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

120 Chapter 4
At this stage it is convenient to cut a 3-mm-diameter disk out of the slice.
For reasonably soft metals (e.g., aluminum, copper), a mechanical punch
may be used. For harder materials, an ultrasonic drill is necessary; it
consists of a thin-walled tube (internal diameter = 3 mm) attached to a
piezoelectric crystal that changes slightly in length when a voltage is applied
between electrodes on its surfaces; see Fig. 4-20a. The tube is lowered onto
the slice, which has been bonded to a solid support using by heat-setting wax
and its top surface pre-coated with a silicon-carbide slurry (SiC powder
mixed with water). Upon applying a high-frequency ac voltage, the tube
oscillates vertically thousands of times per second, driving SiC particles
against the sample and cutting an annular groove in the slice. When the slice
has been cut through completely, the wax is melted and the 3-mm disk is
retrieved with tweezers.
The disk is then thinned further by polishing with abrasive paper (coated
with diamond or silicon carbide particles) or by using a dimple grinder. In
the latter case (Fig. 4-20b), a metal wheel rotates rapidly against the surface
(covered with a SiC slurry or diamond paste) while the specimen disk is
rotated slowly about a vertical axis. The result is a dimpled specimen whose
thickness is 10 � 50 �m at the center but greater (100 � 400 �m) at the
outside, which provides the mechanical strength needed for easy handling.
In the case of biological tissue, a common procedure is to use an
ultramicrotome to directly cut slices � 100 nm or more in thickness. The
tissue block is lowered onto a glass or diamond knife that cleaves the
material apart (Fig. 4-20c). The ultramicrotome can also be used to cut thin
slices of the softer metals, such as aluminum.
Some inorganic materials (e.g., graphite, mica) have a layered structure,
with weak forces between sheets of atoms, and are readily cleaved apart by
inserting the tip of a knife blade. Repeated cleavage, achieved by attaching
adhesive tape to each surface and pulling apart, can reduce the thickness to
below 1 �m. After dissolving the adhesive, thin flakes are mounted on 3-mm
TEM grids. Other materials, such as silicon, can sometimes be persuaded to
cleave along a plane cut at a small angle relative to a natural
(crystallographic) cleavage plane. If so, a second cleavage along a
crystallographic plane results in a wedge of material whose thin end is
transparent to electrons. Mechanical cleavage is also involved when a
material is ground into a fine powder, using a pestle and mortar, for example
(Fig. 4-20d). The result is a fine powder, whose particles or flakes may be
thin enough for electron transmission. They are dispersed onto a 3-mm TEM
grid covered with a thin carbon film (sometimes containing small holes so
that some particles are supported only at their edges).