Available online at www.sciencedirect.com

Thin Solid Films 516 (2008) 2348 – 2357
www.elsevier.com/locate/tsf

Nanoscale dynamics probed by laser-combined scanningtunneling microscopy
Hidemi Shigekawa ⁎, Shoji Yoshida, Osamu Takeuchi, Masahiro Aoyama, Yasuhiko Terada,
Hiroyuki Kondo, Haruhiro Oigawa
Institute of Applied Physics, CREST-JST, 21st COE, University of Tsukuba, Tsukuba 305-8573, Japan
Available online 29 April 2007

Abstract
With the miniaturization of functional devices, the difference in the electronic properties, for example, due to the structural nonuniformity ineach element of nanoscale blocks has an ever more crucial influence on macroscopic functions. Laser-combined scanning tunneling microscopy, apotential method that enables us to probe the photoinduced carrier dynamics on the nanoscale, is reviewed with the latest results.
© 2007 Elsevier B.V. All rights reserved.
Keywords: STM; Ultrashort-pulse laser; Pump-probe technique; Nanotechnology; Ultrafast phenomena

1. Introduction“Smaller” and “faster” are the key concepts in nanoscalescience and technology. Important and interesting phenomena
in various systems, such as functional materials, electronic
devices, signal transfer in biosystems, and chemical reactions,are observed from the several tens of nanometers to the singlemolecule range in space and from the several tens ofpicoseconds to subpicosecond range in time. With the size
reduction in structures, the difference in the electronic properties, for example, due to the structural nonuniformity in eachelement, has an ever more crucial influence on macroscopic
functions. And the direct observation of the characteristics,
which provides us with the basis for the macroscopic analysis of
the results, is of great importance. Thus, for further advances, amethod of exploring the transient dynamics of the local
quantum functions in organized small structures is eagerly
desired. However, it is difficult to obtain spatial and temporal
resolutions simultaneously on this scale. Therefore, it isnecessary to develop a new method, namely, new microscopy.
Scanning tunneling microscopy (STM) [1–4] has an
excellent spatial resolution on the subangstrom scale and has
fulfilled the needs of researchers in various fields. However,⁎ Corresponding author.
URL: http://dora.ims.tsukuba.ac.jp/ (H. Shigekawa).
0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2007.04.109

since its temporal resolution is limited by the circuit bandwidth(∼ 100 kHz), increasing its potential by, for example, combining
its characteristics with those of other techniques has been
desired. One of the promising approaches is to control thematerial conditions, in STM measurement, using the techniquesof quantum optics. Ultrashort optical pulse technology in thenear-infrared to ultraviolet region has enabled us to observetransient phenomena in the femtosecond range, the opticalmonocycle region, which, however, has a drawback of a
relatively low spatial resolution due to the electromagnetic
wavelength. Therefore, since the invention of the STM almost
25 years ago, realizing the time-resolved tunneling current
measurement in the subpicosecond range by developing STM
combined with an ultrashort-pulse laser has been a challenging
subject for obtaining the ultimate spatial and temporal
resolutions simultaneously [5–23].
In this paper, we introduce our latest results obtained using
the laser-combined STM and some related techniques.
2. Electronic structures probed on the nanoscale
In the world of nanofactories, as illustrated in Fig. 1,
elemental blocks of various characteristics are integrated andorganized on a designed stage to produce desired or new
functions in a system on the macroscopic scale. However, torealize such a goal, the characterization and control of theH. Shigekawa et al. / Thin Solid Films 516 (2008) 2348–2357

structures of each element are essential. Let us see the following
examples.
Figs. 2a and b are an STM image of a Si nanoparticle of∼ 3 nm diameter on a graphite surface and the cross sections of
the same region obtained at different sample bias voltages(+ 200 mV, + 50 mV and − 50 mV), respectively. Inhomogeneous structures are observed in the cross sections. Since STM
provides information on the electronic structures at the observed
bias voltage, the results indicate that there are complex
electronic structures even at the single-molecule level (Fig.
2c). How can such a complex local dynamics work to produce asystematic function on the macroscopic scale?
In Fig. 3, contact atomic force microscopy (AFM) (a) and its
tunneling current (b) images of quantum dots, formed on a
GaAs/InP(113) surface, simultaneously obtained using a
conductive cantilever are presented. A tunneling current of
0.2–0.5 nA was measured on quantum dots with a sample bias
voltage of +0.2 V. In general, even though physical parameters
are averaged, when observed using macroscopic techniques,
inhomogeneous characteristic structures are observed; as shownin Fig. 3b. Since control of quantum dots is essential, for future
optical technology, for example, detailed analysis of theelectronic dynamics in each dot is extremely important.
Fig. 4 is a schematic illustration of the surface photovoltage(SPV). When the tip-sample distance is not sufficiently large
and the scanning tunneling microscopy/spectroscopy (STM/
STS) measurement is performed in the dark, tip induced band
bending appears as illustrated in Fig. 4a, where a p-typesemiconductor sample is used. When the sample below the
STM tip is photoilluminated with a sufficient intensity, theintroduction of photoexcited carriers reduces the band bending,resulting in the flat-band condition illustrated in Fig. 4b. Thisband bending change in the MIS structure is defined as an SPV,
which is observed in STM measurement. Since the SPV isrelated to the local carrier dynamics induced by photoillumination [25–28], three-dimensional local electronic structures canbe observed using this technique.
In Fig. 5, the bias-voltage-dependent SPV images of a
Ag/Si(011) surface nanostructure are presented. The Ag islands
formed on the Si(001) surface are marked using green lines inFig. 5a. Fig. 5b to d are the mappings of the SPV measured
at different bias voltages using light-modulated scanning tunneling spectroscopy (LM-STS) [23]. The variations in the SPV
contour are caused by the change in the depletion layer due toFig. 1. Schematic illustration of nanoscale system.

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Fig. 2. STM image of Si nanoparticle (a) and cross section of the same region
obtained at different bias voltages (b). (c) Diagram of the complex structuresinside a nanoparticle.

tip-induced band bending, which is dependent on the appliedbias voltage [24,28,29].
When this technique is applied to analyze a p–n junction
under an operating condition, carrier flow, for example,
depending on the forward bias voltage can be visualized
through the measurement of the forward bias voltage-dependent
SPV. Fig. 6 is an example obtained over a cleaved surface of a
GaAs(110) p–n junction [28]. The change in the color of the left
side, blue to red, shows the flow of holes injected from the ptype region for an increase in the forward bias voltage. The
dynamics of the doped minor carriers plays an essential role infunctional materials and devices. With the miniaturization ofsemiconductor devices, for example, the fluctuation in suchelectronic properties of each element has an ever more crucial
influence on macroscopic functions. However, the electronic
properties have been, in general, analyzed using the macroscopic behavior obtained from the data averaged over theoperating area. These are the first results that provide a solidFig. 3. Contact AFM (a) and tunneling current (b) images of quantum dots
formed on GaAs/InP(113) surface.

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Fig. 4. Schematic illustrations of metal–insulator–semiconductor (MIS)
structures in cases (a) without and (b) with photoillumination (BH: local barrierheight).

basis for elucidating the mechanism of the carrier transport
properties predicted by using the macroscopic analysis.
Can we obtain time-resolved information about the abovementioned nanoscale dynamics using STM? In the next section,some examples of time-resolved measurements by STM arediscussed.
3. Direct observation of dynamics by STM
In STM, a sharp tip is placed close to a target material, andinformation on the region underneath the STM tip is probed
using a tunneling current (Fig. 7). In general, the STM is used
as a probe under an equilibrium condition. However, whensome perturbations, such as in temperature, electric field andphotoillumination, are added from outside, we can analyze thedynamics of the system by observing responses to the change
in the conditions.
Many works have been performed in this respect [2–4]. Fig.
8a is a schematic illustration of the structure of the azobenzene
molecule. This molecule changes its conformation between

Fig. 6. Schematic illustration of a GaAs(100) pn junction and a series of carrierflow mappings of injected holes at different forward bias voltages.

Fig. 5. Topographic STM (a) and bias-voltage-dependent surface photovoltage(SPV) images (b to d) obtained for Ag/Si(011) surface nanostructures(30 nm × 30 nm).

Fig. 7. STM setup with additional perturbation from outside (light this case).

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tions even at 80K [36,37]. Since the tunneling current depends onthe distance between the STM tip and the target material,
structural changes can be determined from the corresponding
change in the tunneling current. Fig. 10b shows the timedependence of the tunneling current measured by STM with the
tip held just above a Si dimer. The tunneling current changes
between two well-defined states, as expected. In this way, we canobserve the dynamics of rapid structural changes. This method
can be used to analyze the change in the dynamics of a single
molecule [30,38,39]. However, as mentioned in the introduction,
the maximum temporal resolution of STM is limited to∼100 kHz. Recently, chemical reaction and molecular motion
induced by ultrashort laser pulses were successfully analyzed[40,41], but these measurements provide only a snapshot of
the dynamics and still leave room for further advancement.
4. Laser-combined STM
4.1. Optical pump-probe techniqueFig. 8. (a) Schematic illustration of photoisomerization of azobenzene molecule.
(b) Illustration of functional control using an azobenzene molecule.

trans and cis forms under visible (Vis, 440 nm) and ultraviolet(UV, 360 nm) lights. Thus, when functional molecules are
modified using this molecule, new functions associated withswitching mechanisms can be added to the original functions
such as molecular recognition, sensing, and memory, as shownin Fig. 8b. Therefore, it is very important to analyze thestructural change of the azo-molecule ([4-(phenyldiazenyl)
phenyl]-N-(2-sulfanylethl)carboxyamide) at the single-molecule level [30–32].
Fig. 9a is an illustration of the STM setup [30]. The STM tip
is placed just above an isolated azo-molecule embedded in a ndodecanethiol (C12) self-assembled monolayer (SAM) film
formed on a Au(111) substrate. Samples were prepared bydipping gold-coated mica substrates into a solution with thetarget azo-molecule, and dodecanthiol molecules that were usedas spacers to isolate azo-molecules. A photoinduced structuralchange was observed using STM under photoillumination as the
wavelength was varied between Vis and UV wavelengths,
alternately. Since an azo-molecule has a longer conformation in
the trans form, it is supposed to be brighter under Vis light.
Fig. 9b is the result of the direct observation of theconformational change. The azo-molecule reversibly changes
its image from bright to dark under Vis and UV lights,
respectively, as expected. This change can be observed using
STM at the single-molecule level. When we change thetemperature, we can observe temperature-dependent phasetransitions of materials at an atomic resolution [2–4,33–35].
However, it requires a long time to image such structuralchanges, for example, one minute for each image.
Without imaging, we can more rapidly observe structuralchanges. Fig. 10a is a schematic of such measurement. An STM
tip is held just above a target, for example, a Si dimer on aSi(011) surface which rapidly flip-flops between two conforma-

Let us see here briefly how fast another method, the opticalpump-probe technique is [21,42]. When ultrashort laser pulses
are produced at 80 MHz, the original pulse train is ∼ 10 ns
between two subsequent pulses, as highlighted in Fig. 11b. Eachpulse is divided into two pulses with a delay time td (Fig. 11a),
to form a train of pulse pairs as illustrated in Fig. 11c. We cancontrol the delay time by changing the optical length of thedelay time system shown in Fig. 11a.

Fig. 9. (a) Schematic of STM measurement setup. (b) Photoisomerization ofazobenzene molecule directly observed by STM. Vis and Uv indicate visible andultraviolet lights, respectively.

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Fig. 12. Schematics of optical pump-probe measurement system (a) andobtained signal (b).

excited state (τcool = 5.3 ps, and τdecay = 400 ps). These dynamics
are attributed to the intra- and inter-band transitions ofphotocarriers [17,42–44].
4.2. Difficulties in combining STM with optical pump-probe
techniqueFig. 10. Schematic of STM setup for observing flip-flop motion of Si dimer (a)and its result (b). Tunneling current changes between two well-defined states.

Now, one may want to combine the two techniques toachieve the ultimate resolutions in space and time simultaneously. However, there are many difficulties we have toovercome. Let us see two examples.

First pulses are used as a pump to excite the sample surface
and second pulses are used as a probe to observe the relaxationof the excited state induced by the pump pulse, as shown in Fig.
12a. Since the reflectivity of the second pulses depends on thesurface conditions, if the reflectivity of such pulses is measured
as a function of the delay time, we can obtain information on therelaxation of the excited state through the change in reflectivity(Fig. 12b). In this case, the time resolution is limited only by thepulse width, in the ∼ 1 fs range.
For comparison, spectra of the GaNAs sample obtained by
the optical pump-probe technique is presented in Fig. 13 [17].
The reflectivity of the second pulses is plotted as a function of
the delay time, which has two relaxation components of theFig. 11. (a) Schematic of optical delay time system. (b) Original laser pulse train
oscillating at 80 MHz. (c) Train of laser pulse pairs (td: delay time).

Fig. 13. (a) Result obtained by optical-pump-probe technique for GaNxAs1−x(x = 0.36%) sample. Excitation: 800 nm, pump (3 mW), probe (0.3 mW). (b)Fitting function. (c) Schematic of the excitation and relaxation process (CB:conduction band, BG: band gap, VB: valence band).

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4.2.1. Temporal resolution of STM
As mentioned, the temporal resolution of STM is limited to∼ 100 kHz, worse than 10 μs. Therefore, although an ultrashortpulse laser has now provided a temporal resolution of ∼ 1 fs, we
still cannot observe the dynamics of rapid changes, for example,
in the picosecond range, by STM (Fig. 14). Since thedevelopment of STM, many researchers have pursued thisgoal [5–22] but have not yet succeeded; probing the dynamics
of rapid changes based on a tunneling current is a critical point.
4.2.2. Thermal expansion of STM tip
Tunneling current depends exponentially on tip-sample
distance, when the distance changes by 0.1 nm, tunnelingcurrent changes by one order. Fig. 15 shows the change intunneling current as a function of time, representing the stabilityof the tunneling current. A tungsten tip and a Au(111) surface
were used for the measurement. As Fig. 15a indicates, the signal
obtained without photoillumination is very stable; however,when the tunnel gap is photoilluminated with chopped light,
tunneling current is modulated at the same frequency due to thethermal expansion and shrinking of the STM tip and the sample
surface, as indicated in Fig. 15b.
As will be discussed in the next section, we need to modulate
the excitation to use a lock-in technique for measuring the weaktarget signal induced by ultrashort laser pulses. Since the change
in tunneling current due to thermal expansion is large, theelimination of this thermal expansion effect is another criticalFig. 15. Schematic of STM setup (upper) and the tunneling current as a functionof time obtained (lower) (a) without photoillumination, (b) under chopped light,(c) by shaken pulse-pair-excited STM.

problem that we have to overcome to combine STM with theultrashort-pulse laser technique [13,22,45–47].
5. How to combine STM with optical pump-probe
technique?
5.1. How to observe fast dynamics by STM?

Fig. 14. (a) Schematic of the setup laser-combined STM and (b) A desiredchange in tunneling current excited by ultrashort-laser pulse.

Since the development of STM, many researchers haveexpended effort to observe fast dynamics by STM. The ultimate
goal of this study is to analyze the electronic and structuraldynamics of materials on the femtosecond scale at an atomic
resolution.
There have been two major concepts proposed for achieving
this goal. One is to introduce an ultrafast photoconductive gate
into the current detection line of STM. This type of microscopy
is called “photoconductively gated STM” [5]. Two laser beams
consisting of a train of laser pulses are used to excite a sample
and switch the photoconductive gate (Fig. 16). The photoconductive gate enables the sampling of instantaneous tunneling current induced on the sample by the excitation laser pulses.
When the current is recorded as a function of the delay time
between excitation and gating, the transient tunneling current ispresumed to be reproduced on a real-time scale. However, thedetected signal is primarily due not to the tunneling current but
to the displacement current generated by the coupling of twostray capacitances; the first at the tunneling junction and theother at the photoconductive gate.
In another approach, a tunneling junction is excited by asequence of laser pulses, and an induced tunneling current is2354

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changed by thinning out the laser pulses from the original pulse
train, the time resolution is limited by the repetition rate of theoriginal pulse train, ∼ 100 MHz (10 ns).
To obtain a time resolution higher than the repetition rate, a
new method, called “pulse-pair-excited STM” was proposed[6–22]. The measurement of the tunneling current using STM
ensures an atomic resolution.
5.2. Pulse-pair-excited STM

measured as a function of interpulse spacing (Fig. 17). As
pioneering work, the carrier relaxation time at the Si(111)-7 × 7
surface was determined with an approximately 10 ns time and a
1-μm spatial resolution [9]. The sample surface just under the
STM tip was irradiated with a train of laser pulses because thesurface potential is modulated by the irradiation due to thesurface photovoltage effect. The displacement current can beprobed using the STM tip when the irradiation is switched fromon to off, and vice versa. Since displacement current depends onthe change in band bending during the interpulse period of thelaser pulses, the signal as a function of the repetition time of thelaser pulses provides information on the band relaxation
mechanism. This technique is applicable, but the spatial resolution is limited to ∼ 1 μm, since the signal is a displacement
current. Moreover, since the repetition rate of laser pulses is

For the conventional optical pump-probe technique, as
shown in Fig. 12a, the electronic structures of a target material
are excited using the pump laser pulses and then relax over time.
The reflectivity of the second laser pulses is measured as afunction of the delay time. Thus, the change in reflectivity withthe delay time is analyzed as a signal associated to the relaxationof the excited state (Fig. 12b).
For a laser-combined STM, the sample surface under the
STM tip is modulated with two laser pulses, and the change inthe tunneling current depending on the delay time is analyzed.
One is called junction-mixing STM (JM-STM), in which thetunneling impedance is electrically modulated with a combination of photoconductive switches and laser pulse pairs; thus, thetunneling current produces a change due to the inherent
nonlinearity of the current-voltage characteristic [6–14]. In
another approach, the surface under the STM tip is photoilluminated with the paired pulses (Fig. 18). Then the change inthe optical pump-probe signal or the average tunneling current
instead of the reflectivity of the second pulses is measured as afunction of the delay time td. The former (JM-STM) is similar tothe conventional optical pump-probe technique and the effect of
the STM tip gives us the local information [15], while the latterprovide us with the time-resolved tunneling current with the
STM spatial resolution.
Let us determine the probe for a pulse-pair-excited STM. In
Fig. 19 the relationship between the tunneling current induced
by a pair of laser pulses and the delay time of the two pulses arepresented. When the delay time is long, change in the tunnelingcurrent induced by the two laser pulses contributes to theaverage tunneling current independently, and their amounts are
the same. For a delay time shorter than the relaxation time of theexcited state induced by the first pulse, and for the nonlinear

Fig. 17. Schematic of pulse-laser-excited STM.

Fig. 18. Schematic of pulse-pair-excited STM.

Fig. 16. Schematics of a photoconductively gated STM system (a) and its
measurement scheme (b).

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Fig. 19. Relationship between tunneling current induced by paired laser pulses
and delay time of the two pulses.

interference between excitations by the two pulses, thetunneling current induced by the second pulses depends onthe delay time. In such a case, the average current also changes
as a function of the delay time. Thus, the dynamics of theelectronic structure of a target material can be probed using thetunneling current, at the pulse width resolution, that is, in the
femtosecond range.
5.3. How to treat thermal expansion?
Since the signal is weak, we need to use a lock-in
detection measurement method. In such a case, excitation ismodulated at a frequency and the corresponding change intunneling current is measured as a signal. However, as
mentioned in Section 4.2.2, intensity modulation (Fig. 20a)strongly influences the stability of the tunneling current (Fig.
15b). A promising way is to modulate the delay time insteadof intensity (Fig. 20b), in which the modulation frequency
can be chosen independently of any noises. Fig. 15c is thestability of the tunneling current measured using this method
under photoillumination. To evaluate the thermal expansion
effect caused by photoillumination, a Au surface was used asa sample in which a negligible photoinduced signal is
expected. In fact, no thermal expansion effect in Fig. 15b isobservable in this case, indicating that the weak target signal
can be measured using this method.

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the femtosecond time-resolved STM that we developed. Eachlaser pulse is divided into two pulses to form a train of pairpulses. The sample just under the STM tip is photoilluminated and the average tunneling current is measured as afunction of delay time. Since the lock-in method is used tomeasure a weak signal with the modulation of delay time, thefirst signal obtained is differentiated tunneling current (dI/dtd),which is a function of delay time. Therefore, after thenumerical integration of the signal, a spectrum could beobtained that was compared with the conventional opticalpump-probe technique shown in Fig. 13a.
Fig. 22 is the result obtained from the SPPX-STM for a
GaNAs sample. The differentiated spectrum of the dashed line
is obtained first as a signal. Using the numerical integration of
the signal, the tunneling current deviation can be estimated fromthe solid line, which can be compared with the result of Fig. 13a.
As shown in Fig. 22, tunneling current changes in thepicosecond range; however, the shape of the spectrum may
differ from that obtained using the optical pump-probe
technique. Because the probe is tunneling current in thiscase, the signal appears even in the negative-delay-time
region. Fig. 22 also shows the fitting function (dashed-dottedline) consistingof two exponential functions andP of the sum STM
a constant,Þ þ constði ¼ 1; 2Þ, that gives
i Ai expðs=si
the best fit to the ΔI curve. The ΔI curve consists of two
decay components that reflect ultrafast phenomena inducedby laser illumination. The faster component (τ1STM = 118 ps)
has a positive amplitude (A1 N 0), while the slower one (τ2STM =550 ps) has a negative amplitude (A2 b 0).
Roughly speaking, there are two different types ofnonlinearity: (i) divergent and (ii) saturation prone [17]. The
divergent and saturation-prone nonlinearities are expressed
here as It (t) ∝ n(t)k (k N 1) and It (t) ∝ n(t)k (k b 1), respectively;
where It is the transient tunneling current and n is thephotocarrier density. Under the assumption of exponential
decay of carrier density, n(t) ∼ exp(− t/τdecay), the divergent
nonlinearity is characterized by a maximum in ΔI at π=0
where a decrease is observed when π is increased, whereas

6. Shaken pulse-pair-excited STM (SPPX-STM)
Here, new microscopy using the method described in 5.3
is explained in more detail [17–22]. Fig. 21 is a schematic ofFig. 20. (a) Intensity and (b) delay-time modulations of light for lock-in
detection.

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Fig. 21. Schematic of shaken pulse-pair-excited STM system.

the saturation-prone nonlinearity is characterized by aminimum in ΔI at π=0 and ΔI increases when π is increased.
From the characteristics of the delay-time dependence of thesignals, the slower one can be attributed, for example, to theprocess of photocarrier recombination, which is in good
agreement with that obtained using the optical pump-probe
technique (Fig. 13) [17].
To confirm that what observed is a tunneling current, thechange in signal intensity was measured as a function of total
tunneling current by changing the distance between the tip
and the sample. As in Fig. 23, there is a clear linear relationship between these parameters, indicating that the signal is atunneling current and not another signal such as photoemission [18]. Since the tunneling current is measured as theprobe in this method, rapid dynamics can be observed at the
STM resolution.
Recently, we have succeeded in developing a new version
where the S/N ratio is 100 times better than that of the present
technique. With this method, we are now conducting the

Fig. 22. Result obtained by SPPX for GaNxAs1−x (x = 0.36%) sample.

direct mapping of photo carrier dynamics in various materials on the nanoscale; details of which will be publishedelsewhere.
7. Conclusion
We have reviewed the basis of laser-combined STM,
especially the femtosecond time-resolved STM that enables
us to observe the dynamics of electronic structures withthe ultimate spatial and temporal resolutions. This newtechnique is expected to contribute to advance future
research in nanoscale science and technology, in terms of
the ultimate temporal and spatial resolutions.
The work about SPPX-STM was supported in part by
a Grant-in-Aid for Scientific research from the Ministry of
Education, Culture, Sports, Science, and Technology ofJapan. We thank Ms. Rie Yamashita in our group at
University of Tsukuba for her help in preparing thispaper.

Fig. 23. Relationship between time-resolved signal intensity and averagetunneling current, measured while varying tip-sample distance.

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