ARTICLE IN PRESS
Ultramicroscopy 99 (2004) 73–85
Measuring the absolute position of EELS ionisation edges
in a TEM
P.L. Potapov*, D. Schryvers
EMAT, University of Antwerp, RUCA, Groenenborgerlaan 171, Antwerp B-2020, Belgium
Received 13 March 2003; received in revised form 10 June 2003; accepted 7 July 2003
Abstract
Measurements of absolute positions of electron energy loss spectroscopy (EELS) core-loss edges in a transmission
electron microscopy (TEM) are hampered by noticeable errors caused by instabilities of the primary energy of the
incident electrons. These instabilities originate from a continuous drift and random ripple of the high tension and are
unavoidable in the present generation of TEM and scanning TEM microscopes. However, more precise measurements
are desired, for instance, to study the shift of the edge onset between atoms of different valency or chemical
environment, the so-called chemical shift. A solution to this problem is presented by collecting a series of short low-loss
acquisitions immediately followed by core-loss ones. To ensure a minimal time lapse between core-loss and low-loss
acquisitions, all operations must be computer controlled. Accumulation of a number of acquisitions and their
summation corrected for energy drift allows to cancel the energy instabilities and to relate the core-loss EELS spectra to
the absolute energy scale. A practical algorithm is presented as well as the necessary calibrations for such a procedure.
Also, examples of spectra collected using this principle and the resulting measured chemical shifts in several metaloxides are presented.
r 2003 Elsevier B.V. All rights reserved.
Keywords: Electron energy loss spectroscopy; Instrument control and alignment
1. Introduction
Today, electron energy loss spectroscopy
(EELS) has become a widespread attachment to
transmission electron microscopes (TEM). The
position and intensity of an EELS ionisation edge
provide chemical information whereas its fine
structure (energy loss near edge structure, ELNES)
contains information on the bonding states in the
*Corresponding author. Tel.: +32-3-218-0472; fax: +32-3218-0257.
E-mail address: [email protected] (P.L. Potapov).
material. It should be realised, however, that the
majority of experimental data on EELS edges, as
reported in the literature and obtained in current
day microscopes, have a precision of 0.5 eV or
worse. However, more accurate data are desired
for several reasons. First, a given ionisation edge is
known to appear at slightly different positions
depending on the actual electronic structure of a
given material [1–8]. This difference in onset
position, also referred to as the chemical shift, in
most cases does not exceed 1 eV and therefore
cannot be properly reproduced by the existing
experimental methods. Still, a chemical shift
0304-3991/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0304-3991(03)00185-2
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P.L. Potapov, D. Schryvers / Ultramicroscopy 99 (2004) 73–85
indicates changes in the relative energy levels of
the inner electron shells with respect to the Fermi
level and might provide important information
complementary to the more widely used ELNES
information. Second, the ongoing progress in
filtered imaging methods asks for the application
of smaller energy windows for valency mapping
and spectrum-imaging. The use of windows less
than 1 eV wide is now under discussion, but this
small window size would require more precise
measurements of the onset positions. Third,
quantitative data of the absolute positions of the
edge onset is important in view of theoretical
understanding of the materials. Until recently, ab
initio calculations could reproduce the real binding
energies only approximately, but new advances in
the calculation methods yield precisions comparable to those of conventional experiments [9,10].
Comparing different theoretical approaches
would definitely require more precise experimental
reference data.
The main reason for the limited precision of the
energy measurements in EELS spectra can be
found in the instability of the energy of the
primary electrons. The only available reference
point for calibration of the origin of the energy
axis is the position of the zero-loss peak, which
consists of those electrons with primary energy.
However, most ionisation edges appear in energy
loss regions of a few to several hundred eV and
cannot be observed in the same spectrum as the
zero-loss peak unless a very rough energy dispersion is used. The common way to combine a high
energy dispersion with an accurate energy calibration is by shifting the energy range between a lowloss region to a core-loss one by a small variation
of the high tension of the microscope or by
applying a voltage to the electrostatic drift of tube
in the spectrometer. However, the precision of
these shifts and the related energy measurements is
limited by the energy instabilities of the currently
available TEM and related scanning TEM
(STEM) instruments. This limitation results in an
unknown extra energy difference between any two
consecutive acquisitions. At present there are only
a few instruments in the world with a dedicated
design in which these energy instabilities are
minimised. For example, it is possible to decelerate
the electrons before the spectral analysis and if the
spectrometer is floating at the cathode potential,
the acceleration and deceleration voltages cancel
out exactly [11,12]. As a result, up to now, any
reliable data about chemical shifts have almost
exclusively been obtained from measurements on
dedicated instruments [7,8]. However, those instruments are difficult in handling and are unlikely
to become widespread in the near future.
In the present work we suggest a new method
enabling spectroscopists to measure precisely the
absolute position of onsets using regular TEM
instruments and standard spectrometers without
the need for any additional hardware installations.
The method is based on an computer script, which
corrects for instabilities in the primary energy of
the electrons. At this point we need to differentiate
clearly between the precision and accuracy of a
measurement. Better precision implies a smaller
spread of the measured data, i.e., higher reproducibility of the measurements in a given instrument,
while better accuracy means closer correspondence
of the measured number to the real absolute value.
In other words, precision deals with random errors
while accuracy deals with systematic errors. Two
other important terms in EEL spectroscopy are
instrumental energy resolution and energy dispersion of the spectrometer. The former characteristic
indicates how much an infinitely narrow peak
(d-function) will be broadened by instrumental
factors. The latter is simply the energy window
covered by a single registration channel. In
principle, the precision of measurements of the
peak positions can be better than the energy
resolution and energy dispersion, provided that
the peak profile is fitted by the proper model and
sufficient counts are collected. The script that has
been developed yields an improved precision as
well as accuracy for the measured position of the
edge onset.
2. Instrumentation
The present work is primarily performed on a
Philips FEG CM30 ultratwin microscope with an
external GIF200 detector. The primary energy of
300 kV is maintained by a Philips high-tension
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generator. The extractor voltage of the gun is
reduced to improve the energy resolution, which is
0.75–0.8 eV in the present experiments. To optimise the precision of the measurements the finest
energy dispersion available in the GIF200, namely
0.05 eV/channel, is chosen. The probe size is about
100 nm and the typical thickness of the investigated samples is 0.6–1.0 times the respective mean
free path of the electron. All spectra are collected
in image mode with the aperture in the objective
lens plane providing a collection angle of 14 mrad.
The second order aberrations in the GIF are
carefully tuned, so that the energy resolution is
almost the same when using a 2 mm or 0.6 mm
GIF entrance aperture. This way, the 2 mm
aperture can be used, increasing the intensity by
a factor of 10 with respect to the 0.6 mm one.
Tests of the energy stability are also conducted
on a Jeol FEG JEM3000F microscope equipped
with a GIF2000 and a Philips FEG Technai
microscope with a monochromator and a high
resolution-GIF detector (National Centre for
HREM, Delft, Netherlands).
The developed scripts are written in the digital
micrograph (DM) code and can be implemented in
any DM environment.
3. Factors affecting the precision and accuracy of
absolute measurements
Fig. 1a shows the variation with time of the
primary electron energy measured through the
GIF spectrometer in the CM30 and JEM3000F
microscopes (lower section). The average energy of
the electrons collected in a single acquisition is
determined as the centre of mass of the zero-loss
peak. This zero-loss peak is collected at a rate of 5
times per second with an exposure time of 0.02 s
for each individual acquisition. In both CM30 and
JEM3000F, a random energy ripple is detected
having a dominant frequency between 1 and 3 Hz
and a peak-to-peak magnitude up to 0.25 eV in the
CM30 and 0.35 eV in the JEM3000F. The top part
in Fig. 1a shows the variation of the high tension
in the CM30 measured 5 times per second by a 8.5
digits voltmeter connected to the reference voltage
in the high tension generator [13]. Taking into
75
account the actual precision of the voltmeter,
which is 70.3 ppm (i.e., 0.09 V) at such a small
integration time, the observed magnitude and
frequency of the high-tension ripple correspond
well to those detected via the zero-loss peak. Thus,
the ripple of the primary energy originates from
micro-instabilities of the high tension and seems to
be attributed to the quality of the electronic
feedback system in the high-tension generators.
On top of this ripple, a continuous energy drift of
0.2 eV/min is observed in both CM30 and
JEM3000F microscopes. Simultaneous measurements of the reference voltage and the zero-loss
peak position reveal that also this drift mainly
originates from the drift of the high tension. More
details on this procedure can be found in a
forthcoming paper [13].
It is interesting to compare these regular TEM
instruments with other instruments specially designed for higher energy stability. Fig. 1a also
shows stability tests performed on the dedicated
Technai microscope at the Technical University
Delft via the zero-loss position measurement. In
this microscope, a monochromator is installed and
the high-tension circuit is significantly improved in
order to meet high-energy resolution requirements.
As a result, the energy ripple is reduced to 0.08 eV
and the drift to 0.04 eV/min.
In practical terms, the apparent magnitude of
the ripple can be reduced by optimising the
exposure time. Fig. 1b shows the effect of a
numerical filter averaging over 5 values from
adjacent channels of the original ripple curve.
This graph demonstrates that an increase of the
exposure time from 0.02 to 1 s would result in a
decrease of the apparent ripple peak-to-peak
magnitude from 0.25 to 0.15 eV due to the
averaging of fast changes within a single 1 s
exposure. Still, random-like energy jumps up to
0.12 eV between any two subsequent acquisitions
would remain. In practice, real experiments with
1 s exposure indicate that the latter also amounts
to 0.15 eV.
During conventional operation of an EELS
experiment a pause between low-loss and coreloss acquisitions lasts more than 1 min because the
operator should manually tune the illumination
conditions and shift the energy region. Also, the
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Fig. 1. (a, bottom) Variation of electron primary energy measured by EELS in different microscopes and (a, top) variation of high
tension in CM30 measured by a 8.5 digits voltmeter; and (b) effect of a smoothing filter on the apparent magnitude of the energy ripple.
The vertical spacing between the horizontal grid lines is 0.1 eV for (a) and 0.05 eV for (b).
exposure time for core-loss acquisitions is usually
chosen to be much larger than that for low-loss
ones so that a new dark current reference image
needs to be collected before each acquisition.
Combining both the ripple and drift factors
determined above, the typical random error in
determining edge positions is expected to be about
0.5 eV or worse when standard microscopes and
operation routines are used. Still, in the case
of relative measurements, for instance when
measuring a chemical shift of a given edge between
two regions in one sample, the precision can be
improved to 0.2–0.3 eV because the intensity does
not have to be retuned and the ripple is partially
averaged by long core-loss acquisitions.
Furthermore, a systematic error of the same
magnitude and related to the imperfect calibration
of the high tension or the drift tube might be
induced. This calibration is normally done by
using the Ni L3 onset in nickel oxide and, taking
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77
Fig. 2. Calibration of the energy dispersion of the CCD camera for different settings and given in number of channels.
the uncertainty in any energy measurement mentioned above into account, is only precise within a
range of 70.5 eV. This calibration error is specific
for each calibration procedure made on a given
instrument. Additionally, in our opinion, the
standard 854 eV value currently adopted by Gatan
for the Ni L3 onset in NiO [14,15] is noticeably
higher than the true value (see below). Thus, the
majority of EELS spectra published in the
literature are systematically shifted to higher
energies. Note that the latter systematic error can
easily be taken into account when comparing
spectra from different sources provided that the
reference point is properly described.
Another factor which can potentially affect the
precision and accuracy of the measurements is a
non-linearity of the energy dispersion of the
spectrometer. Ideally, the zero-loss peak and
the examined core-loss feature should be put at
the same channel by choosing an appropriate
energy shift. In reality, however, even the sharpest
features of ionisation edges are much broader than
the zero-loss peak and are spread over 30–100
channels depending on the nominal energy dispersion used. Moreover, as will be demonstrated
below, the zero-loss peak and core-loss onset
should sometimes be put at different channels to
avoid artefacts in the spectra. Thus, a range of at
least a few hundred channels in the CCD camera
needs to be properly calibrated. To calibrate the
energy dispersion along the CCD camera the zeroloss peak is first precisely positioned at channel
100 (which was taken as a reference) and then
shifted to the desired channel by the drift tube (for
the calibration of the drift tube, see Section 6). The
resulting position of the peak is then measured and
compared with that expected from the nominal
energy dispersion. This procedure was repeated
several times over the same interval of channels to
suppress the energy ripple effect and the smoothed
average curve is shown in Fig. 2. The deviation
from the expected position is maximal for an
energy dispersion of 1 eV/channel and reduces
with decreasing nominal dispersion. For the
0.05 eV/channel dispersion used in the present
measurements, the deviation is negligible up to
channel 500, the region of interest for the present
experiments.
4. Acquisition script minimising the effects of
energy instability
In order to minimise the effects of the energy
instability of the primary electrons a computer
script was written performing several acquisitions
in a row and in the mean time adjusting different
artefacts and performing internal calibrations. To
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equally average the energy ripple during all
acquisitions a 1 s exposure time is adopted for
both low-loss and core-loss acquisitions in the
script. When switching from core-loss to low-loss
acquisitions, the illumination is reduced by changing the current in the C2 condenser lens thus
defocussing the spot. The energy region is shifted
by applying the drift tube, a procedure which is
much faster than changing the high tension. Both
these operations are computer controlled minimising the pause between acquisitions. All acquired
spectra are then gain normalised and corrected for
dark current references, which are prepared at the
start of the execution of the script.
The schematic of the developed script is shown
in Fig. 3. First a spectrum in a low-loss region is
collected followed by the calculation of the
position of the zero-loss peak. Based on this
calculated value, a small drift tube voltage
typically less than 1 V is applied to position the
centre of the zero-loss peak exactly at channel 100.
Then, a number of cycles each consisting of lowloss and core-loss acquisitions is executed. The
pause between low-loss and core-loss acquisitions
is only about 1 s, thus, according to Fig. 1b, the
energy jump is expected to be smaller or equal
to 0.15 eV, which yields a maximum shift
of 3 channels when using the 0.05 eV/channel
dispersion. After each cycle, the centre of the zeroloss peak is again calculated, and typically floats
within the expected window of 3–4 channels
around channel 100. This fluctuation cannot be
avoided and results from the ripple of the high
tension together with the continuous drift as
described above. Then the data of the low-loss
spectrum are shifted by several channels to realign
the zero-loss peak with channel 100 and the same
shift is applied to the subsequent core-loss
spectrum. In those cases when the calculated
deviation exceeds 3 channels, the drift tube
correction is adjusted returning the zero-loss peak
back to channel 100. With this combination of
software and hardware corrections, respectively,
the position of the zero-loss peak in each cycle can
be maintained at channel 100. Thus, the continuous drift is cancelled allowing the script to
execute as long as necessary. Still, the entire ripple
of the primary energy is not completely suppressed
by this operation since energy jumps of 0.15 eV
between two subsequent low-loss and core-loss
acquisitions within one cycle are still possible.
However, this jump is random in nature and
accumulation of a number of low-loss and coreloss spectra can average and minimise the effect of
this feature. In practice, after about 15 cycles the
position of a given peak converges to a fixed value.
Fig. 3. Schematic of the script applied to cancel energy instabilities in a microscope.
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Finally the script yields a cumulative low-loss
spectrum with the zero-loss peak placed at channel
100. The peak centre inside channel 100 is further
calculated more precisely by measuring the centre
of mass of the channels located within the halfwidth of the peak.
Although the primary motivation for applying
cycled acquisitions is improvement of the precision
of measuring the peak positions, the suggested
procedure exhibits another advantage. The instrumental resolution is usually measured in the lowloss region using short acquisitions. Long acquisitions, typical for core-loss regions, can result in
additional energy broadening due to the energy
instabilities in the course of acquisition. For
instance, applying a 1 min acquisition might result
in a further 0.2–0.3 eV deterioration of the
effective energy resolution. In the developed script,
1 s acquisitions are applied and summed while
taking the energy drift between them into account,
resulting in a minimal deterioration of the original
energy resolution. Regardless of the number of
cycles, the energy broadening is determined by the
very negligible energy drift within the 1 s time
period of a single acquisition. Thus, the present
procedure also combines a minimal energy broadening typical for short acquisitions with the good
statistics achieved by long acquisitions.1
The disadvantage of the present version of the
script is that the convergence angle changes with
changing the C2 current. In particular, it implies
that the low-loss spectrum cannot be used for
deconvoluting the core-loss one from plural scattering. Further developments allowing us to register
both low-loss and core-loss spectra under identical
collection and convergence angles and applicable to
both image and diffraction modes are in progress.
5. Artefacts in spectra
Since two subsequent acquisitions are collected
quite rapidly one after the other, the following
1
During preparation of this paper, other work has been
published, which describes the similar script aimed to improve
the energy resolution [16]. With minor changes, their script can
be also employed for measuring the absolute onset positions.
79
artefact has been found. When executing the script
without a sample in the electron beam path, a
small spot is observed in the image of the second
spectrum exactly at the place of the former zeroloss peak. This is shown in Fig. 4a where the
dynamic range for the core-loss case (bottom
image) is stretched by a factor 1000 with respect to
that of the low-loss case (top image). A variation
of the time delay between the low-loss and coreloss acquisitions demonstrates that this echo peak
quickly decays with time and decreases to the noise
level after approximately 5 s. This effect is similar
to the incomplete discharge of the zero-loss peak
observed after oversaturation of a CCD camera.
In that case the charge cannot be read completely
during the first following readout causing a partial
memory of the previous acquisition. However, the
incomplete readout is known to be recovered by
applying a series of short acquisitions. In contrast,
the echo peak observed in the present experiments
decreased only with passing time and not with the
number of readouts preceded the final acquisition.
Fig. 4b shows the intensity of the echo peak after a
series of readouts with different exposure times, as
indicated. Regardless of the number of intermediate readouts, the decay curves practically coincide
with one another and with the case when no
readouts are applied. By fixing the delay time and
varying the intensity of the initial zero-loss peak,
an almost linear relation between the initial and
echo peak intensities is found, as shown in Fig. 4c.
Thus, the echo peak is not related with any
oversaturation of the CCD camera but can
possibly be attributed to the limited time response
of the scintillator layer in front of the CCD array.
Although the echo peak observed after a delay of
1 s is a thousand times lower in intensity than the
original zero-loss peak, it is still noticeable in a
low-intensity core-loss spectrum. The only way to
cancel this artefact is by reducing the intensity of
the zero-loss peak down to 10% of the saturation
level. In this case, the statistics of a low-loss
spectrum are still sufficient to calculate precisely
the centre of the zero-loss peak while the intensity
of the echo peak drops to the noise level as seen in
Fig. 4c. Nevertheless, to avoid any potential
influence of this artefact on the measurements,
the edge onset of the core-loss spectrum was
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Fig. 4. (a) Appearance of an echo peak in the image of a second spectrum, (b) dependency of the echo peak intensity on the pause
between low-loss and core-loss acquisitions (number at each experimental point denotes the amount of intermediate readouts
preceding the final acquisition) and (c) the echo peak intensity as a function of the original zero-loss peak.
placed around channel 200 by the applied drift
tube.
Another disturbing feature is related with
changing the beam intensity between low-loss
and core-loss acquisitions. As described above,
the intensity is tuned by changing the current in
the C2 condenser lens. Unfortunately, it is noticed
that the C2 current slightly affects the position of
the zero-loss peak. This is shown by executing the
script with a zero drift tube value yielding two lowloss spectra, one for defocussed and the other for
focussed illumination. As for the latter C2 setting
the intensity becomes very high, the exposure is
reduced to 0.01 s, which results in a higher random
energy jump between acquisitions as seen in
Fig. 1b. This is then compensated by increasing
the number of cycles up to 120. Fig. 5 shows the
results of the measurements demonstrating that
the apparent electron energy can be shifted by
several tenths of eV when changing the C2 current
with respect to a chosen reference value of
2500 mA. The energy shift is close to linear in the
large range of C2 currents while the intensity
entering the spectrometer, also shown in Fig. 5,
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81
distribution of a random variable, the average
value Emeas measured in N cycles should occur
with a 95% probability within the following
confidence interval of precision sE near the true
value Etrue
s
Eexp ¼ Emeas 71:96 pffiffiffiffiffi ¼ Emeas 7sE ;
ð1Þ
N
where s is the standard deviation of a single
measurement, which is estimated from the variation of the zero-loss peak position Ezl from cycle to
cycle
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
ðEzl E% zl Þ2
s¼
:
ð2Þ
N
Fig. 5. Effect of changing the current in the C2 condenser lens
on the intensity coming into a GIF and the apparent position of
the zero-loss peak.
increases dramatically when the beam passes
through the focus point at which its size becomes
comparable with the spectrometer entrance.
To correct for this C2 effect, the two standard
C2 current settings were chosen at the beginning of
every session, one for the low-loss and another for
the core-loss region, aiming at a large intensity
difference with a minimal energy shift. The
remaining energy shift between the two C2 settings
is measured and accounted for when calculating
the reference point in the core-loss spectra.
However, this shift can be calibrated only with
the precision of 70.03 eV (see below), which
introduces an additional uncertainty in the determination of the core-loss onsets. A possible
solution for this could be to attenuate the intensity
without any changing currents in the microscope
lenses but rather by changes inside the GIF, for
instance by wobbling the beam in the direction
perpendicular to the energy dispersion axis.
6. Precision and accuracy of measuring the
core-loss onsets
As the energy jump between low-loss and coreloss acquisitions is random, the precision of the
present measurements can be estimated by the
standard statistics theory. Assuming a Gaussian
When 1 s acquisitions are used, the standard
deviation s of the zero-loss peak position is
measured to be about 0.1 eV, which agrees well
with the data displayed in Fig. 1b. This results in a
0.05 eV precision sE for a typical series of 20 cycles
as used in the present work. The energy shift
occurring with changing C2 current gives an
additional error in measuring the onset. Calibration of this shift performed within the same script
shows a higher standard deviation, namely
0.15 eV, because shorter exposure times are used.
Since this calibration is needed only once a session,
a higher number of cycles can be applied without
affecting the efficiency of the procedure. Applying
120 cycles reduces the calibration error to 0.03 eV.
As the effective duration of each cycle is only 3 s
and provided that the continuous drift does not
exceed 0.2 eV/min the contribution of the continuous energy drift within a given cycle should be
less than 0.01 eV and can be neglected. Combining
the sE with the calibration error of the C2
adjustment yields a total precision of 0.1 eV.
The systematic error specific for a given instrument comes from the procedure of the drift tube
calibration, which, following the above reasoning,
can be performed with a precision of 0.1 eV. Since
the drift tube can be considered as a linear feature,
its calibration can be performed at large values,
e.g. 1000 V, which yields a relative uncertainty of
0.01% which amounts to much smaller absolute
values when decreasing the applied drift tube
voltage, i.e., for edges with lower binding energies
(e.g., 200 V70.02 V).
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Table 1
Absolute position of the Ni L3 edge measured by dedicated EELS spectrometers [18,21] and XPS [22]
Batson [18]
Fink et al. [21]
Fuggle et al. [22]
Material
Starting point (eV)
Left half-height (eV)
Maximum (eV)
NiO
Ni
NiOa
Ni
NiOa
—
851.9
—
—
—
852.0
852.5
852.9a
852.8
853.0a
852.75
853.0
853.2a
853.4a
853.6a
The numbers for NiO oxide are extrapolated assuming a 0.2 eV chemical shift between Ni metal and NiO.
a
Extrapolated.
A further uncertainty relates with the knowledge
of the exact absolute position of the Ni L3 line in
NiO, which is usually used for the drift tube
calibration. Simple calibration of the voltage with
a voltmeter is not entirely accurate in spectrometers in which an extra potential is applied to the
drift tube, because the electron beam is likely not
to be precisely centred in the drift tube. This
typically creates an extra deflection of the beam as
it is accelerated at the drift tube entrance and
decelerated at the tube exit. The deflection results
in a spurious shift of the energy at which a feature
appears in the spectrum. In Gatan spectrometers,
this effect typically amounts to 1–2 eV at an energy
loss of 1000 eV [17]. At present, the value of
854.0 eV of the Ni L3 onset (defined as the left halfheight of the peak) in NiO is generally adopted for
GATAN spectrometers [14,15]. However, data
from dedicated EELS spectrometers in which
energy instabilities are diminished support a lower
value. Batson, using a dedicated STEM at the
T.J. Watson IBM centre, obtained values of
852.0 eV for the half-height on the left shoulder
of the EELS L3 peak and 852.75 eV for the
maximum of the L3 peak in NiO [18].2 Fink et al.
reported a value of 851.9 eV for the extrapolated
starting point of the Ni L3 edge in pure nickel
measured in a dedicated EELS spectrometer at
.
the Institut fur
. Nukleare Festkorperphysik
in
Karlsruhe [21]. The latter number was derived by
extrapolating the L3 edge to the background line
2
It should be noted that the number 853.2 eV cited by
Egerton [19] unfortunately resulted from a misunderstanding
when copying the respective data from the original source
[18,20].
while the maximum of the peak stayed approximately 1.0 eV above this value [21]. As the
instrumental energy resolution of both spectrometers is similar and as the chemical shift between
nickel metal and nickel oxide is known these
results can be properly compared. In this respect it
is also worth to analyse the data from X-ray
photoelectron emission spectroscopy (XPS), which
is often used for calibration of XAS spectra.
Fuggle et al. [22] reported a 852.8 eV binding
energy for the Ni L3 shell in pure Ni. As this
number is defined relative to the Fermi level it
should be most closely compared with the halfheight of the EELS L3 peak. However, there is a
slight difference in the ionisation event in XPS, in
which an electron is ejected to the vacuum level,
and in EELS, in which the final state locates just
above the Fermi level. A created core-hole rises the
effective attraction between a nuclei and electrons
making all electronic shells deeper in energy and
increasing the apparent binding energy. This effect
is expected to be slightly stronger for XPS as an
electron is ejected to a higher energy than in EELS.
Thus, the XPS data could be considered as an
upper limit for EELS binding energies.
Table 1 lists the measured numbers and extrapolated half-heights and maxima of the L3 peak in
NiO taking the natural lifetime broadening of
edges and the chemical shift between Ni and NiO
into account. As mentioned above, XPS data
represent the upper limit for the EELS number
indicating that the 854.0 eV value for the peak
half-height currently used in the calibration of
Gatan spectrometers is overestimated and should
be revised. Also, the maximum of the L3 peak
instead of the half-height is more relevant for
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P.L. Potapov, D. Schryvers / Ultramicroscopy 99 (2004) 73–85
calibration purposes as it is less dependent on the
instrumental resolution. Finally, the value of
853.0 eV compiled from the results of Batson and
Fink is adopted in the present work as a reference
maximum for the Ni L3 peak in NiO.
7. Examples of collected spectra
Fig. 6a shows oxygen K edges in several metaloxides, obtained using the above script and placed
on an absolute energy scale. Except for a
difference in ELNES as mentioned in the literature, the different positions of the onsets are now
apparent. A chemical shift provides information
complementary to ELNES and helps to differentiate between compounds of different valency.
For instance, the TiO ELNES resembles the
broadened ELNES of TiO2, so that TiO can be
83
confused with TiO2 when no sufficient instrumental energy resolution is available. However, the
onset of TiO is clearly different from that in TiO2
leaving no possibility for wrong identification. In
other cases like nickel and copper oxides, the
oxygen edges differ significantly in both ELNES
and onset position. The measured absolute positions of the first major peaks in NiO, CuO and
TiO2 (rutile) oxides are in excellent agreement with
numbers measured by XAS [23]. In all three cases,
the peak positions in EELS spectra are only 0.1 eV
lower than those in XAS, which is close to the
estimated experimental error.
Examples of a chemical shift in L3 edges are
presented in Fig. 6b–d. Similar to the O K edges
the Ti L edges of TiO2-rutile and TiO2-anatase are
placed at almost the same position while the onset
of TiO is shifted to a lower energy. Thus, rutile and
anatase can be only differentiated by a small
Fig. 6. (a) O K (b) Ti L3 (c) Ni L3 and (d) Cu L3 edges in different metal-oxides measured using the value of 853.0 eV for the Ni L3
peak maximum in NiO as a reference for the absolute energy.
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P.L. Potapov, D. Schryvers / Ultramicroscopy 99 (2004) 73–85
difference in the shape of the second peak, which
was already pointed out by Brydson et al. [5,6].
Comparison of the present data with data [5]
reveals that the chemical shifts are almost identical
although the absolute positions of the onsets in
rutile and anatase measured by us are 0.4 eV lower
than those in Ref. [5]. That can be related with
different reference point for calibration of the drift
tube mentioned in Chapter 6. A chemical shift of
the Cu L3 edges in the series Cu–Cu2O–CuO is in
good agreement with those measured in a dedicated STEM instrument [7]. However, our measurements for the Ni L3 edge contradict with data
by Leapman et al. [2], who reported a 0.2 eV
decrease of the peak position in nickel oxide
compared with nickel metal. In contrast, we have
measured a 0.25 eV increase for this position. On
the other hand, Leapman’s number for a chemical
shift between Cu and CuO [2] agrees with our data
and data by Scheu et al. [7]. The reason for
this discrepancy could relate with the fact that
Leapman et al. used a regular TEM microscope
with the energy instability problem discussed
above. To improve the precision of measurements,
they mounted two samples on the same grid and
moved the probe rapidly from oxide to metal.
Although this procedure improves the results, it is
not always applicable and can still suffer from
instabilities in the microscope.
Strictly speaking, the measured values
of the edge onsets are dependent on the energy
resolution of the instrument. The position of the
first major maximum in the ELNES structure is
actually more reproducible when comparing spectra obtained in different instruments. However, the
peak maximum has often no clear physical meaning while the position of the onset is related with
the mutual location of the inner shell and the
Fermi level. To ensure the correct determination of
the onset positions, the obtained spectra are
deconvoluted using the profile of the zero-loss
peak collected in vacuum. Comparison between
the original and deconvoluted spectra is presented
in Fig. 7. In fact, the profile of ELNES changes
only slightly with deconvolution indicating that
the energy resolution of 0.75–0.8 eV is sufficient
to track the onset positions located in the
400–1000 eV region. Table 2 lists the measured
Fig. 7. Ti L3 edge in TiO2-rutile before and after deconvolution
with the profile of the zero-loss peak.
peak maxima and onset positions for the examined
oxides and metals.
8. Conclusion
A new method for measuring the absolute
positions of EELS ionisation edges has been
developed, which can be employed in regular
TEM microscopes without additional hardware
installation. The precision of the absolute measurements is improved by a factor of 5 compared
with standard routines, thus yielding a precision of
0.1 eV. As an example several chemical shifts in
metal-oxides have been measured and compared
with literature data.
Acknowledgements
The authors like to thank P.E. Batson (IBM
Th.J. Watson Research Centre) for discussions
and providing the unpublished reference data.
Extensive and helpful discussions with O.L.
Krivanek (Nion Co.), R. Rodemeier (Gatan
GmbH) and R.F. Egerton (University of Alberta)
are gratefully acknowledged. Useful advice on DM
scripting was kindly provided by M.Kundmann
(Gatan R&D). J. Verbeeck (University of
Antwerp) is acknowledged for the help with the
high-tension measurements. The possibility to
perform the test of energy stability in the Technai
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P.L. Potapov, D. Schryvers / Ultramicroscopy 99 (2004) 73–85
85
Table 2
Measured positions of the edge onsets and first major maxima in ELNES of different oxides
Edge
Compound
Instrumental
energy
resolution (eV)
Relative
thickness
Onset
(deconvoluted
spectra)
Maximum of the first peak
in ELNES (unprocessed
spectra) (eV)
Ni L3 edge
NiO
Ni
0.75
0.75
0.59
0.69
852.4
852.1
853.0a
852.8
Cu L3 edge
Cu
Cu2O
CuO
0.75
0.75
0.80
0.85
0.91
0.98
932.6
933.0
930.8
933.7
933.7
931.4
Ti L3 edge
TiO2 rutile
TiO2 anatase
TiO
0.80
0.75
0.75
0.97
0.94
0.90
457.2
457.3
456.0
457.8
457.9
457.6
O K edge
NiO
Cu2O
CuO
TiO2 rutile
TiO2 anatase
TiO
0.75
0.75
0.80
0.80
0.75
0.75
0.61
0.87
0.47
1.19
0.93
0.77
531.0
531.6
529.3
529.8
529.9
530.9
531.6
532.5
530.0
530.6
530.7
No sharp peak
The drift tube value is calibrated assuming the maximum of the Ni L3 peak in NiO to be at 853.0 eV.
a
Calibration.
microscope of Technical University of Delft is
appreciated. This work is supported by a GOA
project of the University of Antwerp entitled
‘‘Characterisation of nano-structures by means of
advanced electron energy spectroscopy and filtering’’.
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