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    Lecture TEM

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    Kevin McNally
    TEM provides high resolution imaging of internal structure of thin specimens. Electrons have much shorter wavelengths than visible light, allowing resolutions below 1 nm. TEM forms images using scattered or unscattered electrons to reveal features like crystal structures, defects, and nanoparticles. Diffraction patterns provide crystallographic information. Chemical analysis is possible through techniques like EDS and EELS that detect x-rays or energy losses from the specimen. HRTEM can image individual atomic planes through phase contrast between scattered and unscattered electrons. TEM thus enables versatile characterization of materials structure and composition at nanometer and atomic scales.

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    Lecture TEM

    Uploaded by

    Kevin McNally
    141 views20 pages
    TEM provides high resolution imaging of internal structure of thin specimens. Electrons have much shorter wavelengths than visible light, allowing resolutions below 1 nm. TEM forms images using scattered or unscattered electrons to reveal features like crystal structures, defects, and nanoparticles. Diffraction patterns provide crystallographic information. Chemical analysis is possible through techniques like EDS and EELS that detect x-rays or energy losses from the specimen. HRTEM can image individual atomic planes through phase contrast between scattered and unscattered electrons. TEM thus enables versatile characterization of materials structure and composition at nanometer and atomic scales.

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    Lecture on TEM

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    TEM provides high resolution imaging of internal structure of thin specimens. Electrons have much shorter wavelengths than visible light, allowing resolutions below 1 nm. TEM forms images using scattered or unscattered electrons to reveal features like crystal structures, defects, and nanoparticles. Diffraction patterns provide crystallographic information. Chemical analysis is possible through techniques like EDS and EELS that detect x-rays or energy losses from the specimen. HRTEM can image individual atomic planes through phase contrast between scattered and unscattered electrons. TEM thus enables versatile characterization of materials structure and composition at nanometer and atomic scales.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    141 views20 pages

    Lecture TEM

    Uploaded by

    Kevin McNally
    TEM provides high resolution imaging of internal structure of thin specimens. Electrons have much shorter wavelengths than visible light, allowing resolutions below 1 nm. TEM forms images using scattered or unscattered electrons to reveal features like crystal structures, defects, and nanoparticles. Diffraction patterns provide crystallographic information. Chemical analysis is possible through techniques like EDS and EELS that detect x-rays or energy losses from the specimen. HRTEM can image individual atomic planes through phase contrast between scattered and unscattered electrons. TEM thus enables versatile characterization of materials structure and composition at nanometer and atomic scales.

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    © All Rights Reserved
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    Download as pptx, pdf, or txt
    of 20

    CBE 30361

    Science of Engineering Materials


    Transmission Electron Microscopy (TEM)

    Scale of Structure Organization

    Concept of Resolution

    Rayleigh criterion for visible-light


    Microscope states that the smallest
    distance that can be resolved, d, is
    given approximately by:

    l is the wavelength of the radiation, m the refractive


    index of the viewing medium, and b the semi-angle
    of collection of the magnifying lens. For the
    sake of simplicity we can approximate m sin b to unity

    Visible light l=400 nm R= 200nm


    Electrons l=4 pm
    R= 2 pm << atom diameter

    so the resolution is equal to about half


    the wavelength of light.

    Transmission Electron Microscopy:


    what can be done?
    1.

    TEM gives images of internal structure of a


    specimen sufficiently thin (~1000 ) to allow
    transmission of electrons, typically 100-300 kV.
    2. Electrons diffract. Electron diffraction patterns
    give detailed crystallographic information:
    Crystal orientation
    Lattice parameters
    Specimen thickness
    3. Chemical analysis is also possible with
    available analytical attachments for x-ray or electron
    spectroscopy.

    TEM Imagining
    STANDARD TEM IMAGE MODES
    BRIGHT FIELD (BF) IMAGE:
    Only the transmitted beam is allowed to pass through the objective aperture. Image
    is bright where diffraction in specimen is weak.
    DARK FIELD (DF) IMAGE:
    Only one diffracted beam passes through objective aperture.
    Image is dark where diffraction is weak, bright where diffraction is strong.
    LATTICE IMAGE (High Resolution TEM: HRTEM image):
    Interference of transmitted beam (TB) and diffracted beams (DBs) produces an
    image of the crystal lattice.
    DIFFRACTION PATTERN:
    Intermediate lens adjusted to image the diffraction pattern formed in back focal
    plane (BFP) of objective lens.

    BF & DF Imaging

    Isolated individual Gold Atoms


    around Gold Nanoparticles:
    (left) dark field image,
    (right) bright field image.

    HRTEM: Lattice Imaging

    Lattice-resolution imaging of Ni3Al


    The image shows three grains at a resolution
    where the lines are closely related to planes of
    atoms in the crystalline lattice. One grain
    boundary is being depicted as a series of edge
    dislocations.
    HR-TEM image of Ni3Al
    High-resolution transmission electron microscopy
    is capable of resolving individual planes of atoms
    in the crystalline lattice. The lines in this image are
    closely related to planes of atoms in the crystalline lattice.

    MAJOR IMAGE CONTRAST MECHANISMS


    Mass-thickness contrast: scattering out of transmitted beam creates contrast due to
    difference of atomic number (Z) and/or thickness t; scattering is proportional to Z2t.
    Higher-Z or thicker areas are darker in BF. Applicable to crystalline or amorphous
    materials.
    Diffraction contrast: scattering out of transmitted beam creates contrast due to differences
    in diffracted intensity produces contrast for dislocations, grain boundaries, stacking faults,
    second phase particles etc. Strongly diffracting objects are darker in BF. Applicable only to
    crystalline materials.
    Phase contrast: interference between transmitted and diffracted beam produces lattice
    fringes or atomic structure images (typically referred to as HRTEM (high-resolution
    TEM).

    Mass-Thickness Contrast

    Z-Contrast
    What do the Z-contrast of crystals look like?

    (A) TEM BF image of Bi implanted into Si


    formed from the direct beam: defects with
    the Bi implant are shown but there is no
    contrast associated with Bi;
    (B) Z-contrast image: the Bi implanted area
    is bright, but the defects contrast is not
    preserved

    (A) HRTEM of epitaxial Ge on Si with amorphous SiO2 :


    Si and Ge are undistinguishable;
    (B) HR Z- contrast STEM image also show the atom rows
    bur with strong contrast of at the Si-Ge interface.
    We can think re (B) as a direct map of f(q) variation in
    the foil!!

    Diffraction in TEM

    What is it?
    What can we learn from it?
    Why do we see it?
    What determines the scale?

    Comparison X-ray /Electrons:


    An experimentally observed DP showing
    the central intense, direct beam and array
    of diffraction spots from different atomic
    planes.

    - Electrons have a much shorter wavelength than X-ray


    - Electrons are scattered more strongly
    -Electron beams are easily directed
    However, much of electron D follows directly from X-ray D

    Is the specimen crystalline or amorphous?


    If it is crystalline: what are crystallographic characteristics of the specimen?
    Is the specimen mono-crystalline?
    If not what is the grain morphology and grain size distribution?
    Is more than one phase presented, how are they oriented to each other?

    Ring Patterns

    Amorphous (non-crystalline)
    materials give diffuse rings, as in
    (a) above, which is from a thin
    amorphous carbon support film.
    TEM grids with carbon support
    films are available from several
    microscopy suppliers. Crystalline
    materials give sharp rings, as in
    (c).

    Ring Patterns

    The diffraction pattern from a polycrystalline specimen area contains overlapping spot
    patterns from all grains illuminated by the incident beam. If the number of grains is small, we
    see spotty rings. If the number is large (small grain size) we will see smooth continuous
    rings.

    The ring radii are given by the camera formula:

    R=

    lL
    d

    Where d is a crystal d-spacing; l wave


    length of the electron; L constant of the
    TEM.

    Selected Area Diffraction (SAD)


    - Crystallographic structure from particular areas of a sample.
    - Used to distinguish and identify crystalline (and amorphous) phases in a material.

    SAD pattern:10-10 zone axis pattern


    of a hexagonal GaN/cubic GaN
    heterostructure

    The objective lens forms a diffraction pattern in the back focal plane with
    electrons scattered by the sample and combines them to generate an image
    in the image plane (1. intermediate image). Thus, diffraction pattern and
    image are simultaneously present in the TEM. It depends on the
    intermediate lens which of them appears in the plane of the second
    intermediate image and magnified by the projective lens on the viewing
    screen. Switching from real space (image) to reciprocal space (diffraction
    pattern) is easily achieved by changing the strength of the intermediate lens.

    TEM micrographs taken from


    Ti-15Zr-4Nb-4Ta Alloy specimen :
    (a) the bright field image,
    (b) the magnified image of area,
    (c) the selected area diffraction pattern
    (d) the DF image taken with (012)a reflection

    Chemical analysis: EDS


    Energy Dispersive X-ray Spectroscopy (EDS)
    EDS makes use of the X-ray spectrum emitted by a solid sample bombarded
    with a focused beam of electrons to obtain a localized chemical analysis..
    Spatial resolution on the order of probe size (can be as low as 2-3 )

    Representative example of ED spectrum obtained


    on a ~20 micron grain of titano maghemite from
    submarine basalt. EDAX Phoenix EDS system;15 kV.

    Chemical analysis: EDS

    Nickel-based Supper Alloy

    Chemical analysis: EELS


    Electron energy loss spectroscopy (EELS)

    Transmitted electrons lose energy due to plasmon excitation or excitation of atoms by


    ejecting inner-shell electrons. A bending magnet disperses electrons of different
    energies to different positions; a slit selects electrons of desired energy loss.

    Allows for the determination of valence, bond length,


    nearest neighbor co-ordination and quantification
    of atomic species in minerals

    Chemical analysis: EELS


    Electron energy loss spectroscopy - Valence determination

    TYPICAL STRUCTURAL FEATURES


    STUDIED by TEM
    CRYSTAL STRUCTURE
    Single crystal, polycrystalline or amorphous?
    Determine exact orientation of crystal(s).
    Identify crystal structure (diffraction).
    Evaluate crystal quality (lattice imaging).
    CRYSTAL DEFECTS
    Presence or absence of dislocations, stacking faults,
    grain boundaries, twins
    Dislocation Burgers vectors; nature of stacking faults
    SECOND-PHASE PARTICLES
    Size, shape, and distribution; crystallographic orientation
    relative to surrounding matrix, chemical analysis via
    analytical attachments (EDXS, EELS)
    NANOPARTICLES, NANOWIRES, NANORODS etc.
    Size distribution, crystal structure and orientation, crystal
    perfection

    Conclusions
    TEMs comprise a range of different instruments that make use of the
    properties of electrons, both as particles and as waves.
    The TEM generates a tremendous range of signals so we can obtain
    images, DPs, and several different kinds of spectra from the same
    small region of the specimen.
    If you count up the different imaging, diffraction, and spectroscopic
    operations that are available in a TEM there are almost 40 different
    modes of forming an image, DP, or spectrum, each of which produces
    different information about your specimen.
    No other characterization technique comes close to the combination of
    versatility and quantification that is produced by this remarkable
    instrument, particularly when you consider the enormous range of
    magnifications over which the information is obtainable.

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