Transmissions Electron Microscopy (TEM)€¦ · Transmissions Electron Microscopy (TEM) Basic...

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A.E. Gunnæs MENA3100 V18 Transmissions Electron Microscopy (TEM) Basic principles Diffraction Imaging Specimen preparation

Transcript of Transmissions Electron Microscopy (TEM)€¦ · Transmissions Electron Microscopy (TEM) Basic...

A.E. Gunnæs MENA3100 V18

Transmissions Electron Microscopy (TEM)

Basic principles

Diffraction

Imaging

Specimen preparation

Electron interaction with the (thin) specimen

Specimen

e-

Transmitted electrons

Inelastically scattered

electrons

X-rays

Secondary electrons

Backscattered electrons

Auger electrons

Cathodoluminescence

Gas

Heating

Cooling

Absorbed electrons

EBIC

Elastically scattered

electrons

Typical specimen thickness

~ 100 nm or less

Electrons interacts 100-1000 times stronger with matter than X-rays-need thin samples

Operating modes

Convergent beam Parallel beam

Can be scanned

(STEM mode)

Specimen

Spectroscopy and mapping

(EDS and EELS)

Quartz (1mm)

AZO (sputtering, ~200 nm)

Cu2O (sputtering, 600nm)

TiO2 (ALD, 10 nm)

Example of EDS mapping in STEM mode.

EDS: Energy dispersive spectroscopy

EELS: Electron energy loss spectroscopy

STEM: Scanning transmission electron microscopy

HAADF: High angular annular dark field

S. Gorantla

ImagingDiffraction SpectroscopyWith spatial resolution down to the atomic level (HREM and STEM)

Chemistry and elecronic states (EDS and EELS).Spatial and energy resolution down to the atomic level and ~0.1 eV.

From regions down to a few nm (CBED).

TEM is based on three possible set of techniqes

200 nm

HREM: High resolution electron microscopy

BF: Bright field

CBED: Convergent beam electron diffraction

SAD: Selected area diffraction

SAD pattern BF TEM image

Imaging and resolution

A.E. Gunnæs

Modern TEMs with Cs correctors have sub Å resolution!

Resolution of the eyes:~ 0.1-0.2 mm

Resolution in a visible light microscope: ~200 nm

Defects

Precipitates

Interfaces

Important for material properties

S. Gorantla

CuO

ZnO

HAADF image

Strain analysis around a dislocation core at the CuO-ZnO interface

Local atomic structure and composition,

Electronic structure and chemical bonding

The interesting objects for TEM is local structure and inhomogeneities in specimens

An example of a TEM study:

Identification of an unknown phase in a thin film

A.E. Gunnæs

Specimen: thin film of BiFeO3 + unknown phase

ab

c

BiBi

Fe

O O

Fe

Fe

Bi

O

Bi

Bi

O

Fe

O

O

Bi

O

Fe

Bi

Fe

O

Bi

O

Bi

O

Fe

O

Fe

O

Bi

Bi

O

Fe

O

Bi

Bi

O O

Bi

O

Fe

Fe

O

Fe

BiBi

PowderCell 2 .0

Goal to produce single phase:BiFeO3 with space grupe: R3C and celle dimentions: a= 5.588 Å c=13.867 Å

Metal organic compound on Pt

Heat treatment at 350oC (10 min) to

remove organic parts.

Process repeated three times before final

heat treatment at 500-700 oC (20 min) .

(intermetallic phase grown)

BF TEM image of the cross section of the specimen

200 nm

Si

SiO2

TiO2

Pt

BiFeO3

LimGlue

A.E. Gunnæs

50 nm

Tilting series around a dens row of

reflections in the reciprocal space

0o

19o

25o

40o

52o

Determination of the Bravais-lattice of an unknown crystalline phase

Courtesy: Dr. Jürgen Thomas, IFW-Dresden, Germany

Positions of the

reflections in the

reciprocal space

A. E. Gunnæs

Elastic scattered electronsOnly the direction of v is changing.

(Bragg scattering)

Elastic scattering is due to Coulomb interaction

between the incident electrons and the electric

charge of the electron clouds and the nucleus.

(Rutherford scattering).

The elastic scattering is due to the average

position of the atoms in the lattice.

Reflections satisfying Braggs law:

2dsinθ=nλ

Electrons interacts 100-1000 times stronger with matter than X-rays-can detect weak reflections not observed with XRD technique

Electron Diffraction in TEM

Courtesy: Dr. Jürgen Thomas, IFW-Dresden, Germany

Bravais-lattice and cell parameters

From the tilt series we find that the unknown phase

has a primitive orthorhombic Bravias-lattice with

cell parameters:

a= 6,04 Å, b= 7.94 Å og c=8.66 Å

α= β= γ= 90o

6.0

4 Å

7.94 Å

a

bc

100

110

111

010

011

001101

[011] [100] [101]

d = L λ / R

Chemical analysis by use of EDS and EELS

Ukjent faseBiFeO3

BiFe2O5

1_1evprc.PICT

-0 200 400 600 800 10005

10

15

20

25

30

35

40

Energy Loss (eV)

CC

D c

ounts

x 1

000

Nr_2_1evprc.PICT

-0 200 400 600 800 1000

-0

2

4

6

8

10

12

14

Energy Loss (eV)

CC

D c

ounts

x 1

000

Ukjent faseBiFeO3

Fe - L2,3

O - K

500 eV forskyvning, 1 eV pr. kanal

Published structureA.G. Tutov og V.N. Markin

The x-ray structural analysis of the antiferromagnetic Bi2Fe4O9 and the isotypical combinations Bi2Ga4O9 and Bi2Al4O9

Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy (1970), 6, 2014-2017.

Romgruppe: Pbam nr. 55, celleparametre: 7,94 Å, 8,44 Å, 6.01Å

x y z

Bi 4g 0,176 0,175 0

Fe 4h 0,349 0,333 0,5

Fe 4f 0 0,5 0,244

O 4g 0,14 0,435 0

O 8i 0,385 0,207 0,242

O 4h 0,133 0,427 0,5

O 2b 0 0 0,5

ab

c

O

Bi

Fe

O

Fe

Bi

O

Fe O

O

O

Fe

Fe

OO

O

O

Fe

Bi

O

O

Bi

O

Bi

O

O

Bi

Fe

O

O

OO

Fe

Fe

O

O

O Fe

O

Bi

Fe

O

Fe

Bi

O

PowderCell 2 .0

Celle parameters found with electron diffraction (a= 6,04 Å, b= 7.94 Å and c=8.66 Å) fits reasonably well with the

previously published data for the Bi2Fe4O9 phase. The disagreement in the c-axis may be due to the fact that we

have been studying a thin film grown on a crystalline substrate and is not a bulk sample. The conditions for

reflections from the space group Pbam is in agreement with observations done with electron diffraction.

Conclusion: The unknown phase has been identified as Bi2Fe4O9 with space group Pbam with cell parameters

a= 6,04 Å, b= 7.94 Å and c=8.66 Å.

The construction of a TEM

A.E. Gunnæs

A.E. Gunnæs

Basic TEM

Electron gunApertures

Sample

holder

Fluorescence

screenRecording media

(Cameras, detectors)

Vacuum in the column

better than 10-6 Pa

Sample

1. and 2.

condenser lenses

Objective lens

Intermediate lenses

Projector lens

Similar components as a transmission light microscope

• Two types of emission guns:

• Thermionic emission

• W or LaB6

• Field emission

W ZrO/W

Cold FEG Schottky FEG

The electron source

Thermionic emission

Thermionic gunsFilament heated to give

thermionic emission-Directly (W) or

indirectly (LaB6)

Filament negative

potential to ground

Wehnelt produces a

small negative bias-Brings electrons to

cross over

Field emission gun• The principle:

• The strength of an electric field E is

considerably increased at sharp points.

E=V/r

• rW < 0.1 µm, V=1 kV → E = 1010 V/m

• Lowers the work-function barrier so

that electrons can tunnel out of the tungsten.

• Surface has to be pristine (no contamination or oxide)

• Ultra high vacuum condition (Cold FEG) or poorer vacuum

if tip is heated (”thermal” FE; ZrO surface tratments → Schottky emitters).

Resolution

(JEOL2100F: 0.19 nm)

The point resolution in a TEM is limited by the aberrations of the lenses.-Spherical

- Chromatic

-Astigmatism

Electromagnetic lenses

F= -e(v x B)

A charged particle such as an electron, is deflected by a

magnetic field. The direction and magnitude of the force F,

on the electron is given by the vector equation:

Basic TEM

Electron gunApertures

Sample

holder

Fluorescence

screenRecording media

(Cameras, detectors)

Vacuum in the column

better than 10-6 Pa

Sample

1. and 2.

condenser lenses

Objective lens

Intermediate lenses

Projector lens

Similar components as a transmission light microscope

Simplified ray diagram

Objective lense

Diffraction plane (back focal plane)

Image plane

Sample

Parallel incoming electron beamSi

a

b

c

Po

wd

erCell 2

.0

1,1 nm

3,8

Å

Objective aperture

Selected area aperture

Selected area diffraction

Objective lense

Diffraction pattern

Image plane

Specimen with twocrystals (red and blue)

Parallel incoming electron beam

Selected area aperture

Pattern on the screen

• Diffraction from a single crystal in a polycrystalline sample if the aperture is small enough/crystal large enough.

• Orientation relationships can be determined.

• ~2% accuracy of lattice parameters

– XRD is much more accurate

Poly crystalline sample

The orientation relationship between the phases can be determined with ED.

25

Single Crystals

Interface between two different phases

epitaxially grown

Electron Diffraction in TEM

Amorphous phase

Diffraction with large SAD aperture, ring and spot patterns

Similarities to XRD

SAD

Why do we observe many reflections in one diffraction pattern?

Cu Kalpha X-ray: = 150 pm

Electrons at 200 kV: = 2.5 pm

2dsinθB=λ

Courtesy: Dr. Jürgen Thomas, IFW-Dresden, Germany

Cu Kalpha X-ray: = 150 pm => small k

Illustration with the Ewald Sphere The radius of the Ewald sphere is 1/ (=k)

Resiprocal lattice of a crystal

ko k

Electrons at 200 kV: = 2.5 pm => large k

ED and form effectsThe dimensions of the specimen affects the shape of the resiprocal lattice poins

Real space Resiprocal space

The intensity distribution around each reciprocal

lattice point is spread out in the form of spikes directed

normal to the specimen.2d sinθ = nλ

λ200kV = 0.00251 nm

Θ~1o

I(k’-k)I=(2/λ)sinθB=g

Zone axis and Laue zones

Zone axis [uvw]

(hkl)

uh+vk+wl= 0

Indexing diffraction patternsThe g vector to a reflection is normal to the

corresponding (h k l) plane and IgI=1/dnh nk nl

- Measure Ri and the angles between

the reflections

- Calculate di , i=1,2,3 (=K/Ri)

- Compare with tabulated/theoretical

calculated d-values of possible phases

- Compare Ri/Rj with tabulated values for

cubic structure.

- g1,hkl+ g2,hkl=g3,hkl (vector sum must be ok)

- Perpendicular vectors: gi ● gj = 0

- Zone axis: gi x gj =[HKL]z

- All indexed g must satisfy: g ● [HKL]z=0

(h2k2l2)

Orientations of corresponding

planes in the real space

TEM imaging with parallell incomming beam

A.E. Gunnæs

Imaging / microscopy

200 nm

Si

SiO2

TiO2

Pt

BiFeO3

Glue

Amplitude contrast Phase contrast

The elctron wave can change both its

amplitude and phase as it traverses the specimen

Give rise to contrast

We select imaging conditions so that one of them dominates.

Contrast• Difference in intensity of to adjacent areas:

11

12 )(

I

I

I

IIC

The eyes can not see intensity chanes that is less then 5-10%, however, contrast in images can be enhanced digitally.

NB! It is correct to talk about strong and week contrast but not bright and dark contrast

Use of apertures

Condenser aperture:Limits the number of electrons reaching the specimen (reducing the intensity),

Affecting the convergent of the electron beam.

Selected area aperture:Allows only electrons going through an area on the sample that is limited by the SAD aperture to

contribute to the diffraction pattern (SAD pattern).

Objective aperture:Allows certain reflections to contribute to the image. Increases the contrast in the image.

Bright field imaging (central beam, 000), Dark field imaging (one reflection, g), High resolution

Images (several reflections from a zone axis).

Simplified ray diagram

Objective lense

Diffraction plane(back focal plane)

Image plane

Sample

Parallel incoming electron beamSi

a

b

c

Po

wd

erCell 2

.0

1,1 nm

3,8

ÅObjective aperture

Selected area aperture

A.E. Gunnæs

Objective aperture: Contrast enhancement

No aperture used Central beam selected

Si Ag and Pb

glue(light elements)

hole

Amplitude contrast:

Mass-Density contrast and Diffraction contrast

TEM variables that affect the contrast: -The objective aperture size .-The high tension of the TEM.

Areas of greater Z and/or t scatter electronsmore strongly (in total).

Mass-Density contrast in TEMIncoherent elastic scattering (Rutherford scattering):

peaked in the forward direction, t and Z-dependent

Williams and Carter, TEM, Part 3

Springer 2009

Mass-density contrast

A.E. Gunnæs

50 nm

Diffraction contrast

Objective aperture: Contrast enhancement

Intensity: Dependent on grain orientation

Try to make an illustration to explain why we get

this enhanced contrast when only the central beam

is selected by the optical aperture.

Diffraction contrast

A.E. Gunnæs

Bright field image

Size of objective apertureBright field (BF), dark field (DF) and High resolution EM (HREM)

BF image

Objectiveaperture

DF image

Amplitude/Diffraction contrast

HREM image

Phase contrast

Phase contrast: HREM and Moire’ fringes

http://www.mathematik.com/Moire/

A Moiré pattern is an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different mesh sizes (rotational and parallel Moire’ patterns).HREM image

Long-Wei Yin et al., Materials Letters, 52, p.187-191

Interference pattern

A.E. Gunnæs MENA3100 V18

Transmissions Electron Microscopy (TEM)

Basic principles

Diffraction

Imaging

Specimen preparation

PART II

Some repetiton….

A.E. Gunnæs

Specimen

TEM mode with parallel incoming electron beam

Change the strength of the intermediate lens:

The diffraction pattern or image of the specimen is magnified

Specimen

TEM mode with parallel incoming electron beam

Apertures(No: Blendere)

Condenser

Object

SAD

SAD aperture: used to select an area on the specimen one wantdiffraction date from (on the second intermediat image and projected).

Specimen

TEM mode with parallel incoming electron beam

Apertures(No: Blendere)

Condenser

Object

SAD

Objective aperture: used to select which electron beams willcontribute to the image (on the second intermediat image and projected).

TEM mode with parallel incoming electron beam

Objective aperture: used to select which electron beams willcontribute to the image (on the second intermediat image and projected).

The objective aperture is used to controls the contrast in the image(enhances contrast).

One beam: Amplitude contrast (central (BF) or a scattered beam (DF))

Two or more beams: Phase contrast (+ amplitude)(HREM images (zone axis) or Moire)

Size of objective apertureBright field (BF), dark field (DF) and High resolution EM (HREM)

BF image

Objectiveaperture

DF image

Amplitude/Diffraction contrast

HREM image

Phase contrast

Phase contrast

Amplitude contrast

Diffraction contrast and Mass-density contrast

A TEM image will in most cases show both contrast types

TEM variables that affect the contrast: -The objective aperture size .-The high tension of the TEM.

Areas of greater Z and/or t scatter electronsmore strongly (in total).

Mass-Density contrast in TEMIncoherent elastic scattering (Rutherford scattering):

peaked in the forward direction, t and Z-dependent

Williams and Carter, TEM, Part 3

Springer 2009

Diffraction contrast in the TEM

A.E. Gunnæs

Bright field image

50 nm

The contrast is very sencitive to thespecimen orientation. (In contrast to mass-density contrast)

Effect of specimen tilt on diffraction contrast

Diffraction contrast

Where do you seea)mass-density contrast and b) Diffraction contrast?

200 nm

Si

SiO2

TiO2

Pt

BiFeO3

Glue

Crystal defects

- Effect of bending- Dislocations- Wedges

A.E. Gunnæs

BF image

DF image

DF image

Obj. aperture

Obj. lens

sample

Bending contours

Solberg, Jan Ketil & Hansen, Vidar (2001).

Innføring i transmisjon elektronmikroskopi

Bending contours

Dislocations

Double diffraction, extinction thickness

• Double electron diffraction leads to oscillations in the diffracted intensity with increasing thickness of the sample

• No double diffraction with XRD, kinematical intensities

• Forbidden reflection may be observed

• t0: Extinction thickness

• Periodicity of the oscillations

• t0=πVc/λIF(hkl)I

Incident beam

Diffracted beam Doublydiffracted beam

Transmitted beamWedge shaped TEM sample

t0

Simplified kinematical theory for perfect crystalsBasis of kinematical theory of electron diffraction for imperfect crystals:

Ψg(t)= ∫(πi/ξg) exp(-2πisgz)dz, Ψo=1, t: crystal thickess t

0

Intensity of the scattered beam g (dark field):

Ig= l Ψg(t) l2= sin2 πsgt/(ξgsg)

2

Intensity of the unscattered beam 0 (bright field):

I0= 1-Ig= 1- l Ψg(t) l2= 1 - sin2 πsgt/(ξgsg)

2

Ψg(t)= (i/ξgsg) exp(-πitsg) sinπsgt

A.E. Gunnæs MENA3100 V10

Sample (side view)

e

000 g

t

Ig=1- Io

In the two-beam situation the intensity

of the diffracted and direct beam

is periodic with thickness (Ig=1- Io)

Sample (top view)Hole

Positions with max

Intensity in Ig

Thickness fringes (sg konstant)

Intensity of the scattered beam g: Ig= l Ψg(t) l

2= sin2 πsgt/(ξgsg)2

A.E. Gunnæs MENA3100 V10

Thickness fringes, bright and dark field images

Sample Sample

DF imageBF image

Kikuchi linesOrigin and use

Line pairs in the diffraction plane

Zone axis pattern

Need to have a thick specimen region

Close to a zone axis

Need two scattering events

1. Inelastic2. Elastic

1.

-Angular distribution of inelastic scattered electrons falls of rapidly with angle. I=Iocos2α

Kikuchi pattern

http://www.doitpoms.ac.uk/index.html

http://www.doitpoms.ac.uk/tlplib/diffraction-patterns/kikuchi.php

ExcessDeficient

Excess

lineDeficient

line

2θB

θB

θB

Diffraction plane

Objective lens

1/d

.

1.Ineleastic scattering +

2. Bragg scattering event

What will happen if you tilt the specimen?

Incoherently and inelastically(ΔE~15-25 eV) scattered electrons give rise to diffuse background in the ED pattern

Kikuchi maps

http://www.umsl.edu/~fraundorfp/nanowrld/live3Dmodels/vmapframe.htm

000 g-g

Ig=I-g

Sg<0

Sg=0

Effect of tilting the specimen

Kossel cones

Parabolas

g and –g Kikuchi lines

TEM specimen preparation

What to considder before preparing a TEM specimen

• Ductile/fragile

• Bulk/surface/powder

• Insulating/conducting

• Heat resistant

• Single phase/multi phase

• Etc, etc…….

What is the objectiv of the TEM work?

Specimen preparation for TEM• Crushing

• Cutting• saw, “diamond” pen, ultrasonic drill,

ultramicrotomy

• Mechanical thinning• Grinding, dimpling,

• Tripod polishing

• Electrochemical thinning

• Ion milling

• Coating

• Replica methods

• FIB (Focused ion beam)

• Etc.

•Grids• Several types

• Different materials (Mo, Cu, Ni…)

• Support brittle materials

• Support small particles3 mm

The grid may contribute to the EDS signal.

Preparation of self-supporting discs Top view specimens

• Cutting• Ductile material or not?

• Grinding• 100-200 μm thick

• polish

• Cut the 3mm disc

• Dimple ?

• Final thinning• Ion beam milling

• Electropolishing

A.E. Gunnæs

Grind down/dimple

Cross section TEM sample preparation: Thin films

• Top view

• Cross section

or

Cut out a cylinderand glue it in a Cu-tube

Grind down andglue on support rings

Cut a slice of thecylinder and grindit down / dimple

Ione beam thinning

Cut out cylinder

Ione beam thinning

Cut out slices

Glue the interface of interest face to face together withsupport material

Cut off excessmaterial

• Focused Ion Beam (FIB)

Focused ion beam TEM specimen preparation