The Development of iDPC-STEM and Its Application in Electron Beam Sensitive Materials

Development of a femtosecond analytical electron microscopy based on a Schottky field emission transmission electron microscope

Ultrafast transmission electron microscopy (UTEM) has gained wide applications in the nanoscale dynamics with femtosecond, even attosecond, resolution. The instrument development is still in progress to satisfy the various applications. At Nankai University, we built an UTEM with a laser-driven Schottky field emitter based on a field emission TEM (Talos200i) of Thermo Fisher Scientific. This study comprehensively examines the performance of the UTEM, including the continuous mode and ultrafast photoemission mode. The investigation focuses on optimizing brightness, temporal resolution, energy dispersion, and stability in ultrafast photoemission mode, achieving a temporal resolution of ∼200 fs and an energy dispersion of 0.7 eV with excellent stability through careful adjustments of laser parameters and equipment settings. In scanning transmission electron microscopy mode, the beam size of the photoemission mode is ∼1.4 nm at specific settings with potential for further improvement. As application examples, we illustrate the results of photoinduced structural dynamics of gold film and MoS2 thin flake by ultrafast electron diffraction. We also report the polarization dependent optical interference pattern characterized by the photoinduced near field microscopy effect in a silicon thin film sample prepared by the focused ion beam method. These findings provide valuable insights for researchers aiming to leverage the UTEM's capabilities for advanced electron microscopy applications and pave the way for future advancements in UTEM technology.

In Situ TEM Characterization of Battery Materials

Transmission electron microscopy (TEM) is an indispensable analytical technique in materials research as it probes material information down to the atomic level and can be utilized to examine dynamic phenomena during material transformations. In situ TEM resolves transient metastable states via direct observation of material dynamics under external stimuli. With innovative sample designs developed over the past decades, advanced in situ TEM has enabled emulation of battery operation conditions to unveil nanoscale changes within electrodes, at interfaces, and in electrolytes, rendering it a unique tool to offer unequivocal insights of battery materials that are beam-sensitive, air-sensitive, or that contain light elements, etc. In this review, we first briefly outline the history of advanced electron microscopy along with battery research, followed by an introduction to various in situ TEM sample cell configurations. We provide a comprehensive review on in situ TEM studies of battery materials for lithium batteries and beyond (e.g., sodium batteries and other battery chemistries) via open-cell and closed-cell in situ TEM approaches. At the end, we raise several unresolved points regarding sample preparation protocol, imaging conditions, etc., for in situ TEM experiments. We also provide an outlook on the next-stage development of in situ TEM for battery material study, aiming to foster closer collaboration between in situ TEM and battery research communities for mutual progress.

Defect Chemistry Engineering to Regulate the Excellent ZTave Value of Nonstoichiometric Ca3-xCo4O9 (0 ≤ x ≤ 0.06) Ceramics

Cobalt-based oxides have attracted significant attention as p-type thermoelectric materials due to their wide operational temperature range. However, their low average figure of merit (ZTave) value has hindered service performance. A series of cation vacancies as Ca-active sites were introduced into Ca3-xCo4O9 (0 ≤ x ≤ 0.06) by defect chemistry engineering to regulate the excellent ZTave value. The Ca-active sites of Ca3-xCo4O9 ceramics induced lattice distortion and point defects, which were unequivocally confirmed by the iDPC-STEM results. The power factor from 0.18 mW/(m·K2) to 0.38 mW/(m·K2) and the electrical conductivity from 54.8 to 108.3 S/cm were achieved for the Ca2.96Co4O9 sample. A notable ZT value of 0.32 was obtained at 1073 K. Furthermore, the high ZTave of approximately 0.166 reached within the temperature range of 323-1073 K, representing a 3.25 times improvement compared to pure Ca3Co4O9 ceramics. This study highlights defect chemistry engineering as an effective strategy for optimizing the thermoelectric performance of nonstoichiometric Ca3Co4O9 ceramics, offering promising prospects for the development of Ca3Co4O9-based thermoelectric materials.

Simulation Study of Low-Dose 4D-STEM Phase Contrast Techniques at the Nanoscale in SEM

Phase contrast imaging is well-suited for studying weakly scattering samples. Its strength lies in its ability to measure how the phase of the electron beam is affected by the sample, even when other imaging techniques yield low contrast. In this study, we explore via simulations two phase contrast techniques: integrated center of mass (iCOM) and ptychography, specifically using the extended ptychographical iterative engine (ePIE). We simulate the four-dimensional scanning transmission electron microscopy (4D-STEM) datasets for specific parameters corresponding to a scanning electron microscope (SEM) with an immersive objective and a given pixelated detector. The performance of these phase contrast techniques is analyzed using a contrast transfer function. Simulated datasets from a sample consisting of graphene sheets and carbon nanotubes are used for iCOM and ePIE reconstructions for two aperture sizes and two electron doses. We highlight the influence of aperture size, showing that for a smaller aperture, the radiation dose is spent mostly on larger sample features, which may aid in imaging sensitive samples while minimizing radiation damage.

Revolutionizing the structural design and determination of covalent-organic frameworks: principles, methods, and techniques

Covalent organic frameworks (COFs) represent an important class of crystalline porous materials with designable structures and functions. The interconnected organic monomers, featuring pre-designed symmetries and connectivities, dictate the structures of COFs, endowing them with high thermal and chemical stability, large surface area, and tunable micropores. Furthermore, by utilizing pre-functionalization or post-synthetic functionalization strategies, COFs can acquire multifunctionalities, leading to their versatile applications in gas separation/storage, catalysis, and optoelectronic devices. Our review provides a comprehensive account of the latest advancements in the principles, methods, and techniques for structural design and determination of COFs. These cutting-edge approaches enable the rational design and precise elucidation of COF structures, addressing fundamental physicochemical challenges associated with host-guest interactions, topological transformations, network interpenetration, and defect-mediated catalysis.

Low-cost electron detector for scanning electron microscope

Electron microscopy is an indispensable tool for the characterization of (nano) materials. Electron microscopes are typically very expensive and their internal operation is often shielded from the user. This situation can provide fast and high quality results for researchers focusing on e.g. materials science if they have access to the relevant instruments. For researchers focusing on technique development, wishing to test novel setups, however, the high entry price can lead to risk aversion and deter researchers from innovating electron microscopy technology further. The closed attitude of commercial entities about how exactly the different parts of electron microscopes work, makes it even harder for newcomers in this field. Here we propose an affordable, easy-to-build electron detector for use in a scanning electron microscope (SEM). The aim of this project is to shed light on the functioning of such detectors as well as show that even a very modest design can lead to acceptable performance while providing high flexibility for experimentation and customization.