We demonstrate that semiconductor nanorods can be used to fabricate readily processed and efficient hybrid solar cells together with polymers. By controlling nanorod length, we can change the distance on which electrons are transported directly through the thin film device. Tuning the band gap by altering the nanorod radius enabled us to optimize the overlap between the absorption spectrum of the cell and the solar emission spectrum. A photovoltaic device consisting of 7-nanometer by 60-nanometer CdSe nanorods and the conjugated polymer poly-3(hexylthiophene) was assembled from solution with an external quantum efficiency of over 54% and a monochromatic power conversion efficiency of 6.9% under 0.1 milliwatt per square centimeter illumination at 515 nanometers. Under Air Mass (A.M.) 1.5 Global solar conditions, we obtained a power conversion efficiency of 1.7%.

Colloidal nanocrystals are sometimes referred to as 'artificial atoms' because the density of their electronic states-which controls many physical properties-can be widely and easily tuned by adjusting composition, size and shape. The combination of strongly size-and shape-dependent physical properties and ease of fabrication and processing makes nanocrystals promising building blocks for materials with designed functions 1,2 . But the ability to control the uniformity of the size, shape, composition, crystal structure and surface properties of the nanocrystals is not only of technological interest: having access to defined nanoscale structures is essential to uncovering their intrinsic properties unaffected by sample inhomogeneity. Rigorous understanding of the properties of individual nanocrystals enables exploitation of collective properties of nanocrystal ensembles, making it possible to design and fabricate novel electronic, magnetic and photonic devices and other functional materials based on these nanostructures.Colloidal nanocrystals with a semiconductor as the inorganic material-so-called quantum dots-exhibit size tunable band gaps and luminescence energies due to the quantum size effect 3 . This has led to their use as fluorescent biological labels 4-6 , with colloidal quantum dots now widely employed as targeted fluorescent labels in biomedical research applications. Compared to the organic fluorophores that have been used as biological labels previously, quantum dots are extremely bright and do not photo-bleach, and they provide a readily accessible range of colors. Other applications that could benefit from the combination of low-cost processing with solid-state performance include the use of colloidal quantum dots and rods as possible alternatives to semiconductor polymers in light emitting diodes 7 , lasers 8 , and solar cells 9 . The scope for these applications has prompted intensive study of the synthesis of these materials to optimize colloidal semiconductor nanocrystal fabrication. As a result, many new concepts for controlling the size, shape, and connectivity or coupling of colloidal nanocrystals have been developed first for these materials, but a unified set of synthesis control concepts is now also applied to other classes of materials such as metals and metal oxides. These materials will extend the range of potential applications for colloidal nanocrystals to many other areas, including catalysis.Over the last decade chemists have come to appreciate that from the point of view of synthesis, colloidal inorganic nanocrystals can be viewed as a class of macromolecule, with preparative strategies that are similar in many ways to those employed with artificial organic polymers. For nanocrystals of one to one hundred nanometers diameter, it is possible to define the average and the dispersion of the diameter, as well as the aspect ratio; the degree of precision with which the desired structure is realized is similar to what is achieved with synthetic polymers, where the preparative means at o...

Semiconductor nanocrystals exhibit a wide range of size-dependent properties. Variations in fundamental characteristics ranging from phase transitions to electrical conductivity can be induced by controlling the size of the crystals. The present status and new opportunities for research in this area of materials physical chemistry are reviewed.

The synthesis of epitaxially grown, wurtzite CdSe/CdS core/shell nanocrystals is reported. Shells of up to three monolayers in thickness were grown on cores ranging in diameter from 23 to 39 Å. Shell growth was controllable to within a tenth of a monolayer and was consistently accompanied by a red shift of the absorption spectrum, an increase of the room temperature photoluminescence quantum yield (up to at least 50%), and an increase in the photostability. Shell growth was shown to be uniform and epitaxial by the use of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and optical spectroscopy. The experimental results indicate that in the excited state the hole is confined to the core and the electron is delocalized throughout the entire structure. The photostability can be explained by the confinement of the hole, while the delocalization of the electron results in a degree of electronic accessibility that makes these nanocrystals attractive for use in optoelectronic devices.

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The formation of extremely high aspect ratio CdSe nanorods (30:1), as well as arrow-, teardrop-, tetrapod-, and branched tetrapod-shaped nanocrystals of CdSe, has been achieved by growth of the nanoparticles in a mixture of hexylphosphonic acid and trioctylphosphine oxide. The most influential factors in shape control are the ratio of surfactants, injection volume, and time-dependent monomer concentration.

Nanoscale materials are currently being exploited as active components in a wide range of technological applications in various fields, such as composite materials, chemical sensing, biomedicine, optoelectronics and nanoelectronics. Colloidal nanocrystals are promising candidates in these fields, due to their ease of fabrication and processibility. Even more applications and new functional materials might emerge if nanocrystals could be synthesized in shapes of higher complexity than the ones produced by current methods (spheres, rods, discs). Here, we demonstrate that polytypism, or the existence of two or more crystal structures in different domains of the same crystal, coupled with the manipulation of surface energy at the nanoscale, can be exploited to produce branched inorganic nanostructures controllably. For the case of CdTe, we designed a high yield, reproducible synthesis of soluble, tetrapod-shaped nanocrystals through which we can independently control the width and length of the four arms.

Mergers and Acquisitions The crystallization of small molecules or polymers is often described in terms of a nucleation stage, where initial clusters form, followed by a distinct growth stage. Growth can come from the addition of unbound molecules, or through “Ostwald ripening” where larger crystals grow at the expense of smaller ones due to thermodynamic effects. Zheng et al. (p. 1309 ) studied the growth of platinum nanocrystals inside a transmission electron microscope using a special liquid cell, allowing observation of crystal growth in situ. Both monomer addition to growing particles and the coalescence of two particles were observed. The specific growth mechanism appeared to be governed by the size of each of the particles. The combination of growth processes makes it possible for an initially broad distribution of particles to narrow into an almost uniform one.