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A.E. Gunnæs MENA3100 V18

Transmissions Ele lectron Mic icroscopy (T (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

Operatin ing 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

Imaging Diffraction Spectroscopy

With 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 is based on three possible set of f 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

Th The in interestin ing obje jects for r TE TEM is is lo local l structure and inh inhomogeneit itie ies in in sp specim imens

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

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 Lim Glue 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 electrons

Only 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

6.04 Å 7.94 Å

a b c 100 110 111 010 011 001 101 [011] [100] [101] d = L λ / R

Chemical analysis by use of EDS and EELS

Ukjent fase BiFeO3 BiFe2O5

10 15 20 25 30 35 40 Nr_2_1evprc.PICT

200 400 600 800 1000

2 4 6 8 10 12 14 Energy Loss (eV) CCD counts x 1000

Ukjent fase BiFeO3 Fe - L2,3 O - K 500 eV forskyvning, 1 eV pr. kanal

Published structure

A.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 Fe 4h 0,349 0,333 0,5 Fe 4f 0,5 0,244 O 4g 0,14 0,435 O 8i 0,385 0,207 0,242 O 4h 0,133 0,427 0,5 O 2b 0,5

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

Ba Basic ic TE TEM

Electron gun Apertures Sample holder Fluorescence screen Recording 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 ele lectron source

Thermionic emission

Thermionic guns

Filament 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

Ele lectromagnetic le 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,

• n the electron is given by the vector equation:

Ba Basic ic TE TEM

Electron gun Apertures Sample holder Fluorescence screen Recording 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 beam Si

1,1 nm 3,8 Å

Objective aperture Selected area aperture

Selected area diffraction

Objective lense Diffraction pattern Image plane Specimen with two crystals (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

Dif iffr fractio ion with ith lar large SAD aperture, , rin ring and sp spot patterns

Similarities to XRD

SAD

Why do we observe many reflections in

• ne 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

Il 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 effects

The 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

In Indexin ing dif iffractio ion patterns

The 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

Im 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:

1 1 1 2

) ( I I I I I C    

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 beam Si

1,1 nm 3,8 Å

Objective aperture Selected area aperture

A.E. Gunnæs

Obje jective 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 electrons more strongly (in total).

Mass-Density contrast in TEM

Incoherent 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

Obje jective 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 f objective aperture

Brigh right field field (BF (BF), d dark rk field field (DF (DF) and Hig High res esolu lution EM (HR (HREM)

BF image Objective aperture 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 Ele lectron Mic icroscopy (T (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

• f 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 want diffraction 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 will contribute 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 will contribute 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 f objective aperture

Brigh right field field (BF (BF), d dark rk field field (DF (DF) and Hig High res esolu lution EM (HR (HREM)

BF image Objective aperture 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 electrons more strongly (in total).

Mass-Density contrast in TEM

Incoherent 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 the specimen orientation. (In contrast to mass-density contrast)

Effect of

• f specimen tilt on
• n diffraction contrast

Diffraction contrast

Where do you see a)m )mass-density contrast and b) ) Dif iffraction 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, , ext xtinction 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 Doubly diffracted beam Transmitted beam Wedge shaped TEM sample t0

Sim implif ified kin inematical theory for perfect cry crystals

Basis of kinematical theory of electron diffraction for imperfect crystals: Ψg(t)= ∫(πi/ξg) exp(-2πisgz)dz, Ψo=1, t: crystal thickess

t

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

• f the diffracted and direct beam

is periodic with thickness (Ig=1- Io) Sample (top view) Hole Positions with max Intensity in Ig

Thic ickness fr fringes (s

(sg konstant)

Intensity of the scattered beam g: Ig= l Ψg(t) l2= sin2 πsgt/(ξgsg)2

A.E. Gunnæs MENA3100 V10

Thickness fringes, bright and dark field images

Sample Sample DF image BF image

Kik ikuchi li lines

Origin 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. Inelastic
• 2. 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

Excess Deficient Excess line Deficient 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 consid idder before preparin ing 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 particles

3 mm

The grid may contribute to the EDS signal.

Preparation of self lf-supporting dis iscs Top vie iew 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

in film films

• Top view
• Cross section
• r

Cut out a cylinder and glue it in a Cu-tube Grind down and glue on support rings Cut a slice of the cylinder and grind it down / dimple

Ione beam thinning

Cut out cylinder

Ione beam thinning

Cut out slices Glue the interface

• f interest face to

face together with support material Cut off excess material

• Focused Ion Beam

(FIB)

Focused ion beam TEM specimen preparation