Biomedical Engineering Reference
In-Depth Information
8.5
STEM images of ZnO films deposited on 100 nm SiO
2
spherical
particles.
epoxy resin loaded with the particles of interest. The thickness of the
polymer slices is normally
50 nm. The thickness of the ALD films can be
directly observed by TEM. Scanning transmission electron microscopy
(STEM) utilizes similar equipment and provides a z-contrast image, such
that materials with some difference in molecular weight can be readily
observed. Figure 8.5 is one example of an STEM image, which shows
discrete ZnO ALD films deposited on individual 100 nm SiO
2
particles.
Energy dispersive spectroscopy (EDS) detector unit equipped with TEM can
be used for elemental analysis while imaging.
For ALD thin film coating on particles, the key challenge in trying to
individually coat primary nanoparticles is to overcome their natural
tendency to form soft agglomerates. One method to verify that particles
are coated individually is to check the morphology of the particles before
and after ALD coating, which can be directly observed by SEM. The
particle size distribution can also be measured by a particle size analyzer.
These two methods are very useful for micro-sized particle substrates. For
nanoparticles, one effective method is the determination of the surface area.
If nanoparticles are coated as aggregates, the surface area will dramatically
decrease. Nevertheless, there is an expected change in the surface area of the
nanoparticles as the density and the particle size will change as the thickness
of the ALD films increases. With an increase in the thickness of ALD films,
~
