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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.
Particularly, luminescent nanomaterials are of significant importance in various commercial applications including color displays, bioimaging and solid-state lighting. To achieve high performance of such applications, synthesis of nanomaterials with outstanding optical properties is essential. We aim to synthesize various optically active nanomaterials with bright photoluminescence of tunable colors and high stability under harsh conditions. We also focus on investigating their structures with TEM-based 3D reconstruction methods to establish a structure-optical property relationship at the atomic level. Our research interests encompass InP-based quantum dots, fluorescent / phosphorescent metal nanoclusters and their assembly, and 2D semiconductor materials.
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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 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.
2D Material Synthesis
Since the first discovery of graphene, 2D materials have emerged as attractive candidates for various electronic, optoelectronic, and catalytic applications. Particularly, 2D materials with atomic thickness present unique physical properties in terms of exciton dynamics, or the indirect-to-direct bandgap transition. In addition, the properties of 2D materials can be readily altered by controlling thickness, morphology, and composition. We utilize low pressure chemical vapor deposition (LPCVD) to synthesize atomically thin graphene that can be applied to liquid cells for in situ TEM. Furthermore, we produce transition metal dichalcogenides (TMDs) monolayer with wafer-scale uniformity by using conversion reaction of nanoparticles. Alloyed TMDs monolayer can also be synthesized with controlled composition, which results in a tunable bandgap energy.
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