Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 236

coating in the conventional SEM, a comparison of the resolution of ESEM and
SEM is meaningless.
Recently, TEM detectors that can be mounted underneath the sample have
become available for the SEM. This basically turns the SEM into a STEM, and
is a very good opportunity if no TEM is available or if a quick look at a sample
is required. However, due to the rather low acceleration voltage of the SEM (30 kV)
the penetration of the electrons within the material is rather limited, which
requires very thin samples (see the section on (Scanning) Transmission Electron
Microscopy).
Applications and limits (SEM). Although the resolution of a modern SEM is at
around 1 nm (topography), chemical information results from a much larger volume
due to the larger excitation depth of X-rays. The same holds true for the BSE, which
provide a chemical contrast, although the use of low voltage BSE detectors have
improved the situation. However, if the NPs are well dispersed on the substrate
and chemically distinct from the substrate then the backscattered signal can be used
well to distinguish the NPs. An example is given in Figure 6.11, where mercury
sulfi de (HgS) particles have been located on different substrates (NaCl crystals,
aluminium, carbon) in the ESEM.
The SEM is most often used to investigate bulk materials to get a rough overview
of the sample material. More detailed studies are then performed on TEM samples.
For example, Labrenz et al. (2000) have shown an association between cells and
micrometre-sized mineral aggregates based on SEM investigations. The mineral
aggregates were then investigated in more detail using a TEM. Hochella et al.
(2005) studied samples from riverbeds of the Clark Fork River Superfund complex
(Montana) and reported iron oxide coatings on silicate grains down to less than
100 nm. Kaegi et al. (2008a) visualized the distribution of titanium dioxide particles
on new and aged facades using SEM (low voltage BSE detector) and ESEM studies
(Figures 6.12 and 6.13 ).
Even if the particles are dispersed on a TEM grid and thus can be images with
higher resolution in a TEM, the SE provides information about the sample topog-
raphy which is complementary to the information obtained in the TEM.
SEM investigations are powerful in situations, where a separation of NPs from
the substrate is not feasible or not desired. Low voltage SEM and ESEM allow the
investigation of non-conductive materials without coating which reduces the risk
of artefacts. This allows, for example, the identifi cation of NPs on a non-conductive
substrate. ' Bulk ' samples can be introduced into the sample chamber, which greatly
reduces time and artefacts related to sample preparation, especially when com-
pared to the TEM technology where the samples have to be small enough to be
deposited on a TEM grid.
(Scanning) T ransmission E lectron M icroscopy. In the TEM the sample is irradi-
ated with a parallel beam of electrons and the image of the transmitted electrons
is visualized by either a fl uorescent screen or recorded on a CCD camera. Recent
TEMs are equipped with a CCD camera, which replaces the fl uorescent screen,
meaning that the operator no longer has to sit in a dark room. Crystalline materials
can further be investigated using electron diffraction and related methods such as