12
Laser-Combined STM and Related Techniques
for the Analysis of Nanoparticles/Clusters
Hidemi Shigekawa1, Shoji Yoshida1,
Masamichi Yoshimura2 and Yutaka Mera3
1University of Tsukuba

2Toyota Technological Institute3University of TokyoJapan
1. Introduction
Nanoscale particles and clusters have been attracting considerable attention fromresearchers and engineers from fundamental and practical viewpoints owing to their highpotential for providing an extremely wide range of functional characteristics compared withordinary solid materials, such as chemical reactivity and electrical, magnetic, optical andmechanical properties [1-5]. In fact, nanoparticles with novel functions have been realized invarious fields including catalysis, biology, plasmonics, electronic devices, magnetism and soforth, on the basis of their wide range of properties. The modification of nanoparticle
surfaces is producing further advances in the development of functions including those ofcomposite materials.
For the further development of novel functions based on nanoparticles/clusters and to
optimize their use, it is essential to understand the physics and chemistry of such materials
in relation to their macroscopic functions. However, because nanoparticles/clusters are
generally defined as particles with diameters of 1-100 nm (1-10 nm in the field of
nanotechnology), conventional analysis techniques are considered to average theinformation of nanostructures over the ensemble, limiting the understanding of individual
characteristics. Furthermore, using conventional methods, analysis of the effect of local
structures in each element such as atomic-scale defects, which are considered to determine
the overall characteristics of small materials, is difficult. Therefore, the introduction of new
methods for the analysis of these highly functional small materials is eagerly awaited.
Scanning tunneling microscopy (STM) is one of the most promising techniques for such
purposes. The characteristics of materials can be obtained at the atomic scale not only for
their surface but also for their inner structures including the transient dynamics.
Furthermore, external perturbation such as by thermal, mechanical or electromagnetic
excitation enables advanced measurements. Among the various STM techniques useful forthe study of nanoparticles/clusters, STM in combination with optical technologies, whichenables probing of the response of local electronic structures to optical treatment, is aninteresting approach for considering the future applications of such materials. On the other270

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hand, sample preparation is an important factor in the analysis of nanoscale materials.
Tunneling current and bias voltage can be used to modify target materials to obtain a deeper
understanding of their characteristic properties. In addition, the STM tip plays an essential
role in measurement. A Ag tip, for example, is used to enhance the effect of local excitation,
and a carbon nanotube (CNT) tip is an excellent probe for observing fine nanoscale
structures.
In this chapter, we review and discuss the STM-based techniques developed in combination
with optical technologies and their application to the analysis of nanoscale particles andclusters.

2. Laser-combined STM and related techniques
2.1 Probing methods
In this section, we discuss probing methods of STM combined with optical technologies.
2.1.1 Photoabsorption spectroscopy
Photoabsorption spectroscopy is a major branch of optical spectroscopy used to explore theelectronic states of materials. Photoabsorption spectroscopy using STM (STM-PAS) provides
a high spatial resolution in STM and high spectral accuracy of optical spectroscopy
compared with scanning tunneling spectroscopy (STS). The detected signal in STM-PAS is
the current flowing through the STM tip in response to the modulation of spectroscopic
light. The simplest STM-PAS scheme is based on a lock-in (LI) technique with the intensity
modulation of light while the wavelength is swept. Spatially resolved photoabsorption
spectroscopy using STM was first demonstrated with a resolution of ～50 nm [6]. It was thenshown that STM-PAS enables the nanometer-scale imaging of isolated subsurface defects insemiconductors through the absorption spectra associated with the defects [7].
Here we introduce two advanced STM-PAS schemes.
2.1.1.1 Fourier transform STM-PAS
The simplest STM-PAS scheme has two inherent technical problems. The first is the
spurious spectra that are often generated by temporal instabilities or positional drift of the
STM tip, which cause the tunneling current to change with time because acquisition for LI
detection requires a long time to sweep the wavelength of spectroscopic light. The second
problem is an undesirable excess component in the photomodulated tip current due to thephotothermal expansion of the tip material [8].
Fourier transform STM-PAS (STM-PAS-FT) based on the Fourier transformation technique
was devised to solve such technical problems [9]. This scheme is essentially the same as thatused in Fourier transform near-infrared spectroscopy (FT-NIR). Multiple lights modulatedwith different frequencies corresponding to their wavelengths generate a tip current with aninterferogram caused by the superposition of current components modulated at thedifferent frequencies. The photoabsorption spectrum is computed from the interferogram
via Fourier transformation. In this case, the photothermal expansion is suppressed by thesimultaneous illumination of multiplexed lights.

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Figure 1(a) shows a schematic diagram of the experimental setup of STM-PAS-FT. Figure1(b) shows a typical photoabsorption spectrum obtained around the band gap of Si by the
STM-PAS-FT scheme (a) and the LI scheme (b). Although it took about 100 min to acquire
the spectrum for a single sweep of photon energy from 0.68 to 1.55 eV in the LI scheme, it
took only 16 min to obtain a. high quality STM-FT-NIR spectrum in the range of 0.25–1.85
eV (1s for each scan and 1000 scans totally) [9]. Compared with the long acquisition time forthe LI spectrum, that for a single STM-FT-NIR spectrum is much shorter (1 s), which enables
us to avoid acquiring spurious spectra.

(a)

(b)

Fig. 1. (a) Experimental setup of Fourier transform near-infrared measurement. (b) STM-FTNIR spectrum and LI spectrum of a Si substrate obtained at 94 K. The acquisition time was
16 min for FT-NIR measurement (1s for each scan and 1000 scans totally) and 100 min for LI
measurement (1 scan). The decrease in intensity above 1.3 eV in the LI spectrum is due tothe cutoff filter used in the grating monochromator. [9]*Figure 2 shows an STM topographic image (a) and a two-dimensional map (b) of Si signals
integrated from 1.3 to 1.5 eV in the STM-PAS-FT spectra recorded in the framed area in (a) (8spectra were measured and averaged at each pixel) [9]. The samples were hemisphericalFig. 2. (a) STM image of GeSn nanodots/Si obtained at 98 K. (b) Map of STM-PAS-FT signal
integrated from 1.3 to 1.5 eV for the spectrum obtained from the framed area in (a). [9]*272

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Ge1-xSnx (x=0.1) nanodots epitaxially grown on Si substrates with an ultrathin SiO2 film. The
deposition of Ge and Sn was controlled to 24 monolayers to grow nanodots with diametersof ~40 nm. After the samples were annealed at 770 K, the surface was terminated withatomic hydrogen to suppress surface states. The region of bright contrast in (b) matches theregion without nanodots in (a) reasonably well, i.e., the expected part of the Si substrates.
The contrast indicates the spatial resolution of STM-PAS-FT to be ~10 nm.
Figure 3(a) shows a set of photoabsorption spectra obtained on different GeSn nanodots
with various lateral diameters [10]. The peak indicated in each spectrum by an arrow isobserved at an energy lower than the gap energy of Si (~1.2 eV) and exhibits a clear blue
shift with decreasing dot size, suggesting that the signal is induced by optical transitions
between discrete levels in the quantum dots. The photoabsorption energy of a spherical
nanodot with radius R is given by
Eabs  Ebulk  2 22R 1.7862e2
e2
 0.248
R
8 aB*(1)

Here, aB*  4  2 /  e 2 represents the exciton Bohr radius,  is Planck’s constant,  is thedielectric constant of the nanodot material, e is the electron charge and  is the reduced massof carriers. The first term Ebulk is the bandgap energy of the bulk crystal [11]. The solid
curve in Fig. 3 shows the theoretical curve calculated from Eq. (1). The energy position of theFig. 3. (a) STM-PAS-FT spectra obtained from individual GeSn nanodots of various sizes.
(b) Typical topographic STM image of Ge1-xSnx nanodots. The white arrow indicates the
position where the spectrum for d=30 nm was acquired. (c) Peak energy in (a) as a functionof the lateral diameter of the nanodots. The solid curve is calculated from Eq. (1). [10]Laser-Combined STM and Related Techniques for the Analysis of Nanoparticles/Clusters

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peak is in good agreement with the optical transition energy between discrete levels
theoretically predicted by the size dependence due to the quantum confinement effect.
2.1.1.2 Electric field modulation spectroscopy
Conventional electric field modulation spectroscopy (EFMS) techniques, such as
electroreflectance and photoreflectance, are established tools used for the accurate
measurement of interband transition energies in semiconductors [12]. The principle of EFMS
is based on the fact that applying an electric field to a semiconducting material causes anoscillatory change in the optical absorption coefficient depending on the wavelength, i.e.,
the Franz-Keldysh effect [13]. The spectral line shape of EFMS is closely related to energy
derivatives of the unperturbed dielectric function, and represents features corresponding tointerband transitions. By combining EMFS with STM (STM-EFMS), we can perform EFMS
with nanometer spatial resolution [14].

Fig. 4. Schematic illustrations of STM-EFMS measurement: (a) OM scheme, (b) BM scheme.[15]*Figure 4 illustrates schematics of STM-EFMS measurements using two different schemes forelectric field modulation: (a) optical modulation (OM) and (b) bias modulation (BM). In OM,
electric field modulation is achieved by an SPV periodically induced by chopped lightillumination with energy above the bandgap of the sample from a diode laser. In BM,
electric field modulation is achieved by applying a modulated bias voltage to the sample,
which directly modulates the tip-induced band bending beneath the tip. In STM-EFMS, thechange in  is detected as a change in the STM tip current, which is synchronized with themodulation of the electric field.
Figure 5 shows a typical STM-EFMS spectrum (solid curve) and the spectrum obtained by
the conventional EFMS method (dashed curve). The STM-EFMS spectrum reproduces themain features of the band structure. Two distinct structures observed at photon energies
approximately hv=1.41 and 1.78 eV are ascribed to the interband absorption edge of GaAs
and the spin split-off band absorption, respectively. It was demonstrated that the spatial
resolution of STM-EFMS measurements was of nanometer scale [15, 16]. A typical STM
topographic image of a -FeSi2 nanodot sample is shown in Fig. 6(a). The sample was
epitaxially grown on an n-Si(111) substrate covered with an ultrathin SiO2 film. After thegrowth, the surface was terminated with atomic hydrogen. Figure 6(b) shows STM-EFMS
spectra at 96 K obtained by the two schemes in an energy range lower than the absorption
edge of Si [15].

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Fig. 5. STM-EFMS spectrum obtained for a perfect (110) cleaved surface of GaAs. The broken
curve indicates the EFMS spectrum measured by the conventional electroreflectance method
applied to a macroscopic GaAs sample. E0 and E0+0 denote the interband absorption edgeof GaAs and to the spin split-off band absorption, respectively. [14]*For epitaxially grown sufficiently strained -FeSi2 nanodots on Si, bandgap crossover, i.e.,change from indirect band to direct band, is theoretically predicted [18], which, however,has not been confirmed experimentally despite that the mechanism is of great importancefor application. The two spectra exhibit a common feature from 0.72 to 0.76 eV. The energy
positions of the signals, 0.72–0.76 eV, closely match the absorption thresholds detected bymacroscopic measurements of the photoabsorption coefficient for a bulk -FeSi2 crystal at
100 K [17] and the energy threshold is attributed to optical transitions across the indirect
bandgap. Therefore, these findings strongly indicate that the -FeSi2 nanodot sample
examined was an indirect-gap semiconductor, instead of the theoretical prediction.
Using the STM-EFMS scheme, the band structure of individual nanodots can be exploredwith high accuracy.

Fig. 6. (a) Typical STM topographic image of -FeSi2 nanodot sample. Bright contrasts withheights of 5-10 nm are H-terminated -FeSi2 nanodots grown on Si(111). (b) STM-EFMS
spectra measured on -FeSi2 nanodots by optical modulation (OM) and bias modulation(BM). The common features near 0.72 – 0.76 eV agree well with the absorption thresholds
detected by macroscopic measurements of the photoabsorption coefficient in a bulk-FeSi2
crystal. [15]*Laser-Combined STM and Related Techniques for the Analysis of Nanoparticles/Clusters

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2.1.2 Light-emission spectroscopy
When carriers are injected from an STM tip to a sample, light emission（LE）is induced insome cases. STM luminescence spectroscopy is a measurement scheme in which the emitted
light is collected to explore the local electronic properties of materials (Fig. 7) [19-22].
The mechanism of photon emission depends on the process of measurement such asplasmon polariton (SPP) excitation in conductive samples, carrier recombination insemiconductor samples and the HOMO-LUMO transition in molecular samples.
Information on molecular vibrations can be obtained by analyzing the spectrum [22], suchas by inelastic tunneling spectroscopy [23], which may be used for the analysis of composite
nanoparticles/clusters combined with organic materials.

Fig. 7. Schematic illustrations of LE-STM setup and basic mechanism.

Fig. 8. (a) is the STM light emission spectrum of a single R6G molecule. (b)Photoluminescence (PL) spectrum of R6G on HOPG. The cutoff of the PL spectrum at 2.17
eV is due to the short-wavelength-cutoff filter inserted in the collection optics. [24]*From the distribution of emission intensity on a sample surface (photon map), we caninvestigate the geometry of the electronic structures of the sample. The photon map alsoenables us to estimate the transport properties of minority carriers by considering the*Reprinted with permission from each reference. Copyright American Institute of Physics.

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diffusion length. When a local spectrum is analyzed as a photon map, more detailed
information such as the distribution of elements can be obtained. However, STM
luminescence spectra are affected by various factors other than sample properties, such as
the tip shape, tip material and the characteristics of the substrate used for the experiment;
thus, careful analysis is necessary to determine the physical properties of the target material
from the STM luminescence spectra. In the case of organic materials, damages due to carrier
injection must be avoided.
Figure 8(a) shows the STM luminescence spectrum obtained from a single rhodamine 6G(R6G) molecule on an HOPG surface. The features of the spectrum are in good agreement withthe photoluminescence spectrum of a layer of rhodamine molecules on HOPG (Fig. 8(b)) [24].
2.1.3 Photoexcitation spectroscopy
Dynamical processes have often been studied by a laser pump-probe method where a pumppulse excites a sample and a subsequently arriving probe pulse with a delay time of td isused to track its temporal evolution [25, 26]. The temporal resolution attainable in such
experiments is limited only by the pulse width, which is generally in the femtosecond range.
However, the spatial resolution is determined by the optical diffraction limit, which is large
compared with the typical size of materials and devices currently being developed, andtherefore, the physical observables are obscured by ensemble averaging. Thus, high spatial
resolution in pump-probe experiments would provide new insights into nanoscale
structures and materials and unveil a rich variety of dynamical features of light-sensitive
phenomena in unexplored regimes such as charge transfer, phase transitions, electronictransitions, carrier or spin transport and quantum coherence.
In contrast, STM easily provides atomic-scale spatial resolution despite its low temporal
resolution (typically worse than 100 kHz) [27-31]. Therefore, if the tunneling process directly
produced by optical excitation can be measured, high temporal and spatial sensitivity can be
simultaneously achieved with the atomic-scale resolution of STM [32-41]. A promising setup forachieving this is pulse-pair-excited STM (PPX-STM), in which, in analogy with pump-probe
experiments, a sequence of paired laser pulses with a certain delay time td excites the sample
surface beneath the STM tip, and the tunneling current I is measured as a function of td. To
detect a faint time-resolved tunneling current with a high signal-to-noise ratio, the rectangularmodulation of delay time with a pulse-picking procedure is used (shaken-pulse-pair excited
STM: SPPX-STM), enabling the spatial mapping of time-resolved tunneling current [33].
Figure 9 shows the setup of SPPX-STM. When paired optical pulses arrive at a sample
beneath the STM tip, they generate pulses of raw tunneling current I*, reflecting theexcitation and relaxation of the physical properties of the sample. If these current pulses
decay rapidly compared with the time scale of the STM preamplifier bandwidth, they can be
temporally averaged in the preamplifier but cannot be detected directly in the signal I. Evenin this case, the relaxation dynamics can be probed through the td dependence of I. When td
is sufficiently long, paired optical pulses with the same intensity independently induce twocurrent pulses with the same I*. In contrast, when td is short and the second pulse
illuminates the sample in the excited state induced by the first pulse, the second current
pulse may have a different magnitude, depending on td. A typical process that can beobserved using this mechanism is absorption bleaching in semiconductors; when theLaser-Combined STM and Related Techniques for the Analysis of Nanoparticles/Clusters

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carriers excited by the first optical pulse remain in the excited state, the absorption of thesecond optical pulse is suppressed. In such a case, the current I* induced by the second
current pulse decreases depending on td, reflecting the decay of the excited carriers
excitation by the first-pulse. Signal I also depends on td, because the magnitude difference ofthe second current pulse changes the temporally averaged value of the tunneling current.
Accordingly, the relaxation dynamics of the excited carriers of the target material, namely,
the decay of carrier density after excitation by the first optical pulse, can be probed by STM
at the resolution of the pulse width, that is, in the femtosecond range.

Fig. 9. Schematic illustration of SPPX-STM
In SPPX-STM the sophisticated control of delay-time generation and modulation with apulse-picking procedure is essential. This enables the probing of nanometer-scale structureswith a wide range of relaxation lifetimes. Using the pulse-picking method, a large anddiscrete modification of td can be realized by changing the selection of pulses that transmit
the pulse pickers, which is suitable for modulating td in SPPX-STM. In this method, thedelay time dependence of the tunneling current, I  td   I  td   I   , is accurately probedwith a high acquisition rate, where I(∞) is the tunneling current for a delay time sufficiently
long for the excited state to be relaxed. Accordingly, SPPX-STM has made it possible tovisualize the carrier dynamics in nanometer-scale structures with a wide range of relaxation
lifetimes. Figure 10 shows the capability of wide timescale measurement.

Fig. 10. SPPX-STM signals obtained for various samples. Upper spectra of LT-GaAs, GaNAs
and n-GaAs were obtained by optical pump-probe method (R: reflectivity of probe pulse).

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As an example, SPPX-STM has been applied to the analysis of carrier dynamics in a Co
nanoparticle/GaAs(110) system. When Co is deposited on a GaAs, nanoparticles areformed (Fig. 11(a)). In this system, photoexcited minority carriers (holes) captured at thesurface are recombined with electrons tunneling from the STM tip via the gap states
formed by Co as shown in Fig. 11(c). This is considered to be enhanced by the existence ofgap states at the Co nanoparticle sites. Understanding such a charge transport mechanism
through nanoparticles is of great importance not only for the development of nanoscale
electronic devices but also for their application to the finer control of chemical reactions in
catalysis.

Fig. 11. (a) STM image of Co/GaAs, (b) 2D map of decay constant, (c) schematic model ofrecombination at gap states, (d) cross section along the line in (b), (e) decay constant as afunction of tunneling current, (f) decay constant as a function of Co particle size.
Figure 11(b) shows the overlap of the STM image in Fig. 11(a) with the map of the decay
constant obtained over the surface. The two-dimensional (2D) map of the decay constant
shown in color scale indicates the decay constant of the photoinduced carrier density at eachpoint. The positional agreement is good. As shown in the cross section in Fig. 11(d) obtained
along the line in Fig. 3(b), the decay is rapid in the Co regions. In such regions,photoinduced holes trapped at the surface are recombined with electrons tunneling from the
STM tip at the gap states; thus, there are two limitations in this process: the tunnelingcurrent and hole-capture rate. When the tunneling current is sufficient, the hole-capture rate
becomes the limiting factor of the recombination process. Figure 11(e) shows the relation
between the decay constant and tunneling current. As expected, the decay constantdecreases with increasing tunneling current and has a saturated value of 6.9 ns, whichcorresponds to the hole-capture rate of this system. The decay process should depend on the
gap-state density. Figure 11(f) shows the decay constant as a function of the Co nanoparticle
size. The time constant increases with decreasing nanoparticle size as expected.

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In SPPX-STM, the nonlinear interference between the excitations is essential, which depends
on the material we measure. In SPPX-STM applied to a semiconductor, tip-induced band
bending and surface photovoltage play important roles in the measurement. However, ingeneral, such as dipole formation, charge transfer, changes in conductance, and vibrationthat causes the change in the tip-sample distance are possible mechanisms for producing
SPPX-STM signals. Therefore, SPPX-STM enables the nanoscale probing of transient
dynamics over a wide range of time scales, simultaneously with the observation of local
structures by STM.
Another promising technique is STM combined with synchrotron radiation (SR-STM),
which probes core-level photoemission, enabling the identification of atomic species of thetarget materials [42]. The spatial resolution has been improved to ~10 nm, and therefore in
the near future, in addition to the analysis of isolated nanostructures, probing of the inner
structures of targets may become possible.
2.2 Manipulation for fine measurement
The tunneling current and bias voltage in STM, which are the basic parameters of STM
measurement, can be used for the modification of target materials. Probing, for example, theeffect of atomic-scale defects on local electronic structures enables the clarification of thefundamental mechanism in each element and its relation to macroscopic functions. For
nanoparticles/clusters, such effects are essential for determining the characteristic
properties of their total systems.

Fig. 12. (a) STM image of current-injection-induced polymerized C60 molecules (darkcontrasts), (b) schematic of C60 polymerization, (c) nanoscale patterning of polymerized C60
molecules formed by scanning the tip along the longitudinal direction. [43]†Figure 12(a) shows an STM image of a C60 crystalline film within a thickness of severalmonolayers grown on an HOPG surfaces in ultrahigh vacuum (UHV). Before acquiring theimage, electrons were injected at the point indicated by a cross in Fig. 12(a) at a sample bias
voltage of Vs=+4.2 V. The dark contrasts around the point represent intracluster structureswith a stripe pattern, suggesting the frozen rotation of C60 molecules despite the room
temperature. Namely, the dark sites are C60 molecules polymerized with molecules in theunderlayer. The polymerization was induced by the injection of low-energy electrons fromthe STM probe tip. Figure 12(c) shows a line structure consisting of polymerized C60 clusters
confined in a width as small as ~2 nm, which is a good example of nanoscale electron-beam†

Copyright The Japan Society of Applied Physics.

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patterning [43]. When a template such as nanoscale cavity is used, individual C60 molecules
are stabilized in each cavity even at room temperature. Manipulation of a single C60
molecule using STM tunneling current was successfully carried out (Fig.13) [44.].

Fig. 13. (a) STM image of glycine-nanocavity array (template). (b) Schematic illustration of C60
molecule stabilized in a nanocavity. STM images of C60 molecules stabilized by a glycine
template before (c) and after (d) the injection of tunnel current on the molecule indicated byarrow. (e) Change in tunnel current upon manipulation. (f) Cross sections along the line in (a).
Figures 14(b) and (c) show topographic STM images of a single-walled carbon nanotube(SWNT). After acquiring the image in Fig. 14(b), the STM tip was fixed at the position
marked in Fig. 14(b), and a tunneling current of 0.1 nA with 7.0 V bias voltage was injected.
Figure 13(c) shows the defect generated at the probed site. The finite flat LDOS around the
Fermi level shown in Fig. 14(c), which was measured before defect generation, indicates that

Fig. 14. (a) Schematic illustration of tunnel current injection. (b) STM image of an SWNT
acquired with Vs=1.0 V and It=0.1 nA at 95 K. (c) dI/dV vs bias-voltage curve, obtained at
the position marked in (b), exhibiting the features of a metallic SWNT characterized by afinite flat LDOS in the first van Hove gap. (d) STM image acquired after current injection at
the marked position. (e) dI/dV vs bias-voltage curve, obtained at the position marked in(d), exhibiting a HOMO-LUMO gap of ~0.7 eV. [45]Laser-Combined STM and Related Techniques for the Analysis of Nanoparticles/Clusters

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the tube was initially metallic. Figure 14(e) shows the LDOS measured at the defect, which ischaracterized by a HOMO-LUMO gap that opened across the Fermi level. The HOMOLUMO gap was observed to be over 2 nm along the long axis of the CNT and is consideredto act as a barrier to carrier transport along the metallic SWNT. This result indicates that we
can modify the local electronic properties of a single cluster in a controlled manner using the
STM modification technique [45, 46].
Another method of manipulation is mechanical deformation of clusters by an STM tip. For
example, the change in the HOMO-LUMO gap of C60 molecule due to deformation wasobserved thorough the measurement of tunneling current under the compression of themolecule by STM tip [47].
Combination of STM manipulations with optical techniques enables further analysis ofnanoscale materials.
2.3 Probe technology
For the STM measurement of nanoscale materials, choosing the most suitable STM tip
depending on the specific experiment is important. In this section, silver tips for opticalmeasurement, glass-coated tips for photoemission measurements, molecular tip for chemical
analysis and CNT probes for high resolution imaging will be described.
2.3.1 Insulator-coated metal tips for SR-STM
STM combined with a synchrotron radiation light source (SR-STM) has attracted
considerable attention owing to the possibility of elemental analysis at nanometer resolution
by detecting the core-level electrons of surface atoms. The fabrication of a tip coated with aninsulating thin film is the key to achieving high spatial resolution by reducing thephotoinduced current impinging to the side wall of the tip [48, 49]. For example, a W tip wascoated with glass except for the region less than 5 m from the tip apex using a focused ion
beam (FIB) technique [48]. Using this state-of-the-art STM tip, the photoinduced current was
dramatically reduced by a factor of ~40 compared with that of an untreated tip. Recently,
using this tip in combination with the Lock-in (LI) detection method, a spatial resolution ofas high as ~10 nm was demonstrated on checkerboard-patterned Ni and Fe samples [42].
2.3.2 Silver tips for TERS and STML
Tip-enhanced Raman spectroscopy (TERS) [50] is a promising method of chemical analysis
at the nanometer level. Under external illumination, a sharp tip is used to create a localized
light source and excite a specimen surface. According to classical electromagnetic theory, asharp metal tip is suitable for enhancing the Raman scattering of nearby molecules. It is
known that Ag produces greater enhancement than Au in the visible range because theimaginary part of its permittivity is much smaller. The silver tip is also used in STMinduced luminescence (STML), where STML intensities are enhanced by about one order ofmagnitude compared with those obtained using tungsten tips [51]. There have been many
reports on the fabrication of Ag tips by electrochemical etching with various electrolytes
such as a mixture of perchloric acid and ethanol [52]. Using such a tip, single-molecule tipenhanced Raman spectra from brilliant cresyl blue (BCB) sub-monolayers deposited on a flat

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Au surface were obtained [53]. A highly enhanced electric field was created in the gap of 1
nm between the tip and sample. For STM imaging, the tip apex should be free of oxidationor contaminants. Atomically resolved STM imaging and STML spectra with a high signal-tonoise ratio are obtained using an electrochemically etched Ag tip followed by tip cleaning
by Ar ion sputtering in UHV [54].
2.3.3 Molecular tips
Carboxyl-terminated SWNTs from solution phases can be attached onto Au tips throughself-assembed monolayers for using in STM [55. 56]. In addition to the high-resolution
imaging of molecules (such as diether) on a surface, these CNT tips enable chemicallyselective observation due to electron tunneling through hydrogen-bond interactions
between the atached molecule and carboxyl groups at SWNTs. The differentiation of DNA
bases and chiral recognition on a single-molecule basis have also been demonstrated using
molecular tips [56]. In a similar way, voltage-induced chemical contrast in an STM image
was reported using chemically modified tips with hydrogen-bond donors [57]. Moreover,molecular orbitals of metal phthalocyanines on metal surfaces have been clearly imaged
with an O2-functionalized STM tip, where the observations were supported by theoretical
calculations [58].
2.3.4 CNT probes
CNTs are one of the most intriguing materials in nanotechnology [59]. A CNT has a onedimensional cylindrical structure with sistinct physical characteristics such as a small
diameter, high aspect ratio, high stiffness, high conductivity and so forth. In view of the
shape and electric conductivity required for a high-resolution STM tip, these properties of
CNTs make them ideal as a tip material for probing extremely small objects such as
nanoclusters. [60]. Mechanical attachment, direct growth and dielectrophoresis are methods
employed to fabricate CNT-STM tips. A single CNT tip can be prepared by mechanicalattachment, which enables the high-resolution ghost-free imaging of nanoclusters. However,this method is time-consuming, and the other methods (direct growth anddielectrophoresis) are more suitable for the mass production of CNT tips.
(a) Mechanical attachment method
Following the first approach to fabricating CNT probes under an optical microscope [61]. amore sophisticated method was proposed, where the attachment of a CNT onto a probe is
performed in two independent precise stages under SEM observation, where beam-deposited
amorphous carbon is used as a glue [62]. Although the thus mechanically prepared CNT-STM
tips exhibited atomic resolution on Au(111) reconstruction, the cleaning process of CNTs byheating in UHV was necessary for stable observation. [63]. A metal coating method has been
proposed for improving the electric conductivity between a CNT and the supporting metal tip[64]. Automotive exhaust catalysts consist of metal nanoclusters supported on metal oxidesurfaces. Since catalytic activity can be altered by controlling the size of the nanoclusters
because of its strong size dependence, the precise characterization of metal nanoclusters isessential. High-resolution UHV-STM images of size-selected Pt(n) (n=4,7-10,15) clusters
deposited on TiO2(110)-(1x1) surfaces were obtained using a CNT tip (Fig. 15) [65]. Clusters ofPt(7) (Fig.15(b)) and smaller were oriented flat on the surface with a planar structure, and aplanar-to-three-dimensional transition was observed at n=8 (Fig.15(c)). Color scale shows theLaser-Combined STM and Related Techniques for the Analysis of Nanoparticles/Clusters

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structures of the top layers. Individual Pt atoms are clearly identified, especially for (c) Pt(8)and (d) Pt(9), indicating that the geometry of the clusters is atomically resolved and the details
can be analyzed with a CNT tip.

Fig. 15. STM images of TiO2(110) surface after deposition of size-selected Ptn+ (n = 4,7–10,15)
cluster ions. Images with uppercase letters are 20×20 nm2 views and those with lowercase
letters are 3.5×3.5 nm2 views of a cluster on the same surface. TiO2 surface after thedeposition of [(A), (a)] Pt4+, [(B)(b)] Pt7+, [(C)(c)(c′)] Pt8+, [(D)(d)(d′)] Pt9+, [(E)(e)] Pt10+, [(F)(f)]
Pt15+ [65].
(b) Direct growth method
This method is suitable for the mass production of CNT probes. Chemical vapor deposition(CVD) is commonly used for the synthesis of CNTs [66]. Tips fabricated by the direct growthmethod sometimes consist of numerous CNTs, and selective growth at the apex is required
for stable operation of the tip. For this purpose, several methods of pinpointing catalysis
have been reported [67, 68]. The growth direction of CNTs is also important inmeasurements. Plasma CVD is suitable for controlling the alignment of CNTs [69]. By
optimizing the reaction at the sharp apex, CNT probes can be directly grown on the apex ofa tungsten probe without reducing its sharpness, as shown in Fig.16 [70]. Thin films of Fe or
Co (20–30 nm) are used as a catalyst, and the growth of CNTs with a diameter of ~40 nm has
been observed. Because the magnetic nanoparticles are located at the tip of CNTs, this typeof probe can also be utilized to study the magnetic properties of nanoclusters with higher
spatial resolution [71]. For spin-polarized STM measurement, a magnetic coating of Fe (1020 monolayers) on a cleaned tungsten tip is conventionally used [72], which may be
improved using a CNT tip.

Fig. 16. SEM and TEM images of a grown CNT-STM probe. Two CNTs are grown on theapex of the STM tip. Black contrast corresponds to metal particles used as catalyst.

284

Smart Nanoparticles Technology(c) Dielectrophoresis
When an alternating electric field (~MHz) is applied between asymmetric electrodes (forexample, a metal probe and counter plane electrode) immersed in CNT solution (wheresolvent is water, alcohol, dichloroethane, etc.), the CNTs are polarized and become attachedto the probe (dielectrophoresis). [73]. The high-yield synthesis of conductive CNT tips forthe multiprobe microscope [74-76] was reported using the dielectrophoresis method [77].
After Pt-Ir coating, such a tip were successfully applied for electronic transportmeasurement by multiprobe STM using the four-terminal method.

3. Summary
Laser-combined STM and related techniques have been reviewed and discussed focusing on
the analysis of nanoscale particles and clusters. The addition of optical technologies to STM
provides new approaches to the study of nanoscale-material physics and chemistry. Nearfield optical microscopy (NSOM) and other techniques [78-86], which have not been
discussed in this chapter, are expected to play complementary roles in understanding anddeveloping the physics and chemistry of new nanoparticles/clusters for realizing novel
functional devices.

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