Sol-gel Thin Films Research Articles

Low-cost piezo-MEMS speaker technology

In this paper, we are reporting low-cost piezo-MEMS speakers based on an SOI-free process. The speaker membrane is made of a 1.5 μm thick polyimide passive layer, obviating the need for SOI wafers. The custom-made PZT solution for depositing 1 μm sol-gel PZT thin films has further reduced fabrication costs. The membranes are realized by a backside Deep Reactive Ion etching (DRIE) on the silicon wafer. Laser doppler vibrometer (LDV) characterization has shown the resonance frequency of a single speaker element with a diameter of 2.5 mm to be 30 kHz with a sensitivity of 100 mm/s/V, whereas sensitivity at 4 kHz is around 1.2 mm/S/V. The speaker element can produce around 70–90 dB in-ear Sound Pressure Level (SPL) response in audible frequencies (100 Hz – 10 kHz) at 5 V excitation as shown by a simulation in an ear-coupler which is in line with the LDV measurements. Moreover, on average, the fabricated 4 × 4 arrays have shown a 10 dB higher SPL than a single-speaker element as measured by a microphone.

Rapid laser-induced low temperature crystallization of thermochromic VO2 sol-gel thin films

The thermochromic properties of vanadium dioxide (VO2) offer great advantages for energy-saving smart windows, memory devices, and transistors. However, the crystallization of solution-based thin films at temperatures lower than 400°C remains a challenge. Photonic annealing has recently been exploited to crystallize metal oxides, with minimal thermal damage to the substrate and reduced manufacturing time. Here, VO2 thin films, obtained via a green sol–gel process, were crystallized by pulsed excimer laser annealing. The influence of increasing laser fluence and pulse number on the film properties was systematically studied through optical, structural, morphological, and chemical characterizations. From temperature profile simulations, the temperature rise was confirmed to be confined within the film during the laser pulses, with negligible substrate heating. Threshold laser parameters to induce VO2 crystallization without surface melting were found. With respect to furnace annealing, both the crystallization temperature and the annealing time were substantially reduced, with VO2 crystallization being achieved within only 60 s of laser exposure. The laser processing was performed at room temperature in air, without the need of a controlled atmosphere. The thermochromic properties of the lasered thin films were comparable with the reference furnace-treated samples.

Optical and mechanical characterization of sol-gel thin films of ZrO2 stabilized with different Y2O3-doping mol%

The influence of the molar proportion of Y2O3 dopant (3, 9, and 12 mol%) on the optical and mechanical properties of ZrO2 thin films was studied. For this purpose, the three types of deposited thin films were first characterized microstructurally by spectrophotometry, optical microscopy, scanning electron microscopy, and X-ray diffractometry, and then their optical properties were measured by spectrophotometry and their mechanical properties by nanoindentation. It was found that they all crystallize in the cubic phase, even for dopant contents that would not do so in bulk materials. It was also found that, with increasing Y2O3 dopant, the refractive index increases while the optical absorption and extinction coefficients, the hardness, and the elastic modulus decrease. These trends are due to the poorer crystallinity and greater distortion of the crystal lattice of YSZ thin films with increasing Y2O3 dopant. The results are especially relevant for applications which require certain conditions of reflection and absorption of light energy, while maintaining optimal mechanical properties.

Exploring ferroelectric properties of sol-gel lead zirconate titanate nano-crystalline thin films and ceramics by doping ytterbium for energy-storage application

This work concerns how ytterbium affects the ferroelectric characteristics of sol-gel Yb doped lead zirconate (PYZ) and lead zirconate titanate (PYZT) thin films and ceramics. Lead zirconate Pb(0.9) )-Yb0.01 Zr (0.09 (PYZ) and lead zirconate titanate Pb0.9-Yb0.01-Zr0.045-Ti0.045 (PYZT) doping with ytterbium thin films were prepared on a glass substrate using a sol-gel doctor blade coating technique at low temperature. The structure of the synthesized thin films and ceramic nanocrystals was examined using X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron diffraction (ED). PYZ and PYZT ceramics had 11.8 nm and 30.65 nm respectively, whereas PYZ and PYZT thin films featured nanocrystals that were, respectively, 17.5 nm and 13.87 nm in size. Particle size was determined to be entirely consistent across the TEM and XRD data. Dielectric investigations showed that the dielectric constant was dropping for ceramic samples as compared to thin film samples. According to hysteresis loop estimates, the energy-storage density and efficiency for thin films and ceramics were average approximately 39.35 J cm−3 and 66.02 percent for (PYZ) and 71.33 J cm−3 and 70.91 percent for (PYZT), respectively. The results showed that the ferroelectric, dielectric, and structural properties of materials affected the ytterbium content. (PYZ) and (PYZT) thin films and ceramics may have a prospective use in energy storage devices.

Silica-Titania Integrated Photonics Platform-Based 1 × 2 Demultiplexer Utilizing Two Serially Cascaded Racetrack Microrings for 1310 nm and 1550 nm Telecommunication Wavelengths

Herein, a numerical analysis of a 1 × 2 demultiplexer based on a silica-titania integrated photonics platform is conducted via the finite element method. The structure is composed of two coupled racetrack microrings (RTMRs) and a subwavelength grating (SWG) structure for the demultiplexing of 1310 nm and 1550 nm telecommunication wavelengths. The material platform selected for this design is highly attractive due to its refined optical, physical, and chemical properties. Moreover, silica-titania sol-gel thin-films can be deposited on glass substrates with the dip-coating method. The proposed device has a small footprint of 84 × 125 μm2 and offers crosstalk as low as ~−6.6 dB and ~−9.04 dB for 1550 nm and 1310 nm, respectively. We are convinced that this study promotes the use of the silica-titania platform for the development of low-cost on-chip optical communication devices for signal multiplexing and demultiplexing.

Characterizations of extrinsically doped CZTS thin films for solar cell absorbers fabricated by sol-gel spin coating method

In copper zinc tin sulfide (CZTS) based thin films, copper zinc antisite (CuZn) represents the major p-type acceptor defect with low formation energy. This antisite can create defect-dominated carrier recombination hotspots, which reduces power conversion efficiency. Extrinsic doping in cationic sites of the CZTS thin film during synthesis is an effective way to passivate this defect. It has previously been reported that employing Cd as a dopant can passivate this defect and improve film quality. Injecting toxic Cd into non-toxic CZTS may create more problems than it solves. Simultaneous doping of Cd and other atoms like Mg is hypothesized to accomplish this goal of lowering Cd concentration. For this, it is important to know the effect of doping Cd and Mg separately under analogous experimental conditions. The structural, morphological, and optical properties, as well as the chemical bonding states, of sol-gel spin-coated CZTS thin films were investigated in this study using independent and controlled Cd and Mg doping. The fabrication procedure consisted of two steps: sol-gel spin coating followed by sulfurization. As a solvent for precursors in creating sols for spin coating processes, dimethyl sulfoxide (DMSO) was employed. Each dopant's precursor concentration was chosen to ensure an identical mole percentage of zinc precursor in the solution. The fabricated films were characterized undoped and doped forms using X-ray diffractometry (XRD) with Rietveld refinements, Raman spectroscopy, Field emission scanning electron microscopy (FESEM), 3D profilometry, and Ultraviolet-visible near-infrared (UV-Vis NIR) spectroscopy. The elemental composition ratio and chemical bonding states of the fabricated films were probed by Energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), respectively. Fabricated thin films exhibit distinct synergistic features depending on the dopant types used in the fabrication process. The crystal structure of the doped absorber and the chemical valence states of the other elements (Cu, Zn, Sn, and S) were not changed as shown by XRD, Raman, and XPS analyses. The main diffraction peak at the (112) plane is found at Bragg's diffraction angle of 28.6° for an undoped sample. This peak shifts left upon Cd and Mg doping to 28.2° and 28.4° owing to different ionic radii of dopants relative to Zn ions, which indicates the presence of dopants in the fabricated films. Raman spectroscopy probes the Cu-Sn-S secondary phases: cubic Cu2SnS3 at 305 cm−1 in the undoped sample, tetragonal Cu2SnS3 at 296 cm−1 in the Cd-doped sample, and orthorhombic Cu3SnS4 at 291 cm−1 in the Mg-doped sample. From the EDS results, it appears that 45% of the Zn atoms in both the Cd-doped and Mg-doped samples are partially substituted by Cd and Mg, respectively. The optical band gap for Cd-doped samples decreases from 1.61 eV to 1.56 eV when compared to undoped samples. For Mg-doped samples, the optical band gap shrinks even more to 1.1 eV. This corresponds to shifting of absorbance spectra to higher wavelengths known as “redshift”. Urbach energy is determined to be 284 meV and 1072 meV for Cd and Mg doped samples, respectively, to address band tailing issues. Mg doping enhances crystallite size slightly while decreasing microstrain and dislocation density. The RMS surface roughness of Cd and Mg doped samples is 147 nm and 280 nm, respectively. Nevertheless, the primary peaks of Cu, Zn, Sn, S, Cd, Mg, and O, as well as the secondary photoelectron emission lines of these elements, are all found to be present and identified in both the doped samples. These results imply that Cd and Mg doping in the Zn-cationic site of virgin CZTS thin film can be regulated systematically to get appreciated features as a solar cell absorber.

Photoresponse of solution-processed transparent heterojunction ultraviolet photodetectors composed of n-type ZTO and p-type NiO-based semiconductor thin films

The n-type zinc-tin oxide (ZTO) and p-type nickel oxide (NiO) semiconductor thin films were prepared by the sol-gel spin coating process. The influence of 4 at% Mg and Cu doping on the microstructural, optical, and electrical characteristics of NiO thin films and physical properties of ZTO thin film were investigated. In addition, bi-layered ZTO/NiO thin films were grown onto Corning glasses to develop a visible light transparent p-n heterojunction ultraviolet (UV) photodetector. Mg doping slightly enhances the visible transparency and enlarges the optical bandgap energy, while Cu doping decreases the visible transparency and narrows the optical bandgap energy. The electrical properties of NiO sol-gel thin films were improved through Mg and Cu doping. According to the current-voltage (I–V) characteristic curves, the n-ZTO/p-NiO:Mg photodetector had the highest rectification ratio of 11.7 and on-to-off current ratio of 18.3 at 5 V bias. Under UV illumination, the three kinds of devices all exhibited a self-powered nature, while the Mg- and Cu-doped NiO devices had better photoresponse characteristics than the undoped NiO device. It was found that the Mg-doped NiO devices displayed the best photoconductivity gain and sensitivity, and acceptable response speed and responsivity, with applied reverse bias of 1 V.

Versatile Micro and Nano Patterning Technique for TiO2 and TiN‐Based Sol–Gel Thin Films

AbstractThis paper describes a tailored and versatile technology route to achieve micro and nanopatterning of both TiO2 and TiN thin films using a direct photo patternable sol–gel approach combined with a rapid thermal annealing and nitridation process. The approach consists in multi‐scale structuring TiO2 using laser direct writing or mask/colloidal lithography on a wide range of substrates including materials and of diverse geometry. These patterned TiO2 layers can subsequently be converted into a micro‐structured metallic TiN layer using a direct rapid thermal nitridation process that results in metallic thin films (TiN). This work demonstrates the versatility of this complete chain of soft chemistry based new process for patterning TiO2 and TiN thin films thereby avoiding expensive processes such as etching and lift‐off, while preserving their diffractive properties and high thermal stability, that is, up to 1000 °C under vacuum. The process is compatible with different kinds of substrates of different shapes and sizes. These results pave the way for developing a cost‐effective and time‐saving approach to different cutting‐edge applications such as metasurfaces, sensors, and applications in the luxury and decoration sector.

Defects control and origins of blue and green emissions in sol-gel ZnO thin films

ZnO has broad application prospects in the field of LED lighting and display. However, the unclear luminescence mechanism limits the improvement of luminous efficiency and the realization of controllable luminescence. Herein, intrinsic defects (Zni and VO) in sol-gel ZnO thin films were controlled by post-annealed in different atmospheres (air, N2 and vacuum). The effect of annealing atmospheres on the intrinsic defects were quantitatively studied by XRD, Raman, XPS and EPR spectra, and the relationship between intrinsic defects and visible emissions were systematically investigated by excitation-dependent PL spectra under above bandgap and below bandgap excitations. Results show that N2-annealed ZnO film with more Zni defects and fewer Vo defects exhibits much stronger blue (∼440 nm and ∼450–480 nm) and green emissions (∼510–540 nm). Zinc interstitials states (including Zni and ex-Zni states) are the initial states of the electronic transitions for the blue (∼440 nm and ∼450–480 nm) and green emissions (∼510–540 nm). In addition, transitions from the conduction band to the VO level also emit a green peak at 525 nm, which can only be observed under above bandgap excitation. This work contributes to a better understanding of defect-induced blue and green emission in ZnO, which will help to achieve controllable luminescence and facilitate its commercial application as a luminescent material.

Highly sensitive and selective LPG sensor working below lowest explosion limit (LEL) at room temperature using as-fabricated indium doped SnO2 thin film

In the present work, an extremely responsive and cost-effective Liquified Petroleum Gas (LPG) sensor operating at room temperature has been fabricated with indium-doped SnO2 nanospheres. In/SnO2 was prepared via cost effective sol-gel route and thin films were fabricated using spin-coating technique. Further, the sample was characterized to study the elemental information, crystal structure, phase formation, morphology, optical absorption, etc through EDS, XRD, TEM, SEM, Raman, FTIR and UV–visible spectroscopy. The sensing characteristics of In/SnO2 thin film have been studied and it was found quite useful to detect the LPG below LEL (0.5–2.0 vol%). The maximum value of % sensor response was found 365.11 at 2.0 vol% whereas the minimum response and recovery times were found as 10 s and 14 s for 0.5 vol% of LPG.

A polydimethylsiloxane/deep eutectic solvent sol-gel thin film sorbent and its application to solid-phase microextraction of parabens

A novel, efficient and reproducible thin-film solid-phase microextraction method using the polydimethylsiloxane/deep eutectic solvent sol-gel as coating material was developed for preconcentration of parabens from environmental samples.New sorbent materials consisting of polydimethylsiloxane with nine new synthesized deep eutectic solvents were prepared by the sol-gel method, and these materials were used to prepare thin film coatings on a mesh support. Among obtained compounds, the highest efficiency in the extraction of parabens was demonstrated by a sorbent consisting of trihexyltetradecylphosphonium chloride and n-docosanol in a molar ratio 1:2 mixed with polydimethylsiloxane.For the proposed method, the parameters affecting the extraction efficiency of parabens were optimized, including the coating material type based on polydimethylsiloxane/deep eutectic solvent, the sample volume, the salting-out effect, the pH of the sample, the extraction time, the desorption solvent and time. The limits of detection and quantitation were from 0.023 to 0.062 ng mL−1 and 0.076–0.201 ng mL−1, respectively, and the enrichment factors ranged from 174 to 186. Under optimal conditions, the proposed method based on measurements for river and lake water achieved good precision between 2.7 and 5.9% and recovery ranging from 79.1 to 88.1%. The developed method was successfully applied to determine trace levels of parabens in the samples of surface waters collected from rivers and lakes.

Effect of Al-doping on the thermoelectric properties of CZTS thin film grown by sol-gel method

In this paper, thermoelectric (TE) properties of sol-gel grown Cu2-xAlxZnSnS4 thin films were investigated using Seebeck data. Several thermoelectric parameters such as Seebeck coefficient (71–110 μV/°C), electrical conductivity (29.4–36.98 S/cm) and power factor (1.48 × 10−5- 4.29 × 10−5 Wm−1C−2) were improved with the concentration of Al atoms. The improvement in thermo-electrical properties is linked to the transfer of the electron density to the Cu and Sn atoms, which generated the copper and tin based secondary phases. The formation of secondary phases with the addition of Al atoms in the host CZTS structure is resulted in the simultaneous improvement of Seebeck coefficient and electrical conductivity due to energy filtering effect. Additional measurements such as X-ray Diffraction, Raman Spectroscopy, and Scanning Electron Microscopy were performed to back up this argument. The formation of extra secondary phases with increasing Al atoms concentration was justified by XRD and Raman data. SEM images revealed the irregular formation of grain size having micro and nanoscale.

Antiadherent and antibacterial properties of TiO2-coated and TiO2:Ag-coated stainless steel orthodontic wires against S. mutans bacteria

Purpose Conventional orthodontic treatment with stainless steel orthodontic wires may be detrimental to oral health, as it con-tributes to demineralized lesions and increases adhesion and bacterial biofilm formation, which contributes to cavity development. An alternative that has been investigated to reduce the side effects of orthodontic treatment is the use of coating materials with antimicrobial nanoparticles. This study aims to evaluate the antiadherent and antibacterial properties of TiO2-coated and TiO2:Ag-coated stainless steel orthodontic wires against S. mutans bacteria. Methods In the sol-gel method, TiO2:Ag thin films were deposited on stainless steel orthodontic wires. Coated archwires were analyzed for their antibacterial and antiadherent properties. The evaluation of Streptococcus mutans adhesion to the orthodontic wires’ surface was conducted according to the type of coating used, biofilm formation assay, and measure-ment of the pH of the bacterial community. Results In the microbiological test, the TiO2:Ag coatings revealed a statistically significant difference in terms of microbial adhesion and biofilm formation by Streptococcus mutans. The TiO2:Ag coating on stainless steel wire increased pH levels in the saliva environment. Conclusions It can be concluded that antimicrobial orthodontic wires coated with silver-TiO2 nanoparticles using the sol-gel thin film are a promising choice for improving orthodontic treatment.

Influence of the nature of a polysaccharide on the surface-morphological and physicomechanical properties of sol–gel plates

Thin-film sol–gel plates based on chitosan and glucomannan polysaccharides in a wide range of compositions were obtained. The effect of the polysaccharide component on the SEM surface morphology and physicomechanical properties was evaluated. It was found that the introduction of the polysaccharide glucomannan (and/or Aloe Vera juice) into the composition of chitosan sol–gel plates increased the elastoplastic characteristics of the samples and had a significant effect on the surface relief of their solid phase. The obtained structures can be used for the design of bioadhesive sensor coatings with a high therapeutic effect, which is an urgent area for theranostics and regenerative medicine.

Optical Detection of VOC Vapors Using Nb2O5 Bragg Stack in Transmission Mode

In this study, an emphasis is put on vapor-sensitive Bragg stacks as an important class of optical sensors. All-niobia Bragg stacks were deposited by spin-coating of sol-gel Nb2O5 thin films alternated with mesoporous layers after proper design through optimization of operating wavelength and number of layers in the stack. Mesoporous Nb2O5 films with different morphology and identical structure were obtained using organic templates (Pluronics PE6200 and PE6800) and subsequent annealing. Transmittance measurements were performed as a detection method that offers technological simplicity and accuracy. It was demonstrated that stacks including PE6200 templated films exhibit higher sensitivity than stacks templated with PE6800. It was assumed and verified by computer-aided modelling of experimental data that mesoporous films prepared with addition of PE6200, although less porous, were more stable compared to those templated with PE6800, and did not collapse during the thermal treatment of the stacks. Furthermore, the reproducibility of optical response was studied by sorption and desorption cycles of acetone vapors. The suitability of all-niobia Bragg stacks for optical sensing of VOCs was discussed.

Improvement of Properties of Stainless Steel Orthodontic Archwire Using TiO2:Ag Coating

Orthodontic treatment carries the risk of major complications such as enamel demineralization, tooth decay, gingivitis, and periodontal damage. A large number of elements of fixed orthodontic appliance results in the creation of additional plaque retention sites which increase the risk of biofilm creation. Modification of the surface of orthodontic elements may prevent the formation of bacterial biofilm. In this paper, surface modification of stainless steel orthodontic wires with TiO2: Ag was carried out by the sol-gel thin film dip-coating method. To obtain the anatase crystal structure, substrates were calcined for 2 h at 500 °C. The properties of the obtained coatings were investigated using scanning electron microscopy, X-ray diffraction, and electrochemical tests. Corrosion studies were performed in a Ringer’s solution, which simulated physiological solution. SEM and XRD analyses of the coated surface confirmed the presence of Ag nanoparticles which may have antimicrobial potential.

All-Fiber Magneto-Optical Effect Using Nanoparticles Doped Sol-Gel Thin Film Deposited Within Microstructured Fibers

An innovative functionalization method allowing to greatly increase the Faraday effect of microstructured silica-based optical fibers is described. This method relies on the coating of a sol-gel thin film doped with magnetic field sensitive cobalt ferrite nanoparticles around the suspended core of the fiber. By varying the functionalization process parameters, both the film thickness and nanoparticle concentrations can be tailored to optimize the fiber magneto-optical response by a proper adjustment the interaction between the guided mode and the composite film. Experimental measurements in an all-fiber configuration on several samples diversely functionalized show a significant polar magneto-optical effect. A polarization rotation as large as 10° has been measured at 1550 nm when the most efficiently functionalized fiber is located inside a 0.7 cm air gap electromagnet at 1 Tesla. The present method and the developed all-fiber components are very promising candidates for the design of fiber-based sensors of magnetic fields.

EVALUATION ON THE EGFET PH SENSING PERFORMANCE OF SOL-GEL SPIN COATED TITANIUM DIOXIDE THIN FILM

For this study, TiO2 thin film was fabricated using the sol-gel spin coating method. The fabricated film was then applied as a sensing membrane in an extended gate field effect transistor (EGFET) pH sensor system. The pH sensing performance of the sol-gel spin-coated TiO2 was evaluated in terms of sensitivity, linearity, and hysteresis where the value obtained was 58.70 mV/pH, 0.9922, and 86.17 mV respectively. The drift rate of the sample when being measured for 12 consecutive hours was also determined where measurement in pH 4, pH 7, and pH 10 yield drift rate of 1.72 mV/h, 4.14 mV/h, and 6.05 mV/h respectively. Besides that, the TiO2 was characterized for its thickness (24.32 nm) and surface roughness (5.129 nm). From the results obtained, it was found that sol-gel spin-coated TiO2 thin film with thickness between 20 - 29 nm will have high pH sensitivity (more than 50 mV/pH).

Influence of weathering and temperature on the electrochemical and microscopical characteristics of CeO2 and CeO2:V2O5 sol-gel thin films

Thin films of cerium dioxide (CeO2) and cerium dioxide doped with vanadium pentoxide (CeO2:V2O5) were prepared through sol-gel route and spin coating deposition, for use as smart materials. This work evaluated films that underwent a weathering process in an accelerated weathering test chamber and the changes in their properties. The influence of the temperature was also studied electrochemically. Regarding the weathering process, there were variations in the charge insertion capacity during the investigated period. However, at the end of the 900 simulated days, the CeO2:V2O5 presented interesting values. For the EIS analyses, the curve (Nyquist plot) remained about 45° inclination, suggesting a diffusional process. The morphological and structural analyses demonstrated that by doping the film, occurs a reduction on the degradation, although the structure showed no changes. For different temperatures, the film becomes more resistive as the temperature increases. The chronocoulometry showed that the charge insertion capacity increases with temperature.

Impact of electrode materials on microstructure, leakage current and dielectric tunable properties of lead-free BSZT thin films

Lead-free ferroelectric sol-gel thin films derived from barium strontium titanate (Ba0.85Sr0.15Zr0.1Ti0.9O3, BSZT) were deposited onto two types of electrode: (i) noble metallic Pt- and (ii) conductive oxides LaNiO3- and SrRuO3-coated silicon substrates. The present studies demonstrate that electrode materials impact significantly on morphology, crystallographic orientation, lattice mismatch-induced microstrains and electrical properties of BSZT thin films. Highly (100)-textured BSZT thin films with the highest crystallinity and lowest microstrain were found on conductive oxide electrodes, whereas Pt electrodes showed polycrystalline. Capacitors with oxide LaNiO3 electrode display predominated Schottky emission with a rather low leakage current density ~4.68 × 10−8 A/cm2, while Pt electrode capacitors indicate space-charge limited conduction. Dielectric characterizations in the frequency range of 103–5 × 106 Hz and the dc electric field up to ±800 kV/cm show that the permittivity, loss tangent, and tunability of BSZT films also strongly depend on the electrode materials used. The optimal dielectric tunability with a large figure of merit (FOM ≈ 22.53) and low dielectric loss (tanδ ≈ 2.9%), was found to be 66% for BSZT thin films on the conductive LaNiO3 electrode, suggesting that the as-deposited ferroelectric films are promising candidates for applications in tunable microwave elements and related electronic devices.