Design, Fabrication And Geometric Optimization Of Graphene Electrodes For Electrochemical Detection
Graphene has gained much attention as a biosensing material since its discovery and characterization due to its highly sensitive electronic properties. Reported work on graphene as a biological sensor has focused on solution-gated graphene transistors (SGGFETs) that can measure the perturbed channel conductivity in response to environmental changes in the proximity of the graphene surface. Electrodes present a simpler method of biological detection, both from the operation and the fabrication standpoint. Investigation of graphene's electrochemical properties has reported higher electron transfer kinetics occurring at the edges than at the basal plane of the carbon allotrope. Yet, inconsistencies in sample preparation impede an accurate comparison of electrode performance. This thesis examines the fabrication and characterization of graphene microelectrode arrays made with a variety of graphitic materials that exhibit differences in the number of layers, domain size, defects and substrate. We examine, for the first time, the electrochemical properties of Van der Waals CVD graphene grown on sapphire substrates and electrode arrays made on epitaxial graphene grown on silicon carbide. We find no significant performance differences with mono-, bi- and multilayer graphene, but do observe microelectrode edge effects becoming more dominant in multilayer devices as they are scaled down. CVD graphene on sapphire, with domain sizes as small as 100-200 nm, show higher sensitivity and epitaxial electrodes display the lowest detection limit (1[MICRO SIGN]M) and fastest electron transfer kinetics, with the latter presumed to be effect of the high degree of corrugation in the material and consistent with reports that higher curvature leads to faster kinetics . To further examine the effect of the edges, we patterned electrodes of the same area varying only the perimeter. For clean electrodes, the perimeter to area ratio had little effect on the electrode sensitivity. However, after exposure to a low-power 30-second ozone plasma, the electrode sensitivity and electron kinetics improved, increasing by almost by two-fold with increasing electrode length. This result is consistent with the graphene edges becoming more electroactive through functionalization and result implies that graphene electrode sensitivity can be increased by functionalization and optimization of the electrode geometry.
graphene; electrochemistry; electrode arrays; epitaxial; saphire; optimization
Spencer, Michael G.
Batt, Carl A; Molnar, Alyosha Christopher
Ph.D. of Electrical Engineering
Doctor of Philosophy
dissertation or thesis