Download - TRANSMISSION ELECTRON MICROSCOPY
TRANSMISSION ELECTRON MICROSCOPY (TEM)
BY SUTAPA SAHA MSC. INORGANIC CHEMISTRY 2008-2010 UNIVERSITY OF DELHI
INTRODUCTION TO TEM
COMPONENTS & WORKING OF TEM
SAMPLE PREPARATION
TYPES OF TEM
OUTLINE
INTRODUCTION
TEM IS MUCH LIKE SLIDE PROJECTOR BUT THE BASIC DIFFERENCE IS LIGHT MICROSCOPE USES BEAM OF LIGHT WHEREAS TEM USES BEAM OF ELECTRON
MICROSCOPE RESOLUTION MAGNIFICATION
OPTICAL 200 nm 1000x
TEM 0.2 nm 500000x
IN 1931,WHILE CONDUCTING RESEARCH FOR HIS MASTERS AT THE TECHNICAL COLLEGE OF BERLIN,ERNST RUSKA & MAX KNOLL DESIGNED THE FIRST TEM ERNST RUSKA MAX KNOLL
SPECIMEN MUST BE ULTRATHIN
SIMPLE DIAGRAM TO SHOW THE DIFFERENCE BETWEEN SLIDE PROJECTOR & TEM
DIAGRAM TO REPRESENT TEM’S WORKING
Virtual Source
First Condenser Lens
Second Condenser Lens
Condenser Aperture
Objective Aperture
Selected Area Aperture
Objective Lens
First Intermediate Lens
Second Intermediate Lens
Projector Lens
Main Screen (Phosphor)
Sample
DIFFERENT COMPONENTS OF TEM
1. HIGH TENSION CABLE
2. ELECTRON EMITTER
4. CONDENSER
5. APERTURE CONTROLS
6. SPECIMEN HOLDER
7. OBJECTIVE LENS
3. STEPPER MOTORS FOR CENTERING THE ELECTRON BEAM
8. PROJECTOR LENS
9. OPTICAL LENS
10. FLUORESCENT SCREEN
11. VACUUM PUMP LEADS
12. GONIOMETER
13. VACUUM AND MAGNIFICATION CONTROL
14. FOCUSING CONTROL
SOME TYPICAL TEMs
ELECTRON BEAM SOURCE(ELECTRON GUN)
TUNSTEN(W) FILAMENT
LANTHANUM HEXABORIDE(LaB6 )
ELECTRICAL CONNECTION OF THE GUN
TYPE OF ELECTRON EMISSIONTHERMOIO
NIC EMISSION(T
E)
FIELD EMISSION(F
E)
IN CASE OF (TE) ACCORDING TO RICHARDSON’S LAW
WHERE,Jc=CURRENT DENSITY(Am-2 )k =1.38 ×10 -23 J K-1 (BOLTZMANN’S CONSTANT)TC=CATHODE TEMPERATUREA≈ 12×10⁵ AK-2m-2
Øw =WORK FUNCTION
Jc = ATc2 exp(-øw/ kTc)
IN CASE OF (FE) ACCORDING TO FOWLER NORDHEIM FORMULA
WHERE,E= ELECTRIC FIELDREST OF THE TERMS BEAR MEANING AS USUAL.
Jc = k1 |E|2/øw exp (-k2 øw 3/2/|E|)
W FILAMENT
TC = 2500-3000 K
Øw = 4.5 EV
JC≈(1-3)×10⁴ A/m2 LaB6
TC =1400-2000 K
Øw = 2.7 ev
JC≈(2-5)×10⁵ A/m2
Condenser lensesIT CONTROLS HOW STRONGLY THE BEAM IS FOCUSED ( CONDENSED) ONTO THE SAMPLE.IT DETERMINES THE SIZE OF THE SPOT THAT STRIKES THE SAMPLE
ALIGNMENT
CONDENSER LENSES CONTINUED
CHANGING THE STRENGTH OF THE TWO LENSES WE CAN CHANGE THE POSITION OF THE FOCUS
CONDENSER LENSES CONTINUED
AS WE CHANGE THE EXCITATION OF THE TWO LENSES,THE MAGNIFICATION OF THE IMAGE CHANGES.THIS WAY ADJUSTING THE EXCITATION OF THE TWO LENSES WE CAN VARY THE SPOT SIZE.
HERE WE SEEM TO HAVE BROKEN A RULE, IN THIS DIAGRAM. WE HAVE BENT THE RAYS IN FREE SPACE AT THE PLANES, WHERE THEY REACH FOCUS ACCORDING TO THE PREVIOUS DIAGRAM. SURELY BEAMS JUST CAN’T BEND, WITHOUT HAVING A LENS OR DEFLECTION COIL. TRUE. IN FACT, WHAT WE ARE DOING IS CHANGING OUR ATTENTION FROM ONE SET OF BEAMS THAT PASS THROUGH THE FIRST LENS, TO A 2ND SET OF BEAMS THAT PASS THROUGH THE 2ND LENS.
RESOLUTION IS LIMITED BY LENS ABERRATION
ABERRATION
SPHERICALABERRATION
CHROMATICABERRATION
ASTIGMATISM
SPHERICAL ABERRATION IS CAUSED BY THE LENS FIELD ACTING INHOMOGENOUSLY ON THE OFF AXIS RAYS.
CHROMATIC ABERRATION IS CAUSED BY THE VARIATION OF THE ELECTRON ENERGY & THUS ELECTRON ARE NOT MONOCHROMATIC
Optic Axis
Lens
Spherical Aberration
Marginal Focus
Axial Focus
Disc of minimum confusion
Point is imaged as disc
Optic Axis
Lens
Chromatic Aberration
Focus A Focus B
Disc of minimum confusion
ASTIGMATISM
ASTIGMATISM MEANS THAT THE STRENGTH OF THE LENS IS DIFFERENT IN TWO DIFFERENT DIRECTIONS.THAT MEANS THERE ARE NOW TWO FOCUS POINTS.
THE OVAL IS MEANT TO REPRESENT A PERSPECTIVE VIEW OF THE TOP OF THE LENS.
CORRECTION OF ASTIGMATISM
ASTIGMATISM CAN BE COMPENSATED FOR BY PLACING A SIMPLE STIGMATOR IN THE POLEPIECE BORE OF THE LENS.STIGMATOR WORK BY ADDING A SMALL QUADRUPOLE DISTORTION TO THE LENSES.
IN ORDER TO COPE WITH EVERY POSSIBLE ORIENTATION OF ASTIGMATISM,WE NEED TWO SETS OF QUADRUPOLES MOUNTED AT 45⁰ DEGREES TO ONE ANOTHER.
Line focus in y direction
Line focus in x direction
y
x
CORRECTION OF SPHERICAL & CHROMATIC ABERRATION
SPHERICAL ABERRATION CAN BE COMPENSATED FOR BY A COMBINATION OF MAGNETIC QUARDRUPOLE &
OCTOPOLE LENSES, WHEREAS A COMBINATION OF ELECTROSTATIC & MAGNETIC QUADRUPOLES IS NECESSARY FOR THE CHROMATIC ABERRATION.
IMAGING
BRIGHT FIELD MODE(BF)
DARK FIELD MODE(DF)
IN BF ONLY THE TRANSMITTED PRIMARY BEAM IS ALLOWED TO PASS OBJECTIVE APERTURE TO FORM IMAGES
IN DF ONLY DIFFRACTED BEAMS ARE ALLOWED TO PASS THE APERTURE
PREPARATION OF SAMPLE
GRID IS A SIEVE WOVEN FROM A THIN METAL WIRE,USUALLY NICKEL OR COPPER GRIDS OF 3 mm DIAMETER ARE COMMERCIALLY AVAILABLE WITH DIFFERENT MESH SIZES(GENERALLY OF 100-200 µm SIZE)
SAMPLE MUST BE THIN ENOUGH,SHOULD BE OF THE ORDER OF 100-200 nm, SO THAT IT CAN TRANSMIT AN ELECTRON BEAMTHE THINNER THE SAMPLE,LESS IS THE SCATTERING OF THE ELECTRON BEAM AND BETTER THE IMAGE & ANALYTICAL RESOLUTION.
IN CASE OF MATERIALS OF SMALL DIMENSION LIKE POWDERS OR NANOTUBES,A DILUTE SAMPLE CONTAINING THE SPECIMEN IS DEPOSITED ONTO SUPPORT GRID OR FILMS.
IN CASE OF METALS & SEMICONDUCTORS DIFFERENT TECHNIQUES LIKE ELECTROPOLISHING,CHEMICAL ETCHING ARE USED.
IN CASE OF BIOLOGICAL SAMPLES DIAMOND KNIFE OR ULTRAMICROTOME IS USED TO CUT THIN SECTIONS.
SOMETIME TO INCREASE THE CONTRAST & TO ISOLATE A CERTAIN AREA OF INTEREST STAINING METHOD IS USED.
MORE RECENTLY FOCUSSED ION BEAM METHOD HAVE BEEN USED TO PREPARE SAMPLES.THIS TECHNIQUE MAKES IT POSSIBLE TO MILL VERY THIN MEMBRANES FROM A SPECIFIC AREA OF INTEREST IN A SAMPLE LIKE SEMICONDUCTOR OR METAL
TYPES OF TEM
CONVENTIONAL TEM
HIGH RESOLUTION TEM
ANALYTICAL TEM
HIGH VOLTAGE TEM
THANK YOU