Physical Principles of Electron Microscopy: An Introduction to TEM ...

An Introduction to Microscopy 21
1.7 Analytical Electron Microscopy
All of the images seen up to now provide information about the structure of
a specimen, in some cases down to the atomic scale. But often there is a need
for chemical information, such as the local chemical composition. For this
purpose, we require some response from the specimen that is sensitive to the
exact atomic number Z of the atoms. As Z increases, the nuclear charge
increases, drawing electrons closer to the nucleus and changing their energy.
The electrons that are of most use to us are not the outer (valence) electrons
but rather the inner-shell electrons. Because the latter do not take part in
chemical bonding, their energies are unaffected by the surrounding atoms
and remain indicative of the nuclear charge and therefore atomic number.
When an inner-shell electron makes a transition from a higher to a lower
energy level, an x-ray photon is emitted, whose energy (hf = hc/�) is equal to
the difference in the two quantum levels. This property is employed in an xray
tube, where primary electrons bombard a solid target (the anode) and
excite inner-shell electrons to a higher energy. In the de-excitation process,
characteristic x-rays are generated. Similarly, the primary electrons
entering a TEM, SEM, or STEM specimen cause x-ray emission, and by
identifying the wavelengths or photon energies present, we can perform
chemical (more correctly: elemental) analysis. Nowadays, an x-ray emission
spectrometer is a common attachment to a TEM, SEM, or STEM, making
the instrument into an analytical electron microscope (AEM).
Other forms of AEM make use of characteristic-energy Auger electrons
emitted from the specimen, or the primary electrons themselves after they
have traversed a thin specimen and lost characteristic amounts of energy. We
will examine all of these options in Chapter 6.
1.8 Scanning-Probe Microscopes
The raster method of image formation is also employed in a scanning-probe
microscope, where a sharply-pointed tip (the probe) is mechanically scanned
in close proximity to the surface of a specimen in order to sense some local
property. The first such device to achieve really high spatial resolution was
the scanning tunneling microscope (STM) in which a sharp conducting tip
is brought within about 1 nm of the sample and a small potential difference
(� 1 V) is applied. Provided the tip and specimen are electrically conducting,
electrons move between the tip and the specimen by the process of
quantum-mechanical tunneling. This phenomenon is a direct result of the
wavelike characteristics of electrons and is analogous to the leakage of