When particles or surfaces smaller than the resolution of a
light microscope (∼0.15µm) are encountered, an entirely
different radiant source must be utilized. Since the wave-
length of electrons is less by a factor of about 10−5 than
the wavelength of visible light, it follows that a micro-
scope using a beam of electrons as a source might allow
visualization of much finer detail (i.e., better resolution).
Since electrons are electronegative, they can be focused by
means of electromagnetic or electrostatic lenses. Various
types of electron beam microscopes have been developed,
including the TEM, SEM, STEM, electron probe microan-
alyzer (EPMA), and the field emission microscope, two of
which are discussed: the transmission electron microscope
(TEM) and the scanning electron microscope (SEM).
1. Transmission Electron Microscope
The TEM operates exactly as its name implies: a beam of
electrons is transmitted through the sample. The illuminat-
ing beam of electrons is emitted by a thermionic source,
usually a fine tungsten wire electrically heated at high volt-
ages under fairly high vacuum, typically 10−5 Torr. The
beam then is passed through a series of lenses analogous
to the lens system of a compound microscope. When the
beam passes through the sample, crystal diffraction by thesample causes some of the incident beam to be scattered
away from the normal and some to be absorbed by the sam-
ple. The attenuated beam is then passed through another
series of lenses, known as projector lenses, whose strength
determines the final magnification. This final image is ob-
served directly on a phosphor-coated screen. Resolution
is dependent on wavelength, which, in turn, is a function
of the accelerating voltage. A beam of short wavelength
corresponding to 100 or more kV yields better resolution.
Commercial instruments are available with accelerating
voltages, typically of 100 to 200 kV and some as high as
10,000 kV. It is not uncommon to have resolution on the
order of 2–3 A and magni ˚ fications above 10,000,000×.
Chemical information which can be furnished directly by
the TEM is a diffraction pattern observed at the back focal
plane of the objective lens of the microscope. This pat-
tern is identical in nature to that generated in an X-ray
diffraction camera and it is possible to identify crystalline
materials in this manner. TEM is frequently used to ana-
lyze asbestos collected from water or air.
Sample preparation for TEM is an art, and extreme
care must be taken during sample manipulation to avoid
cross-contamination. Bulk samples must either be crushed
or be sectioned with an ultramicrotome to get samples no
thicker than a few hundred Angstroms. Generally with ˚
crystalline preparations, the sample should merely be
crushed finely and dispersed on a thin carbon film sup-
ported on a 3-mm grid.
2. Scanning Electron Microscope
In contrast to the TEM, the SEM can easily observe nearly
any reasonably sized sample and furnish the chemical mi-
croscopist with much information with little effort. Pri-
mary signals generated in an SEM are secondary elec-
trons (SE), backscattered electrons (BSE), characteristic
X rays, X-ray continuum, Auger electrons (AE), low-loss
electrons (LLE), electron energy loss (EEL), and trans-
mitted electrons. Of value to the chemical microscopist
are primarily the SE, BSE, and X-ray spectrum.
The SEM is related to the TEM only by virtue of the fact
that it also employs a beam of focused electrons to bom-
bard a sample and generate an image; however, once the
beam of electrons passes through the condenser lenses, the
similarity ends. After the beam has been passed through
a final aperture, a set of scanning coils deflects the beam
at various rates across the sample.
The striking three-dimensional images which have pop-
ularized the SEM are the result of secondary electrons
ejected from the surface of the sample by the bombarding
primary electron beam. The SE image is collected by a
scintillator or phosphor-coated light pipe which transmits
the signal to a photomultiplier and finally to a viewing
CRT, which scans simultaneously with the primary elec-
tron beam. Since the area scanned is quite small in rela-
tion to the area of the viewing CRT, magnification is thus
achieved and can be varied by changing the size of the area
scanned across the sample. Resolution and magnification
in the SEM are not as good as with the TEM: commercial
SEMs now routinely work at 60-A resolution and magni ˚ fi-
cations of 500,000× are achievable although the theoreti-
cal MUM is ∼20,000×. Most work on an SEM, however,
is below 5000×, and very high resolution is rarely im-
portant. The lower resolution of the SEM is due partly to
lower accelerating voltages in the SEM (0.5–40 kV), but
mostly to scattering effects of various types of radiation
occurring below the surface of the sample with electrons
and X-rays emerging from an area larger in diameter than
that of the primary beam. Varying the accelerating volt-
age also changes the depth to which the primary beam
penetrates the sample, usually from 1 to 15µm.
The SEM can reveal much chemical information about a
sample; the three-dimensional images can aid in the study
of crystalline material and sample morphology and BSE
images can often be used to determine areas of varying
composition. The signals of most import to the chemical
microscopist, however, are X-rays since their wavelengths
or energies can be measured, thus identifying the individ-
ual chemical elements in the sample.
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