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