Electric shielding films for biased TEM samples and their application to in situ electron holography

Electric shielding films for biased TEM samples and their application to in situ electron holography Abstract We developed a novel sample preparation method for transmission electron microscopy (TEM) to suppress superfluous electric fields leaked from biased TEM samples. In this method, a thin TEM sample is first coated with an insulating amorphous aluminum oxide (AlOx) film with a thickness of about 20 nm. Then, the sample is coated with a conductive amorphous carbon film with a thickness of about 10 nm, and the film is grounded. This technique was applied to a model sample of a metal electrode/Li-ion-conductive-solid-electrolyte/metal electrode for biasing electron holography. We found that AlOx film with a thickness of 10 nm has a large withstand voltage of about 8 V and that double layers of AlOx and carbon act as a ‘nano-shield’ to suppress 99% of the electric fields outside of the sample. We also found an asymmetry potential distribution between high and low potential electrodes in biased solid-electrolyte, indicating different accumulation behaviors of lithium-ions (Li+) and lithium-ion vacancies (VLi−) in the biased solid-electrolyte. transmission electron microscopy, leaking electric field, electric shielding film, electron holography, in situ, solid-electrolyte Introduction In situ analytical measurements are important to investigate the reaction mechanisms of electrochemical devices, such as lithium-ion batteries (LIBs) [1,2]. In general, such electrochemical reactions proceed around electrode/electrolyte interfaces on a micro/nanometer scale. Thus, in situ transmission electron microscopy (TEM) analysis is a promising technique that can provide direct information on the changes in the crystal structure [3–12], chemical/elemental composition [13–16] and electric potential [17,18] during the battery reaction. Electron holography [19–21] has been used to visualize electromagnetic fields through a phase shift in the electron wave that passes through a TEM sample [22–26]. The phase shift in the electron wave is given by an integration of the electromagnetic potentials along the electron trajectory [20]; that is, the phase of the electron wave is shifted not only by the potential inside the TEM sample but also by the potential outside the sample. The influences of the potential outside the sample on the object wave and on the reference wave have to be taken into account for quantitative measurement of the electromagnetic potential inside the sample [27,28]. Tanigaki et al. [29] minimized the perturbation of the reference wave caused by the electric charging of a TEM sample by splitting the incident electron wave into two waves sufficiently separated from each other using a bi-prism located above the sample. However, even with the split-illumination electron holography, suppressing the perturbation of the object wave is impossible due to the potential spreading from the TEM sample along the propagation direction of the electron wave. Aizawa et al. reported a method to measure an electrostatic potential only inside the sample, in which a three-dimensional (3D) electric potential was generated from a biased TEM sample [30,31]. They precisely simulated a projected phase distribution taking into account the 3D potential outside of the sample and compared it with the experimental phase distribution. They found that the potential distribution in the solid-electrolyte (SE) is almost flat except for electric double layer regions, even when a voltage is applied. However, their method requires an exact knowledge of the shape and dielectric constant of each of the multi-layers in the sample, both of which are not very easy to obtain for most of the real TEM samples. Therefore, the most suitable way to determine true electrostatic potential distribution inside the sample is to perform phase measurements such as electron holography using a specimen in which leaking fields are completely suppressed experimentally. In this paper, we present an experimental approach to suppress unwanted perturbations of the electron wave due to an electric field existing outside of the sample. Our method shields the electric field outside of the TEM sample by coating double-layered electrostatic shielding films, which we call a ‘nano-shield’. The details of the shielding technique and the effectiveness of the electrostatic nano-shield on the phase measurement are presented herein. Methods Figure 1a illustrates a model sample on a TEM holder in our experiment. A 50 μm thick Li1+x+yAlx(Ti, Ge)2−xSiyP3−yO12 (LASGTP) [32–36] sheet with Li-ionic conductivity of 1 ×10−4 S/cm at room temperature (Ohara Inc.) was used as a SE, and 250–400 nm copper (Cu) films were deposited on both sides of a LASGTP sheet using a sputtering method. LASGTP is composed of Li1+xAlxGeyTi2−x−yP3O12, Li1+x+3zAlx(Ge,Ti)2−x(SizPO4)3 and AlPO4 [35]. The former two compositions have a high ionic conductivity. The grounded side of the bulk sample dashed box in Fig. 1a was thinned using a focused gallium (Ga)-ion beam (FIB) for TEM observation. Fig. 1. View largeDownload slide Schematic illustration of (a) a model sample in our experiment, (b) cross-section of a conventional TEM sample, (c) electric potential distribution along the propagation direction of the electron wave in a conventional TEM sample, (d) cross-section of a TEM sample with electric shielding film and (e) electric potential distributions along the propagation direction of the electron wave in the case of a TEM sample with the nano-shield. Fig. 1. View largeDownload slide Schematic illustration of (a) a model sample in our experiment, (b) cross-section of a conventional TEM sample, (c) electric potential distribution along the propagation direction of the electron wave in a conventional TEM sample, (d) cross-section of a TEM sample with electric shielding film and (e) electric potential distributions along the propagation direction of the electron wave in the case of a TEM sample with the nano-shield. Figure 1b shows a schematic diagram of a cross-section of a conventional TEM sample. When a voltage is applied between the Cu films by a potentiostat (PS), a biased LASGTP and the Cu films cause an electric field inside and outside of the sample. The broken lines indicate schematic equipotential lines around the sample. Figure 1c shows the distribution of electric potential along the propagation direction of an electron wave, which passes through the biased LASGTP. Because the finite potential distributes due to the applied voltage and the electric charging at a distance far from the sample, the total phase shift in the electron wave is dominated by the potential outside of the sample (oblique line area). Thus, the large contribution of the potential outside of the sample has to be carefully removed from the total phase shift to obtain a correct potential in the sample. Figure 1d shows a schematic diagram of this study’s TEM sample, which is covered with a nano-shield that suppresses superfluous electric fields leaked from biased samples and electric charging on the electrolytes. First, the thin TEM sample was prepared using FIB. Second, the TEM sample was uniformly coated with an insulating film. Third, the sample was coated with a conductor film. When the conductor film is grounded, the superfluous electric fields can be shielded. The insulator film coated between the TEM sample and the conductor film is formed so as to apply a voltage between the Cu electrodes. Figure 1e shows the electric potential distributions along the propagation direction of the electron wave in the case of the TEM sample with the nano-shield. In this case, the total phase shift in the electron wave that passes through the sample is determined by the potential in the thin insulator film and in the TEM sample. In this study, the thicknesses of the insulator film and the TEM sample were controlled to about 20 and 100 nm, respectively. Thus, the insulator film will give a 20% increase in the total phase shift. The insulator and conductor films need the following properties: (1) both films need to be amorphous materials so that electron diffraction does not influence the phase images, (2) dielectric breakdown must not occur in the insulator film when the voltage is applied, and the film should be thin enough to transmit 200–300 keV electrons, (3) macroscopic characteristics of the devices must not change when depositing electrostatic nano-shield films and (4) the leaking fields need to be sufficiently shielded by the electrostatic shielding effect. Considering the aforementioned requirements, we selected amorphous aluminum oxide (AlOx) and amorphous carbon as an insulator film and a conductor film. The details of the device fabrication are as follows. First, we connected electrode plates with the thinned Cu/LASGTP/Cu sample using a silver paste before carbon/AlOx deposition, as shown in Fig. 2a. Second, the Cu/LASGTP/Cu sample and the electrode on the biased side were uniformly coated with an insulating AlOx film using atomic layer deposition (ALD) [37–39] as indicated by the bright gray area in Fig. 2b. Trimethylaluminum and water vapor were used as an aluminum source and oxidizing agent, respectively. The AlOx film was deposited at 300°C. Finally, the Cu/LASGTP/Cu sample with AlOx and the electrode on the grounded side were covered with a conductive carbon film using a sputtering method as indicated by the dark gray area in Fig. 2c. Then, the sample was loaded in the TEM holder. The carbon film was not in contact with the electrode on the biased side but was in contact with the electrode on the grounded side. Therefore, we could apply a voltage between the Cu films while the carbon film was grounded. Fig. 2. View largeDownload slide Schematic illustration of a TEM sample (a) before carbon/AlOx deposition, (b) after AlOx deposition and (c) after carbon/AlOx deposition. Fig. 2. View largeDownload slide Schematic illustration of a TEM sample (a) before carbon/AlOx deposition, (b) after AlOx deposition and (c) after carbon/AlOx deposition. To determine the required characteristics, we conducted the following experiments. (i) The crystal structure and the configuration of the AlOx film were investigated using TEM, annular dark-field scanning TEM (ADF-STEM), energy dispersive X-ray spectroscopy (EDS) and nano-beam electron diffraction (NBD) using a 200 kV transmission electron microscope (JEOL, ARM-200F). (ii) The dielectric breakdown voltage was measured using linear sweep voltammetry on an electrode/AlOx/electrode bulk sample. (iii) The macroscopic current behaviors in the Cu/LASGTP/Cu TEM sample with and without the nano-shields were measured using chronoamperometry and were compared to investigate the influence of the nano-shields on the macroscopic device property. (iv) The electrostatic shielding effects of the nano-shields on the phase measurements were evaluated using electron holography under biasing conditions. The electron holography was performed using a 300 kV transmission electron microscope (Hitachi High-Technologies, HF-3300EH) equipped with multiple bi-prisms and a TEM sample holder having two electrodes for applying voltage to the TEM sample. Results and discussion Crystal structure and configuration of the AlOx film deposited using ALD The shielding film, which coats the TEM sample, has to be very thin and preferably an amorphous material to determine the precise potential of a TEM sample using electron holography because the diffraction effect causes an additional phase change in electron waves. Figure 3a shows a cross-sectional TEM image of the deposited AlOx film on an amorphous-silicon oxide (SiO2)/Si substrate. A protective layer of platinum (Pt) was deposited using sputtering before FIB thinning. The contrast in the AlOx layer is slightly different from that of an amorphous-SiO2 layer (the AlOx granular image looks finer than SiO2). Figure 3b and c show NBD patterns from the AlOx layer and the crystal-Si substrate, respectively. Because diffraction spots are not clearly observable in Fig. 3b, the deposited AlOx film has an amorphous structure. Thus, phase noise due to diffraction effects was minimized when the AlOx film deposited by ALD was used as the insulator shown in Fig. 1d. Fig. 3. View largeDownload slide Crystal structure and configuration of AlOx film on an amorphous-SiO2/Si substrate. (a) Cross-sectional TEM image. (b) and (c) NBD patterns obtained from the AlOx film and the Si substrate. (d) Cross-sectional ADF-STEM image. (e–g) STEM-EDS elemental maps of oxygen (O), aluminum (Al) and silicon (Si). Fig. 3. View largeDownload slide Crystal structure and configuration of AlOx film on an amorphous-SiO2/Si substrate. (a) Cross-sectional TEM image. (b) and (c) NBD patterns obtained from the AlOx film and the Si substrate. (d) Cross-sectional ADF-STEM image. (e–g) STEM-EDS elemental maps of oxygen (O), aluminum (Al) and silicon (Si). To measure the thickness of the deposited AlOx film, we acquired the ADF-STEM image (Fig. 3d) and the STEM-EDS elemental maps of oxygen (O), aluminum (Al) and silicon (Si) (Fig. 3e–g). Note that the weak signals of the Al and Si in the Pt region were due to the peak overlaps of Al-Kα(1.49 keV) with Pt-Mz(1.60 keV) and Si-Kα(1.74 keV) with Pt-Mz(1.60 keV), respectively. The ADF-STEM image and STEM-EDS elemental maps indicate the formation of two layers of AlOx and SiO2. The thickness of the AlOx film is 7 nm. We observed some samples with different AlOx thickness in the same way and obtained an ALD deposition rate of 0.07–0.08 nm/ALD-cycle. Macroscopic characteristics of the AlOx film To evaluate the basic properties of the AlOx film as an insulator, we measured the dielectric breakdown voltage of the film. We prepared a model sample, as illustrated in Fig. 4. An insulating AlOx film with a thickness of about 10 nm was deposited on a Cu substrate using ALD, and then the conductive carbon was coated on part of the film using a common sputtering method. The coated area was about 0.2 mm2. Figure 4 shows the strength of an electric current through the film as a function of the applied voltage between the Cu substrate and the carbon film. When the applied voltage became more than 8 V, the current drastically increased because of the dielectric breakdown. The maximum voltage required for this in situ holography measurement using the Cu/LASGTP/Cu sample was 2 V as described later. Therefore, an AlOx film with a thickness of 10 nm is sufficient to prevent dielectric breakdown. In this study, we deposited AlOx with a thickness of 20 nm to ensure the film had sufficient voltage resistance. Such AlOx film does not seriously degrade the contrast of the TEM images. Fig. 4. View largeDownload slide Schematic of electrode/AlOx/electrode bulk sample for linear sweep voltammetry and I–V plot for a 10-nm thick AlOx film deposited on a Cu substrate. Fig. 4. View largeDownload slide Schematic of electrode/AlOx/electrode bulk sample for linear sweep voltammetry and I–V plot for a 10-nm thick AlOx film deposited on a Cu substrate. Chronoamperometry was performed on the thinned TEM samples of Cu/LASGTP/Cu with and without carbon/AlOx nano-shields to investigate the influence on the macroscopic device characteristics. The thicknesses of the AlOx and carbon films were about 20 and 10 nm, respectively. Figure 5 shows currents as a function of time on both of the TEM samples when the step voltage of 2 V was applied between the Cu electrodes, where the respective current values were normalized with the maximum values at 0 s because the size of each TEM sample was different. The currents were on the nA order in the both samples. The size of the samples was about 1.0 mm long, 1.0 mm wide and 0.05 mm high. When the step voltage was applied, lithium-ions (Li+) and lithium-ion vacancies (VLi−) in a SE moved toward low and high potential electrodes, respectively. As a result, electric double layers having a large potential drop were formed at the electrode/SE interfaces. However, Li+ cannot move quickly in LASGTP, so the current gradually approached 0 on the order of seconds. This macroscopic current property is almost the same in both of the Cu/LASGTP/Cu TEM samples with and without the nano-shields, as shown in Fig. 5. A slight difference is evident between the two data for the longer passage of time. We think this is due to a leaking current through the AlOx film between the Cu/LASGTP/Cu and the carbon film. However, the difference in the two data was sufficiently small, about 0.003 in the vertical scale of Fig. 5, so it did not seriously influence the measurement. This indicates that the AlOx thin film insulates the biased electrode and LASGTP from the carbon film and that the nano-shields did not macroscopically affect the electric and ionic properties of the sample. Fig. 5. View largeDownload slide Comparison of chronoamperometry of Cu/LASGTP/Cu TEM samples with and without the carbon/AlOx film. The respective current values were normalized with the maximum values at 0 s because the size of each TEM sample was different. Fig. 5. View largeDownload slide Comparison of chronoamperometry of Cu/LASGTP/Cu TEM samples with and without the carbon/AlOx film. The respective current values were normalized with the maximum values at 0 s because the size of each TEM sample was different. Electrostatic shielding effect of the carbon/AlOx nano-shields We performed electron holography measurements to determine the electrostatic shielding effect of the nano-shields. Figure 6a and b show holograms around the grounded Cu/LASGTP interfaces without and with the carbon/AlOx nano-shields, respectively, where the Cu electrodes at the other side of the observed Cu/LASGTP interfaces were biased by +2 V. Figure 6c and d show phase images reconstructed from Fig. 6a and b using the Fourier transform method. The spatial resolutions of Fig. 6c and d were estimated to be about 95 and 35 nm, respectively, as commonly determined by the spacings of the interference fringes in the holograms. Fig. 6. View largeDownload slide In situ observation of electric potential distribution inside and outside the TEM sample by electron holography to determine the effect of the screening by the carbon/AlOx films. (a) and (b) Holograms around Cu/LASGTP interfaces without and with the carbon/AlOx film, respectively. (c) and (d) Phase images around Cu/LASGTP interfaces without and with the carbon/AlOx film, respectively. (e) Profiles of phase change without and with the carbon/AlOx film in the vacuum area. (f) Profiles of phase change without and with the carbon/AlOx film in the Cu/LASGTP interface. Fig. 6. View largeDownload slide In situ observation of electric potential distribution inside and outside the TEM sample by electron holography to determine the effect of the screening by the carbon/AlOx films. (a) and (b) Holograms around Cu/LASGTP interfaces without and with the carbon/AlOx film, respectively. (c) and (d) Phase images around Cu/LASGTP interfaces without and with the carbon/AlOx film, respectively. (e) Profiles of phase change without and with the carbon/AlOx film in the vacuum area. (f) Profiles of phase change without and with the carbon/AlOx film in the Cu/LASGTP interface. Figure 6e shows phase profiles along a line of A–B in Fig. 6c and along a line of E–F in Fig. 6d, both of which are outside of the sample. The three-phase profiles along A–B were taken at applied voltages of −2 V, 0 V and +2 V to the other side of the Cu electrode observed in Fig. 6a–d, respectively. The phase profile along A–B shows a significant amount of phase variation—about 31 rad.—even when the sample was not biased. Such a large phase variation along A–B was caused by the electric charging of LASGTP, which does not have good electronic conductivity. The phase profile along A–B changes with the applied voltage as shown in Fig. 6e, indicating that the electric field outside of the sample changes with the applied voltage. A slight difference is evident in the deviations for +2 V and –2 V from that of 0 V. We think that the difference between the two data was caused by a change in electric charging at voltage application. The phase profile along E–F in the vacuum area shows only a small phase variation of 0.26 rad. A 99% reduction in the phase shift corresponds to a 99% suppression of the electrostatic potential, assuming that the suppressed potential has the same spatial distribution as the unshielded potential but is damped by a factor of 1%. Because the superfluous electric fields outside of the sample were shielded, the phase profile along E–F did not change by applying voltage. The electric charging of AlOx and LASGTP was also suppressed by the conductive carbon film. Figure 6f shows the phase profiles along a line of C–D in Fig. 6c (without the nano-shield films) and along a line of G–H in Fig. 6d (with the nano-shield films), both of which were inside of the sample. The three-phase profiles along C–D were taken at applied voltages of −2 V, 0 V, and +2 V. In the case of the sample without the nano-shield films (Fig. 6c), the phase shifted due to the sample and the leaking electric field overlap in the phase profiles. A gradually extended phase slope in along a line of C–D resembled the phase profile along A–B in Fig. 6e. This suggests that the large phase variation was caused by the leaking electric fields. In the phase profile along G–H, a large phase variation was suppressed by the nano-shield. As a result, fine phase variation was observed in the LASGTP and the Cu regions (Fig. 6d). The LASGTP was found to have a poly-crystal and amorphous mixture with different compositions [36]. The diffraction effect somewhat influenced the phase distribution, but mainly the different compositions reflected the phase irregularities in the LASGTP region in Fig. 6d. Thus, the fine phase variations, indicated by the black arrows in Fig. 6f, are caused by the difference in mean inner potentials. In the Cu region, the fine phase irregularities are mainly due to the diffraction effect. The Cu film was deposited using a common sputtering method. Thus, the composition difference is small, but the film has many poly-crystal Cu grains with different orientation. When the bias voltage is applied, the phase change caused by the potential change in LASGTP is much smaller than that by the potential change in the leaking electric field. Thus, the profiles of G–H for 0 V, –2 V, and +2 V almost overlap each other in the vertical scale of Fig. 6f. These results indicate that the nano-shields sufficiently suppressed the electric field leakage and were effective for precisely measuring the electric potential distribution in the samples during any of the biasing experiments as well as in situ electron holography. Quantitative measurement of the electric potential distribution in the biased LASGTP solid electrolyte Here, we present the observation of the potential distribution in the LASGTP SE under biasing conditions. Figure 7a shows a schematic diagram of the TEM sample prepared for the observation of the potential distribution in the SE. The Cu electrode on the left side was grounded. The Cu electrode on the right side was connected to a potentiostat for applying voltage to the LASGTP SE. We used a phase shifting method [40] to reconstruct the phase distribution with higher sensitivity. Fig. 7. View largeDownload slide (a) Schematic of the TEM sample. (b) Profiles of phase change at Cu/LASGTP interface obtained by phase shifting electron holography when +2 V and −2 V were applied. (c) Schematic diagram of potential profile in biased LASGTP with consideration of experimental results. Fig. 7. View largeDownload slide (a) Schematic of the TEM sample. (b) Profiles of phase change at Cu/LASGTP interface obtained by phase shifting electron holography when +2 V and −2 V were applied. (c) Schematic diagram of potential profile in biased LASGTP with consideration of experimental results. The red and blue profiles in Fig. 7b show the phase change at the Cu/LASGTP interface when +2 V and −2 V were applied. To measure the potential change only due to voltage application, we subtracted the phase distribution of the short-circuited (0 V) sample from that of the biased sample. The profiles were taken as a function of the distance, d, from the interfaces. The red profile shows a sharp increase at the interface, followed by a gentle slope. The average of the phase value in the slope region, which is calculated by the phase profile with d > 20 nm, was 0.14 rad. The blue profile shows inverted features to that of the red profiles, that is, a sharp decrease at the interface, followed by a gentle slope. The average of the phase value in the slope region was −0.22 rad., the absolute value of which was slightly larger than the red profile. Gentle slopes in d > 20 nm regions were observed in the red and blue profiles. However, the variations are about 0.04 rad., too small to discuss the physical meaning for these slopes. Next, we consider the potential profile near the interface at an applied voltage of +2 V. The blue profile observed at an applied voltage of −2 V is considered to be the same as the phase profile observed on the right side of the interface between LASGTP and the Cu electrode at an applied voltage of +2 V. This is because this TEM sample has a mirror symmetry about a plane bisecting the two interfacial planes between LASGTP and the left and right Cu electrodes. Thus, we simply flipped the blue profile in the horizontal direction and connected it to the red profile to reconstruct the phase profile near both the interfaces, as shown in the green profile. These profiles could be simplified as shown in Fig. 7c, where the small slopes in the red and green profiles around d > 20 nm were ignored and where the phase values were converted to their ratio. Note that the phase distribution near the Cu/LASGTP interface (within 20 nm from the interface shown by the gray area) was not accurately measured because of the sample drift due to the applied voltage. Thus, we do not discuss fine structures within 20 nm, for example, the thickness of the electric double layer. A further improvement in the stability and resolution in the holography measurement is a key issue for observing the detail near the interface, which will be reported elsewhere. The blue and green profiles were not exactly inverted to each other in the averaged phase values in the slope regions, that is, the potential change at the Cu/LASGTP interface on the low voltage side was smaller than that on the high voltage side. We consider that the asymmetry in the potential distribution in the LASGTP SE was caused by the difference between the densities of the Li+ on the low voltage side and the VLi− on the high voltage side. If the Li+ accumulated on the low voltage side was assumed to be located at interstitial sites in the LASGTP structure, these results suggest that the density of the Li+ occupying the interstitial sites of LASGTP does not exceed the density of the VLi− on the high voltage side at an applied voltage of 2 V. Such an asymmetry in the electric potential distribution in the SE has also been reported in a lithium phosphorus oxynitride (LiPON) SE [30,31]. Further studies are necessary to clarify the details of the distributions and locations of the Li+ and VLi− under applied voltage. Conclusion We developed a new technique to shield an electric field leaked from a TEM sample using a successive coating of an insulating film of AlOx and a conductive film of carbon that was grounded. We found that an AlOx film with a thickness of 10 nm has a large withstand voltage of 8 V that was applied between the TEM sample and the conductive carbon film. We also found that the double layers of AlOx and carbon films suppress the phase shift due to the electric-field leakage by about 99%. A precise electric potential distribution could be successfully observed in the films under biasing conditions. The potential level in the SE region was lower than half of the applied voltage between the Cu electrodes. We consider that the asymmetry in the electric potential profile in the SE was caused by the difference between the densities of the Li+ and the VLi− on the electrode/SE interfaces, indicating the effectiveness of the electric shielding films of AlOx and carbon for the quantitative measurement of the electric potential in the biasing condition. In combination with ‘dynamic electron holography’ [41], we plan to analyze the dynamics of ions in SE. This preparation method of the electric shielding films does not depend on the configuration and composition of TEM samples as long as the double layers of the nano-shield do not react with the TEM sample. The materials of the electric shielding films are not limited to AlOx and carbon and can be selected properly depending on the TEM samples. The key of this method is to coat a whole TEM sample with the insulator uniformly to avoid short-circuits. We consider that ALD is the most suitable for this purpose because ALD is a surface controlled layer-by-layer process that results in the deposition of pinhole-free thin film with high withstand voltage. The method is useful not only for electron holography but also for other phase imaging techniques, such as differential phase contrast STEM [42,43] and electron diffractive imaging [44,45]. Acknowledgements We would like to thank Samco Inc. for preparing the insulator AlOx film using ALD. 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Electric shielding films for biased TEM samples and their application to in situ electron holography

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Oxford University Press
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© The Author(s) 2018. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com
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0022-0744
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1477-9986
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10.1093/jmicro/dfy018
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Abstract

Abstract We developed a novel sample preparation method for transmission electron microscopy (TEM) to suppress superfluous electric fields leaked from biased TEM samples. In this method, a thin TEM sample is first coated with an insulating amorphous aluminum oxide (AlOx) film with a thickness of about 20 nm. Then, the sample is coated with a conductive amorphous carbon film with a thickness of about 10 nm, and the film is grounded. This technique was applied to a model sample of a metal electrode/Li-ion-conductive-solid-electrolyte/metal electrode for biasing electron holography. We found that AlOx film with a thickness of 10 nm has a large withstand voltage of about 8 V and that double layers of AlOx and carbon act as a ‘nano-shield’ to suppress 99% of the electric fields outside of the sample. We also found an asymmetry potential distribution between high and low potential electrodes in biased solid-electrolyte, indicating different accumulation behaviors of lithium-ions (Li+) and lithium-ion vacancies (VLi−) in the biased solid-electrolyte. transmission electron microscopy, leaking electric field, electric shielding film, electron holography, in situ, solid-electrolyte Introduction In situ analytical measurements are important to investigate the reaction mechanisms of electrochemical devices, such as lithium-ion batteries (LIBs) [1,2]. In general, such electrochemical reactions proceed around electrode/electrolyte interfaces on a micro/nanometer scale. Thus, in situ transmission electron microscopy (TEM) analysis is a promising technique that can provide direct information on the changes in the crystal structure [3–12], chemical/elemental composition [13–16] and electric potential [17,18] during the battery reaction. Electron holography [19–21] has been used to visualize electromagnetic fields through a phase shift in the electron wave that passes through a TEM sample [22–26]. The phase shift in the electron wave is given by an integration of the electromagnetic potentials along the electron trajectory [20]; that is, the phase of the electron wave is shifted not only by the potential inside the TEM sample but also by the potential outside the sample. The influences of the potential outside the sample on the object wave and on the reference wave have to be taken into account for quantitative measurement of the electromagnetic potential inside the sample [27,28]. Tanigaki et al. [29] minimized the perturbation of the reference wave caused by the electric charging of a TEM sample by splitting the incident electron wave into two waves sufficiently separated from each other using a bi-prism located above the sample. However, even with the split-illumination electron holography, suppressing the perturbation of the object wave is impossible due to the potential spreading from the TEM sample along the propagation direction of the electron wave. Aizawa et al. reported a method to measure an electrostatic potential only inside the sample, in which a three-dimensional (3D) electric potential was generated from a biased TEM sample [30,31]. They precisely simulated a projected phase distribution taking into account the 3D potential outside of the sample and compared it with the experimental phase distribution. They found that the potential distribution in the solid-electrolyte (SE) is almost flat except for electric double layer regions, even when a voltage is applied. However, their method requires an exact knowledge of the shape and dielectric constant of each of the multi-layers in the sample, both of which are not very easy to obtain for most of the real TEM samples. Therefore, the most suitable way to determine true electrostatic potential distribution inside the sample is to perform phase measurements such as electron holography using a specimen in which leaking fields are completely suppressed experimentally. In this paper, we present an experimental approach to suppress unwanted perturbations of the electron wave due to an electric field existing outside of the sample. Our method shields the electric field outside of the TEM sample by coating double-layered electrostatic shielding films, which we call a ‘nano-shield’. The details of the shielding technique and the effectiveness of the electrostatic nano-shield on the phase measurement are presented herein. Methods Figure 1a illustrates a model sample on a TEM holder in our experiment. A 50 μm thick Li1+x+yAlx(Ti, Ge)2−xSiyP3−yO12 (LASGTP) [32–36] sheet with Li-ionic conductivity of 1 ×10−4 S/cm at room temperature (Ohara Inc.) was used as a SE, and 250–400 nm copper (Cu) films were deposited on both sides of a LASGTP sheet using a sputtering method. LASGTP is composed of Li1+xAlxGeyTi2−x−yP3O12, Li1+x+3zAlx(Ge,Ti)2−x(SizPO4)3 and AlPO4 [35]. The former two compositions have a high ionic conductivity. The grounded side of the bulk sample dashed box in Fig. 1a was thinned using a focused gallium (Ga)-ion beam (FIB) for TEM observation. Fig. 1. View largeDownload slide Schematic illustration of (a) a model sample in our experiment, (b) cross-section of a conventional TEM sample, (c) electric potential distribution along the propagation direction of the electron wave in a conventional TEM sample, (d) cross-section of a TEM sample with electric shielding film and (e) electric potential distributions along the propagation direction of the electron wave in the case of a TEM sample with the nano-shield. Fig. 1. View largeDownload slide Schematic illustration of (a) a model sample in our experiment, (b) cross-section of a conventional TEM sample, (c) electric potential distribution along the propagation direction of the electron wave in a conventional TEM sample, (d) cross-section of a TEM sample with electric shielding film and (e) electric potential distributions along the propagation direction of the electron wave in the case of a TEM sample with the nano-shield. Figure 1b shows a schematic diagram of a cross-section of a conventional TEM sample. When a voltage is applied between the Cu films by a potentiostat (PS), a biased LASGTP and the Cu films cause an electric field inside and outside of the sample. The broken lines indicate schematic equipotential lines around the sample. Figure 1c shows the distribution of electric potential along the propagation direction of an electron wave, which passes through the biased LASGTP. Because the finite potential distributes due to the applied voltage and the electric charging at a distance far from the sample, the total phase shift in the electron wave is dominated by the potential outside of the sample (oblique line area). Thus, the large contribution of the potential outside of the sample has to be carefully removed from the total phase shift to obtain a correct potential in the sample. Figure 1d shows a schematic diagram of this study’s TEM sample, which is covered with a nano-shield that suppresses superfluous electric fields leaked from biased samples and electric charging on the electrolytes. First, the thin TEM sample was prepared using FIB. Second, the TEM sample was uniformly coated with an insulating film. Third, the sample was coated with a conductor film. When the conductor film is grounded, the superfluous electric fields can be shielded. The insulator film coated between the TEM sample and the conductor film is formed so as to apply a voltage between the Cu electrodes. Figure 1e shows the electric potential distributions along the propagation direction of the electron wave in the case of the TEM sample with the nano-shield. In this case, the total phase shift in the electron wave that passes through the sample is determined by the potential in the thin insulator film and in the TEM sample. In this study, the thicknesses of the insulator film and the TEM sample were controlled to about 20 and 100 nm, respectively. Thus, the insulator film will give a 20% increase in the total phase shift. The insulator and conductor films need the following properties: (1) both films need to be amorphous materials so that electron diffraction does not influence the phase images, (2) dielectric breakdown must not occur in the insulator film when the voltage is applied, and the film should be thin enough to transmit 200–300 keV electrons, (3) macroscopic characteristics of the devices must not change when depositing electrostatic nano-shield films and (4) the leaking fields need to be sufficiently shielded by the electrostatic shielding effect. Considering the aforementioned requirements, we selected amorphous aluminum oxide (AlOx) and amorphous carbon as an insulator film and a conductor film. The details of the device fabrication are as follows. First, we connected electrode plates with the thinned Cu/LASGTP/Cu sample using a silver paste before carbon/AlOx deposition, as shown in Fig. 2a. Second, the Cu/LASGTP/Cu sample and the electrode on the biased side were uniformly coated with an insulating AlOx film using atomic layer deposition (ALD) [37–39] as indicated by the bright gray area in Fig. 2b. Trimethylaluminum and water vapor were used as an aluminum source and oxidizing agent, respectively. The AlOx film was deposited at 300°C. Finally, the Cu/LASGTP/Cu sample with AlOx and the electrode on the grounded side were covered with a conductive carbon film using a sputtering method as indicated by the dark gray area in Fig. 2c. Then, the sample was loaded in the TEM holder. The carbon film was not in contact with the electrode on the biased side but was in contact with the electrode on the grounded side. Therefore, we could apply a voltage between the Cu films while the carbon film was grounded. Fig. 2. View largeDownload slide Schematic illustration of a TEM sample (a) before carbon/AlOx deposition, (b) after AlOx deposition and (c) after carbon/AlOx deposition. Fig. 2. View largeDownload slide Schematic illustration of a TEM sample (a) before carbon/AlOx deposition, (b) after AlOx deposition and (c) after carbon/AlOx deposition. To determine the required characteristics, we conducted the following experiments. (i) The crystal structure and the configuration of the AlOx film were investigated using TEM, annular dark-field scanning TEM (ADF-STEM), energy dispersive X-ray spectroscopy (EDS) and nano-beam electron diffraction (NBD) using a 200 kV transmission electron microscope (JEOL, ARM-200F). (ii) The dielectric breakdown voltage was measured using linear sweep voltammetry on an electrode/AlOx/electrode bulk sample. (iii) The macroscopic current behaviors in the Cu/LASGTP/Cu TEM sample with and without the nano-shields were measured using chronoamperometry and were compared to investigate the influence of the nano-shields on the macroscopic device property. (iv) The electrostatic shielding effects of the nano-shields on the phase measurements were evaluated using electron holography under biasing conditions. The electron holography was performed using a 300 kV transmission electron microscope (Hitachi High-Technologies, HF-3300EH) equipped with multiple bi-prisms and a TEM sample holder having two electrodes for applying voltage to the TEM sample. Results and discussion Crystal structure and configuration of the AlOx film deposited using ALD The shielding film, which coats the TEM sample, has to be very thin and preferably an amorphous material to determine the precise potential of a TEM sample using electron holography because the diffraction effect causes an additional phase change in electron waves. Figure 3a shows a cross-sectional TEM image of the deposited AlOx film on an amorphous-silicon oxide (SiO2)/Si substrate. A protective layer of platinum (Pt) was deposited using sputtering before FIB thinning. The contrast in the AlOx layer is slightly different from that of an amorphous-SiO2 layer (the AlOx granular image looks finer than SiO2). Figure 3b and c show NBD patterns from the AlOx layer and the crystal-Si substrate, respectively. Because diffraction spots are not clearly observable in Fig. 3b, the deposited AlOx film has an amorphous structure. Thus, phase noise due to diffraction effects was minimized when the AlOx film deposited by ALD was used as the insulator shown in Fig. 1d. Fig. 3. View largeDownload slide Crystal structure and configuration of AlOx film on an amorphous-SiO2/Si substrate. (a) Cross-sectional TEM image. (b) and (c) NBD patterns obtained from the AlOx film and the Si substrate. (d) Cross-sectional ADF-STEM image. (e–g) STEM-EDS elemental maps of oxygen (O), aluminum (Al) and silicon (Si). Fig. 3. View largeDownload slide Crystal structure and configuration of AlOx film on an amorphous-SiO2/Si substrate. (a) Cross-sectional TEM image. (b) and (c) NBD patterns obtained from the AlOx film and the Si substrate. (d) Cross-sectional ADF-STEM image. (e–g) STEM-EDS elemental maps of oxygen (O), aluminum (Al) and silicon (Si). To measure the thickness of the deposited AlOx film, we acquired the ADF-STEM image (Fig. 3d) and the STEM-EDS elemental maps of oxygen (O), aluminum (Al) and silicon (Si) (Fig. 3e–g). Note that the weak signals of the Al and Si in the Pt region were due to the peak overlaps of Al-Kα(1.49 keV) with Pt-Mz(1.60 keV) and Si-Kα(1.74 keV) with Pt-Mz(1.60 keV), respectively. The ADF-STEM image and STEM-EDS elemental maps indicate the formation of two layers of AlOx and SiO2. The thickness of the AlOx film is 7 nm. We observed some samples with different AlOx thickness in the same way and obtained an ALD deposition rate of 0.07–0.08 nm/ALD-cycle. Macroscopic characteristics of the AlOx film To evaluate the basic properties of the AlOx film as an insulator, we measured the dielectric breakdown voltage of the film. We prepared a model sample, as illustrated in Fig. 4. An insulating AlOx film with a thickness of about 10 nm was deposited on a Cu substrate using ALD, and then the conductive carbon was coated on part of the film using a common sputtering method. The coated area was about 0.2 mm2. Figure 4 shows the strength of an electric current through the film as a function of the applied voltage between the Cu substrate and the carbon film. When the applied voltage became more than 8 V, the current drastically increased because of the dielectric breakdown. The maximum voltage required for this in situ holography measurement using the Cu/LASGTP/Cu sample was 2 V as described later. Therefore, an AlOx film with a thickness of 10 nm is sufficient to prevent dielectric breakdown. In this study, we deposited AlOx with a thickness of 20 nm to ensure the film had sufficient voltage resistance. Such AlOx film does not seriously degrade the contrast of the TEM images. Fig. 4. View largeDownload slide Schematic of electrode/AlOx/electrode bulk sample for linear sweep voltammetry and I–V plot for a 10-nm thick AlOx film deposited on a Cu substrate. Fig. 4. View largeDownload slide Schematic of electrode/AlOx/electrode bulk sample for linear sweep voltammetry and I–V plot for a 10-nm thick AlOx film deposited on a Cu substrate. Chronoamperometry was performed on the thinned TEM samples of Cu/LASGTP/Cu with and without carbon/AlOx nano-shields to investigate the influence on the macroscopic device characteristics. The thicknesses of the AlOx and carbon films were about 20 and 10 nm, respectively. Figure 5 shows currents as a function of time on both of the TEM samples when the step voltage of 2 V was applied between the Cu electrodes, where the respective current values were normalized with the maximum values at 0 s because the size of each TEM sample was different. The currents were on the nA order in the both samples. The size of the samples was about 1.0 mm long, 1.0 mm wide and 0.05 mm high. When the step voltage was applied, lithium-ions (Li+) and lithium-ion vacancies (VLi−) in a SE moved toward low and high potential electrodes, respectively. As a result, electric double layers having a large potential drop were formed at the electrode/SE interfaces. However, Li+ cannot move quickly in LASGTP, so the current gradually approached 0 on the order of seconds. This macroscopic current property is almost the same in both of the Cu/LASGTP/Cu TEM samples with and without the nano-shields, as shown in Fig. 5. A slight difference is evident between the two data for the longer passage of time. We think this is due to a leaking current through the AlOx film between the Cu/LASGTP/Cu and the carbon film. However, the difference in the two data was sufficiently small, about 0.003 in the vertical scale of Fig. 5, so it did not seriously influence the measurement. This indicates that the AlOx thin film insulates the biased electrode and LASGTP from the carbon film and that the nano-shields did not macroscopically affect the electric and ionic properties of the sample. Fig. 5. View largeDownload slide Comparison of chronoamperometry of Cu/LASGTP/Cu TEM samples with and without the carbon/AlOx film. The respective current values were normalized with the maximum values at 0 s because the size of each TEM sample was different. Fig. 5. View largeDownload slide Comparison of chronoamperometry of Cu/LASGTP/Cu TEM samples with and without the carbon/AlOx film. The respective current values were normalized with the maximum values at 0 s because the size of each TEM sample was different. Electrostatic shielding effect of the carbon/AlOx nano-shields We performed electron holography measurements to determine the electrostatic shielding effect of the nano-shields. Figure 6a and b show holograms around the grounded Cu/LASGTP interfaces without and with the carbon/AlOx nano-shields, respectively, where the Cu electrodes at the other side of the observed Cu/LASGTP interfaces were biased by +2 V. Figure 6c and d show phase images reconstructed from Fig. 6a and b using the Fourier transform method. The spatial resolutions of Fig. 6c and d were estimated to be about 95 and 35 nm, respectively, as commonly determined by the spacings of the interference fringes in the holograms. Fig. 6. View largeDownload slide In situ observation of electric potential distribution inside and outside the TEM sample by electron holography to determine the effect of the screening by the carbon/AlOx films. (a) and (b) Holograms around Cu/LASGTP interfaces without and with the carbon/AlOx film, respectively. (c) and (d) Phase images around Cu/LASGTP interfaces without and with the carbon/AlOx film, respectively. (e) Profiles of phase change without and with the carbon/AlOx film in the vacuum area. (f) Profiles of phase change without and with the carbon/AlOx film in the Cu/LASGTP interface. Fig. 6. View largeDownload slide In situ observation of electric potential distribution inside and outside the TEM sample by electron holography to determine the effect of the screening by the carbon/AlOx films. (a) and (b) Holograms around Cu/LASGTP interfaces without and with the carbon/AlOx film, respectively. (c) and (d) Phase images around Cu/LASGTP interfaces without and with the carbon/AlOx film, respectively. (e) Profiles of phase change without and with the carbon/AlOx film in the vacuum area. (f) Profiles of phase change without and with the carbon/AlOx film in the Cu/LASGTP interface. Figure 6e shows phase profiles along a line of A–B in Fig. 6c and along a line of E–F in Fig. 6d, both of which are outside of the sample. The three-phase profiles along A–B were taken at applied voltages of −2 V, 0 V and +2 V to the other side of the Cu electrode observed in Fig. 6a–d, respectively. The phase profile along A–B shows a significant amount of phase variation—about 31 rad.—even when the sample was not biased. Such a large phase variation along A–B was caused by the electric charging of LASGTP, which does not have good electronic conductivity. The phase profile along A–B changes with the applied voltage as shown in Fig. 6e, indicating that the electric field outside of the sample changes with the applied voltage. A slight difference is evident in the deviations for +2 V and –2 V from that of 0 V. We think that the difference between the two data was caused by a change in electric charging at voltage application. The phase profile along E–F in the vacuum area shows only a small phase variation of 0.26 rad. A 99% reduction in the phase shift corresponds to a 99% suppression of the electrostatic potential, assuming that the suppressed potential has the same spatial distribution as the unshielded potential but is damped by a factor of 1%. Because the superfluous electric fields outside of the sample were shielded, the phase profile along E–F did not change by applying voltage. The electric charging of AlOx and LASGTP was also suppressed by the conductive carbon film. Figure 6f shows the phase profiles along a line of C–D in Fig. 6c (without the nano-shield films) and along a line of G–H in Fig. 6d (with the nano-shield films), both of which were inside of the sample. The three-phase profiles along C–D were taken at applied voltages of −2 V, 0 V, and +2 V. In the case of the sample without the nano-shield films (Fig. 6c), the phase shifted due to the sample and the leaking electric field overlap in the phase profiles. A gradually extended phase slope in along a line of C–D resembled the phase profile along A–B in Fig. 6e. This suggests that the large phase variation was caused by the leaking electric fields. In the phase profile along G–H, a large phase variation was suppressed by the nano-shield. As a result, fine phase variation was observed in the LASGTP and the Cu regions (Fig. 6d). The LASGTP was found to have a poly-crystal and amorphous mixture with different compositions [36]. The diffraction effect somewhat influenced the phase distribution, but mainly the different compositions reflected the phase irregularities in the LASGTP region in Fig. 6d. Thus, the fine phase variations, indicated by the black arrows in Fig. 6f, are caused by the difference in mean inner potentials. In the Cu region, the fine phase irregularities are mainly due to the diffraction effect. The Cu film was deposited using a common sputtering method. Thus, the composition difference is small, but the film has many poly-crystal Cu grains with different orientation. When the bias voltage is applied, the phase change caused by the potential change in LASGTP is much smaller than that by the potential change in the leaking electric field. Thus, the profiles of G–H for 0 V, –2 V, and +2 V almost overlap each other in the vertical scale of Fig. 6f. These results indicate that the nano-shields sufficiently suppressed the electric field leakage and were effective for precisely measuring the electric potential distribution in the samples during any of the biasing experiments as well as in situ electron holography. Quantitative measurement of the electric potential distribution in the biased LASGTP solid electrolyte Here, we present the observation of the potential distribution in the LASGTP SE under biasing conditions. Figure 7a shows a schematic diagram of the TEM sample prepared for the observation of the potential distribution in the SE. The Cu electrode on the left side was grounded. The Cu electrode on the right side was connected to a potentiostat for applying voltage to the LASGTP SE. We used a phase shifting method [40] to reconstruct the phase distribution with higher sensitivity. Fig. 7. View largeDownload slide (a) Schematic of the TEM sample. (b) Profiles of phase change at Cu/LASGTP interface obtained by phase shifting electron holography when +2 V and −2 V were applied. (c) Schematic diagram of potential profile in biased LASGTP with consideration of experimental results. Fig. 7. View largeDownload slide (a) Schematic of the TEM sample. (b) Profiles of phase change at Cu/LASGTP interface obtained by phase shifting electron holography when +2 V and −2 V were applied. (c) Schematic diagram of potential profile in biased LASGTP with consideration of experimental results. The red and blue profiles in Fig. 7b show the phase change at the Cu/LASGTP interface when +2 V and −2 V were applied. To measure the potential change only due to voltage application, we subtracted the phase distribution of the short-circuited (0 V) sample from that of the biased sample. The profiles were taken as a function of the distance, d, from the interfaces. The red profile shows a sharp increase at the interface, followed by a gentle slope. The average of the phase value in the slope region, which is calculated by the phase profile with d > 20 nm, was 0.14 rad. The blue profile shows inverted features to that of the red profiles, that is, a sharp decrease at the interface, followed by a gentle slope. The average of the phase value in the slope region was −0.22 rad., the absolute value of which was slightly larger than the red profile. Gentle slopes in d > 20 nm regions were observed in the red and blue profiles. However, the variations are about 0.04 rad., too small to discuss the physical meaning for these slopes. Next, we consider the potential profile near the interface at an applied voltage of +2 V. The blue profile observed at an applied voltage of −2 V is considered to be the same as the phase profile observed on the right side of the interface between LASGTP and the Cu electrode at an applied voltage of +2 V. This is because this TEM sample has a mirror symmetry about a plane bisecting the two interfacial planes between LASGTP and the left and right Cu electrodes. Thus, we simply flipped the blue profile in the horizontal direction and connected it to the red profile to reconstruct the phase profile near both the interfaces, as shown in the green profile. These profiles could be simplified as shown in Fig. 7c, where the small slopes in the red and green profiles around d > 20 nm were ignored and where the phase values were converted to their ratio. Note that the phase distribution near the Cu/LASGTP interface (within 20 nm from the interface shown by the gray area) was not accurately measured because of the sample drift due to the applied voltage. Thus, we do not discuss fine structures within 20 nm, for example, the thickness of the electric double layer. A further improvement in the stability and resolution in the holography measurement is a key issue for observing the detail near the interface, which will be reported elsewhere. The blue and green profiles were not exactly inverted to each other in the averaged phase values in the slope regions, that is, the potential change at the Cu/LASGTP interface on the low voltage side was smaller than that on the high voltage side. We consider that the asymmetry in the potential distribution in the LASGTP SE was caused by the difference between the densities of the Li+ on the low voltage side and the VLi− on the high voltage side. If the Li+ accumulated on the low voltage side was assumed to be located at interstitial sites in the LASGTP structure, these results suggest that the density of the Li+ occupying the interstitial sites of LASGTP does not exceed the density of the VLi− on the high voltage side at an applied voltage of 2 V. Such an asymmetry in the electric potential distribution in the SE has also been reported in a lithium phosphorus oxynitride (LiPON) SE [30,31]. Further studies are necessary to clarify the details of the distributions and locations of the Li+ and VLi− under applied voltage. Conclusion We developed a new technique to shield an electric field leaked from a TEM sample using a successive coating of an insulating film of AlOx and a conductive film of carbon that was grounded. We found that an AlOx film with a thickness of 10 nm has a large withstand voltage of 8 V that was applied between the TEM sample and the conductive carbon film. We also found that the double layers of AlOx and carbon films suppress the phase shift due to the electric-field leakage by about 99%. A precise electric potential distribution could be successfully observed in the films under biasing conditions. The potential level in the SE region was lower than half of the applied voltage between the Cu electrodes. We consider that the asymmetry in the electric potential profile in the SE was caused by the difference between the densities of the Li+ and the VLi− on the electrode/SE interfaces, indicating the effectiveness of the electric shielding films of AlOx and carbon for the quantitative measurement of the electric potential in the biasing condition. In combination with ‘dynamic electron holography’ [41], we plan to analyze the dynamics of ions in SE. This preparation method of the electric shielding films does not depend on the configuration and composition of TEM samples as long as the double layers of the nano-shield do not react with the TEM sample. The materials of the electric shielding films are not limited to AlOx and carbon and can be selected properly depending on the TEM samples. The key of this method is to coat a whole TEM sample with the insulator uniformly to avoid short-circuits. We consider that ALD is the most suitable for this purpose because ALD is a surface controlled layer-by-layer process that results in the deposition of pinhole-free thin film with high withstand voltage. The method is useful not only for electron holography but also for other phase imaging techniques, such as differential phase contrast STEM [42,43] and electron diffractive imaging [44,45]. Acknowledgements We would like to thank Samco Inc. for preparing the insulator AlOx film using ALD. 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MicroscopyOxford University Press

Published: Apr 9, 2018

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