Detection of ammonia in the ppt range based on a composite optical waveguide pH sensor

An optical waveguide (OWG) pH sensor with two thin guiding layers (composite OWG) was fabricated, and its application to sensing extremely low concentrations of ammonia was demonstrated. The highly sensitive element based on a titanium dioxide (TiO₂) film was deposited onto the surface of a potassium ion (K⁺) exchanged glass OWG by RF sputtering. The surface of the TiO₂ film was coated with a thin film of a pH indicator dye (bromothymol blue, BTB) by spin coating. With optimum thickness of BTB film at about 46 nm and of TiO₂ films at 18–20 nm, this system proved to be an extremely sensitive ammonia sensor. The experimental results of the optimum conditions on BTB and TiO₂ film thicknesses were close to theoretically calculated values. The sensor easily detected 1 parts per trillion (ppt) ammonia reversibly, and had a short response time. The present sensor is also characterized by low cost, simple structure and facile fabrication.     

Electrical properties of nickel oxide thin films for flow sensor application

In this work, Ni oxide thin films, with thermal sensitivity superior to Pt and Ni thin films, were formed through annealing of Ni films deposited by a r.f. magnetron sputtering. The annealing was carried out in the temperature range of 300–500 °C under atmospheric conditions. Resistivity of the resulting Ni oxide films were in the range of 10.5 μΩ cm/°C to 2.84 × 10⁴ μΩ cm/°C, depending on the extent of Ni oxidation. The temperature coefficient of resistance (TCR) of the Ni oxide films also depended on the extent of Ni oxidation; the average TCR of Ni oxide resistors, measured between 0 and 150 °C, were 5630 ppm/°C for the 300 °C and 2188 ppm/°C for 500 °C films. Because of their high resistivity and very linear TCR, Ni oxide thin films are superior to pure Ni and Pt thin films for flow and temperature sensor applications.     

Oxygen functionalisation of MWNT and their use as gas sensitive thick-film layers

Multi-wall carbon nanotubes (MWNT) were functionalised in an oxygen-based atmosphere by an inductively coupled RF-plasma at 13.56 MHz. X-ray photoelectron spectroscopy analysis showed that the chemical composition of the nanotube surface depends on the plasma conditions used. The MWNT were then deposited onto microhotplate gas sensor substrates by the drop coating method. A well-adhered thick-film of carbon nanotubes mesh (∼17 μm) was obtained after annealing in air. The influence of the different plasma conditions on the responsiveness of gas sensors fabricated with the functionalised MWNT was studied. The gas sensing properties were investigated both experimentally and theoretically for NO₂, and NH₃ at room temperature.     

Development of a piezoelectric lead titanate thin film process on silicon substrates by high rate gas flow sputtering

Polycrystalline lead titanate (PT) thin films in the range of 3–6 μm were crack and void free deposited on silicon substrates in a high rate gas flow sputtering process. Gas flow sputtering uses the hollow cathode effect which results into high deposition rates of about 120 nm/min. (1 1 1) Textured platinum was used as bottom electrode to assist the nucleation of PT.Material properties of the PT thin films as well as the Pt bottom electrode like topography, morphology, chemical composition, and structure are evaluated. The sputtered PT layers show clearly Perovskite traces in XRD patterns, even the (1 1 1) texture of the Pt is partial transferred. The most difficult part is to fulfil the empirical formula PbTiO₃. This problem is solved by stabilising the process parameters. It was shown that the temperature has got enormous influence at the stoichiometry.     

Orientation and thickness dependence of electrical and gas sensing properties in heteroepitaxial indium tin oxide films

Heteroepitaxial indium tin oxide (ITO) films were grown on three differently oriented yttria-stabilized zirconia (YSZ) substrates ((1 0 0), (1 1 0), (1 1 1)) by rf magnetron sputtering, and their structural characteristics and electrical and gas sensing properties were investigated. The initially formed ITO exhibited an island structure on the very thin layer and became a continuous film after the prolonged deposition. The heteroepitaxial relationships between ITO films and YSZ substrates were confirmed by X-ray diffraction, pole figure, and high resolution transmission electron microscopy (HRTEM). The chemical composition, determined by X-ray photoelectron spectroscopy (XPS), was slightly different at early stage depending on the substrate orientation, but it became similar after the longer deposition. Hall measurements indicated that the electrical resistivity of ITO films decreased with increasing the deposition time (or film thickness) irrespective of the film orientation. The ITO film deposited on (1 1 0) YSZ for 10 s showed the highest electrical resistivity. The gas sensor fabricated from the ITO film on (1 1 0) YSZ deposited for 10 s showed the highest NO₂ gas response at relatively low temperature (100 °C), which was attributed to the higher Sn concentration and higher surface roughness of that film.     

Fabrication and frequency response of dual-element ultrasonic transducer using PZT-5A thick film

Ultrasonic transducers based on PZT-5A thick films deposited onto polycrystalline Al₂O₃ substrates using screen-printing were successfully fabricated. Considering the relatively high sintering temperature of PZT-5A thick films and better impedance matching characteristics with PZT-5A, polished polycrystalline Al₂O₃ were used as substrates. For electrodes, high quality platinum (Pt) was deposited by a thin film process, because the surface state of electrodes greatly affects the quality of piezoelectric films. Applying Pt/PZT-5A/Pt/Al₂O₃ structures, dual-element ultrasonic transducers were assembled. The assembled transducers included a wear plate (normally alumina with 40.21 × 10⁶ kg/m² s of impedance), backing (tungsten carbide-epoxy), electrical matching, an epoxy glue layer, and a housing. The optimum measurement ranges of 5 and 10 MHz ultrasonic transducers were 2.51–300.2 and 2.50–250.1 mm, respectively. From the time and frequency response measurements of the assembled 10 MHz DEUTs, the value of −20 dB level waveform duration and the −6 dB bandwidth was 481.8 ns and 34.4%, respectively. Also, the measurement accuracies of both 5 and 10 MHz DEUTs assembled in this study were below 0.1 and 0.4%, respectively.     

Growth and electrical properties of highly (0 0 1)-oriented Pb(Zr0.52Ti0.48)O3 thin films on amorphous TiN buffered Si(1 0 0)

Pb(Zr_0.52Ti_0.48)O₃ (PZT) ferroelectric thin films with LaNiO₃ (LNO) as bottom electrodes have been grown on amorphous TiN buffered Si(1 0 0) substrates by pulsed laser deposition. It was found that highly (0 0 1)-oriented LNO films could be obtained even if TiN underlayers were amorphous. XRD analyses showed that the subsequently deposited PZT films were also preferentially (0 0 1)-oriented due to the template effect of the perovskite structured LNO films. Dielectric constant of the PZT thin films remained almost constant with frequency in the range from 10³ to 10⁶ Hz, and tangent loss was as small as 0.02 at high frequencies. The remnant polarization and coercive field of an Au/PZT/LNO capacitor were typically 20 μC/cm² and 30 kV/cm, respectively. C–V and I–V characteristics revealed the capacitance and leakage current variations with applied voltage were asymmetric when the bottom electrode was negatively as well as positively biased, indicating that ferroelectric/electrode interfaces and space charges play an important role in the electrical properties of ferroelectric capacitors.     

Characterization of Pb(Zr,Ti)O3 thin films deposited on stainless steel substrates by RF-magnetron sputtering for MEMS applications

Piezoelectric Pb(Zr,Ti)O₃ (PZT) thin films with a composition near the morphotoropic phase boundary were deposited directly on cantilever-shaped stainless steel (SUS) substrates using RF-magnetron sputtering for application of micro-electromechanical systems (MEMS). X-ray diffraction measurements reveal that the PZT thin films have a polycrystalline perovskite structure with a preferential orientation of (1 0 1). Cross-section morphology – observed using a scanning electron microscope – indicates that the PZT films exhibit a dense columnar structure without pores or clacks. The films’ P–E hysteresis loops indicate clear ferroelectricity. Based on the deflection characteristics of the cantilever, the effective piezoelectric coefficient e₃₁ of the PZT films is measured to be −1.35 C/m².     

Moisture sensor at glass/polymer interface for monitoring of photovoltaic module encapsulants

A sensor developed for measurement of water concentration inside glass/polymer encapsulation structures with a particular application area in accelerated aging of photovoltaic module encapsulants is described. An approximately 5 μm thick porous TiO₂ film applied to a glass substrate with a conductive coating acts as the moisture-sensitive component. The response is calibrated with weather chamber experiments for sensors open to the environment and with diffusion experiments for sensors laminated under an encapsulant. For the interpretation of diffusion experiment results, a transport model describing the diffusion of water across the polymer/TiO₂ interface is developed. The logarithm of AC resistance shows a linear dependence on water concentration in both open and encapsulated calibration. The first measurable response from an encapsulated 3.5 mm × 8 mm size sensor is obtained when approximately 10 μg of water has entered the film. Implications of the calibration results for sensor usage in accelerated aging tests are discussed.     

Fluorescence detection of nitrogen dioxide with perylene/PMMA thin films

Thin films of polymethylmethacrylate (PMMA) doped with perylene provide selective, robust and easily prepared optical sensor films for NO₂ gas with suitable response times for materials aging applications. The materials are readily formed as 200 nm thin spin cast films on glass from chlorobenzene solution. The fluorescence emission of the films (λ_max=442 nm) is quenched upon exposure to NO₂ gas through an irreversible reaction forming non-fluorescent nitroperylene. Infrared, UV–VIS and fluorescence spectroscopies confirmed the presence of the nitro adduct in the films. In other atmospheres examined, such as air and 1000 ppm concentrations of SO₂, CO, Cl₂ and NH₃, the films exhibited no loss of fluorescence intensity over a period of days to weeks. Response curves were obtained for 1000, 100 and 10 ppm NO₂ at room temperature with equilibration times varying from hours to weeks. The response curves were fit using a numerical solution to the coupled diffusion and a nonlinear chemical reaction problem assuming that the situation is reaction limiting. The forward reaction constant fitted to experimental data was k_f∼0.06 (ppm min)⁻¹.     

Gas sensing properties of ZnO thin films prepared by microcontact printing

In situ patterned zinc oxide (ZnO) thin films were prepared by precipitation of Zn(NO₃)₂/urea aqueous solution and by microcontact printing of self-assembled monolayers (SAMs) on Al/SiO₂/Si substrates. The visible precipitation of Zn(OH)₂ from the urea containing Zn(NO₃)₂ solution was enhanced by increasing the reaction temperature and the amount of urea. The optimized condition for the ZnO thin films was found to be the Zn(NO₃)₂/urea ratio of 1/8, the precipitation temperature of 80 °C, the precipitation time of 1 h and the annealing temperature of 600 °C, respectively. SAMs are formed by exposing Al/SiO₂/Si to solutions comprising of hydrophobic octadecylphosphonic acid (OPA) in tetrahydrofuran and hydrophilic 2-carboxylethylphosphonic acid (CPA) in ethanol. The ZnO thin film was then patterned with the heat treatment of Zn(OH)₂ precipitated on the surface of hydrophilic CPA. The ZnO gas sensor was exposed to different concentrations of C₃H₈ (5000 ppm), CO (250 ppm) and NO (1000 ppm) at elevated temperatures to evaluate the gas sensitivity of ZnO sensors. The optimum operating temperatures of C₃H₈, CO and NO gases showing the highest gas sensitivity were determined to be 350, 400 and 200 °C, respectively.     

All-optical hydrogen sensor based on a high alloy content palladium thin film

Optical reflectance measurements were performed to determine the hydrogen response characteristics of 20 nm thick Pd_0.6Au_0.4 films. The response time and signal change characteristics were determined as a function of hydrogen concentrations ranging from 0.05% to 4% in a balance of dry CO₂ free air. The detection limit was determined to be 0.05%, with a corresponding response time of 130 s, while at 4% hydrogen concentrations the response time was 5 s at ambient temperatures. A linear decrease of both the signal change and response time was measured within an operating temperature range between 25 °C and 100 °C for a 1% hydrogen in air gas mixture. The sensor response dependence of the Pd_0.6Au_0.4 film with a change in humidity was determined between ambient levels and 95% relative humidity (RH). While the signal change was independent of humidity the response time increased due to water adsorption on the Pd alloy sensing layer. A similar increase in response time was shown for 100 ppm of background CO mixed with 1% hydrogen in nitrogen at room temperature. At an elevated operating temperature of 80 °C, 100 ppm of CO did not affect the sensor response towards 1% hydrogen in a balance of nitrogen. Reliability tests have been performed over a 1-year time period and the sensing specifications have not drifted beyond 2% and 13% of the calibrated signal change and response time, respectively. A response time on the order of seconds and the proven stability of the high alloy content Pd thin film demonstrate the promising attributes of this material for use in an all-optical hydrogen sensor.     

Evaluation of response characteristics of resistive oxygen sensors based on porous cerium oxide thick film using pressure modulation method

In order to reduce the response time of resistive oxygen sensors using porous cerium oxide thick film, it is important to ascertain the factors controlling response. Pressure modulation method (PMM) was used to find the rate-limiting step of sensor response. This useful method measures the amplitude of sensor output (H(f)) for the sine wave modulation of oxygen partial pressure at constant frequency (f). In PMM, “break” response time, which is minimum period in which the sensor responds precisely, can be measured. Three points were examined: (1) simulated calculations of PMM were carried out using a model of porous thick film in which spherical particles are connected in a three-dimensional network; (2) sensor response speed was experimentally measured using PMM; and (3) the diffusion coefficient and surface reaction coefficient were estimated by comparison between experiment and calculation. The plot of log f versus log H(f) in the high f region was found to have a slope of approximately −0.5 for both porous thick film and non-porous thin film, when the rate-limiting step was diffusion. Calculations showed the response time of porous thick film was 1/20 that of non-porous thin film when the grain diameter of the porous thick film was the same as the thickness of non-porous thin film. At 973 K, “break” response time (t_b) of the resistive oxygen sensor was found by experiment to be 109 ms. It was concluded that the response of the resistive oxygen sensor prepared in this study was strongly controlled by diffusion at 923–1023 K, since the experiment revealed that the slope of plot of log f versus log H(f) in the high f region was approximately −0.5. At 923–1023 K, the diffusion coefficient of oxygen vacancy in porous ceria (D_V) was expressed as follows: D_V (m²s⁻¹) = 5.78 × 10⁻⁴ exp(−1.94 eV/kT). At 1023 K, the surface reaction coefficient (K) was found to exceed 10⁻⁴ m/s.     

Preparation of TiO2 coated silicate micro-spheres for enhancing the light diffusion property of polycarbonate composites

The TiO₂ coated silicate micro-spheres (SMS) core–shell particles (SMS@TiO₂) were synthesized using the sol–gel reaction followed by calcination. The SMS@TiO₂ particles were used to enhance the light diffusion property of polycarbonate (PC) composites. Our experimental analysis shows that the TiO₂ was coated on the SMS particles and the optimum parameters of the reaction were 4:1 of the Si/Ti molar ratio and 500 °C of the calcination temperature. The UV–Vis spectra indicate that SMS@TiO₂ can absorb or hinder the UV light, which may prolong the service life of PC light diffusion materials. Compared to that of PC composites physically mixed with SMS + TiO₂, the haze of the PC/SMS@TiO₂ composites was little affected, while the transmittance was obviously enhanced, which can be increased from 55.5% for PC/TiO₂/SMS to 70.3% for PC/SMS@TiO₂ with only 0.6 wt% filler loading.     

Characterisation of PZT thin film micro-actuators using a silicon micro-force sensor

This paper reports on the measurements of displacement and blocking force of piezoelectric micro-cantilevers. The free displacement was studied using a surface profiler and a laser vibrometer. The experimental data were compared with an analytical model which showed that the PZT thin film has a Young's modulus of 110 GPa and a piezoelectric coefficient d_31,f of 30 pC/N. The blocking force was investigated by means of a micro-machined silicon force sensor based on the silicon piezoresistive effect. The generated force was detected by measuring a change in voltage within a piezoresistors bridge. The sensor was calibrated using a commercial nano-indenter as a force and displacement standard. Application of the method showed that a 700 μm long micro-cantilever showed a maximum displacement of 800 nm and a blocking force of 0.1 mN at an actuation voltage of 5 V, within experimental error of the theoretical predictions based on the known piezoelectric and elastic properties of the PZT film.     

Sensor properties and surface characterization of the metal-deposited SPR optical fiber sensors with Au,Ag, Cu,and Al

Metal-deposited optical fiber sensors with Cu and Al with a film thickness of 45 nm based on surface plasmon resonance (SPR) were fabricated for the first time. The response curves and the properties of these sensors were investigated with a comparison of those of the sensors with Au and Ag. The reflection properties of thin films of Au, Ag, Cu, and Al due to the SPR phenomenon were also measured and considered. The metal-deposited SPR optical fiber sensors with Au, Ag, and Cu have high sensitivities and good responses. Though the sensor with Al shows a lower sensitivity, it has a wider response range in the refractivity. The response curve of the sensor with Au calculated from SPR theoretical equations agreed well with that obtained by the experiment. However, the response curves of the sensors with Ag, Cu, and Al have the effects of the surface oxide layers. The surface characterization of these metal films by X-ray photoelectron spectroscopy (XPS) showed the presence of oxide layers on the films of Ag, Cu, and Al. A very thin (about 0.3 nm) oxide layer is present on Ag, while thick (about 2 nm) oxide layers are present on Cu and Al.     

Development of an optical fibre reflectance sensor for copper (II) detection based on immobilised salicylic acid

Immobilized salicylic acid onto XAD-2 (styrene–divinylbenzene cross-linked copolymer) has been attempted in this study as a reagent phase for the development of an optical fibre copper (II) sensor. The measurements were carried out at a given wavelength of 690.27 nm since it yielded the largest divergence different in reflectance spectra before and after reaction with the analyte element. The optimum response was obtained at pH 5.0. The linear dynamic range of Cu(II) was found within the concentration range of 1.0–2.0 mmol L⁻¹ with its LOD of 0.5 mmol L⁻¹. The sensor response from different probes (n = 9) gave an R.S.D. of 8.4% at 0.55 mmol L⁻¹ Cu(II). The effect of interfered ions at 1:1 molar ratio of Cu(II):foreign ion was also studied in this work.     

Gas sensing behavior of polyvinylpyrrolidone-modified ZnO nanoparticles for trimethylamine

Gas sensors based on polyvinylpyrrolidone (PVP)-modified ZnO nanoparticles with different molar ratios of Zn²⁺: PVP were prepared by a sol–gel method. Morphology of the sensors was characterized by field emission-scanning electron microscopy (FE-SEM), which indicated that the sensor with a molar ratio of Zn²⁺: PVP = 5:5 showed uniform morphology. Moreover, the sensor exhibited fairly excellent sensitivity and selectivity to trimethylamine (TMA). The response and recovery time of the sensor were 10 and 150 s, respectively. Finally, the mechanism for the improvement in the gas sensing properties was discussed.     

A wet-chemical process to form palladium oxide sensitiser layer on thin film zinc oxide based LPG sensor

The liquid petroleum gas (LPG) sensitivity characteristics of zinc oxide (ZnO) films have been studied for an optimised level of Pd loading. The sensor element comprises of a layer of chemically deposited ZnO on which an overlayer of palladium (Pd) sensitiser was formed by a chemical dipping technique. The room temperature resistance of the film was found to be a sensitive function of the quantity of palladium loading, which could be optimised for stable and reproducible sensor properties. The sensor characteristics that are dependent also on the operating temperature could be optimised at around 250 °C. A sensitivity of 88% was observed in presence of 1.6 vol.% LPG in air at this optimum temperature with reasonably fast response and recovery times.     

Device characteristics for Pt–Ti–O gate Si–MISFETs hydrogen gas sensors

A novel Pt–Ti–O-gate Si–metal–insulator–semiconductor field-effect transistor (MISFET) hydrogen gas sensor has been proposed by Usagawa and Kikuchi (2010) [1]. The sensors consist of unique gate structures composed of Ti and oxygen accumulated regions around Pt grains on top of a novel mixing layer of nanocrystalline TiO_x and superheavily oxygen-doped amorphous Ti formed on SiO₂/Si substrates. The optimum Pt/Ti thickness and annealing conditions for most hydrogen safety monitoring sensor systems are obtained by annealing Pt(15 nm)/Ti(5 nm)-gate Si–MOS structures in air around 400 °C for 2 h. One of the advantages of the Pt–Ti–O-gate Si–MISFETs after 10 min of air-diluted 1000-ppm hydrogen exposure at 115 °C are reproducible and uniform threshold voltage of V_th in addition to large sensing amplitudes at a practically important hydrogen concentration range between 100 ppm and 1%. The analysis of device characteristics of the Pt–Ti–O-gate Si–MISFETs hydrogen sensors concludes that the oxidation process of the Ti layer is consistently explained by an oxidation model that the oxygen invasion into Ti layer comes from open air through Pt grain boundaries and at the same time Ti will evacuate into the Pt surface through Pt grain boundaries. During the course of this process, the invading oxygen will be balanced with the evacuating Ti so that the Ti layer keeps nearly the same thickness with the as grown states. Ti and oxygen will remains around Pt grains named Ti and oxygen merged corridors.