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This work describes an oxidation process of iron-iron oxide core-shell nanowires at temperatures between 100 °C and 800 °C. The studied nanomaterial was synthesized through a simple chemical reduction of iron trichloride in an external magnetic field under a constant flow of argon. The electron microscopy investigations allowed determining that the as-prepared nanowires were composed of self-assembled iron nanoparticles which were covered by a 3 nm thick oxide shell and separated from each other by a thin interface layer. Both these layers exhibited an amorphous or highly-disordered character which was traced by means of transmission electron microscopy and Mössbauer spectroscopy. The thermal oxidation was carried out under a constant flow of argon which contained the traces of oxygen. The first stage of process was related to slow transformations of amorphous Fe and amorphous iron oxides into crystalline phases and disappearance of interfaces between iron nanoparticles forming the studied nanomaterial (range: 25-300 °C). After that, the crystalline iron core and iron oxide shell became oxidized and signals for different compositions of iron oxide sheath were observed (range: 300-800 °C) using X-ray diffraction, Raman spectroscopy and Mössbauer spectroscopy. According to the thermal gravimetric analysis, the nanowires heated up to 800 °C under argon atmosphere gained 37% of mass with respect to their initial weight. The structure of the studied nanomaterial oxidized at 800 °C was mainly composed of α-Fe2O3 (∼ 93%). Moreover, iron nanowires treated above 600 °C lost their wire-like shape due to their shrinkage and collapse caused by the void coalescence.

Self-organized Ga(Mn)As nanoclusters, embedded in GaAs, were formed during post-growth thermal annealing of Ga1−xMnxAs layers. Structural and magnetic properties of such composites were systematically studied as a function of the annealing temperature. Small (∼3 nm) Mn-rich zinc-blende Mn(Ga)As clusters, coherent with the GaAs matrix, were formed at the annealing temperature of 500 °C. An increase of the annealing temperature of up to 600 °C led to the creation of 10–20 nm large NiAs-type hexagonal MnAs nanocrystals. Magnetization measurements showed that the MnAs nanoprecipitates were superparamagnetic, with a distribution of blocking temperatures that depended on the MnAs cluster size. Some intermediate paramagnetic clusters (structurally disordered clusters) were also observed.

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SummaryThe main goal of this work is to study the structural and magnetic properties of iron nanowires and iron nanoparticles, which have been fabricated in almost the same processes. The only difference in the synthesis is an application of an external magnetic field in order to form the iron nanowires. Both nanomaterials have been examined by means of transmission electron microscopy, energy dispersive X-ray spectrometry, X-ray diffractometry and Mössbauer spectrometry to determine their structures. Structural investigations confirm that obtained iron nanowires as well as nanoparticles reveal core–shell structures and they are composed of crystalline iron cores that are covered by amorphous or highly defected phases of iron and iron oxides. Magnetic properties have been measured using a vibrating sample magnetometer. The obtained values of coercivity, remanent magnetization, saturation magnetization as well as Curie temperature differ for both studied nanostructures. Higher values of magnetizations are observed for iron nanowires. At the same time, coercivity and Curie temperature are higher for iron nanoparticles.

Infrared (IR) shielding materials are commonly used for different applications, such as smart windows or optical filters. Infrared radiation is responsible for about 50% of the energy coming from the sun. During a hot summer or cold winter a lot of energy is needed to keep the optimal temperature inside buildings and means of transport. To reduce the heat transmission and save energy IR shielding materials can be used as coatings made of polymer composites. Graphene oxide (GO) and its reduced forms have interesting IR absorption properties and might be used as a filler in a polymer matrix for IR shielding applications. Graphene oxide can be reduced by different methods. Depending on the reduction method reduced graphene oxide (rGO) with a different content of oxygen can be obtained exhibiting different properties. In this work we propose new polymer nanocomposites with poly(vinyl alcohol) as the matrix and 0.1 wt.% addition of graphene materials with different oxygen content to be used for IR shielding applications. The results show that the properties of the graphene filler strongly influence the infrared shielding properties of the obtained nanocomposites. The best IR shielding properties were obtained for the composites where rGO with the lowest oxygen content was used.

Kinetics of low-temperature activation of acceptors in magnesium-doped gallium nitride epilayers grown by metal-organic vapor-phase epitaxy J. Appl. Phys. 99, 033703 (2006); 10.1063/1.2168232Critical Mg doping on the blue-light emission in p -type GaN thin films grown by metal-organic chemical-vapor deposition J.Comprehensive studies of the electrical properties of Mg-doped bulk GaN crystals, grown by high-pressure synthesis, were performed as a function of temperature up to 750°C. Annealing of the samples in nitrogen ambient modifies qualitatively their resistivity values and the (T) variation. It was found that our material is characterized by a high concentration of oxygen-related donors and that the charge transport in the studied samples is determined by two types of states, one of shallow character ͑Mg-related state, E A Ϸ0.15 eV͒, and the second one much more deep, E 2 Ϸ0.95 eV ͑above the valence band͒. Depending on the effective concentration of either states, different resistivities can be observed: lower resistivity ͑Ͻ10 4 ⍀ cm at ambient temperature͒ in samples with dominant E A states and very high resistivity ͑Ͼ10 6 ⍀ cm at ambient temperature͒ in samples with dominant E 2 states. For the first type of samples, annealing at T ann Ͻ500°C leads to a decrease of their resistivity and is associated with an increase of the effective concentration of the shallow Mg acceptors. Annealing of both types of samples at temperatures between 600 and 750°C leads to an increase of the deep state concentration. The presence of hydrogen ambient during annealing of the low-resistivity samples strongly influences their properties. The increase of the sample resistivity and an appearance of a local vibrational mode of hydrogen at 3125 cm Ϫ1 were observed. These effects can be removed by annealing in hydrogen-free ambient.