Download Transmission Electron Microscope (TEM)

BMFB3263 Materials
Characterization
Transmission Electron
Microscope (TEM)
TEM
Topic outcomes
By the end of this topic you should be
able to:
• List down the differences between
TEM and SEM
• Explain basic principle of TEM
• Explain how bright field and dark
field images can be obtained
• Describe typical sample preparation
steps
TEM
• The most powerful instruments to investigate the
microstructure of materials
• TEM enables the fine-scale microstructure to the
nanoscale to be examined in sufficiently thin
specimens
• Why specimen must be sufficiently thin:
– To facilitate transmission of a beam of
electrons without a great loss of intensity
• Maximum transmittable thickness depends on
atomic number of the materials
– Typical thickness ~250-500 nm
– The higher the electron energy, the better transmission
the better the transmission through the specimen
• Leads to development of TEM with high accelerating
voltages 100keV-3MeV
• TEM allow observation down to
atomic level
• TEM capabilities:
– Physical and chemical analysis
– Micro areas of the specimen
Transmission Electron Microscope
(TEM)
• SEM – image is developed point-by-point, by
collecting signal generated by electron interaction as it
scan the surface.
• TEM – image is focused by objective lens while
imaging lenses enlarge the final image.
• SEM gives info on 3-D topography of surface while
OM and TEM generate 2-D image of thin, planar slice
taken from bulk material.
• Thickness & diameter of TEM sample is limited.
• Prepare thin film – mechanical, electrochemical, or ion
milling. Coating – sputtering carbon.
Transmission Electron Microscope
(TEM)
• Often equipped with energy-dispersive X-ray
spectrometer (EDS) – chemical composition of
micro-volume.
• Cathode, (tungsten filament) heated and high
voltage is passed between it and anode  emit
electrons.
• Electrons accelerated to anode just below cathode.
• Some passed thru a tiny hole in anode, & form
electron beam.
• Electro-magnets, placed at intervals down the
column, focus the electrons mimicking glass lenses
on light microscope.
 electron gun – produce
stream of monochromatic
electrons.
 condenser lenses – focus
into small, thin & coherent
beam.
 condenser aperture –
eliminate high angle
electrons.
 transmitted beam is
focused by objective lens into
image.
Transmission Electron Microscope (TEM)
• Objective aperture – enhance contrast by blocking out high
angle diffracted electrons.
• Selected area aperture – user to examine periodic diffraction of
electrons by ordered arrangement of atoms in sample.
• Intermediate lens – control magnification. Projector lens –
forms a real image on fluorescent screen. Combined both –
enlarge image.
• Image strikes phosphor image screen & light is generated.
Darker areas represent thicker or denser areas, & lighter areas
for thinner.
Alumina nanoparticles
Bamboo fibres cells – higher
magnification and resolution
compared to SEM, lamellation
of the cell wall is clear.
Tungsten sulfide nanotube
GaN grown on sapphire
substrate showing
distribution of
dislocation at grain
boundaries. Arrows show
nanotubes present.
Epoxy matrix reinforced by glass-fibre and fumed
nano-silica – polymer material has relatively high
CTE, silica is added to reduce CTE (silica has low
CTE) while glass-fibre to increase stiffness.
HRTEM micrograph
of Single-walled
Carbon nanotubes
produced by CVD
growth process.
TEM able to get lower resolution, down to 0.2 nm,
even a few angstrom (10-10 m) – very useful tool to
study nanomaterials, or details in near atomic
levels (e.g dislocation, fine precipitate, etc).
Transmission Electron Microscope
(TEM)
• Since material must be exposed to high vacuum, it
must be dried.
• Limited penetrating power of electrons  specimens
must be sliced into 50 – 100 nm thickness.
• Contrast in TEM depends on atomic number of
atoms in specimen ; the higher the atomic number,
more electrons are scattered and the greater the
contrast.
• Biological material composed of very low atomic
number atoms (C, H, N, P & S). Thin sections can be
made visible by selective staining.
Schematic
Drawing of
TEM
• Main features of conventional TEM similar with
SEM
• Major parts
– Evacuated column contains electron source
usually tungsten filament or LaB6 crystal,
– Assembly of condenser, objective and
projector lenses
• For high and ultra-high resolutions, <0.5nm field
emission or extended Schottky emission cathodes
are preferred.
• Current TEM design
– Provide clean and high vacuum system
– Use ions pumps to minimise specimen
contamination
• From Figure- five-lens illumination system
• Trend to increase number of lenses
– To optimise the overall performance of the instrument
– Maximise flexibility and ease of operation
• Smaller value of  reduces effective electron
source size and increases the coherence length of
the beam
– Increase the contrast and resolution of the images and
diffraction patterns
• When passing through a thin foil specimen,
electrons enter the objectives lens whose design
and aberrations critically affect the performance
of the microscope
Parallel incident electron beam
• Diffracted beams leaving the specimen are focused in the
back focal plane of the objective lens
• The diffraction pattern is imaged if the back focal plane is
projected on the viewing screen by reducing the excitation
of the first projector lens
• Interchangeable objectives apertures typically 50 to 200
m diameter, are positioned close to the back focal plane of
the objective lens to enhance image contrast
• If an objective aperture intercepts all the diffracted beams
and allows only the direct beam to pass
– Deficiency contrast occurs and bright field image is formed
• Objective aperture can be used to select a single diffracted
beam to produce dark field image
– Tilt the incident electron beam, the astigmatism of the image
is reduced
Comparison of the ray diagram and mathematical
description of the image formation process
Ray diagrams in a
TEM for diffraction
conditions where the
back focal plane of
the objective lens and
screen are conjugate
Bright field imaging where the
objective aperture is positioned
to allow the direct beam to
form the final image
Typical bright field image of
interphase VC carbide
precipitation in low alloy
½%Cr ½%Mo ¼%V steel
Dark field imaging where the
illumination is tilted to select a
given diffracted beam
Dark field image showing
distribution of interphase
carbide precipitates
Transmission Electron Microscope (TEM)
• Works much like a slide projector. Projector shines a
beam of light through slide, as light passes through it
affected by the structures and objects of slide.
• TEM – shines beam of electron through specimens.
Reveals more information of internal structure.
• Resolving power is approximately three orders of
magnitude (10,000) greater than that of light
microscope. Short wavelength of electron – easy to
obtain resolution of 3 nm.
• Samples limited to less than 3 mm diameter & 100 µm
thick. Thickness is critical aspect.
(a) Replica showing faulted substructure within 1
plate of an age Cu-40%Zn alloy, (b) carbides
extracted from 1%Cr-1/2%Mo steel
Sample preparation
1.
2.
3.
4.
5.
6.
7.
Electrothinning
Ultra-microtomy
Chemical thinning
Cleavage
Solvent casting
Ion bombardment
Focus Ion Beam (FIB)
• Biological samples
– e.g. wood and bone
– Use ultra-microtomy
• Amorphous polymer
– Cut into ultramicrotome with a glass or diamond knife if
temperature reduced below Tg
• Crystalline polymers
– Difficult to prepare
– Improve cutting properties by chemical staining
• Other polymeric materials
– Dissolve in solvent and cast on a glass slides
– When solvent evaporated, the produced film is coated
with thin layer of carbon
– Specimen can be melted and re-crystallised before
stripping from the glass slide in water
• Foils e.g. silica, germanium and magnesium
oxide
– Chemical etching technique
– Hard to produce foils with large and uniform
thin areas
• Ceramics and semiconducting materials
– Use ion bombardment to thin the specimensurface of material is bombarded with ions of
an inert gas, Ar
– Ion bombardment can be used for oxides,
carbides, nitrides, ceramics, glasses, metals
FIB
• The major development in TEM’s
sample preparation
• FIB can be used to machine
specimens from all types of material
• Fast (less than 2 hours per foil),
precise