Page 154 - Materials Science and Engineering An Introduction

126 • Chapter 4 / Imperfections in Solids

Transmission Electron Microscopy

transmission electron The image seen with a transmission electron microscope (TEM) is formed by
microscope (TEM) an electron beam that passes through the specimen. Details of internal microstruc-
tural features are accessible to observation; contrasts in the image are produced by
differences in beam scattering or diffraction produced between various elements of
the microstructure or defect. Because solid materials are highly absorptive to elec-
tron beams, a specimen to be examined must be prepared in the form of a very thin
foil; this ensures transmission through the specimen of an appreciable fraction of
the incident beam. The transmitted beam is projected onto a fluorescent screen or
a photographic film so that the image may be viewed. Magnifications approaching
1,000,000 are possible with transmission electron microscopy, which is frequently
used to study dislocations.
Scanning Electron Microscopy
scanning electron A more recent and extremely useful investigative tool is the scanning electron
microscope (SEM) microscope (SEM). The surface of a specimen to be examined is scanned with an
electron beam, and the reflected (or back-scattered) beam of electrons is collected and
then displayed at the same scanning rate on a cathode ray tube (CRT; similar to a CRT
television screen). The image on the screen, which may be photographed, represents
the surface features of the specimen. The surface may or may not be polished and
etched, but it must be electrically conductive; a very thin metallic surface coating must
be applied to nonconductive materials. Magnifications ranging from 10 to in excess of
50,000 are possible, as are also very great depths of field. Accessory equipment permits
qualitative and semiquantitative analysis of the elemental composition of very localized
surface areas.

Scanning Probe Microscopy
In the past two decades, the field of microscopy has experienced a revolution with
scanning probe the development of a new family of scanning probe microscopes. The scanning probe
microscope (SPM) microscope (SPM), of which there are several varieties, differs from optical and electron
microscopes in that neither light nor electrons are used to form an image. Rather, the
microscope generates a topographical map, on an atomic scale, that is a representation
of surface features and characteristics of the specimen being examined. Some of the
features that differentiate the SPM from other microscopic techniques are as follows:

• Examination on the nanometer scale is possible inasmuch as magnii cations as
9
high as 10 are possible; much better resolutions are attainable than with other
microscopic techniques.

• Three-dimensional magniied images are generated that provide topographical
information about features of interest.
• Some SPMs may be operated in a variety of environments (e.g., vacuum, air,
liquid); thus, a particular specimen may be examined in its most suitable
environment.

Scanning probe microscopes employ a tiny probe with a very sharp tip that is
brought into very close proximity (i.e., to within on the order of a nanometer) of the
specimen surface. This probe is then raster-scanned across the plane of the surface.
During scanning, the probe experiences deflections perpendicular to this plane in
response to electronic or other interactions between the probe and specimen sur-
face. The in-surface-plane and out-of-plane motions of the probe are controlled by
piezoelectric (Section 18.25) ceramic components that have nanometer resolutions.
Furthermore, these probe movements are monitored electronically and transferred