Influence of surface diffusion on the formation of hollow nanostructures induced by the Kirkendall effect: the basic concept

The Kirkendall effect has been widely applied for fabrication of nanoscale hollow structures, which involves an unbalanced counterdiffusion through a reaction interface. Conventional treatment of this process only considers the bulk diffusion of growth species and vacancies. In this letter, a conceptual extension is proposed: the development of the hollow interior undergoes two main stages. The initial stage is the generation of small Kirkendall voids intersecting the compound interface via a bulk diffusion process; the second stage is dominated by surface diffusion of the core material (viz., the fast-diffusing species) along the pore surface. This concept applies to spherical as well as cylindrical nanometer and microscale structures, and even to macroscopic bilayers. As supporting evidence, we show the results of a spinel-forming solid-state reaction of core-shell nanowires, as well as of a planar bilayer of ZnO-Al2O3 to illustrate the influence of surface diffusion on the morphology evolution.     

Fast fabrication of long-range ordered porous alumina membranes by hard anodization

Nanoporous anodic aluminium oxide has been widely used for the development of various functional nanostructures. So far these self-organized pore structures could only be prepared within narrow processing conditions. Here we report a new oxalic-acid-based anodization process for long-range ordered alumina membranes. This process is a new generation of the so-called "hard anodization" approach that has been widely used in industry for high-speed fabrication of mechanically robust, very thick (>100 microm) and low-porosity alumina films since the 1960s. This hard anodization approach establishes a new self-ordering regime with interpore distances, (D(int))=200-300 nm, which have not been achieved by mild anodization processes so far. It offers substantial advantages over conventional anodization processes in terms of processing time, allowing 2,500-3,500% faster oxide growth with improved ordering of the nanopores. Perfectly ordered alumina membranes with high aspect ratios (>1,000) of uniform nanopores with periodically modulated diameters have been realized.     

Electrical Detection and Magnetic Imaging of Stabilized Magnetic Skyrmions in Fe1−xCoxGe (x < 0.1) Microplates

Magnetic skyrmions are topologically protected spin textures that are being investigated for their potential use in next generation magnetic storage devices. Here, magnetic skyrmions and other magnetic phases in Fe_1?xCo_xGe (x < 0.1) microplates (MPLs) newly synthesized via chemical vapor deposition are studied using both magnetic imaging and transport measurements. Lorentz transmission electron microscopy reveals a stabilized magnetic skyrmion phase near room temperature (≈280 K) and a quenched metastable skyrmion lattice via field cooling. Magnetoresistance (MR) measurements in three different configurations reveal a unique anomalous MR signal at temperatures below 200 K and two distinct field dependent magnetic transitions. The topological Hall effect (THE), known as the electronic signature of magnetic skyrmion phase, is detected for the first time in a Fe_1?xCo_xGe nanostructure, with a large and positive peak THE resistivity of ≈32 nΩ cm at 260 K. This large magnitude is attributed to both nanostructuring and decreased carrier concentrations due to Co alloying of the Fe_1?xCo_xGe MPL. A consistent magnetic phase diagram summarized from both the magnetic imaging and transport measurements shows that the magnetic skyrmions are stabilized in Fe_1?xCo_xGe MPLs compared to bulk materials. This study lays the foundation for future skyrmion‐based nanodevices in information storage technologies.     

Atomic layer deposition on biological macromolecules: metal oxide coating of tobacco mosaic virus and ferritin

Decoration of nanoparticles, in particular biomolecules, gathered high attention in recent years.(1-7) Of special interest is the potential use of biomolecules as templates for the fabrication of semiconducting or metallic nanostructures.(1-7,26) In this work we show the application of atomic layer deposition, a gas-phase thin film deposition process, to biological macromolecules, which are frequently used as templates in nanoscale science, and the possibility to fabricate metal oxide nanotubes and thin films with embedded biomolecules.(1-13).     

Cover Picture: Synthesis of Cobalt/Polymer Multilayer Nanotubes (Adv. Eng. Mater. 4/2005)

The cover image shows bundles of colbalt/polymer multilayer nanotubes synthesized by polymer infiltration in highly‐ordered macroporous silicon and released by a selective chemical etch from the template structure. The upper inset shows after polymer removal segments of cobalt nanotubes placed on the top surface of a perfect ordered alumina membrane obtained from imprint lithography. In the lower inset an array of mulitlayer magnetic nanotubes simbedded in a macroporous silicon matrix is presented. More information can be found in the article by K. Nielsch on page 217.     

Cover Picture: Templated Fabrication of Nanowire and Nanoring Arrays Based on Interference Lithography and Electrochemical Deposition (Adv. Mater. 19/2006)

On p. 2593, Ji and co‐workers report on a novel fabrication technique for ideally ordered lateral nanowire and nanoring arrays based on interference lithography and electrochemical deposition. This approach allows the fabrication of metallic and semiconductor nanowire or nanoring arrays over wafer‐scale areas and provides flexible control over shape, arrangement, and thickness of the nanowires and nanorings. The cover shows templated electrodeposited elliptical nanoring arrays and a cross‐section of electrodeposited nanowires.     

Recent Progress in Photonic Processing of Metal‐Oxide Transistors

Over the past few decades, significant progress has been made in the field of photonic processing of electronic materials using a variety of light sources. Several of these technologies have now been exploited in conjunction with emerging electronic materials as alternatives to conventional high‐temperature thermal annealing, offering rapid manufacturing times and compatibility with temperature‐sensitive substrate materials among other potential advantages. Herein, recent advances in photonic processing paradigms of metal‐oxide thin‐film transistors (TFTs) are presented with particular emphasis on the use of various light source technologies for the photochemical and thermochemical conversion of precursor materials or postdeposition treatment of metal oxides and their application in thin‐film electronics. The pros and cons of the different technologies are discussed in light of recent developments and prospective research in the field of modern large‐area electronics is highlighted.     

Ferromagnetism and Morphology of Annealed Fe2O3/CoXOY/ZnO Thin Films

We have studied thermal solid‐state reactions in the Fe₂O₃/Co_XO_Y/ZnO thin film systems grown using the atomic layer deposition technique. The compound produced after annealing at 700 °C is found to be a complex mixture of three different spinel phases: ZnCo₂O₄, CoFe₂O₄, and ZnFe₂O₄. The magnetic properties of the compound strongly depend on the atomic ratio of Fe³⁺ and Co²⁺ atoms, which can be set by choosing the corresponding thicknesses of the Fe₂O₃ and Co_XO_Y films. In addition, we also find a formation of 100 nm voids at the interface between Fe–Co–Zn–O compound and remaining ZnO film after 1h annealing at 700 °C in argon atmosphere. The formation of these voids shows indirectly the preferential outward diffusion of Zn²⁺ ions from ZnO into the Fe₂CoO₄ phase layer what we prove via our magnetic measurements.