Optical studies of capped quantum dots

Abstract

This thesis describes the synthesis and spectroscopy of CdSe and CdTe semiconductor quantum dots (QDs). The first chapter gives an introduction into the unique size dependent properties of semiconductor quantum dots. Highly luminescent QDs of CdSe and CdTe were prepared via a high temperature method in a glovebox. These QDs are soluble in organics but can be transferred into water or ethanol after exchange of the surfactants by various thiols (HS-R). For CdTe the exchange with thiols has a beneficial effect on the luminescence properties: a higher quantum efficiency (up to 60% in water), less defect related emission and a monoexponential decay were obtained (chapters 2 & 3). These results are compared to CdSe QDs: thiols that increase the luminescence of CdTe QDs can quench the luminescence of CdSe QDs. The difference in responding to the thiol molecules of these two comparable semiconductors is explained by the absolute positions of the band gap. Trapping of the photogenerated hole is a well known process for CdSe but can not occur for CdTe since the valence band of CdTe is positioned at higher energy (chapter 4). Hexanethiol capped CdTe QDs were used to investigate the influence of the refractive index on the radiative decay rate. Much theoretical work concerning this relation has been published, but the experimental work was limited to Eu3+ ions (a weak dipole moment transition). We show that QDs (with a strong dipole moment transition) are an ideal test case to study the influence of local field effects on the spontaneous emission rate and find a weak dependence on the refractive index. This is in good agreement with a recent developed ‘fully microscopic’ model (chapter 5). If a luminescing material is cooled down, the quantum efficiency (QE) increases (temperature quenching). Surprisingly we observed a sharp increase in QE (around 250 K) upon heating for alkylamine capped CdSe QDs (temperature antiquenching). The exact transition temperature shifts systematically to higher temperatures as the length of the alkylamine chain increases. This is ascribed to a phase transition in the capping that hampers surface reconstruction which is needed for a high QE. For water-soluble CdTe QDs an even stronger temperature antiquenching is observed: light can be switched on and off in a temperature window of 15 K (around 260 K). Here a phase transition in the solvent (from water to ice) is responsible for this remarkable temperature behaviour (chapter 6 & 7). Finally energy transfer between CdTe QDs in a QD solid was studied as function of temperature. Since the excitonic emission of QDs can occur via the singlet state or a (lower lying) triplet state (comparable to dyes) the luminescence decay rate of a QD is temperature dependent. Efficient ET occurs from QDs with a larger band gap to nearby QDs with a smaller band gap. We show a good agreement between the ET rate and the radiative decay rate as function of temperature since both radiative emission and energy transfer are dependent on the strength of the donor dipole moment (chapter 8).