Antisite Oxygen Research Articles

Toward Versatile Sr2FeMoO6-Based Spintronics by Exploiting Nanoscale Defects.

To actualize the high spintronic application potential of complex magnetic oxides, it is essential to fabricate these materials as thin films with the best possible magnetic and electrical properties. Sr2FeMoO6 is an outstanding candidate for such applications, but presently no thin film synthesis route, which would preserve the magnetic properties of bulk Sr2FeMoO6, is currently known. In order to address this problem, we present a comprehensive experimental and theoretical study where we link the magnetic and half metallic properties of Sr2FeMoO6 thin films to lattice strain, Fe-Mo antisite disorder and oxygen vacancies. We find the intrinsic effect of strain on the magnetic properties to be very small, but also that an increased strain will significantly stabilize the Sr2FeMoO6 lattice against the formation of antisite disorder and oxygen vacancies. These defects, on the other hand, are recognized to drastically influence the magnetism of Sr2FeMoO6 in a nonlinear manner. On the basis of the findings, we propose strain manipulation and reductive annealing as optimization pathways for improving the spintronic functionality of Sr2FeMoO6.

Reversible magnetocaloric response in Sr2FeMoO6 double pervoskite

In the present work, polycrystalline Sr2FeMoO6 double perovskite sample has been synthesized by solid state reaction. The magnetization-temperature curve shows the transition temperature at 358K, which is lower than normal; this is attributed to the anti-site defects and oxygen vacancies present in the sample. A low maximum magnetic entropy change−∆SMmax, and the relative cooling power equal to 0.09Jkg−1K−1 and 8Jkg−1 for ΔH=1T, respectively was observed. On the other hand, Arrott plots and mean field theory analysis reveals the second order of ferromagnetic transition at T=TC. The phenomenon of reversible magnetocaloric effect makes this compound interesting for exploration as a future magnetic refrigerant.

Luminescent, optical and electronic properties of La3Ta0.5Ga5.5O14 single crystals grown in different atmospheres

Luminescent, optical and electronic properties of La3Ta0.5Ga5.5O14 single crystals grown in different atmospheres are presented. The absorption bands at 255, 290, 350 and 480nm were detected; the intensity of bands increases with the concentration of oxygen in the growth atmosphere. It is shown that the shift of the fundamental absorption edge with the temperature obeys Urbach rule. The corresponding fitting allowed to estimate the slope coefficient σ=0.35, which implies self-trapping of excitons in La3Ta0.5Ga5.5O14. Calculations of the band structure, partial densities of states and reflectivity spectra were performed. The bandgap of La3Ta0.5Ga5.5O14 was determined as Eg=5.6eV. The luminescence properties under UV, VUV and X-ray excitation were studied. Intrinsic emission band at 440–450nm is attributed to the excitons self-trapped at TaO6 molecular complexes. Extrinsic emission bands at 410, 440 and 550nm are attributed to the emission of excitons trapped by antisite defects, F-centers and oxygen deficient oxyanionic complexes.

Ab-initio study of magnetism behavior in TiO2 semiconductor with structural defects

Magnetic, electronic and structural properties of titanium dioxide material with different structural defects are studied using the first-principles ab-initio calculations and the Korringa–Kohn–Rostoker method (KKR) combined with the coherent potential approximation (CPA) method in connection with the local density approximation (LDA). We investigated all structural defects in rutile TiO2 such as Titanium interstitial (Tii), Titanium anti-sites (Tio), Titanium vacancies (VTi), Oxygen interstitial (Oi), Oxygen anti-sites (OTi) and oxygen vacancies (Vo). Mechanisms of hybridization and interaction between magnetic atoms are investigated. The transition temperature is computed using the Mean Field Approximation (MFA).Magnetic stability energy of ferromagnetic and disordered local moment states is calculated to determine the most stable state. Titanium anti-sites have a half-metallic aspect. We also studied the change type caused by structural defects in this material.

Influence of Point Defects on the Properties of Undoped and Ga-Doped ZnO Films Grown by Plasma-Assisted Molecular Beam Epitaxy in an O-Rich Environment

Undoped and Ga-doped ZnO films are grown on GaN templates by plasma-assisted molecular beam epitaxy for systematic investigation of the influence of native defects on their electrical properties. The distinct electrical properties of the undoped and Ga-doped ZnO films are observed after thermal treatment in a nitrogen and an oxygen ambient. It is found that the undoped ZnO films show improved characteristics when annealed in an oxygen ambient, and the Ga-doped ZnO films exhibit stable characteristics when annealed in a nitrogen ambient. The variation in electrical properties revealed by the photoluminescence measurements can be attributed to the generation and annihilation of native defects, which are dependent on the ambient of treatments. The properties of the annealed undoped ZnO films grown in an O-rich environment are affected mainly by the oxygen vacancies, antisite oxygen and oxygen interstitials, while the annealed Ga-doped ZnO films are dominated by oxygen interstitials.

Regulation of the magnetic behavior by adjusting oxygen stoichiometry in ZrOx film

The mechanism of ferromagnetic ordering in ZrOx film is investigated by both experimental observation and theoretical calculation. Magnetic measurements reveal that the magnetic properties can be adjusted from diamagnetism to ferromagnetism by varying the oxygen stoichiometry. We find that oxygen-rich defects can be responsible for the observed magnetic properties by taking the measurements of x-ray photoelectron spectroscopy and room temperature photoluminescence spectra. Density functional theory calculations further confirm that the ferromagnetic order is mainly driven by the exchange interaction between the oxygen antisites and the neighboring anion atoms.

A study on native defects and magnetic properties in undoped rutile TiO2 using LDA and LDA+UO p+UTi d methods

The native defects and magnetic properties in undoped rutile TiO2 are studied using local density approximation (LDA) and LDA adding Hubbard parameters (U) schemes. The band gap is adjusted to experimental value of 3.0eV by combination of UTi d=4.2eV and UO p=4.8eV. This LDA+U methodology overcomes the band-gap problem and renders the approach more predictive. The formation energies of oxygen vacancy (VO), oxygen interstitial (Oi), titanium vacancy (VTi), titanium interstitial (Tii), oxygen anti-sites (OTi), and titanium anti-sites (TiO) are investigated by the LDA and LDA+U methods. In addition, some ground state configurations can be obtained by optimization of total spin. It is found that native defects can induce spin polarization and produce magnetic moment.

Solid state synthesis of erbium doped ZnO with excellent photocatalytic activity and enhanced visible light emission

The doped ZnO crystals with different erbium content were prepared by solid state reaction technique. The method is simple, relatively less expense and enables the production in large scale. The samples were characterised by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV/vis diffuse reflectance spectroscopy and photoluminescence techniques. Photocatalytic activity of samples was investigated by the degradation of methylene blue under UV light irradiation and was observed that, small fractions of erbium incorporation have a significant role in the enhancement of photocatalytic activity of ZnO. And the good stability of prepared catalysts makes them suitable for practical applications. Fluorescent property studies of erbium doped ZnO at room temperature show enhanced visible light emission due to the production of induced defects like antisite oxygen OZn and interstitial oxygen Oi. This work reports the co-existence of two mutually exclusive properties such as fluorescence and photocatalytic activity at their best in 0.6wt% erbium doped ZnO.

Energetic, Optical, and Electronic Properties of Intrinsic Electron-Trapping Defects in YAlO3: A Hybrid DFT Study

The formation energies of cation antisite defects (YAl and AlY), oxygen vacancies (VO), and nearest-neighbor defect complexes (YAl–AlY and YAl–VO) in various charge states in the YAlO3 crystal are calculated using density functional theory (DFT) with a modified PBE0 hybrid functional containing 32% Hartree–Fock (HF) exchange. It is found that the formation of YAl is more energetically favorable than AlY under oxygen-poor condition, consistent with the fact that the latter was not observed in experiments. On the basis of calculated optical transition energies associated with the excitons trapped at YAl, VO, and YAl–VO, the two emission bands observed under excitonic excitation at low temperature are identified. Electronic properties of YAl–VO complexes in the neutral and singly negative charge states are finally investigated. It shows that the extra electron added into the negative charge state is mainly localized at 4d orbitals of YAl with a two-component feature of its density distribution extending axia...

Optical properties and photocatalytic activity of Nd-doped ZnO powders

To improve the photocatalytic activity of zinc oxides, ZnO powders doped with different neodymium (Nd) concentrations were prepared via hydrothermal method. X-ray diffraction (XRD) together with X-ray photoelectron spectroscopy (XPS) patterns revealed that Nd atoms were successfully incorporated into the ZnO lattice. XRD pattern also showed some anisotropy of the powders. The photoluminescence (PL) spectrum demonstrated a strong and broad peak in the visible light region, and the intensity of visible light emission was enhanced by Nd-doping. The photocatalytic activity was evaluated by the degradation of methyl orange solution. It is shown that doping of Nd into ZnO induces an increase of the photocatalytic activity and it attains to optimum at 3% (mole fraction) doping concentration. The intense visible light emission and the enhanced photocatalytic activity were explained by the increase in electron hole pairs and induced defects like antisite oxygen OZn and interstitial oxygen Oi, due to the doping of Nd.

Crystallographic comparison of zirconium and molybdenum dioxide-based pyrochlores

For AO1.5–MoO2 and -ZrO2 equimolar systems (A = La, Nd, Sm, Gd, Dy, Y, Er, Lu), oxide compounds samples were prepared by solid-state sintering at 1673 K–1873 K in argon and air, respectively. In addition, oxide samples of yNdO1.5 − (1 − y)ZrO2 with y = 0.3–0.7 were prepared to investigate the pyrochlore–fluorite (P–F) phase transition. X-ray diffraction measurements clarified that molybdenum oxides formed cubic-pyrochlore phase except for A = La, and zirconate pyrochlores formed for A = La, Nd, Sm, Gd, whereas other zirconium oxides exhibited the defect-fluorite structure. With decreasing ionic radius of A3+, the broadening of Raman bands were observed for both pyrochlore systems. This might be due to the decrease of energy for defect cluster formation, i.e. cation antisite reaction and oxygen Frenkel defect formation. Especially for molybdate pyrochlores, it might be also attributed to the fact that the ionic charge of molybdenum was likely to be changeable. The P–F phase transition of the yNdO1.5 − (1 − y)ZrO2 system was clearly confirmed by Raman spectroscopy, and the changes in band shape were influenced mainly by the oxygen environment around Zr, which was elucidated by molecular dynamics calculations.

Comparative study of the luminescence of Y3Al5O12 nanoceramics and single crystals under excitation by synchrotron radiation

Comparative investigation of the luminescent properties of Y3Al5O12 (YAG) nanoceramics with the properties of single crystals counterpart is performed under excitation by synchrotron radiation in the exciton range of YAG host. Analysis of the luminescent properties of such different crystalline forms of YAG allows us to conclude that the behavior of YAG nanoceramics is close to the properties of single crystal analogue with large content of YAl antisite defects (ADs). We also have found that the relative intensity of F+–AD coupled centers is significantly higher in YAG nanoceramics than that for single crystal counterpart. This presupposes the strong coupling of the antisite defects and oxygen vacancy-related centers in YAG nanoceramics, mainly at the boundaries of grains.

Origin of Magnetism from Native Point Defects in ZnO

Based on the first-principle calculations by using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method in connection with the local density approximation (LDA), we study theoretically the electronic and magnetic properties of different point defects in ZnO, which are Zinc interstitials (Zni), Zinc antisites (ZnO), Oxygen interstitials (Oi) and Oxygen antisites (OZn) defects in ZnO. The supercell calculations were also performed using the full potential local-orbital (FPLO) band structure scheme. This work presents detailed information about total and local density of states at some concentrations of these defects; the stability of the ferromagnetic state compared with the spin-glass state is investigated by comparing calculating their total energy. The results show on one hand that Zni and ZnO produce a shallow donor bellow the bottom of the conduction band (CB), while Oi and OZn produces the shallow acceptors above the top of the valence band (VB), and moment magnetic; on other hand that the ferromagnetic state is more stable than the spin-glass in Oxygen interstitials (Oi) and vice versa for oxygen antisites (OZn) of native point defects in ZnO. The other native point defects (Zni, ZnO, VO, and VZn) have a zero magnetic moment. The results show that the Curie temperature increases with the concentration of interstitial oxygen.

Sn-doped polyhedral In2O3 particles: Synthesis, characterization, and origins of luminous emission in wide visible range

Sn-doped octahedronal and tetrakaidecahedronal In2O3 particles were successfully synthesized by simple thermal evaporation of indium grains using SnO as dopant. Structural characterization results demonstrated that the Sn-doped tetrakaidecahedronal In2O3 particle had additional six {001} crystal surfaces compared with the octahedronal one. The luminous properties of both samples were characterized by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. A broad visible luminous emission around 570nm was observed. Studies revealed that the emission consisted of three peaks of 511nm, 564nm, and 622nm, which were attributed to radioactive recombination centers such as single ionized oxygen vacancy, indium interstitial, and antisite oxygen, respectively. We believe that the Sn donor level plays an important role in the visible luminous emission.

Influence of surface defects in ZnO thin films on its biosensing response characteristic

Highly c-axis oriented zinc oxide (ZnO) thin films deposited by rf magnetron sputtering under varying processing pressure (20–50 mT) in a reactive gas mixture of argon and oxygen were studied for biosensing application. The as-deposited ZnO thin films were in a state of compressive stress having defects related to interstitial Zn and antisite oxygen. Glucose oxidase has been chosen as the model enzyme in the present study and was immobilized on the surface of ZnO thin films deposited on indium tin oxide coated Corning Glass substrate. The studies reveal a correlation between the biosensing characteristic and the presence of defects in the ZnO films. The ZnO films deposited under high pressure (50 mT) are found to be more sensitive for biosensing application due to availability of more surface area for effective immobilization of biomolecules and exhibits a suitable microenvironment with good electron transfer characteristic. The obtained results highlight the importance of desired microstate besides availability of suitable native defects in the ZnO thin film for exhibiting enhanced biosensing response.

Polymer/ZnO Micro-Nano Array Composites for Light-Emitting Layer of Flexible Optoelectronic Devices

We prepared PAM/ZnO micro-nano arrays on indium tin oxide (ITO) conductive films based on poly(ethylene terephthalate) (PET) substrates (PET/ITO) by a low cost and low temperature chemical solution approach. The morphology and crystal structure of the nanorod arrays were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that the ordered arrays of the ZnO and PAM/ZnO arrays grew vertically on the substrates and revealed that the nanorods grew along the [0001] direction of the ZnO crystallites. SEM images showed that most of the ZnO arrays had an average diameter of 150 nm and their typical length was about 3 μm. The optical properties of the ZnO and PAM/ZnO micro-nano arrays were characterized by photoluminescence at room temperature. The growth mechanism of the PAM/ZnO arrays and their possible application in flexible optoelectronic devices are discussed. Defect peaks of the blue peak at 457 nm and the green peak at 530 nm were observed in the photoluminescence (PL) spectrum of the ZnO micro-nano arrays in the absence of PAM. The blue and green emissions are attributed to electron transitions from the extended state Zni to the valance band and from the conduction band to antisite oxygen (OZn), respectively. The PAM/ZnO arrays only had a UV peak at 400 nm, and this was caused by electron transitions from the interstitial Zn (Zni) to the valance band. Flexible PAM/ZnO devices with good diode characteristics are suitable for flexible optoelectronic applications.

Structural, optical and magnetic properties of Zn 1− xCo xO dilute magnetic semiconductors thin films by pulsed laser deposition

We fabricated Zn 1− x Co x O ( x=0.05) thin films on sapphire (0 0 0 1) substrates by the pulsed laser deposition (PLD) method at various temperatures in an oxygen-deficient ambient. X-ray diffraction (XRD) spectra, UV–vis transmittance spectra, X-ray photoelectron spectroscopy and photoluminescence (PL) were used for charecterization. All samples possessed the wurtzite structure with the preferential c-axis orientation; PL located at about 418, 450 and 471 nm was observed; Co 2+ ions incorporated into the ZnO lattice with substitution for Zn 2+ ions, oxygen vacancies, oxygen antisites and Zn interstitials existed in the as-samples. Results of magnetic property of the films investigated by an alternating gradient magnetometer (AGM) indicated that all samples take on room temperature ferromagnetism. It suggested that the ferromagnetism originated from the Co 2+–Co 2+ exchange interaction related to oxygen vacancies, oxygen antisites and Zn interstitials.

Effects of proton irradiation on indium zinc oxide-based thin-film transistors

In this research, we performed a high-dose proton-beam irradiation process to investigate the effect of irradiation on the performance of indium-doped zinc oxide-based thin-film transistors (IZO-TFTs) by controlling their electrical and structural properties. The resistivity of IZO thin films dramatically increased with increasing proton-irradiation doses up to ∼ 10 14 cm − 2, after which the resistivity recovered to a level similar to as-deposited IZO thin film. After proton irradiation, the crystallinity of IZO thin films was reduced due to defect-isolation and/or chemical-isolation. The field effect mobility of IZO-TFTs decreased from 2.20 cm 2 V − 1 s − 1 to 1.22 cm 2 V − 1 s − 1, turn-on voltage shifted −7 V to −21 V and subthreshold swing value increased after proton irradiation with a 10 12 cm − 2 dose. With proton-irradiation doses over 10 14 cm − 2, IZO-TFTs device performance recovered to a level similar to that of a pristine device. Moreover, irradiated thin films and devices have low sensitivities to post-thermal annealing due to the creation of antisite oxygen point defects.

Structural and photoluminescence properties of Al-doped ZnO films deposited on Si substrate

Al-doped ZnO (ZnO:Al) thin films were deposited on Si substrates using the radio-frequency reactive magnetron sputtering technique. Effect of Al contents and annealing treatments on the structural and photoluminescence (PL) properties of ZnO films was investigated. The results showed that crystallinity of the films improved first with the increasing of Al content. However, as the Al content further increased more than 0.75 wt %, the crystallinity was deteriorated. Meanwhile, the crystal quality of ZnO:Al thin films (Al content is 0.75 wt %) was markedly improved after annealing at 600 °C for 0.5 h. Four main emission peaks located at 390, 445, 490 and 530 nm were observed from PL spectra. As Al content was increased to 1.14 wt %, the intensity of the blue peak (∼445 nm) and the green peak (∼490 nm) increased and the full width at half maxima (FWHM) decreased, and blue centre blue shifted to 429 nm. A similar result can be obtained after annealing 600 °C for 0.5 h. This is attributed to Al doping and annealing treatment lead to the change of contents of antisite oxygen and interstitial Zn-related defects in the films.

The Al-doping and post-annealing treatment effects on the structural and optical properties of ZnO:Al thin films deposited on Si substrate

Al-doped ZnO (ZnO:Al) thin films with different Al contents were deposited on Si substrates using the radio frequency reactive magnetron sputtering technique. X-ray diffraction (XRD) measurements showed that the crystallinity of the films was promoted by appropriate Al content (0.75 wt.%). Then the ZnO:Al film with Al content of 0.75 wt.% was annealed in vacuum at different temperatures. XRD patterns revealed that the residual compressive stress decreased at higher annealing temperatures. While the surface roughness of the ZnO:Al film annealed at 300 °C became smoother, those of the ZnO:Al films annealed at 600 and 750 °C became rougher. The photoluminescence (PL) measurements at room temperature revealed a violet, two blue and a green emission. The origin of these emissions was discussed and the mechanism of violet and blue emission of ZnO:Al thin films were suggested. We concluded that the defect centers are mainly ascribed to antisite oxygen and interstitial Zn in annealed (in vacuum) ZnO:Al films.