Ceria (CeO 2 ) plays a vital role in emerging technologies for environmental and energy-related applications. The catalytic efficiency of ceria nanoparticles depends on its morphology. In this study, CeO 2 nanoparticles were synthesized by a microwave-assisted hydrothermal method under different synthesis temperatures. The samples were characterized by X-ray diffraction, transmission electron microscopy, Raman scattering spectroscopy, electron paramagnetic resonance spectroscopy and by the Brunauer-Emmett-Teller method. The X-ray diffraction and Raman scattering results indicated that all the synthesized samples had a pure cubic CeO 2 structure. Rietveld analysis and Raman scattering also revealed the presence of structural defects due to an associated reduction in the valence of the Ce 4+ ions to Ce 3+ ions caused by an increasing molar fraction of oxygen vacancies. The morphology of the samples was controlled by varying the synthesis temperature. The TEM images show that samples synthesized at 80 C consisted of spherical particles of about 5 nm, while those synthesized at 120 C presented a mix of spherical and rod-like nanoparticles and the sample synthesized at 160 C consisted of nanorods with 10 nm average diameter and 70 nm length. The microwave-assisted method proved to be highly efficient for the synthesis of CeO 2 nanoparticles with different morphologies.

Ceria (CeO2) is a promising dilute magnetic semiconductor. Several studies report that the intrinsic and extrinsic structural defects are responsible for room temperature ferromagnetism in undoped and transition metal doped CeO2 nanostructures; however, the nature of the kind of defect necessary to promote and stabilize the ferromagnetism in such a system is still a matter of debate. In the work presented here, nanorods from the system Ce1-xCuxO2-δ with x = 0, 0.01, 0.03, 0.05 and 0.10, with the more stable {111} surface exposed were synthesized by a microwave-assisted hydrothermal method. A very careful structure characterization confirms that the Cu in the samples assumes a majority 2+ oxidation state, occupying the Ce (Ce(4+) and Ce(3+)) sites with no secondary phases up to x = 0.05. The inclusion of the Cu(2+) in the CeO2 structure leads to the introduction of oxygen vacancies in a density proportional to the Cu(2+) content. It is supposed that the spatial distribution of the oxygen vacancies follows the Cu(2+) distribution by means of the formation of a defect complex consisting of Cu(2+) ion and an oxygen vacancy. Superconducting quantum interference device magnetometry demonstrated a diamagnetic behavior for the undoped sample and a typical paramagnetic Curie-Weiss behavior with antiferromagnetic interactions between the Cu(2+) ions for the single phase doped samples. We suggest that the presence of oxygen vacancies is not a sufficient condition to mediate ferromagnetism in the CeO2 system, and only oxygen vacancies in the surface of nanostructures would lead to such a long range magnetic order.

We report experimental evidence of excitonic spin-splitting, in addition to the conventional Zeeman effect, produced by a combination of the Rashba spin-orbit interaction, Stark shift and charge screening. The electric-field-induced modulation of the spin-splitting are studied during the charging and discharging processes of p-type GaAs/AlAs double barrier resonant tunneling diodes (RTD) under applied bias and magnetic field. The abrupt changes in the photoluminescence, with the applied bias, provide information of the charge accumulation effects on the device.The effect of the spin-orbit (SO) interaction in quasitwo-dimensional (Q2D) systems has attracted renewed attention in recent years. The topic has been on the focus of many optical and transport investigations of spin-related phenomena in nanoscopic systems [1,2,3], a subject of great fundamental and technological interest [4,5,6,7]. In this letter, we address experimental evidence of electric field coupling to the spin degree of freedom of carriers in RTD; here in particular, the prevailing influence can be attributed to the SO and Stark effects on the hole electronic structure. These interactions are relevant to the study of the internal electric fields and the charge accumulation in the structure. The simultaneous investigation of optical and transport properties at high magnetic and electric parallel fields, has permitted a thorough characterization of the main processes involved in the system response. The novelty of this result consists of the optical detection of electric field modulation of the effective spin-splitting beyond the Zeeman effect and its unambiguous correlation to the transport mechanisms which is responsible for the charge buildup in the states of the RTD.This study is carried out on a symmetric p − i − p GaAs/AlAs RTD, that has been previously used to characterize hole space charge buildup and resonant effects in a magnetic field [8]. The structure is in the form of a 400µm diameter mesa with a metallic AuGe annular top contact to allow optical access. The diode was mounted in a superconducting magnet and the emission spectra were recorded using a double spectrometer coupled to a CCD system with polarizer facilities to select left (right) σ +(−) configurations. When light from an Ar + laser is focused close to the surface, minority electrons are created [8]. As the bias approaches a resonant condition, the carrier density inside the QW increases and then decreases, resulting in the negative differential resistance (NDR) region when the resonance is traversed. The photo-generated electrons tunneling into the QW layer can recombine with the injected holes or tunnel out of the well layer. These processes are represented schematically in the Fig. 1 (a).The I − V characteristics, shown in Fig. 1 (b), displays a series of peaks associated with the injected holes (I dark ) from the hole accumulation layer formed in the outside interface of the diode (see Fig. 1 (a)). Under illumination, an increase of current is observed (I light ) due to ...

Bulk Zn1−xCoxO samples were synthesized via standard solid-state reaction route with different Co molar concentrations up to 21%. A detailed microstructural analysis was carried out to investigate alternative sources of ferromagnetism, such as secondary phases and nanocrystals embedded in the bulk material. Conjugating different techniques we confirmed the Zn replacement by Co ions in the wurtzite ZnO structure, which retains, however, a high crystalline quality. No segregated secondary phases neither Co-rich nanocrystals were detected. Superconducting quantum interference device magnetometry demonstrates a paramagnetic Curie–Weiss behavior with antiferromagnetic interactions. We discuss the observed room temperature paramagnetism of our samples considering the current models for the magnetic properties of diluted magnetic semiconductors.

We have investigated polarization-resolved photoluminescence under applied voltage in p-i-p GaAs/ AlAs double-barrier diodes. We have observed oscillations in the degree of polarization up to 36% at B = 15 T with sign reversals occurring near to the hole subband resonances. At high voltages a polarization saturation up to 25% at B = 15 T is observed. The data are interpreted by using simulations based on a simple theoretical model that considers spin conservation for tunneling and the relaxation processes for carriers at Zeeman states in the quantum well. Our work offers the prospect for the development of voltage-controlled spin filtering systems using standard nonmagnetic semiconductor heterostructures.

Nanostructured Co-doped anatase TiO2 (Ti1–x Co x O2−δ) samples were prepared and studied with particular emphasis on their compositional, structural, and magnetic properties. A detailed microstructural analysis was carried out to investigate the nature of the Co incorporation into the anatase TiO2 matrix. By combining different techniques, we confirmed the replacement of Ti4+ by Co2+ ions in the anatase TiO2 structure. Neither segregated secondary phases nor Co-rich nanocrystals were detected. Co doping was found to introduce oxygen vacancies into the system by means of a charge-compensation process. Superconducting quantum interference device magnetometry demonstrated paramagnetic Curie–Weiss behavior with antiferromagnetic interactions even in the presence of a high density of oxygen vacancies. The fitting of the M(H) curves in the limits of low and high temperatures enable the fractions of isolated and antiferromagnetically coupled Co ions to be extracted. We discuss the observed magnetic behavior of our samples considering the current main theories for the magnetic properties of dilute magnetic oxides.

In this report we present a systematic structural and magnetic analysis of Co-doped ZnO nanoparticles prepared via a microwave-assisted hydrothermal route. The structural data confirm the incorporation of Co ions into the wurtzite ZnO lattice and a Co concentration mainly near/at the surface of the nanoparticles. This Co spatial distribution is set to passivate the surface of the ZnO nanoparticles, inhibiting the nanoparticle growth and suppressing the observation of a ferromagnetic phase. Based on experimental and theoretical results we propose a kinetic-thermodynamic model for the processes of nucleation and growth of the Co-doped ZnO nanoparticles, and attribute the observed ferromagnetic order to a ferromagnetism associated with specific defects and adsorbed elements at the surface of the nanoparticle. Our findings give valuable contribution to the understanding of both the doping process at the nanoscale and the nature of the magnetic properties of the Co-doped ZnO system.

The authors investigate the circular polarization of the electro- and photoluminescence emissions from the quantum well and contact layers of a nonmagnetic GaAs–AlAs p-i-n resonant tunneling diode under an external magnetic field. The contact emission evidences the formation of a spin polarized two-dimensional electron gas at the n-accumulation layer. The quantum well electroluminescence presents a strong σ− degree of polarization, even for null Zeeman splitting energies, and a slight bias dependence. The observed circular polarization is mainly attributed to the spin polarization of the electrons injected into the quantum well from the two-dimensional electron gas.