CdSe/Zn1-X CdX S core/shell heterostructured quantum dots (QDs) with varying shell thicknesses are studied as the active material in a series of electroluminescent devices. "Giant" CdSe/Zn1-X CdX S QDs (e.g., CdSe core radius of 2 nm and Zn1-X CdX S shell thickness of 6.3 nm) demonstrate a high device efficiency (peak EQE = 7.4%) and a record-high brightness (>100 000 cd m(-2) ) of deep-red emission, along with improved device stability.

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Highly uniform large-scale assembly of nanoscale building blocks can enable unique collective properties for practical electronic and photonic devices. We present a two-dimensional (2-D), millimeter-scale network of colloidal CdSe nanorods (NRs) in monolayer thickness through end-to-end linking. The colloidal CdSe NRs are sterically stabilized with tetradecylphosphonic acid (TDPA), and their tips are partially etched in the presence of gold chloride (AuCl3) and didecyldimethylammonium bromide (DDAB), which make them unwetted in toluene. This change in surface wetting property leads to spontaneous adsorption at the 2-D air/toluene interface. Anisotropy in both the geometry and the surface property of the CdSe NRs causes deformation of the NR/toluene/air interface, which derives capillary attraction between tips of neighboring NRs inward. As a result, the NRs confined at the interface spontaneously form a 2-D network composed of end-to-end linkages. We employ a vertical-deposition approach to maintain a consistent rate of NR supply to the interface during the assembly. The rate control turns out to be pivotal in the preparation of a highly uniform large scale 2-D network without aggregation. In addition, unprecedented control of the NR density in the network was possible by adjusting either the lift-up speed of the immersed substrate or the relative concentration of AuCl3 to DDAB. Our findings provide important design criteria for 2-D assembly of anisotropic nanobuilding blocks.

Anisotropic microparticles are promising as a new class of colloidal or granular materials due to their advanced functionalities which are difficult to achieve with isotropic particles. However, synthesis of the anisotropic microparticles with a highly controlled size and shape still remains challenging, despite their intense demands. Here, we report a microfluidic approach to create uniform anisotropic microparticles using phase separation of polymer blends confined in emulsion drops. Two different polymers are homogeneously dissolved in organic solvent at low concentration, which is microfluidically emulsified to produce oil-in-water emulsion drops. As the organic solvent diffuses out, small domains are formed in the emulsion drops, which are then merged, forming only two distinct domains. After the drops are fully consolidated, uniform anisotropic microparticles with two compartments are created. The shape of the resulting microparticles is determined by combination of a pair of polymers and type of surfactant. Spherical microparticles with eccentric core and incomplete shell are prepared by consolidation of polystyrene (PS) and poly(lactic acid) (PLA), and microparticles with single crater are formed by consolidation of PS and poly(methyl methacrylate) (PMMA); both emulsions are stabilized with poly(vinyl alcohol) (PVA). With surfactants of triblock copolymer, acorn-shaped Janus microparticles are obtained by consolidating emulsion drops containing PS and PLA. This microfluidic production of anisotropic particles can be further extended to any combination of polymers and colloids to provide a variety of structural and chemical anisotropy.

In this study, we designed and synthesized photocatalysts for hydrogen evolution from water by coating a thin layer of amorphous TiO2 (a-TiO2) on CdSe nanocrystals (NCs). The thin shell of a-TiO2 serves as a channel for charge carriers otherwise unutilized. Albeit a previous notion that a-TiO2 is a poor photocatalyst, the enhanced photocatalytic activity in the presence of a-TiO2 suggests that the material helps utilize the photogenerated charge carriers when it is in a form of thin shell on CdSe NCs. Type II band offset in CdSe/a-TiO2 appears to allow the electron in the conduction band of CdSe NCs to migrate over to that of a-TiO2, and the electron participates in the hydrogen production from water. Size of CdSe NCs influences the photocatalytic hydrogen evolution rate as the energy difference between the conduction bands of semiconductors becomes larger. Electron transfer from CdSe NCs to a-TiO2 layer is influenced by the level of the conduction-band edge of CdSe NCs: the size dependence indicates that electron injection to TiO2 is facilitated with energy level offset between CdSe and TiO2, while smaller NCs have larger band gap and thus narrower spectral range of absorption. The interplay between charge-transfer rate and absorption cross-section should be considered in designing heterostructure NC-based photocatalysts for water splitting.

We demonstrate photocatalytic reduction of methylene blue in the infrared region, using PbSe/CdSe/CdS core/shell/shell heterostructure nanocrystals (HNCs) with type II or quasi-type II band offsets. Varying deposition rates of the CdS shell result in nanocrystals of diverse morphologies ranging from spheres to pyramids to tetrapods. The faceted shapes enable the selective growth of Au tips, which help increase photocatalytic activity since the Au tips serve as an electron sink. Comparative studies reveal that the photocatalytic activity appears to correlate with the integral overlap of electron and hole wave functions. Tetrapod-shaped HNCs with Au tips show considerably higher photocatalytic activity for the reduction of methylene blue than sphere or pyramid-shaped HNCs of equivalent composition arrangement.

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