Low Drift Reference-less ISFET Comprising Two Graphene Films with Different Engineered Sensitivities

We have engineered pH sensitivity of graphene-based ion-sensitive field-effect transistors (ISFETs) by applying plasma treatment on graphene with two different plasma sources, CF4 and O2 plasma. The CF4 plasma treatment, leaving F-terminated sites on graphene, results in a lower sensitivity of 23 mV/pH, while the O2 plasma treatment, leaving O-terminated sites, results in a higher sensitivity of 40 mV/pH. The validity of the aforementioned plasma treatments on graphene was confirmed by Raman spectra and X-ray photoelectron spectroscopy (XPS). Both plasma-treated graphene samples exhibit good stability with low drift and a small hysteresis width in measuring the pH of the solution, resulting from the two-dimensional (2D) feature of graphene. Finally, a reference-less ISFET that comprises two graphene films with different engineered sensitivities was demonstrated. Both engineered graphene films are directly exposed to the electrolyte solution, with the CF4-treated graphene replacing the reference electrode and the O2-treated graphene as the sensing layer. As surface potentials of the two films are opposite in sign, this device exhibits an overall sensitivity as the difference in sensitivities (i.e., 14.5 mV/pH) of the two individual films. More importantly, the long-term drift behaviors of the two films also cancel each other and the drift rate reaches as low as 0.38 mV/h for this reference-less device. This approach brings opportunities for future high-performance 2D-based ISFETs with extremely low drift and high stability.