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a question from last year final exam

http://emalwww.engin.umich.edu/education_materials/microscopy.htmlhttp://emalwww.engin.umich.edu/education_materials/microscopy.html

A Question from Last Year Final Exam

Recommend an instrumental method that will provide information about the chemical composition and crystal symmetry of precipitates (small black dots of ~0.1m wide) in a polycrystalline sample with micrometer-sized grains as shown below. State your reasons. [10 marks]

Precipitates

http://www.youtube.com/watch?v=yqLlgIaz1L0 see atoms by TEM

limits of om sem spm and xrd

G.B.

NN

Limits of OM, SEM, SPM and XRD

core

G.B.

NN

shell

BT

BT

BT-BaTiO3

NN-NaNbO3

G.B.-Grain boundary

M.G.J.-multiple grain

junction

M.G.J.

EDS

0.2m

NN/BT

  • Lateral resolution: ~m
  • Details of microstructure:

e.g., domain structure,

chemical inhomogeneity

phase distribution, grain

boundaries, interfaces,

precipitates, dislocations,

etc.

why tem
Why TEM?

The uniqueness of TEM is the ability to obtain full morphological (grain size, grain boundary and interface, secondary phase and distribution, defects and their nature, etc.), crystallographic, atomic structural and microanalytical such as chemical composition (at nm scale), bonding (distance and angle), electronic structure, coordination number data from the sample.

TEM is the most efficient and versatile technique for the characterization of materials.

http://www.youtube.com/watch?v=yqLlgIaz1L0 see atoms by TEM

lecture 6 transmission electron microscopy tem scanning transmission electron microscopy stem
Lecture-6 Transmission Electron Microscopy (TEM)Scanning Transmission Electron Microscopy (STEM)
  • What is a TEM?
    • How it works - gun, lenses, specimen holder
    • Resolution
  • What can a TEM do?
    • Imaging and diffraction

Imaging-diffraction and phase contrast

Diffraction-Selected area electron diffraction (SAED)

and Convergent beam electron diffraction (CBED)

    • Chemical analysis

EDS, Electron Energy Loss Spectroscopy (EELS)

Energy Filtered Imaging

http://emalwww.engin.umich.edu/education_materials/microscopy.html

http://www.youtube.com/watch?v=6fX1m2rImiM to~2:40 History & applications

lecture 6 transmission electron microscopy tem scanning transmission electron microscopy stem1

What is TEM?

Lecture-6 Transmission Electron Microscopy (TEM)Scanning Transmission Electron Microscopy (STEM)
  • TEM is an microscopy technique that functions similar to a light microscope, which uses a beam of exited electrons as a light source to provide mophorlogical, compositional and crystallographic information of an ultra thin specimen.
  • The image is formed by the interaction of the electrons transmitted through the specimen, which is then magnified and focused on a fluorescence screen containing a layer of photographic film.
  • What is a TEM?
    • How it works - gun, lenses, specimen stage
    • Resolution

http://www.youtube.com/watch?v=fxEVsnZT8L8 ~2:20-2:40 fluorescence screen

http://www.youtube.com/watch?v=C3uU8c376Aw&list=PLIRAzwu_npNcnPGi2sOk2aaNaS3vzU-N1

Milestones of Science: Ernst Ruska and the Electron Microscope at~5:00-7:15 and ~9:03-9:23

slide7

Comparison of OM and TEM

Principal features of an optical microscope and a transmission electron microscope, drawn to emphasize the similarities of overall design.

slide8

http://www.youtube.com/watch?v=fToTFjwUc5M

Structure and Function of TEM

CM200 (200kV)

Column

Electron Gun

EDS Detector

Condenser

Lens

Objective Lens

Specimen Holder

SAD Aperture

Binocular

Magnifying

Lenses

TV Monitor

Camera

Chamber

Viewing Chamber

Cost:  $4,000,000

http://www.youtube.com/watch?v=6fX1m2rImiM at~2:40-4:40

http://www.youtube.com/watch?v=2wEmsDh_l_A at~0:30

vacuum
Vacuum

The electron microscope is built like a series of vessels connected by pipes and valves separate all the vessels from each other.

The vacuum around the specimen is around 10-7 Torr.

The vacuum in the gun depends on the type of gun, either around 10-7 Torr (the tungsten or LaB6 gun) or 10-9 Torr (for the Field Emission Gun).

The pressure in the projection chamber is usually only 10-5 Torr (and often worse). This pressure is not very good because the projection chamber holds the negatives used to record images. Even though we dry the negatives before putting them in the microscope, they still will give off so many gases that the vacuum in the projection chamber never gets very good.

slide10

How it works? The Lenses in TEM

http://www.youtube.com/watch?v=j2A6KeWrqeM&feature=related

at~0:20-0:44

Condenser lenses(two)-control how

strongly beam is focused (condensed)

onto specimen. At low Mag. spread

beam to illuminate a large area, at high

Mag. strongly condense beam.

Objective lens-focus image (image

formation) and contribute most to

the magnification and resolution of the image.

Magnetic material

Running water

B

Cu coils

Four lenses form magnification

system-determine the magnification

of the microscope. Whenever the

magnification is changed, the currents

through these lenses change.

at~5:50-7:00

http://www.youtube.com/watch?v=C3uU8c376Aw&list=PLIRAzwu_npNcnPGi2sOk2aaNaS3vzU-N1

slide11

How it works? Image Formation in TEM

A disc of metal

Control

brightness,

convergence

under in over

focus focus focus

Control contrast

Schematic of the Optics of a TEM

http://www.youtube.com/watch?v=6fX1m2rImiM at~3:00-4:45

why electrons resolution
Why Electrons? Resolution

In expression for the resolution

(Rayleigh’s Criterion)

r = 0.61/nsin

Green Light

~400nm

n~1.7 oil immersion

r~150nm (0.15m)

Electrons

-wavelength,=[1.5/(V+10-6V2)]1/2 nm

V-accelerating voltage, n-refractive index

-aperture of objective lens, very small in TEM

 sin  and so r=0.61/ ~0.1 radians

200kV Electrons

~0.0025nm

n~1 (vacuum)

r~0.02nm (0.2Å) 1/10th size of an atom!

UNREALISTIC! WHY?

0.1 radians ~ 5.5o

-beam convergence

resolution limited by lens aberrations
Resolution Limited by Lens Aberrations

Chromatic aberration is caused by the variation of the electron energy and thus electrons are not monochromatic.

point is imaged

as a disk.

Spherical aberration is caused by the lens field acting inhomogeneously on the off-axis rays.

rmin0.91(Cs3)1/4

Practical resolution of microscope. Cs–coefficient of spherical aberration of lens (~mm)

point is imaged

as a disk.

beam and specimen interaction
Beam and Specimen Interaction

(EDS)

BF

DF

HREM

Imaging

SAED & CBED

diffraction

(EELS)

scanning transmission electron microscopy

(STEM)

Scanning Transmission Electron Microscopy

JEOL 2000FX

Analytical Electron Microscope

In STEM, the electron beam is rastered (scan coil) across the surface of a sample in a similar manner to SEM, however, the sample is a thin TEM section and the diffraction contrast image is collected on a solid-state (ADF) detector.

Scanning

beam

specimen

HAADF

Detector

ADF

BF

ADF

DF

BF

STEM detector

or EELS

HAADF-high angle

annular dark-field

http://www.youtube.com/watch?v=WJUL22UoCLI Scanning transmission electron holography microscope

http://en.wikipedia.org/wiki/Scanning_transmission_electron_microscopy

specimen holder

Rotation, tilting, heating, cooling and straining

Specimen Holder

beam

holder

a split polepiece

objective lens

Double tilt heating

Heating and straining

Twin specimen holder

http://www.youtube.com/watch?v=j2A6KeWrqeM&feature=related at~0:56-1:42

specimen holder with electrical feedthroughs
Specimen Holder with Electrical Feedthroughs

http://www.youtube.com/watch?v=fxEVsnZT8L8 at~3:00-3:34

specimen preparation destructive
Specimen Preparation-Destructive

Dispersing crystals or powders on a carbon film on a grid

3mm

Making a semiconductor specimen with a Focused Ion Beam (FIB)

1

2

4

5

3

  • a failure is located and a strip of Pt is placed as a protective cover.
  • On one side of the strip a trench is milled out with the FIM.
  • The same is done on the other side of the strip (visible structure).
  • The strip is milled on both sides and then the sides connecting the strip to the wafer are cut through.
  • The strip is tilted, cut at the bottom and deposited on a TEM grid.

http://www.youtube.com/watch?v=F0ZNUykXovk Preparing specimen

specimen preparation 2
Specimen Preparation-2

Ion-milling a ceramic

Ar (4-6keV, 1mm A)

3mm

Ultrasonic cut

grind

Dimple center part

of disk to ~5-10m

ion-mill until a hole appears in disk

Jet-polishing metal

-

+

a thin stream of acid

A disk is mounted in a jet-polishing machine and is electropolished until a small hole is made.

Cut into disks

and grind

Drill a 3mm

cylinder

Ultramicrotomy-using a (diamond) knife blade

Mainly for sectioning biological materials.

To avoid ion-milling damage ultramicrotome can also be used

to prepare ceramic TEM specimens.

http://www.ims.uconn.edu/~micro/Dimple%20Grinding2.pdf

TEM specimen preparation

what can a tem do imaging
What can a TEM do? Imaging

BF and DF imaging

HREM

Objective

Aperture

(OA)

BF - Bright Field

DF - Dark Field

bf df imaging diffraction contrast
BF & DF Imaging – Diffraction Contrast

DDFCDF

OA

OA

Beam tilt

crystal

C-film

amorphous

D

D

T-transmitted

D-diffracted

Objective

aperture

T

Objective

aperture

T

DF image

BF image

Hole in OA

C-film

C-film

crystal

crystal

Diffraction + mass-thicknessContrast

http://micro.magnet.fsu.edu/primer/virtual/virtualzoo/index.html

diffraction thickness and mass contrast
Diffraction, Thickness and Mass Contrast

BF images

Weak diffraction

thinner

thicker

Strong

diffraction

2

G.B.

8

7

thickness

fringes

1

3

thickness

Disk specimen

6

.

.

.

.

.

.

. . .

.

.

.

.

.

.

.

.

.

..

.

.

.

.

4

.

Low

mass

High

mass

5

S

T

T

S

S

8 grains are in different orientations

or different diffraction conditions

Bright Dark

bf and df imaging
BF and DF Imaging

BF imaging-only transmitted beam is allowed to pass objective aperture to form images.

mass-thickness

contrast

Incident beam

BF

specimen

beam

diffracted

DF imaging

only diffracted

beams are

allowed to pass

the aperture to

form images.

Particles in Al-Cu

Alloy.

thin platelets ll e

Vertical, dark

Particles e.

DF

transmitted beam

objective aperture

DF

hole in objective

aperture(10-100m)

phase contrast imaging high resolution electron microscopy hrem
Phase Contrast ImagingHigh Resolution Electron Microscopy (HREM)

T

D

Si

BN

Objective

aperture

Electron diffraction pattern recorded

From both BN film on Si substrate.

Use a large objective

aperture.

Phases and intensities of diffracted and

transmitted beams are combined to form a phase contrast image.

electron diffraction

http://www.matter.org.uk/diffraction/electron/electron_diffraction.htmhttp://www.matter.org.uk/diffraction/electron/electron_diffraction.htm

Geometry for

e-diffraction

Bragg’s Law:l=2dhklsinhkl

Electron Diffraction

e-

=[1.5/(V+10-6V2)]1/2 nm

l=0.037Å (at 100kV)

=0.26o if d=4Å

dhkl

Specimen

foil

e-beam

Zone axis of crystal

l = 2d

e-beam is almost

parallel to {hkl}

L 2

r/L=sin2

as  0

r/L = 2

r/L = l/d or

r = lLx

sample

crystal

X-ray

r

polycrystal

T D

Reciprocal

lattice

1

d

L -camera length

r -distance between T and D spots

1/d -reciprocal of interplanar distance(Å-1)

SAED –selected area electron diffraction

hkl

[hkl] SAED pattern

http://www.youtube.com/watch?v=fxEVsnZT8L8 at~3:00-3:34

reciprocal lattice

A reciprocal lattice is another way of view a crystal lattice and is used to understand diffraction patterns. A dimension of 1/d(Å-1) is used in reciprocal lattices.

Reciprocal Lattice

g – reciprocal lattice vector

2 d reciprocal lattices

http://www.youtube.com/watch?v=iC15RHX4gpQ

2-D Reciprocal Lattices

For every real lattice there is an equivalent  reciprocal lattice.

Real space:

Unit cell vectors: a,b

d-spacing direction

a d10 [10]

b d01 [01]

Reciprocal space:

Unit cell vectors:a*,b*

magnitude direction

a* 1/d10b

b* 1/d01a

A reciprocal lattice can be built using reciprocal vectors. Both the real and reciprocal construc-tions show the same lattice, using different but equivalent descriptions.

[01]

[10]

(10)

b*

a*

01

(01)

02

10

11

12

20

21

22

Note:each point in the reciprocal lattice represents a set of planes.

http://www.matter.org.uk/diffraction/geometry/2d_reciprocal_lattices.htm

3 d reciprocal lattice
3-D Reciprocal Lattice

Real space:

Unit cell vectors: a,b,c

magnitude direction

a d100 [100]

b d010 [010]

c d001 [001]

Reciprocal space:

Unit cell vectors:a*,b*

magnitude direction

a* 1/d100b and c

b* 1/d010aand c

c* 1/d001aand b

Orthorhombic

Note:as volume of unit cell in real space increases the volume of unit cell in reciprocal space decreases, and vice versa. a*,b* and c* are parallel to corresponding a,b and c, and this is only true for the unit cells of cubic, tetragonal and orthorhmbic crystal systems.

http://www.matter.org.uk/diffraction/geometry/3d_reciprocal_lattices.htm

http://www.matter.org.uk/diffraction/geometry/reciprocal_lattice_exercises.htm

lattice vectors
Lattice Vectors

Real space lattice vector

corresponds to directions in crystal and it can be defined as:

r=ua+vb+wc

a,b and care unit cell vectors,

u,vandware components of

the direction index[uvw].

A reciprocal lattice vector

can be written as:

g*=ha*+kb*+lc*

a*,b*andc*are reciprocal unit vectors, and h,k and lare the Miller indices of the plane (hkl).

effect of spacing of planes in real space on length of reciprocal vector g
Effect of Spacing of planes in Real Space on Length of Reciprocal Vector, g

-

[111]

-

(111)

-

d111

In a crystal of any structure, ghkl is normal to the (hkl)

plane and has a length inversely proportional to the interplanar spacing of the planes.

http://www.matter.org.uk/diffraction/geometry/reciprocal_vector_g.htm

why are there so many spots ewald sphere and diffraction pattern
Why are there so many spots?Ewald Sphere and Diffraction Pattern

SAED pattern XRD pattern

Reciprocal Lattice

k – wave vector

lkl = 1/

 – wavelength of electron

slide34

Reciprocal Lattice

The Ewald Sphere and Diffraction Pattern

Ewald Sphere Construction

1/

A set of real lattice planes

D

T

k – wave vector

lkl = 1/

 – wavelength of electron

Bragg’s Law

http://www.matter.org.uk/diffraction/geometry/ewald_sphere_diffraction_patterns.htm

slide35

R=1/

XRD

R

Why there are so many diffraction spots in ED?

SAED

R

R=1/

slide36

parallelbeam

A TEM technique to reduce both the area and intensity of the beam contributing to a diffraction pattern by the insertion of an aperture into the image plane of the objective lens. This produces a virtual diaphragm in the plane of the specimen.

Virtual

SAED

Selected Area Electron Diffraction

aperture

specimen

Objective

lens

Diffraction

pattern

Back focal

plane

SAD

aperture

focusing saed pattern at fixed screen
Focusing SAED Pattern at Fixed Screen

by changing magnetic lens strength

specimen

lens

screen

Diffracted beam

Transmitted beam

SAED gives 2-D information

Spot pattern

http://www.matter.org.uk/diffraction/electron/electron_diffraction.htm

saed patterns of single crystal polycrystalline and amorphous samples
SAED Patterns of Single Crystal, Polycrystalline and Amorphous Samples

a

b

c

020

110

200

r1

r2

  • Single crystal Fe (BCC) thin film-[001]
  • Polycrystalline thin film of Pd2Si
  • Amorphous thin film of Pd2Si. The diffuse

halo is indicative of scattering from an

amorphous material.

diffraction spot intensity
Diffraction Spot Intensity

Spot intensity: Ihkl lFhkll2

Fhkl - Structure Factor

N

Fhkl =  fj exp[2i(hu+kv+lw)]

j=1

fj – atomic scattering factor

fjZ, sin/

h,k,l are Miller indices and u,v,w fractional coordinates

slide40

_

[013]

131 (311)?

200

SAED

_

slide41

SAED Patterns

dhkl = lL/rhkl

SAED

the table

50nm

next lecture
Next Lecture

TEM

Convergent beam electron diffraction (CBED)

Chemical analysis

EDS, Electron Energy Loss Spectroscopy (EELS)

Energy Filtered Imaging

Secondary Ion Mass Spectroscopy (SIMS)