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Thanks for your patience

Electron Microscopy for Material

Characterization

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Outline

Introduction to Electron Microscopy

Why Electron Microscopy came in existence?

Physical Principle Involved Components of Electron Microscope

Comparison of LM, TEM and SEM

Scanning Electron Microscope -Overview

Transmission Electron Microscope Overview

Disadvantages

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Introduction to Electron Microscopy For imaging of nanoscale objects

Electron microscopes uses electrons instead of

photons

Electrons have a much shorter wavelength than

photons

Observe matter with atomic resolution

Two general types of electron microscopes:1.Scanning Electron Microscope (SEM)

2.Transmission Electron Microscope (TEM)

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Magnifications achievable by the different microscopes

Variation in magnifications achieved by optical,scanning and transmission electron microscope

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Comparison of Transmission and Scanning

Electron Microscopic images

The electron range increases with beam energy.

Fig.e.g.. The internal structure of the Au deposits examined athigh electron beam energies in SEM and TEM.

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Incident high KeV electron beam

Auger Electrons

Characteristic

X-Rays

BremstrahlungSecondary & Backscattered

Electrons for SEM

Visible Light

Direct Beam for TEM

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SEM

Electron beam is focused and scanned over thesample surface

Secondary electrons or back scattered electrons and

characteristic x-rays from each point of interaction is

collected by the detector2-D image of spatial distribution of these signals are

mapped.

TEMTransmitted electron beam (through sample) forms

an enlarged image.

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Instrumentation of Electron Microscope

Source

Condenser lens

Objective lens

Projector lens (TEM)

Suitably prepared specimen

Metallic coating for SEM - conducting

Thin section for TEM - transmission

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Condenser Lens

Collects light to direct it at the small area of theobject.

It makes the object brighter (better contrast). Enables to control the angle at which the

illumination reaches the object.

Converge the light beam on object or canilluminate it with parallel rays.

Condenser aperture: controls the area ofspecimen to be illuminated.

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Comparison of LM, TEM and SEM

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The Scanning Electron Microscope

Produces a 3-dimensional image of specimenssurface features

The interactions of the electrons with surface are

registered, and electrons reflected from surface

create image.

Electron beam is scanned back and forth over

the specimen, imaging only one point at a time.

PE energy is kept relatively low (1-30 keV) to

limit the interaction volume in the specimen so

high sensitivity to surface composition (cannot

penetrate far into the sample).

Can be used for thicker specimens

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Electron Beam Interaction

Primary electrons generate low energysecondary electrons, emphasize the topographicnature of the specimen

Primary electrons backscattered produces

images with a high degree of atomic number(Z) contrast

Ionized atoms can relax by electron shell-to-shell transitions, which lead to either X-rayemission or Auger electron ejection.

The X-rays emitted are characteristic of theelements in the top few m of the sample.

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Scanning Electron Microscopy (SEM) &

Energy-dispersive X-ray Microanalysis

(EDAX)

Impinging electrons interact with the samplesmolecular composition.

The energy of the impinging electrons is inproportion to the type of electron interactiongenerated from the sample.

A series of measurable electron energiesproduced are analyzed -creates a spectrum of

the unique elements .

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SEM

Fig. Schematic presentation of SEM

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Range of Applications of SEM

Classification of materials

Failure and defect analysis

Examination of surface morphology (including

stereo imaging) Analysis and identification of surface and

airborne contamination

Powder morphology, particle size and analysis

Cleaning problems and chemical etching

Welding and joining technology

Paints and coating failures

Identification and elimination of corrosion andoxidization problems.

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Applications of SEM

Fig. The image is of the surfaceof a metal stamper.

Fig.Fracture section through a pellet oftungsten powder sintered and then

sputter coated with an alloy of osmiumand ruthenium.

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EDAX analysis

Fig.Intermetallic particles from an aluminium alloy and

EDAX analysis of an Al-rich intermetallic phase.

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The Transmission Electron Microscope

Characterization of materials crystal structure

and microstructure simultaneously by

diffraction and imaging techniques.

Electrons scatter when they pass through thin

sections of a specimen

Denser regions in specimen, scatter more

electrons and appear darker

Transmitted electrons (those that do not

scatter) are used to produce image

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TEM

Fig. Transmission Electron Microscope

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Common Modes ofOperation of TEM

Bright Field (BF) Microscopy

Selected Area Diffraction

Dark Field (DF)

Weak Beam (Special case of DF)

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Bright field & Dark field Image

Select Direct Beam Select Scattered Electrons

Resultant Image Resultant Image

Bright-Field Image Dark-Field Image

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TEM Images

Fig. Bright Field & Dark field images

of a grain in TEM

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TEM Sample Preparation

Bulk ceramics

Mechanical grinding, Polishing, Focused ion

thinning

Metals

Mechanical Grinding, Polishing,Electrolytic thinning

Organic MaterialsFreeze drying, Ultramicrotomy (cryo)

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Diffraction Pattern In TEM

Electrons through specimen are diffracted

according to Bragg's law,

n = 2d sin ,

forming a diffraction pattern.

Diffraction pattern is Fourier transform of the

periodic crystal lattice. Information on the periodicities in the lattice,

and hence the atomic positions.

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Diffraction pattern in TEM

Diffraction pattern :

The planar section of the reciprocal lattice

perpendicular to beam direction. Also the zone of planes appearing as spots in the

pattern.

Different types:

1. Kikuchi Patterns2.Convergent-Beam Electron Diffraction (CBED)

3. Ring Patterns

4.Spot Patterns

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Diffraction Pattern in TEM

Ring

Pattern

Kikuchi

Pattern

Spot

PatternCBED

Pattern

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Imaging in TEM

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Disadvantages of EM

Larger ,expensive and destructive technique.

Some materials are sensitive to electron beam

radiation, resulting in a loss of crystallinity and mass

(TEM).

Sample may be damaged by the electron beam,

particularly in the case of biological materials.

Field of view is relatively small- the region analysedmay not be characteristic of the whole sample.

Sample preparation is very time consuming, sample

dimension small .