Unravelling the Elusive Antiferromagnetic Order in Wurtzite and Zinc Blende CoO Polymorph Nanoparticles

Although cubic rock salt‐CoO has been extensively studied, the magnetic properties of the main nanoscale CoO polymorphs (hexagonal wurtzite and cubic zinc blende structures) are rather poorly understood. Here, a detailed magnetic and neutron diffraction study on zinc blende and wurtzite CoO nanoparticles is presented. The zinc blende‐CoO phase is antiferromagnetic with a 3rd type structure in a face‐centered cubic lattice and a Néel temperature of T_N (zinc‐blende) ≈225 K. Wurtzite‐CoO also presents an antiferromagnetic order, T_N (wurtzite) ≈109 K, although much more complex, with a 2nd type order along the c‐axis but an incommensurate order along the y‐axis. Importantly, the overall magnetic properties are overwhelmed by the uncompensated spins, which confer the system a ferromagnetic‐like behavior even at room temperature.     

Synthesis and photoluminescence of a full zinc blende phase ZnO nanorod array

A single-crystalline ZnO nanorod array with rectangular cross-sections has been synthesized, in which the as-obtained products are a complete metastable zinc blende (ZB) phase. X-ray powder diffraction, electron microscopy, and elemental maps have been used to show that the ZB-ZnO samples have a lattice constant a = 4.580??, and are free from contamination by hexagonal wurtzite (HW) ZnO. Based on our experimental data, the associated growth mechanism is tentatively suggested. In addition, the photoluminescence (PL) spectrum (about 400?nm (3.1?eV)) of the as-fabricated ZB-ZnO products was detected; this is the first experimental report of the optical properties of ZB-ZnO nanorod arrays.     

Lattice expansion in ZnSe quantum dots

Solid ZnSe quantum dots (QDs) have been prepared via chemical route. The QDs have been characterized by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Photoluminescence (PL) and Ultraviolet-Visible Spectroscopy (Uv-Vis). The QD sizes were found to vary from 2.5 to 9.5 nm. The XRD measurement reveals an increase in the interplanar spacing in QDs as compared to their bulk counterpart. This observation is further supported by Rietveld analysis which establishes the formation of single phase zinc blende ZnSe QDs and confirms 3.9% lattice expansion. Calculations based upon the thermodynamical theory yield 8.7% concentration of vacancies due to the lattice expansion. We observe various peaks in the PL spectra which may arise either due to the QD size variations or the defects due to the vacancies.     

MnTe and ZnTe grown on sapphire by molecular beam epitaxy

We report on growth of MnTe layers by molecular beam epitaxy on Al₂O₃ substrates and of ZnTe layers on hybrid MnTe/Al₂O₃ substrates. The aim of our work was to prepare hexagonal phases of epitaxial thin films of these two materials. In the case of MnTe, the hexagonal NiAs-type phase was prepared by depositing the film directly on Al₂O₃ substrates. On the other hand, the crystal structure of ZnTe layers grown on hybrid MnTe/Al₂O₃ substrates was found to depend on the layer thickness: layers thinner than 0.05 μm grew in a metastable hexagonal wurtzite structure, but with further increases of the thickness, the cubic zinc blende phase of ZnTe tended to appear. The structural properties of MnTe and ZnTe layers were characterized by high energy electron and X-ray diffraction methods. Electrical properties of MnTe films were assessed by the Hall effect measurements. The topography and microstructure were analyzed by atomic force microscope. The Néel temperature and magnetic domains structure of antiferromagnetic hexagonal MnTe layers were obtained from neutron experiments.     

Stable structure of platinum carbides: A first principles investigation on the structure,elastic, electronic and phonon properties

A comprehensive first principles study of structural, elastic, electronic, phonon and thermodynamical properties of novel metal carbide, platinum carbide (PtC) is reported within the density functional theory scheme. The ground state properties such as lattice constant, elastic constants, bulk modulus, shear modulus and finally the enthalpy of PtC in zinc blende (ZB) and rock-salt (RS) structures are determined. The energy band structure and electron density of states for the two phases of PtC are also presented. Of these phases zinc blende phase of PtC is found stable and phase transition from ZB to RS structure occurs at the pressure of about 37.58 GPa. The phonon dispersion curves and phonon DOS are also presented. All positive phonon modes in phonon dispersion curves of ZB-PtC phase indicate a stable phase for this structure. Within the GGA and harmonic approximation, thermodynamical properties are also investigated. All results reveal that the synthesized PtC would favor ZB phase. The compound is stiffer and ductile in nature.     

Unit cell parameters of wurtzite InP nanowires determined by x-ray diffraction

High resolution x-ray diffraction is used to study the structural properties of the wurtzite polytype of InP nanowires. Wurtzite InP nanowires are grown by metal-organic vapor phase epitaxy using S-doping. From the evaluation of the Bragg peak position we determine the lattice parameters of the wurtzite InP nanowires. The unit cell dimensions are found to differ from the ones expected from geometric conversion of the cubic bulk InP lattice constant. The atomic distances along the c direction are increased whereas the atomic spacing in the a direction is reduced in comparison to the corresponding distances in the zinc-blende phase. Using core/shell nanowires with a thin core and thick nominally intrinsic shells we are able to determine the lattice parameters of wurtzite InP with a negligible influence of the S-doping due to the much larger volume in the shell. The determined material properties will enable the ab initio calculation of electronic and optical properties of wurtzite InP nanowires.     

Comparison of wurtzite and zinc blende III–V nitrides field effect transistors: a 2D Monte Carlo device simulation

An ensemble Monte Carlo method is used to compare the potentialities of zinc blende and wurtzite GaN for field effect transistor applications. First, bulk material electron transport properties are compared and we find that mobility, steady state velocity and velocity overshoot are at the advantage of zinc blende GaN. Then, zinc blende GaN and wurtzite GaN MESFET with very short gate length (L_g=0.12 μm) are investigated using a 2D Monte Carlo device simulation. A 50% gain in performance is obtained for the zinc blende GaN MESFET as compared with the wurtzite one. A zinc blende AlGaN/GaN HEMT is also simulated and exhibits a current density of 900 mA mm⁻¹, a transconductance of 480 mS mm⁻¹ and a cut-off frequency of 180 GHz.     

Difference between mechanochemical and thermal processes of polymorphic transformation of ZnS and PbO

Mechanochemical and thermal processes of the polymorphic transformations, litharge ? massicot and wurtzite ? zinc blende were compared. Massicot transformed readily to litharge with a slight mechanical stimulation at room temperature whereas the mechanochemical transformation in the opposite direction was almost impossible. The wurtzite ? zinc blende transformation was similar. On the other hand, simple heating of litharge at temperatures higher than 673K enabled the transformation into massicot without any difficulty. Wurtzite was also formed on heating zinc blende without any mechanical aids. The reason for this kind of apparent irreversibility of the transformation, being topotactic and including layer structures, is discussed.     

Atomistic simulation of epitaxial interfaces and polytypes

In this paper we discuss howab initio local density electronic structure calculations can be used to investigate extended defects such as interfaces and polytypes. LMTO-supercell calculations have been performed to understand the nature of bonding in epitaxial metal/ceramic interfaces such as Ag/MgO(001) and Ti/MgO(001). Cohesive and electronic properties of hexagonal polytypes of diamond, with different stacking sequences, have been predicted for the first time and compared with the available experimental data. The relative stabilities of 4H, 6H and 8H diamond polytypes have been calculated using a generalized version of force theorem.     

Estimates of internal stresses about interstitials in a bcc lattice

The atomic volume of interstitial cubic and hexagonal interstitial compounds has been found to correlate with the size of the filled octahedron in corresponding dilute solutions. Data from binary systems containing N, C, and O in V, Nb, Ta, Cr, Mo, W, and Fe lattices were examined. Systematic correlations enable estimates to be made of the components of the dipole tensor for interstitials in octahedral sites within selected bcc lattices. The volume change in filling an interstitial site is used to determine the sum of the diagonal tensor components, while their ratio is obtained from the second-neighbour displacement of a lattice atom about a filled site. Estimates from crystallographic data are obtained using either a volume correlation or a common shape factor along with the second-neighbour displacement. A filled octahedron tends to give equal first and second-neighbour distance parameters making it nearly regular. Estimates are made from isotropic and anisotropic elasticity. Lattice Green function calculations support the use of the second neighbour as a core displacement parameter. As expected, core displacements obtained from lattice theory can differ greatly from those obtained by elastic calculations except for the second-neighbour displacement. The influence of crystal anisotropy on the long range elastic field is examined. In niobium the first neighbour is displaced along an elastically soft direction and one finds the largest displacements of any system examined.     

Lattice parameter and microhardness investigations on CdTe1-xSex

CdTe1-xSex anion mixed crystals have been grown from non-stoichiometric melts and by means of a modified vertical travelling-heater method. Selenium has been found to have an essentially higher affinity for cadmium than tellurium. The cubic phase (zincblende structure) exists in the composition range 0xCdSe0.45, the hexagonal phase (wurtzite structure) in the range 0.65xCdSe1. In order to compare the lattice parameters of the cubic and hexagonal modifications, the mean distances between next-neighbouring anions and cations, da-c, were calculated from the lattice parameters. The relation between da-c values and composition x roughly obeys Vegard's rule. By means of Vickers indentation microhardness measurements, the solid-solution hardening (SSH) behaviour was investigated for the first time and has been compared with the SSH of II–VI cation solid solutions. Regarding the hexagonal CdTe1-xSex modifications, one has to pay attention to the large anisotropy of microhardness depending on the surface orientation. © 1998 Kluwer Academic Publishers     

Magnetic properties of Cd(1 - x)Mn(x)Te/C nanocrystals

Mn doped CdTe nanocrystals coated by carbon (Cd(1 - x)Mn(x)Te/C) were synthesized by a one-step, kinetically controlled solid state reaction under autogenic pressure at elevated temperatures. Electron microscopic analysis confirmed that the 40-52 nm Cd(1 - x)Mn(x)Te core was encapsulated by a 6-9 nm carbon shell. The efficient doping by Mn(2+) in the zinc blende Cd(1 - x)Mn(x)Te lattice, up to an atomic ratio of Mn/Cd of 0.031, was confirmed from electron paramagnetic resonance (EPR) experiments. In the case of higher doping, it is likely that manganese is partially expelled to the nanocrystal surface. All the doped samples exhibit ferromagnetism at room temperature. The lowest doped sample has the highest magnetic moment (1.91 ± 0.02 μ(B)/Mn). The more concentrated samples exhibit weaker ferromagnetic interactions, probably due to an incomplete coupling between carriers in the host CdTe semiconductor and dopant spins.     

Symmetry and crystal structure of Ti3SiC2

Convergent beam electron diffraction (CBED) and X-ray diffractometry (XRD) have been used to examine the symmetry, structure and lattice parameters of Ti₃SiC₂. The CBED analysis confirms that the material is hexagonal with a point group of 6/mmm. The XRD diffraction peak positions and intensities are in good agreement with calculations based on previously proposed atomic positions.     

Ferromagnetic and weak superparamagnetic like behavior of Ni-doped ZnS nanocrystals synthesized by reflux method

In this report we have studied the magnetism induced in the undoped and Ni-doped ZnS nanocrystals synthesized via low temperature reflux method. The average diameter of the undoped and Ni-doped nanoparticles is ~3 nm as revealed from transmission electron microscopy and X-ray diffraction (XRD) studies. From XRD studies, the structure of Ni-doped nanocrystals was observed as cubic zinc blende with lattice parameter, a = 0.539 nm. The band gap of the undoped and Ni-doped ZnS nanocrystals, analyzed with UV–visible spectroscopy, was found to be blue shifted as compared to the bulk counterpart. Quenching in photoluminescence spectra was observed at higher Ni concentrations as compared to undoped counterpart. The induced magnetization as analyzed from vibrating samples magnetometer indicated a weak superparamagnetic like behavior in 1 % Ni-doped ZnS nanocrystals, whereas; at 5 and 10 % Ni-doping concentrations, ferromagnetic behavior is indicated.     

Atomic Scale Elastic Textures Coupled to Electrons in Superconductors

We present an atomic scale theory of lattice distortions using strain-related variables and their constraint equations. Our approach connects constrained atomic length scale variations to continuum elasticity and can describe elasticity at all length scales. We apply the general approach to a two-dimensional square lattice with a monatomic basis, and find the atomic scale elastic textures around a structural domain wall and a single defect, as exemplar textures. We clarify the microscopic origin of anisotropic gradient terms, some of which are included phenomenologically in Landau–Ginzburg theory. The obtained elastic textures are used to investigate the effects of elasticity-driven lattice deformation on the nanoscale electronic structure in superconductor by solving the Bogliubov–de Gennes equations with the electronic degrees of freedom coupled to the lattice ones. It is shown that the order parameter is depressed in the regions where the lattice deformation takes place. The calculated local density of states suggests the electronic structure is strongly modulated as a response to the lattice deformation—the elasticity propagates the electronic response over long distances. In particular, the trapping of low-lying quasiparticle states around the defects is possible. These predictions could be directly tested by STM experiments in superconducting materials.     

Effect of lattice constant of zinc oxide on antibacterial characteristics

Zinc oxide powders were heated in different atmospheres at 800 and 1400 degrees C, of which the characterization and the antibacterial activity were studied by X-ray diffractometry and the measurement of the change in electrical conductivity with bacterial growth. The diffraction peaks corresponding to zinc oxide with hexagonal type structure were detected in all samples, which shifted in low-angle side with the increase in the oxidizability of atmosphere during heat-treatment. From the results of calculating lattice constants, a0 and c0, it was found that the value of c0 in hexagonal structure increased with the increase in the oxidizability of atmosphere. On the samples heated at 1400 degrees C, the changes of the c0 value were less than those at 800 degrees C. However, no change of the a0 value showed, irrespective of atmosphere and temperature. Hydrogen peroxide that contributes to the occurrence of antibacterial activity was found to generate from all samples, and the generation amount increased with the increase of c0 value; incidently the amount in the samples heated at 1400 degrees C was less than that at 800 degrees C. The antibacterial activity of zinc oxide increased with the increase of c0 value; that is, it was found that the value of c0 in crystal structure affected the antibacterial activity of zinc oxide.     

Microstructure and Optical Properties of Yttrium-doped Zinc Oxide(YZO) Nanobolts Synthesized by Hydrothermal Method

In this work,yttrium-doped zinc oxide(YZO) nanopowder was synthesized via hydrothermal precipitationmethod. The microstructure and optical properties of yttrium-doped zinc oxide nanopowder were characterized,which confirmed the well-crystalline wurtzite hexagonal phase of Zn O. The yttriumdoped zinc oxide nanopowder grains formed the nanobolts of ~400 nm in length and ~900 nm in width.High resolution-transmission electron microscopy(HR-TEM) of the nanobolts revealed uniform lattice fringes and no visible faults and/or distortions. X-ray photoelectron spectroscopy(XPS) analysis confirmed the presence of yttrium in the zinc oxide lattice,proving the contribution of yttrium on the microstructural and optical properties of the material. A strong ultra violet(UV) emission peak of the YZO exhibited a red shift compared to pure zinc oxide,which was ascribed to the defects and the formation of a shallow energy level caused by the incorporation of yttrium.     

Scanning Tunneling Luminescence of Individual CdSe Nanowires

The local luminescence properties of individual CdSe nanowires composed of segments of zinc blende and wurtzite crystal structures are investigated by low‐temperature scanning tunneling luminescence spectroscopy. Light emission from the wires is achieved by the direct injection of holes and electrons, without the need for coupling to tip‐induced plasmons in the underlying metal substrate. The photon energy is found to increase with decreasing wire diameter due to exciton confinement. The bulk bandgap extrapolated from the energy versus diameter dependence is consistent with photon emission from the zinc blende‐type CdSe sections.     

Studies on polycrystalline Cd0.96Zn0.04Te thin films prepared by vacuum evaporation

Polycrystalline Cd_0.96Zn_0.04Te thin films are deposited onto glass substrates (Corning 7059) kept at room temperature by vacuum evaporation. The films exhibit zinc blende structure with predominant (1 1 1) orientation. The rms roughness of the films evaluated by atomic force microscope is 3.7 nm. The band gap energy of the films measured by optical transmittance measurement is 1.539 eV. The photoluminescence (PL) spectrum of the films shows intense emission due to free and bound exciton recombination and no emission associated with crystal imperfection and PL line shapes give indications of the high quality of the layers. These films have been implanted with properly mass analyzed Boron ions (¹⁰B⁺) and the effect of implantation has been analyzed by X-ray diffraction, Raman scattering and optical transmittance measurements and the results are explained on the basis of the implantation induced surface roughness and lattice disorder.     

Effect of Trivalent Ion Substitution on the Physical Properties of M-Type Hexagonal Ferrites

Aluminum-substituted barium hexagonal ferrite particles BaAl _x Fe_12-x O₁₉ with 0 ≤ x ≤ 3.5 have been prepared by solid state reaction method. The qualitative phase analysis of studied powder samples and the morphology of powders after milling were determined using the x-ray diffraction method and scanning electron microscopy, respectively. The barium hexagonal ferrite phase appeared to be the main component of the samples. The crystal size of BaFe₁₂O₁₉ phase is above 25 nm. The scanning electron microscopy images showed irregular shape and size of powder particles. According to the analytical method findings, the type of crystal lattice was confirmed to be hexagonal and the parameters of unit cell volume and x-ray density were determined. It is shown that such parameters decrease with increasing Al substitution from 699.019 to 696.702 Å³ and 5.258 to 4.828 gm/Cm³, respectively. The values of lattice parameters, grain size, microstrain, and dislocation density of all samples were calculated. The c/a value obtained from the x-ray indicates that notable changes of the atomic lattice anisotropy were induced by the Al-substitution and preheat treatments. Characteristics such as the interchain distance and interplanar distance parameter, which were obtained in the analytical method calculations, decrease with increasing Al substitution, in addition to the fact that they are related to the binding energy.