Transmission Electron Microscopy Column Research Articles

On-Chip Electrochemical Analysis Combined with Liquid-Phase Electron Microscopy of Zinc Deposition/Dissolution

Dendrite growth of Zn on the anode of Zn-based rechargeable batteries can cause short-circuiting. To avoid the formation of dendrites, the Zn deposition/dissolution behaviors and their dependence on the electrochemical conditions should be clarified. In this study, in situ transmission electron microscopy (TEM) observations using an electrochemical chip (e-chip) were conducted to visualize the initial stage of the electrodeposition of Zn on an anode. The electrochemical data corresponding to the in situ TEM observations were precisely and extensively analyzed. The combined optimized use of a potentiostat and transmission electron microscope enabled electrochemical electrodes to be isolated completely from the potential of the TEM column. This environment stabilized the electrodeposition process during the in situ TEM observations. Under constant-current mode, the electric potential was varied, resulting in the deposition of various amounts of Zn onto the Pt working electrode. Controlling the surface materials of the electrodes and the electrochemical conditions was important for in situ TEM observations of electrochemical reactions.

In Situ Scanning Transmission Electron Microscopy Study of MoS2 Formation on Graphene with a Deep-Learning Framework.

Atomic-scale information is essential for understanding and designing unique structures and properties of two-dimensional (2D) materials. Recent developments in in situ transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) enable research to provide abundant insights into the growth of nanomaterials. In this study, 2D MoS2 is synthesized on a suspended graphene substrate inside a TEM column through thermolysis of the ammonium tetrathiomolybdate (NH4)2MoS4 precursor at 500 °C. To avoid misinterpretation of the in situ STEM images, a deep-learning framework, DeepSTEM, is developed. The DeepSTEM framework successfully reconstructs an object function in atomic-resolution STEM imaging for accurate determination of the atomic structure and dynamic analysis. In situ STEM imaging with DeepSTEM enables observation of the edge configuration, formation, and reknitting progress of MoS2 clusters with the formation of a mirror twin boundary. The synthesized MoS2/graphene heterostructure shows various twist angles, as revealed by atomic-resolution TEM. This deep-learning framework-assisted in situ STEM imaging provides atomic information for in-depth studies on the growth and structure of 2D materials and shows the potential use of deep-learning techniques in 2D material research.

In Situ TEM Crystallization of Amorphous Iron Particles

Even though sub-micron and nano-sized iron particles generally display single or polycrystalline structures, a growing interest has also been dedicated to the class of amorphous ones, whose absence of a crystal structure is capable of modifying their physical properties. Among the several routes so far described to prepare amorphous iron particles, we report here about the crystallization of those prepared by chemical reduction of Fe3+ ions using NaBH4, with sizes ranging between 80 and 200 nm and showing a high stability against oxidation. Their crystallization was investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and in situ heating transmission electron microscopy (TEM). The latter technique was performed by the combined use of electron diffraction of a selected sample area, and bright and dark field TEM imaging, and allowed determining that the crystallization turns the starting amorphous particles into polycrystalline α-Fe ones. Also, under the high vacuum of the TEM column, the crystallization temperature of the particles shifted to 550 °C from the 465 °C, previously observed by DSC and XRD under 105 Pa of Ar. This indicates the pivotal role of the external pressure in influencing the starting point of phase transition. Conversely, upon both the DSC/XRD pressure and the TEM vacuum conditions, the mean size of the crystal domains increases as a consequence of further thermal increase, even if with some pressure-related differences.

Beam-induced oxidation of mixed-valent Fe (oxyhydr)oxides (green rust) monitored by STEM-EELS

Analytical transmission electron microscopy (TEM) is often used to investigate morphologies, crystal structures, chemical compositions and oxidation states of highly reactive mixed-valent mineral phases. Of prime interest, due to its potential role in toxic metal remediation, is green rust sulphate (GRSO4) an FeII-FeIII layered double hydroxide. In this study, we quantified the effects that TEM analysis has on GRSO4 in order to ensure the measured material properties are a result of synthesis and reaction kinetics, and not due to sample preparation and analysis technique. To do this, we compared two sample preparation techniques (anoxic drop-cast with drying, and frozen-hydrated cryogenic) and exposed samples to the electron beam for several minutes, acquiring fluence series between ca. 40 e− Å−2 and 10,000 e− Å−2. TEM imaging and electron diffraction showed that the hexagonal plate-like morphology and crystal structure of GRSO4 were largely unaffected by sample preparation and analysis technique. However, quantitative analysis of a series of monochromated Fe L3,2-edge electron energy loss spectra (EELS) showed that electron irradiation induces oxidation. We measured an Fe(II)/Fe(III) ratio of 1.94 (as expected for GRSO4) at 50 e− Å−2. However, above this fluence, the ratio logarithmically decreased and dropped to ca. 0.5 after 1000 e− Å−2. This trend was approximately the same for both sample preparation techniques implying that it is the beam alone which causes valence state changes, and not exposure to oxygen during transfer into the TEM or the vacuum of the TEM column. Ultimately this work demonstrates that GR valence can be quantified by EELS provided that the sample is not over exposed to electrons. This also opens the possibility of quantifying the effect of redox-sensitive toxic metals (e.g., As, Cr, Se) on Fe oxidation state in GR phases (relevant to the treatment of contaminated soils and water) with a higher spatial resolution than other techniques (e.g., Mössbauer spectroscopy).

In situ TEM observations of initial oxidation behavior in Fe-rich Fe-Cr alloys

Two nanocrystalline alloys of Fe-4Cr and Fe-16Cr were sputter deposited and formed a solid solution A2 phase. Each alloy was then in situ annealed up to 800 °C with the phase and microstructure quantified by transmission electron microscopy (TEM). The low partial pressure of oxygen in the TEM column (~2.6 × 10−6Pa) oxidized both films at temperatures at and above 700 °C. However, the oxide morphologies were quite different. The Fe-4Cr revealed nanosphere Cr oxide clusters within the grains which have been explained by the thermodynamic preference for Cr to be in solution. The Fe-16Cr precipitated distinct Cr2O3 grains which have been described by Cr phase separation at the grain boundaries. Iron oxide was not observed and demonstrating Cr's strong preference for oxygen, even in very low oxygen environments. Using precession electron diffraction (PED), the grain characters of these boundaries were determined to reveal a significant effect of high angle grain boundaries on the growth and precipitation behavior of the oxides. The collective results reveal the initial stages of oxidation evolution in nanocrystalline grains, the role of Cr content on such behavior.

Thermal Stability and Anisotropic Sublimation of Two-Dimensional Colloidal Bi2Te3 and Bi2Se3 Nanocrystals.

The structural and compositional stabilities of two-dimensional (2D) Bi2Te3 and Bi2Se3 nanocrystals, produced by both colloidal synthesis and by liquid phase exfoliation, were studied by in situ transmission electron microscopy (TEM) during annealing at temperatures between 350 and 500 °C. The sublimation process induced by annealing is structurally and chemically anisotropic and takes place through the preferential dismantling of the prismatic {011̅0} type planes, and through the preferential sublimation of Te (or Se). The observed anisotropic sublimation is independent of the method of nanocrystal’s synthesis, their morphology, or the presence of surfactant molecules on the nanocrystals surface. A thickness-dependent depression in the sublimation point has been observed with nanocrystals thinner than about 15 nm. The Bi2Se3 nanocrystals were found to sublimate below 280 °C, while the Bi2Te3 ones sublimated at temperatures between 350 and 450 °C, depending on their thickness, under the vacuum conditions in the TEM column. Density functional theory calculations confirm that the sublimation of the prismatic {011̅0} facets is more energetically favorable. Within the level of modeling employed, the sublimation occurs at a rate about 700 times faster than the sublimation of the {0001} planes at the annealing temperatures used in this work. This supports the distinctly anisotropic mechanisms of both sublimation and growth of Bi2Te3 and Bi2Se3 nanocrystals, known to preferentially adopt a 2D morphology. The anisotropic sublimation behavior is in agreement with the intrinsic anisotropy in the surface free energy brought about by the crystal structure of Bi2Te3 or Bi2Se3.

Double-wall TiO2 nanotube arrays: enhanced photocatalytic activity and in situ TEM observations at high temperature.

By decreasing the water content in an NH4F and glycerol-water electrolyte, the transition from single-wall to double-wall TiO2 nanotube arrays was successfully achieved using an anodization method. The double-wall TiO2 nanotube structures exhibited better photocatalytic activity than the typical single-wall structures. After modification with platinum nanoparticles, the photocatalytic activity of both the single- and double-wall TiO2 nanotubes was improved further. In situ observations at the annealing temperature of the TiO2 nanotubes were performed using a transmission electron microscopy (TEM) system. A slower structural failure of the nanotubes was obtained with the introduction of oxygen gas into the TEM column compared with the structural changes observed under high-vacuum conditions without the introduction of oxygen gas. These behaviors suggest that oxygen injection is an important factor in stabilizing the TiO2 nanotubes during the in situ TEM annealing process. The high-magnification TEM images of the double-wall TiO2 nanotubes revealed that the sintering of the inner wall can draw a clear distinction between the inner and outer walls.

In Situ Analytical Electron Microscopy for Probing Nanoscale Electrochemistry

Oxides and their tailored structures are at the heart of electrochemical energy storage technologies and advances in understanding and controlling the dynamic behaviors in the complex oxides, particularly at the interfaces, during electrochemical processes will catalyze creative design concepts for new materials with enhanced and better-understood properties. Such knowledge is not accessible without new analytical tools. New innovative experimental techniques are needed for understanding the chemistry and structure of the bulk and interfaces, more importantly how they change with electrochemical processes in situ. Analytical Transmission Electron Microscopy (TEM) is used extensively to study electrode materials ex situ and is one of the most powerful tools to obtain structural, morphological, and compositional information at nanometer scale by combining imaging, diffraction and spectroscopy, e.g., EDS (energy dispersive X-ray spectrometry) and Electron Energy Loss Spectrometry (EELS). Determining the composition/structure evolution upon electrochemical cycling at the bulk and interfaces can be addressed by new electron microscopy technique with which one can observe, at the nanometer scale and in situ, the dynamic phenomena in the electrode materials. In electrochemical systems, for instance in a lithium ion battery (LIB), materials operate under conditions that are far from equilibrium, so that the materials studied ex situ maymore » not capture the processes that occur in situ in a working battery. In situ electrochemical operation in the ultra-high vacuum column of a TEM has been pursued by two major strategies. In one strategy, a 'nano-battery' can be fabricated from an all-solid-state thin film battery using a focused ion beam (FIB). The electrolyte is either polymer based or ceramic based without any liquid component. As shown in Fig. 1a, the interfaces between the active electrode material/electrolyte can be clearly observed with TEM imaging, in contrast to the composite electrodes/electrolyte interfaces in conventional lithium ion batteries, depicted in Fig.1b, where quantitative interface characterization is extremely difficult if not impossible. A second strategy involves organic electrolyte, though this approach more closely resembles the actual operation conditions of a LIB, the extreme volatility In Situ Analytical Electron Microscopy for Probing Nanoscale Electrochemistry by Ying Shirley Meng, Thomas McGilvray, Ming-Che Yang, Danijel Gostovic, Feng Wang, Dongli Zeng, Yimei Zhu, and Jason Graetz of the organic electrolytes present significant challenges for designing an in situ cell that is suitable for the vacuum environment of the TEM. Significant progress has been made in the past few years on the development of in situ electron microscopy for probing nanoscale electrochemistry. In 2008, Brazier et al. reported the first cross-section observation of an all solid-state lithium ion nano-battery by TEM. In this study the FIB was used to make a 'nano-battery,' from an all solid-state battery prepared by pulsed laser deposition (PLD). In situ TEM observations were not possible at that time due to several key challenges such as the lack of a suitable biasing sample holder and vacuum transfer of sample. In 2010, Yamamoto et al. successfully observed changes of electric potential in an all-solid-state lithium ion battery in situ with electron holography (EH). The 2D potential distribution resulting from movement of lithium ions near the positive-electrode/electrolyte interface was quantified. More recently Huang et al. and Wang et al. reported the in situ observations of the electrochemical lithiation of a single SnO{sub 2} nanowire electrode in two different in situ setups. In their approach, a vacuum compatible ionic liquid is used as the electrolyte, eliminating the need for complicated membrane sealing to prevent the evaporation of carbonate based organic electrolyte into the TEM column. One main limitation of this approach is that EELS spectral imaging is not possible due to the high plasmon signal of the ionic liquid. To this end, we have developed a novel in situ instrumental system combining analytical electron microscopy with advanced spectroscopy to probe the dynamic phenomena in an all solid-state nano-battery. In situ electron microscopy is a versatile technique that yields insights into challenging questions that could not be obtained using other techniques. However, in order to fully exploit the capabilities, a very carefully thought-out plan of action is essential. It is important to recognize that this is not just a simple characterization tool, but a collection of tools that make up a complete experimental set-up: the choice of FIB operation conditions, specimen holder for biasing, grid materials and design as well as microscope environment must be thoroughly considered before performing an experiment.« less

Synthesis and Characterization of Hydrocarbon Coating Prepared by in Situ Electron Beam Deposition on ZnSe Nanowire

Hydrocarbon coating has been performed on a single ZnSe nanowire to construct a core/shell nanostructure using transmission electron microscopy (TEM) via in situ electron beam induced deposition. Hydrocarbon contamination resulting from the pump oil vapor of the vacuum system of the TEM column chamber was used as source precursor for the coating. The thickness of the coating layer strongly depends on vacuum level of TEM column chamber, deposition time, and electron beam intensity and, thus, is controllable under TEM inspection. Microstructures of the hydrocarbon coating characterized by TEM demonstrated the amorphous characteristics. Electrical characteristics of such a core/shell ZnSe/hydrocarbon nanostructure were examined and compared with the naked ZnSe nanowire. The significant reduction in the threshold voltage suggests the amorphous hydrocarbon shell has a semiconducting nature but relatively lower Schottky barrier height.

Structural Stability and Phase Transitions in WO3 Thin Films

Tungsten oxide (WO3) thin films have been produced by KrF excimer laser (lambda = 248 nm) ablation of bulk ceramic WO3 targets. The crystal structure, surface morphology, chemical composition, and structural stability of the WO3 thin films have been studied in detail. Characterization of freshly grown WO3 thin films has been performed using X-ray diffraction (XRD), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy (RS), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) measurements. The results indicate that the freshly grown WO3 thin films are nearly stoichiometric and well crystallized as monoclinic WO3. The surface morphology of the resulting WO3 thin film has grains of approximately 60 nm in size with a root-mean-square (rms) surface roughness of 10 nm. The phase transformations in the WO3 thin films were investigated by annealing in the TEM column at 30-500 degrees C. The phase transitions in the WO3 thin films occur in sequence as the temperature is increased: monoclinic --> orthorhombic --> hexagonal. Distortion and tilting of the WO6 octahedra occurs with the phase transitions and significantly affects the electronic properties and, hence, the electrochemical device applications of WO3.

TEM Observation of Natural Rubber Thin Films Crystallized under Molecular Orientation

Abstract Morphology of natural rubber (NR) thin films crystallized under molecular orientation was examined by transmission electron microscopy (TEM). A thin film of NR (RSS#1) was made by casting its n-hexane (or benzene) solution onto the water surface, and prestretched up to a desired amount of strain (%). Each of the specimens thus prepared was introduced into a TEM column and isothermally crystallized at −25 °C with a cryo-transfer specimen-holder. Morphological observations in bright- and dark-field imaging modes revealed directly that the length of the lamellae, which were oriented perpendicular to the stretching direction, decreased with increasing strain. The result seems to be related to the population density of “γ-filaments” (named by Andrews) generated only by prestretching. Time-resolved bright-field observations by in situ TEM of the thin film prestretched up to a strain of 200% revealed the following. Though the film gave crystalline reflections in its selected-area electron diffraction even before isothermal crystallization, no well-defined lamellae were recognized. Subsequent isothermal crystallization for 2 h, however, formed well-defined lamellae oriented perpendicular to the stretching direction. This observation, conceivably, suggests that “γ-filaments” were formed immediately by prestretching and then the lamellae were nucleated on the γ-filaments at −25 °C.

Morphology of melt-crystallized poly(ethylene 2,6-naphthalate) thin films studied by transmission electron microscopy

Thin films of poly(ethylene 2,6-naphthalate) (PEN) were isothermally crystallized at 190 °C after being melted at 300 °C. Morphological observation by transmission electron microscopy (TEM) showed the spherulitic texture in the films. Selected-area electron diffraction (SAED) indicated that the crystals in the films are the a form, as expected from our thermal condition for crystallization. The SAED pattern from the untilted specimen was characterized by the fairly intense reflection ring accompanied by other weak rings, and this intense ring was indexed as 010. A series of SAED patterns, which were obtained from the same specimen area tilted at various angles in the TEM column, suggested that the crystallites are oriented with their (001) planes being preferentially parallel to the film surface. Subsequently, a set of the dark-field images of the two-dimensional spherulite taken by using two different parts of the 010 reflection ring revealed that most of the crystallites in such a spherulite are oriented with their (010) planes being parallel in its radial direction. In addition, the spherulites in small pieces (0.05–0.08 mm thick) of PEN, which had been crystallized under the same thermal condition as above, were determined to be negatively birefringent by polarizing light microscopy.

Preferential Orientation of Poly(ethylene 2,6-naphthalate) Melt-Crystallized on the Friction-Transfer Layer of PTFE.

Solution-cast thin films of poly (ethylene 2,6-naphthalate) (PEN) were isothermally crystallized at 190°C on the highly oriented ultra-thin friction-transfer layer of polytetrafluoroethylene (PTFE), after being melted at 300°C. Transmission electron microscopy (TEM) of the films showed that PEN crystallized in an oriented fashion on the layer. Selected-area electron diffraction (ED) indicated that the crystals in the PEN films are mostly the α-form, as expected from our thermal condition for crystallization. The ED pattern from the untilted specimen was characterized by the fairly intense are-shaped 01 0 reflection on the equator. A series of ED patterns, which were obtained from the same specimen area tilted at various angles around the axis parallel or perpendicular to the chain axis of l'TFE in a TEM column, suggested that the crystallites of PEN α-form are preferentially oriented with their chain axis being parallel to the chain axis of PTFE and moreover, with their (100) planes being parallel to the surface of the PTFE layer.