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Methods for Structural and Chemical Characterization of Nanomaterials 129

a small circular metal disk. Nonconducting carbon tape is commonly
used for these purposes. One of the distinct advantages of AFM is that
measurements may be carried out in both air and water. For analysis
of powder materials they must be deposited onto a surface and fixated
to prevent movement during imaging.

Limitations. Samples for AFM analysis are generally constrained to
rather smooth samples with average roughness values of less than
around several microns. This constraint results from the need to main-
tain contact between the scanning probe and the surface and limitations
in the change in height that the probe can accommodate. Particles in
size from around 1
m to several 10s of microns may be attached to AFM
cantilevers for force measurements. The minimum particle size contin-
ues to decrease with improving methods of particle attachment. For
example, nanotubes are now being used as tethers for attaching parti-
cles to cantilevers serving to reduce the minimum particle size. The
principle constraint here results from the use of epoxy to fix the parti-
cle to a tipless cantilever. Below a critical size the epoxy covers the
sample particle and thus alters the interaction chemistry.

Form and size characterization
Form and size of nanoparticles can be determined using various tech-
niques, but scattering techniques are particularly well adapted for this
purpose.

Scattering experiments. Scattering experiments may be used to char-
acterize particle suspension in situ. Light or X-ray scattering experi-
ments may be used to measure the size, shape, agglomeration state, and
dynamic properties (diffusion coefficient) of nanoparticles in a sample.
These properties are averaged over the whole scattering volume, and
thus over a large number of nanoparticles or nanoparticle aggregates. The
statistical relevance and the nondestructive in situ nature of scattering
experiments represent significant advantages of these techniques over
the electron scattering techniques previously discussed. The operating
principles of a typical scattering apparatus are illustrated in Figure 4.15.
One monochromatic beam (photons or neutrons) is incident on a sample.
Typically, both the incident and scattered beam are shaped by optics
adapted to the radiation characteristics (e.g., apertures, slits, and lens).
Part of the incident beam crosses the sample unaffected and some is scat-
tered. Adetector measures at an angle the scattered intensity I( , t). The
volume illuminated by the incident beam and analyzed by the detector
is the scattering volume V.
From the general experimental setup shown in Figure 4.15, three dif-
ferent types of measurements can be performed: dynamic, static, and