Molecular interaction in graphene and its influence in sensing applications

Abstract

Graphene is suitable for use as a high-performance sensor material due to its unique atomically-thin structure and excellent electrical properties. Until recently, graphene has been actively studied in gas, solution and biosensors. In the early stages, the sensing characteristics of a graphene sensor vary from case to case and some devices exhibit performance beyond the theoretical limit. However, the electrostatic gating effect at that time, the sensing mechanism, could not explain this. To solve this, a hypothesis called chemical doping has been added. In chemical doping, ions/molecules are adsorbed through the defects and surface of graphene, resulting in drastic changes in the electrical properties. By introducing chemical doping, the results of different performances can be explained by different defect densities, and the performance has been improved with defect patterning. However, understanding of chemical doping of the graphene sensor is still insufficient and performance deterioration is possible. In this dissertation, I focus on ion/molecule interactions in graphene defects or surface and investigate the effect on graphene device transfer characteristics. A fast but irreversible response exhibited when a engineered defect was introduced to the graphene solution sensor with a bottom-up graphene mesh growth process. This phenomenon has been shown to be due to engineered graphene defects and strong adsorption reactions between ions, and defect control can be a significant issue because irreversible responses can affect the reliability of sensor performance. One way to control an irreversible response is to introduce a receptor-assisted structure that greatly inhibits the irreversible response through a reversible response through the receptor. In particular, the structure in which the receptor is directly bonded to the graphene defects greatly improves the sensing performances because the graphene defect plays a role in improving charge transfer between the receptor and the graphene. The electrical double layer (EDL) formed spontaneously at the interface between the graphene and the solution has a structure in which ions of opposite charges are arranged in two parallel layers. In this experiments, the EDL structure at the graphene interface was more affected by underlying substrate than the graphene. On the other hand, it was confirmed that the EDL serves as a dielectric layer of the graphene sensor, and the EDL controlled by the substrate influences the charge transport characteristics of the graphene through the changed dielectric properties. Ion/molecular interaction through graphene defects and interfaces show not only improved device performance, but also a strong relationship with irreversible reaction, deterioration and device breakdown. However, through such basic research, it is expected that the cause of degradation occurring in conventional graphene device can be found and suppressed, and further, it can be utilized for improving the device performances.