Synthesis and characterization of lead-based core-shell-shell quantum dots and studies on excitation-dependent quantum yield measurement




Cao, Jieming

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Nano-sized semiconductors, known as quantum dots (QDs), are one of the hottest research areas in recent years. The energy gaps of QDs change with their diameters, giving them size-dependent optical properties. By controlling reaction conditions, people are able to make QDs that can emit in certain wavelength ranges. So far, QDs have shown great potential in telecommunication, bio-imaging, single-photon laser source, etc. This thesis starts with Chapter 1, which first introduces the finding of QDs and why they have such special properties. The quantum confinement and energy gap are discussed, followed by the absorption and emission of QDs. Moreover, the synthesis methods and mechanism involved are reviewed in brief. Chapter 2 presents the synthesis of lead-based core, core-shell and core-shell-shell QDs and previous work by other people. A few techniques including transmission electron microscopy (TEM), UV-absorption, photoluminescence (PL) measurement, and X-ray photoelectron spectroscopy (XPS) were used and shown in this chapter. Core-shell and core-shell-shell QDs are shown to present excellent stability over 20 months. The ZnS shell was proved by energy-dependent XPS and TEM measurements. A detailed discussion on quantum yield (QY) is given in Chapter 3. Absolute and relative QY measurements and some standard dyes are discussed. After that, Chapter 4 shows systematic QY measurements on lead-based core and core-shell QDs. For each type of QDs, at least two batches are selected with their emission spectra presented as well. It is revealed by collected data that they have excitation-dependent QYs. The QY drops as the excitation light increases in energy (higher wavelength), which is due to non-radiative decays from higher excited states or the Auger effects. QY of PbS and PbSe QDs can be as high as 50%. Eventually, the conclusion and future work are included in Chapter 5. All experimental work is described in detail in the experimental section.



Quantum dot, Quantum yield