Activation of high-TC ferromagnetism in Co2+:TiO2 and Cr3+:TiO2 nanorods and nanocrystals by grain boundary defects

Colloidal Co(2+)- and Cr(3+)-doped TiO(2) nanorods and nanocrystals were synthesized and studied by X-ray powder diffraction, electronic absorption spectroscopy, magnetic circular dichroism spectroscopy, magnetic susceptibility, and transmission electron microscopy. The nanorods were paramagnetic as colloids but showed room-temperature ferromagnetism when spin-coated aerobically into films. Crystalline domain size, thermal annealing, and dopant or defect migration are not the dominating factors converting the doped TiO(2) nanocrystals from the paramagnetic state to the ferromagnetic state. The most important factor for activating ferromagnetism is found to be the creation of grain boundary defects, proposed to be oxygen vacancies at nanocrystal fusion interfaces. These defects are passivated and the ferromagnetism destroyed by further aerobic annealing. These results not only help elucidate the origins of the TM(n+):TiO(2) DMS ferromagnetism but also represent an advance toward the controlled manipulation of high-T(C) DMS ferromagnetism using external chemical perturbations.     

Evidence of superparamagnetic co clusters in pulsed laser deposition-grown Zn0.9Co0.1O thin films using atom probe tomography

Nanosized Co clusters (of about 3 nm size) were unambiguously identified in Co-doped ZnO thin films by atom probe tomography. These clusters are directly correlated to the superparamagnetic relaxation observed by ZFC/FC magnetization measurements. These analyses provide strong evidence that the room-temperature ferromagnetism observed in the magnetization curves cannot be attributed to the observed Co clusters. Because there is no experimental evidence of the presence of other secondary phases, our results reinforce the assumption of a defect-induced ferromagnetism in Co-doped ZnO diluted magnetic semiconductors.     

Influence of Rare Earth Ho3+ Doping on Structural,Microstructure and Magnetic Properties of ZnO Bulk and Thin Film Systems

We have investigated the doping behavior of rare earth element holmium (Ho³⁺) in ZnO semiconductor. The structural, microstructure, and magnetic properties of Zn_1-xHo_xO (x=0.0, 0.04, and 0.05) thin films deposited on Si(100) substrate by thermal evaporation technique were studied. The ceramic targets were prepared by conventional solid state ceramic technique. The pallets used as target were final sintered at 900 oC in the presence of N2 atmosphere. The experimental results of X-ray diffraction (XRD) spectra, surface morphology, and magnetic properties show that the Ho³⁺ doped ZnO thin films has a strong influence on the materials properties. The higher angle shift in peak position and most preferred (101) orientation were observed in XRD pattern. These spectra confirmed the substitution of Ho3+ in ZnO lattice. The surface morphology and stoichiometry for both bulk and thin films were analyzed by scanning electron microscopy and energy dispersive spectroscopy. It was observed that grain size decreases with the increase of Ho³⁺. Room temperature ferromagnetism was observed for Zn_0.95Ho_0.05O films. The ferromagnetism might be attributed to the substitution of Ho ions for Zn²⁺ in ZnO lattices.     

Integrating Photoluminescence and Ferromagnetism in Carbon Quantum Dot/ZnO by Interfacial Orbital Hybridization for Multifunctional Bioprobes

Integrating ferromagnetism (FM) and photoluminescence (PL) into one particular nanostructure as biological probe plays an irreplaceable role in accurate clinical diagnosis combining magnetic resonance and photoluminescence imaging technology. However, magnetic emergence generally needs a spin polarization at Fermi level to display a half-metallic electronic feature, which is not beneficial for preserving radiation recombination ability of photo-excited electron-hole carriers. To overcome this intrinsic difficulty, we propose a feasible atomic-hybridization strategy to anchor carbon quantum dots (CQDs) onto ZnO microsphere surface via breakage of C=O bonds at CQDs and subsequent Zn-3d and C-2p orbital hybridization, which not only ensures the carrier recombination but also leads to a room-temperature magnetism. Herein, the photoluminescence and magnetism coexist in this multifunctional heterojunction with outstanding biocompatibility. This work suggests that integration of magnetism and photoluminescence could be accomplished by particular interfacial orbital hybridization.     

Stable photogenerated carriers in magnetic semiconductor nanocrystals

We report the preparation and investigation of charged colloidal Co2+:ZnO and Mn2+:ZnO nanocrystals. Although both charged and magnetically doped colloidal semiconductor nanocrystals have been reported previously, colloidal charged and magnetically doped semiconductor nanocrystals as described herein have not. Conduction band electrons were introduced into colloidal ZnO diluted magnetic semiconductor (DMS) nanocrystals photochemically, and the resulting TM2+-e-CB interactions were observed by electron paramagnetic resonance spectroscopy (TM2+ = Co2+ or Mn2+). This new motif of colloidal charged magnetic semiconductor nanocrystals reveals attractive new opportunities for studying spin effects in DMS nanostructures relevant to proposed spintronics technologies.     

Synthesis of colloidal Mn2+:ZnO quantum dots and high-TC ferromagnetic nanocrystalline thin films

We report the synthesis of colloidal Mn(2+)-doped ZnO (Mn(2+):ZnO) quantum dots and the preparation of room-temperature ferromagnetic nanocrystalline thin films. Mn(2+):ZnO nanocrystals were prepared by a hydrolysis and condensation reaction in DMSO under atmospheric conditions. Synthesis was monitored by electronic absorption and electron paramagnetic resonance (EPR) spectroscopies. Zn(OAc)(2) was found to strongly inhibit oxidation of Mn(2+) by O(2), allowing the synthesis of Mn(2+):ZnO to be performed aerobically. Mn(2+) ions were removed from the surfaces of as-prepared nanocrystals using dodecylamine to yield high-quality internally doped Mn(2+):ZnO colloids of nearly spherical shape and uniform diameter (6.1 +/- 0.7 nm). Simulations of the highly resolved X- and Q-band nanocrystal EPR spectra, combined with quantitative analysis of magnetic susceptibilities, confirmed that the manganese is substitutionally incorporated into the ZnO nanocrystals as Mn(2+) with very homogeneous speciation, differing from bulk Mn(2+):ZnO only in the magnitude of D-strain. Robust ferromagnetism was observed in spin-coated thin films of the nanocrystals, with 300 K saturation moments as large as 1.35 micro(B)/Mn(2+) and T(C) > 350 K. A distinct ferromagnetic resonance signal was observed in the EPR spectra of the ferromagnetic films. The occurrence of ferromagnetism in Mn(2+):ZnO and its dependence on synthetic variables are discussed in the context of these and previous theoretical and experimental results.     

Structural and magnetic properties of nano-crystalline FeCoNiN thin films

Iron cobalt nickel nitride (FeCoNiN) thin films are prepared by sol-gel spin coating route. The structural, magnetic and surface properties of the thin films are evaluated. The crystalline nature of thin films was enhanced upon annealing, leading to increased crystallite size. The X-ray diffraction shows mixed phases with crystallite size in the range of 20–26.93 nm. Thin films show ferromagnetism at room temperature. Coercivity and saturation magnetization are in the range of 642–716 Oe and 2.5–7.5 emu/cm³ respectively. Both coercivity and saturation magnetization increased with annealing of thin films. Magnetic properties are related to the crystallinity of thin films. The increase in crystallite size results into an increase of magnetic properties. Rectangular shaped particles are seen on the surface of thin films. The same type of grains can be seen on the surface of thin films which confirmed the formation of FeCoNiN as predicted by XRD. These novel thin films might be used in memory devices and optoelectronic applications.     

Effect of Doped Boron on the Properties of ZnO Thin Films Prepared by Sol-gel Spin Coating

Transparent conductive boron-doped ZnO thin films were prepared by sol-gel spin coating method. The effect of doped boron concentration on the properties of the films was systematically discussed. The films were characterized by X-ray diffraction, atomic force microscopy, spectrophotometry, and Hall effect measurement system. All the doped and undoped ZnO films were of a single hexagonal structure, and showed a preferred orientation of （002）. The particle size and surface roughness of the films decreased with increased doped boron concentration. All the films exhibited an average transmittance of approximate 90% in visible-light region and an energy gap of about 3.3 cV. The maximum carrier concentration, the highest carrier mobility and the lowest resistivity were observed at a doped boron concentration of 0.5%（molar fraction）. Based on these results, we suggested that the saturation concentration of doped boron in ZnO film is 0.5%（molar fraction）.     

Room temperature ferromagnetism in sol–gel deposited un-doped ZnO films

Dilute magnetic semiconductors are fast emerging spintronic materials where advantage of magnetic properties of semiconductor materials (usually doped with small quantities of magnetic ions) is being explored. Sol–gel technique, being low-cost simple and application oriented method, has been used in the present case. ZnO films of <150 nm thickness have been deposited by spin coating onto single crystal p-type Si substrates. The optimized sol is of paramagnetic nature, whereas, mixed para- dia-magnetic phase is observed for the as-prepared films. A complete ferromagnetic phase transition has been observed after heating the films in vacuum at a temperature of 300 °C. These sol–gel prepared films exhibit hexagonal wurtzite structure as observed by X-ray diffraction. After the magnetic field annealing in vacuum the films showed strengthened magnetic as well as structural properties. This work presents a clear evidence of ferromagnetic behavior of the un-doped ZnO films deposited by sol–gel at room temperature. It is also pointed out that Zn vacancies rather than oxygen deficiency are responsible for ferromagnetism in these sol–gel deposited ZnO thin films, whereas, the experimental evidence has been substantiated with the theoretical calculations using density functional theory.     

Correlation between magnetic properties of electrodeposited Co(P) and NH4Cl concentrations in the electrolyte

The effect of ammonium chloride on the electrodeposition process and resulting magnetic properties of Co(P) alloy films was investigated. It was observed that the magnetic properties of the films varied extensively with the ammonium chloride contents in the solution. Cyclic voltammetry was employed as an electroanalytical technique to characterize the variation of interfacial polarization at different electrolyte compositions. In addition, microstructural characterization of electrodeposited Co(P) thin films by transmission electron microscopy revealed a significant increase in both grain size and preferred orientation of Co(P) grains with ammonium chloride contents. Results of characterizations indicated that the addition of ammonium chloride controlled the kinetics of electrocrystallization of Co(P) by modifying the charge transfer process and thus gave rise to increased grain size and preferred orientation in the Co(P) thin films. These microstructural features provide reasonable explanation on the variation of the magnetic properties of electrodeposited Co(P) films with the amount of ammonium chloride in the solution.     

Polymerization Strategies to Construct a 3D Polymer Passivation Network toward High Performance Perovskite Solar Cells

The spontaneously formed uncoordinated Pb²⁺ defects usually make the perovskite films demonstrate strong n-type with relatively lower carrier diffusion length and serious non-radiative recombination energy loss. In this work, we adopt different polymerization strategies to construct three-dimensional passivation frameworks in the perovskite layer. Thanks to the strong C≡N⋅⋅⋅Pb coordination bonding and the penetrating passivation structure, the defect state density is obviously reduced, accompanied by a significant increase in the carrier diffusion length. Additionally, the reduction of iodine vacancies also changed the Fermi level of the perovskite layer from strong n-type to weak n-type, which substantially promotes the energy level alignment and carrier injection efficiency. As a result, the optimized device achieved an efficiency exceeded 24 % (the certified efficiency is 24.16 %) with a high open-circuit voltage of 1.194 V, and the corresponding module achieved an efficiency of 21.55 %.     

Conductivity of ZnO nanowires, nanoparticles, and thin films using time-resolved terahertz spectroscopy

The terahertz absorption coefficient, index of refraction, and conductivity of nanostructured ZnO have been determined using time-resolved terahertz spectroscopy, a noncontact optical probe. ZnO properties were measured directly for thin films and were extracted from measurements of nanowire arrays and mesoporous nanoparticle films by applying Bruggeman effective medium theory to the composite samples. Annealing significantly reduces the intrinsic carrier concentration in the ZnO films and nanowires, which were grown by chemical bath deposition. The complex-valued, frequency-dependent photoconductivities for all morphologies were found to be similar at short pump-probe delay times. Fits using the Drude-Smith model show that films have the highest mobility, followed by nanowires and then nanoparticles, and that annealing the ZnO increases its mobility. Time constants for decay of photoinjected electron density in films are twice as long as those in nanowires and more than 5 times those for nanoparticles due to increased electron interaction with interfaces and grain boundaries in the smaller-grained materials. Implications for electron transport in dye-sensitized solar cells are discussed.     

Fabrication of surface-patterned ZnO thin films using sol–gel methods and nanoimprint lithography

Surface-patterned ZnO thin films were fabricated by direct imprinting on ZnO sol and subsequent annealing process. The polymer-based ZnO sols were deposited on various substrates for the nanoimprint lithography and converted to surface-patterned ZnO gel films during the thermal curing nanoimprint process. Finally, crystalline ZnO films were obtained by subsequent annealing of the patterned ZnO gel films. The optical characterization indicates that the surface patterning of ZnO thin films can lead to an enhanced transmittance. Large-scale ZnO thin films with different patterns can be fabricated by various easy-made ordered templates using this combination of sol–gel and nanoimprint lithography techniques.     

Orientation degree dependence of magnetic properties of Co doped ZnO thin films by sol–gel process

Co-doped ZnO films were successfully fabricated on Si substrates by sol–gel process. The effects of Co concentration and preheating temperature on the structure and magnetic properties of the Zn_1−x Co _x O films were systematically studied. The results revealed that the films were highly c-axis oriented and contained no impurity phase. With preheating temperature increasing, the orientation degree of the films decreased. When the Co concentration and oxygen vacancy of the films are almost changeless, the enhancement of ferromagnetism in films originates from the orientation degree of the films increasing.     

Structure and morphology of nanostructured zinc oxide thin films Prepared by dip-vs. spin-coating methods

In this study, we use dipping and spinning methods to coat glass slides with sol-gel ZnO thin films, composed of zinc acetate dihydrate, monoethanolamine (MEA), de-ionized water and isopropanol. The effect of the annealing temperature on the structural morphology and optical properties of these films is investigated. These ZnO films were preheated at 275 °C for 10 min and annealed either at 350, 450 or 550 °C for 60 min. As-deposited films, formed by amorphous zinc oxide-acetate submicron particles, are transformed into a highly-oriented ZnO after thermal treatment. The surface morphology, phase structure and optical properties of the thin films were investigated by scanning electron microscopy, X-ray diffraction (XRD) and optical transmittance. Both techniques produced nanostructured ZnO thin films with well-defined orientation. The annealed films were transparent in the visible range with an absorption edge at about 375 nm and a transmittance of ca 85–90% with an average diameter of 40 nm. XRD results show the film was composed of polycrystalline wurtzite, with a preferential c-axis orientation of (002) and a single sharp XRD peak at 34.40, corresponding to the hexagonal ZnO. The grain size is increased by the annealing temperature. Both coating techniques create sol-gel ZnO films with the potential for application as transparent electrodes in optic and electronic devices.     

p-Type behaviour of electrodeposited ZnO:Cu films

Cu-doped ZnO (ZnO:Cu) thin films and ZnO/ZnO:Cu homojunction devices were electrodeposited on conductive glass substrates in a non-aqueous electrolyte containing Cu and Zn salts. The Cu content of the films is proportional to the Cu/Zn precursor ratio in the deposition electrolyte. ZnO:Cu was found to be of a hexagonal wurtzite structure with (002) preferred orientation. A transition from n-type to p-type was observed for ZnO:Cu films with a Cu/Zn ratio higher than 2% as inferred from the change in the direction of the photocurrent. The rectifying characteristics shown by homojunction devices further confirm the p-type conductivity of ZnO:Cu layers.     

Supramolecular Coordination of Pb2+ Defects in Hybrid Lead Halide Perovskite Films Using Truxene Derivatives as Lewis Base Interlayers

Truxene derivatives, due to their molecular structure and properties, are good candidates for the passivation of defects when deposited onto hybrid lead halide perovskite thin films. Moreover, their semiconductor characteristics can be tailored through the modification of their chemical structure, which allows-upon light irradiation- the interfacial charge transfer between the perovskite film and the truxene molecules. In this work, we analysed the use of the molecules as surface passivation agents and their use in complete functional solar cells. We observed that these molecules reduce the non-radiative carrier recombination dynamics in the perovskite thin film through the supramolecular complex formation between the Truxene molecule and the Pb²⁺ defects at the perovskite surface. Interestingly, this supramolecular complexation neither affect the carrier recombination kinetics nor the carriers collection but induced noticeable hysteresis on the photocurrent vs voltage curves of the solar cells under 1 sun illumination.     

Characterization of superparamagnetic "core-shell" nanoparticles and monitoring their anisotropic phase transition to ferromagnetic "solid solution" nanoalloys

The structure, magnetism, and phase transition of core-shell type CoPt nanoparticles en route to solid solution alloy nanostructures are systematically investigated. The characterization of Co(core)Pt(shell) nanoparticles obtained by a "redox transmetalation" process by transmission electron microscopy (TEM) and, in particular, X-ray absorption spectroscopy (XAS) provides clear evidence for the existence of a core-shell type bimetallic interfacial structure. Nanoscale phase transitions of the Co(core)Pt(shell) structures toward c-axis compressed face-centered tetragonal (fct) solid solution alloy CoPt nanoparticles are monitored at various stages of a thermally induced annealing process and the obtained fct nanoalloys show a large enhancement of their magnetic properties with ferromagnetism. The relationship between the nanostructures and their magnetic properties is in part elucidated through the use of XAS as a critical analytical tool.     

电沉积铁族薄膜的磁性圆二色研究

采用循环伏安法在GaAs(100)单晶表面电沉积了铁族金属单质薄膜. 扫描电子显微镜(SEM)结果显示, Fe族金属薄膜的晶粒较小, 薄膜平整度较高。通过X射线衍射(XRD)谱分析了Fe, Co, Ni在GaAs(100)晶面上的外延生长. 使用磁光克尔效应装置研究了Fe族金属薄膜的宏观磁性, 用同步辐射圆偏振软X射线测量了铁族单质磁性薄膜的吸收谱, 获得了X射线磁性圆二色谱, 并通过加和定则计算了磁性薄膜中Fe族金属原子的轨道磁距和自旋磁矩.     

ZnO自组装薄膜的可控生长及其光催化性能

通过改变工艺参数, 制得了粒径可控的ZnO自组装薄膜. 该薄膜在可见光区域出现了光子带隙. 以染料甲基橙的光催化降解为模型评价了ZnO自组装薄膜的光催化活性. 利用X射线衍射仪(XRD)和扫描电子显微镜(SEM)表征了ZnO的晶体结构和微观形貌. 实验结果表明, ZnO自组装薄膜在太阳光照射下表现出良好的光催化性能, 其光催化活性随着ZnO颗粒粒径的减小而提高. ZnO自组装薄膜光催化降解甲基橙的反应符合一级反应动力学规律.