PHYSICS PhD DISSERTATION DEFENSE: John Bartel

Ph.D. Candidate:  John Bartel

Research Advisor:  Prof. David Goldhaber-Gordon

Date: Thursday, October 18th 2018
Time: 3:30 PM
Location: Allen 101X

Title: Tuning the Surface Properties of Two-Dimensional Materials with a Solid Electrolyte Back Gate

Abstract:
 
Electrostatic gates are widely used to modify materials properties for electronics and for fundamental research. For example, the ability to tune carrier density in a semiconductor has enabled new understanding of electron transport as well as functional technologies. However, traditional gates are limited by dielectric breakdown, and the highest density modulations have only been achieved by gating with the mobile ions of an electrolyte. In two-dimensional materials such as graphene, surface properties like the work function can also be tuned with a gate. These materials are therefore well suited for applications like efficient thermionic energy converters, which require low work function anodes. However,  the scanning probe techniques used to study such surface properties are often incompatible with conventional liquid electrolyte top gates. Thus, a device architecture that gates from the back is necessary to enable scanning probe studies of electrolyte gate tunable surface properties.
 
In this presentation, I will discuss our work to realize an electrolyte back gate by transferring CVD graphene onto a lithium-ion-conducting glass-ceramic (LICGC) substrate. In order to understand the gating process, we conducted electrochemical impedance spectroscopy measurements, and established voltage-dependent bounds on the double layer capacitance. We then used an in-situ combination of scanning Kelvin probe force microscopy, cyclic voltammetry, and two-terminal resistance measurements to characterize how the work function of graphene changes with gating. We performed these measurements both in air and in an argon glovebox. Additionally, we correlated our results with changes in the Raman spectra of measured samples, suggesting an electrochemical doping mechanism. Our data suggest that this device architecture is capable of inducing tunable contact potential changes on the order of 1 V, corresponding to carrier density changes on the order of 1 x 1014 cm-2, but this seems to depend on factors such as back-gate contact material, initial doping of the graphene, and the details of the electrochemical reactions that can occur.