Sol-gel Thin Films Research Articles

The Influencing Factors for the Thickness of Silica Sol-Gel Film by the Dip Coating Process

The porous sol-gel silica thin films were prepared on silicon substrate by the dip coating process. The surface roughness and microstructure of the film was measured by optical microscopy and AFM .Coating thickness and refractive index were measured by ellipsometry method. Influence of withdrawal speed and concentration of colloid was investigated for the thickness of silica. The relation of the thickness and withdrawal speed was fitted by the linear and power functions, and the results were analyzed and compared. It was found that the films with the same thickness prepared by different concentrations of colloid have different refractive indexes. The experimental results indicated that the thickness and refractive index of the film can be controlled by changing the withdrawal speed and colloid concentration.

Study of sol-gel Cu-doped Al2O3 thin films: structural and optical properties

We present a study of of copper-doped Al2O3 films prepared by sol-gel deposition. The films were spin-coated on Si and quartz substrates and a high-temperature treatment was conducted in the range 500 – 800 °C in oxygen or nitrogen. The impact was followed of the annealing procedures on the AlxCu1-xO3 films's properties. XRD was used to determine the films' structure; it revealed a mixture of amorphous and crystalline phases. Optical characterization was performed by UV-VIS spectroscopy. The sol-gel films prepared are very transparent. The band gaps of the Al-Cu-O films were estimated from the optical data, with the values ranging from 3.4 to 4.8 eV depending on the Cu content, the annealing and the gas ambients.

Effect of different technological approaches on the optical properties of ZnO sol-gel thin films

The work presents a sol-gel approach with two types of solvents for ZnO films deposition in view of varying the films' structural and optical properties. The ZnO films were characterized by X-Ray diffraction (XRD) and found to have a polycrystalline wurtzite phase structure. The grain sizes and the lattice parameters were determined; it was established that the ZnO crystallites' size increases with the annealing temperatures from 25 nm to 36 nm. The ZnO films possess high transmittance in the visible spectral range. Further, the optical band gap values were estimated. The material's vibration properties were analyzed by Fourier Transform Infrared (FTIR) spectroscopy. The two technological sol-gel approaches for deposition of ZnO films, using two different solvents, proved to be successful in producing structures with different structural and optical properties. The thin films fabricated were very smooth and uniform and exhibited a high transparency in the visible spectral range.

Simulation of Negative Differential Resistivity in Thin Ferroelectric Films

I-V dependences of sol-gel PZT thin films containing areas with negative differential resistivity are studied. For their simulation a modified model taking into account simultaneous existence of different leakage components (the Dawber-Scott diffusion current, the ohmic one, and space charge limited one) is proposed. The proposed model extends the simulation range to the high-voltage values, and increases the accuracy determination of the oxygen vacancies concentration, as well as depletion width regardless of the test voltage rate in comparison with the Dawber-Scott model.

A Single Chip Integrated Sol-Gel Thin Film LC Sensor for Measuring Moisture in ppm Level

A simple single chip integrated moisture sensor capable of wireless operation through inductive coupling has been proposed. The structure consists of planar inductor and parallel plate capacitor fabricated by sol-gel thin film of γ-Al2O3 (5 μm thick). The sensor chip is capable of measuring ppm level moisture in gaseous environment. The resonance frequency shift of the sensor in the presence of moisture excited by a high frequency ac signal is measured by impedance analyzer (4294A Agilent). The frequency sensitivity of the structure for a moisture range from 100 to 5000 ppm is nearly 119 Hz/ppm. The response and recovery times are 7 and 24 s, respectively. The experimental results show that the output is linear (4.2%), reproducible and stable. Device can be applied for wireless moisture measurement in ppm level.

Free-standing SnO2/MWCNT nanocomposite anodes produced by different rate spin coatings for Li-ion batteries

Free standing SnO2/multiwalled carbon nanotube (MWCNT) nanocomposite anode materials were prepared for Li-ion batteries by sol–gel technique. Firstly, SnO2 precursor sols were synthesized after removing chloride ions. Then the sols coated on MWCNT buckypapers, which used as substrates to form nanocomposite electrodes by spin coating method. Sintered nanocomposite structures were then characterized by field emission gun-scanning electron microscopy (FEG-SEM), energy dispersive X-ray spectrometer (EDS), and X-ray diffraction (XRD) analyses. Electrochemical tests were performed for the produced electrodes, assembled as CR2016 cells. The effect of spin rate on the anode capacity was investigated. Coating on the MWCNT buckypapers was thought to use as mechanical support to prevent electrode failure and prevented the formation of cracks of the sol–gel thin film on the MWCNT surfaces. The results showed beneficial effects to prevent mechanical disintegration and subsequent anode pulverization of SnO2 anodes because of huge volume increase during lithium intercalation. The results indicated that the nanocrystalline SnO2/MWCNT composites are suitable for applying as an anode electrode for Li-ion batteries to increase electrochemical energy storage performance.

Photoconductive UV detectors based heterostructures of Cd and Mg doped ZnO sol gel thin films

The photoconductive properties of single (CdZnO/MgZnO, Li:CdZnO/MgZnO) and double (MgZnO/ZnO/MgZnO, CdZnO/ZnO/MgZnO) heterostructures were studied by sol gel method at different annealing temperatures. These structures were analysed by X-ray diffraction (XRD) and photoluminescence (PL). The photoconductive measurements were carried out by semiconductor parametric analyzer. The hexagonal wurtzite structure was retained with good crystalline nature. The UV related strong near band edge (NBE) emission, ultra violet and blue green emission peaks were observed from PL spectra. Photocurrent was observed to have enhanced as a result of annealing temperature and Li substitution in the doped ZnO lattice. The increase of responsivity at ten orders of magnitude was found. Meanwhile, the dark current was found to have increased slightly due to the adsorption of O2 and H2O molecule on the surface of the film. Sub-linear behaviour of photocurrent was also observed.

Aluminum-doped zinc oxide sol–gel thin films: Influence of the sol's water content on the resistivity

Thin films of indium tin oxide (ITO) have gained substantial interest due to their optical and electrical properties. Since ITO is an expensive material and indium is a rare element, considerable attempts have been made to replace it by, e.g., aluminum-doped zinc oxide (ZnO:Al). The production of ZnO:Al is less cost-intensive, especially if the sol–gel technique is applied, while its properties are comparable to those of ITO. In this study, we demonstrate that the electrical properties of ZnO:Al thin films can be improved considerably by the addition of small amounts of ultrapure water to the dip coating solution during the preparation. The lowest resistivity obtained with a film prepared from a sol containing 0.2M water is 2.8·10−3Ωcm. Optical modeling thus indicates an improvement of the free carrier mobility of films prepared from sols in the presence of additional water. The films prepared have an average thickness of 340nm and a solar transmittance above 85% after annealing in a forming gas atmosphere. Clearly, the addition of water to the sol has a positive impact on the resistivity of the final ZnO:Al thin film. We suggest the observed increase of the free carrier mobility to be due to an improved electron transfer at the grain boundaries between the spherical nanoparticles.

Hybrid sol–gel thin films doped with a pH indicator: effect of organic modification on optical pH response and film surface hydrophilicity

Optical sensors for application in innovative wearable sensing systems such as textile-integrated systems and wireless sensor platforms rely on the development of low-cost multifunctional materials compatible with standard fabrication technologies. We are developing optically responsive pH sensitive sol–gel coatings for integration with a mobile wireless smart tag sensing system. For this application, we have fabricated a range of thin pH sensitive films using bromocresol green (BCG) indicator immobilised in inorganic–organic silica hybrid matrices prepared by a sol–gel method and deposited by spin-coating onto glass substrates. The surface hydrophilicity of the films were varied by using the inorganic sol–gel precursor tetraethoxysilane together with either methyltriethoxysilane, ethyltriethoxysilane, phenyltrimethoxysilane or glycidoxypropyltrimethoxysilane as organically modified sol–gel precursors, co-polymerised in different ratios. Spectral characterisation of the films was performed using visible absorption spectroscopy. The shift in absorption maxima and other spectral changes of the different matrices have been identified, and the apparent pK app values of the immobilised BCG pH indicator determined. The surface wettability properties of the films have been studied by measuring the contact angle of water, formamide and diiodomethane which has allowed the estimation of the surface free energy (SFE) using three different models: Owens–Wendt, Wu and van Oss-Chaudhury-Good. It is shown that the SFE of the hybrid films is directly related to the type and the degree of organic modification, which in turn has a significant effect on the pH response-time of these sensing films.

A third-generation hydrogen peroxide biosensor based on horseradish peroxidase immobilized by sol–gel thin film on a multi-wall carbon nanotube modified electrode

A third-generation H2O2biosensor was developed by using tetraethoxy-silicone sol–gel film to immobilize horseradish peroxidase (HRP) on a multi-walled carbon nanotubes (MWNTs) modified glass carbon electrode.

Conductive sol–gel films

We report that silver nanowires (AgNW) entrapped in a series of sol–gel thin films – zirconia, alumina, titania and silica – provide conductive, highly flexible, highly adhesive, transparent thin films with excellent thermal and electrical stability. These films were subjected to intensive study and characterization including resistance, percolation analysis, critical bending angle analysis, critical thermal stability, transparency, and more. Remarkably, when resistance is kept constant, all of the studied parameters follow the same order – either increasing or decreasing – of ZrO2, SiO2, Al2O3, and TiO2. A detailed interpretation, based on the particle-sizes of the elementary sol–gel building blocks, is provided. In particular we found that the AgNWs re-enforce the ceramic matrices, thus providing excellent electroconductive stability with low silver concentration and high degree of homogeneity of the dopant distribution. Having in mind that current materials for films with these properties are often accompanied by disadvantages (such as brittleness, scarcity and poor adhesion) we offer the new conductive films as an interesting alternative to be considered.

In vitro Assessment of Photocatalytic Titanium Oxide Surface-modified Stainless Steel and Nickel Titanium Orthodontic Wires for its Antiadherent and Antibacterial Properties against Streptococcus mutans

Objective: To assess the antiadherent and antibacterial pro­ perties of surface-modified stainless steel and nickel titanium orthodontic wires with photocatalytic titanium oxide against Streptococcus mutans (S. mutans). Materials and methods: This study was done on 80 specimens of orthodontic wires each of 5 cm in length. The specimens were divided into eight test groups, four groups were used for anti ­ adherent test, and other four groups were used for antibacterial test. Each group consisted of 10 specimens. The groups contain­ ing uncoated stainless steel and uncoated nickel titanium wires acted as control group for their respective experimental group, containing coated stainless steel and nickel titanium wires. Surface modification of both stainless steel and nickel titanium orthodontic wires with photocatalytic titanium oxide was carried out by sol gel thin film dip coating method. Orthodontic wires were then subjected to microbiological tests for assessment of the antiadherent and antibacterial properties of photocatalytic titanium oxide coating against S. mutans. Results: Photocatalytic TiO2 coating on the wires prevented the adhesion of S. mutans to the orthodontic wires, hence demonstrating antiadherent property against S. mutans. Also, photocatalytic TiO2 coating on the surface of orthodontic wires demonstrated antibacterial effect against S. mutans. Conclusion: Surface modification of orthodontic wires with photocatalytic titanium oxide was found to have antiadherent and antibacterial properties against S. mutans.

Protein-templated gold nanoclusters sequestered within sol–gel thin films for the selective and ratiometric luminescence recognition of Hg2+

Sequestration of bovine serum albumin (BSA)-stabilized gold nanoclusters (AuNCs@BSA) prepared using microwave assistance within sol-gel-derived mesoporous silica films permits the selective and highly sensitive quenchometric detection of aqueous Hg(2+) (limit of detection = 600 pM) with luminescence signal arising from oxidized BSA allowing for an analytically robust and reliable ratiometric detection. Overall, this work highlights a number of important advances, including the highest luminescence quantum yield reported to date for a protein-templated luminescent noble metal nanocluster (13%) made possible using a microwave-mediated synthesis followed by cold incubation. We also demonstrate the clear advantage of exploiting the luminescence signal arising from oxidized BSA as an internal reference to generate selectivity of response to Hg(2+). A careful Stern-Volmer quenching analysis reveals the persistence of two unique quenching sites for AuNCs@BSA entrapped within a sol-gel-derived glass, a minor population of which is unquenchable. Finally, based on these AuNCs@BSA nanosensors, we advise a path forward for paper-based indicator strip detection of heavy metals in aqueous streams, the implementation of which can be performed using the unaided eye, making it a meaningful approach for routine screening and in resource-limited situations.

Improvement of the corrosion performance of 304L stainless steel by a nanostructure hybrid coating/henna extract

Purpose – The purpose of this investigation was to study the corrosion protection and structural characteristics of a hybrid organic-inorganic thin film preloaded with green corrosion inhibitor for anticorrosive protection of stainless steel 304L. Design/methodology/approach – An ethanol solution of the polymerized 3-methacryloxypropyltrimethoxysilan and tetraethylorthosilicate was mixed with henna extract to give homogeneous sols. The morphology, composition and adhesion of hybrid sol-gel coatings were examined by SEM, EDX and pull-off tests, respectively. The surface chemistry of the hybrid sol-gel coatings was investigated with polarization scans and electrochemical impedance spectroscopy (EIS) in the physiological saline solution. Findings – The polarization curves and EIS data were in agreement. Henna extract additions significantly increased the corrosion protection capability of the sol-gel thin film to greater than 85 percent in the physiological saline solution. In addition, the doped hybrid coating on stainless steel 304L was useful in 3.5 percent NaCl solution. Originality/value – There have been few reports on the hybrid organic-inorganic thin films preloaded with corrosion inhibitor, as described in the paper, and this environmentally friendly coating on stainless steel 304L was found to be highly effective for industrial applications.

High-Mobility Pentacene-Based Thin-Film Transistors With Synthesized Strontium Zirconate Nickelate Gate Insulators

Strontium zirconate nickelate [(SZN); Sr0.69Ni0.47Zr0.085O3.75] was synthesized through a sol-gel method by adding nickel (II) acetylacetone instead of titanium isopropoxide, which can effectively make thin films smoother and further act as gate insulators applied in pentacene-based thin-film transistors. Sol-gel SZN thin films were investigated by X-ray photoelectron spectroscopy and atomic force microscopy to confirm the chemical composition and smooth surface roughness, and the latter property was found to be compatible for pentacene thin film growth. Thus, the electrical characteristics of pentacene-based transistors exhibited a high mobility of 10.04 cm2 V-1s-1 low threshold voltage of -1.51 V,, and low subthreshold swing slope of 350 mV/decade. To realize high mobility mechanisms, intermolecular coupling and reorganization energy of pentacene thin films were introduced. Another purpose of this introduction was to evaluate the higher hopping rate of carriers between the adjacent pentacene molecules on SZN derived from the March-Hush equation than other high-κ traditional sol-gel insulators.

Sol–gel silica controlled release thin films for the inhibition of methicillin-resistant Staphylococcus aureus

The incidence of methicillin-resistant Staphylococcus aureus (MRSA) infection has significantly increased. Generally, the success of this bacterium as a pathogen is attributed to its ability to adhere to surfaces and remain there, under the protection of an extracellular matrix known as biofilm. To combat MRSA with regular doses of vancomycin, efforts are continuously underway to increase its effectiveness. A promising technique is to use combinational therapeutics. In vitro experiments showed that farnesol can be used as an adjuvant with conventional antibiotics. Farnesol is a natural sesquiterpenoid and quorum-sensing molecule. The biggest obstacle to using this concept is that farnesol is highly water insoluble. This compromises its bioavailability if it were to be used along with vancomycin at the site of infection when the treatment needs to be administered in vivo. Herein we designed an efficient therapeutic strategy for the simultaneous delivery of both antibiotic and adjuvant in order to treat MRSA infections. We demonstrate that sufficient quantities of both vancomycin and farnesol can be incorporated into sol–gel silica applied as thin films on an implant surface. The incorporation of the hydrophobic farnesol does not affect the stability of the thin films and neither does it affect the controlled release of vancomycin. The data demonstrate the potent adjuvant effect of farnesol on vancomycin in inhibiting MRSA infection. In vitro experiments show the complete inhibition (106 fold reduction in growth compared to control) of methicillin-sensitive S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA) when the ratio of vancomycin to farnesol in the sol–gel silica films is optimized. The local delivery of antibiotics minimizes the need for systemic antibiotics. The incorporation of vancomycin and farnesol into thin sol–gel films represents a new treatment paradigm for the topical delivery of antibiotics with adjuvant. The potential clinical benefits are significant and include avoiding the need for revision surgery, preventing surgical site infection and controlling healthcare costs.

Modification of polyurethane‐coated fabrics by sol–gel thin films

AbstractSol–gel derived silica and hybrid films from tetraethoxysilane (TEOS) and 3‐glycidyloxypropyltrimethoxysilane (GLYMO) were deposited by dip‐coating, in order to find the best treatment. GLYMO‐based treatments preserved textile feel of the material. The coatings were characterized by infrared spectroscopy (IR), scanning electron microscopy (SEM), energy‐dispersive X‐ray spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and water contact angle measurement. IR spectra confirmed the existence of silicate network and successfully conducted modification for both precursors. DSC and TGA showed that the conditions of hydrolysis had greater influence on TEOS than GLYMO‐based treatments. Both treatments shifted the degradation onset to higher temperatures. SEM images showed that polyurethane surface and pores were completely covered and filled by silica or hybrid thin films forming a composite organic–inorganic coating. Hydrophobic effect was preserved for all the samples, except for a single treatment of the GLYMO, due to its polar epoxy group. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2014,131, 39914.

Strain Measurement and Mechanical Property Evolution in Sol-Gel PZT Thin Films

Cracking is a notorious issue with sol-gel PZT thin films, and film failure in this manner is difficult to reliably predict due to constantly evolving mechanical properties. In this work, two non-contact experimental methods are used to quantify the stiffness response of PZT sol-gel during thermal loading and the influence of solution additives, such as lactic acid and gycerol. The mechanical properties are determined using a digital image correlation method to measure strain and wafer curvature measurements to estimate bi-axial stress. Results show the transition from a compliant response to more glassy behavior. The importance of this changing stiffness is shown for a specific application of a buckled MEMS actuator and in relation to adhesion based failure modes.

An investigation into relative humidity measurement using an aluminosilicate sol–gel thin film as the active layer in an integrated optical Bragg grating refractometer

We have demonstrated that a planar integrated optical Bragg grating sensor chip, fabricated using a direct UV-writing approach, can perform as an all-optically accessed relative humidity sensor. Using a simple mesoporous silica thin film enabled operation across the range 0–100%RH, however a thickness decrease of 25% caused by hydrolytic degradation of the silica thin film resulted in a permanent decrease in sensitivity after 1–2 weeks use. Grafting the thin-film with aluminium oxide is shown here to prevent hydrolytic degradation with long-term sensitivity maintained. Aluminium oxide grafting offers the additional benefit of enhanced sensitivity, up to 3.5 times that of the silica analogue; the sensitivity of the aluminium oxide modified thin film device=0.69±0.05%RH/pm, in the range 0–60%RH within a gas flow system, whilst the sensitivity of the silica thin film sensor device=2.47±0.18%RH/pm. Long-term measurements, in static ambient lab conditions, were in excellent agreement with a commercial humidity sensor and also indicated that the thermal compensation afforded by the on-chip reference gratings was sufficient to prevent any drift in calibration. As with all humidity sensors, contamination of the sensor occurred. But for the aluminium oxide device, this was easily removed by rinsing with common organic solvents and the sensitivity was repeatedly and reproducibly restored. The device is accessed using optical fibre technology and is intrinsically safe in flammable environments.

Sol–gel thin-film based mesoporous silica and carbon nanotubes for the determination of dopamine, uric acid and paracetamol in urine

This work describes the preparation, characterization and application of a hybrid material composed of disordered mesoporous silica (SiO2) modified with multiwalled carbon nanotubes (MWCNTs), obtained by the sol–gel process using HF as the catalyst. This hybrid material was characterized by N2 adsorption–desorption isotherms, X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission microscopy (HR-TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). This new hybrid material was used for the construction of a thin film on a glassy carbon electrode. The modified electrode using this material was designated SiO2/MWCNT/GCE. The electrocatalytic properties of the electrode toward dopamine, uric acid and paracetamol oxidation were studied by differential pulse voltammetry. Well-defined and separated oxidation peaks were observed in phosphate buffer solution at pH 7.0, in contrast with the ill-defined peaks observed with unmodified glassy carbon electrodes. The electrode had high sensitivity for the determination of dopamine, uric acid and paracetamol, with the limits of detection obtained using statistical methods, at 0.014, 0.068 and 0.098µmolL−1, respectively. The electrode presented some important advantages, including enhanced physical rigidity, surface renewability by polishing and high sensitivity, allowing the simultaneous determination of these three analytes in a human urine sample.