Field emission scanning electron microscopy a high-resolution ...

198 J. M. Huggett and H. F. Shaw
50-
(nm)
~10
"-I
O
03
CE
1 9
W-hairpin filament
LaBs
FEG
I I I I I I I
5 10 15 20 25 30 35
Accelerating voltage (kV)
FIG. 1. Accelerating voltage vs. resolution for different types of electron source (JEOL, 1994).
narrower range of emitted electron energies than is
obtained with thermal emission guns, permits high
resolution at low kV (Fig. 1). The ability to work at
low kV is particularly valuable for ultra-thin clay
mineral particles which would appear to be
'electron transparent' at high kV. For FESEM
examination it is not always necessary to apply a
conductive coating, particularly if working at very
low kV, but for most clay work 10 kV is about the
optimum and a conductive coating is preferred. The
specimens should have either gold-palladium or
platinum coatings of 30-40 nm thickness as the
gold coating used in conventional SEM is too
coarse and can be observed as a mosaic-like pattern
on the sample surface thus obscuring morphological
features. This is demonstrated by comparing the
micrographs in Figs. 2 and 3. A Polaron 5000 series
coater with a two minute coating time and normal
FIG. 2. FESEM of chlorite-smectite in a Palaeocene mudrock. This sample was coated with gold rather than
platinum and the mosaic-like pattern of the coating is clearly visible on all surfaces.