Molybdenum disulfide (MoS2) of single- and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The number of S−Mo−S layers of the samples was independently determined by contact-mode atomic force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.

/pdf/anomalous-lattice-vibrations-of-single-and-few-layer-mos2-1w8dp2s904.pdf

Anomalous lattice vibrations of single- and few-layer MoS2.

This feature article highlights work from the authors' laboratories on the synthesis, assembly, reactivity, and optical applications of metallic nanoparticles of nonspherical shape, especially nanorods. The synthesis is a seed-mediated growth procedure, in which metal salts are reduced initially with a strong reducing agent, in water, to produce ∼4 nm seed particles. Subsequent reduction of more metal salt with a weak reducing agent, in the presence of structure-directing additives, leads to the controlled formation of nanorods of specified aspect ratio and can also yield other shapes of nanoparticles (stars, tetrapods, blocks, cubes, etc.). Variations in reaction conditions and crystallographic analysis of gold nanorods have led to insight into the growth mechanism of these materials. Assembly of nanorods can be driven by simple evaporation from solution or by rational design with molecular-scale connectors. Short nanorods appear to be more chemically reactive than long nanorods. Finally, optical applica...

Anisotropic metal nanoparticles: Synthesis, assembly, and optical applications

All battery technologies are known to suffer from kinetic problems linked to the solid-state diffusion of Li in intercalation electrodes, the conductivity of the electrolyte in some cases and the quality of interfaces. For Li-ion technology the latter effect is especially acute when conversion rather than intercalation electrodes are used. Nano-architectured electrodes are usually suggested to enhance kinetics, although their realization is cumbersome. To tackle this issue for the conversion electrode material Fe3O4, we have used a two-step electrode design consisting of the electrochemically assisted template growth of Cu nanorods onto a current collector followed by electrochemical plating of Fe3O4. Using such electrodes, we demonstrate a factor of six improvement in power density over planar electrodes while maintaining the same total discharge time. The capacity at the 8C rate was 80% of the total capacity and was sustained over 100 cycles. The origin of the large hysteresis between charge and discharge, intrinsic to conversion reactions, is discussed and approaches to reduce it are proposed. We hope that such findings will help pave the way for the use of conversion reaction electrodes in future-generation Li-ion batteries.

High rate capabilities Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery applications.

This review covers important advances in recent years in the physics of thin-film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin-film form. The authors introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this the review covers the enormous progress that has been made in the first-principles computational approach to understanding ferroelectrics. The authors then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, this review ends with a look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in nonconventional nanoscale geometries.

/pdf/physics-of-thin-film-ferroelectric-oxides-3zmqphtpv1.pdf

Physics of thin-film ferroelectric oxides

Plasmon-induced photoelectrochemistry in the visible region was studied at gold nanoparticle−nanoporous TiO2 composites (Au−TiO2) prepared by photocatalytic deposition of gold in a porous TiO2 film. Photoaction spectra for both the open-circuit potential and short-circuit current were in good agreement with the absorption spectrum of the gold nanoparticles in the TiO2 film. The gold nanoparticles are photoexcited due to plasmon resonance, and charge separation is accomplished by the transfer of photoexcited electrons from the gold particle to the TiO2 conduction band and the simultaneous transfer of compensative electrons from a donor in the solution to the gold particle. Besides its low-cost and facile preparation, a photovoltaic cell with the optimized electron mediator (Fe2+/3+) exhibits an optimum incident photon to current conversion efficiency (IPCE) of 26%. The Au−TiO2 can photocatalytically oxidize ethanol and methanol at the expense of oxygen reduction under visible light; it is potentially appli...

Mechanisms and Applications of Plasmon-Induced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles

Many biomolecules are chiral--they can exist in one of two enantiomeric forms that only differ in that their structures are mirror images of each other. Because only one enantiomer tends to be physiologically active while the other is inactive or even toxic, drug compounds are increasingly produced in an enantiomerically pure form using solution-phase homogeneous catalysts and enzymes. Chiral surfaces offer the possibility of developing heterogeneous enantioselective catalysts that can more readily be separated from the products and reused. In addition, such surfaces might serve as electrochemical sensors for chiral molecules. To date, chiral surfaces have been obtained by adsorbing chiral molecules or slicing single crystals so that they exhibit high-index faces, and some of these surfaces act as enantioselective heterogeneous catalysts. Here we show that chiral surfaces can also be produced through electrodeposition, a relatively simple solution-based process that resembles biomineralization in that organic molecules adsorbed on surfaces have profound effects on the morphology of the inorganic deposits. When electrodepositing a copper oxide film on an achiral gold surface in the presence of tartrate ion in the deposition solution, the chirality of the ion determines the chirality of the deposited film, which in turn determines the film's enantiospecificity during subsequent electrochemical oxidation reactions.

Enantiospecific electrodeposition of a chiral catalyst

ZnO nanopillars have been electrodeposited epitaxially onto Au(111), Au(110) and Au(100) single-crystal substrates. The nanopillars grow with the same [0001] out-of-plane orientation on all three substrates. The in-plane orientation was probed by X-ray pole figure analysis. The pole figures had six peaks on Au(111) and twelve peaks on Au(110) and Au(100). Scanning electron microscopy revealed aligned hexagonal nanopillars of ZnO with an average grain size of 85 nm on Au(111). There were two sets of hexagonal grains with an average size of 85 nm on Au(110) and 95 nm on Au(100) that were rotated 90° with respect to each other. Rocking curve analysis showed that the ZnO on Au(100) had the smallest mosaic spread.

Epitaxial Electrodeposition of Zinc Oxide Nanopillars on Single-Crystal Gold

Single-crystal films are essential for devices because the intrinsic properties of the material, rather than its grain boundaries, can be exploited. Cubic bismuth oxide has the highest known oxide ion mobility, which makes it useful for fuel cells and sensors, but it is normally only stable from 729° to 825°C. The material has not been previously observed at room temperature. Single-crystal films of the high-temperature cubic polymorph of bismuth oxide were epitaxially electrodeposited from an aqueous solution onto single-crystal gold substrates. The 35.4 percent lattice mismatch was accommodated by forming coincidence lattices in which the bismuth oxide film was rotated in relation to the gold substrate. These results provide a method for producing other nonequilibrium phases that cannot be accessed by traditional thermal processing.

https://science.sciencemag.org/content/sci/284/5412/293.full.pdf

Electrodeposited ceramic single crystals

Epitaxial electrodeposition was used to crystallize a 0.5 μm thick film of Cu2O onto single-crystal Au(100) from an alkaline copper lactate solution at room temperature. The film showed strong out-of-plane and in-plane orientation, with no rotation of the respective crystallographic directions of the Cu2O overlayer relative to the Au substrate. The technique should be amenable to the deposition of other metal oxides, allowing the simple and inexpensive selection of surfaces that show high catalytic activity or crystallographic directions that have desired electrical or magnetic properties.

Epitaxial Electrodeposition of Copper(I) Oxide on Single-Crystal Gold(100)

Epitaxial cuprous oxide, Cu2O, nanocrystals are deposited electrochemically onto n-InP(001). The lattice of the Cu2O is rotated 45° around the common [001] axis normal to the substrate plane. The shape of the Cu2O nanocrystals depends on the pH of the solution. Nanopyramids form at pH 9, and nanocubes form at pH 12.

Eric W. Bohannan

Papers

Enantiospecific electrodeposition of a chiral catalyst

Epitaxial Electrodeposition of Zinc Oxide Nanopillars on Single-Crystal Gold

Electrodeposited ceramic single crystals

Epitaxial Electrodeposition of Copper(I) Oxide on Single-Crystal Gold(100)

Shape Control in Epitaxial Electrodeposition: Cu2O Nanocubes on InP(001)