Sol-gel Thin Films Research Articles

Optimization of the production process of BZT–BCT sol–gel thin films obtained from a highly stable and green precursor solution

ABSTRACT Barium Zirconate Titanate–Barium Calcium Titanate (BZT–BCT) is an environmentally friendly piezoelectric-ferroelectric ceramic that is a promising alternative to lead zirconate titanate (PZT), whose toxicity is well known. Producing thin films by sol–gel deposition with properties comparable to their bulk counterparts is quite a difficult task, because of the substrate constraints which typically generate residual stresses during the production process, leading to the onset of cracks. Moreover, typically employed solutions contain toxic and harmful substances or are unstable over time. Recently, we produced a highly stable BZT–BCT precursor solution based on nontoxic and non-carcinogenic solvents. In this work, we optimized the BZT–BCT sol–gel deposition process by varying the spin coating velocity, which influences the thickness of each deposited layer, and the pyrolysis temperature, which impacts on the formation of cracks. Moreover, we explored the possibility to add a third heat treatment step to the deposition process with the aim of releasing the residual stresses, further improving the film properties. As a result, 250 nm thick films were obtained with optimized morphology and limited presence of cracks, thus enhancing the ferroelectric properties (Pr = 33.26 μC cm−2, Ec = 220.6 kV cm−1, ε = 1817).

Effect of storage and surface roughness on the SiO2 thin films adhesion behavior

The thin film elaboration and use, commonly related by the substrate properties and quality, are of great interest in the optical field. Indeed, storage conditions and duration, in addition to the surface quality, can have an effect on the materials and their performances, especially if they are functioning as optical materials. For this purpose, this study aims to contemplate both the storage duration of optical glasses and their surface quality, as well as the adhesion behavior of silica thin film deposition. Therefore, optical glasses were developed and stored for 3 years in the same solution (colloidal silica premixed [CMP] slurry), then they were characterized before the study of their sol-gel thin films adhesion behavior. Furthermore, surfaces were treated with several techniques and their surface quality was measured. The adhesion phenomenon was studied via droplet technique. The results show the influence of the storage duration on the optical properties of the optical glasses (refraction index, Abbe number) and the surface roughness. Moreover, the surface roughness has a great influence on the adhesion of the thin films. In addition to that, sol-gel prepared with 20% TEOS has the highest adhesion of the optical glass when the surface roughness is low.

Transparent piezoelectric thin-film devices: Pb(Zr, Ti)O3 thin films on glass substrates

To realize transparent piezoelectric microdevices, Pb(Zr,Ti)O3 (PZT) thin films are deposited on fluorine-doped tin oxide (FTO)-coated glass substrates, and the resultant piezoelectric properties are evaluated. PZT thin-film deposition is conducted by radio frequency (RF) magnetron sputtering and sol-gel methods. X-ray diffraction (XRD) measurements indicate that both sputtering and sol-gel PZT thin films on FTO-coated glass exhibit a polycrystalline perovskite structure with sufficient transparency. The relative dielectric constant and dielectric loss for the PZT thin films are 344 and 3.3 % for sputtering and 482 and 8.2 % for sol-gel deposition, respectively. In addition, the PZT thin films exhibit typical ferroelectric P–E hysteresis curves. The converse piezoelectric coefficient is measured from the tip displacement of the transparent unimorph cantilevers composed of PZT thin films on glass substrates. The sputtering and sol-gel depositions both produce transparent PZT thin-film devices with a clear converse piezoelectric effect, the converse piezoelectric coefficient |e31,f| of which ranges between 2.8 and 5.0 C/m2.

Sol-Gel Thin Film Processing for Integrated Waveguide Sensors

Integrated opto-chemical sensors present great advantages in comparison to the current lab equipment. They bring portability, reduced costs, facilitate in-situ measurements, as well as largely reduced sample volumes. In this quest, standard processing protocols over established materials, such as silicon nitride, silicon, silicon dioxide, titanium oxide, and even a wide variety of polymers have so far been the key toward on-chip devices. However, if very specific materials in terms of composition and tailored properties are required, the deposition via a solution represents a viable alternative. In this review, we highlight the role of sol-gel chemistry and top-down processing of sol-gel thin film layers in the design of waveguide-based optical sensors. In particular, we stress the advantages of porous sol-gel based materials as a new approach to increase sensitivity and selectivity, first when used as claddings, and, more recently, as waveguides with enhanced light–analyte interaction. We finally discuss the future perspectives of such devices to increase specificity in complex matrices, which is of utmost importance for bio-sensing.

Effect of infrared laser irradiation on electrical conductivity and ethanol sensitivity of sol-gel ZnO thin films

Zinc oxide thin films are prepared by spin-coating sol-gel method. Warm air flow is used as a first step in the drying procedure during the preparation of a part of the films. Post-deposition irradiation of some films with infrared pulse laser is also applied. Spectroscopic ellipsometry, X-ray diffraction and conductivity measurements are used to get information about the influence of the laser irradiation and the usage of the warm air flow on the properties of the films. It is shown that all films are nanocrystalline and the treatment with warm air flow and laser beam irradiation improves films crystallinity and reduces their conductivity. The results are related to lower porosity and smaller amount of defects in treated films.

The investigation of corrosion behavior of ZrO2–Al2O3-inhibitor/AA2024 nanocomposite thin film using sol-gel and AHP-TOPSIS method

Nowadays, the sol-gel thin films doped with corrosion inhibitor have suggested enhancing corrosion resistance of materials. In the present work, ZrO2 and ZrO2–Al2O3 thin films with benzotriazole (BTA) and cerium nitrate inhibitors were applied on the aluminum alloy substrate by using dip-coating process, and heat treated at 150 °C for 2 h with heating rate of 1 °C/min. Then, the crystalline phase, chemical composition, the microstructure and topography of thin films were evaluated by grazing incidence x-ray diffraction, energy dispersive spectroscopy, field emission scanning electron microscopy, and atomic force microscopy, and the average thickness of thin films was measured by ellipsometry technique. In addition, electrochemical impedance spectroscopy, and potentiodynamic polarization investigations were used to study the corrosion protection properties of thin films. Eventually, the experimental results related to the three independent coating conditions (sample A, B, D) were investigated by analytic hierarchy process (AHP) and technique for order preference by similarity to ideal solution (TOPSIS). Results indicate the formation of composite thin films without microcracks comparison to the single-component coating, and electrochemical impedance tests after 4 day immersion in the corrosive environment illustrates an improvement of corrosion resistance of sample C (with the first and outside layer of ZrO2 and two middle layers of ZrO2, Al2O3 and BTA) more than 5 times compare to the bare AA2024 aluminum alloy. Additionally a decrease in the corrosion current density from 3.1 μA/cm2 for the uncoated aluminum to 0.2 μA/cm2, which was observed for sample C. Moreover, the data evaluation was studied by AHP and TOPSIS methods, which was presented the best thin films conditions for the sample B (Four layers of ZrO2 – Al2O3- BTA thin films on the AA2024 alloy) which was in good agreement with the experimental results.

Nitrogen doped carbon nanotubes grafted TiO2 rutile nanofilms: Promising material for dye sensitized solar cell application

The TiO2 based solar cells are an exciting 3rd class generation photovoltaic, since they hold promise for the realization of large area devices that can be fabricated by room temperature solution processing, that can lead to high photoactive performance. Here, we reported similar efficiencies by focusing our work on obtaining sol-gel thin films belonging to N-CNT/TiO2 system by dispersing nitrogen doped carbon nanotubes (N-CNTs) powders in titanium tetra-isopropoxide using sol gel route. The film samples were evaluated in terms of structure morphology and optical properties using XRD analysis, atomic force microscopy (AFM) for TiO2, thin film scanning electron microscopy (SEM) for TiO2 and N-CNT/TiO2 thin films as well as UV–vis absorption spectroscopic techniques. X-ray diffraction analysis of films produced by the sol gel route and deposited on ITO glass has shown that they are also amorphous before annealing. However, XRD spectra showed only the peak of the rutile phase after the high annealing temperatures starting from 300 °C. The SEM micrographs showed the presence of grains of very small sizes, the grain size increases with the annealing temperature. The measurements made by AFM revealed that the thickness and roughness increase with the rise of annealing temperature of the film. 300 °C is the optimum temperature. This finding was assessed by the experimental investigation of annealing temperature effect through determination of specific area and pores diameter using BET and BJH methods respectively.Moreover, it was confirmed by SEM observations that the grains of N-CNT/TiO2 thin films are homogeneously dispersed and transmission electron microscopy clearly indicated that TiO2 grains are adsorbed on N-CNTs. Optical transmittance of N-CNT/TiO2 indicated a decrease with increasing the incorporated wt% N-CNTs. This behavior is due to charge carriers increase, which lead to band gap narrowing. The findings concerning the optical gap are assessed by electrical measurements. After careful analysis of the results presented in this work, we have deduced that the mentioned method is found to be simple and more efficient in the calculations of direct band gap.

Physico-mechanical properties and SEM of sol–gel plates based on chitosan L-(D-)ascorbate

In our previous studies, it was shown that the isomeric form of ascorbic acid (L-or D-AscA) used to obtain the salt form of chitosan had a significant effect on the total space charge, size and general chirality of chitosan ascorbate macromolecules in solution, their stereomeric ordering in lyophilically isolated solid phase, as well as the kinetics of sol–gel synthesis of materials based thereon. This study examines the effect of L- and D-AscA on the physico-mechanical and morphological properties of thin-film sol–gel plates based on chitosan L-or D-ascorbate without and with Aloe Vera juice added. It has been established that the AscA isoform has a significant effect on the elastoplastic characteristics of the samples and the surface relief of their solid phase. The addition of Aloe Vera also leads to significant texturing of the surface relief. The latter could be used to design specific optical sensors and optodes of medico-biological purposes.

Effect of sulfur amount during post-growth sulfurization process on the structural, morphological and thermoelectric properties of sol-gel grown quaternary chalcogenide Cu2ZnSnS4 thin films

In this work, the CZTS thin films (TFs) have been grown using sol-gel spin coating method on Si substrate. The effect of sulfur concentration on the structural, morphological and thermoelectric properties was investigated using various amounts of sulfur (100–400 mg) during post growth sulfurization process. XRD data evident that the best crystallinity of the CZTS thin film was obtained when the sample was sulfurized using 400 mg amount of sulfur. The vibrational modes of CZTS thin films were observed using Raman spectroscopy with major A1 mode at 331 cm−1 and secondary B(TO) and B(LO) modes at 292 cm−1 and 367 cm−1 respectively. SEM images showed that higher amount of sulfur contents resulted in the compact and smooth surface when compared with the samples sulfurized at lower concentration of sulfur. The thermoelectric properties such as Seebeck coefficient, electrical conductivity and power factor have improved from (249–334 μV/°C), (69–117 S/cm) and (4.28 × 104–1.305 × 103 Wm−1C−2) respectively with increasing the amount of sulfur during sulfurization process. This behavior of thermoelectric properties explained in detail using standard formulas and available literature.

Effect of laser processing parameters on spectral characteristics of silver-impregnated titanium dioxide thin films

Subject of Research. Local and precise control of nanocomposite material optical properties become possible due to lasers. Laser irradiation can be used as an instrument for the fabrication and modification of such materials. However, for practical applications, it is necessary to know how laser processing parameters impact on spectral characteristics of composite materials which, as a rule, are related to sizes and distribution of nanoparticles. The paper presents research results of the laser processing parameters impact on the reflection spectra of nanocomposite material based on titanium dioxide. Methods. Sol-gel titanium dioxide thin films impregnated with small (less than 5–7 nm) silver nanoparticles on glass slides were exposed to a 405 nm ultraviolet laser. Changes in the sample reflection spectra after laser processing in the continuous wave mode were studied via optical spectrophotometry in the range of 350–760 nm. Main Results. An array of laser tracks on the sample surface was recorded with such processing parameters as the scanning speed and average radiation power. Each track had central and edge areas which were visually observed. Experimental data analysis showed that there was a shift in the reflection spectra peak position in these two areas in the range of 380–440 nm. In order to determine the reasons for such spectral changes, numerical modeling was carried out using the effective medium model in the Bruggemann-Bergman approximation. It was found that the size and distribution of silver nanoparticles at the edges and in the center of the laser processed area may vary. The scanning speeding-up and the average radiation power decrease leads to an increase of nanoparticles size. These size changes occur due to various temperature distributions. Practical Relevance. Control methods for spectral characteristics of sol-gel silver-impregnated titanium dioxide thin films via local laser resizing of nanoparticles are demonstrated. The obtained results are promising for a number of applications: integrated optics, photonics devices, biosensors, photocatalytic devices, and security labels.

Double layer SiO2-TiO2 sol-gel thin films on glass for antireflection, antifogging, and UV recoverable self-cleaning.

Double layer thin films, mechanically stable and adhering to glass, were produced through the sol-gel process, using tetraethyl orthosilicate and titanium butoxide as precursors. The refractive index of the titania and silica-titania composite layers were typically 2.1 and 1.7, and their physical thicknesses were approximately 65 nm and 81 nm, respectively, as determined by ellipsometry. These optical constants allowed attainment of quarter-wave optical thicknesses at the center of the visible spectrum (550 nm) as designed, with an increase of 3.4% in transmittance. The nanometric surface roughness, measured by optical profilometry, was effective to decrease light scattering and water contact angles to below 10°. As novelty in dip-coated sol-gel films, superhydrophilicity for self-cleaning, antifogging, and antireflection in the mid-visible spectrum were simultaneously attained with durability of 9 weeks in the dark. Further application of UV light allowed regeneration of contact angles for self-cleaning.

Microstructure and Electrical Properties of Low-temperature Solution-processed Sol-gel KNN Thin Films

We report on an environmentally friendly and versatile chemical solution deposition route to K0.5Na0.5NbO3 (KNN) thin films. The excess amounts of K and Na in KNN precursor solutions was found to be strong influence on perovskite KNN single-phase thin films. It was revealed from Raman spectroscopic analysis data that a change in scattering mode was observed for the KNN thin films fabricated under various processing conditions. This change was due to the chemical composition fluctuation of K and Na in the KNN thin films during heat treatment. The leakage current and ferroelectric properties of the thin films were strongly affected by the excess amounts of K and Na as well. KNN thin films with 20 mol% excess K and Na exhibited a leaky ferroelectric polarization–electric field (P–E) hysteresis. Leakage current density of the film was 3.85´10-8 A/cm2 at applied field of -60 kV/cm.

Preparation, Fabrication and Characterization of Sol-Gel ZnO Thin Films for Organic Solar Cells

In this work, ZnO has been prepared by the sol-gel method and thin films have been deposited onto the ITO (Indium-Tin-Oxide) coated glass substrates by spin coating method at different ZnO concentration and spin parameters. For this, Sol-gel ZnO was synthesized by Zinc acetate dehydrate, 2-methoxethanol and ethanolamine as a starting material, solvent and stabilizer respectively. The study of deposition parameters on the structural, optical and electrical properties of the ZnO thin films was carried out. The Roughness and thickness were calculated by Profilometer. X-ray diffraction (XRD) analysis of the films showed the polycrystalline nature of the prepared films. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) was used to describe the surface morphology and optical properties were studied using UV-VIS-IR Spectroscopy. The fabricated results showed that ZnO thin films is crystalline and low-cost techniques with good features that will be useful for Organic Solar Cells (OSCs) device as an electron transport layer.

High conductivity along with high visible light transparency in Al implanted sol-gel ZnO thin film with an elevated figure of merit value as a transparent conducting layer

Abstract In this work, sol–gel ZnO thin films were implanted with Al ions of various fluences to study their electrical transport properties as transparent conducting oxide (TCO) layers. Additionally, formation of structural defects and modifications of the optical emission properties due to implantation have been investigated extensively to correlate the resultant electrical properties. By varying the ion fluence over two orders of magnitude from 1 × 1013 to 6 × 1015 ions/cm2, interestingly the electrical transport properties have been seen to be degraded in an unexpected manner as soon as Al is implanted and beyond a certain fluence (5 × 1013 ions/cm2), the electrical properties begin to improve. For the highest fluence (6 × 1015 ions/cm2), the sheet resistance is found to be 156 Ω/sq with an average visible transmission of 82%. The figure of merit value of our Al implanted ZnO film for an Al fluence of 5 × 1015 ions/cm2 is the highest till date, which may be prospective as a TCO layer. Detailed investigation on the evolution of the electrical and photoluminescence properties with an increase in the Al ion fluence reveals that zinc vacancy type defects are formed in large number which on healing produces further higher conductivity in the films.

Enhancing the Corrosion Protection of AA2024-T3 Alloy by Surface Treatments Based on Piperazine-Modified Hybrid Sol–Gel Films

The aim of this study was to develop new chrome-free surface pretreatments for AA2024-T3 aluminum alloy. These pretreatments were based on hybrid organic–inorganic sol–gel thin films prepared from mixtures of γ-methacryloxypropyltrimethoxysilane (MAPTMS) and tetramethylorthosilicate (TMOS). Different MAPTMS/TMOS molar ratios were used for optimizing the physical–chemical characteristics of the sol–gel films. The formulation of a set of these sols was modified by incorporating piperazine (1,4-diazacyclohexane) as a corrosion inhibitor. The resulting sol–gel films were characterized by using Fourier transform infrared spectroscopy (FTIR), liquid-state 29Si nuclear magnetic resonance spectroscopy (29Si-NMR) and viscosity measurements. The corrosion performance of the sol–gel films was analyzed by using electrochemical impedance spectroscopy (EIS) and local electrochemical impedance mapping (LEIM). The characterization techniques indicated that piperazine behaved as a catalyst for the condensation reaction during the formation of the MAPTMS/TMOS organopolysiloxane network and produces an increase of the crosslinking degree of the sol–gel films. EIS and LEIM results showed that piperazine is an effective corrosion inhibitor, which can be used to enhance the active corrosion protection performance of sol–gel films.

Solution-Processed Flexible Gas Barrier Films for Organic Field-Effect Transistors

The solution-processed gas barrier film was fabricated and used for organic field-effect transistors (OFETs). Organic-inorganic hybrid sol-gel materials and cyclic transparent optical polymer (CYTOP) were used as the bottom and top layers of the barrier films, respectively, to effectively protect against gas permeation through the barrier films. The organic-inorganic hybrid material includes sol-gel precursors and amphiphilic polymers. The conventional sol-gel precursors form siloxane bonds by sol-gel reaction and form densely-packed rigid part in thin films. The alkoxysilane-functionalized amphiphilic polymer in the sol-gel precursor solutions has two hydrophobic segments and a hydrophilic segment. The amphiphilic polymer with reactive alkoxysilane groups at both ends of the hydrophobic segments can be involved in the sol-gel reaction, and they can act as surfactants to surround the conventional precursors stabilizing the nanoparticles formed by the hydrolytic condensation reaction of precursors. The amphiphilic polymer also provides flexibility for hybrid sol-gel thin films. CYTOP was used to introduce hydrophobicity on top of the organic-inorganic hybrid thin films. The barrier films containing the organic-inorganic hybrid and hydrophobic CYTOP layers were applied to OFETs and exhibited notable gas barrier properties, high transparency, and flexibility. The encapsulated OFETs with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) as an organic semiconductor showed a slight decrease in hole mobility from 0.13 to 0.11 cm2 V-1 s-1, while the OFETs without barrier films showed a mobility decrease from 0.11 to 0.03 cm2 V-1 s-1.

Understanding the effects of electric-field-induced phase transition and polarization loop behavior on the energy storage performance of antiferroelectric PbZrO3 thin films

Antiferroelectric PbZrO3 (PZO) thin-films were fabricated by pulsed laser deposition (PLD) and sol-gel techniques to investigate the effect of antiferroelectric-ferroelectric (AFE-FE) phase transition on the energy storage performance. The (100)-oriented PLD thin-films have a square-double polarization-electric field (P-E) hysteresis loop with a sharp-phase transition and the (111)-oriented sol-gel thin-films have a slanted P-E loop with a diffused-phase transition. The difference in the phase transition fields between PLD and sol-gel thin-films could be attributed to their difference in crystalline orientations. In this case, the antipolar AFE phase is along the [110]-direction of the original tetragonal-cell under zero electric-field, while the antipolar AFE phase switches into the polar FE phase under a strong enough electric-field and the tetragonal-cell becomes rhombohedral with the polar direction of [111]. Due to the larger maximum polarization and slimmer P-E loop, the higher recoverable energy-storage (Ureco) of 19.4 J/cm3 and larger energy-efficiency (η) of 70.8%, are achieved in sol-gel thin-films. More importantly, this film shows excellent charge-discharge cycling endurance of both Ureco and η values after 1010 cycles and good thermal-stability under a wide operating temperature. From the viewpoint of applications, the sol-gel method is considered to be a promising approach towards producing low-cost PZO thin-films for high-efficiency energy-storage devices over a broad temperature range.

Controllable doping and passivation of ZnO thin films by surface chemistry modification to design low-cost and high-performance thin film transistors

Solution-processed metal oxide thin-film transistors have become more popular as they can be used to fabricate transparent and flexible electronics at low cost. However, additional and complex processes for trap-site passivation and doping hinder the potential of the low-cost solution process. This study introduces a surface passivation process involving treatment with 3-aminopropyltriethoxysilane (APTES) that can enhance the electrical properties of ZnO sol–gel thin films. Optical, chemical, and structural analyses of ZnO sol–gel thin films revealed that their trap sites were passivated successfully through APTES treatment under basic conditions. Taking advantage of this process, high-mobility and negligible-hysteresis ZnO thin-film transistors were successfully fabricated, showing an Ion/Ioff of 105, hysteresis as low as 1.13 V, and mobility of up to 0.117 cm2/Vs. Furthermore, a characteristic transition of ZnO sol–gel thin films from semiconductive to semimetallic was observed during investigations with various APTES concentrations.

Synthesis of N-CNT/TiO2 composites thin films: surface analysis and optoelectronic properties

In this study, we have investigated the surface analysis and optoelectronic properties on the synthesis of N-CNT/TiO2 composites thin films, using sol gel method for a dye synthetized solar cell (DSSC) which is found to be simple and economical route. The titanium dioxide based solar cells are an exciting photovoltaic candidate; they are promising for the realization of large area devices. That can be synthetized by room temperature solution processing, with high photoactive performance. In the present work, we stated comparable efficiencies by directing our investigation on obtaining Sol Gel thin films based on N-CNT/TiO2, by dispersing nitrogen (N) doped carbon nanotubes (N-CNTs) powders in titanium tetraisopropoxyde (TTIP). The samples were assessed in terms of optical properties, using UV—visible absorption spectroscopic techniques. After careful analysis of the results, we have concluded that the mentioned route is good and more efficient in terms of optoelectronic properties. The gap of “the neat” 0.00w% N-CNT/TiO2 is of 3eV, which is in a good agreement with similar gap of semiconductors. The incorporated “w%NCNTs” led to diminishing the Eg with increasing N-CNTs amount. These consequences are very encouraging for optoelectronic field.

Micro/Nanostructure Engineering of Epitaxial Piezoelectric α-Quartz Thin Films on Silicon.

The monolithic integration of sub-micron quartz structures on silicon substrates is a key issue for the future development of piezoelectric devices as prospective sensors with applications based on the operation in the high-frequency range. However, to date, it has not been possible to make existing quartz manufacturing methods compatible with integration on silicon and structuration by top-down lithographic techniques. Here, we report an unprecedented large-scale fabrication of ordered arrays of piezoelectric epitaxial quartz nanostructures on silicon substrates by the combination of soft-chemistry and three lithographic techniques: (i)laser interference lithography, (ii) soft nanoimprint lithography on Sr-doped SiO2 sol-gel thin films, and (iii) self-assembled SrCO3 nanoparticle reactive nanomasks. Epitaxial α-quartz nanopillars with different diameters (from 1 μm down to 50 nm) and heights (up to 2 μm) were obtained. This work demonstrates the complementarity of soft-chemistry and top-down lithographic techniques for the patterning of epitaxial quartz thin films on silicon while preserving its epitaxial crystallinity and piezoelectric properties. These results open up the opportunity to develop a cost-effective on-chip integration of nanostructured piezoelectric α-quartz MEMS with enhanced sensing properties of relevance in different fields of application.