APPLICATION EXAMPLES

Phase imaging can reveal microphaseseparations occurring in block
copolymers. Obtaining this information
with alternative techniques involves
complications, such as chemical
staining for TEM. With TappingModephase imaging, AFM can provide
the visualization of the microphaseseparation pattern directly from images
obtained in ambient conditions of an
untreated thin ﬁlm. Figure 2 shows
topography and phase images of aPS-b-PB-b-PS triblock copolymer (PS,polystyrene; PB, polybutadiene)1. Bothchannels clearly show the expectedworm-like microphase separation
pattern. The microphase domains
exhibit a width of ~ 35nm. This can beseen most clearly in the 2-D Fourier
Transform shown in Figure 3. Notethat the intensity maximum is perfectly
circular, as it should be given theoverall isotropy with no preferred
block orientation and no dependence ofblock width on azimuthal angle. These
images were acquired using closedloop scan control, ensuring calibrated,
undistorted measurements.
For a phase image to reﬂect differencesin viscoelasticity or modulus of amaterial, the AFM probe needs
to penetrate the material. Moreprecisely, the probe needs to penetrate
sufﬁciently far such that the tipsample interactions are inﬂuenced bymaterial properties from the layer ofinterest. In the case of a PS-b-PB-bPS ﬁlm, an amorphous, PB-enriched
top-layer is usually present. Thus,
the combination of a soft cantilever(e.g., FESP, k ~ 2–5N/m) with very lighttapping conditions would fail to uncover
the microphase separation pattern.
Images such as those shown in Figure
2 are usually obtained with fairly hardtapping conditions, that is, fairly highratios of free amplitude to amplitude
setpoint. On Innova, probe tuning at
moderate amplitude (input gain setting~ 8 or 10) and signiﬁcant increase of
the cantilever drive amplitude uponengaging will yield the desired result.
As phase imaging is sensitive to local
variations in mechanical properties,
it can afford an efﬁcient means
for mapping out the distribution ofcomponents in composite samples.
Figure 4 shows topography and phase
images of a cross-sectioned multilayer
polyethylene sample, composed ofalternate high and low density layers.

The topography image is dominatedby the large scale, low frequency
height undulation that has apparently
resulted from cross-sectioning by
cryo-microtoming. The phase image
has a different appearance, clearlyproviding complementary information.
It is dominated by an alternating setof stripes, obviously representing thesought after alternation in material
properties and thus component
layers. In addition, the phase image
reveals topographic ﬁne features thatare much less apparent in the height
image. In particular, small droplets
can be discerned with well-deﬁned
phase contrast indicating distinct local
mechanical properties. The formationand coalescence of small droplets on
microtomed polyethylene samples
indicate an ageing surface. Notethat the droplets are not distributed
randomly. Rather, some of them appear
to form along lines, presumably small
scratches imparted on the sample by
the microtoming process.
In both, Figures 2 and 4, the phase
image brings out the material
contrast most clearly and separates
it from information about large scale
topographic information. However,

Figure 2: Topography (left) and phase image (right) of PS-b-PB-b-PS triblock copolymer. Sufﬁciently hard tapping conditions have ensured probe penetration into
the subsurface layer, where a wormlike microphase separation pattern is present as can be seen clearly in both channels. Image size 2.0µm. Closed loop active.
1

Samples courtesy of N. Erina

PAGE 2 ADVANCED NANOSCALE CHARACTERIZATION WITH INNOVATM. PHASE IMAGING OF POLYMER MATERIALS

in both cases, the material contrast
seems to be also partly containedin the topographic image. The partsof the sample appearing with brightcontrast in the phase image appear
raised in the topographic image. Thiscan be rationalized as a reﬂection ofmaterial stiffness. Under hard tapping
conditions, the probe penetrates into
the material. The regions appearing
bright in the phase image are stiffer,leading to less probe penetrationand thus a raised topography relative
to the softer parts at high force. A

Figure 3: 2-D Fourier transform of the phase data shown in Figure 2. The ring-shaped intensity maximum
indicates that the phase separation pattern is isotropic with a well-deﬁned repeat distance of r=35nm as
indicated at the bottom of the dialog and as expected for this triblock copolymer.

more complete explanation of thiseffect has to include the nature of
the feedback in tapping mode. Strong
positive phase shifts in hard tapping
conditions indicate signiﬁcant upshifts in the resonance frequency ofthe cantilever – while drive frequency
and amplitude setpoint chosen for
feedback remain constant and werechosen at the resonance frequencyof the free cantilever. In the regionswith the most positive phase shift, theresonance frequency effectively shifts
furthest away from the drive frequency.
Being furthest off-resonance, thecantilever is being driven less efﬁciently(while the amplitude setpoint remains
unchanged), leading effectively tolighter tapping, which contributes to theappearance of a raised proﬁle.
Aside from compositional mapping
and the visualization of microphaseseparations, phase imaging can aid
in the detection of ﬁne structures. In
the case of the MLPE sample shownin Figure 4, interesting ﬁne structure
can be observed in higher resolution
images of the layer boundaries whenslightly lighter tapping conditions are
employed. As can be seen in Figure
5, hair-like structures appear toextend from the boundary into thelayer appearing with darker phase

Figure 4: Topography (left) and phase image (right) of a cryo-microtomed multilayer polyethylene sample. While topography is dominated by large-scale
undulations, phase provides a clean view of the layered structure. Additional ﬁne structure shows the presence of small droplets. Image size 35µm. Closed
loop active.

ADVANCED NANOSCALE CHARACTERIZATION WITH INNOVATM. PHASE IMAGING OF POLYMER MATERIALS PAGE 3

Figure 5: Phase image of a cryo-microtomed
multilayer polyethylene sample. Hair-like ﬁne
structure can be seen near the layer-interface.
Image size 5µm. Closed loop active.

contrast. Closer inspection of Figure 5
reveals lamellar structures throughout
the lower density (darker phase)component visible in the right half of theimage and a gradual loss of alignment
with increasing distance from theinterface.
Figure 6 shows AFM images of anoriented ﬁlm of isotactic polypropylene,
also known as the microporous
membrane Celgard. Both, topography
and phase clearly show the patternof oriented ﬁbrillar structures thatis characteristic of this sample. With

the overall height scale (~ 200nm)
dominated by large variations, ﬁner
structures are not evident in the
topography data. In contrast, thephase image exhibits very clear andwell-deﬁned additional features. Finelamellar structures (only ~ 20nm wide)
are seen to be present in between therows of ﬁbrils. The lamellar structuresare seen to be oriented perpendicular
to the larger ﬁbrillar structures. As thephase signal is sensitive to deviationsof the oscillation amplitude from theamplitude setpoint, it can serve as
an edge detection technique and thus
highlights such ﬁne structures that areeasily overlooked in the topography
channel. The accurate imaging
of ﬁne structures on this delicate
sample beneﬁts greatly from Innova’s
combination of excellent force control
and low noise closed-loop scan control.
The appearance of ﬁne structurein phase images complements thesensitivity to material properties. By
identifying components in composite
samples, the appearance of ﬁne
structure in phase images aids incompositional imaging, see (AN103)
Phase Imaging of Polymer Materials
with diCaliber. Even higher resolution
phase images can reveal length scales

associated with the self-assembly of
individual molecules in monolayer ﬁlms
and their relationship to the substrate.
Figure 7 shows topography and phase
images of a self-assembled monolayerof C60H122 alkane molecules on asubstrate of highly-oriented pyrolytic
graphite (HOPG).
Both images clearly show the presenceof patches (domains) separated bysharp (mostly straight) borders. Closer
inspection reveals a pattern of straightlines within each domain. This lamellar
pattern is much more obvious in thehigher resolution phase image shownin Figure 8. The lamellae are seen tohave a well-deﬁned spacing as well as
preferred directions. This is conﬁrmed
in Figure 9, which shows the Fourier
Transform of the phase image shownin Figure 7. The Fourier Transform
shows very clearly the hexagonal
symmetry of the self-assembly patternthat reﬂects the hexagonal symmetryof the underlying graphite substrate.
When forming self-assembled
monolayers, C60H122 alkane retains aﬁxed relationship to the high symmetry
axes of graphite.
The special relationship of the

Figure 6: Topography (left) and phase image (right) of Celgard. While oriented ﬁbrillar structures are evident in topography, the phase image additionally reveals
lamellar ﬁne structure. Image size 2.5µm. Closed loop active.

PAGE 4 ADVANCED NANOSCALE CHARACTERIZATION WITH INNOVATM. PHASE IMAGING OF POLYMER MATERIALS.

Figure 7: Topography (left) and phase image (right) of a C 60H122 monolayer self-assembled on graphite. Both images clearly show self-assembled domains, each ofwhich is composed of parallel lines. Image size 1.5µm. Closed loop active.

more on probing the self-assemblyof alkanes and their derivatives ongraphite see (AN85) Scanning TunnelingMicroscopy: A Tool for Studying SelfAssembly and Model Systems for
Molecular Devices.

Figure 8: Phase image of a C 60H122 monolayer selfassembled on graphite clearly showing lamellar
ﬁne structure associated with the self-assembly.
Image size 390nm. Closed loop active.

Figure 9: 2-D Fourier Transform of the phase
image shown in Figure 7. The hexagonal symmetry
is clearly visible. As shown near the bottom, theperiodicity is measured to be r⯝7.5nm.

adsorbate structure with the graphite
substrate is consistent with theassumption, that the layer probed
here is actually the molecular layerthat is in direct contact with thesubstrate. The question arises because
the preparation of the C60H122 alkane
sample cannot be assumed to resultin a single molecular layer. Indeed,
judicious choice and excellent controlof tapping forces is required to reveal
the structures shown here. The probe
has to penetrate through partiallydisordered, soft adsorbate multilayers
without destroying that “ﬁrst”

molecular layer that is in direct contact
with the substrate and is thereby
subject to additional stabilization.
As indicated in Figure 9, the spatial
periodicity is seen to be about
7.5nm. C60H122 alkane is known toself-assemble on graphite such thateach molecule assumes an extended
all-trans conformation with its
backbone parallel to the substrate
and perpendicular to the lamella axis.
Therefore, the lamella width equals
the length of a single C60H122 alkane
molecule, which is about 7.5nm. For

The interrogation of self-assembly inmonolayers on ﬂ at substrates depends
critically on good force control for
nondestructive imaging, in conjunction
with high stability and low stage drift
to enable the required high resolution
performance. Obviously, the analysis
presented above also requires correctscanner calibration. Closed-loop scan
control can ensure correct calibration
but is often associated with excessive
noise levels. Not so on Innova. In fact,
all images presented in this applicationnote (including the 390nm image shownin Figure 8) were acquired on Innova
with closed-loop control active, thus
ensuring accurate measurements.
SUMMARY

With TappingMode phase imaging,
the Innova system can efﬁciently
and nondestructively map variations
in sample properties at the highest
resolution. As TappingMode is often
the preferred imaging mode fordelicate samples, phase imaging cancomplement other modes such as

ADVANCED NANOSCALE CHARACTERIZATION WITH INNOVATM. PHASE IMAGING OF POLYMER MATERIALS. PAGE 5

force modulation and lateral force
microscopy, often with superior image
detail. Phase imaging applications
include the characterization ofcomposite materials, mappingof variations in adhesion andviscoelasticity, and identiﬁcationof surface contamination. Highest
resolution phase images open thedoor to studies of molecular selfassembly. The combination of excellent
high resolution performance andTappingMode phase imaging makes
Innova a powerful tool for the study ofmaterial properties at thenanometer scale.
ABOUT THE IMAGING TECHNIQUES

Conventional Approaches
Two conventional AFM scanningmodes – contact mode and noncontact
mode have been used for some timewith varying success. Each has its
limitations, particularly for imaging
delicate samples in ambient conditions,see also (AN04) TappingMode Imaging
Applications and Technology.
Contact mode AFM represents thesimplest imaging technique. The
sample is simply moved laterally
relative to the probe such that the probe
is dragged across the surface. While
this technique has been successful formany samples, it is subject to serious
drawbacks. In essence, the dragging
motion of the probe combined withadhesive tip-surface forces can leadto substantial damage to probe andsample, creating artifacts in theimage and often degrading theresolution severely.
Under ambient air conditions, mostsurfaces are covered by a ﬂuid
layer, composed of water and othercontaminants, which is typically several
nanometers thick. An AFM tip touching
this layer will cause a meniscus toform and surface tension will pull
the tip onto the surface. Additional
adhesive forces can arise from trappedelectrostatic charges (see Figure 10).
Tip-sample forces are thus larger thanthe cantilever deﬂection would seemto indicate. Correspondingly, lateralmotion during contact mode imaging is

associated with larger lateral forces,
resulting in severe tip or sample
damage or involuntary displacementof weakly bound surface adsorbates.
Capillary forces can be eliminated bycompletely submersing the sample andprobe tip in liquid. However, sample
surfaces are often either less robust in
liquid (e.g., adsorbates are more weakly
bound) or are not compatible with aliquid environment at all.

local mechanical properties (see thepreceding sections of this applicationnote for many examples of probing
local mechanical properties with
TappingMode phase imaging). In
practice, the probe is frequently drawn
onto the sample surface and remains
trapped there while the scanningmotion proceeds, leading to unusable
data and sample damage similar to thatcaused by contact mode.

Non-contact mode represents anattempt to overcome the deleterious
tip-surface forces associated withcontact mode. Unfortunately, ambient
air conditions are rarely conducive to
non-contact AFM imaging.

TappingMode Imaging in Air

Non-contact mode is based upon
the detection of weak attractive vander Waals forces that exist between
tip and sample a few nanometers
above the surface. It is important tonote that the ﬂuid layer present inambient conditions partially shields
these forces and occupies a large
fraction of their useful range, i.e.,
when compared to operation in ultrahigh vacuum, where no ﬂuid layer ispresent. Due to the limited range ofany residual van der Waals forces,
their detection necessitates very small
Cantilever with tip

Fluid layer

Electrostatic charge
Figure 10: In contact mode AFM, electrostatic
and/or surface tension forces from the adsorbed
ﬂuid layer lead to destructive lateral shear
forces. From application note AN04, TappingMode
Imaging Applications and Technology.

oscillation amplitudes. At the sametime, a cantilever operated at verysmall amplitudes is easily trapped
inside the ﬂuid layer, once it touches.
In the ideal case, operation remains
outside the ﬂuid layer – and therefore
several nanometers away from thesample surface. The consequences
are substantially degraded resolution(as compared with TappingMode) andan inability to map out variations in

Veeco’s patented TappingMode
imaging overcomes the limitations of
the conventional scanning modes byalternately placing the tip in contact
with the surface to provide highresolution and then lifting the tip off
the surface to avoid dragging the tip
across the sample. TappingMode
imaging is implemented in ambient air
by oscillating the cantilever at or nearits fundamental ﬂexural resonance,
usually a frequency in the range of 50
to 500 kHz. “Free air” amplitudes are
typically greater than 20nm. During
the engage process, the tip-sample
separation is reduced until the tip
begins to interact with the surface.
The interaction with the surface(“tapping”) leads to energy loss anda reduced oscillation amplitude, see
Figure 11 and (AN04) TappingMode
Imaging Applications and Technology.
Deviations of the amplitude from a
setpoint value serve as error signal
in the feedback loop that drives the
Z-motion to track surface features.
In phase imaging, the phase lag ofthe cantilever oscillation, relative tothe drive signal, is simultaneously
monitored with topography data. As
the phase lag is inﬂuenced by energy
dissipation experienced during the”Free” Amplitude

Fluid layer”Tapping”
Amplitude reduced

Figure 11: TappingMode cantilever oscillation
amplitude in free air and during scanning. Fromapplication note AN04, TappingMode Imaging
Applications and Technology.

PAGE 6 ADVANCED NANOSCALE CHARACTERIZATION WITH INNOVATM. PHASE IMAGING OF POLYMER MATERIALS.

Figure 12: Selecting drive frequency and phase in preparation for TappingMode imaging using the autotune
function in the Cantilever Tune dialog.

oscillation cycle, it is very sensitive tomaterial properties such as adhesion,
viscoelasticity, and modulus. Phase
imaging can also act as a real-time
contrast enhancement technique.
Because phase imaging highlights
edges and is not affected by largescale height differences, it provides forclearer observation of ﬁne features,such as grain edges, which can be
obscured by rough topography.

Cantilever Tuning dialog (see Figure12). Choice of drive frequency andadjustment of lock-in parameters forproper phase imaging are all takencare of by the AutoTune function. The
user chooses the desired cantilever
oscillation amplitude by selecting atarget amplitude and lock-in sensitivity(“input gain”) factor. As shown in
Figure 12, both, amplitude and phaseare displayed in the Cantilever Tuning
dialog.

TappingMode prevents the tip fromsticking to the surface and causing
damage during scanning. In contrast
to non-contact mode, the oscillation
amplitude and energy is sufﬁcient
to overcome tip-sample adhesion in
every oscillation cycle. In addition,
TappingMode provides a large
linear operating range, i.e., linear
dependence of cantilever amplitude
on tip-sample separation, making the
feedback system highly stable andallowing routine reproducible sample
measurements at high resolution.
Selection of the optimal cantilever
oscillation frequency is a critical stepin preparing for TappingMode imaging.
On Innova this is accomplished in the

ADVANCED NANOSCALE CHARACTERIZATION WITH INNOVATM. PHASE IMAGING OF POLYMER MATERIALS. PAGE 7