Ji-Hwan Lee

Solution-based synthesis of anisotropic metal chalcogenide nanocrystals and their applications

This article reviews recent advances in solution phase synthesis to generate 1-D and 2-D anisotropic metal chalcogenide (MC) nanostructures with a focus on using different growth mechanisms to control the shapes of the MCs. Four different synthetic approaches have been reviewed: naturally favoured growth due to its intrinsically anisotropic crystal structure, modified anisotropic growth by changing surface energies or utilizing organic templates, oriented attachment of small nanocrystal building blocks to form nanowires or nanosheets, and chemical transformation from existing nanostructures into new species. We discuss current understanding of the thermodynamic and kinetic aspects associated with the mechanisms of forming these anisotropic MC nanostructures. We provide examples of representative applications of anisotropic chalcogenide nanomaterials that are expected to be practically meaningful in the near future. The applications include electrodes for lithium ion batteries, photodetectors, thermoelectric devices, and solar cells. A brief review of other potential applications (oxygen reduction reaction, localized surface plasmon resonance, topological insulator, superconductor) is provided as well. This review ends with discussions on the challenges to be investigated thoroughly in the solution-based synthesis of anisotropic nanomaterials, which includes surface energy control, correcting the nucleation & growth mechanism, removal of the organic surfactant, kinetic study on the chemical transformation, scale-up of production, and eco-friendly synthesis.

Influence of Rb/Cs Cation-Exchange on Inorganic Sn Halide Perovskites: From Chemical Structure to Physical Properties

CsSnI3 is a potential lead-free inorganic perovskite for solar energy applications due to its nontoxicity and attractive optoelectronic properties. Despite these advantages, photovoltaic cells using CsSnI3 have not been successful to date, in part due to low stability. We demonstrate how gradual substitution of Rb for Cs influences the structural, thermodynamic, and electronic properties on the basis of first-principles density functional theory calculations. By examining the effect of the Rb:Cs ratio, we reveal a correlation between octahedral distortion and band gap, including spin–orbit coupling. We further highlight the cation-induced variation of the ionization potential (work function) and the importance of surface termination for tin-based halide perovskites for engineering high-performance solar cells.

The effect of Se doping on the growth of Te nanorods

In this study, we successfully conducted a series of computationally-assisted experiments, regarding the morphology control and chemical transformation of Te nanorods. The morphology of Te nanorods is controlled by introducing a minute amount of isovalent Se dopant. Density-functional theory calculations predicted the Gibbs surface free energy change due to the adsorbent Se on the major facets of Te nanorods. Encouraged by the theoretical prediction, we conducted experiments on Te nanorod growth and did find significant variation of the morphology of Te nanorods due to Se injection. Furthermore, we demonstrated the chemical transformation of the shape-controlled Te nanorods to binary thermoelectric compounds such as PbTe and Bi2Te3 without losing the tailored morphology. The transformed PbTe and Bi2Te3 have nanoscale grain boundaries as seen from the cross-section HRTEM image. We emphasize that the robust production of morphology-controlled thermoelectric nanorods can be an optimal approach to develop an advanced thermoelectric composite material, by which the multiscale phonon scattering effect can be maximized.

Anisotropic vacancy-mediated phonon mode softening in Sm and Gd doped ceria

Ceria doped with Sm and Gd (SDC and GDC) has been suggested as a promising candidate for the electrolyte used in solid oxide fuel cells (SOFCs), since it has relatively high oxygen ion conductivity at intermediate temperature. There have been many previous experimental and computational studies to investigate the properties, structure, and effect of vacancies, etc. for SDC and GDC. However, in these previous studies, it is commonly assumed that the interaction between oxygen vacancies is negligible and many focus only on the mono-vacancy system. In addition, the possibility of anisotropic vibrational motion of the oxygen ions around vacancies is often neglected. In this paper, using both first-principle density-functional theory and classical molecular dynamics calculations, we investigate the structural and vibrational properties of the optimized SDC and GDC structures, such as bonding analysis, phonon density-of-state and mean-square-displacement of the oxygen ions. Also, we report the direction-dependent vibrations at the specific frequency of the oxygen ions near the vacancies, activation energies, and diffusion coefficients of SDC and GDC which can extend our understanding of diffusion dynamics in doped ceria-based electrolytes for SOFC applications.