Nanocrystalline Zn Research Articles

Room temperature ferromagnetism and photoluminescence of nanocrystalline Zn1 − xCoxO thin films prepared by sol–gel MOD

Zn1 − xCoxO (ZC) [x = 0, 1, 3, 5, 7 and 9 mol%] thin films were prepared by sol–gel combined metallo-organic decomposition method. The films were deposited on the Si substrate with spin-coating technique and annealed at 600 °C for 3 h. X-ray diffraction pattern shows the formation hexagonal wurtzite phase and distortion (c/a) decreases with increasing Co concentration in ZnO. The average grain size is measured using Scherer relation. Atomic force microscopy is used to confirm the formation of nanograins resulted by the use of polyethylene glycol as surfactant. The photoluminescence was recorded by using He-Cd laser of excitation wavelength 325 nm in wavelength region of 350–650 nm which exhibits some influence of Co doping on the multiplication of defects such as O vacancies, Zn interstitials and grain boundary defects. All thin films show room temperature ferromagnetism except pure ZnO which is diamagnetic and 9 mol% of Co shows paramagnetism. This behaviour is interpreted as due to fluctuations in the magnetic ordering, depending on grain size and site location in grain boundaries or oxygen vacancies.

Structural and electrical characterizations of nanocrystalline Zn1−xCdxS (0≤x≤0.9) prepared by low cost dip coating

Thin films of nanocrystalline Zn1−xCdxS (0≤x≤0.9) were deposited by a dip coating method on glass substrates from aqueous solution containing cadmium acetate, zinc acetate and thiourea at 200°C. The morphological, structural and electrical properties of the deposited Zn1−xCdxS thin films were studied by atomic force microscopy (AFM), X-ray diffractometer (XRD), selected area electron diffraction (SAED) and photoluminescence (PL). To understand the predominant conduction mechanism of the nanocrystalline Zn1−xCdxS (0≤x≤0.9) thin films, DC electrical conductivity was measured in the temperature range of 300–420K. These measurements revealed that the DC behavior of the films can be described by a one-dimensional variable range hopping (VRH) model in the entire temperature range instead of a three-dimensional variable range hopping (VRH) model. The current density–voltage (J–V) characteristics of Zn1−xCdxS (0≤x≤0.9) thin films shows that the current conduction is ohmic type at the low-voltage region while the charge transport phenomenon appears to be space charge limited current (SCLC) at the higher-voltage regions. The latter conduction is attributed to the presence of a discrete trapping level. Various electrical parameters were determined and studied as a function of Cd-content such as electron mobility (μ0), density of states in conduction band (Nc), Fermi energy (EF), trap energy (Et) and trap electron density (Nt).

Crystallite size estimation and photosensitivity characterization of nanocrystalline Zn1−xCdxS (0 ⩽ x ⩽ 0.9) based heterojunctions prepared by simple dip-coating

Nanocrystalline Zn1−xCdxS (0⩽x⩽0.9) thin films were successfully prepared by simple dip-coating method. The structural characteristics of Zn1−xCdxS were achieved by X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The X-ray peak broadening analysis was used to evaluate the crystallite size by using uniform deformation (UDM), uniform deformation stress (UDSM) and uniform deformation energy density (UDEDM) models. The UDEDM model was found to be in good agreement with the results obtained from TEM. Heterojunctions of n-Zn1−xCdxS/p-Si exhibit good rectifying characteristics, especially for x=0.9. The photocurrent characteristics of n-Zn1−xCdxS/p-Si heterojunctions depend on illumination intensity and increase with increasing power intensity. The transient photocurrent results indicate that photocurrent under illumination increases with increasing light intensity which can be explained by continuous distribution of traps.

CoO doping effects on the ZnO films through EBPDV technique

Nanometric Zn1−xCox O( x = 0.020, 0.025 and 0.030 in mol.%) nanopowders were obtained from low temperature calcination of a resin prepared using the Pechini's method. Firing the Zn1−xCoxO resin at 400 ◦ C/2 h a powder with hexagonal structure was obtained as measured by X-ray diffraction (XRD). The powder presented average particle size of 40 nm observed by field emission scanning electronic microscopy (FE-SEM) micrographs and average crystallite size of 10 nm calculated from the XRD using Scherrer's equation. Nanocrystalline Zn1−xCoxO films with good homogeneity and optical quality were obtained with 280-980 nm thicknesses by electron beam physical vapour deposition (EBPVD) under vacuum onto silica substrate at 25 ◦ C. Scanning electron microscopy with field emission gun showed that the film microstructure is composed by spherical grains and some needles. In these conditions of deposition the films presented only hexagonal phase observed by XRD. The UV-visible-NIR and diffuse reflectance properties of the films were measured and the electric properties were calculated using the reflectance and transmittance spectra.

Studies on the synthesis and characterization of co-precipitated nanocrystalline Zn–Pb–O

Zinc lead oxide (Zn2PbO4) nanopowders were synthesized by a simple co-precipitation method using zinc nitrate and lead nitrate precursors in 2:1 proportion by carefully controlling the preparative parameters. Ammonium hydroxide was used to precipitate Zn2+ and Pb4+ cations simultaneously. X-ray diffraction study of prepared samples indicates both orthorhombic and tetragonal phase formation of Zn2PbO4. The microstructural features are examined by scanning electron microscopy. Frequency dependent dielectric constant (ε′) shows the usual dispersion behavior, while linear variation of AC conductivity (σAC) with frequency confirms the conduction due to small polaron. The DC resistivity decreases with increase in temperature indicating semiconducting behavior of samples. To understand the conduction mechanism, complex impedance measurements are carried out.

Synthesis and soft magnetic properties of Zn0.8−xNixMg0.1Cu0.1Fe2O4 (x = 0.0−0.8) ferrites prepared by sol–gel auto-combustion method

Single phase nanocrystalline Zn0.8−xNixMg0.1Cu0.1Fe2O4 (x=0.0−0.8), were synthesized by sol–gel auto-combustion method without post-preparation treatment and, effect of Ni content on structural and magnetic properties is studied by X-ray diffraction (XRD), magnetic measurements and cation distribution. Scherrer’s grain diameter range between 23.5 and 35.1nm. Experimental and theoretical lattice parameter values suggest that the estimated cation distribution is close to the real distribution. Change in Ni-content leads to variation in cation distribution at site A and B. Ni2+ ions occupy both A and B site, but when Zn2+ is fully replaced by Ni2+, it totally goes to A site. Both coercivity measured at 50Hz and quasi static coercivity value is found to increase with nickel content. Same trend is observed for anisotropy constant values. Best magnetization value of 25.04emu/g was obtained for the specimen with x=0.60. Variation of magnetic properties and other parameters (obtained via cation distribution) can be understood in terms of observed changes in distribution of cation(s) on A and B site.

Photoelectrochemical splitting of water with nanocrystalline Zn1−xMnxO thin films: First-principle DFT computations supporting the systematic experimental endeavor

Photoelectrochemical splitting of water with nanocrystalline Zn1−xMnxO thin films was investigated. ZnO thin films with 1, 3, 5 and 7% at. Mn incorporation were synthesized by sol–gel method and characterized by X-Ray Diffraction (XRD) analysis, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), X-ray Photoelectron spectroscopy (XPS), High Resolution Transmission Electron Microscopy (HR-TEM) and UV–Vis spectroscopy. Mn incorporation coupled with variation in sintering temperature led to significant microstructural changes, which tentatively influenced the magnitude of optical absorption and charge carrier mobility, thereby impacting the performance of such systems towards photoelectrochemical splitting of water. Electronic structure computations based on first principle density functional theory (DFT) revealed electronic states of Mn being responsible for the marginally recorded red shift in bandgap energy. Photoelectrochemical measurements using thin films of 1% at. Mn:ZnO sintered at 600 °C yielded 3 times enhanced photocurrent at zero bias due to improved optical absorption. Plausible explanations for the effect have also been offered.

Synthesis and structural, magnetic characterization of nanocrystalline Zn1−xMnxO diluted magnetic semiconductors (DMSs) synthesized by combustion reaction

Nanocrystalline Mn2+ doped Zn1−xMnxO, where x=0.1, 0.15, 0.2, 0.25, 0.3 and 0.4mol of Mn2+ diluted magnetic semiconductors (DMSs), were synthesized by the combustion reaction for spintronic applications. The effect of Mn2+ ion doping on the structural, morphological and magnetic properties of ZnO was investigated. The products of the reactions were characterized by X-ray diffraction (XRD), nitrogen adsorption (BET), transmission electron microscopy (TEM), and magnetic measurements (VSM). XRD spectra data revealed the formation of a ZnO phase at all the Mn2+ doping concentrations used, indicating that the synthesis was efficient in diluting the Mn2+ ions in the ZnO lattice. Increasing the Mn2+ ion concentrations reduced the maximum reaction and ignition temperature and contributed to reduce crystallite and particle sizes. The samples showed the typical behavior of soft magnetic materials at all the Mn2+ concentrations evaluated here. The Curie temperature (Tc) was higher than room temperature at all the Mn2+ concentrations.

Room temperature ferromagnetism in Mn-doped ZnS nanocrystalline thin films grown by sol–gel dip coating process

Nanocrystalline Zn1−xMnxS (x = 0.00, 0.01, 0.02, 0.03, 0.05, and 0.1) thin films having different Mn content were grown by the sol–gel dip coating process. The effect of Mn content on the structural, optical and magnetic properties of Zn1−xMnxS nanocrystalline thin films were investigated. X-ray diffraction results showed the presence of single hexagonal phase corresponding to ZnS with a preferred orientation along the ZnS (002) hexagonal plane direction without any detectable secondary phase, suggesting the incorporation of Mn ions into the ZnS lattice. Scanning electron microscope revealed the surface of the nanocrystalline films to be homogeneous and dense and the grains of the film surface were randomly scattered. In ultraviolet–visible measurements, the band gap energy corresponding to the absorption edge estimated were found to be 3.59–3.23 eV depending on the Mn doping ratio. Magnetic measurements showed that a paramagnetic behavior at 5 K and ferromagnetic behavior at 300 K.

Nanocrystalline Zn<SUB>1–<I>x</I></SUB>Co<SUB><I>x</I></SUB>O (0.05 ≤ <I>x</I> ≤ 0.2) Powders Produced by Novel Auto-Combustion Method and Their Characterization

Nanocrystalline Zn<SUB>1–<I>x</I></SUB>Co<SUB><I>x</I></SUB>O (0.05 ≤ <I>x</I> ≤ 0.2) Powders Produced by Novel Auto-Combustion Method and Their Characterization

Nanocrystalline Zn1−x Ag x O y thin films evolved through electrodeposition for photoelectrochemical splitting of water

Nanocrystalline Zn1 − x Ag x O y (x = 10− 3 − 50 × 10− 3) thin films evolved through electrodeposition over ITO substrate have been investigated for photoelectrochemical splitting of water. Samples were characterized by XRD, EDX, SEM, AFM, UV–visible optical absorption, and Mossbauer spectral analysis. Ag incorporation led to a decrease in the band gap energy and alterations in microstructural and semiconductor properties. Raising Ag concentration in samples up to 1 % at. caused a significant reduction in density and electrical resistivity, enhanced absorption along with red shift to the band gap energy, anodic shift in flat band potential, and increased charge carrier density, enabling 1 % at. Ag-incorporated ZnO films most photosensitive by yielding highest short-circuit current, photocurrent density, and applied bias photon to current efficiency. Plausible reasons have been offered.

Nanocrystalline Zn1−x−yBexMgyO thin films synthesized by the sol–gel method: structural and near infrared photoluminescence properties

In this paper, we reports on the structural and optical properties of Zn1−x−yBexMgyO thin films prepared by sol–gel method, which are new materials for optoelectronic and ultraviolet-light-emitting devices. The crystal structure and core level spectra of these films are studied by X-ray diffraction and X-ray photoelectron spectroscopy. Surface morphology of the films is analyzed by scanning electron microscope images and the surface is composed of spherical shaped grains. Micro-photoluminescence shows a near edge band emission and the peak values tuned from 3.26 eV for the undoped to 3.4 eV for the doped ZnO film. Near infrared emission is observed in the region 1.64–1.67 eV for pure and co-doped ZnO films. In micro-Raman spectra, multiple-order Raman bands originating from ZnO-like longitudinal optical (LO) phonons are observed. A Raman shift of about 5–18 cm−1 is observed for the first-order LO phonon. A comparative study was made on Raman band for BeZnO, MgZnO and BeMgZnO nanocrystals with the LO phonon band of bulk ZnO. The ultraviolet resonant Raman excitation at room temperature shows multi-phonon LO modes up to the fourth order. Deformation energy of all the films is calculated and BeMgZnO film has the minimum deformation energy.

Spectroscopic ellipsometry of Zn1−xCuxO thin films based on a modified sol–gel dip-coating technique

Nanocrystalline Zn1−xCuxO thin films (x=0, 0.01, 0.02, 0.03, 0.04 and 0.05) were synthesized by sol–gel dip-coating technique on a quartz substrate. These films were annealed at 350°C for 2h. The X-ray diffraction showed a hexagonal crystal structure with high intensity peak for the (002) reflection plane indicating preferential growth along the c-axis of the crystal lattice. The peak position related to the (002) peak was shifted as a result of the copper ion incorporation, confirming the interstitial substitution of the zinc ions by the copper ions. This interstitial substitution leads to a decrease of an average crystallite size and lattice constants and an increase of the micro-strain up to 2at.% of the copper amount. The surface morphology was explored by scanning electron microscopy which confirmed the homogenous distribution of nanoparticles in the deposited films along the quartz substrates. The energy dispersion X-ray spectroscopy revealed absence of impurities in the as-deposited films. The high resolution electron microscopy and selected area electron diffraction depicted that the films have polycrystalline nature. The film thickness and optical constants of the Zn1−xCuxO thin films were estimated by fitting the spectroscopic ellipsometric data (ψ and Δ) using three different models. The refractive index was fitted using harmonic oscillator model from which the oscillator and the dispersive energies were found. The dielectric constant, dielectric loss, energy loss functions were also determined.

The Electrical Resistivity and ac Conductivity of Co Substituted Li–Ni–Zn Nanocrystalline Ferrites Prepared by a Chemical Method

The Electrical Resistivity and ac Conductivity of Co Substituted Li–Ni–Zn Nanocrystalline Ferrites Prepared by a Chemical Method

Influence of cadmium content on the microstructure characteristics of dip coated nanocrystalline Zn1−xCdxS (0 ⩽ x ⩽ 0.9) and their heterojunction applications

Nanocrystalline Zn1−xCdxS (0⩽x⩽0.9) thin films were prepared by dip-coating method. The compositional analyses were made by the energy dispersive X-ray technique (EDX). X-ray diffraction analyses (XRD) of the prepared samples were performed to investigate the crystalline structure and determine the main structural parameters using Williamson–Hall (W–H) model. The surface morphology and crystalline structure of nanocrystalline Zn1−xCdxS thin films were studied using the scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The estimated average crystallite size of Zn1−xCdxS by W–H analysis were compared with TEM results. It is found that the average crystallite size measured by W–H analysis is in good agreement with TEM results. The capacitance–voltage (C–V) characteristics of n-Zn1−xCdxS/p-Si heterojunction were measured at different temperatures ranging from 300 to 400K under dark condition. The dependence of C−2 on V for the heterojunctions was found to be almost linear which indicates that the prepared heterojunctions are abrupt and then the essential junction parameters were obtained. The built-in potential (Vb), the donor carrier concentration (ND) and the width of the depletion region (W) were studied as a function of Cd-content.

Physical properties of antiferromagnetic Mn doped ZnO samples: Role of impurity phase

Structural, morphological, optical, and magnetic properties of nanocrystalline Zn1−xMnxO samples (x=0.01, 0.02, 0.04, 0.06, 0.08 and 0.10) prepared by the sol–gel route are studied by X-ray diffraction (XRD), Scanning electron microscopy (SEM), UV–visible absorption spectroscopy, Superconducting quantum interference device (SQUID) magnetometry and positron annihilation lifetime spectroscopy (PALS). XRD confirms formation of wurzite structure in all the Mn-substituted samples. A systematic increase in lattice constants and decrease in grain size have been observed with increase in manganese doping concentration up to 6at% in the ZnO structure. An impurity phase (ZnMnO3) has been detected when percentage of Mn concentration is 6at% or higher. The optical band gap of the Mn-substituted ZnO samples decrease with increase in doping concentration of manganese whereas the width of the localized states increases. The antiferromagnetic exchange interaction is strong in the samples for 2 and 4at% of Mn doping but it reduces when the doping level increases from 6at% and further. Positron life time components τ1 and τ2 are found to decrease when concentration of the dopant exceeds 6at%. The changes in magnetic properties as well as positron annihilation parameters at higher manganese concentration have been assigned as due to the formation of impurity phase.

Effects of oxide distributed in grain boundaries on microstructure stability of nanocrystalline metals

Nanocrystalline copper and zinc prepared by high-pressure compaction method have been studied by positron lifetime spectroscopy associated with X-ray diffraction. For nanocrystalline Cu, mean grain sizes of the samples decrease after being annealed at 900 °C and increase during aging at 180 °C, revealing that the atoms exchange between the two regions. The positron lifetime results indicate that the vacancy clusters formed in the annealing process are unstable and decomposed at the aging time below 6 hours. In addition, the partially oxidized surfaces of the nanoparticles hinder the grain growth during the ageing at 180 °C, and the vacancy clusters inside the disorder regions which are related to Cu2O need longer aging time to decompose. In the case of nanocrystalline Zn, the open volume defect (not larger than divacancy) is dominant according to the high relative intensity for the short positron lifetime (τ1). The oxide (ZnO) inside the grain boundaries has been found having an effect to hinder the decrease of average positron lifetime (τav) during the annealing, which probably indicates that the oxide stabilizes the microstructure of the grain boundaries. For both nanocrystalline copper and zinc, the oxides in grain boundaries enhance the thermal stability of the microstucture, in spite of their different crystal structures. This effect is very important for the nanocrystalline materials using as radiation resistant materials.

Room-temperature ferromagnetism in Zn1−x Ni x S diluted magnetic semiconducting nanocrystalline thin films

This study reports the structure and the magnetic properties of Zn1−xNixS nanocrystalline films with Ni composition ratios of 0.05 ≤ x ≤ 0.2. Thin films of Zn1−xNixS were deposited on Corning glass substrates by using an electron-beam evaporation method. X-ray diffraction patterns revealed single- phase films with a cubic zinc-blend-type structure with preferred crystallographic orientations along the (111) and the (220) planes. Evidence for the nanocrystalline nature of the films was observed from the investigations of the surface morphology by using scanning electron microscopy and atomic force microscopy. Magnetic domains were observed by using magnetic force microscopy at room temperature, indicating the existence of ferromagnetism over the film’s surface. The magnetic measurements at 5 K revealed a superparamagnetic behavior. However, room-temperature ferromagnetism was observed in all the nanocrystalline Zn1−xNixS films. The saturation magnetization for the Zn1−xNixS films was found to increase with increasing dopant concentration (x). An exchange interaction between local spin-polarized electrons (Ni2+ ions) and conductive electrons according to the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction mechanism is proposed as a possible mechanism for the ferromagnetism. These results show that the Ni-doped ZnS nanocrystalline films are promising materials for applications in spintronic and magnetic sensor devices.

A comparative study on ethanol gas sensing properties of ZnO and Zn0.94Cd0.06O nanoparticles

Nanocrystalline ZnO and Zn0.94Cd0.06O particles were synthesized using sol–gel method and were characterized for their structural and morphological properties by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDXS). The results confirmed the formation of pure and spherical ZnO and Zn0.94Cd0.06O nanoparticles with wurtzite crystal phase and particle size of 20–25nm obtained by TEM. Additionally, the ethanol sensing properties of ZnO and Zn0.94Cd0.06O based sensors were investigated for different operating temperatures/concentrations and they were compared with commercial ZnO microparticles. Comparative results demonstrated that Zn0.94Cd0.06O nanoparticles exhibit higher and faster response to 250ppm of ethanol at 250°C. Results on response/recovery time, sensing mechanism, conductance variation and thermodynamic/kinetic investigation of ethanol sensing mechanism is also studied and discussed.

Plastic Deformation of Nanocrystalline Zinc Investigated by Positron Annihilation Lifetime Spectroscopy

The plastic deformation of nanocrystalline Zn is investigated by positron annihilation lifetime spectroscopy. The deformation is carried out by exerting a pressure on the surface of the samples, which causes a thickness reduction of the samples. The different behavior of the relative thickness reduction with the increasing pressure between the nanocrystalline sample and the conventional bulk sample indicates that a distinct plastic deformation mechanism is operating in the nanocrystalline sample. The positron lifetime results confirm indirectly that the mechanism is grain boundary based, such as grain boundary sliding. Once the nanocrystalline sample is annealed at high temperature, the changes in the thickness reduction and the positron lifetime results become similar to those of the conventional bulk sample. Furthermore, vacancy clusters inside the nanocrystalline sample are found to be an impediment to the plastic deformation process.