Tuning the pH Response of Monolayer Hexagonal Boron Nitride/Graphene Field-Effect Transistors

Scaling the fabrication of 2D devices for pH sensing will have implications in materials and life sciences. Monolayer hexagonal boron nitride has recently reached commercial wafer-scale transferability on monolayer graphene and is hypothesized to preserve graphene quality, reducing device-to-device variation, while simultaneously screening charge density at the liquid-solid interface resulting in attenuation of pH sensitivity of the graphene transducer. The pH-dependencies of field-effect transistors derived from monolayer hexagonal boron nitride/graphene were compared to monolayer graphene on four-inch SiO2/p-type Si wafers. Photoresistless fabrication of the two-dimensional devices relied on shadow masking for metallization, and the sensing areas were defined using microcentrifuge tube masking and reactive ion etching to produce quasi-pure sensing areas. Microcentrifuge tubes sealed the devices and were opened for experimentation where the liquid-gated Dirac voltages were studied as a function of pH in 10 mM phosphate solutions. The sensitivity of the shift in the Dirac voltage versus pH of hexagonal boron nitride/graphene devices (-40 mV/pH) was smaller than bare graphene (-47 mV/pH) with greatest attenuation in the acidic regime. Moreover, triplicating this experiment revealed smaller standard deviations for the hexagonal boron nitride/graphene transistors. Then, electron beam and atomic layer deposition of AlxOy nanofilms were employed before encapsulation to study the tunability of the pH response of hexagonal boron nitride/graphene and revealed thickness-dependent enhancement, with the greatest sensitivity on 8.6 nm AlxOy/hBN/graphene/SiO2 (-100 mV/pH). Then, reversion of the pH response upon dissolving the AlxOy was characterized. In this work, nanoscale dielectrics enabled tuning of the electrical response of graphene-based pH sensors.