Environmental Engineering Reference
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of high-energy electrons to generate a variety of signals at the surface of solid
specimens. The signals that derive from electron-sample interactions reveal infor-
mation about the sample including external morphology (texture), chemical com-
position, and crystalline structure and orientation of materials making up the
sample. In most applications, data are collected over a selected area of the surface
of the sample, and a 2-dimensional image is generated that displays spatial varia-
tions in these properties.
Areas ranging from approximately 1 cm to 5
m in width can be imaged in a
scanning mode using conventional SEM techniques. The SEM is also capable of
performing analyses of selected point locations on the sample; this approach is
especially useful in qualitatively or semiquantitatively determining chemical com-
positions, crystalline structure, and crystal orientations [ 4 ].
μ
13.2.2 Atomic Force Microscopy (AFM)
Atomic force microscopy (AFM) is a high-resolution surface imaging technique
which operates by scanning a sharp probe over the surface of a sample, while
measuring the forces experienced by the probe [ 5 ]. In the most commonly used
imaging mode, the probe and sample are brought in contact and the probe is
scanned over the surface while the interaction force is kept constant. Alternatively,
the deflection of the cantilever can be measured while the probe scans at constant
height. This provides an image of the surface topography with (sub) nanometer
scale resolution. A major advantage of the AFM over classical microscopy tech-
niques is that it can simultaneously provide information on local physical proper-
ties, such as mechanical properties and interaction forces. In particular, force-
distance curves, made by recording the deflection of the cantilever while the sample
is moved up and down, allow one to measure directly surface forces in aqueous
environments, such as van der Waals and electrostatic forces, solvation forces,
steric forces, and intermolecular forces between complementary molecules.
AFM can be applied to a wide variety of samples (conductors, insulators) and
may be operated in various environments (vacuum, air, liquids) which make it
possible to examine biological systems (biomolecules, cells) under physiological
conditions. In this laboratory, AFM is used to probe the nanoscale organization of
organic surfaces, including polymers, lipid films, adsorbed protein layers, and
living cells. Besides topographic imaging, a special emphasis is put on the quanti-
tative measurement of molecular interactions using functionalized probes.
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