Optical properties of bulk Zn1−xCoxO magnetic semiconductors

Polycrystalline Zn_1−xCo_xO (x=0, 0.02, 0.05, 0.10 and 0.15) oxides have been synthesized by solid state reaction via sintering ZnO and Co powders in open air. X-ray diffraction analyses using Rietveld refinement indicate that a stoichiometric single phase with a wurtzite-like structure was found in Zn_1−xCo_xO samples with x up to 0.10. The elemental mapping using energy dispersive X-ray spectroscopic analyses presents a uniform distribution of Co. Optical transmittance measurements show that several extra absorption bands appear in the Co-doped ZnO, which is due to the transitions between the crystal-field-split 3d levels of tetrahedral Co²⁺ substituting Zn²⁺ ions. Raman measurements show that limited host lattice defects are induced by Co doping. Magnetization measurements reveal that the Co-doped ZnO samples are paramagnetic due to the absence of free carriers and in low temperature the dominant magnetic interaction is nearest-neighbor antiferromagnetic.     

Structural, electrical, optical and magnetic properties of Co0.2AlxZn0.8−xO films

Co_0.2Al_xZn_0.8−xO films prepared with different molar ratio of aluminum nitrate to zinc acetate were deposited on substrates by the sol-gel technique. X-ray diffraction, photoluminescence and ferromagnetism measurements were used to characterize the Co_0.2Al_xZn_0.8−xO diluted magnetic semiconductors. The authors found that the intensity of the acceptor-related photoluminescence increased with increasing aluminum concentration and an increase in the number of the acceptor-like defects (zinc vacancies especially) in the Co_0.2Al_xZn_0.8−xO film might lead to the enhancement of the magnetic properties. This implies that controls of the aluminum concentration and the number of the acceptor-like defects are important factors to produce strong ferromagnetism Co_0.2Al_xZn_0.8−xO films prepared by the sol-gel method.     

Study on the doping stability and electronic structure of wurtzite Zn1−xCdxO alloys by first-principle calculations

An ab initio calculation based on density functional theory is applied to study the doping stability and electronic structure of wurtzite Zn_1−xCd_xO alloys. It is found that the different alloy configurations of Zn_1−xCd_xO with a given Cd content are possible thermodynamically, but having different band gaps. With increasing Cd content, the formation enthalpy of Zn_1−xCd_xO alloy increases sharply. The Cd-content dependence of the band-gap values can be fitted with a second-order polynomial. The reduction of band gap can be attributed to the contributions of the hybridization of Zn-4s and Cd-5s, the enhancement of p-d repulsion, and the tensile strain due to Cd-doping.     

Low-temperature thermoelectric properties of Bi85Sb15−xNbx alloys prepared by high-press sintering

The nanocrystalline materials with the general formula Bi₈₅Sb_15−xNb_x (x=0, 0.5, 1, 2, 3) were prepared by mechanical alloying and subsequent high-pressure sintering. Their transport properties involving electrical conductivity, Seebeck coefficient and thermal conductivity have been investigated in the temperature range of 80-300 K. The absolute value of Seebeck coefficient of Bi₈₅Sb₁₃Nb₂ reaches a maximum of 161 μV/K at 105 K, which is 69% larger than that of Bi₈₅Sb₁₅ at the same temperature. The power factor and figure-of-merit are 4.45×10⁻³ WK⁻²m⁻¹ at 220 K and 1.79×10⁻³ K⁻¹ at 196 K, respectively. These results suggest that thermoelectric properties of Bi₈₅Sb₁₅ based material can be improved by Nb doping.     

High temperature thermoelectric properties of Nb-doped ZnO ceramics

Solid-state reaction processing technique was used to prepare Zn_xNb_1−xO (0≤x≤0.02) polycrystalline bulk samples. In the present study, we find that their lattice parameters a and c tend to decrease with increasing amount of Nb additive. The electrical conductivity of all the Zn_1−xNb_xO samples increased with increasing temperature, indicating a semiconducting behavior in the measured temperature range. The addition of Nb₂O₅ to ZnO led to an increase in the electrical conductivity and a decrease in the absolute value of the Seebeck coefficient. The best performance at 1000 K has been observed for nominal 0.5 at% Nb-doped ZnO, with an electrical resistivity of about 73.13 (S cm⁻¹) and Seebeck coefficient of ∼257.36 μV K⁻¹, corresponding to a power factor (S²σ) of 4.84×10⁻⁴ Wm⁻¹ K⁻². The thermal conductivity, κ, of the oxide decreased as compared to pure ZnO. The figure of merit ZT values of ZnO-doped Nb₂O₅ samples are higher than the ZnO pure sample, demonstrating that the Nb₂O₅ addition is fairly effective for enhancing thermoelectric properties.     

Structural and photoluminescence analysis of Zn1 − xMnxO nanocrystalline powders

This Letter reports on structural and photoluminescence properties of Zn_1 − xMn_xO nanocrystalline powders, which were synthesized by using oxalate precursor decomposition method. From the XRD features, we have noticed that all samples exhibit wurtzite crystal structure. The origin of photoluminescence properties of Mn doped and undoped ZnO have been discussed.     

Phase transformation behavior and dielectric characteristics of BaTi1−x(Al1/2Nb1/2)xO3 (0.01≤x≤0.40) ceramics

Microstructure, phase transformation behavior and dielectric properties of BaTi_1−x(Al_1/2Nb_1/2)_xO₃ (0.01≤x≤0.40) ceramics were investigated. A high level of (Al_1/2Nb_1/2)⁴⁺ substitution for Ti⁴⁺ ions was not conducive to the stability of the perovskite structure and resulted in the formation of BaAl₂O₄. As x was increased, lattice constants and unit cell volume decreased, reached a minimum at x=0.10 and then increased. The BaTi_1−x(Al_1/2Nb_1/2)_xO₃ ceramics at room temperature experienced a transformation from ferroelectric to paraelectric phase with increasing (Al_1/2Nb_1/2)⁴⁺ concentration. Meanwhile, permittivity of the BaTi_1−x(Al_1/2Nb_1/2)_xO₃ ceramics was markedly reduced, while Q value was slightly increased. Frequency dispersion of dielectric peak was obviously increased as x was increased from 0.01 to 0.10. It is of great interest that a dielectric abnormity represented by a broad dielectric peak at 200-400 K was observed for the composition with x=0.40.     

On the structural analysis and magnetic behavior of heavily doped Zn1−xMnxO nanopowders synthesized by wet chemistry method

Nanocrystalline Zn_1−xMn_xO(x=0−0.1) powders are prepared by polymeric precursor method and their structural and magnetic properties carefully studied. X-ray diffraction studies and Raman spectroscopy reveal that Mn²⁺ ions have substituted the Zn²⁺ ion without changing the würtzite structure of pristine ZnO up to Mn concentrations x≤0.05. The presence of a secondary phase, related to the solubility of Mn in ZnO is evident for higher Mn-doping concentrations. The negative value obtained for the Curie–Weiss temperature indicates that the interactions between the Mn ions are predominantly antiferromagnetic. Thus, no bulk ferromagnetism is evident in any of the studied samples.     

Observation of optical properties related to room-temperature ferromagnetism in co-sputtered Zn1−xCoxO thin films

We report on the analysis of optical transmittance spectra and the resulting ferromagnetic characteristics of sputtered Zn_1−xCo_xO films. Zn_1−xCo_xO films were prepared on (0001)-oriented Al₂O₃ substrates by the radio-frequency (rf) magnetron co-sputtering method. The XRD results showed that the crystallinity of films was properly maintained up to x=0.30 and no second phase peaks were detected up to x=0.40. The transmittance spectra showed both the increase of the absorption band intensity and the red shift of the absorption peak as well as the band edge with increasing x. We have proved experimentally that these changes depend on Co concentration. These optical properties suggest that sp-d exchange interactions and typical d-d transitions become activated with increasing x, which leads to the enhancement of ferromagnetic properties in Zn_1−xCo_xO films as shown in the AGM results. Therefore, it is concluded that the ferromagnetism derives from the substitution of Co²⁺ for Zn²⁺ without changing the wurtzite structure.     

Influence of Co doping content on its valence state in Zn1−xCoxO (0 ≤ x ≤ 0.15) thin films

Zn_1−xCo_xO (0 ≤ x ≤ 0.15) thin films grown on Si (1 0 0) substrates were prepared by a sol-gel technique. The effects of Co doped on the structural, optical properties and surface chemical valence states of the Zn_1−xCo_xO (0 ≤ x ≤ 0.15) films were investigated by X-ray diffraction (XRD), ultraviolet-visible spectrometer and X-ray photoelectron spectroscopy (XPS). XRD results show that the Zn_1−xCo_xO films retained a hexagonal crystal structure of ZnO with better c-axis preferred orientation compared to the undoped ZnO films. The optical absorption spectra suggest that the optical band-gap of the Zn_1−xCo_xO thin films varied from 3.26 to 2.79 eV with increasing Co content from x = 0 to x = 0.15. XPS studies show the possible oxidation states of Co in Zn_1−xCo_xO (0 ≤ x ≤ 0.05), Zn_0.90Co_0.10O and Zn_0.85Co_0.15O are CoO, Co₃O₄ and Co₂O₃, with an increase of Co content, respectively.     

Structural and optical properties of ZnO nanorods grown on MgxZn1−xO buffer layers

ZnO nanorod arrays were synthesized by chemical-liquid deposition techniques on Mg_xZn_1−xO (x = 0, 0.07 and 0.15) buffer layers. It is found that varying the Mg concentration could control the diameter, vertical alignment, crystallization, and density of the ZnO nanorods. The X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) data show the ZnO nanorods prefer to grow in the (0 0 2) c-axis direction better with a larger Mg concentration. The photoluminescence (PL) spectra of ZnO nanorods exhibit that the ultraviolet (UV) emission becomes stronger and the defect emission becomes weaker by increasing the Mg concentration in Mg_xZn_1−xO buffer layers.     

Effect of La substitution on conductivity and dielectric properties of Bi1−xLaxFeO3 ceramics: An impedance spectroscopy analysis

Bismuth ferrite (BFO) and La-substituted BFO with composition Bi_1−xLa_xFeO₃ (x=0.05, 0.1 and 0.15) (BLFO_x=0.05-0.15) ceramics were prepared using the solid state reaction route. A structural phase transition from rhombohedral phase to triclinic phase was observed for BLFO_x=0.05-0.15 ceramics. Modulus spectroscopy reveals the deviation of dielectric behavior from ideal Debye characteristics and the dependence of conductivity on ion hopping in BFO and BLFO_x=0.05-0.15 ceramics. The conductivity of the BFO ceramics decreases for La content of 5 mol%, followed by a subsequent increase with 10 and 15 mol% of lanthanum doping. The typical values of the activation energies at high temperature reveal the contribution of short range movement of doubly ionized oxygen vacancies to the conduction process in BFO and BLFO_x=0.05 ceramics. Both short range and long range motion of oxygen vacancies are responsible for large conductivity in BLFO_{x=0.1 and 0.15} ceramics.     

A study on the Raman spectra of Al-doped and Ga-doped ZnO ceramics

Al_xZn_1−xO and Ga_yZn_1−yO ceramics were synthesized through a solid-state reaction technique. The crystal phase of the samples was identified by an X-ray diffraction experiment. For each sample, the electrical resistivity was determined. The Al 2-mol%-doped and Ga 0.5-mol%-doped ZnO ceramics had the lowest resistivity. Raman measurement was performed to study the doping effects in the ZnO ceramics including ZnO single crystal as a reference. The line-shape parameters, q₁ and Γ₁, at the same certain doping rate and the solubility limit of Al (2 mol%) and Ga (0.5 mol%) in ZnO ceramics, are strongly related to the each other, and that the solubility limit plays an important role. The second-order Raman peak at 1162 cm⁻¹ of the ZnO ceramics was fitted by Fano formalism. The Fano’s fitting parameters, such as the lifetime of phonon and the degree of asymmetry degree of the second-order Raman peak changed as the amounts of dopants were varied.     

Cu(In,Ga)Se2 superstrate-type solar cells with Zn1−xMgxO buffer layers

Superstrate-type Cu(In,Ga)Se₂ (CIGS) thin film solar cells were fabricated using Zn_1−xMg_xO buffer layers. Due to the diffusion of Cd into CIGS during the growth of the CIGS layer, the conventional buffer material of CdS is not suitable. ZnO is a good candidate because of higher thermal tolerance but the conduction band offset (CBO) of ZnO/CIGS is not appropriate. In this study, the Zn_1−xMg_xO buffer layers were used to fulfill both the requirements. The superstrate-type solar cells with a soda-lime glass/In₂O₃:Sn/Zn_1−xMg_xO/CIGS/Au structure were fabricated with different band gap energies of the Zn_1−xMg_xO layer. The CIGS layers [Ga/(In + Ga)∼0.25] were deposited by co-evaporation method. The substrate temperature during the CIGS deposition of 450 °C did not cause the intermixing of the Zn_1−xMg_xO and CIGS layers. The conversion efficiency of the cell with Zn_1−xMg_xO was higher than that with ZnO due to the improvement of open-circuit voltage and shunt resistance. The results well corresponded to the behavior of the adjustment of CBO, demonstrating that the usefulness of the Zn_1−xMg_xO layer for the CBO control in the superstrate-type CIGS solar cells.     

The influence of microscopic parameters on the ionic conductivity of SrBi2(Nb1−xVx)2O9−δ(0≤x≤0.3) ceramics

Layered SrBi₂(Nb_1−xV_x)₂O_9−δ (SBVN) ceramics with x lying in the range 0-0.3 (30 mol%) were fabricated by the conventional sintering technique. The microstructural studies confirmed the truncating effect of V₂O₅ on the abnormal platy growth of SBN grains. The electrical conductivity studies were centred in the 573-823 K as the Curie temperature lies in this range. The concentration of mobile charge carriers (n), the diffusion constant (D₀) and the mean free path (a) were calculated by using Rice and Roth formalism. The conductivity parameters such as ion-hopping rate (ω_p) and the charge carrier concentration (K′) term have been calculated using Almond and West formalism. The aforementioned microscopic parameters were found to be V₂O₅ content dependent on SrBi₂(Nb_1−xV_x)₂O_9−δ ceramics.     

Enhanced ferroelectric, magnetic and magnetoelectric properties of Bi1−xCaxFe1−xTixO3 solid solutions

We report on the enhanced electromechanical, magnetic and magnetoelectric properties of Bi_1−xCa_xFe_1−xTi_xO₃ solid solutions. The crystal structure of the x≈0.25 compounds are close to the rhombohedral-orthorhombic phase boundary, and the solid solutions are characterized by increased electromechanical properties due to the polarization extension near the polar-nonpolar border. The homogenous weakly ferromagnetic state is established at x>0.15 doping. The chemical doping shifts the magnetic transition close to room temperature, thus enlarging the magnetic susceptibility of the compounds. The solid solutions at the morphotropic phase boundary exhibit a nearly twofold increase in piezoelectric response, whereas the magnetoelectric coupling shows five times enhancement in comparison with the parent bismuth ferrite.     

Properties of MgxZn1−xO thin films sputtered in different gases

Mg_xZn_1−xO alloy films were prepared on sapphire substrates using Ar and N₂ as the sputtering gases. The effect of the sputtering gas on the structural, optical and electrical properties of the Mg_xZn_1−xO films was studied. By using N₂ as the sputtering gas, the Mg_xZn_1−xO film shows p-type conductivity and the band gap is larger than that employing Ar as the sputtering gas. The reason for this phenomenon is thought to be related to the reaction between N-O or N-Zn, and the N-doping.     

Effect of fabrication parameters on the electrical conductivity of YxSr1−xTiO3 for anode materials

Yttrium-doped strontium titanate (Y_xSr_1−xTiO₃), as probable anode material for SOFC, was prepared by solid-state reaction. The solubility of yttrium in SrTiO₃ at different temperatures was examined and the electrical conductivities of Y_xSr_1−xTiO₃ were measured from 500 to 1000 °C. The effects of doping amount, fabrication atmosphere, and sintering temperature on the electrical conductivity of Y_xSr_1−xTiO₃ were investigated. Y_xSr_1−xTiO₃ with x=0.08 was found to give the maximum electrical conductivity, 71 S/cm at 800 °C in pure hydrogen. Reducing atmospheres and appropriate sintering temperatures play a positive role in improving the electrical conductivity.     

Inelastic X-ray scattering investigations of lattice dynamics in SmFeAsO1−xFysuperconductors

We report measurements of the phonon density of states as probed with inelastic X-ray scattering in SmFeAsO_1−xF_y powders. An unexpected strong renormalization of phonon branches around 23 meV is observed as fluorine is substituted for oxygen. Phonon dispersion measurements on SmFeAsO_1−xF_y single crystals allow us to identify the 21 meV A_1g in-phase (Sm,As) and the 26 meV B_1g (Fe,O) modes to be responsible for this renormalization, and may reveal unusual electron-phonon coupling through the spin channel in iron-based superconductors.     

The effect of growth ambient on the structural and optical properties of MgxZn1−xO thin films

Mg_xZn_1−xO (x = 0.0-0.20) thin films have been deposited by sol-gel technique on glass substrates and the effect of growth ambient (air and oxygen) on the structural, and optical properties have been investigated. The films synthesized in both ambient have hexagonal wurtzite structure. The c-axis lattice constant decreases linearly with the Mg content (x) up to x = 0.05, and 0.10 respectively for air- and oxygen-treated films, above which up to x = 0.20, the values vary irregularly with x. The change in the optical band gap values and the ultraviolet (UV) peak positions of Mg_xZn_1−xO films show the similar change with x. These results suggest that the formation of solid solution and thus the structural and optical properties of Mg_xZn_1−xO thin films are affected by the growth ambient.