Polymer-Free 2D Heterostructure Transfer onto Quartz Crystal Microbalance (QCM) Electrode Surface: Method and Sensing Characteristics

There has been tremendous interest to employ 2-D materials as sensing medium in acoustic wave sensors such as quartz crystal microbalance (QCM). We report a method to transfer polymer-free 2-D materials onto QCM through a tailor-made automated process system and resultant enhancement in the sensing characteristics of QCM. A customized polytetrafluoroethylene (PTFE) structure was designed for the automated transfer of a 2-D heterostructure material made of hexagonal boron nitride (hBN)/graphene onto the QCM. The design ensured that the 2-D material adhered to the most sensitive region of the QCM, at the center of the electrode surface. The transferred 2-D material was characterized using Raman spectroscopy and scanning electron microscopy. Also, the $S_{11}$ characteristics of the QCM were measured before and after the transfer process using a network analyzer. We observed increment in return loss and loaded $Q$ -factor ( $Q_{L}$ ) in the presence of the 2-D material. In order to investigate the sensing characteristics of the 2-D material integrated QCM, relative humidity (RH) was taken as an example measurand, and experiments were carried out to study the adsorption and desorption of moisture. The 2-D material integrated QCM showed good sensitivity with excellent signal-to-noise ratio for RH variations as evidenced from the significant changes in return loss and conductance measurements. On the other hand, the bare QCM exhibited poor signal-to-noise ratio and inconsistent response to RH changes.