TEM Flashcards
reasons to index a diffraction pattern and the utility of knowing directions
- crystal structure information including crystallinity, lattice parameters, symmetry, grain size, fabric
- directions give proper phase identification and can give clues to thermodynamic origins
how to obtain highest practical resolution
highest acceleration voltage with the smallest spherical aberration coefficient
do the laue equations or braggs law better describe electron diffraction?
we typically use Braggs law to describe a special condition of the Laue equations which describe the process of elastic scattering in transmission. Braggs law does not properly represent electron diffraction because it assumes scattering at a glancing angle. Laue’s equations take into account the the agnle of incidence and angle of the diffracted beam
how to get thickness from an EELS spectrum
log of the total intensity in the low loss region divided by the intensity of the zero loss peak multiplied, multiplied by the mean free path (
Bragg’s law
2 d sin (theta) = n lambda
3 systems of a TEM
illumination (gun and condenser lenses), image forming system (objective lenses), imaging recording lenses (projector lenses)
astigmatism
nonuniformity of lens distorts probe into an ellipse
occurs when electrons are affected by a non-uniform magnetic field as they spiral
spherical aberrations
inability of the lens to focus electron beam in the same image plane, smears out point into a disc. electrons that pass through the lens further from the center are refracted more than those passing through the center
chromatic aberrations
energy spread of beam and as electrons scatter into sample, close to the focal length of the lens, caused by gun and sample, only relevant after 5th order Cs correction. slower electrons are diffracted more strongly than nominal electrons
electromagnetic radiation of different energies converging at different focal planes
eucentric position
horizontal center of the objective lens, point where the sample should not translate while tilting in alpha
where are diffraction patterns formed?
back focal plane of the objective lens
selected area electron diffraction
a field-limiting aperture that is placed into the image plane of the objective lens to create a virtual aperture above the sample
imaging type in TEM mode
bright field imaging in TEM mode does not allow for constructive interference, giving us amplitude contrast
diffraction vs imaging mode in TEM mode
in diffraction mode, the BFP of the objective lens is the object plane of the intermediate lens
in image mode, the image plane of the objective lens is the object plane of the intermediate lens
relationship between camera length and DP
Rd=lambdaL
diffraction pattern
records the distribution of scattered electrons around the forward scattered beam, gives d-spacings of cell
atomic scattering factor
measure of the scattering amplitude of electrons scattered from isolated atom, depends on scattering angle, lambda, and atomic number
structure factor
tells us if to expect reflections in our DP based on the symmetry of the unit cell, measure of the amplitude scattered by the unit cell of a crystal. amplitude of scattering is impacted by the type, position, and atomic planes of the atom
fluorescence yield
ratio of x-ray emissions to inner shell ionization events
plasmons
incident electrons cause oscillations in the free electron gas
phonons
shaking of the lattice as a result of striking by incident electrons causes lattice to heat up due to vibrations
3 types of beam damage
- radiolysis: breaking of chemical bonds
- knock-on damage: displacement of atoms from their preferred lattice positions, resulting in point defects
- heating due to phonons
dead time
how long detector is shut off to evaluate the charge pulse
Cliff-Lorimer method
concentration of element A (in wt%) over element B is equal to the sensitivity factor of the elements times the intensity of element A over element B