Electrical and optical properties of ZnO/Si heterojunctions as a function of the Mg dopant content

Undoped and Mg-doped ZnO thin films prepared by a sol–gel process were deposited on p-Si and glass substrates via spin coating. The electrical and optical properties of the films were investigated. Atomic force microscopy images revealed that the ZnO films are formed from fibers consisting of nanoparticles. The electrical conductivity mechanism of the films was investigated. The I–V characteristics of Al/ZnO/p-Si samples showed rectification behavior with a rectification ratio that depended on the applied voltage and the Mg doping ratio. ZnO/p-Si heterojunction diodes exhibited non-ideal behavior with an ideality factor greater than unity that could be ascribed to the interfacial layer, interface states, and series resistance. The barrier height for undoped and Mg-doped ZnO/p-Si diodes was in the range 0.78–0.84 eV. The results demonstrate that the electrical properties of ZnO/p-Si heterojunction diodes are controlled by the Mg dopant content and suggest that the optical bandgap of these ZnO films can be tuned using the Mg level.     

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.     

Sol–gel fabrication and enhanced optical properties,photocatalysis, and surface wettability of nanostructured titanium dioxide films

TiO₂ photocatalytic film, annealed at temperatures of 500 °C and 700 °C, was prepared on SiO₂ pre-coated glass via sol–gel technique for photocatalytic purposes and effects of catalyst-type on its properties were investigated by an X-ray diffractometer (XRD), Scanning Electron Microscope, UV–vis spectrophotometer, and contact angle measurements. The XRD results showed that present phases depend upon catalyst used in the solution and phase transformation behaves in a temperature-dependent manner. For the layers derived from sols containing acidic catalysts, the anatase structure dominated and exhibited much better photocatalytic activity. The results indicated that the sample derived from sol comprises H₂SO₄ as catalyst, and exhibits anatase grains with the lowest size. This could be the reason for its better photocatalytic activity. Finally, samples derived from sol containing acidic catalysts showed superhydrophilicity and superior cleaning ability.     

TiO2 nanoparticle aggregations prepared by nitro-functionalized tripodal ligand as promising candidates for dye-sensitized solar cells

Tripodal tetraamine ligands were nominated as complexing agent in preparation of TiO₂ nanoparticle via a two-step sol–gel method. The effect of complexing agents was investigated on the particle and aggregation size as well as optical and photovoltaic properties. The products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron spectroscopy (TEM), scanning electron microscopy (SEM), and UV–vis spectroscopy. The results showed that the symmetry of ligands has a crucial effect on the size of the particles and the aggregations. Three different tripodal ligands were applied as complexing agents. Larger aggregation size with tiny nanoparticles was obtained from the sample prepared with Nitro-funtionalized ligand and led to improved photovoltaic performance due to enhanced photon scattering in the sample.     

Comparison of structural,spectral and magnetic properties of NiO nanofibers obtained by sol–gel electrospinning from two different polymeric binders

NiO is a p-type semiconductor with wide band gap energy. In this study, nickel oxide nanofibers were fabricated by sol–gel electrospinning followed by high temperature calcination, using two sacrificial polymeric binders. Poly(2-ethyl-2-oxazoline) (PEtOx) in water and styrene-acrylonitrile random copolymer (SAN) in N,N- dimethylformamide (DMF) along with nickel (II) acetate tetrahydrate (NATH), as metal oxide precursor, were the two distinct polymeric systems used in this study. The morphological and structural properties of NiO fibers obtained from the aforementioned systems were compared with each other. The degradation behavior of the sacrificial polymeric binder imparted a significant effect on the properties of the obtained NiO fibers. The grain sizes and the activation energies for grain growth of NiO fibers from two systems were different. The non-stoichiometric NiO fibers obtained from the SAN/NATH system had a better ferromagnetic behavior as compared with that produced from the PEtOx/NATH system. This non-stoichiometry made a difference also in the optical band gap energies of the NiO nanofibers.     

Sol–gel production of ZnO:CO: Effect of post-annealing temperature on its optoelectronic properties

Thin films of ZnO:Co were prepared by a sol–gel process. The microstructure and optoelectronic properties as a function of sintering temperature were studied extensively by optoelectronic characterizations. It was observed from the scanning electron microscopy images that the introduction of Co eliminated the commonly observed wrinkle effect in sol-gel derived films. Structurally, XRD measurements revealed that the derived film were c-axis oriented that enhances as sintering temperature increases upto 500 °C. Electrical measurements confirm that the deposited ZnO:Co thin film is n-type with decreasing resistivity as sintering temperature increases. Optical measurements revealed that the derived films exhibit good transmittance ~82% with a wide band gap ~4.01 eV.     

Effect of TiO2 nanoparticle content in sol–gel derived TiO2 paste on the photovoltaic properties of TiO2 photoanode of dye-sensitized solar cells

In the present work, the effect of the amount of TiO₂ nanoparticles, added to the sol–gel derived paste, on the photovoltaic properties of fabricated dye-sensitized solar cells (DSSCs) was investigated. A titanium sol (Ti-sol) was synthesized using a Pechini type sol–gel method, and different pastes were prepared by adding various amounts of TiO₂ nanoparticles to the obtained Ti-sol. The pastes were used to fabricate the mesoporous TiO₂ semiconducting layers for DSSCs. It was observed that by increasing the mass ratio (MR) of TiO₂ nanoparticles to Ti-sol the thickness of TiO₂ layer increases. This led to the more adsorption of dye molecules per unit area of active TiO₂ layer, which were determined by UV–vis spectrophotometry. Also, micro-cracks were observed in TiO₂ layers obtained from pastes with low MR values. But their amount and size decreased with increasing MR, which was due to the decrease of paste surface tension (σ). As a result, short circuit current density (I_sc) showed continuous increase with increasing MR, which was due to the more dye adsorption. Open circuit voltage (V_oc) first increased and then decreased by enhancing MR, which was explained by considering the electron–hole recombination rate. Finally, the DSSC fabricated from the paste with MR=0.65 showed the maximum conversion efficiency (η).     

Photocatalyst Cr-doped titanium oxide nanoparticles: Fabrication,characterization, and investigation of the effect of doping on methyl orange dye degradation

The Cr/TiO₂ nanoparticles were synthesized via sol–gel technique by using tetraisopropylorthotitanate (TIPOT) and chromium nitrate (Cr(NO₃)₃·9H₂O) as precursors. The framework substitution of Cr into TiO₂ nanoparticles was evaluated by X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive X-ray microanalysis (SEM-EDX), transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR) techniques. XRD results showed that pure TiO₂ and Cr/TiO₂ nanoparticles own anatase phase with some rutile phase. SEM and TEM images confirmed the successful doping of Cr³⁺ ions into TiO₂ structure. The result of photocatalytic degradation of methyl orange (MO) demonstrated that appropriate Cr³⁺ doping can greatly enhance photocatalytic activity of TiO₂, which was attributed to the "red-shift" in ultraviolet–visible light (UV–vis) absorption spectra of TiO₂ nanoparticles while doped with Cr³⁺ ions and also to the improved transfer efficiency of photogenerated electrons and holes caused by incorporation of Cr³⁺ ions. The optimal Cr³⁺ concentration to obtain the highest photocatalytic activity was 5% mol. The relatively high photocatalytic activity of Cr/TiO₂ nanoparticles, suggests that it may have a promising future for water and wastewater purifications.     

Synthesis based structural and optical behavior of anatase TiO2 nanoparticles

The effect of synthesis method on optical and photoconducting properties of titanium dioxide (TiO₂) nanoparticles has been investigated. Sol–gel and co-precipitation methods have been employed to prepare pure anatase phased TiO₂ nanoparticles calcinated at different temperatures below 500 °C. The optimized value of average crystallite size is within the range of 19−21 nm for a common calcination temperature of 400 °C for both the methods. The redshift in optical band gap of 0.9 eV has been observed for the sample synthesized by co-precipitation method with respect to the sol–gel method. The photoluminescence spectrum exhibits broad visible emission in both routes of synthesis while photoconductivity shows fast growth and decay of photocurrent in TiO₂ prepared by co-precipitation method as compared to TiO₂ prepared by the sol–gel method under visible illumination. Crystal structure based Rietveld refinement of X-ray diffraction data, scanning electron microscopy as well as photoluminescence and photoconductivity measurements were performed to characterize nanocrystalline anatase TiO₂.     

Sol–gel derived nanocrystalline TiO2 thin films: A promising candidate for self-cleaning smart window applications

In the present work, anatase TiO₂ films are prepared by sol–gel spin coating method. The structural and optical properties of the films have been studied at different post-annealing temperatures. The photocatalytic activity and electrochromic performance of the films are investigated. The films annealed at 400 °C exhibit the highest photocatalytic activity with a rate constant of 4.56×10⁻³ min⁻¹. The electrochromic performance for the films annealed at 400 °C expressed in terms of difference in optical density (ΔOD) at 550 nm between coloured and bleached state is 0.5493. This combination of photocatalysis and electrochromism makes the sol–gel derived titania thin films as promising candidates for self-cleaning smart window applications.     

High performance sol–gel spin-coated titanium dioxide dielectric based MOS structures

High-κ TiO₂ thin films have been fabricated using cost effective sol–gel and spin-coating technique on p-Si (100) wafer. Plasma activation process was used for better adhesion between TiO₂ films and Si. The influence of annealing temperature on the structure-electrical properties of titania films were investigated in detail. Both XRD and Raman studies indicate that the anatase phase crystallizes at 400 °C, retaining its structural integrity up to 1000 °C. The thickness of the deposited films did not vary significantly with the annealing temperature, although the refractive index and the RMS roughness enhanced considerably, accompanied by a decrease in porosity. For electrical measurements, the films were integrated in metal-oxide-semiconductor (MOS) structure. The electrical measurements evoke a temperature dependent dielectric constant with low leakage current density. The Capacitance–voltage (C–V) characteristics of the films annealed at 400 °C exhibited a high value of dielectric constant (~34). Further, frequency dependent C–V measurements showed a huge dispersion in accumulation capacitance due to the presence of TiO₂/Si interface states and dielectric polarization, was found to follow power law dependence on frequency (with exponent ‘s’=0.85). A low leakage current density of 3.6×10⁻⁷ A/cm² at 1 V was observed for the films annealed at 600 °C. The results of structure-electrical properties suggest that the deposition of titania by wet chemical method is more attractive and cost-effective for production of high-κ materials compared to other advanced deposition techniques such as sputtering, MBE, MOCVD and ALD. The results also suggest that the high value of dielectric constant ‘κ‘ obtained at low processing temperature expands its scope as a potential dielectric layer in MOS device technology.     

Nb-doped TiO2 sol–gel films for CO sensing applications

Nb doped titania (TiO₂:Nb) multilayered films (1–10 layers) with anatase structure were obtained by the low-cost sol–gel and dipping method on microscope glass substrates, followed by thermal treatment at 450 °C for 1 h. After each layer deposition, an intermediate annealing step was performed at 300 °C for 30 min. Doping TiO₂ sol–gel films with a low amount of Nb (0.8 at%) allows obtaining an improved CO sensor able to operate under environmental atmosphere (air). It was found that the sensor sensitivity is less dependent on the film thickness but is significantly influenced by Nb doping at the optimal working temperature of 400 °C. Good recovery characteristics were obtained for a wide CO detection range, between 0 and 2000 ppm. The gas-sensing behavior of the films was correlated with the structural, chemical and morphological properties of the multi-layered structures.     

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.     

p-type LiCr0.33V0.33Mn0.33O2 semiconductor as a cathode electrode for high rate Li-ion batteries

Layered LiCr_0.33V_0.33Mn_0.33O₂ oxides have attracted attention as cathode materials for lithium ion batteries. These materials are good candidates to replace LiCoO₂ used in the commercially available lithium ion batteries. In this study, a systematic work has been performed to investigate the structural and electrochemical behaviors of LiCr_0.33V_0.33Mn_0.33O₂ oxide structures via sol–gel method. In order to increase the conductivity, the surfaces of the as-synthesized LiCr_0.33V_0.33Mn_0.33O₂ oxide structures were coated with Cu via electroless deposition techniques. Powder X-ray diffraction (XRD) was performed on a Rigaku DMAX 2200 diffractometer (Cu Kα radiation, λ=1.5418 Å) between 10° and 90° (2θ) by steps of 0.02° (2θ) with a constant counting time of 10 s/step. Scanning electron microscopy (SEM) was carried out with a Jeol 6060 LV microscope. The electrochemical performances of the LiCr_0.33V_0.33Mn_0.33O₂ samples were measured in the 3.0–4.3 V potential range. Their discharge capacity reached 174 and 181 mA h g⁻¹ at 1 C. This structural stability during the cycling combined with the obtained electrochemical features make these materials convenient for the lithium ion batteries applications.     

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%.     

Determination of effective electrode configuration for electrical measurements of carbon thin conductive coatings

When electrical characterization of thin conductive coating is carried out, it requires an effective electrode configuration that prevents design problems due to low thickness, such as short circuits or electrode diffusion. In this work, two electrode configurations were carried out, surface and volume configurations, in order to compare and demonstrate the efficiency of surface electrode configuration for thin films. Highly conductive coatings based on carbon black (CB) have been prepared by the sol–gel method, and electrical properties were measured for both configurations, indicating that when the thickness of the film is low enough, the electrical conduction with the electrodes disposed on the surface is produced for the entire volume. This effect facilitates the design and treatment of the coating. This configuration could be very useful for many technological applications.     

Investigation of characteristics of ZnO:Ga nanocrystalline thin films with varying dopant content

In this study, undoped and Ga doped ZnO thin films were synthesized by the sol–gel spin coating technique. The effect of Ga contribution on the structural, morphological and optical properties of the ZnO thin films was examined. XRD results showed that all films had a hexagonal wurtzite crystal structure with polycrystalline nature. The intensity of the (002) peak changed with the variable Ga content. The scanning electron microscopy (SEM) results revealed that the surface morphology of the ZnO thin films was affected by Ga content. Moreover, it consisted of nanorods as a result of the increased function of the Ga content. Additionally, the presence of Ga contributions was evaluated by energy dispersive x-ray (EDX) measurements. Although the transparency and the optical band gap of the ZnO thin films increased with Ga contribution, Urbach energy values decreased from 221 meV to 98 meV. In addition, these steepness parameters increased with the increased Ga content from 0% to 6%. The correlation between structural and optical properties was investigated and significant consistency was found.