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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 .