TEM TRANSMISSION ELECTRON MICROSCOPE

1. Transmission Electron Microscopy (TEM)
Invention and Evolution of the Modern TEM
• In 1932, invented by E. Ruska et al.
• In 1986, Ruska received the Nobel Prize
Ruska at Nobel prize meet
Max Knoll and Ernst Ruska
Modern day TEM

2. Milestones in the History of Electron Microscopy
• 1897 Discovery of the electron by J.J. Thompson
• 1924 P. De Broglie: particle/wave dualism
• 1927 Hans Busch: Electron beams can be focused in an inhomogeneous magnetic field
• 1931 Max Knoll and Ernst Ruska built the first TEM
• 1938 Scanning transmission electron microscope (M. von Ardenne)
• 1939 First commercial TEM by Siemens (Ruska, von Borries)
• 1943 Electron energy-loss spectroscopy EELS (J. Hillier)
• ~1940 Basic theoretical work on electron optics and electron lenses (W. Glaser, O. Scherzer)
• 1951 X-ray spectroscopy (R. Castaing)
• 1956 First lattice image (J. Menter)
• 1957 Multi-slice method (J. Cowley, A. Moodie)
• 1964 First commercial SEM by Cambridge Instruments
• ~1970 HRTEM microscopes with a resolution better than 4 Å
• 1986 Nobel prize for E.Ruska (together with G. Binning and H. Rohrer, who developed the Scanning Tunneling
Microscope)

3. What we covered on electron microscopy
1. Difference between Electron Microscope and Optical Microscope
2. Basic Working principle of SEM and analysis of SEM images
3. Outcomes of SEM images analysis
During this discussion we have covered
1. Why electrons
2. How we define resolution and magnification
3. EM lenses
Because of these differences, the microscope construction will also be different
What we to covered in TEM
1. Difference between SEM, TEM, and HRTEM
2. Basic difference between XRD and SAED (Selected Area Electron
Diffraction pattern)
3. Basic Working principle of TEM (including the basic components),
sample preparation and different imaging modes
4. Analysis of TEM images
5. Major outcomes of TEM analysis of a specimen

4. Why Electrons?
In the expression for the resolution (Rayleigh’s Criterion)
r = 0.61 /n sin
-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
Green Light 200kV Electrons
~400nm ~0.0025nm
n~1.7 oil immersion n~1 (vacuum)
r~150nm (0.15m) r~0.02nm (0.2Å)
Resolution Limited by Lens Aberrations

5. Types of Electron Microscope
Transmission Electron Microscope (TEM) uses a wide beam of
electrons passing through a thin sliced specimen to form an
image. This microscope is analogous to a standard upright or
inverted light microscope
Scanning Electron Microscope (SEM) uses focused beam of
electrons scanning over the surface of thick or thin specimens..
Images are produced one spot at a time in a grid-like raster
pattern. (secondary electrons generated due to elastically
scattered electrons)
High Resolution Transmission Electron Microscope (HRTEM)
uses both the transmitted and the scattered beams to create an
interference image. It is a phase contrast image and can be as
small as the unit cell of crystal. HRTEM has used for analysing
crystal structures and lattice imperfections in various kinds of
advanced materials on an atomic resolution scale. It can be used
for the characterization of point defects, stacking faults,
dislocations, precipitates grain boundaries, and surface structures.

7. What is a Transmission Electron Microscope (TEM)?
Electrons pass through a (very thin) sample (i.e. are transmitted) to form an
image.
Simplistically, in its operation a TEM can be thought of as analogous to a slide
projector:-
slide
Objective lens
Condenser lens
Light source
Slide projector
Projection screen
Fluorescent
screen
Objective lens Condenser lens
Specimen (thin)
aperture
Electron source
Electron beam
TEM

8. •In a slide projector white light from the bulb is focussed onto the slide by a
condenser lens.
•This produces an image which is focussed by the objective lens to a point
(crossover).
• The image is then projected onto a screen and magnified by varying the
distance from the crossover to the screen.
•In a TEM the electron beam is focussed on the sample using the condenser
lens system.
•This produces an image which is focussed by the objective lens to a point
(crossover).
•This image is then magnified by a series of projector lenses to vary the size
of the image on a fluorescent screen.
•Changing the current of an electromagnetic lens alters its focal length
altering magnification.

9. •TEMs’ have basically the same design independent of manufacturer or age:-
Anatomy of a TEM
Apertures
Viewing
Screen
Electron
Gun
Condenser
lens system
Apertures
Sample
Sample
Viewing
Screen
a) b) c)
Examples of Transmission Electron
Microscopes, a)Philips EM420 (1980s), b)
Cut through Schematic of Philips CM 200
(1990s) and c) JEOL 2100 (2000s)
• JEOL 2100 has digital imaging and
computer assisted operation.

10. The TEM Column
• Gun emits electrons
• Electric field accelerate
• Magnetic (and electric) field
control path of electrons
• Electron wavelength @ 200KeV
 2x10-12 m
• Resolution normally achievable
@ 200KeV  2 x 10-10 m  2Å

11. Electron Gun
W hairpin
LaB6 crystal
FEG

12. Electron Guns
•Two main types of gun – Thermionic and Field Emission.
•Thermionic sources produce electrons when heated.
•Field emission sources produce electrons when exposed to an intense
electric field.
•FEG’s give much more brightness than thermionic systems.
•FEG’s give a more monochromatic electron source and finer probe (i.e. better
resolution).
Comparison of the three types of source operating at 100kV

13. Electromagnetic lens

14. C1 controls the spot size
C2 changes the convergence of the beam
Condenser-lens system

15. The Condenser System
•The Wehnelt (or 2nd anode in a FEG) focuses the
beam to a crossover which is accelerated down the
column.
•The first condenser de-magnifies the crossover to
give a smaller point source – this is referred to as C1
or spot size.
•The second condenser lens (C2) is used to either
converge or spread the beam of illumination on the
sample (intensity (Philips) or brightness (JEOL)).
•A condenser aperture is placed in the beam path
to remove electrons far from the optic axis which
would reduce resolution.
•The smaller the aperture the better the
resolution, but there is an associated decrease in
brightness – need to compromise.

16. The Sample Preparation
•Samples are typically 3mm in diameter and <80µm thick. (it depends)
•Sample preparation is tricky and time consuming.
•For powders:-
•Grindmix the powder in chloroform or reagent grade methanol to create a
suspension.
•Pipette a drop of suspension onto a support grid with a Carbon film coating.
•Solvent will evaporate leaving powder dispersed on the grid.
•Allow to dry and observe.
Thin films are directly coated on support grids.
Copper
coated
Carbon
Coated
TEM sample Grids

18. Image Formation
•All rays from a point in the object are gathered
by the lens and converge to a point in the
image.
•All parallel rays are focused in the focal plane.
•The back focal plane of the objective lens
contains groupings of rays that have left the
object at the same angle.
•The back focal plane contains the diffraction
pattern of the sample.
•Diffraction pattern and image are both formed
in the imaging process
•The intermediate lens is then focused on
either the image plane (for the image), or the
back focal plane (for the diffraction pattern).
sample
Objective
lens

19. IMAGING MODES In the bright field (BF) mode of the TEM, an aperture
is placed in the back focal plane of the objective lens,
allowing only the transmitted or direct beam to pass.
The diffracted beams are blocked. In this case,
diffraction contrast contributes to image formation.
Crystalline regions that are oriented so as to strongly
diffract intensity away from the transmitted beam
will appear dark. In addition, Z-contrast occurs when
regions with heavy atoms scatter more of the
incident beam and appear darker.
Therefore, interpretation of contrast should be done
with care.
In dark field (DF) images, a selected diffracted beam
is allowed to pass through the objective aperture.
This is accomplished by electromagnetic lenses that
effectively tilt the incident beam from the optical axis
by an amount 2θ hkl such that the hkl diffracted
beam exits the sample parallel to the TEM optical
axis.
In contrast to the direct beam, the diffracted beam
has interacted strongly with the specimen, and very
useful information is often present in DF images, e.g.,
about planar defects, stacking faults, or particles.

20. In many analyses, correlation between DF and BF images provides useful
information about structure and morphology, including layer thickness, grain size,
grain orientation, and defect orientations. In general, one or both of these imaging
modes are used in conjunction with electron diffraction analysis.

21. Selected Area Diffraction
Similar to BF and DF imaging—an image of the diffracted beams is brought into
focus at the back focal plane of the objective lens.
Once this real image is formed, it can be projected onto the viewing screen by the
intermediate and projection lens system.
In many cases, one desires to obtain crystallography information about a specific
area of the sample or about a secondary phase present in the sample. For such
cases, selected area diffraction (SAD) requires that an intermediate aperture is
placed at the first intermediate image focal plane to specify the area from which
the diffraction image is acquired.
Through use of this method, crystallographic information is obtained from small
features and from small volumes of materials such as precipitates.

22. Under diffraction conditions, a portion of
the incident radiation makes the appropriate
angle with a specific set of (hkl) planes such
that Bragg’s Law is satisfied .
Given that the typical operation wavelength
can vary between 0.37 and 0.87 pm, the
diameter of the Ewald sphere (ES) is very
large when compared with the size of the
unit cell in reciprocal space. Hence, near the
origin of reciprocal space, the curvature of
the ES is so small that it is essentially a plane
perpendicular to the direction of the
incident radiation.
Hence, the ES passes through several points
in reciprocal space that satisfy Bragg’s Law.
The function of the magnification lenses is
to project the image that is in the back focal
plane, and that image is a set of points on
the surface of the Ewald sphere. The image
in fact is a planar section of the reciprocal
lattice, perpendicular to the incident
radiation.

23. A single-crystal sample produces spot patterns associated with a
specific zone axis for that crystal. Under diffraction conditions,
polycrystalline samples give rise to ring patterns, which are
actually the superposition of many single-crystal patterns.
In general, correlation between DF and BF images can provide
extensive information about structure and morphology, including
layer thickness, grain size, grain orientation, and defect
orientation. Typically, however, one or both of these imaging
modes are used in conjunction with electron diffraction analysis.

24. Contrast
•Conventionally, in order to get contrast in an image an objective aperture is placed in the
beam path at the back focal plane of the objective lens.
•Any electrons which are scattered by interaction with the sample are prevented from
recombining in the image by the aperture.
•Any regions where electrons are scattered will appear dark in the image.
•The smaller the aperture used the greater the contrast, but at the expense of brightness.
•Another method of achieving contrast is the ‘Mass thickness
Contrast’ mechanism.
•Thicker (or higher Z) areas of the sample scatter more electrons
than thinner (lower Z areas).
•Therefore fewer electrons from the dark region fall onto the
screen, so the region appears darker in a Bright Field image.
Mechanism of mass
thickness contrast

25. Projection - Magnification
•A series of projector lenses are then used to magnify the image formed by the
intermediate lens onto a viewing screen.
•Electron Microscope lenses are Electro-magnetic in nature.
•They consist of a cylindrical soft metal core (polepiece) with a hole drilled through
it (bore) wound with copper wire.
•When a current is passed through the coils a magnetic field is created in the bore.
•Changing the current in the windings changes
the magnetic field and effectively changes the
focal length of the lens.

26. ViewingRecording Images
•The image, comprising of a beam of focused electrons falls onto a fluorescent screen
therefore producing a visible image.
•This image can be recorded onto photographic film using the camera located underneath
the viewing screen.
•Modern machines can capture images digitally using CCD cameras.
•The signal from the beam can be output to external analysis equipment.
Digital CCD camera
Fluorescent viewing
screen

27. Indexing Diffraction Patterns (Single-Crystal Diffraction Patterns)
Single-crystal diffraction patterns consist of a series of bright spots, with the
transmitted beam being the brightest and the others bright spots being the
diffracted beams. The first step in the analysis of electron diffraction patterns is to
measure the dhkl values.
The diffracted beam makes an angle of 2 relative to the direct beam and falls on
the screen at a distance R from the transmitted spot.
The net effect of the lens settings is encompassed
in the camera length (L), which is an effective
distance between the sample and the
photographic plate (and/or viewing screen).
When diffraction spots appear, the Ewald sphere
intersects the center spot (i.e., the origin of 1/R
space) and also intersects the 1/R-lattice vector
d∗
hkl , so Bragg’s Law is satisfied.

28. When Bragg’s Law is satisfied during ED, the wavelength is small (∼ tens of
picometers) and the Bragg angle is also small (0.5◦). From Figure, tan(2) = R/L
Since θ is small, tan(2) ≈ 2 (in radians); this gives us the relationship 2 = R/L .
Similarly for Bragg’s Law, sin() ≈ θ and hence λ = 2 dhkl  or λ/dhkl = 2.
R/L = 2 = λ/dhkl ,
R dhkl = L λ,
where λL is known as the camera constant and (like L) varies with the lens settings
in the microscope. The units of the camera constant are typically expressed in
units of mm-nm or cm-nm or m-nm, where R is the distance measured on the
screen or photographic plate (e.g., mm or cm or m) and dhkl (e.g., nm) is the
interatomic spacing of the planes (hkl).
The camera constant is a function of the lens settings. It is these lens settings that
actually control the optical system’s magnification, and the electron energy
determines the value of the wavelength.
The accuracy of the calculated dhkl is limited by uncertainty in the diffraction spot
positions and of the camera constant. The diffraction spots are often diffuse, and
hence the determination of the center position is an approximation. The camera
constant is used to determine a known standard sample for a given electron
energy setting. The resulting diffraction pattern is indexed, and the calculated
camera constant is loaded into the microscope’s memory.

29. For diffraction in electron microscope:
The single crystal electron
diffraction pattern is a
series of spots equivalent
to a magnified view of a
planar section through the
reciprocal lattice normal to
the incident beam.
specimen
Ewald sphere
1/
Camera
Length
(L)
r
r
rdhkl=L, L - camera constant

30. Indexing the SAED pattern (spot pattern):
h1k1l1
h2k2l2
h3k3l3
1
2
R3
R1
R2

31. Measure R values for at least three reflections

32. Index other reflections by vector sums, differences
Next find zone axis from cross product of any
two (hkl)s
(202) x (220) ——> [4̄44] ——> [1̄11]

33. Ring pattern:
The reciprocal lattice becomes a series of sphere
concentric with the origin of the reciprocal lattice.
beam
O
hkl sphere
D
The main steps of indexing ring patterns:
1) Measuring ring diameters D1, D2, D3 …….
2) Calculation of the dhkl (using the expression: rdhkl=L)
3) Use some structure database to index each ring.

34. SAED Patterns of Single Crystal,
Polycrystalline and Amorphous Samples
a b c
a. Single crystal Fe (BCC) thin film-[001]
a. Polycrystalline thin film of Pd2Si
b. Amorphous thin film of Pd2Si. The diffuse
halo is indicative of scattering from an
amorphous material.
SAED Patterns of Single Crystal, Polycrystalline and
Amorphous Samples
a. Single crystal Fe (BCC) thin film-[001]
b. Polycrystalline thin film of Pd2Si
c. Amorphous thin film of Pd2Si. The diffuse
halo is indicative of scattering from an
amorphous material.

35. Comparison of SAED and XRD
a. Similarities (Arrangement and function of components are similar)
1) Illumination system: produces required radiation and directs it onto the
specimen. Consists of a source, which emits the radiation, and a
condenser lens, which focuses the illuminating beam (allowing variations
of intensity to be made) on the specimen.
2) Specimen stage: situated between the illumination and imaging
systems.
3) Imaging system: Lenses which together produce the final magnified
image of the specimen. Consists of i) an objective lens which focuses the
beam after it passes through the specimen and forms an intermediate
image of the specimen and ii) the projector lens(es) which magnifies a
portion of the intermediate image to form the final image.
4) Image recording system: Converts the radiation into a permanent image
(typically on a photographic emulsion) that can be viewed.

36. b. Differences
1) 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.
2) Magnification in the LM is generally changed by switching between different power objective lenses
mounted on a rotating turret above the specimen. It can also be changed if oculars (eyepieces) of
different power are used. 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.
3) The LM has a small depth of field, thus different focal levels can be seen in the specimen. The large
(relative) depth of field in the TEM means that the entire (thin) specimen is in focus simultaneously.
4) Mechanisms of image formation vary (phase and amplitude contrast).
5) TEMs are generally constructed with the radiation source at the top of the instrument: the source is
generally situated at the bottom of LMs.
6) 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 !!).
7) TEM specimens (biological) are rapidly damaged by the electron beam.
8) TEMs can achieve higher magnification and better resolution than LMs.
9) Price tag!!! (100x more than LM)