Nanotubes and nanorods on CMOS substrates for gas sensing

In this paper we discuss the combined use of integrated CMOS microhotplates employing nanomaterial sensing layers for intelligent, compact gas sensors with increased sensitivity, selectivity and fast response times. We first review the status of nanomaterial‐based gas sensors, their operating principles, discussing growth issues and their compatibility with CMOS substrates. We then describe Multiwall (MW) Carbon Nanotubes (CNTs) and ZnO Nanowires (NW) growth∕deposition onto CMOS microhotplates. The paper continues by discussing the response of these nanomaterial sensing layers to vapours and gasses. Finally we discuss the future prospects of nanomaterial‐based CMOS gas sensors, highlighting on one hand their future potential and on the other hand their present shortcomings and future challenges that need to be addressed before they can be released commercially

Selective Detection of Nitrogen-Containing Compound Gases

N-containing gaseous compounds, such as trimethylamine (TMA), triethylamine (TEA), ammonia (NH3), nitrogen monoxide (NO), and nitrogen dioxide (NO2) exude irritating odors and are harmful to the human respiratory system at high concentrations. In this study, we investigated the sensing responses of five sensor materials—Al-doped ZnO (AZO) nanoparticles (NPs), Pt-loaded AZO NPs, a Pt-loaded WO3 (Pt-WO3) thin film, an Au-loaded WO3 (Au-WO3) thin film, and N-doped graphene—to the five aforementioned gases at a concentration of 10 parts per million (ppm). The ZnO- and WO3-based materials exhibited n-type semiconducting behavior, and their responses to tertiary amines were significantly higher than those of nitric oxides. The N-doped graphene exhibited p-type semiconducting behavior and responded only to nitric oxides. The Au- and Pt-WO3 thin films exhibited extremely high responses of approximately 100,000 for 10 ppm of triethylamine (TEA) and approximately −2700 for 10 ppm of NO2, respectively. These sensing responses are superior to those of previously reported sensors based on semiconducting metal oxides. On the basis of the sensing response results, we drew radar plots, which indicated that selective pattern recognition could be achieved by using the five sensing materials together. Thus, we demonstrated the possibility to distinguish each type of gas by applying the patterns to recognition techniques

SnO₂ Nanoslab as NO₂ Sensor: Identification of the NO₂ Sensing Mechanism on a SnO₂ Surface

Among the various metal oxides, SnO₂ has been most widely exploited as a semiconductor gas sensor for its excellent functionalities. Models illustrating the sensing mechanism of SnO₂ have been proposed and tested to explain experimentally derived “power laws”. The models, however, are often based on somewhat simplistic assumptions; for instance, the net charge transfer from an adsorbate to a sensor surface site is assumed to occur only by integer values independent of the crystallographic planes. In this work, we use layer-shaped SnO₂ crystallites with one nanodimension (1ND-crystallites) as NO₂ gas sensing elements under flat band conditions, and derive appropriate “power laws” by combining the dynamics of gas molecules on the sensor surface with a depletion theory of semiconductor. Our experimentally measured sensor response as a function of NO₂ concentration when compared with the theoretically derived power law indicates that sensing occurs primarily through the chemisorption of single NO₂ molecules at oxygen vacancy sites on the sensor surface

ZnO film thickness effect on surface acoustic wave modes and acoustic streaming

Surface acoustic wave(SAW) devices were fabricated on ZnO thin films deposited on Si substrates. Effects of ZnOfilm thickness on the wave mode and resonant frequency of the SAWs have been investigated. Rayleigh and Sezawa waves were detected, and their resonant frequencies decrease with increase in film thickness. The Sezawa wave has much higher acoustic velocity and larger signal amplitude than those of Rayleigh mode wave.Acoustic streaming for mixing has been realized in piezoelectric thin filmSAWs. The Sezawa wave has a much better efficiency in streaming, and thus is very promising for application in microfluidics