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The experiment in AFM begins with sample preparation, a probe selection,
a setting up of a microscope. One should be aware about imaging
artifacts that appear as a sharp change of contrast or appearance
on identical shapes in various parts of the image. These artifacts
can be caused by change of the tip condition and shape due to picking
an occasional contamination particle or breaking a tip apex that
happens when the tip hit a big structures whose height is larger
than a vertical range of the scanner employed in the instruments.
Therefore, one should be careful with storage of the samples to
avoid various contaminations, and use of the scanners with large
range of vertical displacements is preferable, especially, for examination
of rough surfaces.
The precise positioning of the probe, which is controlled with
an optical viewing of the probe and the sample, may be required
for imaging the same area using different probes, or when the probe
needs to be in a small sample area (such as single carbon fiber
~ 10 micron in width). In this case it is important to know a position
of the tip with respect to the cantilever free end. GP probes are
preferable for this reason as the tip is positioned at the end of
the cantilever, while in other probes it is offset about 10 micron
from the end.
The probe engagement is the important procedure to prevent the
tip damage. An example of the careful engagement procedure is described
in high-resolution imaging page.
Of course, in quantitative AFM studies it is important to have
the microscope scanners properly calibrated. We suggest using calibration
gratings TGX01 and step-structures of the TGZ series for proper
adjustments of X, Y and Z sensitivity of the scanners.
Depending on a particular task, imaging routines have their specifics.
A few of them are described below.
Topograpgy imaging
Visualization of surface topography is the main function of AFM.
The success of the experiment depends not only on a correctly chosen
probe but to a large extent on optimal imaging routine. A minimal
tip-sample force, a properly adjusted feedback and an appropriate
scanning rate are a combination of important parameters for most
precise topographic imaging.
At the end of the topography studies of the particular area, it
might be useful to make a final scan on a large area that includes
the location where the scans were made. In some cases you might
find that imaging of the smaller area left a "window"
seen in the large scan. This will be a strong indication that tip-force
should be made smaller. How to control the tip-sample force in tapping
mode is described in some detais on the topography
imaging page.
Exploring Materials Morphology and Structural Order
Besides high-resolution topographic information, AFM imaging of
ordered structures and heterogeneous materials substantially provides
visualization of surface morphology and nanostructures thus complementing
other microscopic and diffraction techniques.
In many cases, visualization of ordered structures and compositional
imaging of individual components in complex materials are achieved
by imaging at elevated forces. This happens in visualization of
lamellar order of alkanes and semicrystalline polymers, microphase
separation patterns of block copolymers, morphology of elastomer
blends, etc. It is not rare that top surfaces of multicomponent
polymer materials are enriched of low-molecular weight material
and ordered lamellar structures are hidden underneath. In this case,
scanning at low tip-force will reveals only stucture of the top
layer formed of amorphous polymer and imaging at elevated forces
will lead to images showing sub-surface ordered structures. This
is the case when imaging at elevated forces might provide images
with well-resolved structures contrary to imaging at low forces.
For sample with rubbery top layer, which can be penetrated by an
AFM probe at elevated forces, one would like to know at what sub-surface
depth a particular image was obtained. This information can be gained
from analysis of force-curves.
You can find more details on the materials
composition mapping page.
Examination of local mechanical properties
AFM-nanoindentation experiments can be conducted as follows. First,
a sample area where one would like to make indents should be imaged
and preferably in the tapping mode to avoid a tip-induced sample
modification. When a homogeneous material is studied this allows
checking its topography and avoiding occasional surface imperfections
that might influence indents. In case of heterogeneous samples a
choice of area is important when mechanical properties at interfaces
between different components are examined.
While making indents the force curve are collected and later used
for extracting force versus penetration curves. Usually, the indents
and corresponding force curves are collected in a large number of
locations and various levels of the tip-force. The tip indents should
be made in different locations, which should be spaced far from
each other to exclude interference of the indents. For homogeneous
material, this procedure provides the statistically-sound force
curve data leading to reliable estimates of elastic modulus. In
the examination of the heterogeneous materials, one faces more challenges.
First of all, they related to compromises between spacing of indent
and strong wish to examine the interfaces between different materials.
Second, heterogeneities in bulk demand a more careful assignment
of the force curves obtained at different locations. In many a thickness
of top material is unknown and the force data might be influenced
by another material situated underneath.
After the indents are made, their shapes are visualized with AFM
either using the same probe that was employed for indenting or using
new and sharper probe. The observation of the indented locations
provides important information about the nature of the indent. In
case of elastic deformation the material will practically recover
and the pits will be barely seen. A partial recovery of the indent
takes places for non-elastically deformed materials and a presence
of pile-ups at the indent pit indicates a plastic flow developed
during indenting. This information should be considered for selection
an appropriate model for extracting of quantitative mechanical data
from force curves.
In addition to the choice of theoretical model, the quantitative
analysis of the force curve requires also knowledge about the prove
stiffness, tip geometry and optical sensitivity of the instrument
for the particular probe. The probe stiffness is usually measured
from probe thermal excitation, cantilevers with pre-measured spring
constant are also available (-F Series). The geometry of the tip
can be deduced from TEM or SEM micrographs either provided by the
probe manufacturer or measured by the AFM user. This is much easy
to do for the probe with round-shaped tip apex. The optical sensitivity
is usually measured force curves obtained on a rigid substrate (e.g.
sapphire) after the indenting experiments. This procedure should
be made with an extreme caution to avoid tip damage. Again, in this
procedure it will be much easy to handle the probe with rounded
tip apex.
The reason why compositional imaging and indenting experiments in
AFM are often made on polymer materials is related to the fact that
in general stiffness of commercial probes and stiffness of AFM probes
are the same range. There are two additional consequences. First
of all, due to wide spread of stiffness of polymer materials there
should also a wide selection of AFM probes with different stiffness
in order to choose the most appropriate probe for compositional
imaging of a particular polymer multicomponent material. Second,
for nanomechanical examination of biological objects and, particularly,
cells one should use the AFM probes which operate at much smaller
loads. Due to definite limitations in reducing stiffness of the
cantilever one might concentrate on the use of round-shaped beads,
which can be attached to the cantilever instead of tips. The beads
of micron size can be glued to tip-less probes, like NSC12 and CSC12
cantilevers.
You can find more details on the mechanical properties page.
Studies at Different Temperatures
AFM imaging of polymer samples at different temperatures is used
for visualization of phase transitions (melting, crystallization,
etc) and direct monitoring of structural changes following thermal
annealing. Practically, such imaging is performed after the sample
temperature has changed and stabilized. To prevent moisture condensation
dry environment of the sample is needed at sub-0C temperatures.
This is achieved by purging the heating cell with dry gas. At high
temperature purging with inert gas might be need to avoid sample
oxidation. In the imaging at elevated temperatures the probe is
heated through direct tip-sample contact in the contact mode. In
the tapping mode, heating of the probe is made with an additional
heater and this prevents a condensation of moisture on the probe
that is colder than the sample and condensation of volatile components
that might exude from a sample. The heating of the probe make difficult
to use the cantilevers with reflected Al coating due to the cantilever
bending (bimetallic effect). Additionally, during scanning at high
temperature one should be aware about possible increase of sample
adhesion. This might restrict a use of soft Si probes. Therefore,
for temperature measurements we recommend non-coated Si probes with
stiffness ~ 5 N/m and 40 N/m.
Imaging in Different Environments
AFM imaging in gas or vapor atmosphere is usually performed with
the same probes that studies at ambient conditions. The situation
is different in imaging under liquid, especially, when studies are
performed with oscillatory or tapping modes. Due to the fact that
driving of the probe in liquid is usually proceeds at the clean
resonant frequency the tip-force is not well-controlled. Therefore,
in studies under liquid one is limited to soft Si probes with stiffness
below 5 nm. Therefore, our probes in the stiffness range from 0.3
N/m to 5 N/m can be used in oscillatory modes in measurements under
liquid.
Further reading
Topography imaging
Material composition map
Mechanical properties
Electric properties
Magnetic properties
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