Detection Of Heavy Metal Ions Based On Silicon-based Nanobeam Resonance Technology

NEMS?Nanoelectromechanical system?is a system with feature sizes on nanoscale.It has many new properties,such as ultrahigh frequency,high quality factor,low energy consumption and small size.Therefore,NEMS devices have been found in varieties of important applications,including ultrasensitive mass sensing and ultra-low power radio frequency signal generation.The analysis and detection of heavy metal ions in waters have attracted extensive attention from researchers in the world.Current detection methods have some problems.For example,some methods are not able to accomplish in-situ measurement,some are with low sensitivities.Nanobeams/nanowires are typical structures of NEMS.Trace detection of heavy metal ions in water environment based on the frequency shift of the vibration of silicon nanobeams is a new sensing method with ultra-high mass sensitivity and ultra-small sensing device.The main work of this article is as follows:The frequency shift of the vibration of silicon nanobeams is deduced and analyzed.Methods of actuation and detection of vibration frequency of NEMS resonators are investigated and therefore electrostatic actuation and piezoresistive detection are chosen when silicon nanobeam resonators are designed.The doubly clamped nanobeam,cantilever beam,U-beam and H-beam structure are designed.ANSYS software is used to simulate the vibration mode and resonant frequency to determine the theoretical value of the resonant frequency of the nanobeam.Vibration of doubly clamped silicon nanobeams in[111],[110]and[112]orientation with rectangular cross-sections are studied by the molecular dynamics method.Results indicate that there is big deviation of resonant frequency between simulation results and calculation results based on classical continuum theory in every orientation.The first-order resonant frequency changes with its size similar to the macroscopic law.Completely reverse rank of resonant frequencies in three directions between simulation and calculation is also obtained,which is attributed to the different surface to volume ratio of three orientations of silicon nanobeam and thus discrepant surface effects.Modification materials of silicon-based nanobeam for adsorbing heavy metal lead ions are prepared.Because thiol-functionalized mesoporous silica nanoparticles have good adsorbability with with Pb²⁺and silicon material,they were prepared and characterized by TEM,SEM and EDS.Results show that the nanoparticles modified on the silicon-gold composite sheet are dense,well-distributed,uniform in size?30nm in diameter approximately?,and capable of adsorbing a sufficient amount of Pb²⁺.Layouts and the process flow of four kinds of the silicon-based nano-resonant beam are designed and key processes are discussed and analyzed carefully.Nanobeams are fabricated in Suzhou Nano Institute of the Chinese Academy of Sciences.This thesis also discusses various problems in the process,such as the problem that the aluminum gold electrode is etched by side etching due to unprotected coating,which leads to side-turning,and the etching of buried oxide layer SiO₂ in the release process is incomplete and realize nanobeams finally.Polytec laser Doppler vibrometer is used to measure the vibration of doubly clamped nanobeams and results show that the first-order resonance frequencies are lying between 0.736MHz and 1.608 MHz.