Nondestructive Thickness Quantification for Nanoscale Coatings on Li

Feb 3, 2017 - Analytical Sciences, DOW Chemical Company, Midland, Michigan 48667, United States. Anal. Chem. , 2017, 89 (5), ... For the model sample ...
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Nondestructive Thickness Quantification for Nanoscale Coatings on Li-Ion Battery Cathode Material Wuye Ouyang*,† and Clifford S. Todd‡ †

Analytical Sciences, DOW Chemical Company, Shanghai, 201203, China Analytical Sciences, DOW Chemical Company, Midland, Michigan 48667, United States

‡

ABSTRACT: Nickel manganese cobalt oxide (NMC) is a high energy capacity cathode material that attracts the interest of many research groups. Coating a protection layer on the NMC surface is one approach to improve its cycling and safety performance. However, there is no standard and consistent way to characterize the coating performance (thickness) of this protection layer, especially due to the nanoscale of primary particle and spherical morphology of the secondary particle. In this paper, a novel empirical method based on energy dispersive X-ray spectroscopy (EDX) analysis at low accelerating voltage is proposed to evaluate the protection layer thickness on the scale of tens of nanometers. The layer thickness is characterized by measuring the intensity decrease of a substrate element due to absorption by overlying coating layers. An internal standard coating (metal layer) is applied to mimic the morphology influence and improve the accuracy of thickness quantitation. For the model sample evaluation, carbon layer coatings of 1 to 10 nm thickness were successfully quantified by this method.

L

diameter. These are composed of small primary particles (∼1 μm) (Figure 1). The thickness of coating layers is typically in the range of several to dozens of nanometers. Therefore, there

ayer thickness evaluation is a common analytical request in various research fields. Many techniques and devices have been developed for different purposes and samples, including ellipsometry, profilemetry, X-ray fluorescence, microscopy, and so on. Each of them has their advantages and disadvantages. For example, ellipsometry is powerful for planar samples with nanosized films, but it is not suitable for micrometer sized granular samples. Profilemetry and X-Ray Fluorescence Spectrometer are suitable for planar samples but may have errors for nonplanar samples or even be incapable for characterization on micrometer sized granular samples. Electron microscopy is suitable for nanosized coatings, but the sample preparation (e.g., section process, Focused Ion Beam) can be complicated. Recently, with the large demands for the development of high energy storage capacity materials, many research groups and companies have launched projects on coating fine powders,1 including The DOW Chemical Company (DOW). DOW successfully developed lithium nickel manganese cobalt oxide (NMC) materials for enhanced capacity lithium-ion batteries through process and formulation optimization. The morphology characterization for battery materials has been discussed in the literature.2 However, cycling performance and safety issues are still of concern for these materials, especially for electric vehicle applications. An ultrathin coating layer (e.g., magnesium oxide, aluminum fluoride, etc.) on the NMC surface is one possible approach for this issue, but the ability to identify the thickness of films coated on cathode materials has been a challenge in characterization. Because of its production process, the final cathode material, e.g., nickel manganese cobalt oxide (NMC), is a roughly secondary spherical particle with a 10−50 μm © 2017 American Chemical Society

Figure 1. (a) SEM image of NMC material. Field of view is 28 μm. (b) Illustration of electron beam scanning and EDX detection on the NMC particle. Received: September 28, 2016 Accepted: February 3, 2017 Published: February 3, 2017 2816

DOI: 10.1021/acs.analchem.6b03818 Anal. Chem. 2017, 89, 2816−2822

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coated and scratched surfaces in a line across an AFM image. Meanwhile, the real samples, NMC and nickel oxide (NiO) particles, were created in the lab of Dow Chemical. Monte Carlo Simulation. Monte Carlo simulations were performed via CASINO v2.4 software.17,18 The number of layers, composition, thickness, and accelerating voltage were set according to the test purpose. The number of electrons to simulate was set 20 000, and the takeoff angle for the X-ray detector was initially set to 40° and changed on the basis of the requirements. The output of the program gives the number of simulated characteristic X-rays of each element generated and the number of X-rays that leave the sample taking into account the absorption characteristics of the overlying layers. In our simulations, the K-line was selected for elements silicon and carbon, and L-line was used for nickel. Characterization. Scanning Electron Microscope (SEM). FEI Nova Nano630 was used to characterize the prepared NMC samples and the thickness of carbon coated onto the glass slides. These were observed by secondary electron detectors at a varied accelerating voltage and working distance of around 9.0 mm. Before observation, carbon or iridium was sputtered onto the samples prepared on glass electrical conductivity. Energy Dispersive X-ray Spectrum (EDX). X-rays were measured using a Silica Drift SDD based EDX detector (Bruker XFlash 5030). Generally, the working distance of EDX analysis was 9.0 mm, and the beam current was 0.90 nA at 5 keV. The scanning area for each sample was the same: 3 × 2 μm2; the accelerating voltage was set between 2 and 15 keV. X-ray counts were collected for 100 s. Because most of the tests were performed at 5 keV or even lower, the L-line for Ni was used for layer-thickness quantification in order to get a sufficient overvoltage and better peak-to-background ratio. During measurement, any surface geometric difference (nonflat) will impact the X-ray absorption. In order to mimic this for method validation in this paper, planar samples were tilted, toward the EDX spectrometer. Atomic Force Microscope (AFM). The prepared carbon- or iridium-coated silicon wafers were characterized in tapping mode using a Bruker NanoScope V, Dimension 3100 at ambient condition. The tip was antimony doped Si (f: 130−250 kHz; k: 48 N/m). Image analysis (step size) was completed via SPIP 6.0 software. Transmission Electron Microscope (TEM). Cathode powders were briefly ground in methanol using a silicon nitride mortar and pestle and then transferred to a lacey carbon-coated TEM grid. TEM analysis work was conducted using a JEOL 2010F 200 keV microscope. A 5 min beam exposure was used to help stabilize organics.

was no suitable analytical method available to quickly characterize the layer thickness that had the potential to be implemented in industrial production control. Hence, there was an analytical gap to develop a method for nanosized thickness evaluation on granular samples with diameters of about 10 to 50 μm. Thin film analysis based on electron probe X-ray microanalysis (EPMA) has been well-known for many years;3−10 the theoretical understanding is based on the development of reliable models and dedicated software. Thin film composition,10 thickness,11 density,12 porosity,13 etc. all impact the calculations. To evaluate the thickness of a film with known composition, the experimental factor k-ratio, defined as ratio of intensities between sample and standard, is measured. It can be transferred to a mass concentration by a correction procedure, e.g., ZAF function. Recently, with more theoretical models and software developments, the analysis based on ionization depth dependence ϕ(ρz) was also reported14−16 and widely used. The MSG model from Packwood and Brown14 and PAP and XPP models from Pouchou and Pichoir16 were targeted to provide a better quantitative accuracy for light elements and film samples, where the film interface and tilt angle of specimen were also considered. Generally, the k-ratio can be expressed as k=

Isample Istd

(1)

where I is the measured or emitted X-ray intensity, which is defined as I = Φ(Δρz)

∫0

R max

Φ(ρz)exp( −χρz)d(ρz)

(2)

where ρz is mass depth, χ is the absorption factor, which is the product of the mass absorption coefficient and X-ray takeoff angle (TOA), and Rmax is equal to ultimate ionization depth in PAP mode and equal to infinity in the XPP model. These models and applications based on these models are powerful for many planar samples, but they are difficult to implement for nonplanar samples, e.g., particles. In this paper, an empirical method based on electron excitation and detection by energy dispersive X-ray spectrometry (EDX) for thickness quantitation of a carbon coating on granular samples was developed and implemented. In order to systematically understand such method, the details in this paper are described from simulation work to real product quantification, from planar samples to spherical samples. In this paper, a carbon layer with nanometer thickness will be used as demonstration for this characterization method.

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EXPERIMENTAL SECTION Materials. In this report, carbon-coated or iridium-coated silicon wafers and nickel foils with ca. 200 μm thickness (purchased from Sinoreagent) were prepared via carbon evaporation (EMITECH K950) or iridium sputtering (EMITECH K575). The different thickness of carbon or iridium layer was controlled by varied evaporation repeats (carbon coating) and sputtering duration (iridium coating). The current for iridium sputtering is 15 mA, and sputtering duration is set from 30 s to 3 min. The final coating thickness was evaluated by atomic force microscopy (AFM) using the scratching method: The blade was used to make a clear scratch mark to ensure all coating materials were removed in the mark. The final coating thickness can be obtained via comparing the height between

RESULTS AND DISCUSSION The purpose of this work is to develop a method based on EDX analysis for thickness evaluation of thin coatings on spherical samples. The emitted substrate X-ray intensity is inversely related to coating thickness and impacted by coating composition. A calibration curve, film layer thickness vs measured X-ray intensity of substrate, was developed on the basis of test samples and Monte Carlo simulations. In this work, the analysis was first tested on planar samples for method exploration and then applied to spherical NMC powders. Monte Carlo Simulation. Monte Carlo simulation for electron microscope analysis has been reported in many publications.19,20 It is a useful tool for researchers and operators 2817

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only suitable for high accelerating voltage (>10 keV) and LLine is more reliable for low accelerating voltage. Layer Coated on Planar Surface. On the basis of the previous discussion, there are two possible ways to perform the analysis to evaluate the coating thickness on a planar surface via EDX. One is to measure the X-ray signal of the coating element directly. The other way is to measure the X-ray signal of a substrate element and indirectly characterize the layer thickness by its absorption effect; these have been called the “coating method”21 and the “substrate method”22 for multilayer film analysis in the past. The indirect (substrate) method can be more favorable for coating layers composed of light elements C/H/O/Li/B due to their low X-ray yield. Since the purpose of this work is to evaluate carbon layer thickness, the indirect method is the target of this work. First, simulations were done for preliminary understanding, in which silicon wafer was selected as a model substrate. Figure 3a shows the simulation

to understand beam−sample interactions in order to choose favorable experimental conditions for lab experiment design. Figure 2a shows the simulation result of different accelerating

Figure 2. (a) Monte Carlo modeling results showing beam electron paths in nickel under varied accelerating voltages. (b) Relationship between X-ray intensities and accelerating voltages. Simulations of iron (Fe) K- and L-lines are shown by ▲ and ■. Simulation and experimental results for aluminum (Al) are shown by ◆ and ◇.

voltage beams interacting with nickel. It can be seen that at higher accelerating voltage primary electrons penetrate deeper and there is a larger ionization volume. The detected X-ray signal generated at different accelerating voltage is also obtained by the simulation (Figure 2b). It is clearly seen that the detected X-ray signal has a linear relationship with the accelerating voltage in the relative low voltage range. However, with further voltage increases, the ionization depth becomes greater and X-rays are generated deeper in the sample. Due to absorption, a higher percentage of those deep X-rays do not escape from the sample, which results in a plateau, or even a decrease, in X-ray intensity at higher accelerating voltage. Additional insight into beam−sample interaction can be found in reference texts.6 These simulation results were experimentally confirmed on aluminum metal using the same conditions (Figure 2b). The most suitable voltage range for layer thickness evaluation work in this paper is the linear region, since linearity is a simple and reliable relationship to build a calibration curve for thickness and X-ray signal correlation. Additionally, due to the X-ray excitation and absorption properties of different elements,7 K-Line for generated X-ray signal is feasible for light elements at low accelerating voltage (1−10 keV in Figure 2b), while with heavier elements (e.g., Iron in Figure 2b), K-Line is

Figure 3. (a) Simulation result for Si X-ray intensity from Si-wafer substrate at 5 keV accelerating voltage as a function of carbon or iridium layer thickness. (b) Simulation result for X-ray intensity from Si-wafer substrate at 5 keV accelerating voltage as a function of carbon layer thickness, where the overlying iridium layer has a 5 nm thickness.

results for carbon or iridium coated silicon substrate at 5 keV accelerating voltage. It can be seen that the measured Si X-ray intensity has a good linear correlation with the thickness of carbon or iridium. It is noted here that the selection of suitable acceleration voltage is a key factor for consideration, especially for indirect layer thickness measurement of thin coatings. Normally, a low accelerating voltage and soft characteristic lines are preferred for very thin film analysis.7 Low accelerating voltage would result in a lower overvoltage and less ionization cross section for samples; for example, Monte Carlo simulations indicate that, if a carbon layer thickness increases from 10 to 100 nm, there is about a 70% X2818

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correlation between Si signal and carbon or iridium layer thickness at 5 keV. Although the simulation results do not match the absolute values of the experiments, both show linear correlations. Thus, the method can be implemented using an empirical calibration curve. Layer Coated on Spherical Surface. The previous experiments gave strong evidence that EDX works well for layer thickness evaluation from several nanometers to micrometers on planar surfaces. However, there is a big deviation for nonplanar or rough samples which is due to the geometry/ morphology.6,23,24 To understand more details, simulation work based on a simplified model system was performed (Figure 5). The methodology for this work is to use a tilted flat

ray intensity loss of Si-substrate at 3 keV, while there is only 7% loss at 15 keV. Therefore, low accelerating voltage is advantageous for thin carbon layer analysis, which was also evidenced by Pouchou’s finding.7,8 Regarding thickness analyses of a strong X-ray absorption layer (composed of heavy elements or a thick layer), the benefits of using low accelerating voltage are not necessarily significant. In some cases, low voltage should be avoided, especially for those heavy elements, because the beam energy is not sufficient to penetrate the coating and generate substrate X-rays, which would result in a bigger error for the calibration curve. For example, regarding the calibration curve for iridium layers with thicknesses from 3 to 27 nm, the obtained correlation factor R2 values between Xray signal of Si substrate and carbon layer thickness increased from 0.917 (4 keV) to 0.969 (5 keV) to 0.999 (8 keV). 3 keV was too weak to pass through the Ir layer and generate a substrate signal. Therefore, the higher voltage is favorable for direct measurement of a heavy metal layer. If the iridium X-ray intensity is directly quantified for thickness analysis, a similar trend is also observed. Higher accelerating voltage results in a better linearity of the calibration curve. Additionally, simulations also show the linear relationship between carbon layer thickness and Si substrate signal for a bilayer coating (Figure 3b), where the carbon layer is in between the substrate and an iridium coating of constant thickness (5 nm). To validate the simulation results, two samples were prepared by sputtering the corresponding materials (carbon and iridium) on clean silicon wafers. AFM was used to measure the iridium or carbon layer thickness. Figure 4 shows the

Figure 5. Methodology to mimic the geometric influence by changing the specimen tilt angle.

specimen to mimic the geometric effect on the particle surface, which is meant to represent geometry differences, e.g., different locations on the spherical sample. Low tilt angles of a planar sample represent locations close to the apex of spheres (Position A in Figure 5), and high tilt angles represent locations that are away from the apex (Position B in Figure 5). Figure 6 shows the simulation result of the Ni X-ray intensity at different sample tilts. It can be seen from the figure that Ni Xray intensity decreases with increasing tilt. This is due to a

Figure 4. (a) Si X-ray intensities from Si-wafer substrate by EDX measurement and simulation under 5 keV accelerating voltage as a function of carbon coating thickness. R2 is calculated correlation factor. (b) Si X-ray intensities from Si-wafer substrate by EDX measurement and simulation under 5 keV accelerating voltage as a function of iridium coating thickness. R2 is calculated correlation factor.

Figure 6. Simulation result of detected nickel X-ray intensity at 5 keV on nickel substrate with different tilts. 2819

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impossible to scan exactly the same location on different sample particles manually, where the errors are more related to the scanning area/position changes. Besides the work above on the tilted planar sample, one trial experiment based on spherical NiO samples with different diameters was performed. Figure 9a

higher backscatter electron yield on the high tilt specimen, resulting in fewer electrons remaining in the sample to generate X-rays. Generally, there is a constant value of nickel X-ray intensity when tilt angle changes from 0° to 20°. On the higher tilt specimen, it decreases. For iridium-coated nickel substrate (Figure 7), the X-ray intensity of Ni substrate decreases with

Figure 7. Simulation result of detected Ni and Ir X-ray intensity at 5 keV on iridium coated nickel foil with different tilts, where the thickness of the iridium layer is 20 nm.

increasing sample tilt angle, which is similar to the data for noniridium coated simulation, while X-ray intensity of iridium increases with tilt angle, which is due to a longer path length for electrons in the iridium layer and more iridium X-rays generated. For validation, Ni X-rays were measured from coated and tilted nickel foils. Results support the simulation in that nickel X-ray intensity (net count) slightly increased and then decreased as tilt angle increased (Figure 8). The Ni intensity

Figure 9. (a) SEM image of prepared NiO particles. (b) Relationship between RSD values and scanning areas on different sized NiO particles.

shows an SEM image of the NiO particles. Three groups of particles with either 10, 20, or 30 nm diameters were selected and analyzed by EDX. All analysis parameters were the same except the amount of area scanned. A point analysis and two area scans with 4 × 4 or 8 × 8 μm2 on the dome of the spherical NiO were compared. Figure 9b shows the RSD value of measured X-ray intensity on three different sized particles, where at least three repeats were performed for one trial to get an RSD value. Obviously, there is no difference for point scanning on different sized particles, since there is no geometry influence for one single point. However, both a scanning area increase and a particle size decrease can result in a high RSD value, which would result in a large deviation of obtained Ni Xray intensity from the edges of the scanned area. In the experiments, incidence angles of electron beam on the corners (Figure 1b) of the scan were approximately 11° for 4 × 4 μm2 scans on 30 μm particles, 17° for 4 × 4 μm2 scans on 20 μm particles, and 35° for 4 × 4 μm2 scans on 10 μm particles. This resulted in an increase in RSD from 0.15% to 0.60% as scanning area increased. The RSD for 8 × 8 μm2 scanned areas was even larger than the 4 × 4 μm2 case. Since the incidence angle of this experiment is analogous to tilt angle in the previous test using planar nickel foils, combined with the results from Figures 6 and 7, it can be deduced that measurement precision can be severely negatively impacted by sample geometry. The only possible scenario for low variability is that all sample particles have uniform geometry and morphology and the scanning location is geometrically the same. Such conditions are rare. However, in the previous simulation work of Figure 6, it was found that the RSD for tilts from 0° to 60° is ca. 4% and the RSD for tilts from 0° to 20° is only 0.3%. Hence, one possible

Figure 8. EDX experiment result of detected Ni X-ray intensity at 5 keV on iridium coated nickel foil as a function of tilt.

increase at low tilt is larger than the simulation results, and there are differences at large tilt angle; however, the trends are similar. Iridium X-ray intensity also shows the same trend as the simulation results, which increases with increasing specimen tilts. Interestingly, the Ni/Ir ratio is relatively constant within the tilt angle range up to 20°. If uncertainties for nickel and iridium intensities based on counting statistics are propagated to the Ni/Ir ratio, it is found that, in the tilt range of 0−20°, the uncertainty calculated on the basis of relative standard deviation (RSD) of Ni/Ir is less than half of that of nickel (1.5% for Ni/Ir and 6.5% for Ni, respectively). This indicates that using the ratio of Ni/Ir has an advantage to compensate for the influence of particle geometry. On the basis of these observations, it is shown that the geometry difference can greatly impact coating thickness measurement by EDX. In the real situation, it is difficult to find samples with exactly the same geometry and it is also 2820

DOI: 10.1021/acs.analchem.6b03818 Anal. Chem. 2017, 89, 2816−2822

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Analytical Chemistry approach for this method is to use a relatively flat area for EDX analysis: choose bigger particles and use a small scanning area. Carbon Layer Coated on Spherical NMC Surface. According to the previous discussion, it is possible to apply this method to evaluate carbon coating thickness via EDX analysis if a large particle is selected and small scanning area is used. In the experiment, a NMC particle with 20 μm diameter (Figure 1a) was used and a 3 × 2 μm2 area was chosen for EDX analysis, where the incidence angle of the corner of the scan is smaller than 20° (Figure 1b). The coating thicknesses of five carbon coated NMC samples were independently evaluated by TEM (Figure 10). Due to the limitation of carbon coating

for all five tilt angles is about 3%, while the RSD for the first three (0° to 10°) is only ca. 0.3%. In contrast, the RSD value for Ni intensity alone for the first three locations is higher than 8%. Such an observation indicates that using Ir as an internal normalization standard can reduce the experimental variability that comes from complicated geometry and morphology. One precaution should be noted: it is important that the incidence angle of the beam on the sample is kept smaller than 10°. For this example of 20 μm NMC particles, the maximum scanning area was calculated to be 3 × 2 μm2. There is a good linear relationship between Ni/Ir X-ray signal and carbon layer thickness (Figure 12) for five samples,

Figure 10. TEM images for five carbon coated NMC particles, named as samples A, B, C, D, and E.

Figure 12. Relationship between carbon layer thickness obtained by TEM analysis and Ni/Ir ratio done by EDX.

technology, there is variation of the coating coverage on the NMC surface, especially the poor coverage for sample B. Therefore, the average carbon thickness value is estimated for further analysis. However, no correlation was found between average coating thickness by TEM and Ni intensity by SEMEDX. A model system with double layers on the substrate was discussed earlier: a metal layer coated on a carbon layer. The thickness of the carbon layer can be estimated if the metal layer thickness is constant and its coverage is homogeneous (Figure 3b). Also, it was found that the Ni/Ir ratio could minimize the detrimental influence of morphology and geometry (Figure 8). Therefore, a metal layer (iridium) was sputtered onto the NMC sample to apply a constant-thickness iridium layer on the particles. An experiment was performed to evaluate the iridium intensity as a function of the tilt angle of the sample stage (Figure 11). Although the Ir intensity varied, Ir and Ni intensities changed simultaneously for small stage tilts (0−10°). Similar to the result shown in Figure 8, the RSD value of Ni/Ir

A, B, C, D, and E. However, due to the coverage issue of the carbon layer on the sample B surface (Figure 10), there is one big deviation in the fitting curve (Point B in Figure 12). Interestingly, if sample B is excluded for fitting, then a better calibration curve is obtained, where the correlation factor R2 increases from 0.964 for A, B, C, D, and E samples to 0.985 for A, C, D, and E samples. Although the coating homogeneity or coverage is one source of deviation for quantitation, from these data, it still can be concluded that thickness analysis by EDX is reasonably reliable for these spherical samples. Theoretical Discussion. Previous experimental data shows the possibility to leverage Ni/Ir ratio as one effective parameter for carbon layer thickness prediction, which can reduce the error from geometric influence. The main approach is to use one known thin layer with homogeneous composition and constant thickness as a standard to mimic the geometric changes, which is aligned with Castaing’ theory15 for X-ray quantitation that quantitative analysis for element concentrations should be equal to the ratio (k-ratio) of characteristic X-ray intensity for a given element from the specimen and standard (eq 1). The measured intensity is based on the function of (ϕ(ρz)). This theory works quite well for many thin film analyses.14−16 However, due to the geometric influence (particle analysis), the electron scattering and X-ray propagation would strongly depend on the particle size and composition, as well as incident beam energy. On the basis of the conventional matrix correction ZAF function,6 the influence can be physically assigned to three parts: particle mass effect (Z), particle absorption effect (A), and particle fluorescence effect (F). The mass effect would always result in a lower intensity for a particle samples; especially, it would become significant for particle diameters below 5 μm, since the X-ray absorption follows an exponential relation:

Figure 11. EDX experiment result of detected Ni and Ir X-ray intensity at 5 keV on a carbon- and iridium-coated NMC particle with different tilts. 2821

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μ

I = I0exp−( ρ )(ρz cosecψ )

(3)

Article

AUTHOR INFORMATION

Corresponding Author

*Tel: 0086-21-38512627. E-mail: [email protected].

where I is intensity emitted, I0 is intensity generated, μ/ρ is mass absorption coefficient, ρ is density, z is ionization depth, and ψ is take off angle for X-ray detection. The takeoff angle strongly depends on the geometry of the particles; then, X-ray absorption would be strongly affected by geometric influence. Meanwhile, the generated intensity (I0) and ionization depth ϕ(ρz) both are the function of the takeoff angle. Therefore, if particle absorption effect becomes significant, the light element, e.g., oxygen or carbon, will appear at a significantly higher concentration as expected. In addition, the particle fluorescence effect is the loss of possible intensity produced by the secondary fluorescence from the characteristic X-ray, which results in a lower intensity value than the bulk sample. However, compared to the other two effects, this one generally contributes only small errors. Besides the particle geometric effects, in this paper, the NMC particle would be more complicated, because of the presence of many small particles on the surface of the big NMC particle (Figure 1). Such influence can be considered as a morphology influence or roughness influence, but it also can be treated as an accumulation of numerous particle geometric influences. Normally, the correction method for particle geometric effects would be the nominalization process, which can eliminate most of the influence from the particle mass effect, but it is less effective for the particle absorption and fluorescence effects, which are mainly due to their energy dependence properties. Another way for one to correct is to use the peak-to-background method, which is based on the fact that the peak-to-background ratio between characteristic X-ray and continuum X-ray at the same electron beam energy is less sensitive even though the characteristic X-ray strongly depends on particle geometry. Here in this paper, the peak-tobackground ratio can be leveraged and transferred to the Ni/ Ir ratio, since the sputtered Ir layer has good homogeneity and can be treated as the standard layer for background reference. However, this empirical method is based on the quality of the prepared standard/background layer; thus, only the restricted area on the particle surface is available for thickness quantitation purposes. Furthermore, the uncertainty of this method is mainly due to that, as well as the homogeneity of the studied target (carbon layer in this study). Meanwhile, the parameters of incident beam energy, EDX position, and characteristic line selection all contribute to the final uncertainty.

ORCID

Wuye Ouyang: 0000-0001-8945-6212 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Michael Behr for the TEM analysis, Xiangyang Tai for the NMC/NiO sample preparation, and Phillip Lin, Myra Zhai, Greg Meyers, and an anonymous journal reviewer for their helpful suggestions.

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REFERENCES

(1) Sun, Y. K.; Myung, S. T.; Park, B. C.; Prakash, J.; Belharouak, I.; Amine, K. Nat. Mater. 2009, 8, 320−324. (2) Carpenter, G. J. C.; Wronski, Z. Microsc. Microanal. 2015, 21, 1433−1442. (3) Pouchou, J. L.; Pichoir, F. Electron probe X-ray microanalysis applied to thin surface films and stratified specimens. Scanning Microscopy Suppl. 1993, 7, 167−189; ISSN 0892-953X. (4) Pouchou, J. L. Anal. Chim. Acta 1993, 283, 81−97. (5) Armigliato, A.; Rosa, R. Microsc. Microanal. 2009, 15, 99−105. (6) Goldstein, J. I. Scanning Electron Microscopy and X-ray Microanalysis, 3rd ed.; Springer: New York, 2007. (7) Pouchou, J. L. Microchim. Acta 2002, 138, 133−152. (8) Pouchou, J. L. Anal. Chim. Acta 1993, 283, 81−97. (9) Stary, V.; Jurek, K. Microchim. Acta 2002, 139, 179−184. (10) Rinaldi, R.; Llovet, X. Microsc. Microanal. 2015, 21, 1053−1069. (11) Babazadeh, M.; Movla, H.; Jouneghani, F. G.; Salami, D. Optik 2015, 126, 1040−1043. (12) Prencipe, I.; Dellasega, D.; Zani, A.; Rizzo, D.; Passoni, M. Sci. Technol. Adv. Mater. 2015, 16 (2), 025007. (13) Ortel, E.; Hertwig, A.; Berger, D.; Esposito, P.; Rossi, A. M.; Kraehnert, R.; Hodoroaba, V. Anal. Chem. 2016, 88, 7083−7090. (14) Packwood, R. H.; Brown, J. D. X-Ray Spectrom. 1981, 10, 138− 146. (15) Castaing, R. Application of Electron Probes to Local Chemical and Crystallographic Analysis, Thesis, University of Paris, 1951. (16) Pouchou, J. L.; Pichoir, F. Electron Probe Quantitation; Plenum Press: New York, 1991. (17) Drouin, D.; Couture, A. R.; et al. Scanning 2007, 29, 92−101. (18) Starý, V. Thin Solid Films 2003, 433, 326−331. (19) Todd, C.; Beyer, D. Microsc. Microanal. 2015, 21, 472−479. (20) Todd, C.; Kuznetsova, V. Microsc. Microanal. 2011, 17, 772− 778. (21) Schumacher, B. W.; Mitra, S. S. Microelectron. Reliab. 1962, 1, 321. (22) Bentzon, M. D.; Nielsen, P. S.; Eskildsen, S. S. Diamond Relat. Mater. 1993, 2, 893. (23) Poirier, D.; Gauvin, R. Scanning 2011, 33, 126−134. (24) Anderhalt, R.; Chan, D.; Lupu, M. Microsc. Microanal. 2009, 15, 530−531.

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CONCLUSIONS In this report, a fast nondestructive method for quantification of nanoscale carbon coating thickness on spherical NMC samples was discussed. This method was developed on the basis of EDX analysis comparing the X-ray intensity of nickel buried under the carbon coating layer with a calibration curve to obtain the carbon layer thickness. The key parameter for success was to use an internal standard, a sputtered nanosized iridium layer, to normalize the detrimental influence of geometry/morphology of imperfectly spherical NMC particles. A calibration curve was developed and validated against independent samples. Because the method is nondestructive, it has a high potential to become a thickness analysis method for mass production if the SEM has automation capabilities. 2822

DOI: 10.1021/acs.analchem.6b03818 Anal. Chem. 2017, 89, 2816−2822