ELECTRON MICROSCOPYWednesdays at One

UNL REU in Nanomaterials and Nanoscience

Jeff ShieldDepartment of Mechanical Engineering

University of Nebraska

What we can discoverWhat we can discover . . .IMAGING

M ifi ti f l illi•Magnifications of several million times•Resolution approaching 0.1 nm

DIFFRACTIONDetermine crystal pp g

•See defects such as grain boundaries, dislocations, stacking faults etc

structureSmall areas (<1 nm)3-D symmetrystacking faults, etc.

•3-D imaging3 D symmetry

CHEMISTRYDetermine what elements are thereDetermine concentrationDetermine concentrationSmall areas (<1 nm)

The PartsThe Parts . . .

1 Electron source1. Electron source2. Electromagnetic lenses3 D t ti3. Detection

The Electron SourceThe Electron Source

“W k F ti ”e-“Work Function”

e-

e

e-

Free e-

e-

e-

e-

Two techniques to overcome the work function

Vacuum

Solid

1. Thermionic emission2. Field emission

Thermionic EmissionThermionic Emissione- Use thermal energy

e-

e

e-

Use thermal energy

iFree e-

e-

e-

e-

i

Vacuum

Solide-ee-Electron beam

Field EmissionField Emissione- Use electric field

e-

e

e-

Use electric field--modifies barrier

e

Free e-

e-

e-

e-

Vacuum

Solide-ee-Electron beam

Field v ThermionicField v. Thermionic

Fi ldField•Smaller virtual source (=smaller beam size)•More uniform electron energy (easier to control)

Thermionic•Less stringent vacuum requirementsLess stringent vacuum requirements•Cheaper

Field emission sources provide higher resolution, better spectroscopy

EM LensesEM Lenses

• CondensorCondensor– Demagnify electron beam

Objective• Objective– Focus probe onto sample (SEM)

( )– Focus image (TEM)• Magnifying (only in TEM)

Scanning Electron Microscopy (SEM)

SEMSEM

Electron GunX-ray DetectorElectron Detector(s)

Column (lenses)

Sample Chamber

Detection: SEMDetection: SEMX-rays

Backscattered electronsSecondary electrons Auger electrons

Electron beam

Can be microns!

Interaction volume depends on:Interaction volume—depends on:•Accelerating voltage•Atomic number of sample•Density of material

SEM OperationSEM OperationElectron Beam

Scan Area

Secondary electronselectrons

Low energyUsed to form most iimages

Auger electronsLow energyElement-specificElement specificCarry chemical information

More secondary electron images

Backscattered electronsHigh energy beam electronsg gyCarry atomic number information

Al Ca Cl

Fe K Mg Na

X-raysCarry chemical information

(images from EDAX, Inc.)

SEM: SummarySEM: Summary

• Magnification depends on scanned areaMagnification depends on scanned area• Resolution depends on probe size and

electron specimen interactionelectron-specimen interaction• Good for fracture surfaces, microstructure,

ti l tparticles, etc.• Easy to interpret images!

Transmission Electron Microscopy (TEM)

Transmission Electron Microscopy (TEM)

El t GElectron Gun

Condenser Lenses

S l Ch b /Obj ti LSample Chamber/Objective Lens

Magnification Lenses

Detection: TEMDetection: TEMX-rays

Backscattered electronsSecondary electrons Auger electrons

Electron beam

<100 nm!

Interaction volume

<100 nm!

Interaction volume•Much smaller than in SEM!!!

•Able to probe smaller regions!!!Diffracted electrons

Transmitted electrons

ImagingImagingCondensor lens

<100 nm!

Beam now hits large area

<100 nm!

Diffraction Plane

Image Plane

Diffraction Plane

Magnifying lenses

“Regular” ImagingRegular Imaging“Bright Field”

Contrast due to:•Differences in grain orientation•Imperfections in crystals

100 nm

“Regular” ImagingRegular Imaging

(a) (b)

High-Resolution ImagingHigh Resolution ImagingContrast due to wave interaction•“See” columns of atoms• See columns of atoms

From Urban, MRS Bulletin 32, 946 (2007)

High-Resolution ImagingHigh Resolution Imaging

DiffractionSelected area electron diffraction

Nanoprobe and/or convergent beam electron diffraction

Crystal structure determination

Small region ORSmall region OR3D symmetry (point group)

Chemical AnalysisChemical Analysis

e- e-ΔEe

e-

eΔE

e-

e-

Incoming electrons ionize atomsRelaxation leads to emission of x ray

e-

e-

Relaxation leads to emission of x-rayCharacteristic of atom•Collect x-rays and categorize them by energy

X-ray

“ENERGY DISPERSIVE X-RAY SPECTROSCOPY (EDS)”

An ExampleAn Example . . .1000

ount

s

Ti

500

mbe

r of C

o

00 10 20 30 40

Nu

10.00

nts

100 nmElectron probe 6.00

8.00

nds

of C

ou Fe

1.5 nm2.00

4.00Th

ousa

n

0.000 10 20 30 40

Position (nm)

Electron Energy Loss Spectroscopy d E Filt d I iand Energy-Filtered Imaging

e- e-e

e-

eE = Eo - Eion

e-

e-

Incoming electrons ionize atomsIncoming electron loses energy corresponding to the

e-

e-

Incoming electron loses energy corresponding to the ionization energy of the atom

“Energy loss spectra”•Form image using specific electrons

Electron Energy Loss Spectrumgy pns

ityIn

ten

Peaks due to energy loss during Fe K shell ionization

Energy loss

EELS provides information about:Elements in samples (especiall good for lo Z elements)•Elements in samples (especially good for low Z elements)

•“Neighborhood” of each atom (bonding type, valence, etc)

Energy Filtered TEM (EFTEM)Energy Filtered TEM (EFTEM)Select specific electrons to form imageThen, see where those electrons have been

Bright fieldBright field

Energy nsity Fe windowFe window

50 nm50 nm

gy“window”

Inte

n

Fe50 nm50 nm

Energy loss

TEM: SummaryTEM: Summary

• Resolution depends on lenses electronResolution depends on lenses, electron source

• Good for seeing small things• Good for seeing small things• Good for analyzing imperfections• Combine microstructure (imaging), crystal

structure analysis (diffraction) and chemical analysis (EDS/EELS) in one instrument

Electron MicroscopyElectron Microscopy

Lots more stuff possible!!!Lots more stuff possible!!!• Electron Holography• Electron energy loss spectroscopy• Scanning Transmission Electrong

Microscopy (STEM)