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Transmission Electron MicroscopyPhys 590BNovember 9, 2018

Matthew J Kramermjkramer@ameslab.gov

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Atomic Resolution imaging

Atomic species mapping

STEM analysis

Scanning Transmission Electron Microscopy (S/TEM) and the Dual-Beam Focused Ion Beam (FIB) are critical tools for the characterization of materials at the nanoscale.

Accelerating advanced material design and development

2 nm

100nmElemental mapping

Co

Fe

2.9Å

• While all use electron to image, the information and its resolution very greatly!

SEM, TEM and STEM

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Why TEM?

• Why use electrons?

– Advantages• Very short wavelength

– Resolution , δ, is proportional to the wavelength of the radiation, λ , divided by the refractive index of the viewing medium, μ , and β the semi-angle of collection of the lens

» For light δ ~ 300nm while 100 keV < 1 Å

– Depth of Field and Depth of Focus

• Charged particles

– E is in electron volts and λ in nm.

– Disadvantages• Charged particles

– Short mean free path

– Interactions with sample

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Interactions of Electrons with the Sample• What do you ‘see’ in the

TEM– Elastically scattered

– Inelastically scattered

– Characteristic X-rays

– Electron Energy Losses

– Requires a ‘thin sample’!

Williams and Carter, 1996

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Forest or the Leaves6

Imaging Mechanisms

• SEM– Secondary or Backscattered

electron

• (S)TEM– Contrast!

• Diffraction– Two-beam

» Phase or strain contrast

– Multi-beam

» Lattice imaging

• Mass-thickness contrast– Increases with Z and t

» Alter Eo and β.

• Z-contrast

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Resolution:

rmin = 0.61 3/4 Cs1/4 200KV: λ ~ 0.025Å

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Interaction with matter

• Mean free path

– Measure of the probability of an electron interacting with the sample

– How to thin your sample and how thin to make it

– Affect diffraction contrast

– Determine accuracy of your spectroscopy

MoO3 on C support film

Sample Preparation

• How the sample is prepared for TEM analysis is of prime importance. – Limits the techniques– Introduce artifacts

• Form of the sample– Crystalline or amorphous– Inorganic or organic

• Matrix or not?– Dispersed powder– Inclusions

• Coherent or incoherent

– Grain Boundary phases

Pt-Sn

W. Haung

PdTe with nanodot inclusions

M. Kanatzidis, Northwestern U andB Cook

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Sample Preparation Methods

• Crush and Float– Quick and easy

• Lose matrix relationships• Introduce defects• Oxidation

• Microtome– Relatively easy

• ‘soft matter’ or ductile• Sometimes cooling is required

• Electropolishing– Need the right chemistry

• Lose second phases• Introduce reaction byproducts

• Ion Milling– Flexible but time consuming

• Can introduce artifacts w/ low Z materials– Can cool

• Difficult to perforate at specific localities

• Focused Ion Beam (milling)– Limited and expensive

• Precise location• Can be very damaging!

• CryoPlunge– Freezing liquids/polymers

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2 0 n m2 0 n m

EBSD to locate grain of the right orientation

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• Electromagnetic lenses allow for switching from Imaging to diffraction

• Navigate to regions of interest through translation and specific orientations through tilting.

– Amount of tilting is dependent on the holder and the microscope.

What Can I ‘see’ in the (S)TEM

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• Since the wavelength is small, the reciprocal space is ‘flat’ (2D)

Image & its Diffraction

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• TEM mode– An aperture used to

selected regions

Image & its Diffraction

Image Formation

HRTEM

phase contrast

objective aperture

diffraction contrast

objective aperture

Electron diffraction patterns

objective aperture

High Resolution Transmission Electron Microscopy (HRTEM)

objective aperture

lattice image

specimen

beam

diffraction pattern

objective lens

Practical Electron Microscopy in Materials Science, J. W. Edington

Nanocrystalline phase in amorphous matrix

TEM-DFTEM-BF

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Bright Field 2-beam imaging

• Use imaging and diffraction together– Strain contrast varies

with orientation of the defect relative to the electron beam

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• Major crystallographic axis aligned to the beam

Lattice imaging (high resolution TEM) a

b

c

xy

z

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• Energy dispersive and energy loss detectors provide concurrent chemical analysis while imaging

Adding Spectroscopy Incident parallel beam

specimen

CCD detector

Energy Dispersive Detector(s)

Fe Co

Sm

• Major contrast mechanism depends on configuration– Scattered electrons for near

field detectors

– Atomic mass for far field detectors

• Techniques– Bright Field, Dark Field, PED,

Z-contrast imaging

– Convergent beam diffraction

– EDS/EELS mapping

– Lorentz

– Tomography

STEM modes

Incident converged beam

specimen

BF detector < 10 mrad

ADF detector10-50 mrad

HAADF detector

> 50 mrad off-axis

Energy Dispersive Detector(s)

STEM ImagingIncident converged beam

specimen

BF detector < 10 mrad

ADF detector10-50 mrad

HAADF detector

> 50 mrad off-axis

Energy Dispersive Detector(s)

Alnico magnet alloy

50 nm5 n m

Multiple Imaging and mapping is done simultaneously

STEM atomic resolution

Li0.33La0.56TiO3

Intensity is ~ to the average z along the beam direction

• Samples with large variation in z need multiple detectors

STEM atomic resolution Incident converged beam

specimen

BF detector < 10 mrad

ADF detector10-50 mrad

HAADF detector

> 50 mrad off-axis

Energy Dispersive Detector(s)

HAADF: Intensity Z1.5-1.8

(Li, O invisible)ABF: Intensity Z1/3

(Li, O visible)

• EDS and EELS can be done concurrently imaging

STEM mapping

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Atomic Resolution Mapping

• Aberration correction

– Eliminates most of the spherical distortions of the lenses

– Sub-Å spatial resolution 2.9Å

a b cA

B

A

B

EDS mapping of Fe and Co

Interfaces and defects

5 n m

dislocation

εxx

Interfacial misfit dislocation

Li0.33La0.56TiO3 /LixTiyOz

Cu –rich precipitate distorted during heat treatment in an

alnico magnet

Resolution < 0.6 Ȧenables precise measurements of local lattice distortions

• Precesses the electron beam to better index• Records pattern while scanning the sample

Precession Electron Diffraction (PED)

Iman Ghamarian, Yue Liu, Peyman Samimi, Peter C. Collins, Acta Materialia 79 (October 2014), pp. 203-215, DOI: 10.1016/j.actamat.2014.06.063.

Misorientation in quaternion color map Grain Boundary Character

Distributions (GBCD) of nanosizedgrains

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N

S

N

S

B~2T

Objective lens on

Conventional TEM mode

N

S

N

S

Objective lens off

Lorentz lens

Lorentz mode

TEM mode vs Lorentz mode

• Magnetic domains

– Lower resolution since objective pole piece is turned off

Lorentz Microscopy

2 µ m

FeSi 6.5 Si

2 µ m

@ 5° tilt

Domain wall

Formation of skyrmion in FeGe

Lorentz

Stripe Stripe + skyrmions Skyrmions

0 . 2 µ m

0.2 µm

0 mT

0 . 2 µ m

62.5 mT 87.8 mT

263 K

Induction map

• Uses a charged wire to split the beam

– Recovers both phase and amplitude• Electrostatic potentials

• Magnetic fields

Holography

5 0 n m

1 µ m

200nm

hologramLorentz

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Domain Imaging alnico 9• Well aligned grain result in primarily 180°

domain walls

200nm

Color induction map from

holographic phase image.

Lorentz image

SIF Suite of Instruments

• FEI Teneo LoVac Field Emission SEM (FE-SEM)

• FEI Helios G3 UC Dual-Beam Instrument

Focused Ion Beam (FIB) and FE-SEM

• FEI Tecnai G2 F20 Scanning Transmission Electron Microscope (STEM)

• FEI Titan Themis 300 Cubed 300 STEM/TEM

SIF Suite of Instruments

• Schottky field emitter– From 200 eV to 30 keV

– With beam current of 1 pA to 400 nA

• Resolution – 0.8 nm at 30 kV

– 1.4 nm at 1 kV

• Secondary, backscattered, and in-lens detectors

• Oxford energy dispersive and backscattered electron detectors

• Quick loader– 12.5 mm to 32 mm size sample stubs

• Remote operation

FEI Teneo LoVac Field Emission SEM (FE-SEM)

• Electron optics– Schottky field emitter

• 350 V to 30 kV

• With beam current of 0.8 pA – 100 nA

– Resolution

• 0.6 nm at 2 – 15 kV

• 1.0 nm at 500 V

• Ion optics– liquid Gallium ion emitter

• 500 V to 30 kV

• With beam current of 0.1 pA - 65 nA

– Resolution

• 4.0 nm at 30 kV

• 5-axes motorized x-y-z-rotate-tilt eucentric stage

FEI Helios G3 UC Dual-Beam Instrument

• Secondary, backscattered and in-lens detectors

• Oxford energy dispersive spectrometer

• Quick loader

• Remote operation

• MultiChem Gas Delivery System

• EasyLift EX NanoManipulator

– For TEM and 3D atom probe sample preparation

• Autoslice and view for 3D reconstructions

• Integrated plasma cleaner

FEI Helios G3 UC Dual-Beam Instrument

• Conventional TEM– BF, DF, and Lattice imaging

• Spectroscopy– Point, line, and mapping using

• Electron energy loss

• Energy dispersive scanning

– BF, DF, and HAADF

• Energy Filtered Imaging– Elemental imaging

– Thickness imaging

– Energy filtered diffraction

FEI Tecnai G2 F20 Scanning Transmission Electron Microscope (STEM)

Nd

• Scanning TEM

• Tomography

• Lorentz Microscopy

• Holography

FEI Tecnai G2 F20 Scanning Transmission Electron Microscope (STEM)

The move will include moving the large format OriusCCD from the CM30 for better field of view and scan rates up to 15 fps

Primary uses – training, routine microscopy or in situ experiments not requiring atomic resolution imaging.

200nm

• Electron optics– Schottky Field Emission Gun equipped with

monochromator

– Fully aligned at 80, 200 and 300 kV

– Cs DCOR Probe Corrector

– Symmetric S-TWIN objective lens

• α-tilt (35° double tilt, 70° tomography), β-tilt 30°

• Easy switching between TEM and STEM modes

• Symmetric S-TWIN objective lens – Symmetric design allows changing of the accelerating

voltage without the need for adjusting the stage

z-position

– Easy switching between TEM and STEM modes

FEI Titan Themis 300 Cubed STEM/TEM

• Detectors

– Ceta 16M (CMOS) camera w/ high speed upgrade

• 20 fps at 4kx4k readout

– FEI Super-X detector for high speed and high quality

data acquisition with dwell times below 10 μs

• Four SSD’s are perfectly integrated in the S-TWIN objective lens and offer maximum collection efficiency with a solid angle of 0.7 srad

• High sensitivity allows detection of all elements down to and including boron

– Gatan Quantum ER/965

• 0.1 ev resolution

– The On-axis Bright-Field/Dark-Field STEM detector

• Useful for Z-contrast imaging and EELS

• Simultaneous acquisition of the signal of (a) the

bright-field detector and (b) one of the dark-field

detectors.

FEI Titan Themis 300 Cubed STEM/TEM

200nmAl Fe Cu

live measurement of intrinsic magnetic and electric fields

+α+β

-α-β

1 2

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Double tilt TEM holder

• Capabilities

– Remote operations

– BiPrism

• Bipolar power supply -400V to -14V and +14V to +1250V

– Lorentz Lens

– Tomography

– Low-dose exposure

• Holders– Low background single and double tilt holders

• +/- 40° alpha and +/- 30° beta

– Gatan 70° FOV Cryo holder (< -170°C at specimen)

– Single tilt tomography holder (+/- 70°)

– Hysitron picoindenter with field biasing

– NanoEx heating and biasing (4 contact)

• Up to 1200°C, consumable MEMS chips

FEI Titan Themis 300 Cubed STEM/TEM

All existing FEI holders are cross compatible with both instruments.

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• To become a SIF User:

1. Complete the Materials and Experiment Safety Assessment (MESA) form

2. Complete all on-line training modules in Cyber Train

3. Receive approval from SIF Program Coordinator detailing your arrival instructions

4. Arrive on-site the day you’re scheduled, complete emergency awareness training, and meet with SIF researcher

• All users must become an Ames Laboratory associate

– Foreign Nationals must register and check-in

• All materials

– must be checked for DOT transportation compliance

– must be checked for hazard waste/proper disposal

• All charging accounts must be verified

• All users must be cleared by their Group Leaders/PIs for access, training, and charging

Accessing the SIF