Si-based Li-ion battery anodes have recently received great attention, as they offer specific capacity an order of magnitude beyond that of conventional graphite. The applications of this transformative technology require synthesis routes capable of producing safe and easy-to-handle anode particles with low volume changes and stable performance during battery operation. Herein, we report a large-scale hierarchical bottom-up assembly route for the formation of Si on the nanoscale--containing rigid and robust spheres with irregular channels for rapid access of Li ions into the particle bulk. Large Si volume changes on Li insertion and extraction are accommodated by the particle's internal porosity. Reversible capacities over five times higher than that of the state-of-the-art anodes (1,950 mA h g(-1)) and stable performance are attained. The synthesis process is simple, low-cost, safe and broadly applicable, providing new avenues for the rational engineering of electrode materials with enhanced conductivity and power.

Hierarchical nanostructures of lead sulfide nanowires resembling pine trees were synthesized by chemical vapor deposition. Structural characterization revealed a screwlike dislocation in the nanowire trunks with helically rotating epitaxial branch nanowires. It is suggested that the screw component of an axial dislocation provides the self-perpetuating steps to enable one-dimensional crystal growth, in contrast to mechanisms that require metal catalysts. The rotating trunks and branches are the consequence of the Eshelby twist of screw dislocations with a dislocation Burgers vector along the 110 directions having an estimated magnitude of 6 +/- 2 angstroms for the screw component. The results confirm the Eshelby theory of dislocations, and the proposed nanowire growth mechanism could be general to many materials.

Microporous carbons, produced by selective etching of metal carbides in a chlorine-containing environment, offer narrow distribution of micropores and one of the highest specific capacitances reported when used in electrical double layer capacitors (EDLC) with organic electrolytes. Previously, the small micropores in these carbons served as an impediment to ion transport and limited the power storage characteristics of EDLC. Here we demonstrate, for the first time, how the preparation and application of templated carbide-derived carbon (CDC) can overcome the present limitations and show the route for dramatic performance enhancement. The ordered mesoporous channels in the produced CDC serve as ion-highways and allow for very fast ionic transport into the bulk of the CDC particles. The enhanced transport led to 85% capacitance retention at current densities up to approximately 20 A/g. The ordered mesopores in silicon carbide precursor also allow the produced CDC to exhibit a specific surface area up to 2430 m(2)/g and a specific capacitance up to 170 F/g when tested in 1 M tetraethylammonium tetrafluoroborate solution in acetonitrile, nearly doubling the previously reported values.

The optical and structural properties of high-quality single-crystal epitaxial MgZnO films deposited by pulsed-laser deposition were studied. In films with up to ∼36 at. % Mg incorporation, we have observed intense ultraviolet band edge photoluminescence at room temperature and 77 K. The highly efficient photoluminescence is indicative of the excitonic nature of the material. Transmission spectroscopy was used to show that the excitonic structure of the alloys was clearly visible at room temperature. High-resolution transmission electron microscopy, x-ray diffraction, and Rutherford backscattering spectroscopy/ion channeling were used to verify the epitaxial single-crystal quality of the films and characterize the defect content. Post-deposition annealing in oxygen was found to reduce the number of defects and to improve the optical properties of the films. These results indicate that MgZnO alloys have potential applications in a variety of optoelectronic devices.

The performances of heterojunction-based electronic devices are extremely sensitive to the interfacial electronic band structure. Here we report a largely enhanced performance of photoelectrochemical (PEC) photoanodes by ferroelectric polarization-endowed band engineering on the basis of TiO2/BaTiO3 core/shell nanowires (NWs). Through a one-step hydrothermal process, a uniform, epitaxial, and spontaneously poled barium titanate (BTO) layer was created on single crystalline TiO2 NWs. Compared to pristine TiO2 NWs, the 5 nm BTO-coated TiO2 NWs achieved 67% photocurrent density enhancement. By numerically calculating the potential distribution across the TiO2/BTO/electrolyte heterojunction and systematically investigating the light absorption, charge injection and separation properties of TiO2 and TiO2/BTO NWs, the PEC performance gain was proved to be a result of the increased charge separation efficiency induced by the ferroelectric polarization of the BTO shell. The ferroelectric polarization could be switched by external electric field poling and yielded PEC performance gain or loss based on the direction of the polarization. This study evidence that the piezotronic effect (ferroelectric or piezoelectric potential-induced band structure engineering) holds great promises in improving the performance of PEC photoelectrodes in addition to chemistry and structure optimization.

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The power density and charge-discharge time of electrical double layer capacitors are largely determined by how fast the electrolyte ions can travel within the carbon electrode particles. Our systematic studies using zeolite-templated carbons show that an enhancement in ion transport rate by more than 2 orders of magnitude is possible by minimizing the micropore tortuosity. Very uniform carbon deposition was achieved using a well-controlled process involving the decomposition of acetylene precursor at a reduced pressure of 10 Torr and under a constant flow rate of 100 sccm. Selected carbon samples with well-aligned, straight micropores demonstrate high specific capacitance of up to 300 F/g and outstanding frequency response of up to 10 Hz for 250 microm thick electrodes, indicating an attractive combination of high specific energy and high specific power in electrical double layer capacitors. Such properties are critical for many peak-power hungry applications, such as the leveling of subsecond disturbances in power lines. Our findings provide guidance for the optimal design of porous carbons with greatly improved power storage characteristics.

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