A. Urbieta

Growth and luminescence properties of micro- and nanoneedles in sintered CdSe

Sintering CdSe powder under argon flow at temperatures in the range 800–900 °C produces the formation of needles on the sample surface. Bundles of parallel needles of a diameter of about 50 nm give rise to a domain-like appearance in the sample. In addition, rods and needles with a wide range of dimensions up to several microns appear distributed in the surface. The influence of ball milling of the starting powder on the formation of the needles is investigated. Cathodoluminescence in the scanning electron microscope has been used to characterize the sintered samples.

Study of structure and luminescence of CdSe Nanocrystals obtained by ball milling

The effect of mechanical milling on the structure and luminescent properties of CdSe powder has been investigated by means of cathodoluminescence (CL) in the scanning electron microscope, transmission electron microscopy (TEM) and x-ray diffraction. The starting powders were ball milled for times between 5 and 20h. TEM of the milled samples reveals the presence of nanocrystals with a wide range of sizes including nanocrystals of 10nm and smaller. The CL spectra of the untreated samples show the characteristic band edge emission centered at 1.8eV. In the milled samples a band centered at about 2.5eV is observed which appears to be associated to the presence of nanocrystals. The evolution of defect structure with milling time has been monitored by recording infrared cathodoluminescence spectra. A partial phase transformation from wurtzite to zinc blende structure also has been observed.

Cathodoluminescence and scanning tunnelling spectroscopy of ZnO single crystals

Bulk ZnO single crystals grown by the hydrothermal (HTT) and alkali flux methods have been investigated by means of scanning tunnelling spectroscopy (STS) and time resolved cathodoluminescence (CL). Measurements were performed in the different crystalline faces. The results from these measurements show that both, surface electrical properties and luminescent characteristics depend on the face studied. Polar O-terminated surfaces show an intrinsic conduction behaviour with a surface band gap ranging from 0.4 to 0.8 eV. Zn-terminated surfaces show mainly n-type conduction. The non-polar faces present either intrinsic or p-type behaviour. CL spectra show that the relative intensity of the different components of the deep level band also depends on the atomic structure of the face under study. This complex behaviour is clearly revealed from the time resolved spectra. The differences observed are attributed to the nature of the defects present in each case and, in particular, to different impurity incorporation processes that could be mainly controlled by the atomic configuration and polarity of the planes.

Scanning electron microscopy characterization of ZnSe single crystals grown by solid-phase recrystallization

ZnSe single crystals were grown from n-type microcrystalline boules by a Solid Phase Recrystallization (SPR) method. The recrystallizations were performed under different atmospheres, Ar or Se, and pressures to investigate the influence of growth conditions on the structural features of the resulting crystals. The samples were mechanically and mechano-chemically polished in a bromine methanol solution and, then, etched in HCl for a short time, before characterization. The homogeneity and the nature of defects in the crystals were studied by Cathodoluminescence (CL) in the scanning electron microscope (SEM). CL measurements show the existence of slip bands in the recrystallized samples, likewise CL spectra show that on these samples the dislocation related Y band is enhanced and the emission bands appearing in the 2.0-2.5 eV region depend on the annealing conditions. In addition during SPR, twinned regions appear with different electronic recombination properties.

Scanning tunnelling spectroscopy characterization of ZnO single crystals

Bulk ZnO single crystals grown by the hydrothermal and flux methods have been characterized by scanning tunnelling spectroscopy performed in the different crystalline faces. Normalized differential conductance has been found to depend on the face considered. Polar O-terminated surfaces show an intrinsic conduction behaviour and surface bandgap in the range 0.4-0.8 eV, which depends on the position probed. The Zn polar surfaces show mainly n-type conduction. The non-polar m regions present either intrinsic or p-type behaviour. The differences observed are attributed to the nature of impurities and defects appearing in the polar and non-polar surfaces during crystal growth.

Growth and Luminescence of Nanowires and Oriented Nanoplate Arrays of Mg Doped ZnO

Sintering of a ZnO-MgO powder mixture under argon flow leads to the growth of elongated Mg doped ZnO structures such as stacks of nanoplates and rods with secondary nanowires in a comb-like arrangement. The nanoplates grow perpendicular to a central axis and have a six-fold symmetry. The hierarchical growth is related to the presence of Mg. Cathodoluminescence (CL) in the scanning electron microscope shows a blue-shift of the ZnO exciton peak as well as changes in the deep level bands due to the influence of Mg.

Deformation-induced defect levels in ZnSe crystals

The influence of defects, in particular the deformation-induced defects, on the luminescence of bulk ZnSe single crystals has been investigated by cathodoluminescence (CL) in the scanning electron microscope. Deformation has been found to cause a reduction of the total CL intensity of the sample. CL images of deformed samples reveal dark slip bands. The CL spectrum of an undeformed crystal shows the near-band-edge emission at 2.8 eV and a broad band peaked at 2.2 eV with a shoulder at about 2 eV. Deformation at low strain causes only slight spectral changes while in a heavily deformed crystal a strong relative enhancement of the deep level band in the range 2-2.2 eV is observed. The relation of these spectral changes to the deformation-induced defects is discussed. The infrared spectra show the existence of broad bands at 0.95 and 1.27 eV. However, these emissions were found to be rather insensitive to deformation.

Scanning electron microscopy study of twins in ZnSe single crystals grown by solid-phase recrystallization

ZnSe single crystals were grown from n-type microcrystalline boules by a solid phase recrystallization (SPR) method. During SPR, twinned regions appear with different electronic recombination properties. The recrystallizations were performed under different atmospheres, Ar or Se, and pressures to investigate the influence of growth conditions on these structural features. Recombination properties were studied by means of cathodoluminescence (CL) and remote-electron beam induced current (REBIC). Wavelength dispersive X-ray (WDX) mappings were also performed to analyze possible differences in stoichiometry related to the presence of extended defects.

From spent alkaline batteries to ZnxMn3−xO4 by a hydrometallurgical route: synthesis and characterization

A series of Zn/Mn binary oxides with different molar ratios (1.4–11) were synthesized via co-precipitation from a solution obtained through the acidic (HCl) leaching of a black mass originating from the mechanical recycling of spent alkaline and Zn–C batteries. The oxides obtained were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Magnetic properties of the samples were also investigated. The Raman spectroscopy results showed all the binary metallic oxides belong to the ZnxMn3−xO4 (0.25 ≤ x ≥ 1.75) type. All showed a spinel crystalline structure. The saturation magnetization decreases with the Zn/Mn molar ratio; a maximum of 13.19 emu g−1 was found for the molar ratio of 11 at the Curie temperature (25.5 K). XPS showed that all the synthesized compounds contained Mn2+, Mn3+ and Mn4+. Mn2+ was the most prominent at a molar ratio of 11, Mn3+ was most common at a molar ratio of 2, and Mn4+ at 1.4.

Fast growth of undoped and Sn- and Tb-doped ZnO nanowires by Joule heating of Zn

Joule heating of Zn wires by flowing a high electric current density has been used to grow, in times as short as tens of seconds, undoped and Sn- or Tb-doped ZnO nanowires. The contact of tin oxide powder with the starting Zn wire during heating leads to the growth of branched ZnO nanostructures by a catalytic effect of Sn. The growth treatment in the presence of terbium oxide gives rise to a complex ZnO arrangement of nanoparticles attached to nanowires, which show luminescence emission of Tb3+ transition due to the incorporation of a small amount of Tb into the ZnO nanoparticles during the growth process. The contribution of electromigration, associated with the high electric current density, to the rapid growth of the nanowires, as compared with other thermal-based techniques, is discussed.