Biomedical Engineering Reference
In-Depth Information
200 nm
figure 6.6
Transmission electron micrograph of gold “nanocages” within the root system
of a royal bean plant. The plant sample underwent fixation, polymerization, ultrathin sec-
tioning, and negative staining prior to imaging.
6.3.2
sem
SEM is a surface imaging technique that is also widely used for characterizing
nanoparticle size and shape. In a SEM, electrons are fired from an electron beam at
voltages ranging from less than 1 kV to about 30 kV toward a sample in a vacuum
chamber; the electrons collide with the sample and are reflected back toward detec-
tors located above and to the sides of the sample where a CCD camera collects an
image. an example of the instruments is shown in figure 6.7. Most SEM imaging
is accomplished by collecting the signal from low-energy secondary electrons.
These images have a three-dimensional (3D) appearance as contrast is produced
primarily by contours and surface features of the sample that alter the trajectory of
the reflected secondary electrons and hence alter the intensity of detected electrons
at any one point on the sample. While SEM can provide subnanometer spatial reso-
lution, it is most useful for imaging particles tens of nanometers to several microns
in size (an example is shown in fig. 6.8). The magnification range on an SEM
extends from about 10 times to greater than 100,000 times and can provide a much
larger field of view than TEM and so is more useful for determining statistically
relevant particle sizes.
Particle sample preparation for SEM involves securing particles to a flat, conduc-
tive substrate. for particles in a dry powder form, carbon tape is often used to secure
particles to a standard aluminum stub. Particles in liquid suspensions are usually
drop cast or spin coated on a piece of silicon wafer (fig. 6.9) and allowed to dry
before imaging. SEM is very sensitive to charging effects; therefore, nonconductive
samples that would accumulate charge are usually sputter coated with a thin layer of
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