Chemical sensors based on dielectric response of functionalized mesoporous silica films
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Dielectric response of mesoporous silica films was monitored as a function of several gas-phase chemical species. The effects of humidity, ammonia, and methane on dielectric constant and dissipation factor of films subjected to different chemical treatments are described. Dielectric constant and dissipation factor of partially dehydroxylated films were found to be highly sensitive to both water vapor and ammonia in air. The capacitive devices based on mesoporous silica films show potential for use in chemical sensors.
I. INTRODUCTION
Since the discovery of molecularly-templated processing for producing silica materials with ordered mesoscale (1 nm < pore size < 100 nm) porosity,1,2 a number of ceramic materials containing surfactant-templated porosity have been considered for applications in chemical sensors. Surfactant-derived “mesoporous” silica materials have been evaluated in piezoelectric and optical sensor devices.3,4 Chemical sensors based on infrared spectroscopy of absorbent nanoporous films have also been investigated.5 In addition, changes in electrical resistance in conventional semiconducting sensor materials such as tin oxide, prepared in a porous form, have been used to detect reducing gases.6 More recently, the semiconductor-type properties of transition metal oxide based molecular sieves (“NbTMS1”) have been exploited for detecting changes in gas concentration (e.g., humidity) in a capacitive-type chemical sensor.7 The calibrated changes in dielectric properties of such thin mesoporous films with gas phase concentration can potentially form the basis for many new chemical sensors. The dielectric behavior of mesoporous films has been of recent interest8–10 because of their potential use as interlevel films in semiconductor interconnects. A key aspect of this application development, apart from controlling the porosity and pore structure, is reducing the dielectric constant of the material by decreasing the number of hydroxyl groups in the film.9 The presence of highly polar hydroxyl groups can significantly increase the dielectric constant, and also render
a)
Currently with Massachusetts Institute of Technology, Cambridge, MA. b) Address all correspondence to this author. e-mail: [email protected] 2810
http://journals.cambridge.org
J. Mater. Res., Vol. 16, No. 10, Oct 2001 Downloaded: 16 Mar 2015
the electrical parameters highly susceptible to water vapor in the ambient air. For advanced semiconductor applications where a low (Ⰶ2.0) stable value of dielectric constant is required, it is desirable to completely dehydroxylate the pore surfaces. For other important applications such as gas sensors, it is possible to take advantage of the strong susceptibility of partially dehydroxylated or functionalized mesoporous films to ambient conditions. In this paper, we demonstrate the use of mesoporous silica films in capacitive gas-phase chemical sensors. Variations in the dielectric properties with concentration of water vapor, ammonia, and methane in air are presented for films subjected to bot
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