Heterostructure nanocrystals (NCs) of gamma-Fe(2)O(3) and MS (M = Zn, Cd, Hg) are synthesized. The large lattice mismatch between gamma-Fe(2)O(3) and MS NCs leads to noncentrosymmetric structures. Crystallographic planes at the heterojunctions are identified by high-resolution transmission electron microscopy. Preferential formation of trimers and higher oligomers for ZnS and dimers or isolated particles for CdS and HgS with gamma-Fe(2)O(3) NCs are observed and explained by changes in the effective mismatch between the coincidence lattices of the most commonly observed junction planes.

Colloidal nanocrystal heterostructures in which two or more chemically distinct inorganic components are epitaxially fused together provide new opportunities in developing multifunctional building block materials. The ability to synthesize structurally and chemically well-defined nanocrystal heterostructures can provide novel combinations of unique properties arising at the nanometer length scale. Here, we examine the structural evolution of inverse spinel iron oxide/CdS nanocrystal heterostructures with respect to the sizes of both components. The crystal structure and the crystallinity of the initial iron oxide are first identified by a combination of X-ray diffraction and Raman scattering measurements. Studies on the size effect suggest lattice-strain-induced limitations on the achievable sizes of CdS within the heterostructures. Because of this limitation, increasing the amount of Cd/S reagents leads to multiple particle nucleation on individual iron oxide nanocrystals rather than continued growth. Larger sizes and a limited amount of the CdS component can be achieved by starting with small iron oxide nanocrystals. These results suggest that exploiting lattice strain may be a viable approach to obtaining heterostructured colloids with nanoscale precision.

Diffractive imaging has the potential to succeed in structure determination of single nanoparticles using probes such as pulsed X-rays 1 or medium-energy electrons 2 where an atomicresolution imaging lens is not available and radiation damage can be remedied 3 . Although diffractive imaging has been demonstrated for particles 4 and single cells 5,6 at several nanometres in resolution, ultimately, atomic resolution is required to determine their three-dimensional structure. A major difficulty in atomic-resolution diffractive imaging is the loss of weak coherent scattering signals in recorded diffraction patterns. Here, we show that this can be overcome using information from low-resolution images. By combining information from both diffraction and imaging, we succeeded in phasing experimental electron diffraction patterns of individual CdS quantum dots at sub-ångström resolution. The low-resolution image provides the starting phase, realspace constraint, missing information in the central beam and essential marks for aligning the diffraction pattern, and diffraction provides high-resolution information. We show that for CdS nanocrystals, the improved image resolution enables determination of their atomic structures. As lowresolution images can be obtained from different sources, the technique developed here is general and provides a basis for imaging the three-dimensional atomic structure of single nanoparticles, where correct orientation of the recorded diffraction patterns is critical 7 . Diffractive imaging uses diffraction intensity and phase retrieval to form real-space images. By bypassing the need for imaging lenses and their associated aberrations, the resolution, in principle, is limited only by the amount of high-angle scattering [8][9][10] . For this reason, diffractive imaging has the potential to achieve atomic resolution for hard X-rays or other short-wavelength particles. For example, high-resolution diffractive imaging has been proposed for imaging biological molecules using ultrashort and extremely bright coherent X-ray pulses 3 . Ultrafast X-ray diffraction is intended to record diffraction patterns before the start of significant radiationinduced damage 1,3 . For electrons, the knock-on damage limits the number of images that can be recorded by high-resolution electron microscopes 11 . Imaging at below the knock-on damage threshold is critical for the determination of three-dimensional (3D) atomic structure. In such cases, diffractive imaging provides real-space information at resolutions that are not available otherwise.The principle of diffractive imaging is based on coherent diffraction of an isolated object and recording of diffraction patterns at a spatial frequency (f ) smaller than the reciprocal of the object size 12 (1/S). The product of (Sf ) −n , where n is the dimension of the diffraction pattern, is called the oversampling ratio 8 . Oversampling increases the amount of information about the object recorded in the diffraction pattern, which is necessary for retrieving the phase requ...

Triblock copolymers [poly(ethylene oxide) (PEO) and polypropylene oxide (PPO)], Pluronic F127 with 100 PEO blocks on each end, and 65 blocks of PPO in the center were examined in aqueous solution. The "sol" and "gel" phase diagram was determined as a function of concentration and temperature. For further study, the concentration was fixed at 20 wt %, and the temperature dependence of the dynamic viscosity differed from the temperature dependence of fluorescence emission spectra and the microviscosity probed by the fluorescence depolarization kinetics of rhodamine 123 dye, which was dissolved in the continuous hydrophilic phase. The depolarization measurements used single-photon counting after two-photon excitation with a Tisapphire femtosecond laser. Although the viscoelastic modulus increased by an order of magnitude when the sol-to-gel transition was crossed, the microviscosity of the hydrophilic continuous medium showed only minor changes. At different temperatures the fluorescence lifetime was the same with a single-exponential time constant, but the fluorescence depolarization displayed a double-exponential decay. After comparison with fluorescence depolarization of the dye in PPO melt and PEO whose molecular weight and aqueous concentrations were varied, the relative proportions of faster and slower components of the fluorescence depolarization were tentatively attributed to varying ratios of the dye in free solution and associated with micelles.

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