Mass detection of elastically distributed ultrathin layers using piezoresponse force microscopy

Abstract

Piezoresponse force microscopy (PFM) is utilized in this paper as a new technique for the identification of elastically distributed thin layers on top of microcantilever sensors. Using the conventional actuation methods such as base excitation, the ratio of stiffness over the layer mass per unit length affects the resonant frequencies of the system as a single parameter. However, due to the tip/sample elastic contact in PFM, these two parameters can be separately identified using the frequency shifts before and after attaching the layer. The concept is theoretically proven here using the modal analysis of the system. For practical verification, three gold-coated AFM microcantilevers, well suited for PFM application, were primarily tested for their initial resonant frequencies using a periodically poled lithium niobate (PPLN) piezoelectric sample. The focused ion beam (FIB) technique was then employed to deposit thin layers of Pt-based material in different configurations on the cantilevers' surfaces. The microcantilevers were then reexamined for their new resonances, and the properties of the deposits were identified using a robust system identification procedure. Results indicate acceptable estimation of the cantilevers' added mass and stiffness, making the technique suitable for detection of elastically distributed biological species. The framework proposed here can serve as an alternative approach for precise characterization of the solutions containing biomolecules (e.g., unknown DNA), their assemblies or small organisms.