Thin Film Sensor Devices Research Articles

Thiacalixarenes with Sulfur Functionalities at Lower Rim: Heavy Metal Ion Binding in Solution and 2D-Confined Space.

Sulfur-containing groups preorganized on macrocyclic scaffolds are well suited for liquid-phase complexation of soft metal ions; however, their binding potential was not extensively studied at the air–water interface, and the effect of thioether topology on metal ion binding mechanisms under various conditions was not considered. Herein, we report the interface receptor characteristics of topologically varied thiacalixarene thioethers (linear bis-(methylthio)ethoxy derivative L2, O2S2-thiacrown-ether L3, and O2S2-bridged thiacalixtube L4). The study was conducted in bulk liquid phase and Langmuir monolayers. For all compounds, the highest liquid-phase extraction selectivity was revealed for Ag+ and Hg2+ ions vs. other soft metal ions. In thioether L2 and thiacalixtube L4, metal ion binding was evidenced by a blue shift of the band at 303 nm (for Ag+ species) and the appearance of ligand-to-metal charge transfer bands at 330–340 nm (for Hg2+ species). Theoretical calculations for thioether L2 and its Ag and Hg complexes are consistent with experimental data of UV/Vis, nuclear magnetic resonance (NMR) spectroscopy, and single-crystal X-ray diffractometry of Ag–thioether L2 complexes and Hg–thiacalixtube L4 complex for the case of coordination around the metal center involving two alkyl sulfide groups (Hg2+) or sulfur atoms on the lower rim and bridging unit (Ag+). In thiacrown L3, Ag and Hg binding by alkyl sulfide groups was suggested from changes in NMR spectra upon the addition of corresponding salts. In spite of the low ability of the thioethers to form stable Langmuir monolayers on deionized water, one might argue that the monolayers significantly expand in the presence of Hg salts in the water subphase. Hg2+ ion uptake by the Langmuir–Blodgett (LB) films of ligand L3 was proved by X-ray photoelectron spectroscopy (XPS). Together, these results demonstrate the potential of sulfide groups on the calixarene platform as receptor unit towards Hg2+ ions, which could be useful in the development of Hg2+-selective water purification systems or thin-film sensor devices.

Tripod-Loop Metasurfaces for Terahertz-Sensing Applications: A Comparison

The high electric field intensity achieved on the surface of sensors based on metasurfaces (metasensors) makes them an excellent alternative for sensing applications where the volume of the sample to be identified is tiny (for instance, thin-film sensing devices). Various shapes and geometries have been proposed recently for the design of these metasensors unit-cells (meta-atoms) such as split ring resonators or hole arrays, among others. In this paper, we propose, design, and evaluate two types of tripod metasurfaces with different complexity in their geometry. An in-depth comparison of their performance is presented when using them as thin-film sensor devices. The meta-atoms of the proposed metasensors consist of a simple tripod and a hollow tripod structure. From numerical calculations, it is shown that the best geometry to perform thin-film sensing is the compact hollow tripod (due to the highest electric field on its surface) with a mean sensitivity of 3.72 × 10−5 nm−1. Different modifications are made to this structure to improve this value, such as introducing arms in the design and rotating the metallic pattern 30 degrees. The best sensitivity achieved for extremely thin film analytes (5–25 nm thick) has an average value of 1.42 × 10−4 nm, which translates into an extremely high improvement of 381% with respect to the initial hollow tripod structure. Finally, a comparison with other designs found in the literature shows that our design is at the top of the ranking, improving the overall performance by more than one order of magnitude. These results highlight the importance of using metastructures with more complex geometries so that a higher electric field intensity distribution and, therefore, designs with better performance can be obtained.

Synthesis, Characterization, and Study of NO2Gas Sensing Behavior of CSA Doped PANi-Ta2O5Nanocomposite

Polyaniline in its emeraldine salt form was obtained by chemical oxidative polymerization of aniline monomer. PANi thus obtained was mixed with tantalum pentoxide (Ta2O5) nano powder in 50 wt% to get PANi-Ta2O5 nanocomposite. The nanocomposite was further mixed with Camphor Sulphonic Acid (CSA) with concentrations of 20wt%, 30wt% and 40wt% respectively for solid-state doping. All the samples were characterized by FESEM, TEM, XRD, FTIR and UV-Vis spectroscopy. Thin film sensor device was developed taking each of the samples as the sensing material to study the gas sensing properties towards NO2 gas at room temperature. The CSA doped nanocomposite possessed highly aggregated globular morphology that led to high surface area. Doping of CSA notably favored the gas sensing process. PANi-Ta2O5-CSA40wt% exhibited the best performance with sensor response above 80% towards 500 ppm NO2, a faster response time of 55 seconds and a recovery time of 160 seconds. The device showed good repeatability and high selectivity to NO2. The sensor response was studied under 38%, 57% and 65% relative humidity conditions respectively and it was found to remain stable under different humidity levels.

Thin Films Sensor Devices for Mycotoxins Detection in Foods: Applications and Challenges

Mycotoxins are a group of secondary metabolites produced by different species of filamentous fungi and pose serious threats to food safety due to their serious human and animal health impacts such as carcinogenic, teratogenic and hepatotoxic effects. Conventional methods for the detection of mycotoxins include gas chromatography and high-performance liquid chromatography coupled with mass spectrometry or other detectors (fluorescence or UV detection), thin layer chromatography and enzyme-linked immunosorbent assay. These techniques are generally straightforward and yield reliable results; however, they are time-consuming, require extensive preparation steps, use large-scale instruments, and consume large amounts of hazardous chemical reagents. Rapid detection of mycotoxins is becoming an increasingly important challenge for the food industry in order to effectively enforce regulations and ensure the safety of food and feed. In this sense, several studies have been done with the aim of developing strategies to detect mycotoxins using sensing devices that have high sensitivity and specificity, fast analysis, low cost and portability. The latter include the use of microarray chips, multiplex lateral flow, Surface Plasmon Resonance, Surface Enhanced Raman Scattering and biosensors using nanoparticles. In this perspective, thin film sensors have recently emerged as a good candidate technique to meet such requirements. This review summarizes the application and challenges of thin film sensor devices for detection of mycotoxins in food matrices.

Hydrogen sensing performance of a nickel oxide (NiO) thin film-based device

An interesting nickel oxide (NiO) thin film-based hydrogen sensor device, prepared by a low-powered (50 W) radio-frequency (RF) sputtering process, is studied and demonstrated. The studied device shows improved performance including a very high hydrogen sensing response ratio (416 (ΔR/R)), an extremely low detecting limit (<50 ppm H2/air), a high sensing response speed (7 s), a lower operating temperature (≦350 °C) and a widespread sensing range of hydrogen concentration (50–10,000 ppm H2/air). In addition, the device demonstrates benefits of low cost, easy fabrication and chemical stability. Based on these advantages, therefore, the studied NiO thin film sensor device shows promise for high-performance hydrogen sensing applications.

Analyte detection with Cu-BTC metal-organic framework thin films by means of mass-sensitive and work-function-based readout.

Metal-organic frameworks (MOFs) constitute a new generation of porous crystalline materials, which have recently come into focus as analyte-specific active elements in thin-film sensor devices. Cu-BTC--also known as HKUST-1--is one of the most theoretically and experimentally investigated members of the MOF family. Its capability to selectively adsorb different gas molecules renders this material a promising candidate for applications in chemical gas and vapor sensing. Here, we explore details of the host-guest interactions between HKUST-1 and various analytes under different environmental conditions and study the vapor adsorption mechanism by mass-sensitive and work-function-based readouts. These complementary transduction mechanisms were successfully applied for the detection of low ppm (2 to 50 ppm) concentrations of different alcohols (methanol, ethanol, 1-propanol, and 2-propanol) adsorbed into Cu-BTC thin films. Evaluation of the results allows for the comparison of the amounts of adsorbed vapors and the contribution of each vapor to the changes of the electronic properties of Cu-BTC. The influence of the length of the alcohol chain (C1-C3) and geometry (1-propanol, 2-propanol) as well as their polarity on the sensing performance was investigated, revealing that in dry air, short chain alcohols are more likely adsorbed than long chain alcohols, whereas in humid air, this preference is changed, and the sensitivity toward alcohols is generally decreased. The adsorption mechanism is revealed to differ for dry and humid atmospheres, changing from a site-specific binding of alcohols to the open metal sites under dry conditions to weak physisorption of the analytes dissolved in surface-adsorbed water reservoirs in humid air, with the signal strength being governed by their relative concentration.

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.

PZT 박막의 압전특성에 미치는 공정변수의 효과

We have studied the effect of crystallization temperature, composition and film thickness, which are the fundamental processing parameters of lead zirconate titanate(PZT) thin film fabrication, in the respect of the piezoelectric properties by our pneumatic loading method(PLM). A great deal of research has been done in the field of characterization for piezoelectric thin films after the first report on the measurement for the piezoelectric coefficient of thin films in 1990. Even though the piezoelectric properties of thin films are very critical factors in the micro-electro mechanical system(MEMS) and thin film sensor devices, a few reports for the piezoelectric characterization are provided for the last decade unlikely the bulk piezoelectric devices. We have found that the piezoelectric properties of thin films are improved as the increase of crystallization temperature up to 750<TEX>$\^{C}$</TEX> and this behavior can be also explained by the analysis of dielectric polarization hysteresis loop, X-ray diffraction and scanning electron microscopy. The effect of Zr/Ti composition has been also studied. This gives us the fact that the maximum piezoelectricity is found near Morphotropic Phase Boundary(MPB) as bulk PZT system does.

The effect of processing parameter on the piezoelectric d-coefficients of PZT thin films

We have studied the effect of crystallization temperature, which is one of the most fundamental processing parameters of leads zirconate titanate (PZT) thin film, on the piezoelectric properties by our pneumatic loading method. Even though the piezoelectric properties of thin films are very critical factors in the micro-electro mechanical system (MEMS) and thin film sensor devices, very few reports are provided for the last decade unlikely on bulk piezoelectric devices. We have found that the piezoelectric properties of thin films are improved as the increase of crystallization temperature up to 750°C and this behavior can be also explained by the analysis of dielectric polarization hysteresis loop, crystal structures and scanning electron microscopy.

Nanoparticles and polarons: active centers in thin film sensor devices

The spectroscopic characterization of localized charges in thin film devices is addressed. We demonstrate their existence and show their particular contribution to the operation of sensor devices. Examples in oxidic and polymeric thin films systems are given in which the localized states dominate the electronic properties.

Perovskite‐Type Oxide‐Based Electrode: A New Sensor for Hydrogen‐Phosphate Ion

A perovskite‐type oxide‐based electrode showed good properties of amperometric sensing to hydrogen‐phosphate ion. The anodic current of carbon‐based electrodes loaded with La‐Co‐based perovskite‐type oxide, the sensing signal, increased with increasing concentration of hydrogen‐phosphate ion at +0.85 to + 1.0 V vs. a saturated calomel electrode. The carbon electrode loaded with showed remarkable selectivity to among the examined anions of , , , , , , , and , although it received serious interference from . The use of a thin film electrode of perovskite‐type oxide drastically improved amperometric sensing to hydrogen‐phosphate ion. The thin film sensor device responded to at concentrations between and , and the 90% response time was as short as ca. 5 min or ca. 10 s at the low or high concentration range of hydrogen‐phosphate ion, respectively. The thin film sensor device showed the highest selectivity to among the examined anions. © 2000 The Electrochemical Society. All rights reserved.

Hydrothermally treated sol solution of tin oxide for thin-film gas sensor

A sol solution containing SnO 2 particles (crystallites) of 6 nm in average diameter was prepared by a hydrothermal treatment. The hydrothermal treatment was found to be very effective for suppressing the thermal growth of SnO 2 grains. A thin-film sensor device, fabricated by spin-coating from the sol solution, consisted of SnO 2 crystallites with 6 nm in mean diameter after calcination at 600°C. The device proved for better properties in sensitivity and response transient than a conventional sintered-block sensor device using SnO 2 precipitated gel.

Preparation of Temperature-Sensitive Magnetic Ferrite Thin-Film for Sensor Device

Temperature-sensitive magnetic bulk cores and thick films have been fabricated for thermal sensors. These magnetic substances are used in large size instruments, but cannot be adapted for miniaturization. The authors have previously been proposed a temperature-sensitive magnetic thin film annealed in oxygen atmosphere, but the saturation magnetization is below 1/5 of a bulk core. Then a temperature-sensitive Mn-Zn ferrite thin film sensor device (TMS) was prepared by sputtering and annealing in argon atmosphere. TMS's saturation magnetization is nearly equal to that of an Mn-Zn bulk ferrite. The magnetization has a marked temperature dependence like that of a bulk core or a thick film. A TMS with a small heat capacitor responds sharply to a minute temperature variation, and is therefore expected to be applicable in pyromagnetic photosensors, photo pattern sensors, and enzyme sensors.

Significance of Microstructure for a MOCVD-Grown YSZ Thin Film Gas Sensor

AbstractWe report the fabrication and characterization of a low temperature (200°C-400°C) thin film gas sensor constructed from a MOCVD-grown yttria-stabilized zirconia (YSZ) layer sandwiched between two platinum thin film electrodes. A reproducible gas-sensing response is produced by applying a cyclic voltage which generates voltammograms with gas-specific current peaks and shapes. Growth conditions are optimized for preparing YSZ films having dense microstructures, low leakage currents, and maximum ion conductivities. In particular, the effect of growth temperature on film morphology and texture is discussed and related to the electrical and gas-sensing properties of the thin film sensor device.

TiO2 and Bi2O3 Thin Film Oxygen Sensor Materials

AbstractThe electrical conductivity of polycrystalline TiO2 and Bi2O3, deposited on sapphire substrate by various methods, has been determined as a function of the film thickness, the oxygen partial pressure and the temperature. As a rule, TiO2 films with d ≲ 30 nm are incoherent after heat treatment, thereby restricting the miniaturization of the planar sensor device. Larger films essentially behave like bulk material with fully ionized oxygen defects V2•o (or Ti4•i interstitials) as major intrinsic defects. In case of extrinsic p‐type films, the conductivity contribution of the TiO2/sapphire substrate and TiO2/gas phase interfaces is clearly enhanced with respect to the bulk contribution, mainly due to segregation effects. Doubly ionized oxygen vacancies V2•o and M'Ti defects (trivalent acceptor atoms at Ti‐sites) appear to be the main intrinsic and extrinsic defects involved in the segregation process and in the oxygen exchange reaction. In contrast to TiO2, thin films of Bi2O3 react strongly with the substrate ultimately forming a new stable phase, Al4Bi2O9. Along with this structural change, the conductivity decreases by several orders of magnitude, classifying Bi2O3 as an unsuitable material for such thin film sensor devices.