Halide-hydrogen vapor transport for growth of ZnO single crystals with controllable electrical parameters

The advantages of HCl+H₂ gas mixture as a chemical vapor transport agent for ZnO single crystals growth in the closed growth chambers are shown in comparison with Cl₂, HCl and H₂ by the thermodynamic analysis. The influence of the growth temperature, density of HCl+H₂ transport agent and undercooling were investigated experimentally on the rate of ZnO mass transport. It was shown that HCl+H₂ gas mixture provides (i) a rather high growth rate (up to 1 mm per day), (ii) a minimization of wall adhesion effect and deformations during a post-growth cooling, (iii) stable and reproduced seeded growth of the void-free single crystals with controllable conductivity and charge carrier concentration varied in the range of 2–22 (Ω cm)⁻¹ and (1–31)·10¹⁷ cm⁻³, respectively. The characterization by the photoluminescence spectra, the transmission spectra and the electrical properties, as well as energy spectra of stable Cl-containing defects are analyzed.     

Temperature dependent study of basal plane stacking faults in Ag:ZnO nanorods by Raman and photoluminescence spectroscopy

We report the specific features of basal plane stacking faults (BSFs) in ZnO nanorods (NRs), studied by temperature dependent photoluminescence and Raman spectroscopy. At low temperature (4 K) the intense band of emission at 3.321 eV is attributed to the presence of BSFs defects and Ag as an acceptor dopant in ZnO. This specific peak red-shifts with the temperature increase, occupying the position 3.210 eV at RT. The nature of the emission is explained as exciton recombination of the electrons, confined in the homo-heterojunction QW, with the holes, localized near the Ag atoms close to SFs. Raman spectroscopy revealed that Ag:ZnO nanorods have slightly downshifted positions of the modes 330 cm⁻¹ and 440 cm⁻¹ by 4 cm⁻¹, which we explain as due to the presence of BSFs. It was also observed, that the longitudinal optical phonon mode A^LO, which is common polar mode for ZnO, was not detected by Raman spectroscopy in the samples with high BSFs density. This feature can be explained as due to existence of the bound charge induced by the BSFs in the NRs.     

Magnetron-sputtered high performance Y-doped ZnO thin film transistors fabricated at room temperature

In this report, sputtered-grown undoped ZnO and Y-doped ZnO (ZnO:Y) thin film transistors (TFTs) are presented. Both undoped ZnO and ZnO:Y thin films exhibited highly preferred c-axis oriented (002) diffraction peaks. The ZnO:Y thin film crystallinity was improved with an increase of (002) peak intensity and grain size. The electrical properties of ZnO:Y TFTs were significantly enhanced relative to undoped ZnO TFTs. ZnO:Y TFTs exhibited excellent performance with high mobility of 38.79 cm² V⁻¹ s⁻¹, small subthreshold swing of 0.15 V/decade, and high I_on/I_off current ratio of the order of 8.17 × 10⁷. The O_1s X-ray photoelectron spectra (XPS) showed oxygen vacancy-related defects present in the ZnO:Y TFTs, which contributed to enhancing the mobility of the TFTs.     

Influence of plasma treatment of ITO surface on the growth and properties of hole transport layer and the device performance of OLEDs

Surface energy of indium tin oxide (ITO) surfaces treated by different plasmas, including argon (Ar–P), hydrogen (H₂–P), carbon tetrafluoride (CF₄–P), and oxygen (O₂–P), was measured and analyzed. The initial growth mode of hole transport layers (HTLs) was investigated by atomic force microscope observation of thermally deposited 2 nm thick N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) on the plasma treated ITO surfaces. The results show that different plasma treatments of ITO influence the growth of HTLs in significantly different ways through the modification of surface energy, especially the polar component. The O₂–P and CF₄–P were found to be most effective in enhancing surface polarity through decontamination and increased dipoles, leading to more uniform and denser nucleation of NPB on the treated ITO surfaces. It was further found that increased density of nucleation sites resulted in a decreased driving voltage of OLEDs. Under the same fabricating conditions, a lowest driving voltage of 4.1 V was measured at a luminance of 200 cd/m² for the samples treated in CF₄–P, followed by the samples treated in O₂–P (5.6 V), Ar–P (6.4 V), as-clean (7.0 V) and H₂–P (7.2 V) plasma, respectively. The mechanisms behind the improved performance were proposed and discussed.     

Characteristics of ZnOV2O5MnO2Nb2O5Er2O3 semiconducting varistors with sintering processing

The effects of sintering temperature on the microstructure, electrical properties, and dielectric characteristics of ZnOV₂O₅MnO₂Nb₂O₅Er₂O₃ semiconducting varistors have been studied. With increase in sintering temperature the average grain size increased (4.5–9.5 μm) and the density decreased (5.56–5.45 g/cm³). The breakdown field decreased with an increase in the sintering temperature (6214–982 V/cm). The samples sintered at 900 °C exhibited remarkably high nonlinear coefficient (50). The donor concentration increased with an increase in the sintering temperature (0.60×10¹⁸–1.04×10¹⁸ cm^?3) and the barrier height exhibited the maximum value (1.15 eV) at 900 °C. As the sintering temperature increased, the apparent dielectric constant increased by more than four-fold.     

Synthesis and semiconducting properties of Na2MnPO4F. Application to degradation of Rhodamine B under UV-light

Na₂MnPO₄F is synthesized by hydrothermal route at 453 K and the physical properties and photo-electrochemical characterizations are reported. The compound crystallizes in a monoclinic system (SG: P 2₁/n) with the lattice constants: a=13.7132 Å, b=5.3461 Å, c=13.7079 Å, β=119.97°. The UV–visible spectroscopy shows an indirect optical transition at 2.68 eV; a further direct transition occurs at 3.70 eV, due to the charge transfer O²⁻: 2p → Mn²⁺: e_g. The thermal variation of the electrical conductivity is characteristic of a semiconducting behavior with activation energy of 39 meV and an electron mobility (µ_318 K=5.56×10⁻⁴ cm² V⁻¹ s⁻¹), thermally activated. The flat band potential (+0.47 V_SCE) indicates that the valence band derives mainly from O²⁻: 2p orbital with a small admixture of F⁻ character while the conduction band is made up of Mn²⁺: t_2g orbital. The electrochemical impedance spectroscopy shows the contribution of both the bulk and grains boundaries. The photocatalytic performance of Na₂MnPO₄F for the degradation of Rhodamine B (RhB) is demonstrated on the basis of the energy diagram. 88% of the initial concentration is degraded under UV light and the oxidation follows a first order kinetic with a rate constant of 0.516 h⁻¹. Neither adsorption nor photolysis is observed. The photoactivity results from the electron transition from the hybridized band (O²⁻, F⁻) to the Mn²⁺: e_g orbital, occurring in the UV region. The catalyst was subjected to three successive photocatalytic cycles, thus proving its long term stability.     

Investigation of the properties of ferromagnetic ZnO:Cr2O3 nanocomposites

Present work focuses on the structural, optical and magnetic properties of ZnO:Cr₂O₃ nanocomposites. ZnO nanoparticles were synthesized and the structure was confirmed using powder x-ray diffraction. ZnO nanoparticles was grown in the hexagonal wurtzite structure with the preferential orientation along (101) plane. ZnO:Cr₂O₃ composites have been synthesized by doping different concentration of Cr₂O₃ (1, 3 and 5 wt%) into ZnO. The incorporation of Cr₂O₃ was confirmed using Fourier transformed infrared spectroscopy. UV–visible absorption spectra have been observed and interpreted for the determination of optical constants of ZnO:Cr₂O₃ composites. The optical constants like optical band gap, refractive index were determined and the effect of Cr₂O₃ on these constants was investigated. Relation between optical band gap and the refractive index were obtained. Magnetic studies using vibrating sample magnetometer reveal the ferromagnetism at 150 K in the composites with 3 and 5 wt% of Cr₂O₃.     

Preparation and physical properties of the layered niobate Cu0.5Nb3O8: Application to photocatalytic hydrogen evolution

The new layered niobate Cu_0.5Nb₃O₈ is synthesized by soft chemistry in aqueous electrolyte via Cu²⁺→H⁺ exchange between copper nitrate and HNb₃O₈·H₂O. The characterization of the exchanged product is made by means of thermal gravimetry, chemical analysis, X-ray diffraction and IR spectroscopy. Thermal analysis shows a conversion to anhydrous compound above 500 °C. The oxide displays a semiconductor like behavior; the thermal variation of the conductivity shows that d electrons are strongly localized and the conduction is thermally activated with activation energy of 0.13 eV. The temperature dependence of the thermopower is indicative of an extrinsic conductivity; the electrons are dominant carriers in conformity with an anodic photocurrent. Indeed, the Mott–Schottky plot confirms n-type conduction from which a flat band potential of −0.82 V_SCE, an electronic density of 8.72×10¹⁹ m⁻³ and a depletion width of 4.4 nm are determined. The upper valence band, located at ~5.8 eV below vacuum is made up predominantly of Cu²⁺: 3d with a small admixture of O²⁻: 2p orbitals whereas the conduction band consists of empty Nb⁵⁺: 5s level. The energy band diagram shows the feasibility of the oxide for the photocatalytic hydrogen production upon visible light (29 mW cm⁻²) with a rate evolution of 0.31 mL g⁻¹ min⁻¹.     

Structural,optical and transport properties of SxZnO

In this paper, S-doped ZnO (S_xZnO) was prepared using sol-gel method at different S amounts. The structural, optical and transport properties were investigated. The introduction of S atoms into the ZnO network was found to lower the crystallization level which results in reducing the crystallite size up to x=0.3. The doping process is confirmed by the observed peak at ~610 cm⁻¹ in the ATR spectrum related to the Zn-S linking. EDX mapping shows a homogeneous distribution of S atoms on the particles surface. The best compromise between the band gap (Eg=2.96 eV), the charge carriers (N_A=2.139×10²² cm⁻³), the conductivity (σ=5.56×10⁻⁴ Ω⁻¹ m⁻¹) and the mobility (µ=16.26×10⁻¹⁴ m² V⁻¹ s⁻¹) is obtained for x=0.1. The conduction mechanism is assumed by small hopping polaron. The S-doping has impacted positively the photocatalytic activity of ZnO, with particularly high performance for S_0.2ZnO.     

Synthesis of highly selective and sensitive Cu-doped ZnO thin film sensor for detection of H2S gas

Copper (Cu) doped zinc oxide (ZnO) thin films were successfully prepared by a simple sol-gel spin coating technique. The effect of Cu doping on the structural, morphology, compositional, microstructural, optical, electrical and H₂S gas sensing properties of the films were investigated by using XRD, FESEM, EDS, FTIR, XPS, Raman, HRTEM, and UV–vis techniques. XRD analysis shows that the films are nanocrystalline zinc oxide with the hexagonal wurtzite structure and FESEM result shows a porous structured morphology. The gas response of Cu-doped ZnO thin films was measured by the variation in the electrical resistance of the film, in the absence and presence of H₂S gas. The gas response in relation to operating temperature, Cu doping concentration, and the H₂S gas concentration has been systematically investigated. The maximum H₂S gas response was achieved for 3 at% Cu-doped ZnO thin film for 50 ppm gas concentration, at 250 °C operating temperature.     

Analysis of localized vibration of nitrogen complexes in CZ silicon

Nitrogen doping reduces secondary defects in CZ silicon. It is necessary to establish the infrared measurement method of nitrogen (N) and nitrogen–oxygen (N–O) point defect (N–V) complex concentrations. In this study, we examine the vibration of N–O and N–V complexes by the molecular orbital calculation. First, we analyze the NNO and NNOO in as-grown crystal. The structures used for calculation are Si₄₀N₂O(and O₂)H₅₄. Vibration mode and induced dipole moment by localized vibration are obtained by vibrational calculation. We have shown that the sum of absorption of N and N–O peaks is closely related to the total N concentration measured by SIMS. It is to be noted that oscillator strength is proportional to square of induced-dipole. Weighted sum of absorption coefficient inversely proportional to oscillator strength is used in determining the conversion coefficient to N concentration. Next, we calculate the frequency and dipole moment of normal vibration modes of NNV and NNVV which are thought to be formed and affect the defect formation at high temperature. The structures used to calculate are Si₃₄N₂H₃₆ and Si₄₂N₂H₄₂. H₂O-type and BF₃-type asymmetric stretch vibrations are obtained. The frequency of NNV is 940 cm⁻¹ in H₂O-type and 867 cm⁻¹ in BF₃-type. In NNVV 909 and 920 cm⁻¹ are obtained corresponding to H₂O-type and the BF₃-type vibrations, respectively. We are trying to find these peaks experimentally.     

Defect modification in ZnInSnO transistor with solution-processed Al2O3 dielectric by annealing

The properties of solution-processed Al₂O₃ thin films annealed at different temperatures were thoroughly studied through thermogravimetry–differential thermal analysis, UV–vis-NIR spectrophotometer measurements, scanning electron microscopy, X-ray diffraction, atomic force microscopy and a series of electrical measurements. The solution-processed ZnInSnO thin films transistors (TFTs) with the prepared Al₂O₃ dielectric were annealed at different temperatures. The TFTs annealed at 600 °C have displayed excellent electrical performance such as the field-effect mobility of 116.9 cm² V⁻¹ s⁻¹ and a subthreshold slope of 93.3 mV/dec. The performance of TFT device could be controlled by adjusting the annealing temperature. The results of two-dimensional device simulations demonstrate that the improvement of device performance are closely related with the reduction of interface defects between channel and dielectric and subgap density of stats (DOS) in the channel layer.     

Electrochemical growth of tin(II) oxide films: Application in photocatalytic degradation of methylene blue

We have investigated the semiconducting and photoelectrochemical properties of SnO films grown potentiostatically on tin substrate. The oxide is characterized by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The anodic process corresponds to the formation of SnO·nH₂O pre-passive layer that is removed upon increasing potential due to surface etching at the metal/oxide interface. SnO films deposited for long durations (>50 mn) are uniform and well adhered; they thicken up to ~50 nm by diffusion-controlled process and the growth follows a direct logarithmic law. The thickness is determined by coulometry and the X-ray diffraction indicates the tetragonal SnO phase (SG: P4/mmm) with a crystallite size of 32 nm. The Mott–Schottky plot is characteristic of n type conductivity with an electrons density of 5.72×10¹⁸ cm⁻³, a flat band potential of −0.09 V_SCE and a depletion width of ~10 nm. The valence band, located at 5.91 eV below, vacuum is made up of hybridized O²⁻:2p Sn²⁺:5s while the conduction band (4.45 eV) derives from Sn²⁺:5p orbital. The electrochemical impedance spectroscopy (EIS) measured in the range (10⁻²–10⁵ Hz) shows the contribution of the bulk and grain boundaries. The energy band diagram predicts the photodegradation of methylene blue on SnO films. 67% of the initial concentration (10 mg L⁻¹) disappears after 3 h of exposure to visible light (9 mW cm⁻²) with a quantum yield of 0.072.     

Chemical,vibrational and optical signatures of nitrogen in ZnO nanowires

ZnO nanowires with various concentrations of nitrogen molecules have been fabricated by remote plasma annealing. X-ray absorption near-edge spectroscopy (XANES) reveals that nitrogen exists mainly in two chemical states: atomic nitrogen substituting oxygen (N_O) and molecular nitrogen (N₂) weakly bound to the ZnO lattice; the latter state increases substantially with prolonged plasma time. Cathodoluminescence microanalysis of individual nanowires reveals a broad emission band at 3.24 eV at 10 K, attributable to the recombination of a shallow donor and a N₂ acceptor state. The Raman modes at 547 and 580 cm⁻¹ from the N-doped nanowires are found to rise in proportion to the N₂ concentration, indicating they are related to N₂ molecules or defects caused by the incorporation of N₂ in the nanowires.     

Polymer films with multilayer low-E coatings

The paper presents the experimental results on depositing a multilayer low-emissivity (low-E) coating with oxide–metal–oxide structure on polyethylene terephthalate (PET) and polyethylene (PE) films by magnetron sputtering. The TiO₂/ZnO:Ga/Ag/ZnO:Ga/TiO₂ coating on the PET film with high water-resistance and capability to be used outside of sealed double-glazed panes was proposed. The optimal thickness of coating layers was experimentally determined. The coating with the optimal structure has 82% transmittance over the visible spectrum and 91% reflection over the infrared spectrum. The window with a PET film and low-E coating was investigated in terms of heat engineering. It was revealed that heat transfer resistance increased up to 0.73 m² °C W⁻¹ for the windows with a PET film and low-E coating. Heat transfer resistance of the windows without a polymer film was 0.38 m² °C W⁻¹. The water-resistant ZnO:Ga/Ag/ZnO:Ga/SiO₂ coating on a PE film with 77% transmittance and 91–92% reflection in the IR range was proposed to be used as greenhouse covering material. The possibility of using the PE film with a low-E coating to reduce heat loss in greenhouses and enhance yielding capacity was demonstrated.     

Innovative impregnation process for production of γ-Fe2O3–activated carbon nanocomposite

An immobilized superparamagnetic nanocomposite comprising γ-Fe₂O₃ and activated carbon was synthesized via a facile thermal decomposition route. To prepare the magnetically functionalized nanocomposite, treated activated carbon (TAC) loaded with lepidocrocite (γ-FeOOH) nanoparticles (MAC-1) was first produced via a wet chemical method. Then magnetic activated carbon (AC/γ-Fe₂O₃, MAC-2) was fabricated by thermal decomposition of MAC-1 at 250 °C under argon gas for 1 h. Characterization analyses confirmed that superparamagnetic spherical maghemite nanoparticles of 21±2 nm in size were homogeneously dispersed on the TAC. The specific surface area was 643.8 m² g⁻¹ for TAC, 289 m² g⁻¹ for MAC-1, and 303.5 m² g⁻¹ for MAC-2. The industrially friendly nanocomposite was applied as an adsorbent for pollutant removal from aqueous solution.     

Sol–gel derived nanocrystalline ZnO photoanode film for dye sensitized solar cells

Nanocrystalline ZnO was synthesized from zinc (II) acetate/oxalate mixture using a facile sol–gel synthesis and is characterized by techniques such as powder XRD, FTIR and Raman spectroscopy, TEM and SEM. The TEM and SEM study showed that the nanocrystalline ZnO powder and film have an average particle size of 25 nm. This material has been successfully applied as photoanodes in dye sensitized solar cells (DSCs) constructed with standard N719 dye and conventional iodide/triiodide (I⁻/I₃⁻) electrolytes. A systematic investigation of the performance of DSCs with film thickness and dyeing time had also been carried out. Among the five different film thicknesses 4, 8, 12, 16 and 20 μm prepared, the best result was obtained for the film thickness of 16 μm for 2 h dying showing an efficiency of 2.2% with a J_SC of 4.7 mA cm⁻² and a very high fill factor of >73%.     

Organic photovoltaics with V2O5 anode and ZnO nanoparticles cathode buffer layers

We report the hybrid inorganic–organic photovoltaics incorporating vanadium pentoxide (V₂O₅) as hole and zinc oxide (ZnO) nanoparticles (NPs) as electron extraction layers. This device demonstrates high open circuit voltage of about 0.89 V with considerably high short-circuit current density of 10.13 mA/cm² along with fill factor of about 61.03%. Combining all these parameters, the power conversion efficiency is 5.53% which is higher compared to that (3.6%) of the cell without ZnO NPs.     

Sulfur stoichiometry driven chalcopyrite and pyrite structure of spray pyrolyzed Cu-alloyed FeS2 thin films

We report on fabrication of Cu_xFe_1−xS₂ (CFS) thin films using chemical spray pyrolysis followed by post-sulfurization. Post-sulfurized CFS films were grown with compact and good crystalline texture. The sulfur stoichiometry in CFS films was found to be crucial for determination of its crystal structure. The sulfur deficient CFS films were driven to chalcopyrite CFS (CH-CFS) structure whereas the sulfur cured CFS films were grown with Cu-incorporated pyrite CFS (P-CFS) structure which was confirmed by X-ray diffraction and Raman spectroscopy analysis along with UV–vis spectroscopy measurement. Electrical characterizations of both types of CFS films revealed p-type conductivity with carrier concentration in the range of 10¹⁸–10²⁰ cm⁻³ and mobility of 0.5–9 cm² V⁻¹ s⁻¹. The band gaps of CFS films of CH-CFS structure (0.885–0.949 eV) were found to be less than that of P-CFS structure (0.966–1.156 eV), which indicates its potential application for thermoelectric and photovoltaic devices.     

Post-annealing modification in structural properties of ZnO thin films on p-type Si substrate deposited by evaporation

Zinc oxide (ZnO) films of thickness ∼380 nm were deposited on p-type Si (1 1 1) substrate maintained at 300 °C under 3×10⁻⁶ Torr by a radio frequency (RF) heating source. Transmission Fourier transform infrared (FTIR) spectrum exhibited a clear Zn–O bond excitation frequency of ∼408 cm⁻¹. X-ray diffraction spectrum demonstrated four peaks (P₁–P₄) at 2θ (deg) ∼36±0.06, 40±0.09, 82±0.17 and 86±0.2, which originated from (1 0 0), (0 0 2), (2 0 1) and (0 0 4) hexagonal planes, respectively. P₂ being the highest intensity peak indicated that the growth of ZnO predominantly occurred along the c-axis i.e. (0 0 2) plane. Micrographs of the samples obtained from scanning electron microscopy (SEM) and atomic force microscopy (AFM) identically displayed scattered nanocrystallites, which grew bigger with the increase of sample annealing temperature (°C) in the range of 400–1000. AFM pictures, in particular, exposed the hexagonal structure of the deposited films along with voids. However, ZnO composition ∼6:1 (Zn:O) as calculated from the energy dispersive spectrum (EDS) revealed that the formation of ZnO was not stoichiometric, rather of Zincsuboxide structure ZnO_x (x<1). Arrhenius plot of the resistivity data yielded a donor level (zinc interstitial and/or Zn–on–O site) with ionization energy E_c–1.26 eV, thereby it supports our measured results, in general.