School of Chemistry - Theses
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ItemSpectroelectrochemistry of Semiconductor Nanocrystals( 2021)Semiconductor nanocrystals exhibit well-known, size-dependent optical and electronic properties. Control over the charge carriers in semiconductor nanocrystals enables the possibility to tune the optical response. One way to achieve this is through electrochemistry. Carrier modulation through electrochemical methods allows more precise control over electron transfer compared to methods such as photocharging and chemical redox reactions. By combining electrochemistry with spectroscopic techniques, the charged states in semiconductor nanocrystals can be studied in detail. A spectroelectrochemical setup has been developed to study the charging of semiconductor nanocrystals in solution and its influence on absorption and photoluminescence (PL). A negative trion state can be generated in CdSe quantum dots (QDs) and stabilised for hours under an applied cathodic potential. By monitoring both the absorbance and fluorescence changes, one can determine whether charge carriers are free or trapped. The total number of electrons injected into the QDs can be estimated from current and coulometry measurements. Hole injection into CdSe QDs induces corrosion of the lattice, whereas injection into nanocrystals shelled with CdS induces bleaching. Coupling the spectroelectrochemical setup with time-resolved PL measurements reveals the trion lifetime of CdSe/CdS QDs as a function of shell thickness. In the last section of the thesis, the effects of charge injection on CdSe nanoplatelets (NPLs) is explored. In contrast to QDs, hole injection into the NPLs enhances the photoluminescence.
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ItemSynthesis and Modification of ZnSe Nanoplatelets( 2021)Nanoplatelets (NPLs) are a class of nanoparticles which have garnered significant interest in the research community. Unfortunately, much of the focus has been on the usual workhorse material: cadmium selenide. Zinc selenide is a close relative of cadmium selenide, both belonging to the II-VI family of semiconductors, but little research exists on ZnSe NPLs beyond its initial synthesis. In this thesis, ZnSe NPLs are addressed from the bottom up. In Chapter 2, the formation mechanism of ZnSe NPLs and MSCs is investigated. The evolution of nanoparticles in the reaction is monitored while exploring the reaction space. It is demonstrated that the concept of surface reversibility can be used to predict the formation of NPLs and MSCs. Additionally, it is found that MSCs and NPLs compete in the reaction, and selective formation can be induced by varying selenium precursor and the ripening behaviour of the ligand. Along the way, five unreported ZnSe magic-sized clusters (MSCs) are found. Chapter 3 of the thesis is a demonstration of Mn 2+ doping into the ZnSe and ZnS NPLs. Mn 2+dopant incorporation can be confirmed via its photoluminescence and photoluminescence excitation spectra and the photostability is measured. Additionally, the unique Mn 2+ emission is used a probe to investigate the evolution of various ZnSe species. Finally, Chapter 4 is concerned with the post-synthetic shelling of ZnSe nanoplatelets. ZnSe NPLs as synthesized from literature are colloidally and photo-unstable. A common solution to this is to coat the surface of the nanocrystal with a suitable semiconductor material. By modifying the process introduced for CdSe NPLs, the synthesized ZnSe NPLs are shelled successfully via colloidal atomic layer deposition (C-ALD). This allows us to improve its photoluminescence properties and observe unique features associated with Type-II ZnSe/CdS heterostructures.