Sensing Properties of Oxidized Nanostructured Silicon Surface on Vaporized Molecules

Porous silicon has been intensely studied for the past several decades and its applications were found in photovoltaics, biomedicine, and sensors. An important aspect for sensing devices is their long–term stability. One of the more prominent changes that occur with porous silicon as it is exposed to atmosphere is oxidation. In this work we study the influence of oxidation on the sensing properties of porous silicon. Porous silicon layers were prepared by electrochemical etching and oxidized in a tube furnace. We observed that electrical resistance of oxidized samples rises in response to the increasing ambient concentration of organic vapours and ammonia gas. Furthermore, we note the sensitivity is dependent on the oxygen treatment of the porous layer. This indicates that porous silicon has a potential use in sensing of organic vapours and ammonia gas when covered with an oxide layer

FABRICATION AND CHARACTERIZATION OF CARBON NANOTUBES FOR PRESSURE SENSING APPLICATION

Carbon nanotubes (CNTs) can be utilized to replace Si as the pressure sensing element since they have better electromechanical properties that remain stable even at 250oC. With Si-based sensor, the intrinsic limitation of P-N junction causes it to operate below 120oC. Thus the work of developing CNTs as the pressure sensing element would require synthesizing good quality, highly aligned CNTs array with specific characteristics to enhance their sensing properties before integrating them into the testing configuration in order to establish the working principle of the pressure sensor. Aligned multiwalled carbon nanotubes (MWCNTs) array was synthesized via thermal chemical vapor deposition (CVD) and characterized using Raman spectroscopy, scanning electron spectroscopy (SEM) and transmission electron microscopy (TEM). Studies were performed to investigate the effect of different methods of Al2O3 buffer layer preparations and Fe precursor used on the MWCNTs produced. It was found that direct deposition of Al2O3 produced MWCNTs with highest degree of crystallinity (ratio of disorder peak to graphitic peak, ID/IG = 0.98) whilst oxidation of Al thin film in air exhibited the highest growth rate resulting in the thickest MWCNTs film (~60 m). Both forms of Fe catalyst, pellet and powder produced similar film thickness (typically 47 m) but Fe pellet gives cleaner MWCNTs array without the presence of particles. Piezoresistive measurement was performed on the as-grown MWCNTs array without transferring to a conductive substrate as the buffer layer is made electrically conductive. Electrical resistance of the MWCNTs array, R decreased with applied weights (50 g to 500 g) and is more responsive towards rapid changes in the applied loading (interval of 10 s). At constant weight loading (100 g and 500 g), the resistance decreases linearly with temperature, T (from 30oC to 180oC) while the slope, dR/dT remains independent of the weight. In summary, good quality, highly aligned MWCNTs array has been successfully synthesized, characterized and developed as the pressure sensing element. Results of piezoresistive measurement have established its prospect as the pressure sensing element suitable for operation beyond the temperature limit of Si-based sensors

Confined hydration in nanometer-graded plasma polymer films: Insights from surface-enhanced infrared absorption spectroscopy

To shed light on recently explored long-range surface forces generated by subsurface-confined water, the structural characteristics of water molecules penetrating into nanoporous homogeneous and nanograded siloxane plasma polymer films (PPFs) over the time scale of 24 hours are studied by surface-enhanced IR spectroscopy (SEIRAS). Chemically graded PPFs, with embedded hydrophobic-to-hydrophilic gradient, are found to significantly change the average interfacial water orientation due to a unique nanoporous morphology and silanol group coordination. Diffusion of water through the hydrophobic SiO:CH matrix creates an evolution of the coordination of matrix silanol groups, which are eventually deprotonated as soon as the hydration network connects to the aqueous environment. This occurs after -6 hours of water immersion and coincides with the change of average interfacial water orientation. Both effects are present on hydrophobic samples, but are significantly amplified by the presence of the subsurface vertical amphiphilic gradient (Vgrad), whereas enhanced water uptake in oxygen-plasma modified graded PPFs is covering such effects

Broadband Dielectric Spectroscopic Detection of Ethanol: A Side-by-Side Comparison of ZnO and HKUST-1 MOFs as Sensing Media†,‡

The most common gas sensors are based on chemically induced changes in electrical resistivity and necessarily involve making imperfect electrical contacts to the sensing materials, which introduce errors into the measurements. We leverage thermal- and chemical-induced changes in microwave propagation characteristics (i.e., S-parameters) to compare ZnO and surface-anchored metal–organic-framework (HKUST-1 MOF) thin films as sensing materials for detecting ethanol vapor, a typical volatile organic compound (VOC), at low temperatures. We show that the microwave propagation technique can detect ethanol at relatively low temperatures (<100 °C), and afford new mechanistic insights that are inaccessible with the traditional dc-resistance-based measurements. In addition, the metrological technique avoids the inimical measurand distortions due to parasitic electrical effects inherent in the conductometric volatile organic compound detection

Broadband Dielectric Spectroscopic Detection of Ethanol: A Side-by-Side Comparison of ZnO and HKUST-1 MOFs as Sensing Media

The most common gas sensors are based on chemically induced changes in electrical resistivity and necessarily involve making imperfect electrical contacts to the sensing materials, which introduce errors into the measurements. We leverage thermal- and chemical-induced changes in microwave propagation characteristics (i.e., S-parameters) to compare ZnO and surface-anchored metal-organic-framework (HKUST-1 MOF) thin films as sensing materials for detecting ethanol vapor, a typical volatile organic compound (VOC), at low temperatures. We show that the microwave propagation technique can detect ethanol at relatively low temperatures (\u3c100 \u3e°C), and afford new mechanistic insights that are inaccessible with the traditional dc-resistance-based measurements. In addition, the metrological technique avoids the inimical measurand distortions due to parasitic electrical effects inherent in the conductometric volatile organic compound detection

Strategies for Enhancing the Performance of Chemical Sensors Based on Microcantilever Sensors

Microcantilever (MC) based chemical sensors have become more widely used during the past 10 years due to the advantages they possess over other chemical sensors. One of the most significant characteristics is their extremely high surface to volume ratio. This key facet allows surface forces that can be ignored on a macroscale to become a significant sensing transduction mechanism. MC based sensors also exhibit a higher mass sensitivity to adsorbates than do many other chemical sensor platforms. Under many conditions, MC based sensors directly translate changes in Gibbs free energies due to analyte-surface interactions into mechanical responses. However, the widespread application of MCs in the field of sensors has yet to be fully realized. This is primarily due to the lack of a unifying methodology and instrumentation that would allow various research groups to benefit from a combined wealth of knowledge on the subject. The underlying goal of this research is to broaden the depth and scope of knowledge of MC based chemical sensors. By working on several areas in a coherent order, the limitations of MC based sensors have been determined and largely overcome. The information gathered in all aspects of this project will be useful to present and future researchers in this field. The initial research was focused on the application of various chemical films to MC sensors to be able to measure a wide range of chemical species. In one case, thin films of polymeric gas chromatography (GC) phases were deposited onto V-shaped MCs. A main strength to using GC phases was that the responses of the analytes could be predicted before hand by using the McReynolds constants of the phases used. This allowed for the detection and quantification of various chemical species using these moderately selective phases. vi During this phase of research it was discovered that methods for enhancing MC response were needed to overcome some of the traditional problems facing MC based sensors. By employing a new type of underlying nanostructured metallic film, MC response was greatly enhanced. This resulted in a better limit of detection and wider dynamic range relative to previous results with smooth surface MCs. In addition to advances resulting from nanostructuring, important advances were made in MC coating strategies. The widely used and well-characterized process of physical vapor deposition was used to deposit both organic and polymeric materials onto the MC surface. This process allowed for uniform films to be deposited with tailored thicknesses and for individual MCs on a single chip to be coated selectively. Another approach involving the immersion of MCs into fused silica capillaries containing solutions of thiolated materials was also developed. This method also allowed for individual MCs in an array to be selectively coated. Finally, out of these results and a developing trend of using sensor arrays came the need to increase the robustness and selectivity of MC based systems. Two different systems for achieving these goals were developed. First, a simple differential system based upon dual diode lasers was constructed in order to eliminate common sources of noise and non-specific interactions that decrease the dynamic range of these sensors. This system was also applied to the quantification of individual components in a binary mixture. While this system has met only limited success, it has been a beneficial first step towards MC systems of higher order. Towards that goal, a system designed to measure multiple MCs simultaneously using an array of vertical cavity surface emitting lasers was also used. This system measures the responses of multiple MCs exposed to an vii analyte in a single run and provides unique response patterns for that analyte. This allowed for the qualitative analysis of a simple mixture to be performed

Gas Sensors Based on Conducting Polymers

Since the discovery of conducting polymers (CPs), their unique properties and tailor-made structures on-demand have shown in the last decade a renaissance and have been widely used in fields of chemistry and materials science. The chemical and thermal stability of CPs under ambient conditions greatly enhances their utilizations as active sensitive layers deposited either by in situ chemical or by electrochemical methodologies over electrodes and electrode arrays for fabricating gas sensor devices, to respond and/or detect particular toxic gases, volatile organic compounds (VOCs), and ions trapping at ambient temperature for environmental remediation and industrial quality control of production. Due to the extent of the literature on CPs, this chapter, after a concise introduction about the development of methods and techniques in fabricating CP nanomaterials, is focused exclusively on the recent advancements in gas sensor devices employing CPs and their nanocomposites. The key issues on nanostructured CPs in the development of state-of-the-art miniaturized sensor devices are carefully discussed. A perspective on next-generation sensor technology from a material point of view is demonstrated, as well. This chapter is expected to be comprehensive and useful to the chemical community interested in CPs-based gas sensor applications

Highly Sensitive and Selective Gas Sensors Based on Vertically Aligned Metal Oxide Nanowire Arrays

Mimicking the biological olfactory systems that consist of olfactory receptor arrays with large surface area and massively-diversified chemical reactivity, three dimensional (3D) metal oxide nanowire arrays were used as the active materials for gas detection. Metal oxide nanowire arrays share similar 3D structures as the array of mammal\u27s olfactory receptors and the chemical reactivity of nanowire array can be modified by surface coatings. In this dissertation, two standalone gas sensors based on metal oxide nanowire arrays prepared by microfabrication and in-situ micromanipulation, respectively, have been demonstrated. The sensors based on WO3 nanowire arrays can detect 50 ppb NO2 with a fast response; well-aligned CuO nanowire array present a new detection mechanism, which can identify H2S at a concentration of 500 ppb. To expand the material library of 3D metal oxide nanowire arrays for gas sensing, a general route to polycrystalline metal oxide nanowire array has been introduced by using ZnO nanowire arrays as structural templates. The effectiveness of this method for high performance gas sensing was first investigated by single-nanowire devices. The polycrystalline metal oxide coatings showed high performance for gas detection and their sensitivity can be further enhanced by catalytic noble metal decorations. To form electronic nose systems, different metal oxide coatings and catalytic decorations were employed to diversify the chemical reactivity of the sensors. The systems can detect low concentrated H2S and NO2 at room temperature down to part-per-billion level. The system with different catalytic metal coatings is also capable of discriminiating five different gases (H2S, NO2, NH3, H2 and CO)

Nanocomposite Films for Gas Sensing

Nanocomposite films are thin films formed by mixing two or more dissimilar materials having nano-dimensional phase(s) in order to control and develop new and improved structures and properties. The properties of nanocomposite films depend not only on the individual components used but also on the morphology and the interfacial characteristics. Nanocomposite films that combine materials with synergetic or complementary behaviours possess unique physical, chemical, optical, mechanical, magnetic and electrical properties unavailable from that of the component materials and have attracted much attention for a wide range of device applications such as gas sensors.NRC publication: Ye

FABRICATION AND CHARACTERIZATION OF CARBON NANOTUBES FOR PRESSURE SENSING APPLICATION

Carbon nanotubes (CNTs) can be utilized to replace Si as the pressure sensing element since they have better electromechanical properties that remain stable even at 250oC. With Si-based sensor, the intrinsic limitation of P-N junction causes it to operate below 120oC. Thus the work of developing CNTs as the pressure sensing element would require synthesizing good quality, highly aligned CNTs array with specific characteristics to enhance their sensing properties before integrating them into the testing configuration in order to establish the working principle of the pressure sensor. Aligned multiwalled carbon nanotubes (MWCNTs) array was synthesized via thermal chemical vapor deposition (CVD) and characterized using Raman spectroscopy, scanning electron spectroscopy (SEM) and transmission electron microscopy (TEM). Studies were performed to investigate the effect of different methods of Al2O3 buffer layer preparations and Fe precursor used on the MWCNTs produced. It was found that direct deposition of Al2O3 produced MWCNTs with highest degree of crystallinity (ratio of disorder peak to graphitic peak, ID/IG = 0.98) whilst oxidation of Al thin film in air exhibited the highest growth rate resulting in the thickest MWCNTs film (~60 m). Both forms of Fe catalyst, pellet and powder produced similar film thickness (typically 47 m) but Fe pellet gives cleaner MWCNTs array without the presence of particles. Piezoresistive measurement was performed on the as-grown MWCNTs array without transferring to a conductive substrate as the buffer layer is made electrically conductive. Electrical resistance of the MWCNTs array, R decreased with applied weights (50 g to 500 g) and is more responsive towards rapid changes in the applied loading (interval of 10 s). At constant weight loading (100 g and 500 g), the resistance decreases linearly with temperature, T (from 30oC to 180oC) while the slope, dR/dT remains independent of the weight. In summary, good quality, highly aligned MWCNTs array has been successfully synthesized, characterized and developed as the pressure sensing element. Results of piezoresistive measurement have established its prospect as the pressure sensing element suitable for operation beyond the temperature limit of Si-based sensors