Hybrid Shape Memory Alloy-Based Nanomechanical Resonators for Ultrathin Film Elastic Properties Determination and Heavy Mass Spectrometry.

Stachiv Stachiv,Gan Gan

doi:10.3390/ma12213593

Abstract

Micro-/nanomechanical resonators are often used in material science to measure the elastic properties of ultrathin films or mass spectrometry to estimate the mass of various chemical and biological molecules. Measurements with these sensors utilize changes in the resonant frequency of the resonator exposed to an investigated quantity. Their sensitivities are, therefore, determined by the resonant frequency. The higher resonant frequency and, correspondingly, higher quality factor (Q-factor) yield higher sensitivity. In solution, the resonant frequency (Q-factor) decreases causing a significant lowering of the achievable sensitivity. Hence, the nanomechanical resonator-based sensors mainly operate in a vacuum. Identification by nanomechanical resonator also requires an additional reference measurement on the identical unloaded resonator making experiments, due to limiting achievable accuracies in current nanofabrication processes, yet challenging. In addition, the mass spectrometry by nanomechanical resonator can be routinely performed for light analytes (i.e., analyte is modelled as a point particle). For heavy analytes such as bacteria clumps neglecting their stiffness result in a significant underestimation of determined mass values. In this work, we demonstrate the extraordinary capability of hybrid shape memory alloy (SMA)-based nanomechanical resonators to i) notably tune the resonant frequencies and improve Q-factor of the resonator immersed in fluid, ii) determine the Young’s (shear) modulus of prepared ultrathin film only from frequency response of the resonator with sputtered film, and iii) perform heavy analyte mass spectrometry by monitoring shift in frequency of just a single vibrational mode. The procedures required to estimate the Young’s (shear) modulus of ultrathin film and the heavy analyte mass from observed changes in the resonant frequency caused by a phase transformation in SMA are developed and, afterward, validated using numerical simulations. The present results demonstrate the outstanding potential and capability of high frequency operating hybrid SMA-based nanomechanical resonators in sensing applications that can be rarely achieved by current nanomechanical resonator-based sensors.

Highlights

Because of their small mass, high operating frequencies and the extraordinary sensitivity to external stimuli, micro-/nanomechanical resonators are used in various sensing devices including infrared thermal [1], fluid viscosity [2,3], force [4,5] and mass sensors [6,7,8,9,10]
We demonstrate the extraordinary capability of hybrid shape memory alloy (SMA)-based nanomechanical resonators to i) notably tune the resonant frequencies and improve Q-factor of the resonator immersed in fluid, ii) determine the Young’s modulus of prepared ultrathin film only from frequency response of the resonator with sputtered film, and iii) perform heavy analyte mass spectrometry by monitoring shift in frequency of just a single vibrational mode
2 Ei Vi (n) where ∆ fF_i = fF _i − fF_i, fF_i and fF _i are the loaded by analyte and unloaded resonant frequencies of the n-th flexural vibrational mode of the SMA nanomechanical resonator, subscript i stands for the resonator with SMA film at a given particular value of transformation temperature, madd, Eadd and V add are the analyte mass, stiffness and volume, x0 is the analyte mass position of attachment, V is the resonator volume, κ(n) is the normalized mode shape curvature and θ is the dimensionless parameter depending on the analyte shape and contact area [11]

Summary

Introduction

Because of their small mass, high operating frequencies and the extraordinary sensitivity to external stimuli, micro-/nanomechanical resonators are used in various sensing devices including infrared thermal [1], fluid viscosity [2,3], force [4,5] and mass sensors [6,7,8,9,10]. Hybrid SMA-based nanomechanical resonators made of an elastic substrate and NiTi in form of thin film were proposed [29,30] These hybrid resonators were proven to operate at high resonant frequency ranges (i.e., up to tens of MHz) that can be significantly tuned up and down making them suitable for applications, such as radio frequency filters, infrared and temperature sensors. We show that variable effective elasticity of hybrid SMA-based nanomechanical resonators enable determination of the Young’s and shear moduli of prepared ultrathin film from only measured changes in the resonant frequency of resonator with sputtered film, and the heavy analyte mass from detected frequency shift of just a single vibrational mode. Procedures of the ultrathin film elastic (shear) modulus and analyte mass determinations by means of hybrid SMA-based nanomechanical resonators are developed and their validity are reinforced using numerical simulations

Theory

Results and Discussion

Conclusions