Date of Award

Fall 11-17-2018

Document Type


Degree Name

Doctor of Philosophy (PhD)


Micro and Nanoscale Systems

First Advisor

Dr. Adarsh D. Radadia


The electrical characterization on two-dimensional carbon-based graphene and nanodiamond materials was performed to improve charge transport properties for the label-free electrical biosensors. The charge transport in solution-gated graphene devices is affected by the impurities and disorders of the underlying dielectric interface and its interaction with the electrolytes. Advancement in field-effect ion sensing by introducing a dielectric isomorph, hexagonal boron nitride between graphene and silicon dioxide of a solution-gated graphene field-effect transistor was investigated. Increased transconductance due to increased charge carrier mobility is accompanied with larger ionic sensitivity. These findings define a standard to construct future graphene devices for biosensing and bioelectronics applications.

Furthermore, we demonstrated selectivity for sensing several ions using ionophoretic membranes over the graphene channel. Selectivity is obtained from the shift in the Dirac voltage (VDirac) and the transconductance (gm) with varying the electrolyte concentration. We demonstrated graphene ion-sensitive field-effect-transistors with more than 99% repeatability and over 98% reproducibility over 60 days testing period. Using hexagonal boron nitride as an underlying layer for graphene transistors, the sensitivity of the desired ions enhanced significantly while the sensitivity for the undesired ions remained unchanged. Subsequently, we reported frequency domain sensing of K+ and Ca2+ ions using a solution-gated graphene field-effect transistor. The sensitivity at 2nd and 3rd harmonics was found to be higher for Ca2+ than K+, or Na+ due to the higher ionic charges. By introducing hexagonal boron nitride as a graphene support-substrate, the sensitivity was increased as well as the device-to-device reproducibility.

Next, we reported our findings on preparing a graphene oxide-based gas sensor for sensing fast pulses of volatile organic compounds with a better signal-to-noise ratio. We found that the dielectrophoresis yielded films were uniform in terms of graphene oxide coverage with better sensor responses compared to the solvent evaporated film properties. Contrary to prior reports, we found that if we sonicated the sensor in acetone, we created a sensor with a few flakes of reduced graphene oxide with higher signal-to-noise ratio. Modeling showed the sensor’s response was due to the one-site Langmuir adsorption or an overall single exponent adsorption process. Impedance spectroscopy of the pure water was carried out with the interdigitated electrodes to reveal the role of the detonated nanodiamonds (DNDs). Results show that the DNDs (having a positive zeta potential) at the IDEs reduce the geometrical resistance to less than half of its initial value, and the time constant reduced for the HN relaxation to less than one-fourth of its initial value. These changes propose doubled diffusion coefficient and mobility of the charged species, and reduced dielectric relaxation time constant. It is hypothesized that the DNDs, which have a positive zeta potential, when seeded on gold and oxide surfaces with a negative zeta potential, reduce the electrostatic force acting on the diffuse layer ions, and increase their mobility.

We believe our findings in two-dimensional nanodiamonds and graphene-based materials’ electrical characterization will add values in label-free electrical biosensing in pathological diagnostic applications.