Research
We are interested in studying physical and chemical reactions in multi-dimensional approaches to illustrate reactions in a single molecule/particle level. In situ dynamics, structural evolution in solution phase reaction and on surfaces/interfaces are studied.
Liquid phase / in situ TEM study for nanoparticle chemistry
Because individual nanoparticles have a different surface structure and morphology, the individual particles exhibit different chemical properties. We use liquid-phase TEM to investigate various chemical properties of individual nanoparticles at the level of a single particle, which is typically difficult to be analyzed using conventional methodologies that only provide averaged properties of multiple objects. The liquid-phase TEM enables the visualization of chemical reactions and fundamental physics occurring in nanoscale, such as growth, degradation, etching, corrosion, inter-particle coalescence, and diffusion dynamics. We expect that the direct observation of individual chemical processes will deepen the understanding on the fundamental material chemistry from macroscopic to nanoscale point of view.
In addition, to obtain statistically meaningful mechanistic information from the in situ/liquid phase TEM studies, we develop unique methods for image processing, automated data acquisition, statistical analysis, and big-data analysis. This allows us to study the integrated mechanism of nucleation, growth, and interactions between nanoparticles. We also investigate the redox-induced surface reaction and dissolution of oxide nanoparticles in liquid. The morphological evolution and the crystallographic information of nanomaterials during electrical biasing is also studied to find important factors in the performance of rechargeable batteries. In addition, we also utilize Cryo-EM or low-dose TEM to study beam-sensitive materials such as battery materials and proteins. Specific research areas include:
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Integrated mechanistic study of nanoparticle growth
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Scaling nanoparticle interactions and coalescence events
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Diffusion of nanoparticles
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Etching, corrosion, dissolution chemistry of redox active nanoparticles
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In operando TEM experiment of rechargeable battery
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Cryo-EM for soft or beam-sensitive materials
3D structures of nanomaterials
Determining the three-dimensional (3D) atomic structure of nanomaterial in a native environment is important to understand their structure-property relationship. Conventional structural analysis methods such as, X-ray diffraction, high-resolution transmission electron microscopy, electron tomography, and cryogenic electron microscopy have limitations in identifying the exact 3D structure of materials in their pristine phases. Developing a new structure analysis method that can resolve the 3D structures of individual materials at native environment such as liquid with atomic resolution would provide allow us with different level of structural understanding of materials both in material and biological sciences.
Our group develops 3D SINGLE (Structure Identification of Nanoparticles by Graphene Liquid cell Electron microscopy), a direct method to resolve 3D structures of individual nanocrystals in liquid. 3D SINGLE utilizes a sub-angstrom resolution TEM, a 3D structural reconstruction algorithm typically used for bio-molecules, and a graphene liquid cell TEM which provides highest spatial resolution of specimens in liquid. 3D SINGLE method maps structural information of single solvated nanocrystal with atomic details. Specific research area includes:
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Phase transitions in nanomaterials
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3D structure of individual metal, alloy, and semiconductor nanocrystals
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3D structure of soft materials in liquid
Nanomaterial synthesis and engineering applications
Understanding how nanomaterials grow and behave in operando condition and visualizing their structure in atomic level can serve as a key idea to develop the new synthetic strategy for nanomaterials with desired properties. Utilizing the mechanistic information learned from advanced TEM analysis and reconstructed 3D structure (Section 1 and 2), we develop precise synthesis of nanomaterials that are applied in catalytic, electronic, and optical fields. For catalysis, we delicately engineer the nanoparticle formation step during the synthesis to make heterogeneous catalysts with selective exposure of highly active surfaces and important grain boundaries. We also tackle and engineer the origin of the high catalytic activity by directly visualizing the critical catalytic events relevant to active site formation, sintering, and interface formation. Our synthetic approaches based on mechanistic behaviors of nanoparticles can be further applied to design optically active materials such as atomic clusters and 2D nanomaterials. Specific research areas include:
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Correlation between nanoparticle atomic structure and catalytic activity
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Direct observation of structural evolution during catalysis using in situ TEM
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Precise structure control of nanoparticles for high catalytic activity
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Synthesis and analysis of 2D nanomaterials for optical/electronic applications
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Single particle spectroscopy of optically active nanoparticles
Development of MEMS devices for advance electron microscopy
Cryogenic electron microscopy (cryo-EM) is a state-of-the-art technique to preserve and characterize the native structure of soft macromolecules in a liquid environment, whereas structures of the macromolecules usually tend to denature at normal dry imaging condition. By rapidly ‘fixing’ these molecules in vitreous ice at cryogenic temperature, structures of interest are maintained as if they are suspended in a native liquid environment. While cryo-EM has been mainly used for proteins and virus, it can be also exploited to characterize other materials whose structures are easily degraded by highly energetic electrons in TEM, throughout the field of biomedicine, battery, and heterogeneous catalyst.
Liquid-phase TEM is another main methodology used for directly visualizing nanoscale structures and chemistries in liquid condition. To preserve liquid specimens from high-vacuum conditions inside a TEM, we fabricate and use ‘liquid cells’ made up of either electron-transparent windows. Although these technologies are becoming matured, there are still bottlenecks to improve limiting the wide utilization in many research areas. Our lab is focusing on developing advanced MEMS-based devices for reliable and reproducible cryo-EM and liquid-phase TEM, as well as a computational algorithm for data analysis. Specific research areas include:
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Development of MEMS devices for the next-generation cryo-EM of materials in structural biology and battery science
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Development of advanced GLC for quantitative and reproducible liquid phase TEM
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Development of automated image analysis algorithm for big-data processing