Luminescence mechanism of colloidal graphene quantum dots and its correlation with conjugated oxygen-functional groups

Abstract

Graphene quantum dot is defined as a graphene sheet with lateral dimension less than 100 nm and with few layers. The graphene quantum dots have a lot of advantages, including not only an eco-friendly property, a low toxicity, a good solubility, and a high biocompatibility for various bio-appications, but also a broad range of optical absorptivity and a stable photoluminescence without blinking for optoelectronic devices. However, the GQDs have been leaving many questions unanswered. Therefore, it is very important to reveal a comprehensive luminescence origin and mechanisms in the GQDs to improve quantum efficiency and design a new strucutral GQD for applications. In this thesis, we have studied the luminescence mechanisms and origins in the GQDs by both ways of experimental approaches through various optical measurements and theoretical approaches through density functional theory. As a result, we suggested that the GQDs mostly have two luminescence origins, which called as an ‘extrinsic state’ formed by oxygen-functional groups and an ‘intrinsic state’ based on a crystalline structure of graphene with nanoscale. Our research is to verify the luminescence mechanisms in the GQDs through the various optical characterizations such as PL, PL excitation, and time-resolved PL, the theoretical calculation of structural model, and the design of GQDs structure for selectively controlling of oxygen-functional groups. Our results can lead to future development of novel strategies for guiding the synthesis of the GQDs and engineering the properties of GQDs toward various applications such as bioimaging, sensor, biological labeling, and functional nanoscale devices.