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Application Note #133
Introduction to Bruker’s ScanAsyst and
PeakForce Tapping AFM Technology
PeakForce Tapping™ (PFT) and ScanAsyst™ (SA) are two
Atomic Force Microsocope (AFM) imaging techniquesthat have been recently introduced by Bruker. In thisapplication note we will explain the underlying physicalbackground, fit PFT into the framework of existing AFM
modes, and show the benefits of these new modes throughapplication examples.

Why Were AFMs Previously Difficult to Use?
Let’s examine briefly the actual workflow necessary to run
an AFM experiment. A typical session starts with sample
preparation and AFM mode selection. The latter sometimes
dictating the former. Once the sample is ready, the AFM
has to be set up (i.e., the sample has to be mounted, thescan mode selected, a tip inserted, and the detection
system aligned). After that, the system has to be brought
into “feedback” and the feedback constantly tuned by theoperator according to the scan conditions to ensure proper
operation. Following the successful acquisition of an image
it has to be analyzed and presented.

PeakForce Tapping Monitor

If one analyzes this typical workflow across applications anduser experience, the crucial step is the actual adjustmentof the AFM feedback parameters. It is by far the mosttime-consuming and nuanced part of an AFM experiment,
and thus offers the most potential benefit to the user for
automation. In short, ScanAsyst automatically provides
you with consistent, expert-quality results independent ofuser experience.
To put the capability of ScanAsyst into context, it isuseful to examine the existing AFM scan modes to better
understand how ScanAsyst, and its underlying mode
PeakForce Tapping fit into the modes hierarchy.

Fundamentals of AFM Operation
In atomic force microscopy a sharp probe is brought
into close proximity to a sample. Probe and sample aresubsequently moved relative to each other in a raster
pattern, and a quantity is measured in a serial fashion at
discreet locations (pixels). Figure 1 shows a schematic of aprobe in an AFM system.

External
displacement
sensorCan be mounted
to a piezoelectric
tranducer

Fixed end of
the cantilever

Free end of
the cantilever
Cantilever

Probe

Tip of theprobe
Specimen

Direction of tip
displacement

Probe
height

Probe
width

Figure 1. Schematic of the cantilever-tip assembly used in an AFM.

The cantilever and tip are typically manufactured as one
unit from silicon. Common dimensions are about 100µm
for the length of the cantilever, with <10nm for the tip
radius and springs constants from 10mN/m to 100N/m.1
The tip itself can have various coatings to enable its
sensitivity to a certain interaction to be measured – ranging
from a metal for conductivity to a ligand for biological
specificity. Any interaction between the tip and sample
surface is measured by monitoring the displacement of
the free end of the attached cantilever. There are several
schemes to accomplish that task, including beam-bounce,
capacitive sensor, interferometry. A beam-bounce scheme,
where a laser beam is reflected off the cantilever into asegmented photodetector is arguably the most commonand established for a variety of reasons.2 The fixed endof the cantilever can be mounted either static or on asmall actuator to enable dynamic imaging modes. During
operation the cantilever/probe is part of a modified classicalclosed-loop feedback system (see figure 2).

set value–

PID controller

Z-piezodemodulator

sensoractual value

external
disturbancestablized
oscillator

Figure 2. Block diagram of the feedback loop controlling theinteraction force in an AFM.

2

The external disturbance is the tip-sample interaction as
measured through the cantilever displacement sensor.
The setpoint value is a user input that determines themagnitude of the tip-sample interaction. For example, inconventional AFM the setpoint represents the imaging
force. The resulting error signal (or difference between the
setpoint and actual value) is processed by a ProportionalIntegral-Differential (PID) feedback controller that drives thez-piezo to minimize the error signal, thereby realizing thedesired setpoint.
Contact Mode
Contact mode is the easiest AFM mode to understand, and
it is also the fundamental basis of such additional modes as
Scanning Capacitance Mode (SCM), Scanning Spreading
Resistance Mode (SSRM), etc. A typical AFM cantilever isshown in figure 3.

→

D

→

→

G

Z

SampleFigure 3. Deflection of a cantilever caused by tip-sample forces.

Here the tip-sample gap (G) is G = Z-D. D is the deflectionof the free end of the lever caused by tip-sample
interactions. (For a closer look into tip sample interaction
look into Mie potential in common physics textbooks.).3 G
is directly proportional to the applied force action on the
cantilever.
The small (angular) movement of the lever is commonlymeasured by a laser beam that is reflected off the cantileverand directed onto a split photodetector, as shown infigure 4. With that setup, the lever motion becomes
proportional to the movement of the laser beam on the
split photodetector amplified by B = 3s/l, with s being the
distance between the cantilever and photodetector, andl the cantilever length.4 It is, however, important to notethat one can not simply increase the distance from thecantilever to the detector to achieve more sensitivity. By
increasing the distance, the spot size of the beam on thedetector also increases, in turn making the actual sensitivityof the system independent of l and proportional to 1/s.5,6

Figure 4. Schematic of light source, cantilever, and photo detector
reassembling the basic components of the light-lever AFM
detection system.

A basic AFM operation, which helps explain contact mode,is the force-distance curve. Here the cantilever is broughtfrom a location above the surface but within the range ofthe z-piezo (Z < Zpiezorange) toward the surface until thetip contacts the surface. Any further movement of thez-piezo toward the sample surface will result in an upward
deflection of the lever and/or sample deformation. The
z-scanner position is commonly generated by a triangular
waveform applied to the z-piezo. A schematic of a force
curve is depicted in figure 5.

Figure 5. Force distance curve. The approach (red) and withdraw(blue) curves are shown on the right. Note that the total contact
force is dependent on the adhesion as well as the applied load.

It should be noted that to obtain a force curve, thez-feedback normally used to keep the cantilever deflection
constant has to be turned off. Points 2 and 4 describe
two important occurrences in a loading curve. These are
the points where the force generated by the tip-sample
interaction force is not balanced by the restoring force ofthe cantilever, i.e., dF/dx > k at point 2. Setpoint denotes
the deflection value used for the z-feedback. To convert thevertical axis from the photodetector output in volts to unitsof force, the system must be calibrated. The first step is tocalibrate the photodetector output to the actual cantileverdisplacement, commonly referred to as “deflection
sensitivity.” That step is simply carried out on a hard sample
with the assumption that the tip-sample gap is zero (G=0).
The vertical axis now has the units of length. The second
step is the determination of the cantilever spring constant .
Force curves in themselves can reveal a variety of sample
properties, such as adhesion and compliance. As an aside,
there is an imaging mode called Force-Volume that is basedon pixel-by-pixel analysis of force curves, however, due toits slow speed, it is not often used. The most common useof force curves is in combination with any of the forms of
SPM imaging in a “point-and-shoot” fashion.
Contact mode imaging is carried by simply keeping the
setpoint (point 3 in the force curve diagram) constant whileraster scanning the tip and sample relative to each other.
The movement of the z-piezo then becomes the sample
topography that is plotted as a function of xy. The user has
to make sure that the feedback loop is fast enough to allow
the z-piezo to respond to changes in sample topography
but slow enough to avoid oscillations of the system. Even
though reasonably easy to operate, contact mode has theinherent drawback that lateral force exerted on the sample
can be quite high. This can result in sample damage or the
movement of relatively loosely attached objects. A solution
to that problem was to oscillate the cantilever during
imaging, which led to TappingMode™ Imaging.
TappingMode
The solution to the problem of having high lateral forces
between the cantilever and surface, but still maintaining
very high lateral resolution, is to having the tip touch thesurface only for a short time, thus avoiding the issue oflateral forces and drag across the surface. This mode washence referred to as TappingMode AFM.7 In TappingMode
operation, the cantilever is oscillated at or near its resonancefrequency normal to the sample surface. Typical amplitudes
of oscillation are in the range of tens of nanometers, andthus very small compared to the cantilever length.

3

In order to explain TappingMode operation, it can be
useful to examine the dynamics of operation. Assuming
air damping is the dominant factor, the movement of thecantilever can be described using the (sinusoidal) driven
damped harmonic oscillator model:3

m*

∂2 z
∂ t2

∂z

+b

∂t

+ kz = F0

with m* being the effective mass of the cantilever, z thedisplacement of the lever, b the damping coefficient, k the
spring constant, and F0 the driving force (F0 = k A 0). With thenatural frequency:

ω0 =

k
m*

m*

and the relaxation time τ0 =

ωd
ω0

2

+

1
(ω0 τ)2ωdω0Distance2(a)Modulating
Piezo

ƒ0
∆ƒ0

(b)

∆A

Modulating
Piezo

Specimen

Tapping Mode

Attractive forces –van der Waals interaction

An important parameter to consider here is the quality factor“Q” of the system. Q is the ratio of the energy stored in thesystem divided by the energy loss per cycle. In the case of
a lightly damped system, the Q can be written as Q= ω0
τ. The maximum amplitude at resonance then becomes:
Ar = A0 Q.

Specimen

Repulsive forces –short range Coulomb interaction

Force response curve

A0

A (ωe) =Forceb

one can write the amplitude of the lever as:

1–

A typical response curve of a cantilever is shown infigure 6. Typical TappingMode operation is carried out using
amplitude modulation detection with a lock-in amplifier.
This means a frequency close to the cantilever resonance isselected, and the tip-sample spacing is changed to maintainƒ0´ ƒ0Figure 6. Resonance curve of a TappingMode cantilever above
and close to the surface. Note that the resonance shifts to lower
frequencies and exhibits a drop in amplitude.

Figure 7. Force curve highlighting the motion of an oscillating
cantilever in TappingMode.

a constant cantilever amplitude without changing the drive
frequency. Similar to contact mode, the movement of thez-piezo when plotted as a function of xy becomes oursample topography.
It is important to emphasize that we are not measuring a
direct force in TappingMode. The curve shown in figure 7
is constructed by adding the short range repulsive and longrange attractive forces.
When the probe approaches the sample, it experiences anattractive force and is pulled toward the surface until contact
is made. From that point on, the repulsive interaction forces
dominate the response. The probe can then be retracted
and additional information can be extracted from that
trace. The TappingMode AFM, while experiencing theseinteractions, does not actually measure this force curve, nor
the direct forces between the tip and the sample for thatmatter. The TappingMode AFM oscillates back and forthon this curve, interacting without being in direct controlof the force, and reporting only an average response ofmany interactions though the lock-in amplifier. One cancertainly measure the reduction of cantilever amplitude as
tip and sample approach each other, as is shown in figure
8, but it must be understood that each point on that curve
represents an average value and not a single interaction.
While this is in no way detrimental to basic imaging, it
restricts the information beyond sample topography that can4the tapping dynamics depend strongly on the sample
properties. A well-tuned feedback loop for the soft part ofthe sample can cause feedback oscillation for the hard partof the sample, rendering optimization of the parameters
for every part of the sample very difficult. Furthermore,
the long time constant (milliseconds) of the cantileverresonance also prevents instantaneous optimization at each
imaging point. Finally, the direct force control of contact
mode imaging and thus added information available arelost in TappingMode. TappingMode does however offer theundeniable benefit of lateral force free imaging, which has
made it the dominant imaging mode in AFM to date.

Amplitude

Distance
Figure 8. Force curve showing the reduction in cantilever amplitude
versus tip-sample distance.

be gained and unambiguously assigned to a certain sample
property. This is unlike the previously shown force-distancecurve, during which one has direct control and is able toextract useful sample information.
Additionally, the inherently unstable feedback situation in
TappingMode operation makes it quite difficult to automate
some of the scan adjustments. Forces can vary when goingaway from a steady-state situation. This will occur while
scanning rough surfaces, as the amplitude error at the
sharp edges can correspond to interaction forces one orderof magnitude higher than that of steady-state. Amplitude
error incurred force is the leading cause of tip damage, andsuch damage occurs because the feedback is not directly
controlling interaction force. On samples exhibiting highadhesion forces, a tip amplitude has to be selected that ishigh enough to ensure that the tip is actually leaving thesample surface. The higher the tip amplitude, the higher theenergy stored in the lever and subsequently the imaging
forces (see Appendix for a simple example). Operation in
fluids suffers from drift due to temperature changes and/orchanging fluid levels.
At this point, we have established that the adjustment of
the feedback system is a task that is essential to achieving
reliable information from the AFM. It is easier to control
a contact mode scan when compared to a TappingModescan due to the added complexity of the oscillating system.
While past attempts have been made to adjust imaging
parameters automatically in TappingMode, no method has
proven competitive with an experienced user for the broad
range of samples commonly studied with AFMs. This isbecause TappingMode operates at cantilever resonant
frequency, where the cantilever dynamics are relatively
complicated. For example, the cantilever dynamics can bedramatically changed by changing the amplitude set-point.
This causes the highest usable gain to change, which inturn requires the optimal set-point to change. Additionally,

PeakForce Tapping
PeakForce Tapping operates similarly to TappingMode inthat it avoids lateral forces by intermittently contacting thesample. However, it is very different from TappingModein that it operates in a non-resonant mode. The PeakForceTapping oscillation is performed at frequencies well below
the cantilever resonance, thus avoiding the filtering effectand dynamics of a resonating system. In PeakForceTapping, we now have an oscillating system that combines
the benefits of contact and TappingMode imaging: direct
force control and avoidance of damaging lateral forces.
The differences to a conventional force curve (and force
volume imaging) are that the z-position is modulated by asine wave and not a triangular one, thus avoiding unwanted
resonances at the turnaround points. A triggering at thepeak force and an algorithm to extract the force curves fromeven noisy backgrounds complete the package. Continuous
force curves can now be executed at frequencies between
1kHz and 10kHz, which in turn enables imaging speedsthat are comparable to TappingMode imaging. The general
operation is illustrated in figure 9.

Tip trajectory

ApproachWithdraw

Peak tapping force

Peak tapping forcetime

Z-position

Figure 9. Experimental data of force curves for a cantilever
operated in PeakForce Tapping. The lever is driven by a sinusoidal
wave and the curves are displayed as force versus time and force
versus distance.
5All the curves shown are experimental data obtained using
a silicon probe approaching a silicon surface. The top curve
represents tip trajectory as it approaches the sample.
Depicted underneath are plots of deflection (or force) as
a function of time and z-position, respectively. The latterbeing the more familiar representation. The dashed line is
the zero force (no load) reference, established when tip is
not interacting with the sample. As the tip approaches the
sample surface, it will experience long-range van der Waals
attraction until dF/dx > k, causing the cantilever to jump
into contact with the sample. After contact, the short range
repulsive forces dominate the interaction, leading to thepeak point at the approaching curve. When the tip begins
to unload it goes through an adhesion minimum, usuallycaused by capillary meniscus and finally becomes free.
PeakForce Tapping refers to the proprietary control
method that uses the individual peak force points as
triggering mechanisms to force the the z-piezo to retract.
Its effectiveness can be seen from the force curves in
the left column. The feedback algorithm recognized the
local peak force even though the setpoint is below thebaseline. Operating below the baseline allows operation atvery low forces, which in turn is crucial for obtaining highresolution data on soft samples. It is important to note forcomparison purposes that all curves shown were obtainedusing a cantilever with a spring constant of 40N/m, as iscommonly used in TappingMode operation. With a softer
cantilever, the controlled interaction force can of course be
much lower due to the higher force sensitivity. The typical
repetition rate of 2kHz allows for imaging speed speedsthat are comparable to TappingMode operation. An example
of clearly resolved atomic steps on graphene is shown infigure 10.

ScanAsyst
ScanAsyst uses the previously described PeakForceTapping mechanism, which decouples cantilever response
from resonance dynamics, to automatically adjust all criticalimaging parameters. Because the peak force feedback
directly controls the interaction force, there is no distinction
for peak force for soft or hard parts of the sample. Directinteraction force control enables the possibility of producing
a uniformly optimized feedback loop for all the points of
the inhomogeneous sample. Using a patent pending image
correlation algorithm, feedback oscillation is detected andeliminated in a matter of milliseconds. A real-time feedback
loop constantly monitors and adjusts the gain to keep thedata quality within a predefined noise level (unlike manualgain adjustment where usually one gain is used for a whole
image). ScanAsyst optimizes the gain according to current
sample condition at different locations. The ScanAsyst
algorithm also optimizes the set-point to the minimum
force required to track sample surfaces, controls the scan
6

Figure 10. 2µm scan of a sample of graphene obtained in PeakForceTapping operation. Several monoatomic steps and small islands can
be clearly identified.

rate, and can automatically lower the z limit if necessary.
This results in extremely high-quality images without user
adjustment of imaging parameters and without the normallyproblematic AFM user interfaces exhibiting numerous andoften confusing parameters.
A basic ScanAsyst interface is shown in figure 11. The
whole “Feedback” section is on “autopilot” and isconstantly updated. In addition, ScanAsyst also sets theactual scan speed and the z-limits. The only task left for
the user is the selection of the actual scan area, which canbe achieved by either typing the appropriate values in thecorresponding fields or by simply using the mouse anddrawing a box inside the image acquired. If the flexibilityof partial or fully manual operation is desired, the user can
set the AutoControl field to individual and choose whichparameters should be selected by the system and which by
the human operator.
The underlying calculations enabling ScanAsyst happens
on the fast FPGA chips implemented in Bruker controllers.
One of the key parameters controlling the auto-optimization
procedure is the “noise threshold” parameter. Noise
threshold is determined by analyzing the high-frequency
components of the AFM data and adjusting, for example,
the feedback gains accordingly. The noise threshold isalso automatically adjusted by the AFM itself after it hascompleted a full frame, with values typically ranging from
0.1 to 1. The reasoning behind this feature is that not allscans require the ultralow noise that Bruker AFMs areFigure 11. Screen shot of the basic ScanAsyst interface. All feedback
settings and the scan rate are automatically calculated by the AFM.
Figure 12. 80nm scan of C18H38 alkane chains obtained in PeakForceTapping. The inter-lamellar distance is only 2nm!

capable of achieving (e.g., the Dimension® Icon® has a noiselevel of 30pm!). If a sample exhibits features hundreds ofnanometers high, picometers of noise are simply not visible.
It is, however, quite important to properly follow the often
steep sample features without bringing the actual scan
speed to a crawl. ScanAsyst consequently uses a moreaggressive feedback gain for imaging. On flat samples likesilicon wafers, a very low noise threshold is chosen by theauto-optimization routine, which results in less aggressive
gain parameters to take full advantage of the ultra-low noiselevels of the AFM. By selecting a discrete threshold we areasking the AFM to adjust feedback and imaging speed toget a certain result instead of manipulating the feedback
loop trying to figure out what the actual result is!
The data that ScanAsyst produces is often better thanwhat even a seasoned AFM expert can produce. More so,this data is often generated during the first time imaging
a sample, is reproducible, and is independent of the skillof the AFM operator. The 80x80nm2 image of the alkane
C18H38 deposited on HOPG in figure 12 strikingly illustrates
that point.
ScanAsyst does have an amazing amount of flexibility
built-in that allows users to fully or partially control the
PeakForce Tapping operation. An example of an expanded
ScanAsyst user interface is shown in figure 13.
In this specific example, an advanced user elected tomanually control all relevant feedback parameters, with the
exception of the imaging speed. At the bottom of the dialog

Figure 13. Screen shot of the expanded ScanAsyst interface.
If desired, ScanAsyst allows flexibility for parameters to be
adjusted manually.

are two previously not mentioned parameters. “Peakforce
Amplitude” refers to the actual z-excursion of the AFM
tip during operation. The value is pre-set for the specificexperiment selected, e.g., operation in air or fluids, but
can be changed if desired. “Lift Height” refers to a valuethat is used to determine any background signals affecting
the data (it is not related to the LiftMode feature, where
the tip follows a previously scanned line). This becomes
especially important for operation in fluids where small
volume changes due to evaporation or fluctuations intemperature can have significant effects on AFM operation.
7

During ScanAsyst/PeakForce Tapping imaging the user canconstantly monitor the integrity of operation by looking at
the built-in force monitor shown in figure 14. Here the top
graph depicts the familiar force versus z plot, whereas thebottom shows force versus time.
PeakForce Tapping also avoids another phenomenathat plagues TappingMode. The cantilever amplitude
in conventional TappingMode can change as a resultof both long range and short range interactions. As aresult, the height data represents the system response
to a combination of interactions. PeakForce Tapping, on

the other hand, only responds to short range interaction.
The long range interactions (adhesive and electrostaticforces) are basically ignored for height control. Short range
interactions are the key to high-resolution imaging. By
consistently controlling the short range interaction forces,
PeakForce Tapping enables image quality control with fewer
artifacts linked to complication of the tip surface interactions
and cantilever dynamics.
An example of a structure difficult to image in regular scanmodes is shown in figure 15, which depicts a cross-sectionof a sample with very narrow trenches. This type of sample
is very difficult to image in TappingMode because thegeometry would cause the AFM to “stick” to the sidewalls,
thus damping the tapping vibration and preventing the tipfrom reaching the bottom of the trench. A researcher using
TappingMode would likely conclude that the trenches do
not have flat bottoms or that they are significantly shallower
than shown. Peak Force Tapping is insensitive to the effects
that geometries like this have on a resonating systemand therefore has no difficulty reaching the bottom of
the trench.
Another type of sample where PeakForce Tapping solves
problems commonly encountered in TappingMode arenested structures with steep and often high topographies.
As one can clearly see in figure 16, the PeakForce Tapping
image on the left is flawless, whereas the TappingMode
image on the right shows streaking and some tip
parachuting. This is again a direct effect of not operating atresonance, which enables direct force control and thus not
being affected by the Q-factor of the cantilever.

Figure 14. Real-time shot of the force-monitor during imaging with
ScanAsyst. This allows the user to constantly monitor the integrity of
the imaging process.

Examples for operation in changing environments include
heating and cooling experiments and scanning in fluids.
Heating and cooling can be useful for learning about
dynamics in melting and crystallization experiments.
Imaging in fluids is a common and necessary operating
mode for an AFM. Besides the obvious benefit of being
able to study a sample under low force due to the lack ofFigure 15. 160nm linescan of steep trenches. The flat bottom indicates that the probe reached all the way.
8

Figure 16. 30µm scan of a Teflon membrane in PeakForce (left) and regular TappingMode (right). Artifacts visible in TappingMode operation
are not present in the PeakForce Tapping data.

usually high capillary forces, a lot of studies simply require
the sample being immersed in fluids. Examples include
the study of biological specimens under physiologicalrelevant conditions or the examination of electrochemical
phenomena for corrosion or battery research, to namejust a few. It is often interesting or necessary to image
samples under fluid or at temperatures above or below
room temperature. ScanAsyst-PeakForce Tapping works
well in these environments with several benefits overTappingMode. For one, it is not necessary to tune thecantilever at all, as the cantilever is operated at a fixed
frequency. Consequently there is no re-tuning required
when the temperature is changed or when changing
from air to fluid operation. In TappingMode on the other
hand, a temperature change causes changes in resonant
frequency and Q of the cantilever, making it imperative thatthe drive amplitude and frequency are adjusted whenever
the temperature is changed significantly. With PeakForceTapping, the system is not being driven at the cantileverresonance, so it is not sensitive to changes in probe
resonant frequency and Q. Any background changes causedby temperature or fluid level fluctuations that can influence
operation are subtracted in real-time by the ScanAsyst
software, allowing imaging forces as low as a few tensof pico-newtons. An example of a high-resolution image
in buffer solution is shown in figure 17. One can clearly
see the square origami DNA assemblies, the connection
points on each corner, and the single DNA strand building
blocks. An example of ScanAsyst operation at different

Figure 17. 1µm scan of Origami DNA in buffer solution using
ScanAsyst. Single strands of DNA comprising the square structure
are clearly discernible.

9Figure 18. 500nm images of C 60H122 at room temperature and 70°C.

temperatures is given in figure 18. The 500x500nm2 imagesof C60H122 alkane with a lamella spacing of 7.5nm wereobtained at room temperature and 70°C, respectively. At
the elevated temperature, the alkane chains are able to slide
into a more favorable energetic lower arrangement dictatedby the underlying graphite substrate.
One of the strong points about ScanAsyst is thatforce curves are indeed available to the user to extract
additional material specific information if desired. Using
the High-Speed Data Capture (HSDC) functionality of
the NanoScope® V Controller, a researcher can gather
thousands of force-distance curves in one shot without
interrupting the imaging process. The resulting force-curves
can be extracted, correlated to the sample topography, andanalyzed using NanoScope analysis or custom software.
Bruker uses the capability of obtaining multiple forcedistance curves at each image location in its PeakForceQNM™ package, which performs nanomechanical analysis
in real-time. Figure 19 shows the resulting curves from a
HSDC of 100ms on the top, and one selected curve on
the bottom.
ScanAsyst and PeakForce Tapping have changed thelandscape for AFM operation of basic imaging andnanomechanical analysis but this is far from the end. A

10

Figure 19. Result of a HSDC during the imaging process. The force
curves that enable the imaging can be extracted and also be used
for further analysis.

Figure 20. PFT-TUNA image of carbon nanotubes. Sample topography on the left and conductivity map on the right. Sample courtesy of Prof.
Hague, Rice University.

variety of modes, traditionally carried out in contact mode
operation, can greatly benefit from combination with
PeakForce Tapping. Electrical modes such as Scanning
Capacitance Microscopy (SCM) or Tunneling AFM (TUNA)
are two that would get a performance boost by reducing
the lateral components stemming from imaging in contact
mode. An example of a TUNA image obtained by combining
the ScanAsyst/PeakForce Tapping is shown in figure 20.
A scan of carbon nanotubes clearly shows the sample
topography on the left and the current map on the right,
highlighting the electrical connectivity of the nanotubes.
This data is not obtainable using the traditional TUNAcontact mode combination due to excessive lateral tipsample forces.

Conclusions
Tapping has long dominated the world of AFM. Its main
advantage has been the lack of lateral forces that are
inherent to contact imaging. At the same time, it hashindered the advancement of atomic force microscopy
in some ways, due to the inherently complex nature ofoperation that has prevented automation of the adjustmentof the feedback loop, arguably the most critical step.
This application note shows that not only can PeakForceTapping generate data that is equal and often better thanTappingMode images, but that this data can be obtained
reliably by a new user using ScanAsyst without sacrificing
full flexibility for experienced researchers.
11Appendix

References

Another way to look at the fact that we are not measuring
a force but rather a derivative is shown here. Following
Newton’s second law, the motion of the cantilever can be
described as:

1 www.brukerafmprobes.com

∂Fext
∂x

with k e known as the effective spring constant. Thus, theeffective resonance frequency becomes:

ωe =5. Fukuma T., Jarvis, S.P., Rev. Sci. Instrum. 77 (2006) 043701.
6. Putman, C.A.J., de Grooth, B.G., van Hulst, N.F., Greve, J. Ultamicroscopy
42-44 (1992) 1509.
7. Zhong, Q., Innis, D., Kjoller, K., Elings, V. Surface Sci. Lett. 290 (1993).
8. Cleveland, J., Anczykowski, B., Schmid, A.E., Elings, V. Appl. Phys. Lett 72(1998) 2613.

1
me k0 –

4 Dror, S: Scanning Force Microscopy. Oxford Univ. Press (1991).

∂Fext
∂x

As the effective resonance frequency is the parameter the
feedback signal is based upon, the force derivative, and not
the force itself, is the crucial parameter here.
It may be beneficial to get an idea of the actual energy
stored in the cantilever. Again Hooke’s law lets us calculate
the elastic potential energy Ep for a cantilever. Using atypical spring constant for a TESP lever of 40N/m and 10nm
vibration amplitude, we get Ep = ½ k z2 = 2 • 10 –15 [Joule] =12.5 [keV].

9. Garcia, R., Gomez, C.J., Martinez, N.F., Patil, S., Dietz, C., Amgerle, R.
Phys. Rev. Lett. 97 (2006) 016103.
10. Hashemi, N., Paul, M.R., Dankowicz, H., Lee, M., Jhe, W. J. Appl. Phys.
104 (2008) 063518.

Authors
Stefan B. Kaemmer, Ph.D., Bruker Nano Surfaces Division(Stefan. Kaemmer@bruker-nano.com)

Acknowledgements
I would like to thank my colleagues Steve Minne, Natalia Erina, Bede
Pittenger, Adam Mednick, and Peter DeWolf for their help. The origami DNA
solution is courtesy of Paul Rothermund/CalTech.

At 10% reduction in amplitude, the set-point results in anecessary energy dissipation of 0.1 • 12.5 keV = 1.3 keV.
That number may look high, but one has to consider thatthe energy is dissipated over a large area when entering
the contamination layer (see the literature for additional
calculations).8-10

Bruker Nano Surfaces Division
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2. Meyer, G., and Amer, N.M. Appl. Phys. Lett. 53 (1988) No. 12, 1045-47; andMeyer, G., and Amer, N.M. Appl. Phys. Lett. 53 (1988) No. 24, 1045.