University of G
Center for Advanced Ultrastructural Research
Electron Microscopy
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Why bother electron? Better resolution!
1.
Point resolution: the smallest distance between two points
2.
For human being’s eyes is about 0.1-0.2 mm.
3.
For light microscope is defined by Rayleigh criterion: r = 0.61l/ n sin a
For simplicity it is approximately: r ~ 0.5l
Green light, λ~550 nm, the best resolution for a optical microscope is about 300 nm.
Electrons 200 kV, λ ~ 0.0025 nm, n ~ 1 (vacuum): the theoretical resolution r ~ 0.02 nm
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
Ernst Abbe
1840 - 1905
University of G
0.61 λ
R.P. = ---------N.A.
N.A. = n (sin α)
n = index of refraction
α = half angle of illumination
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
What is a TEM microscope
FEI Tecnai 20
Point resolution: 0.2 nm
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Electron Source
•Three sources of electrons
–Tungsten hairpin
–Lanthan hexaboride
–Field emission
LaB6
W
•W and LaB6 work on
thermionic emission
•FE strong electrical field.
FE
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
Electron Sources
Increasing the filament current will increase the
beam current but only to the point of saturation at
which point an increase in the filament current
Department of Physics and Astronomy
will only shorten the life of the emitter
Center for Advanced Ultrastructural Research
University of G
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Electromagnetic Lens
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
Transmission Electron Microscope
Optical instrument in that it uses a
lens to form an image
Scanning Electron Microscope
Not an optical instrument (no image
forming lens) but uses electron optics.
Probe forming-Signal detecting
device.
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
1. What is a SEM? .. Surface characterization by
SEM
2. What is a TEM?
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
A number of different detectors can be
incorporated into the chamber surrounding
the specimen.
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
What is a SEM microscope ?
LEO 982 SEM
Point resolution: ~ 10 nm
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
How does an image form in SEM?
Interaction of electron with specimen
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
How to define the resolution of SEM?
The SEM is a probe forming (e- beam) and signal detecting device.
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
Gold particles on E. coli appear as bright
white dots due to the higher percentage of
backscattered electrons compared to the low
atomic weight elements in the specimen
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
SEM Image of Nano-Gold Particles
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
SEM Image of Multi-Wall Carbon Nanotubes
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
1. What is a SEM? .. Surface characterization by
SEM
2. What is a TEM?
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
What is a TEM microscope
FEI Tecnai 20
Point resolution: 0.2 nm
Thin Specimen
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Total magnification in the TEM is a
combination of the magnification from the
objective lens times the magnification of the
intermediate lens times the magnification of
the projector lens. Each of which is capable
of approximately 100X.
Mob X Mint X Mproj = Total Mag
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Wave functions for elastically-scattered, forward electrons
(r) - electron
(r) - potential
sample
From potential (r) to exit-wave
function q(r) :
1). Weak Phase Object (single
scattering):
Objective lens
q(r) = 1 - i(r), where =/lU.
q(r) - exit-wave function
Fourier
transform
2). Phase Object:
q(r) = exp[i(r)].
I(H)=|Q(H)|2
3). Dynamic Scattering:
Multislice method
Backfocal Plane
Q(H)
transfer
function
Q(H)T(H)
Diffraction
Inverse Fourier
transform
Bloch-wave method.
I(r)=|(r)|2
Image
image
Plane
I(r)
(r)
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Selected Area Electron
Diffraction (SAED):
SAED use parallel illumination and limits the
sample volume by an aperture in the image
plane of the low objective lens.
A SAED pattern of a crystal.
Lattice plane have spacing of d
D
 tan   tan2 B ;
L
2dSin B  l
1
D

d
Ll
Camera length
Ewald Sphere
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
Electron Diffraction of
Amorphous
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Electron Diffraction of tiny crystals: Ring pattern
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Electron Diffraction of single crystals: spots pattern
[111]
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Tilting crystals in a TEM and collect the Selected
Area Electron Diffraction (SAED) patterns:
SAED use parallel illumination and limits the
sample volume by an aperture in the image
plane of the low objective lens.
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Image formation in TEM: Mass contrast
Different specimen regions have different thickness. Or different specimen
regions consist of different elements. The contrast forms by different
electron absorption in different specimen areas.
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
TEM contrast is mostly diffraction contrast
Different specimen regions generate Bragg reflections of different intensity.
The contrast forms by either Bragg reflections or transmitted beam do not
contribute to the image. Thus the atomic resolution can not be realized.
Dislocation network between two phases
in a single crystal superalloy : TEM
picture , original magnification 21000X
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
A TEM image is
made up of
nonscattered electrons
(which strike the
screen) and scattered
electrons which do
not and therefore
appear as a dark area
on the screen
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
(r) - electron
Phase contrast transfer
function:
(r) - potential
sample
q(r) - exit-wave function
T(H)=sin(Csl3H4/2+flH2).
Cs: Spherical aberration constant.
Objective lens
Fourier
transform
f: defocus value.
I(H)=|Q(H)|2
Backfocal Plane
Q(H)
transfer
function
Q(H)T(H)
Inverse Fourier
transform
I(r)=|(r)|2
image
Plane
I(r)
(r)
Phase contrast transfer function calculated at
f=-61 nm with Cs=1.0 mm.
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Wave functions for elastically-scattered, forward electrons
(r) - electron
High-Resolution Electron
Microscopy:
(r) - potential
sample
For a weak phase object, the observable
image intensity is:
I=1 - 2(r)  FFT[T(H)].
represents convolution.
Objective lens
q(r) - exit-wave function
Fourier
transform
This shows a pure phase-contrast image.
atoms
I(H)=|Q(H)|2
Backfocal Plane
Q(H)
transfer
function
Q(H)T(H)
Inverse Fourier
transform
Sample
thickness
Illustration of electron wave passing through
under phase object approximation. The phase
changes (contrast) is imaged.
I(r)=|(r)|2
image
Plane
I(r)
(r)
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
High-Resolution Electron
Microscopy: Carbon nanotube
Discovery of the carbon nanotube
S. Iijima, Nature 354, 56 (1991).
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Shape Determination of Au Nanoparticles
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
High-Resolution Electron
Microscopy: Stacking fault
and nanotwins
A HREM image of SrRuO3 crystal along the [110]
direction shows an isolated {111} intrinsic stacking
fault. The dislocation at the end of the fault is identified
as a Shockley partial dislocation Burgers vectors of
a/3<112>.
A HREM image of a {111} nanotwin, which have a wider
thickness of the ‘fault planes’.
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
High-Resolution Electron
Microscopy: Interfaces
a=0.3905 nm
Misfit=0.64%
a=0.3982 nm
HREM image the coherent SrTiO3/SrRuO3 interface.
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
Aberrations
University of G
• Three types of
aberrations.
– Spherical (Aperture size)
– Chromatic (Different
energies)
– Astigmatism (Lens
defect)
Astigmatism aberration
Aberrations are why
resolutions
not 0.2
Department
of is
Physics
andÅ.
Astronomy
University of G
Center for Advanced Ultrastructural Research
Super Resolution Scheme:
Aberration Correction
Cs=1.0 mm
1/4.17=0.24 nm
1/7.4=0.135 nm
Cs=0 mm
Up: Phase contrast transfer function with
Cs=1.0 mm (f=-61 nm).
Down: Phase contrast transfer function with
Cs=0 mm (f=-7 nm).
Philips CM200FEG ST with Cs-corrector
at Juelich Germany
r

r = 2Cs 3
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
Super Resolution Scheme II:
Aberration Correction
“Seeing is Believing”
Direct imaging of light O atoms at
resolution of 0.138 nm.
C.L. Jia, M. Lentzen and K. Urban,
Science 299, 870 (2003).
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Convergent Beam Electron
Diffraction (CBED)
Advantages:
•Small probe
•Rocking curve information:
Condenser II
Upper
Objective
Specimen
Low er
Objective
Back Focal Plan
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
Convergent Beam Electron
Diffraction (CBED)
High Voltage Calibration using HOLZ lines
200kV
201kV
Si [331] 200kV, -180 °C, Energy-filtered
Department
of Physics
and Astronomy
(13,-13,3)
(-13,13,3)
University of G
Center for Advanced Ultrastructural Research
Scanning Transmission Electron
Microscopy (STEM)
Beam energy
50-300 keV .
STEM Imaging Modes:
1). Bright Field: detector is placed on the optic axis (atom
images are superimposed on a bright background).
Field-emission electron source.
X-ray detector
Aberration-corrected magnetic lens.
Sample thickness
< 100nm
Probe diameter 0.15nm. Current 0.2 nA.
Complete spatial coherence .
Detector for elastically scattered electrons,
to form image.
2). Dark field: shift the stationary diffraction pattern and
make scattered beam is on the optic axis. (use the same BF
detector).
3). Annular Dark Field: an annular detector is used to collect
the intensity outside the central disk.
4). High-angle annular dark-field (HAADF, Z-contrast): an
annular detector is used to collect the intensity far away from
the central disk. This mode has become popular because it
allows convenient collection of EELS spectra, while
minimizing thickness and focus dependence of the images.
The probe-formation process remains coherent, but the
detector geometry renders the imaging partially coherent.
B field
into page
Slow e
Oxygen K edge
Fast e
Elastic peak
ELS spectrum proportional to empty density of
states,or Im(1/( (,k)). Similar to XANES.
Simplified ray diagram for a modern STEM instrument. In reality
several probe-forming lenses may be used, in addition to lenses
after the sample. A CCD detector is used to record the EELS
spectrum dispersed by a magnetic-sector bending magnet with the
important advantage of parallel detection.
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Other Applications
1.
Gatan heat stage: room temperature – 950 deg
2.
Gatan Cryo-TEM transfer system.
3.
Micro-CT: 3D tomography using X-ray with few
um resolution.
An energy-filtered image of TMV virus
embedded in vitreous ice.
Unfiltered
filtered, 10 ev Slit
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Conclusion
Transmission electron microscopy is the most
powerful technique for nanostructure
characterizations. It is the only technique that
can provide real space images at atomic
resolution of the defects within materials.
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Contacting information:
Jinsong Wu
Barrow Hall, Room 152,
Tel: 706-542-3435
Email: jswu@physast.uga.edu
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Sample Preparation
SEM
1. Place a drop of sample on
holder.
2. Dry.
3. Sputter with gold or
platinum.
TEM
1. Cut a thin slab.
2. Mechanically polish
to ~ 10 um
3. Ion-milling to ~10-50
nm.
4. Glue to a TEM grid.
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
SEM
TEM
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
l– wavelength
r = 0.61l/ n sin a
a– aperture of objective lens
V- acceleration voltage
n- refractive index
l = [ 1.5/ V +10-6 V2] ½ nm
Green light
Electrons
l ~ 400 nm
200 kV ~ 0.0025 nm
n ~ 1.7 oil immersion
n ~ 1 (vacuum)
r ~ 150 nm (0.15 mm)
r ~ 0.02 nm (0.2 Å)
Department of Physics and Astronomy
Unrealistic but Why?
Center for Advanced Ultrastructural Research
University of G
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Backscattered Electron
When the electron beam strikes the sample some of the electrons will
interact with the nucleus of the atom in much the same way a space
craft will interact with the gravity of a planet. The negatively-charged
electron will be attracted to the positive nucleus but if the angle is just
right instead of being captured by the "gravitational pull" of the
nucleus it will circle the nucleus and come back out of the sample
without slowing down. These electrons are called backscattered
electrons because they come back out of the sample. Because they are
moving so fast, they travel in straight lines. In order to form an image
with BSE (backscattered electrons), a detector is placed in their path.
When they hit the detector a signal is produced which is used to form
the TV image.
All the elements have different sized nuclei. As the size of the atom
nucleus increases, the number of BSE increases. Thus, BSE can be
used to get an image that showed the different elements present in a
sample.
Department of Physics and Astronomy
University of G
Center for Advanced Ultrastructural Research
Secondary Electron
Sometimes beam electrons interact with the electrons present in the
atom rather than the nucleus. Since all electrons are negatively
charged, the beam electrons will repel the electrons present in the
sample. This interaction causes the beam electrons to slow down as it
repels the specimen electrons, The repulsion may be so great that the
specimen electrons are pushed out of the atom, and exit the surface of
the sample, these are called secondary electrons. Unlike the BSE, the
secondary electrons are moving very slowly when they leave the
sample. Since they are moving so slowly, and are negatively charged,
they can be attracted to a detector which has a positive charge on it.
This attraction force allows you to pull in electrons from a wide area
and from around corners in much the same way that a vacuum pulls in
dust particles. The ability to pull in electrons from around corners is
what gives secondary electron images a 3-dimensional look.
Department of Physics and Astronomy
Center for Advanced Ultrastructural Research
University of G
After an inner shell excitation
an atom has an energy above its
ground state. It can relax and
lose some of this energy in
several ways, of which two are
described here. Both start with
an outer electron jumping in to
fill the vacancy in the inner
shell.
Characteristic X-ray emission.
Energy is given off as a single
X-ray photon.
Auger electron emission.
Energy is given off by one of
the outer electrons leaving. It
carries a characteristic kinetic
energy.
Department of Physics and Astronomy