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

174 Chapter 6
A typical energy-loss spectrum (Fig. 6-10) contains a zero-loss peak,
representing electrons that were scattered elastically or remained unscattered
while passing through the specimen. Below 50 eV, one or more peaks
represent inelastic scattering by outer-shell (valence or conduction) electrons
in the specimen. This scattering can take the form of a collective oscillation
(resonance) of many outer-shell electrons, known as a plasmon excitation.
Inelastic excitation of inner-shell electrons causes an abrupt increase in
electron intensity (an ionization edge) at an energy loss equal to an innershell
ionization energy. Because this ionization energy is characteristic of a
particular chemical element and is known for every electron shell, the energy
of each ionization edge indicates which elements are present within the
specimen. Beyond each ionization threshold, the spectral intensity decays
more gradually toward the (extrapolated) pre-edge background that arises
from electron shells of lower ionization energy. The energy-loss intensity
can be integrated over a region of typically 50 � 100 eV and the background
component subtracted to give a signal IA that is proportional to the
concentration of the element A that gives rise to the edge. In fact, the
concentration ratio of two different elements (A , B) is given by:
nA/nB = (IA/IB) (�B /�A) (6.11)
where �A and �B are ionization cross sections that can be calculated,
knowing the atomic number, type of shell, and the integration range used for
each
element.
Because the magnetic prism has focusing properties, an electron
spectrometer can be incorporated into the TEM imaging system and used to
form an image from electrons that have undergone a particular energy loss in
the specimen. Choosing this energy loss to correspond to the ionization edge
of a known element, it is possible to form an elemental map that represents
the
distribution of that element, with a spatial resolution down to about 1 nm.
Other information is present in the low-loss region of the spectrum
(below 50 eV). In a single-scattering approximation, the amount of inelastic
scattering is proportional to specimen thickness, as in Eq. (4.16). Therefore,
the intensity of the plasmon peak relative to the zero-loss peak (Fig. 6-10)
can be used to measure the local thickness at a known location in a TEM
specimen.
Also, the energy-loss spectrum contains detailed information about the
atomic arrangement and chemical bonding in the specimen, present in the
form of fine structure in the low loss region and the ionization edges. For
example, the intensities of the two sharp peaks at the onset of the Ba M-edge
in Fig. 6-10 can be used to determine the charge state (valency) of the
barium atoms. For further details, see Brydson (2001) and Egerton (1996).