Quantitative nanoscale tracking of oxygen vacancy diffusion inside single ceria grains by in situ transmission electron microscopy

Oxygen vacancy formation and migration in ceria is critical to its electrochemical and catalytic properties in systems for chemical and energy transformation, but its quantification is rather challenging especially at atomic-scale because of disordered distribution. Here we report a rational approach to track oxygen vacancy diffusion in single grains of pure and Sm-doped ceria at −20 °C to 160 °C using in situ (scanning) transmission electron microscopy ((S)TEM). To create a gradient in oxygen vacancy concentration, a small region (∼30 nm in diameter) inside a ceria grain is reduced to the C-type CeO_1.68 phase by the ionization or radiolysis effect of a high-energy electron beam. The evolution in oxygen vacancy concentration is then mapped through lattice expansion measurement using scanning nano-beam diffraction or 4D STEM at a spatial resolution better than 2 nm; this allows direct determination of local oxygen vacancy diffusion coefficients in a very small domain inside pure and Sm-doped ceria at different temperatures. Further, the activation energies for oxygen transport are determined to be 0.59, 0.66, 1.12, and 1.27 eV for pure CeO₂, Ce_0.94Sm_0.06O_1.97, Ce_0.89Sm_0.11O_1.945, and Ce_0.8Sm_0.2O_1.9, respectively, implying that activation energy increases due to impurity scattering. The results are qualitatively supported by density functional theory (DFT) calculations. In addition, our in situ TEM investigation reveals that dislocations impede oxygen vacancy diffusion by absorbing oxygen vacancies from the surrounding areas and pinning them locally. With more oxygen vacancies absorbed, dislocations show extended strain fields with local tensile zone sandwiched between the compressed ones. Therefore, dislocation density should be reduced in order to minimize the resistance to oxygen vacancy diffusion at low temperatures.     

In situ transmission electron microscopy of light-induced photocatalytic reactions

Transmission electron microscopy (TEM) makes it possible to obtain insight into the structure, composition and reactivity of photocatalysts, which are of fundamental interest for sustainable energy research. Such insight can be used for further material optimization. Here, we combine conventional TEM analysis of photocatalysts with environmental TEM (ETEM) and photoactivation using light. Two novel types of TEM specimen holder that enable in situ illumination are developed to study light-induced phenomena in photoactive materials, systems and photocatalysts at the nanoscale under working conditions. The technological development of the holders is described and two representative photo-induced phenomena are studied: the photodegradation of Cu?O and the photodeposition of Pt onto a GaN:ZnO photocatalyst.     

Controlled growth of nanoparticles from solution with in situ liquid transmission electron microscopy

Direct visualization of lead sulfide nanoparticle growth is demonstrated by selectively decomposing a chemical precursor from a multicomponent solution using in situ liquid transmission electron microscopy. We demonstrate reproducible control over growth mechanisms that dictate the final morphology of nanostructures while observing growth in real-time with subnanometer spatial resolution. Furthermore, while an intense electron beam can initiate nanoparticle growth, it is also shown that a laser can trigger the reaction independently of the imaging electrons.     

Understanding all solid-state lithium batteries through in situ transmission electron microscopy

Owing to the use of solid electrolytes instead of flammable and potentially toxic organic liquid electrolytes, all solid-state lithium batteries (ASSLBs) are considered to have substantial advantages over conventional liquid electrolyte based lithium ion batteries(LIBs) in terms of safety, energy density, battery packaging, and operable temperature range. However, the electrochemistry and the operation mechanism of ASSLBs differ considerably from conventional LIBs. Consequently, the failure mechanisms of ASSLBs, which are not well understood, require particular attention. To improve the performance and realize practical applications of ASSLBs, it is crucial to unravel the dynamic evolution of electrodes, solid electrolytes, and their interfaces and interphases during cycling of ASSLBs. In situ transmission electron microscopy (TEM) provides a powerful approach for the fundamental investigation of structural and chemical changes during operation of ASSLBs with high spatio-temporal resolution. Herein, recent progress in in situ TEM studies of ASSLBs are reviewed with a specific focus on real-time observations of reaction and degradation occurring in electrodes, solid electrolytes, and their interfaces. Novel electro-chemo-mechanical coupling phenomena are revealed and mechanistic insights are highlighted. This review covers a broad range of electrode and electrolyte materials applied in ASSLBs, demonstrates the general applicability of in situ TEM for elucidating the fundamental mechanisms and providing the design guidance for the development of high-performance ASSLBs. Finally, challenges and opportunities for in situ TEM studies of ASSLBs are discussed.     

Straight and kinked InAs nanowire growth observed in situ by transmission electron microscopy

Live observations of growing nanowires using in situ transmission electron microscopy （TEM） is becoming an increasingly important tool for understanding the dynamic processes occurring during nanowire growth. Here we present observations of growing InAs nanowires, which constitute the first reported in situ growth of a In-V compound in a transmission electron microscope. Real time observations of events taking place over longer growth lengths were possible due to the high growth rates of up to I nm/s that were achieved. Straight growth （mainly in 〈111〉B directions） was observed at uniform temperature and partial pressure while intentional fluctuations in these conditions caused the nanowires to form kinks and change growth direction. The mechanisms behind the kinking are discussed in detail. In situ observations of nanowire kinking has previously only been reported for nonpolar diamond structure type materials （such as Si）, but here we present results for a polar zinc blende structure （InAs）. In this study a closed cell with electron and X-ray transparent a-SiN windows was used in a conventional high resolution transmission electron microscope, enabling high resolution imaging and compositional analysis in between the growth periods.     

Individual cathodoluminescence and photoluminescence spectroscopy of zinc oxide nanoparticles in combination with in situ transmission electron microscopy

The functions of scanning near-field optical microscopy (SNOM) were installed in high-resolution transmission electron microscopy (TEM) for cathodoluminescence spectroscopy and photoluminescence spectroscopy of individual nanostructures. Optical fiber probes used in SNOM were allowed to approach nanoparticles by piezomanipulation with simultaneous observation by TEM. As an application of this method, cathodoluminescence and photoluminescence from zinc oxide nanoparticles were measured at room temperature and 130 K. It was demonstrated that the present method directly provides the relationships between structural features of individual nanoparticles and spectra.     

Local temperature measurements on nanoscale materials using a movable nanothermocouple assembled in a transmission electron microscope

A nanoscale thermocouple consisting of merged Cu and Cu-Ni tips is developed for local temperature measurements on advanced nanomaterials by using a probing technique in a high-resolution transmission electron microscope (TEM) equipped with a double probe scanning tunneling microcopy (STM) unit. The fabricated nanothermocouple works as the so-called T-type thermocouple and displays a quick response and high spatial and thermal resolutions. A generated thermoelectromotive force which reflects rapid temperature changes controlled by electron beam intensity alternations on a metal nanoelectrode proves the technique's usefulness for high-precision local temperature measurements. The developed method demonstrates the effectiveness while also measuring temperature changes in Joule heated multi-walled carbon nanotubes (CNTs) and in a modeled electrical conductive composite nanosystem.     

Low-temperature nanocrystal unification through rotations and relaxations probed by in situ transmission electron microscopy

Through the mechanism of "oriented attachment", small nanocrystals can fuse into a wide variety of one- and two-dimensional nanostructures. This fusion phenomenon is investigated in detail by low-temperature annealing of a two-dimensional array of 10 nm-sized PbSe nanocrystals, in situ in the transmission electron microscope. We have revealed a complex chain of processes; after coalescence, the connected nanocrystals undergo consecutive rotations in three-dimensional space, followed by drastic interfacial relaxations whereby full fusion is obtained.     

High resolution transmission electron microscopy study of nanoscale Ni-rich NiAl films evaporated onto NaCl and KCl

The atomic and microstructure of Ni-richB2 Ni_xAl_{100 − x} thin fllms with nanoscale dimensions, for thickness as well as lateral grain size, and produced by evaporation onto NaCl and KCl (001) surfaces are investigated by high resolution electron microscopy. Different textures and orientation relationships with the substrate are found exhibiting grains with [001], [011] and [111] zones parallel with the film normal. Local distortions including precursor-like domains and surface shearing as well as dislocations are observed in the grains. No micromodulations as in bulk martensite precursors are visible. No martensitic transformation was observed when cooling the free standingfilms to −170 °C.     

An in situ hot stage transmission electron microscopy study of the decomposition of Fe-C austenites

Hot stage transmission electron microscopy is applied to determine the growth mechanism during decomposition of austenite in hypo-eutectoid Fe-C austenites. The austenite-ferrite interface is mostly curved and moves sluggishly with periods of acceleration and deceleration. In some cases the interface is nearly straight and effectively immobile. Then, migration takes place by means of ledges which displace parallel to the immobile straight interface. The ledges migrate at a rate equal to the migration rate predicted for diffusion controlled migration. The highest migration rates observed for the curved interface are nearly equal to that calculated for diffusion controlled growth. The observed succession of periods of acceleration and deceleration for the curved interface is not predicted in the common theories for interface mobility during phase transformation. Detailed examination of region around the interface indicate that stress build up and stress relaxation are responsible for the deceleration and acceleration respectively. The stresses are due to the volume misfit between the ferrite formed and the parent austenite.     

Atomic-scale insights into the formation of 2D crystals from in situ transmission electron microscopy

Nano Research - Two-dimensional (2D) crystals are attractive due to their intriguing structures and properties which are strongly dependent on the synthesis conditions. To achieve their superior...     

Mechanical and field-emission properties of individual ZnO nanowires studied in situ by transmission electron microscopy

The mechanical and field-emission properties of individual ZnO nanowires,grown by a solid-vapour phase thermal sublimation process,were studied in situ by transmission electron microscopy(TEM)using a home-made TEM specimen holder.The mechanical resonance is electrically induced by applying an oscillating voltage,and in situ imaging has been achieved simultaneously.The mechanical results indicate that the elastic bending modulus of individual ZnO nanowires were measured to be～58 GPa.A nanobalance was buil...     

Verification of unzipping models of electromigration in gold nanocontacts by in situ high-resolution transmission electron microscopy

We observed in situ the electromigration process of gold (Au) nanocontacts (NCs) by high-resolution transmission electron microscopy. The structural dynamics of the interior and surfaces of the NCs were investigated at the atomic level. In particular, we directly verified the evidence of the unzipping model of electromigration with the in situ observation of surface-edge movement. The fundamental parameters of NCs, i.e., conductance and tensile force, were also measured during in situ lattice imaging of electromigration. Atoms migrating from the negative electrode accumulated at the most constricted regions of the NCs, leading to expansion. As a result, the NCs were compressed by the two electrodes. We demonstrated the magnitude of the force acting on the NCs during electromigration. The critical voltage of electromigration was approximately 80 mV, and the current density at the critical voltage was 60 TA m(-2). We found that Au nanogaps could be fabricated by applying this bias voltage to Au NCs.     

Recent developments in dynamic transmission electron microscopy

One of the current major driving forces behind instrument development in transmission electron microscopy (TEM) is the ability to observe materials processes as they occur in situ within the microscope. For many processes, such as nucleation and growth, phase transformations and mechanical response under extreme conditions, the beam current in even the most advanced field emission TEM is insufficient to acquire images with the temporal resolution (∼1 μs to 1 ns) needed to observe the fundamental interactions taking place. The dynamic transmission electron microscope (DTEM) avoids this problem by using a short pulse laser to create an electron pulse of the required duration through photoemission which contains enough electrons to form a complete high resolution image. The current state-of-the-art in high time resolution electron microscopy in this paper describes the development of the electron optics and detection schemes necessary to fully utilize these electron pulses for materials science. In addition, developments for future instrumentation and the experiments that are expected to be realized shortly will also be discussed.     

Thermal stability of Al/nanocrystalline-Si bilayers investigated by in situ heating energy-filtered transmission electron microscopy

In situ heating energy-filtered transmission electron microscopy was employed to investigate the interfacial intermixing/reactions during thermal annealing of Al/nanocrystalline-Si (nc-Si) bilayers in the temperature range of 150–500 °C. In comparison with the Al/amorphous-Si (a-Si) bilayer, the Al/nc-Si bilayers were found to be much more stable against thermal annealing. Wetting and c-Si growth processes along Al grain boundaries, which take place during annealing of Al/a-Si bilayers, do not occur in Al/nc-Si bilayers, because of the lack of thermodynamic driving forces in the latter case. As a consequence, also in contrast with Al/a-Si bilayers, no layer exchange occurs in Al/nc-Si bilayers, not even after annealing at 500 °C. Instead, intermixing of Al/nc-Si is realized at the Al/nc-Si interface by the formation of Al spikes growing into the nc-Si sublayers at temperatures higher than 300 °C. The relatively low Al-spike formation temperature in Al/nc-Si systems, as compared with that for Al/single-crystalline Si systems, is ascribed to the higher Gibbs energy of nanocrystalline Si as compared to single-crystalline Si.     

Review: transmission scanning electron microscopy

The availability of scanning attachments for transmission microscopes and the advent of very high resolution scanning microscopes now enables materials to be studied in both the back scattered and transmission scanning modes. It is the purpose of this review to present in outline the subject of transmission scanning microscopy, the advocated advantages in comparison with conventional transmission microscopy and some of the achieved and potential applications.     

Carbon nanotube nanoelectronic devices compatible with transmission electron microscopy

We report on a novel method to fabricate carbon nanotube (CNT) nanoelectronic devices on silicon nitride membrane grids that are compatible with high resolution transmission electron microscopy (HRTEM). Resist-based electron beam lithography is used to fabricate electrodes on 50 nm thin silicon nitride membranes and focused-ion-beam milling is used to cut out a 200 nm gap across a gold electrode to produce the viewing window for HRTEM. Spin-coating and AC electrophoresis are used as methods to deposit small bundles of carbon nanotubes across the electrodes. We demonstrate the viability of this approach by performing both electrical measurements and HRTEM imaging of solution-processed CNTs in a device.     

Measurement of foil thickness in transmission electron microscopy

Methods for the measurement of the thickness of thin-foil specimens used in transmission electron microscopy are either difficult to carry out or have been subject to criticism. In particular, the contamination spot method is said to overestimate the thickness because the region of rapidly changing contrast marking the apparent edge of the spot is not on the foil surface but is on a broad contamination deposit whose thickness is changing much more slowly. A new method for measuring foil thickness is proposed, based on contamination deposits on the foil surfaces. The problems of the contamination spot method, in which the deposit is of circular form, are avoided by using one of the condenser lenses to focus the electron beam in a thin line on the foil during deposition. Adequate contrast can be obtained with a line whose width is one-third to one-fifth of the foil thickness and having a height equal to or less than its width. The error, being a fraction of the line width, is then very small. After rotation of the foil, the lines separate into two and the corresponding edges of the lines provide distinct features whose separation can be measured to determine thickness. The axis of rotation, perpendicular to which the separation of the lines has to be measured to calculate foil thickness, is determined by depositing two contamination lines at right angles. The method allows a number of measurements of thickness covering a relatively large area of foil to be made per contamination experiment. Near the edge of the foil, the upper and lower lines of contamination can join around the foil edge to form a U shape which can be used to measure thickness profile of the foil right up to the edge.     

Metallurgical applications of scanning transmission electron microscopy

It is shown that scanning transmission electron microscopy (STEM) has followed two main lines of development, the pure STEM based upon a field emission electron source in which the emphasis is given to high resolution, and a combined system in which STEM is an attachment to a conventional transmission microscope (TEM + STEM). When used in combination with an energy dispersive X-ray spectrometer, the combined TEM + STEM system is shown to be extremely versatile and possibly the more useful for the applied metallurgist. The high vacuum requirements of pure STEM, however, make this system suitable to be used in conjunction with an Auger spectrometer. Examples of the various microanalysis facilities of STEM are given in the article, including micro-diffraction, rocking-beam channelling patterns, qualitative and quantitative X-ray spectroscopy analysis, particle analysis and in situ experimentation. The controversial subject of whether thicker specimens can be studied in STEM compared with conventional TEM is also discussed.