Fluorine doping induced fluorescence emission redshift of CDs

Elemental doping is an effective way to modify the optical properties of carbon dots (CDs). In this work, the effects of fluorine-doped (F-doped) types and configurations on the structural stability and fluorescence properties of CDs were investigated based on density functional theory and time-dependent density functional theory. Circumcoronene was selected as an undoped structure, and a series of F-doped CD structures including edge and basal plane fluorine-doping (F-doping) types, and CF, CF2 and CF3 configurations were constructed. The calculation results show that the edge CF groups play a more positive role in the structural stability of CDs than the CF2 and basal plane CF groups. Different from the three configurations, the CF3 groups cannot contribute to the structural stability. For optical properties, the edge CF and CF3 groups have less effect on the electron distribution of CDs, hence only a slight fluorescence emission redshift occurs in two F-doping cases. In contrast, the CF2 and basal plane CF groups can evidently influence the highest occupied molecular orbital of the CDs, which induces the charge transfer transition and the fluorescence emission is redshifted to the near-infrared region. This work is helpful to predict the F-doped types and configurations in CDs, reveals the modulation mechanism of F-doping on the fluorescence properties and provides a theoretical guidance for the controlled synthesis of CDs in the future.