High-Bandwidth HighGain Current Amplifier

PeakForce Tapping
Mode

Application Note #132
Simultaneous Electrical and Mechanical
Property Mapping at the Nanoscale with
PeakForce TUNA
AFM-based conductivity measurements are a powerful
technique for nanometer-scale electrical characterization ona wide range of samples. Traditionally, these measurements
have been categorized into two classes: Conductive AFM(CAFM), which covers the higher current range (sub-nA
up to µA), and Tunneling AFM (TUNA), which covers thelower current range (sub-pA up to nA). Because of practicallimitations, most conductive AFM measurements have been
restricted to the Contact Mode of AFM operation. As for
TUNA, it has become common to use the term to refer toboth the sensing module and the measurement technique,regardless of the current-level. As a technique, TUNA has
3 key elements: 1) the current sensor, also known as the
TUNA module, 2) the conductive AFM probe, and 3) thebase mode of AFM operation. Each of these elements
contributes to the technique’s capabilities, but also to its
limitations. Improvement to any of these areas has thepotential to improve the technique’s overall performanceand applications. Bruker has developed an enhanced TUNA
module with its proprietary PeakForce Tapping™ modeof operation1 that makes significant improvements to all
three of these elements to enable 1) exquisite tip-sample
force control, which is ideal for soft delicate samples, 2)
quantitative nano-mechanical material property mapping

Simultaneous HeightCurrent-Adhesion-Modulus
Imagesthrough PeakForce QNM™, 3) correlated nanoscale
electrical property characterization through TUNA, and4) extreme ease of use through the ScanAsyst™ image
optimization algorithms. A special probe has also been
designed for use on particularly challenging samples. This
note discusses the basics of PeakForce TUNA, compares it
to standard Contact Mode–based TUNA, and provides data
demonstrating the unique capabilities and differentiatedapplications enabled through the combination of PeakForceTapping and AFM conductivity measurement.

AFM and Conductivity Mapping
The overall performance of AFM-based conductivity
mapping parallels the performance of its underlying mode ofoperation, carrying over both its benefits and its limitations.
Contact Mode Based TUNA (Contact TUNA)
Conductivity measurements at the nanoscale were firstenabled with a Contact Mode AFM equipped with aconductive tip and a current-sensing module. Contact-TUNA
has been applied in many research and manufacturinglaboratories for the analysis of a wide range of materials,
for a wide range of applications. For example, traditional
Contact Mode TUNA has been used to localize and image

electrical defects in semiconductor and data storage
devices, to evaluate the uniformity and integrity of thindielectric films, to characterize piezoelectric and ferroelectricmaterials, conducting polymers, nanotubes, biomaterials,
and others. However, the use of Contact Mode for
topographic feedback has proven to be a severe limiting
factor. For samples that require low imaging forces in either(or both) the vertical or lateral directions, Contact Mode
imaging is not possible, and therefore neither is traditional
TUNA imaging. The Contact Mode limitation applies tothe study of many conductive polymers, organics or othersoft conducting materials, or loosely bound samples suchas nanowires.
TappingMode Point Contact IV (Tapping-TUNA)
AFM has achieved tremendous benefits from a variety
of oscillating tip modes of operation, most notably
TappingMode®. During TappingMode imaging, the AFM
cantilever is oscillated at its fundamental flexural resonance.
This has the advantage of largely eliminating lateral forcesthat tend to damage the tip and/or sample when imaging
in Contact Mode. The vertical interaction force is also
substantially reduced due to the high mechanical Q of thecantilever, permitting imaging of soft or delicate samples.
When imaging in TappingMode, the AFM tip oscillates
relative to the sample by tens of nanometers, and only
spends a few percent of that oscillation in contact with thesurface. This is advantageous for eliminating tip wear andsample damage, but presents a problem for conductivity
measurements. In order to measure the current signal insuch a short time duration (~microseconds) with reasonable
signal-to-noise ratio, the current amplifier would need abandwidth in the MHz range at a high gain (10 9~1011 V/A).
This is beyond the reach of the current technology. To
circumvent this challenge, point-contact current imaging
was introduced. In this configuration, TappingMode is used
for topographic imaging, and then current-voltage curves aretaken at selected points in a Contact Mode fashion.2
Torsional Resonance TUNA (TR-TUNA™)
It is known that AFM cantilevers can oscillate in manydifferent modes, including higher order flexural and torsionalmodes. Imaging with these higher order modes enables thestudy of a wide range of tip-surface interactions. Consider
the case of imaging with the cantilever oscillating in thefirst torsional resonance mode. In this mode, lateral forcesthat act on the tip cause a change in the torsional resonant
frequency, amplitude, and/or phase of the cantilever.
AFM measurements at torsional resonances have many
advantages, but the key advantage for TR-TUNA is theability to achieve low-force scanning while maintaining
the tip in the near-field. 3 The use of TR-Mode® extends
the sample space available to TUNA to include thosesamples traditionally believed to be too soft or delicate for2

Contact Mode (and hence TUNA). This includes conducting
polymers, loosely bound nano-materials and organic
thin films, as well as more traditional thin film samples
such as nano-pillar based low-k materials. However, with
TR-TUNA there is concern that the tip-sample contact ischanging during each torsional oscillation cycle, and thatthese deviations are likely to cause variability in the current
measurement. To mitigate this, the torsional resonanceamplitude is kept very small (less than a few angstroms),
but stable operation at these low amplitudes can be
extremely challenging. This operational limitation has
prevented the wide spread use of TR-TUNA.

PeakForce TUNA Principles
PeakForce TUNA (PF-TUNA) builds on PeakForceTapping, and in doing so, also acquires the capability of
PeakForce quantitative nanomechanical measurements(PeakForce QNM).

Laser Diode

Topography
CMT Modulus

Photodetector

Adhesion
Dissipation
Deformation
Current Amplifier

Conductive AFM Probe

Conductivity

SampleX,YScannerZ

Figure 1. Illustration of PeakForce TUNA setup for simultaneous
topography, mechanical and electrical property mapping.

PeakForce Tapping
As in TappingMode, in PeakForce Tapping, the probe andsample are intermittently brought into contact while thetip is scanned across the sample. This eliminates lateralforces during imaging. Unlike TappingMode, where thefeedback loop keeps the average cantilever vibration
amplitude constant, in PeakForce Tapping the feedback loop
controls the maximum force on the tip (Peak Force) for eachindividual cycle. Because the force measurement bandwidthof a cantilever is approximately equal to its fundamental
resonant frequency, by choosing a modulation frequency
significantly lower than the cantilever’s resonant frequency,
the PeakForce Tapping control algorithm is able to directly
respond to the tip-sample force interaction. This direct force
control protects the tip and the sample from damage, butmore importantly, allows every tip-sample contact to becontrolled and recorded for additional mechanical property
analysis. In the current implementation, the modulation
frequency is 1 to 2kHz.

Quantitative Mechanical Property Mapping
The foundation of material property mapping with
PeakForce QNM is the ability of the system to acquireand analyze the individual force curves from eachtip-sample interaction that occurs during the imaging
process. The curves are analyzed in real-time to obtain
quantitative mechanical properties of the sample, including
adhesion, modulus, deformation, and dissipation. Thesematerial property maps are treated as conventional AFM
channels, and can be displayed and analyzed together
with topography.
PeakForce TUNA Module
It is important to note that the PeakForce Tapping,oscillation frequency (1kHz–2kHz) falls nicely between theTappingMode (>50kHz), and Contact Mode (DC) interaction
cycles. In fact, this mid-band operation is the single mostimportant element for TUNA to work in an intermittent
contact mode. In each tapping cycle, the tip is in contact
with the sample only for a fraction of the cycle (tens tohundreds of microseconds). The TUNA module must beable to pick up a current signal during this time periodwith acceptable signal-to-noise ratio. A rule of thumb isthat the bandwidth of the TUNA module must be 10x
greater than the tapping frequency at the chosen gain. At
TappingMode frequencies this is far beyond the reach ofcurrent technology. At PeakForce Tapping speeds, it is anattainable challenge. The released PeakForce TUNA module
is engineered to have a bandwidth around 15kHz acrossa range of gains from 107 V/A to 1010 V/A, with the noise
below 100fA on cycle-averaged current.
The PF-TUNA module has 6 gain settings (107 V/A, 10 8
V/A, 5 x 10 8 V/A, 10 9 V/A, 1010 V/A, 5 x 1010 V/A), adjustablethrough a combination of hardware and software switches.
The integration of a wide range of gains on one single
module eliminates the need to change modules while
searching for the optimal gain to match the conductivity
level of a sample, or when different gains are needed toreveal all different conductivity levels present in one single
sample. It is noteworthy that the offset at each different
gain setting is automatically zeroed out upon each engage,or gain change, to assure measurement accuracy. The
PeakForce TUNA module, while designed to work with
PeakForce Tapping mode, is compatible with Contact Mode,and provides equal or better noise performance in Contact
Mode compared to existing TUNA modules.
PeakForce TUNA Quantities
Figure 2 illustrates what happens when the periodicallymodulated PeakForce Tapping probe interacts with thesurface. The top line represents the Z-position of thecantilever base, as a function of time, as it goes through one
period. The middle line represents the force measured by

the probe during the approach (blue) and withdraw (red) of
the tip to the sample. The bottom line (green) represents
the detected current passing through the sample. Since themodulation frequency is about 1kHz, the time from point A
to point E is about 1ms.
When the tip is far from the surface (point A), there islittle or no force on the tip. As the tip approaches thesurface, the cantilever is pulled down toward the surface
by attractive forces (usually van der Waals, electrostatic,or capillary forces) as represented by the negative force(below the horizontal axis). At point B, the attractive forces
overcome the cantilever stiffness and the tip is pulled tothe surface. The tip then stays on the surface and the force
increases until the Z position of the modulation reaches
its bottom-most position at point C. This is where thepeak force occurs. The peak force (force at point C) during
the interaction period is kept constant by the system
feedback. The probe then starts to withdraw and the force
decreases until it reaches a minimum at point D. Adhesionis measured by the force at this point. The point where thetip comes off the surface is called the pull-off point. This
often coincides with the minimum force. Once the tip hascome off the surface, only long range forces affect the tip,
so once again, the force is very small or zero when the tipsample separation is at its maximum (point E).
From the current-time plot, the PeakForce TUNA algorithm
extracts three measurements: 1) peak current, 2) cycleaveraged current, and 3) contact-averaged current. Peakcurrent is the instantaneous current at point C, coinciding
with peak force. This corresponds to the current measured
at a defined force. Peak current may be, but is notnecessarily, the maximum current, since the limited risetime (imposed by the bandwidth of the TUNA module or
the resistance-capacitance of the sample) may cause alag in the current response. Cycle-averaged current is theaverage current over one full tapping cycle, from point A topoint E. This includes both the current measured while tip
is in contact with the surface, and while it is off the surface.
Contact-averaged current is the average current only when
the tip is in contact with the surface, from the snap-in atpoint B to the pull-off at point D.
The current-time plot usually has different characteristics
than the force-time plot. It does not always have a peak(as in the force-time plot); current can reach a plateau
after a certain force threshold. There is also an AC
current component of the measurement, part of which is
capacitive charging, which is removed from the output.
The tip (including cantilever) and the sample essentiallyform a capacitor, and the modulation of z-position causes
its capacitance to modulate. At a constant DC bias,
charging/discharging current at the tapping frequency will
occur. Again, this is considered parasitic, and removed as3

Z position
C
Force
A

E
B

approachD

withdraw

Current

Time
Figure 2. Plots of Z position, Force, and Current as a function of time
during one PeakForce Tapping cycle, with critical points including (B)jump-to-contact, (C) peak force, (D) adhesion labeled.

mechanical properties maps (when using PeakForce QNM).
The observed current can be used as a measure of the local
conductivity, or electrical integrity, of the sample understudy. Since the system can acquire up to eight channels
simultaneously, it is possible to map the major mechanicalproperties such as deformation, adhesion, DMT modulus,
dissipation, and electrical properties such as cycle-averaged
current and peak current together with topography ina single pass. Offline analysis functions can calculate
statistics of the electrical properties of different regions,sections through the data showing the spatial distributionof the properties, and/or correlation between mechanical,
topographic and electrical properties.
Here are several tips for using PeakForce TUNA in theimaging mode: 1) use smaller PeakForce setpoints forsoft or delicate samples; 2) PeakForce setpoint willaffect all three reported current quantities (peak current,
cycle-averaged current and contact-averaged current), 3)
Decreasing PeakForce Tapping amplitude will increase
contact time within each tapping cycle, resulting in higher
cycle-averaged current and higher contact-averaged current.
4) If simultaneous mechanical properties are desired, aPeakForce setpoint sufficient to attain a few nanometersof deformation is necessary for an accurate DMT
Modulus reading.
• I-V Spectroscopy mode

background. The dynamic change of the current (together
with deflection) can be captured with NanoScope’s “HighSpeed Data Capture” function at any time during scanning,and can later be analyzed and correlated (with force
for instance).
PeakForce TUNA Operation Mode
PeakForce TUNA can be operated in either imaging
or spectroscopy mode. In the imaging mode, maps ofthe electrical current are obtained with topography andmechanical properties. In the spectroscopy mode, one cancollect current-voltage (I-V) curves.
• Imaging mode
In the imaging mode, an electrically conductive probe isscanned over the sample surface in PeakForce Tapping
mode as the feedback loop keeps the maximum force (peakforce) exerted on the tip at a constant value by adjusting theextension of the Z piezo. This protects the tip and sample
from damage while allowing the tip sample contact area
to be minimized. During scanning, the user can apply a DC
bias between the tip and the sample. The TUNA module, alow-noise, high-bandwidth linear current amplifier, senses
the resulting current passing through the sample. This data
is presented simultaneously with the topography image and4

In addition to the imaging mode, PeakForce TUNA alsomeasures local current-voltage (I-V) spectra using thespectroscopy mode. In order to obtain I-V spectra, theimaging scan is stopped and the tip is held in a fixedlocation while the sample bias is ramped up or down.
In spectroscopy mode, the feedback is switched toContact Mode, a constant deflection is maintained by
the feedback loop while the sample bias is ramped. Thisassures tip-sample contact is fixed while I-V curve istaken. The resulting current through the sample is plottedversus the applied bias. The software can either record
a single spectrum or average over multiple spectra. The
higher bandwidth of the PeakForce TUNA module allows
I-V curves to be taken at higher speeds; and it expands
the bandwidth of AC based dI/dV measurements, forinstance, using the “Generic Lock-in” feature offered with
the NanoScope® V Controller. I-V curves can also be takenusing the “Point & Shoot” feature. The “Point & Shoot”
feature offers the option of drawing a line or a box on animage, defining a number of points, and then the AFM
tip will automatically move to those locations to capture
one or multiple I-V curves at each point. While this is a
powerful automation feature, it often can be more useful
to “manually” choose a few spots of interest at specificregions on the sample.

PF-TUNA

Contact-TUNA

Tapping TUNA

TR-TUNA

Conductivity mapping

Yes

Yes

No

Yes

Minimum peak force<100 pN<10 nN<3 nN—

Quantitative Mechanical Property Mapping

Yes

No

No

No

Ease of use

Yes

Yes

Yes

No

Figure 3. Comparison of AFM-based conductivity measurement techniques.

Summarizing Conductivity Mapping Modes

mechanical flexibility. However, their widespread adoption
is hindered by the efficiency of such organic photovoltaic(OPV) devices, which are now below the threshold forcommercial viability. The key component of an organic
solar cell is a blend of a donor and an acceptor materials toform bi-continuous networks, named a bulk heterojunction(BHJ).4 The donor/acceptor pair can comprise of two
different conjugated polymers, but more often a conjugatedpolymer such as poly(2-methoxy-5-(3’,7’-dimethyloctyloxy))-p-phenylene vinylene (MDMO-PPV) or poly3(hexylthiophene) (P3HT) as the donor and a soluble
fullerene derivative such as [6,6]-phenyl C61 - butyric acidmethyl ester (PCBM, a C60-derivative) as the acceptor. To
fabricate a device, powders of the donor and the acceptor
are dissolved in an organic solvent followed by spincasting this solution onto an indium tin oxide (ITO) coatedglass substrate (a semitransparent conductive substrate).
Subsequently, aluminum electrodes are deposited atop theactive layer using a shadow mask and a thermal evaporator.
When light is shined on the device through the ITO side, theactive layer absorbs light creating excitons (bound electron-

Figure 3 summarizes the nanoscale conductivity mapping
techniques available with AFM. Of the conductivity mapping
techniques, PeakForce TUNA has the best capability inforce control, quantitative mapping, and ease of use while
retaining the current mapping resolution of Contact-TUNA.
While Contact-TUNA remains a useful mode for robust
samples, PeakForce TUNA is the mode of choice forsoft delicate samples, such as conducting polymers andloosely bound nanostructures. Conductivity mapping on
hard samples also benefits from the light forces used in
PeakForce TUNA. Tips can be preserved from wearing,
rupture or break-off, which is in the interest of highresolution and measurement consistency.

Complementary Environmental Control

B
LUMO 3.5eVS

A glove-box is often a necessity for handling air sensitive
materials, such as organic solar cell and lithium battery
materials. Bruker provides complete environmental
control by integrating Bruker’s AFM systems (currently
MultiMode® 8, Dimension® Icon, andDimension Edge™) with M-Braun’s glove
box. Feed-through ports are engineeredA
to guarantee the performance of the glove
box, providing sub-ppm levels of O2 andH2O. Vibration isolation is addressed witha custom engineered table, along withan active vibration isolation table in the
glove box (passive tables also available).
This enables measurement to be donehv
without compromising the integrity of thesample nor the performance of the AFM.
The controlled environment also benefitsCmeasurement consistency and resolution
when no water condenses on the sample or
Acceptoraround the tip
phase
S

60

LUMO 3.75eV

O
OCH3

S

+

ITO 4.7eVSeS

Au 5.3eV

PEDOT 6.0eV

HOMO 5.2eV

S

HOMO 6.1eVSS

Donor:
P3HT

Acceptor:
PCBM

Al-CathodeAbsorber
PEDOT-Anode

Applications in Organic Solar Cells
Organic solar cells have been regarded
as a promising candidate in harvesting
solar energy due to their great potentialof low-cost production, light weight, andITO
Donorphase

Substrate

Figure 4. (a) A common donor/acceptor pair used in organic solar cells: poly3(hexylthiophene) (P3HT) as the donor (p-type) and [6,6]-phenyl C61 - butyric acidmethyl ester (PCBM, a C60-derivative) as the acceptor (n-type). (b) HOMO and LUMO
levels of P3HT and PCBM in comparison with the work functions of Au, PEDOT and
ITO. (c) The stacking of an organic bulk heterojunction solar cell.
5hole pairs) which are typically separated into free charges
at the donor/acceptor interface (the junction). Although theactive layer needs to be 100~200nm thick to capture mostof the incident light, the diffusion length of an exciton is only
10~20nm. To yield an efficient energy conversion device,
the donor and acceptor domains must be around 20nm.
The charge generation and charge transport in organic solar
cells depend strongly on the nanoscale morphology andthe degree of the donor and acceptor phase separation.
The efficiency of the solar cell is largely determined by themorphology of the heterojunction, thus, it is crucial to probethese nanoscale properties.
Conductive AFM has proven to be a useful tool in revealing
the morphology on the nanoscale and detecting the
conductivity on the same location for direct correlation.
It lends unique insight into the underlying heterogeneityof organic solar cell materials or devices, and provides ananoscale basis for understanding the interplay between
its morphology and performance. However, Conductive
AFM has been largely based on Contact Mode, which is
not well suited for imaging polymer samples. The verticaland lateral forces involved in Contact Mode imaging
inevitably cause damage to the sample and jeopardize
data integrity. Conductive tips with small spring constant(~0.2N/m) and small setpoint value are often used tominimize destruction of the polymer layer while keeping the
conductive coating free from surface contamination. Even
with all these measures taken, reliable traditional ContactTUNA remains a challenge. As pointed out by Ginger, whocoined photo-current AFM (pcAFM), which is derived fromContact Mode-based Conductive AFM, “Perhaps one of
the most significant practical challenges to using pcAFM is
obtaining a good electrical image without causing significant
damage to the sample. Patience and a willingness tosacrifice many AFM cantilevers in the name of science, areoften necessary.”5
Example 1: Thermal Annealing Effect on P3HT Thin Film
Figure 5 shows PeakForce TUNA data taken on a P3HT
deposited atop poly(3,4-ethylenedioxythiophene) (PEDOT)
and ITO. In this example, the effect of thermal annealingon P3HT (a common donor) thin film was examined. The
substrate was a glass slide coated with transparent ITO
and modified by spin-coating a PEDOT layer. A thin filmof P3HT was spin-cast on top in a N2-filled glove box (<1ppm O2 and H2O) and annealed at 120°C. Various annealing
approaches have been reported to affect the polymer
ordering, resulting in changes in morphology and chargetransport behavior. The taller features in topography, withassociated higher modulus, may be an indication of areas
with more order. It is not surprising that most of those6

Figure 5. PeakForce TUNA images of P3HT thin film spin-coatedon glass/ITO/PEDOT substrate, and annealed at 120 °C. Shownare (a) topography, scale 10nm; (b) peak current, scale 300pA; (c)DMT modulus, scale 15MPa (d) the overlay of conductivity map on
topography. Image size is 2 µm × 2 µm, taken at 1 nN PeakForce, 3V
DC bias, using Bruker’s PeakForce TUNA probe (Au coating, spring
constant of 0.4N/m) on a MultiMode 8 AFM in a glove box withbelow 1 ppm O 2 and H 2O. Sample courtesy of Prof. Nguyen, UCSB.

ordered areas show higher conductivity. Some of them,
however, are poorly conductive, implying there may be apoorly ordered layer underneath that act as traps. Some“hot” spots from flat areas on the surface may have someconductivity enhancing ordered structure lying underneath.
Note the conductive spots have similar round shapes,
suggesting ordered aggregates tend to be cylindrical.
Example 2: P3HT:PCBM Organic Solar Cell
Figure 6 shows PeakForce TUNA data taken on a
P3HT:PCBM bulk heterojunction solar cell with the AFM
tip in place of the cathode. P3HT:PCBM thin films (~100nm
thick) were prepared by spin coating from a toluene
solution of the polymers onto ITO-coated glass substrates
modified with a thin PEDOT layer. PF-TUNA was used toimage the P3HT:PCBM BHJ networks, and their respective
domains. Variations in conductivity (Figure 6b) can be clearlyseen in Cycle-averaged Current. The closer match of theworkfunction of Au (tip coating) and ITO to the HOMO ofP3HT (p-type) determines that the majority of the current
comes from hole transport along the P3HT phase. Thus
it is postulated the higher conductivity regions are P3HT
rich, whereas the poorer conductive regions are PCBM
rich. The conductivity through the active layer indicates theFigure 6. PeakForce TUNA images of P3HT:PCBM solar cell with a PEDOT modified ITO/glass anode. Shown are (a) topography, scale 10nm;(b) Cycle-averaged Current, scale 5pA; (c) Adhesion, scale 8 ~10nN and (d) the overlay of conductivity map on topography. Image size is 2µm
× 2µm, taken at 2.5V DC bias, a net-negative PeakForce of -1.5nN is shown in the force curve (e). Bruker’s Multimode 8 AFM is used with
Bruker’s PeakForce TUNA probe (Au coating, spring constant 0.4N/m) in a glove box with below 1ppm O 2 and H 2O. Sample courtesy of Prof.
Nguyen, UCSB.

formation of vertical conductive networks. A close look of
the current image also reveals fiber-like features, evidence
that BHJ networks also exist laterally. Nguyen6 using
conductive AFM (Contact Mode based) to image boththe top surface and the cross section of the same device,
revealed the nanoscale three-dimensional interpenetrating
networks of P3HT and PCBM. The topography (Figure6a) shows some granular structures that can be polymer
aggregates. The adhesion map (Figure 6d) shows features
measuring 10~50nm that are quite uniformly dispersed
across the whole surface, the length scale falls closely
to the hypothesized exciton diffusion length of 6~20nm.
This can be a useful criterion in the further optimization ofactive layer formulation processes. Average conductivity
over a certain scan area, and hole and electron motilities
extracted from I-V curves were reported to agree with theconversion efficiency of organic solar cells in the generaltrend.6 It is worth noting that, as a net-negative PeakForce
was used for the imaging, the same tip could last for morethan 6 hours without perceivable degradation in resolution
and conductivity signal, a stark contrast to Contact Mode–
based conductive-AFM.

Applications in Lithium Ion Batteries
Lithium-based battery, as an energy storage device, hasenjoyed wide acceptance for its light weight and highenergy density. Its applications in consumer electronics
are ubiquitous; lithium battery powered vehicles havestarted to appear on the market. Despite the tremendous
achievements in the past decades, only about 10% of thetheoretical capacity of lithium has been realized.7 Muchattention is being directed to improve lithium battery
chemistry, formulation, and structuring of cathode andanode materials to improve energy density, power density,
safety, shelf/cycle life, and cost. Electrode materials areoften engineered into micrometer to nanometer sized
structures (particles, fibers, pillars and so forth) and held
together by additives for mechanical support while leavingsome space (measured in porosity) around them forlithium ions to pass. This three-dimensional nanostructureassembly has the following benefits: (1) the large surface
area increases capacity; (2) the small size of each individual
particle shortens the diffusion length for lithium ion to get in
and out of the hosting material, leading to higher charging/discharging rate, or power density. As Tarascon and Armand
vividly put, “the arrival of nanomaterials gave lithium-ion

7

Figure 7. PF-TUNA images of a Li[Ni1/3Mn1/3Co1/3]O2 composite cathode, on the top row are topography, DMT modulus, adhesion and current
maps. The overlay of a current map on topography is shown on the bottom row. Images were taken on a Dimension ICON AFM in ambientconditions, with a DDESP probe (spring constant was calibrated to be 93N/m), 50µm scan at a DC sample bias of 500mV. Sample courtesyof Dr. Zheng and Battaglia, Lawrence Berkeley National Laboratory.

batteries a new lease of life.”8, 9 Characterization on thenanoscale is therefore of fundamental importance. The
following two examples demonstrate the unique capabilities
of PeakForce TUNA in this regard.

8

L333

90

PVDF+AB
Conductive
coverage 56%

L333

2

60

1

30

0

0

20

40

60

80

100

Current (a.u.)

Figure 8. Bearing analysis of the Current map in figure 7.

0

Bearing %Lithium-ion battery cathode materials often come in acomposite form. Li[Ni1/3Mn1/3Co1/3]O2 (L333) is one of themost used cathode materials. Besides this active material,
polyvinylidene difluoride (PVDF) was added as the binder
polymer to hold L333 particles together; and acetylene
black (AB) as the conductive additive to enhance electronic
conductivity. PF-TUNA was used to visualize the distributionof each component, and to characterize the elastic modulus
and the formation of a conductive network that is intended
to connect all the L333 particles together and connect
them to the current collector (in this case Al foil). From thetopography, particle sizes measuring 3~15µm are seen,
which represent the size of L333 particles, as PVDF+AB
contribute little given their smaller sizes (AB: ~50nm) andpercentage (see figure 7). In the current image, two distinct
conductivity levels can be readily seen. The less conductive
regions (shown in dark purple color, encircled within the3
Histogram %Example 1: Material assignments in Li[Ni1/3Mn1/3Co1/3]O2
Composite Cathode

PVDF+AB

3.2% (AB+PVDF)
12.8% (AB+PVDF)
24% (AB+PVDF)

Histogram %dotted green line) can be assigned to L333 particles thatare not covered with AB+PVDF. This assignment is further
supported by the higher modulus seen in the same location.
The more conductive regions (light pink color) suggest
where the top surface is covered with PVDF+AB. PVDF
itself is not conductive, but becomes so when mixed witha sufficient amount of AB nanoparticles (i.e., when thenanoparticles connect with each other to form conductive
networks). Those regions also showed smaller elastic
modulus and smaller adhesion. The overlay of the current
map (denoted by color) atop the topography (denoted byheight) clearly shows which L333 particles are covered with
PVDF+AB and which are not. Those uncovered particles
are electrically isolated from the collecting electrode, andthus do not contribute to the battery capacity and becomea dead weight. Offline bearing analysis of the current data
map shows two distinct peaks corresponding to L333
(left peak) and PVDF+AB (right peak), the coverage of the
conductive network is 56% (see figure 8).

0

20

40

60

80

100

Current (a.u.)

Figure 9. Bearing analysis of the current maps (not shown) of
LiNi0.8Co 0.15Al0.05O2 composite cathode containing 3.2%, 12.8%
and 24% PVDF+AB. Sample courtesy of Dr. Zheng and Battaglia,
Lawrence Berkeley National Laboratory.

Example 2: Optimizing PVDF+AB Content in
LiNi0.8Co0.15Al0.05O2 Cathode

120

60

100

50

80

40

60

30

40

20

20

10
0

0
0

5

10

15

20

25

30

Weight Percent of (PVDF+AB) (AB: PVDF=0.6:1)

15

150

10

100

5

50

Average Young’s Modulus/GPA

70

Most Probable Young/s Modulus/GPA

140

Average Conductiveity (a.u.)

Conductive Coverage %LiNi0.8Co0.15Al0.05O2 (LiNCA) is another cathode material thatis being actively pursued to optimize its performance.10 The
variation of PVDF+AB content on the characteristics of the

0
0
5
10
15
20
25
30
Weight Percent of (PVDF+AB) (AB: PVDF=0.6:1)
0

Figure 10. Plot of the conductive network coverage and average conductivity (a) and average elastic modulus (b) over 50µm scan area as afunction of the percentage content of PVDF+AB in LiNi0.8Co 0.15Al0.05O2 composite on the same samples used in figure 9.

9composite was studied using PeakForce TUNA. Figure 9shows the bearing analysis of a series of samples containing
3.2%, 12.8% and 24% PVDF+AB (a fixed PVDF:AB ratio
1:0.6). The conductivity peak corresponding to PVDF+AB
rapidly increases as more (AB+PVDF) was added.
Conductive network coverage approaches completion with
12.8% PVDF+AB (see figure 10a). As a validation of the
conductivity measurement, we found good agreement
between average conductivity over a 50µm scan area with
4-point probe measurement on the millimeter scale. Higher
conductivity translates to less internal resistance of thebattery, benefiting high power density. The average elastic
modulus decreased with more PVDF+AB content meaning
the cathode becomes more flexible, which is important toaccommodate volume changes upon lithium-ion insertion
into and extraction from the cathode (see figure 10b).
Keep in mind that the addition of those inactive materials
lowers the energy density, so optimization is needed tobalance different performance aspects of the battery.
PeakForce TUNA measurements with other complementary

techniques can provide direction of optimization to satisfy
different application needs.
Although the above examples are on lithium-ion battery
cathodes, PeakForce TUNA is similarly useful in the studyof anode materials, for example on their aging over timeor upon charging/discharging cycling when associatedmechanical degradation and resistance increase may occur.

Applications for Nanostructures
Various nanostructures are viewed as the building blocks
for the ever shrinking electronic devices. Their electricalbehaviors have been the subject of intensive study. The
following two examples demonstrate the superiority of
PeakForce TUNA for imaging this class of fragile samples,
which has been a challenge for Contact Mode AFM-based
conductivity measurement.
Figure 11 shows the topography and current map
simultaneously taken with PeakForce TUNA of carbon
nanotubes connected to conductive pads laid on an

Figure 11. PeakForce TUNA images (a) topography (b) current map of carbon nanotubes lying flat on a SiO2 /Si sample. Images were taken on
Bruker’s Dimension® Icon® AFM in ambient conditions, with an SCM-PIT probe (spring constant ~4N/m), 5µm scan at a DC sample bias of
500mV. Sample courtesy of Prof. Hague, Rice University.

10insulating SiO2 /Si substrate. All the nanotubes present in
the topography image are clearly seen in the current map,suggesting they are all conductive and all connected to the
conductive pads. The densely packed nanoparticles areresidues produced during the sample preparation. Theirconductivity can not be assessed as they are not electricallyconnected to the conductive pads. This is shown as they
are clearly not represented in the current map. However, thevariation in the conductivity along the tubes may be due totheir presence on, or along the tubes. It is worth noting thatwhile the nanotubes are fragile and can be pushed about
with an AFM tip (if in Contact Mode), the substrate is hard.
With PeakForce TUNA, the SCM-PIT tip (platinum-iridium
coating) endured for hours without the coating being worn
off by the substrate. For comparison, the same sample wasimaged with TR-TUNA. With TR-TUNA the conductivity
trace was wider, and it is hypnotized that this increase isdue to the lateral dithering of the probe when operating inthis mode.

Figure 12a and b show PeakForce TUNA images of avertical multi-walled carbon nanotube mat on a conductive
substrate. Seen are the end caps of the nanotubes. Initially,
it was expected that all of the multi-walled nanotubes
would be conductive. However, as observed in the peakcurrent map, this is not the case, but instead the different
bundles exhibit different levels of conductivity(see figure12b). Two possible interpretations are that there is variability
in how the nanotubes are connected to the underlying
substrate, or that the capping of the tubes is affecting themeasured conductivity even if the cylindrical part of the
tube is conductive and base attached. The same sample
was tried with Contact Mode–based TUNA, however nostable images could be attained. TR-TUNA gave a current
image that looked quite different (see figure 12c,d). Several
discontinued conductive spots appeared on single tubes,
and this is against intuition as we expect more uniformity
on each individual tube. The lateral twisting motion of
the probe in the Torsional Resonance Mode is likely to becausing an intermittent electrical contact to the surface.

PeakForce TUNA Probe Selection Guide
The spring constant and the conductive coating material are
important considerations when selecting a probe. Bruker’s
newly introduced PeakForce TUNA probes are specificallydesigned for optimal electrical and mechanical performanceand lifetime on soft delicate samples such as conductive
polymers, bio-materials, or loosely bound nanostructures;
these probes have Au coated tips and a spring constants of~0.4N/m. SCM-PIT probes which have a Platinum-Iridium
coating and a spring constant of ~3N/m, work well forfragile samples, for instance, loosely bound nanostructures,and hard samples, for instance SiO2 dielectric films.
DDESP probes, which have doped diamond coatings andspring constants of ~40N/m, are well suited for samples
containing hard components. To attain accurate mechanicalproperty measurements, as a rule of thumb, the match of
the spring constant to the elastic modulus of the sample
is to be observed. Since the instrument can operate in an
inert atmosphere, it is possible to use aluminum or low
work-function metal coated silicon probes for organic solar
cell characterization.
Figure 12. PeakForce TUNA images (a) topography 50nm scale(b) peak current map (1 nA scale) of a vertical multi-walled carbon
nanotube mat on a conductive substrate. Images were taken on
Bruker’s MultiMode 8 AFM in ambient, with SCM-PIT probe (spring
constant ~4N/m), 1µm scan at a peak force of 10nN, and DC bias of-1V. TR-TUNA images (c) topography scale 100nm (d) current map(scale 1nA) for comparison.

Conclusions
Through an innovative high-bandwidth, high-gain, lownoise current amplifier design, PeakForce TUNA couples
AFM conductivity measurements with Bruker’s exclusive
PeakForce Tapping technology. Using the unparalleled force
control of PeakForce Tapping, PeakForce TUNA enables,
for the first time, current imaging on extremely soft and11

8. M. Armand and J.-M. Tarascon, “Building better batteries,” Nature 451(2008): 652-57.
9. J.-M. Tarascon and M. Armand, “Issues and challenges facing rechargeable
lithium batteries,” Nature 414(2001):359-367.
10. G. Liu, H. Zheng, S. Kim, Y. Deng, A. M. Minor, X. Song, and V. S.
Battaglia, “Effects of Various Conductive Additive and Polymeric Binder
Contents on the Performance of a Lithium-Ion Composite Cathode,”
J. Electrochem. Soc., 155(2008): A887-A892.

Authors
Chunzeng Li, Stephen Minne, Bede Pittenger, and Adam Mednick,
Bruker Nano Surfaces Division

References
1. B. Pittenger, N. Erina, and C Su, “Quantitative Mechanical Property
Mapping at the Nanoscale with PeakForce QNM,” Bruker application note
AN128, Rev. A0 (2010).
2. Y. Otsuka, Y. Naitoh, T. Matsumoto and T. Kawai, “A Nano Tester: A New
Technique for Nanoscale Electrical Characterization by Point- Contact CurrentImaging Atomic Force Microscopy,” Jpn. J. Appl. Phys. 41 (2002): L742-L744.
3. P. Harris, L. Huang, and C. Su, “Electrical Testing of Soft Delicate Samples
Using Torsional Resonance Mode and TUNA,” Bruker application note AN107,
Rev. A0 (2007).
4. M. Niggemann, Y. Thomann, H. Hoppe, and A. Gombert,” AFM
Investigation of the Absorber Morphology of Bulk Heterojunction Organic
Solar Cells,” Bruker application note AN93, Rev. A0 (2006).
5. R. Giridharagopal, G. Shao, C. Groves and D.S. Ginger “New SPM
Techniques for Analyzing OPV Materials,” Materials Today 13 (2010): 50-56.
6. M. Dante, J. Peet, and T. Nguyen, “Nanoscale Charge Transport andInternal Structure of Bulk Heterojunction Conjugated Polymer/Fullerene Solar
Cells by Scanning Probe Microscopy,” J. Phys. Chem. C 112 (2008): 7241-49.
7. G. Nazri, and G. Pistoia (editors), Lithium Batteries: Science and Technology,
ISBN 1-4020-7628-2 (2009).

Bruker Nano Surfaces Division
Santa Barbara, CA • USA+1.805.967.1400/800.873.9750
productinfo@bruker-nano.com
12

www.bruker.com

NanoScope, MultiMode, Dimension, Edge, and Icon are trademarks of Bruker Corporation. AN132, Rev. A0.

Michele Guide and Thuc-Quyen Nguyen, University of California, Santa Barbara©2011 Bruker Corporation. All rights reserved. PeakForce Tapping, PeakForce QNM, ScanAsyst, TappingMode, TR-TUNA,

delicate samples, as well as, superior tip lifetime for current
imaging on hard samples. In both cases, the enhanced
force control improves the repeatability and resolutionof conductive AFM imaging. The ScanAsyst algorithm,
which is included with PeakForce TUNA, dramatically
improves ease of operation by automatically optimizing
the AFM’s scan parameters (including feedback gain
settings). PeakForce TUNA also includes the quantitative
nanomechanical property mapping suite of PeakForce
QNM, thereby providing electrical information simultaneouswith topography and mechanical property information(deformation, adhesion, DMT modulus, and dissipation).
Having all of these orthogonal data channels available ina single scan, brings out the PeakForce TUNA’s uniqueability to correlate the different properties of the sample atthe nanometer scale. This technique is complemented by
Bruker’s AFM specific glove-box offering, enabling proper
handling of air-sensitive materials. Both individually andcombined, PeakForce TUNA and the ppm capable gloveboxes, will be useful tools in the characterization of fragile
samples such as loosely bound nanostructures, organic
solar cells, lithium ion batteries, fuel cells, and many others.