Effects of Cd concentration on structure and optical properties of the ternary Zn1−xCdxO nanopowder prepared by sol–gel method

Zn_1−xCd_xO nanocrystalline powder with different Cd contents (0≤x≤1) has been prepared by new facile sol–gel route. The crystal structure and optical properties were investigated by X-ray diffraction patterns, Transmission electron microscope, X-ray photoelectron spectroscopy, Photoluminescence. As x varied from x=0 to 0.25, the Zn_1−xCd_xO nanopowder exhibits a hexagonal wurtzite structure of pure ZnO without any significant formation of a separated CdO phase. For the samples with 0.5≤x≤0.85, the Zn_1−xCd_xO nanopowder exhibits the coexistence of hexagonal ZnO and cubic CdO phase, meanwhile, the content of ZnO phase decreases while that of CdO increases with increasing the Cd content x. The ultra-violet near-band-edge emission of the Zn_1−xCd_xO nanopowder was monotonously red-shifted from 389 nm (x=0) to 406 nm (x=0.25) due to the direct modulation of band gap caused by Cd substitution.     

Structural and optical properties of Mn-doped ZnO nanocrystalline thin films with the different dopant concentrations

The transparent nanocrystalline thin films of undoped zinc oxide and Mn-doped (Zn_1−xMn_xO) have been deposited on glass substrates via the sol–gel technique using zinc acetate dehydrate and manganese chloride as precursor. The as-deposited films with the different manganese compositions in the range of 2.5–20 at% were pre-heated at 100 °C for 1 h and 200 °C for 2 h, respectively, and then crystallized in air at 560 °C for 2 h. The structural properties and morphologies of the undoped and doped ZnO thin films have been investigated. X-ray diffraction (XRD) spectra, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and microstructure of the thin films. Optical properties of the thin films were determined by photoluminescence (PL) and UV/Vis spectroscopy. The analyzed results indicates that the obtained films are of good crystal quality and have smooth surfaces, which have a pure hexagonal wurtzite ZnO structure without any Mn related phases. Room temperature photoluminescence is observed for the ZnO and Mn-doped ZnO thin films.     

Dielectric study on Zn1−x Mg x O ceramic materials prepared by the solid-state route

The present work investigates the structural and dielectric properties of Zn_1?x Mg _x O composites prepared by the standard sintering method at 1200 °C during 24 h and doped with different weight percentages of MgO (x = 0–40 %). For this purpose, the scanning electron microscopy (SEM) was used to study the effect of the magnesium’s proportion on the morphology and crystallinity of the obtained samples. The SEM observations have shown rougher surfaces of the samples covered by grains having prismatic shapes and different sizes. The dielectric properties of the ceramics were investigated by spectroscopic impedance at different temperatures and frequencies, thus showing a frequency-dependent dispersion of the permittivity constants and dielectric losses. From these measurements, the relaxation processes were identified and their activation energies extracted. Dielectric responses were correlated with the microstructure and chemical composition of the ZnMgO composites. The mechanisms of ac conductivity are controlled by the polaron hopping and the electron tunneling models. Concerning the tunneling model, two types corresponding to the overlapping large polaron tunneling model for the composites Zn_0.9Mg_0.1O and Zn_0.8Mg_0.2O and the small polaron tunneling model for the composites Zn_0.64Mg_0.36O (in the frequency range 1.7 × 10⁴ Hz–1 MHz) and Zn_0.6Mg_0.4O were observed. Besides, one type of hopping model corresponding to the correlated barrier hopping for the composites ZnO and Zn_0.64Mg_0.36O (in the frequency range 6 × 10²–1.7 × 10⁴ Hz) was noted.     

Effect of Fe doping on the structural and gas sensing properties of ZnO porous microspheres

Fe-doped ZnO porous microspheres composed of nanosheets were prepared by a simple hydrothermal method combined with post-annealing, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller N₂ adsorption–desorption measurements and photoluminescence (PL) spectra. In this paper we report Fe doping induced modifications in the structural, photoluminescence and gas sensing behavior of ZnO porous microspheres. Our results show that the crystallite size decreases and specific surface area increases with the increase of Fe doping concentration. The PL spectra indicate that the 4 mol% Fe-doped ZnO has higher ratio of donor (V_O and Zn_i) to acceptor (V_Zn) than undoped ZnO. The 4 mol% Fe-doped ZnO sample shows the highest response value to ppb-level n-butanol at 300 °C, and the detected limit of n-butanol is below 10 ppb. In addition, the 4 mol% Fe -doped ZnO sample exhibits good selectivity to n-butanol. The superior sensing properties of the Fe-doped porous ZnO microspheres are contributed to higher donor defects contents combined with larger specific surface area.     

Synthesis of Ni1−xZnxO hollow structures by a facile method

By a sol–gel method, we obtained Ni_1−xZn_xO polycrystalline powders. The obtained Ni_1−xZn_xO was characterized by scanning electron microscope and transmission electron microscope techniques. The characterized results showed that the obtained Ni_1−xZn_xO showed hollow structure. The grain sizes of the obtained hollow Ni_1−xZn_xO were quite uniform, typically in the range of 400–500 nm. This method is a simple and effective method for large-scale synthesis of hollow Ni_1−xZn_xO quasi-spheres. The present method is very promising for large-scale production because the method is only involves commercial Zn(AC)₂·2H₂O and NiCl₂·6H₂O powders and the reaction is achieved in an open system free of autoclave and organic chemical reagents. This template-free method is facile but effective and therefore it is very promising for large-scale industrial production.     

Optical and structural properties of Zn1−x Mg x O ceramic materials

This paper reports structural, optical and cathodoluminescence characterizations of sintered Zn_1?x Mg _x O composite materials. The effects of MgO composition on these film properties have been analyzed. X-ray diffraction (XRD) confirms that all composites are polycrystalline with prominent hexagonal wurtzite structure along two preferred orientations (002) and (101) for the crystallite growth. Above doping content x = 10 %, the formation of the hexagonal ZnMgO alloy phase and the segregation of the cubic MgO phase start. From reflectance and absorption measurements, we determined the band gap energy which tends to increase from 3.287 to 3.827 eV as the doping content increases. This widening of the optical band gap is explained by the Burstein–Moss effect which causes a significant increase of electron concentration (2.89 × 10¹⁸?5.19 × 10²⁰ cm^?3). The luminescent properties of the Zn_1?x Mg _x O pellets are studied by cathodoluminescence (CL) at room and liquid nitrogen temperatures under different electron beam excitations. At room temperature, the CL spectra of the Zn_1?x Mg _x O composites exhibit a dominant broad yellow-green light band at 2.38 eV and two ultraviolet emission peaks at 3.24 and 3.45 eV corresponding to the luminescence of the hexagonal ZnO and ZnMgO structures, respectively. For the doped ZnO samples, it reveals also new red peaks at 1.72 and 1.77 eV assigned to impurities’ emissions. However, the CL spectra recorded at 77 K show the presence of excitonic emission peaks related to recombination of free exciton (X _A), neutral donor-bound excitons (D⁰X) and their phonon replicas. The CL intensity and energy position of the green, red and ultraviolet emission peaks are found to depend strongly on the MgO doping content. The CL intensity of the UV and red emissions is more enhanced than the green light when the MgO content increases. CL imaging analysis shows that the repartition of the emitting centers in Zn_1?x Mg _x O composites is intimately connected to the film composition and surface morphology.     

Influences of Al doping concentration on structural,electrical and optical properties of Zn0.95Ni0.05O powders

Nanocrystalline Zn_0.95−xNi_0.05Al_xO (x = 0.01, 0.02, 0.05 and 0.10) diluted magnetic semiconductors have been synthesized by an auto-combustion method. X-ray diffraction measurements indicate that all Al-doped Zn_0.95Ni_0.05O samples have the pure wurtzite structure. Transmission electron microscope analyses show that the as-synthesized powders are of the size 40–45 nm. High-resolution transmission electron microscope, energy dispersive spectrometer and X-ray photoemission spectroscope analyses indicate that Ni²⁺ and Al³⁺ uniformly substitute Zn²⁺ in the wurtzite structure without forming any secondary phases. The Al doping concentration dependences of cell parameters (a and c), resistance and the ratio of green emission to UV emission have the similar trends.     

Effect of Ce and Cu co-doping on the structural,morphological, and optical properties of ZnO nanocrystals and first principle investigation of their stability and magnetic properties

Ce, Cu co-doped ZnO (Zn_1−2xCe_xCu_xO: x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05) nanocrystals were synthesized by a microwave combustion method. These nanocrystals were investigated by using X-ray diffraction (XRD), UV–visible diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The stability and magnetic properties of Ce and Cu co-doped ZnO were probed by first principle calculations. XRD results revealed that all the compositions are single crystalline. hexagonal wurtzite structure. The optical band gap of pure ZnO was found to be 3.22 eV, and it decreased from 3.15 to 3.10 eV with an increase in the concentration of Cu and Ce content. The morphologies of Ce and Cu co-doped ZnO samples confirmed the formation of nanocrystals with an average grain size ranging from 70 to 150 nm. The magnetization measurement results affirmed the antiferro and ferromagnetic state for Ce and Cu co-doped ZnO samples and this is in agreement with the first principles theoretical calculations.     

A study on the Raman spectra of Al-doped and Ga-doped ZnO ceramics

Al_xZn_1−xO and Ga_yZn_1−yO ceramics were synthesized through a solid-state reaction technique. The crystal phase of the samples was identified by an X-ray diffraction experiment. For each sample, the electrical resistivity was determined. The Al 2-mol%-doped and Ga 0.5-mol%-doped ZnO ceramics had the lowest resistivity. Raman measurement was performed to study the doping effects in the ZnO ceramics including ZnO single crystal as a reference. The line-shape parameters, q₁ and Γ₁, at the same certain doping rate and the solubility limit of Al (2 mol%) and Ga (0.5 mol%) in ZnO ceramics, are strongly related to the each other, and that the solubility limit plays an important role. The second-order Raman peak at 1162 cm⁻¹ of the ZnO ceramics was fitted by Fano formalism. The Fano’s fitting parameters, such as the lifetime of phonon and the degree of asymmetry degree of the second-order Raman peak changed as the amounts of dopants were varied.     

PPV/PVA/ZnO nanocomposite prepared by complex precursor method and its photovoltaic application

In this article, we describe a new method to prepare a ZnO and conjugated polymer nanocomposite and its application in bulk-heterojunction solar cells. The composite was composed of zinc oxide (ZnO) and poly(phenylene vinylene)/poly(vinyl alcohol) (PPV/PVA). For the preparation, the composite was prepared first through the complex reaction between Zn²⁺ ion and –OH of the PVA–PPV precursor by simply mixing zinc salts and a PVA–PPV precursor aqueous solution at 70 °C. By addition of a concentrated aqueous ammonia into the system, highly regular Zn(OH)₂ nanodots were formed and dispersed in the PVA/PPV precusor mixed solution. The PVA/PPV precursor can well bind Zn²⁺ ion through complex interaction, so act as a template to direct the distribution of ZnO in the process. The nanocomposite films were finally obtained by solution casting and subsequently treated by heating samples at 160 °C for 6 h. TEM observations showed that ZnO nanodots uniformly dispersed in PVA–PPV mixtures. The resulting nanocomposite films possess a large interfacial area between the electron donor and acceptor of the bulk-heterojunction. Improved charge seperation and collection are evidenced by the large photoluminescence intensity difference between pure PPV and composites films, which result in the increase in both open circuit voltage and short circuit current of the hybrid solar cells.     

I–V characteristics of ZnO/Cu2O thin film n–i–p heterojunction

ZnO/Cu₂O thin film n–i–p heterojunctions were fabricated by magnetron sputtering. The microstructure, optical, and electrical properties of n-type (n‐) ZnO, insulating (i‐) ZnO, and p-type (p‐) Cu₂O films deposited on glass substrates were characterized by X-Ray diffraction (XRD), spectrophotometer, and the van der Pauw method, respectively. XRD results show that the mean grain size of i-ZnO film is much larger than that of n-ZnO film. The optical band gap energies of n-ZnO, i-ZnO, and p-Cu₂O film are 3.27, 3.47, and 2.00 eV, respectively. The carrier concentration of n-ZnO film is two orders of magnitude larger than that of p-Cu₂O film. The current–voltage (I–V) characteristics of ZnO/Cu₂O thin film n–i–p heterojunctions with different i-ZnO film thicknesses were investigated. Results show that ZnO/Cu₂O n–i–p heterojunctions have well-defined rectifying behavior. All ideality factors of these n–i–p heterojunctions are larger than 2.0. The forward bias threshold voltage and ideality factor increase when i-ZnO layer thickness increases from 100 to 200 nm. An energy band diagram was proposed to analyze the I–V characteristics of these n–i–p heterojunctions.     

Sonocatalytic degradation of a textile dye over Gd-doped ZnO nanoparticles synthesized through sonochemical process

The present study was performed to sonochemically synthesize Gd_xZn_1 − xO (x = 0–0.1) nanoparticles for sonocatalysis of Acid Orange 7 (AO7) in an aqueous medium. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) analysis confirmed proper synthesis of Gd-doped sonocatalyst. 5% Gd-doped ZnO nanoparticles with band gap of 2.8 eV exhibited the highest sonocatalytic decolorization efficiency of 90% at reaction time of 90 min. The effects of initial dye concentration and sonocatalyst dosage on decolorization efficiency were evaluated. In the presence of sodium sulfate, sodium carbonate and sodium chloride the decolorization efficiency decreased from 90 to 78, 65 and 56%, respectively. Among various enhancers, the addition of potassium periodate improved the decolorization efficiency from 90 to 100%. The highest decolorization efficiency was obtained at pH value of 6.34 (90%). The decolorization efficiency decreased only 6% after 4 repeated runs. Therefore, Gd-doped ZnO nanoparticles can be used as a promising catalyst for degradation of organic pollutants with great reusability potential.     

Above-gap and subgap differential conductance anomaly in concentrated magnetic semiconductor Zn0.32Co0.68O1 − v/Pb superconductor hybrid junctions

The Zn_0.32Co_0.68O_1 − v/Pb hybrid junctions were prepared, where the concentrated magnetic semiconductor Zn_0.32Co_0.68O_1 − v is in the region of variable range hopping transport instead of the ballistic or diffusive transport. The high differential conductance peak at gap voltage and two above-gap peaks were observed below the superconducting critical temperature. Moreover, both the zero bias conductance peak and the finite bias conductance peak were observed below the gap voltage. All these differential conductance peaks systematically evolve and finally disappear as the temperature or the magnetic field increases. These transport phenomena were explained by phase coherent Andreev reflection in the presence of strong disorder, magnetic impurity scattering, and spin polarization.     

Memory characteristics and tunneling mechanism of Pt nano-crystals embedded in HfAlOx films for nonvolatile flash memory devices

A non-volatile flash memory device based on metal oxide semiconductor (MOS) capacitor structure has been fabricated using platinum nano-crystals(Pt–NCs) as storage units embedded in HfAlO_x high-k tunneling layers. Its memory characteristics and tunneling mechanism are characterized by capacitance–voltage(C–V) and flat-band voltage-time(ΔV_FB-T) measurements. A 6.5 V flat-band voltage (memory window) corresponding to the stored charge density of 2.29 × 10¹³ cm⁻² and about 88% stored electron reserved after apply ±8 V program or erase voltage for 10⁵ s at high frequency of 1 MHz was demonstrated. Investigation of leakage current–voltage(J–V) indicated that defects-enhanced Pool-Frenkel tunneling plays an important role in the tunneling mechanism for the storage charges. Hence, the Pt–NCs and HfAlO_x based MOS structure has a promising application in non-volatile flash memory devices.     

Influence of substrate temperature on physical properties of sprayed Zn0.85Mn0.15O films

Zn_1−xMn_xO thin films have been synthesized by chemical spray pyrolysis at different substrate temperatures in the range, 250–450 °C for a manganese composition, x = 15%, on corning 7059 glass substrates. The as-grown layers were characterized to evaluate their chemical and physical behaviour with substrate temperature. The change of dopant level in ZnO films with substrate temperature was analysed using X-ray photoelectron spectroscope measurements. The X-ray diffraction studies revealed that all the films were strongly oriented along the (0 0 2) orientation that correspond to the hexagonal wurtzite structure. The crystalline quality of the layers increased with the increase of substrate temperature up to 400 °C and decreased thereafter. The crystallite size of the films varied in the range, 14–24 nm. The surface morphological studies were carried out using atomic force microscope and the layers showed a lower surface roughness of 4.1 nm. The optical band gap of the films was ∼3.35 eV and the electrical resistivity was found to be high, ∼10⁴ Ω cm. The films deposited at higher temperatures (>350 °C) showed ferromagnetic behaviour at 10 K.     

The study on the ZnO and Zn0.95Mn0.05O added YBCO system: Investigation of microstructure and transport properties

The effect of nano-size Zn_0.95Mn_0.05O and ZnO (30 nm) addition on the microstructure and the normal state transport properties of polycrystalline YBa₂Cu₃O_y (YBCO) was systematically studied. Samples were synthesized in air using a standard solid state reaction technique by adding nano-sized particles up to 10 wt.%. When Zn_0.95Mn_0.05O and ZnO are added to the YBCO the orthorhombic structure maintained even at the highest concentration. TEM and EDS analyses show the presence of inhomeginities embedded in the superconducting matrix. To interpret the normal state properties of the samples, the percolation theory based on localized states is applied. A cross-over between variable-range hopping and Coulomb gap mechanisms is observed as a result of increasing the nano-particles concentration. The ZnO addition modifies the electrical behavior of samples from metallic to insulating with a much lower concentration comparatively to Zn_0.95Mn_0.05O addition. The calculated values of the localization length, d, are greater in the case of Zn_0.95Mn_0.05O addition. This result can be interpreted by the internal structure defects.     

Properties of Mg-based materials for hydrogen storage

In the presented paper, microstructures of as-cast MgNi_14.0 and MgNi_27.8 alloys are described. The alloys are composed of α(Mg) phase and Mg₂Ni intermetallic (space group P6₂22) of primary and/or eutectic nature. The α(Mg)+Mg₂Ni eutectic is characterized by an asymmetric zone of coupled growth. Primary Mg₂Ni phase shows a branched dendritic morphology and eutectic Mg₂Ni phase forms narrow and interconnected lamellae. The MgNi_27.8 alloy was electrochemically hydrided in 6 M KOH. XRD proved that the hydriding led to a transformation of the Mg₂Ni phase into Mg₂NiH_x (x=0–0.3) solid solution. It was shown that the hydriding rate increased with bath temperature and that the optimum voltage between cathode and anode was 1.5 V. Higher voltages resulted in H₂ evolution which reduced hydriding efficiency. An 1 h hydriding at 90 °C and 1.5 V was able to produce almost 20 μm layer of the Mg₂NiH_0.3 phase. Further hydriding probably led to a formation of Mg₂NiH₄ hydride which retarded the inward diffusion of H.     

White-light emission from annealed ZnO:Si nanocomposite thin films

As grown ZnO:Si nanocomposites of different compositional ratios were fabricated by thermal evaporation techniques. These films were subjected to post-deposition annealing under high vacuum at a temperature of 250 °C for 90 min. The photoluminescence (PL) spectra of annealed samples have shown marked improvements both in terms of intensity and broadening. Structural and Raman analyses show formation of a Zn–Si–O shell around ZnO nanoclusters wherein on heating Zn₂SiO₄ compound forms resulting in huge UV, orange and red peaks at 310, 570 and 640 nm in PL. The new emissions due to Zn₂SiO₄ completes white light spectrum. The study not only suggests that 1:2 ratio is the best suited for material manipulation but also shows process at the interface of ZnO nanoclusters and silicon matrix leads to new PL emissions.     

Zn1−x Mg x O (0≤x≤0.05) nanowalls grown on catalyst-free sapphire substrates by high-pressure PLD and their photoluminescence properties

We report the growth of Zn_1?x Mg _x O (x=0, 0.02, 0.05 at.%) nanowalls on sapphire substrates without any metal catalysts by a high-pressure pulsed laser deposition (PLD). The influences of the experimental parameters like growth pressure, temperature, and target-substrate distance on the growth of Zn_1?x Mg _x O nanowalls were systemically studied and the growth mechanism was discussed. It was found that large area and uniform Zn_1?x Mg _x O nanowalls with c-orientation can be grown on sapphire substrates when the growth temperature and pressure were 950 °C and 400 Torr at a target-substrate distance of 2 cm. A thin layer assisted vapor-solid (VS) process was proposed for Zn_1?x Mg _x O nanowalls growth. The photoluminescence spectrum shows the bandgap of Zn_1?x Mg _x O nanowall was effectively expanded together with defect-related levels formation in a forbidden gap, which also induced enhancement of visible emission.     

Cu(In,Ga)Se2 superstrate-type solar cells with Zn1−xMgxO buffer layers

Superstrate-type Cu(In,Ga)Se₂ (CIGS) thin film solar cells were fabricated using Zn_1−xMg_xO buffer layers. Due to the diffusion of Cd into CIGS during the growth of the CIGS layer, the conventional buffer material of CdS is not suitable. ZnO is a good candidate because of higher thermal tolerance but the conduction band offset (CBO) of ZnO/CIGS is not appropriate. In this study, the Zn_1−xMg_xO buffer layers were used to fulfill both the requirements. The superstrate-type solar cells with a soda-lime glass/In₂O₃:Sn/Zn_1−xMg_xO/CIGS/Au structure were fabricated with different band gap energies of the Zn_1−xMg_xO layer. The CIGS layers [Ga/(In + Ga)∼0.25] were deposited by co-evaporation method. The substrate temperature during the CIGS deposition of 450 °C did not cause the intermixing of the Zn_1−xMg_xO and CIGS layers. The conversion efficiency of the cell with Zn_1−xMg_xO was higher than that with ZnO due to the improvement of open-circuit voltage and shunt resistance. The results well corresponded to the behavior of the adjustment of CBO, demonstrating that the usefulness of the Zn_1−xMg_xO layer for the CBO control in the superstrate-type CIGS solar cells.