The Omicron NanoTechnology
Newsletter Vol. 15 No. 1 December 2011

Joint Future
Towards Wafer Scale Fabrication of Graphene Based Devices

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Robust and open tailored supramolecular networks controlled by the template effect ofa Silicon Surface. Author: Frank Palmino et al.

An Oxford Instruments Company

Real-Space Imaging ofTransient Carrier Dynamics byFemtosecond Time-Resolved STMYasuhiko Terada, Shoji Yoshida, Osamu Takeuchi, and Hidemi Shigekawa
Institute of Applied Physics, University of Tsukuba, Tsukuba 305-8573 Japan"Smaller and faster" are the key
concepts in the progress of current
nanoscience and technology. Thus, amethod for exploring the transient
dynamics in small organized structures is eagerly desired. Real-space
imaging of nanoscale carrier dynamics with a wide range of lifetimes
has been realized by pulse-lasercombined scanning tunneling microscopy (STM) with a noble delay time
modulation method developed from apulse-picking technique.

Fig. 1 (a) (b) Schematic illustrations of SPPX-STM. Optical pulses from the two synchronizedTi-sapphire lasers are selectively transmitted using pulse pickers to produce a sequence ofSDLUHGSXOVHVZLWKDFHUWDLQGHOD\WLPHWG3XOVHSDLUVZLWKDFHQWUDOZDYHOHQJWKRIQP
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The real space observation of atomic-scalestructures by STM has lifted the veil fromvarious longstanding problems and is extending the frontiers of science and technology.
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development of ultrashort-pulse laser technology has enabled us to observe ultrafast
dynamics, currently in the femtosecond range.
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ited by the pulse wavelength. Therefore, since
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most challenging goals has been to combine
STM with ultrashort-pulse laser technology.
6tunnel current is measured as a function of td. (c) Relationship between raw tunneling current
I* and delay time td. (d) Measured tunneling current I as a function of delay time, where thedelay times labeled (i), (ii) and (iii) correspond to those of (i), (ii) and (iii) in (c).

Figure 1 shows schematic
illustrations of our newly
developed microscopy technique, shaken-pulse-pairexcited STM (SPPX-STM), andits basic probing mechanism.
The surface of a sample
beneath the STM tip is excitedby a sequence of paired laser
pulses with a certain delay

time td and the tunneling current I is measured as a function of td (Fig. 1a). The opticalpulses give rise to current
pulses in the raw tunnelingcurrent I* )LJF UHʏHFWLQJthe excitation and relaxation of the sample. When td
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optical pulses with the same
intensity independently

3LFR9RO1RIURP2PLFURQ1DQR7HFKQRORJ\induce two current pulses
with the same magnitude in I*)LJF L ,QFRQWUDVWZKHQ
td is short and the secondpulse illuminates the sample
in the excited state induced
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current pulse may have a different height, depending on td(Figs. 1c-(ii) and 1c-(iii)). The
signal I therefore also depends
on td, because the height difference in the second current
pulse changes the temporally averaged value of thetunneling current (Fig. 1d).
Accordingly, the relaxation
dynamics of the excited statein the target material can be
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the spatial and temporal
resolutions of a microscopy
technique are limited by thoseof the probes used, that is,tunneling current (atomicscale) and optical pulse width(femotosecond range) in thecase of SPPX-STM.
To realize SPPX-STM, wedeveloped a method enabling
the discrete and large modulation of the delay time by using
a pulse-picking technique)LJE ZKLFKIRUWKHʎUVWtime, has allowed the prob-)LJ633;670VSHFWUDREWDLQHGIRUORZWHPSHUDWXUHJURZQ*D$V /7*D$V *D1$VQ*D$VDQGK\GURJHQWHUPLQDWHG6LVDPSOHV

ing of carrier dynamics innanometer-scale structures
over a wide range of timescales. Furthermore, thismethod reduces the measurement time and hence enables
the spatial mapping of timeresolved signals, which has
been desired for a long time.
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spectra obtained for various
semiconductors. With the newmethod, carrier dynamics canbe measured using a single
microscope over a wide rangeof time scales. We can evaluate the transient dynamics
in small organized structuresconsisting of composite mate-

3LFR9RO1RIURP2PLFURQ1DQR7HFKQRORJ\rials with the spatial resolution of STM.
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of carrier dynamics in the nanosecond range
modulated on the nanoscale. Photoexcitedminority carriers (holes) captured at the surIDFHRI*D$VDUHUHFRPELQHGZLWKHOHFWURQVtunneling from the STM tip via the gap states
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(Fig. 3(a)). Understanding such a charge transport mechanism involving nanoparticles is ofgreat importance not only for the developmentof nanoscale electronic devices but also for
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cal reactions in catalysis.
Figure 3(c) shows the overlap of the STM
image in Fig. 3(b) with the map of the decay
constant of photoinduced carrier density
obtained over the surface. As shown in thecross section in Fig. 3(d) obtained along theP.8

Real-Space Imaging ofTransient Carrier Dynamics byFemtosecond Time-Resolved STM
The Research Group

3URI+LGHPL6KLJHNDZDAssociate Prof.

Assistant Prof.

e-mail:Osamu Takeuchi

Shoji Yoshidahidemi@ims.tsukuba.ac.jp

Fig. 3 (a) Schematic of the carrier recombination mechanism via the gap states. (b) STM image
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E  H +ROHFDSWXUHWLPHDVDIXQFWLRQRIQDQRSDUWLFOHVL]Hline in Fig. 3(c), the decay is
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to the enhancement of carrier
recombination via the gapstates.
Figure 3(e) shows the decay
constant as a function of the&RQDQRSDUWLFOHVL]H7KH
time constant increases withdecreasing nanoparticle size
as expected.

3D representation of the STM image of&RQDQRSDUWLFOHV*D$V

8SPPX-STM enables us to study
ultrafast dynamics simultaneously with the observationof local structures by STM.
,QJHQHUDOSKHQRPHQDVXFKas dipole formation, chargetransfer, changes in conductance, modulation of local
potential, and vibration arepossible mechanisms for producing signals. Using an elecWURPDJQHWLFʎHOGFRPELQHG
with polarized light, SPPXSTM also has the potential tobe used to investigate spindynamics.
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3URIHVVRU+6KLJHNDZDOHDGVDJURXSVSHFLDO
izing in nanoscale science and technology basedon scanning probe microscopy and related techniques. The group also has experience in synchrotron radiation experiments and has been engaged
in research on ultrashort-pulse laser technology
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research backgrounds, group members have beendeveloping new microscopy techniques and investigating applications of these techniques. The current research interests of group members includesemiconductor physics, molecular physics, lowtemperature physics and low-dimensional physics.
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