Effect of annealing temperatures on properties of sol‐gel grown ZnO‐ZrO2 films

Mixed ZnO‐ZrO₂ films have been obtained by sol‐gel technology. By using spin coating method, the films were deposited on Si and glass substrates. The influence of thermal annealings (the temperatures vary from 400 °C to 750 °C) on their structural properties has been studied. The structural behavior has been investigated by the means of XRD and FTIR techniques. The results revealed no presence of mixed oxide phases, the detected crystal phases were related to the hexagonal ZnO and to crystalline ZrO₂. The sol‐gel ZnO‐ZrO₂ films showed polycrystalline structure with a certain degree of an amorphous fraction. The optical transmittance reached 91% and it diminished with increasing the annealing temperatures. The optical properties of the sol‐gel ZnO‐ZrO₂ films, deposited on glass substrates are excellent with high transparency and better then those of pure ZrO₂ films, obtained at similar technological conditions. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and growth mechanism of ZnO rod‐like nanostructures by a microwave‐assisted low‐temperature aqueous solution route

A zinc oxide (ZnO) nanoarray (rod‐like nanostructure) was successfully synthesized through a low‐temperature aqueous solution and microwave‐assisted synthesis using zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetramine (HMTA) as raw materials, and using FTO glass as substrate. The effects of parameters in the preparation process, such as solution concentration, reaction temperature and microwave power, on the morphology and microstructure of ZnO nanoarray were studied. Phase structure and morphology of the products were characterized by X‐ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results indicated that hexagonal wurtzite structure ZnO nanoarray with good crystallization could be prepared through a low‐temperature solution method. When the concentration of the mixed solution was 0.05 M, the reaction temperature was 95 °C, and the reaction time was 4 h, high‐density ZnO regular nanorods of 200 nm diameter were obtained. A possible mechanism with different synthesis methods and the influence of microwave processing are also proposed in this paper.     

Structural and morphological studies on Mn substituted ZnO nanometer‐sized crystals

Mn substituted ZnO nanocrystals synthesized by a co‐precipitation method. X‐ray diffraction (XRD) studies confirms the presence of wurtzite (hexagonal) crystal structure similar to un doped ZnO, suggesting that doped Mn ions go at the regular Zn sites. The lattice parameters a and c are increasing with increasing Mn content. The unit cell volume increases with increasing Mn concentration, indicating the homogeneous substitution of Mn²⁺ for the Zn²⁺. The lattice distortion parameter (ε_v) is evaluated from XRD data and found that it enhances as Mn content increases. Transmission electron microscopy photographs show that the size of the ZnO crystals is in the range of 20‐50 nm. The SAED pattern confirms the hexagonal and crystalline nature of the samples which are in agreement with X‐ray analysis. The chemical groups of the samples have been identified by FTIR studies (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of microscopic defects on optical damage resistance of Zn:Fe:LiNbO3 crystals

Influence of defect structure on the infrared transmission spectra of OH^‐ in Zn:Fe:LiNbO₃ crystals with various ZnO concentration and different Li/Nb ratios was investigated. It indicates that above the Zn concentration threshold the OH^‐ absorptions bands successively shift from 3482cm^‐1 to 3504cm^‐1 and 3529cm^‐1. The intensity of the 3504cm^‐1 band increases with ZnO concentration increasing. The optical damage resistance of the Zn:Fe:LiNbO₃ crystals increases rapidly when the ZnO concentration exceeds a threshold value. This result contributed to the site alteration from the Li sites to Nb sites due to Zn‐doping in crystal. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Synthesis and optical properties of ZnO nanorods

ZnO nanorods were prepared on the silicon (100) substrates using the chemical solution deposition method (CBD) without catalyst under a low temperature (90°C). The cool water was used to dissolve the mixture of zinc nitrate hexahydrate (Zn (NO₃)₂·6H₂O) and methenamine (C₆H₁₂N₄) in order to decrease the size of ZnO nanorods. From the X‐ray diffraction (XRD) results, it can be seen that the growth orientation of the as‐prepared ZnO nanorods was (002). Scanning electron microscopy (SEM) results illustrated that the nanorods had a hexagonal wurzite structure and average diameter of about 120nm. The average diameter of nanorods prepared by the cool water process was much smaller than that by the room‐temperature (RT) water process we always used. Photoluminescence (PL) measurements were also carried out. The result showed that a blue shift in UV emission band appeared in the PL spectrum of the sample grown with cool water process, which was mainly due to the reduction of tensile strain when the diameter of the ZnO nanorods decreased. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Facile synthesis of pencil‐like ZnO nanostructures at low temperature

Pencil‐like ZnO nanostructure was synthesized by directly oxidizing granular Zn films, which was thermal deposited in a nitrogen atmosphere from Zn powder in a horizontal tube furnace. The formation of the pencil‐like structure, including a hexagonal rod and a sharp tip with diameter about 60 nm, highly depend on the thickness of the initial zinc film and the temperature of the oxidizing process. ZnO nanorods were formed in a relatively low temperature, while thicker zinc film was apt to form a dense ZnO film with tubular structures. The different structured ZnO materials showed distinguishing optical properties which indicate the intrinsic defects forming in the different growth conditions. The pencil‐like ZnO structures exhibit a relatively strong green emission attributed to the high concentrations of oxygen vacancies and its taper tip has great prospects in field‐emission devices.     

Growth and optical damage properties of In:Zn:LiNbO3 waveguide substrate

In:Zn:LiNbO₃ crystals doped with different indium concentrations were grown by Czochralski technique. The optical damage threshold value and ultraviolet‐visible absorption spectra of the In:Zn:LiNbO₃ crystals were measured. The In:Zn:LiNbO₃ crystals were made into optical waveguide substrates using hexanedioic acid as proton exchange agent. The optical damage resistant ability of those optical waveguide substrates was investigated by the m‐line method. The optical damage threshold values of In(2mol.%):Zn(3mol.%):LiNbO₃ crystal and optical waveguide substrate are two orders of magnitude higher than those of pure LiNbO₃. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical absorption spectra of LiNbO3, Fe:LiNbO3, and Zn:Fe:LiNbO3 single crystals grown by Bridgman method

The optical absorption spectra of LiNbO₃ (LN), Fe:LiNbO₃ (Fe:LN), and Zn:Fe:LiNbO₃ (Zn:Fe:LN) single crystals grown by Bridgman method were measured and compared. The absorption characteristics of the samples and the effects of growth process conditions on the absorption spectra were investigated. The Fe, Zn and Li concentrations in the crystals were analyzed by inductively coupled plasma (ICP) spectrometry. The results indicated that the overall Fe ion and Fe²⁺ concentration in Fe:LN and Zn:Fe:LN crystals increased along the growing direction. The incorporation of ZnO in Fe:LN crystal induced increase of Fe²⁺ in the crystal. Among Fe‐doped and Zn:Fe‐codoped LN single crystals, 3 mol% ZnO doped Fe:LN had a biggest change of Fe²⁺ ion concentration from bottom to top part of crystal. The effects of technical conditions (atmosphere and thermal history) on Fe²⁺ ion concentration were discussed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of radial‐like ZnO structure by hydrothermal method with ZnSO4·7H2O and Zn(CH3COO)2·2H2O as zinc sources

Radial‐like ZnO structures were prepared using zinc sulfate (ZnSO₄·7H₂O) and zinc acetate [Zn(CH₃COO)₂·2H₂O] as zinc sources by a facile template‐free hydrothermal method in this paper. Structural and optical properties of radial‐like ZnO structures are characterized by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV‐vis spectrophotometer and photoluminescence measurement (PL). It has been found that the distinct surface morphologies of radial‐like ZnO structures grown by different zinc sources. Slim radial‐like ZnO with a hexagonal wurtzite structure is grown by using ZnSO₄·7H₂O as zinc sources, whereas coarse radial‐like ZnO with zincite structure is achieved by zinc acetate. The UV‐vis absorption spectra of them both display an obvious and significant absorption in the ultraviolet region. The room temperature PL spectra of ZnO structures grown by two different zinc sources possess a common feature that consists of a strong ultraviolet (UV) peak and visible emission band.     

Synthesis of ZnO nanostructures composed of nanosheets with controllable morphologies

ZnO nanostructures composed of nanosheets have been synthesized by a facile low temperature reaction of Zn(OH)₂ and NaOH without the aid of any organic molecular templates. The influences of the reaction parameters, such as the concentrations of Zn(NO₃)₂, reaction temperatures, and reaction time on the morphologies of ZnO have been investigated. The thickness of ZnO nanosheets can be adjusted from 10–20 nm to 30–40 nm by altering the reaction temperatures from 80 °C to 180 °C. ZnO nanosheets are single crystals and the growth direction is perpendicular to [1100]. A possible gradual nucleation – rapid growth formation mechanism of ZnO nanosheets is proposed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterisation of Zn:LiNbO3/Mg:LiNbO3 multilayer thin films grown by liquid phase epitaxy

1, 3 and 5 mol% ZnO doped LiNbO₃ film and 2 mol% MgO doped LiNbO₃ multilayer films were grown on the LiNbO₃ (001) substrate by liquid phase epitaxy (LPE) method with a Li₂O‐V₂O₅ system. We examined the optical transmission spectra of the Zn:LiNbO₃ by Fourier Transform‐Infrared Spectrophotometer (FT‐IR). The crystallinity and the lattice mismatch between the Zn:LiNbO₃ film and Mg:LiNbO₃ film was confirmed by x‐ray rocking curve (XRC) and observed the ZnO and MgO distribution in the cross‐section of the multilayer thin films by electron probe micro analyzer (EPMA). Furthermore, the surface morphology of the films was observed using atomic force microscopy (AFM). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical investigations on the existence of phase transition in ZnO:Li thin films prepared by DC sputtering method

We investigated the effect of temperature on the absorption spectra of Zn_0.8Li_0.2O thin films (ZnO:Li), deposited at 573 K, in the wavelength range 190‐800 nm. The films were deposited on sapphire, MgO or quartz substrates by DC sputtering method. The results show a shift of the optical energy gap (E_g), with direct allowed transition type near the fundamental edge, to lower wavelengths as the temperature increases. The temperature rate of E_g changes considerably showing an anomaly around 320 K depending on type of substrate. The founded results indicated that replacement of Zn ions with Li ions induces a ferroelectric phase in the ZnO wurtzite‐type semiconductor. The exponential dependence of the absorption coefficient on the incident photon energy suggests the validity of the Urbach rule. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microstructure and optical properties of Zn:Mn:Fe:LiNbO3 crystals

Zn:Mn:Fe:LiNbO₃ crystals were prepared by Czochralski technique. Its microstructure was measured and analyzed by UV‐Vis absorption spectra. The optical damage resistance of Zn:Mn:Fe:LiNbO₃ crystals was characterized by the transmitted beam pattern distortion method. It increases remarkably when the concentration of ZnO is over a threshold concentration. Its value in Zn(7.0 mol%):Mn:Fe:LiNbO₃ crystal is about three orders of magnitude higher that in the Mn:Fe:LiNbO₃ crystal. The dependence of the defects on the optical damage resistance was discussed. The non‐volatile holographic storage was realized in all crystals, and the sensitivity of the Zn(7.0 mol%):Mn:Fe:LiNbO₃ crystal is much higher than that of others. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature dependence of growth orientation and surface morphology of Li‐doped ZnO thin films on SiO2/Si substrates

ZnO thin films doped with Li (ZnO:Li) were deposited onto SiO₂/Si (100) substrates by direct‐current sputtering technique in the temperature range from room temperature to 500 °C. The crystalline structure, surface morphology and composition, and optical reflectivity of the deposited films were studied by X‐ray diffraction (XRD), Scanning Electron Microscopy (SEM), X‐ray Photoelectron Spectroscopy (XPS) and optical reflection measurements. Rough surface p‐type ZnO thin film deposition was confirmed. The results indicated that the ZnO:Li films growed at low temperatures show c‐axis orientation, while a‐axis growth direction is preferable at high temperatures. Moreover, the optical reflectivity from the surface of the films matched very well with the obtained results. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fast synthesis of Cu‐doped ZnO nanosheets at ambient condition

A new, fast and low cost method to produce Cu‐doped ZnO nanosheets is reported for the first time in this paper. Zinc foil specimens were immersed into CuSO₄ aqueous solutions with various concentrations for 3 seconds and then dried at ambient condition. The immersed specimens were characterized with a scanning electron microscope, an X‐ray diffractometer and a transmission electron microscope. The results show that Cu‐doped ZnO nanosheets with a multilayer structure on a cupper layer are formed. Cu‐doped ZnO nanosheets show hexagonal crystalline structure and comprises polycrystalline grains with diameters of 5∼10 nm. A physical modal is suggested to explain the prepared Cu‐doped ZnO nanosheet structure, based on the chemical reactions and a metallurgical cell.     

Effect of substrate temperature on the properties of vacuum evaporated indium selenide thin films

Thin films of InSe were obtained by thermal evaporation techniques on glass substrates maintained at various temperatures (T_sb = 30°, 400°C). X‐ray diffraction analysis showed the occurrence of amorphous to polycrystalline transformation in the films deposited at higher substrate temperature (400°C). The polycrystalline films were found to have a hexagonal lattice. Compositions of these films have been characterized by EDAX and the surface analysis by scanning electron microscopy. Optical properties of the films, investigated by using spectrophotometer transmittance spectra in the wavelength range (300 – 1100 nm), were explained in terms of substrate temperatures. Films formed at room temperature showed an optical band gap (E_g^opt) 1.56 eV; where as the films formed at 400°C were found to have a E_g^opt of 1.92 eV. The increase in the value of E_g^opt with T_sb treatment is interpreted in terms of the density of states model as proposed by Mott and Davis. The analysis of current ‐Voltage characteristics, based on space charge limited currents (SCLC) measurements, confirms the exponential decrease of density of states from the conduction band edge towards the Fermi level for both the amorphous and polycrystalline films. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Morphology and photoluminescence properties of ZnO nanostructures fabricated with different given time of Ar

ZnO nanostructures were grown on Au‐coated Si (100) substrates by carbonthermal reduction method with the help of Ar at the beginning of growth. The structural and optical properties of ZnO nanostructures strongly depended on the supply time of Ar. When the given time of Ar gas current was 90s, sample was ZnO nanowires with hexagonal morphology. The Raman spectroscopy revealed the low level of oxygen vacancies and Zn interstitials in samples. Room temperature photoluminescence (PL) spectra exhibited the intensity of green emission increased on the condition of rich oxygen (decrease given time of Ar) and the nanowire had strongest intensity of UV emission compared with other nanostructures. Green emission is ascribed to the electron transition from the bottom of the conduction band to the antisite defect O_Zn level. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth,electrical and optical behaviour of nanocrystalline Ag2Cu2O3 films produced by RF magnetron sputtering

Thin films of Ag₂Cu₂O₃ were formed on glass substrates by RF magnetron sputtering technique under different oxygen partial pressures in the range 5 × 10^‐3 – 8 × 10^‐2 Pa using mosaic target of Ag₇₀Cu₃₀. The influence of oxygen partial pressure on the core level binding energies, crystallographic structure, and electrical and optical properties of the deposited films was studied. The atomic ratio of copper to silver in the films was 0.302. The oxygen content was in correlation with the oxygen partial pressure maintained during the growth of the films. The films formed at oxygen partial pressures < 2 × 10^‐2 Pa was mixed phase of Ag₂Cu₂O₃ and Ag. The films deposited at 2 × 10^‐2 Pa were single phase of Ag₂Cu₂O₃. The crystallite size of the films formed at 2 × 10^‐2 Pa was 12 nm, while those films annealed at 473 K was 16 nm. The nanocrystalline Ag₂Cu₂O₃ films formed at oxygen partial pressure of 2 × 10^‐2 Pa showed electrical resistivity of 8.2 Ωcm and optical band gap of 1.95 eV. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of ZnO nanorods grown on Si substrates coated with NiCl2

ZnO nanorods were synthesized on NiCl₂‐coated Si substrates via a chemical vapor deposition (CVD) process. The as‐fabricated nanorods with diameters ranging from 150 nm to 200 nm and lengths up to several tens of micrometers grew preferentially arranged along [0001] direction, perpendicular to the (0002) plane. The clear lattice fringes in HRTEM image demonstrated the growth of good quality hexagonal single‐crystalline ZnO. Room temperature photoluminescence (PL) spectra illustrated that the ZnO nanorods exhibit strong UV emission peak and green emission peak, peak centers located at 388 nm and 506 nm. A possible growth mechanism based on the study of our X‐ray diffraction (XRD), electron microscopy and PL spectroscopy was proposed, emphasizing the effect of NiCl₂ solution (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation and characterization of ZnO-TiO2 films obtained by sol-gel method

The sol-gel route has been applied to obtain ZnO-TiO₂ thin films. For comparison, pure TiO₂ and ZnO films are also prepared from the corresponding solutions. The films are deposited by a spin-coated method on silicon and glass substrates. Their structural and vibrational properties have been studied as a function of the annealing temperatures (400-750 °C). Pure ZnO films crystallize in a wurtzite modification at a relatively low temperature of 400 °C, whereas the mixed oxide films show predominantly amorphous structure at this temperature. XRD analysis shows that by increasing the annealing temperatures, the sol-gel Zn/Ti oxide films reveal a certain degree of crystallization and their structures are found to be mixtures of wurtzite ZnO, Zn₂TiO₄, anatase TiO₂ and amorphous fraction. The XRD analysis presumes that Zn₂TiO₄ becomes a favored phase at the highest annealing temperature of 750 °C. The obtained thin films are uniform with no visual defects. The optical properties of ZnO-TiO₂ films have been compared with those of single component films (ZnO and TiO₂). The mixed oxide films present a high transparency with a slight decrease by increasing the annealing temperature.