Direct observation of elemental segregation in InGaN nanowires by X‐ray nanoprobe

Using synchrotron radiation nanoprobe, this work reports on the elemental distribution in single In_x Ga_1–xN nanowires (NWs) grown by molecular beam epitaxy directly on Si(111) substrates. Single NWs dispersed on Al covered sapphire were characterized by nano‐X‐ray fluorescence, Raman scattering and photoluminescence spectroscopy. Both Ga and In maps reveal an inhomogeneous axial distribution inside sin‐ gle NWs. The analysis of NWs from the same sample but with different dimensions suggests a decrease of In segregation with the reduction of NW diameter, while Ga distribution seems to remain unaltered. Photoluminescence and Raman scattering measurements carried out on ensembles of NWs exhibit relevant signatures of the compositional disorder. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photo‐capacitance spectroscopy investigation of deep‐level defects in AlGaN/GaN hetero‐structures with different current collapses

We have investigated electronic deep levels in two AlGaN/GaN hetero‐structures with different current collapses grown at 1150 and 1100 °C by a photo‐capacitance spectroscopy technique, using Schottky barrier diodes. Three specific deep levels located at ～2.07, ～2.80, and ～3.23 eV below the conduction band were found to be significantly enhanced for severe current collapse, being in reasonable agreement with photoluminescence and capacitance–voltage characteristics. These levels probably originate in Ga vacancies and residual C impurities and are probably responsible for the current collapse phenomena of the AlGaN/GaN hetero‐structures. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal expansion behaviors of hcp and fcc Co nanowire arrays

The lattice parameters of as-prepared and annealed Co nanowires with hcp and fcc structures have been measured using the in situ high-temperature x-ray diffraction method. The hcp and fcc Co nanowires have been fabricated within the porous anodic alumina membranes by a direct-current electrodeposition technique. The results indicate that the variational quantity of the interplanar spacing for hcp Co nanowire arrays is bigger than that for fcc Co nanowire arrays in spite of as-prepared and annealed samples. The structural difference between hcp and fcc Co nanowires results in the different thermal expansion behaviors.     

Structural evolution of self‐assisted GaAs nanowires grown on Si(111)

GaAs nanowires are grown on Si(111) by self‐assisted molecular beam epitaxy, and the ratio between wurtzite and zinc‐blende phases is determined as function of nanowire length using asymmetric X‐ray diffraction. We show that under the applied growth conditions, nanowires grow in both phases during the initial stage of growth, whereas the zinc‐blende content increases with growth time and dominates in long nanowires. Compared to the zinc‐blende units, the vertical lattice parameter of the wurtzite segments is 0.7% larger, as measured by the positions of respective diffraction peaks. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Strong blue emission from ZnSe nanowires grown at low temperature

We report optimized photoluminescence of ZnSe nanowires grown by molecular beam epitaxy, obtained by lowering the growth temperature down to 300 °C. The low‐temperature growth method has been developed using Si(111) and GaAs(111)B substrates. On the latter, vertical oriented blue‐emitting nanowires have been obtained. The growth mecha‐ nism is discussed with the help of in‐situ and ex‐situ electronic and structural measurements. We also report strong blue luminescence from ZnSe nanowires grown on ITO‐coated glasses, demonstrating that ZnSe nanowires are optimal candidates for transparent optoelectronics. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic and structural properties of Fe–Co nanowires fabricated by matrix synthesis in the pores of track membranes

Fe_1-x Co _x nanowires are obtained by electrochemical deposition into the pores of track-etched membranes. The characteristics of the growth process that allow controlling the length and aspect ratio of the nanowires are established. The elemental composition and magnetic properties of the nanowires depend on the diameter of the track-etched pores, which varies from 30 to 200 nm, and the electrochemical potential U (650–850 mV), which determines the nanowire growth rate. According to the results of elemental analysis and the Mössbauer spectroscopy data, the Co content in Fe_1-x Co _x lies in the range of x=0.20?0.25. It is found that the orientation of the magnetic moment of Fe–Co nanoparticles in the wires depends both on the track pore size d and on the nanowire growth rate. Thus, the magnetic moments in nanowires grown in 50-nm-diameter pores are oriented within 0°–40° with respect to the nanowire axis. The magnetic properties of the nanowires are explained in the framework of a theoretical model describing the magnetic dynamics of nanocomposites, which was extended to include the relaxation of the magnetization vector and to take into account interaction between the particles. The key physical parameters important for the technological applications of the nanowires are determined, their dependence on the nanowire growth conditions is traced, and the possibility of controlling them is established.     

Studies on the composition of ancient Indian punch‐marked silver coins

Punch‐marked coins are the oldest known numismatic objects used in ancient India. Seven punch‐marked silver coins were analysed by using the non‐destructive multi‐elemental PIXE technique. A 3 MeV proton beam from a pelletron accelerator was used for irradiation of these samples to obtain the spectral data and GUPIX software was employed to derive the elemental concentrations. The results reveals that silver, copper, gold and lead are significant constituents of the punch‐marked silver coins and there are also traces of elements such as Ti, Cr, Mn, Fe, Co, Ni, As and Y, which seems to imply that PIXE can be used effectively for the non‐destructive quantitative analysis of ancient coins. Copyright © 2004 John Wiley & Sons, Ltd.     

Aligned silicon nanowires with fine‐tunable tilting angles by metal‐assisted chemical etching on off‐cut wafers

The morphology control of aligned silicon nanowires (SiNWs) is highly desirable as SiNWs demonstrated high prospect in a variety of applications. Besides the control over length, shape and distribution of aligned SiNWs, the fine‐tuning of tilting angles thereof also attracted intense interest. Up to now, only several discrete tilting angles have been reported. In this Letter, the ability to fine‐tune the tilting angle of SiNWs is demonstrated and the range that can be achieved is identified. Our technique employs the anisotropic characteristic of the etching process using custom‐produced off‐cut Si wafers of various orientations as substrates. With this technique, a uniquely favoured etching direction can result and the tilting angle can be precisely controlled. Tilted SiNWs with tilting angles from 0° to 50° relative to the wafer normal were obtained. The mechanism of the tilting angle manipulation is also discussed. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Strain control in germanium nanowires: the use of a silicon nitride shell

The core–shell geometry is a strong tool for inducing and controlling strains in nano‐objects in order to tune their optoelectronic properties. We synthesized and characterized core–shell nanostructures by depositing a non‐epitaxial silicon nitride shell around germanium nanowires. Scanning electron microscopy as well as energy dispersive X‐ray spectroscopy confirms the structural integrity of the heterostructures, and grazing incidence X‐ray diffraction measurements reveal the presence of a radial tensile strain in the Ge nanowires. A control of this strain is then demonstrated up to 0.3% by adjusting the SiN_x shell thickness versus Ge nanowire diameter. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interaction of deep impurities with radiation defects in n-Si at γ-irradiation

This work presents the results of research on peculiarities of radiation defect formation in single crystal n-Si, doped by deep level impurities (Cu, Ni, Ir, Rh, Pt and Au), at irradiation by γ-quanta of ⁶⁰Co. A property of γ-irradiation to create only vacancies and self-interstitial atoms is used to understand the nature of deep levels with participation of these impurities and primary elemental radiation defects. The role of covalence radii and diffusion coefficients in efficiency of radiation defect formation is discussed.     

Nanowire manipulation on surfaces through electrostatic self‐assembly and magnetic interactions

A method for fabricating aligned nanowire arrays on surfaces is shown. Gold and segmented Au/Ni/Au nanowires of high aspect ratio have been prepared by template electrosynthesis, and functionalized with charged short alkanethiols that can be ionized in aqueous solutions. Different distributions of funtionalized nanowires could be obtained on large surfaces from nanowire aqueous suspensions, avoiding aggregation due to electrical repulsion. Due to the high magnetic anisotropy of segmented Au/Ni/Au nanowires chaining of aligned nanowires could be obtained by applying a low magnetic field. While electrostatics favours side wire interactions due to the high aspect ratio, concurrent electrostatics and applied magnetic field yields end‐to‐end interaction and linear alignment without bifunctionalization. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of metallic coatings on the photoluminescence properties of ZnO nanowires

We report on low‐temperature photoluminescence studies of ZnO nanowires coated with thin metallic films. For all analyzed metals (Al, In, Au, Ni, Cu), we find an increased relative intensity of the green deep‐level emission. This is accompanied by a significant reduction of the relative intensity of the surface exciton band. The observed effects are most likely related to the formation of metal induced gap states in the surface region of the ZnO nanowires. A model for the band structure in the surface region of the metal‐coated nanowires is proposed that successfully explains the changes in the photoluminescence spectra after the coating process. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of Gap Spacing between Dielectric Barriers in Atmospheric Pressure Discharges

The breakdown activity in helium atmospheric pressure dielectric barrier discharge (DBD) plasma is strongly modified by introducing small impurities (nitrogen (N₂) and air in ppm), although its precise implications for the behavior of discharge plasma is not evident under several constraints. In this simulation study, we investigate the influence of gap spacing between the dielectric barriers to explore the dynamic modification in the structure of discharge plasma in distinct phases of the discharge current pulse using a two‐dimensional fluid model in He‐air gas mixture. Specifically, the impact of nitrogen and air impurities is contrasted by exploring the spatial distributions of electrons in the breakdown phase under similar operating conditions. The filamentary mode of DBD plasma in He‐N₂ is transformed into uniform glow discharge in He‐air gas mixture by the dominant effect of Penning ionization. Finally, the outcomes of two‐dimensional fluid model are validated by comparing with three‐dimensional fluid model to provide the reliability of numerical simulations. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural,electronic and magnetic properties of Fe(1−x)Cox alloy nanowires encapsulated inside (10,0) boron nitride nanotube

The structural, electronic and magnetic properties of Fe–Co alloy nanowires encapsulated inside zigzag (10,0) boron nitride nanotube (BNNT) are investigated by ab initio calculations. Similar to pristine nanotube, the opposite directional relaxations for the N atoms (move outwards) and B atoms (move inwards) from their initial positions are observed for outside BNNT although with the Fe–Co alloy nanowires inside, but the outward relaxations of the N atoms bonding to the Fe or Co atoms are smaller due to their attractions. The combining processes of Fe–Co/BNNT composites are endothermic when Co concentration x≤0.6 and exothermic x>0.6, and the most stable Fe–Co/BNNT composite is at Co concentration x=0.8. So the semiconducting (10,0) BNNT can be used to shield the Co-rich Fe–Co nanowires. The charges are transferred from Fe–Co nanowires to BNNT and the formed Co–N bonds have covalent bond as well as slight ionic bond characters. Although (10,0) BNNT is nonmagnetic and a decrease in the magnetic moment is found after Fe–Co nanowires are encapsulated inside (10,0) BNNT, the Fe–Co/BNNT composites still have large magnetic moment, reflecting they can be utilized in magnetic storage and ultra high-density magnetic recording devices.     

Piezoelectric effect in GaAs nanowires: experiment and theory

The anomalous piezoelectric effect in GaAs nanowires was detected (the piezoelectric strain coefficient up to d₃₃ ≈ 26 pC/N) that is 12 times larger than the theoretically estimated value. This result is explained by the dominant content of the phase with the wurtzite‐type crystal structure in GaAs nanowires and an increased pressing force by the contact layer on the nanowire. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Growth and properties of self‐catalyzed (In,Mn)As nanowires

Mn‐assisted molecular beam epitaxy is used for the growth of (In,Mn)As nanowires (NWs) on GaAs(111)B. The transmission electron microscopy measurements revealed that despite the relatively high growth temperature regime this technique can be used to obtain (In,Mn)As NWs with high crystalline quality without any crystal defects, such as dislocations, stacking faults or precipitates inside the investigated NWs or on their side‐walls, although the growth processes of NWs were accompanied by the formation of MnAs precipitates between the NWs at the interface of the wetting layer. The results obtained are of importance for the realization of new spintronic nanostructured materials. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Orientation-controlled synthesis and magnetism of single crystalline Co nanowires

Orientation control and the magnetic properties of single crystalline Co nanowires fabricated by electrodeposition have been systematically investigated. It is found that the orientation of Co nanowires can be effectively controlled by varying either the current density or the pore diameter of AAO templates. Lower current density or small diameter is favorable for forming the (1 0 0) texture, while higher current values or larger diameter leads to the emergence and enhancement of (1 1 0) texture of Co nanowires. The mechanism for the manipulated growth characterization is discussed in detail. The orientation of Co nanowires has a significant influence on the magnetic properties, resulting from the competition between the magneto-crystalline and shape anisotropy of Co nanowires. This work offers a simple method to manipulate the orientation and magnetic properties of nanowires for future applications.     

Light‐induced degradation in copper‐contaminated gallium‐doped silicon

To date, gallium‐doped Czochralski (Cz) silicon has constituted a solar cell bulk material free of light‐induced degradation. However, we measure light‐induced degradation in gallium‐doped Cz silicon in the presence of copper impurities. The measured degradation depends on the copper concentration and the material resistivity. Gallium‐doped Cz silicon is found to be less sensitive to copper impurities than boron‐doped Cz silicon, emphasizing the role of boron in the formation of copper‐related light‐induced degradation. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

SSRL Workshop on STXM and X-ray Nanoprobe Capabilities and Needs in the Environmental,Geological, and Biomedical Sciences

Scanning transmission X-ray microscopy (STXM) has emerged as an important technique for chemical imaging and spectromicroscopy on the <100nm spatial scale in the environmental, geological, and biomedical sciences. Much has transpired technologically and scientifically in the 20-plus year interval that zone-plate STXM technology has been in development. Yet STXM/nanoprobe facilities are few in number, beam time remains exceedingly tight, and numerous scientific, technical, and infrastructure challenges continue to present significant barriers to the widespread use of these techniques in these three areas of science.     

First‐principle investigation of strain effects on the electronic properties of germanium nanowires

Using the GGA functional in density functional theory, the effects of axial strain on the band structure and effective mass of narrow [110] and [100] germanium nanowires are investigated. It is observed that both compressive and tensile strain cause indirect‐to‐direct bandgap transitions. One percent of tensile strain can cause a 40 meV change in the bandgap of [110] nanowires. Effective masses of electrons and holes are subject to a change of 3–4 times in the strain‐induced transition point. This change translates into a density of state modulation which opens new possibilities for the construction of Ge nanowire‐based sensors. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)