INSTITUTE OF PHYSICS PUBLISHINGNANOTECHNOLOGY

Nanotechnology 18 (2007) 084001 (10pp)

doi:10.1088/0957-4484/18/8/084001

Japan AFM roadmap 2006
Seizo Morita1 , Hirofumi Yamada2 and Toshio Ando3
1
Department of Electrical, Electronic and Information Engineering, Graduate School ofEngineering, Osaka University, Suita, Osaka 565-0871, Japan2
Department of Electronic Science and Engineering, Kyoto University, Katsura, Nishikyo,
Kyoto 615-8510, Japan3
Department of Physics, Faculty of Science, Kanazawa University, Kakuma-machi,
Kanazawa, 920-1192, Japan

Received 21 July 2006, in final form 25 October 2006
Published 18 January 2007
Online at stacks.iop.org/Nano/18/084001
Abstract
This article reviews recent progress in Japan in the techniques andapplications of dynamic mode atomic force microscopy (AFM), specificallyfocusing on three topics: (1) mechanical discrimination of intermixed
two-atom species and their manipulation by frequency modulation (FM)AFM under ultrahigh vacuum; (2) sub-molecular and atomic-resolution
imaging in liquids by FM-AFM; and (3) high-speed imaging of biologicalmacromolecules in solution by amplitude modulation (AM) AFM. After
reviewing the state of the art of these high-performance AFMs and various
techniques that have enabled them, we present future prospects for these
AFMs and for technological innovations that will be led by them. These
prospects are given from the viewpoint of Japanese groups that have been
involved in these three topics.

studied. In the present article, we review the techniquesthat have made these studies possible, briefly introduce somestudies that are currently underway for further progress, andthen present roadmaps for these three topics from our point ofview. Roadmaps for a vast range of scanning probe microscopy
are given by [9].

1. Introduction
In 1986, the atomic force microscope (AFM) was invented asa novel atomic-resolution microscope [1], which was capableof imaging even insulating surfaces, unlike the scanningtunnelling microscope. In the early stages, however, static
contact AFM could not achieve stable and reproducible atomic
resolution because of the destruction of the surface and tip
apex due to a strong interaction force between them. In
order to reduce this interaction force, dynamic mode AFM [2],including non-contact AFM [3], was devised, wherein the
cantilever is oscillated. At present, using dynamic mode
AFM, true atomic-resolution imaging under ultrahigh vacuum(UHV) [4] is routinely carried out.
Various technological developments and applied studies
have continued using dynamic mode AFM, and this progress is
expected to continue further. It seems worthwhile to consider,at the current stage, its prospects for potential developments
and directions to go forward. From a wide range of studies
using dynamic mode AFM, here we consider three recent
topics in which the authors (three different Japanese groups)
have been actively involved: chemical identification of single
atoms [5] and selected-atom manipulation [6] in UHV; highresolution imaging in a liquid [7]; and high-speed imaging ina liquid [8]. These three lines of investigation have become
important issues in AFM and are currently being actively
0957-4484/07/084001+10$30.00

2. Atomic discrimination, manipulation, andassembly
The frequency modulation (FM) detection method, combinedwith the large oscillation amplitude of the cantilever [2],has enabled the achievement of true atomic resolution using
dynamic mode AFM in UHV [3, 4] (UHV-FM-AFM). To
expand the ability of UHV-FM-AFM at room temperature(RT) further toward atomic discrimination, manipulation andassembly, the stabilities of both the imaging area and the tip–
sample distance are crucial issues. In the case of conventional
UHV-FM-AFM, the typical value of thermal drift over theimaging area is nearly 10 Å min−1 , which would disturb
the positioning for atom discrimination, manipulation andassembly. In the case of our home-built UHV-FM-AFM,
however, thermal drift is typically 3 Å min−1 even at RT, andsometimes close to 1 Å min−1 . Furthermore, we used an atomtracking technique to compensate for thermal drift, in addition
to using our home-built UHV-FM-AFM with low thermal drift.
1© 2007 IOP Publishing Ltd Printed in the UKNanotechnology 18 (2007) 084001

S Morita et al

dim substituted Si adatoms. Thus, this AFM topography
image shows chemical contrast by atom-selective imaging
between the In and Si adatoms. Such chemical contrast utilizes
the height difference in the topographic image, and hence
requires an atomically flat surface for chemical discriminationof atom species. Recently, we succeeded in measuring precise
frequency shift as a function of tip–sample distance (frequency
shift curve) of selected atoms even at RT [5] by compensating
for thermal drift using the atom-tracking technique and by
averaging many frequency shift curves of selected atoms [10].
By converting a site-specific frequency shift curve to a sitespecific force curve and simultaneously subtracting the longrange force contribution due to the van der Waals force [5],we have obtained precise short-range force curves of selected
In and Si adatoms, indicated by the white arrows in theinset in figure 1(a). From figure 1(b), it was confirmed that
the Si atom has a larger covalent bonding force than the In
atom. Therefore, in principle, such short-range force curvesof selected atoms enable us to discriminate between chemical
species of individual atoms. This method thus is topographybased atom discrimination, since it needs a topographic imageof the surface with atomic resolution for choosing the target
atom. The advantage of this method is that it enables
us to clarify the chemical covalent force over each species
independently of the tip–surface distance.

1.0

(b)

In

Short-Range Force F [nN]

0.5

Si
0.0

(a)
(a)

-0.5

Si

In

-1.0
-1.5
-2.0
0

5

10

15

20

Tip-Sample Distance Z[Å]Figure 1. √Chemical√ discrimination of In and Si adatoms on a
In/Si(111) 3× 3 pure phase (In 1/3 ML) surface using (a)non-contact AFM topography (inset image: 4.2 nm × 4.2 nm), and(b) short-range force curves of selected In and Si atoms indicated by
the white arrows in (a) at RT.

It was shown that the precision of positioning control at RT
is better than 0.2 Å peak-to-peak [10], with lateral stability
comparable to that in cryogenic inelastic electron tunnelling
spectroscopy measurements [11].

2.2. Roadmap towards chemical identification of atom species
2.1. Topography-based atom discrimination

Figure 2 shows a roadmap (status and future prospects)
towards chemical identification of atom species by AFM.
The results achieved thus far include chemical discriminationof periodic lattice atoms, such as In and As atoms in
InAs(110) [12], and Ca and F atoms in CaF2 (111) [13], andthat of intermixed aperiodic atoms such as Cl and Br atoms√
in KCl0.6 Br0.4 (100) [14], Si and Sb atoms in Si(111)5 3 ×

UHV-FM-AFM under attractive force was able to achieve
chemical discrimination of atom species using topographybased atom-selective imaging based on the atomic force ofindividual atoms [5]. The inset in figure 1 shows non-contact√√
AFM topography of In/Si(111) 3 × 3 pure phase (In 1/3
monolayer, ML) at RT. We can clearly observe a few small,

Chemical Identificationof Atom Species by AFM
2020
Amorphous

Achieved
2010
5 atom species

Achievable
Indeterminate

2006
3 atom species
Force curve2003 [15]Si and Sb
in Si(111)5√3×5√3Sb: KPFM
2002 [14]14

KCl0.6Br0.4
NCNC-AFM

2005 [6]Sn/Ge(111)-c(2×8)-c
NCNC-AFM

2012 Isolated 3
adsorbed atom
species2015
2010
3-D recognition
2 atom species
NMR
Dissipation

2007
2 atom species
NCNC-AFM in liquid

2004 [16]Sn/Si(111) 2006
2 atom species√3×√3
NCNC-AFM in gas
NCNC-AFM

1999 [12] 2001 [13]InAs(110) CaF (111))
2
NCNC-AFM NCNC-AFM
K
78K&14K

2000“6 years ago”

2015 3 atom species
in atom cluster

Atomicallyflat surface

2006“Now””

Intermixed
aperiodic atom
distribution

Rough (oramorphous) Periodic atom
surface
distribution

2010“4 years later”

2015“9 years later”

Year“Further”

Figure 2. Roadmap toward chemical identification of atom species by AFM.

2

Nanotechnology 18 (2007) 084001

S Morita et al√√√
5 3-Sb [15], Sn and Si atoms in Sn/Si(111) 3 × 3 [16],and Sn and Ge atoms in Sn/Ge(111)-c(2 × 8) [6]. NCAFM, KPFM and the force curve represent non-contact AFM
topography, Kelvin probe force microscopy and site-specificforce curve, respectively. Thus, chemical discriminationof atom species has developed from easier systems withperiodic atoms to more difficult ones with intermixed aperiodic
atoms. The mainstream until 2010 related to chemical
identification of atom species will be the achievement ofchemical discrimination using other methods such as force
curve and energy dissipation, and the realization of chemical
discrimination in non-UHV environments such as a gas ora liquid. Chemical discrimination of insulator and metallic
atoms will also become important subjects to widen the range
of applicable materials. After 2010, the target of chemical
identification will move from easier samples with atomicallyflat surfaces to more difficult ones with rough surfaces, such as
isolated adsorbed atoms and atom clusters for the assembly ofcomplex compound nanostructures. Chemical contrast in noncontact AFM topography, however, utilizes height differencesin the topographic image and hence requires an atomically flat
surface. Therefore, we cannot use this method for a rough (orcorrugated) sample surface such as a three-dimensional atomcluster. On the other hand, short-range force curves of single
atoms can discriminate between atom species using not theheight difference but the force difference, so that we can use
this method even for rough (or corrugated) sample surfaces.
However, short-range force curves of single atoms as well as
non-contact AFM topography use the atomic force. Therefore,chemical discrimination of individual atoms on an amorphoussurface, whose atomic force will have a wide variation due
to the amorphous structure, appears to be a very difficult andchallenging subject. Further, true chemical identification, thatis, universal chemical identification of unknown single atoms isthe most important issue. Not only true chemical identification
but also three-dimensional recognition (tomography) may
be achievable using a nuclear magnetic resonance (NMR)
technique such as magnetic resonance force spectroscopy(MRFM) [17].

Sn Atom

Ge Atom

Figure 3. Protruded ‘atom inlay’, that is, the atom letters ‘Sn’
consisting of a slightly large 19 Sn adatoms embedded in a little
small Ge adatoms on Sn/Ge(111)-c(2 × 8) surface at RT.

interchange but also the appearance of such a phenomenon
itself depends on the tip.
2.4. Roadmap for mechanical atom manipulation andassembly
Figure 4 shows a roadmap for mechanical atom manipulation
and assembly by AFM. Results achieved thus far include conventional vertical [3, 18] and lateral [18] atom manipulations in
UHV at LT that moved only one kind of atom species, and lateral atom-interchange manipulation at RT that moved heterogeneous two-atom species by tip-induced directional atom interchange. Thus, mechanical atom manipulation by AFM wasdeveloped from simpler manipulation of single-atom species
to more complex manipulation of multi-atom species. Veryrecently, atomistic manipulations on insulating surfaces wereachieved [20, 21]. A manipulated atomic-scale object, however, seems to be an atomic-scale defect or adsorbed molecule.
Further, site-by-site positioning by atom manipulation is still
incomplete. In 2006, site-by-site manipulation of a single atom
selected from multi atom species on insulating surface may beachievable. By 2010, chemical identification of intermixed heterogeneous atoms on an insulating surface and following selected atom manipulation will become important subjects in
UHV. Besides, the manipulation of single atoms in a gas andliquid will also become an important subject. After 2010, theassembly of atom clusters and atom devices with novel functions from multi-atom species at RT will become important.
Selected atom manipulation of intermixed multi-atom species
and artificial nanostructuring in gases and liquids will also become important. Furthermore, the development of a techniquethat can arbitrarily construct designed molecules from individual atoms using observation, chemical identification and manipulation of atoms is the most difficult and challenging issue
for atom manipulation. Such a technology may enable us toconstruct complex molecules and high polymers from multiatom species in the future. The assembly of molecules using
two-atom species is the first step in this direction.

2.3. Atom manipulation and assembly from multi-atom species
UHV-FM-AFM was used to achieve not only verticalatom manipulation, such as atom extraction and atom
deposition [18], but also lateral atom manipulation [19] of
selected individual atoms by precisely controlling the lateraland vertical tip position around the near-contact region [19]at low temperature (LT). Recently, we found a lateral atominterchange manipulation phenomenon between a substituted
Sn adatom and an adjacent Ge adatom on a Sn/Ge(111)-c(2 ×8) surface at RT [6]. Using the vector scan, we succeeded
in directional atom interchange between a selected Sn adatom
and a selected adjacent Ge adatom. We were then able toconstruct embedded atom letters or an atom inlay, by forming‘Sn’ (the symbol for a tin atom) on the surface consisting of
19 slightly larger Sn atoms embedded in the smaller Ge atoms
of a Ge(111) substrate, as shown in figure 3. To construct theatom inlay shown in figure 3, we needed 9 h and 120 times
the lateral atom interchange between Sn and Ge adatoms. It
should be remarked that not only the minimum force for atom

3. Atomic resolution in a liquid
Atomic-scale FM-AFM imaging of atomically flat samples in
UHV environments is now becoming routine. High-resolution
imaging in a liquid has been greatly anticipated, especially for3

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S Morita et al

Mechanical Atom Manipulation by AFM

Achieved2020 Assembly ofmolecule using 2 atom
species

2006 Atom
manipulation on
insulating surface

2015 Assembly of
atom cluster using
3 atom species
Indeterminate
2011 Assembly byatom manipulation in2006 Lateral atom
liquidmanipulation in 3
atom species sample
2012 Assembly ofMulti atom
atom device using
3 atom species
species 2005 [6] Lateral atom2010
interchange manipulation of Sn
Assembly by atom
and Ge adatoms and “Atom
Inlay” in Sn/Ge(111)-c(2×8):
Sn/Ge(111)-c(2 8): RT manipulation in gas
2009 Atom2005 [19] Lateral atom
manipulation
manipulation of single
in liquid
adsorbed atom on
2008Single atom Ge(111)-c(2×8):
Ge(111)-c(2 8): 80K
Atom manipulationin gasspecies 2003 [18] Extraction anddeposition of Si adatom 2006 Atomisticon Si(111)7×7
Si(111)7 7 by vertical manipulation on
atom manipulation: 78K insulating surface

Achievable2006 Verticalatom-interchange
manipulation: RT

2000“6 years ago”

2006
“Now”

2010
“4 years later”

2015

Year

“9 years later”“Further”

Figure 4. Roadmap for mechanical atom manipulation by AFM.

power with a high-frequency (HF) signal whose frequency
was typically 300–500 MHz was effective in reducing thecoherence and hence the noise [24]. We applied this techniqueto the deflection sensor as shown in figure 5. Figure 6
shows the frequency spectra of cantilever Brownian motion
measured using the improved deflection sensor in water. The
solid line shows an experimentally measured curve using theimproved deflection sensor, while the dotted curve shows atheoretical curve calculated using the following equation forthe displacement noise density Nth ( f ):
1
2k B TNth ( f ) = 
(1)


2
2 2
π
f0 k Q
1 − ( f / f0 ) + f / ( f 0 Q)

biological studies. However, the imaging capability in liquids
by FM-AFM is severely hindered by the extreme reductionof the cantilever Q -factor due to hydrodynamic interaction
between the cantilever and the liquid. The Q -factors in UHV
usually exceed 10 000, while those in liquid environments are
smaller than 10. In fact, it was not certain that FM-AFM would
work in such an environment with a low Q -factor, because
a high Q -factor of the cantilever is indispensable for stable
operation of an FM-AFM with an electromechanical resonator
for its self-oscillation circuit.
The use of the small-amplitude mode and large noise
reduction in the cantilever deflection sensor have resulted ingreat progress in FM-AFM imaging in liquids [7, 22, 23].
Short-range chemical interactions between a tip and a sample
surface occur only when the tip is in close proximity to
the surface. Thus, the force sensitivity can be increased
by FM detection with small-amplitude oscillation becauseof the increase in the effective duration of the proximity
interactions. Note that the small-amplitude mode can only be
used when the noise in the deflection sensor (usually in thelaser beam deflection method) is sufficiently reduced to thelevel of thermal fluctuations in the cantilever. On the other
hand, we have to consider the increase in the phase noiseof the FM detector caused by a decrease in the oscillation
amplitude, since the noise is proportional to the reciprocal of
the amplitude.

where f , f 0 , k , kB and T are the vibration frequency, resonancefrequency, spring constant of a cantilever, Boltzmann constantand temperature, respectively.
The peaks observed in the spectra correspond to the
Brownian vibration at the cantilever resonance, while thebackground white noise (grey horizontal line in figure 6) comesfrom the deflection sensor noise. The results reveal that the
deflection noise densities arising from our deflection sensor
1
1
were 17 fm Hz− 2 in air and 39 fm Hz− 2 in liquid. These values
are much smaller than the deflection noise densities obtainedwith a deflection sensor in commercially available AFMs
1
(typically 100–1000 fm Hz− 2 in air). Since the bandwidthof the FM detector ( BFM ) is usually less than 1 kHz, thedeflection noise components, which induce frequency noises,
should be in the frequency range from f0 to f 0 + 1 kHz. In
this frequency regime, the experimentally measured values arenearly the same as the theoretically calculated values in both
air and liquid environments.
Consequently, true atomic-resolution imaging in a liquid
was successfully achieved by FM-AFM [7, 22]. Figure 7 showsan FM-AFM image of a cleaved (001) surface of muscovite

3.1. Development of low-noise FM-AFM
The shot noise in the laser beam deflection sensor, as shown infigure 5, gives the theoretical limit of the minimum noise forsignal detection. Apart from the shot noise, other dominantnoises in the sensor are the ‘optical feedback noise’ and‘optical interference noise’, both of which originate in thehigh coherence of the laser light. The modulation of laser
4

Nanotechnology 18 (2007) 084001

S Morita et albias-Tfm
300~500 MHz

LD

laser driver
collimatorphase
comparator

I/V converter
amplifierfrequency
converter

frequency shift
loop filter

Δff IF = f L– f0
4.5 MHzPZT platef0
100~300 kHz

phase shifterθθ ∼π / 2

Local oscillator
fL

VCXO

energy dissipation

AGC
band pass filter

Figure 5. Schematic of an experimental setup in FM-AFM.

3.2. Roadmap of FM-AFM imaging in liquids
AFM imaging in liquids has been applied to studies of
biological samples for a long time, since the early stagesof its development. There has been tremendous and rapid
progress in AFM imaging techniques in liquids such as the Qcontrol method and high-resolution imaging using FM-AFM.
Figure 9 shows a technology roadmap for AFM measurements
in a liquid, based on possible technological developments aswell as user demand. For practical imaging applications,the imaging of a sample with a large height difference in
topography is unavoidable. However, in FM-AFM imaging ofsuch a sample, the cantilever oscillation tends to be unstable orstops, in the worst case. When the oscillation stops, the AFM
imaging signal is inevitably lost; in addition, the tip can crashanytime into the sample. By setting the frequency shift to asmall value (the tip is positioned far from the sample surface),
the risk of stopping of the oscillation can be eliminated at
the severe sacrifice of imaging resolution. A new intelligent
control method for the frequency shift, which changes flexibly
according to the sample topography, is required.
Several FM-AFM methods, such as electrostatic force
microscopy and force spectroscopy, are powerful tools foratomic-scale investigations of material properties, therefore
applications of these methods to molecular-scale studies of
biological functions are strongly expected. In particular,
local charge density mapping of biological molecules ina liquid can be a unique method for these studies, sinceinteractions between biological molecules are often based onelectrostatic forces. Although high-resolution electrostaticforce microscopy (EFM) measurements in UHV have been
established, measurements in liquids cannot be conductedeasily because of the existence of charged ions, which can bevaried depending on the electrostatic potentials of the AFM tip
and the sample. The electrically conductive part of the tip must
be shielded except for the tip apex, which can be realized by theapplication of various MEMS techniques as well as advancesin liquid AFM technology.

Figure 6. Frequency spectra of cantilever Brownian motion. Solid
curve: spectrum measured using the improved deflection sensor inwater (k = 40 N m−1 , Q = 8). Dotted curve: theoretical spectrum
curve calculated using equation (1). Grey horizontal line: estimated1
sensor noise level (39 fm Hz− 2 ).

mica obtained in pure water. Atomic-scale features with aperiod of 0.52 nm are clearly imaged. These features werestably obtained with frequency shifts in the range from +200
to +500 Hz. The vertical resolution of the FM-AFM wascalculated from the measured frequency shift curve and thefrequency noise with an FM bandwidth of 1 kHz. The obtained
value of the vertical resolution was 2–6 pm. Since the verticalcorrugation of the atomic surface is typically 20–100 pm, avertical resolution of 2–6 pm is sufficiently small to achieve
true atomic resolution. Figure 8 shows an FM-AFM image ofa purple membrane containing bacteriorhodopsin (bR) protein
molecules on a mica substrate in a phosphate buffer solution.
We can clearly see the regular hexagonal arrangement of sets ofthree bright dots corresponding to bR trimers, as well as some
defects in the trimers (indicated by the white arrows).
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Figure 7. Left: FM-AFM image of the cleaved (001) surface of muscovite mica taken in water.  f = +200 Hz, A = 0.6 nm. Right: thecrystal structure of muscovite mica (cleaved surface).

Figure 8. FM-AFM image of a purple membrane on a mica substrate taken in a phosphate buffer solution (10 mM PBS, 50 mM KCl).
 f = +54 Hz, A = 1.6 nm. Right: schematic of hexagonal array structure of bR trimers.

In addition, studies of hydration structures have been
carried out by AFM force spectroscopy in liquids [25]. Since
the hydration structure measurement strongly depends on
the shape and the chemical properties of the AFM tip, thepreparation of tips suitable for the solvation/hydration study is
crucial. Further developments in the measurement resolution
and sensitivity of liquid AFM will possibly expand present
AFM studies of hydration structures at a single position
into a novel mapping method of three-dimensional hydration
structures.

4.1. Various devices optimized for high-speed scanning
The maximum scan speed is determined by two factors:(1) the maximum frequency at which the scanner can be
driven without producing unwanted vibrations; and (2) the
bandwidth of feedback operation to maintain the tip–sample
interaction force. The scanner has macroscopic dimensions
and hence its structural resonant frequency is low. Thus, thescanner is the device that is most difficult to optimize forhigh-speed scanning. Recent efforts have been successful inovercoming this difficulty. The impulsive forces produced by
the quick displacement of piezoactuators are counterbalancedby simultaneous displacement of additional piezoactuators in
respective opposite directions [26, 27]. A new techniquefor actively damping structural and piezoactuator vibrations
successfully eliminates their resonant vibrations and at thesame time enhances the response speed of piezoactuators [28].
This technique uses a mock scanner (or a mock actuator)
whose transfer function is similar to that of the real system.

4. High-speed AFM
High-speed AFM has enormous potential. It can be used
to visualize moving objects in the nanometre-scale world.
In addition, it allows us to assess and/or process a large
surface area by scanning its divided areas in rapid succession.
Since these innovative abilities will surely create new scienceand nanotechnology areas, various studies have focused onachieving a high imaging rate.
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Technological Progress2015 ~
Mapping of 3-dimensional
solvation structures

Achieved~ 2010FM-AFM high-speed imaging
Frame rate > 1 frame/sec

Achievable
Indeterminate

2015
Charge density mapping~ 2010Novel force-sensing devices with high Q-values in liquid
Force sensitivity: 0.1 pN
0.01pN
Imaging of samples with large height difference2010 single-nanometre
single-na
resolution
2015 subnanometre resolution

2005
True atomic resolution AFM
2004
True molecular resolution AFM
1998
Q-control

2001
FM-AFM imaging in liquid

Year
2000
6 years ago

2006
present

2010
in 4 years

2015
in 9 years

2020
in 14 years

Figure 9. Technology roadmap for FM-AFM imaging in liquid.

However, since the capacity of available piezo-actuators islimited, the bandwidth of the z-scanner is limited to ∼150 kHz
at present.
The feedback bandwidth is determined by delays that
occur in various steps in the feedback loop. In the dynamic
mode, a cantilever is oscillated at (or close to) its resonant
frequency. Since it takes at least a half cycle of the oscillation
to read the amplitude signal even with the fastest detection
technique, the feedback bandwidth (defined by the feedback
frequency that gives a 45◦ phase delay) can never exceed
1/4 (in practice ∼1/8) of the cantilever’s resonant frequency.
Therefore, the resonant frequency must be very high, while
its spring constant should be kept as small as possible, which
requires small cantilevers [29]. Olympus has developed smallcantilevers that have resonant frequencies of 3 MHz in air and
1.2 MHz in water, and a spring constant ∼0.2 N m−1 [30].
The response time of a cantilever is proportional to the quality
factor ( Q ). In water, small cantilevers have Q = 2–3,
resulting in a response time of ∼0.66 μs. A main delay
among others is produced in ‘parachuting’, which occurs
after the cantilever tip completely detaches from the sample
surface, especially at a steep downhill region of the sample(i.e. the tip cannot quickly land on the surface again).
Complete detachment can be avoided by setting the amplitude
set-point deeper, but this setting increases the tip–sample
interaction force. The feedback bandwidth quickly decreases
with a shallowing setpoint and hence the allowable scan speed
depends on the nature of the sample. For hard samples or
samples firmly fixed onto the substratum, it is relatively easy
to increase the scan speed using a deep setpoint. However, forexample, for observations of dynamic interactions of biologicalmacromolecules in a liquid, the tip should barely touch thesample and, accordingly, the setpoint should be very shallow(very close to the cantilever’s free oscillation amplitude). Thus,
high-speed imaging of the dynamic behavior of fragile samples
is quite difficult. A newly developed PID controller (dynamic

PID controller) can change its gain parameters automatically,
depending on the tip–sample interaction, which eliminates
the sharp dependence of the feedback bandwidth on the
setpoint [31]. With these devices all together, the feedback
bandwidth has reached ∼70 kHz. Therefore, protein moleculesmoving on mica in solution can be captured clearly by video at
60 ms/frame without damaging the sample (see figure 10, left
panel). Several movies can be viewed at the following URL:
www.s.kanazawa-u.ac.jp/phys/biophys/roadmap.htm.
4.2. Current attempts toward the next generation ofhigh-speed AFM
Various attempts are now underway to enhance the scan
speed and to expand the functionality of high-speed AFM. (1)A new method is being developed to enhance the resonant
frequency of piezoactuators beyond their natural resonant
frequencies without changing the mechanics; the driving
signal is manipulated so as to extend the bandwidth of themechanical transfer function. This is a kind of inverse
transfer function compensation and can create an approximate
inverse-transfer function for an arbitrary transfer function(figure 11). (2) Direct actuation of a cantilever is usually
performed by applying a magnetic field to a cantilever coatedwith a magnetic material. Because of this coating, theresonant frequency is lowered. Self-actuation cantilevers [32]are the other alternative, but require complicated fabrication
processes, which at present results in a relatively low resonant
frequency and a large spring constant. Photo-thermal actuationof a cantilever was pioneered by Umeda et al [33]. When
this technique was applied to our small cantilevers using
405 nm laser light, the dc deflection sensitivity was about
10 nm mW−1 . Since no additional coating is required, it does
not lower the natural resonant frequency. One drawback is theslow deflection response to changes in the laser power becauseof slow heat transmission. However, this was successfully
eliminated by inverse transfer function compensation [34].
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Figure 10. AFM images of myosin V and purple membrane captured by high-speed AFM. Left panel: Clippings of images of myosin V
captured at 60 ms/frame for a scan range of 250 nm. The attached numbers are the frame numbers. Scale bar is 37 nm. Right image:
Bacteriorhodopsin lattice structure of purple membrane captured by high-speed FM-AFM at 1.9 s/frame. Scan range is 25 nm.

demonstrated simultaneous in-liquid imaging of topography
and phase contrast at ∼70 ms/frame [35]. Even with highspeed FM-AFM, where the Q value of the cantilever wasinevitably elevated, relatively fast imaging was demonstrated
without using a phase-locked loop (PLL) circuit. A large shift(> +50 kHz) in the resonant frequency was observed when
the oscillating small-cantilever tip touched the mica surface.
As shown in figure 10 (right image), a high-resolution imageof the lattice structure formed by bacteriorhodopsin molecules
on purple membranes was imaged at 1.9 s/frame.

Figure 11. A circuit diagram for producing the inverse transfer
function from an arbitrary transfer function. G(s) is a given transfer
function, L 1 (s) and L 2 (s) are low-pass filters representing limited
bandwidth of amplifiers used in the circuit, and ‘g ’ is the gain
parameter. The transfer function of this circuit becomes 1/G(s),when L 1 (s), L 2 (s) and g are 1. Because of the limited bandwidth of
amplifiers used in the circuit, the gain has to be less than 1. To
approach 1/G(s) more closely, similar circuits can be connected inseries with phase compensation by the (1 + differential) operation.

4.3. Roadmap for high-speed AFM
Figure 12 shows the roadmap for high-speed AFM. The
first generation of high-speed AFMs appeared in 2001 [26].
Although moving protein molecules were captured at
80 ms/frame for a scan range of 250 nm, the tip–sample
interaction force was too large to image fragile biomolecules
without damaging or disturbing their physiological functions.
This problem has recently been overcome to some extent
by optimizing various devices as mentioned above, which
has resulted in successful imaging of protein molecules atwork [34, 36]. Video-rate imaging was demonstrated by giving
up the electronic feedback operation [37]. Such a high-speed
AFM without electronic feedback control can be applied tohard samples with a small height. Video-rate AFM withfeedback control will likely be realized soon, probably in
one to three years, although the scan size is still limited to∼250 nm. High-speed AFM with this capacity or somewhat
better capacity (∼10 ms/frame) appears to be most usefulfor studying protein molecules at work, since a protein’sfunctional processes generally take place in this time domain
and because dynamic structural changes of protein molecules
at work represent the aspect most lacking currently, despite
the recent developments of advanced techniques for biologicalsciences. In ten years, high-speed AFM for biological science
will likely be widely distributed in the world, which should
activate biological studies and accelerate our understanding of
the molecular mechanisms of protein functions.
Thus far, high-speed AFM for industrial applications has
not been developed. For industrial uses, the sample stagecannot be scanned; instead, the cantilever has to be scannedThis actuation has great potential to enhance the feedback
bandwidth, because the resonant frequency of small cantilevers
is much higher than those of available piezoactuators with
sufficient displacement. By controlling the tip–sample distanceby modulating the laser power, a feedback bandwidth of
100 kHz was recently achieved, which promises video rate
imaging for a scan range of ∼150 nm. (3) One of theunique features of dynamic mode AFM is the availabilityof additional signals for detecting the tip–sample interaction;
the phase and resonant frequency of an oscillating cantileverchange as a result of the interaction. These signals canafford information on the physico-chemical properties of thesample. Conventional cantilevers need to be oscillated with alarge Q for sensitive detection of these signals. In addition,a slow detector, typically a lock-in amplifier, is required
to detect small changes in these signals. Therefore, highspeed phase-contrast imaging has been impossible. Since
small cantilevers undergo relatively small damping from thesurrounding medium, the weak damping that occurs between
the tip and the sample is reflected and amplified in the
cantilever oscillation. Also, because of the large ratio of thehigh resonant frequency to the small spring constant, a large
shift in the resonant frequency is expected to take place on
the tip–sample interaction. A recent attempt with high-speed
AM-AFM using small cantilevers (with a small Q ∼ 2)
8

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S Morita et al

Figure 12. Roadmap 2006 for high-speed AFM. The numbers in square brackets are the reference numbers.

(1) Chemical discrimination of intermixed two-atom
species and atom-interchange manipulation of heterogeneous
two-atom species at RT made by frequency modulation (FM)AFM under ultrahigh vacuum. By converting a precise
site-specific frequency shift curve to a site-specific force
curve, we have obtained precise short-range force curves of
selected single atoms. This method can clarify the chemical
covalent force over each species independent of the tip–surface
distance. Further, using directional atom interchange, we haveconstructed embedded atom letters, ‘Sn’ (the symbol for a
tin atom) consisting of 19 Sn atoms embedded in a Ge(111)substrate. Using these methods, we can assemble compound
semiconductor nanostructures atom-by-atom.
(2) Submolecular and atomic-resolution imaging evenin a liquid by FM-AFM. The use of the small-amplitude
mode and large noise reduction in the cantilever deflection
sensor has resulted in great progress in FM-AFM imaging
in a liquid. Consequently, true atomic-resolution imaging of
the cleaved (001) surface of muscovite mica in pure waterand submolecular imaging of a purple membrane on a mica
substrate in a phosphate buffer solution were demonstrated.
Submolecular and atomic-resolution imaging in a liquid is now
possible using FM-AFM.
(3) High-speed imaging of biological molecules moving
on a substrate in solution by dynamic mode AFM. With several
new techniques all together, the feedback bandwidth hasreached ∼70 kHz. A recent attempt at high-speed AM-AFM
using small cantilevers demonstrated simultaneous in-liquid
imaging of topography and phase contrast at ∼70 ms/frame.
In addition, a high-resolution image of the lattice structure
formed by bacteriorhodopsin molecules on purple membranes
was obtained at 1.9 s/frame. High-speed AFM with innovative

over a large sample surface. In this situation, the system fordetecting the cantilever deflection has to be miniaturized togain high resonant frequency. Self-sensing and self-actuationcantilevers [38] appear to be ideal for such a miniaturized‘cantilever scanner’. This type of cantilever is alreadyavailable. However, the resonant frequency in air (∼50 kHz)is not sufficiently high. Thus, MEMS technology appears to
be key in breaking the present situation to realize innovative
high-speed AFMs for industrial uses. Before this innovation,
which could occur in 10–15 years, the first version of a highspeed AFM for industrial use will likely appear in five years.
Its main target will probably be the semiconductor industry,where high-speed AFMs can play an active role in evaluating
semiconductor wafers without damaging them.
Various AFM-related techniques, such as nano-printing
and nano-lithography, have been developed for creating nanostructures on surfaces. Also, as mentioned in section 2,
the manipulation at room temperature of selected individual
atoms with an AFM probe has become possible. The powerof these advanced techniques has been demonstrated at thelaboratory level. However, at present, the surfaces createdcannot be applied for practical use, since the area size of thesurfaces created is quite limited because of slow processing andmanipulation. Therefore, high-speed AFM will make a great
contribution to this field. Newly created surfaces with enough
dimensions will become targets to be studied in materialscience and will also be used by industries that require wellcontrolled nano-patterns for manufacturing advanced products.

5. Summary
In this paper, recent progress using dynamic mode AFM has
been reviewed, focusing on three topics.
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S Morita et alabilities is expected to result in the creation of new science andnew nanotechnology.
After reviewing the state of the art of these highperformance AFMs and various techniques that have enabled
them, the status and future prospects were reviewed from theviewpoint of Japanese groups that have been involved in workin these three topics.

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