285
Figure 8.55
Electron energy loss spectroscopy (EELS)
spectrum.
Intensity
Energy (eV)
Low loss
Core loss
x 500 gain change
0
500
1000
Plasmons
Zero loss
Zero loss: Elastically scattered electrons
Plasmons: Collective excitation of valence electrons
Core loss: Inelastic scattering with inner shell electrons
Characterization of Nanomaterials
electrons decay to their relaxed states, X-rays are emitted with
energies that are unique to the ionized atom. Therefore, by measuring the energies or the wavelengths of the X-ray emitted from the
sample, the composition of the material from the region where the
electron probe is positioned can be determined. The actual measurement of the X-ray energies is performed by a semiconductor
detector. The detector generates first a charge pulse proportional to
the X-ray energy. Subsequently, it converts the pulse into a voltage,
which is then amplified and digitalized.
The spatial resolution of the EDS technique is a function of the size
of the electron probe and the volume of interaction between the
beam and the sample. In this regard, an FEG emitter can provide
finer probes, whereas the STEM mode is recommended for accuracy. In addition, because the samples for TEM and STEM are much
thinner than those typically used for SEM, the spatial resolution in
TEM and STEM is considerably better. In terms of minimum detectability, a careful analysis can measure levels of elements below 0.1
at%, particularly if a high-brightness source, high voltage, and thin
foils are used. Currently, with the introduction of aberration-free
correctors, the probe size has been considerably reduced, leading to
improved spatial and energy resolution.
The EELS system is common in both TEMs and STEMs but not
SEMs. It is also fundamentally different from EDS spectroscopy.
The basic concept of EELS is to measure the energy loss of inelastic
scattered electrons. In both the TEM and STEM, these are electrons
that suffer energy loss and change of momentum upon interacting with other electrons in the sample. According to their energy
losses, the transmitted electrons are dispersed by a spectrometer,
and a spectrum is produced (see Figure 8.55). The first peak that
appears in the spectrum, called the zero-loss peak, consists mainly
of electrons that retained their energy. The zero-loss peak is usually
a problem due to its high intensity rather than providing useful
information. Next in the spectrum is the low-loss region. This range
corresponds to energy-loss electrons up to about 50 eV. This region
is related to longitudinal oscillations of valence electrons, called
plasmons. These plasmon losses are dependent on the thickness of
the specimen and are more predominant in materials with freeelectron structures, such as metals. In the case of insulator materials, a further mechanism occurs in the low-loss region, which is the
possibility of exciting valence electrons to states above the Fermi
level. These processes lead to shifts in the plasmon peak that reflect
the type of bonding present in the material. The high loss region
of the EELS spectrum (above 50 eV) corresponds to the inelastic
Figure 8.55
Electron energy loss spectroscopy (EELS)
spectrum.
Intensity
Energy (eV)
Low loss
Core loss
x 500 gain change
0
500
1000
Plasmons
Zero loss
Zero loss: Elastically scattered electrons
Plasmons: Collective excitation of valence electrons
Core loss: Inelastic scattering with inner shell electrons
Characterization of Nanomaterials
electrons decay to their relaxed states, X-rays are emitted with
energies that are unique to the ionized atom. Therefore, by measuring the energies or the wavelengths of the X-ray emitted from the
sample, the composition of the material from the region where the
electron probe is positioned can be determined. The actual measurement of the X-ray energies is performed by a semiconductor
detector. The detector generates first a charge pulse proportional to
the X-ray energy. Subsequently, it converts the pulse into a voltage,
which is then amplified and digitalized.
The spatial resolution of the EDS technique is a function of the size
of the electron probe and the volume of interaction between the
beam and the sample. In this regard, an FEG emitter can provide
finer probes, whereas the STEM mode is recommended for accuracy. In addition, because the samples for TEM and STEM are much
thinner than those typically used for SEM, the spatial resolution in
TEM and STEM is considerably better. In terms of minimum detectability, a careful analysis can measure levels of elements below 0.1
at%, particularly if a high-brightness source, high voltage, and thin
foils are used. Currently, with the introduction of aberration-free
correctors, the probe size has been considerably reduced, leading to
improved spatial and energy resolution.
The EELS system is common in both TEMs and STEMs but not
SEMs. It is also fundamentally different from EDS spectroscopy.
The basic concept of EELS is to measure the energy loss of inelastic
scattered electrons. In both the TEM and STEM, these are electrons
that suffer energy loss and change of momentum upon interacting with other electrons in the sample. According to their energy
losses, the transmitted electrons are dispersed by a spectrometer,
and a spectrum is produced (see Figure 8.55). The first peak that
appears in the spectrum, called the zero-loss peak, consists mainly
of electrons that retained their energy. The zero-loss peak is usually
a problem due to its high intensity rather than providing useful
information. Next in the spectrum is the low-loss region. This range
corresponds to energy-loss electrons up to about 50 eV. This region
is related to longitudinal oscillations of valence electrons, called
plasmons. These plasmon losses are dependent on the thickness of
the specimen and are more predominant in materials with freeelectron structures, such as metals. In the case of insulator materials, a further mechanism occurs in the low-loss region, which is the
possibility of exciting valence electrons to states above the Fermi
level. These processes lead to shifts in the plasmon peak that reflect
the type of bonding present in the material. The high loss region
of the EELS spectrum (above 50 eV) corresponds to the inelastic