:: AZoNanotechnology Article
By AZoNano
Background
Application Examples
Block Copolymers
Viscoelasticity and Material Modulus
Mechanical Properties
Material Contrast and Topographic Information
Summary
Background
TappingMode imaging has proved to be the most versatile mode of atomic
force microscopy (AFM)
in ambient conditions where the presence of a fluid layer (condensed
water vapor and other contaminants) severely limits the applicability
of both, contact mode and non-contact techniques. Overcoming the
challenges posed by friction, adhesion, and other issues, TappingMode
has provided a means of greatly extending AFM
applications. Phase Imaging is an important extension of TappingMode
imaging.
By mapping out the phase of the oscillating cantilever, phase
imaging goes beyond simple topographical mapping. Being sensitive to
variations in adhesion and viscoelasticity, phase imaging can provide
information about sample composition and microphase separation.
First class high resolution performance is critical for taking full
advantage of TappingMode phase imaging. With its stable, low-drift
platform, ultralow noise closed loop scan control, and excellent force
control the new Innova SPM (see Figure 1) is an ideal instrument
for high resolution imaging of delicate samples. In addition, the Innova
combines this outstanding core performance with generous data
acquisition bandwidth and facile signal access, thus enabling a wide
range of demanding research applications.
Figure
1. The new Innova
Application Examples
Block Copolymers
Phase imaging can reveal microphase separations occurring in block
copolymers. Obtaining this information with alternative techniques
involves complications, such as chemical staining for TEM. With
TappingMode phase imaging, AFM
can provide the visualization of the microphase separation pattern
directly from images obtained in ambient conditions of an untreated
thin film. Figure 2 shows topography and phase images of a PS-b-PB-b-PS
triblock copolymer (PS, polystyrene; PB, polybutadiene). Both channels
clearly show the expected worm-like microphase separation pattern. The
microphase domains exhibit a width of ~ 35nm.
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 2.0ìm. Closed loop active.
This can be seen most clearly in the 2-D Fourier Transform shown in
Figure 3. Note that the intensity maximum is perfectly circular, as it
should be given the overall isotropy with no preferred block
orientation and no dependence of block width on azimuthal angle. These
images were acquired using closedloop scan control, ensuring
calibrated, undistorted measurements.
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.
Viscoelasticity and Material Modulus
For a phase image to reflect differences in viscoelasticity or
modulus of a material, the AFM
probe needs to penetrate the material. More precisely, the probe needs
to penetrate sufficiently far such that the tipsample interactions are
influenced by material properties from the layer of interest. In the
case of a PS-b-PB-b- PS fi lm, 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 light tapping conditions would fail to
uncover the microphase separation pattern.
Images such as those shown in Figure 2 are usually obtained with
fairly hard tapping conditions, that is, fairly high ratios of free
amplitude to amplitude setpoint. On Innova, probe tuning at moderate
amplitude (input gain setting ~ 8 or 10) and signifi cant increase of
the cantilever drive amplitude upon engaging will yield the desired
result.
Mechanical Properties
As phase imaging is sensitive to local variations in mechanical
properties, it can afford an efficient means for mapping out the
distribution of components in composite samples. Figure 4 shows
topography and phase images of a cross-sectioned multilayer
polyethylene sample, composed of alternate high and low density layers.
The topography image is dominated by the large scale, low frequency
height undulation that has apparently resulted from cross-sectioning by
cryo-microtoming. The phase image has a different appearance, clearly
providing complementary information. It is dominated by an alternating
set of stripes, obviously representing the sought after alternation in
material properties and thus component layers. In addition, the phase
image reveals topographic fine features that are 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 formation and coalescence of small droplets on
microtomed polyethylene samples indicate an ageing surface. Note that
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.
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 fi ne structure shows the presence of
small droplets. Image size 35ìm. Closed loop active.
Material Contrast and Topographic Information
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, in both cases, the material
contrast seems to be also partly contained in the topographic image.
The parts of the sample appearing with bright contrast in the phase
image appear raised in the topographic image. This can be rationalized
as a reflection of material 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 penetration and thus
a raised topography relative to the softer parts at high force. A more
complete explanation of this effect 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 of
the cantilever – while drive frequency and amplitude setpoint chosen
for feedback remain constant and were chosen at the resonance frequency
of the free cantilever. In the regions with the most positive phase
shift, the resonance frequency effectively shifts furthest away from
the drive frequency.
Being furthest off-resonance, the cantilever is being driven less
efficiently (while the amplitude setpoint remains unchanged), leading
effectively to lighter tapping, which contributes to the appearance of
a raised profile. Aside from compositional mapping and the
visualization of microphase separations, phase imaging can aid in the
detection of fine structures. In the case of the MLPE sample shown in
Figure 4, interesting fine structure can be observed in higher
resolution images of the layer boundaries when slightly lighter tapping
conditions are employed. As can be seen in Figure 5, hair-like
structures appear to extend from the boundary into the layer appearing
with darker phase contrast. Closer inspection of Figure 5 reveals
lamellar structures throughout the lower density (darker phase)
component visible in the right half of the image and a gradual loss of
alignment with increasing distance from the interface.
Figure
5. Phase image of a cryo-microtomed multilayer polyethylene
sample. Hair-like fine structure can be seen near the layer-interface.
Image size 5ìm. Closed loop active.
Figure 6 shows AFM
images of an oriented film of isotactic polypropylene, also known as
the microporous membrane Celgard. Both, topography and phase clearly
show the pattern of oriented fibrillar structures that is
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, the phase image exhibits very clear and
well-defi ned additional features. Fine lamellar structures (only ~
20nm wide) are seen to be present in between the rows of fibrils. The
lamellar structures are seen to be oriented perpendicular to the larger
fibrillar structures. As the phase signal is sensitive to deviations of
the oscillation amplitude from the amplitude setpoint, it can serve as
an edge detection technique and thus highlights such fine structures
that are easily 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.
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 2.5ìm.
Closed loop active.
The appearance of fine structure in phase images complements the
sensitivity to material properties. By identifying components in
composite samples, the appearance of fine structure in phase images
aids in compositional imaging. 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
monolayer of C60H122 alkane molecules on a
substrate of highly-oriented pyrolytic graphite (HOPG).
Figure
7. Topography (left) and phase image (right) of a C60H122
monolayer self-assembled on graphite. Both images clearly show
self-assembled domains, each of which is composed of parallel lines.
Image size 1.5ìm. Closed loop active.
Both images clearly show the presence of patches (domains)
separated by sharp (mostly straight) borders. Closer inspection reveals
a pattern of straight lines within each domain. This lamellar pattern
is much more obvious in the higher resolution phase image shown in
Figure 8. The lamellae are seen to have a well-defined spacing as well
as preferred directions. This is confirmed in Figure 9, which shows the
Fourier Transform of the phase image shown in Figure 7. The Fourier
Transform shows very clearly the hexagonal symmetry of the
self-assembly pattern that reflects the hexagonal symmetry of the
underlying graphite substrate. When forming self-assembled monolayers, C60H122
alkane retains a fixed relationship to the high symmetry axes of
graphite.
Figure
8. Phase image of a C60H122 monolayer
selfassembled on graphite clearly showing lamellar fine structure
associated with the self-assembly. Image size 390nm. Closed loop active.
The special relationship of the adsorbate structure with the
graphite substrate is consistent with the assumption, that the layer
probed here is actually the molecular layer that is in direct contact
with the substrate. The question arises because the preparation of the C60H122
alkane sample cannot be assumed to result in a single molecular layer.
Indeed, judicious choice and excellent control of tapping forces is
required to reveal the structures shown here. The probe has to
penetrate through partially disordered, 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 to
self-assemble on graphite such that each 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.
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,
the periodicity is measured to be r=.5nm.
The interrogation of self-assembly in monolayers 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 correct scanner 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 application note (including the
390nm image shown in 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 for
delicate samples, phase imaging can complement other modes such as
force modulation and lateral force microscopy, often with superior
image detail. Phase imaging applications include the characterization
of composite materials, mapping of variations in adhesion and
viscoelasticity, and identification of surface contamination. Highest
resolution phase images open the door to studies of molecular
selfassembly. The combination of excellent high resolution performance
and TappingMode phase imaging makes Innova a powerful tool for the
study of material properties at the nanometer scale.
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This information has been sourced, reviewed and adapted from materials provided by Bruker AXS.
For more information on this source please visit Bruker AXS.