Semiconducting nanocrystals: synthesis, colloidal stability, and charge carrier dynamics

Abstract

Colloidal nanocrystals (NCs) with controllable morphologies and sizes are at the core of nanomaterials research. Among different NCs (zero-dimensional quantum dots, one-dimensional nanorods, etc.), two-dimensional cadmium selenide (CdSe) nanoplatelets (NPLs) with lateral extension of over 100 nm2 and atomically precise thickness are one of the most promising semiconductors for optoelectronic applications. However, several issues remain for implementing CdSe NPLs for applications, including: (1) containing toxic and heavy elements, and (2) lacking a fundamental understanding of the colloidal stability and the process of charge carrier generation in solids. This thesis aims to synthesize environmentally friendly alternatives, such as indium phosphide (InP) NCs with varied shapes, and shed light on colloidal stability and charge carrier generation and dynamics using conventional CdSe NPLs as the model system. The introductory Chapter 1 briefly presents an overview of nanomaterials and nanotechnology, including quantum confinement in NCs, quantum dots, InP NCs, the synthesis status of CdSe NPLs, colloidal NCs stabilization in dispersion, photophysics of CdSe NPLs, ligand exchange method, and the thermal annealing method. In Chapter 2, we introduce terahertz (THz) spectroscopy basics, including the generation and detection mechanism, experimental setups, and the conductivity models for data analysis to infer charge transport properties in materials of interest. Chapters 3-5 present the key results of this thesis. In Chapter 3, we synthesize InP NCs with different shapes and sizes by utilizing triphenyl phosphite as the phosphorus source. We show that InP NCs can be synthesized and formed from In2O3 NCs. Our findings open new synthetic possibilities by utilizing a cost-effective, non-pyrophoric, and non-toxic phosphorus precursor. In Chapter 4, we combine THz photoconductivity measurements with simulations to study the colloidal stability and aggregation process in CdSe NPL dispersion. We demonstrate that increasing the facet area of NPLs and the solvent length can reduce the critical concentration at which NPLs start aggregating. Our simulation attributes the effect to the solvation force-dominated colloidal stability for the NPL system; the solvation force is enhanced by increasing the NPL lateral area and the length of solvent molecules. In Chapter 5, we report that thermal annealing constitutes an effective strategy to control the optical absorption and electrical properties of CdSe NPLs by tuning the inter-NPL distance. We observe a direct correlation between the temperature-dependent ligand decomposition and the NPL-NPL distance shortening (by TEM). This leads to a strong red-shift in the absorption band edge (by UV-vis studies) and enhanced electrical transport Abstract in NPL films. Our results illustrate a straightforward manner to control the interfacial electronic coupling strength for developing functional optoelectronics through thermal treatments.