Phase stability in organic and inorganic compounds using in-situ transmission electron microscopy

Abstract

All matter on Earth is about to exist in the most energy stable state. This results in a large number of reactions and changes in the direction of lowering one's energy, much of which is understood only from a static thermodynamic perspective. To be understood from a dynamic kinetic point of view, the reaction needs to be observed in real-time and quantitative measurements. We focused on discussing the stability of the materials according to temperature and atmospheric control using in-situ transmission electron microscopy (TEM) that can real-time observe their structure, the growth process of nanomaterials, and analyze composition at the high-resolution where atoms are visible. Through real-time quantitative analysis of morphology, growth rate, and crystallinity changes, we want to carry out fundamental materialistic research that needs to be understood in order to achieve more breakthrough properties. We selected as the subjects, Organic and inorganic thin films and nano-compounds (SrTiCoO3, MAPI3, VO2) used in three representative fields (transistors, solar cells, fuel cells). In the first part of the dissertation, we carried out a precise control of the size, density, and distribution of metal nanoparticles dispersed on functional oxide supports, which is critical for promoting catalytic activity and stability in renewable energy and catalysis devices. We measure the growth kinetics of individual Co particles ex-solved on SrTi0.75Co0.25O3‑δ polycrystalline thin films under a high vacuum, and at various temperatures and grain sizes using in-situ TEM. The ex-solution preferentially occurs at grain boundaries and corners which appear essential for controlling particle density and distribution, and enabling low temperature ex-solution. The particle reaches a saturated size after a few minutes, and the size depends on temperature. Quantitative measurements with a kinetic model determine the rate limiting step, vacancy formation enthalpy, ex-solution enthalpy, and activation energy for particle growth. The ex-solved particles are tightly socketed, preventing interactions among them over 800 °C. Furthermore, we obtain the first direct clarification of the active reaction site for CO oxidation - the Co-oxide interface, agreeing well with density functional theory calculations. In the second part of the dissertation, we conducted the study for the requirement of a deeper understanding and control of the materials and processing in perovskite solar cells offering remarkable performance. We fabricate the first single crystal nanorods of intermediate phase (MAI-PbI2-DMSO), allowing us to directly observe the phase evolution while annealing in situ in a high-vacuum TEM, which lets up separate thermal effects from other environmental conditions such as oxygen and moisture. We attain the first full determination of the crystal structures and orientations of intermediate phase, evolving perovskite, precipitating PbI2 and e-beam induced PbI2 during phase conversion and decomposition. Surprisingly, the perovskite decomposition to PbI2 is reversible upon cooling, critical for long-term device endurance due to the formation of MAI-rich MAPbI3 and PbI2 upon heating. Quantitative measurements with a thermodynamic model suggest the decomposition is entropically driven. The single crystal MAPbI3 nanorods obtained via thermal cycling exhibit excellent mobility and trap density, with full reversibility up to 100 °C (above the maximum temperature for solar cell operation) under high vacuum, offering unique potential for high-performance flexible solar cells. In the last part of the dissertation, we developed low-temperature processing methods targeting for sol-gel metal oxides. In most cases, it has been hard to use amorphous metal oxide films because a much higher temperature is required for crystalline functional metal oxide formation. We report the photocombustion developed to dramatically reduce the crystallization temperature of sol-gel VO2 film from 600 to 250 °C by combining deep ultraviolet irradiation with carbon-free oxidizer for reactive radical generation. The morphological transition and crystal evolution of the sol-gel films annealed via various conditions including combustion, photoactivation, and photocombustion were thoroughly investigated using in-situ and ex-situ TEM techniques, allowing us to separate the regimes of amorphous and crystal solid formation. TGA-DTA, UV-vis spectroscopy and EDX analyses reveal that at [AN]/[VO(acac)2] of 1.5, impurity removal and V-O-V polycondensation are most effective, and suggest that oxygen vacancies generated during reduction play a key role in accelerating crystallization. Functional VO2 films were successfully deposited on polyimide substrates for large-area flexible phase-transition device arrays, while their excellent uniformity and phase-transition reliability were confirmed under various bending conditions. We believe that the low-temperature crystallization via DUV-assisted photocombustion is useful for strain-sensitive low temperature devices of crystalline metal oxide films fabricated via sol-gel routes.