Nanoparticle engineering and control of microhotplate gas sensor performance

Tin Oxide (SnO₂) based microhotplate gas sensors developed at the National Institute of Standards and Technology (NIST) are considered as potential candidates for “electronic nose” applications. Three performance parameters that govern their use in electronic noses are sensitivity, selectivity and long term stability in different analytes. This dissertation reports on experimental work using MEMS-based microhotplate arrays as material development platforms to explore techniques for enhancing and controlling these three performance parameters. Metallic nanoparticles are explored as ultra thin seed layers to control SnO₂ growth, microstructure, and improve sensitivity, stability, and selectivity.; The experimental work consists of (1) evaporative deposition of nanoparticles seed layers (Fe, Ni, Co, Ag, & Cu) onto the microhotplates, (2) self-aligned SnO₂ deposition in a cold wall CVD reactor, (3) microstructure characterization using SEM, and (4) testing in different hydrocarbon gases to monitor sensitivity, selectivity an stability.; In general, nanoparticle seeding with high temperature metals (Fe, Co, & Ni) resulted in faster SnO₂ growth, smaller SnO₂ grain size and higher sensitivity for different analytes. The rapid thermal capability of microhotplates was used to facilitate monitor temperature programmed selectivity (TPS) characterization for the different analytes. Large-scale arrays of microhotplates were employed to monitor materials programmed selectivity (MPS). Limited TPS and enhanced MPS were observed for the nanoparticle seeded microhotplate gas sensors. The merits and de-merits; of MPS and TPS for effective gas identification are discussed.; The stability of the nanoparticle seeded SnO₂ sensors was monitored continuously for more than 70 hours. The enhanced sensitivity found for the nickel seeded SnO₂ sensors together with very good stability for over 70 hours makes it a good choice for hydrocarbon sensing applications. Recommendations are placed for sensors developed in this thesis and future work that can improve the current status of microhotplate technology are discussed.