our group at University of Tsukuba, for her
help in preparing this paper.

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