Simultaneous secondary electron microscopy in the scanning transmission electron microscope with applications for in situ studies.

Scanning/transmission electron microscopy (STEM) is a powerful characterization tool for a wide range of materials. Over the years, STEMs have been extensively used for in situ studies of structural evolution and dynamic processes. A limited number of STEM instruments are equipped with a secondary electron (SE) detector in addition to the conventional transmitted electron detectors, i.e. the bright-field (BF) and annular dark-field (ADF) detectors. Such instruments are capable of simultaneous BF-STEM, ADF-STEM and SE-STEM imaging. These methods can reveal the 'bulk' information from BF and ADF signals and the surface information from SE signals for materials <200 nm thick. This review first summarizes the field of in situ STEM research, followed by the generation of SE signals, SE-STEM instrumentation and applications of SE-STEM analysis. Combining with various in situ heating, gas reaction and mechanical testing stages based on microelectromechanical systems (MEMS), we show that simultaneous SE-STEM imaging has found applications in studying the dynamics and transient phenomena of surface reconstructions, exsolution of catalysts, lunar and planetary materials and mechanical properties of 2D thin films. Finally, we provide an outlook on the potential advancements in SE-STEM from the perspective of sample-related factors, instrument-related factors and data acquisition and processing.

In-situ TEM study from the perspective of holders.

During the in situ transmission electron microscopy (TEM) observations, the diverse functionalities of different specimen holders play a crucial role. We hereby provide a comprehensive overview of the main types of holders, associated technologies and case studies pertaining to the widely employed heating and gas heating methods, from their initial developments to the latest advancement. In addition to the conventional approaches, we also discuss the emergence of holders that incorporate a micro-electro-mechanical system (MEMS) chip for in situ observations. The MEMS technology offers a multitude of functions within a single chip, thereby enhancing the capabilities and versatility of the holders. MEMS chips have been utilized in environmental-cell designs, enabling customized fabrication of diverse shapes. This innovation has facilitated their application in conducting in situ observations within gas and liquid environments, particularly in the investigation of catalytic and battery reactions. We summarize recent noteworthy studies conducted using in situ liquid TEM. These studies highlight significant advancements and provide valuable insights into the utilization of MEMS chips in environmental-cells, as well as the expanding capabilities of in situ liquid TEM in various research domains.

In situ observation of the deterioration process of sulfide-based solid electrolytes using airtight and air-flow TEM systems.

Sulfide-based solid electrolytes (SEs) exhibiting high ionic conductivity are indispensable battery materials for next-generation all-solid-state batteries. However, sulfide-based SEs have a major drawback in their low chemical stability in air. When exposed to H2O or O2 gas, toxic H2S is generated, and their ionic conductivity considerably declines. However, their degradation mechanism caused by air exposure has not been understood yet. To clarify the degradation process, in this study, we developed a transmission electron microscope (TEM) system to evaluate the air stability of battery materials. Using a vacuum transfer double-tilt TEM holder with a gas-flow system, the in situ observation of the degradation process was conducted for a sulfide-based Li4SnS4 glass ceramic under an air-flow environment. Consequently, electron diffraction (ED) patterns and TEM images could clearly capture morphological changes and the amorphization process caused by air exposure. Moreover, based on the analysis of ED patterns, it is observed that Li4SnS4 is likely to decompose because of the reaction with H2O in air. Therefore, this airtight and air-flow TEM system should be effective in clarifying the process of the deterioration of sulfur-based SEs during exposure to air.

Observing and Modeling the Sequential Pairwise Reactions that Drive Solid-State Ceramic Synthesis.

Solid-state synthesis from powder precursors is the primary processing route to advanced multicomponent ceramic materials. Designing reaction conditions and precursors for ceramic synthesis can be a laborious, trial-and-error process, as heterogeneous mixtures of precursors often evolve through a complicated series of reaction intermediates. Here, ab initio thermodynamics is used to model which pair of precursors has the most reactive interface, enabling the understanding and anticipation of which non-equilibrium intermediates form in the early stages of a solid-state reaction. In situ X-ray diffraction and in situ electron microscopy are then used to observe how these initial intermediates influence phase evolution in the synthesis of the classic high-temperature superconductor YBa2 Cu3 O6+ x   (YBCO). The model developed herein rationalizes how the replacement of the traditional BaCO3 precursor with BaO2 redirects phase evolution through a low-temperature eutectic melt, facilitating the formation of YBCO in 30 min instead of 12+ h. Precursor selection plays an important role in tuning the thermodynamics of interfacial reactions and emerges as an important design parameter in planning kinetically favorable synthesis pathways to complex ceramic materials.

Nanoparticle size and 3D shape measurement by electron tomography: An Inter-Laboratory Comparison.

Electron tomography (ET) has been used for quantitative measurement of shape and size of objects in three dimensions (3D) for many years. However, systematic investigation of repeatability and reproducibility of ET has not been evaluated in detail. To assess the reproducibility and repeatability of a protocol for measuring size and three-dimensional (3D) shape parameters for nanoparticles (NPs) by ET, an inter-laboratory comparison (ILC) has been performed. The ILC included six laboratories and six instruments models from three instrument manufacturers following a standard measurement protocol. A technical specification describing the normative steps of the protocol is published by the International Standards Organization (ISO). Gold NPs with 30 nm nominal diameter contained within a rod-shaped carbon support were measured. The use of a rod-shaped sample support eliminated the missing wedge effect in the experimental tilt series of projected images for improved quantification. A total of 443 NPs were initially measured by NRC-NANO and then 115 out of the 443 NPs were measured by five other labs to compare measurands such as the Volume (V), maximum Feret diameter (Fmax), minimum Feret diameter (Fmin), volume-equivalent diameter (Deq) and aspect ratio (Frat) of the NPs. The results of the five labs were compared with the results obtained at NRC-NANO. The maximum disagreement in measurements of Fmin and Fmax obtained by the participating labs did not exceed 7 %. The measured Deq was between 27.5 nm and 30.3 nm in agreement with the NP manufacturer's specification (28 nm-32 nm). In addition to the above, the influence of the missing wedge effect and beam-induced NP movement was quantified based on the differences of the results between labs.

Development and Application of a Sample Holder for In Situ Gaseous TEM Studies of Membrane Electrode Assemblies for Polymer Electrolyte Fuel Cells.

Polymer electrolyte fuel cells hold great potential for stationary and mobile applications due to high power density and low operating temperature. However, the structural changes during electrochemical reactions are not well understood. In this article, we detail the development of the sample holder equipped with gas injectors and electric conductors and its application to a membrane electrode assembly of a polymer electrolyte fuel cell. Hydrogen and oxygen gases were simultaneously sprayed on the surfaces of the anode and cathode catalysts of the membrane electrode assembly sample, respectively, and observation of the structural changes in the catalysts were simultaneously carried out along with measurement of the generated voltages.

High-resolution environmental transmission electron microscopy: modeling and experimental verification.

Quantitative modeling for high-resolution (phase contrast) gas cell environmental transmission electron microscopy (ETEM) imaging is presented in this paper. Concepts of pre-specimen scattering object (PreSO) and post-specimen scattering object (PoSO) are introduced to explain electron scattering caused by gas and window membranes associated with the gas environmental cell (E-cell). PreSO preserves the structural phase information and the effect can be evaluated by averaging the contrast transfer functions (CTFs) over random electron scattering. PoSO is treated as information loss and the unscattered electrons play a major role in determining the ETEM image quality. The theoretical model is compared and matched well with our systematic gas ETEM experimental results under various gas pressures. Extension of our approach to the aberration-corrected ETEM is discussed.

Development of a technique for in situ high temperature TEM observation of catalysts in a highly moisturized air atmosphere.

To clarify the influence of moisture on the structural changes of heated nano materials, in situ high temperature transmission electron microscopy (TEM) has been carried out using a conventional analytical TEM combined with a gas injection-specimen heating holder. Air with high moisture content, above 94% relative humidity (RH), from a humidifier was directly injected onto the heated platinum catalyst dispersed on carbon black (Pt/CB), and the morphological changes of the specimens were observed at high magnification dynamically. The result of the experiment was compared with a result obtained from an experiment using air with a low moisture content, 34% RH. Active movement of the Pt particles, leading agglomeration and grain growth, occurred prior to degradation of the CB support at high moisture content. In contrast, the degradation of the CB support leading agglomeration and grain growth of the Pt particles occurred before the displacement of the Pt particles on the CB supports in a low humidity environment.

Development of a high temperature-atmospheric pressure environmental cell for high-resolution TEM.

An environmental cell for high-temperature, high-resolution transmission electron microscopy of nanomaterials in near atmospheric pressures is developed. The developed environmental cell is a side-entry type with built-in specimen-heating element and micropressure gauge. The relationship between the cell condition and the quality of the transmission electron microscopic (TEM) image and the diffraction pattern was examined experimentally and theoretically. By using the cell consisting of two electron-transparent silicon nitride thin films as the window material, the gas pressure inside the environmental cell is continuously controlled from 10(-5) Pa to the atmospheric pressure in a high-vacuum TEM specimen chamber. TEM image resolutions of 0.23 and 0.31 nm were obtained using 15-nm-thick silicon nitride film windows with the pressure inside the cell being around 5 × 10(-5) and 1 × 10(4) Pa, respectively.

Observation of three-dimensional elemental distributions of a Si device using a 360 degrees -tilt FIB and the cold field-emission STEM system.

A technique for preparation of a pillar-shaped specimen and its multidirectional observation using a combination of a scanning transmission electron microscope (STEM) and a focused ion beam (FIB) instrument has been developed. The system employs an FIB/STEM compatible holder with a specially designed tilt mechanism, which allows the specimen to be tilted through 360 degrees [T. Yaguchi, M. Konno, T. Kamino, T. Hashimoto, T. Ohnishi, K. Umemura, K. Asayama, Microsc. Microanal. 9 (Suppl. 2) (2003) 118; T. Yaguchi, M. Konno, T. Kamino, T. Hashimoto, T. Ohnishi, M. Watanabe, Microsc. Microanal. 10 (Suppl. 2) (2004) 1030]. This technique was applied to obtain the three-dimensional (3D) elemental distributions around a contact plug of a Si device used in a 90-nm technology. A specimen containing only one contact plug was prepared in the shape of a pillar with a diameter of 200nm and a length of 5mum. Elemental maps were obtained from the pillar specimen using a 200-kV cold-field emission gun (FEG) STEM model HD-2300C equipped with the EDAX genesis X-ray energy-dispersive spectrometry (XEDS) system through a spectrum imaging technique. In this study, elemental distributions of minor elements with weak signals were enhanced by applying principal component analysis (PCA), which is a superior technique to extract weak signals from a large dataset. The distributions of elements, especially the metallization component Ti and minor dopant As in this particular device, were successfully extracted by PCA. Finally, the 3D elemental distributions around the contact plug could be visualized by reconstruction from the tilt series of maps.

In situ high temperature TEM observation of interaction between multi-walled carbon nanotube and in situ deposited gold nano-particles.

Interaction between multi-walled carbon nanotubes (MWNTs) and deposited gold nano-particles has been dynamically observed in a 200 kV transmission electron microscope (TEM) using a specimen heating holder. Gold particles with diameters of several tens of microns were mixed with MWNTs to mount on the heating element of a specimen heating holder. The gold particles were instantaneously heated to 1373 K to deposit gold nano-particles on the MWNTs from a very short distance. The MWNTs were then heated to 1073 K to observe interaction between the deposited gold nano-particles and MWNTs. Some gold nano-particles drilled through the wall of the MWNT and entered the capillary space of the MWNTs. To characterize the mechanism of the transition of the gold nano-particles into the capillary space of the MWNT, high resolution TEM observation of the deformed wall of MWNT was also carried out.

Development of a technique for high resolution electron microscopic observation of nano-materials at elevated temperatures.

A technique for high resolution transmission electron microscopic (TEM) observation of nano-materials at very high temperatures has been developed. A spirally wound tungsten wire, normally used as the heating element of a high resolution-high temperature-specimen heating holder, was coated with a thin carbon film and the carbon film was used as the substrate of nanometer-sized specimen. The carbon film was securely self-adhered on the heater and the form of the carbon film remained unchanged until the tungsten heater is heated to around 1173 K. Temperature distribution on the carbon film has been measured by observing the sublimation of ZnS particles. Behavior of gold atoms on a surface of gold nano-particles dispersed on the carbon film has been clearly observed at 773 K in a scanning transmission electron microscope (STEM).

Development of a gas injection/specimen heating holder for use with transmission electron microscope.

A new gas injection/specimen heating holder is developed for the purpose of in situ observation of gas reaction of materials at high temperatures in a transmission electron microscope at near-atomic resolution. A fine tungsten wire is employed as a heating element of the holder and a battery is used as the power source. Gas was injected onto specimens in the form of particles lying on the heating element via a nozzle. The maximum pressure near specimens was middle of 10(-2) Pa, while the pressure in the electron-gun chamber was kept to 2 x 10(-4) Pa. This gas injection/specimen heating holder was applied to observe solid-gas reactions. The reactions observed include oxidation of pure In into In2O3, reduction of SiO2 into Si and re-oxidation of Si into SiO2.

Improvements in performance of focused ion beam cross-sectioning: aspects of ion-sample interaction.

A gallium (Ga) focused ion beam (FIB) has been applied increasingly to 'site-specific' preparation of cross-sectional samples for transmission electron microscopy (TEM), scanning TEM, scanning electron microscopy and scanning ion microscopy. It is absolutely required for FIB cross-sectioning to prepare higher-quality samples in a shorter time without sacrificing the site specificity. The present paper clarifies the parameters that impose limitation on the following performances of the FIB cross-sectioning: milling rate, cross-sectioning at a right angle with respect to the sample surface, curtain structures formed on the cross sections, ion implantation and ion damage. All of these are discussed from the viewpoint of ion-sample interaction. Improvements for these performances achieved by diminishing their limiting origins or by correcting the resultants are described. Especially, the FIB scanning speed is significantly utilizable to improve the milling rate. A microsampling method, which allows the FIB incidence in a sidewards or upwards direction as well as downwards with respect to the microsample surface, is very effective to minimize the curtain structures.

Evaluation of TEM samples of an Mg-Al alloy prepared using FIB milling at the operating voltages of 10 kV and 40 kV.

Transmission electron microscopy (TEM) samples of an Mg-Al alloy has been prepared using a Ga-focused ion beam (FIB) milling at two different operating voltages of 10 kV and 40 kV to investigate the influence of the FIB energy on the sample quality. The fine structures of the samples have been studied using a high resolution TEM, and the concentration of the implanted Ga was analysed using an energy dispersive X-ray (EDX) analysis. The result of the TEM observation revealed that point defects were introduced to the sample finally milled at 40 kV but not at 10 kV. However, crystal lattice images and electron diffraction patterns were clearly observed on both the samples. The typical influence of the FIB energy was indicated in the elemental analysis. The relative Ga concentration in the thin sample finally milled at 10 kV was 1.0-2.0 at% that is less than half of 4.0-6.0 at% of the Ga concentration in the sample finally milled at 40 kV. A comparison between the experimental results of the Ga concentration measurement with simulation was also discussed.

Application of a FIB-STEM system for 3D observation of a resin-embedded yeast cell.

The combination of a focused ion beam (FIB) system and a scanning transmission electron microscope (STEM) has been applied to the three-dimensional (3D) observation of a resin-embedded yeast cell. Using a FIB microsampling technique, a sample with a thickness of tens of micrometres was extracted from a resin-embedded block sample. The extracted sample was transferred to a FIB-STEM-compatible specimen rotation holder and trimmed by FIB milling for 3D STEM observation. Although the FIB milling was carried out at an operating voltage of 40 KV, the sample was cross sectioned without forming a harmful damage layer on its surface. Cell structures, such as cell wall, cell membrane, mitochondria, peroxisomes, endoplasmic reticulum and vacuoles, were observed clearly in a pillar-shaped sample of 20 microm long, 4 microm wide and 3 microm deep.

A method for multidirectional TEM observation of a specific site at atomic resolution.

A new technique has been developed for the three-dimensional structure characterisation of a specific site at atomic resolution. In this technique, a focused ion beam (FIB) system is used to extract a specimen from a desired site as well as to fabricate the electron transparent specimen. A specimen holder with a specimen stage rotation mechanism has also been developed for use with both an FIB system and a high-resolution transmission electron microscope (TEM). The specimen holder allows both the FIB milling of a specimen and its observation in TEM without remounting the specimen from the specimen holder. A specimen for the three-dimensional TEM observation is extracted using the FIB micro-sampling technique and shaped into a pillar to mount on a tip of a needle stub enabling a multidirectional observation. The technique was applied to the multidirectional observation of the crystal structure of an Si single crystal at atomic resolution. The crystal lattice fringes of the two Si(111) planes with distances of 0.31 nm as well as the lattice fringes of the Si(200) with distances of 0.19 nm were clearly observed.

Formation and structural observation of cesium encapsulated single-walled carbon nanotubes.

Cesium encapsulation inside single-walled carbon nanotubes (SWNTs) is for the first time realized by ion irradiation of SWNTs immersed in a magnetized alkali-metal plasma, the configuration of which is confirmed to comprise three varieties by field emission type transmission electron microscopy (FE-TEM) and scanning TEM (STEM) observation.

Method for Cross-sectional Thin Specimen Preparation from a Specific Site Using a Combination of a Focused Ion Beam System and Intermediate Voltage Electron Microscope and Its Application to the Characterization of a Precipitate in a Steel.

In this study, we discuss a method for cross-sectional thin specimen preparation from a specific site using a combination of a focused ion beam (FIB) system and an intermediate voltage transmission electron microscope (TEM). A FIB-TEM compatible specimen holder was newly developed for the method. The thinning of the specimen using the FIB system and the observation of inside structure of the ion milled area in a TEM to localize a specific site were alternately carried out. The TEM fitted with both scanning transmitted electron detector and secondary electron detector enabled us to localize the specific site in a halfway milled specimen with the positional accuracy of better than 0.1 &mgr;m. The method was applied to the characterization of a precipitate in a steel. A submicron large precipitate was thinned exactly at its center for the characterization by a high-resolution electron microscopy and an elemental mapping.