155,372 research outputs found
With No Deliberate Speed: The Segregation of Roma Children in Europe
In this study, by taking the advantage of both inorganic ZnO nanoparticles and the organic material chitosan as a composite seed layer, we have fabricated well-aligned ZnO nanorods on a gold-coated glass substrate using the hydrothermal growth method. The ZnO nanoparticles were characterized by the Raman spectroscopic techniques, which showed the nanocrystalline phase of the ZnO nanoparticles. Different composites of ZnO nanoparticles and chitosan were prepared and used as a seed layer for the fabrication of well-aligned ZnO nanorods. Field emission scanning electron microscopy, energy dispersive X-ray, high-resolution transmission electron microscopy, X-ray diffraction, and infrared reflection absorption spectroscopic techniques were utilized for the structural characterization of the ZnO nanoparticles/chitosan seed layer-coated ZnO nanorods on a gold-coated glass substrate. This study has shown that the ZnO nanorods are well-aligned, uniform, and dense, exhibit the wurtzite hexagonal structure, and are perpendicularly oriented to the substrate. Moreover, the ZnO nanorods are only composed of Zn and O atoms. An optical study was also carried out for the ZnO nanoparticles/chitosan seed layer-coated ZnO nanorods, and the obtained results have shown that the fabricated ZnO nanorods exhibit good crystal quality. This study has provided a cheap fabrication method for the controlled morphology and good alignment of ZnO nanorods, which is of high demand for enhancing the working performance of optoelectronic devices
Oxidation mechanism in metal nanoclusters: Zn nanoclusters to ZnO hollow nanoclusters
Zn nanoclusters (NCs) are deposited by Low-energy cluster beam deposition
technique. The mechanism of oxidation is studied by analysing their
compositional and morphological evolution over a long span of time (three
years) due to exposure to ambient atmosphere. It is concluded that the
mechanism proceeds in two steps. In the first step, the shell of ZnO forms over
Zn NCs rapidly up to certain limiting thickness: with in few days -- depending
upon the size -- Zn NCs are converted to Zn-ZnO (core-shell), Zn-void-ZnO, or
hollow ZnO type NCs. Bigger than ~15 nm become Zn-ZnO (core-shell) type: among
them, NCs above ~25 nm could able to retain their initial geometrical shapes
(namely triangular, hexagonal, rectangular and rhombohedral), but ~25 to 15 nm
size NCs become irregular or distorted geometrical shapes. NCs between ~15 to 5
nm become Zn-void-ZnO type, and smaller than ~5 nm become ZnO hollow sphere
type i.e. ZnO hollow NCs. In the second step, all Zn-void-ZnO and Zn-ZnO
(core-shell) structures are converted to hollow ZnO NCs in a slow and gradual
process, and the mechanism of conversion proceeds through expansion in size by
incorporating ZnO monomers inside the shell. The observed oxidation behaviour
of NCs is compared with theory of Cabrera - Mott on low-temperature oxidation
of metal.Comment: 9 pages, 8 figure
High quality ZnO layers with adjustable refractive indices for integrated optics applications
Thin ( 1 μm) crystalline ZnO films with a good optical quality and good (0002) texture are grown under two considerably different process parameter sets using a r.f. planar magnetron sputtering unit. The optical parameters of the two corresponding ZnO layers are distinctly different: high refractive index ( 2.0 at λ = 632.8 nm) ZnO films resembling the single crystal form, and ZnO films with considerably lower (typical difference 0.05) refractive indices. The refractive index of the latter ZnO layers is adjustable ( 1.93–1.96 at λ = 632.8 nm) through the process deposition parameters. It is shown that the difference in refractive index between the two ZnO types most probably results from a difference in package density of the crystal columns. The optical waveguide losses of both ZnO types are typically 1–3 dB/cm at λ = 632.8 nm, however the low refractive index ZnO layers need a post-deposition anneal step to obtain these values. The two ZnO types are used to fabricate optical channel-and slab waveguides with small refractive index differences.\u
The growth of ZnO crystals from the melt
The peculiar properties of zinc oxide (ZnO) make this material interesting
for very different applications like light emitting diodes, lasers, and
piezoelectric transducers. Most of these applications are based on epitaxial
ZnO layers grown on suitable substrates, preferably bulk ZnO. Unfortunately the
thermochemical properties of ZnO make the growth of single crystals difficult:
the triple point 1975 deg C., 1.06 bar and the high oxygen fugacity at the
melting point p_O2 = 0.35 bar lead to the prevailing opinion that ZnO crystals
for technical applications can only be grown either by a hydrothermal method or
from "cold crucibles" of solid ZnO. Both methods are known to have significant
drawbacks. Our thermodynamic calculations and crystal growth experiments show,
that in contrast to widely accepted assumptions, ZnO can be molten in metallic
crucibles, if an atmosphere with "self adjusting" p_O2 is used. This new result
is believed to offer new perspectives for ZnO crystal growth by established
standard techniques like the Bridgman method.Comment: 6 pages, 6 figures, accepted for J. Crystal Growt
Room temperature ferromagnetism in chemically synthesized ZnO rods
We report structural and magnetic properties of pure ZnO rods using X-ray
diffraction (XRD), magnetization hysteresis (M-H) loop and near edge x-ray fine
structure spectroscopy (NEXAFS) study at O K edge. Sample of ZnO was prepared
by co-precipitation method. XRD and selective area electron diffraction
measurements infer that ZnO rods exhibit a single phase polycrystalline nature
with wurtzite lattice. Field emission transmission electron microscopy, field
emission scanning electron microscopy micrographs infers that ZnO have rod type
microstructures with dimension 200 nm in diameter and 550 nm in length. M-H
loop studies performed at room temperature display room temperature
ferromagnetism in ZnO rods. NEXAFS study reflects absence of the oxygen
vacancies in pure ZnO rods.Comment: 8 Pages, 3 Figure
Electronic, Mechanical, and Piezoelectric Properties of ZnO Nanowires
Hexagonal [0001] nonpassivated ZnO nanowires are studied with density
functional calculations. The band gap and Young's modulus in nanowires which
are larger than those in bulk ZnO increase along with the decrease of the
radius of nanowires. We find ZnO nanowires have larger effective piezoelectric
constant than bulk ZnO due to their free boundary. In addition, the effective
piezoelectric constant in small ZnO nanowires doesn't depend monotonously on
the radius due to two competitive effects: elongation of the nanowires and
increase of the ratio of surface atoms
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