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.     

Formation of room-temperature ferromagnetic Zn1−xCoxO nanocrystals

Optical and magnetic properties of Co²⁺-doped ZnO nanocrystals were studied. Optical measurements confirm the incorporation of Co²⁺ in ZnO lattice with tetrahedral geometry. Optical absorption spectra also reveal the partial bleaching of the excitonic feature attributable to an increase in electron concentration. Magnetization measurements indicate the ferromagnetic ordering in Co²⁺-doped ZnO nanocrystals with saturation magnetization . No structural changes were observed in lightly doped ZnO nanocrystals. The present investigations are important in obtaining the ferromagnetic Zn_1−xCo_xO nanocrystals.     

Electronic structure and ferromagnetism of polycrystalline Zn1−xCoxO (0≤x≤0.15)

The electronic structure of polycrystalline ferromagnetic Zn_1−xCo_xO (0.05≤x≤0.15) and the oxidation state of Co in it, have been investigated. The Co-doped polycrystalline samples are synthesized by a combustion method and are ferromagnetic at room temperature. XPS and optical absorption studies show evidence for Co²⁺ ions in the tetrahedral symmetry, indicating substitution of Co²⁺ in the ZnO lattice. However, powder XRD and electron diffraction data show the presence of Co metal in the samples. This give evidence to the fact that some Co²⁺ ion are incorporated in the ZnO lattice which gives changes in the electronic structure whereas ferromagnetism comes from the Co metal impurities present in the samples.     

Magnetic and Raman scattering studies of Co-doped ZnO thin films grown by pulsed laser deposition

Phase pure Zn_1?x Co _x O thin films grown by pulsed laser deposition have transmittance greater than 75 % in the visible region. Raman studies confirm the crystalline nature of Zn_1?x Co _x O thin films. Zn_0.95Co_0.05O thin films show room temperature ferromagnetism with saturation magnetization of 0.4μ _B /Co atom. The possible origin of paramagnetism at higher Co doping concentrations can be attributed to the increased nearest-neighbor antiferromagnetic interactions between Co²⁺ ions in ZnO matrix. XPS confirms the substitution of Co²⁺ ions into the ZnO host lattice.     

Structure and ferromagnetic properties of Co-doped ZnO powders

Single-phase Zn_1−xCo_xO (x=0.02, 0.04) powders were synthesized by a simple co-precipitation technique. X-ray diffraction analysis reveals that the Co-doped ZnO crystallizes in a wurtzite structure. The lattice constants of Co-doped ZnO powders decrease slightly when Co is doped into ZnO. Optical absorption spectra show a decrease in the bandgap with increasing Co content and also give an evidence of the presence of Co²⁺ ions in tetrahedral sites. Raman spectra indicate that Co doping increased the lattice defects and induced another Raman vibration mode around at 538 cm⁻¹, which is an indicator for the incorporation of Co²⁺ ions into the ZnO host matrix. Magnetic measurement reveals that the Zn_1−xCo_xO (x=0.02, 0.04) powders clearly exhibit room-temperature ferromagnetic behavior, which makes them potentially useful as building components for spintronics.     

Effect of Co-doping content on hydrothermal derived ZnO array films

Cobalt doped ZnO films are synthesised using a hydrothermal process. The effect of Co²⁺ concentration on morphology, phase composition, crystallisation and spectroscopic characteristics of ZnO films is investigated. The results indicate that both the structure and morphology of the ZnO films evolve with the concentration of cobalt ions incorporated into the lattice. In the presence of a small amount of Co²⁺ ions, films are formed that comprise hexagonal ZnO nanorods, oriented with the c-axis perpendicular to the substrate. With increasing amount of Co²⁺, cracks in the ZnO nanorods can be observed and growth in the [0 0 1] direction is significantly inhibited. When the Co²⁺ concentration exceeds 0.010 M, ZnO rods with the typical hexagonal structure are no longer observed and instead, ZnO films comprising close-packed grains with an irregular polygonal structure are formed. The epitaxial growth of ZnO films is nearly completely inhibited when the concentration of Co²⁺ is increased above 0.050 M. This behaviour can be explained by the selective adsorption of the organic substances in the solution onto the (0 0 1) ZnO crystal face, thus inhibiting growth in the [0 0 1] direction and disrupting the crystallisation of ZnO films. Increasing the Co content deteriorates the crystallisation of ZnO rods and increases tensile stresses present in the ZnO films.     

Optical characterization of Mn, Ni and Co ions doped zinc lead borate glasses

This paper reports on the development and optical characterization of heavy metal oxide (HMO)-based transparent glasses in the chemical composition of 15PbO-40B₂O₃-(45−x) ZnO−x TM²⁺ (=Mn²⁺ or Ni²⁺ or Co²⁺) (where x=0.2, 0.5 mol%). For these glasses both absorption and emission spectra have been measured, in order to understand their optical performances. The XRD profiles have confirmed their glassy nature and the FTIR spectral features have been analyzed. From the emission spectra, a bright green emission (538 nm) from Mn²⁺-glasses, an intense red emission (670 nm) from Ni²⁺ and from Co²⁺ (625 nm) glasses have been noticed very clearly. Based on the UV-absorption spectra of these materials, both direct and indirect bond gaps have been computed. Apart from the spectral analysis, different physical properties of these glasses have also been carried out. Due to the presence of both PbO and ZnO, these glasses are found to be good moisture-resistant optical systems. Both optical and physical properties have been found to be more encouraging towards their use as novel luminescent optical materials.     

Structural and optical properties of cobalt doped ZnO nanocrystals

Zn_1−xCo_xO nanocrystals with nominal Co doping concentrations of x = 0–0.1 were synthesized through a simple solution route followed by a calcining process. The doping effects on the structural, morphological and optical properties were investigated by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman, absorption and luminescence spectroscopy. The results indicated that a small amount of Co ions were incorporated into ZnO lattice structure, whereas the secondary phase of Co₃O₄ was segregated and precipitated at high Co doping concentrations, the solid solubility of Co ions in ZnO nanocrystals could be lower than 0.05. The spectra related to transitions within the tetrahedral Co²⁺ ions in the ZnO host crystal were observed in absorption and luminescence spectra.     

Structure and magnetic properties of Zn<Subscript>0.9</Subscript>Co<Subscript>0.1</Subscript>O nanorods synthesized by a simple sol–gel method using metal acetylacetonate and poly(vinyl alcohol)

Room-temperature ferromagnetism was observed in Zn_0.9Co_0.1O nanorods with diameters and lengths of ∼100–200 nm and ∼200–1000 nm, respectively. Nanorods were synthesized by a simple sol–gel method using metal acetylacetonate powders of Zn and Co and poly(vinyl alcohol) gel. The XRD, FT-IR and SAED analyses indicated that the nanorods calcined at 873–1073 K have the pure ZnO wurtzite structure without any significant change in the structure affected by Co substitution. Optical absorption measurements showed absorption bands indicating the presence of Co²⁺ in substitution of Zn²⁺. The specific magnetization of the nanorods appeared to increase with a decrease in the lattice constant c of the wurtzite unit cell with the highest value being at 873 K calcination temperature. This magnetic behavior is similar to that of Zn_0.9Co_0.1O nanoparticles prepared by polymerizable precursor method. We suggest that this behavior might be related to hexagonal c-axis being favorable direction of magnetization in Co-doped ZnO and the 873 K (energy of 75 meV) being close to the exciton/donor binding energy of ZnO.     

Raman scattering investigations on Co-doped ZnO epitaxial films: Local vibration modes and defect associated ferromagnetism

Raman scattering spectroscopy has been performed on high quality Co-doped ZnO epitaxial films, which were grown on Al₂O₃ (0001) by oxygen-plasma assisted molecular beam epitaxy. Raman measurements revealed two local vibration modes (LVMs) at 723 and 699 cm⁻¹ due to the substitution of Co²⁺ in wurtzite ZnO lattice. The LVM at 723 cm⁻¹ is found to be an elemental sensitive vibration mode for Co substitution. The LVM at 699 cm⁻¹ can be attributed to enrichment of Co²⁺ bound with oxygen vacancy, the cobalt–oxygen vacancy–cobalt complexes, in Zn_1−xCo_xO films associated with ferromagnetism. The intensity of LVM at 699 cm⁻¹, as well as saturated magnetization, enhanced after the vacuum annealing and depressed after oxygen annealing.     

Research on La–Co-substituted strontium ferrite magnets for high intrinsic coercive force

M-type strontium ferrites with substitution of Sr²⁺ by rare-earth La³⁺, and a little amount of Fe³⁺ by Co²⁺ according to the formula Sr_1−xLa_xFe_12−xCo_xO₁₉, are prepared by the ceramic process. Effects of the substituted amount of La³⁺ and Co²⁺ on structure and magnetic properties of Sr_1−xLa_xFe_12−xCo_xO₁₉ compounds have systematically been investigated by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and B–H hysteresis curve measurements. In our results, the suitable amount of La³⁺–Co²⁺ substitution may remarkably increase saturation magnetization. Intrinsic coercive force (H_cj) of Sr ferrite magnets is evidently increased without significant decrease in residual flux density (B_r) by La³⁺–Co²⁺ substitution.     

Tuning the bandgap of ZnO by substitution with Mn, Co and Ni

ZnO doped with Mn²⁺, Co²⁺ and Ni²⁺ have been prepared by the decomposition of acetate solid solutions. Electronic absorption spectroscopy has been used to measure the bandgaps of the doped ZnO samples and the variation of the bandgap with dopant concentration has been examined.     

Sensitized luminescence through nanoscopic effects of ZnO encapsulated in SiO2:Tb sol gel derived phosphor

Terbium (1 mol%) doped ZnO-SiO₂ binary system was prepared by a sol-gel process. Nanoscopic effects of ZnO on the photoluminescence (PL) and the cathodoluminescence (CL) properties were studied. Defects emission from ZnO nanoparticles was measured at 560 nm and the line emission from Tb³⁺ ions in SiO₂:Tb³⁺ and ZnO-SiO₂:Tb³⁺ with a major peak at 542 nm was measured. The PL excitation wavelength for 542 nm Tb³⁺ emission was measured at ∼320 nm in both SiO₂:Tb³⁺ and ZnO-SiO₂:Tb³⁺. The CL data showed quenched luminescence of the ZnO nanoparticles at 560 nm from a composite of ZnO-SiO₂:Tb³⁺ and a subsequent increase in 542 nm emission from the Tb³⁺ ions. This suggests that energy was transferred from the ZnO nanoparticles to enhance the green emission of the Tb³⁺ ions. The PL and CL properties of ZnO-SiO₂:Tb³⁺ binary system and possible mechanism for energy transfer from the ZnO nanoparticles to Tb³⁺ ions are discussed.     

Sensitizing effects of ZnO quantum dots on red-emitting Pr-doped SiO2 phosphor

In this study, red cathodoluminescence (CL) (λ_emission=614 nm) was observed from Pr³⁺ ions in a glassy (amorphous) SiO₂ host. This emission was enhanced considerably when ZnO quantum dots (QDs) were incorporated in the SiO₂:Pr³⁺ suggesting that the ZnO QDs transferred excitation energy to Pr³⁺ ions. That is, ZnO QDs acted to sensitize the Pr³⁺ emission. The sol–gel method was used to prepare ZnO–SiO₂:Pr³⁺ phosphors with different molar ratios of Zn to Si. The effects of the ZnO QDs concentration and the possible mechanisms of energy transfer from ZnO to Pr³⁺ are discussed. In addition, the electronic states and the chemical composition of the ZnO–SiO₂:Pr³⁺ phosphors were analyzed using X-ray photoelectron spectroscopy (XPS).     

Fe,Co共掺杂ZnO薄膜结构及发光特性研究

采用溶胶凝胶法在玻璃衬底上制备了Fe,Co共掺Zn0.9FexCo0.1-xO(x=0,0.03,0.05,0.07)系列薄膜.通过扫描电镜(SEM)、X射线衍射(XRD)、X射线光电子谱(XPS)和光致发光(PL)谱对薄膜样品的表面形貌、晶体结构、成分和光学性能进行了研究.XRD结果表明所有ZnO薄膜样品都呈六方纤锌矿结构,在样品中没有观察到与Fe和Co相关的团簇,氧化物及其他杂相的衍射峰,表明共掺杂改善了Fe或Co在ZnO的分散性.XPS测试结果揭示样品中Co离子的价态为+2价;Fe离子的价态为+2价和+3价共存,但Fe相对浓度的增大导致Fe3+含量增加.所有样品的室温光致发光谱(PL)均观察到紫外发光峰和蓝光双峰,其中Fe,Co共掺ZnO薄膜的紫外发光峰较本征ZnO出现蓝移,蓝光双峰峰位没有变化,但发光强度有所减弱;而掺杂ZnO薄膜的绿光发光峰几乎消失.最后,结合微结构和成分分析对薄膜样品的发光机理进行了讨论.     

Effect of indium incorporation in Zn1−xCoxO thin films

In the present paper, the preliminary investigations of a series of ZnO thin films co-doped with indium and cobalt with an objective to elucidate the correlation, if any, between the carrier concentration and the induced room temperature ferromagnetism (RTFM), are presented. The single-phasic (Zn_99.5In_0.5)_1−xCo_xO thin films are deposited by spray pyrolysis. The substitution of Zn²⁺ by Co²⁺ has been established by optical transmission analysis of these films. The films are ferromagnetic at room temperature; and the magnetization has higher value for indium and cobalt co-doped thin film as compared with Zn0₉₀Co_0.1O thin film (having no indium).     

PL and EL characterizations of ZnO:Eu, Li films derived by sol-gel process

ZnO:Eu³⁺, Li⁺ films prepared by the dip-coating method were characterized by photoluminescence (PL) and electroluminescence (EL). When the ZnO:Eu³⁺, Li⁺ films were excited using UV light with energy corresponding to the band-to-band excitation of the host matrix, the PL spectra showed emissions from both ZnO and Eu³⁺ ions, while their EL spectra showed emissions only from Eu³⁺ ions, and no emission from ZnO could be detected. It is found that the EL emission intensity B is dependent on the applied voltage, B=B_o exp(−bV^−1/2). With increasing frequency, the EL intensity dramatically increases at lower frequencies (<1000 Hz), and then increases gradually at higher frequencies (>1000 Hz).     

Magnetic structure of geometrically frustrated compound Co2Cl(OH)3 determined by proton NMR

The magnetic structure of a geometrically frustrated system Co₂Cl(OH)₃ is determined by comparing the observed proton NMR spectrum with numerical calculations based on various magnetic models. The best fit is obtained with a model that the magnetic moments of Co²⁺ ions in the triangular plane are parallel to the principal axis of local crystal field and those of Co²⁺ ions in the kagome lattice plane are randomly disordered in the a-b plane, which nearly bisects the angle between the principal axis of the local field and a line pointing towards the body center of the tetrahedron. The coexistence of the ferromagnetic order in the triangular plane and the random disorder in the kagome plane is consistent with the results of measurements by Zheng et al. However, the magnetic moments of Co²⁺ ions are not directed towards the body center of the tetrahedron as characteristic in the “spin ice” magnetic structure. Furthermore, the Co²⁺ ions in the triangular plane have a smaller magnitude of magnetic moment than those in the kagome plane. Thus, our result suggests that the transition metal compound Co₂Cl(OH)₃ is different from the “spin ice” in magnetic structure, although it is similar to rare-earth pyrochlores in crystal structure.     

Synthesis and characterization of ZnO:Co<Superscript>2+</Superscript> nanoparticles

This work investigates cobalt doped ZnO nanoparticles prepared by using wet chemical methods. The nanoparticles have a typical size of 3–8 nm. The electronic structure as well as the optical and magnetic properties of Co²⁺ have been characterized. X-ray diffraction spectra of the powder show wurtzite ZnO with no secondary Co phases. In the energy range below the bandgap, the optical absorption spectra show the internal d–d transitions related to Co²⁺ incorporated on the Zn lattice site in ZnO. Low temperature photoluminescence measurements confirm these results. Based on the analysis of the g-valuesfor bulk ZnO:Co., electron paramagnetic resonance measurements coincide with the simulation of Co-doped ZnO powder. Thus far, no evidence for ferromagnetism has been obtained. PACS 61.46.Df; 76.30.Fc; 78.67.Bf     

Optical characterization of fourfold (Td)- and sixfold (Oh)-transition-metal species in MgAl2O4:Co by time-resolved spectroscopy

This work investigates the origin of novel visible photoluminescence (PL) bands observed in the spinel MgAl₂O₄:Co²⁺. Besides the well-known fourfold-coordinated Co²⁺(T_d) PL at 670 nm [N.V. Kuleshov, V.P. Mikhailov, V.G. Scherbitsky, P.V. Prokoshin and K.V. Yumashev, J. Lumin. 55 (1993) 265.], a rich structured PL band at 686 nm was also observed that we associate with uncontrolled impurities of sixfold coordinated Cr³⁺(O_h) by time-resolved spectroscopy and lifetime measurements and their variation with temperature. We also show that the lifetime of the Co²⁺(T_d) emission at 670 nm varies from τ=6.7 μs to 780 ns on passing from T=10 to 290 K. This unexpected behaviour for T_d systems is related to the excited-state crossover (⁴T₁↔²E), making the emission band to transform from a narrow-like emission from ²E at low temperature to a broad structureless band from ⁴T₁ at room temperature.