Synthesis of transition metal-doped tungsten oxide nanostructures and their optical properties

In the present work, we report the solvothermal synthesis of transition metal-doped tungsten oxide nanostructures and their optical properties. Uniform, well-defined Co-doped tungsten oxide nanoblocks consisting of ultrathin nanowires, lenticular bundled Zn-doped tungsten oxide nanorods, and plate-shaped or flower-like Fe-doped tungsten oxide particles have been successfully prepared with the addition of different dopants. The doping of Co and Zn ions may prohibit the oriented growth of the tungsten oxide nanowires and favor their self-assembly, leading to the formation of bundled nanoblocks and nanorods. The Fe-doped tungsten oxide flowers may result from the ordered arrangement of single nanoplates. The band gaps of undoped, Fe-, Zn-, and Co-doped tungsten oxides are found to be 2.92, 2.78, 2.62, and 2.52 eV, respectively.     

Electronic transport properties of Fe-doped CoSb3 prepared by encapsulated induction melting

Fe-doped CoSb₃ skutterudites were prepared by encapsulated induction melting and their thermoelectric and electronic transport properties were investigated. The positive signs of Seebeck and Hall coefficients for all Fe-doped specimens revealed that Fe atoms successfully acted as p-type dopants by substituting Co atoms. Carrier concentration increased with increasing Fe doping content and the Fe dopants could affect the electronic structure of CoSb₃ and generate excess holes. However, carrier mobility decreased with increasing doping content, which indicates that the hole mean free path was reduced by the impurity scattering. Seebeck coefficient and electrical resistivity were almost independent of carrier concentration between 5.8 × 10¹⁹ and 2.0 × 10²⁰ cm^− 3 because the increase in carrier concentration by doping was competitive with the decrease in carrier mobility by the impurity scattering. Seebeck coefficient showed a positive value at all temperatures examined and it increased as the temperature increased. Temperature dependence of electrical resistivity suggested that Co_1 − xFe_xSb₃ is a highly degenerate semiconducting material. Thermal conductivity was considerably reduced by Fe doping and the lattice contribution was dominant in the Fe-doped CoSb₃ skutterudites.     

Sol–gel synthesis and properties of YBa2(Cu1−xMx)4Oy (M=Co,Ni) and effects of additional replacement of yttrium by calcium

In this work the influence of replacing Cu by Ni or Co on the structural and superconducting properties of YBa₂Cu₄O₈ is studied by means of XRD, TGA, SEM, EDX and resistivity measurements. The samples are prepared by the acetate–tartrate sol–gel method, which has proved to be very appropriate for the synthesis of Y-124 at 1 atm with even very small amounts of dopants. It is demonstrated that the transition temperature drastically decreases upon Co or Ni substitution, and that superconductivity is lost at 3% Ni and 6% Co doping. XRD measurements show that Ni and Co produce different structural effects to the orthorhombic YBa₂Cu₄O₈ phase, possibly because they are introduced in different copper positions. Interesting results are obtained by additional substitution of Y by Ca in the Co- and Ni-doped samples: It is shown that the superconducting properties are enhanced in the samples with less than 3% Ni or 6% Co, and that superconductivity is even recovered in samples with more than 3% Ni or 6% Co.     

Microscopic annealing process and its impact on superconductivity in T'-structure electron-doped copper oxides

High-transition-temperature superconductivity arises in copper oxides when holes or electrons are doped into the CuO(2) planes of their insulating parent compounds. Whereas hole doping quickly induces metallic behaviour and superconductivity in many cuprates, electron doping alone is insufficient in materials such as R(2)CuO(4) (R is Nd, Pr, La, Ce and so on), where it is necessary to anneal an as-grown sample in a low-oxygen environment to remove a tiny amount of oxygen in order to induce superconductivity. Here we show that the microscopic process of oxygen reduction repairs Cu deficiencies in the as-grown materials and creates oxygen vacancies in the stoichiometric CuO(2) planes, effectively reducing disorder and providing itinerant carriers for superconductivity. The resolution of this long-standing materials issue suggests that the fundamental mechanism for superconductivity is the same for electron- and hole-doped copper oxides.     

Magnetic properties and phase change features in Fe-doped Ge-Sb-Te

The relationship between magnetic property and phase change features in Fe-doped Ge-Sb-Te has been studied. Fe-doped Ge-Sb-Te is a phase change magnetic material, which exhibits a fast phase change feature and different magnetic, optical and electrical properties between amorphous and crystalline states. However, the crystallization temperature increases and crystallization rate drops with an increase of Fe doping content. Fe doping content should be less than the solid solubility limit so that Fe-doped Ge-Sb-Te has both magnetic property and phase change features. Fe-doped Ge-Sb-Te at crystalline state shows p-type conduction and has a high hole concentration. The Ruderman-Kittel-Kasuya-Yosida indirect interaction via carriers is the origin of the ferromagnetism in Fe-doped Ge-Sb-Te.     

Effects of Co and Fe addition on the properties of lanthanum strontium manganite

The effects of Co and Fe dopants with the amount of 20 and 40 mol% on the properties of La_0.84Sr_0.16MnO₃ were investigated. All compositions were prepared by conventional mixed oxide process and sintered at 1450 °C. The structure of undoped and Co-doped compositions was found to be monoclinic. In addition, the second phase was observed in these sintered compositions. The conductivity of doped materials decreased as compared to that of La_0.84Sr_0.16MnO₃. The SEM microstructure showed the decrease of grain size as Co content increased. The thermal expansion coefficient (TEC) tended to increase as Co content increased. In contrast, the monoclinic and orthorhombic structures were found in 20 and 40 mol% Fe-doped La_0.84Sr_0.16MnO₃. The amount of second phase in sintered composition depends on the amount of Fe content. The conductivity at 1000 °C decreased, but the grain size increased as Fe content increased. The thermal expansion coefficient slightly changed with Fe addition.     

Effect of Strain on the Magnetism of Transition Metal-Doped ZnO: The First-Principles Calculations

The electronic structures and magnetic properties of transition metal (Mn, Fe, Co)-doped ZnO under the in-plane biaxial strains are investigated systematically by the first-principles calculations. It is found that the augmented in-plane biaxial strains can mediate the Fe-doped ZnO ferromagnetism effectively, but have less influence on Mn- and Co-doped systems. Therefore, the strain may be an effective means in inducing ferromagnetism in ZnO:Fe system.     

Switching in structural, optical, and magnetic properties of self-assembled Co-doped ZnO: effect of Co-concentration

Here, we report on the self-assembled Co-doped ZnO nanoparticles synthesized by using sol–gel technique. It has been observed that when Co is introduced in the solution, nanoparticles arrange themselves into a particular pattern. This self-assembly of Zn_1?xCo_xO, were found to be consistent with the change in Co-doping concentration. Using electron microscopy these systems were studied to confirm the assembly formation with doping. Optical study of these assemblies suggests the decrease in direct band gap of these systems with the increase in doping concentration. In addition to it, the Urbach energy was found to be increased, indicating the redistribution of states in between conduction and valence band with doping. Magnetic measurement shows the introduction of paramagnetic behavior with Co-doping in this self-assembled Co-doped ZnO.     

Oxygen chain disorder and compensation by Ca in Y(1-2-3) cuprates

Several M dopants such as Al, Fe, and Co at the Cu site destroy the superconductivity of YBa₂Cu₃O_6+z . However, superconductivity is restored by substituting Ca at the Y site. Arguments are developed to show that the oxygen chain disorder is not the only cause for destroying the superconductivity. A universal relation seems to exist between the net hole density as a result of Ca substitution andT _c . To stabilize the perovskite structure of YSr₂Cu₃O_6+z , it is necessary to substitute Cu by certain elements. Examples are given on Ti and Re substitution. Again, Ca cosubstitution increasesT _c . Further, the irreversibility line is enhanced by Ca, indicating improved pinning in these materials in spite of the oxygen disorder.     

Unintended Phosphorus Doping of Nickel Nanoparticles during Synthesis with TOP: A Discovery through Structural Analysis

We report the discovery of unintentional phosphorus (P) doping when tri-n-octylphosphine (TOP) ligands are used in Ni nanoparticle synthesis, which is the most common method for monodisperse Ni nanoparticle synthesis. The nanoparticles appear pure face-centered cubic (fcc) Ni in X-ray diffraction despite the surprisingly high level (5 atomic %) of P. We find that the P doping follows a direct relationship with increased reaction time and temperature and that the P doping can be estimated with the degree of lattice expansion shown from a peak shift in the XRD spectrum. Through EXAFS modeling and density-functional (DFT) calculations of defect formation energies we find that the P atoms are preferentially located on the fcc lattice as substitutional dopants with oxidation state of zero. Magnetic and catalytic properties are shown to be greatly affected by this doping; DFT calculations show magnetization losses in the Ni system, as well as in Fe and Co systems. These findings are likely relevant for other metal syntheses that employ phosphine ligands.     

Preparation and characterization of doped WO3 photocatalyst powders

WO₃ semiconductor particles, useful in solar energy conversion processes, were doped with transition metal ions, Ti(III), V(IV), Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Ru(III) by a high-temperature sintering technique. The method of preparation of these photocatalysts is described in detail. The structural changes effected during sintering were investigated by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The XRD analysis indicated that the monoclinic crystal structure of WO₃ was not altered during sintering. SEM studies showed that the sizes of the particles ranged from 1 to 10 μm and the crystallinity was increased due to doping. The dopants were found to be mostly distributed on the surface of WO₃ particles.     

Positron Annihilation Study of Fe- and Zn-Doped SmBa2Cu3O7−δ Systems

Polycrystalline SmBa₂Cu_3?x (Fe, Zn) _x O_7?δ (x=0.0–0.4) samples are prepared by the traditional solid-state reaction technique. To make clear how the different substitution sites and magnetism of doped ions influence the physical mechanism of SBCO superconductivity, we investigated systemically polycrystalline samples SmBa₂Cu_3?x (Fe, Zn) _x O_7?δ by means of X-ray diffraction, the test of superconductivity and Positron Annihilation Spectroscopy (PAS). The results show that Fe doping induces the localization of carriers (electron and hole) in the Cu–O chains and weakens the function of carrier reservoir, while Zn doping makes the electronic localization in the CuO₂ planes and increases the electronic density in SmBa₂Cu₃O_7?δ systems. Compared with the Fe doping, Zn doping suppresses the superconductivity stronger. Combining the results of the electronic density n _e and the transition temperature T _c with Fe and Zn doping, we can arrive at the conclusion that the localized carriers in Cu–O chains and CuO₂ planes have enormous influence on superconductivity by affecting the paring and the charge transfer between the reservoir layers and the CuO₂ planes. In addition, the present results indicate that T _c depression has no direct correlation with the magnetism of Fe and Zn ions.     

Fe dopants enhancing ethanol sensitivity of ZnO thin film deposited by RF magnetron sputtering

Pure and 5 % Fe-doped ZnO thin films (TFs) have been successfully deposited on Al₂O₃ substrate from pre-doped target material by RF magnetron sputtering technique. X-ray diffraction (XRD) patterns confirm the formation of both films in single phase wurtzite structure without any extra impurity peak. The calculated average crystallite sizes are found to be 22 and 17 nm for pure and Fe-doped ZnO TFs, respectively. The broadening in XRD peaks of Fe-doped ZnO TF occurs due to decrease in crystallite size and increase in lattice strain. Field emission scanning electron microscopy images exhibit that the particles growth in Fe-doped ZnO TF is more uniform and smaller than pure ZnO. Energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy results confirm the existence of Fe dopants into ZnO matrix. The doping effect enhances the sensitivity of ZnO sensor almost three times for ethanol gas sensing, the improvement in the response time and recovery time is noticeable as the size reduction effect increases the surface to volume ratio, and resulting more numbers of ethanol gas molecules are adsorbed to produce a higher concentration of oxygen ions which leads a larger deviation in capacitance.     

Catalytic Properties of CuMoO<Subscript>4</Subscript> Doped with Co,Ni, and Ag

Modified copper molybdate phases which differ in unit-cell parameters from undoped CuMoO₄ have been prepared using the pyrolysis of mixtures of Cu, Mo, and Co (Ni, Ag) extracts. Increasing the Co or Ni content from 1 to 7 at % increases the content of the modified copper molybdate in the samples, and their catalytic activity for the oxidation of carbon black decreases. In contrast, doping with 1 at % Ag markedly improves the catalytic performance of the molybdate. The synthesized copper molybdate phases are capable of reducing the CO yield in catalytic carbon black combustion. When the process was catalyzed by Co-doped CuMoO₄, no carbon monoxide was detected among the combustion products.     

Synthesis of Co-Doped MoS2 Monolayers with Enhanced Valley Splitting

Internal magnetic moments induced by magnetic dopants in MoS₂ monolayers are shown to serve as a new means to engineer valley Zeeman splitting (VZS). Specifically, successful synthesis of monolayer MoS₂ doped with the magnetic element Co is reported, and the magnitude of the valley splitting is engineered by manipulating the dopant concentration. Valley splittings of 3.9, 5.2, and 6.15 meV at 7 T in Co-doped MoS₂ with Co concentrations of 0.8%, 1.7%, and 2.5%, respectively, are achieved as revealed by polarization-resolved photoluminescence (PL) spectroscopy. Atomic-resolution electron microscopy studies clearly identify the magnetic sites of Co substitution in the MoS₂ lattice, forming two distinct types of configurations, namely isolated single dopants and tridopant clusters. Density functional theory (DFT) and model calculations reveal that the observed enhanced VZS arises from an internal magnetic field induced by the tridopant clusters, which couples to the spin, atomic orbital, and valley magnetic moment of carriers from the conduction and valence bands. The present study demonstrates a new method to control the valley pseudospin via magnetic dopants in layered semiconducting materials, paving the way toward magneto-optical and spintronic devices.     

Photoluminescence decay kinetics of doped ZnS nanophosphors

Doped nanophosphor samples of ZnS:Mn, ZnS:Mn, Co and ZnS:Mn, Fe were prepared using a chemical precipitation method. Photoluminescence (PL) spectra were obtained and lifetime studies of the nanophosphors were carried out at room temperature. To the best of our knowledge, there are very few reports on the photoluminescence investigations of Co-doped or Fe-doped ZnS:Mn nanoparticles in the literature. Furthermore, there is no report on luminescence lifetime shortening of ZnS:Mn nanoparticles doped with Co or Fe impurity. Experimental results showed that there is considerable change in the photoluminescence spectra of ZnS:Mn nanoparticles doped with X (X = Co, Fe). The PL spectra of the ZnS:Mn, Co nanoparticle sample show three peaks at 410, 432 and 594?nm, while in the case of the ZnS:Mn, Fe nanoparticle sample the peaks are considerably different. The lifetimes are found to be in microsecond time domain for 594?nm emission, while nanosecond order lifetimes are obtained for 432 and 411?nm emission in ZnS:Mn, X nanophosphor samples. These lifetimes suggest a new additional decay channel of the carrier in the host material.     

Effects of As Doping in Tl0.4K0.4Fe2−y Se2 Superconductor

The effects of As doping on the superconductivity and magnetism in the Tl_0.4K_0.4Fe_2?y Se₂ superconductor are studied. We find that the As doping does not significantly change the occupancy at the Fe-site, meaning a certain amount of Fe ions changes its valence from 2+ to 3+. The superconductivity is gradually suppressed with increasing As doping. The magnetic measurements suggest that though the As doping does not destroy the antiferromagnetic order in the system, it induces an increase in the staggered magnetic moment. These results suggest that the Tl_0.4K_0.4Fe_2?y Se₂ system is a system with intrinsic phase separation. The change in the valence state of Fe could be a possible reason for the suppression of superconductivity.     

Evidence for Fe(2+) in wurtzite coordination: iron doping stabilizes ZnO nanoparticles

First-principles calculations are used to investigate the structural and electronic properties of Fe-doped ZnO nanoparticles. Based on extensive validation studies surveying various density functionals, the hybrid functional PBE0 is employed to calculate the structures, formation energies, and electronic properties of Fe in ZnO with Fe concentrations of 6.25, 12.5, and 18.75 at%. Substitution of Zn by Fe, zinc vacancies, and interstitial oxygen defects is studied. High-resolution inner-shell electron energy loss spectroscopy measurements and X-ray absorption near-edge structure calculations of Fe and O atoms are performed. The results show that Fe-doped ZnO nanoparticles are structurally and energetically more stable than the isolated FeO (rocksalt) and ZnO (wurtzite) phases. The Fe dopants distribute homogeneously in ZnO nanoparticles and do not significantly alter the host ZnO lattice parameters. Simulations of the absorption spectra demonstrate that Fe(2+) dominates in the Fe-doped ZnO nanoparticles reported recently, whereas Fe(3+) is present only as a trace.