Microstructure and electro-optical properties of sol–gel derived Cd-doped ZnO films

The microstructure, and the electrical and optical properties of undoped zinc oxide (ZnO) and cadmium-doped ZnO (CZO) films deposited by a sol–gel method have been investigated. The films have a polycrystalline structure with hexagonal wurtzite ZnO. Scanning electron microscopy (SEM) images indicated that the films have a wrinkle network with uniform size distributions. The elemental analyses of the CZO films were carried out by energy dispersive X-ray analysis. The fundamental absorption edge changed with doping. The optical band gap of the films decreased with Cd dopant. The optical constants of the films such as refractive index, extinction coefficient and dielectric constants changed with Cd dopant. A two-probe method was used to investigate the electrical properties, and the effect of Cd content on the electrical properties was investigated. The electrical conductivity of the films was improved by incorporation of Cd in the ZnO film.     

Growth of vertically aligned one-dimensional ZnO nanowire arrays on sol–gel derived ZnO thin films

Vertically aligned one-dimensional ZnO nanowire arrays have been synthesized by a hydrothermal method on sol–gel derived ZnO films. Sol–gel derived ZnO films and corresponding ZnO nanowire arrays have been characterized by X-ray diffraction and field-emission scanning electron microscopy. The effect of sol–gel derived ZnO film surface on the morphology of ZnO nanowire arrays has been investigated. The authors suggest from our investigation that sol–gel derived ZnO films affect the growth of one-dimensional ZnO nanostructures. Not only crystalline ZnO films but also amorphous ones can act as a scaffold for ZnO nucleus. Tilted ZnO micro-rods are grown on ZnO gel films, whereas vertically aligned ZnO nanowire arrays are grown on nanometer-sized ZnO grains. The average diameter of ZnO nanowire arrays are correlated strongly with the grain size of sol–gel derived ZnO films.     

Effect of dopants on the structural,optical and electrical properties of sol–gel derived ZnO semiconductor thin films

Undoped, Ga-, In-, Zr-, and Sn-doped ZnO transparent semiconductor thin films were deposited on alkali-free glasses by sol–gel method. 2-methoxyethanol (2-ME) and diethanolamine (DEA) were chosen as a solvent and a stabilizer, respectively. The doping concentration was maintained at 2 at.% in the impurity doping precursor solutions. The effects of different dopants on the structural, optical, and electrical properties of ZnO thin films were investigated. XRD results show that all annealed ZnO-based thin films had a hexagonal (wurtzite) structure. ZnO thin films doped with impurity elements obviously improved the surface flatness and enhanced the optical transmittance. All impurity doped ZnO thin films showed high transparency in the visible range (>91%). The Ga- and In- doped ZnO thin films exhibited higher Hall mobility and lower resistivity than did the undoped ZnO thin film.     

Effect of the excimer laser irradiation on sol–gel derived tungsten–titanium dioxide thin films

A novel technique based on the excimer laser induced crystallization and modification of TiO₂ thin films is being reported. W⁺⁶ ions loaded TiO₂ (WTO) precursor films were prepared by a modified sol–gel method and spin-coated onto microscopic glass slides. Pulsed KrF (248 nm, 13 ns) excimer laser was used to irradiate the WTO amorphous films at various laser parameters. Mesoporous and nanostructured films consisting of anatase and rutile were obtained after laser irradiation at room temperature. The effect of varying W⁺⁶ ions concentrations on structural and optical properties the WTO films was analyzed by X-ray diffraction, field-emission scanning electron microscope, UV-Vis spectrophotometer and transmission electron microscope before and after laser treatment. Films irradiated for 10 pulses at 65–75 mJ/cm² laser fluence, exhibited anatase whereas higher parameters promoted the formation of rutile. XPS results revealed WO₃ along with minor proportion of WO₂ compounds after laser irradiation. Photo-absorbance of the WTO films was increased with increase in W⁺⁶ ions concentration in the film. TEM results exhibited a crystallite size of 15 nm which was confirmed from SEM results as well.     

Azobenzene-containing small molecules organic–inorganic hybrid sol–gel materials for photonic applications

Azobenzene-containing germania-ormosil hybrid materials are prepared by combining a low-temperature sol–gel technique with a spin-coating process, which can be used for the simple and low cost fabrication of waveguide devices for photonic applications. The planar waveguide and structural properties of the hybrid waveguide films are characterized by a prism coupling technique and Fourier transform infrared spectroscopy. The effects of azobenzene content and heat treatment temperature on the photo-responsive properties of the hybrid films are also studied by photoirradiation with UV light. The results indicate that the azobenzene in hybrid materials can undergo trans–cis–trans photoisomerization efficiently by photoirradiation with UV light, and surface pattern structure induced due to UV light photoirradiation can be easily observed on such azobenzene-doped hybrid materials. Thus, this as-prepared organic–inorganic hybrid sol–gel material shows promising candidates for optical switch applications and allows for directly integrating on a single chip waveguide device with optical data storage and optical switching devices.     

Nanostructures and luminescence properties of porous ZnO thin films prepared by sol–gel process

ZnO thin films containing nano-sized pores were synthesized on solid substrates through a sol–gel process by accommodating cetyl-trimethyl-ammonium bromide (CTAB) as an organic template in the precursor solution. By X-ray diffraction the resultant ZnO films were found to possess ordered pore arrays forming lamellar structure with the spacing between two adjacent pores being ∼3.0 nm. Photoluminescence measurements indicated that the surfactants effectively passivated the surface defects of the ZnO films responsible for the green emission. Al doping was found to improve not only the lamellar structure of the pore arrays but also the near-band-gap emission intensity while the suppression effect of CTAB on the green emission remained undisturbed. With a proper control of doping level, the optical property as well as the structural integrity can be tailored to augment the potential of ZnO films for the optoelectronics and sensor applications.     

Electrical and optical properties of Sb-doped ZnO thin films synthesized by sol–gel method

Sb-doped ZnO thin films with different values of Sb content （from 0 to 1.1 at.%） are deposited by the sol-gel dip- coating method under different sol concentrations. The effects of Sb-doping content, sol concentration, and annealing ambient on the structural, optical, and electrical properties of ZnO films are investigated. The results of the X-ray diffraction and ultraviolet-visible spectroscopy （UV-VIS） spectrophotometer indicate that each of all the films retains the wurtzite ZnO structure and possesses a preferred orientation along the c axis, with high transmittance （〉 90%） in the visible range. The Hall effect measurements show that the vacuum annealed thin films synthesized in the sol concentration of 0.75 mol/L each have an adjustable n-type electrical conductivity by varying Sb-doping density, and the photoluminescence （PL） spectra revealed that the defect emission （around 450 nm） is predominant. However, the thin films prepared by the sol with a concentration of 0.25 mol/L, despite their poor conductivity, have priority in ultraviolet emission, and the PL peak position shows first a blue-shift and then a red-shift with the increase of the Sb doping content.     

Sol–gel synthesis,magnetic and magneto-optical properties of CoFe2−xTbxO4 nanocrystalline films

The nanocrystalline thin films of terbium-doped cobalt ferrite were fabricated by a sol–gel method, and the effects of crystallization conditions on the phase, morphology, magnetic and magneto-optical (MO) properties of products were investigated. Due to its large radius, the doping content, x, of Tb³⁺ ion inside cobalt spinel cannot exceed 0.2. The CoFe_2−xTb_xO₄ films consist of the grains with the average size smaller than 50 nm even annealed up to 800°C. Saturation magnetization, coercive force and MO rotation are strongly dependent on the annealing temperature.     

Sol–gel nonhydrolytic synthesis of a hybrid organic–inorganic electrolyte for application in lithium-ion devices

A new inorganic–organic hybrid electrolyte was synthesized by a nonhydrolytic sol–gel simple route without specific treatment of the reagents. The hybrid ion conductor is prepared with citric acid (CA), tetraethyl-orthosilicate (TEOS) and ethylene glycol (EG), forming polyester chains. The time-consuming drying step, required in most of the chemical syntheses, is not necessary for the preparation of the present hybrid electrolyte, pertaining to the polyelectrolyte class because only Li⁺ is mobile in the polymeric chain. The effect of the concentration of Li is investigated in terms of the Li-ionic conductivity. The new hybrid conductor is shown to be fully amorphous at room temperature with the vitreous transition temperature around 228 K (−45 °C). The material is solid, transparent and displays an ionic conductivity above 10⁻⁵ (Ω cm)⁻¹, besides presenting a great reproducibility of all these characteristics.     

Sol–gel synthesis and optical improvement of novel red-emitting [LiY(1−x)Eux][MoyW(1−y)O4]2 powder for white LED applications

In this work, we report on sol–gel synthesis of red-emitting [LiY_0.88Eu_0.12] [Mo_0.1W_0.9O₄]₂ phosphor and its luminescent properties at room temperature. X-ray diffraction patterns indicate that it presents one main phase with a tetragonal (space group I4₁/a (88)) structure. Luminescent spectra show that the main emission peaks lies at 613.2 nm and 616.2 nm deriving from ⁵D₀→⁷F₂ transition peak splitting due to Mo⁶⁺ and W⁶⁺ concentration waving in the host. All those exhibit the excellent prospect in light emitting diodes application in the future.     

Sol–gel deposition and luminescence properties of lanthanide ion-doped Y2(1-x)Gd2xSiWO8 (0≤x ≤1) phosphor films

Y_2(1-x)Gd_2xSiWO₈:A (0x1; A=Eu³⁺, Dy³⁺, Sm³⁺, Er³⁺) phosphor films have been prepared on silica glass substrates through the sol–gel dip-coating process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), atomic force microscope (AFM), scanning electron microscopy (SEM) and photoluminescence spectra as well as lifetimes were used to characterize the resulting films. The results of the XRD indicated that the films began to crystallize at 800 °C and crystallized completely at 1000 °C. The AFM and SEM study revealed that the phosphor films, which mainly consisted of closely packed grains with an average size of 90–120 nm with a thickness of 660 nm, were uniform and crack free. Owing to an efficient energy transfer from the WO₄^2- groups to the activators, the doped lanthanide ion (A) showed its characteristic f–f transition emissions in crystalline Y_2(1-x)Gd_2xSiWO₈ (0x1) films. The optimum concentrations for Eu³⁺, Dy³⁺, Sm³⁺, Er³⁺ were determined to be 21, 5, 3 and 7 mol % of Y³⁺ in Y₂SiWO₈ films, respectively. The above lanthanide ions showed higher emission intensity for 02(1-x)Gd_2xSiWO₈ films. PACS 73.63.Bd; 78.55.Hx; 78.66.Nk; 81.15.Lm; 81.20.Fw     

Photoluminescence studies of ZnO thin films on R-plane sapphire substrates grown by sol–gel method

Zinc oxide (ZnO) thin films on R-plane sapphire substrates were grown by the sol–gel spin-coating method. The optical properties of the ZnO thin films were investigated using photoluminescence. In the UV range, the asymmetric near-band-edge emission was observed at 300 K, which consisted of two emissions at 3.338 and 3.279 eV. Eight peaks at 3.418, 3.402, 3.360, 3.288, 3.216, 3.145, 3.074, and 3.004 eV, which respectively correspond to the free exciton (FX), bound exciton, transverse optical (TO) phonon replica of FX recombination, and first-order longitudinal optical phonon replica of FX and the TO (1LO+TO), 2LO+TO, 3LO+TO, 4LO+TO, and 5LO+TO, were obtained at 12 K. From the temperature-dependent PL, it was found that the emission peaks at 3.338 and 3.279 eV corresponded to the FX and TO, respectively. The activation energy of the FX and TO emission peaks was found to be about 39.3 and 28.9 meV, respectively. The values of the fitting parameters of Varshni's empirical equation were α=4×10^?3 eV/K and β=4.9×10³ K, and the S factor of the ZnO thin films was 0.658. With increasing temperature, the exciton radiative lifetime of the FX and TO emissions increased. The temperature-dependent variation of the exciton radiative lifetime for the TO emission was slightly higher than that for the FX emission.     

Optical band gap engineering of ZnO nanophosphors via Cu incorporation for ultraviolet–violet LED

The European Physical Journal Plus - For the Earth, water is at the core of sustainable development and at the heart of adaptation to climate change. For the Enceladus, the sixth-largest moon of...     

Fabrication and characterization of Al–Mn superconducting films for applications in TES bolometers

Superconducting transition edge sensor(TES) bolometers require superconducting films to have controllable transition temperatures T_c in different practical applications.The value of T_c strongly affects thermal conductivity and thermal noise performance of TES detectors.Al films doped with Mn(Al-Mn) of different concentrations can accomplish tunable T_(c.)A magnetron sputtering machine is used to deposit the Al-Mn films in this study.Fabrication parameters including sputtering pressure and annealing process are studied and their influences on T_c and superconducting transition width ΔT_c are optimized.The Al-Mn films with ΔT_c below 1.0 mK for T_c in a range of 520 mK-580 mK are successfully fabricated.     

Nanostructured ZnO thin films by chemical bath deposition in basic aqueous ammonia solutions for photovoltaic applications

This paper presents further insights and observations of the chemical bath deposition (CBD) of ZnS thin films using an aqueous medium involving Zn-salt, ammonium sulfate, aqueous ammonia, and thioure. Results on physical and chemical properties of the grown layers as a function of ammonia concentration are reported. Physical and chemical properties were analyzed using scanning electron microscopy (SEM), X-ray energy dispersive (EDX), and X-ray diffraction (XRD). Rapid growth of nanostructured ZnO films on fluorine-doped SnO₂ (FTO) glass substrates was developed. ZnO films crystallized in a wurtzite hexagonal structure and with a very small quantity of Zn(OH)₂ and ZnS phases were obtained for the ammonia concentration ranging from 0.75 to 2.0 M. Flower-like and columnar nanostrucured ZnO films were deposited in two ammonia concentration ranges, respectively: one between 0.75 and 1.0 M and the other between 1.4 and 2.0 M. ZnS films were formed with a high ammonia concentration of 3.0 M. The formation mechanisms of ZnO, Zn(OH)₂, and ZnS phases were discussed in the CBD process. The developed technique can be used to directly and rapidly grow nanostructured ZnO film photoanodes. Annealed ZnO nanoflower and columnar nanoparticle films on FTO substrates were used as electrodes to fabricate the dye sensitized solar cells (DSSCs). The DSSC based on ZnO-nanoflower film showed an energy conversion efficiency of 0.84%, which is higher compared to that (0.45%) of the cell being constructed using a photoanode of columnar nanoparticle ZnO film. The results have demonstrated the potential applications of CBD nanostructured ZnO films for photovoltaic cells.     

Er3+-doped sol–gel SnO2 for optical laser and amplifier applications

Erbium-doped tin dioxide (SnO₂:Er³⁺) was obtained by the sol–gel method. Spectroscopic properties of the SnO₂:Er³⁺ are analyzed from the Judd–Ofelt (JO) theory. The JO model has been applied to absorption intensities of Er³⁺ (4f¹¹) transitions to establish the so-called Judd–Ofelt intensity parameters: Ω₂, Ω₄, and Ω₆. With the weak spectroscopic quality factors Ω₄/Ω₆, we expect a relatively prominent infrared laser emission. The intensity parameters are used to determine the spontaneous emission probabilities of some relevant transitions, the branching ratios, and the radiative lifetimes of several excited states of Er³⁺. The emission cross section (1.31×10^-20 cm²) is evaluated at 1.54 μm and was found to be relatively high compared to that of erbium in other systems. Efficient green and red up-conversion luminescence were observed, at room temperature, using a 798-nm excitation wavelength. The green up-conversion emission is mainly due to the excited state absorption from ⁴ I _11/2, which populates the ⁴ F _3/2,5/2 states. The red up-conversion emission is due to the energy transfer process described by Er³⁺ (⁴I_13/2)+Er³⁺(⁴I_11/2)→Er³⁺(⁴F_9/2)+Er³⁺ (⁴ I _15/2) and the cross-relaxation process. The efficient visible up-conversion and infrared luminescence indicate that Er³⁺-doped sol–gel SnO₂ is a promising laser and amplifier material. PACS 71.20.Eh; 74.25.Gz; 78.55.-m     

Band gap control and photoluminescence properties of Ba(Co2x Ti1−x )O3 thin films prepared by Sol–gel method

This paper reports on the preparation, characterization and optical properties of transparent Ba(Co_2x Ti_1?x )O₃ (0 ≤ x ≤ 0.06) thin films prepared by sol–gel method and deposited on fused quartz substrate by spin-coating technique. Their formation is confirmed by X-ray diffraction patterns, energy dispersive X-ray spectrometry and Fourier transformed infrared measurements. Hitherto unreported near-band-gap photoluminescence in ultraviolet, at 378 nm (3.28 eV), of exciton origin is observed which remains unaffected with change in excitation wavelength from 320 to 350 nm. A weak defect emission appears in green region. For larger excitation wavelength, i.e., 488 nm, emission arising from localized states again occurs in green region but with lower energy. The occurrence of efficient violet–blue PL emission is related to ‘direct’ band gap and shallow levels with high optical band gap values. Analysis of band gap variation with dopant concentration, determined using Tauc’s plot assuming them both of ‘direct’ and ‘indirect’ nature, also indicates the ‘direct’ nature. Co⁺² ions as dopants promote a decrease of band gap of films linearly. Scanning electron micrographs show the granular and flakes-like surface growth. Atomic force microscopy images show the presence of ribbon-like nanostructured grains throughout the surface of the films which is smooth with small values of surface roughness.