Transmission Electron Microscope, Slides of Physics

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Direct Imaging of Crystal

Structure Using

Transmission Electron

Microscopy( TEM)

Scale of Structure

Organization

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. TEM Imagining

BF & DF Imaging Isolated individual Gold Atoms around Gold Nanoparticles: (left) dark field image, (right) bright field image.

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 Z^2 t. 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

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

TYPICAL STRUCTURAL FEATURES

STUDIED by TEM

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.