Excitons in polycrystalline and disordered films of organic semiconductors have been shown to diffuse over distances of 10-50 nm. Here, using polarization- and wavelength-dependent photoconductivity in the highly ordered organic semiconductor rubrene, we show that the diffusion of triplet excitons in this material occurs over macroscopic distances (2-8 μm), comparable to the light absorption length. Dissociation of these excitons at the surface of the crystal is found to be the main source of photoconductivity in rubrene. In addition, we observe strong photoluminescence quenching and a simultaneous enhancement of photoconductivity when the crystal surface is functionalized with exciton splitters. In combination with time-resolved measurements, these observations strongly suggest that long-lived triplet excitons are indeed generated in molecular crystals by fission of singlets, and these triplets provide a significant contribution to the surface photocurrent generated in organic materials. Our findings indicate that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors.

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Alloy nanocrystals provide an additional degree of freedom in selecting desirable properties for nanoscale engineering because their physical and optical properties depend on both size and composition. We report the pyrolytic synthesis of homogeneously alloyed CdS(x)Se(1-x) nanocrystals in all proportions. The nanocrystals are characterized using UV-visible absorption spectroscopy, transmission electron microscopy, X-ray diffractrometry, and Rutherford backscattering spectrometry to determine precisely structure, size, and composition. The dependence of band gap on nanocrystal size and composition is elucidated, yielding a bowing constant of 0.29, in agreement with bulk values. In addition, the morphology of the resultant nanocrystals can be altered by changing the reaction conditions, generating structures ranging from homogeneous, spherical nanocrystals to one-dimensional gradient nanorods.

Aberration-corrected Z-contrast scanning transmission electron microscopy of core/shell nanocrystals shows clear correlations between structure and quantum efficiency. Uniform shell coverage is obtained only for a graded CdS/ZnS shell material and is found to be critical to achieving near 100% quantum yield. The sublattice sensitivity of the images confirms that preferential growth takes place on the anion-terminated surfaces. This explains the three-dimensional "nanobullet" shape observed in the case of core/shell nanorods.

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Nanostructures, with their very large surface to volume ratio and their non-planar geometry, present an important challenge to surface scientists. New issues arise as to surface characterization, quantification and interface formation. This review summarizes the current state of the art in the synthesis, composition, surface and interface control of CdSe nanocrystal systems, one of the most studied and useful nanostructures. KeywordsCadmium; Selenium; Selenide; CdSe; nanocrystal; quantum dot; nanoparticle; surface; synthesis; electron microscopy; Z-STEM; RBS; alloy; core/shell; fluorescence; quantum yield; growth mechanism; expitaxy; spectroscopy 1.0 Introduction BackgroundWe are currently witnessing an explosion of research in nanoscale science, engineering, and biotechnology. As the nano-revolution proceeds, basic research is leading to the development of true nanotechnologies which have the potential to impact fields from health care and opto-electronics to energy and the environment. One particular type of nanostructure, semiconductor nanocrystals (also known as quantum dots), may ultimately be employed in technologies in each of these fields. For example, antibody-conjugated CdSe/ ZnS core-shell nanocrystals have been used to detect respiratory syncytial virus. 1 The high brightness of the quantum dots enables the virus to be detected in a matter of minutes, whereas the previous assay required 4 days. The ramification of this new technology is that anti-viral drugs can be given early in the infection when they are most effective, reducing Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access Author ManuscriptSurf Sci Rep. Author manuscript; available in PMC 2011 April 6. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript hospitalizations and deaths. In the field of opto-electronics CdSe nanocrystals have been incorporated in light emitting diodes2 -6 and it has been demonstrated that they can serve as the gain media in a laser.7 , 8 Nanocrystals are also poised to play a role in solar energy conversion.9 -14 A specific example is a device in which elongated CdSe nanocrystals (nanorods) are mixed with a conducting polymer and sandwiched between aluminum and indium tin oxide electrodes to create a device with a power conversion efficiency of 1.7 %. 12 Nanocrystals may also have a positive impact on the environment, as it has been shown that they can be used to sequester CO 2 . 15 The spark that ignited two decades of intense research in nanocrystal synthesis, properties, and applications was Louis Brus' demonstration that qua...

The optical and morphological characteristics of vanadium dioxide nanoparticles and thin films during their nucleation and growth phases have been studied by correlating the temperature and sharpness of the transition with the processing parameters. Thermal annealing results in grain growth and improved crystallinity. Normally, larger crystallites show smaller hysteresis, as there is a greater probability of finding a nucleating defect in the larger volume. But at the same time, this improved crystal perfection, which accompanies the thermal annealing and grain growth, tends to a larger hysteresis, as there are fewer nucleating defects within the volume. We show that the width and shape of the hysteresis cycle are thus determined by the competing effects of crystallinity and grain size.