S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Real time investigation of structural and chemical analysis of the nanoparticles in the liquid using custom-built liquid cell stage
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- Transmission electron microscopy; liquid phase; in-situ observation; Energy dispersive spectroscopy; growth and dissolution of the nanoparticles; micro electro mechanical systems; Silicon nitride membrane
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2015. 8. 김영운.
- As the range of applications of nanomaterials has expanded, the microstructural analysis of nanomaterials has become a major issue for interpreting the mechanisms of synthesis. Transmission electron microscopy (TEM)-based studies on nanoparticles have been widely used because of the superior lateral resolution and ability to analyze crystallographic, chemical, and electronic structural information. Even though ex-situ TEM analysis could help identify the material kinetics after the experiment, in-situ TEM analysis has an additional benefit of enabling direct observation of the dynamic nucleation, growth, and dissolution of nanoparticles.
A variety of methods for preparing nanoparticles from solutions have been reported. In contrast to the bulk materials, properties of nanomaterials are significantly affected by size, distribution, aspect ratio, and faceting of the crystallographic surfaces. Because of the lack of information regarding the nanoparticle synthesis mechanism, many research groups attempted to control the specific morphology and size of nanoparticles by conducting numerous experiments empirically, with the aim to produce nanoparticles with specific properties. If the synthesis of nanoparticles is directly observed in a liquid with high spatial resolution, it could help in understanding the synthesis mechanism. In this thesis, we develop a custom-built in-situ liquid cell TEM stage, to directly observe growth and dissolution of nanoparticles, with a high spatial resolution.
Recently, in-situ liquid cell TEM has been widely used to observe nanoparticle dynamics. However, in most liquid cell studies, only structural analysis is possible. Since a SiNx membrane is used to encapsulate the liquid in TEM, it causes multiple scattering of characteristic x-ray signals due to structural restrictions. In this thesis, we redesign the previously reported SiNx membrane by fabricating the asymmetrical SiNx membrane for structural and chemical analysis at the same time. The critical size of the window was confirmed by energy dispersive spectroscopy (EDS) peaks. From the experimental results, the critical escape angle of x-ray signals, which could reach the EDS detector, was calculated. By conducting the in- situ experiment in various solution systems, sufficient x-ray signals were successfully obtained in the redesigned SiNx membrane.
Due to inelastic collisions, the energetic electrons used in TEM form images and diffraction patterns to generate radiolysis species while passing through the liquid. The variation in the electron number density inside the illumination area does not have a significant impact on the microstructure observation in the solid thin foil sample, if high resolution is not required. In the liquid sample, however, radiolysis species, generated with the illuminated electrons, could influence the reaction rates and product distribution. Inhomogeneous growth and dissolution of silver nanoparticles prior to stabilization were observed in the in situ liquid TEM. It is believed that the key player in this sequence of reactions is the inhomogeneous distribution of eaq-, originating from the non-uniform illumination intensity of electron beam. The inhomogeneous growth is the result of the diffusion of radiolysis species from a high concentration region to a lower concentration region.