Ana Cremades

Growth and luminescence properties of micro- and nanotubes in sintered tin oxide

Sintering SnO2 under argon flow at temperatures in the range of 1350–1500°C causes the formation of wires, rods, and tubes on the sample surface. At high temperatures of the mentioned range, microwires with lengths of hundreds of microns are formed. At lower temperatures the formation of micro- and nanorods as well as micro- and nanotubes takes place. The influence of ball milling of the starting powder on the formation of tubes is investigated. The local cathodoluminescence measurements show a different defect structure in the tubes than in the sample background.

Growth and luminescence of elongated In2O3 micro- and nanostructures in thermally treated InN

Indium oxide elongated micro- and nanostructures have been grown by thermal treatment of InN powder. Chains of nanopyramids connected by nanowires, forming a necklace-like structure, as well as cubes and arrow-like structures consisting of a long rod with a micron size pyramid on the top, grow at temperatures in the range 600–700°C in a catalyst free process. The structures have been characterized by scanning electron microscopy and cathodoluminescence.

The effect of annealing in different atmospheres on the luminescence of polycrystalline TiO2

Polycrystalline samples of titanium oxide were prepared by thermal sintering, in argon and air atmospheres at temperatures ranging from 1100 to 1500??C, from powders of anatase and rutile phases. The samples sintered in argon were further treated in oxygen at the temperature of 800??C for intervals of time up to 8?h. The luminescence emission of the initial powders was situated in the green region, with the peak at 580?nm. The sintering treatment in argon leads to a decrease of the luminescence intensity that appears as a broad band peaked at 550?nm in the case of anatase and as a band peaked at 450?nm in the case of the rutile phase. The sintering treatment in air causes the quenching of the emission in the visible region. In both cases, a sharp and intense emission appears in the infrared region at 800?nm for rutile and 820?nm for the anatase phase. The blue emission is sensitive to the treatment in oxygen atmosphere which causes mainly an intensity increase of the band at 450?nm. The treatment in oxygen results in the reconstruction of the polycrystalline sample surface as hexagonal shape protrusions and large terraces.

Cathodoluminescence study of defects in deformed (110) and (100) surfaces of TiO2 single crystals

In this work, the effect of different surface orientations on the defect structure of TiO2 single crystals and the evolution of the luminescence properties under plastic deformation are investigated by cathodoluminescence (CL) microscopy. The main features of the spectra are an infrared band at 1.53 eV, and a complex band in the visible range, whose peak position depends on the electron beam energy, and is attributed to oxygen vacancy related defects. Comparison of the intensity of these bands in the spectra recorded at low electron beam voltage indicates that the Ti3+ defects present a higher concentration at the (110) surface, whereas the (100) surface presents a more complex defect structure related to the oxygen vacancies. Competition is observed between the visible and the infrared emission centres during plastic deformation of the samples in agreement with the different diffusion mechanisms of the defects involved in the emissions.

Cathodoluminescence of defects in sintered tin oxide

Cathodoluminescence (CL) in the scanning electron microscope (SEM) has been used to investigate the luminescence mechanisms in tin oxide. Sintered material prepared from high purity powder has been found to show a strong dependence of the CL emission on the thermal treatments applied during sample preparation. SEM images show the presence of nano and microcrystalline grains. The correlation of the grain size and morphology with the optical emission is analyzed by CL microscopy and spectroscopy. The main emission bands appear centered at about 2.58, 2.25, and 1.94 eV depending on the sintering treatment. CL images reveal that the 2.25 and the 2.58 eV bands are associated at specific crystal faces. The evolution of the luminescence bands with mechanical milling shows a complex evolution of the 1.94 and 2.58 eV emissions which is explained by formation and recovery of defects during milling.

Cathodoluminescence study of GaN epitaxial layers

Abstract GaN epitaxial layers have been investigated by cathodoluminescence (CL) in the scanning electron microscope (SEM). The most prominent feature of the spectra is a complex band at 2.2 eV, whose evolution with temperature and excitation density suggests emission mechanisms involving a deep center and donor-donor or donor-acceptor pairs. Time resolved photoluminescence (TRPL) measurements confirm the involvement of a deep center in the emission. CL images reveal that the centers responsible for this emission decorate grain boundaries. Emission bands at 2.87 eV and 1.31 eV have been also detected in the films.

Carrier Recombination at Screw Dislocations in n-Type AlGaN Layers

We analyse the recombination and scattering at dislocations in Si doped AlGaN layers by transmission electron microscopy (TEM), electron beam induced current (EBIC) and cathodoluminescence. We show that c-type screw dislocations are nonradiative recombination centres. From temperature dependent measurements of the EBIC contrast we find a shallow acceptor state 20 meV above the valence band. The level can be understood iri terms of one-dimensional dislocation bands split off by the dislocation deformation field. In addition piezoelectric potentials play a role.

Influence of Fe and Al doping on the stabilization of the anatase phase in TiO2 nanoparticles

Anatase TiO2 nanoparticles doped with Al or Fe have been synthesized via a modified Pechini method which allows us to reach high control in size and composition. Microstructural analysis confirms the good crystallinity of the doped anatase nanoparticles with average sizes around 5 nm and dopant cationic concentrations up to 30%. The anatase to rutile transition (ART) has been thermally driven and analyzed as a function of the doping. Thermo-diffraction measurements indicate that the phase transition can be either promoted or inhibited by Fe or Al doping, respectively. The influence of Al and Fe doping on the phase transition has been discussed by means of Raman spectroscopy, photoluminescence and X-ray photoelectron spectroscopy, with special attention paid to the role played by Ti3+ at the surface. The anatase phase has been stabilized up to temperatures above 900 °C by appropriate Al doping.

Effects of phase separation and decomposition on the minority carrier diffusion length in AlxGa1−xN films

Combined electron beam induced current and transmission electron microscopy (TEM) measurements have been performed on both undoped and Si-doped AlGaN epitaxial films with aluminum contents x ranging from x=0 to x=0.79, in order to correlate the electrical and structural properties of the films. The diffusion length of holes in the films ranges between 0.3 and 15.9 μm, and the estimated lifetime of holes for doped samples varies between 0.2 ns and 16 μs. Different effects contribute to the observed increase in the diffusion length with increasing aluminum content. Among others, dislocations seem to be active as nonradiative recombination sites, and phase separation and decomposition as observed by TEM in Al-rich alloys lead to the formation of a spatially indirect recombination path due to the piezoelectric field in the films. Potential fluctuations associated with these phase irregularities could also give rise to electron induced persistent conductivity contributing to the increase of the diffusion lengt...

Thermal growth, luminescence and whispering gallery resonance modes of indium oxide microrods and microcrystals

Indium oxide microcrystals with shapes of truncated cubes and pyramids as well as microrods with square, triangular or hexagonal cross-sections are grown by a catalyst free evaporation deposition method. Size and shape control of the crystals as a function of the treatment parameters is discussed. Their luminescence properties are investigated and some features in the cathodoluminescence images are suggested to be related to the possible formation of a core dislocation. The waveguiding behaviour of the rods as well as whispering gallery resonances with improved Q-factor are demonstrated for hexagonal and square cross-section rods. The effect of tapering of the rods on the resonance wavelength is also studied and fitted to a theoretical model.