Structural Transformations in self-assembled Semiconductor Quantum Dots as inferred by Transmission Electron Microscopy

Transmission electron microscopy studies in both the scanning and parallel illumination mode on samples of two generic types of self-assembled semiconductor quantum dots are reported. III-V and II-VI quantum dots as grown in the Stranski-Krastanow mode are typically alloyed and compressively strained to a few %, possess a more or less random distribution of the cations and/or anions over their respective sublattices, and have a spatially non-uniform chemical composition distribution. Sn quantum dots in Si as grown by temperature and growth rate modulated molecular beam epitaxy by means of two mechanisms possess the diamond structure and are compressively strained to the order of magnitude 10 %. These lattice mismatch strains are believed to trigger atomic rearrangements inside quantum dots of both generic types when they are stored at room temperature over time periods of a few years. The atomic rearrangements seem to result in long-range atomic order, phase separation, or phase transformations. While the results suggest that some semiconductor quantum dots may be structurally unstable and that devices based on them may fail over time, triggering and controlling structural transformations in self-assembled semiconductor quantum dots may also offer an opportunity of creating atomic arrangements that nature does not otherwise provide

Structural Transformations in self-assembled Semiconductor Quantum Dots as inferred by Transmission Electron Microscopy

Transmission electron microscopy studies in both the scanning and parallel illumination mode on samples of two generic types of self-assembled semiconductor quantum dots are reported. III-V and II-VI quantum dots as grown in the Stranski-Krastanow mode are typically alloyed and compressively strained to a few %, possess a more or less random distribution of the cations and/or anions over their respective sublattices, and have a spatially non-uniform chemical composition distribution. Sn quantum dots in Si as grown by temperature and growth rate modulated molecular beam epitaxy by means of two mechanisms possess the diamond structure and are compressively strained to the order of magnitude 10 %. These lattice mismatch strains are believed to trigger atomic rearrangements inside quantum dots of both generic types when they are stored at room temperature over time periods of a few years. The atomic rearrangements seem to result in long-range atomic order, phase separation, or phase transformations. While the results suggest that some semiconductor quantum dots may be structurally unstable and that devices based on them may fail over time, triggering and controlling structural transformations in self-assembled semiconductor quantum dots may also offer an opportunity of creating atomic arrangements that nature does not otherwise provide

Ferrimagnetic Heusler tunnel junctions with fast spin-transfer torque switching enabled by low magnetization

Magnetic random access memory that uses magnetic tunnel junction memory cells is a high performance, non-volatile memory technology that goes beyond traditional charge-based memories. Today its speed is limited by the high magnetization of the memory storage layer. Here we show that fast and highly reliable switching is possible using a very low magnetization ferrimagnetic Heusler alloy, Mn3Ge. Moreover, the tunneling magnetoresistance is the highest yet achieved for a ferrimagnetic material at ambient temperature. Furthermore, the devices were prepared on technologically relevant amorphous substrates using a novel combination of a nitride seed layer and a chemical templating layer. These results show a clear path to the lowering of switching currents using ferrimagnetic Heusler materials and, therefore, to the scaling of high performance magnetic random access memories beyond those nodes possible with ferromagnetic devices.Comment: main manuscript 14 pages, 4 main figures and supplementary information. Submitted to Nature naotechnolog

Analyzing semiconductor nanostructures by scanning transmission electron microscopy.

Analyzing semiconductor nanostructures by scanning transmission electron microscopy

One-pot synthesis of graphene-molybdenum oxide hybrids and their application to supercapacitor electrodes

Electrochemical double-layer capacitor (EDLC) electrodes are conventionally based on carbon materials, which provide increased chemical stability and the possibility of a large number of charge and discharge cycles. However, their specific capacitance is generally much lower than pseudo-capacitors based on metal oxide or conductive polymer electrodes. Carbon-based electrodes for electrochemical devices can be hybridized with metal oxide functionalities in order to provide catalytic activity that increases their electrochemical performances. We report the preparation of a conductive hybrid electrode of reduced graphene oxide and molybdenum oxide by a facile one-pot hydrothermal synthesis. A three-dimensional network structure comprising graphene and molybdenum oxide was obtained when phosphomolybdic acid was used as a precursor for molybdenum oxide. The hybrid material contains polycrystalline nanoparticles of molybdenum (IV) oxide (MoO2) that covers the surface of reduced graphene oxide. Compared to graphene electrodes, the hybrid electrodes showed significantly improved specific capacitance, as good as three times higher (381 vs. 140 F/g), and considerable reduction of the equivalent series resistance (by about half) when they were used in a supercapacitor with sodium containing aqueous electrolyte

Chiral domain wall motion in unit-cell thick perpendicularly magnetized Heusler films prepared by chemical templating

Heusler compounds are of great interest for spintronic applications. Here the authors report current driven domain wall motion in unit cell thick perpendicularly magnetized Heusler films with low current densities and show the velocity is dominated by the bulk chiral Dzyaloshinskii–Moriya exchange interaction

Tetragonal Mn3Sn Heusler films with large perpendicular magnetic anisotropy deposited on metallic MnN underlayers using amorphous substrates

Tetragonal Heusler compounds that exhibit large perpendicular magnetic anisotropy are promising materials for advanced spintronic devices. A prerequisite are thin films whose tetragonal axis is oriented perpendicular to the plane of the films. Here we show that highly textured, (001) oriented, tetragonal Mn3Sn layers can be prepared using metallic zinc-blende (ZB) MnN as underlayers. Moreover, we show that these layers can be deposited on amorphous substrates using reactive magnetron sputtering. The ferrimagnetic Mn3Sn layers exhibit perpendicularly magnetized hysteresis loops with coercive fields of ∼2 T. Stoichiometric ZB-MnN underlayers share an “equivalent” Mn-Mn layer at the interface with Mn3Sn, thus promoting their oriented growth. Other nitride underlayers are not effective due to their rock-salt (RS) crystal structure and the absence of Mn. Density functional theory calculations confirm that tetragonal Mn3Sn Heusler films are energetically stable when interfaced with ZB-MnN underlayers and not with any of the other RS nitride underlayers considered here. Such Heusler compounds have much promise as electrodes for magnetic tunnel junction memory elements for deeply scaled magnetic random access memories