Cobalt-doped ZnO Research Articles

Effect of Co-doping on the structure and optical properties of ZnO nanostructure prepared by RF-magnetron sputtering

Cobalt-doped ZnO nanorods were successfully synthesized on SiO2/Si substrate using RF-magnetron sputtering at room temperature. The undoped and Co-doped ZnO nanostructures were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). The results showed that Co2+ replaced Zn2+ in the ZnO lattice without changing the wurtzite structure. The ZnO structure became high crystallite and was gradually converted into nanorods without extra phases as increased cobalt doping levels to 3at.% and 4at.%. The as-synthesized nanorod arrays were dense and vertically grew on the substrate with lengths of approximately 341 and 382.3nm for 3at.% and 4at.% CO, respectively. The energy bandgap values increased as the Co concentration increased to 4at.% to reach 3.3eV compared with undoped ZnO (3.26eV).

Ultrasonically sprayed ZnO:Co thin films: Growth and characterization

In this work, undoped and cobalt-doped ZnO thin films were deposited at 275±5°C on glass substrates by the ultrasonic spray pyrolysis technique. The structural, electrical, optical and surface characterization of the films as a function of the cobalt concentration in the spraying solution were studied by means of x-ray diffractometer, current–voltage characteristics, UV–vis spectrophotometer and atomic force microscope, respectively. X-ray diffraction reveals that the films are polycrystalline in nature with preferred orientations of (002) for the ZnO:Co (0, 2, 4at.%) and (100) for the ZnO:Co (6at.%). The optical transmittance of all films was studied as a function of wavelength in the range of 300–1100nm. They exhibit high transparency in the visible wavelength region with some interference fringes and sharp ultraviolet absorption edges. The optical band gap and Urbach energy values of the films were found in the range of 3.250–3.301eV and 90–230meV, respectively. The electrical studies for all films were carried out by using conductivity-temperature measurements and it was seen that the electrical conductivity of ZnO films decreases slightly depending on the increasing of Co doping. Also, Co doping increases both energies of donor-like traps and activation energy for ZnO films. The surface morphology was analyzed by atomic force microscope and a strong dependence on the cobalt incorporation was found.

Synthesis and characterization of bulk cobalt-doped ZnO and their thin films

(ZnO)1−x(Co3O4)x≤0.05 thin-film series were deposited in vacuum Ar–O2 and N2 environment using the pulsed-laser deposition (PLD) technique. Bulk pellets were prepared by the solid-state reaction technique by mixing ZnO and Co3O4 powder and further utilized for deposition of thin films. XRD patterns of pellets exhibit homogeneous ZnCoO phase formation with no other impurity phases. Single-phase (002) oriented ZnCoO thin films were formed without any cobalt segregation. The cobalt core peaks confirmed the existence of +2 valence state of cobalt in thin-film samples, and the binding energy difference between the cores peaks showed the homogeneous doping of cobalt in ZnO matrix. The magnetic ordering was achieved in thin films samples which were deposited in vacuum and N2 ambience.

Cobalt doped ZnO quantum dots in a monolayer: do the bands depend on the alignment of the magnetic domain?

We show that energy of conduction and valence band-edges of ferromagnetic quantum dots in a monolayer changes when the nanocrystals' magnetic moments are aligned or oriented. Here we characterize a monolayer of cobalt-doped ZnO nanoparticles with scanning tunnelling microscopy. We observed that when their magnetic domains are aligned or oriented, transport gap of the quantum dots decreases. The decrease in the gap is primarily due to an increase in valence band-edge energy. We vary the content of cobalt in the doped-nanoparticles and also the degree of alignment of their magnetic moments. Measurements of band-edges and transport gap of such nanocrystals in a monolayer support our observation. The results hence provide a new route to control transport gap and band-edge energies of ferromagnetic quantum dots through the process of alignment of magnetic domains of the nanocrystals.

Cobalt-doped ZnO nanocrystals: quantum confinement and surface effects from ab initio methods

Cobalt-doped ZnO nanocrystals were studied through ab initio methods based on the Density Functional Theory. Both quantum confinement and surface effects were explicitly taken into account. When only quantum confinement effects are considered, Co atoms interact through a superexchange mechanism, stabilizing an antiferromagnetic ground state. Usually, this is the case for high quality nanoparticles with perfect surface saturation. When the surfaces were considered, a strong hybridization between the Co atoms and surfaces was observed, strongly changing their electronic and magnetic properties. Our results indicated that the surfaces might qualitatively change the properties of impurities in semiconductor nanocrystals.

Electron transport mechanisms in individual cobalt-doped ZnO nanorods

This study examined carrier transport in single cobalt-doped zinc oxide (Co:ZnO) nanorods in temperatures ranging from 80 to 405 K. Measurements were taken on single nanorods deposited on a Si template, where two- and four-point metallic contacts were previously made using e-beam lithography, dielectrophoresis, and focused ion beam. In both two- and four-point probe measurements, the current–voltage curves were clearly linear and symmetrical with respect to both axes. The electrical measurements were carried out according to three different measuring methods to accurately determine the resistivity of the single Co:ZnO nanorods. The Co-doped nanorods exhibited ferromagnetic behavior at room temperature. The remanence permanent magnet of the nanorods increased with increasing Co concentration, however, this was accompanied by the decrease in electrical resistivity. The transport properties were dominated by the thermal activation of electrons from the Fermi level to the conduction band for a temperature above 140–160 K, and to the impurity band at a lower temperature. The electronic transport of the nanorods was influenced by the surface states when exposed in the air.

Effect of doping concentration on absorbance, structural, and magnetic properties of cobalt-doped ZnO nano-crystallites

Abstract Controlled conduction of magnetic spins is desired for data processing in modern spintronic devices. Transition metal-doped ZnO is a potential candidate for this purpose. We studied the effects of cobalt doping on structural, absorbance, and magnetic properties of ZnO nano-particles. Different compositions (Zn0.99Co0.1O, Zn0.97Co0.3O, and Zn0.95Co0.5O) of cobalt-doped ZnO were fabricated using metallic chlorides by co-precipitation method. XRD revealed standard ZnO wurtzite crystal structure without lattice distortion due to impurities but showed presence of additional phases at higher doping ratios. Fourier transformed infrared spectroscopy also confirmed the standard ZnO profiles at lower doping ratios but additional phases at higher doping. Vibrating sample magnetometer showed soft ferromagnetic behavior for low impurity samples and harder ferromagnetic behavior for higher doping at room temperature. A simultaneous differential scanning calorimetry/thermo gravimetric analysis was performed to study the phase variations during crystallization.

Solution-based synthesis of cobalt-doped ZnO thin films

Undoped and cobalt-doped (1–4wt.%) ZnO polycrystalline, thin films have been fabricated on quartz substrates using sequential spin-casting and annealing of simple salt solutions. X-ray diffraction (XRD) reveals a wurzite ZnO crystalline structure with high-resolution transmission electron microscopy showing lattice planes of separation 0.26nm, characteristic of (002) planes. The Co appears to be tetrahedrally co-ordinated in the lattice on the Zn sites (XRD) and has a charge of +2 in a high-spin electronic state (X-ray photoelectron spectroscopy). Co-doping does not alter the wurzite structure and there is no evidence of the precipitation of cobalt oxide phases within the limits of detection of Raman and XRD analysis. Lattice defects and chemisorbed oxygen are probed using photoluminescence and Raman spectroscopy — crucially, however, this transparent semiconductor material retains a bandgap in the ultraviolet (3.30–3.48eV) and high transparency (throughout the visible spectral regime) across the doping range.

Absence of free carrier and paramagnetism in cobalt-doped ZnO nanoparticles synthesized at low temperature using citrate sol–gel route

Cobalt-doped ZnO nanoparticles have been synthesized using a simple citrate sol–gel auto-combustion method. The XRD confirms nano-single phase and Wurtzite structure. Increased cobalt solubility from 15 to 20 % was observed in ZnO matrix due to low temperature synthesis. Room temperature paramagnetic contribution is observed for all the samples, since cobalt is a neutral dopant and no free carriers are produced. Normally, ferromagnetism is observed in ZnO due to carrier mediated interaction between transition metal ions and free carriers. Hence no ferromagnetism was observed and only paramagnetism was observed due to non-availability of free carriers for long range ferromagnetic interaction in our system. At higher doping, peak broadening of the highly intense XRD peak (101) was observed indicating formation of cobalt cluster (metal–metal) which reduces paramagnetism due to antiferromagnetic interaction and the magnetization value decreases to 0.00456 from 0.0076 emu/g. Absence of photoluminescence peak at 520 nm due to oxygen related defects also supports, the presence of paramagnetism in our samples, since oxygen defects are the another source of ferromagnetism in ZnO. Indirect evidence for the presence of cobalt clustering is also obtained from the photoluminescence studies which lead to concentration quenching of peaks. Photoluminescence studies exhibit NBE peak at 412 nm and defect peaks at 471 and 672 nm. The intensity of red emission peak at 672 nm remains constant whereas the intensity of the peaks at 412 and 471 nm increases and then decreases due to doping induced disorder leading to concentration quenching.

Growth and photoluminescence properties of inclined ZnO and ZnCoO thin films on SrTiO3(110) substrates

ZnO thin film growth prefers different orientations on the etched and unetched SrTiO3(STO)(110) substrates. Inclined ZnO and cobalt-doped ZnO (ZnCoO) thin films are grown on unetched STO(110) substrates using oxygen plasma assisted molecular beam epitaxy, with the c-axis 42° inclined from the normal STO(110) surface. The growth geometries are ZnCoO[100]//STO[11̄0] and ZnCoO[11̄1]//STO[001]. The low temperature photoluminescence spectra of the inclined ZnO and ZnCoO films are dominated by D0X emissions associated with A0X emissions, and the characteristic emissions for the 2E(2G)→4A2(4F) transition of Co2+ dopants and the relevant phonon-participated emissions are observed in the ZnCoO film, indicating the incorporation of Co2+ ions at the lattice positions of the Zn2+ ions. The c-axis inclined ZnCoO film shows ferromagnetic properties at room temperature.

Synthesis, electronic property and photocatalytic applications of mesoporous cobalt-doped ZnS and ZnO nanoplates

Mesoporous cobalt-doped ZnS and ZnO nanoplates were fabricated by calcination of a Zn0.95Co0.05S(en)0.5 complex (en=ethylenediamine), which was hydrothermally synthesized using ethylenediamine as a single solvent and chelating agent. When the as-prepared Zn0.95Co0.05S(en)0.5 complex was calcined, mesoporous nanoplates of wurtzite Zn0.95Co0.05S were formed, which then transformed to Zn0.95Co0.05O platelets upon further oxidation. Photocatalytic performance of the as-prepared materials was investigated for decomposition of the azo dye (acid red 14) and photoelectrochemical current generation in aqueous Na2S/Na2SO3 solution as probe reactions. The Zn0.95Co0.05S calcined at 500°C exhibited the highest photocatalytic activity under UV irradiation and also showed the photocatalytic performance under visible light irradiation.

Ferromagnetism and point defects in ZnCoO

We report on optical and electrical properties of oxygen-vacancy (VO) related defects in cobalt-doped ZnO materials grown using molecular beam epitaxy (MBE). We probe the concentration of the VO in samples with various cobalt contents, which were annealed under different conditions, and correlate it to the magnetic properties of the samples. Our results suggest that the oxygen vacancy is a major player in the observed ferromagnetism of Co-doped ZnO.

Cobalt-doped ZnO nanowires on quartz: Synthesis by simple chemical method and characterization

The synthesis of cobalt-doped ZnO nanowires is achieved using a simple, metal salt decomposition growth technique. A sequence of drop casting on a quartz substrate held at 100°C and annealing results in the growth of nanowires of average (modal) length ∼200nm and diameter of 15±4nm and consequently an aspect ratio of ∼13. A variation in the synthesis process, where the solution of mixed salts is deposited on the substrate at 25°C, yields a grainy film structure which constitutes a useful comparator case. X-ray diffraction shows a preferred [0001] growth direction for the nanowires while a small unit cell volume contraction for Co-doped samples and data from Raman spectroscopy indicate incorporation of the Co dopant into the lattice; neither technique shows explicit evidence of cobalt oxides. Also the nanowire samples display excellent optical transmission across the entire visible range, as well as strong photoluminescence (exciton emission) in the near UV, centered at 3.25eV.

Enhanced gas sensing performance of Co-doped ZnO hierarchical microspheres to 1,2-dichloroethane

A chlorinated compound sensor was developed with cobalt-doped ZnO hierarchical microspheres as the sensing materials, which were prepared by a template-free hydrothermal method via salt basic carbonate precursor combined with subsequent calcination. The calcination of the precursor produces hierarchical microspheres composed of interconnected nanosheets with high porosity resulting from the thermal decomposition process. Field emission scanning electron microscopy (FESEM) reveals that the doped ZnO microspheres have the same microstructure and size as pristine ZnO microspheres. That the dopant Co2+ has substituted Zn2+ sites in ZnO crystal lattice was determined by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV–vis absorption spectrum. These hierarchical cobalt-doped ZnO architectures are highly promising for gas sensor applications. A potential application of Co-doped ZnO microspheres as gas sensors to 1,2-dichloroethane was investigated. It was found that the hierarchical Co-doped ZnO microspheres exhibit dope-induced enhancement of sensing performance with good reversibility and high selectivity. The sensors also show fast response and recovery time of 30s and 5s, respectively. It is believed that this precursor route can be extended to fabricate other doped metal oxide nanostructure with enhanced gas sensing performance.

Room-temperature gas sensing properties of cobalt-doped ZnO Nanobelts with visible light irradiation

Cobalt-doped ZnO (Co:Zn is 1, 3 and 5 mol%) nanobelts were prepared by a multi-components precursor self-assembled method, and characterized by the scanning electronic microscopy (SEM), X-ray diffraction (XRD), surface photocurrent spectroscopy and surface photovoltage spectroscopy. The room-temperature gas sensing of oxygen and ethanol vapor was measured with irradiation of visible light. The sensitivity is decreased on increasing the doped concentration. The cobalt-doped ZnO (1%) showed higher sensitivity and faster response rate to ethanol than to oxygen. The results are due to a large amount of photo-generated holes left in the surface of cobalt-doped ZnO (1%) nanobelts. And the changed photocurrent intensity was linear as function of the ethanol vapor.

A high performance cobalt-doped ZnO visible light photocatalyst and its photogenerated charge transfer properties

Highly photocatalytically active cobalt-doped ZnO (ZnO:Co) nanorods have been prepared by a facile hydrothermal process. X-ray diffraction, X-ray photoelectron spectroscopy, Raman scattering and UV-vis diffuse reflectance spectroscopy confirmed that the dopant ions substitute for some of the lattice zinc ions, and furthermore, that Co2+ and Co3+ ions coexist. The as-prepared ZnO:Co samples have an extended light absorption range compared with pure ZnO and showed highly efficient photocatalytic activity, only requiring 60 min to decompose ∼93% of alizarin red dye under visible light irradiation (λ > 420 nm). The photophysical mechanism of the visible photocatalytic activity was investigated with the help of surface photovoltage spectroscopy. The results indicated that a strong electronic interaction between the Co and ZnO was present, and that the incorporation of Co promoted the charge separation and enhanced the charge transfer ability and, at the same time, effectively inhibited the recombination of photogenerated charge carriers in ZnO, resulting in high visible light photocatalytic activity. Open image in new window

Hydrothermal Synthesis and Characterizations of Cobalt-Doped ZnO Nanostructures

Pure and Cobalt-doped ZnO nanostructures have been synthesized by a hydrothermal process. The samples were characterized by X-ray diffraction (XRD), UV-Vis diffusion reflectance spectra (DRS) and X-ray energy dispersion spectroscopy (EDS). The structure and morphology analyses show that Co doping can influence the nanostructures morphology, but cannot change the crystal structures of ZnO samples. The DRS spectra showed that Co dopant enhanced the ability of visible light absorption of the ZnO samples

Origination of electron magnetic chiral dichroism in cobalt-doped ZnO dilute magnetic semiconductors

Electron magnetic chiral dichroism (EMCD) relates the microstructure of a single nanostructure with the magnetic property, indicating that the ferromagnetism observed in cobalt-doped ZnO dilute magnetic semiconductor is intrinsic. First-principles calculations were carried out and the electronic states contributing to the spin imbalance are identified. An exact model is developed, allowing one to visualize the origination of dichroism to be the difference of transition intensity from spin-polarized 2p states to Co-3d states in cobalt-doped ZnO nanostructures.

Combustion synthesis of Co-doped zinc oxide nanoparticles using mixture of citric acid–glycine fuels

In this study, cobalt-doped ZnO nanoparticles were synthesized by combustion method. Mixtures of citric acid and glycine were used as fuel. As-prepared powders were characterized by XRD, XPS, SEM-EDX, TEM and spectrophotometer. XRD patterns indicated that combustion reaction by different fuel mixture resulted in the formation of pure ZnO phase. However, citric acid combustion alone led to amorphous powder. Scherrer's equation demonstrated that the crystallite size increases with citric acid/glycine (C/G) ratio (38–61 nm). SEM and TEM images illustrated that the morphology of the powder depends on the C/G ratios and changes from rod-like to spongy hexagonal particles. Reflectance spectra showed that by higher C/G ratios, deeper green colors are obtained.

Low-temperature hopping and absence of spin-dependent transport in single crystals of cobalt-doped ZnO

Long needle-shaped single crystals of ${\text{Zn}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{O}$ were grown at low temperatures using a molten salt solvent technique, up to $x=0.10$. The conduction process at low temperatures is determined to be Mott variable range hopping. Both pristine and cobalt-doped crystals clearly exhibit a crossover from negative to positive magnetoresistance as the temperature is decreased. The positive magnetoresistance of the ${\text{Zn}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{O}$ single crystals increases with increased Co concentration and reaches up to 20% at low temperatures (2.5 K) and high fields $(>1\text{ }\text{T})$. Superconducting quantum interference device magnetometry confirms that the ${\text{Zn}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{O}$ crystals are predominantly paramagnetic in nature and the magnetic response is independent of Co concentration. The results indicate that cobalt doping of single crystalline ZnO introduces localized electronic states and isolated ${\text{Co}}^{2+}$ ions into the host matrix but that the magnetotransport and magnetic properties are decoupled.