(2011!12!02) HRTEM and CBED for Analysis Materials

8/3/2019 (2011!12!02) HRTEM and CBED for Analysis Materials

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Physics departmentPhysics department

Principle & practice of transmission electron

diffraction.

Tran Ngoc Cuong

Physics department.,

Dongguk University, KOREA

Date: 2011/12/03


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Principal operation

Transmission electron microscopy (TEM) is a microscopy technique whereby a

beam of electrons is transmitted through an ultra thin specimen, interacting with the

specimen as it passes through. An image is formed from the interaction of the

electrons transmitted through the specimen; the image is magnified and focused ontoan imaging device, such as a fluorescent screen, on a layer of photographic film, or

to be detected by a sensor such as a CCD camera.

The operating modes:

Transmission electron microscopy

Scanning electron microscopy

Scanning TEM


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Components of the TEM

optical components in a basic TEM

Electron gun: connecting this to a high voltage

source (typically ~100-300 kV).

Aperture: mechanical devices which allow for

the selection of different aperture sizes.

Specimen port: high vacuum, TEM grid sizes

is a 3.05 mm diameter ring.

Objective lens focuses on the specimen.

Projector lens: transfers the diffraction pattern

onto the viewing screen.


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Three basic operating modes of an electron microscope

Three types of TEM contrast for a specimen including a crystalline region (left half) and

an amorphous region(right half).

Three microstructural characterization techniques known as Selected Area Electron

Diffraction (SAED), Conventional Transmission Electron Microscopy (CTEM) and

High-Resolution Transmission Electron Microscopy (HRTEM).

(a) bright-field image (b) dark-field image (c) phase contrast, high-resolution image


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Take selected-area diffraction pattern

(a) Image formation (a) For selected are diffraction

Alter strength of

intermediate lens and

focus diffraction pattern

on to screen

By tilting a crystalline

sample to low-index

zone axes, SAED

patterns can be used to

identify crystal

structures and measure

lattice parameters.


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Convergent-beam electron diffraction

Basic principle

Convergent beam electron diffraction, generally referred to as CBED, is one of the

most powerful techniques for the determination of crystal structure in the field of

transmission electron microscopy (TEM).

The geometry of electron diffraction is different in these two modes, namely

SAED and CBED


7/11Physics departmentPhysics department

Convergent-beam electron diffraction

Instead of parallel illumination with selected-area aperture, CBED uses highly

converged illumination to select a much smaller specimen region

(a) SAED (a) CBED

Diffraction spots of the SAED are enlarged

into CBED discs.



Comparing SAED and CBED

SAED consists of a set of spots CBED consists of a set of discs



Effect of Convergence Angle

Each spot becomes a disc which variations in intensity, they contain a wealth of

information about the symmetry and thickness of the crystal and are widely used in TEM

The electrons are scattered though

2 B.

Electrons are scattered from all the

directions in the convergent conical

illumination.

Each point in the direct beam disc is

one direction of illumination so each

point in the disc can be scattered by

the same 2 B.



Advanced CBED

Patterns from dynamical scattering in direct and diffraction discs allow determination

of:

- Polarity of non-centrosymmetric crystals

- Sample thickness

simulated and experimental CBED patterns for GaN and ZnO

g = hkl means that the (hkl) reflection satisfies its Bragg condition



Advanced CBED

Inelastic scattering which can go

in any direction

Elastic scattering which can go

only in specific directions.

Documents

(2011!12!02) HRTEM and CBED for Analysis Materials