Highly luminescent CdSe and CdTe quantum dots (QDs) are prepared in a hot solvent of capping molecules (TOP/TOPO/HDA for CdSe and TOP/DDA for CdTe). The influence of exchange of the capping molecules with different types of thiol molecules (amino ethanethiol, (3-mercaptopropyl)trimethoxysilane, hexanethiol, 2-propenethiol, and 4-mercaptophenol) is investigated for both CdSe and CdTe QDs. A remarkable difference is observed: capping exchange with thiol molecules results in an increased luminescence efficiency for CdTe QDs but induces quenching of the excitonic emission of CdSe QDs. The striking difference between the two types of II-VI QDs is explained by the difference in the energy of the valence band top. The lower energetic position of the valence band for CdSe results in hole trapping of the photogenerated hole on the thiol molecule, thus quenching the luminescence. For CdTe the valence band is situated at higher energies with respect to the redox level of most thiols, thus inhibiting hole trapping and maintaining a high luminescence efficiency.

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.

First published as an Advance Article on the web 9th 2000 No¿emberThe luminescence properties of nanocrystalline ZnSe : Mn2`prepared via an inorganic chemical synthesis are described. Photoluminescence spectra show distinct ZnSe and Mn2`related emissions, both of which are excited via the ZnSe host lattice. The Mn2`emission wavelength and the associated luminescence decay time depend on the concentration of Mn2`incorporated in the ZnSe lattice. Temperature-dependent photoluminescence spectra and photoluminescence lifetime measurements are also presented and the results are compared with those of Mn2`in bulk ZnSe.

A comprehensive investigation is presented on the factors governing the photoluminescence (PL) quantum yields (QYs) and size dispersion of colloidal CdSe nanocrystals. The temporal evolution of the ensemble PL properties (absorption and luminescence spectra, QYs and lifetimes) during growth at different temperatures (170-310°C) and different Cd:Se ratios was followed for several hours (2-6 h). The QY values increase during the growth to a maximum and, after a variable time interval (from minutes to hours, depending on the growth temperature), decrease gradually. Low QYs are due to poor passivation, surface disorder, and surface degradation, which arise at different stages of the growth. High QYs can be achieved and maintained only under an ideal combination of growth temperature, solvent composition, and Cd:Se ratio, which leads to an optimum surface. The overgrowth of a fresh surface layer restores high QYs to CdSe nanocrystals with decreased efficiencies because of surface degradation. The insight gained in this investigation enabled us to achieve a high degree of reproducibility and control over the emission color (green to red), bandwidth (90 meV), lifetimes (30 ns), and quantum yields (50-85%) of colloidal CdSe nanocrystals without any postpreparative treatment.

The influence of the synthesis conditions on the properties of nanocrystalline ZnS:Mn 2+ is discussed. Different Mn 2+ precursors and different ratios of the precursor concentrations [S 2-]/[Zn 2+ ] were used. The type of Mn 2+ precursor does not have an effect on the luminescence properties in the synthesis method described. On going from an excess of [Zn 2+ ] to an excess of [S 2-] during the synthesis, the particle diameter increases from 3.7 to 5.1 nm, which is reflected by a change in the luminescence properties. Photoluminescence measurements also showed the absence of the ZnS defect luminescence around 450 nm when an excess [S 2-] is used during the synthesis. This effect is explained by the filling of sulfur vacancies. The ZnS luminescence is quenched with an activation energy of 62 meV, which is assigned to the detrapping of a bound hole from such a vacancy.Over the past few years, considerable interest in the novel optical and electrical properties of doped semiconductor nanocrystals (NC) has emerged. 1-5 These structures are interesting from a physical and chemical point of view mainly because several of their properties are very different from those of bulk materials. 3 Especially, the significant sizedependent shift in the band gap has attracted much attention. This so-called quantum-size effect allows one to tune the emission and excitation wavelengths of a nanocrystal by tuning the crystal radius r. A quite good first-order approximation to calculate the energy of the band gap is given by the Brus equation. 1 In the case of zinc blende ZnS, the bulk values of all the materials parameters are known. 6 For nanocrystalline ZnS this results in a relation between the particle radius r, in nanometers, and the band gap E, in electronvolts, as follows:Manganese-doped materials represent a class of phosphors that have already found their way into many applications. The 4 T 1 f 6 A 1 transition within the 3d 5 configuration of the divalent manganese ion (Mn 2+ ) has been studied extensively and its orange-yellow luminescence in ZnS is well documented. This luminescence was also observed in nanocrystalline ZnS:Mn 2+ 7,8 and applications have already been suggested. 9-11 Different types of Mn 2+ centers are present in nanocrystalline ZnS:Mn 2+ . 12,13 The orange luminescence originates from Mn 2+ ions on Zn 2+ sites, where the Mn 2+ is tetrahedrally coordinated by S 2-.Previous workers have always used equal concentrations of Zn 2+ and S 2-precursors in the synthesis of these ZnS: Mn 2+ nanocrystals. This letter will focus on the effect of the synthesis conditions on several properties of these nanocrystals. Two different Mn 2+ precursors were used and the influence of the ratio of [Zn 2+ ] to [S 2-] was investigated. The experimental comparison will include measurements of the diameter, reflectivity, temperature-dependent photoluminescence (PL) emission and excitation as well as luminescence lifetimes. From these results, new insights into the ZnS-related luminescence are obtained and a qualitative...