E-beam Characterization:

a primerOverview of the Modern STEM

Phys 590B

April 4, 2014

Matthew J Kramer

mjkramer@ameslab.gov

1

Overview

• State-of-the-Art Analytical Scanning Transmission Electron Microscope combines:– Conventional TEM

• BF, DF and Lattice imaging

• SADP, CBED

– Spectroscopy• Point, line and mapping using

– Electron Energy Loss Spectroscopy

– Energy Dispersive Spectroscopy

– Scanning• BF, DF and HAADF

– Energy Filtered Imaging• Elemental imaging

• Thickness imaging

• Energy filtered diffraction

– Lorentz Microscopy• Free lens control of the Objective lens

– Direct imaging of the magnetic domains

– in situ dynamic materials response

2

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

3

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

4

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

Sm2Fe17J.P. Liu, U

Texas

PdTe with

nanodot

inclusions

M.

Kanatzidis,

Northwestern

U and

B Cook 5

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

(a)

(b)

(c)Shear band

Pt overlayer

20 µm

10 µm

2 µm

Shear band

Pt

overlayer

(a)

(b)

(c)Shear band

Pt overlayer

20 µm

10 µm

2 µm

Shear band

Pt

overlayer

Zr based metallic glass

X. Tang and D. Sordelet

AlCu DS sample, R.

Trivedi and S. David

(ORNL)

6

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

7

Defects at the Nanoscale

2

1

8

Def = -200ÅDef = -100ÅDef = 0ÅDef = 100Å

Computer simulation on HRTEM image:

Why simulation?

Do the bright spots correspond to the atom positions?

HRTEM

Computer Simulations

9

Simulation of [001] f.c.c. Cu (2x2 cells):

S.G.: Fm3m (225); a = 3.615 Å

a

b

c

x

y

z

a

b

c

x

y

z [001]

Do the bright spots correspond to the atom positions?

By thickness

20 Å 40 Å 60 Å 80 Å 100 Å

By defocus

-200 Å -100 Å 0 Å 100 Å 200 Å

Computer Simulations

10

Computer simulation on HRTEM image:

Why simulation?

As a result of the strong scattering and of the transfer of information by the

microscope, HRTEM images, that are interference images, depend mainly on the

thickness of the crystal and the transfer function of the microscope (defocus).

Do the bright spots correspond to the atom positions?

➢ Lattice image and structure image.

➢ For known structure, to interpret the image with atom configuration.

➢ For unknown structure, to test the proposed structure model.

Computer Simulations

11

Image Interpretation

• Gd2Te5– Crystal

perfection

– Check lattice

over a wide

region

– Zoom into a

thin region and

model

structure

a

b

c

xy

z

I. Fisher, Stanford 12

HRTEM VS STEM Imaging• HRTEM

– Planer

illumination

– Multi-beam

scattering

– Image

contrast

• Thickness

• defocus

• Z-contrast

– Scans a fine

probe

– Electrons are

scattered to

an annular

detector

– Strength of

the scattering

~ Z

From Eiji Abe and An Pang Tsai

13

STEM Imaging

• Scan a fine probe across the sample– Resolution increases w/ decreasing probe

size• Reduce S:N

– 3 different detector positions• BF, similar to multi-beam image

• DF, similar to conical DF in TEM

• HAADF, higher Z contrastAu on a C film

STEM BF

STEM DF

14

Incident

converged

beam

specimen

BF detector < 10 mrad

ADF detector10-50 mrad

HAADF

detector> 50 mrad

off-axis

Energy

Dispersive

Detector

STEM vs TEM

• Co-block

polymer

– Weak

contrast

• Use

heavy

metal

oxides to

enhance

contrast

Co-block Polymer, E Cochran

TEM - BFSTEM - HAADF

15

Interfaces and Defects

• HRTEM – Delocalization effects due to

strongly scattered electrons introduces further complications in image interpretation

Si twins in a

directionally

solidified Al-Si

alloy

Ralph

Napolitano and

Halim Meco

16

Grain Boundaries

I. Anderson, R.W. McCallum, Y Wu and W. Tang

STEM-HAADF

HRTEM

STEM-BF

17

Chemistry and Structure

• STEM Imaging

– Allows for

better control

of the fine

probe

• EDS or EELS

can be done

– Point

– Line

– Area

• Convergent

beam pattern

collection

0

20

40

60

80

100

0 50 100 150 200

Position (nm)A

tom

ic %

-■- Cr-■- Co-▲- Sm-●- Pd

18

Electron Energy Loss Spectroscopy

• EELS

– Zero Loss• Sample thickness = ∑Io/ ∑ I(eV)

– Low Loss Region < 100 eV• Plasmons

– longitudinal oscillations of free electrons, which decay either in photons or phonons

– High Loss Region ~ > 100 eV• ionization energy of the inner-shell

e’s

• More sensitive to lighter elements

• Balances EDS

– Has different energy spectrum that X-rays

– In addition to composition can be used to determine• Thickness

• Binding/oxidation states

– Harder to quantify

– More sensitive to thickness effects

0.2

0.4

0.6

0.8

1.0

170 190 210 230 250 270

energy loss (eV)

I/Io

Hex BN stnd

Cubic BN

Fe Co

Sm

19

EFTEM Imaging

20

EFTEM Imaging

• Uses

quadropole

to bend the

image and

select out a

narrow

energy

window

carbonoxygen

Mesoporous SiO2 spheres on a Lacey C grid

V. Lin

21

Energy Filtered Imaging

• Zero Loss

• Plasmon

– thickness

• 3 window

method

• Spectrum

Imaging

EELS line scan

Pre-edge 1 Pre-edge 2 Post-edge

22

Atomic Resolution

• Aberration correction

– Eliminates most of the

spherical distortions of the

lenses

– Sub-Å spatial resolution

23

2.9Å

a b c

e

A

B

A

B

EDS mapping of Fe and Co

N

S

N

S

B~2T

Objective lens on

Conventional TEM mode

N

S

N

S

Objective lens off

Lorentz lens

Lorentz mode

24

CTEM mode vs Lorentz mode

Lorentz Microscopy

image

e- e-e-

Deflection of

electron trajectory

by Lorentz force.

• Lorentz microscopy: direct observation of

magnetic domains with high spatial

resolution.

25

0 . 5 µ m

➢ Stripe shape domains: ~500nm wide.

Lorentz image

alnico 5-7

26

Holography• Recover

phase objects

in the TEM

– Electrostatic

potentials

– Magnetic

fields

27

Electric fields, magnetic fields, and potentials change the electron path but do

NOT affect the wave amplitude.

=> Effects are not visible in recorded image intensity

Off-axis electron holography

x, y( )= CEV x, y( )t x, y( )−e

hB⊥st x, y( )

Magnetic phase shift for a uniformly thick slab of

magnetic material in the absence of a fringing field

is a uniform (±) slope in the phase.

Mean inner potential+ electric field

In-plane magnetic component

M.R. McCartney & D.J. Smith, Annu. Rev. Mater. Res. 37 (2007) 729

• Nanoscale phase imaging of electrostatic and

magnetic fields

28

Tecnai F20

FEG

Hologram

Sample

CCD camera

Lorentz lens

Biprism

(for coherence)

(to overlap waves)

(for digital recording)

M.R. McCartney & D.J. Smith, Annu. Rev. Mater. Res. 37 (2007) 729

How it is done

29

alnico 5-7

30

200nm200nm

Color induction map from

holographic phase image.

HAADF STEM image

➢ Hue: in-plane induction direction; saturation value: magnitude.

➢ Single domain FeCo-rich rods coupled and form 180º micro-

magnetic domain.

Tomography

• Collect a series of images over a wide range

of tilts

• Bring the series into registry

• Assemble into a movie or deconstruct into

3D slices

31

2-14-1

2-14-1

TiC

Gets around the inherent 2D perspective of TEM32

Summary

• (S)TEM – Powerful tool for

• Structure– HRTEM

– Z-Contrast

• Chemistry– EDS/EELS

» Point

» Line

» Mapping

– EFTEM

– Must know your system• Sample preparation

• Contrast mechanisms– Amorphous, low Z materials

– Matrix, particles and defects

33

Concluding Remarks

• 200 keV FEG source provides– High brightness, < 0.2 nm probe

– Narrow energy spread, < 0.7 ev

– Point-to-point resolution, < 0.25 nm

• Configured as an analytical tool with minimal compromising HRTEM– Scanning capabilities

• HAADF (Z-contrast)

• EELS

• EDS

– Imaging• BF/DF

• Energy filtered imaging

• Fluctuation Electron Microscopy

– Diffraction• SA, energy filtered SA

• Convergent beam

– Lorentz

– Tomography

– Holography

34