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Carbon nanotube isolation layer enhancing in-liquid quality-factors of thin film bulk acoustic wave resonators for gravimetric sensing

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Rughoobur, Girish 
Sugime, Hisashi 
DeMiguel-Ramos, Mario 
Mirea, Teona 
Zheng, Shan 


A thickness longitudinal mode (TLM) thin film bulk acoustic resonator biosensor is demonstrated to operate in water with a high quality-factor, Q. This is achieved using a layer of carbon nanotubes (CNTs) on top of the resonator which has a significantly different acoustic impedance to either the resonator or liquid whilst being susceptible to the binding of biological molecules. This allows the resonance to be decoupled from direct energy loss into the liquid, although still retaining mass sensitivity. AlN solidly mounted resonators (SMRs) having a thickness shear mode (TSM) at 1.1 GHz and TLM at 1.9 GHz are fabricated. CNTs with different forest densities are grown by chemical vapor deposition on the active area with Fe as catalyst and resulting devices compared. High forest density CNTs are shown to acoustically decouple the SMRs from the water and in-liquid TLM Q values higher than 150 are recorded even exceeding TSM SMRs without CNTs. The TLM Q in water is remarkably improved from 3 to 160 for the first time by dense CNT forests, rendering the large-scale fabrication of TLM SMRs for liquid-phase sensing applications possible. Despite this partial isolation, SMRs with CNT forests ~15 μm tall can still detect binding of bovine serum albumin.



Carbon nanotubes, Bulk wave resonators, Gravimetric sensing, Acoustic wave decoupling, In-liquid sensors

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Sensors and Actuators, B: Chemical

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European Commission Horizon 2020 (H2020) Industrial Leadership (IL) (636820)
This work was supported by the European Community's Horizon 2020 Programme [grant number SPIRE-01-2014-636820 (RECOBA)]; and the Ministerio de Economía y Competitividad del Gobierno de España [grant number MAT2013-45957-R]. G.R. and S.Z. also wish to acknowledge funding from the Cambridge Commonwealth, European and International Trust.