We demonstrate that hollow nanocrystals can be synthesized through a mechanism analogous to the Kirkendall Effect, in which pores form due to the difference in diffusion rates between two components in a diffusion couple. Cobalt nanocrystals are chosen as a primary example to show that their reaction in solution with oxygen, sulfur or selenium leads to the formation of hollow nanocrystals of the resulting oxide and chalcogenides. This process provides a general route to the synthesis of hollow nanostructures of large numbers of compounds. A simple extension of this process yields platinum-cobalt oxide yolk-shell nanostructures which may serve as nanoscale reactors in catalytic applications.

Lattice mismatch strains are widely known to control nanoscale pattern formation in heteroepitaxy, but such effects have not been exploited in colloidal nanocrystal growth.We demonstrate a colloidal route to synthesizing CdS-Ag 2 S nanorod superlattices through partial cation exchange. Strain induces the spontaneous formation of periodic structures. Ab initio calculations of the interfacial energy and modeling of strain energies show that these forces drive the self-organization. The nanorod superlattices exhibit high stability against ripening and phase mixing. These materials are tunable near-infrared emitters with potential applications as nanometer-scale optoelectronic devices.

ABSTRACT. hcp Co disk-shaped nanocrystals were obtained by rapid decomposition of cobalt carbonyl in presence of linear amines. Other surfactants, in addition to the amines, like phosphine oxides and oleic acid were used to improve size dispersion, shape control and nanocrystal stability. Co disks are ferromagnetic in character and they spontaneously self assemble into long ribbons. X-ray and electron diffraction, electron microscopy and SQUID magnetometry have been employed to characterize this material.

The deformation behavior of BCC metals is being investigated by x-ray microdiffraction measurements (µXRD) for the purpose of characterizing the dislocation structure that results from uniaxial compression experiments. The high brilliance synchrotron source at the Advanced Light Source (Lawrence Berkeley National Lab) and the micron resolution of the focusing optics allow for the mapping of Laue diffraction patterns across a sample. These measurements are then analyzed in order to map the distribution of residual stresses in the crystal. An important finding is the observation of Laue spot "streaking", which indicates localized rotations in the lattice. These may represent an accumulation of same-sign dislocations. Theoretical modeling of the diffraction response for various slip systems is presented, and compared to experimental data. Preliminary results include orientation maps from a highly strained Ta bicrystal and a less highly strained Mo single crystal. The orientation maps of the bicrystal indicate a cell-like structure of dense dislocation walls. This deformation structure is consistent with previous OIM studies of the same crystal. The results suggest that µXRD may be a particularly useful tool for microscale studies of deformation patterns in a multi-scale investigation of the mechanisms of deformation that ranges from macroscopic deformation tests to high resolution TEM studies of dislocation structures.

Sustained progress in nanocrystal synthesis has enabled recent use of these materials as inorganic, macromolecular precursors that can be chemically transformed into new nanostructures. 1, 2 The literature now contains several cases with chemical transformations being accompanied by varying degrees of modification of properties, including crystal structure and particle shape. [3][4][5] As a recent example, we demonstrated that as-synthesized metallic nanocrystals yield, upon oxidation, nanostructures with modified morphologies such as hollow particles. 6 This morphological change derives from directional material flows due to differing diffusivities for the reacting atomic species, in a nanoscale version of the well-known Kirkendall Effect. This general methodology has since been extended by other groups to produce nanostructures with various compositions and shapes. [7][8][9][10] Galvanic replacement reactions have been demonstrated to also produce hollow nanostructures. Xia et al. reported cases where a replacement reaction took place uniformly around Ag cubes of ~100 nm size, leading to formation of Au nanoboxes. The exterior shape of the nanoboxes tends to largely reproduce that of the sacrificial Ag counterparts. 11-15 Here, we report that performing the same replacement reaction on silver nanocrystals that are an order of magnitude smaller leads to significant changes in the external morphology of the Au shells as the reaction proceeds, while still creating a central void in each particle. Specifically, single crystalline silver nanoscrystals with a spherical shape act in the presence of Au 3+ as precursors for formation of hollow Au nanocrystals with truncated octahedral shape. The growth of significantly faceted particles from spherical precursors is made possible by the enhanced role of surface effects in our smaller nanocrystals. Production of hollow Au nanocrystals with faceted geometry allows increased tunability of optical properties as the surface plasmon resonance spectra of a hollow metallic nanocrystal depends strongly not only on the shell thickness, but also on the detailed shape. 16,17 To produce Ag nanocrystals with reduced sizes and improved monodispersity, we performed the synthesis using a modification of the polyol process in an organic solvent and at high temperature. 6,18 The silver salt AgNO 3 was reduced by a long chain polyol such as 1,2-hexadecanediol using an organic solvent, o-dichlorobenzene (DCB). Oleylamine was present as a surfactant. Near instant formation of silver nanocrystals upon reaction was indicated by the originally colorless solution turning dark brown. The transformation of solid nanocrystals into hollow ones was performed through galvanic replacement by dropwise addition of gold (III) chloride solution to diluted silver colloidal solution until the solution changed in color from dark yellow to blue. In this reaction, oleylamine likely serves two purposes. First, it solubilizes the precursors AgNO 3 and AuCl 3 in DCB. Second, it acts as a surfactant that controls t...

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Monolithic cathodes of optimized porosity prepared by sintering LiCoO2 powders provide high volume utilization and surprising stability under electrochemical cycling. Combined with a novel packaging approach, ultrahigh energy densities in small volumes are enabled. The microbatteries have volumes <6 mm3 and provide sustained ∼2.5 h discharges with energy densities of 400–650 W h L−1.

The low cost, significant reduction potential and relative safety of the zinc electrode is a common hope for a reductant in secondary batteries, but it is limited mainly to primary implementation due to shape change. In this work, we exploit such shape change for the benefit of static electrodes through the electrodeposition of hyper-dendritic nanoporous zinc foam. Electrodeposition of zinc foam resulted in nanoparticles formed on secondary dendrites in a three-dimensional network with a particle size distribution of 54.1-96.0 nm. The nanoporous zinc foam contributed to highly oriented crystals, high surface area and more rapid kinetics in contrast to conventional zinc in alkaline mediums. The anode material presented had a utilization of 88% at full depth-of-discharge (DOD) at various rates indicating a superb rate capability. The rechargeability of Zn 0 /Zn 2+ showed significant capacity retention over 100 cycles at a 40% DOD to ensure that the dendritic core structure was imperforated. The dendritic architecture was densified upon charge-discharge cycling and presented superior performance compared with bulk zinc electrodes.