Large attention has been directed toward carbon nanotubes as material for chemical sensors. However, little attention was paid toward the different behavior of the metallic and semiconductive carbon nanotubes as optical sensing materials.
Semiconductive or metallic Single Wall Carbon Nanotubes (SWCNTs) have been deposited on gold nanoparticles (NPs) monolayer and used as plasmonic based gas sensor.
The coupling between SWCNTs and Au NPs has the aim of combining the reactivity of the nanotubes towards hazardous gases, such as H2, CO, NO2, with the Localized Surface Plasmon Resonance (LSPR) of gold NPs. The LSPR is known to be extremely sensitive to the changes in the dielectric properties of the surrounding medium, a characteristic that has been widely exploited for the preparation of sensing devices. While the use of SWCNTs for gas sensing has been covered in multiple reports, to the best of our knowledge this is the first time that SWCNTs are used as sensing material in an optical sensor for the detection of reducing and oxidizing gases.
Two different techniques, ink-jet printer and dropcasting, were used for depositing the transparent CNTs film on the plasmonic layer. Both the deposition techniques proved to be effective for the development of transparent optical sensing films.
Metallic SWCNTs showed high sensitivity toward H2 at low temperature and an enhancement of performance at 300°C with the detection of low concentration of H2 and NO2. On the contrary, the semiconductive SWCNTs displayed very poor gas sensing properties, especially for the thinner film.
The desirable electrical properties of InO<sub>x</sub> thin films and their response towards oxidizing gases has promoted InO<sub>x</sub> to be recognized as a promising material for gas sensors. In this study, InO<sub>x</sub> films in the thickness range of 10-1000 nm were deposited onto Corning 7059 glass substrates by dc magnetron sputtering. Their structural, electrical, and O<sub>3</sub> and NO<sub>2</sub> sensing properties were analyzed. Structural investigations carried out by XRD and AFM showed a strong correlation between crystallinity, surface topology and gas sensitivity. Moreover, the electrical conductivity exhibited a change of over six orders of magnitude during the processes of photoreduction and oxidation. The films deposited on alumina transducers were calibrated towards O<sub>3</sub> and NO<sub>2</sub> at temperatures from 50-300 °C. The sensors show promising characteristics as they exhibited reproducible and stable responses. The 50 nm thin film had a response of over 10 towards 50 ppb of ozone operating at 50°C, while the 20 nm film had a response of over 22 towards 0.1 ppm of NO<sub>2</sub> at 100°C.
Molybdenum trioxide - tungsten trioxide (MoO<sub>3</sub>-WO<sub>3</sub>) and titanium oxide - molybdenum oxide (TiO<sub>2</sub>-MoO<sub>3</sub>) binary metal oxide thin films have been prepared by the sol-gel process and by PVD. The films were deposited using the spin coating technique and by sputtering onto alumina substrates with interdigital electrodes. MoO<sub>3</sub>-WO<sub>3</sub> film morphology is composed of MoO<sub>3</sub> needle like grains and WO<sub>3</sub> spherical grains when annealed at 450°C. MoO<sub>3</sub>-TiO<sub>2</sub> film morphology consists of a well-developed crystal structure for the sol-gel film and a porous morphology for the sputtered films when annealed at 800°C. The films exhibited selective gas sensing characteristics at an operating temperature of 300°C towards nitrogen dioxide (NO<sub>2</sub>). The RF and SG fabricated MoO<sub>3</sub>-TiO<sub>2</sub> possess different gas sensing properties attributed to the fabrication and resulting morphological difference of the thin films.
Layered Surface Acoustic Wave (SAW) devices that allow the propagation of Love mode acoustic waves will be studied in this paper. In these devices, the substrate allows the propagation of Surface Skimming Bulks Waves (SSBWs). By depositing layers, that the speed of Shear Horizontal (SH) acoustic wave propagation is less than that of the substrate, the propagation mode transforms to Love mode. Love mode devices which will be studied in this paper, have SiO2 and ZnO acoustic guiding layers. As Love mode of propagation has no movement of particles component normal to the active sensor surface, they can be employed for the sensing applications in the liquid media.
MoO<SUB>3</SUB>-WO<SUB>3</SUB> thin films have been fabricated via the sol-gel method. FESEM, TEM, RBS and SIMS analysis techniques have been employed to analyse the films and material properties for use as gas sensors to detect CO and NO<SUB>2</SUB>. FESEM shows the film made up of segregated molybdenum crystals. TEM highlights the nano-sized grains sructure and crystallinity. RBS analysis confirmed the films are stoichimetric and that the Mo component of the system decreases as the annealing temperature is increased. SIMS illustrates the interesting elemental depth profiles of the films. The films were exposed to CO and NO<SUB>2</SUB>. MoO<SUB>3</SUB>-WO<SUB>3</SUB> shows better NO<SUB>2</SUB> sensitivity and selectivity compared to its single metal oxide constituents.
Binary metal oxide MoO<SUB>3</SUB>-TiO<SUB>2</SUB> thin films have been prepared by the sol-gel process. These films were deposited on sapphire substrates with interdigital electrodes and single crystal silicon substrates. The films were annealed at different temperatures of 400 degrees C, 500 degrees C, and 600 degrees C for 1 hour. The morphology, crystalline structure and chemical composition of the films have been analyzed using SEM, XRD, RBS and XPS techniques. The SEM analysis showed that the films annealed at 500 degrees C are smooth and uniform with nanosized grains and probes. RBS and XPS characterizations have revealed that the films are nearly stoichiometric. In this work, we have investigated the sensitivity of this material for oxygen and ozone gases. The MoO<SUB>3</SUB>-based gas sensor is capable of detecting O<SUB>2</SUB> down to 50 ppm with a very fast response time. Adding TiO<SUB>2</SUB> to MoO<SUB>3</SUB> thin films tremendously reduced the resistance, which assisted the measurement of ozone gas sensing.