Environmental Engineering Reference
In-Depth Information
introduction to both SEM and TEM techniques, the textbook by Egerton (2007)
is recommended.
The conventional SEM requires high vacuum conditions, therefore the samples
need to be conductive in order to prevent charging effects due to the electron
bombardment. Newer devices equipped with fi eld emission guns (FEG), however,
can be operated at low acceleration voltages, which enable the investigation of
uncoated, non-conductive samples. The resolution of the SEM can go down to
1 nm; however, this parameter has to be used cautiously. The resolving power on
an SEM is determined either by using standard materials (gold particles on carbon)
or by referring to the smallest beam diameter than can be achieved. The resolution
can thus be used to compare the performance of different SEMs. In practice,
however, the properties of the samples will limit the resolution that can be achieved.
In addition, it has to be kept in mind that this resolution limit is only valid for the
SE image. Due to the higher escape depth (which describes the maximum depth
from where the electron can still reach the surface and leave the material) the reso-
lution of the BSE signal is much lower than the SE signal. The resolution of the
BSE signal can be increased by lowering the acceleration voltage and using a low
voltage BSE detector. However, a certain acceleration voltage has to be applied to
get enough BSE signal. The same holds true for the lateral resolution of the X-ray
signal.
SEMs that are capable of working at a low vacuum (so-called variable pressure
or low vacuum SEMs where the pressure in the sample chamber can be a few mil-
libar) can be used to investigate materials that are not stable under high vacuum
conditions. The gas in the sample chamber can be chosen according to the specifi c
applications, and most often H 2 O(g) is used. The working principle of the detectors
is based on the gas ionisation, where negatively charged ions are accelerated
towards the detector and the positive charges maintain the charge equilibrium of
the sample (charge suppression). Specifi c detectors can be operated at even higher
chamber pressures, up to about 20 mbar, which then refers to the environmental
SEM (ESEM). In the ESEM the triple point of water (0.01 ° C, 6.11 mbar) can be
reached and, thus, water can be condensed on the sample surface by cooling the
sample (for example with a peltier stage). This allows not only the investigation of
non-conductive samples but also wet samples. An in depth description of the ESEM
technology can be found in Danilatos (1988). For a shorter introduction into this
topic Danilatos (1991) and Danilatos (1997) can be consulted.
Due to the gas present in the sample chamber, the primary electron beam is
scattered before it reaches the sample surface, which produces an electron ' skirt ' .
However, for the image formation (SE image) only the signal resulting from the
unscattered fraction of the electron beam is used. The SE resulting from the scat-
tered primary electrons originate from a much larger surface and thus only provide
a featureless noise to the image. The resolution in the ESEM is therefore primarily
dependent on the electron optics used, and the presence of a gas in the sample
chamber is of secondary importance. The resolution of the ESEM is degraded,
when investigating organic materials, but this decreased resolution is not caused by
the ESEM technique itself but rather by the properties/stability of the sample
under the electron beam. As the same material cannot be investigated without
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