Core–Shell GaAs-Fe Nanowire Arrays: Fabrication Using Electrochemical Etching and Deposition and Study of Their Magnetic Properties

The preparation of GaAs nanowire templates with the cost-effective electrochemical etching of (001) and (111)B GaAs substrates in a 1 M HNO3 electrolyte is reported. The electrochemical etching resulted in the obtaining of GaAs nanowires with both perpendicular and parallel orientations with respect to the wafer surface. Core–shell GaAs-Fe nanowire arrays have been prepared by galvanostatic Fe deposition into these templates. The fabricated arrays have been investigated by means of scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). The magnetic properties of the polycrystalline Fe nanotubes constituting the shells of the cylindrical structures, such as the saturation and remanence moment, squareness ratio, and coercivity, were analyzed in relation to previously reported data on ferromagnetic nanowires and nanotubes

Magnetic Properties of GaAs/NiFe Coaxial Core-Shell Structures

Uniform nanogranular NiFe layers with Ni contents of 65%, 80%, and 100% have been electroplated in the potentiostatic deposition mode on both planar substrates and arrays of nanowires prepared by the anodization of GaAs substrates. The fabricated planar and coaxial core-shell ferromagnetic structures have been investigated by means of scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). To determine the perspectives for applications, a comparative analysis of magnetic properties, in terms of the saturation and remanence moment, the squareness ratio, and the coercivity, was performed for structures with different Ni contents

Core–Shell Structures Prepared by Atomic Layer Deposition on GaAs Nanowires

GaAs nanowire arrays have been prepared by anodization of GaAs substrates. The nanowires produced on (111)B GaAs substrates were found to be oriented predominantly perpendicular to the substrate surface. The prepared nanowire arrays have been coated with thin ZnO or TiO2 layers by means of thermal atomic layer deposition (ALD), thus coaxial core–shell hybrid structures are being fabricated. The hybrid structures have been characterized by scanning electron microscopy (SEM) for the morphology investigations, by Energy Dispersive X-ray (EDX) and X-ray diffraction (XRD) analysis for the composition and crystal structure assessment, and by photoluminescence (PL) spectroscopy for obtaining an insight on emission polarization related to different recombination channels in the prepared core–shell structures

Electrochemical nanostructuring of (111) oriented GaAs crystals: From porous structures to nanowires

A comparative study of the anodization processes occurring at the GaAs(111)A and GaAs(111)B surfaces exposed to electrochemical etching in neutral NaCl and acidic HNO3 aqueous electrolytes is performed in galvanostatic and potentiostatic anodization modes. Anodization in NaCl electrolytes was found to result in the formation of porous structures with porosity controlled either by current under the galvanostatic anodization, or by the potential under the potentiostatic anodization. Possibilities to produce multilayer porous structures are demonstrated. At the same time, one-step anodization in a HNO3 electrolyte is shown to lead to the formation of GaAs triangular shape nanowires with high aspect ratio (400 nm in diameter and 100 μm in length). The new data are compared to those previously obtained through anodizing GaAs(100) wafers in alkaline KOH electrolyte. An IR photodetector based on the GaAs nanowires is demonstrated. © 2020 Monaico et al

Aero-TiO2 Prepared on the Basis of Networks of ZnO Tetrapods

In this paper, new aeromaterials are proposed on the basis of titania thin films deposited using atomic layer deposition (ALD) on a sacrificial network of ZnO microtetrapods. The technology consists of two technological steps applied after ALD, namely, thermal treatment at different temperatures and etching of the sacrificial template. Two procedures are applied for etching, one of which is wet etching in a citric acid aqua solution, while the other one is etching in a hydride vapor phase epitaxy (HVPE) system with HCl and hydrogen chemicals. The morphology, composition, and crystal structure of the produced aeromaterials are investigated depending on the temperature of annealing and the sequence of the technological steps. The performed photoluminescence analysis suggests that the developed aeromaterials are potential candidates for photocatalytic applications

Structural and vibrational properties of CdAl2S4 under high pressure: Experimental and theoretical approach

"This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/jp5037926.”The behavior of defect chalcopyrite CdAl2S4 at high pressures and ambient temperature has been investigated in a joint experimental and theoretical study. High-pressure X-ray diffraction and Raman scattering measurements were complemented with theoretical ab initio calculations. The equation of state and pressure dependences of the structural parameters of CdAl2S4 were determined and compared to those of other AB(2)X(4) ordered-vacancy compounds. The pressure dependence of the Raman-active mode frequencies is reported, as well as the theoretical phonon dispersion curves and phonon density of states at 1 atm. Our measurements suggest that defect chalcopyrite CdAl2S4 undergoes a phase transition above 15 GPa to a disordered-rocksalt structure, whose equation of state was also obtained up to 25 GPa. In a downstroke from 25 GPa to 1 atm, our measurements indicate that CdAl2S4 does not return to the defect chalcopyrite phase; it partially retains the disordered-rocksalt phase and partially transforms into the spinel structure. The nature of the spinel structure was confirmed by the good agreement of our experimental results with our theoretical calculations. All in all, our experimental and theoretical results provide evidence that the spinel and defect chalcopyrite phases of CdAl2S4 are competitive at 1 atm. This result opens the way to the synthesis of spinel-type CdAl2S4 at near-ambient conditions.Financial support from the Spanish Consolider Ingenio 2010 Program (Project CSD2007-00045) is acknowledged. This work was also supported by Spanish MICCIN under Project MAT2010-21270-C04-03/04 and by Vicerrectorado de Investigacion de la Universitat Politecnica de Valencia under Projects UPV2011-0914 PAID-05-11 and UPV2011-0966 PAID-06-11. Supercomputer time was provided by the Red Espanola de Supercomputacion (RES) and the MALTA cluster. J.A.S. acknowledges the Juan de la Cierva fellowship program for financial support. AM. and P.R.-H. acknowledge S. Munoz Rodriguez for providing a data-parsing application.Sans Tresserras, JÁ.; Santamaría Pérez, D.; Popescu, C.; Gomis, O.; Manjón Herrera, FJ.; Vilaplana Cerda, RI.; Muñoz, A.... (2014). Structural and vibrational properties of CdAl2S4 under high pressure: Experimental and theoretical approach. Journal of Physical Chemistry C. 118(28):15363-15374. https://doi.org/10.1021/jp5037926S15363153741182

Core–Shell GaAs-Fe Nanowire Arrays: Fabrication Using Electrochemical Etching and Deposition and Study of Their Magnetic Properties

The preparation of GaAs nanowire templates with the cost-effective electrochemical etching of (001) and (111)B GaAs substrates in a 1 M HNO3 electrolyte is reported. The electrochemical etching resulted in the obtaining of GaAs nanowires with both perpendicular and parallel orientations with respect to the wafer surface. Core–shell GaAs-Fe nanowire arrays have been prepared by galvanostatic Fe deposition into these templates. The fabricated arrays have been investigated by means of scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). The magnetic properties of the polycrystalline Fe nanotubes constituting the shells of the cylindrical structures, such as the saturation and remanence moment, squareness ratio, and coercivity, were analyzed in relation to previously reported data on ferromagnetic nanowires and nanotubes

Multilayer porous structures on GaN for the fabrication of Bragg reflectors

We report on the development of electrochemical etching technology for the production of multilayer porous structures |(MPS) allowing one to fabricate Bragg reflectors on the basis of GaN bulk substrates grown by Hydride Vapor Phase Epitaxy |(HVPE). The formation of MPS during anodization is caused by the spatial modulation of the electrical conductivity throughout the surface and the volume of the HVPE-grown GaN substrate, which occurs according to a previously proposed model involving generation of pits and their overgrowth. We found that the topology of the porous sheets constituting the MPS is different in the vicinity of N-face and Ga-face of the bulk wafer, it being of conical shape near the N-face and of hemispherical shape near the Ga-face. The composition of electrolytes, their concentration as well as the anodization potential applied during electrochemical etching are among technological parameters optimized for designing MPS suitable for Bragg reflector applications. It is shown also that regions with various porosities can be produced in depth of the sample by changing the anodization potential during the electrochemical etching

Spin-Coating and Aerosol Spray Pyrolysis Processed Zn1−xMgxO Films for UV Detector Applications

A series of Zn1−xMgxO thin films with x ranging from 0 to 0.8 were prepared by spin coating and aerosol spray pyrolysis deposition on Si and quartz substrates. The morphology, composition, nano-crystalline structure, and optical and vibration properties of the prepared films were studied using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), and optical and Raman scattering spectroscopy. The optimum conditions of the thermal treatment of samples prepared by spin coating were determined from the point of view of film crystallinity. The content of crystalline phases in films and values of the optical band gap of these phases were determined as a function of the chemical composition. We developed heterostructure photodetectors based on the prepared films and demonstrated their operation in the injection photodiode mode at forward biases. A device design based on two Zn1−xMgxO thin films with different x values was proposed for extending the operational forward bias range and improving its responsivity, detectivity, and selectivity to UV radiation

Magnetic Properties of GaAs/NiFe Coaxial Core-Shell Structures

Uniform nanogranular NiFe layers with Ni contents of 65%, 80%, and 100% have been electroplated in the potentiostatic deposition mode on both planar substrates and arrays of nanowires prepared by the anodization of GaAs substrates. The fabricated planar and coaxial core-shell ferromagnetic structures have been investigated by means of scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). To determine the perspectives for applications, a comparative analysis of magnetic properties, in terms of the saturation and remanence moment, the squareness ratio, and the coercivity, was performed for structures with different Ni contents