Semiconductor nanoplatelet heterostructures enhanced via combinations of colloidal atomic layer deposition and hot injection shell growths

Abstract

One of the most promising families of semiconductor nanocrystals in colloidal optoelectronics and nanophotonics is considered to be colloidal quantum wells, also commonly referred to as nanoplatelets (NPLs). Possessing an atomically flat structure, NPLs feature unique properties including spectrally-resolved and tunable light-holeand heavy-hole transitions accompanied by their respective giant oscillator strengths. CdSe, CdS and CdTe, making the first colloidal NPLs synthesized in core-only structure, portray distinct qualities necessary for light-harvesting and -generating applications. However, going beyond the core structure, there are many properties that are highly enhanced by growing crown and/or shell layers around core NPLs. While the crown growth takes place anisotropically in lateral directions, the shell layer covers the entire NPL surface, combinations of which enable NPL heterostructures in new architectures. Depending on the electronic alignment of parts of the NPL heterostructure and the resulting confinement of electron-hole wave functions, these hetero-NPLs can be type-I or type-II. In type-I electron-hole pairs are confined in the core-NPL and recombination occurs in a direct pathway. In type-II electron-hole wave function is separated into different semiconductor layers, resulting in spatially indirect recombination. In this thesis, we synthesized and showed thin- and thick-shell grown heterostructures of type-I CdSe/ZnS NPLs using hot-injection (HI) for the first time particularly for these semiconductor NPLs. Unlike the typical colloidal atomic layer deposition (c-ALD) technique, which produces NPL heterostructures with low quantum yield (QY) and low chemical and optical stability, our approach yields CdSe/ZnS NPLs of almost unity (100%) quantum yield (QY) and improved chemical stability, tested by washing the same samples rigorously up to 6 times with ethanol with little change observed in the QY. Additionally, unparalleled thermal and optical aging endurances is achieved in aging tests. These tests experimentally demonstrated that, elevated to 400 K, HI thick-shelled NPLs can retain up to 65% of their emission intensity in the colloidal form and 52% of that in the film. This level of high stability creates a great opportunity for employing these NPLs for high-temperature applications. Also, in the thesis, we synthesized and studied CdS/CdSe core/crown, CdS/CdZnS core/c-ALD shell-grown and CdS/CdSe/CdZnS core/crown/c-ALD shell-grown heterostructures of NPLs. Here the starting-template CdS NPLs are considered to be unique in terms of their emission in the blue region, which may open up new opportunities for NPL lasing in this spectral region. Nominally CdS NPLs are folded due to great lateral sizes. However, in this research work, when coated with crown and shell layer, these particles unfold. The unrolled CdS/CdSe core/crown NPLs are found to exhibit relatively higher QY up to 15-20% in its class of CdS core-seeded NPLs. The findings of this thesis reveal that such heterostructures of the NPLs are very rich in terms of variety of the quantum architectures one can achieve using them as working model systems.