Exciton Dynamics in InP Quantum Dots: Fine Structure and Radiative Recombination Processes

Abstract

Colloidal quantum dots (QDs) are semiconductor nanocrystals with size- and shape- tunable optical properties, fundamentally different from those of the bulk, and of significant interest in optoelectronics. For decades, CdSe-based QDs have been the workhorses in this field and they have reached a very mature level with a bright and size-tunable photoluminescence in the visible range, making them very suitable for applications in LEDs, displays and TV screens. However, Europa’s legislation does not allow the use of Cd-containing materials for opto-electronic applications. For this reason, the implementation of QDs in commercial devices requires to replace these QDs by toxicologically harmless materials, which should have a similar optical performance. In this respect, the development of III-V QDs, especially of InP, has received increasing attention in recent years. Colloidal hetero-nanocrystals composed of InP cores show a reasonable photoluminescence quantum yield, a size-tunable emission spectrum in the visible range and a good chemical stability, provided that they are coated with a suitable inorganic shell material. At present, suspensions of well-passivated InP core/shell QDs are prepared by wet-chemical synthesis and this synthesis can be scaled up to industrial production. They show bright emission ranging from the near-IR (1.5 eV) to the green (2.5 eV) spectral range with near-unity photoluminescence quantum yields. It is thus very important to investigate the energy-level structure and optical properties of this promising class of nanocrystals. The optoelectronic properties of InP core/shell QDs are determined by the chemical nature of both the core and the shell materials, the strain induced on the core due to core/shell epitaxy, and the relative offsets between the conduction and valence band edges of the core and shell materials. The electron and hole can be both localized within the core, or one carrier can be localized in the core and the other in the shell. For all these reasons, the nature of the shell has a strong influence on the radiative processes. This thesis aims to investigate these aspects in detail, providing a fundamental understanding of the physical mechanisms of spontaneous emission by unravelling the exciton fine-structure of InP core/shell nanocrystal quantum dots, and by studying how the exciton recombination depends on the size of the nanocrystals, the shell composition and core/shell structure, with an emphasis on the active role of the different types of phonons interacting with dark exciton states.