Page 135 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Methods for Structural and Chemical Characterization of Nanomaterials 121

elements strongly affects the signal. For example, the presence of iron
in a system leads to a strong decrease of the peak intensity and there-
fore limits the application of NMR in these cases.

Application in the particular case of nanoparticles. For nanoparticles, NMR is of
high interest since this element-specific technique does not require long-
range order and may be used to characterize nanoparticle surface prop-
7
erties like XAS. For example, Li NMR of polycrystalline nano-LiMn O 4
2
indicated that the lithium ion occupies the 8a position, however, it also
revealed that it has two different distances to neighboring manganese
and oxide ions [Hon et al., 2002]. Such information is difficult to extract
from XRD due to the small size of the product. More specifically, in the
55
case of quantum tunneling in nanomagnets such as Mn, Mn NMR
[Kubo et al., 2001] can provide unique information on the magnetic
behavior of these nanoparticles. This is evidence of phonon-activated res-
onant quantum tunneling.

Microscopy
SEM and TEM. Spectroscopic techniques can provide detailed infor-
mation about the structure and size of nanoparticles. The most common
examples of electron microscopy techniques used for characterizing
nanoparticles include scanning electron microscopy (SEM) and trans-
mission electron microscopy (TEM). A potential drawback to these tech-
niques is that in some cases particle shape can induce indirect
modification of the spectroscopic signal and is thus a source of error in
these types of measurements. Nevertheless, these microscopy tech-
niques allow for direct visualization of nanoparticles, and thereby pro-
vide information about particle size, shape, and structure. With this in
mind, both SEM and TEM imaging are highly versatile and powerful
techniques for characterizing nanoparticles.

Operating principle. The general operating principles and the compo-
nents that make up the respective instruments of TEM and SEM imag-
ing are summarized in Figure 4.11.
The interaction between an electron beam and a solid surface results
in a number of elastic or inelastic scattering processes (backscattering
or reflection, emission of secondary electrons, X-rays or optical photons,
and transmission of the undeviated beam along with beams deviated as
a consequence of elastic—single atom scattering, diffraction—or inelas-
tic phenomena). The operational principle for a scanning electron micro-
scope (SEM) is based on the scanning of finely focused beams of electrons
onto a surface. When using an SEM, there are a number of different visu-
alization techniques that can be used. During scanning, the incident elec-
trons are completely backscattered, reemerging from the incident surface