Very High Frequency Nano-electro-mechanical Resonators and Their Chemical Sensing Applications

Nanoelectromechanical systems (NEMS) have been proven to be ultrasensitive sensors for a variety of physical variables with unprecedented sensitivity, including force, mass, electrical charge, magnetic field, pressure, and heat. This thesis is intended to discuss using NEMS devices as chemical gas sensors, in a portable and compact total chemical analysis system. An integrated transduction method using piezoresistive metallic thin film is described, which enables both fabrication and operation of nanoscale NEMS resonator devices with resonance frequency up to very high frequency (VHF). The advantages over using traditional doped semiconductor film as piezoresistive material is discussed. Performance and noise properties of the devices are carefully characterized. The dependence between quality factor, device dimension, and pressure is studied, and very high quality factor is obtained with devices at nanoscale dimensions, indicating advantages over their microscale counterparts. Subsequently, the resonator devices are employed as a mass sensor, demonstrating attogram scale mass sensitivity in ambient conditions. Application of these devices as detectors in a gas chromatographic (GC) system is then described, together with method of coating them with functional polymeric film. Detection of multiple analytes of nerve gas simulants with ultrahigh speed, superior sensitivity, and excellent selectivity is achieved. The replacement of conventional bulky detectors with an NEMS detector makes fully integrated microscale gas analysis system possible, which has promising potential applications in health care, medical science, and environmental science.