Post on 03-Sep-2019
AP 5301/8301Instrumental Methods of Analysis
and Laboratory
Lecture 4
Microscopy (III): Transmission Electron Microscopy (TEM)
Prof YU Kin Man
E-mail: kinmanyu@cityu.edu.hk
Tel: 3442-7813
Office: P6422
1
mailto:kinmanyu@cityu.edu.hk
Lecture 4: Outline Introduction:
─ Development of transmission electron microscope
─ Essential parts and functions
─ Operation principles
TEM specimen preparation
Imaging modes: brief field, dark field and high resolution
TEM diffraction
─ Diffraction basics
─ TEM diffraction patterns
─ Selected area electron diffraction
─ Convergent beam electron diffraction
Scanning transmission electron microscopy (STEM)
─ Z-contrast imaging
Electron probe microanalysis
─ Electron energy loss spectroscopy
─ Energy dispersive and wavelength dispersive x-ray spectroscopy
2
OM TEM SEM
Magneticlenses
detector
CRTCathode Ray Tube
Light sourceSource of electrons
Condenser
Specimen
Objective
Eyepiece
Projector Specimen
Optical and electron microscopes3
Transmission electron microscope4
TEM: an introduction
E (keV) Wavelength (pm)
50 5.36
80 4.18
100 3.70
200 2.51
300 1.97
5
Electrons at 300 keV have a 𝜆~2 𝑝𝑚 and a diffraction limited resolution ~1 pm
In practice TEM resolution is far from these
limits
Imperfections (aberrations) of magnetic lenses
are the limiting factor
A short history:
1897 J. J. Thompson Discovers the electron
1924 Louis de Broglie: identifies the wavelength for electrons as 𝜆 = ℎ/𝑚𝑣
1926 H. Busch: magnetic or electric fields act as lenses for electrons
1929 E. Ruska: Ph.D thesis on magnetic lenses
1931 Knoll & Ruska: built the 1st electron microscope (EM)
1931 Davisson & Calbrick: properties of electrostatic lenses
1934 Driest & Muller: surpass resolution of the Light Microscope
1938 von Borries & Ruska: first practical EM (Siemens) - 10 nm resolution
1940 RCA: commercial EM with 2.4 nm resolution
2000 new developments, cryomicroscopes, primary energies up to 1 MeV
Comparison: SEM and TEM
TEM SEM
Electron beam Broad, static beam Beam focused to fine point and
scan over specimen
Electron path passes through thin specimen. scans over surface of specimen
Specimens Specially prepared thin
specimens supported on TEM
grids.
Sample can be any thickness and is
mounted on an aluminum stub.
Specimen stage Located halfway down column. At the bottom of the column.
Image formation Transmitted electrons collectively
focused by the objective lens and
magnified to create a real image
Beam is scanned along the surface
of the specimen to build up the
image
Image display On fluorescent screen. On TV monitor.
Image nature Image is a two dimensional
projection of the sample.
Image is of the surface of the
sample
Magnification Up to 5,000,000x ~250,000x
Resolution ~0.2 nm ~2-5 nm
6
Advantages
TEMs offer very powerful magnification and resolution.
TEMs have a wide-range of applications and can be utilized in a variety of
different scientific, educational and industrial fields
TEMs provide information on element and compound structure.
Images are high-quality and detailed.
Chemical information with analytical attachments
Disadvantages
TEMs are large and very expensive (USD 300K to >1M)
Laborious sample preparation.
Operation and analysis requires special training.
Samples are limited to small size (mm) and must be electron transparent.
TEMs require special housing and maintenance.
Images are black and white .
TEM: advantages and disadvantages7
Transmission electron microscopy (TEM)8
Two unique features of transmission electron microscopy (TEM) are its high
lateral spatial resolution (better than 0.2 nm) and its capability to provide
both image and diffraction information from a single sample.
Hence TEM can be used to obtain full morphological, crystallographic,
atomic structural and microanalytical such as chemical composition (at
nm scale), bonding (distance and angle), electronic structure,
coordination number data from the sample.
Diffraction
SpectroscopyImaging
TEM: operation principle Primary electrons generated by electron
gun and focused by stages of condenser
lenses into bundles
Electrons illuminate the sample:
─ at low magnification, a spread beam is used
to illuminate a large area
─ at high magnification, a strongly condensed
beam is used
The pattern of electrons leaving the object,
reaches the objective lens forms the image.
The image is greatly enlarged by a projector
lens.
The traversing electrons (transmission)
reach the scintillator plate at the base of the
column of the microscope.
The scintillator contains phosphor
compounds that can absorb the energy of
the striking electrons and convert it to light
flashes, forming an image
9
Control brightness,
convergence
Control contrast
A disc of metal
TEM: operation principle10
TEM: essential parts and functions11
Electron Gun
EDS Detector
Condenser
LensSpecimen
HolderObjective Lens
Magnifying Lenses
CM200 (200kV)
SAD Aperture
Fluorescenc
e screen
Cost: $4,000,000
Column
Binocular
LN2
Specimen Holder
a split polepiece objective lens
holder
beam
Heating and strainingTwin specimen holder
Double tilt heating
Rotation, tilting, heating, cooling and straining
TEM: specimen preparationTEM is a microscopy technique whereby a beam of electrons is transmitted through an ultrathin specimen, interacting with the specimen as it passes through it. Materials for TEM must be specially prepared to thicknesses which allow electrons to transmit through the sample (~10-200 nm).
13
In addition to be thin, samples have to be:
─ Electrically conductive
─ Stable under vacuum
─ Free from hydrocarbon contaminants
─ No artefacts
For nanoparticles or thin foils, e.g. graphene, disperse crystals or powders on
a carbon film on a Cu grid
Thin foil
TEM: specimen preparation
For solid samples, there are different methods:
Mechanical:─ Mechanical polishing down to electron
transparency
─ Cleavage
─ Ultramicrotomy-using a (diamond) knife
blade
─ Crushing
Mechanical+ionic/chemical─ Grinding, dimpling, ion milling
Using keV Ar ions focused on the sample to
thin it down
─ Focused ion beam (FIB)
─ Electro-chemical polishing
─ Chemical polishing or etching
14
ion milling
Focused ion beam
(FIB)
TEM: Cross-section specimen preparation15
Cross sectional TEM:
characterization of multilayer materials
layers thickness measurement
layers and interfaces structure analysis
Cross-sectional TEM image of a silicate-
titanate film containing 10 nm gold particles
http://www.nanoanalysis.co.jp/en/busin
ess/case_example_49.html
TEM operation16
TEM offers two methods of specimen observation, diffraction mode and image
mode. The objective lens forms a diffraction pattern in the back focal plane with
electrons scattered by the sample and combines them to generate an image in the
image plane.
Whether the diffraction pattern or the image
appears on the viewing screen depends on
the strength of the intermediate lens.
The diffraction pattern is entirely equivalent
to an X-ray diffraction pattern.
The image mode produces an image of the
illuminated sample area
In image mode, the post-specimen lenses
are set to examine the information in the
transmitted signal at the image plane of the
objective lens.
There are three primary image modes that
are used in conventional TEM work, bright-
field microscopy, dark-field microscopy,
and high-resolution electron microscopy.
Use of apertures Condenser aperture:
─ Limit the beam divergence (reducing the
diameter of the discs in the convergent
electron diffraction pattern).
─ Limit the number of electrons hitting the
sample (reducing the intensity)
Objective aperture:
─ Control the contrast in the image.
─ Allow certain reflections to contribute to the
image.
─ Bright field imaging (central beam, 000),
─ Dark field imaging (one reflection, g),
─ High resolution Images (several reflections
from a zone axis).
Selected area aperture:
─ Select diffraction patterns from small (>
1µm) areas of the specimen.
─ Allows only electrons going through an area
on the sample that is limited by the SAD
aperture to contribute to the diffraction
pattern (SAD pattern).
TEM imaging: bright field
Bright field (BF): a small objective aperture is used to block all diffracted beams and to pass only the transmitted(undiffracted) electron beam.
─ Contrast arises in a bright-field image when thickness or compositional variations or structural anomalies are present.
─ Regions in which intensity is scattered (defects) appear dark
─ High-Z material appear darker than the low-Z material
─ In crystalline materials, dark contrast regions in bright-field usually originate from areas that are aligned for Bragg diffraction
18
In image mode, the post-specimen lenses are set to examine the information in the
transmitted signal at the image plane of the objective lens. The scattered electron
waves finally recombine, forming an image with recognizable details related to the
sample microstructure (or atomic structure). There are three primary image modes:
TEM BF image of
microcrystalline ZrO2.
some crystals appear with
dark contrast since they
are oriented (almost)
parallel to a zone axis
(Bragg contrast).
TEM imaging: dark field Dark field (DF): a small objective aperture is
used to select a diffracted beam and block all other beams.
─ Undistorted crystal lattice appears dark since little scattered intensity arises from these regions to contribute brightness.
─ dislocations (defects) appear as brightlines on a dark background
19
In the DF image (right), some
of the microcrystals appear with
bright contrast, namely such
whose diffracted beams partly
pass the objective aperture
(a) Bright-field (BF) micrograph of
multilayer cross-section sample Ni/Co
multilayera; (b) Dark-field (DF) TEM
image.
TEM imaging: high resolution Phase contrast or high resolution (HREM): use
the non-diffracted and at least one diffracted
beams by using a large (or none) objective
aperture and add them back together, phase and
intensity to form an image
─ When viewed at high-magnification, it is
possible to see contrast in the image in the
form of periodic fringes that represent direct
resolution of the Bragg diffracting planes
─ The contrast is referred to as phase contrast
20
TDSi
BN
Objective
aperture
Electron diffraction pattern recorded from both BN film on Si substrate.
High resolution TEM
image of a RuO2
nanorod
High Resolution Transmission Electron
Microscope (HRTEM) Image of a Grain
Boundary Film in Strontium-Titinate
TEM diffraction
Electrons like X-rays are scattered by atoms
and can be used to analyze crystal structures
in a similar way.
As in X-ray diffraction (XRD), the scattering
event can be described as a reflection of the
beams at planes of atoms (lattice planes)
There are however fundamental differences:
─ Electrons have a much shorter wavelength
than the X-rays
─ X-rays are scattered by the electrons that make
up the bulk of the atom. Electrons are charged
particles and interact with the electrons
surrounding atoms and also the nucleus.
─ The elastic cross section of the electron is ca.
106 times larger than that of X-rays.
─ Electron beams can be focused using
electromagnetic lenses
21
TEM: electron diffraction22
𝜆 = 2𝑑ℎ𝑘𝑙 sin 𝜃ℎ𝑘𝑙Bragg’s law:
x-ray electrons
𝜆 = 1.54Å (Cu K) 𝜆 = 0.037Å (100kV)
A wide range of 𝜃ℎ𝑘𝑙 𝜃 = 0.26𝑜 𝑓𝑜𝑟 𝑑 = 4Å
For electron diffraction, the incident beam has to
be almost parallel to the planes for diffraction to
occur, so that 𝜆 = 2𝑑ℎ𝑘𝑙𝜃ℎ𝑘𝑙𝑟
𝐿=
𝜆
d→ 𝑟 = 𝜆𝐿
1
𝑑
L is the camera length (mm)
r is the distance between T and D spots
1/d is the reciprocal of interplanar distance (Å−1)
Specimen foil
e-
L 2
r
e-beamZone axis
of crystal
sample
𝑑ℎ𝑘𝑙
𝑟
𝐿= sin 2𝜃 ≈ 2𝜃
hkl
Real lattice
[001]
For electrons: 𝜆 𝑛𝑚 =1.5
𝑉+10−6𝑉2
Reciprocal latticeReciprocal lattice is another way to view a crystal lattice and is used to
understand diffraction patterns. A dimension of 1/d (Å-1) is used in
reciprocal lattices.
g – reciprocal lattice vector─
23
TEM: diffraction intensity
Spot (ring) intensity: 𝐼ℎ𝑘𝑙 ∝ 𝐹ℎ𝑘𝑙2
24
𝐹ℎ𝑘𝑙 =
𝑏𝑎𝑠𝑖𝑠
𝑓𝑗𝑒𝑥𝑝 2𝜋𝑖(ℎ𝑢𝑗 + 𝑘𝑣𝑗 + 𝑙𝑤𝑗)
where 𝑓𝑗 is the atomic scattering factor, and is dependent on atomic number
𝑢𝑗,𝑣𝑗 , 𝑤𝑗 are the fractional distances within the unit cell
ℎ, 𝑘, 𝑙 is the Miller indices of the plane
Atomic scattering factor:
𝑓 𝜃 ∝𝜆
sin𝜃2
𝑍
where Z is the atomic number of the atom
Structure Factor:
lattice
+
basis Crystal structure
We can consider the BCC structure as a simple
cubic lattice with a two atom basis, with atoms at
[000] and [½½½]
𝐹ℎ𝑘𝑙 =
𝑏𝑎𝑠𝑖𝑠
𝑓𝑗𝑒𝑥𝑝 2𝜋𝑖(ℎ𝑢𝑗 + 𝑘𝑣𝑗 + 𝑙𝑤𝑗)
𝐹ℎ𝑘𝑙 = 𝑓𝑒𝑥𝑝 𝑖0 + 𝑓𝑒𝑥𝑝 2𝜋𝑖(1
2ℎ +
1
2𝑘 +
1
2𝑙)
𝐹ℎ𝑘𝑙 = 𝑓 1 + 𝑒𝑥𝑝 𝑖𝜋(ℎ + 𝑘 + 𝑙)
Hence: 𝐹ℎ𝑘𝑙 = 2𝑓 𝑖𝑓 ℎ + 𝑘 + 𝑙 𝑖𝑠 𝑒𝑣𝑒𝑛0 𝑖𝑓 ℎ + 𝑘 + 𝑙 𝑖𝑠 𝑜𝑑𝑑
For a monatomic BCC crystal diffraction from (111), (003), (201), (221), etc. are missing and these are the forbidden diffractions
TEM: structure factor (example)
(000)
( 1 2 1
2 1
2)
25
(010)
(100)
TEM: diffraction pattern26
Real lattice
reciprocal lattice
2𝜃
T D
For a simple cubic structure
𝑑ℎ𝑘𝑙 =𝑎
ℎ2 + 𝑘2 + 𝑙2
T
r
r010
100 110
𝑟ℎ𝑘𝑙 = 𝐿𝜆/𝑑ℎ𝑘𝑙
Diffraction pattern: points with space distance proportional to the reciprocal
of the interplanar spacing (1/d) in the direction of the normal to the plane
Polycrystalline materials
The electron diffraction pattern is a set of rings, with some spots depending on
the crystallite sizes.
Nano to Amorphous materials
As the crystal size get smaller (nm) the
rings get more diffuse and eventually
become halo-like when the material
becomes amorphous
TEM: diffraction pattern
Al single crystalPolycrystalline Pt
silicide (PtSi)
Silicon with epitaxial nickel
silicides ( Si - NiSi - NiSi2)
Polycrystalline nickel mono
silicide (NiSi) on top of
single crystalline silicon
(Si).
Amorphous GaNAsnanocrystalline GaNAs
27
TEM: selected area electron diffraction (SAED)
Combined with sample tilting,
diffraction images of single
crystallites can be obtained in
various orientations.
Single crystals of a few
hundred nm in size can be
examined in this way.
28
Selected Area Electron Diffraction SAED is probably the most commonly used
TEM technique.
A selected area aperture is located underneath the
sample holder and can be adjusted to block parts of
the beam so as to examine just selected areas of the
sample.
SAED aperture
Many grains covered by SAED aperture
TEM: diffraction pattern
Each grain is a single crystal
A single grain Two grainsAnother grain
(different orientation)More grains Many grains
29
TEM: convergent beam electron diffraction
Parallel beam (SAED) Convergent beam (CBED)
disksT D
Convergence angle
Spatial
resolution
beam size
[hkl]
30
http://www.feic.com/support/tem/silicon.htmhttp://www.feic.com/support/tem/silicon.htm
TEM: convergent beam electron diffraction31
Convergent Beam Electron Diffraction (CBED): converging the electrons in a
cone onto the specimen, one can in effect perform a diffraction experiment over
several incident angles simultaneously. This technique can reveal the full three-
dimensional symmetry of the crystal. Each spot in SAED then becomes a
disk within which variations in intensity
can be seen.
CBED patterns contain a wealth of
information about symmetry and
thickness of specimen.
The information is generated from
small regions beyond reach of other
techniques (
TEM: convergent beam electron diffraction32
The convergence semiangle, α, can be
adjusted by changing the C2 aperture. The
size of the diffraction disk depends on α.
Depending on α different patterns are
produced.
Electrons are scattered in all directions in the
convergent conical illumination.
Each point in the disc can be scattered by the
same 2θ. Therefore the diffracted electrons
also form discs, one for each Bragg reflection.
Weaknesses:
Limited to crystalline specimens
Complicated analysis, normally compared to
computer simulated pattern.
The focused beam gives a very high current
density which causes damage to the sample.
Specimens are typically cooled with LN2
CBED: phase identification in BaAl2Si2O8
200oC 400oC 800oC
Hexagonal Orthorhombic Hexagonal
6mm 2mm 6mm
CBED (top) and SAED (bottom) patterns6 - rotation axis (rotation about axis by 360/6 degrees) m – mirror plane
mm
33
Scanning Transmission Electron Microscopy (STEM)34
In a STEM the electron beam is focused into a narrow spot which is scanned
over the sample in a rastering mode.
With STEM we can use many more of these signals in a highly spatially
resolved way than we can with TEM
Z-contrast image
EELS
EDS
CL
SEM
Scanning Transmission Electron Microscopy (STEM)
The rastering of the beam across
the sample makes these
microscopes suitable for analysis
techniques such as mapping by
energy dispersive X-ray (EDX)
spectroscopy
electron energy loss
spectroscopy (EELS)
annular dark-field imaging
(ADF).
By using a high-angle detector
(high angle annular dark-field
HAADF), atomic resolution
images where the contrast is
directly related to the atomic
number (z-contrast image) can
be formed.
SAED =0.26o or ~6.4 mrads
I Z2
35
X-rays
EDX detector
luminescence
STEM: Z-contrast imaging36
Low angle scattering: Coulombic
interaction with the electron cloud
Higher angle scattering: Coulombic
interaction with the nucleus─Rutherford
scattering with cross section 𝜎𝑅
𝜎𝑅(𝜃) ∝ 𝑍2
Rutherford scattering will dominate when the
scattering angle > screening parameter 𝜃𝑜
𝜃𝑜 =0.117𝑍1/3
𝐸𝑜1/2
, 𝐸𝑜 𝑖𝑛 𝑘𝑒𝑉
e.g. Cu for 200 keV e-beam, 𝜃𝑜 ≈ 25 𝑚𝑟𝑎𝑑
Z-sensitive electrons can be collected by
using a detector/camera length combination
that gives large collection angles (e.g. β
>80–100 mrad): high-angle annular dark-
field (HAADF)
STEM: HAADF images37
STEM HAADF micrographs of 2 layers
of Bi absorbed along the general GBs of
a Ni polycrystal quenched from 700oC
http://www.jeol.co.jp/en/products/
detail/JEM-2800.html
HREM-TEM HR Z-contrast STEM http://www.microscopy.ethz.ch/HD-1.htm
Pt pn TiO2 Pt on C foil
(a) HRTEM and (b) HAADF-STEM images of Pt
nanoparticles (diameter 1-2 nm) dispersed on
ceria. Krumeich and Müller
SrTiO3
Electron probe microanalysis (EPMA)38
Electrons lose energy through inner-shell
ionizations are useful for detecting the
elemental components of a material.
In Electron Energy Loss Spectroscopy
(EELS) characteristic spectral signature,
termed the edge profile, is derived from the
excitation of discrete inner shell levels to
empty states above the Fermi level.
By studying the detailed shape of the
spectral profiles measured in EELS, the
electronic structure, chemical bonding, and
average nearest neighbor distances for each
atomic species detected can be derived.
Quantitative elemental concentration
determinations can be obtained for the
elements 3 ≤ 𝑍 ≤ 35 using a standard-less data analysis procedure
Electron energy loss spectroscopy (EELS)
EPMA: electron energy loss process39
Measures the changes in the energy
distribution of an electron beam
transmitted through a thin specimen.
The energy loss process is the primary
interaction event.
All other sources of analytical information
( i.e. X-rays, Auger electrons, etc.) are
secondary products of the initial inelastic
event.
EPMA: EELS spectrum40
Region 1: zero loss peak,
represents electrons that have
passed through the specimen
suffering either negligible or no
energy losses
Region 2 (~1-50 eV): low loss
regime, exhibits a series of broad
spectral features related to inelastic
scattering with the valence electron
structure of the material.
─ In metallic systems these peaks arise
due to a collective excitation of the
valence electrons, and are termed
plasmon oscillations or peaks
Region 3 (extending to 100-1000 eV): a series of “edges” resulting from
electrons that have lost energy corresponding to the creation of vacancies in
the deeper core levels of the atom (K, L, M shells).
─ Edge energies are characteristic for each element and therefore can identify
different elements and their quantity (edge height).
EPMA: EELS elemental mapping41
a) HREM image of a carbon
nanotube.
b) Carbon map at the same
region.
c) EELS spectrum
d) Intensity profile of carbon map
perpendicular to the tube axis.
The intensity profile
corresponds well to the
calculated number
distribution of carbon atom
(solid line) based on the size
and the shape of nanotube.
The intensity dip at center
part corresponds to 20
carbon atoms.
http://eels.kuicr.kyoto-u.ac.jp/eels.en.html
EPMA: EELS spectrum42
The inner shell edge profile in
EELS varies with the edge type
(K, L, M, etc.), the electronic
structure, and the chemical
bonding. The details of the profile
is a measure of the empty local
density of states above the
Fermi level of the elemental
species being studied.
For example, Carbon edge from
graphite, C60 and diamond show
very different fine structures.
Comparing spectra with data
library or computation can reveals
the bonding state and local
electronic structure of the
particular sample.
http://eels.kuicr.kyoto-u.ac.jp/eels.en.html
EELS: examples43
Two-dimensional EELS elemental
mapping of Fe (red) and Pt (green) in a
PtFe nanowire
Zhu et al. JACS, 137 (32 (2015)
a) Ti L2,3-edges elemental map; b) La M4,5-edges
elemental map; c) Sr L2,3-edges at 1940 eV
elemental map; d) Mn L2,3-edges elemental map;
e) colorized map using the color scheme from
Figures 9a-d.
SrTiO3/SrLaMnO3 interface
http://www.gatan.com/atomic-level-eels-mapping-using-high-energy-edges-dualeels-mode
Ti La
Sr Mn
SrLaMnO3SrTiO3
EPMA: Energy dispersive x-ray spectroscopy44
Energy-dispersive X-ray spectroscopy (EDS, EDX, or XEDS), sometimes called
energy dispersive X-ray analysis (EDXA) or energy dispersive X-ray microanalysis
(EDXMA), is an analytical technique used for the elemental analysis or chemical
characterization of a sample. It relies on an interaction of some source of X-ray
excitation and a sample.
A high-energy beam of charged particles such as electrons or protons (PIXE),
or a beam of X-rays (XRF), is focused into the sample.
The incident beam excites an electron in an inner shell, ejecting it from the
shell while creating an electron hole.
An electron from an outer shell fills the
hole, and the difference in energy
between the two shells may be released
in the form of an X-ray.
The emitted x-rays are characteristic to
specific elements and can be measured
by an energy-dispersive spectrometer
giving information on the identity and
amount of the atoms in the sample.
EDS detectors: Si(Li), Ge(Li)45
A ED-spectrometer is p-n junction (or Schottky) of a high purity Si or Ge semiconductor crystal (typically compensated with Li).
A high negative voltage is applied over the crystal (500-1000 V) create a depletion width larger than the x-ray penetration depth (mm).
When x-rays enter the crystal electron-hole pairs are formed and the number is proportional to the energy of the x-ray.
The 𝑒 − ℎ pairs are swept across the semiconductor creating a current pulse with an amplitude proportional to the energy.
The crystal is cooled (using a LN2 dewar or thermal-electric cooled) to reduce thermal excitation (noise).
Measuring the amplitude and counting produces the ED-spectrum.
Energy resolution ~100-150 eV
EDS: characteristic x-rays46
Characteristic x-ray line energy= 𝐸𝑓𝑖𝑛𝑎𝑙 − 𝐸𝑖𝑛𝑖𝑡𝑖𝑎𝑙
Relative intensities of major x-ray lines
𝐾𝛼1 = 100 𝐿𝛼1 = 100 𝑀𝛼1,2 = 100
𝐾𝛼2 = 50 𝐿𝛼2 = 50 𝑀𝛽 = 60
𝐾𝛽1 = 15 − 30 𝐿𝛽1 = 50
𝐾𝛽2 = 1 − 10 𝐿𝛽2 = 250
𝐾𝛽3 = 6 − 15 𝐿𝛽3 = 1 − 6
𝐿𝛽4 = 3 − 5
𝐿𝛾1 = 1 − 10
EDS: in SEM/TEM/STEM47
SEM-EDS analysis: example48
Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy
(EDS or EDX) microanalysis for calcium oxalate (CaOx) crystals.
Chen et al. Kidney intnl. 80, 369 (2011)
SEM-EDS elemental mapping49
Fe3O4/graphene prepared at a low concentration of Fe2+ ions
Lim et al., in Advanced Topics on Crystal Growth, Chapter 12 (2013) ISBN 978-953-51-1010-1
EDS vs EELS mapping50
Fast joint EELS / EDS color map across a 32 nm
transistor device
http://www.gatan.com/techniques/edsedx
EELS / EDS color map of a SrTiO3 crystal
Wavelength dispersive x-ray spectroscopyWavelength-dispersive X-ray spectroscopy (WDXRF or WDS) analyzes the wavelength (instead of the energy in EDS) of the emitted x-rays.
51
Note that: 𝐸 𝑒𝑉 =ℎ𝑐
𝜆𝑛𝑚 𝑜𝑟 𝜆 Å =
12.26/𝐸 𝑘𝑒𝑉
So we can either measure the energy or
wavelength of an emitted x-ray
Wavelength Dispersive Spectrometers
measure by diffraction from a crystal
utilizing Braggs’ law:
n𝜆 = 2𝑑 sin 𝜃 𝑤ℎ𝑒𝑟𝑒 𝑛 = 1,2,3…
In WDS the emitted X-rays are diffracted by
a crystal and counted by a detector.
The intensity of the diffracted X-rays is
recorded as a function of the diffraction
angle.
WDS can achieve superb energy resolution
of a few eV.
WDS52
Zr L-line portion of an ED spectrum of zirconia
(ideally, ZrO2) containing Y acquired using 15kV.
Blue: WDS energy scan of the same spectral region
EPMA: WDS vs EDS53
WDS EDS
Spectra acceptance One element/run Entire spectrum in one shot
Collection time > 10 mins Mins
Sensitive elements Better for lighter elements
(Be, B, C, N, O)
Resolution ~few eV ~130 eV
Probe size ~200 nm ~5 nm
Max count rate ~50000 cps
EPMA: EELS vs EDS54
EELS EDS
Energy resolution ~0.1 eV ~130 eV
Energy range 0-3000 eV 1-50 keV
Element range Better for light elements Better for heavy
elements
Ease of use Medium high
Spatial resolution Good beam broadening
Information Elemental, coordination,
bonding
Only elemental
Quantification Easy Easy
Peak overlap No Can be severe
Related techniques: x-ray fluorescence55
X-ray fluorescence (XRF) is the emission of characteristic X-rays from a material that has been excited by bombarding with high-energy X-rays. Characteristic x-rays can be measured either in energy or wavelength dispersive mode.
Hot cathode tube (Coolidge tube) is the most
common x-ray source.
electrons are produced by thermionic effect from a
tungsten filament heated by an electric current.
A high voltage potential is applied between the cathode
and the anode, the electrons are thus accelerated
The anode is usually made out of tungsten or
molybdenum. So the x-ray generated are characteristic
x-rays of the anode materials
High intensity sources: rotating anode, synchrotron
Comparison: XRF and EPMA56
SEM-EDS
(STEM)
ED-XRF
probe Electron X-ray
Sample
applicability
Conductive samples Conductive or
insulating
Vacuum
requirement
Yes (10
Analysis
time
Minutes Minutes