presence of an adsorbate, the qualitative analysis may reveal information about
the size and rotational alignment of the adsorbate unit cell with respect to the
substrate unit cell.
2. Quantitatively, where the intensities of diffracted beams are recorded as a function of incident electron beam energy to generate the so-called I-V curves. By
comparison with theoretical curves, these may provide accurate information on
atomic positions on the surface at hand.
Reflection high-energy electron diffraction (RHEED) is a technique used to
characterise the surface of crystalline materials. RHEED systems gather information
only from the surface layer of the sample, which distinguishes RHEED from other
characterisation methods for materials that also rely on diffraction of high-energy
electrons.
7.5 Electron Microscopy
An electron microscope is a microscope that uses a beam of accelerated electrons as
a source of illumination. As the wavelength of an electron can be up to 100,000 times
shorter than that of visible light photons, electron microscopes have a higher
resolving power than light microscopes and can reveal the structure of smaller
objects but not quite at the atomic scale. Electron microscopes use shaped magnetic
fields to form electron optical lens systems that are analogous to the glass lenses of
an optical light microscope. Electron microscopes are used to investigate the ultrastructure of a wide range of biological and inorganic specimens including microorganisms, cells, large molecules, biopsy samples, metals and crystals. Industrially,
electron microscopes are often used for quality control and failure analysis. The
technique was pioneered in 1933 by Ernst Ruska, and he was awarded a Nobel Prize
in 1986. Modern electron microscopes produce electron micrographs using
specialised digital cameras and frame grabbers to capture the images. The transmission electron microscope (TEM) uses a high-voltage electron beam to illuminate the
specimen and create an image. The electron beam is produced by an electron gun,
commonly fitted with a tungsten filament cathode as the electron source. The
electron beam is accelerated by an anode typically at +100 keV (40–400 keV)
with respect to the cathode, focused by electrostatic and electromagnetic lenses
and transmitted through the specimen that is in part transparent to electrons and in
part scatters them out of the beam. When it emerges from the specimen, the electron
beam carries information about the structure of the specimen that is magnified by the
objective lens system of the microscope. The spatial variation in this information
(the “image”) may be viewed by projecting the magnified electron image onto a
fluorescent viewing screen coated with a phosphor or scintillator material such as
zinc sulphide. Alternatively, the image can be photographically recorded by exposing a photographic film or plate directly to the electron beam, or a high-resolution
phosphor may be coupled by means of a lens optical system or a fibre optic light
Early History of X-Ray Crystallography
35
the size and rotational alignment of the adsorbate unit cell with respect to the
substrate unit cell.
2. Quantitatively, where the intensities of diffracted beams are recorded as a function of incident electron beam energy to generate the so-called I-V curves. By
comparison with theoretical curves, these may provide accurate information on
atomic positions on the surface at hand.
Reflection high-energy electron diffraction (RHEED) is a technique used to
characterise the surface of crystalline materials. RHEED systems gather information
only from the surface layer of the sample, which distinguishes RHEED from other
characterisation methods for materials that also rely on diffraction of high-energy
electrons.
7.5 Electron Microscopy
An electron microscope is a microscope that uses a beam of accelerated electrons as
a source of illumination. As the wavelength of an electron can be up to 100,000 times
shorter than that of visible light photons, electron microscopes have a higher
resolving power than light microscopes and can reveal the structure of smaller
objects but not quite at the atomic scale. Electron microscopes use shaped magnetic
fields to form electron optical lens systems that are analogous to the glass lenses of
an optical light microscope. Electron microscopes are used to investigate the ultrastructure of a wide range of biological and inorganic specimens including microorganisms, cells, large molecules, biopsy samples, metals and crystals. Industrially,
electron microscopes are often used for quality control and failure analysis. The
technique was pioneered in 1933 by Ernst Ruska, and he was awarded a Nobel Prize
in 1986. Modern electron microscopes produce electron micrographs using
specialised digital cameras and frame grabbers to capture the images. The transmission electron microscope (TEM) uses a high-voltage electron beam to illuminate the
specimen and create an image. The electron beam is produced by an electron gun,
commonly fitted with a tungsten filament cathode as the electron source. The
electron beam is accelerated by an anode typically at +100 keV (40–400 keV)
with respect to the cathode, focused by electrostatic and electromagnetic lenses
and transmitted through the specimen that is in part transparent to electrons and in
part scatters them out of the beam. When it emerges from the specimen, the electron
beam carries information about the structure of the specimen that is magnified by the
objective lens system of the microscope. The spatial variation in this information
(the “image”) may be viewed by projecting the magnified electron image onto a
fluorescent viewing screen coated with a phosphor or scintillator material such as
zinc sulphide. Alternatively, the image can be photographically recorded by exposing a photographic film or plate directly to the electron beam, or a high-resolution
phosphor may be coupled by means of a lens optical system or a fibre optic light
Early History of X-Ray Crystallography
35