Fabrication, Characterization and Applications of Nanomechanical Resonators

Abstract

Nanomechanics is a branch of nanotechnology where fundamental mechanical properties of nanomaterials are studied, for example nanomechanical resonators, tiny vibrating devices. These mechanical resonators are of interest and have a large number of potential sensing applications. In particular nanomechanical strings recently made attention for their high quality factor ($ Q $) and simple sinusoidal mode shapes. These high-$ Q $ resonators show great promise in sensing applications and the simple mode shapes make it easy to characterize the device. Here work was performed to make one dimensional hybridized arrays of high-$ Q $ nanostrings and to show the sensing applications of different nanostrings. To accomplish this, a standard fabrication process is established to make nanostrings from high stress silicon nitride. These resonators provide high-$ Q $ and show string like behavior because of the intrinsic high stress of the material. The dissipation mechanism of a high stress silicon nitride nanostring is studied to find the loss channel of the device. This provides a new way to engineer the anchor points of the nanostring to increase the $ Q $ of the device. Then one dimensional arrays are fabricated from high-$ Q $ strings that are joined end-to-end by tiny post in between them. These arrays show strong coupling between individual connecting nanostrings and hence provide remote sensing opportunity, $ i.e.$ measurement performed on only one string of the array and it can acquire full information of the array. It is a state of the art device and technique in sensing applications. To prove the concept of sensing applications of the nanostrings, explosive molecules called RDX are detected by a single high stress silicon nitride nanostring on the order of femtogram by IR absorption spectroscopy (which is the lowest amount to date by this method). In addition, a multimode analysis is developed to show better accuracy in mass sensing applications. However these devices are limited in chemical sensing applications due to the inert nature of the silicon nitride material. Therefore, a metalic layer is added to the bare silicon nitride nanostring to activate this for chemical sensing applications by a particular thiol intermediary technique. It is shown that the addition of the metallic layer does not change the quality factor of the device adversely for the fundamental mode. These devices are functionlized by thiol species to have self-assembled monolayers (SAMs). SAMs provide the opportunity to adsorb specific molecules. Here it is shown that a functionalized gold coated string, by a particular thiol species, can be used to capture acetone. After showing the potential applications of individual strings, I propose an experimental procedure for future works of my thesis, to use hybridized arrays in real sensing applications.