Synthesis,structure, and room-temperature ferromagnetism of Ni-doped ZnO nanoparticles

In this paper, Ni-doped ZnO (Zn_1−x Ni _x O, in which 0 ≤ x ≤ 0.05) diluted magnetic semiconductors nanoparticles are prepared by an ultrasonic assisted sol–gel process. Transmission electron microscopy shows sphere-like nanoparticles with an average size of about 25 nm. From the analysis of X-ray diffraction, the Ni-doped ZnO nanoparticles are identified to be a wurtzite structure, but impurity phases are observed when the Ni content x reaches 0.05. Sample structures are further studied by Raman spectra, from which a broad and strong Raman band in the range of 500–600 cm⁻¹ is observed in Zn_1−x Ni _x O. With the increment of x, wurtzite structures degrade gradually. The magnetic properties are measured using superconducting quantum interference device at room temperature; the Zn_1−x Ni _x O (x ≤ 0.02) nanoparticles show ferromagnetism. However, for the sample of Zn_0.95Ni_0.05O, paramagnetism is observed, which may be ascribed to ferromagnetic–antiferromagnetic competition.     

Visible photoluminescence of hydrothermal synthesized Sn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> nanostructures

Sn_1−x Ni _x O₂ nanostructures such as nanocubes, nanospheres and hollow spheres were synthesized by a simple hydrothermal method. Room temperature photoluminescence spectra of the as-synthesized samples display a strong yellow emission at about 600 nm and a weak blue emission at about 430 nm. The as-prepared and annealed Sn_1−x Ni _x O₂ (x = 0, 0.01, 0.02, 0.04) were characterized by X-ray diffraction, field emission scanning electron microscopy, Raman spectrum, UV–Vis absorption spectra, and room temperature photoluminescence spectra. By investigating the relationship between the Raman band centered at 560 cm⁻¹ and the photoluminescence of the samples, we suggest that the broad yellow emission and weak blue emission primarily originate from singly ionized oxygen vacancies and tin interstitials, respectively.     

Investigation of structural,electrical and optical properties of nickel substituted CdSe thin films

The technologically important Cd_1−xNi_xSe thin films with variable compositions (0 ≤ x ≤ 1) have been developed by a chemical deposition method. The structural, compositional, optical and electrical properties were studied by X-ray diffraction, Scanning electron microscopy, Atomic absorption spectroscopy, UV–visible double beam spectrophotometer and d.c. two probe method. XRD studies indicate polycrystalline in nature with hexagonal phase for all the samples. The lattice constants decrease with increase in nickel content in CdSe host lattice. The surface morphology study of all samples reveals uniform and spherical grains. An optical study of the samples shows that band gap value decreases with nickel content. Electrical measurements depict semiconducting properties of all samples.     

Structure and photoluminescence of SiC/ZnO nanocomposites prepared by radio frequency alternate sputtering

SiC/ZnO nanocomposites were prepared by radio frequency alternate sputtering followed by annealing in N₂ ambient. Well-crystallized ZnO matrix was obtained after annealed at 750 °C according to X-ray diffractometer patterns. Transmission electron microscopy analyses indicated that the SiC thin layer aggregated to form SiC nanoclusters with the average size of 7.2 nm when the annealing temperature was 600 °C. When the annealing temperatures increased above 900 °C, some of the SiC nanoclusters changed into SiC nanocrystals and surfacial atoms of the SiC nanoparticles were surrounded by a layer of SiO _x (x ≤ 2) according to the Fourier transform infrared spectrums. The SiC/ZnO nanocomposites annealed at 750 °C exhibit strong photoluminescence bands ranging from 250 to 600 nm. UV light originates from the near band edge emission of ZnO and the blue emission peaked at around 465 nm (2.7 eV) may be due to the formation of emission centers caused by the defects in Si–O network, while the green-emission peak at around 550 nm (2.3 eV) may be attributed to the deep level recombination luminescence caused by the vacancies of oxygen and zinc.     

Magnetic Properties of Zn1?x Ni x O (0.25≤x≤ 0.50) Prepared by Solid-State Reactions

In this study the origin of ferromagnetism in ZnO-based bulk systems has been investigated using Ni-doped ZnO samples, Zn_1−x Ni _x O with 0.25≤x≤0.50, prepared by solid-state reactions. The structural characterizations indicated that the Ni²⁺ ions almost uniformly distributed in all the samples, and the samples have hexagonal wurtzite structure; however, when x is increased toward 0.50, a new NiO phase is formed. A ferromagnetism (FM) has been observed for all the samples at and below the room temperature. In other words, the room temperature results of (M–H) curves show that the FM observed is intrinsic for all the Ni-doped ZnO samples. However, the saturated magnetizations decrease gradually with increasing Ni concentration. This indicates that, in addition to FM, the excessive doping of Ni in ZnO also causes an antiferromagnetic (AFM) contribution which increases with increasing Ni amount. This result is also supported by the magnetization against temperature measurements. Furthermore, the trend of the ac-susceptibility (χ) versus temperature curves, measured under an ac-magnetic field of 100 Oe, also support our conclusion about the antiferromagnetic contribution to ferromagnetism in our samples.     

Morphology controlled synthesis of ZnO nanostructures by varying pH

ZnO nanostructures have been synthesized in a controlled manner by varying the pH of the precursor solution using hydrothermal technique. The morphological changes of the prepared ZnO nanostructures have been investigated in the range of pH 5–10. Radial hexagonal rod-like shape is formed at lower pH values of 5 and 6 whereas, flower-like shape is obtained for higher pH values of 9 and 10. Flake-like structure is observed at moderate pH of 8. The prepared ZnO nanostructures have been characterized using X-ray diffraction technique (XRD), energy dispersive X-ray analysis, scanning electron microscope and FTIR spectroscopy. XRD results show that the prepared ZnO nanostructures exhibit hexagonal wurtzite structure. The growth mechanism suggests that the supersaturation of the precursor results in various nucleation habits, which induce the formation of ZnO nanostructures with different morphologies. UV–Vis spectroscopy and photoluminescence were applied to study the optical properties. The photoluminescence spectrum demonstrated two emission bands, a near band edge emission in the UV region and a strong deep band emission in the visible region. The change in pH from 5 to 10 results in band gap variations of 3.47–3.97 eV and blue-shift in the peak emission of visible PL from 560 to 460 nm.     

Nanostructure, optical and photoluminescence properties of Zn1−xNixO nanoclusters by co-precipitation method

Zn_1?xNi_xO (x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) nanoclusters have been successfully synthesized by co-precipitation method. The synthesized samples have been characterized by powder X-ray diffraction, energy dispersive X-ray spectra, UV–visible spectrophotometer and Fourier transform infrared spectroscopy. The XRD and SEM measurements reveal that the prepared undoped and Ni-doped nanoclusters have different microstructure without changing a hexagonal wurtzite structure. The calculated average crystalline size from XRD measurement decreases from 37.5 to 26.6 nm for x = 0 to 0.05 which was confirmed by SEM micrographs. The change in lattice parameters, micro-strain, shift of XRD peaks and the blue shift of energy gap from 3.18 to 3.33 eV (ΔE_g = 0.15 eV) for Ni = 0–0.02 and red shift of E_g from 3.33 to 3.14 eV (ΔE_g = 0.19 eV) for Ni = 0.02 to 0.05 reveal the substitution of Ni²⁺ ions into Zn–O lattice. The presence of functional groups and the chemical bonding are confirmed by FTIR spectra. The shift of NBE UV emission between 374 and 395 nm, the shift of green band emission between 517 and 531 nm, the change in intensity and the broadening effect in the photoluminescence spectra confirms the substitution of Ni²⁺ ions into the Zn–O lattice. Ni-doped ZnO system shows a great pledge for the fabrication of nano-optoelectronic devices like tunable light emitting diode in the near future.     

Structure and photocatalytic activity of Ni-doped ZnO nanorods

The one-dimensional (1D) Zn_1−xNi_xO (x = 0, 0.02, 0.05, 0.10) nanorods have been synthesized by a simple hydrothermal method. New bands show at ∼130 cm⁻¹ in the Raman spectra of Ni-doped ZnO nanorods and their relative intensity depends on the doping concentration of nickel. The optical band gap of the ZnO nanorods have been tuned by Ni-doping, which is revealed by absorption spectra. The photocatalytic activity of Zn_1−xNi_xO was studied by comparing the degradation rate of rhodamine B (RB) under UV-light irradiation. It was found that Zn_0.95Ni_0.05O exhibited the highest photocatalytic degradation efficiency among the samples.     

Room Temperature Ferromagnetism in Ni Doped ZnO Powders

Zn_1−x Ni _x O (x=0.1%, 0.4%, 0.7%, 1.0%) powders were prepared using the sol-gel technique, and the structural, optical, and magnetic properties of the samples were investigated. X-ray diffraction measurements show that all samples have a wurtzite structure and that the c-axis lattice constant decreases as the Ni content increases. X-ray photoelectron spectroscopy studies reveal that the doped Ni ions are in divalent states in all samples. Optical absorption spectra show that the band energy of Zn_1−x Ni _x O powders decreases with increasing Ni concentration. Photoluminescence measurements of Zn_1−x Ni _x O show a broad peak at a wavelength centered at about 467 nm which indicates the presence of a fraction of defects in ZnO. The room temperature ferromagnetism observed in all samples is intrinsic in nature.     

Dominancy of antiferromagnetism in Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O diluted magnetic semiconductors

The outline of magnetic interactions in DMSs was determined using Zn_1−x Co _x O particles, where “x” was changed as 0.01, 0.05, 0.10, 0.15, and 0.20. The syntheses were accomplished though mechanical milling and thermal treatment, known as solid state reaction. The formation of each synthesis was monitored by differential thermal and thermo gravimetric methods (DT-TGA). Substitution of Co²⁺ ions with Zn²⁺ host atoms in a ZnO lattice was analyzed using X-ray diffraction (XRD) patterns, Fourier transform infrared (FT-IR) spectroscopy, energy dispersive X-ray spectrometry (EDS) data, transmission electron microscopy (TEM) figures, scanning area electron diffraction (SAED) patterns, and X-ray photo spectroscopy (XPS) spectrum. The measured Co contents in ZnO lattice were found to be ~0.7% less than the expected result. In addition to Zn_1−x Co _x O particles, tungsten (W) contaminations were noticed in the variations of 1.5 ± 0.2%, as originating from the abrasion between the miller and balls. The progressive replacement of Co²⁺ with Zn²⁺ host ions in ZnO lattice from 1% to 20% decreased the band edge from 3.03 ± 0.01 eV to 2.95 ± 0.01 eV, respectively. Co doping has also changed the magnetic nature of the ZnO. Although having both interactions (ferromagnetic and antiferromagnetic), dominance of ferromagnetic behavior was only observed for Zn_0.99Co_0.01O with the coercivity of ~154 ± 50 Oe and positive Curie–Weiss temperature as 79 ± 1 K. However, the calculated \frac2J_\textex k_\textB {\frac{{2J_{\text{ex}} }}{{k_{\text{B}} }}} values have proved that the higher Co²⁺ concentrations in ZnO lattice have increased the efficiency of antiferromagnetic interactions. Surprisingly, there was no rapid change at \frac2J_\textex k_\textB {\frac{{2J_{\text{ex}} }}{{k_{\text{B}} }}} values as mentioned in previous works.     

Band gap modified Al-doped Zn1?xMgxO and Zn1?yCdyO transparent conducting thin films

Al-doped Zn_1−x Mg _x O and Zn_1−y Cd _y O thin films were prepared on glass substrates by sol–gel method. The codoping thin films showed preferential c-axis orientation, and the lattice constant c evaluated from the shift of the position of (002) peak displayed an increasing evolution from x = 8 at.% to y = 8 at.%, indicating a roughly statistical substitution of Mg²⁺ and Cd²⁺ for Zn²⁺ in their solid solution. The effects of narrowing and widening band gap (E _g) on conductivity of (Cd, Al) and (Mg, Al) codoped ZnO thin films were simultaneously investigated using transmission spectra and electrical measurements. The transmittances of these films are obviously decreased by vacuum annealing to 50–60%. However, the carrier concentration and Hall mobility both increase, and resistivity decreases with narrowing band gap in 1 at.% Al-doped Zn_1−x Mg _x O and Zn_1−y Cd _y O thin films from x = 8 at.% to y = 8 at.%. It is revealed that the conductivity of Al-doped ZnO thin films could be enhanced by this simple band gap modification.     

Effect of Cr doping on the structure,optical and magnetic properties of multiferroic BiFeO<Subscript>3</Subscript> thin films

Multiferroic BiCr _x Fe_1−x O₃ (BCFO) (0 ≤ x ≤ 0.12) thin films were fabricated on silicon substrates by sol–gel technique. The microstructure and properties of the films are characterized using X-ray diffraction, spectroscopic ellipsometry, micro-Raman spectrometry and a Modular Control system. The BCFO films are the rhombohedral structure with the Cr content up to 7%. Raman scattering spectra of the BCFO films demonstrate the transformation of structure with the Cr content exceeding 10%. The band gap of the BCFO films is from 2.53 to 2.82 eV with the Cr content being from 0 to 12%. The magnetization of the BCFO films is significantly enhanced with the increasing of the Cr content.     

Optical and dielectric properties of lanthanide titanium tantalate and niobate ceramic composites

The xPrTiTaO₆ (1 − x) YTiNbO₆ dielectric ceramic composites are fabricated through the solid state ceramic route. The compositions are calcined in the temperature range 1,200–1,260 °C and sintered in the range 1,350–1,410 °C. Structural analysis of the materials is done using X-ray Diffraction analysis. The composites contain both aeschynite and euxenite orthorhombic phases. The surface morphology of the sintered pellets is examined by scanning electron microscopy. The dielectric constant, conductance and loss factor are measured in the radio frequency region. The UV–visible spectra are recorded and the band gap is calculated. The photoluminescence spectra of the compositions are recorded and the transitions causing emission are identified. The elemental composition of the composites is confirmed using energy dispersive spectroscopy. The materials are suitable for substrate and optoelectronic applications.     

Optical and electrical properties of Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O thin films by sol–gel method

Mg _x Zn_1−x O (0 ≤ x ≤ 0.35) thin films have been deposited by sol–gel technique and the composition related structural, electrical, and optical properties are investigated. All the films have hexagonal wurtzite structure and the separation of MgO phase occurs when x = 0.3 and 0.35. With the increase of Mg content, the densification of the films decrease and band gap values increase. The maximum band gap value reaches 3.56 eV when x = 0.15. After Mg doping the conductivities of the Mg _x Zn_1−x O films are reduced greatly and the electrical current–voltage (I–V) characteristics show nonlinearity for x > 0.15.     

Applicability of the polymeric precursor method to the synthesis of nanometric single- and multi-layers of Zn1?xMnxO (x?=?0–0.3)

Polymeric precursor method (Pechini) was employed to fabricate single- and multilayers of Zn_1−x Mn _x O (x = 0–0.3) on glass substrates. X-ray diffraction measurements revealed that crystal structure of Zn_1−x Mn _x O multilayers is the typical hexagonal würzite structure of pristine ZnO. A reduced peak intensity and widened full width half maximum (FWHM) value of prominent peaks suggested that the Mn²⁺ ions have substituted the Zn²⁺ ion without changing the würzite structure of pristine ZnO up to Mn concentrations x ≤ 0.2. A distinct redshift of the absorption edge was observed as the Mn concentration x was increased. Additionally, the absorption edge was less sharp due, probably, to s–d and p–d interactions, which give rise to band gap bowing. Nevertheless, amorphous states appearing in the band gap as a consequence of reduced crystallinity may also be responsible for the shrinking of the band gap in this material. Interestingly, the field dependence of the magnetization showed typical paramagnetic behavior for all the chosen Mn concentrations with no evidence of ferromagnetic ordering. Probably, the absence of ferromagnetism in the studied Zn_1−x Mn _x O films is strongly related to defects (say Mn impurities at the interface between nano-crystallites) in ZnO due to partial substitution of host Zn ions by Mn ions.     

Effect of ZnO on the microstructure and electrical properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> lead-free piezoelectric ceramics

K_0.5Na_0.5NbO₃–x ZnO (KNN–xZn) lead-free ceramics have been prepared using the conventional sintering technique and the effects of ZnO addition on the phase structure and piezoelectric properties of the ceramics have been studied. Our results reveal that a small amount of ZnO can improve the density of the ceramics effectively. Because of the high density and ZnO doping effects, the piezoelectric and dielectric properties of the ceramics are improved considerably. The good piezoelectric and dielectric properties of d ₃₃ = 114 pC/N, k _p = 0.36, ε _r = 395, and Q _m = 68 were obtained for the KNN ceramics doped with 1 mol% ZnO. Therefore, the KNN-1.0 mol%Zn ceramics is a good candidate for lead-free piezoelectric application.     

Preparation and properties of ZnO thin films deposited by sol-gel technique

Zinc oxide (ZnO) thin films were deposited on (100) Si substrates by sol-gel technique. Zinc acetate was used as the precursor material. The effect of different annealing atmospheres and annealing temperatures on composition, structural and optical properties of ZnO thin films was investigated by using Fourier transform infrared spectroscopy, X-ray diffraction, atomic force microscopy and photoluminescence (PL), respectively. At an annealing temperature of 400°C in N₂ for 2 h, dried gel films were propitious to undergo structural relaxation and grow ZnO grains. ZnO thin film annealed at 400°C in N₂ for 2 h exhibited the optimal structure and PL property, and the grain size and the lattice constants of the film were calculated (41.6 nm, a = 3.253 ? and c = 5.210 ?). Moreover, a green emission around 495 nm was observed in the PL spectra owing to the oxygen vacancies located at the surface of ZnO grains. With increasing annealing temperature, both the amount of the grown ZnO and the specific surface area of the grains decrease, which jointly weaken the green emission. Translated from Journal of Lanzhou University (Natural Science), 2006, 42(1): 67–71 [译自: 兰州大学学报 (自然科学版)]     

Photocatalytic activities of LaFe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> nanocrystals prepared by sol–gel auto-combustion method

Nanophotocatalysts LaFe_1−x Zn _x O₃ (x = 0, 0.05, 0.1, 0.3, 0.5) were successfully prepared by sol–gel auto-combustion method. The samples were characterized by X-ray diffraction (XRD), ultraviolet/visible absorption spectra (UV–vis), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The photocatalytic activities of the prepared samples were investigated for the photodegradation of methylene blue (MB). The results show that the lattice constant of LaFe_1−x Zn _x O₃ nanocrystals increases due to the substitution of Zn for Fe, which leads to the lattice distortion. The absorption edges of Zn-doped LaFeO₃ display a red shift with a significant absorption between 400 and 500 nm. Doping with the Zn ions enhances the photodegradation rate of LaFeO₃ for MB. The LaFe_0.7Zn_0.3O₃ particles are spherical with mean grain size of about 20–30 nm, which exhibits the highest degradation rate of 75% under irradiation time of 150 min.     

Synthesis and characterization of copper telluride nanowires via template-assisted dc electrodeposition route

Using polycarbonate track-etch membranes (Whatman), copper telluride (Cu_1.75Te) nanowires of diameter 100 nm and 50 nm have been synthesized electrochemically via template-assisted electrodeposition technique on indium tin oxide (ITO) coated glass from aqueous acidic solution of copper (II) sulphate (CuSO₄·5H₂O) and tellurium oxide (TeO₂) at room temperature (30 °C). Scanning electron microscopy (SEM) reveals the morphology of the nanowires having uniform diameter equal to the diameter of the template used. X-ray diffraction (XRD) pattern showed the structure corresponding to the hexagonal structure of copper telluride and single-crystalline. Using UV–visible spectrometry, the optical band gap of copper telluride nanowires was found to be 3.092 eV for 100 nm and 3.230 eV for 50 nm diameters. The photoluminescence (PL) studies shows higher intensity and broad spectrum in the blue region (450–475 nm) of visible light spectrum.     

Photovoltaic properties and photoconductivity in multilayer Ge/Si heterostructures with Ge nanoislands

Interband optical transitions in multilayer heterostructures with SiGe nanoislands were investigated using photocurrent spectroscopy and photo-emf. The n-p heterostructures containing Ge nanoislands in the area of the potential barrier were prepared by molecular-beam epitaxy at the temperature about 500 °C. It was shown that electron transitions from the ground state of the valence band in a nanoislands to the conduction band of Si surrounding made the main contribution into the vertical photo-emf in the range 0.75–1.05 eV, which is below the interband absorption edge of Si. The lateral photoconductivity observed in the range 0.63–0.8 eV at 77 K can be attributed to indirect interband transitions from the ground state of a nanoisland to L-state of the conduction band of a nanoisland. Analysis of Raman scattering spectra revealed that the Ge composition x in a nanoisland is about 0.87, while elastic deformation value amounts to ε _xx  = −0.016. The calculated energies of interband transitions from the ground state of a nanoisland to the conduction band of Si surrounding (0.63 eV) and to L-state of the conduction band of a nanoisland (0.81 eV) fit the experimental data with a rather good accuracy.