Transmission Electron Microscopy Skills:Electron detection Lecture 5

8/8/2019 Transmission Electron Microscopy Skills:Electron detection Lecture 5

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Electron detectionlectron detectionLecture 5ecture 5


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OutlineutlineDetector characteristics & definitionsetector characteristics & definitions Gain, noise, DQE, resolution, PSFain, noise, DQE, resolution, PSF

Types of electron detectors and how they workypes of electron detectors and how they work Viewing screeniewing screen Photographic filmhotographic film CCD camerasCD cameras There are several more (wehere are several more (well discuss in MSE 640)l discuss in MSE 640)


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Detector characteristicsetector characteristicsGain:ain: Magnitude of signal amplificationagnitude of signal amplification

Shot noise:hot noise: Random noise fluctuationsandom noise fluctuations Provides a natural limitrovides a natural limit

Detection quantum efficiency (DQE) (noiseetection quantum efficiency (DQE) (noiseamplification):mplification):

DQE =S

out

Nout

2S

in

Nin

2

Dynamic range:ynamic range: Ratio of brightest possible pixel / to dimmest pixelatio of brightest possible pixel / to dimmest pixel

Point Spread Functionoint Spread Function - real resolutioneal resolution Measurable is theeasurable is the MTFTF - modulation transfer functionodulation transfer function

Resolution:esolution: Related toelated to size of the imaging pixelize of the imaging pixel


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Some termsome termsCathodoluminescenceathodoluminescence (CL)CL) Production of visible lightroduction of visible light

from the impact of highrom the impact of highenergy electronsnergy electronsScintillationcintillation Light emission caused byight emission caused byionizing radiationonizing radiationFluorescenceluorescence Rapidapid emission (mission (nanosecsanosecs)Phosphorescencehosphorescence Slowlow emission (mission (secsecs)


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Viewing screeniewing screena.k.a..k.a. fluorescent screenluorescent screenZnSnS orr ZnSnS / CdSdS powder on a backing plateowder on a backing plateCathodoluminescenceDope to get to 550 nm, best eye sensitivityGrain sizes on the order of 100 to 50 m, 10 m onthe focusing screen

Intensity is proportional to current densityntensity is proportional to current densityTime constant on order of 10ime constant on order of 10-5 to 10o 10-3 secsecs

Phosphoresces for a second or so afterwardsVery intense transmitted beam can damage theery intense transmitted beam can damage thescreen (over time)creen (over time)


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Photographic filmhotographic filmEmulsionmulsion of silver halide in gel (f silver halide in gel (AgBrgBr; AgClgCl) Presently use a polymer backingresently use a polymer backing Used to be glasssed to be glass --- hence the termence the term plateslatesElectrons strike silver halide grains, ionize the grain,lectrons strike silver halide grains, ionize the grain,convert to silveronvert to silver Have to develop to create the negativeave to develop to create the negative

Materials scienceaterials science - fast speed (Kodak SOast speed (Kodak SO-163)63) Minimize drift during imaginginimize drift during imaging Larger grains (5arger grains (5 m vs. 4vs. 4 m, but that, but thats not a big deal)not a big deal)

Resolutionesolution grain sizerain size bloominglooming to 20o 20m sizesizeAdvantages: large # of pixels 10dvantages: large # of pixels 109; high DQE (0.85high DQE (0.85 - 0.9).9)Disadvantages: analog, low dynamic range; not linearisadvantages: analog, low dynamic range; not linear


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Chargeharge-coupled devices (oupled devices (CCDCDs)operation principlesperation principlesBased on accumulation ofased on accumulation ofcharge in a MOS capacitorharge in a MOS capacitor

Instead, apply a voltagenstead, apply a voltagepulseulse Transient condition:ransient condition: deepeepdepletionepletion Photohoto-injected electronsnjected electronsstored in this welltored in this well

Apply a steadypply a steady-statetatepositive gate voltageositive gate voltage Surface potential wellurface potential well

Thermal generation w/ timehermal generation w/ time Inversion conditionnversion condition


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Chargeharge-coupled devices (oupled devices (CCDCDs)operation principlesperation principlesClocking scheme used tolocking scheme used totransfer charge from oneransfer charge from oneMOS capacitor to the nextOS capacitor to the next

t1

t2

t3

t4

V1 (+)+)

V1,VV2 (+)+)

V1 < VV2

V2 (+)+)

Must happen beforeust happen beforethermal generationhermal generationCan have either serial oran have either serial orfull frame readoutull frame readout


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Chargeharge-coupled devices (oupled devices (CCDCDs)camera setupamera setupNeed to convert eeed to convert e- to ho hThin aluminum to reflecthin aluminum to reflectany stray light (ny stray light ( 100 nm)00 nm)

Binninginning Average several pixelsverage several pixels

YAGAG scintillatorcintillator Convertsonverts e- to ho h Must be relatively thick toust be relatively thick togetet enough conversionnough conversion Can result in a significantan result in a significant

bloominglooming Measured aseasured as point spreadoint spreadfunctionunction


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CCD Camera useCD Camera useNeed to prepareeed to prepare GainainReferenceeference This is a good low noisehis is a good low noisepicture of the cameraicture of the camera Used to subtract out thesed to subtract out thefiber optic honeycomb, andiber optic honeycomb, and

defectsefects This is stored and used withhis is stored and used witheach imageach image

Need to acquireeed to acquire darkarkreferenceeference imagemage Even w. LN cooling, someven w. LN cooling, someshot noise presenthot noise present Allows subtraction ofllows subtraction of noiseoiseimagemage This image was taken without gain subtraction. Note thehoneycomb pattern from the fiber-optic coupling & dead pixels


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CCD CamerasCD CamerasAdvantages:dvantages:Good DQE, even at low input signal levelsood DQE, even at low input signal levelsHigh dynamic rangeigh dynamic range

Diffraction patternsiffraction patternsLinearinearConvenientonvenient

Disadvantages:isadvantages:Less resolution than filmess resolution than film

Filmilm - 6e9 image pointse9 image points 1k x 1kk x 1k - 1e6; 2k x 2ke6; 2k x 2k - 4e6; 4k x 4ke6; 4k x 4k - 1e7e7

Expensive (initiallyxpensive (initially - cheaper in the long runheaper in the long run ) 1k x 1kk x 1k - $50k50k 2k x 2kk x 2k - $80k, 4k x 4k80k, 4k x 4k - $150k150k

Generally have slow readoutsenerally have slow readouts - 2 frames per sec at bestframes per sec at best But, improving. I just bought a 1k x 1k with 15fpsut, improving. I just bought a 1k x 1k with 15fps


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CCD CamerasCD CamerasIndividual pixels canndividual pixels canhandle only so manyandle only so manyincident electronsncident electronsCreates too manyreates too many e-/hh+pairs, which spill over intoairs, which spill over intoadjacent pixelsdjacent pixels Shows up as either a largehows up as either a largeintense region or a streakntense region or a streak

In addition to being ugly, itn addition to being ugly, itmakes quantitativeakes quantitativediffraction difficultiffraction difficult


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Which method for whichhich method for whichexperiment?xperiment?Filmilm Large area imagesarge area images Diffraction contrast imagesiffraction contrast images

More dynamic range helpsore dynamic range helpsCCD:CD: Quantitative diffraction and imaginguantitative diffraction and imaging HREM, electron diffraction, most imagingREM, electron diffraction, most imaging

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Transmission Electron Microscopy Skills:Electron detection Lecture 5