Characterization and Application of Functional Ⅲ-Ⅴ Semiconductor Magic Sized Clusters

Abstract

Comprehending the transition between molecular and bulk materials has great importance in the field of discovering new materials and novel applications. Semiconductor magic sized clusters (MSCs) often form as extremely small -sized intermediate species during the growth of semiconductor quantum dots (QDs) and manifest definitive structures and unique characteristics which lie between molecules and larger sized QDs. Thus, investigation of MSCs can provide profound insights about the transition from molecules to QDs. Furthermore, MSCs have specific electronic structures with enhanced stability compared to similar sized QDs with a diameter ranging 1-2 nm, opening up the possibility of exploiting MSCs as photocatalysts. Main subject of this thesis is characterization and application of functional III-V semiconductor MSCs. At first, the roles of MSCs in the transition from molecules to QDs are studied. Two different zinc incorporated InP MSCs (InP:Zn MSCs), F393-InP:Zn MSCs and F360-InP:Zn MSCs, were exploited as precursors for synthesis of QDs. To initiate a reaction, typical molecular precursors containing indium, zinc, phosphorus were reacted with F393-InP:Zn MSCs or F360-InP:Zn MSCs. During QD synthesis, MSCs can mainly deliver indium-octadecylphosphonate (In-ODPA) complex, which was used for the growth of QDs. In the heat up process, F393-InP:Zn MSCs can release In-ODPA complex at a higher temperature than F360-InP:Zn MSCs because F393- InP:Zn MSCs have higher thermal stability than F360-InP:Zn MSCs. Then, In-ODPA complex could suppress the Ostwald ripening and yield narrow size distributed QDs. F360-InP:Zn MSCs provided In-ODPA complex at an earlier stage of QD synthesis and MSC recreation occurred. Low concentration of F360-InP:Zn MSCs produced relatively narrow size distributed QDs due to the In-ODPA complex effect. But for high concentration, more MSCs generated smaller and broader size distributed QDs by the competitive precursor consumption. For an excessively large amount, MSC recreation exceeded QD nucleation and completely blocked the QD production. Due to atomically precise features of MSCs, MSCs have been used as model systems for understanding interrelations between structure/composition and electronic property. InP MSCs series (F360-InP, 386-InP, and F399-InP MSCs) and InP:Zn MSCs series (F360- InP:Zn, F393-InP:Zn, and F408-InP:Zn MSCs) were studied by the cyclic voltammetry (CV) method to investigate how their size, composition, ligand, and structure affect the electronic structures. We found that the existence of zinc in InP:Zn MSCs alter electronic structures significantly compared to InP MSCs. Lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of InP:Zn MSCs are positioned at lower levels compared to InP MSCs. In addition, zinc in InP MSCs resulted in extra hole states above HOMO energy levels by a p-type dopant effect producing additional reduction peaks. A highly diastereoselective [2+2] cycloaddition of olefin substrates is demonstrated using MSCs as the photocatalysts without any sacrificial agents and co-catalysts. MSCs surfaces make the substrates of 4-vinylbenzoic acid align in syn-direction leading to high diastereoselectivities up to 99% and it exceeded the result of iridium photocatalysts. Also, due to the extremely small size of MSCs, MSCs can efficiently drive the triplet energy transfer to catalyze the cycloaddition reaction showing the product yields up to 96%, comparable to the results of QDs having more ligand binding sites than MSCs. Various MSCs types which have carboxylate, halide, or phosphonate ligands were used to catalyze the cycloaddition reaction. It was found that the strongly bound halide or phosphonate ligands could reduce the catalytic activities or the number of active sites on the surface of MSCs, reducing the product yields. One of the MSC catalysts with phosphonate ligand were recyclable at least two cycles and surface modification such as shell-overcoating can enhance the recyclability of the MSC photocatalysts.