Glossary

Abrasive Media

Extremely hard materials (Diamond, SiC, Al2O3, etc.), usually in a very fine particulate form [<15 micrometers], used to grind or polish specimens to desired thickness or finish.

Å:Angstrom

A metric unit of length measurement:

  • 1 x 10-10 meters or 0.0000000001 meters
  • 4 x 10-9 inches or 0.000000004 inches
  • Atomic diameters range between 1 and 2 Angstroms

Aperture

A thin (<100 micrometer thick) disk or strip of metal (usually Pt) with a small (2-100 micrometers) circular through-hole. Used to restrict electron beams and filter out unwanted scattered electrons before image formation.

Bright Field Imaging

A imaging mode in a TEM that uses only Unscattered Electrons to form the image. Contrast in such an image is due entirely to thickness and density variations in a sample.

Backscattered Electron

Produced by an incident electron colliding with an atom in the specimen which is nearly normal to the incident's path. The incident electron is then scattered "backward" 180 degrees with no appreciable loss of energy, an elastic collision.

Backscattered Electron Imaging

The production of backscattered electrons varies directly with the specimen's atomic number. This differing production rates causes higher atomic number elements to appear brighter than lower atomic number elements. This interaction is utilized to differentiate parts of the specimen that have different average atomic number. (See Interaction Volume) An example is shown in the SEM output section, specifically the mechanically alloyed specimen micrograph. Detection of Backscattered Electrons is achieved by using a "donut" shaped solid state semiconductor device mounted on the bottom of the objective lens. When Backscattered Electrons strike the detector electron-hole pairs are created which are then counted. This quantity is translated into a pixel intensity and displayed on the CRT, forming the image. See the SEM page for details on image formation.

CBED: Convergent Beam Electron Diffraction

An electron probe is tightly focused on a TEM specimen and the resulting pattern of diffracted electrons is observed. The patterns contains information on the crystal symmetry and atomic and electronic structure of the sample. Regions as small as 0.2 nm may be examined.

CFEM: Central Facility for Electron Microscopy

A centralized collection of Electron Microscopy resources in Nebraska; for more information visit the Nebraska Center for Materials and Nanoscience webpage.

Characteristic X-ray

An X-ray having a unique energy that is emitted by an atom during its de-energization after ionization of one of its electrons.

Composition

The elements that a specimen is composed of and their relative ratios.

Crystal Structure

The order in which atoms are arranged in a material. These arrangements have a direct effect on the physical properties of the material. These arrangements commonly take the form of cubes, rectangular solids, hexagonal solids, etc.

Dark Field Imaging

Using a single diffracted beam to form the image in a TEM. This causes all regions of the specimen not of the same crystal structure and orientation as the region which produced the diffracted beam to be represented as very dark in the final image; allowing phase differentiation visually in the TEM.

Diffraction

Periodic scattering of moving objects when they collide with an orderly pattern of fixed objects. Diffraction always follows Bragg's Law:

nλ = 2d sinθ

λ - wavelength
d - spacing of fixed objects
θ - angle of scattering
n - is an integer

This periodic scattering is useful in determining the spacing of the fixed objects since all other variables in the equation are know. In Electron Microscopy the spacings being determined are those between atoms in a lattice or crystal.

ECP: Electron Channeling Patterns

A pattern formed by the periodic backscattering of electrons by the specimen lattice in a SEM. Allows determination of crystal structure and lattice parameters in crystals >20 micrometers diameter.

EDS: Energy Dispersive Spectroscopy

A technique that is based on the characteristic X-ray peaks that are generated when an energetic electron beam interacts with the specimen. Each element produces characteristic x-rays that may be used to identify the presence of that element in the region being examined. Comparison of the relative intensities of x-ray peaks may be used to determine the relative concentrations of each element in the specimen. Elements with an atomic number less than that of carbon carbon (Z=5) are not generally detectable.

Electron

An sub-atomic particle which allows the atom to react with its environment and form bonds with other atoms. Electrons and their orbits around the nucleus of an atom account for most of the physical properties of the element. Used extensively in Electron Microscopy.

Electron Gun

A cathode/anode device intended to produce a stream of electrons. A more detailed explanation and diagram is available.

Emergency Operating Procedure

Written instructions detailing procedures that are to be carried out in case of any emergency (fire, explosion, chemical spill, etc.) for a specific piece of equipment.

Hi-resolution Imaging

Direct imaging of the specimens lattice in a HRTEM. This is accomplished by allowing some of the diffraction image to overlay the bright field image, enhancing the contrast along the lattice lines. Allows direct measurement of lattice parameters, inspection of individual defects and grain orientation.

Ion

An atom in a charged state, either having a deficiency (a positive charge) or a surplus (having a negative charge).

Lattice

An orderly arrangement of atoms in a material, usually thought of as an infinite series of "cells" forming a "latticework" of atoms in 3-dimensions. A thorough treatment of lattices and other materials issues can be found at the Visualizations in Materials Science WWW site.

Magnetic Lens

Circular electro-magnets capable of projecting a precise circular magnetic field in a specified region. The field acts like an optical lens, having the same attributes (focal length, angle of divergence, etc.) and errors (spherical aberration, chromatic aberration, etc.). They are used to focus and steer electrons in an Electron Microscope.

Micrometer: µm

A metric unit of length measurement:

  • 1 x 10-6 meters or 0.000001 meters
  • 4 x 10-5 inches or 0.00004 inches
  • Human hair is approximately 20 micrometers in diameter

Monochromatic

All objects in a group (electrons, light, X-rays, etc.) have the same energy/wavelength.

Morphology

The shape and size of the particles making up the object; direct relation between these structures and materials properties (ductility, strength, reactivity, etc.).

NCMN: Nebraska Center for Materials and Nanoscience

A research establishment founded by the state of Nebraska to promote materials research; more detailed information is available at www.unl.edu/ncmn.

nm: Nanometer

A metric unit of length measurement:

  • 1 x 10-9 meters or 0.000000001 meters
  • 4 x 10-8 inches or 0.00000004 inches
  • about 10 average atoms in a lattice

Phase

A specific crystal structure, usually given a name.

Secondary Electron

Produced by an incident electron passing "near" an atom in the specimen, near enough to impart some of its energy to a lower energy electron (usually in the K-shell). This causes a slight energy loss and path change in the incident electron and the ionization of the electron in the specimen atom. This ionized electron then leaves the atom with a very small kinetic energy (5eV) and is then termed a "secondary electron". Each incident electron can produce several secondary electrons.

Secondary Electron Imaging

Production of secondary electrons is very topography related. Due to their low energy, 5eV, only secondaries that are very near the surface (<10nm, see Interaction Volume) can exit the sample and be examined. Any changes in topography in the sample that are larger than this sampling depth will change the yield of secondaries due to collection efficiencies. Collection of these electrons is aided by using a "collector" in conjunction with the secondary electron detector. The collector is a grid or mesh with a +100V potential applied to it which is placed in front of the detector, attracting the negatively charged secondary electrons to it which then pass through the grid-holes and into the detector to be counted. When a Secondary Electrons collide with the solid-state semiconductor detector an electron-hole pairs are created which are then counted. This quantity is translated into a pixel intensity and displayed on the CRT, forming the image. See the SEM page for details on image formation.

SAED: Selected Area Electron Diffraction

An aperture is used to define the area from which a diffraction pattern is formed in a TEM specimen. The resulting patterns contain information about phases present (lattice spacing measurement) and sample orientation.

Specimen Interaction

Reactions that occur inside the specimen when being struck with a beam of energetic electrons. A more detailed explanation and diagram is available.

Specimen Interaction Volume

The volume inside the specimen in which all specimen interactions occur during electron beam irradiation. A more detailed explanation and diagram is available.

Standard Operating Procedure

A written set of instructions detailing the normal procedures for operating a piece of equipment.

Topography

The surface features of an object or "how it looks" its texture; direct relation between these features and materials properties (hardness, reflectivity, etc.).

Torr

A unit of measure of pressure, 760 Torr is standard pressure at sea level.

UNL: University of Nebraska–Lincoln

The main campus for the University of Nebraska. For more information, visit www.unl.edu.