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.     

Synthesis and characterization of CuO–SnO2 nanocomposite and its application as liquefied petroleum gas sensor

Present paper reports the synthesis of CuO–SnO₂ nanocomposite via sol–gel route as a sensing material for a liquefied petroleum gas (LPG). X-ray diffraction analysis confirmed the formation of CuO–SnO₂ nanocomposite. Crystallite size was found 5 nm. The optical band gap of the nanocomposite was found 4.1 eV. The thin/thick films were fabricated using spin coating and screen printing technology respectively and investigated with the exposition of LPG at room temperature (25 °C). Surface morphology of the thin film exhibits that it has a number of gas adsorption sites. The sensitivities of the thick and thin film sensors were found 4.1 and 42 respectively. The response and recovery times of the fabricated film sensor were 180 and 200 s respectively. Maximum sensor response of thin film sensor was found 4100. Better sensitivity and percentage sensor response, small response and recovery times, and good reproducibility and stability recognize the fabricated thin film of CuO–SnO₂ as a challenging material for the detection of LPG.     

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.     

Synthesis of ZnO: Ge nanocomposite thin films by plasma gas condensation

In this work, we introduce a new method for the synthesis of Ge nanoparticles embedded ZnO thin films that are considered to be a potential candidate for photovoltaic applications. As opposed to current techniques, for the independent preparation of Ge nanoparticles, we propose using Cluster Deposition Source (CDS), which utilizes gas condensation of sputtered Ge atoms. For the synthesis of ZnO thin film host material conventional sputtering technique is employed. In the proposed technique independently synthesized Ge nanoparticles and ZnO thin films are combined into a composite structure on (100) oriented Si substrates. X-ray diffraction (XRD) patterns of the samples have revealed that Ge nanoparticles preferentially settle on (113) planes on top of the (002) oriented ZnO layer. It is realized that Ge nanoparticles with sizes ranging from 16 nm to 20 nm could be embedded into a well-defined ZnO matrix. In fact, transmission electron microscopy (TEM) studies performed on Ge nanoparticles captured on a Cu grids placed just above the substrate during deposition for about 60 s have manifested that Ge nanoparticles reach to ZnO matrix as clusters composed of particles with sizes of about 7–8 nm and then eventually grow larger due to substrate heating implemented during capping layer deposition. Optical absorption measurements have revealed that Ge nanoparticle inclusion lead to an additional absorption edge at about 2.75 eV along with 3.17 eV edge resulting from ZnO host.     

Preparation,structural and optical investigations of ITO nanopowder and ITO/epoxy nanocomposites

Indium Tin Oxide (ITO) nanopowder was synthesized by a sol–gel method. It was characterized by X-ray diffraction (XRD) and transmission electron microscopy. ITO/epoxy nanocomposites (ITO–EP-NCs) were prepared by mechanically dispersing the as-prepared ITO nanopowder into epoxy matrix. The XRD patterns show structural changes depending on ITO content. The interdependence of structural, morphological, optical properties and the dispersed concentration of ITO nanoparticles were investigated. The UV–visible absorption spectra revealed that the ITO–EP-NCs exhibit enhanced UV light absorption properties and wide absorption bandwidth ranging up to 400 nm from 2 wt% ITO loading. Thus, it indicated that UV and IR-shielding properties have been improved with the incorporation of ITO nanoparticles into the epoxy matrix.The gap energy of epoxy matrix was reduced by adding the ITO-NPs, leading to the improvement of its electrical conductivity. Indeed, the AC electrical conductivity of ITO–EP-NCs showed a critical percolation threshold p_c=0.21 wt% ITO. For low loading (<2 wt% ITO), the ITO–EP-NCs have combined good transparency in the visible range and enhanced electrical conductivity, which are required for optoelectronics devices.     

Effect of Al dopant concentration on structural,optical and photoconducting properties in nanostructured zinc oxide thin films

Thin films of undoped and doped ZnO, with different Al concentrations (1–5 wt%) were deposited onto glass substrates, by the sol–gel spin coating method. Grazing incidence X-ray diffraction (GIXRD) studies confirmed the nature of films as poly-crystalline, with typical hexagonal wurtzite structure. The films showed variation in crystallite size and change in relative intensities, upon different Al doping concentrations. The surface morphology of the films examined using FE-SEM, showed the grain size becoming smaller upon Al doping. The influence of Al with different concentrations, onto ZnO on the optical absorption and transmittance was studied using UV–Vis–NIR spectrophotometer in the wavelength range 300–2500 nm. The UV absorption shifted towards shorter wavelength upon Al doping. The average transmittance in the visible region increased for Al doped films up to 1–2 wt% and decreased for other concentration. The dark and photo conductivity measurements of the films indicated increase in the current values upon doping up to 1–2 wt% of Al and decreased for further concentrations. The rise and decay time measured from the photoresponse study, indicate larger values of rise time for the doped films compared to undoped ZnO. However, the film with 1–2 wt% doping of Al showed better response within the doping concentration. The thermal activation energy obtained from temperature-dependant conductivity showed decrease in the value upon Al doping up to 2 wt% and increased beyond this concentration in the temperature range 300–400 K.     

ZnO nano-ridge structure and its application in inverted polymer solar cell

We report a unique nano-ridge structure of zinc oxide (ZnO) and its application in high performance inverted polymer solar cells. The ZnO nano-ridge structure was formed by a sol–gel process using a ramp annealing method. As the solvent slowly evaporated due to the low heating rate, there was sufficient time for the gel particles to structurally relax and pile up, resulting in a dense and undulated film. Nano-ridges with peak as high as 120 nm and valley to valley distance of about 500 nm were formed. This film provided an effective hole blocking layer and also an increased interfacial area for electron collection. An inverted bulk heterojunction polymer solar cell was fabricated using the ZnO nano-ridge film as the electron collecting layer. The device showed a high power conversion efficiency of 4.00%, an improvement of about 25% over similar solar cells made with a planar film of ZnO nanoparticles.     

Sol–gel synthesized Titania nanoparticles deposited on porous polycrystalline silicon: Improved carbon dioxide sensor properties

Titania nanoparticles (TNPs) were synthesized by a sol–gel method in our laboratory using titanium tetrachloride as the precursor and isopropanol as the solvent. The particles׳ size distribution histogram was determined using ImageJ software and the size of TNPs was obtained in the range of 7.5–10.5 nm. The nanoparticle with the average size of 8.5 nm was calculated using Scherrer׳s formula. Homogeneous and spherical nanoparticles were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy (UV–vis). The X-ray powder diffraction analysis showed that the prepared sample (TNPs) has pure anatase phase. TNPs were deposited on porous polycrystalline silicon (PPS) substrate by electron beam evaporation. The TNPs thickness was 23±2 nm at 10⁻⁵ mbar pressure at room temperature. Porosity was performed by an anodization method. Since polycrystalline silicon wafers consist of different grains with different orientations, the pore size distribution in porous layer is non-uniform [1]. Therefore, the average diameter of pores can be reported in PPS layer analysis. Average diameter of pores was estimated in the range of 5 μm which was characterized by FESEM. The nanostructured thin films devices (Al/Si/PPS/TNPs/Al and Al/Si/PPS/Al) were fabricated in the sandwich form by aluminum (Al) electrodes which were also deposited by electron beam evaporation. Electrical measurements (I–V curves) demonstrated the semiconducting behavior of thin film devices. The gas sensitivity was studied on exposure to 10% CO₂ gas. As a result, conductivity of devices increased on exposure to CO₂ gas. The device with TNPs thin film (Al/Si/PPS/TNPs/Al) was more sensitive and, had better response and reversibility in comparison with the device without TNPs thin film (Al/Si/PPS/Al).     

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.     

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.     

Work function engineering of electrodes via electropolymerization of ethylenedioxythiophenes and its derivatives

The work functions of two commonly used electrodes, Au and indium-tin-oxide (ITO), can be modified by surface-electropolymerization of ethylenedioxythiophenes (EDOT) or its derivatives. The resulting work function is tunable continually by varying the electropolymerization condition. The value of work function of ITO (or Au) electrodes can be adjusted to decrease by 0.6 eV (or increase by 0.4 eV) when the deposited EDOT film is thin (<40 nm). By contrast, when the EDOT film deposited with thickness greater than 40 nm, the work function is controlled by the intrinsic property of the polymers regardless of the electrode materials. A diode devices with EDOT–COOH modified on ITO surface demonstrates the possibility of improving contact property in organic electronics using the proposed method.     

A plasmonically enhanced polymer solar cell with gold–silica core–shell nanorods

We report the use of chemically synthesized gold (Au)–silica core–shell nanorods with the length of 92.5 ± 8.0 nm and diameter of 34.3 ± 4.0 nm for the efficiency enhancement of bulk heterojunction (BHJ) polymer solar cells. Silica coated Au nanorods were randomly blended into the BHJ layers of these solar cells. This architecture inhibits the carrier recombination at the metal/polymer interface and effectively exploits light absorption at the surface plasmon resonance wavelengths of the Au–silica nanorods. To match the two plasmon resonant peaks of the Au–silica nanorods, we employed a low bandgap polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) to construct a solar cell. The absorption spectrum of PCPDTBT:[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) is relatively wide and matches the two plasmon resonance peaks of Au–silica nanorods, which leads to greater plasmonic effects. We also constructed the poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PC₆₀BM) cells for comparison. The absorption spectrum of P3HT:PC₆₀BM only overlaps one of the plasmon resonance peak of Au–silica nanorods. The efficiency of the P3HT:PC₆₀BM device incorporating optimized Au–silica nanorods is enhanced by 12.9% from 3.17% to 3.58%, which is due to the enhanced light absorption. Compared with the P3HT:PC₆₀BM device with Au–silica nanorods, the PCPDTBT:PC₇₀BM device with 1 wt% Au–silica nanorods concentration has a higher efficiency of 4.4% with an increase of 26%.     

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 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.     

Controlling the diameter of silicon nanowires grown using a tin catalyst

Silicon nanowires were grown on ITO-coated glass substrates via a pulsed plasma enhanced chemical vapor deposition method, using tin as a catalyst. The thin films of catalyst, with different thicknesses in the range 10–100 nm, were deposited on the substrates by a thermal evaporation method. The effect of the thickness of the thin film catalyst on the morphology of the silicon nanowires was investigated. The scanning/transmission electron microscopy images showed that the wire diameter increased as the thickness of the thin film catalyst increased. The nanowires grown using a thin film thickness of 10 nm were inhomogeneous in diameter, whereas the other thicknesses led to an increase in the homogeneity of the diameters of the nanowires. The dominant wire diameter of the grown silicon nanowires ranged from 70 to 80 nm with 10 nm catalyst thin film thickness, and increased to a range of 190–200 nm with 100 nm catalyst thin film thickness.     

Effect of calcination temperature on Cu doped NiO nanoparticles prepared via wet-chemical method: Structural,optical and morphological studies

In the present study, NiO and Cu-doped NiO nanoparticles were successfully synthesized by wet chemical method at room temperature using sodium hydroxide (NaOH) as precipitating agent. The as-prepared Cu-doped NiO powder samples were subjected to three different calcination temperatures such as, 350 °C, 450 °C and 550 °C in order to investigate the impact of calcined temperatures on the phase formation, particle size and band gap evolution. The phase formation and crystal structure information of the prepared nanomaterials were examined by X-ray powder diffraction (XRD). XRD revealed the face-centered cubic (FCC) structure. Average crystalline size of pure and doped samples estimated using Scherer formula was found to be 15 nm and 9 nm respectively. With increase in the calcination temperature from 350 °C to 550 °C for the Cu doped NiO samples the particle size of the nanoparticles was found to increase from 4 nm to 9 nm respectively. The optical study for both pure and doped NiO nanoparticles was performed using an UV–Vis spectrophotometer in the wavelength range of 200–800 nm. The strong absorption in the UV region confirms the band gap absorption in NiO and was estimated from the UV–Vis diffuse reflectance spectra via Tauc plot. Systematic studies were also carried out to study the effect of calcination on the optical transmittance. Samples were also investigated using Raman and Fourier Transform Infrared Spectroscopy (FTIR). Furthermore, morphology of the pure NiO and Cu-doped NiO Nanoparticles were examined by scanning electron microscope (SEM).     

Fabrication of ZnO-NiO nanocomposite thin films and experimental study of the effect of the NiO,ZnO concentration on its physical properties

ZnO-NiO nanocomposites thin films were elaborated at different mixing concentrations using sol gel and spin coating methods. Their structural and morphological evolutions as well as the optical and electrical properties were investigated. XRD diffraction and Raman spectra allowed phase identifications of ZnO (zinc oxide) and NiO (nickel oxide) with no appearance of secondary phases and the crystallinity of elaborated nanocomposite films improved with doping concentration increase. The grain sizes of obtained ZnO-NiO nanocomposites are investigated by AFM (Atomic force microscopy); they increase in the range (10–65 nm) and they are observed to affect the optical and electrical properties. In fact, ZnO-NiO nanocomposites thin films optical reflectivity decreased in the range (10–5%) with the increasing of mixing proportion and their resistivity decreased up to 1.4 10² Ω cm. The optical band gaps were in the range (3.3–4 eV). The values obtained by UV–Vis spectroscopy and ellipsometry are quite similar. We remarked also that the NiO concentration increase on to the nanocomposite induced a red shift of the gap value while the ZnO increase led toward a blue shift     

Optical,electrical and photovoltaic properties of thermally evaporated 3-amino-2-[(2-nitrophenyl)diazinyl]-3-(piperidin-1-yl)acrylonitrile thin films

Thin films with various thickness of 3-amino-2-[(2-nitrophenyl)diazinyl]-3-(piperidin-1-yl)acrylonitrile (ANPA) were produced by conventional thermal evaporation. The dependence of the optical, electrical, and photovoltaic properties on the film thickness was investigated. X-ray diffraction showed that the films have an amorphous structure. The optical properties were investigated in terms of transmittance and reflectance in the spectral range 200–2500 nm. Spectral distribution analysis of the absorption coefficient revealed that the films have an indirect allowed electronic interband transition. The optical bandgap decreased from 2.47 to 2.1 eV for an increase in film thickness from 105 to 265 nm. The direct current electrical conductivity of the films was measured for sandwich-structured samples as a function of the heating temperature and film thickness. The films exhibited semiconductor behavior and electrical conduction was attributed to hopping of charge carriers in localized states. ANPA films of differing thickness were deposited on p-type Si single-crystal substrates. The influence of film thickness on the electrical and photovoltaic parameters of Au/ANPA/p-Si/Al heterojunction solar cells was investigated.     

Structural,surface morphology and optical properties of sputter-coated CaCu3Ti4O12 thin film: Influence of RF magnetron sputtering power

This study focusses on the investigation of RF power variations (100–300 W) effects on structural, morphological and optical properties of CaCu₃Ti₄O₁₂ thin film deposited on ITO/glass substrate in a non-reactive atmosphere (Ar). The increase of RF power from 100 W to 300 W led to evolution of (112), (022), (033), and (224) of CCTO XRD peaks. The results indicated that all the films were polycrystalline nature with cubic structure. The crystallite size increased from 20 nm to 25 nm with increasing RF power. FESEM revealed that the films deposited were uniform, porous with granular form, while the grain size increased from 30 to 50 nm. AFM analysis confirmed the increment in surface roughness from 1.6 to 2.3 nm with increasing film grain size. Besides, optical transmittance values decreased to minimum 70% with increasing RF power while optical energy bandgap increased from 3.20 eV to 3.44 eV. Therefore, favorable CCTO thin film properties can be possibly obtained for certain application by controlling RF magnetron sputtering power.     

Ultraviolet photodetectors based on low temperature processed ZnO/PEDOT:PSS Schottky barrier diodes

In this work, vertical Schottky barrier diodes (SBDs) were fabricated using a thin film of ZnO (50 nm) and PEDOT:PSS deposited by RF Sputtering and micro-drop casting, respectively. ITO and Au were used as ohmic contacts to ZnO and PEDOT:PSS films, respectively. The final structure consisted on Glass/ITO/ZnO/PEDOT:PSS/Au. The SBDs performance was characterized under dark and four different wavelengths conditions. From current–voltage characteristics, under dark and ambient conditions, a diode ideality factor of 1.4; a saturation current density of 1×10⁻⁹ A/cm²; a Schottky barrier height of 0.9 eV and a rectification ratio of 5 orders of magnitude at ±1 V were obtained. A carrier density of 5×10¹⁷ cm⁻³ for the ZnO film was estimated from capacitance–voltage measurements. For their characterization as photodiodes, the SBDs were illuminated with an ultra-bright UV (~380 nm) LED. A maximum UV responsivity of 0.013 A/W was obtained. The transient response of the SBDs was also analyzed with the UV LED connected to a pulsed signal of 0.5 Hz, demonstrating rise and fall times in the order of 200 ms. With a low temperature processing (<80 °C), visible-blind and UV photon-detection characteristics, the fabricated SBDs are candidates for flexible optoelectronics devices such as optical receivers for digital signal processing and measurement of light intensity.