Spatially Resolved Functional Group Analysis of OLED Materials Using EELS and ToF-SIMS.

Electron energy-loss spectroscopy (EELS) is widely used in analyzing the electronic structure of inorganic materials at high spatial resolution. In this study, we use a monochromator to improve the energy resolution, allowing us to analyze the electronic structure of organic light-emitting diode (OLED) materials with greater precision. This study demonstrates the use of the energy-loss near-edge structure to map the nitrogen content of organic molecules and identify the distinct bonding characteristics of aromatic carbon and pyridinic nitrogen. Furthermore, we integrate EELS with time-of-flight secondary ion mass spectrometry for molecular mapping of three different bilayers composed of OLED materials. This approach allows us to successfully map functional groups in the by-layer OLED and measure the thickness of two OLED layers. This study introduces spatially resolved functional group analysis using electron beam spectroscopy and contributes to the development of methods for complete nanoscale analysis of organic multilayer architectures.

Graphene Liquid Cell Electron Microscopy: Progress, Applications, and Perspectives.

Graphene liquid cell electron microscopy (GLC-EM), a cutting-edge liquid-phase EM technique, has become a powerful tool to directly visualize wet biological samples and the microstructural dynamics of nanomaterials in liquids. GLC uses graphene sheets with a one carbon atom thickness as a viewing window and a liquid container. As a result, GLC facilitates atomic-scale observation while sustaining intact liquids inside an ultra-high-vacuum transmission electron microscopy chamber. Using GLC-EM, diverse scientific results have been recently reported in the material, colloidal, environmental, and life science fields. Here, the developments of GLC fabrications, such as first-generation veil-type cells, second-generation well-type cells, and third-generation liquid-flowing cells, are summarized. Moreover, recent GLC-EM studies on colloidal nanoparticles, battery electrodes, mineralization, and wet biological samples are also highlighted. Finally, the considerations and future opportunities associated with GLC-EM are discussed to offer broad understanding and insight on atomic-resolution imaging in liquid-state dynamics.

Hydrogen-Assisted Fast Growth of Large Graphene Grains by Recrystallization of Nanograins.

Chemical vapor deposition has been highlighted as a promising tool for facile graphene growth in a large area. However, grain boundaries impose detrimental effects on the mechanical strength or electrical mobility of graphene. Here, we demonstrate that high-pressure hydrogen treatment in the preannealing step plays a key role in fast and large grain growth and leads to the successful synthesis of large grain graphene in 10 s. Large single grains with a maximum size of ∼160 µm grow by recrystallization of nanograins, but ∼1% areal coverage of nanograins remains with 28-30° misorientation angles. Our findings will provide insights into mass production of high-quality graphene.

Real-Time Observation of CaCO3 Mineralization in Highly Supersaturated Graphene Liquid Cells.

The mineralization dynamics of calcium carbonate is investigated under highly supersaturated conditions using graphene liquid cell transmission electron microscopy. We demonstrate that the mineralization process has three steps: nucleation, diffusion-limited growth, and Ostwald ripening/coalescence. In addition, we show that the polymorphs precipitate in a specific order, from metastable aragonite to stable calcite, thus proving Ostwald's rule of stages. In highly supersaturated solutions, the aragonite phase crystallizes in a stable manner, in addition to the calcite phase.

Live Cell Electron Microscopy Using Graphene Veils.

As a promising tool over the optical resolution limits, liquid electron microscopy is practically utilized to visualize the structural information on wet biological specimens, such as cells, proteins, and nucleic acids. However, the functionality of biomolecules during their observation is still controversial. Here we show the feasibility of live-cell electron microscopy using graphene veils. We demonstrate that the electron dose resistivity of live bacterial cells increases to 100-fold in graphene veils, and thus they maintain their structures and functions after electron microscopy experiments. Our results provide the guidelines and show possibilities for the electron microscopy imaging of live cells and functional biomolecules.

Graphene Liquid Cell Electron Microscopy of Initial Lithiation in Co3O4 Nanoparticles.

As it governs the overall performance of lithium-ion batteries, understanding the reaction pathway of lithiation is highly desired. For Co3O4 nanoparticles as anode material, here, we report an initial conversion reaction pathway during lithiation. Using graphene liquid cell electron microscopy (GLC-EM), we reveal a CoO phase of the initial conversion product as well as morphological dynamics during Co3O4 lithiation. In accordance with the in situ TEM observation, we confirmed that the Co3O4 to CoO conversion is a thermodynamically favorable process by calculating the theoretical average voltage based on density functional theory. Our observation will provide a useful insight into the oxide electrode that undergoes conversion reaction.

Pulverization-Tolerance and Capacity Recovery of Copper Sulfide for High-Performance Sodium Storage.

Finding suitable electrode materials is one of the challenges for the commercialization of a sodium ion battery due to its pulverization accompanied by high volume expansion upon sodiation. Here, copper sulfide is suggested as a superior electrode material with high capacity, high rate, and long-term cyclability owing to its unique conversion reaction mechanism that is pulverization-tolerant and thus induces the capacity recovery. Such a desirable consequence comes from the combined effect among formation of stable grain boundaries, semi-coherent boundaries, and solid-electrolyte interphase layers. The characteristics enable high cyclic stability of a copper sulfide electrode without any need of size and morphological optimization. This work provides a key finding on high-performance conversion reaction based electrode materials for sodium ion batteries.

Phonon-glass electron-crystals in ZnO-multiwalled carbon nanotube nanocomposites.

We propose a strategy for enhancing thermoelectric performance through the realization of a 'phonon-glass electron-crystal' (PGEC) by interface control using multiwalled carbon nanotubes (MWCNTs). By the consolidation of undoped ZnO nanoparticles with MWCNTs (0.5, 1, and 2 wt%) using spark plasma sintering, we fabricated the interface-controlled ZnO-MWCNT nanocomposites, in which ZnO grains were surrounded with a MWCNT network. Both single crystal-like charge transport (electron-crystal) and considerably reduced thermal conductivity (phonon-glass) were achieved simultaneously thanks to the beneficial effects of the MWCNT network, and this led to the enhancement of the thermoelectric figure of merit. We discussed these findings on PGECs in the ZnO-MWCNT nanocomposites from the viewpoint of interface control in detail, and our strategy may provide a promising way to the realization of PGEC in other hybrid thermoelectric materials.

A gigantically increased ratio of electrical to thermal conductivity and synergistically enhanced thermoelectric properties in interface-controlled TiO2-RGO nanocomposites.

We report synergistically enhanced thermoelectric properties through the independently controlled charge and thermal transport properties in a TiO2-reduced graphene oxide (RGO) nanocomposite. By the consolidation of TiO2-RGO hybrid powder using spark plasma sintering, we prepared an interface-controlled TiO2-RGO nanocomposite where its grain boundaries are covered with the RGO network. Both the enhancement in electrical conductivity and the reduction in thermal conductivity were simultaneously achieved thanks to the beneficial effects of the RGO network, and detailed mechanisms are discussed. This led to the gigantic increase in the ratio of electrical to thermal conductivity by six orders of magnitude and also the synergistic enhancement in the thermoelectric figure of merit by two orders. Our results present a strategy for the realization of 'phonon-glass electron-crystals' through interface control using graphene in graphene hybrid thermoelectric materials.