40
T. B. Asafa et al.
generated by electron interactions with samples do not lead to volume loss of the
sample indicating that the same materials can be analyzed repeatedly (Beane 2004).
For SEM, there are two basic types of electron sources that can be used for the
formation of the electron beam (Goldstein et al. 1992). Conventional SEMs utilize
thermionic emission sources that are equipped with tungsten, lanthanum hexaboride
(LaB 6 ), or more recently with a cerium filament (CeBix). With thermionic emission,
electrons become adequately energetic to leave the metal cathode after the wire or
rod is heated beyond a specific temperature. The tungsten filament is the simplest
in development and operation, less costly, and highly reliable, and its properties are
comprehensible. However, the tungsten cathode has relatively low brightness, and
the operating time is constrained. Meanwhile, the LaB 6 cathode has higher brightness
and the operating lifetime is longer, but is prone to contamination, more expensive,
and the processing is more difficult. The best is the CeBix filament which offers
equal performance with better stabilization and longer life than the LaB 6 filament.
Careful preparation of specimen is essential for good electron microscopy analysis, as well as some additional time used to optimize specimen preparation and
cleanliness can prevent days of wasted effort in conducting microscopy and image
analysis (Echlin 1999). In other words, contaminated or dirty specimens result to the
production of SEM images of the surface dirt. Best practices for the avoidance of
contamination include specimen storage in clean environments (sealed boxes, desiccators, and vacuum packs), humidity reduction in desiccating media, and soaking of
oil-contaminated specimen in solvents and ultrasonic agitation (Inkson 2016). After
cleaning, non-conducting specimens often require ultrathin coating using carbon,
gold, and platinum, which creates a path to conduct away accumulated electrostatic
charge. Sample size is often limited to 10 cm in horizontal dimension and 40 mm
in vertical dimensions. Samples must not be oxidized when placed in a vacuum of
about 10
−5 to 10
−6 torr and should not outgas. Conventional SEM scans surfaces
within approximately 1 cm to 5 microns while magnification ranges between 20X
and 30,000X with a spatial resolution of 50–100 nm (Beane 2004; Egerton 2005).
Asides surface scanning, SEM can also perform analyses of a chosen point across
the surface of the sample for the purpose of qualitative or semi-quantitative determination of chemical compositions (using EDS), crystalline structure, and crystal
orientations (using EBSD).
Most of the studies in nanotechnology research and related fields require extensive utilization of scanning electron microscope to quantify and qualify the surface
structures of developed materials. Figure 19 presents the images of carbon nanotubes
and CoPt multilayer nanowires (Kohl and Reimer 2008), silicon germanium (SiGe),
nanocantilevers (Asafa et al. 2013a, b), and nanosized silicon (Si) (Durowoju
et al. 2019). In these images, the structure of the materials, orientation, size, and
distribution are well quantified.
T. B. Asafa et al.
generated by electron interactions with samples do not lead to volume loss of the
sample indicating that the same materials can be analyzed repeatedly (Beane 2004).
For SEM, there are two basic types of electron sources that can be used for the
formation of the electron beam (Goldstein et al. 1992). Conventional SEMs utilize
thermionic emission sources that are equipped with tungsten, lanthanum hexaboride
(LaB 6 ), or more recently with a cerium filament (CeBix). With thermionic emission,
electrons become adequately energetic to leave the metal cathode after the wire or
rod is heated beyond a specific temperature. The tungsten filament is the simplest
in development and operation, less costly, and highly reliable, and its properties are
comprehensible. However, the tungsten cathode has relatively low brightness, and
the operating time is constrained. Meanwhile, the LaB 6 cathode has higher brightness
and the operating lifetime is longer, but is prone to contamination, more expensive,
and the processing is more difficult. The best is the CeBix filament which offers
equal performance with better stabilization and longer life than the LaB 6 filament.
Careful preparation of specimen is essential for good electron microscopy analysis, as well as some additional time used to optimize specimen preparation and
cleanliness can prevent days of wasted effort in conducting microscopy and image
analysis (Echlin 1999). In other words, contaminated or dirty specimens result to the
production of SEM images of the surface dirt. Best practices for the avoidance of
contamination include specimen storage in clean environments (sealed boxes, desiccators, and vacuum packs), humidity reduction in desiccating media, and soaking of
oil-contaminated specimen in solvents and ultrasonic agitation (Inkson 2016). After
cleaning, non-conducting specimens often require ultrathin coating using carbon,
gold, and platinum, which creates a path to conduct away accumulated electrostatic
charge. Sample size is often limited to 10 cm in horizontal dimension and 40 mm
in vertical dimensions. Samples must not be oxidized when placed in a vacuum of
about 10
−5 to 10
−6 torr and should not outgas. Conventional SEM scans surfaces
within approximately 1 cm to 5 microns while magnification ranges between 20X
and 30,000X with a spatial resolution of 50–100 nm (Beane 2004; Egerton 2005).
Asides surface scanning, SEM can also perform analyses of a chosen point across
the surface of the sample for the purpose of qualitative or semi-quantitative determination of chemical compositions (using EDS), crystalline structure, and crystal
orientations (using EBSD).
Most of the studies in nanotechnology research and related fields require extensive utilization of scanning electron microscope to quantify and qualify the surface
structures of developed materials. Figure 19 presents the images of carbon nanotubes
and CoPt multilayer nanowires (Kohl and Reimer 2008), silicon germanium (SiGe),
nanocantilevers (Asafa et al. 2013a, b), and nanosized silicon (Si) (Durowoju
et al. 2019). In these images, the structure of the materials, orientation, size, and
distribution are well quantified.