1 introduction - electron microscopy and diffraction

8/9/2019 1 Introduction - Electron Microscopy and Diffraction

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Do Minh Nghiep

Materials Science Center

Electron MicroscopyElectron Microscopyand Diffractionand Diffraction

1. Introduction1. Introduction


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ContactContact

Lecturer: Minh Nghip

-

Tel. 38691332

e-mail: n hie mail.hut.edu.vn Class time: Mon 14:50-17:20

Class room: D6-106

01/01/2009 Handouts-MSE4346 -K51 KHVL 2


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ourse ou comesourse ou comes

n erstan ng o mage orma on y g ass anelectromagnetic lenses.

.

Understanding of the construction of various types of

electron microscopes, the function of the variousparts and methods of image formation.

Understanding of methods ofsample preparation for

. Ability to utilize EDS and WDS results forelemental

(microchemical) analysis.



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on enon en

Introduction to EM Electron optics

Electron source and vacuum system

Electron-matter interaction SEM

TEM

Electron diffraction

EDS and WDS

01/01/2009 4Handouts-MSE4346 -K51 KHVL


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Lab worksLab works

1. Preparation of an alloy specimen to observeits microstructure by SEM

2. Determination and imaging of chemicalcomposition of the alloy specimen by SEMand EDS

. repara on o an spec men o mageits microstructure and to analyze ED pattern



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ea ngea ng

Textbook: Handouts

References:1. D. B. Williams and C. B. Carter, Transmission electron microscopy, Books

1 to 4, Plenum Press, 1996

2. P. Hirsch, et al.; Electron microscopy of thin crystals; Huntington, N. Y., R.. . .,

3. Elizabeth M. Slayter, Henry S. Slayter; Light and electron microscopy;Cambridge (England), New York, Cambridge University Press, 1992

.formation and microanalysis; Berlin, New York, Springer-Verlag, 1993

5. John M. Cowley; Diffraction physics; Amsterdam, New York: North-HollandPub. Co., New York: Sole distributors for the U.S.A. and Canada, ElsevierNorth-Holland, 1981



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ra ngra ng

The weighting factors used to determinet e na gra e:

Lab work and reports: 20 % Mid-term test and final exam: 70 %



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Week 1 Why electron microscope - a brief history

Week 2 Electron sources, Vacuum

Week 3 Electron optics, Electromagnetic lenses, Resolution limits

Week 4 Electron beam -specimen interactions

Week 5 Mid-term test

Week 6 SEM: Scanning system, Detectors, SE image,

Backscattered image, Resolution - , ,

Diffraction, Phase)

Week 8 Microprobe analysis: Detection systems (EDS, WDS), Qualitative

and quantitative analysis

Week 9 Practical lab for SEM, EDS

Week 10 Electron diffraction, Diffraction patterns

Week 11 Practical lab for TEM, ED

Week 12 Final exam



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IntroductionIntroduction

a er a s c arac er za ona er a s c arac er za on

HistoryHistory Why EM ?Why EM ?

Overview of SEM and TEMOverview of SEM and TEM



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characterizationcharacterization mean?mean?

Character: a sum of qualities that make a person / thing different fromothers

Characterize: to indicate / describe / investigate / express the characterof a person / thing (action)

Materials characterization:

- in specifying the internal microstructure of an engineering materials

including the chemistry, the crystallography, the structural morphology- in terms of engineering properties of materials, and reflects the need toknow the physical, chemical and mechanical properties of the materialsbefore we can design an engineering system or manufacture its components



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characterization is important?characterization is important?

It is believed that optimization of material properties through control of


.


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structure and propertiesstructure and properties

-

Atomic level: Physics and Chemistry

Microscopic level: Chemistry and Materials Sci..

Macroscopic level: Mechanical and Materials Eng.



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of materialsof materials

Mechanical properties

em ca proper es

Physical properties- Thermal property

- Optical property

- Electrical property- Magnetic property



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EquipmentsEquipments

analysisanalysis

molecular levels)molecular levels)



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EquipmentsEquipments

analysisanalysislevels)levels)



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HistorHistor



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Optical/light microscope (OM): visible light limits magnification of max1000-2000X and resolution of 0,2 mm. Electron microscope (EM) is

developed for overcoming these limitations. 1932 - 1938:

- The first TEM (1932, idea of EM, Max Knoll and Ernst Ruska,Germany)

- The first SEM (1938, laterSTEM - TEM with scanning coil, Knoll and,

1940 -1952:- The first SEM for thick sample (1942, Zworykin et al., RCA

a ora or es . . , reso u on mm.- The first Field Emission electron source (1942, FE Gun)- The SEM with resolution of 50 nm (1952, Oatley and McMullan,

ng an



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1959-1967:

- SEM with stereoscan (Wells)- Performance of SE detector (1960, Everhart and

Thornley)- The first commercial EM (1965)- Electron-channeling contrast for crystal orientations

(1967)

[Oatley (1982), J. Appl. Phys. 53, R1]



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1932 theory of EMAntonie van Leeuwenhoek,

th


1986 Nobel Prize winners.,


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James Hillier



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EM19401938 - First SEM is produced



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The prototype of thefirst Stereoscansupplied by the

Company to theduPont Company,

U.S.A. (Stewart and

McMullans originalmicroscope



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Wh EM ?Wh EM ?



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e n one n on

Microscope - A device with a lens or series of lensesthat enlarge (magnify) the appearance of an object.

Image - Perception of an object using your eyes (vision).

One can sense an object without vision (touch, etc..).Requires visible light.

Lens - A lens is an optical componentwhich is used to

.made of a glassy material, whereas non-uniformelectromagnetic fields are used as lens for electrons.



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Ma nification - The ratio between ima e size to the

object size. Can be varied by changing the distancebetween the object and the final lens (of the eye) or by.

Wavelength - Distance between peaks of the waveform.



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Resolution

- RP is the smallest

points at which two or moreobjects can be

distin uished as se arate.

- Resolution is the abilityofa lens to distinguish

at infinity, when they areviewed in the image plane.



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In 1870, Ernst Abbe (1840-1905) derived mathematical

RP (1/2)

expression for resolution of microscope: Resolution is

limited to 0.5 the wavelength of illuminating source.

, RP= ----------------

NA = n.sin

- . -n - index of refraction - half angle of illumination

increasing the half angle of illumination, b) increasing the refractiveindex of the lens by using Crown glass, and c) decreasing the


.


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Increase sin decr. workin distance incr. size of the lens :

Increase refractive index n (oil refractive index ishigher than air)



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Replacing visible lightReplacing visible lightby electron beamby electron beam



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e sma est stance etween two po nts t at can e

resolved by

uman eye: . - . mm

Light microscope: 0.2 m

SEM: 1-2 nm

TEM: 2

This high resolution is achieved by TEM thanks to theuse of a high energy electron beam (small wavelength).



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ew ng op onsew ng op ons

Sample

thickness

Sampleenvironment

Resolution[m]MagnificationInstrument

crye

ThickAir15-1002-10

Magnifying

glass

ThickAir or Oil0.2200-1300Optical

microscope

cacuum.-

ThinVacuum0.00014103-1.5X106TEM



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ElectronElectron-- dual (wavedual (wave--particle) character:particle) character:Light particlesLight particles -- Matter wavesMatter waves

Hermann Busch (1924): Axial magnetic fields refract electrons

h - Planck constant (6.624 X 10-27 erg/s)

v- electron velocity (p = mv: momentum)V- accelerating voltagemo - rest electron mass


- re a v s c e ec ron mass . x - gram = o pro on


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1/2

e ro g e re at ons pe ro g e re at ons p

Resolution limit of light microscope:

- can ecrease o nm- n.sin is limited to 1.6- Thus the maximum resolution is about 200 nm

Cannot go beyond this even with better optics.Solution ? Use illumination ofshorter wavelength



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eso u on o m croscopeseso u on o m croscopes

Resolution of electron microscope:

- can decrease to 10-3 nm- n.sin is very small, because n 1 and 0.1 radians

- ,of 0.0389 and a theoretical resolution of 0.0195 !

- But in practice most TEMs will only have an actual

resolution 2.4 at 100 kV



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refraction/bendingrefraction/bending

on a ray of illumination entering amedium of differing density causing

.

In the vacuum environment of an electron microsco e the indexof refraction is 1.0 and thereforeNA depends solely on the half

angle of illumination.

In electron microscopy therefractive index cannot exceed

Refractive index:

n = tani/tanr

. , e a ang e s very sma ,and thus the only thing that can beadjusted is decreasing the



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en ng y ensesen ng y enses

Converging (convex/positive) lens: bendsrays toward the axis. It has a positive focallength. Forms a real inverted image of anobject placed to the left of the first focal pointand an erect virtual image of an object placedbetween the first focal point and the lens.

Diverging (concave/negative) lens: bendsthe light rays away from the axis. It has anegative focal length. An object placed

in an erect virtual image. It is not possible toconstruct a negative magnetic lens althoughnegative electrostatic lenses can be made.



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Glass lenses (curved glassor mirror) forvisible light

concaveconcave convexconvex

Electromagnetic lenses(solenoid/coil) for

(electron, protons,positrons)



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Equipments for structural analysisEquipments for structural analysis

m croscop c eve sm croscop c eve s



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ScanningScanning ElectronElectron MicroscopesMicroscopes - SEMVersions: Environmental (ESEM) / Low Voltage (LVSEM) /

ar e ressure eva e ressure ,Field Emission (FESEM)

-Versions: High Resolution (HRTEM), Scanning- (STEM),Field Emission (FETEM), Energy Filtering (EFTEM)

AnalyticalAnalytical ElectronElectron MicroscopeMicroscope - AEMVersions: SEM-EDS/WDS, STEM-EDS-EELS



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Overview of EMsOverview of EMs



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nnin El r nnnin El r nMicroscopeMicroscope -- SEMSEM

SEM is an instrument using electron light to image andcontrol the material sample in very fine scale.

What is imaged and controlled: Surface topography (SE: microstructure roughness) ,

and size) Composition contrast (BSE)

Elemental composition (EDS/WDS: qualitative andquantitative analysis)



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The SEM is a distinctly

image formation than is anoptical microscope and a TEM.

In the SEM, the probeexamines the object one spot at

a time, then gives out an arrayn - o e resu s rommany spots.

The optical microscope, (also

TEM) conversely, takes thesignals from simultaneously-examined spots, and gives


em ac a a once.


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Construction of SEMConstruction of SEM



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Construction of SEMConstruction of SEM

JEOL SEM 6335F



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unc ons o ma n par sunc ons o ma n par s

Electron gun (filament/cathode, Wehnelt

cylinder, anode): generating/ emittingelectron beam

Lens system (condense and objectivelenses): focusing/linking electron beam

Scanning coil: scanning electron beamover specimen surface

Detectors: collecting electrons and/x-rays

Mearurement system (Cathode RayTube-CRT, electronic devices): dataamplifying, acquisition and processing



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ormat on o acce erate e ectron eam towar

specimen by a positive voltage (kV)

narrow beam with electromagnetic lenses andmetallic diaphragms/apertures

Focusing and scanning electron beam intospecimen surface through electromagneticlenses and scannin coil

Beam-specimen interaction Data acquisition and imaging



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In environmental SEMIn environmental SEM

Ionizing of rest gas to eliminate electric charge on- ,

- A low vacuum is used

- -avaiable

- Hydrated specimens are allowed

- Chemical composition is analyzed for onlyspecimen (without coating)



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Surface topography,

econ ary ec ron mag ng(SEI)

(BSI)

Compositional contrast

,

Scannin ElectronScannin Electron

Transmitted Electron Imaging(TEI)

Electron BackscatteredDiffraction (EBSD)

MicroscopeMicroscope(SEM)(SEM)

Internal ultrastructure Energy-Dispersive X-raySpectrometry (EDS)

Crystallographic info


Elemental composition, mapping and linescans


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rosros strengt s :strengt s :

- Higher resolution and depth of field than that of OM (the surface of

- Allows for direct observation of surface morphology- In-situ environmental studies are possible (e.g. catalysts in an

atmosphere)- Microanalysis (composition of small areas)- Typically a low power technique, so organic material can be studied- Very good at looking at an average of sizes and arrangements in a

- Easy operation and maintenance- Negligible damage of specimen (through coating, heating during

radiation



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Pros and cons of SEMPros and cons of SEM

ConsCons (weaknesses/limitations)(weaknesses/limitations)::- Specimens must be suitable in vacuum- False/artificial outputs through sample preparation- Coating non-conductive samples is required

- The crystallinity cannot be determined

- Resolution is often not sufficient to tell all of the surface features

- Scanning process is slow and so sample may move leading to

distorted images



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Electrons path in TEM:

condenser lens(es) and

aperture sample objective lens(es) andaperture projectorlens es screen



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TEM is designed to project a

onto screen (we see only the

shadowof the specimen). Its thanks to contrast(intensity difference) of

ransm e unsca ere anforward diffracted / scattered

beam.



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--

--

Each particle image represents a 2Dprojection of the 3D object

A single projection image isplainly insufficient to infer thestructure of an ob ect.



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--

--

Watch out!

A cover slide!


High ResolutionHigh Resolution


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High ResolutionHigh Resolution

TEMTEM -- HRTEMHRTEMContrast intensit difference on ima e lane:

Mass-thickness contrast (BF imaging)

Diffraction contrast (BF, DF imaging)

Phase contrast / HREM and Moirefringes (HR imaging)

HREM image

Interference pattern



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comparison of parameterscomparison of parameters


TEM ca abilitiesTEM ca abilities


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TEM ca abilitiesTEM ca abilities

Electron DiffractionBright- and Dark-Field Imagingmag ng

Crystallographic info Internal ultrastructure Nanostructure dispersion Defect identification

High-ResolutionTransmission ElectronMicroscopy (HR-TEM)

Electron Energy LossSpectroscopy (EELS)

MicroscopeMicroscope

(TEM)(TEM)

Interface structure Defect structure Energy-DispersiveX-ray Spectrometry

(EDS)

Chemical composition Other bonding info


Elemental composition, mapping and linescans


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SEMSEM High resolution for thick bulk TEMTEM High resolution (1,2-1,5 )samples: 20-50 (mostcommercial SEM) and < 10 Ao

for very thin samples (mostcommercial TEM)

- 3D-imaging due to large

depth of field Information about crystal

structure (crystal lattice ma magn ca on ava a e

as for light microscope

an or en a on,

dislocation, twinning,)


1 introduction - electron microscopy and diffraction

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