By AZoNano
Topic List
Background
Application Examples
Block Copolymers
Distribution of Composite Components
Fine Structure Detection
Summary
About the Imaging Techniques
Conventional Approaches
Contact Mode AFM
Non-Contact Mode AFM
Tapping Mode Imaging
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.
TappingMode phase imaging is a standard feature on atomic
force microscopes from Bruker (see Figure 1). The AFM models also
include proprietary lownoise electronic control, closedloop scanning
tip technology, and comprehensive software including full control over
the two integrated dual lock-in amplifiers, thus enabling a suite of
imaging modes, including phase imaging.
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 fi lm. Figure 2 shows two 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. The stiffer PS lamellae appear bright in both, topography
(implying taller features) and phase (implying more positive phase
angle). 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 suffi ciently 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 light
tapping conditions would fail to uncover the microphase separation
pattern. Images such as those shown in Figure 2 are usually obtained
with hard tapping conditions, that is, fairly high ratios of free
amplitude to amplitude setpoint. Probe tuning at moderate
amplitude (input gain setting ~ 8) and significant increase of the
cantilever drive amplitude upon engaging, in conjunction with high
proportional feedback gains, will yield the desired result.
Figure
1. 5µm (left) and 500nm (right) phase image 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. Images acquired with Closed-loop active.
Distribution of Composite Components
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 3 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
cryomicrotoming. The phase image has an entirely different appearance,
clearly providing complementary information. The phase image is
dominated by an alternating set of stripes, obviously representing the
sought after alternation in material properties and thus component
layers. In addition, topographic fine features are readily apparent,
such as droplets, indicating an ageing sample surface. The droplets are
clearly not distributed randomly. Rather, they appear to form along
lines, presumably small scratches imparted on the sample by the
microtoming process.
Figure
2. 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 55µm.
While the phase image in Figure 3 provides a particularly clean map
of alternating density components, the differing material properties
can also have an effect on the observed AFM topography, depending on
the choice of imaging mode, cantilever, and other factors.
Fine Structure Detection
Aside from compositional mapping and the visualization of
microphase separations, phase imaging can aid in the detection of fine
structures. Figure 4 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 fi brillar
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
clearly reveals fi ner, partially oriented lamellar structures (~ 20nm
wide) in between the rows of fibrils. 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.
Figure
3. Topography (left) and phase image (right) of Celgard. While
oriented fi brillar structures are evident in topography, the phase
image additionally reveals lamellar fi ne structure. Image size 3.5µm.
The appearance of fine structure in phase images not only
complements the sensitivity to material properties. By identifying
components in composite samples, the appearance of fine structure in
phase images aids in compositional imaging. Figure 5 shows topography
and phase images of a thermoplastic vulcanizate, a multicomponent
material consisting of isotactic polypropylene, rubber, and carbon
black filler.
Figure
4. Topography (left) and phase image (right) of thermoplastic
vulcanizate. The phase image clearly shows lamellar fine structure,
indicating enrichment of the polypropylene component in this region.
Image size 7.6µm.
A lamellar fine structure can be discerned in the topography image
and is much more clearly seen in phase, indicating that this region is
enriched in the polypropylene component. Other AFM imaging modes can
help to obtain additional information about this composite sample. In
particular, electric force microscopy can uncover the distribution of
carbon black filler material near the surface.
Summary
With TappingMode phase imaging, the Bruker
AFM systems can efficiently map variations in sample properties at
high
resolution. 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. Phase
imaging makes AFM a powerful tool for the study of material
properties at the nanometer scale.
About the Imaging Techniques
Conventional Approaches
Two conventional AFM scanning modes – contact mode and noncontact
mode have been used for some time with varying success. Each has its
limitations, particularly for imaging delicate samples in ambient
conditions.
Contact Mode AFM
Contact mode AFM represents the simplest 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 for many samples, it is subject to serious drawbacks. In
essence, the dragging motion of the probe combined with adhesive
tip-surface forces can lead to substantial damage to probe and sample,
creating artifacts in the image and often degrading the resolution
severely.
Under ambient air conditions, most surfaces are covered by a fluid
layer, composed of water and other contaminants, which is typically
several nanometers thick. An AFM tip touching this layer will cause a
meniscus to form and surface tension will pull the tip onto the
surface. Additional adhesive forces can arise from trapped
electrostatic charges (see Figure 6).
Tip-sample forces are thus larger than the cantilever defl ection
would seem to indicate. Correspondingly, lateral motion during contact
mode imaging is associated with larger lateral forces, resulting in
severe tip or sample damage or involuntary displacement of weakly bound
surface adsorbates. Capillary forces can be eliminated by completely
submersing the sample and probe 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 a liquid environment at all.
Figure
6. 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. Non-contact mode represents an attempt to overcome the
deleterious tip-surface forces associated with contact mode.
Unfortunately, ambient air conditions are rarely conducive to
non-contact AFM imaging.
Non-Contact Mode AFM
Non-contact mode is based upon the detection of weak attractive van
der Waals forces that exist between tip and sample a few nanometers
above the surface. It is important to note that the fluid layer present
in ambient conditions partially shields these forces and occupies a
large fraction of their useful range, i.e., when compared to operation
in ultra-high vacuum, where no fluid layer is present. Due to the
limited range of any residual van der Waals forces, their detection
necessitates very small oscillation amplitudes. At the same time, a
cantilever operated at very small amplitudes is easily trapped inside
the fluid layer, once it touches.
Tapping Mode Imaging
In the ideal case, operation remains outside the fluid layer – and
therefore several nanometers away from the sample surface. The
consequences are substantially degraded resolution (as compared with
TappingMode) and an inability to map out variations in local mechanical
properties. In practice, the probe is frequently drawn onto the sample
surface and remains trapped there while the scanning motion proceeds,
leading to unusable data and sample damage similar to that caused by
contact mode.
Bruker’s TappingMode imaging overcomes the
limitations of the conventional scanning modes by alternately placing
the tip in contact with the surface to provide high resolution 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 near its fundamental fl exural
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 and a reduced oscillation amplitude (see Figure 7
and application note 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 of the cantilever
oscillation, relative to the drive signal, is simultaneously monitored
with topography data. As the phase lag is infl uenced by energy
dissipation experienced during the oscillation cycle, it is very
sensitive to material 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 for clearer observation
of fine features, such as grain edges, which can be obscured by rough
topography.
Figure
7. TappingMode cantilever oscillation amplitude in free air and
during scanning..
TappingMode prevents the tip from sticking to the surface and
causing damage during scanning. In contrast to non-contact mode, the
oscillation amplitude and energy is suffi cient 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 and allowing routine reproducible sample measurements at high
resolution.
Selection of the optimal cantilever oscillation frequency is a
critical step in preparing for TappingMode imaging. This can be
accomplished in the Cantilever Tuning dialog (see Figure 8).
Choice of drive frequency and adjustment of lock-in parameters for
proper phase imaging are all taken care of by the autotune function.
The user just chooses the desired cantilever oscillation amplitude by
selecting a target amplitude and lock-in sensitivity (“input gain”)
factor. As shown in Figure 8, both, amplitude and phase are displayed
in the Cantilever Tuning dialog.
Figure
8. Selecting drive frequency and phase in preparation for
TappingMode imaging using the autotune function in Bruker's powerful
AFM software.
This information has been sourced, reviewed and adapted from
materials provided by Bruker AXS.
For more information on this source please visit Bruker AXS.