Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 239

spectroscopy (EDX)) and high resolution applications, much thinner samples are
required. As a rule of thumb morphological investigations can be carried out on
micron sized particles and for analytical or high resolution applications the sample
thickness should be 100 nm. Thus, investigating NPs within the TEM does in

general not require any additional thinning, as the NPs are already thin enough.
The conventional TEM can be equipped with a scanning unit, which leads to a
scanning transmission electron microscope (STEM). In the STEM a focused elec-
tron probe is scanned over the sample and, thus, the STEM requires a convergent
beam. Therefore, the operating principle of a STEM is fundamentally different
from a conventional TEM, where a parallel beam is required, meaning STEM is
more related to the principle of the SEM. A highly focused electron probe is
required for the quantitative elemental analysis, which is discussed in the next
section.

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Applications and limits. For a general investigation of the morphology of the
particles, the TEM is operated in the conventional mode. Perret et al. (1994) sepa-
rated particles from the River Rhine into different size fractions and then used
TEM to visualize these size fractions. If the particles are homogeneously dispersed
on the TEM grid, then the size distribution of the particles can be derived by apply-
ing image analysis tools to several TEM images. Absolute number concentrations
can also be derived if the separation procedures are well documented (Kaegi et al. ,
2008b, 2008a). A comparison between images recorded in the SEM and in the TEM
(conventional mode) is given in Figure 6.14. The SEM image is similar to a topo-
graphic image (surface) and the TEM image is a projection caused by a density
and/or thickness contrast.
The strong contrast of heavy elements can be used to detect nanoparticles con-
sisting of elements such as silver, platinum or gold. In Figure 6.15 a TEM image of

Figure 6.14 Left: High resolution SEM (SE) image of a lepidocrocite recorded in a Hitachi
S-4800 SEM. The low voltage (5 kV) allows the investigation of non (or poorly) conductive
samples without treatment. Right: TEM image particles from the same sample recorded with
a CM30 operated at 300 kV.