Thin Films For Gas Sensing Applications Research Articles

Developing GLAD Parameters to Control the Deposition of Nanostructured Thin Film.

In this paper, we describe the device developed to control the deposition parameters to manage the glancing angle deposition (GLAD) process of metal-oxide thin films for gas-sensing applications. The GLAD technique is based on a set of parameters such as the tilt, rotation, and substrate temperature. All parameters are crucial to control the deposition of nanostructured thin films. Therefore, the developed GLAD controller enables the control of all parameters by the scientist during the deposition. Additionally, commercially available vacuum components were used, including a three-axis manipulator. High-precision readings were tested, where the relative errors calculated using the parameters provided by the manufacturer were 1.5% and 1.9% for left and right directions, respectively. However, thanks to the formula developed by our team, the values were decreased to 0.8% and 0.69%, respectively.

Surface-area-controlled synthesis of porous TiO2 thin films for gas-sensing applications

Surface-area-controlled porous TiO2 thin films were prepared via a simple sol–gel chemical route, and their gas-sensing properties were thoroughly investigated in the presence of typical oxidizing NO2 gas. The surface area of TiO2 thin films was controlled by developing porous TiO2 networked by means of controlling the TiO2-to-TTIP (titanium isopropoxide, C12H28O4Ti) molar ratio, where TiO2 nanoparticles of size ∼20 nm were used. The sensor’s response was found to depend on the surface area of the TiO2 thin films. The porous TiO2 thin-film sensor with greater surface area was more sensitive than those of TiO2 thin films with lesser surface area. The improved sensing ability was ascribed to the porous network formed within the thin films by TiO2 sol. Our results show that surface area is a key parameter for obtaining superior gas-sensing performance; this provides important guidelines for preparing and using porous thin films for gas-sensing applications.

RF magnetron sputtered Cd doped ZnO thin films for gas-sensing applications

ABSTRACTRF magnetron sputtering has been employed to manufacture undoped and Cd doped (3%, 10%, and 20%) ZnO (CZO) films. X-ray diffraction (XRD) results revealed that the films have preferential orientation along (002) plane with hexagonal structure, and there are no secondary peaks. It is observed from the Hall Effect measurement that CZO films posses n-type conductivity, and the carrier concentration does not vary much with Cd concentration. On the other hand, the band gap estimated from Tauc’s plot shows that band gap decreased with increase in Cd concentration. Red shift was observed in photoluminescence spectrum. Ethanol, methanol, and ammonia at gas concentration range of 50–200 ppm were exposed on the grown CZO films at different temperatures. Among the various concentrations of the films, CZO having 20% Cd exhibited better gas response toward methanol, while CZO with 10% Cd showed maximum gas response toward ammonia gas. This has been explained by grain boundary barrier (GBB) and double Schottky potential barrier mechanisms.

Monolayer Co3 O4 Inverse Opals as Multifunctional Sensors for Volatile Organic Compounds.

Monolayers of periodic porous Co3 O4 inverse opal (IO) thin films for gas-sensor applications were prepared by transferring cobalt-solution-dipped polystyrene (PS) monolayers onto sensor substrates and subsequent removal of the PS template by heat treatment. Monolayer Co3 O4 IO thin films having periodic pores (d≈500 nm) showed a high response of 112.9 to 5 ppm C2 H5 OH at 200 °C with low cross-responses to other interfering gases. Moreover, the selective detection of xylene and methyl benzenes (xylene+toluene) could be achieved simply by tuning the sensor temperature to 250 and 275 °C, respectively, so that multiple gases can be detected with a single chemiresistor. Unprecedentedly high ethanol response and temperature-modulated control of selectivity with respect to ethanol, xylene, and methyl benzenes were attributed to the highly chemiresistive IO nanoarchitecture and to the tuned catalytic promotion of different gas-sensing reactions, respectively. These well-ordered porous nanostructures could have potential in the field of high-performance gas sensors based on p-type oxide semiconductors.

Thermal treatment stabilization processes in SnO/sub 2/ thin films catalyzed with Au and Pt

We examined the thermal treatment stabilization processes in SnO/sub 2/-based thin films for gas-sensing applications in order to understand their influence on structural properties and possible relations with functional behavior of the sensors. The films of SnO/sub 2/ were deposited by magnetron sputtering, following the rules of the rheothaxial growth and thermal oxidation (RGTO) technique and annealed in humid synthetic air at 600/spl deg/C. Afterward, a thin overlayer of gold or platinum was deposited on the oxidized films. The samples were characterized soon after catalyzer deposition and after one-hour aging process at 400/spl deg/C, in order to evaluate stabilization process influence as a function of catalyzer. X-ray photoelectron spectroscopy (XPS) and scanning tunnelling microscopy (STM) demonstrated that a short aging affects the microstructure of gold-catalyzed samples, while it does not influence platinum catalyzed ones. The results of the microstructural characterization were confirmed by electrical measurements of gas-sensitivity: the short-aging influences the stability of gas-sensing parameters only in gold-catalyzed samples.

Nanosized Ti-doped MoO 3 thin films for gas-sensing application

Development of MoO 3 as a novel material for gas sensing was addressed. Thin films were produced by reactive rf sputtering assisted by annealing. Doping with Ti was performed to enhance the conductance of the MoO 3 film. It came out that the layers were two orders of magnitude more conductive than undoped material. Good and reversible response to CO was achieved at 300°C, which fell off at higher temperatures. An interesting feature of the films was a considerably fast response for both CO and NO 2 despite low operating temperature. Doped films were found to operate at best about 100°C below than for pure MoO 3 layers.

The effects of oxygen doping and annealing on the surface and bulk electrical conductivity in planar copper phthalocyanine thin films for gas-sensing applications

In the development of gas-sensing devices based on phthalocyanine materials, it has become evident that the current drawn depends on previous exposure to oxygen, which acts as an acceptor impurity, and on the relative importance of the bulk and surface regions. The present work focuses on the effects of oxygen on α-phase thin films of copper phthalocyanine (CuPc) having a planar configuration. In unannealed samples both ohmic and space-charge-limited conductivity (SCLC) regions were observed in films of thickness 0.1 μm, whereas for films of thickness 1 μm, SCLC was not observed below a field strength of 2×10 6 V m −1. These variations may be explained in the context of the differing effects of oxygen on the surface and bulk regions of the films. Exposure to oxygen has the effect of enhancing the conductivity and increasing the threshold voltage between the ohmic and SCLC regions. In the thinner films, samples lightly doped with oxygen had a trap distribution exponentially distributed in energy, whereas long exposure to oxygen resulted in the establishment of a discrete dominant trap level. While annealing was found to reduce the conductivity due to oxygen desorption, the resulting activation energy was found to be dependent on the concentration of oxygen within the films. Observations of oxygen desorption from films of different thickness revealed that the binding energy of oxygen to the CuPc molecule is lower on the surface than in the bulk of the films. It was concluded that the adsorbed or absorbed oxygen molecules resulted in the establishment of a partial charge-transfer complex, and that the hole concentration on the surface of the films is lower than that in the bulk under vacuum.