Electric double layer gated field effect transistors (EDL FETs) are devices commonly used to investigate the fundamental properties of new materials, such as two-dimensional (2D) layered materials. Despite this usefulness and further potential for integration into devices and circuits, EDL FETs have the possibility of undergoing electrochemical reactions during device operation. Most often, this electrochemistry goes unmonitored. Part of the challenge of detecting electrochemical reactivity within EDL FETs resides in a knowledge gap between the device and analytical electrochemistry communities; that is, what type of reference electrode should be used, how should one be used, and what does it mean to monitor one?
This work addresses this issue by introducing a silver metal quasi-reference electrode in a graphene EDL FET with a solid polymer electrolyte- polyethylene oxide lithium perchlorate (PEO:LiClO4)- serving as the ion-conducting dielectric. The hypothesis was that Li+ ion intercalation in graphene would drive irreversible changes to device transfer characteristics and be detectable by reference electrode monitoring. The reference electrode was used in two experiments in which the gate window of an EDL FET device was either increased with each measurement (starting from -2.5 V ≤ VSG ≤ 2.5 V and expanding to -2.5 V ≤ VSG ≤ 10 V) or fixed (-2.5 V ≤ VSG ≤ 7 V). Changes to the transfer characteristics of the devices after each experiment- an increase in average drain current, Dirac points shifted negative vs. VSG in both forward and reverse transfer sweeps, an increased ON/OFF ratio, and higher-sloped side-gate current shifted negative vs. VSG- were noted as possible evidence of electrochemical reactivity within the graphene channels. However, thanks to the Ag/Ag+ reference electrode, the data from these experiments provide evidence that intercalation of Li+ in graphene was highly unlikely. Possible alternative explanations will be discussed, including reduction of water, PMMA, and graphene oxide defects
