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For example, layers of normal alkanes (CnH2n+2) on HOPG (Fig.1.)
are build of lamellar structures whose alignment mimics 3-fold symmetry
of the substrate. Different size and mobility of -CH3 end groups
compared to chain groups -CH2- is responsible for a stripped pattern
observed in the AFM images recorded in tapping mode. The spacing
is defined by a length of the alkane molecules and varies from 1.4
nm in C18H38 to 49 nm in C390H782. Imaging of C60H122 alkane layers
(the spacing is ~ 7.5 nm) is routinely used for demonstration of
high-resolution of commercial AFM microscopes.
Imaging of films of block copolymers such as SBS is also common
for routine checks of the microscopes or scanners. These samples
have regular patterns with averaged spacing in the 20-40 nm range
and do not bring problems for imaging. The patterns are clearly
resolved in height and phase images when the blocks are plastic
and rubbery. The observation of the high-contrast height and phase
images is related to difference in mechanical properties of these
blocks. In such cases, the assignment of the height images to surface
topography can be misleading because the contrast of height images
can be caused by a cross-talk with the phase pattern that reflects
a difference of tip interactions with dissimilar blocks. In this
case, a minimization of the tip-sample force by choosing soft probes
such as DP09/GP (Stiffness below 1 N/m), is needed for a recording
of true sample topography.
The samples mentioned above have relatively flat surfaces and demand
less attention to optimization of feedback gains and scanning rate.
The situation is different when examining such samples as latex
arrays (see Latex systems) or microporous
surface like that of Celgard membrane made of isotactic polypropylene.
The alternation of nanofibrils, which are separated by nanoscale
cavities, and elevated lamellar regions characterizes morphology
of this sample. It takes some efforts to get tapping mode images
of this material with nm-scale resolved features without disturbing
the interfibrillar cavities. Si probes with stiffness ~ 5 N/m (DP14/GP)
is the right choice for such topography studies. A non-destructive
imaging of this sample in the contact mode is only possible under
water.
Three of the described samples: mica, HOPG and commercial Celgard
film can be studied with AFM without any preparation. Mica and HOPG
are layered materials and their fresh surfaces are prepared by cleaving
with a sticky tape. Other samples need some kind of preparation.
Hot rubbing of PTFE on different substrates is used for making sheets
of oriented polymer chains. Spin casting of block copolymer solution
on a substrate leads to a formation of films of different thickness
in which microphase separation layer originate during solvent evaporation.
Annealing of these samples at high temperature or in solvent vapors
improved this order and this process can be monitored by AFM. Other
deposition methods such as dipping, droplet deposition and the use
of Langmuir-Blodgett trough are also employed for preparation of
single macromolecules on different substrates and different self-assembled
layers. A proper choice of solution concentration, a nature of substrate
and annealing procedure are helpful for preparation of the optimal
sample.
More elaborative is a preparation of polymer bulk materials for
AFM studies of their morphology. This is usually done with a use
of ultramicrotome. A choice of a diamond knife, cutting speed and
temperature are crucial for preparation of smooth surface with minimal
morphology disturbance. This can be achieved and lamellar structures
with dimensions in 10-20 nm range can be observed on microtomed
surfaces. Examples of the AFM studies of complex polymer materials
on samples prepared with the ultramicrotome are given in Heterogeneous
Polymer Systems.
Further reading
Robust samples
Soft, fragile and near-liquid samples
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