Ultrafast exciton dynamics in InAs/ZnSe nanocrystal quantum dots.

Colloidal nanocrystal quantum dots with a band gap in the near infra-red have potential application as the emitters for telecommunications or in vivo imaging, or as the photo-absorbing species in next generation solar cells or photodetectors. However, electro- and photoluminescence yields and the efficiency with which photo-generated charges can be extracted from quantum dots depend on the total rate of recombination, which can be dominated by surface-mediated processes. In this study, we use ultrafast transient absorption spectroscopy to characterise the recombination dynamics of photo-generated charges in InAs/ZnSe nanocrystal quantum dots. We find that recombination is dominated by rapid, sub-nanosecond transfer of conduction band electrons to surface states. For the size of dots studied, we also find no evidence of significant multiple exciton generation for photon energies up to 3.2 times the band gap, in agreement with our theoretical modelling.

Investigation of the internal heterostructure of highly luminescent quantum dot-quantum well nanocrystals.

In this paper, we report on the growth and characterization of quantum dot-quantum well nanostructures with photoluminescence (PL) that is tunable over the visible range. The material exhibits a PL efficiency as high as approximately 60% and is prepared by reacting ZnS nanocrystals in turn with precursors for CdSe and ZnS in an attempt to form a simple "ZnS/CdSe/ZnS quantum-well structure". Through the use of synchrotron radiation-based photoelectron spectroscopy in conjunction with detailed overall compositional analysis and correlation with the size of the final composite nanostructure, the internal structure of the composite nanocrystals is shown to consist of a graded alloy core whose composition gradually changes from ZnS at the very center to CdSe at the onset of a CdSe layer. The outer shell is ZnS with a sharp interface, probably reflecting the relative thermodynamic stabilities of the parent binary phases. These contrasting aspects of the internal structure are discussed in terms of the various reactivities and are shown to be crucial for understanding the optical properties of such complex heterostructured nanomaterials.

A highly luminescent ZnS/CdSe/ZnS nanocrystals-tetrapeptide biolabeling agent.

Stable colloidal ZnS/CdSe/ZnS nanocrystals capped with hexadecylamine were transferred into water by encapsulation in a protective shell of an amphiphilic polymer. The properties of the products have been investigated by dynamic lighting scattering, absorption, and photoluminescence spectroscopy. These nanocrystals, when conjugated with tuftsin in aqueous suspension, formed an effective labeling reagent for macrophages or lymphocytes. The labeling of cells was demonstrated by confocal microscopy. Hybrid bio-inorganic nanoconjugates are potentially powerful fluorescent tracking tools in biological systems.

Photorefractive performance of a CdSe/ZnS core/shell nanoparticle-sensitized polymer.

We report the photorefractive performance of a polymer composite sensitized by CdSe/ZnS core/shell nanoparticles, and also comprising poly(N-vinylcarbazole) and an electro-optic chromophore. The nanoparticles are characterized by absorption and photoluminescence spectroscopy, elemental analysis, transmission electron microscopy, and powder x-ray diffraction. The electro-optic response of the composite is measured independently of the photorefractive effect by transmission ellipsometry. An asymmetric two-beam coupling gain of 30.6+/-0.4 cm(-1) is obtained, confirming photorefractivity. Degenerate four-wave mixing is used to assess photorefractive performance and, at a poling field of 70 V microm(-1), yields a diffraction efficiency of 4.21%+/-0.03%, a holographic contrast of 3.05 x 10(-4)+/-1 x 10(-6), a space-charge rise time of 25+/-2 s, and a sensitivity of 4.7 x 10(-5)+/-4 x 10(-6) cm3 J(-1). These results constitute a significant improvement on the performance of previous nanoparticle-sensitized photorefractive polymer composites.

Quantum dot-labelled polymer beads by suspension polymerisation.

CdSe quantum dots with polymerisable ligands have been incorporated into polystyrene beads, via a suspension polymerisation reaction, as a first step towards the optical encoding of solid supports for application in solid phase organic chemistry.

New synthetic routes for quantum dots.

Various methods for the synthesis of quantum dots of compound semiconductors are discussed. Emphasis is placed on methods involving the decomposition of chalcogenide-containing precursors in tri-n-octylphosphine oxide. The use of simple coordination complexes can avoid the use of pyrophoric precursors. Approaches based on melts or the use of imino-bisdichalcogeno-diphosphinates are also discussed. These routes may be suitable for the production of large quantities of quantum dots.