Author ORCID Identifier

https://orcid.org/0000-0003-1416-5212

Date Available

12-21-2023

Year of Publication

2022

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department/School/Program

Chemical and Materials Engineering

First Advisor

Prof. Fuqian Yang

Abstract

Colloidal semiconductor quantum dots (QDs), also called as nanocrystals (NCs), are a class of functional materials with extraordinary properties, which are different from their bulk counterparts and benefit from their exclusive quantum confinement (size) effect. Semiconductor exhibits the quantum confinement effect when the characteristic size of the semiconductor is comparable to or smaller than the de Broglie wavelength of the electron wave function and/or the exciton Bohr diameter of the bulk semiconductor. In recent years, metal halide perovskite NCs, as next-generation semiconductor materials for lighting and display, have aroused a wide attention due to their excellent optoelectronic properties. However, traditional methods for synthesizing these materials involve toxic solvents and pose a potential threat to the environment, health and security. Alternatively, using environmentally benign solvents is the best approach to address this issue from the viewpoint of sustainability. In this dissertation, green-route approaches are implemented for the preparation of both lead and lead-free halide perovskite NCs and kinetic analysis for the synthesis of lead-based NCs in a microfluidic platform is presented. As a potential component of flexible liquid crystal displays (LCD) backlights, perovskite nanocrystal films are facing the challenge issue of deformation-induced shift of photoluminescence (PL), leading to an image distortion. In sight of this issue, environmental-friendly solvent (deionized (DI) water)-driven CsPbBr3 NCs and MAPbBr3 (MA = CH3NH3) NCs were prepared. Both NCs possess high photoluminescence quantum yields (PLQY) of ~80% and ~94%, respectively. The poly(methyl methacrylate) (PMMA)-NC films exhibit bending-endurable (under a tensile strain of 3.97%) and compression-resistant (under a compressive stress of 1.6 MPa) PL for the films made from the prepared CsPbBr3 NCs and MAPbBr3 NCs, respectively. The PMMA-MAPbBr3 NC films integrated with a red-emitting film and a blue-emitting light source achieve an LCD backlight of ~114% color gamut of National Television System Committee (NTSC) 1953 standard. As an alternative to replace the toxic element of lead (Pb) in the metal halide perovskites, blue emitting Sn-based lead-free halide perovskite NCs are becoming more and more promising in the area of lighting and display. Using DI water and small amount of inorganic acid as solvent, large-scale synthesis of Cs2SnCl6 and Cs2SnI6 NCs is achieved with subsequent ultrasonication. The highest PLQY is 13.4% for the Cs2SnCl6 NCs with a volume ratio of oleic acid (OA) to oleylamine (OAm) of 40 μL to 10 μL. The PL-peak wavelength, PL-peak intensity and quantum yield can be tailored by changing the volume ratio of OA to OAm. The Cs2SnI6 NCs with a PLQY of ~7% possess a PL peak at 440 nm. Both the Cs2SnCl6 and Cs2SnI6 NC solutions exhibit long-term optical stability over 60 and 50 days, respectively, in ambient environment. A microfluidic platform was employed for continuous fabrication of CsPbBr3 NCs. A simple relation between total flow rate and the bandgap of the NCs (without the contribution of Coulomb interaction to the quantum confinement effect) for the synthesis of the NCs is proposed to analyze the growth behavior of the NCs. It is concluded that a larger total flow rate results in a smaller average size of the CsPbBr3 NCs and a lower temperature and a larger total flow rate will narrow the size distribution of the NCs.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2022.456

Funding Information

This study was support by the NSF (National Science Foundation) through the grant CMMI-1854554 (start from 07/01/2019) and CBET- 2018411 (start from 08/01/2020).

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