| dc.description.abstract | Gas sensors play a vital role in today’s world; be it in pollution monitoring, breath analysis, food quality monitoring or in agriculture. A variety of methods are being explored to develop reliable gas sensor systems such as optical, acoustic, electrochemical and chemical. Among these methods of gas detection, the chemical gas sensors based on conductometric change of the sensing materials; mainly semiconducting metal oxides, are gaining much attention due to their simplicity, easy fabrication and low cost. However, the metal-oxide based gas sensors still pose issues in terms of cross-sensitivity, reproducibility, and sensor life.
Researchers have explored many avenues including processing methods, nanostructuring and doping to develop reliable metal oxide gas sensors. Gas sensors based on nanostructures of metal oxides have shown a lot of promise due to their high surface to volume ratio; high surface energies and specificity of crystallographic planes. The scope of this thesis is to explore different fabrication methods for developing nanodimensional gas sensors which meet the figures of merit namely sensitivity, selectivity and stability. Furthermore, an effort is also made to develop a low cost and low power sensor array platform on a flexible substrate, using CMOS processes, to enable the application of these gas sensors in wearable electronics.
In this thesis, plasma oxidation of different metallic structures to form metal-metal oxide core-shell type sensors is investigated for achieving the best hydrogen sulfide sensor. Platinum, widely known for its catalytic nature, is plasma oxidized at optimum conditions to fabricate Pt-PtOx core-shell nanowire sensor. The Pt-PtOx sensor shows degradation in response when used for H2S sensing, which is due to surface contamination by the sulphur species. To recover the sensor, deep ultra-violet light (UV) treatment is studied as a promising recovery method for sulphur contaminated sensor surfaces. Further, plasma oxidation of tungsten nanodiscs decorated W nanowire is carried out to achieve high sensitive H2S sensor with fast response and recovery times and good response repeatability over a study span of 6 months. The nanostructured W-WOx nanowire sensor is highly selective towards H2S with low order of detection of 10 ppb which is one of the lowest values reported in literature. The tungsten nanodiscs are patterned using electron beam lithography process which is known to be expensive and time consuming. Thus, an effort is also made to integrate the inexpensive process such as colloidal lithography to pattern the
nanostructures on the metal nanowires, later plasma oxidized to form the core shell sensor. Colloidal lithography assisted nanostructure-based palladium-palladium oxide sensor is
fabricated and tested for sensing of H2S gas. The fabricated sensor shows a detection limit of 10 ppb but lacks its performance in terms of high response and high selectivity when compared to nanostructured W-WOx sensor. In summary, plasma oxidation of metallic structures is explored to fabricate H2S gas sensors, with low order of gas detection of 10 ppb. The surface poisoning during H2S sensing is tackled through UV exposure to recover the sulphur poisoned sensors. Colloidal lithography is investigated for fabricating nanostructures as an inexpensive alternate for electron beam lithography. Thus, we believe that plasma oxidation of metallic structures and nanostructuring using colloidal lithography are two important methods which need to be explored further to develop gas sensors which meet the SSS figure of merit with low cost fabrication methodology.
We also explored a low cost and low temperature process for synthesizing nanostructured metal oxide through microwave radiation. Microwave synthesis of NiO is optimized for detection of NO2 resulting in room temperature response all the way down to 200 ppb.
In sensor community the manufacturing cost of gas sensors which is a big concern has led to the exploration of new substrates to fabricate sensors. Motivated by this we developed a sensor array platform with integrated microheater on flexible and low-cost plastic substrate using CMOS compatible fabrication processes. The sensor array consisted of four sensors with individually controlled microheater deposited on nanogap created using electromigration process. Due to flexible nature of substrate, the bending angle dependent microheater characteristics and sensing performance show the potential of the sensor platform in low power wearable electronics. | en_US |
