Inferring mechanical resonances in micro- and nanocantilevers using the harmonic detection of resonance (HDR) method to develop a novel sensing platform.

摘要

During the past two decades, advances in microelectromechanical systems (MEMS) have spurred efforts worldwide to develop sensing platforms based on smart microcantilevers. A microcantilever beam is one of the simplest MEMS structures which forms the basis for portable, fast and highly sensitive schemes that are capable of measuring small deflections in static or dynamic response due to changes in external parameters such as mass, pressure, charge, etc.;In this dissertation, I mainly focus on MEMS sensors with transducers in the form of microcantilevers. Variations in the microcantilever's response such as resonant frequency, amplitude, phase and quality factor when exposed to external stimuli are measured. Recently, we have developed a fully electrical sensing platform called the harmonic detection of resonance (HDR) method by which a silicon microcantilever (or a multiwalled carbon nanotube) can be electrically actuated and its resonance parameters electrically detected [4, 5] through capacitance changes. It is well known that a large interfering signal coming from the inherent parasitic capacitance in the circuit at the driving frequency O, is present in the platforms which use the capacitive readout method. However, we found that by driving the cantilever at O and detecting its response at higher harmonics of O, the parasitic capacitance can be avoided, facilitating the measurement of dynamic capacitance with high sensitivity in micro and nano-cantilevers [1, 2]. A significant part of this dissertation is devoted to the study of the nonlinear dynamics of microcantilevers under varying gas environments and pressures using HDR [3]. I also discuss the characteristics of an electrostatically driven microcantilever which exhibits Duffing-like behavior using HDR. The first experimental demonstration of its potential use as a highly sensitive sensing platform is discussed. [4]. We also discuss the behavior of an unfunctionalized microcantilever sensor which can be used for active sensing of gaseous species under ambient conditions. Our sensing platform measures the changes in the mechanical response (in amplitude and/or phase) of the vibrating microcantilever in air at its resonant frequency when exposed to several vapors and gases [5]. Finally I present the preliminary results on sensing toxic gases using functionalized microcantilevers.;In the final chapter, I present evidence for the fact that HDR method is scaleable and can be adapted for nanoscale cantilevers. In particular, I introduce the reader to bending modulus measurements of multiwalled carbon nanotubes performed in Prof. Rao's group. One of the key factors in these measurements is an accurate knowledge of density of carbon nanotubes. I provide in-depth discussion of the gradient sedimentation technique which enables one to measure the density of both single- and multi-walled carbon nanotubes.