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Abstract

Colloidal semiconductor nanocrystals (NCs) or Quantum Dots (QDs) have seen tremendous development in the past 25 years as solution processed semiconductors for optoelectronic applications. While colloidal semiconductors of the II-VI family have reached the stage of commercialization, their III-V cousins, arguably the most important class of semiconductors, are still lagging behind in their colloidal form. One such class of semiconductors is the III-V compounds such as InP, InAs, GaAs, GaP etc. In this thesis, I present my work on the development of new synthetic methodologies for a variety of III-V semiconductors such as InAs, GaAs, InxGa1-xP, InxGa1-xAs etc. These new methodologies enable the preparation of high quality III-V QDs whose structural and optical properties are then studied from the perspective of their applications in optoelectronic devices. In Chapters 2 and 3, novel syntheses of InAs QDs from aminoarsine precursors are discussed. In Chapter 2, the activation of aminoarsines using a reducing agent for the synthesis of InAs QDs is discussed and this general synthetic route for the synthesis of other arsenide materials is explored. In Chapter 3, the role of reducing agents in governing the precursor conversion kinetics is elucidated and an improved reaction scheme for the synthesis of monodisperse InAs QDs is shown. The emission properties of these near-IR emitting materials are also demonstrated by growing appropriate wide band gap shells on them. In Chapter 4, the synthesis and properties of colloidal GaAs NCs are discussed. It describes a novel synthesis protocol for colloidal GaAs nanocrystals for a range of sizes and discusses the structural properties of the NCs obtained via this approach. The anomalous optical properties of GaAs are rationalized from a structural point of view. Chapter 5 is devoted to the exploration of nanocrystal colloids in molten salts and ionic liquids. The dispersions of NCs, III-V and others, are prepared in a variety of molten salt eutectics and their physical state is probed in depth via synchrotron X-ray scattering studies. These measurements help gain a better understanding of the mechanism of stabilization of NCs in molten salts and ionic liquids and demonstrate their potential as non-traditional solvents for colloidal chemistry. In Chapters 6 and 7, I discuss the applications of molten salts as high temperature solvents for the synthesis and processing of colloidal III-V QDs. A defect annealing protocol for GaAs QDs using molten salts as high temperature solvents is proposed in Chapter 6. The annealing of defects at high temperature was shown to improve the optical properties of GaAs NCs. Chapter 7 describes the synthesis of ternary III-V alloy QDs (InxGa1-xP and InxGa1-xAs) in molten salts via a cation exchange approach. It is demonstrated that molten salts enable the formation of these alloys which are otherwise difficult to synthesize via direct colloidal routes. These QDs show quantum confined absorption and emission features. The optical properties of these alloy nanocrystals are studied in depth and the potential of InGaP QDs as a material for display applications is assessed in comparison to InP QDs.

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