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Transition-metal high-entropy oxides (HEOs) are promising electrode materials for lithium-ion batteries (LIBs) due to their superior electrochemical properties and excellent long-term cycling stability. The performance of HEOs for LIBs is highly correlated with their microstructures, especially their evolution during charging/discharging. However, there is limited information regarding this topic in the literature. In this study, the unique transition behavior of a spinel HEO, (CrMnFeCoNi)3O4, at various states of charge and cycle numbers is examined in detail for the first time. Although the elemental segregation of lithiated HEO particles is observed, the crystal structure remains spinel, leading to great cyclability. Mn nanocrystals form at 0.5 V lithiation and metallic Cr, Fe, Ni, and Co particles form at 0.01 V lithiation. The spinel CrxFe3-xO4 and LiNixCo1-xO2 phases act as seeds that grow by devouring surrounding metal nanoparticles during delithiation. The Mn can reversibly move at least dozens of nanometers across the oxide during lithiation/delithiation. The detailed cycling mechanism is examined using electron energy-loss spectroscopy. The reversible valence state variations of the constituent elements are observed. The results provide an in-depth understanding of the fundamental lithiation/delithiation mechanism of HEO, which will facilitate the development of better multi-element HEOs for Li+ storage applications.

One-dimensional polycrystallineTa2O5nanostructures are synthesized by the annealing of theSiO2nanowires at 950°C in a reductive Ta vapor ambiance. The formation mechanism ofTa2O5nanostructures is discussed and illustrated in detail. The nucleation and grain growth ofTa2O5crystals were investigated during the formation of theSiO2/Ta2O5core-shelled structures. The diffusion-controlled growth is suggested to be the rate-determining step for the diffusion of the Ta atoms through the ash layer to react with O atoms and substitute Si atoms.

Large-area MoS2 atomic layers are synthesized on SiO2 substrates by chemical vapor deposition using MoO3 and S powders as the reactants. Optical, microscopic and electrical measurements suggest that the synthetic process leads to the growth of MoS2 monolayer. The TEM images verify that the synthesized MoS2 sheets are highly crystalline., Comment: First submitted on 12-Dec-2011. Accepted in Adv. Mater

Magnetic patterning, with designed spatial profile of the desired magnetic properties, has been a rising challenge for developing magnetic devices at nanoscale. Most existing methods rely on locally modifying magnetic anisotropy energy or saturation magnetization and thus post stringent constraints on the adaptability in diverse applications. We propose an alternative route for magnetic patterning: by manipulating the local intergranular exchange coupling to tune lateral magnetic properties. As demonstration, the grain boundary structure of Co/Pt multilayers is engineered by thermal treatment, where the stress state of the multilayers and thus the intergranular exchange coupling can be modified. With Ag passivation layers on top of the Co/Pt multilayers, we can hinder the stress relaxation and grain boundary modification. Combining the pre-patterned Ag passivation layer with thermal treatment, we can design spatial variations of the magnetic properties by tuning the intergranular exchange coupling, which diversifies the magnetic patterning process and extends its feasibility for varieties of new devices.

Low-temperature synthesis of GaN nanowires is successfully achieved by silica-enhanced processes. GaN nanowires were synthesized by reaction of metal gallium vapor with ammonia and hydrogen gases at the temperature of 700 °C on the amorphous SiO2 substrate. From scanning electron microscopy images, the morphologies of GaN nanowires are wirelike with a length up to 5μm and the average diameter of GaN nanowires measured by transmission electron microscopy (TEM) is about 25 nm. The x-ray diffraction analysis of as-synthesized products indicates that the nanowires have the hexagonal wurtzite structure of GaN crystal. The corresponding electron diffraction pattern also indicates that the as-synthesized GaN nanowires exhibited a single-crystal feature with uniform oxygen doping characterized by electron energy loss spectroscopy. The compositional line profile of TEM analysis reveals that GaN nanowires are terminated by Au nanoparticles, which infer an evidence that the vapor-liquid-solid model is the major grow...

The two-dimensional layer of molybdenum disulfide (MoS2) has recently attracted much interest due to its direct-gap property and potential applications in optoelectronics and energy harvesting. However, the synthetic approach to obtain high quality and large-area MoS2 atomic thin layers is still rare. Here we report that the high temperature annealing of a thermally decomposed ammonium thiomolybdate layer in the presence of sulfur can produce large-area MoS2 thin layers with superior electrical performance on insulating substrates. Spectroscopic and microscopic results reveal that the synthesized MoS2 sheets are highly crystalline. The electron mobility of the bottom-gate transistor devices made of the synthesized MoS2 layer is comparable with those of the micromechanically exfoliated thin sheets from MoS2 crystals. This synthetic approach is simple, scalable and applicable to other transition metal dichalcogenides. Meanwhile, the obtained MoS2 films are transferable to arbitrary substrates, providing great opportunities to make layered composites by stacking various atomically thin layers., manuscript submitted on 11-Dec-2011, revision submitted on 16-Feb-2012

Formation of Pt nanoparticles or nanoisland films as a function of annealing temperature, initial thickness, underlying substrates, and annealing process is investigated. Using microphotoluminescence (PL) measurement, we find great enhancement of self-emission in visible spectrum from Pt nanoparticles. The integral intensity of the micro-PL of the 49.38nm Pt nanoparticles is 38 times of that of the Pt thin film. In addition, the peak wavelength varies from 554to615nm as the surface morphology of Pt changes due to different annealing parameters. Spectral analyses suggest that this enhancement of micro-PL from Pt is due to the local field enhancement mechanism analogous to that of PL from noble metals.

GaN nanowires were synthesized by reaction of metal gallium vapor with ammonia and hydrogen gases at the temperature of 700 °C on the amorphous SiO2 substrate. The morphologies of GaN nanowires are wirelike with a length up to 5 μm and the average diameter of GaN nanowires measured by transmission electron microscopy (TEM) is about 25 nm. The x-ray diffraction analysis of as-synthesized products indicates that the nanowires have the hexagonal wurtzite structure of GaN crystal. The corresponding electron diffraction pattern also indicates that the as-synthesized GaN nanowires exhibited a single-crystal feature with uniform oxygen doping characterized by electron energy loss spectroscopy.

A 15-nm-thick GdO x membrane in an electrolyte-insulator-semiconductor (EIS) structure shows a higher pH sensitivity of 54.2 mV/pH and enzyme-free hydrogen peroxide (H2O2) detection than those of the bare SiO2 and 3-nm-thick GdO x membranes for the first time. Polycrystalline grain and higher Gd content of the thicker GdO x films are confirmed by transmission electron microscopy (TEM) and X-ray photo-electron spectroscopy (XPS), respectively. In a thicker GdO x membrane, polycrystalline grain has lower energy gap and Gd2+ oxidation states lead to change Gd3+ states in the presence of H2O2, which are confirmed by electron energy loss spectroscopy (EELS). The oxidation/reduction (redox) properties of thicker GdO x membrane with higher Gd content are responsible for detecting H2O2 whereas both bare SiO2 and thinner GdO x membranes do not show sensing. A low detection limit of 1 μM is obtained due to strong catalytic activity of Gd. The reference voltage shift increases with increase of the H2O2 concentration from 1 to 200 μM owing to more generation of Gd3+ ions, and the H2O2 sensing mechanism has been explained as well.