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

Recent Developments 183
one type of gun monochromator, in this case a Wien filter in which a
magnetic field Bx and an electric field Ey are applied perpendicular to the
beam, such that they introduce equal and opposite forces on the electron.
The net force is zero when
(�e)vBx + (�e)Ey = 0 (7.1)
Equation (7.1) is satisfied only for a single electron speed v; faster or slower
electrons are deflected to one side and removed by the energy-selecting slit.
A convenient feature of the Wien filter is that the electrons allowed through
it travel in a straight line, unlike a magnetic prism, which changes the
direction of the optic axis.
Monochromators are expected to be useful for revealing additional fine
structure in an energy-loss spectrum, as illustrated in Fig. 7-4. This fine
structure can sometimes be used as a “fingerprint” to identify the chemical
nature of very small volumes of material. In addition, good energy resolution
is important for resolving energy losses below 5 eV, in order to measure the
energy gap (between valence and conduction bands) in a semiconductor or
insulator and to investigate energy levels within the gap, which may be
associated with localized defects such as dislocations.
Figure 7-4. Part of the electron energy-loss spectrum recorded from a vanadium pentoxide
sample using several FEI microscopes. The CM20 has a thermionic electron source, giving an
energy spread of about 1.5 eV. The TF20 has a field-emission source and when fitted with a
gun monochromator, it achieves an energy resolution below 0.2 eV, revealing additional fine
structure just above the vanadium L-ionization threshold (� 515 eV). Although the energy
resolution obtained by x-ray absorption spectroscopy (using synchrotron radiation) is even
better, the TEM offers greatly superior spatial resolution. From D.S. Su et al., Micron 34
(2003) 235�238, courtesy of Elsevier Ltd. and F. Hofer, Technical University of Graz.