Zn1 xCoxO Research Articles

Defect induced room temperature ferromagnetism in high quality Co-doped ZnO bulk samples

The nature of the often reported room temperature ferromagnetism in transition metal doped oxides is still a matter of huge debate. Herein we report on room temperature ferromagnetism in high quality Co-doped ZnO (Zn1−xCoxO) bulk samples synthesized via standard solid-state reaction route. Reference paramagnetic Co-doped ZnO samples with low level of structural defects are subjected to heat treatments in a reductive atmosphere in order to introduce defects in the samples in a controlled way. A detailed structural analysis is carried out in order to characterize the induced defects and their concentration. The magnetometry revealed the coexistence of a paramagnetic and a ferromagnetic phase at room temperature in straight correlation with the structural properties. The saturation magnetization is found to increase with the intensification of the heat treatment, and, therefore, with the increase of the density of induced defects. The magnetic behavior is fully explained in terms of the bound magnetic polaron model. Based on the experimental findings, supported by theoretical calculations, we attribute the origin of the observed defect-induced-ferromagnetism to the ferromagnetic coupling between the Co ions mediated by magnetic polarons due to zinc interstitial defects.

Tuned magnetic properties of Co-doped ZnO/B-doped graphene PN junction

Co-doped ZnO(Zn1−xCoxO)/B-doped graphene(BG) PN junction with quasi-core-shell nanostructure is designed and fabricated by a facile chemical process. The interplay between the Zn1−xCoxO core and the BG shell is discussed. X-ray Photoelectron Spectroscopy (XPS) and photoluminescence (PL) spectra confirm that B and Co2+ ions are doped successfully. The C1s XPS spectra suggest the formation of the Zn(Co)OCO bonds, which can efficiently transfer the holes from BG to Zn1−xCoxO. X-ray diffraction (XRD) displays that Co doping hardly changes the wurtzite structure of ZnO. It can be clearly seen that edge of the BG covers Zn1−xCoxO to form the quasi-core-shell structure by High-resolution transmission electron microscopy (HRTEM). The position of the G band in Raman spectrum indicates that BG is P-type. The interaction between the Zn1−xCoxO and BG can be found by the red-shifts of E2H band of Zn1−xCoxO and broader 2D of BG as well. Magnetization measurement demonstrates that the increasing of the ferromagnetic phases in Zn1−xCoxO/BG nanoparticles with the increasing temperature. The observed ferromagnetic phases can be due to the exchange of the electron/hole at interface in the Zn1−xCoxO/BG PN junction.

Unveiling the Co2+ Ion Doping-Induced Hierarchical Shape Evolution of ZnO: In Correlation with Magnetic and Photovoltaic Performance

A sustainable, rapid microwave-assisted glycothermal (MW-GT) method has been adopted for the synthesis of pristine ZnO and a series of Zn1–xCoxO (x = 0, 0.02, 0.03, 0.05, 0.07, 0.10) within 15 min at 180 °C using ethylene glycol (EG) as solvent. The XRD results reveal that the altering of lattice parameters of ZnO by introduction of Co2+ ions and crystalline sizes of Co2+ doped ZnO samples decreased with increasing Co2+ ion content. A spectacular morphological change of ZnO from well-defined hexagonal prismoid to hierarchical flower-like 1-D nanorods-assembly upon increasing Co2+ ion concentration was perceived using FE-SEM and TEM analyses. After Co2+ ion inclusion into pristine ZnO, the width of the M–H loop significantly changes, where the diamagnetic behavior of ZnO changes from ferromagnetic to paramagnetic upon further increase in Co2+ ion content. Particularly, 5 mol % Co2+ ion doped ZnO sample shows enhanced photovoltaic performance in dye-sensitized solar cells (DSSCs) due to nanoscale level inte...

Structural, optical, dielectric, magnetic and magnetoelectric properties of Co-doped ZnO nanoparticles

Zn1–xCoxO (x = 0.0, 0.06, 0.12, 0.18) nanoparticles were prepared by chemical precipitation method using hydroxyoxalate type precursors and thermal decomposition process. All the samples crystallize in hexagonal wurtzite structure. Cobalt incorporation into the ZnO structure was confirmed by electron diffraction X-ray, transmission electron microscopy and UV–Vis spectroscopy. The increase in the observed band gap of the ZnO nanoparticles compared to that of the bulk ZnO is explained on the basis of size effect. The fluorescence spectra confirm the excitonic peak at 366 nm corresponding to near band edge emission for undoped and cobalt doped ZnO. The ferromagnetism of the lowest concentration cobalt doped ZnO nanoparticle has been evidenced by SQUID. Dielectric properties and magnetoelectric coupling have also been analyzed.

Dynamics of the incorporation of Co into the wurtzite ZnO matrix and its magnetic properties

Bulk Co-doped ZnO (Zn1−xCoxO) samples were prepared and studied with particular emphasis on their compositions, structures, and magnetic properties. A detailed microstructural analysis was conducted to investigate the nature of Co incorporation into the wurtzite ZnO matrix. The Zn1−xCoxO ceramic samples were prepared using the standard solid-state reaction method with different Co molar concentrations of up to 30%. A Co solubility limit of approximately 23% was determined. For samples with Co concentrations greater than 23%, a segregated phase identified as Zn-doped CoO (Co1−yZnyO with y=0.23) was observed. The magnetic characterization for the single-phase samples revealed a paramagnetic behavior with antiferromagnetic coupling of Co2+ ions within the ZnO matrix. In order to get more insight about the magnetic results Monte Carlo simulations were conducted for different magnetic couplings and different Co distributions over the volume of the samples. We discussed the observed magnetic behavior of our samples by considering the main theories regarding the magnetic properties of dilute magnetic oxides. An inhomogeneous distribution of Co ions within the grains was inferred from the microstructural and magnetic characterization.

Room temperature ferromagnetism and photoluminescence of nanocrystalline Zn1 − xCoxO thin films prepared by sol–gel MOD

Zn1 − xCoxO (ZC) [x = 0, 1, 3, 5, 7 and 9 mol%] thin films were prepared by sol–gel combined metallo-organic decomposition method. The films were deposited on the Si substrate with spin-coating technique and annealed at 600 °C for 3 h. X-ray diffraction pattern shows the formation hexagonal wurtzite phase and distortion (c/a) decreases with increasing Co concentration in ZnO. The average grain size is measured using Scherer relation. Atomic force microscopy is used to confirm the formation of nanograins resulted by the use of polyethylene glycol as surfactant. The photoluminescence was recorded by using He-Cd laser of excitation wavelength 325 nm in wavelength region of 350–650 nm which exhibits some influence of Co doping on the multiplication of defects such as O vacancies, Zn interstitials and grain boundary defects. All thin films show room temperature ferromagnetism except pure ZnO which is diamagnetic and 9 mol% of Co shows paramagnetism. This behaviour is interpreted as due to fluctuations in the magnetic ordering, depending on grain size and site location in grain boundaries or oxygen vacancies.

Effect of Cobalt Doping on Structural, Optical, and Magnetic Properties of ZnO Nanoparticles Synthesized by Coprecipitation Method

Zn1–xCoxO (x = 0, 0.05, 0.10, and 0.15) nanoparticles (NPs) were synthesized by a coprecipitation method. The crystalline sizes of synthesized samples were calculated from the powder XRD patterns, which were found to decrease with the increase of cobalt content. The FT-IR spectra confirmed the Zn–O stretching bands at 468, 456, 452, and 461 cm–1 for the respective ZnO NPs. SEM images demonstrated the distinct flowerlike morphology. The photoluminescence spectra of all the samples exhibited a broad emission in the visible range. XPS studies were carried out for Zn0.90Co0.10O NPs. The carriers (donors) bound on the Co sites were observed from the micro-Raman spectroscopic studies. The pure and Co-doped ZnO NPs showed significant changes in the M–H loop where the diamagnetic behavior of ZnO changes to ferromagnetic nature when doping with Co. Oxygen vacancies and zinc interstitials were found to be the main reasons for room-temperature ferromagnetism in the Co-doped ZnO NPs with the support of the results obtained from the EPR, photoluminescence, and micro-Raman studies.

Anionic polymerization in Co and Fe doped ZnO: Nanorods, magnetism and photoactivity

The growth mechanism of Zn1−xCoxO (ZC) and Zn1−xFexO (ZF) nanorods, and resulting magnetic and optical properties have been studied. The ZC and ZF nanorods were prepared by sol–gel synthesis route. X-ray diffraction results in polycrystalline phase with wurtzite structure of ZC and ZF nanorods. The transmission electron microscopy images show the formation of nanorods. The growth mechanism of nanorods is explained on the basis of agglomeration of Zn2+ with OH− ions which is react with poly vinyl alcohol involve anionic polymerization of oriented growth. Magnetic measurement of ZC and ZF nanorods exhibit superferromagnetic behavior and the large value of saturation magnetization observed at room temperature. The magnetization below room temperature measurement confirms the origin of observed magnetism. Raman and photoluminescence spectra show good photoactivity. The observed Raman active modes show wurtzite structure belongs to C6v symmetry group. Photoluminescence measurements of ZC and ZF nanorods exhibit ultraviolet peaks at 413.90 nm (∼3 eV) due to free exciton emission and at 546.31 nm (∼2.27 eV) due to transition from deep donor states which arises from oxygen vacancy.

Influence of Substrate Temperature on Stress and Morphology Characteristics of Co Doped ZnO Films Prepared by Laser-Molecular Beam Epitaxy

Zn1–xCoxO (x = 0.05) thin films are deposited on sapphire (0001) substrates by laser-molecular beam epitaxy technique at different substrate temperatures. The structural, stress and morphology evolution features are investigated by means of X-ray diffraction and atomic force microscopy. The surface parameters of roughness exponent α, root mean square (RMS) roughness w and autocorrelation length ξ are calculated and the surface parameters are preliminarily analyzed. The values of α vary from 0.7 to 0.9. The RMS roughness w is less than 2.2 nm, and it increases with increasing Ts from 300 to 400 °C, and then decreases when Ts is 500 °C. The autocorrelation length ξ decreases monotonously with the increase in Ts from 300 to 500 °C, which indicates that the increase in Ts restrains the spread of the surface fluctuations until Ts is higher than 400 °C.

XRD, XPS, optical, and Raman investigations of structural changes of nanoCo-doped ZnO

The mixed oxides Zn1−xCoxO (ZCO) (0.0⩽x⩽3.0) were prepared as nanopolycrystalline powders and thin films by a simple sol–gel process and dip coating method. Structural and microstructural analysis was carried out applying X-ray diffraction (XRD) and Rietveld method. Analysis showed that for x<0.12, Co2+ replaces Zn2+ substitutionally yielding ZCO single phase, while for x⩾0.15 two phases are identified; ZCO and Co3O4. Replacing Zn+2 by Co+2 affects the lattice parameters in opposite ways, the parameter c decreases while a increases with an overall decrease in the ratio c/a of the wurtzite ZCO, which deviates the lattice gradually from the hexagonal structure as Co+2 increases. The IR analysis shows red shift entirely dependent on Co concentration. With increased concentration of cobalt the E2high Raman mode exhibits a redshift attributed to the phonon softening caused by the ab-plane lattice expansion, while the E2low mode shows a blueshift due to the reduced mass effect caused by cation replacement.

Synthesis and characterization of ferromagnetic Zn1−xCoxO films

High-quality Zn1−xCoxO (ZnCoO) thin films with different Co concentrations (x=0−0.07) were synthesized on Al2O3 substrates using a radio-frequency magnetron sputtering process. The structural, chemical, optical, and electrical properties of the films were characterized using various techniques. High-resolution X-ray diffraction measurements showed that the ZnCoO films had a c-oriented hexagonal wurtzite structure with no extra phases. The lattice constants of the films increased slightly with increasing the Co concentration ratio. X-ray photoelectron spectra show the incorporation of Co2+ ions in the ZnO host without the formations of Co clusters. Hall measurements revealed that the films were n-type with a charge carrier density of ∼2.6×1018 per cm3. A neutral-donor bound exciton peak (D0X) from all specimens was observed at 3.36eV using photoluminescence measurements at 10K. The physical and chemical properties of the films suggested that their ferromagnetic properties originated from a direct exchange interaction between magnetic Co ions.

Comparative Study of Ni- and Co-Substituted ZnO Nanoparticles: Synthesis, Optical, and Magnetic Properties

Ni- and Co-substituted ZnO nanoparticles were synthesized using forced hydrolysis of acetate metallic salts in a polyol medium. The X-ray diffraction patterns show a hexagonal wurtzite structure (space group P63mc). The characteristic absorption bands of UV–vis-IR spectra are correlated with the d–d transitions of tetrahedrally coordinated Co2+ and Ni2+ ions in octahedral and tetrahedral sites. The photoluminescence spectra exhibited a typical ZnO UV-exitonic emission band around 380 nm and a broad band between 400 and 500 nm that might be ascribed to the intrinsic defects in the ZnO material. The transmission electron microscopy displays spherical particles with a diameter between 20 and 30 nm. The magnetic measurements reveal that Zn1–xNixO and Zn1–xCoxO nanoparticles show, respectively, ferromagnetic and paramagnetic behavior at 5 K. Homogeneous distributions of Co and Ni ions in the particles observed by filter imaging analysis indicates that there is no evidence of Co or Ni metal throughout the powders.

Sputtering-induced Co formation in x-ray photoelectron spectroscopy of nanocrystalline Zn1-xCoxO spinodal enrichment models

Nanoscale enrichments resulting from spinodal decomposition have been proposed to contribute to the interesting magnetic properties of diluted magnetic oxides such as cobalt-doped ZnO (Zn1-xCoxO), but little is known experimentally about the electronic structures or physical properties of such enrichments. Here, x-ray photoelectron spectroscopy (XPS) is used to examine wurtzite Zn1-xCoxO crystallites over the full composition range (0.0≤x≤1.0) that serve as models of the proposed spinodal decomposition nanostructures within Zn1-xCoxO bulk materials. With increasing x, the valence band edge shifts to smaller binding energies and the cobalt 2p peaks shift to greater binding energies, providing spectroscopic signatures that may allow identification of spinodal decomposition in bulk Zn1-xCoxO. Reduction of Co2+ to Co0 by argon ion (Ar+) sputtering was also found to become markedly more facile with increasing x, suggesting that locally-enriched Zn1-xCoxO is at greater risk of yielding false-positive Co0 XPS signals than uniformly dilute Zn1-xCoxO with the same overall composition.

Monte Carlo simulations of diluted magnetic semiconductors using ab initio exchangeparameters

Co doped ZnO (Zn1−xCoxO) is studied as a prototype material for transition metal doped II–VI diluted magneticsemiconductors (DMSs) from first-principles and Monte Carlo simulations. The exchangeinteractions are calculated using the Korringa–Kohn–Rostoker (KKR) Green’s functionmethod. The exchange coupling constants thus obtained are treated in the classicalHeisenberg model and the magnetic phase transitions are studied by the MonteCarlo technique. Our results show that the defect free substitutional DMSs ofZn1−xCoxO do not sustain magnetization at low concentration. At high concentration, we findlayered magnetic structures. Ferromagnetism, with Curie temperature below roomtemperature, is stable at intermediate Co concentrations. First-principles studies with thegeneralized gradient approximation (GGA) and the GGA together with the HubbardU are discussed with respect to structural and electronic properties of ZnO.

Structural and electronic properties of Co‐doped ZnO nanocrystals synthesized by co‐precipitation method

AbstractZnO nanocrystals doped with Co were synthesized by co‐precipitation method combined with post‐thermal treatment. The synthesized crystals formed high‐quality wurtzite ZnO crystal lattice with the dimension about 10 nm, substituting Co ions into Zn sites in the composition range of [Co] &lt; 3.0%. The position of Co2+/1+ charge transfer (CT) level composed by O 2p and Co 3d orbitals was observed at 3.0 eV by optical absorption measurements. Raman spectra showed the enhancement of LO phonon due to resonant Raman effect caused by photo‐excitation of free carriers mediated the CT level. And, Photoluminescence spectra showed UV emission at ∼3.3 eV suppressed and shifted to higher energy side with increase in Co doping level, indicating increase in nonradiative recombination centers and the possibility of band‐gap engineering in Zn1–xCoxO nanocrystals, respectively. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Cathodic electrodeposition of ZnCoO thin films

AbstractWe report on the characterization of ternary Zn1–xCoxO alloy crystalline films grown by electrodeposition onto FTO‐coated substrates. The Zn1–xCoxO films have hexagonal wurtzite structure as shown by X‐ray diffraction measurements. The Co incorporation into the ZnO lattice is confirmed by the presence of absorption peaks assigned to Co in trigonal crystal field. X‐ray photoemission spectroscopy indicates that as‐grown films have a considerable concentration of not‐fully oxidized metallic Co in the surface that correlates with the O concentration in the surface. Finally, Raman measurements of as‐grown films indicate that they are polycrystalline with grains of nanometric size showing short‐range order but no long‐range order. The Raman spectra show no trace of first‐order phonons and resemble the one‐and two‐phonon density of states of ZnO (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Back Cover: phys. stat. sol. (a) 205/8

AbstractThe back cover shows results from ab initio studies of a diluted magnetic semiconductor system by S. K. Nayak et al. (p. 1839). Depicted is the electronic density of states of Co‐doped ZnO (Zn1–xCoxO, x = 0.125), corrected by adding Hubbard‐like correlations to the Zn‐d and Co‐d orbitals with 7.5 eV and 5.0 eV, respectively. The additional incorporation of Hubbard‐like correlations into the electronic structure of ZnO improves the description of orbital hybridization by shifting the bands to correct locations below the Fermi energy. The experimental peak position of the Co 3d‐states at –3 eV and –7 eV, as obtained from valence‐band ultraviolet photoemission measurements available in the literature, thus agree with the corresponding spectral weight obtained in the calculations. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structural Evidence for Zn Intersititials in Ferromagnetic Zn1–xCoxO Films

Structural Evidence for Zn Intersititials in Ferromagnetic Zn_1–_<i>x</i>Co_<i>x</i>O Films

Anisotropy of magnetization and magnetoresistance of (Zn,Co)O films grown by pulsed laser deposition

We report the anisotropy in magnetic and magneto-transport properties of c - and a -plane Zn1–xCoxO (x = 0.05, 0.1 and 0.15) thin films grown on c - or r -surface sapphire by PLD technique. The magnetization and magnetoresistance (MR) measurements were performed with magnetic fields parallel or perpendicular to the film plane. It was found that both the c - and a -plane films were paramagnetic down to the lowest temperature of 2K. The magnitudes of the magnetization and the positive MR were larger under magnetic fields along a - or b-axis than along c-axis for both the c - or a -plane films. These anisotropic behaviours are interpreted as intrinsic properties originating from Co2+ spins substituting the Zn site in ZnO. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancement of ferromagnetism in Zn 0.98 Co 0.02 O by quenching from high temperatures

Co-doped ZnO samples with above-room-temperature Curie point were fabricated by the standard solid-state reaction method. Ferromagnetic behavior varied with different quenching temperatures ranging from 1173 K to 573 K after calcination. Extensive analyses on structural and composition revealed that cobalt is homogeneously incorporated into the ZnO lattice forming solid solution instead of precipitation when x ≤ 0.02 in Zn1–xCoxO. In the case of Zn0.98Co0.02O, ferromagnetism was enhanced by decreasing the quenching temperature until 573 K. The coercive field measured by a vibrating sample magnetometer (VSM) at room temperature is around 60–80 Oe. The implication of the effect of quenching temperature in bulk Zn0.98Co0.02O is also discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)