TRANSMISSION ELECTRON MICROSCOPy (TEM)

COMPONENTS & WORKING OF TEM OUTLINE INTRODUCTION TO TEM COMPONENTS & WORKING OF TEM SAMPLE PREPARATION TYPES OF TEM

INTRODUCTION TEM IS MUCH LIKE SLIDE PROJECTOR BUT THE BASIC DIFFERENCE IS LIGHT MICROSCOPE USES BEAM OF LIGHT WHEREAS TEM USES BEAM OF ELECTRON 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 MICROSCOPE RESOLUTION MAGNIFICATION OPTICAL 200 nm 1000x TEM 0.2 nm 500000x 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 Sample Objective Lens Objective Aperture Selected Area Aperture First Intermediate Lens Second Intermediate Lens Projector Lens Main Screen (Phosphor)

DIFFERENT COMPONENTS OF TEM 1. HIGH TENSION CABLE 2. ELECTRON EMITTER 3. STEPPER MOTORS FOR CENTERING THE ELECTRON BEAM 4. CONDENSER 11. VACUUM PUMP LEADS 5. APERTURE CONTROLS 12. GONIOMETER 6. SPECIMEN HOLDER 13. VACUUM AND MAGNIFICATION CONTROL 7. OBJECTIVE LENS 8. PROJECTOR LENS 14. FOCUSING CONTROL 9. OPTICAL LENS 10. FLUORESCENT SCREEN

SOME TYPICAL TEMs

ELECTRON BEAM SOURCE(ELECTRON GUN) TUNSTEN(W) FILAMENT LANTHANUM HEXABORIDE (LaB6 ) ELECTRICAL CONNECTION OF THE GUN

TYPE OF ELECTRON EMISSION THERMOIONIC EMISSION(TE) FIELD EMISSION(FE) 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 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 TEMPERATURE A≈ 12×10⁵ AK-2m-2 Øw =WORK FUNCTION IN CASE OF (FE) ACCORDING TO FOWLER NORDHEIM FORMULA WHERE, E= ELECTRIC FIELD REST OF THE TERMS BEAR MEANING AS USUAL. Jc = k1 |E|2/øw exp (-k2 øw 3/2/|E|) Jc = ATc2 exp(-øw/ kTc)

Condenser lenses ALIGNMENT IT CONTROLS HOW STRONGLY THE BEAM IS FOCUSED ( CONDENSED) ONTO THE SAMPLE.IT DETERMINES THE SIZE OF THE SPOT THAT STRIKES THE SAMPLE

CONDENSER LENSES CONTINUED CHANGING THE STRENGTH OF THE TWO LENSES WE CAN CHANGE THE POSITION OF THE FOCUS

CONDENSER LENSES CONTINUED 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. 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.

Disc of minimum confusion Disc of minimum confusion RESOLUTION IS LIMITED BY LENS ABERRATION ABERRATION SPHERICAL CHROMATIC ASTIGMATISM Spherical Aberration Marginal Focus Axial Focus Optic Axis Disc of minimum confusion Chromatic Aberration Lens Focus A Focus B Point is imaged as disc Optic Axis SPHERICAL ABERRATION IS CAUSED BY THE LENS FIELD ACTING INHOMOGENOUSLY ON THE OFF AXIS RAYS. Disc of minimum confusion Lens CHROMATIC ABERRATION IS CAUSED BY THE VARIATION OF THE ELECTRON ENERGY & THUS ELECTRON ARE NOT MONOCHROMATIC

CORRECTION OF ASTIGMATISM ASTIGMATISM MEANS THAT THE STRENGTH OF THE LENS IS DIFFERENT IN TWO DIFFERENT DIRECTIONS.THAT MEANS THERE ARE NOW TWO FOCUS POINTS. 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. y x Line focus in y direction Line focus in x direction IN ORDER TO COPE WITH EVERY POSSIBLE ORIENTATION OF ASTIGMATISM,WE NEED TWO SETS OF QUADRUPOLES MOUNTED AT 45⁰ DEGREES TO ONE ANOTHER. THE OVAL IS MEANT TO REPRESENT A PERSPECTIVE VIEW OF THE TOP OF THE LENS.

CORRECTION OF SPHERICAL & CHROMATIC ABERRATION SPHERICAL ABERRATION CAN BE COMPENSATED FOR BY A COMBINATION OF MAGNETIC QUARDRUPOLE & CORRECTION OF SPHERICAL & CHROMATIC ABERRATION OCTOPOLE LENSES, WHEREAS A COMBINATION OF ELECTROSTATIC & MAGNETIC QUADRUPOLES IS NECESSARY FOR THE CHROMATIC ABERRATION. IN BF ONLY THE TRANSMITTED PRIMARY BEAM IS ALLOWED TO PASS OBJECTIVE APERTURE TO FORM IMAGES IMAGING BRIGHT FIELD MODE(BF) DARK FIELD MODE(DF) IN DF ONLY DIFFRACTED BEAMS ARE ALLOWED TO PASS THE APERTURE

PREPARATION OF SAMPLE SAMPLE MUST BE THIN ENOUGH,SHOULD BE OF THE ORDER OF 100-200 nm, SO THAT IT CAN TRANSMIT AN ELECTRON BEAM THE 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 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)

TYPES OF TEM HIGH RESOLUTION TEM ANALYTICAL TEM HIGH VOLTAGE TEM CONVENTIONAL TEM HIGH RESOLUTION TEM ANALYTICAL TEM HIGH VOLTAGE TEM

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