Synthesis of cobalt-doped cadmium sulphide nanocrystals and their optical and magnetic properties

Cobalt-doped cadmium sulphide (CdS) nanocrystals (NCs) were synthesized with three different cobalt concentrations by aqueous chemical coprecipitation method. Dopant incorporation was recognised using X-ray diffraction (XRD) analysis with a shift in the diffraction peaks and compression in the lattice. The size and crystallinity of the cobalt-doped CdS NCs were studied by high resolution transmission electron microscopy (HRTEM). The blue shift in the spectra and the band gap value of Co-doped CdS NCs was estimated using reflectance UV spectrophotometer. Variation in the luminescence properties was studied by fluorescence spectroscopy. The change in the optical properties supports the Co incorporation in the CdS NCs. The BET measurement revealed that the powders had a relatively high specific surface area of 131.30, 106.93 and 102.04 m²/g for 2, 4 and 6% Co:CdS, respectively. Room temperature magnetisation was studied using vibrating sample magnetometer for both 4 and 6% cobalt-doped CdS NCs, which revealed a weak ferromagnetic signal and strong ferromagnetic hysteresis respectively.     

Fabrication of Co-doped ZnO nanowires and their temperature-dependent ultraviolet emission properties

Co-doped ZnO nanowires have been fabricated through a high temperature vapor–solid deposition process. The temperature-dependent ultraviolet emission properties of Co-doped ZnO nanowires under 10–300 K were reported. The results show that there are multipeak emissions situated at the ultraviolet region. The investigation of the excitonic transition in Co-doped ZnO nanowires shows that there is an intensive ultraviolet periodic emission of Co-doped ZnO nanowires under low temperature. The oscillatory structure has an energy periodicity about 70 meV. The oscillatory structure is mainly attributed to the longitudinal optical phonon replicas of the free exciton. The ultraviolet emission shows an obvious redshift with the increasing temperature.     

Effect of oxygen vacancy defect on the magnetic properties of Co-doped ZnO

The influence of oxygen vacancy on the magnetism of Co-doped ZnO has been investigated by the first-principles calculations.It is suggested that oxygen vacancy and its location play crucial roles on the magnetic properties of Co-doped ZnO.The exchange coupling mechanism should account for the magnetism in Co-doped ZnO with oxygen vacancy and the oxygen vacancy is likely to be close to the Co atom.The oxygen vacancy (doping electrons) might be available for carrier mediation but is localized with a certain length and can strengthen the ferromagnetic exchange interaction between Co atoms.     

Cobalt-doping effects in single crystalline and polycrystalline EuFe_(2-x)Co_xAs_2 compounds

A series of Co-doped EuFe2-xCoxAs2 compounds were prepared in both of single crystalline and polycrystalline forms.The Co-doping effects on the crystal structure,electrical resistivity and magnetic susceptibility were systematically studied.Superconductivity was found in polycrystalline Co-doped samples from zero resistivity effects,with the highest onset superconducting transition temperature at 26 K in the optimum doped EuFe1.84Co0.16As2 compound.While due to the stronger competition between the superconducting order and the Eu2+ magnetic order,the zero resistivity effect is absent in the Co-doped single crystal samples.     

Spin manipulation in Co-doped ZnO

We report the clearly observed tunneling magnetoresistance at 5 K in magnetic tunnel junctions with Co-doped ZnO as a bottom ferromagnetic electrode and Co as a top ferromagnetic electrode prepared by pulsed laser deposition. Spin-polarized electrons were injected from Co-doped ZnO to the crystallized Al2O3 and tunnelled through the amorphous Al2O3 barrier. Our studies demonstrate the spin polarization in Co-doped ZnO and its possible application in future ZnO-based spintronics devices.     

Origins of ferromagnetism in transition metal doped diamond

Using first-principles calculations, we investigated the electronic and magnetic properties of Mn-doped, Fe-doped, and Co-doped diamond. It is found that the Mn-, Fe-, and Co-doped diamond are stabilized in ferromagnetic configurations. The origins of the magnetic ordering are explained successfully by the phenomenological band coupling model based on the p–d and d–d level repulsions between the dopant ions and host elements. According to Heisenberg model, high Curie temperature may be expected for Mn-, Fe-, and Co-doped diamond if there are no native defects or other impurities.     

Study on field-emission characteristics of electrodeposited Co-doped ZnO thin films

Co-doped ZnO films were fabricated using electrodeposition method on the ITO substrates. The structure of the Co-doped ZnO films was analyzed by X-ray diffraction and scanning electron microscope. The field-emission characteristics of the prepared Co-doped ZnO films were examined using diode structure in a vacuum chamber. The examined results indicate that the Co-doping cause the turn-on field increasing by increasing the concentration of the Co-dopant, probably due to the band gap changing, which could attributed to the sp-d exchange interactions between the band electrons and the localized d electrons of the Co²⁺ ions substituting Zn ions in the films.     

Magnetic properties of Co-doped SnO: first-principles calculations

Based on density functional theory calculations,the electronic and magnetic properties of Co-doped SnO are investigated.It is found that the spin-polarized state,with a magnetic moment of about 1.0 μ B per Co-dopant,is more favorable in energy than the non-spin-polarized state.Moreover,the origin of the ferromagnetism in Co-doped SnO is found to be the double exchange mechanism.Our results indicate that Co-doped SnO is a possible candidate of the p-type spintronics material.     

Magnetic properties of Co-doped SnO2 at different carrier concentrations

Magnetic properties of Co-doped wide-gap semiconductor SnO₂ were studied theoretically by using the PPLCAO first-principles computational scheme. Since the carrier plays an important role on magnetic properties about diluted magnetic semiconductors (DMS) materials, we discuss the origin of magnetic moments and the magnetic ordering mechanism with different carrier concentration in Co-doped SnO₂ based on calculated spin density distribution. It is found that, the RKKY interaction is dominated in the magnetic coupling in Co-doped SnO₂.     

Effects of temperature and atmosphere on the magnetic properties of Co-doped ZnO rods

Co-doped ZnO (Zn_0.95Co_0.05O) rods are fabricated by co-precipitation method at different temperatures and atmospheres. X-ray diffraction, Energy dispersive X-ray spectroscopy and Raman results indicate that the samples were crystalline with wurtzite structure and no metallic Co or other secondary phases were found. Raman results indicate that the Co-doped ZnO powders annealed at different temperatures have different oxygen vacancy concentrations. The oxygen vacancies play an important role in the magnetic origin for diluted magnetic semiconductors. At low oxygen vacancy concentration, room temperature ferromagnetism is presented in Co-doped ZnO rods, and the ferromagnetism increases with the increment of oxygen vacancy concentration. But at very high oxygen vacancy concentration, large paramagnetic or antiferromagnetic effects are observed in Co-doped ZnO rods due to the ferromagnetic-antiferromagnetic competition. In addition, the sample annealed in Ar gas has better magnetic properties than that annealed in air, which indicates that O₂ plays an important role. Therefore, the ferromagnetism is affected by the amounts of structural defects, which depend sensitively on atmosphere and annealing temperature.     

Mn2+、Mn4+共掺杂Zn2GeO4电子结构和光学性质的第一性原理的研究

利用第一性原理的局域密度近似（LDA）方法, 对Zn2GeO4, Mn2+掺杂Zn2GeO4，Mn2+、Mn4+共掺杂Zn2GeO4的光电性质进行了理论研究。结果表明，Mn2+、Mn4+掺杂可以提高Zn2GeO4的载流子浓度，从而改善Zn2GeO4的导电性。Mn2+离子的掺杂导致Zn2GeO4对光的吸收由紫外区域扩展到可见光区域， Mn2+、Mn4+共掺杂促进Zn2GeO4晶体对可见光的吸收能力大幅增加，因此Mn2+、Mn4+共掺杂Zn2GeO4可以用于制备高效率的光催化剂和发光材料。     

Structure and room-temperature ferromagnetism of Co-doped ZnO DMS films

Co-doped ZnO diluted magnetic semiconductor films were prepared on Si(100) substrates by magnetron sputtering system and the Co content varies from 0.01 to 0.15. The X-ray diffraction results showed ZnO of the wurtzite structure. The ferromagnetism was observed at room temperature. The X-ray near-edge absorption spectroscopy revealed that Co substitutes for Zn²⁺ ions in the valence of +2 state in the Co-doped ZnO films.     

Mn2+、Mn4+共掺杂Zn2GeO4电子结构和光学性质的第一性原理的研究

利用第一性原理的局域密度近似（LDA）方法, 对Zn2GeO4, Mn2+掺杂Zn2GeO4,Mn2+、Mn4+共掺杂Zn2GeO4的光电性质进行了理论研究。结果表明,Mn2+、Mn4+掺杂可以提高Zn2GeO4的载流子浓度,从而改善Zn2GeO4的导电性。Mn2+离子的掺杂导致Zn2GeO4对光的吸收由紫外区域扩展到可见光区域, Mn2+、Mn4+共掺杂促进Zn2GeO4晶体对可见光的吸收能力大幅增加,因此Mn2+、Mn4+共掺杂Zn2GeO4可以用于制备高效率的光催化剂和发光材料。     

Co掺杂ZnO纳米棒的共振拉曼光谱和发光特性

采用X射线衍射(XRD)和透射电子显微镜(TEM)手段对微乳液法合成的Zn0.9Co0.1O纳米棒进行了表征.通过室温下的共振拉曼光谱和光致发光光谱手段,研究了所合成纳米材料的共振拉曼光谱和发光特性,并与体相ZnO的研究结果对比,发现合成的材料具有四阶声子紫外共振拉曼散射,而体相材料只有两阶,并观察到在紫外和可见区域所...     

Effects of oxygen vacancies on the room-temperature ferromagnetism of Co-doped polycrystalline CeO2

Pure and Co-doped single-phase CeO₂ crystals were synthesized by a solid-state reaction method. Samples of different oxygen vacancy concentration were studied, including (1) as-sintered crystals, (2) powders ground from the same crystal, and (3) a cold-pressed pellet from the ground powder that was unannealed and annealed at 800 °C. By analyzing the magnetic behaviors, surface/volume ratio and O vacancy concentration, the effects of oxygen vacancies on the room-temperature ferromagnetism (RT-FM) of Co-doped CeO₂ were systematically investigated. The results confirm that the RT-FM observed in Co-doped CeO₂ has a direct relationship with the oxygen vacancy concentration, and support the oxygen vacancy mediated FM mechanism.     

Structural and magnetic properties of single-crystalline Co-doped barium titanate nanoparticles

Undoped and Co-doped BaTiO₃ nanoparticles were synthesized by a one-step sol-precipitation method. For all the samples, X-ray diffraction showed characteristic diffraction lines for BaTiO₃ without the indication of secondary phases. High-resolution transition electron microscopy images showed that BaTiO₃ nanoparticles exhibit the nature of single-crystal. Magnetometry revealed that all the Co-doped BaTiO₃ samples show paramagnetic behaviors and Co ions in BaTiO₃ are present as isolated paramagnetic centers. This is contrasted to several reported cases of ferromagnetism in Co-doped BaTiO₃.     

Ferromagnetism from Co-Doped ZnO Nanocantilevers above Room Temperature

At low temperature （400℃）, chemical vapour deposition （CVD） is employed to make comb-like Co-doped ZnO nanocantilever arrays （NAs）. The magnetization curves of the as-synthesized Co-doped ZnO NAs indicate the existence of above-room-temperature ferromagnetism （ARTFM） （Curie temperature, Tc 〉 300 K） whereas undoped ZnO NAs does not. The corresponding ferromagnetic source mechanism is discussed, in which defects play an important role due to the strong green light emission.     

Studies on structural and magnetic properties of Co-doped pyramidal ZnO nanorods synthesized by solution growth technique

Large-area arrays of highly oriented Co-doped ZnO nanorods with pyramidal hexagonal structure are grown on silica substrates by wet chemical decomposition of zinc–amino complex in an aqueous medium. In case of undoped ZnO with an equi-molar ratio of Zn²⁺/hexamethylenetetramine (HMT), highly crystalline nanorods were obtained, whereas for Co-doped ZnO, good quality nanorods were formed at a higher Zn²⁺/HMT molar ratio of 4:1. Scanning electron microscope (SEM) studies show the growth of hexagonal-shaped nanorods in a direction nearly perpendicular to the substrate surface with a tip size of ～50 nm and aspect ratio around 10. The XRD studies show the formation of hexagonal phase pure ZnO with c-axis preferred orientation. The doping of Co ions in ZnO nanorods was confirmed by observation of absorption bands at 658, 617 and 566 nm in the UV–vis spectra of the samples. The optical studies also suggest Co ions to be present both in +2 and +3 oxidation states. From the photoluminescence studies, a defect-related emission is observed in an undoped sample of ZnO at 567 nm. This emission is significantly quenched in Co-doped ZnO samples. Further, the Co-doped nanorods have been found to show ferromagnetic behavior at room temperature from vibrating sample magnetometer (VSM) studies.     

Co掺杂ZnO薄膜的结构和磁学性能

研究了用单束脉冲激光沉积法制备的Co掺杂ZnO薄膜的结构和磁学性能。XRD表征结果表明制备的薄膜是具有沿c轴择优取向的纤锌矿点阵结构。然而,进一步的高分辨电子显微镜结果显示整个样品上的晶体取向并不完全相同。很难说明形成了单晶。结果分析表明Co占据了部分Zn的格点,并对电子结构产生了影响。室温下观察到了磁滞回线,显示掺杂Co可以实现ZnO的磁性翻转,但磁性比较小。该薄膜与我们以前用双束脉冲激光沉积法制备的Co掺杂ZnO薄膜具有相似的性能,提示我们其内部的机制可能相似。     

Structure, magnetic and optical properties of polycrystalline Co-doped TiO2 films

Co-doped TiO₂ films were fabricated under different conditions using reactive facing-target magnetron sputtering. Co doping improves the transformation of TiO₂ from anatase phase to rutile phase. The chemical valence of doped Co in the films is +2. All the films are ferromagnetic with a Curie temperature above 340 K. The average room-temperature moment per Co of the Co-doped TiO₂ films fabricated at 1.86 Pa decreases from 0.74 μ_B at x=0.03 to 0.02 μ_B at x=0.312, and decreases from 0.54 to 0.04 μ_B as x increases from 0.026 to 0.169 for the Co-doped TiO₂ films fabricated at 0.27 Pa. The ferromagnetism originates from the oxygen vacancies created by Co²⁺ dopants at Ti⁴⁺ cations. The optical band gaps value (E_g) of the Co-doped TiO₂ films fabricated at 1.86 Pa decreases linearly from 3.35 to 2.62 eV with the increasing x from 0 to 0.312. For the Co-doped TiO₂ films fabricated at 1.86 Pa, the E_g decreases linearly from 3.26 to 2.53 eV with increasing x from 0 to 0.350.