Far-Field Technique for Visualization of Broadband Surface Plasmons Developed
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crystals, through the incorporation of nonlinear optical nanoparticles. Other possibilities mentioned by the researchers include holograms with magnetic nanocrystals for magneto-optic applications and the incorporation of inorganic nanocrystals for multifunctional lightemitting devices. STEVEN TROHALAKI
Sequential Synthesis of Colloidal Type-II Core/Shell CdTe/CdSe Semiconductor Nanocrystals Demonstrated Semiconductor nanocrystals have been the subject of recent scientific and technological interest due to their promising potential applications, including photovoltaics and bio-imaging. In heterostructured nanocrystals, a higher-bandgap shell material can be grown onto a core material with a lower bandgap. Type II core–shell nanocrystals are materials engineered by their band offset (∆Ev), and in these structures, the band offsets are such that the energies of the conduction and valence bands of the shell are either both higher or both lower than those of the core. These materials can spatially separate and confine photo-generated holes and electrons. A common-cation system, CdTe (core)/CdSe (shell) nanocrystals are type II materials, with the majority of photogenerated holes confined in the CdTe core and the electrons in the CdSe shell. Recently, K. Yu and co-workers from the National Research Council of Canada have developed an efficient approach to the synthesis of high-quality CdTe/CdSe nanocrystals. For the CdSe shell, by control of various factors, the technique avoids the formation of a thicker CdSe nanocrystal layer resulting in a thinner shell, leading to a higher photolumines-
Figure 1. Photoemission spectra of CdTe(core)/ CdSe(shell) nanocrystals showing that the thicker the CdSe shell is, the more the emission redshifts. The inset is a transmission electron micrograph of one core–shell nanocrystal in an ensemble exhibiting 833 nm emission.
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cence efficiency. The researchers report their results in the March issue of Small. Colloidal type II CdTe/CdSe nanocrystals were synthesized by the sequential addition of a tri-n-octylphosphine telluride (TOPTe)/tri-n-octylphosphine (TOP) solution and several shell-precursor solutions to a cadmium oxide (CdO)/TOP solution. The shell-precursor solutions consist of CdO and tri-n-octylphosphine selenide (TOPSe) in TOP. This synthetic approach is simple and does not involve either the addition of any acids, amines, or traditional tri-n-octylphosphine oxide (TOPO), or precipitation of core CdTe nanocrystals. During the synthesis of the type-II CdTe/CdSe nanocrystals, the researchers monitored the temporal evolution of the optical properties during the growth of the CdSe shell, as shown in the figure. The emission redshifts with increasing CdSe shell thickness. This synthetic approach, similar to that of sequential anionic polymerization for well-defined block copolymers, is based on the knowledge gained during the search for synthetic routes for providing slow growth rates for high-quality CdSe and CdTe nanocrystals in large-scale production. It has been acknowledg
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