Colloidal CdTe quantum dots prepared in TOP/DDA (trioctylphosphine/dodecylamine) are transferred into water by the use of amino− ethanethiol•HCl (AET) or mercaptopropionic acid (MPA). This results in an increase in the photoluminescence quantum efficiency and a longer exciton lifetime. For the first time, water-soluble semiconductor nanocrystals presenting simultaneously high band-edge photoluminescence quantum efficiencies (as high as 60% at room temperature), monoexponential exciton decays, and no observable defect-related emission are obtained.

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The growth of highly luminescent CdTe nanocrystals synthesized in a mixture of tri-octylphosphine and dodecylamine was investigated. The CdTe nanocrystals grow in time to a constant size which is dependent on the reaction temperature. In the initial stage of the growth particles show a broad defect emission band which is replaced by an efficient excitonic emission after a few minutes of growth. Quantum yields up to 45% were obtained. The life time of the exciton emission increases with quantum yield and reaches a constant value of about 10 ns for the most efficient particles. The life time increase is explained by the reduction of the (fast) non-radiative decay for samples with a higher quantum yield. In CdTe samples aged at room temperature sharp absorption peaks at discrete energies were observed. These were ascribed to small clusters of CdTe which form at room temperature from unreacted precursors. r

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Luminescence Luminescence D 6540Luminescence and Growth of CdTe Quantum Dots and Clusters. -Highly luminescent CdTe quantum dots (2.4-2.9 nm in diameter) are prepared from CdMe2 and Te with a Cd/Te ratio of about 1.25 in a mixture of dodecylamine and tri-octylphosphine at three different temperatures (145, 165, and 180°C). The growth of the particles as a function of time is monitored by absorption and emission spectroscopy and by quantum efficiency and luminescence life time measurements. Highly luminescent quantum dots with a quantum efficiency of up to 45% are obtained after several hours. Particle growth is fast in the first 30 min. The final size of the particles is reached after three hours and is controlled by the reaction temperature. The CdTe particles with the highest quantum efficiency have a luminescence lifetime of ≈10 ns. -(WUISTER*, S. F.; VAN DRIEL, F.; MEIJERINK, A.; Phys.

Control over spontaneous emission of light is of great importance in quantum optics. It is essential for diverse applications ranging from miniature lasers and light-emitting diodes, to single-photon sources for quantum information, and to solar-energy harvesting. It was realized by Yablonovitch and John that 3D periodic dielectric structures, known as photonic crystals, can be used to control spontaneous emission as well as propagation of light.We present experimental studies on spontaneous emission of CdSe quantum dots (QDs) embedded in 3D photonic crystals consisting of air spheres in titanium dioxide (see Fig. 1). The influence of Bragg diffraction on the QD emission is confirmed by close agreement between measured direction-dependent spectra and a recently developed theory [1]. Performing time-resolved experiments, we show that the photonic crystals control the emission decay rate of excitons confined in the QDs [2]. By varying the lattice parameter of the photonic crystals, we demonstrate both broadband inhibition and enhancement of the decay rate for an ensemble of QDs (see Fig. 2). The decay curves are modeled by a distribution of decay rates, from which we extract an averaged decay rate and the width of the distribution. The variations in the width of the distribution with lattice parameter indicate that individual QDs experience even larger decay-rate modifications than the ensemble average. 2 -reference sample 3 3 -enhanced decay rate --1 0 0 0 -0 20 40 60 80 Fig. 1. Scanning electron microscope image Time [ns]ofthe (I 1) face of a titania inverse opal. Fig. 2. Fluorescence decay curves of quantum dots inside three The air spheres are seen as grey holes in the photonic crystals with different lattice parameters. The solid white TiO2 backbone material.lines are fits of the decay curves with a decay-rate distribution model.[1] I.S.Nikolaev, P.Lodahl, and W.L.Vos, submitted: http://arxiv.org/abs/physics/0410056.