Novel materials based on 2D and 3D arrays of nanocrystalline building blocks can be prepared from colloidal suspensions by controlled processing. There is a large effort in this field due to the exciting electrical, [1][2][3] magnetic [4,5] and optical [6,7] properties of such systems, which can be tuned by the size of the building blocks and the interactions between them. Ordered arrays of uncharged, sterically stabilized nanocrystals have been successfully obtained by a subtle destabilization of the suspension [8] or by assembly at a liquid/air [9][10][11] or liquid/liquid interface.[12] There is, however, an important class of suspensions that are stable owing to the nanocrystal surface charge. Assembly of uncapped, charged nanocrystals into arrays is considerably more difficult as destabilization of a suspension usually leads to uncontrolled coagulation.[13]Herein, we report surprising results that may lead to novel routes for the controlled fabrication of materials from chargestabilized nanocrystal colloids. We have observed that gold nanocrystals spontaneously form a monolayer at the water/oil interface if the surface charge of the nanocrystals is gradually reduced. The separation between the nanocrystals in the layer is smaller than the width of the diffuse electrical double layer. Nevertheless, coagulation of the particles into clumps does not occur. The monolayers are remarkably robust and can be easily transferred to substrates, opening the way to technological applications. The spontaneous 2D assembly of charged nanocrystals is qualitatively described in terms of a reduction of the water/oil interfacial energy upon particle adsorption counteracted by electrostatic repulsion in the film.Suspensions of sterically stabilized and supposedly uncharged gold nanocrystals have been used as model systems for the study of self-assembly. A wealth of gold nanocrystal structures have been reported [9,11,14] and their optoelectrical properties are still under investigation. The synthesis of charge-stabilized gold sols is well established. The processing of such sols has been focused on the capping of charged nanocrystals with organic molecules to allow transfer to a

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.

Semiconductor nanocrystals produced by means of colloidal chemistry in a solvent medium are an attractive class of nanometer-sized building blocks from which to create complex materials with unique properties for a variety of applications. Their optical and electronic properties can be tailored easily, both by their chemical composition and particle size. While colloidal nanocrystals emitting in the infrared region have seen a burst of attention during the last decade there is clearly a paucity of review articles covering their synthesis, assembly, spectroscopic characterization, and applications. This Review comprehensively addresses these topics for II-VI, III-V, and IV-VI nanocrystals, examples being HgTe and Cd(x)Hg(1-) (x)Te, InP and InAs, and PbS, PbSe, and PbTe, respectively. Among the applications discussed here are optical amplifier media for telecommunications systems, electroluminescence devices, and noninvasive optical imaging in biology.

Monodisperse spherical, star-shaped, and octahedral PbSe nanocrystals were synthesized via a hot injection method. We show that the shape and size of the colloidal PbSe nanocrystals are determined by the concentration of acetate and that only acetate-free reaction mixtures result in spherical nanocrystals. The presence of acetate leads to efficient oriented attachment of smaller PbSe nanoparticles along the 100 crystal axis. Comparing different synthesis procedures from the recent literature with our observations, we propose that the acetate, naturally present in insufficiently dried reaction mixtures, is responsible for many of the PbSe crystal shapes reported in the literature. In addition we show that it is possible to synthesize these star-shaped nanocrystals so monodisperse that they form ordered monolayers with crystal alignment.