Probing viscoelasticity of nanometer thick self-assembled layers

In this paper, we will demonstrate the applications of micromachined bulk acoustic wave resonators for the systematic investigation of adsorbing films and nanomaterials on their surface. By careful design and development of glass etching technology, we have successfully micromachined quartz shear wave resonator arrays with frequencies in the range of 60 to 90 MHz (25-18 μm in thickness) with quality factors exceeding 30,000. The response of the resonators to various surface loads has been carefully modeled assuming a dynamically forming viscoelastic film under a fluidic overlayer. Making systematic measurements of the resonator frequency and quality factor response at the fundamental and third overtone and using nonlinear model fits, we have deduced the density, thickness, viscosity, and elasticity of various globular proteins adsorbing on hydrophobic, hydrophilic and charged hydrophilic surfaces. Furthermore, we will show that the proposed method is sensitive enough to study the copper underpotential deposition (UPD) and stripping in 1 mM CuSO₄ + 0.5 M H₂SO₄ solution. The results provide a comprehensive characterization of the viscoelastic properties of the electrical double layer in the copper UPD system through the use of high frequency QCM resonators and demonstrate a highly useful approach for achieving a deeper understanding of aqueous electrochemical systems.