Research article
Received: 16 February 2016
Revised: 24 April 2016
Accepted: 2 May 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/sia.6049
Nanoscale imaging of Li and B in nuclear waste
glass, a comparison of ToF-SIMS, NanoSIMS,
and APT
Zhaoying Wang,a,b† Jia Liu,b† Yufan Zhou,b,c James J. Neeway,d
Daniel K. Schreiber,d Jarrod V. Crum,d Joseph V. Ryan,d Xue-Lin Wang,c
Fuyi Wanga* and Zihua Zhub*
It has been very difficult to use popular elemental imaging techniques to image Li and B distribution in glass samples with
nanoscale resolution. In this study, time-of-flight secondary ion mass spectrometry, nanoscale secondary ion mass spectrometry,
and atom probe tomography (APT) were used to image the distribution of Li and B in two representative glass samples, and their
performance was comprehensively compared. APT can provide three-dimensional Li and B imaging with very high spatial resolution
(≤2 nm). In addition, absolute quantification of Li and B is possible, although there remains room for improving accuracy. However,
the major drawbacks of APT include poor sample compatibility and limited field of view (normally ≤100 × 100 × 500 nm3).
Comparatively, time-of-flight secondary ion mass spectrometry and nanoscale secondary ion mass spectrometry are samplefriendly with flexible field of view (up to 500 × 500 μm2 and image stitching is feasible); however, lateral resolution is limited to only
about 100 nm. Therefore, secondary ion mass spectrometry and APT can be regarded as complementary techniques for nanoscale
imaging of Li and B in glass and other novel materials. Copyright © 2016 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: ToF-SIMS; NanoSIMS; APT; nanoscale imaging; lithium; boron; nuclear waste glass
Introduction
The current main-stream techniques of nuclear waste treatment are
immobilizing nuclear wastes into glass or glass ceramic and then
storing the nuclear waste glass or glass ceramic at pre-selected
disposal facilities.[1,2] Although silicon oxide is the major matrix
material in nuclear waste glasses and glass ceramics, boron oxide
and lithium oxide are two important components, too. The presence
of boron in glass as a network component reduces the thermal
expansion coefficient, making it more resistant to thermal shock. In
addition, boron as a modifier gives the glass a sharp dependence
of viscosity on temperature, which is beneficial for vitrification.
Lithium may present in some nuclear waste, but more lithium (with
other alkali metal, such as Na, K, Ru, and Cs) is intentionally added
into glass as modifier to facilitate nuclear glass fabrication and waste
incorporation.[3] Therefore, nanoscale imaging of Li and B in nuclear
waste glasses and glass ceramics is of great interest. Reasons for this
are due, firstly, to glass ceramics showing separate phase structures,
where the phase sizes can range from nanometers to tens of
micrometers. Therefore, it is very important to confirm that Li and
B stay in desirable phases so the stability of the whole system can
be maximized. The second reason why Li and B imaging is of interest
is related to understanding the corrosion behavior of glass and glass
ceramic materials because radionuclides initially encapsulated in the
waste forms will be released into the surrounding environment upon
contact with ground water in the geological repository.[4–7] Therefore, understanding the behavior of different components (including
Surf. Interface Anal. (2016)
Li and B) during glass corrosion and developing a reasonable model
to predict glass corrosion behavior in a geological repository are of
great importance. It has been reported that the elemental profiles
of corrosion frontlines may be irregular and many nanometer scale
structures with different physical and chemical properties may
exist.[5,8,9] Therefore, techniques that can provide two-dimensional
* Correspondence to: Zihua Zhu, Environmental Molecular Sciences Laboratory,
Pacific Northwest National Laboratory, Richland, WA 99352, USA. Fuyi Wang,
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of
Analytical Chemistry for Living Biosystems, Beijing Centre for Mass Spectrometry,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
E-mail: [email protected]; [email protected]
†
Equal contribution.
a Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of
Analytical Chemistry for Living Biosystems, Beijing Centre for Mass
Spectrometry, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, China
b Environmental Molecular Sciences Laboratory, Pacific Northwest National
Laboratory, Richland, WA, 99352, USA
c School of Physics, State Key Laboratory of Crystal Materials & Key Laboratory of
Particle Physics and Particle Irradiation (MOE), Shandong University, Jinan
250100, China
d Energy and Environment Directorate, Pacific Northwest National Laboratory,
Richland, WA, 99352, USA
Copyright © 2016 John Wiley & Sons, Ltd.
Z. WANG ET AL.
and three-dimensional (3-D) elemental and isotopic distributions
with nanometer scale spatial resolution would be a major benefit
to the field.
Currently, scanning electron microscope (SEM) and transmission
electron microscope are the two most important techniques for
nanoscale imaging of elements in scientific research. Combined
with energy dispersive spectroscopy (EDX), SEM can perform elemental imaging with a lateral resolution of ≥1 μm. However, the
major drawback of SEM/EDX is poor sensitivity for light elements,
such as B and Li. In addition, 1-μm lateral resolution is not sufficient
for the nanoscale imaging need for accurately locating Li and B in
the pristine glass ceramic or altered glass structure. Scanning transmission electron microscope (STEM) can combine with EDX to provide better spatial resolution (down to 0.1 nm is possible), but it still
suffers from EDX’s intrinsic low sensitivity for light elements. STEM
can combine with electron energy loss spectroscopy (EELS), and
this combination has been widely used for Li and B imaging with
very high spatial resolution (down to 0.1 nm is possible). A few successful cases using STEM-EELS in glass corrosion research have
been reported,[8,10] where B corrosion frontline can be clearly
observed with ~30–50 nm lateral resolution, but no Li imaging data
were reported. However, charging effects, relatively low concentrations of B and Li, and interference from other components make
imaging of B and Li using STEM-EELS in glass a challenging task
(especially, for Li imaging). X-ray photoelectron spectroscopy is also
a very popular surface analysis technique, but it can only perform
elemental imaging with a lateral resolution of ~10 μm, which is
far away from nanoscale imaging necessary to identify the multiple
phases present in glass ceramics. Additionally, the technique is only
sensitive to the first few nanometers of the surface, and this limits
its usefulness in terms of understanding the behavior and location
of different elements during the glass corrosion process. Lastly, in
principle, Auger electron spectroscopy may be used for B and Li
imaging glass samples with sub-micrometer lateral resolution;
however, severe charging effects make this task impractical.
Time-of-flight secondary ion mass spectrometry (ToF-SIMS) has
been used for elemental and isotopic depth profiling of leached
glass samples for over 10 years.[4,10–19] It can also be used for
elemental imaging (including B and Li) with ≥100 nm lateral
resolution.[20,21] Recently, nanoscale secondary ion mass spectrometry (NanoSIMS) and atom probe tomography (APT) were used for
elemental imaging (including B and Li) glass samples.[5,8,9,22] As
far as we know, very few other laboratory tools can provide B and
Li nanoscale imaging for glass samples. Therefore, it is reasonable
to expect that ToF-SIMS, NanoSIMS, and APT will play a more and
more important role in glass research in the future. However, so
far, a comparison study of these three techniques in glass imaging
is not available. This creates a challenge for scientists in the field to
choose the right tools for their scientific research to answer the specific questions that they are asking. In addition, because Li and B
play a very important role in numerous novel nanomaterials (e.g.
novel electrode materials used in future Li ion batteries,[23,24] boron
nitride nanotubes and nanosheets,[25] etc.), therefore, a comparison
study of ToF-SIMS, NanoSIMS, and APT for Li and B imaging will
benefit such fields, too.
Time-of-flight secondary ion mass spectrometry, NanoSIMS, and
APT are unique techniques, and a rare ensemble of instruments
co-located at the Environmental Molecular Sciences Laboratory.
Environmental Molecular Sciences Laboratory is a Department of
Energy national scientific user facility at Pacific Northwest National
Laboratory and might be the only place worldwide equipped with
all three instruments (until December 2015). We have used these
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instruments extensively in glass research in the last several
years[8,9,16,17,19,21,22] and are in a unique position to perform the
comparison study of ToF-SIMS, NanoSIMS, and APT for Li and B imaging in glass samples. In this study, we report a complementary
approach using these three techniques to image the distribution
of Li and B in two representative samples (a glass-ceramic sample
and a water-corroded glass sample). The strengths and limitations
of each technique will be discussed.
Experiment
Sample preparation
Glass-ceramic sample
The glass ceramic was fabricated using traditional crucible-scale
melting. Details of the preparation procedure are available
elsewhere.[22] A piece of glass ceramic (~10 mm × 10 mm × 3 mm)
was cut from the location of interest and immobilized into resin
with one of the 10 mm × 10 mm sides exposed for polishing. After
polishing, this sample was used for SEM, ToF-SIMS, and NanoSIMS
analysis. Cutting, resin immobilization, and polishing details have
been depicted in our previous paper.[22]
Specimens for APT analysis were extracted from the polished
glass-ceramic sample using standard focus ion beam (FIB) lift-out
procedures for APT specimens with a dual-beam SEM/FIB (FEI
Helios Nanolab 600, Hillsboro, OR, USA).[26] In brief, a Pt capping
protection layer of 2 × 20 μm2 was deposited over a region of
interest consisting of approximately 50 nm-thick electron beamdeposited Pt followed by 150 nm-thick ion beam-deposited Pt.
Two rectangular trenches were made along the Pt cap at 60 ° with
respect to the specimen surface to create a triangular lamellar
wedge. The tungsten needle of a micromanipulator was then attached to one end of the wedge followed by cutting two attached
ends releasing the wedge. Individual sections of the wedge were
then attached to commercially available (CAMECA Instruments
Inc., Madison, WI, USA) Si microposts. Specimens were sharpened
into the conical geometry using annular mill patterns with a
30-keV Ga+ beam, and final specimen cleanup was performed with
a 2-keV Ga+ beam to minimize ion-beam damage. The radius at the
apex of final tip was ~50 nm.
Corroded glass sample
The glass used in this study was the Advanced Fuel Cycle Initiative
(AFCI) glass, and the entire composition is described in a previous
publication.[27] The mass percentage of B2O3 and Li2O in the glass
was 9.65% and 4.50%, respectively. Briefly, the glass batch was
prepared by first weighing the proper mass of each reagent-grade
chemical in the form of metal oxides, carbonates, H3BO3, and salts.
Batches were then homogenized by mixing in an agate mill and
then melting in a platinum crucible for 1 h. The melts were then
quenched by casting them onto clean stainless steel plates. The
quenched glass was crushed in an agate mill, remelted for 1 h,
and again quenched onto clean stainless steel plates.
The corrosion procedure consisted of placing AFCI glass
powder (32–75 μm) and six coupons (~10 mm × 10 mm × 1 mm
each) in a polytetrafluoroethylene (PTFE) vessel enclosed in a
steel pressure vessel and contacting the glass with high-purity
water (18.2 MΩ/cm) for a glass-surface-to-solution-volume ratio
of roughly 20 000 m 1. The pH was unconstrained. The samples
were placed in a convection oven held at 90 ± 2 °C for 358 days
Copyright © 2016 John Wiley & Sons, Ltd.
Surf. Interface Anal. (2016)
Nanoscale imaging of Li and B in nuclear waste glass
at which point the glass coupons were removed, rinsed with
water and then ethanol, and allowed to dry.
For NanoSIMS imaging of the corrosion layer of the AFCI sample,
a wedged crater was prepared by an FIB instrument. A brief
introduction of the procedure is shown in the Supporting Information (Figure S1). The details of the preparation procedure of the
trench can be seen in our previous publication.[9] As shown in
Figure S1, the slope of the wedged crater was about 5 μm wide.
One side of the slope was sample surface, and the depth of the
other side of the slope was about 1 μm.
Instrumentation and data analysis
Time-of-flight secondary ion mass spectrometry imaging was
performed using an IONTOF TOF.SIMS5 spectrometer. The glassceramic sample was coated with 10 nm Au to reduce charging
effects. Before imaging analysis, the direct current (DC) beam from
a 25-keV primary Bi ion source was used to scan an area of
12 × 12 μm2 for ~60 s to remove the Au coating. To collect image
data, a 25-keV Bi+ analysis beam was used to scan an area of
10 × 10 μm2 at the center of the DC crater with a frame of
256 × 256 pixels. The 25-keV Bi+ beam was focused to a diameter
of ~100 nm with a current of ~0.12 pA. It should be noted that only
unit mass resolution spectra could be obtained because our ToFSIMS instrument cannot provide high mass resolution and high spatial resolution, simultaneously. The collection time for each frame
was about 2 s, and a typical collection time for one measurement is
about 3000 s. A low-energy (~10 eV) electron beam was used for
charge compensation. Charge compensation tuning was tricky, and
relevant parameters (e.g. reflector voltage) needed to be carefully adjusted before each measurement.[16] The data analysis software is
SURFACELAB 6.4, which was provided by the instrument manufacturer.
Nanoscale secondary ion mass spectrometry imaging was performed using a CAMECA NanoSIMS 50 L. The glass samples were
coated with 10 nm Au to reduce charging. A 16.0-keV Cs+ beam
(0.65 pA) was used as the primary beam, which was focused to a
spot size around 70 nm. The Cs+ beam was used to scan an
8 × 8 μm2 area (for the glass-ceramic sample) or a 7 × 7 μm2 area
(for the corroded AFCI sample) with 256 × 256 pixels per frame,
and the data collection time was about 11 min. Low-energy
(≤10 eV) electrons were introduced to the sputter interface to compensate surface charging so that reasonable signal intensity could
be obtained. 7Li , 18O , 11B16O , and 27Al16O were collected
simultaneously. The 16O signal was too strong, and it led to some
signal saturation problems. All secondary ion images were acquired
after a pre-sputtering process. The pre-sputtering was performed
using the 16.0-keV Cs+ beam with an ~160 pA current over a
10 × 10 μm2 area. The pre-sputtering time was about 100 s. This
process was carried out to remove the Au-coating layer, clean
surface contamination, and also prepare a mature analysis area
with enough Cs implantation for optimal intensities of negative
ion signals. Image processing was carried out using the software
IMAGEJ 1.46r (Wayne Rasband, National Institute of Health, USA,
http://rsbweb.nih.gov.ij/index.html) equipped with the OpenMIMS
plug-in (http://www.nrims.harvard.edu.software.php).
The APT analyses were performed with a CAMECA LEAP 4000X
HR in laser pulsing mode (λ = 355 nm).[28,29] The instrumental setting used in this research was similar with that used in our previous
study of corroded SON68 glass samples.[8] In brief, the laser pulse
repetition rate was set at 160 kHz, the laser pulse energy was
80 pJ per pulse, and the target evaporation rate was 3 detected ions
per 1000 pulses. The APT specimen was held at 40 K during analysis
Surf. Interface Anal. (2016)
in ultrahigh vacuum (<1 × 10 8 Pa). A minimum of 5 million ions
were collected during the analysis.
Atom probe tomography reconstruction parameters were estimated based on the analysis volume measured by SEM imaging
of each specimen prior to and after APT analysis. The data analysis
software was integrated visualization and analysis software (version
3.6.8), which was provided by CAMECA. An iso-concentration surface (4 at.% Li) was used to define the topography of the droplet
phase and matrix interface and thereby maximized the accuracy
of the compositional and interfacial width measurements using
the proximity histogram method.[29,30] Because of the complex
nature of the glass-ceramic sample and limited mass resolution
(M/ΔM ~800 for 16O+) in APT mass spectra, the major challenges
in data reconstruction were peak identification and peak
deconvolution. The most significant peak convolution in this study
was 11B16Ox with 27Al16Ox 1. This convolution was partially recovered using the natural isotopic abundances of 10B (19.9%) and 11B
(80.1%), but this recovery was insufficient for absolute compositional accuracy. More details on the APT analysis of the glass
specimens have been discussed in a separate article.[8]
Results and discussion
Glass-ceramic sample
In order to reveal the microstructure of the glass-ceramic sample after quenching, a cross section of the specimen was examined with
SEM to observe the morphology and distribution of multiple
phases. Figure 1 shows a representative microstructure of the
amorphous region from glass ceramic. The phase with dark contrast and spherical shape was uniformly distributed in the lightcontrast background. The contrast difference presented in the
two phases indicates a liquid phase separation during the vitrification process. In this study, the spherical-shape phase was called the
droplet phase, while the background phase was called the matrix
phase. It was difficult to use SEM/EDX to do elemental imaging
for this sample because the size of the droplet phase (around
200–500 nm) was smaller than the best lateral resolution of
SEM/EDX (normally ~1.0 μm). STEM/EDX can provide elemental distribution with higher spatial resolution. The previously obtained
STEM/EDX images show that Al, Si, and Cs are enriched in the droplet phase and the alkaline earths and lanthanides are enriched in
the matrix phase.[22] However, STEM/EDX cannot provide B and Li
distributions because of the intrinsically weak signal of Li and B.
2 µm
Figure 1. A scanning electron microscope image, showing the dropletmatrix phase separation in the glass-ceramic sample.
Copyright © 2016 John Wiley & Sons, Ltd.
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Z. WANG ET AL.
STEM-EELS is sensitive to B and Li, and it has been used to image B
distribution in a leached SON68 sample.[8,10] We tried STEM/EELS;
however, our result (data not shown here) did not provide any
useful signal of Li and B from this glass-ceramic sample because
of high background noise, which can be attributed to charging
effects, relatively low concentration of B and Li, and interference
from other components.
Atom probe tomography
Normalized to total counts
Atom probe tomography is a unique analysis technique that can provide 3-D chemical and structural information on a nanometer scale.
The basic principle of this technique is detailed elsewhere.[31,32]
Typical analyses may contain 1–10 s of millions of atoms reconstructed within volumes of approximately 100 × 100 × 500 nm3.
From an APT analysis, the mass spectrum provides the chemical
identity for each detected ion, which provides a means to quantify
the composition based on the number of counts, while a correctly
scaled 3-D atom reconstruction provides a means to quantify the
spatial distribution of individual atoms and assess atomic-scale
heterogeneity. Figure 2 shows the element-specific atom maps
from a subsection of APT data reconstruction from the glassceramic specimen. Each dot in this image represents one detected
ion. The curvature of surface contour indicates that the region on
the left side of the interface represents the droplet phase and the
region on the right side of the interface represents the matrix
phase. The data clearly show Li and B were enriched in the matrix
phase. It should be noted that the composition analysis of several
representative heavy elements (data will be reported in another
paper) shows Al and Cs were enriched in the droplet phase, which
is consistent with our previous STEM/EDX results.[22]
Atom probe tomography analysis shows some ‘matrix effect’ for
the droplet-matrix sample, which suggested by the observation
that the atomic density of all detected total ions in the matrix phase
was higher than that in the droplet phase. This situation may be
attributed to the different evaporation fields of materials with various compositions; therefore, normalization seems very necessary to
obtain the absolute concentration of Li and B in the two phases. In
addition, it should be noted that many complex ion peaks, e.g. BOx+
and AlOx+, were observed in the APT mass spectrum. During data
analysis, the complex ions were decomposed to obtain atomic
composition.
Figure 3 shows the proximity histogram calculated across the
iso-concentration interface to quantify the compositional
differences.[33] The APT instrument we used can provide a depth
resolution of 0.2 nm and a lateral resolution of 0.3 nm, when a desirable reference sample is tested. We therefore assigned distance
uncertainty to 0.5 nm, which was double the typical resolution for
this APT instrument. The concentrations calculated for Li, B, and O
(including all atoms in both monatomic and complex ions) were
normalized to the total number of counts with background
subtracted and presented in Table 1. For convenience, all concentration reported in APT analysis was atomic percentage. It is seen
that oxygen atoms showed similar concentration in the two phases
(56% in the droplet phase and 53% in the matrix phase, as atomic
percentage), consistent with our expectation. The Li atoms showed
1.8% in the droplet phase and 5.5% in the matrix phase; the B
atoms showed 2.2% in the droplet phase and 3.8% in the matrix
phase. In addition, despite the visual similarity of the B and Li atom
images, the quantitative widths of these composition gradients
were strikingly different. Using the ASTM International standard,
the interfacial width can be regarded as the distance, over which
(b)
(a)
20 nm
0.70
Droplet
0.60
Matrix
0.50
Li
O
B
0.06
0.04
0.02
0.00
-10
-5
0
5
Distance (nm)
10
Figure 3. A proximity histogram calculated across the iso-concentration
interface, showing the concentration change of Li, B, and O across the
droplet-matrix interface.
(c)
Table 1. Concentrations (atomic percentage) and interfacial widths of
Li, B, and O atoms obtained from the atom probe tomography analysis
of the glass-ceramic sample
(d)
Figure 2. Element-specific maps of a 10-nm-thick subsection of the atom
probe tomography data from a specimen lifted from the droplet-matrix
phase separation region of the glass-ceramic sample. (a) Total ion; (b)
6 + 7 +
10 + 11 + 10 2+ 11 2+
16 +
Li + Li ; (c) B + B + B + B ; and (d) O . The scale bar is
equal to 20 nm.
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O
Li
B
Droplet phase
Matrix phase
Interfacial width (nm)
56%
1.8%
2.2%
53%
5.5%
3.8%
—
2.4
4.8
Copyright © 2016 John Wiley & Sons, Ltd.
Surf. Interface Anal. (2016)
Nanoscale imaging of Li and B in nuclear waste glass
there is a 16–84% change in signal intensity. If so, the interfacial
widths were found to be 2.4 ± 0.5 nm for Li and 4.8 ± 0.5 nm for B.
The data clearly show the advantage of APT on high resolution
measurement of interfacial widths. In addition, it was demonstrated
that each species may exhibit a significantly different interfacial
width, which might only be observed using the APT method.
Time-of-flight secondary ion mass spectrometry
Figure 4 presents the ToF-SIMS images of Li+, B+, O+, and Al+, the
overlay image of Li+ and Al+, and the overlay image of B+ and Al+.
The results of this analysis show that Li+ was enriched in the matrix
phase, whereas Al+ was enriched in the droplet phase, consistent
with APT analysis. Such distributions could be highlighted by the
overlay image of Li+ (red) and Al+ (green), where the contrast of
red and green colors was vivid. However, the B+ image looked fuzzy
because of relatively low counts, although careful examination
showed it might share a similar distribution with Li+. Also, due to
low counts of B+ signal, the overlay image of B+ (red) and Al+
(green) was dominated by Al+.
In Fig. 4, signals showed some fall-off at the top, right, and
bottom edges, which can be attributed to uneven charging in the
analysis area. It should be noted that a low-current (tens to hundreds
of pA) 1.0-keV O2+ beam, which was scanned on a 600 × 600 μm2 over
Figure 4. Time-of-flight secondary ion mass spectrometry positive secondary ion images, showing the distribution of Li, B, O, and Al in the droplet-matrix
6 +
7 +
10 +
11 +
16 +
27 +
phase separation region of the glass-ceramic sample. (a) Sum-counts image of Li and Li ; (b) sum-counts image of B and B ; (c) O ; (d) Al ; (e)
overlay of image (a) and (d); and (f) overlay of image (b) and (d). The black spots are most probably the bubbles in the sample.
Surf. Interface Anal. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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Z. WANG ET AL.
the imaging area during data acquisition, could be used to reduce
such intensity fall-off. However, because the analysis area was small
(10 × 10 μm2), such intensity fall-off was difficult to fully eliminate
during measurement.
In principle, semi-quantitative analysis is feasible for ToF-SIMS
analysis. Normally, a reference signal is required, and it can be used
to reduce ‘matrix effect’. Unfortunately, in this system, it was not
easy to find a reference signal. Oxygen was expected to share
similar concentrations in both matrix and droplet phases, but the
O+ signal was too low to be used. All other elements showed some
preference in the two phases, and they also could not be used.
Therefore, only a very rough estimation of the Li and Al concentration ratios between the two phases was obtained based on
Table 2. A comparison of lateral resolution and concentration ratios of Li,
B, and Al in the matrix phase over the droplet phase in the glass-ceramic
sample using APT, ToF-SIMS, and NanoSIMS
Lateral resolution
Limatrix/Lidroplet
Bmatrix/Bdroplet
Almatrix/Aldroplet
APT
ToF-SIMS
NanoSIMS
≤2 nm
~
110–
130 nm
1.3 ± 0.2
1.5 ± 0.3
0.29 ± 0.06
~150 nm
3.0 ± 0.4
1.7 ± 0.2
0.34 ± 0.04
2.2 ± 0.3
3.6 ± 0.6
0.57 ± 0.07
APT, atom probe tomography; ToF-SIMS, time-of-flight secondary ion
mass spectrometry; NanoSIMS, nanoscale secondary ion mass
spectrometry.
signal intensity ratios between the two phases across line-scan
analysis (Figure S2). As shown in Table 2, Limatrix/Lidroplet ≈ 1.3 and
Almatrix/Aldroplet ≈ 0.3 were observed. It should be noted that the
B+ counts were so low that the line-scan analysis did not provide
any meaningful result.
Lateral resolution is an important parameter for SIMS imaging.
Several line-scan profiles across the interface between the matrix
phase and droplet phase indicate the lateral resolution of ToF-SIMS
was about 130 nm if using Al+ image (Figure S2). If a strong signal
was used, e.g. Na+, the lateral resolution could be as good as
~100 nm (Figure S3). This value is very good because it is close to
the specification value of the best lateral resolution of this ToF-SIMS
instrument (~100 nm).
BO2 and BO were very strong signals in ToF-SIMS negative ion
spectra. So the negative ion mode may provide better B images.
Figure 5 shows the images of 16O signal, the sum-counts image
of 10B16O , 11B16O , and 10B16O2 signals, the normalized image
of the sum-counts of 10B16O , 11B16O , and 10B16O2 signals over
16
O signal, and the 7Li image. The O and B images showed some
edge fall-off, too, because of uneven surface charging. However,
the normalized B image did not show any noticeable edge fall-off.
More importantly, after normalization, semi-quantitative analysis
(i.e. determination of relative B concentration between the droplet
and matrix phases) was expected to be more reasonable. Figure S4
shows a line scan in the normalized B image, and data show that
the concentration of B in the matrix phase was about 1.5 times
the B concentration in the droplet phase, qualitatively consistent
with the APT data. Also, a very good lateral resolution (110–120 nm)
could be obtained based on line-scan data, which was similar with
the lateral resolution values observed in ToF-SIMS positive ion images.
Figure 5. Time-of-flight secondary ion mass spectrometry negative secondary ion images, showing distribution of B, O, and Li in the droplet-matrix phase
10 16
11 16
10 16
16
B O ,
B O , and
B O2 ; (b)
O ; (c) the image of
separation region of the glass-ceramic sample. (a) Sum-counts image of
10 16
11 16
10 16
16
7
B O + B O + B O2 normalized to O ; and (d) Li .
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Copyright © 2016 John Wiley & Sons, Ltd.
Surf. Interface Anal. (2016)
Nanoscale imaging of Li and B in nuclear waste glass
Nanoscale secondary ion mass spectrometry
Nanoscale secondary ion mass spectrometry is another new imaging technique that was introduced for glass imaging during the last
3–4 years. Similar with ToF-SIMS, either negative secondary ions or
positive secondary ions can be collected using NanoSIMS. The
specifications of our NanoSIMS indicate that the best resolution of
negative ion mode can be as low as 50 nm, much better than the
200 nm for positive ion mode; therefore, only negative ion mode
was tested in this study. It should be noted that a new O source
has been developed for high lateral resolution (e.g. 40 nm) positive
secondary ion imaging using NanoSIMS[34]; however, so far, only
prototype instrument is available.
Compared with ToF-SIMS, NanoSIMS can provide high spatial
resolution and high mass resolution simultaneously, so it potentially can provide ion images with lower background and better
quantitative performance than ToF-SIMS. Figure 6 shows the ion
images of 7Li , 11B16O , 27Al16O , and 18O . It is very clear that Li
and B were enriched in the matrix phase, and the Al was enriched
in the droplet phase. To obtain a semi-quantitative result, normalization was necessary. The 18O signal could serve as a good
reference signal because oxygen atoms were expected to share
similar concentrations in the two phases. After normalized to the
18
O signal, the Li and B still showed enrichment in the matrix phase,
and the Al enrichment in the droplet phase looked more vivid.
To obtain the lateral resolution and relative concentration of Li, B,
and Al in these two phases, a few representative locations were
selected for line-scan analysis. Figure S5 shows a typical line-scan
result. The lateral resolution was found to be around 150 nm. This
value was considerably poorer than the Cs+ beam diameter
measured on a Si grid reference sample (~70 nm). This degradation
of lateral resolution may be attributed to charging effects. Charging
effects led to low signals and degraded image quality, even when
low-energy electron charge compensation was applied. In addition,
significant image distortion and shifting were observed during
imaging collection, which can be attributed to uneven charging
at sputter interface.
The line-scan data show that the concentrations of B, Li, and Al in
the matrix phase were about 2.2, 3.6, and 0.57 times those in the
droplet phase, respectively (Table 2). The results of B and Li enrichment in the matrix phase and Al enrichment in the droplet phase
are consistent with the results from APT and ToF-SIMS data.
Figure 6. Nanoscale secondary ion mass spectrometry negative secondary ion images, showing distribution of Li, B, O, and Al in the droplet-matrix phase
7
11 16
27 16
18
7
18
11 16
separation region of the glass-ceramic sample. (a) Li ; (b) B O ; (c) Al O ; (d) O ; (e) the image of Li normalized to O ; (f) the image of B O
18
27 16
18
11 16
7
normalized to O ; (g) the image of Al O normalized to O ; (h) overlay of normalized B O and normalized Li ; and (i) overlay of normalized
11 16
27 16
B O and normalized Al O .
Surf. Interface Anal. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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Z. WANG ET AL.
Corroded Advanced Fuel Cycle Initiative glass sample
The corroded AFCI glass is a very interesting sample, due to a porous
alteration layer observed in SEM images (Fig. 7). In this study, APT
was used for analysis of elemental distributions in these corrosion
layers. However, the highly porous sample broke easily during APT
measurement when a high electric field was applied. As a comparison, the electric field in SIMS measurement was relatively gentle,
and the electric field-induced sample breakage was rarely observed.
Sample preparation is a key issue for imaging glass corrosion
layers. Generally, a corroded glass sample is immobilized in a resin
matrix, the sample is vertically cut by a diamond saw, and then the
Figure 7. A top-view scanning electron microscope image on the surface
of the corroded Advanced Fuel Cycle Initiative glass sample, showing the
morphology of the focus ion beam-prepared wedge, in which the
nanoscale secondary ion mass spectrometry imaging measurement was
2
performed (the red square area, 7 × 7 μm ).
cross section is polished for imaging analysis. However, the best lateral resolution of SIMS imaging is about 50–100 nm. If the corrosion
layers are thin (e.g. ≤100 nm), it may be difficult for SIMS imaging to
distinguish between the corrosion layers. Therefore, we developed
a ‘wedged crater’ strategy and observed a fivefold increase in depth
information using SIMS imaging.[9] The details of the preparation of
a wedged crater have been introduced in our previous publication,
where the corrosion layers of a SON68 glass sample were imaged
by NanoSIMS.[9] In this study, we used the same strategy to analyze
the corrosion layers of an AFCI glass sample using NanoSIMS.
Figure 8 shows the NanoSIMS ion images of 6Li + 7Li , 16O ,
10 16
10 16
B O2 ,
(6Li + 7Li )/16O ,
B O2 /16O ,
and
6
7
10 16
6
7
( Li + Li )/ B O2 . The images of Li + Li , 16O , and 10B16O2
show some signal increasing at the glass surface and may be due
to the Cr layer deposited on the sample surface.[9] In addition, the
16
O image shows some low-intensity locations at the shallow
region (region I in Fig. 8b), possibly due to topographic issues
(porous structure). Therefore, to obtain semi-quantitative information
of Li and B, normalization was performed. The 16O signal could
serve as a good reference because oxygen atoms were expected
to share similar atomic densities in different corrosion layers. After
normalization, the most characteristic feature was an irregular distribution of B. Generally speaking, more B depletion occurred in the
shallow region (region I in Fig. 8b), and less depletion occurred at
the deep region (region II in Fig. 8b). However, some strong depletion occurred at the deep region (e.g. 2# location in Fig. 8e), and
some weak depletion occurred at the shallow region (e.g. 1# location
in Fig. 8e). Apparently, the measured corrosion depth was deeper
than 1 μm, but some pristine glass could exist at the shallow region
(much less than 1 μm). This situation made it very difficult to define
the corrosion front line.
Another interesting observation came from the ratio image of Li
over B (Fig. 8f). This image shows that the Li behavior was different
Figure 8. Nanoscale secondary ion mass spectrometry negative secondary ion images, showing distribution of Li, B, and O in the focus ion beam-prepared
6
7
16
10 16
wedge at the surface of the corroded Advanced Fuel Cycle Initiative glass sample. (a) Sum-counts image of Li and Li ; (b) O ; (c) B O ; (d) the image of
6
7
16
10 16
16
6
7
10 16
Li + Li normalized to O ; (e) the image of B O normalized to O ; and (f) the image of Li + Li normalized to B O .
wileyonlinelibrary.com/journal/sia
Copyright © 2016 John Wiley & Sons, Ltd.
Surf. Interface Anal. (2016)
Nanoscale imaging of Li and B in nuclear waste glass
from the B behavior in the corrosion layers. The Li/B ratio in the Bdepletion region was higher than the Li/B ratio in the pristine glass
region, indicating that B is easier to be dissolved into solution than Li.
A comparison of atom probe tomography, time-of-flight secondary ion mass spectrometry, and nanoscale secondary ion
mass spectrometry
Time-of-flight secondary ion mass spectrometry, NanoSIMS, and
APT can perform nanoscale imaging of Li and B in glass samples;
however, they have their own merits as well as limitations (summarized in Table 3). APT provides very high lateral resolution (≤2 nm in
this research) with an intrinsic 3-D mode analysis. As a comparison,
lateral resolutions of both ToF-SIMS and NanoSIMS are no better
than 100 nm, which is far inferior to that of APT analysis. It should
be noted that SIMS can also perform 3-D imaging analysis with very
decent depth resolution (1–2 nm is possible) if combined with
depth profiling; however, the poor lateral resolution makes 3-D
SIMS analysis less appealing than APT analysis.
Quantification performance of APT seems better than SIMS. For
example, our APT analysis can provide absolute concentrations of
Li and B in the droplet and matrix phases: 1.8%/5.5% for Li and
2.2%/3.8% for B (all atomic percentage) in the droplet/matrix
phases, which cannot be obtained by SIMS analysis. The Li concentration values were in a very reasonable range, because the nominal
concentration value of Li atoms in the entire glass-ceramic system
was about 3%. However, the B concentration values were much
lower than the nominal value (~12% atomic percentage) in the system (partially due to the overlap of 11B16Ox and 27Al16Ox 1 peaks).
Therefore, the accuracy of quantification using APT needed to be
improved for B.
Quantification has long been a challenge in SIMS analysis.[35]
Absolute quantification is normally impossible unless a reference
sample is available. In this study, only concentration ratios between
droplet and matrix phases were obtained. Table 2 shows that the
concentration ratios of Li in the matrix phase over the droplet phase
were about 1.3 ± 0.2 (ToF-SIMS) and 2.2 ± 0.3 (NanoSIMS), respectively. If compared with the value of 3.0 ± 0.4 from APT analysis,
we could see considerable difference between these values; however, the good news is that they all showed some Li enrichment
in the matrix phase. A similar situation was found for B and Al. This
observation indicates that all the three techniques in this study may
be affected by matrix effect in some degree. Because APT analysis
can provide absolute atomic concentrations and SIMS cannot, the
ratio data from APT analysis may be more reliable. On the other
hand, although the ratio values from SIMS analysis may not be as
accurate as APT data, SIMS can give a quick estimation of relative
enrichment of Li and B in different phases.
The biggest drawback of APT is sample compatibility. Our study
shows that APT measurement might be very difficult for some
porous samples, such as the corroded AFCI sample. As a comparison, SIMS imaging was easily applied for this sample. In addition,
it should be noted that the wavelength of the APT laser plays an
important role for glass sample analysis. Our experience shows that,
for some glass samples, laser absorption was so weak that APT analysis did not work (data not shown here). Obviously, the laser
absorption issue does not exist in SIMS analysis. Therefore, SIMS is
generally more sample-friendly than APT.
Another drawback of APT is limited field of view. Although APT
can provide very nice 3-D spatial resolution, the field of view is
normally no larger than a size of 100 × 100 × 500 nm3. As a comparison, the field of view of SIMS imaging can vary from sub-μm to
500 × 500 μm2. In addition, one more advantage for SIMS imaging
is that image stitching is available. Therefore, it has been very convenient to use SIMS to image many interesting structures with a
size range of several microns to tens of microns on the glassceramic sample,[22] but it is a big challenge for APT.
Generally speaking, time cost is not a major consideration if
some desirable information is very important. However, if a large
amount of samples need to be tested, time cost may be an issue
for APT analysis. For example, a common method of sample preparation for APT analysis is FIB cutting and polishing, which requires
an FIB instrument and is very time-consuming. As a comparison,
sample preparation for SIMS analysis is relatively simple, which is
similar to traditional SEM sample preparation. Also, although FIB
was used in this study to prepare a wedged crater on an AFCI
sample for NanoSIMS analysis, the time cost for preparation of a
wedged crater (2–3 h) was much less than the time cost for preparation of a set of APT samples from one interesting location
(typically 5–10 h).
It has been observed that ToF-SIMS and NanoSIMS provide
similar lateral resolution for Li and B nanoscale imaging on the
glass-ceramic sample. NanoSIMS can simultaneously image Li and
B in negative secondary ion mode with high mass resolution, and
Table 3. A detailed comparison of advantages and disadvantages of ToF-SIMS, NanoSIMS, and APT for nanoscale imaging of Li and B in nuclear water
glass samples
Best spatial resolution
Mass resolution
Quantification
Field of view
Sample preparation
Typical time cost for sample preparation
Sample compatibility
ToF-SIMS
NanoSIMS
~110 nm (2-D)
Unit mass
~150 nm (2-D)
≥6000
Need a reference
sample
2
Up to 500 × 500 μm
2
(ToF-SIMS) and 200 × 200 μm
(NanoSIMS), image stitching
feasible
Easy
3–5 h per sample
Regularly available
APT
≤2 nm (three-dimensional)
~800
Feasible
3
≤100 × 100 × 500 nm
Time-consuming, focus ion beam is required.
5–10 h per interesting location
Lack of experience, sample-to-sample case
APT, atom probe tomography; ToF-SIMS, time-of-flight secondary ion mass spectrometry; NanoSIMS, nanoscale secondary ion mass spectrometry; 2-D,
two-dimensional.
Surf. Interface Anal. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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Z. WANG ET AL.
semi-quantitative analysis is easily available. As a comparison,
ToF-SIMS needs to perform both positive and negative polarities
to obtain reasonable quality Li (positive) and B (negative) images.
It should be noted that quantification result from ToF-SIMS positive
ion mode is less reliable than negative ion mode. Nonetheless,
ToF-SIMS has some advantages, too. For example, charging effects
cause less trouble in ToF-SIMS measurement than that in NanoSIMS
measurement, because NanoSIMS uses line-by-line scan mode
during data collection, while random imaging mode can be applied
in ToF-SIMS measurement. However, a more important advantage
of ToF-SIMS may be that ToF-SIMS instrumentation is much more
available than NanoSIMS. Therefore, ToF-SIMS may be a more
convenient tool for Li and B imaging than NanoSIMS, while
NanoSIMS can provide better data quality.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
Conclusions
[16]
We report a comparative study of ToF-SIMS, NanoSIMS, and APT in
nanoscale imaging of Li and B in glass and glass-ceramic samples.
Our findings show that SIMS is more sample-friendly than APT,
while spatial resolution and quantification performance of APT are
better than SIMS. In addition, although both SIMS and APT can
perform nanoscale Li and B imaging, their field of view is different.
Therefore, the relationship between SIMS and APT is complementary and not competitive. On the one hand, it is very important to
choose the right techniques based on the scales of scientific
questions. Our suggestion is APT should be the first choice if the
desirable field of view is smaller than 1 × 1 × 1 μm3; otherwise, SIMS
may be a better choice. On the other hand, to better understand
the structure of a glass sample, chemical images from various scales
are all necessary. For example, SIMS can be used as a screening tool
to examine a large area to find interesting locations for APT analysis.
It is very reasonable to expect that the combination of SIMS and
APT will become more and more important in glass research.
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
Acknowledgements
The research was performed at the Environmental Molecular
Sciences Laboratory, a national scientific user facility located at
Pacific Northwest National Laboratory (PNNL), and sponsored by
the Department of Energy’s (DOE) Office of Biological and Environmental Research. PNNL is operated for DOE by Battelle Memorial
Institute under contract number DE-AC06-76RLO-1830. F. Y. W.
and Z. Y. W. thank the National Natural Science Foundation of China
(NSFC grant nos. 21127901, 21135006, and 21321003) for support.
We also appreciate Linda H. Burk’s English editing.
[27]
[28]
[29]
[30]
[31]
[32]
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Supporting information
Additional supporting information may be found in the online
version of this article at the publisher’s web site.
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