Material Synthesis

Nanomaterials have attracted enormous interest during the last decades due to their distinct size-dependent properties. Various physical properties of nanomaterials can be controlled not only by their size but also by their morphologies, crystal structures, or surface states. Based on mechanistic information obtained from our advanced microscopic techniques, we aim to precisely synthesize various nanomaterials with desired structure and properties, and apply them in catalytic, electronic, and optical systems. 

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Nanocluster Synthesis

Metal nanoclusters have gained tremendous interest due to both their atomic precision and molecule-like optical properties. To produce nanoclusters with desired structure, it is important to identify the synthetic mechanisms of metal nanoclusters thoroughly in the molecular-level. We investigate the synthetic mechanisms of thiolate-protected Au nanoclusters with a special focus on analyzing the metal-ligand complexes. Understanding from these studies can be used to synthesize nanoclusters composed of various metals such as Ag, Pd, or Pt. We also focus on producing metal nanoclusters with desired luminescent properties and investigating their 3D structures with TEM-based 3D reconstruction methods to establish a structure-property relationship at the atomic level.

Structure Dynamics of MOFs and Their Applications

Flexible metal-organic frameworks (MOFs) have great potential for various applications including chemical sensing, drug delivery, and gas storage. Flexibility originates from reversible structural rearrangement of metal ions and organic ligands in response to external stimuli (guest adsorption, pressure, and light irradiation). Transformation behaviors of the flexible MOFs have widely been explored by theoretical calculations, gas sorption isotherms, and in situ X-ray diffraction. Although in situ transmission electron microscopy (TEM) is suitable for analyzing the structural dynamics, direct observation of the transition is still lacking because flexible MOFs are sensitive to the electron beam. Low electron dose can prevent structural deformation and trigger volume expansion/contraction. By using low-dose TEM imaging technique, we investigate the reversible transformation of flexible MOF in real-time. 

Structure and Stability of Quantum Dots

Colloidal quantum dots (QD) are promising candidate materials for next-generation light emitting devices. However, QDs typically suffer from suppression of luminescence properties by trap state emission, mainly originating from surface defects. Therefore, it is important to understand the surface structure of QDs and its effects on luminescence properties. We develop comprehensive analytical methods based on identical location transmission electron microscopy (ILTEM) and liquid phase TEM (LPTEM), which enable correlating photo-quenching processes with structural deformation. Furthermore, we are interested in revealing the complex 3D structures of individual QDs via Brownian one-particle reconstruction based on high-resolution liquid cell TEM. 

MOF-Nanocluster Composites

Metal-organic frameworks (MOFs), porous materials consisting metal nodes and organic ligand linkers, are highly capable of forming composite materials with functional properties. Photo-luminescent metal nanoclusters are to be used in various ways but enhancing efficiency and stability in luminescence is important for their use in applications. Our research goal is to achieve high PL efficiency in a wide range of wavelengths by synthesizing MOF-nanocluster composite materials. We are also interested in tuning metal-to-ligand ratios, which are expected to show various PL properties depending on the extent of freely-rotating ligands.