Characterisation and vapour sensing properties of spin coated thin films of anthracene labelled PMMA polymer

In the present article thin films of poly (methyl methacrylate) (PMMA) polymer labelled with anthracene (Ant-PMMA) prepared by spin coating are characterised by UV–visible spectroscopy, surface plasmon resonance (SPR), spectroscopic ellipsometry (SE) and Atomic Force Microscopy (AFM) and their organic vapour sensing properties are investigated. Ant-PMMA films' thickness are determined by performing theoretical fitting to experimental data measured using SPR and SE. Results obtained show that the spin-cast films are of good uniformity with an average thickness of 6–8 nm. Organic vapour sensing properties are studied using SPR technique during exposures to different volatile organic compounds (VOCs). Ant-PMMA films' response to the selected VOCs has been examined in terms of solubility parameters and molar volumes of the solvents, and the films were found to be largely sensitive to benzene vapour compared to other studied analytes.     

Room temperature H2S gas sensing characteristics of platinum (Pt) coated porous alumina (PoAl) thick films

The study reports H₂S gas sensing characteristics of platinum (Pt) coated porous alumina (PoAl) films. The porous alumina (PoAl) thick layers were formed in the dark on aluminum substrates using an electrochemical anodization method. Thin semitransparent platinum (Pt) films were deposited on PoAl samples using chemical bath deposition (CBD) method. The films were characterized using energy dispersive X-ray analysis (EDAX) and scanning electron microscopy (SEM). The thicknesses of coated and bare films were measured using ellipsometry. The sensing properties such as sensitivity factor (S.F.), response time, recovery time and repeatability were measured using a static gas sensing system for H₂S gas. The EDAX studies confirmed the purity of Pt–PoAl film and indicated the formation of pure platinum (Pt) phase. The ellipsometry studies revealed the thickness of PoAl layer of about 15–17 μm on aluminum substrates. The SEM studies demonstrated uniform distribution of spherical pores with a size between 0.250 and 0.500 μm for PoAl film and nearly spherical platinum particles with average particle size ∼100 nm for Pt–PoAl film. The gas-sensing properties of these samples were studied in a home-built static gas characterization system. The H₂S gas sensing properties of Pt–PoAl at 1000 ppm of H₂S gave maximum sensitivity factor (S.F.) = 1200. The response time and recovery time were found to be 2–3 min and ∼1 min respectively. Further, the measurement of H₂S gas sensing properties clearly indicated the repeatability of gas sensing response of Pt–PoAl film. The present study indicated the significant potential of Pt coated PoAl films for H₂S gas sensing applications in diverse areas.     

Indium oxide thin film based ammonia gas and ethanol vapour sensor

A sensor for ammonia gas and ethanol vapour has been fabricated using indium oxide thin film as sensing layer and indium tin oxide thin film encapsulated in poly(methyl methacrylate) (PMMA) as a miniature heater. For the fabrication of miniature heater indium tin oxide thin film was grown on special high temperature corning glass substrate by flash evaporation method. Gold was deposited on the film using thermal evaporation technique under high vacuum. The film was then annealed at 700 K for an hour. The thermocouple attached on sensing surface measures the appropriate operating temperature. The thin film gas sensor for ammonia was operated at different concentrations in the temperature range 323–493 K. At 473 K the sensitivity of the sensor was found to be saturate. The detrimental effect of humidity on ammonia sensing is removed by intermittent periodic heating of the sensor at the two temperatures 323K and 448 K, respectively. The indium oxide ethanol vapour sensor operated at fixed concentration of 400 ppm in the temperature range 293–393 K. Above 373 K, the sensor conductance was found to be saturate. With various thicknesses from 150–300 nm of indium oxide sensor there was no variation in the sensitivity measurements of ethanol vapour. The block diagram of circuits for detecting the ammonia gas and ethanol vapour has been included in this paper.     

Tungsten Oxide Thin Film Characterizations for Acetone Gas Detection

The gas sensing properties and topology of tungsten oxide thin films deposited by reactive-ion radio frequency magnetron sputtering at room temperature have been investigated. The abnormalities in behaviour of sensing film are observed when acetone gas is flowed over surface. The reduction reaction of surface and oxidation reaction of acetone gas have been studied. As the gas comes in contact with the surface, the molecules tend to reduce the surface, hence decreasing the resistance. The sensing film was annealed to 500 °C for 1 h for the purpose of achieving a suitable grain size for sensing to take place. Operational optimum temperature for sensing has been computed to be 260 °C. A grain size of 7.3 nm has been computed through analysis of AFM image and a film thickness of 100 nm has been calculated through surface profiler. The SEM image of the film demonstrates the grains developed on the surface. The XRD patterns reveal that the oxide showed up as WO₂. It has been observed that the response percentage is approximately 30% for acetone vapour concentration of 20 ppm and approximately 18% for the concentration of 15 ppm. The response time of the sensor is approximately 5 min and the recovery time is 4 min.     

Design of low cost gas sensor based on SrTiO3 and BaTiO3 films

We have prepared SrTiO3/BaTiO3 multilayer film on alumina substrates by a sol-gel technique and investigated their response for sensing ethanol vapor. The surface morphology of the films were characterized by atomic force microscope (AFM) showing that the grain size of the films increase up to 40 nm as the annealing temperature increased to 1000 degrees C. The ethanol sensors based on SrTiO3/BaTiO3 thin films were fabricated by applying interdigitated gold electrodes by sputtering technique. The ethanol sensing characteristics of SrTiO3/BaTiO3 thin films were quantified by the change in resistance of the sensors when they were exposed to ethanol. The optimum operating tempearature of these sensors was found to be 350 degrees C. In addition, the film annealed at 1000 degrees C exhibited p-type gas sensing behavior with the best sensitivity of 30-100 for low ethanol concentration in the range of 100-1000 ppm.     

Effect of thickness on nanostructured SnO2 thin films by spray pyrolysis as highly sensitive H2S gas sensor

Nanostructured SnO2 thin films were prepared by spray pyrolysis technique onto glass substrates with different thickness by varying quantity of precursor solution. The structural, optical and electrical properties of these films have been studied. The crystallographic structure of the films was studied by X-ray diffraction (XRD). It is found that the films are tetragonal with (110) orientation. The grain size increases with thickness. Atomic Force Microscopy (AFM) showed that the nanocrystalline nature of the films with porous nature. The grain size increased 14 to 29 nm with increase in film thickness. The studies on the optical properties show that the direct band gap value decreases from 3.75 to 3.50 eV. The temperature dependence of the electrical conductivity was studied. The activation energies of the films are calculated from the conductance temperature characteristics. The nanostructured SnO2 thin films were used as sensing layers for resistive gas sensors. The dependence of gas sensing properties on the thickness of SnO2 thin films was investigated. The gas response of the SnO2 thin films towards the H2S gas was determined at an operating temperature of 150 degrees C. The sensitivity towards H2S gas is strongly depending on surface morphology of the SnO2 thin films.     

Sterically hindered phthalocyanines: solid-phase interactions with carbon monoxide in matrix-entrapped thin functional films

A thin film comprising highly sterically hindered iron phthalocyanine derivatives incorporated into sol–gel networks shows effective sensitivity to carbon monoxide gas, when monitored by UV–visible absorption spectroscopy at 670 nm. The absorbance of a sol–gel thin film incorporating octaphenyl-di-tert-butylphthalocyanine iron(II) was particularly sensitive to carbon monoxide gas and showed a change in absorption after exposure to 1000 ppm CO gas for 1 min, while repetitive exposures produced only a limited loss of response. The optical response of the thin film gradually decreases with increasing temperature, and the binding energy for this sensing phenomenon was calculated to be 0.401 eV. The spectrum recovers at room temperature some 5 min after exposure to the gas.     

Ethanol and hydrogen sensors based on ZnO nanoparticles and nanowires

Hydrogen sensing characteristics of thick films of nanoparticles of ZnO and 3% Mn or Co doped ZnO with and without impregnation of 1% Pt have been investigated over a wide concentration range. The sensor characteristics of the Pt impregnated ZnO and 3% Mn or Co-doped ZnO nanoparticles are found to be excellent with the sensitivity reaching values in the 1000-1500 range for 1000 ppm of H2 at 125 degrees C or lower. The sensitivity is quite high even for 10 ppm of H2. The response and recovery times are generally less than 10 s in the region of high sensitivity. These films exhibit very good sensing characteristics for ethanol, the sensitivity being well above 2000 at 50 degrees C for 1000 ppm of vapour. Nanowires of Pt-impregnated ZnO also exhibit satisfactory sensing characteristics, but the values of the sensitivity are somewhat lower. Humidity has negligible effect on the performance of these sensors.     

Alcohol sensing of tin oxide thin film prepared by sol-gel process

The present paper describes the alcohol sensing characteristics of spin coated SnO₂ thin film deposited by using sol-gel process. The sensitivity of the film was measured at different temperatures and different concentrations of alcohol at ppm level. Alcohol detection result shows peak sensitivity at 623 K. The variation of sensitivity and ethanol concentration has shown a linear relationship up to 1150 ppm and after that it saturates. The response time measurement of the sensor was also observed and it was found that the response time is 30 sec. The results obtained favour the sol-gel process as a low cost method for the preparation of thin films with a high sensing characteristic.     

Indium Tin Oxide thin film gas sensors for detection of ethanol vapours

Indium Tin Oxide (ITO: In₂O₃ + 17% SnO₂) thin films grown on alumina substrate at 648 K temperatures using direct evaporation method with two gold pads deposited on the top for electrical contacts were exposed to ethanol vapours (200-2500 ppm). The operating temperature of the sensor was optimized. The sensitivity variation of films having different thickness was studied. The sensitivity of the films deposited on Si substrates was studied. The response of the film with MgO catalytic layer on sensitivity and selectivity was observed. A novel approach of depositing thin stimulating layer of various metals/oxides below the ITO film was tried and tested.     

ZnO thin films deposited by a CVT technique in closed ampoules

ZnO thin films were prepared on quartz glass, Si (100), and sapphire (001) substrates by a chemical vapour transport (CVT) technique. During the growing processes, the source and substrate temperatures were maintained at 1000 °C and 600 °C, respectively. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements showed that the crystalline qualities of ZnO thin films were sensitively dependent on substrates. ZnO thin film deposited on sapphire substrate exhibited the best morphology with the largest crystallite size of more than 20 μm. Meanwhile, the XRD patterns showed that ZnO thin film deposited on sapphire substrate was strongly c-axis preferred-oriented with high crystalline quality. The optical properties of ZnO thin films were investigated by photoluminescence (PL) spectroscopy at room temperature (RT). The results suggested that the optical properties of ZnO thin films were highly influenced by their crystalline qualities and surface morphologies.     

Optical properties and sensing applications of lithium iron phosphate thin films

A composite optical waveguide sensor, consisting of lithium iron phosphate (LiFePO₄, LFP) as the sensing material, was constructed and utilized for the detection of volatile organic compound gases. Nano-LFP powder was prepared via the hydrothermal method and was subsequently utilized in a dip-coating procedure for the fabrication of LFP thin films. The effect of heat treating temperature on the refractive index of the thin films was studied. A glass optical waveguide gas sensor was fabricated by coating an LFP thin film on the surface of single-mode tin-diffused glass optical waveguide. The sensor was found to exhibit a linear response to xylene in the range of 50-1000 ppm, with response times of less than 5 s.     

Thickness optimization of metal films for the development of surface-plasmon-based sensors for nonabsorbing media

The surface-plasmon resonance (SPR) effect in metals is highly sensitive to fluctuations in the optical properties of the interface and has been frequently employed in the Kretschmann configuration for optical sensing. The operating conditions required for using the SPR effect for probing nonabsorbing media under maximum sensitivity are derived analytically under the Lorentzian approximation. It is found that the film thickness that maximizes sensitivity occurs when the radiation damping of the oscillation is half the intrinsic damping. Numerical results are presented for the spectral dependence of the optimum thickness as well as of the SPR parameters of gold, copper, silver, and aluminum films, useful for the design of optical sensors for both gaseous and aqueous environments.     

聚甲基丙烯酸甲酯掺杂剂对7,7,8,8-四氰基苯醌二甲烷薄膜特性的影响

采用旋涂法制备了7,7,8,8-四氰基苯醌二甲烷(TCNQ)及聚甲基丙烯酸甲酯(PMMA)掺杂后的TCNQ敏感薄膜,探讨了PMMA掺杂剂及不同转速对薄膜性能的影响.结果表明,TCNQ/PMMA复合薄膜中TCNQ颗粒变得更细小和均匀,但对NH3的响应不稳定;纯TCNQ薄膜对NH3具有相当高的灵敏度,但响应和恢复时间较长.     

Optical properties and instrumental performance of thin gold films near the surface plasmon resonance

The optical properties of very thin gold films have been evaluated by Fresnel analysis, with optical boundary conditions pertaining to the surface plasmon resonance (SPR) at the gold-water interface. The experimental SPR characteristic was evaluated in the angular interrogation mode. Film morphology was characterized by high resolution transmission electron microscopy. The magnitude of the resonance, i.e., the SPR signal, sensitively depends on, and is affected by film thickness and morphology. A sharply defined thickness of 55 ± 5 nm is required, to achieve optimum SPR excitation conditions, and instrumental sensitivity. With decreasing film thickness, below 40 nm, the resonance angle starts to shift to larger values. A substantial increase of the intrinsic resonance broadening parameter is observed below 70 nm, associated with an increasingly asymmetric SPR line shape. A similar effect occurs in the presence of a very thin chromium adhesion layer. Surface roughness and film thickness modulations determine the experimentally observed line broadening parameter. Instrumental noise levels largely depend on accuracy and quality at which the resonance angle can be determined. Substantial improvement and instrumental sub-pixel resolution is achievable by optimum fitting routines, accounting for drastic noise reduction and improved instrumental sensitivity, up to two orders of magnitude over the inherent geometric sensor pixel resolution.     

Enhanced acetone detection using Au doped ZnO thin film sensor

Zinc oxide (ZnO) thin films are prepared using sol–gel method for acetone vapor sensing. Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) was taken as starting material and a stable and homogeneous solution was prepared in ethanol by deliquescing the zinc acetate and distinct amount of monoethanolamine as a stabilizing agent. The prepared solution was then coated on silicon substrates by spin coating method and then annealed at 650 °C for preparing ZnO thin films. The thickness of the film was maintained at 410 nm. The structural, morphological and optical studies were done for the synthesized ZnO thin films. The operating temperature and sensor response is considered to be an important parameter for the gas sensing behavior of any material. Therefore, the present study examined the effect of sensing behavior of 3% v/v gold (Au) doped ZnO thin films as a sensor. The response characteristics of 410 nm ZnO thin film for temperature ranging from 180 to 360 °C were determined for the acetone vapors. The reported study provides a significant development towards acetone sensors, where a very high sensitivity with rapid response and recovery times are reported with lowered optimal operating temperature as compared to bare ZnO nano-chains like structured thin films. In comparison to the bare ZnO thin films giving a response of 63 at an operating temperature of 320 °C, a much better response of 132.3 was observed for the Au doped ZnO thin films at an optimised operating temperature of 280 °C for a concentration of 500 ppm of acetone vapors.     

Instabilities in nanoporous media

Rayleigh instabilities in thin polymer films confined within nanoporous alumina membranes were studied. Thin films of poly(methyl methacrylate) (PMMA) were prepared by filling cylindrical nanopores in an anodic aluminum oxide (AAO) membrane with a PMMA solution in chloroform followed by solvent evaporation. When the PMMA nanotubes were annealed above the glass transition temperature (Tg), undulations in the film thickness were observed that were induced by a Rayleigh instability. The amplitude of the undulations increased with time and eventually bridged across the cylindrical nanopore in the AAO membrane, resulting in the formation of polymer nanorods with periodic encapsulated holes. A similar behavior was observed when PMMA films were confined within carbon nanotubes (CNTs). The Rayleigh instabilities in these confined geometries offer a novel means of controlling and fabricating the polymer nanostructures. These compartmentalized nanorods may have potential applications as delivery devices.     

Indium Tin Oxide Thin-Film Sensor for Detection of Volatile Organic Compounds (VOCs)

Indium tin oxide (ITO) (In₂O₃ + 17% SnO₂) thin films were grown on glass substrate by direct evaporation method. Two thick gold pads were deposited to take out contacts. The response of these films at different operating temperatures, when exposed to various volatile organic compounds (VOCs) such as methanol, ethanol, butanol, and acetone in the concentration range 200-2500 ppm was evaluated. Additionally, the effect of film thickness on the response charateristics of methanol and acetone was studied. The linearity and sensitivity of the sensors were measured. The ITO thin-film sensors showed a sensitivity of 0.256 ohms/ppm to acetone vapors, which was almost linear in the range 200-2500 ppm. In order to improve sensitivity and selectivity, a thin layer of various metal and metal oxides such as Cu and PbO was deposited on the sensor surface to work as catalytic layer and the effect on the performance of the sensor was studied. The response and recovery times of the sensor were determined for acetone vapors and were found to be 155 sec and 110 sec, respectively.     

Solution grown nanocrystalline ZnO thin films for UV emission and LPG sensing

Nanocrystalline ZnO thin films were successfully deposited by a simple and inexpensive solution growth technique. Photoluminescence (PL) and liquefied petroleum gas (LPG) sensing properties were investigated. Films were found to be uniform, pinhole free, and well adherent. As deposited and heat treated (at 673 K for 2 h) films were characterized by XRD, SEM, and EDAX. The dc electrical resistivity and LPG sensing property were measured. The change in morphology, from spherical particle to rod-like, was observed after air annealing. XRD results revealed that the obtained films were nanocrystalline and had a hexagonal wurtzite structure. The absorption edge was found to be at around 366 nm for the as-deposited film and 374 nm for the annealed film. The band gaps were found to be 3.29 and 2.9 eV for the as-deposited and annealed films, respectively. PL spectra of ZnO thin films showed strong peak at 384 nm, which corresponds to near band edge emission (UV emission) and a relatively weak peak at 471 nm. Further, the annealed film was used for detection of LPG in air. Maximum response was observed at 673 K. The maximum sensitivity of sensor was found to be 4.5 for 0.6 vol.% LPG. Sensing response got saturated after 0.6 vol.% of gas concentration. A possible mechanism of LPG sensing has been explained.     

ZnO thin films for VOC sensing applications

ZnO thin films were prepared by reactive RF sputtering on thermally oxidized Si for gas sensing applications. Three VOC vapors were chosen to investigate the response behavior of the prepared ZnO. Acetone, isopropanol and ethanol were tested, and the sensitivity of the sensor toward acetone was the highest (S ∼ 100) for 500 ppm acetone at 400 °C. The largest sensitivity was achieved at 400 °C for all the above vapors. The sensor shows a stable, reversible and repeatable behavior in the acetone concentration ranging from 15 up to 1000 ppm. The mechanism of the sensing was explained according to the ionosorption model.