C. Díaz-Guerra

Cathodoluminescence and photoluminescence spectroscopy of NiO

Cathodoluminescence (CL) and photoluminescence (PL) of nickel oxide (NiO) have been investigated. The observed emission bands in the visible and near infrared spectral ranges are attributed to Ni^(2+) intraionic transitions. Some of the features observed in the CL spectra recorded from vacuum annealed samples are tentatively attributed to electronic transitions involving defect-induced states located inside the charge-transfer gap of this transition metal oxide.

Exchange bias in single-crystalline CuO nanowires

Exchange anisotropy has been observed and investigated in single-crystalline CuO nanowires grown by thermal oxidation of Cu. The exchange bias field decreases by increasing temperature and can be tuned by the strength of the cooling field. A training effect has also been observed. The obtained results can be understood in terms of a phenomenological core-shell model, where the core of the CuO nanowire shows antiferromagnetic behavior and the surrounding shell behaves as a spin glass-like system due to uncompensated surface spins.

Optical and magnetic properties of CuO nanowires grown by thermal oxidation

CuO nanostructures with different morphologies, such as single-crystal nanowires, nanoribbons and nanorods, have been grown by thermal oxidation of copper in the 380?900??C temperature range. Cathodoluminescence spectra of the nanostructures show a band peaked at 1.31?eV which is associated with near band gap transitions of CuO. Two additional bands centred at about 1.23 and 1.11?eV, suggested to be due to defects, are observed for nanostructures grown at high temperatures. The magnetic behaviour of nanowires with lengths in the range of several micrometres and diameters of 50?120?nm has been investigated. Hysteresis loops of the nanowires show ferromagnetic behaviour from 5?K to room temperature.

Structural and cathodoluminescence study of mechanically milled silicon

The structural and luminescent properties of nanocrystalline silicon produced by high-energy ball milling of Si single crystals have been investigated using transmission electron microscopy (TEM), x-ray diffraction (XRD) and cathodoluminescence (CL) in a scanning electron microscope. XRD measurements show that the average size of the nanocrystals in the milled samples is about 30 nm but TEM reveals a wide range of size distribution including crystallites with the dimension of few nanometres. Ball milling causes the appearance of a visible luminescence band at 1.61 eV, attributed to the presence of nanocrystals, and a near-infrared band peaked at about 0.79 eV which is suggested to be related to the high density of extended defects formed during the mechanical treatment. These bands, attributed to processes in Si, are not observed in the cathodoluminescent spectra of untreated and ball-milled SiO2 powder.

Laser irradiation-induced α to δ phase transformation in Bi2O3 ceramics and nanowires

The α-Bi2O3 to δ-Bi2O3 phase transformation has been locally induced by laser irradiation in ceramic samples and single-crystal nanowires of this oxide. The threshold power densities necessary to induce this transformation, as well as the corresponding transformation kinetics and its temporal stability, have been investigated in both kinds of samples by micro-Raman spectroscopy. The appearance of the δ phase was also monitored by spatially resolved photoluminescence spectroscopy. An emission band peaked near 1.67 eV, not observed in α-Bi2O3, is tentatively attributed to δ-Bi2O3 near band gap transitions.

Structural, magnetic and luminescent characteristics of Pr3+-doped ZrO2 powders synthesized by a sol–gel method

The structural, magnetic and luminescence properties of praseodymium-doped zirconia powders of compositions Pr0.03Zr0.97O2 and Pr0.05Zr0.95O2 synthesized by a sol?gel process have been investigated. X-ray diffraction patterns indicate that these materials crystallize in a tetragonal fluorite-type structure. Scanning electron microscopy shows that the powders exhibit an agglomerated microcrystalline structure and the grain size may be in the order of 5?20??m. The study of the magnetic properties of these doped metal oxides indicates a Curie?Weiss behaviour in the temperature range (100?300)?K that allow us to estimate an effective magnetic moment of 3.51??B, which indicates the presence of Pr3+ in the grown samples. Cathodoluminescence spectra recorded at temperatures between 85 and 295?K show emission peaks that can be attributed to transitions between different states within the 4f2 configuration of Pr3+ ions incorporated in the zirconia crystal lattice. Thermoluminescence measured at temperatures ranging from 373 to 773?K and at 550?nm wavelength show an intense and broad peak around 653?K for the Pr-doped zirconia which is not observed in the undoped material.

Intense luminescence emission from rare-earth-doped MoO3 nanoplates and lamellar crystals for optoelectronic applications

Strong and stable room-temperature photoluminescence (PL) emission is achieved in MoO3 nanoplates and lamellar crystals doped with Er and Eu by ion implantation and subsequent annealing. Micro-Raman and PL spectroscopy reveal that optical activation of the rare earth ions and recovery of the original MoO3 structure are achieved for shorter annealing treatments and for lower temperatures in nanoplates, as compared with lamellar crystals. Er seems to be more readily incorporated into optically active sites in the oxide lattice than Eu. The influence of the dimensionality of the host sample on the characteristics of the PL emission of both rare earth dopants is addressed.

Time-resolved cathodoluminescence assessment of deep-level transitions in hydride-vapor-phase-epitaxy GaN

The temporal behavior of deep-level luminescence emissions in undoped hydride-vapor-phase-epitaxy GaN layers of different thicknesses has been investigated by time-resolved cathodoluminescence (TRCL). The complex nature of the yellow luminescence is revealed in the TRCL spectra by the presence of two bands peaked at 2.22 and 2.03 eV. A red band with a decay time of 700 μs, centered at about 1.85 eV, dominates spectra recorded for long delay times. Exponential transients with associated decay times of hundreds of μs were measured at 87 K for all the deep-level emissions found in the layers.

Spatially resolved cathodoluminescence of GaN nanostructures fabricated by photoelectrochemical etching

The emission properties of GaN nanostructures created by photoelectrochemical etching have been investigated by cathodoluminescence (CL) in the scanning electron microscope. Columnar structures with diameters of 150-250 nm formed near the surface of the as-grown GaN layers branch into nanowires with diameters of 20-60 nm, while islands with coral-like relief were observed at the bottom of the etched areas. CL emission of the observed nanostructures is dominated by free electron to acceptor transitions. Local CL spectra provide direct evidence of the existence of either compressive or tensile stress in different nanostructures. No free exciton luminescence was observed in GaN nanowires, supporting their relation to threading dislocations.

Magnetic transitions in α-Fe2O3 nanowires

Magnetic transitions in single-crystal α-Fe2O3 (hematite) nanowires, grown by thermal oxidation of iron powder, have been studied in the range of 5–1023K with a superconducting quantum interference device below room temperature and with a vibrating sample magnetometer at higher temperatures. The broad temperature range covered enables us to compare magnetic transitions in the nanowires with the transitions reported for bulk hematite. Morin temperatures (TM) of the nanowires and of hematite bulk reference powder were found to be 123 and 263K, respectively. Also the Neel temperature (TN) of the nanowires, 852K, was lower than the bulk TN value. Measurements of the magnetization as a function of temperature show an enhanced signal in the nanowires, which suggests a decrease in the antiferromagnetic coupling. A coercive field observed below TM in the hysteresis loops of the nanowires is tentatively explained by the presence of a magnetic phase.