Chapter 3 Transmission Electron Microscopy (TEM)

Chapter 3
Transmission Electron Microscopy
(TEM)
Today, let’s study TEM
Why TEM?
Higher resolution. As high as 0.1-0.3nm.
Allow for the study of internal structures of
materials.
Electron diffraction patterns allow for
structural analysis.
What is TEM?
Transmission electron microscopy (TEM) is a
microscopy technique whereby a beam of electrons
is transmitted through an ultra thin specimen,
interacting with the specimen as it passes through.
An image is formed from the interaction of the
electrons transmitted through the specimen; the
image is magnified and focused onto an imaging
device, such as a fluorescent screen.
Outlines
3.1 Applications of TEM.
3.2 Brief working principle of TEM.
3.3 Instrumentation.
3.4 Sample Preparation.
3.5 Selected area electron diffraction
(SAED)
What is TEM used for?
 Morphology
The size, shape and arrangement of the particles which
make up the specimen as well as their relationship to
each other on the scale of atomic diameters.
 Crystallographic Information
The arrangement of atoms in the specimen and their
degree of order, detection of atomic-scale defects in
areas a few nanometers in diameter
 Compositional Information (if so equipped)
The elements and compounds the sample is composed
of and their relative ratios, in areas a few nanometers in
diameter
Applications of TEM—
Morphology
ZnO Nanowires
Applications of TEM—Morphology
Carbon nanotubes
Applications of TEM—Morphology
Crystallographic Information
Crystallographic Information
Crystallographic Information
CdSe quantum dot
Crystallographic Information
Selected area diffraction patterns
Now, Let’s think about:
How can these TEM images
be produced?
Do you still remember how
SEM images are produced?
Electron-Solid Interactions
When electrons passing through a
specimen, will they change their
direction or not?
Because of the wave-particle
duality, the electrons will be
scattered or diffracted!
Rutherford Scattering
Mass-thickness contrast
More electrons are scattered in
thick than in thin areas.
More electrons are scattered in
heavy atoms.
Contrast= (I1-I 2)/I1
Thickness contrast
Mass Contrast
Heavy metal
staining
in biomaterials and
Polymers.
Wave-particle Duality of Electrons
Bragg’s Law
n λ = 2 d sin θ
0 < Θ < 2.
Eg: 100kV, l=0.037Å
sinq = l/2dHKL=10-2,
q≈10-2<1o
Electron diffraction
Diffraction contrast
If the sample has crystalline areas, many
electrons are strongly scattered by Bragg
diffraction ,and this area appears with dark
contrast in the BF image as well.
Diffraction contrast is affected by defects,
bending, dislocations etc. It can reflect the
crystal information of a sample.
Diffraction contrast image
Phase contrast
Phase contrast : non-uniform distribution
of electrons based on interference
between direct wave and phase shifted
diffracted waves (multi-beam imaging or
high-resolution TEM)
Working modes of SEM
Imaging modes:
– Bright field mode
– Dark field mode
– High resolution image mode
Diffraction mode:
Bright field mode
 An aperture is placed in
the back focal plane of
the objective lens which
allows only the direct
beam to pass.
 In this case, the image
results from a weakening
of the direct beam by its
interaction with the
sample.
 Therefore, mass-thickness
and diffraction contrast
contribute to image
formation
Dark field mode
 The direct beam is
blocked by the aperture
while one or more
diffracted beams are
allowed to pass the
objective aperture.
 Since diffracted beams
have strongly interacted
with the specimen, very
useful information is
present in DF images, e.g.,
about planar defects,
stacking faults or particle
size.
Examples of BF and DF images
High resolution lattice images
– A large objective
aperture has to be
selected that allows
many beams including
the direct beam to pass.
– The image is formed
by the interference of
the diffracted beams
with the direct beam
(phase contrast)
Let’s see how TEM works!
How TEM works?
- A electron beam is focused by 2
condenser lenses, restricted by a
condenser aperture;
- The beam strikes a specimen and
part of it is transmitted;
- This transmitted portion is focused
by objective lens into an image;
- The image is passed down through
enlarge lenses and a projector lens,
being enlarged all the way;
- The image strikes the phosphor
image screen and light is generated,
allowing user to see the image.
Overview of a TEM Instrument
Four Main Components:
•Illuminating System
•Specimen Manipulation
System
•Imaging System
•Vacuum System
TEM vs. OM: Similarities
 Illumination system: produces required
radiation and directs it onto the specimen.
 Specimen stage: situated between the
illumination and imaging systems.
 Imaging system: Lenses which together
produce the final magnified image of the
specimen. Consists of i) an objective lens and ii)
the projector lens.
 Image recording system: Converts the
radiation into a permanent image (typically on a
photographic emulsion) that can be viewed.
TEM vs. OM: Differences

Optical lenses are generally made of glass
with fixed focal lengths, whereas magnetic
lenses are constructed with ferromagnetic
materials and windings of copper wire
producing a focal length which can be
changed by varying the current through the
coil.

Magnification in the OM is generally
changed by switching between different
power objective lenses. In the TEM the
magnification (focal length) of the objective
remains fixed while the focal length of the
projector lens is changed to vary
magnification.
TEM vs. OM: Differences

Mechanisms of image formation vary (phase
and amplitude contrast).

TEMs are generally constructed with the
radiation source at the top of the instrument:
the source is generally situated at the bottom
of OMs.

TEM is operated at high vacuum (since the
mean free path of electrons in air is very
small) so most specimens (biological) must
be dehydrated (i.e. dead !!).

TEMs can achieve higher magnification and
better resolution than OMs.
Materials Sample Preparation for TEM
Electron Transparency (thickness < 100 nm)
Initial form of the material: particulate or bulk
material
Particulate materials (powders, nanoparticles,
nanowires)
Desirable particle size is about 500 nm or less. Powders
that are more coarse than this should be ground with a
mortar and pestle. The specimen preparation consists in
transferring a suspension of the particles in a solvent
such as isopropanol to a carbon coated grid and letting
the solvent evaporate
Bulk material
Simple metals or single-phase alloys can often be electro polished
with an appropriate electrolytic solution. Even multiple phase
alloys can sometimes be prepared in this fashion.
More commonly, samples from bulk material are thinned with an ion
beam. Before the final ion-beam thinning, however, the sample
should be first mechanically thinned by lapping and polishing so that
the final thickness in the center of the sample is about 30 microns
Supporting Film
TEM Sample Preparation
 Ion Milling
 Electrochemical polishing
 Focused Ion Beam
Technique
TEM Sample Preparation: FIB
TEM Sample Preparation: FIB
TEM Sample Preparation: FIB
TEM Sample Preparation: FIB
TEM Sample Preparation: FIB
TEM Sample Preparation: FIB
TEM Sample Preparation: FIB
5 nm
50 nm
Pan, Liu, Fujita, et al. Phys. Rev. B (2010).