Al-doped and pure ZnO thin films elaborated by sol–gel spin coating process for optoelectronic applications

Pure and aluminum-doped zinc oxide thin films were grown by spin coating at room temperature. As a starting material, zinc acetate was used. The dopant source was aluminum nitrate; the dopant molar ratio was varied between 1 and 10%. Structural analysis reveals that all films consist of single hexagonal wurtzite phase ZnO, and a preferential orientation along c-axis. They have a homogeneous surface. The measurements show that the films are nanostructured. The transmittance is greater than 75% in the visible region. The band gap energy decreases with the addition of dopant (Al) in prepared thin films and the resistivity decreases significantly.     

Optical,structural, and electrochromic properties of cobalt oxide films prepared via the sol–gel route

Cobalt oxide films were prepared by the sol–gel dip-coating method on glass and conducting (fluorine-doped tin oxide) glass substrates for electrochromic applications. XRD studies revealed that coated films comprising two to 10 layers were amorphous. FTIR spectra exhibited peaks attributable to Co–O vibrations. SEM images revealed the formation of nanopores in the surface of four-layered films. The electrochromic properties of the films were studied using cyclic voltammetry and in situ spectroelectrochemical techniques. The four-layered CoO film exhibited the highest anodic diffusion coefficient among all the films (3.50×10^?13 cm²/s) and optical transmission variation of 26%.     

Effect of annealing atmosphere on the electrical properties of nickel oxide/zinc oxide p–n junction grown by sol–gel technique

Zinc oxide (ZnO) and nickel oxide (NiO) thin films were prepared on glass substrates by a sol–gel method. Spin coating was used to fabricate a p-NiO/n-ZnO junction. The influence of the post annealing atmosphere (air or nitrogen) on the microstructure and surface morphology of NiO and ZnO thin films and the p-NiO/n-ZnO junction are examined. The structural properties are characterized by X-ray diffraction (XRD) and the surface morphology of the thin films and the p–n junction are investigated by atomic force microscopy (AFM). Optical properties are investigated by UV–visible spectroscopy and the electrical properties, such as I–V photocurrent, are characterized by a voltage source meter instrument. XRD patterns show that the films are polycrystalline with preferred orientation in the (002) direction for the ZnO films and the (200) direction for the NiO films. The AFM results indicate that the morphology of the ZnO and NiO films and the p-NiO/n-ZnO junction are mainly influenced by the annealing atmosphere. All films have a high optical transmittance of about 80% in the visible region and a sharp absorption edge. The optical band gaps of the two materials change with the annealing atmosphere (air or nitrogen). The p-NiO/n-ZnO heterojunction device has an average transmittance of over 80% in the visible region, which lies between the transmittance of the ZnO and NiO films separately. The ideality factor, barrier height, and series resistance of the heterojunction treated in different annealing atmospheres are determined by using conventional forward bias I–V characteristics and also Norde׳s and Cheung׳s methods.     

Structural,optical and electrical properties of Fe-doped SnO2 fabricated by sol–gel dip coating technique

Nanosized Fe³⁺-doped SnO₂ thin film was prepared by the sol–gel dip coating (SGDC) technique on quartz class substrate and sintered at 800 °C. The microstructures, surface morphology and optical properties of these films were then characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical absorption measurements, respectively. Electrical properties were analyzed, and resistivity, type and number of carrier concentration, Hall mobility measured as a function of Fe³⁺ doping and temperature. The XRD spectrum shows the decrease in peak heights as a result of Fe³⁺-doping while SEM images reveal reduction in crystallite size with increase in Fe³⁺ content. The optical studies showed a direct band gap reducing with increase in Fe³⁺-doping from 3.87 to 3.38 eV. From the electrical measurements, it was found that the resistivity initially increased with Fe³⁺-doping before reducing at higher doping level. Hall mobility measurements showed n-type conductivity at lower Fe³⁺-doping levels and p-type at higher levels. The increase in conductivity with temperature ascertained the semiconducting behavior of these films.     

Characterization of ZrTiO4 thin films prepared by sol–gel method

The electrical properties, memory switching behavior, and microstructures of ZrTiO₄ thin films prepared by sol–gel method at different annealing temperatures were investigated. All films exhibited ZrTiO₄ (111) and (101) orientations perpendicular to the substrate surface, and the grain size increased with increasing annealing temperature. A low leakage current density of 1.47×10^?6 A/cm² was obtained for the prepared films. The I–V characteristics of ZrTiO₄ capacitors can be explained in terms of ohmic conduction in the low electric field region and Schottky emission in the high electric field region. An on/off ratio of 10² was measured in our glass/ITO/ZrTiO₄/Pt structure with an annealing temperature of 600 °C. Considering the primary memory switching behavior of ZrTiO₄, ReRAM based on ZrTiO₄ shows promise for future nonvolatile memory applications.     

Optical properties of electrochemically synthesized polypyrrole thin films:the electrolyte effect

Polypyrrole thin films are prepared by the potentiostatic mode of electrodeposition at ＋0.7 V versus a saturated calomel electrode （SCE）. The polypyrrole films are prepared in the presence of different electrolytes such as： p-toluene sulphonic acid （PTS）, oxalic acid and H2SO4. The prepared films are characterized by UV- vis absorption spectroscopy and normal reflectance measurements. The electrochemically synthesized films are semiconductor in nature. The band gap energy ofpolypyrrole thin films is found to be 1.95, 1.92 and 1.79 eV for H2 SO4, oxalic acid and p-toluene sulphonic acid, respectively. The normal reflectance spectroscopy of polypyrrole films shows that the maximum reflectance is in the presence of p-toluene sulphonic acid; this is may be due to a more distinct microstructure than the others. The optical constants such as the extinction coefficient, refractive index, optical conductivity, etc. are calculated and studied with various electrolytes.     

Properties of fluorine-doped SnO2 thin films by a green sol–gel method

Fluorine doped tin oxide (FTO) films were fabricated on a glass substrate by a green sol–gel dip-coating process. Non-toxic SnF₂ was used as fluorine source to replace toxic HF or NH₄F. Effect of SnF₂ content, 0–10 mol%, on structure, electrical resistivity, and optical transmittance of the films were investigated using X-ray diffraction, Hall effect measurements, and UV–vis spectra. Structural analysis revealed that the films are polycrystalline with a tetragonal crystal structure. Grain size varies from 43 to 21 nm with increasing fluorine concentration, which in fact critically impacts resultant electrical and optical properties. The 500 °C-annealed FTO film containing 6 mol% SnF₂ shows the lowest electrical resistivity 7.0×10⁻⁴ Ω cm, carrier concentration 1.1×10²¹ cm⁻³, Hall mobility 8.1 cm²V⁻¹ s⁻¹, optical transmittance 90.1% and optical band-gap 3.91 eV. The 6 mol% SnF₂ added film has the highest figure of merit 2.43×10⁻² Ω⁻¹ which is four times higher than that of un-doped FTO films. Because of the promising electrical and optical properties, F-doped thin films prepared by this green process are well-suited for use in all aspects of transparent conducting oxide.     

Structural and optical properties of (Al,K)-co-doped ZnO thin films deposited by a sol–gel technique

Thin films of Al-doped ZnO (AZO) and (Al, K)-co-doped ZnO (AKZO) were synthesized by sol–gel spin coating and their structural and optical properties were investigated. All the films had a preferential orientation in which the c-axis was perpendicular to the substrate. The optical bandgap increased after Al doping, but decreased after K doping at a given Al doping concentration. UV emission and a broad visible emission band were observed in photoluminescence (PL) spectra. The intensity of both emission bands decreased after Al and K co-doping. PL excitation (PLE) spectra of the blue emission band indicate that the initial state is possibly the same for all the samples and a similar case occurs for the orange–red emission band. The green emission can be attributed to electronic transitions involving oxygen vacancies. A possible process for the orange–red emission of the thin films is radiative recombination of an electron trapped in a zinc interstitial defect with a hole deeply trapped in interstitial oxygen.     

Effect of annealing and electrochemical properties of sol–gel dip coated nanocrystalline V2O5 thin films

Nanocrystalline vanadium pentoxide (V₂O₅) thin films were deposited on glass substrates by a simple and cost effective sol–gel dip coating method. The effect of annealing on microstructure and optical properties of V₂O₅ thin films were investigated. Formation of nanorods with the average diameter of 500–750 nm after annealing is observed by scanning electron microscopy. X-ray diffractometry indicates that an orthorhombic structured thin film is transformed to β-V₂O₅ nanorods by subsequent annealing at 500 °C. It was also confirmed that the growth of nanorods strongly correlates with annealing conditions; nanorod formation can be explained by surface diffusion phenomenon. The electrochemical performance of the V₂O₅ nanorods was investigated by cyclic voltammetry.     

Influence of Pb doping on the structural,morphological and optical properties of sol–gel ZnO thin films

In this work, undoped and lead (Pb)-doped ZnO thin films were deposited on glass substrate using the sol–gel dip-coating process. The effects of different Pb doping concentrations on the structural, morphological, optical, electrical and photoluminescence properties of such films were investigated by X-ray diffraction (XRD), energy-dispersive X-rays (EDS), atomic force microscopy (AFM), UV–vis–NIR spectrophotometry, Hall effect measurement and Photoluminescence (PL) spectroscopy. XRD patterns of the synthesized films exhibited hexagonal wurtzite crystal structure with a c-axis preferred (002) orientation. AFM images showed that film morphology and surface roughness were influenced by the Pb doping level. Incorporation of Pb was confirmed from elemental analysis using EDS. The UV–vis–NIR spectroscopy characterizations demonstrated that all the films were highly transparent with average visible transmission values ranging from 75% to 85%. Electrical measurement shows that carrier concentration and resistivity are dependent on Pb content. Room temperature PL spectra clearly indicated a great dependence of the UV, green and red emissions on the Pb concentration. In particular, the red emission at 2 eV is quenched after introduction of Pb atoms.     

Spectroscopic studies of sol–gel grown CdS nanocrystalline thin films for optoelectronic devices

CdS is one of the highly photosensitive candidate of II–VI group semiconductor material. Therefore CdS has variety of applications in optoelectronic devices. In this paper, we have fabricated CdS nanocrystalline thin film on ultrasonically cleaned glass substrates using the sol–gel spin coating method. The structural and surface morphologies of the CdS thin film were investigated by X-ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM) respectively. The surface morphology of thin films showed that the well covered substrate is without cracks, voids and hole. The round shape particle has been observed in SEM micrographs. The particles sizes of CdS nanocrystals from SEM were estimated to be～10–12 nm. Spectroscopic properties of thin films were investigated using the UV–vis spectroscopy, Photoluminescence and Raman spectroscopy. The optical band gap of the CdS thin film was estimated by UV–vis spectroscopy. The average transmittance of CdS thin film in the visible region of solar spectrum found to be～85%. Optical band gap of CdS thin film was calculated from transmittance spectrum ～2.71 eV which is higher than bulk CdS (2.40 eV) material. This confirms the blue shifting in band edge of CdS nanocrystalline thin films. PL spectrum of thin films showed that the fundamental band edge emission peak centred at 459 nm also recall as green band emission.     

Microstructure and electrical properties of nitrogen doped 3C–SiC thin films deposited using methyltrichlorosilane

The growth, microstructure and electrical properties of in-situ nitrogen doped 3C–SiC (111) thin films for sensor applications are presented in this paper. These thin films are deposited at a pressure of 2.5 mbar and temperature of 1040 °C on thermally oxidized Si (100) substrates from methyltrichlorosilane (MTS) precursor using a hot wall vertical low pressure chemical vapor deposition (LPCVD) reactor. Ammonia (NH₃) is used as the nitrogen doping gas. The sensor response depends on chemical composition, structure, morphology and operating temperature. The above properties are investigated for all in situ nitrogen doped (0, 9, 17 and 30 at% of nitrogen) 3C–SiC thin films using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and four probe method. The XRD patterns of the 3C–SiC thin films show a decrease in the crystallinity and intensity of the peak with increase in dopant concentration from 0 to 17 at%. AFM investigations show an improvement in the grain size of the nitrogen doped 3C–SiC thin films with increase in nitrogen concentration from 0 to 17 at%. The sheet resistance of nitrogen doped 3C–SiC thin films is measured by the four probe technique and it is found to decrease with increase in temperature in the range of 40–550 °C. The resistivity and average temperature coefficient of resistance (TCR) of doped 3C–SiC thin film deposited with 17 at% of nitrogen concentration are found to be 0.14 Ω cm and −103 ppm/°C, respectively and this can be used as a sensing material for high temperature applications.     

Optical and electrochemical gas sensing properties of yttrium–silver co-doped lithium iron phosphate thin films

The new sensing material, LiFe_0.995Y_0.0025Ag_0.0025PO₄ was synthesized using hydro-thermal methods, and characterized by X-ray diffraction, energy dispersive spectroscopy and X-ray photoelectron spectroscopy. The as prepared products were subsequently utilized in a self assembled optical waveguide gases testing apparatus and a WS-30A electro-chemical gas sensing apparatus for xylene detection. A glass optical waveguide gas sensor was fabricated by spin-coating a LiFe_0.995Y_0.0025Ag_0.0025PO₄ thin film on the surface of single-mode tin-diffused glass Optical Waveguide. The sensing elements for electro-chemical gas sensor were made by dip-coating a LiFe_0.995Y_0.0025Ag_0.0025PO₄ thin film on the surface of an alumina ceramic tube, assembled with platinum wire. The experimental results indicated that, at room temperature, LiFe_0.995Y_0.0025Ag_0.0025PO₄ thin film/tin-diffused optical waveguide sensing element exhibited higher response to xylene in the range of 0.1–100 ppm; at an optimum operating temperature (300 °C), the response (S_r) of LiFe_0.995Y_0.0025Ag_0.0025PO₄ to 100 ppm of xylene was 5.29, as measured by the WS-30A electro-chemical gases sensing apparatus.     

Investigations on the structural and optical properties of Li,N and (Li,N) co-doped ZnO thin films prepared by sol–gel technique

Pure and doped zinc oxide thin films have been deposited on sapphire substrates by using the sol–gel method and spin coating technique. The X-ray diffraction pattern showed that the deposited films exhibit hexagonal zinc oxide structure. Room temperature photoluminescence measurements show the presence of two emission bands. The predominant near band edge ultraviolet emission is at 3.28 eV and a suppressed broad band of deep level emission in the range of 2.1–2.5 eV. The incorporation of nitrogen is indicative of p-type behavior as observed from X-ray photoelectron spectrum of nitrogen in the doped samples. The p-type conduction of Li, N:ZnO may be attributed to the formation of a LiZn–N complex acceptor.     

Synthesis of TiO2 thin films with highly efficient surfaces using a sol–gel technique

Three different strong acid catalysts were used in a simple sol–gel synthesis to produce TiO₂ thin films with increased homogeneity and enhanced photocatalytic activity on their mesoporous surfaces. Various techniques were used to characterize the samples, including UV–visible spectrophotometry, X-ray diffraction, micro-Raman spectrometry, photobleaching, scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The band gaps varied from 3.73 to 3.75 eV and the transmittance was >80%. An anatase phase was obtained in all the samples and the crystal size varied from 20 to 45 nm as a function of the annealing temperature. The increase in the efficiency of the surface of the TiO₂ thin films was evaluated by photodegradation of methylene blue in water. The results showed that the acid catalysts used in the synthesis had an important effect on the morphology and photocatalytic activity of the thin films, resulting in more efficient surfaces. Synthesis with hydrofluoric acid produced thin films with a homogenous mesoporous structure and improved the photodegradation of the methylene blue dye to 92% in 2.5 h.     

Structural properties of size-controlled SnO2 nanopowders produced by sol–gel method

SnO₂ nanoparticles were synthesized by sol–gel method with different sol concentrations and the effect of sol concentration on the structural properties of SnO₂ was investigated. The aim of this work is synthesizing of SnO₂ nanoparticles from SnCl₂·2H₂O (tin (II) chloride dihydrate) precursor to obtain high quality powders for using as Li-ion anode material. For this purpose, during the SnO₂ precursor solution preparation, chloride ions were removed from the solution and then the sol–gel synthesis was applied. Produced SnO₂ nanopowders were characterized by x-ray diffraction (XRD), field emission gun scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive x-ray spectroscopy (EDS) analyses. TG-DTA and FT-IR analysis were performed on the synthesized sol. Grain size, crystal index and lattice strains of SnO₂ particles were calculated. The results showed that the grain size of particles has increased by the increasing of sol concentration, and the crystallinity has been improved. The smallest crystallite size (6.03 nm) was obtained from the SnO₂ sample of 6 mmole concentration sol and maximum size (9.65 nm) from 14 mmole sol according to W–H analysis.     

Effect of Co2+ ions on structural,morphological and optical properties of ZnO nanoparticles synthesized by sol–gel auto combustion method

Undoped and Co²⁺ doped ZnO nanoparticles have been successfully synthesized by sol–gel auto combustion method. The ratio of metal nitrates to citric acid was taken at 1:1.11. The synthesized nanoparticles were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), Fourier transform infrared and Uv–visible spectroscopy techniques. The X-ray powder diffraction analysis revealed the formation of single phase having hexagonal wurtzite structure. The lattice constant ‘a’ increases while ‘c’ decreases as Co²⁺ concentration ‘x’ increases. The average crystallite size obtained from XRD data is in the range of 19–15 nm. The X-ray density, atomic packing factor, strain, surface area to volume ratio, etc was obtained using XRD data. SEM analysis showed that the prepared nanoparticles are in nano regime, nearly spherical and loosely agglomerates. EDAX analysis showed that composition obtained is near stoichiometries. In order to understand functional group and vibrational frequency band position of synthesized nanoparticles FTIR technique was used. FTIR analysis results observed that vibrational frequency band position of Zn–O shifted to higher frequency band with Co²⁺ ion increasing host semiconductor nanoparticles. Uv–visible absorption spectra showed that absorption edge shifted to higher wavelength with increasing Co²⁺ concentration while corresponding energy band gap of semiconductor nanoparticles decreases with increasing Co²⁺ concentration.     

Enhanced photo-degradation methyl orange by N–F co-doped BiVO4 synthesized by sol–gel method

Using ammonium fluoride (NH₄F) as the source of nitrogen (N) and fluorine (F), N–F co-doped bismuth vanadate (BiVO₄) visible-light-driven photocatalysts have been synthesized via a sol–gel method. The resulting materials were characterized by a series of techniques: X-ray photoelectron spectroscopy, X-ray diffractometry, scanning electron microscopy, Brunauer–Emmett–Teller (BET) surface analysis, and UV–vis diffuse reflectance spectroscopy. Compared with BiVO₄, the N–F co-doped BiVO₄ photocatalysts exhibited much higher photocatalytic activity for methyl orange (MO) degradation under visible light irradiation. It was revealed that N and F atoms were doped into the lattice of BiVO₄. The doped N atoms existed as O–N or V–O–N bonding, and the F atoms replaced some O atoms to form the O–V–F structure, which can be attributed to the appearance of more active species V⁴⁺ and oxygen vacancies. The doped N and F atoms resulted in a red shift in the absorption edge. However, the N and F doping only slightly changed the morphologies and BET special surface areas of the samples. The photocatalytic activity of BiVO₄ significantly depended on the N–F doping content and the calcination temperature. The maximum activity was observed for the catalyst obtained with calcination at 500 °C, a molar ratio of NH₄F to Bi(NO₃)₃ was 6%.     

Improving magnetic and structural properties of Zn1–xCuxFeCrO4 by substituting copper synthesized by citrate gel autocombustion route

The nanocrystalline Zn_1−xCu_xFeCrO₄ (x=0.0, 0.25, 0.50, 0.75 and 1.0) have been synthesized by citrate-gel autocombustion method. X-ray diffraction (XRD) confirmed formation of cubic spinel structure for all the compositions. The lattice constant (a) shows a decreasing trend with the increase in Cu content. Thermal analysis of precursor after autocombustion was carried out by using thermo gravimetric analysis (TG–DTA). Formation of spherical nanoparticles was revealed by transmission electron microscopy (TEM). The elemental analysis by SEM–EDAX was in close agreement with the expected composition of the starting composition used for the synthesis. The room temperature magnetic properties studied by using the vibrating sample magnetometer (VSM) indicate ferrimagnetic nature of the samples. The role of copper in the formation of modified structural and magnetic properties of these samples has been explained.     

Phase transformation kinetics and optical properties of Ga–Se–Sb phase-change thin films

Phase transformation kinetics in Ga₂₅Se_75?xSb_x glasses have been determined by non-isothermal differential scanning calorimetric measurements at heating rates of 5, 10, 15, 20 and 25 K/min. The values of glass transition (T_g) and crystallization temperature (T_c) are found to be composition and heating rate dependent. The activation energy of crystallization and glass transition have been determined from the dependence of T_c and T_g on the heating rate. Thin films of Ga₂₅Se_75?xSb_x glasses have been prepared by vacuum evaporation technique with thickness 400 nm. These thin films were crystallized by thermal annealing and laser-irradiation. The phase change phenomena have been studied by measuring optical absorption of as-prepared and crystallized thin films in the wave length region 400–900 nm. The optical absorption data indicate that the absorption mechanism is non-direct transition. Optical band gap values decrease with increase in Sb contents in Ga–Se as well as with increase in annealing temperature and laser-irradiation time. The optical band gap is shifted due to crystallization by annealing/laser-irradiation. As the phase of the films changes from amorphous to crystalline, a non sharp change of the optical band gap is observed. This gradual decrease in optical band gap was explained to be a result of an amorphous–crystalline phase transformation.