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The energy resolution of conventional EEL spectrometers can be limited by both the aberrations of spectrometers and the energy spread of incident electron beams.
A monochromator for the electron source or data deconvolution is necessary in the frontiers of TEM-EELS if the energy spread of the available electron source in TEM is larger than the intrinsic fine structures of spectra.
For instance, because all the electron sources have significantly larger energy spreads (marked in green in Table 3628), monochromators are essential to obtain an energy resolution of about 0.1 eV in local spectroscopy. In this case, the monochromator is used to reduce the energy width of the incident electron beam by blocking all electrons whose energy deviation exceeds a given limit.
Table 3628. Operating parameters and characteristics of electron sources.
Type of source |
Tungsten
thermionic |
LaB6
thermionic |
Schottky
emission |
Cold field
emission |
|
Material |
W |
LaB6 |
ZrO/W |
W |
|
ds (µm) |
≈40 |
≈10 |
≈0.02 |
≈0.01 |
|
ΔE (eV) |
1.5 |
1.0 |
0.5 |
0.3 |
|
φ (eV) |
4.5 |
2.7 |
2.8 |
4.5 |
|
T (K) |
2700 |
1800 |
1800 |
300 |
|
E (V/m) |
Low |
Low |
≈108 |
>109 |
|
Je (A/m2) |
≈104 |
≈106 |
≈107 |
≈109 |
|
β (Am-2/sr-1) |
≈109 |
≈1010 |
≈1011 |
≈1012 |
|
Vacuum (Pa) |
<10-2 |
<10-4 |
<10-7 |
≈10-8 |
|
Lifetime (hours) |
100 |
1000 |
104 |
104 |
|
** ds -- Effective (or virtual) source
diameter; ΔE -- Energy spread of the electron beam; φ -- Work function; T -- Operating temperature; E -- Electric field; Je -- current density of electron beam; β -- Electron-optical brightness at the cathode.
|
