Theoretical Study on Crystal Structure and Morphology Control of Semiconductor Nanomaterials via Multiscale Simulation

Abstract

Crystalline solids are prevalent materials used in industrial applications and daily life. By controlling the crystal structure and morphology of the materials, the manufacturing process of high-value-added products can be improved. Crystal structure and morphology are influenced by both internal and external factors, where internal factors refer to the modification of the chemistry at a molecular level and external factors refer to the aspects beyond the crystal itself (i.e., temperature, pressure, concentration, solvent types, and ligand interaction). Depending on the specific molecular chemistry, the crystal structure and morphology of a material can be altered. Meanwhile, external factors can be manipulated to change the structure or morphology of a substance during crystallization. Moreover, they influence the thermodynamics (i.e., interaction between crystal surface and ligand) and kinetics (i.e., reaction rate) of reaction, resulting in different crystallization paths. During crystallization process composed of nucleation and crystal growth stages, numerous physicochemical properties of the materials are determined. Consequentially, thermodynamic, electronic, optical, and catalytic properties are dependent on the crystal structure and morphology. In this dissertation, the crystal structure and morphology of semiconductor nanomaterials applied in electrochemical energy storage and optoelectronic fields were extensively studied. Various processes were used for the modification of these properties and in each case the principal mechanisms involved was investigated using multiscale simulation. In Chapter 1, a brief background on crystallization is given, including information on internal and external factors that influence the crystal structure and morphology during crystallization. We discuss how a variety of physicochemical phenomena can be investigated using a multiscale simulation approach that includes density functional theory (DFT) calculation, Monte Carlo (MC) simulation, molecular dynamics (MD) simulation, and morphology model. In Chapter 2, we designed the crystal structures of these nanomaterials (i.e., solid solution and co-crystal methods) by focusing on contorted hexabenzocoronene (cHBC), fluorinated cHBC, and fullerene (C60) molecules. Also, the detailed Li-ion storage mechanisms of each electrode materials were explored. Crystal structures vary depending on the molecules and the specific elemental composition, which results in materials exhibiting different electrochemical behaviors. These differences can be attributed to the difference in adsorption sites of Li-ions. In Chapter 3, using solvent engineering, the crystal structure and morphology were controlled by manipulating the crystallization process in perovskite materials. In dimethylformamide (DMF) solvent environment, DMF molecules have a relatively strong coordination with PbI2 due to the carbonyl group, resulting in PbI2·DMF intermediate phase. Through slow nucleation and crystal growth in closed system, the intermediate phase of PbI2·DMF crystal exhibited the one dimensional (1D) granular wire shape. From this granular wire morphology, many defects and grain boundaries appeared on the surface. Due to its surface characteristics of granular wire morphology, the ultrahigh photo-detectivity can be obtained from the easy generation of deep trap states at surface and upward band bending at grain boundaries. In Chapter 4, the morphological changes of zinc-blended structures in ZnSe, the shell material of quantum dots, were ascribed to the interaction of Zn with the oleate ligand. Due to the strong adsorption of oleate on the (111) surface, a tetrahedron shape around the (111) surface was formed. As the reaction temperature increased by heating-up method, the stabilization effect of the oleate decreased, resulting in morphological changes in the form of a truncated tetrahedron. Depending on the morphology, different optical performance of QDs can be expected due to morphology-dependent quantum confinement effect. Overall, these studies demonstrated the design and control strategies of crystal structure and morphology of semiconductor materials. Crystal engineering, solvent engineering, and morphological engineering are expected to improve the electrical and optical performances of each semiconductor material.