Localized states in carbon dots: Structural and optical investigation of three systems with varying degrees of carbonization

Abstract

Carbon dots (CDs) have been widely researched in recent years, mainly to investigate their potential in various applications such as drug delivery, photocatalysis, and sensing. However, understanding of their fundamental properties, including physical and electronic structure, has lagged behind their development in applied sciences. To address this, it is necessary to use novel methods which go beyond the current level of characterization in the literature. In this work, we utilize time of flight-secondary ion mass spectrometry (ToF-SIMS) to generate specific knowledge of the structural components of three CDs generated in our lab. This work revealed that black CDs (B-CDs) possess a highly carbonic structure with nitrogen and oxygen functionalization throughout the particle structure. Carbon nitride dots (CNDs) possess some of these same carbonic structures, but also show the presence of more organic structures which would be expected through a bottom-up approach. In terms of carbonization, CNDs lie between B-CDs and the third sample, yellow-CDs (Y-CDs). Y-CDs are believed to be almost completely polymeric/organic in structure and the groups detected through this mass analysis supports this idea. The structural information from ToF-SIMS is compared with other structural techniques. Additionally, the optical properties of CDs before and after oxidation and reduction are used to craft a proposed photoluminescence (PL) mechanism for each system. The analysis contained herein enables further understanding of the structure of these three samples, and the attained understanding of the surface structure is particularly important for future biomedical applications.