Title: Physics of Sensing for Graphene Solution Gated Field Effect Transistors

Author: Mauricio Bedoya

Date: July 6th at 10:00am

Location: MS&E 3201A

Thesis Advisor: Dr. Jennifer Curtis

Abstract:

Graphene is a promising material for chemical sensing applications. It has the largest possible surface area per volume which maximizes the sensitivity of its physical properties to environmental effects, it is biocompatible, and it is possible to use standard fabrication techniques to make devices in a commercial scale. In this work epitaxial graphene (EG) and graphene produced by chemical vapor deposition (CVD) are used to fabricate solution-gated field effect transistors (SGFETs) that can be used to measure the concentration of chemical species and charged biomolecules in liquid solutions.

Although many studies have focused on incorporating graphene into sensors, in particular into SGFETs, deeper understanding of how graphene conductivity responds to ionic solutions and charged molecules is necessary in order to engineer an optimized sensor. The purpose of this work is to clarify the physics governing the surface interaction of graphene in SGFETs with ions and charged molecules. With a clearer understanding of how these interactions register in the conductivity of graphene, it then may be possible to design the ultrasensitive sensors that are so often predicted to be possible when using graphene.

To achieve these goals, first we performed a detailed characterization of the performance of graphene SGFETs including reproducibility, reversibility, and device-to-device consistency.  The loss of performance when the leakage current is high was also investigated. To get a clearer picture of the electrostatic gating effect in ionic solutions, we analyzed our data combining two models: the electrical double layer model, which accounts for the distribution of ions inside the solution, and a chemical ionization model that accounts for ionizable groups on the graphene surface. The simultaneous solution of the two models gives us quantitative information about the surface charge ionization under different salt concentrations. These calculations provide us an insight into the influence of ions -both freely diffusing ions in the liquid and charged groups fixed to the surface- on the gating effect which is fundamental to the performance of SGFETs as sensors.

The relationship between the charge due to ionizable groups on the graphene's surface that can play a role as impurities and other charged impurities in graphene FETs is still not fully understood. There are two regimes in the conductivity of graphene in which charged impurities were considered. For high carrier densities, we estimated the surface impurity concentration corresponding to several ionic concentrations using our experimental data in that regime. We found that the ionizable impurities have an effect on the conductivity. For small carrier densities, we modeled the carriers for the first time using a self-consistent approximation (SCA) that also includes charged impurities. From our experimental data, we estimated the values for these impurities in the low carrier density regime. The analysis of the conductance data in the two regimes of carrier density provided a first systematic insight into the roles of the different impurities in electrical conductivity and sensing.

We also performed experiments of protein adsorption to graphene to study the role of these charged molecules as dopants and to gain fundamental understanding of their interaction with graphene in order to optimize sensing with graphene SGFETs. It is apparent from our studies that the ionic environment and its interaction with the ionizable groups at the surface have an effect on the conductivity that needs to be accounted for the design of SGFET sensors. The theoretical models applied shed a light on this effect and a quantitative way to understand them. The experiments and analysis performed in this work give a guide for the development of EG and CVD SGFETs sensors.

Committee Members:

Professor Jennifer Curtis, Advisor
School of Physics
Georgia Institute of Technology

Professor Hang Lu
School of Chemical & Biomolecular Engineering
Georgia Institute of Technology

Professor Elisa Riedo
School of Physics
Georgia Institute of Technology

Professor Victor Breedveld
School of Chemical & Biomolecular Engineering
Georgia Institute of Technology

Professor Phillip First
School of Physics
Georgia Institute of Technology