Mater. Res. Soc. Symp. Proc. Vol. 901E © 2006 Materials Research Society

0901-Ra15-01.1

How to realize ultimate spatial and temporal resolutions by laser-combined scanning tunnelingmicroscopy?
Hidemi Shigekawa*, Osamu Takeuchi, Masahiro Aoyama, Yasuhiko Terada,
Hiroyuki Kondo and Haruhiro Oigawa
Institute of Applied Physics, CREST, 21st COE, University of Tsukuba,
Tsukuba 305-8573 Japan* http://dora.ims.tsukuba.ac.jp/
Abstract
By combining scanning tunneling microscopy (STM) and the optical pump-probe technique using afemtosecond pulse laser, we have developed a new microscopy, shaken pulse-pair-excited STM (SPPX-STM), thatenables us to observe the dynamics of electronic structures with the ultimate spatial and temporal resolutions.
1. Introduction
At present, smaller and faster are the keywords in nanoscale science and technology. Actually,
important and interesting phenomena occurring in various systems, such as functional materials, electronic
devices, signal transfer in biosystems, and chemical reactions, are observed at several tens of nanometer to thesingle-molecule level in space and in the several tens of picosecond to subpicosecond range in time. However,
it is extremely difficult to realize spatial and temporal resolutions simultaneously on this scale. Therefore, todetermine the possibilities of realizing such resolutions, it is necessary to develop a new method. That is what
we have been challenging to do, namely, the development of a new microscopy, the laser-combined scanningtunneling microscopy (STM) [1-6].
STM has an excellent spatial resolution on the subangstrom scale, but, its temporal resolution is limited
to ~100kHz. In contrast, the ultrashort pulse laser technique enables the observation of electronic dynamics inthe femtosecond range, but its spatial resolution is generally limited by wavelength. Therefore, it is desired to
combine these two techniques to realize the ultimate spatial and temporal resolutions simultaneously.
2. Electronic structures on nanoscale.
In the world of nanofactories, as illustrated in Fig. 1, elemental blocks with various characteristics areintegrated and organized on a designed stage to produce desired or new functions in a system on themacroscopic scale. However, to realize such a goal, the characterization and control of the structures of each

Figure 1. Schematic model of nanoscale
system.

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element are essential. Let us see two examples.
Figures 2(a) and 2(b) show an STM image of a Si nanoparticle of ~3nm diameter on a graphite surface
and the cross sections of the same region obtained at different bias voltages, respectively. Inhomogeneous
structures are observed in the cross sections. Since STM provides information on the electronic structures atthe observed bias voltage, the results indicate that there are complex electronic structures even at thesingle-molecule level. How can complex local dynamics work to produce a systematic function on themacroscopic scale?

(a)

(b)

+200mV

Figure 2. STM image of Si nanoparticle(a) and cross section of the same region
obtained at different bias voltages (b).

+50mV

-50mV

Figure 3 shows bias-voltage-dependent surface photovoltage (SPV) images of a Ag/Si(011) surface
nanostructure. The Ag islands formed on the Si(001) surface are marked by green lines in Fig. 3(a). Figures
3(b) to 3(d) show the mappings of SPV measured at different bias voltages by light-modulated scanningtunneling spectroscopy (LM-STS) [7,8]. The differences are variations in the depth of the depletion layer withapplied bias voltage due to tip-induced band bending [9]. Since SPV is related to the local carrier dynamics
induced by photoillumination, three-dimensional electronic structures can be observed by this technique. Whatis desired next is to probe the dynamics of fast change of electronic structures on the nanoscale.
Can we obtain information on the above-mentioned nanoscale dynamics by STM?
3. Observation of dynamics of electronic structures by STM

Figure 3. Bias-voltage-dependent surface photovoltage (SPV) images obtained for Ag/Si(011)
surface nanostructures (30nm x 30nm).

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In STM, we place a sharp tip close to the target material. Information on the region underneath the
STM tip is probed by tunneling current. What is imaged by STM is in general a structure under an equilibrium
condition. However, when some perturbations in conditions, such as temperature, electric field andphotoillumination, are added from outside, we can analyze the dynamics of the system by observing
responses to the change in the conditions.
Figure 4(a) shows the structure of the azobenzene molecule. This molecule changes its conformation
between trans and cis forms under visible (Vis, 440nm) and ultraviolet (UV, 360nm) lights. Therefore, whenfunctional molecules are modified using this molecule, new functions associated with switching mechanisms
can be added to the original functions such as molecular recognition, sensing, and memory, as shown in Fig.
4(b). Thus, it is very important to analyze the structural change of the azo-molecule([4-(phenyldiazenyl)phenyl]-N-(2-sulfanylethl)carboxyamide) at the single-molecule level.

Figure 4. (a) Schematic of photoisomerization of azobenzene molecule. (b) Schematic offunctional control using an azobenzene molecule.

Figure 5(a) shows a schematic of the STM setup [10]. The STM tip is placed just above an isolated

Figure 5. (a) Schematic of STM measurement setup. (b) Photoisomerization of azobenzene
molecule directly observed by STM. Vis and Uv indicate visible and ultraviolet lights, respectively.

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azo-molecule embedded in a n-dodecanethiol (C12) self-assembled monolayer (SAM) film formed on aAu(111) substrate. Samples were prepared by dipping gold-coated mica substrates into a solution with thetarget azo-molecule, and dodecanthiol molecules that were used as spacers to isolate azo-molecules. A
photo-induced structural change was observed by STM under photoillumination with the wavelength varied
between Vis and UV wavelengths alternately. Since an azo-molecule has a longer conformation in the transform, it is supposed to be brighter under Vis light.
Figure 5(b) shows the result of direct observation of the conformational change. The azo-molecule
reversibly changes its image from bright to dark under Vis and UV lights, respectively, as expected. Thischange can be observed by STM at the single-molecule level. When we change temperature, we can observe
temperature-dependent phase transition of materials at atomic resolution [9, 11-16]. However, it requires along time to image such structural changes, for example, one minute per each image.
Without imaging, we can more rapidly observe structural changes. Figure 6(a) shows a schematic ofsuch measurement. An STM tip is held just above a target, for example, a Si dimer on a Si(011) surface which
rapidly flip-flops between two conformations even at 80K [17]. Since tunneling current depends on the
distance between the STM tip and the target material, structural changes can be determined from thecorresponding change in tunneling current. Figure 6(b) shows the time dependence of tunneling current
measured by STM with the tip held just above the same Si dimer. Tunneling current changes between two
well-defined states, as expected. In this way, we can observe the dynamics of faster structural changes.
This method can be used to analyze the change in the dynamics of a single molecule [10]. However, as
mentioned in the introduction, the maximum temporal resolution of STM is limited to ~100kHz.

Fig. 6. 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.

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4. Optical pump-probe technique
Let us see here briefly how fast another method, namely, the optical pump-probe technique, is [5, 18].
When ultrashort laser pulses are produced at 80MHz, the original pulse train has a ~10ns time distance
between two subsequent pulses, as shown in Fig. 7(b). Each pulse is divided into two pulses with a certain
delay time (Fig. 7(a)) to form a train of pulse pairs, as shown in Fig. 7(c). We can control delay time bychanging the optical length of the delay time system shown in Fig. 7(a).
First pulses are used as a pump to excite the sample surface and second pulses are used as a probe toobserve the relaxation of the excited state induced by the pump pulse, as shown in Fig. 8(a). Since the(a)Figure 7. (a) Schematic of optical delay timesystem. (b) Original laser pulse train oscillating
at 80MHz. (c) Train of laser pulse pairs.

probe
pump
Delay Timeoriginal pulse train

~10ns

(b)
td

(c)
train of pulse pairs

reflectivity of the second pulses depends on the surface conditions, if the reflectivity of such pulses is measured
as a function of delay time, we can obtain information on the relaxation of the excited state through the change
in reflectivity (Fig. 8(b)). In this case, time resolution is limited only by the pulse width, namely, in the ~1fs
range.

(b)

Figure 8. Schematics of
opticalpump-probe
measurement system (a)and obtained signal (b).

Probe signal(a)Delay time td

For comparison, a spectrum of GaAs sample obtained by the optical pump-probe technique is shownin Fig. 9. The reflectivity of the second pulses is plotted as a function of delay time, which shows the relaxationof the excited state in the picosecond range. The signal consists of two components’ lifetimes which are ~0.8ps
and ~10 ps. These dynamics are attributed to the intra- and inter-band transitions of photocarriers [18].

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Figure 9. Result obtained
optical-pump-probe technique
GaNxAs1-x (x=0.36%) sample.

byfor

5. Difficulties in combining STM with optical pump-probe techniqueNow, one may want to combine the two techniques to achieve the ultimate resolutions in space andtime simultaneously. However, there are many difficulties we have to overcome [5]. Let us see two examples.
5-1. Temporal resolution of STM
As has been mentioned, the temporal resolution of STM is limited to ~100kHz, worse than 10μs.
Therefore, although an ultrashort pulse laser has now provided a temporal resolution of ~1fs, we still cannot
observe the dynamics of a fast change, for example, in the picosecond range, by STM (Fig. 10). Since thedevelopment of STM, many researchers have pursued this goal [5, 19-23], but have not yet succeeded
completely. Thus probing the dynamics of a fast change based on tunneling current is a critical point.

Figure 10. Change in tunneling current excitedby ultrashort laser pulse.

5-2. Thermal expansion of STM tip
Tunneling current depends exponentially on
tip-sample distance, namely, when the distance changes by 0.1nm, tunneling current changes one order.
Therefore, it is a critical problem that we have to solve [1, 2, 5].
Figure 11 shows the change in tunneling current as a function of time, representing the stability of0901-Ra15-01.7

tunneling current. A tungsten tip and a Au(111) surface were used for the measurement. As Fig. 11(a) shows,the signal obtained without photoillumination is very stable; however, when the tunnel gap is photoilluminatedwith chopped light, tunneling current is modulated at the same frequency due to the thermal expansion andshrinking of the STM tip and the sample surface, as shown in Fig. 11(b).
As will be discussed in the next section, we need to modulate the excitation to use a lock-in techniquefor measuring the weak target signal induced by ultrashort laser pulses. Since the change in tunneling current
due to thermal expansion is large, the elimination of this thermal expansion effect is another critical problem
that we have to overcome to realize the combination of STM with the ultrashort pulse laser technique.

Figure 11. Tunneling current withtime obtained (a) without
photoillumination,(b) under
chopped light, (c) by our method,
shaken pulse-pair-excited STM.

6. How to combine STM with optical pump-probe technique?
6-1. How to observe fast dynamics by STM?
Since the development of STM, many researchers have expended effort to observe fast dynamics by STM.
The ultimate goal is to analyze the electronic and structural dynamics of materials on the femtosecond scale atatomic resolution.
There have been two major concepts proposed for achieving this goal [19-25]. One is to introduce anultrafast photoconductive gate into the current detection line of STM. This type of microscopy is called“photoconductively gated STM”. Two laser beams consisting of a train of laser pulses are used to excite asample and switch the photoconductive gate. 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 afunction of the delay time between excitation and gating, transient tunneling current is presumed to be
reproduced on a real-time scale. However, the detected signal is primarily due not to tunneling current, but tothe displacement current generated by the coupling of two stray capacitances, one at the tunneling junction andthe other at the photoconductive gate.
In another approach, a tunneling junction is excited by a sequence of laser pulses and induced tunneling0901-Ra15-01.8

current is measured as a function of interpulse spacing.
As pioneering work, the carrier relaxation time at the Si(111)-7x7 surface was determined with anapproximately 10 ns time resolution and a 1 µm spatial resolution. The sample surface just under the STM tip
was irradiated with a train of laser pulses. Because surface potential is modulated by the irradiation due to thesurface photovoltage effect, displacement current can be probed using the STM tip when the irradiation isswitched from on to off, and vice versa. As 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 laser pulses
provides information on the band relaxation mechanism. This technique is applicable, but the spatial resolution
is limited to ~1µm, since the signal is displacement current. Moreover, since the repetition rate of laser pulses ischanged by thinning out laser pulses from the original pulse train, time resolution is limited by the repetition
rate of the original pulse train, ~100MHz (10ns).
To obtain a time resolution higher than the repetition rate, a new method, called “pulse-pair-excited
STM” was proposed [2, 5, 6]. Tunneling current measurement by STM ensures atomic spatial resolution.
6-2. Pulse-pair-excited STM
For the conventional optical pump-probe technique, as shown in Fig. 8(a), the electronic structures of atarget material are excited by the pump laser pulses and then relax with time. The reflectivity of the second
laser pulses is measured as a function of delay time. Thus, the change in the reflectivity with delay time isanalyzed as a signal associated with the relaxation of the excited state (Fig. 8(b)) [5, 18].
For a laser-combined STM, the sample surface just under the STM tip is similarly photoilluminatedwith the paired pulses. Average tunneling current is measured as a function of delay time instead of thereflectivity of the second pulses. Let us determine the probe for a pulse-pair-excited STM. Figure 12 shows therelationship between the tunneling current induced a pair of laser pulses and the delay time of the two pulses.
When the delay time is large, change in the tunneling current 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(a)Figure 12. Relationship between
tunneling current induced by pairedlaser pulses and delay time of thetwo pulses.

(b)
Measuredcurrent leveltd0901-Ra15-01.9

relaxation time of the excited state induced by the first pulse, and for the nonlinear interference between
excitations by the two pulses, the tunneling current induced by the second pulses depends on delay time. In
such a case, average current also changes as a function of the delay time. Thus, the dynamics of the electronic
structure of a target material can be probed on the basis of tunneling current, at the pulse width resolution,
namely, in the femtosecond range.
6-3. How to treat thermal expansion?
Since the signal is very weak, we need to use a lock-in detection measurement. In such a case,
excitation is modulated at a certain frequency and the corresponding change in tunneling current is measured
as a signal. However, as mentioned in section 5-2, intensity modulation strongly influences the stability oftunneling current. What we did is to modulate delay time instead of intensity, in which modulation frequency
was chosen independently of that of any noises. Figure 11(c) shows the stability of the tunneling current
measured by this method under photoillumination. To evaluate the thermal expansion effect caused byphotoillumination, we used a Au surface as a sample in which little photoinduced signal is expected to occur.
Actually, no thermal expansion effect shown in Fig. 11(b) was observed in this case, indicating the possibility
for measuring the weak target signal with our method.
6-4. Shaken pulse-pair-excited STM (SPPX-STM)
Figure 13 shows a schematic of the femtosecond time-resolved STM that we developed. Each laser
pulse is divided into two pulses to form a train of pair pulses. The sample just under the STM tip is
photoilluminated and average tunneling current is measured as a function of delay time. Since the lock-in
method is used to measure a weak signal with the modulation of delay time, the first signal obtained isdifferentiated tunneling current, which is a function of delay time. Therefore, after the numerical integration of
the signal, we can obtain a spectrum that we can compare with that obtained by the conventional opticalpump-probe technique.

Figure 13. Schematic of shaken
pulse-pair-excited STM system.

7. Results of SPPX-STM
Figure 14 shows the result obtained by SPPX-STM for a GaAs sample. As is explained in section 6-3,

0901-Ra15-01.10

differentiated spectrum is obtained first as shown in Fig. 14 by black lines for in-phase (upper) andout-of-phase (lower) signals. By the numerical integration of the in-phase signal, tunneling current deviation
can be obtained as drwan in the red graph, which can be compared with that in Fig. 9.
As shown in Fig. 14, tunneling current changes in the picosecond range; however, the shape of the
Figure 14. Result obtained by SPPX
for GaNxAs1-x (x=0.36%) sample.

spectrum may differ from that obtained by the optical pump-probe technique. Because the probe is tunnelingcurrent in this case, the signal appears even in the negative-delay-time region. The asymmetry of the signals
between the positive- and negative-delay-time regions is due to the difference in intensity between the pumpand probe pulses. The signal consists of two components’ lifetimes are ~0.5ps and ~50ps, which are in good
agreement with these obtained by the optical pump probe technique. To confirm that what we observed istunneling current, we measured the change in signal intensity as a function of total tunneling current bychanging the distance between the tip and the sample. As shown in Fig. 15, there is a clear linear relationshipbetween these parameters, indicating that the signal is tunneling current and not other signals, i.e., those causedby photoemission observed in previous work [2,5,19-25].
Since tunneling current is measured as probe in this method, fast dynamics can be observed principallyat STM resolution.

150

Figure 15. Relationship between
time-resolved signal intensity andaverage tunneling current, measured
while varying tip-sample distance.

100
50
0
0

100

200

300

400

500

600

Reference Current [pA]

8. Conclusion
We have been developing a new femtosecond time-resolved STM, shaken pulse-pair-excited STM(SPPX-STM), and have obtained partial results. We hope that our new technique will contribute to advancefuture research in nanoscale science and technology, in terms of the ultimate temporal and spatial resolutions.

0901-Ra15-01.11

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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