Electron spectroscopic imaging and electron energy loss spectroscopy (EELS)

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Electron spectroscopic imaging and electron
energy loss spectroscopy (EELS) Can we see a
colored image in a TEM? The forth dimension ---
when electrons are measured by their energy.
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Every primary electron has one of three
possibilities in terms of its interactions with
atoms of the specimen.
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The energy of a transmitted or elastically
scattered electrons, even diffracted ones,
remains relatively unchanged. The energy of an
inelastically scattered electron is always less
than that of the primary electron.
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For most of the electrons, the change in energy
is not random but is directly related to which
electron, from which atom, from which orbital
shell the inelastic collision took place.
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This specific loss of energy is known as Electron
Energy Loss Spectroscopy or EELS
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One important thing Unlike x-rays (EDX), an
electron beam traversing a thin sample may lose
any amount of energy. One good reference
Egerton R F 1996 Electron Energy-Loss
Spectroscopy in the Electron Microscope (New
York Plenum)
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How the EELS signal is measured
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As in light optics the resultant polychromatic
illumination can be broken down based on the
wavelength, which in tern, is determined by the
energy of the beam.
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What a measured EELS spectrum look like?
The energy loss spectrum can be displayed and the
loss profiles be used to identify elements in a
specimen.
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EELS Spectra
Process of inelastic electron scattering
The Zero-Loss Peak 1). It is the main feature
in EELS spectra of thin specimens. 2). Originates
from electrons that have lost NO energy on their
way through the specimen (except for small losses
due to phonon scattering). 3). Corresponds to
undiffracted beam in the diffraction pattern. 4).
Width of the zero-loss peak energy spread of the
electron source. 5). Zero loss peak contains less
analytical information about the sample. What is
phonon? -- Phonons are lattice vibrations, which
are equal to heating the specimen. This effect
may lead to a damage of the sample.
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Low-Loss area Plasmon 1). It is the region with
energy losses up to 50 eV. 2). It reflects
excitation of plasmons and interband
transitions. What is plasmons Plasmons are
longitudinal oscillations of free electrons,
which decay either in photons or phonons. It is
caused by weakly bonded or quasi-free electrons.
It depends on local density of the weakly bonded
electrons. The typical lifetime of plasmons is
about 10-15 s. It is localized to about 10 nm and
mean free path length for electron scattering
(100 keV) at plasmons is about 100 nm.
Thin
Thick
In the EELS spectra, plasmon losses always occur,
except the ultra-thin specimens. Thus it can be
used to estimate the thickness of the sample.
However, when the specimen is quite thick,
multiple plasmon losses will make the
straightforward analysis impossible.
t ?p Ln(Ip/I0), ?p the electron mean free path
for plasmon excitation
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Low-Loss area longitudinal oscillations of the
valence or conduction band electrons in both the
surface and the bulk gives characteristic plasmon
energy losses. This energy loss region also
contains energy losses from the excitation of
inter- and intraband transitions. Band-band
transitions It is the excitation of electrons to
an orbital of higher quantum number. Quantitative
analysis of the low-loss peak is still difficult.

Simplified diagram of band structure of an
insulator or semiconductor
The low-loss peaks of the Al atoms in different
environments.
Illustration of inter- and intraband transitions
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High-loss Region 1). It is the region with
energy losses E gt 50 eV. 2). It reflects
inelastic scattering in inner regions of atoms,
i.e. ionization of inner shells (K, L, M, ).
3). A specific minimum energy, the critical
ionization energy EC or ionization threshold,
must be transferred from the incident electron to
the expelled inner-shell electron, which leads to
ionization edges in the spectrum at energy losses
that are characteristic for an element. What
is ionization? The high-energy electrons of the
incident beam can transfer a critical amount of
energy to an inner-shell electron of an atom,
leading to the ejection of this electron. The
ionization energy is provided by the incident
electron, reducing its energy. This leads to an
ionization edge in the electron energy loss
spectrum. The closer a primary electron
approaches the nucleus of an atom, the larger can
be the energy loss during inelastic scattering.
EELS spectrum of BN (boron nitride)
Illustration of ionization of inner shells
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Why is it ionization (absorption) edge? 1).
Electrons should lose at least the energy equal
to the critical ionization energy EC for
ionization (this energy is the binding energy of
the inner-shell electron to the nucleus of
atom). 2). At the same time, however, ionization
occurs also with larger energy losses E gt
Ec. 3). Intensity of the absorption edge
decreases with increasing energy loss 4). The
edge has a high background. The background is
originated from multiple inelastic electron
scattering extension of previous absorption
edges 5). The most important thing is from the
edge the critical ionization energy Ec can be
identified. We then can identify the elements in
our sample from the energy.
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Like X-ray microanalysis EELS offers a method by
which an electron beams interaction with the
specimen can yield specific information about
which elements are present in a specimen.
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Two more things about the ionization (absorption)
edge. 1. ELNES, electron loss near-edge
structure features in the spectrum with energy
loss E Ec to Ec 50 eV (by definition). It
contains information on local density of empty
states, oxidation state. 2. EELFS, extended
energy-loss fine structure features in the
spectrum with energy loss E gt Ec 50 eV. It
contains information on local coordination of the
respective atom. multiple inelastic
scattering ionization followed by plasmon
scattering modulation at Ec 15..25 eV
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Diamond, graphite and fullerene all consist of
only carbon. All of these specimens have
absorption peaks around 284 eV in EELS
corresponding to the existence of carbon atoms.
From the fine structure of the absorption peak,
the difference in bonding state and local
electronic state can be detected. The sharp peak
at absorption edge corresponds to the excitation
of carbon K-shell electron (1s electron) to empty
anti-bonding pi-orbital. It is not observed for
diamond, because of no pi-electron in it.
eels.kuicr.kyoto-u.ac.jp/eels.en.html
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Comparisons of EELS and EDS (X-ray
Energy-Dispersive Spectrometer)
1). EELS has higher spatial resolution than EDS.
EDAX may be affected by backscattering
electron, And fast secondary electron within the
sample. 2). EELS has higher energy resolution
than EDS. (around 1 eV) 3). EELS is better in
detecting light element. 4). EELS contains
information of electronic structure. 5). EDS is
easy to operate and quick for a qualitative
composition analysis.
However, EELS Spectra from thick specimens
(gt50nm) may be difficult to interpret because of
plural scattering. Interpretation of fine
structure sometimes requires sophisticated
calculations.
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Electron Spectroscopic Imaging (ESI)It is a
contrast-enhancement technique. It improves
contrast in images and diffraction patterns by
removing inelastically scattered electrons that
produce heavy background.It is a mapping
technique. It creates elemental (chemical) maps
by forming images with inelastically scattered
electrons.It is an analytical technique. It
records electron energy-loss spectra (and maps)
to provide precise chemical analysis of the
samples.
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When a monochromatic electron beam interacts with
a specimen it becomes polychromatic due to
multiple scattering events.
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An electromagnetic prism is needed to separate
the resultant wavelengths. After bouncing off of
an electrostatic mirror the selected wavelength
of electrons is used to produce an image of the
specimen.
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Zero-loss images are those created by only using
the transmitted, and thus no energy loss,
electrons. They have increased contrast due to
the elimination of scattered electrons but retain
high resolution because an objective lens
aperture is not needed to eliminate scattered
electrons.
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Zero-loss image of a 1.0 mm thick section at
100KeV
Multiple scattering events are the primary reason
why thick sections cannot normally be imaged in
conventional TEM. Zero-loss imaging allows for
imaging of thick sections.
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Muscle cell Fibroblast
Stereo-pair images from thick sections imaged
with zero-loss imaging.
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Cryoimage of cationic vesicles
Increased contrast from low contrast images such
as unstained sections or cryo-samples.
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Energy filtering can also be used to improve
diffraction patterns eliminating scattered, but
not diffracted, electrons from the image. Like
transmitted electrons diffracted electrons have
no energy loss.
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One can create an image using only those
electrons that were slowed down by their
interactions with a specific element. Electron
Spectroscopic Imaging or ESI
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ESI is usually accomplished by increasing the
accelerating voltage of the TEM by precisely the
additional energy needed (e.g. 100,250 eV vs.
100,000 eV).
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Conventional image
Same sample imaged with electrons that have lost
250eV corresponding to Fe
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Element specific images can be created but these
are not maps because they are not made scanning
the beam or collecting the signal as is done with
an X-ray map.
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High resolution elemental distribution imaging
using elemental spectroscopic imaging (ESI) is an
optimal complement to high resolution imaging. It
provides important additional information about
specimen structures. This highly sensitive and
fast imaging method can show element demarcations
and element distributions with a resolution in
the nanometer range within minutes.
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Because EELS is not dependent on signal
collection even light elements such as Boron can
be imaged.
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LEO Omega system uses four prisms and no
electrostatic mirrors.
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LEO holds the patent on in-column energy
filtering devices.
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The Gatan Imaging Filter (GIF) is an add-on
device that can be attached to any TEM. Like the
in-column prisms it can separate the signal based
on changes in wavelength (energy) and use the
results for either EELS or ESI.
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At 200K the GIF is not for every lab and it
occupies the port normally used for high
resolution digital cameras.
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Advantages of EELS, ESI, and Zero-loss
imaging Elemental Analysis Elemental Imaging
and Mapping Improved contrast without loss of
resolution Thick specimen imaging Disadvantages
Elemental analysis requires very thin specimens
(10-20nm) Mucho (LEO TEM or GIF on a STEM)
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Atomic Electronic Structure
Atomic Diameter Electron Probe
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• STEM EELS makes essential connection between
  physical electronic structure, both at atomic
  resolution

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Annular Dark Field (ADF) detector
Increasing energy loss
Electron Energy Loss Spectrometer